Many neurological disorders, including neurodevelopmental disorders, report hypersynchrony of neu... more Many neurological disorders, including neurodevelopmental disorders, report hypersynchrony of neuronal networks. These alterations in neuronal synchronization suggest a link to the function of inhibitory interneurons. In Fragile X Syndrome (FXS), it has been reported that altered synchronization may underlie hyperexcitability, cognitive dysfunction and provide a link to the increased incidence of epileptic seizures. Therefore, understanding the roles of inhibitory interneurons and how they control neuronal networks is of great importance in studying neurodevelopmental disorders such as FXS. Here, we present a review of how interneuron populations and inhibition are important contributors to the loss of excitatory/inhibitory balance seen in hypersynchronous and hyperexcitable networks from neurodevelopmental disorders, and specifically in FXS.
Proceedings of the National Academy of Sciences of the United States of America, Jan 23, 2014
Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular s... more Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular signals that generate normal cortical networks remain poorly understood. Semaphorin 7A (Sema7A) is an atypical member of the semaphorin family that is GPI-linked, expressed principally postnatally, and enriched in sensory cortex. Significantly, SEMA7A is deleted in individuals with 15q24 microdeletion syndrome, characterized by developmental delay, autism, and sensory perceptual deficits. We studied the role that Sema7A plays in establishing functional cortical circuitry in mouse somatosensory barrel cortex. We found that Sema7A is expressed in spiny stellate cells and GABAergic interneurons and that its absence disrupts barrel cytoarchitecture, reduces asymmetrical orientation of spiny stellate cell dendrites, and functionally impairs thalamocortically evoked synaptic responses, with reduced feed-forward GABAergic inhibition. These data identify Sema7A as a regulator of thalamocortical a...
In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane... more In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane expression, composition and function of GABAA receptors (GABAARs). For this, the tissue of chronically epileptic rats was collected within 3h of seizure occurrence (≤3h group) or at least 24h after seizure occurrence (≥24h group). A retrospective analysis of seizure frequency revealed that selecting animals on the bases of seizure proximity also grouped animals in terms of overall seizure burden with a higher seizure burden observed in the ≤3h group. A biochemical analysis showed that although animals with more frequent/recent seizures (≤3h group) had similar levels of GABAAR at the plasma membrane they showed deficits in inhibitory neurotransmission. By contrast, the tissue obtained from animals experiencing infrequent seizures (≥24h group) had increased plasma membrane levels of GABAAR and showed no deficit in inhibitory function. Together, our findings offer an initial insight into t...
The dysfunction of parvalbumin-positive, fast-spiking interneurons (FSI) is considered a primary ... more The dysfunction of parvalbumin-positive, fast-spiking interneurons (FSI) is considered a primary contributor to the pathophysiology of schizophrenia (SZ), but deficits in FSI physiology have not been explicitly characterized. We show for the first time, that a widely-employed model of schizophrenia minimizes first spike latency and increases GluN2B-mediated current in neocortical FSIs. The reduction in FSI first-spike latency coincides with reduced expression of the Kv1.1 potassium channel subunit which provides a biophysical explanation for the abnormal spiking behavior. Similarly, the increase in NMDA current coincides with enhanced expression of the GluN2B NMDA receptor subunit, specifically in FSIs. In this study mice were treated with the NMDA receptor antagonist, MK-801, during the first week of life. During adolescence, we detected reduced spike latency and increased GluN2B-mediated NMDA current in FSIs, which suggests transient disruption of NMDA signaling during neonatal de...
The cellular and molecular mechanisms that govern the response of the perinatal brain to injury r... more The cellular and molecular mechanisms that govern the response of the perinatal brain to injury remain largely unexplored. We investigated the role of white matter astrocytes in a rodent model of diffuse white matter injury produced by exposing neonatal mice to chronic hypoxia -a paradigm which mimics brain injury in premature infants. We demonstrate absence of reactive gliosis in the immature white matter following chronic hypoxia, as determined by astrocyte proliferation index and glial fibrillary acidic protein levels. Instead, Nestin expression in astrocytes is transiently increased, and the glial-specific glutamate transporters GLAST and GLT-1 are reduced. Finally, we demonstrate that JAK/STAT signaling -which is important in both astrocyte development and response to injury -is reduced in the white matter following hypoxia, as well as in primary astrocytes exposed to hypoxia in vitro. Hypoxia and JAK/STAT inhibition reduce glutamate transporter expression in astrocytes, but unlike hypoxia JAK/STAT inhibition downregulates GLAST expression without affecting GLT-1, as demonstrated in vitro by treatment with JAK Inhibitor I and in vivo by treatment with the JAK/STAT inhibitor AG490. Our findings: i) demonstrate specific changes in astrocyte function after perinatal hypoxia, which might contribute to the particular pathogenesis of perinatal white matter injury, ii) provide evidence that at least part of these changes result from a disturbance of the JAK/STAT pathway by hypoxia, and iii) identify JAK/STAT signaling as a potential therapeutic target to restore normal GLAST expression and uptake of glutamate after perinatal brain injury.
Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by variable cognitive imp... more Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by variable cognitive impairment and behavioral disturbances such as exaggerated fear, anxiety and gaze avoidance. Consistent with this, findings from human brain imaging studies suggest dysfunction of the amygdala. Underlying alterations in amygdala synaptic function in the Fmr1 knock-out (KO) mouse model of FXS, however, remain largely unexplored. Utilizing a combination of approaches, we uncover profound alterations in inhibitory neurotransmission in the amygdala of Fmr1 KO mice. We demonstrate a dramatic reduction in the frequency and amplitude of phasic IPSCs, tonic inhibitory currents, as well as in the number of inhibitory synapses in Fmr1 KO mice. Furthermore, we observe significant alterations in GABA availability, both intracellularly and at the synaptic cleft. Together, these findings identify abnormalities in basal and action potential-dependent inhibitory neurotransmission. Additionally, we reveal a significant neuronal hyperexcitability in principal neurons of the amygdala in Fmr1 KO mice, which is strikingly rescued by pharmacological augmentation of tonic inhibitory tone using the GABA agonist gaboxadol (THIP). Thus, our study reveals relevant inhibitory synaptic abnormalities in the amygdala in the Fmr1 KO brain and supports the notion that pharmacological approaches targeting the GABAergic system may be a viable therapeutic approach toward correcting amygdala-based symptoms in FXS.
Neuronal rhythmic activities within thalamocortical circuits range from partially synchronous osc... more Neuronal rhythmic activities within thalamocortical circuits range from partially synchronous oscillations during normal sleep to hypersynchrony associated with absence epilepsy. It has been proposed that recurrent inhibition within the thalamic reticular nucleus serves to reduce synchrony and thus prevents seizures. Inhibition and synchrony in slices from mice devoid of the ␥-aminobutyric acid type-A (GABA A ) receptor  3 subunit were examined, because in rodent thalamus,  3 is largely restricted to reticular nucleus. In  3 knockout mice, GABA A -mediated inhibition was nearly abolished in reticular nucleus, but was unaffected in relay cells. In addition, oscillatory synchrony was dramatically intensified. Thus, recurrent inhibitory connections within reticular nucleus act as "desynchronizers."
Proceedings of the National Academy of Sciences, 1998
The relative abundance of alternatively spliced long (gamma2L) and short (gamma2S) mRNAs of the g... more The relative abundance of alternatively spliced long (gamma2L) and short (gamma2S) mRNAs of the gamma2 subunit of the gamma-amino butyrate type A (GABAA) receptor was examined in dorsolateral prefrontal cortex of schizophrenics and matched controls by using in situ hybridization histochemistry and semiquantitative reverse transcription-PCR (RT-PCR) amplification. A cRNA probe identifying both mRNAs showed that the transcripts are normally expressed at moderately high levels in the prefrontal cortex. Consistent with previous studies, overall levels of gamma2 transcripts in prefrontal cortex of brains from schizophrenics were reduced by 28.0%, although this reduction did not reach statistical significance. RT-PCR, performed under nonsaturating conditions on total RNA from the same blocks of tissue used for in situ hybridization histochemistry, revealed a marked reduction in the relative proportion of gamma2S transcripts in schizophrenic brains compared with controls. In schizophrenics, gamma2S transcripts had fallen to 51.7% (+/-7.9% SE; P < 0.0001) relative to control levels. Levels of gamma2L transcripts showed only a small and nonsignificant reduction of 16. 9% (+/-12.0% SE, P > 0.05). These findings indicate differential transcriptional regulation of two functionally distinct isoforms of one of the major GABAA receptor subunits in the prefrontal cortex of schizophrenics. The specific reduction in relative abundance of gamma2S mRNAs and the associated relative increase in gamma2L mRNAs should result in functionally less active GABAA receptors and have severe consequences for cortical integrative function.
In GABAAβ3 knockout mice (β3−/−), inhibitory synapses between thalamic reticular (RE) neurons are... more In GABAAβ3 knockout mice (β3−/−), inhibitory synapses between thalamic reticular (RE) neurons are selectively impaired. Consequently, oscillations in thalamic slices from β3−/− mice last longer and are more synchronized than those from wild-type mice (Huntsman et al., Science 283 (1999) 541). We used computational models to study mechanisms by which intra-RE inhibition abbreviates and desynchronizes intrathalamic oscillations. Intra-RE inhibition could produce the former effect by shunting recurrent excitation. Intra-RE ...
The development of the amygdala, a central structure of the limbic system, remains poorly underst... more The development of the amygdala, a central structure of the limbic system, remains poorly understood. We found that two spatially distinct and early-specified telencephalic progenitor pools marked by the homeodomain transcription factor Dbx1 are major sources of neuronal cell diversity in the mature mouse amygdala. We found that Dbx1-positive cells of the ventral pallium generate the excitatory neurons of the basolateral complex and cortical amygdala nuclei. Moreover, Dbx1-derived cells comprise a previously unknown migratory stream that emanates from the preoptic area (POA), a ventral telencephalic domain adjacent to the diencephalic border. The Dbx1-positive, POA-derived population migrated specifically to the amygdala and, as defined by both immunochemical and electrophysiological criteria, generated a unique subclass of inhibitory neurons in the medial amygdala nucleus. Thus, this POA-derived population represents a previously unknown progenitor pool dedicated to the limbic system.
Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (T... more Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (TC) nuclei are important in sustaining oscillatory activity within thalamic and thalamocortical circuits. The biophysical properties and pharmacological sensitivities of these IPSCs both depend on the subunit combination of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors. Recombinant GABA(A) receptors containing the delta subunit (heavily expressed in TC nuclei) have been shown to exhibit slowed desensitization rates and high affinity for GABA in heterologous expression systems. We tested whether the GABA(A)-mediated synaptic inhibition in TC neurons would be affected by loss of the delta subunit. Spontaneous and evoked IPSCs were recorded from neurons in the ventral basal complex (VB) of the thalamus from brain slices of wild-type (delta(+/+)) and homozygous delta subunit deficient mice (delta(-/-)). Spontaneous IPSCs (sIPSCs) from delta(-/-) mice had no significant differences in amplitude, duration, or frequency compared with their delta(+/+) counterparts. However, baseline noise (63% of control) and the relative contribution of the slow component to overall decay (79% of control) were significantly lower in delta(-/-) VB recordings. Evoked IPSCs (eIPSCs) in delta(-/-) neurons showed no difference in peak amplitude, but had an accelerated slow decay component (40- vs. 55-ms time constant). We further tested whether neurosteroid modulation of GABA(A) receptors was dependent on the presence of the delta subunit, as previously reported in recombinant systems. Pregnenolone sulfate (PS) significantly reduced eIPSC peak amplitude (-30%) and increased duration in delta(-/-), but not in delta(+/+) mice. sIPSCs were not affected in any neurons, delta(-/-) or delta(+/+). In contrast, 3-alpha,5-alpha-tetrahydrodeoxycorticosterone (THDOC) increased the durations of eIPSCs and sIPSCs in both delta(-/-) and delta(+/+) VB neurons. Our findings show that although the delta subunit confers a striking PS insensitivity to eIPSCs in VB neurons, it plays only a minor role in the synaptic inhibition of VB neurons. This suggests delta subunit containing GABA(A) receptors may be functionally limited to an extrasynaptic locus in VB neurons.
conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at... more conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at varying degrees across different cell types. Despite a rich history of cortical interneuron diversity, little is known about tonic inhibition in the different classes of cells in the cerebral cortex. We therefore examined the cell-type specificity and functional significance of tonic inhibition in layer 4 of the mouse somatosensory barrel cortex. In situ hybridization and immunocytochemistry showed moderatesubunit expression across the barrel structures. Whole-cell patch-clamp recordings additionally indicated that significant levels of tonic inhibition can be found across cell types, with differences in the magnitude of inhibition between cell types. To activate tonic currents, we used 4,5,6,7-tetrahydroisothiazol-[5,4-c]pyridin-3-ol (THIP, a superagonist at -subunit containing GABA A receptors) at a concentration which did not affect synaptic decay kinetics. THIP produced greater shifts in baseline holding current in inhibitory cells (low threshold spiking (LTS), 109±17pA; fast spiking (FS), 111±15pA) than in excitatory cells (39±10pA; p<0.001). In addition to these differences across cell-types, there was also variability within inhibitory cells. FS cells with faster action potentials had larger baseline shifts. As FS cells are known mediators of feedforward inhibition, we tested whether THIP induced tonic conductance selectively controls feedforward circuits. THIP application resulted in the abolishment of the IPSP in thalamic-evoked disynaptic responses in a subset of excitatory neurons. These data suggest multiple feedforward circuits can be differentiated by the inhibitory control of the presynaptic inhibitory neuron. Agmon A and Connors BW. Correlation between intrinsic firing patterns and thalamocortical synaptic responses of neurons in mouse barrel cortex. J Neurosci 12: 319-329, 1992. Agmon A and Connors BW. Thalamocortical responses of mouse somatosensory (barrel) cortex in vitro. Neuroscience 41: 365-379, 1991. Ali AB, Bannister AP and Thomson AM. Robust correlations between action potential duration and the properties of synaptic connections in layer 4 interneurones in neocortical slices from juvenile rats and adult rat and cat. J Physiol 580: 149-169, 2007. Bacci A, Rudolph U, Huguenard JR and Prince DA. Major differences in inhibitory synaptic transmission onto two neocortical interneuron subclasses. J Neurosci 23: 9664-9674, 2003. Barnard EA, Skolnick P, Olsen RW, Mohler H, Sieghart W, Biggio G, Braestrup C, Bateson AN and Langer SZ. International Union of Pharmacology. XV. Subtypes of gammaaminobutyric acidA receptors: classification on the basis of subunit structure and receptor function. Pharmacol Rev 50: 291-313, 1998. Beierlein M, Fall CP, Rinzel J and Yuste R. Thalamocortical bursts trigger recurrent activity in neocortical networks: layer 4 as a frequency-dependent gate. J Neurosci 22: 9885-9894, 2002. Beierlein M, Gibson JR and Connors BW. A network of electrically coupled interneurons drives synchronized inhibition in neocortex. Nat Neurosci 3: 904-910, 2000. Beierlein M, Gibson JR and Connors BW. Two dynamically distinct inhibitory networks in layer 4 of the neocortex. J Neurophysiol 90: 2987-3000, 2003. Brickley SG, Cull-Candy SG and Farrant M. Development of a tonic form of synaptic inhibition in rat cerebellar granule cells resulting from persistent activation of GABAA receptors. J Physiol 497 ( Pt 3): 753-759, 1996. Brown N, Kerby J, Bonnert TP, Whiting PJ and Wafford KA. Pharmacological characterization of a novel cell line expressing human alpha(4)beta(3)delta GABA(A) receptors. Br J Pharmacol 136: 965-974, 2002. McCartney MR, Deeb TZ, Henderson TN and Hales TG. Tonically active GABAA receptors in hippocampal pyramidal neurons exhibit constitutive GABA-independent gating. Mol Pharmacol, 2006. Mortensen M and Smart TG. Extrasynaptic {alpha}{beta} subunit GABAA receptors on rat hippocampal pyramidal neurons. J Physiol 577: 841-856, 2006. Nusser Z, Sieghart W and Somogyi P. Segregation of different GABAA receptors to synaptic and extrasynaptic membranes of cerebellar granule cells. J Neurosci 18: 1693-1703, 1998. Ortinski PI, Turner JR, Barberis A, Motamedi G, Yasuda RP, Wolfe BB, Kellar KJ and Vicini S. Deletion of the GABA(A) receptor alpha1 subunit increases tonic GABA(A) receptor current: a role for GABA uptake transporters. J Neurosci 26: 9323-9331, 2006. Pirker S, Schwarzer C, Wieselthaler A, Sieghart W and Sperk G. GABA(A) receptors: immunocytochemical distribution of 13 subunits in the adult rat brain. Neuroscience 101: 815-850, 2000. Porter JT, Johnson CK and Agmon A. Diverse types of interneurons generate thalamusevoked feedforward inhibition in the mouse barrel cortex. J Neurosci 21: 2699-2710, 2001. Saxena NC and Macdonald RL. Assembly of GABAA receptor subunits: role of the delta subunit. J Neurosci 14: 7077-7086, 1994. Schubert D, Kotter R, Zilles K, Luhmann HJ and Staiger JF. Cell type-specific circuits of cortical layer IV spiny neurons. J Neurosci 23: 2961-2970, 2003. Semyanov A, Walker MC and Kullmann DM. GABA uptake regulates cortical excitability via cell type-specific tonic inhibition. Nat Neurosci 6: 484-490, 2003. Semyanov A, Walker MC, Kullmann DM and Silver RA. Tonically active GABA A receptors: modulating gain and maintaining the tone. Simons DJ. Response properties of vibrissa units in rat SI vibrissa cortex. J Neurophysiol 41: 798-820, 1978. Simons DJ and Carvell GE. Thalamocortical response transformation in the rat vibrissa/barrel system. J Neurophysiol 61: 311-330, 1989. Stell BM, Brickley SG, Tang CY, Farrant M and Mody I. Neuroactive steroids reduce neuronal excitability by selectively enhancing tonic inhibition mediated by delta subunitcontaining GABAA receptors. Sun QQ, Huguenard JR and Prince DA. Barrel cortex microcircuits: thalamocortical feedforward inhibition in spiny stellate cells is mediated by a small number of fast-spiking interneurons. J Neurosci 26: 1219-1230, 2006. Swadlow HA. Efferent neurons and suspected interneurons in S-1 vibrissa cortex of the awake rabbit: receptive fields and axonal properties. J Neurophysiol 62: 288-308, 1989. Swadlow HA. Fast-spike interneurons and feedforward inhibition in awake sensory neocortex. Cereb Cortex 13: 25-32, 2003. Swadlow HA. Influence of VPM afferents on putative inhibitory interneurons in S1 of the awake rabbit: evidence from cross-correlation, microstimulation, and latencies to peripheral sensory stimulation. J Neurophysiol 73: 1584-1599, 1995. Swadlow HA, Beloozerova IN and Sirota MG. Sharp, local synchrony among putative feedforward inhibitory interneurons of rabbit somatosensory cortex. J Neurophysiol 79: 567-582, 1998. Swadlow HA and Gusev AG. Receptive-field construction in cortical inhibitory interneurons.
Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, a... more Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits of the gamma-aminobutyric acid A (GABAA) receptor were used for in situ hybridization histochemistry of the primary motor, somatosensory, and anterior parietal areas of the cerebral cortex in macaque monkeys. mRNAs for the alpha 1, beta 2, and gamma 2 subunit polypeptides, which form receptors with the full range of classical properties, are expressed at much higher levels in all areas and show laminar- and sublaminar-specific concentrations. alpha 2, alpha 4, alpha 5, and beta 1 subunit transcripts are expressed at much lower levels but also display individual, laminar-specific concentrations; alpha 5 expression, in particular, is highly expressed in layer IV in the somatosensory and parietal areas and in a layer IV-like band in the motor cortex. In layers in which expression of a particular transcript is high, all neurons may express the gene, but in layers in which expression is moderate, it is possible to detect differences in the degree of labeling of individual neurons for a particular mRNA, and some neurons may not express certain subunit transcripts in detectable amounts. These findings indicate the variability in expression of different GABAA receptor subunits in the cerebral cortex. Laminar differences may indicate the assembly of functional receptors from different arrangements of available subunits in different classes of cells.
Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate recep... more Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate receptor subtypes. Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) receptor subunits (GluR1-4; GluR5-7; NR1, NR2A-D) on a nucleus- and cell type-specific basis was examined by quantitative in situ hybridization histochemistry and by immunocytochemical staining for receptor subunits and colocalized gamma-aminobutyric acid (GABA) or calcium binding proteins. Levels of NMDA subunit expression, except NR2C, are higher than for the most highly expressed AMPA (GluR1,3,4) and kainate (GluR6) receptor subunits. Expression of NR2C, GluR2, GluR5, and GluR7 is extremely low. Major differences distinguish the reticular nucleus and the dorsal thalamus and, within the dorsal thalamus, the intralaminar and other nuclei. In the reticular nucleus, GluR4 is by far the most prominent, and NMDA receptors are at comparatively low levels. In the dorsal thalamus, NMDA receptors predominate. Anterior intralaminar nuclei are more enriched in GluR4 and GluR6 subunits than other nuclei, whereas posterior intralaminar nuclei are enriched in GluR1 and differ among themselves in relative NMDA receptor subunit expression. GABAergic intrinsic neurons of the dorsal thalamus express much higher levels of GluR1 and GluR6 receptor subunits than do parvalbumin- or calbindin-immunoreactive relay cells and low or absent NMDA receptors. Relay cells are dominated by NMDA receptors, along with GluR3 and GluR6 subunits not expressed by GABA cells. High levels of NR2B are found in astrocytes. Differences in NMDA and non-NMDA receptor profiles will affect functional properties of the thalamic GABAergic and relay cells.
In situ hybridization histochemistry and immunocytochemistry were used to map distributions of ce... more In situ hybridization histochemistry and immunocytochemistry were used to map distributions of cells expressing mRNAs encoding alpha, beta, gamma, and delta isoforms of type II calcium/calmodulin-dependent protein kinase (CaMKII), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA)/ kainate receptor subunits, (GluR1-7), and N-methyl-D-aspartate (NMDA) receptor subunits, NR1 and NR2A-D, or stained by subunit-specific immunocytochemistry in the dorsal lateral geniculate nuclei of macaque monkeys. Relationships of specific isoforms with particular glutamate receptor types may be important elements in neural plasticity. CaMKII-alpha is expressed only by neurons in the S laminae and interlaminar plexuses of the dorsal lateral geniculate nucleus, but may form part of a more widely distributed matrix of similar cells extending from the geniculate into adjacent nuclei. CaMKII-beta, -gamma, and -delta isoforms are expressed by all neurons in principal and S laminae and interlaminar plexuses. In principal laminae, they are down-regulated by monocular deprivation lasting 8-21 days. All glutamate receptor subunits are expressed by neurons in principal and S laminae and interlaminar plexuses. The AMPA/kainate subunits, GluR1, 2, 5, and 7, are expressed at low levels, although GluR1 immunostaining appears selectively to stain interneurons. GluR3 is expressed at weak, GluR 6 at moderate and GluR 4 at high levels. NMDA subunits, NR1 and NR2A, B, and D, are expressed at moderate to low levels. GluR4, GluR6 and NMDA subunits are down-regulated by visual deprivation. CaMKII-alpha expression is unique in comparison with other CaMKII isoforms which may, therefore, have more generalized roles in cell function. The results demonstrate that all of the isoforms are associated with NMDA receptors and with AMPA receptors enriched with GluR4 subunits, which implies high calcium permeability and rapid gating.
Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in... more Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.
The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in t... more The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in the adult monkey lateral geniculate nucleus. In situ hybridization histochemistry was used to localize mRNAs of the genes encoding monkey-specific al, a2, a4, a5, p l , p2, and 72 subunits. The highest levels of expression in the nucleus were for a l , p2, and y2 subunit transcripts. The levels were substantially higher in the magnocellular than in the parvocellular layers. Alpha-2, 014, 015, and p l subunit mRNAs were expressed at much lower levels, and magno-and parvocellular layers had approximately equal levels of expression.
Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disabilit... more Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.
The prefrontal cortex of schizophrenics is hypoactive and displays changes related to inhibitory,... more The prefrontal cortex of schizophrenics is hypoactive and displays changes related to inhibitory, GABAergic neurons, and GABAergic synapses. These changes include decreased levels of glutamic acid decarboxylase (GAD), the enzyme for GABA synthesis, upregulation of muscimol binding, and downregulation of benzodiazepine binding to GABAA receptors. Studies in the visual cortex of nonhuman primates have demonstrated that gene expression for GAD and for several GABAA receptor subunit polypeptides is under control of neuronal activity, raising the possibility that similar mechanisms in the hypoactive prefrontal cortex of schizophrenics may explain the abnormalities in GAD and in GABAA receptor regulation. In the present study, which is the first of its type on human cerebral cortex, levels of mRNAs for six GABAA receptor subunits (alpha 1, alpha 2, alpha 5, beta 1, beta 2, gamma 2) and their laminar expression patterns were analyzed in the prefrontal cortex of schizophrenics and matched controls, using in situ hybridization histochemistry and densitometry. Three types of laminar expression pattern were observed: mRNAs for the alpha 1, beta 2, and gamma 2 subunits, which are the predominant receptor subunits expressed in the mature cortex, were expressed at comparatively high levels by cells of all six cortical layers, but most intensely by cells in lower layer III and layer IV. mRNAs for the alpha 2, alpha 5, and beta 1 subunits were expressed at lower levels; alpha 2 and beta 1 were expressed predominantly by cells in layers II, III, and IV; alpha 5 was expressed predominantly in layers IV, V, and VI. There were no significant changes in overall mRNA levels for any of the receptor subunits in the prefrontal cortex of schizophrenics, and the laminar expression pattern of all six receptor subunit mRNAs did not differ between schizophrenics and controls. Because gene expression for GABAA receptor subunits is not consistently altered in the prefrontal cortex of schizophrenics, the previously reported upregulation of muscimol binding sites and downregulation of benzodiazepine binding sites in the prefrontal and adjacent cingulate cortex of schizophrenics are possibly due to posttranscriptional modifications of mRNAs and their translated polypeptides.
Many neurological disorders, including neurodevelopmental disorders, report hypersynchrony of neu... more Many neurological disorders, including neurodevelopmental disorders, report hypersynchrony of neuronal networks. These alterations in neuronal synchronization suggest a link to the function of inhibitory interneurons. In Fragile X Syndrome (FXS), it has been reported that altered synchronization may underlie hyperexcitability, cognitive dysfunction and provide a link to the increased incidence of epileptic seizures. Therefore, understanding the roles of inhibitory interneurons and how they control neuronal networks is of great importance in studying neurodevelopmental disorders such as FXS. Here, we present a review of how interneuron populations and inhibition are important contributors to the loss of excitatory/inhibitory balance seen in hypersynchronous and hyperexcitable networks from neurodevelopmental disorders, and specifically in FXS.
Proceedings of the National Academy of Sciences of the United States of America, Jan 23, 2014
Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular s... more Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular signals that generate normal cortical networks remain poorly understood. Semaphorin 7A (Sema7A) is an atypical member of the semaphorin family that is GPI-linked, expressed principally postnatally, and enriched in sensory cortex. Significantly, SEMA7A is deleted in individuals with 15q24 microdeletion syndrome, characterized by developmental delay, autism, and sensory perceptual deficits. We studied the role that Sema7A plays in establishing functional cortical circuitry in mouse somatosensory barrel cortex. We found that Sema7A is expressed in spiny stellate cells and GABAergic interneurons and that its absence disrupts barrel cytoarchitecture, reduces asymmetrical orientation of spiny stellate cell dendrites, and functionally impairs thalamocortically evoked synaptic responses, with reduced feed-forward GABAergic inhibition. These data identify Sema7A as a regulator of thalamocortical a...
In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane... more In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane expression, composition and function of GABAA receptors (GABAARs). For this, the tissue of chronically epileptic rats was collected within 3h of seizure occurrence (≤3h group) or at least 24h after seizure occurrence (≥24h group). A retrospective analysis of seizure frequency revealed that selecting animals on the bases of seizure proximity also grouped animals in terms of overall seizure burden with a higher seizure burden observed in the ≤3h group. A biochemical analysis showed that although animals with more frequent/recent seizures (≤3h group) had similar levels of GABAAR at the plasma membrane they showed deficits in inhibitory neurotransmission. By contrast, the tissue obtained from animals experiencing infrequent seizures (≥24h group) had increased plasma membrane levels of GABAAR and showed no deficit in inhibitory function. Together, our findings offer an initial insight into t...
The dysfunction of parvalbumin-positive, fast-spiking interneurons (FSI) is considered a primary ... more The dysfunction of parvalbumin-positive, fast-spiking interneurons (FSI) is considered a primary contributor to the pathophysiology of schizophrenia (SZ), but deficits in FSI physiology have not been explicitly characterized. We show for the first time, that a widely-employed model of schizophrenia minimizes first spike latency and increases GluN2B-mediated current in neocortical FSIs. The reduction in FSI first-spike latency coincides with reduced expression of the Kv1.1 potassium channel subunit which provides a biophysical explanation for the abnormal spiking behavior. Similarly, the increase in NMDA current coincides with enhanced expression of the GluN2B NMDA receptor subunit, specifically in FSIs. In this study mice were treated with the NMDA receptor antagonist, MK-801, during the first week of life. During adolescence, we detected reduced spike latency and increased GluN2B-mediated NMDA current in FSIs, which suggests transient disruption of NMDA signaling during neonatal de...
The cellular and molecular mechanisms that govern the response of the perinatal brain to injury r... more The cellular and molecular mechanisms that govern the response of the perinatal brain to injury remain largely unexplored. We investigated the role of white matter astrocytes in a rodent model of diffuse white matter injury produced by exposing neonatal mice to chronic hypoxia -a paradigm which mimics brain injury in premature infants. We demonstrate absence of reactive gliosis in the immature white matter following chronic hypoxia, as determined by astrocyte proliferation index and glial fibrillary acidic protein levels. Instead, Nestin expression in astrocytes is transiently increased, and the glial-specific glutamate transporters GLAST and GLT-1 are reduced. Finally, we demonstrate that JAK/STAT signaling -which is important in both astrocyte development and response to injury -is reduced in the white matter following hypoxia, as well as in primary astrocytes exposed to hypoxia in vitro. Hypoxia and JAK/STAT inhibition reduce glutamate transporter expression in astrocytes, but unlike hypoxia JAK/STAT inhibition downregulates GLAST expression without affecting GLT-1, as demonstrated in vitro by treatment with JAK Inhibitor I and in vivo by treatment with the JAK/STAT inhibitor AG490. Our findings: i) demonstrate specific changes in astrocyte function after perinatal hypoxia, which might contribute to the particular pathogenesis of perinatal white matter injury, ii) provide evidence that at least part of these changes result from a disturbance of the JAK/STAT pathway by hypoxia, and iii) identify JAK/STAT signaling as a potential therapeutic target to restore normal GLAST expression and uptake of glutamate after perinatal brain injury.
Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by variable cognitive imp... more Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by variable cognitive impairment and behavioral disturbances such as exaggerated fear, anxiety and gaze avoidance. Consistent with this, findings from human brain imaging studies suggest dysfunction of the amygdala. Underlying alterations in amygdala synaptic function in the Fmr1 knock-out (KO) mouse model of FXS, however, remain largely unexplored. Utilizing a combination of approaches, we uncover profound alterations in inhibitory neurotransmission in the amygdala of Fmr1 KO mice. We demonstrate a dramatic reduction in the frequency and amplitude of phasic IPSCs, tonic inhibitory currents, as well as in the number of inhibitory synapses in Fmr1 KO mice. Furthermore, we observe significant alterations in GABA availability, both intracellularly and at the synaptic cleft. Together, these findings identify abnormalities in basal and action potential-dependent inhibitory neurotransmission. Additionally, we reveal a significant neuronal hyperexcitability in principal neurons of the amygdala in Fmr1 KO mice, which is strikingly rescued by pharmacological augmentation of tonic inhibitory tone using the GABA agonist gaboxadol (THIP). Thus, our study reveals relevant inhibitory synaptic abnormalities in the amygdala in the Fmr1 KO brain and supports the notion that pharmacological approaches targeting the GABAergic system may be a viable therapeutic approach toward correcting amygdala-based symptoms in FXS.
Neuronal rhythmic activities within thalamocortical circuits range from partially synchronous osc... more Neuronal rhythmic activities within thalamocortical circuits range from partially synchronous oscillations during normal sleep to hypersynchrony associated with absence epilepsy. It has been proposed that recurrent inhibition within the thalamic reticular nucleus serves to reduce synchrony and thus prevents seizures. Inhibition and synchrony in slices from mice devoid of the ␥-aminobutyric acid type-A (GABA A ) receptor  3 subunit were examined, because in rodent thalamus,  3 is largely restricted to reticular nucleus. In  3 knockout mice, GABA A -mediated inhibition was nearly abolished in reticular nucleus, but was unaffected in relay cells. In addition, oscillatory synchrony was dramatically intensified. Thus, recurrent inhibitory connections within reticular nucleus act as "desynchronizers."
Proceedings of the National Academy of Sciences, 1998
The relative abundance of alternatively spliced long (gamma2L) and short (gamma2S) mRNAs of the g... more The relative abundance of alternatively spliced long (gamma2L) and short (gamma2S) mRNAs of the gamma2 subunit of the gamma-amino butyrate type A (GABAA) receptor was examined in dorsolateral prefrontal cortex of schizophrenics and matched controls by using in situ hybridization histochemistry and semiquantitative reverse transcription-PCR (RT-PCR) amplification. A cRNA probe identifying both mRNAs showed that the transcripts are normally expressed at moderately high levels in the prefrontal cortex. Consistent with previous studies, overall levels of gamma2 transcripts in prefrontal cortex of brains from schizophrenics were reduced by 28.0%, although this reduction did not reach statistical significance. RT-PCR, performed under nonsaturating conditions on total RNA from the same blocks of tissue used for in situ hybridization histochemistry, revealed a marked reduction in the relative proportion of gamma2S transcripts in schizophrenic brains compared with controls. In schizophrenics, gamma2S transcripts had fallen to 51.7% (+/-7.9% SE; P &amp;amp;lt; 0.0001) relative to control levels. Levels of gamma2L transcripts showed only a small and nonsignificant reduction of 16. 9% (+/-12.0% SE, P &amp;amp;gt; 0.05). These findings indicate differential transcriptional regulation of two functionally distinct isoforms of one of the major GABAA receptor subunits in the prefrontal cortex of schizophrenics. The specific reduction in relative abundance of gamma2S mRNAs and the associated relative increase in gamma2L mRNAs should result in functionally less active GABAA receptors and have severe consequences for cortical integrative function.
In GABAAβ3 knockout mice (β3−/−), inhibitory synapses between thalamic reticular (RE) neurons are... more In GABAAβ3 knockout mice (β3−/−), inhibitory synapses between thalamic reticular (RE) neurons are selectively impaired. Consequently, oscillations in thalamic slices from β3−/− mice last longer and are more synchronized than those from wild-type mice (Huntsman et al., Science 283 (1999) 541). We used computational models to study mechanisms by which intra-RE inhibition abbreviates and desynchronizes intrathalamic oscillations. Intra-RE inhibition could produce the former effect by shunting recurrent excitation. Intra-RE ...
The development of the amygdala, a central structure of the limbic system, remains poorly underst... more The development of the amygdala, a central structure of the limbic system, remains poorly understood. We found that two spatially distinct and early-specified telencephalic progenitor pools marked by the homeodomain transcription factor Dbx1 are major sources of neuronal cell diversity in the mature mouse amygdala. We found that Dbx1-positive cells of the ventral pallium generate the excitatory neurons of the basolateral complex and cortical amygdala nuclei. Moreover, Dbx1-derived cells comprise a previously unknown migratory stream that emanates from the preoptic area (POA), a ventral telencephalic domain adjacent to the diencephalic border. The Dbx1-positive, POA-derived population migrated specifically to the amygdala and, as defined by both immunochemical and electrophysiological criteria, generated a unique subclass of inhibitory neurons in the medial amygdala nucleus. Thus, this POA-derived population represents a previously unknown progenitor pool dedicated to the limbic system.
Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (T... more Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (TC) nuclei are important in sustaining oscillatory activity within thalamic and thalamocortical circuits. The biophysical properties and pharmacological sensitivities of these IPSCs both depend on the subunit combination of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors. Recombinant GABA(A) receptors containing the delta subunit (heavily expressed in TC nuclei) have been shown to exhibit slowed desensitization rates and high affinity for GABA in heterologous expression systems. We tested whether the GABA(A)-mediated synaptic inhibition in TC neurons would be affected by loss of the delta subunit. Spontaneous and evoked IPSCs were recorded from neurons in the ventral basal complex (VB) of the thalamus from brain slices of wild-type (delta(+/+)) and homozygous delta subunit deficient mice (delta(-/-)). Spontaneous IPSCs (sIPSCs) from delta(-/-) mice had no significant differences in amplitude, duration, or frequency compared with their delta(+/+) counterparts. However, baseline noise (63% of control) and the relative contribution of the slow component to overall decay (79% of control) were significantly lower in delta(-/-) VB recordings. Evoked IPSCs (eIPSCs) in delta(-/-) neurons showed no difference in peak amplitude, but had an accelerated slow decay component (40- vs. 55-ms time constant). We further tested whether neurosteroid modulation of GABA(A) receptors was dependent on the presence of the delta subunit, as previously reported in recombinant systems. Pregnenolone sulfate (PS) significantly reduced eIPSC peak amplitude (-30%) and increased duration in delta(-/-), but not in delta(+/+) mice. sIPSCs were not affected in any neurons, delta(-/-) or delta(+/+). In contrast, 3-alpha,5-alpha-tetrahydrodeoxycorticosterone (THDOC) increased the durations of eIPSCs and sIPSCs in both delta(-/-) and delta(+/+) VB neurons. Our findings show that although the delta subunit confers a striking PS insensitivity to eIPSCs in VB neurons, it plays only a minor role in the synaptic inhibition of VB neurons. This suggests delta subunit containing GABA(A) receptors may be functionally limited to an extrasynaptic locus in VB neurons.
conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at... more conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at varying degrees across different cell types. Despite a rich history of cortical interneuron diversity, little is known about tonic inhibition in the different classes of cells in the cerebral cortex. We therefore examined the cell-type specificity and functional significance of tonic inhibition in layer 4 of the mouse somatosensory barrel cortex. In situ hybridization and immunocytochemistry showed moderatesubunit expression across the barrel structures. Whole-cell patch-clamp recordings additionally indicated that significant levels of tonic inhibition can be found across cell types, with differences in the magnitude of inhibition between cell types. To activate tonic currents, we used 4,5,6,7-tetrahydroisothiazol-[5,4-c]pyridin-3-ol (THIP, a superagonist at -subunit containing GABA A receptors) at a concentration which did not affect synaptic decay kinetics. THIP produced greater shifts in baseline holding current in inhibitory cells (low threshold spiking (LTS), 109±17pA; fast spiking (FS), 111±15pA) than in excitatory cells (39±10pA; p<0.001). In addition to these differences across cell-types, there was also variability within inhibitory cells. FS cells with faster action potentials had larger baseline shifts. As FS cells are known mediators of feedforward inhibition, we tested whether THIP induced tonic conductance selectively controls feedforward circuits. THIP application resulted in the abolishment of the IPSP in thalamic-evoked disynaptic responses in a subset of excitatory neurons. These data suggest multiple feedforward circuits can be differentiated by the inhibitory control of the presynaptic inhibitory neuron. 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Pharmacological characterization of a novel cell line expressing human alpha(4)beta(3)delta GABA(A) receptors. Br J Pharmacol 136: 965-974, 2002. McCartney MR, Deeb TZ, Henderson TN and Hales TG. Tonically active GABAA receptors in hippocampal pyramidal neurons exhibit constitutive GABA-independent gating. Mol Pharmacol, 2006. Mortensen M and Smart TG. Extrasynaptic {alpha}{beta} subunit GABAA receptors on rat hippocampal pyramidal neurons. J Physiol 577: 841-856, 2006. Nusser Z, Sieghart W and Somogyi P. Segregation of different GABAA receptors to synaptic and extrasynaptic membranes of cerebellar granule cells. J Neurosci 18: 1693-1703, 1998. Ortinski PI, Turner JR, Barberis A, Motamedi G, Yasuda RP, Wolfe BB, Kellar KJ and Vicini S. Deletion of the GABA(A) receptor alpha1 subunit increases tonic GABA(A) receptor current: a role for GABA uptake transporters. J Neurosci 26: 9323-9331, 2006. Pirker S, Schwarzer C, Wieselthaler A, Sieghart W and Sperk G. GABA(A) receptors: immunocytochemical distribution of 13 subunits in the adult rat brain. Neuroscience 101: 815-850, 2000. Porter JT, Johnson CK and Agmon A. Diverse types of interneurons generate thalamusevoked feedforward inhibition in the mouse barrel cortex. J Neurosci 21: 2699-2710, 2001. Saxena NC and Macdonald RL. Assembly of GABAA receptor subunits: role of the delta subunit. J Neurosci 14: 7077-7086, 1994. Schubert D, Kotter R, Zilles K, Luhmann HJ and Staiger JF. Cell type-specific circuits of cortical layer IV spiny neurons. J Neurosci 23: 2961-2970, 2003. Semyanov A, Walker MC and Kullmann DM. GABA uptake regulates cortical excitability via cell type-specific tonic inhibition. Nat Neurosci 6: 484-490, 2003. Semyanov A, Walker MC, Kullmann DM and Silver RA. Tonically active GABA A receptors: modulating gain and maintaining the tone. Simons DJ. Response properties of vibrissa units in rat SI vibrissa cortex. J Neurophysiol 41: 798-820, 1978. Simons DJ and Carvell GE. Thalamocortical response transformation in the rat vibrissa/barrel system. J Neurophysiol 61: 311-330, 1989. Stell BM, Brickley SG, Tang CY, Farrant M and Mody I. Neuroactive steroids reduce neuronal excitability by selectively enhancing tonic inhibition mediated by delta subunitcontaining GABAA receptors. Sun QQ, Huguenard JR and Prince DA. Barrel cortex microcircuits: thalamocortical feedforward inhibition in spiny stellate cells is mediated by a small number of fast-spiking interneurons. J Neurosci 26: 1219-1230, 2006. Swadlow HA. Efferent neurons and suspected interneurons in S-1 vibrissa cortex of the awake rabbit: receptive fields and axonal properties. J Neurophysiol 62: 288-308, 1989. Swadlow HA. Fast-spike interneurons and feedforward inhibition in awake sensory neocortex. Cereb Cortex 13: 25-32, 2003. Swadlow HA. Influence of VPM afferents on putative inhibitory interneurons in S1 of the awake rabbit: evidence from cross-correlation, microstimulation, and latencies to peripheral sensory stimulation. J Neurophysiol 73: 1584-1599, 1995. Swadlow HA, Beloozerova IN and Sirota MG. Sharp, local synchrony among putative feedforward inhibitory interneurons of rabbit somatosensory cortex. J Neurophysiol 79: 567-582, 1998. Swadlow HA and Gusev AG. Receptive-field construction in cortical inhibitory interneurons.
Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, a... more Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits of the gamma-aminobutyric acid A (GABAA) receptor were used for in situ hybridization histochemistry of the primary motor, somatosensory, and anterior parietal areas of the cerebral cortex in macaque monkeys. mRNAs for the alpha 1, beta 2, and gamma 2 subunit polypeptides, which form receptors with the full range of classical properties, are expressed at much higher levels in all areas and show laminar- and sublaminar-specific concentrations. alpha 2, alpha 4, alpha 5, and beta 1 subunit transcripts are expressed at much lower levels but also display individual, laminar-specific concentrations; alpha 5 expression, in particular, is highly expressed in layer IV in the somatosensory and parietal areas and in a layer IV-like band in the motor cortex. In layers in which expression of a particular transcript is high, all neurons may express the gene, but in layers in which expression is moderate, it is possible to detect differences in the degree of labeling of individual neurons for a particular mRNA, and some neurons may not express certain subunit transcripts in detectable amounts. These findings indicate the variability in expression of different GABAA receptor subunits in the cerebral cortex. Laminar differences may indicate the assembly of functional receptors from different arrangements of available subunits in different classes of cells.
Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate recep... more Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate receptor subtypes. Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) receptor subunits (GluR1-4; GluR5-7; NR1, NR2A-D) on a nucleus- and cell type-specific basis was examined by quantitative in situ hybridization histochemistry and by immunocytochemical staining for receptor subunits and colocalized gamma-aminobutyric acid (GABA) or calcium binding proteins. Levels of NMDA subunit expression, except NR2C, are higher than for the most highly expressed AMPA (GluR1,3,4) and kainate (GluR6) receptor subunits. Expression of NR2C, GluR2, GluR5, and GluR7 is extremely low. Major differences distinguish the reticular nucleus and the dorsal thalamus and, within the dorsal thalamus, the intralaminar and other nuclei. In the reticular nucleus, GluR4 is by far the most prominent, and NMDA receptors are at comparatively low levels. In the dorsal thalamus, NMDA receptors predominate. Anterior intralaminar nuclei are more enriched in GluR4 and GluR6 subunits than other nuclei, whereas posterior intralaminar nuclei are enriched in GluR1 and differ among themselves in relative NMDA receptor subunit expression. GABAergic intrinsic neurons of the dorsal thalamus express much higher levels of GluR1 and GluR6 receptor subunits than do parvalbumin- or calbindin-immunoreactive relay cells and low or absent NMDA receptors. Relay cells are dominated by NMDA receptors, along with GluR3 and GluR6 subunits not expressed by GABA cells. High levels of NR2B are found in astrocytes. Differences in NMDA and non-NMDA receptor profiles will affect functional properties of the thalamic GABAergic and relay cells.
In situ hybridization histochemistry and immunocytochemistry were used to map distributions of ce... more In situ hybridization histochemistry and immunocytochemistry were used to map distributions of cells expressing mRNAs encoding alpha, beta, gamma, and delta isoforms of type II calcium/calmodulin-dependent protein kinase (CaMKII), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA)/ kainate receptor subunits, (GluR1-7), and N-methyl-D-aspartate (NMDA) receptor subunits, NR1 and NR2A-D, or stained by subunit-specific immunocytochemistry in the dorsal lateral geniculate nuclei of macaque monkeys. Relationships of specific isoforms with particular glutamate receptor types may be important elements in neural plasticity. CaMKII-alpha is expressed only by neurons in the S laminae and interlaminar plexuses of the dorsal lateral geniculate nucleus, but may form part of a more widely distributed matrix of similar cells extending from the geniculate into adjacent nuclei. CaMKII-beta, -gamma, and -delta isoforms are expressed by all neurons in principal and S laminae and interlaminar plexuses. In principal laminae, they are down-regulated by monocular deprivation lasting 8-21 days. All glutamate receptor subunits are expressed by neurons in principal and S laminae and interlaminar plexuses. The AMPA/kainate subunits, GluR1, 2, 5, and 7, are expressed at low levels, although GluR1 immunostaining appears selectively to stain interneurons. GluR3 is expressed at weak, GluR 6 at moderate and GluR 4 at high levels. NMDA subunits, NR1 and NR2A, B, and D, are expressed at moderate to low levels. GluR4, GluR6 and NMDA subunits are down-regulated by visual deprivation. CaMKII-alpha expression is unique in comparison with other CaMKII isoforms which may, therefore, have more generalized roles in cell function. The results demonstrate that all of the isoforms are associated with NMDA receptors and with AMPA receptors enriched with GluR4 subunits, which implies high calcium permeability and rapid gating.
Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in... more Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.
The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in t... more The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in the adult monkey lateral geniculate nucleus. In situ hybridization histochemistry was used to localize mRNAs of the genes encoding monkey-specific al, a2, a4, a5, p l , p2, and 72 subunits. The highest levels of expression in the nucleus were for a l , p2, and y2 subunit transcripts. The levels were substantially higher in the magnocellular than in the parvocellular layers. Alpha-2, 014, 015, and p l subunit mRNAs were expressed at much lower levels, and magno-and parvocellular layers had approximately equal levels of expression.
Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disabilit... more Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.
The prefrontal cortex of schizophrenics is hypoactive and displays changes related to inhibitory,... more The prefrontal cortex of schizophrenics is hypoactive and displays changes related to inhibitory, GABAergic neurons, and GABAergic synapses. These changes include decreased levels of glutamic acid decarboxylase (GAD), the enzyme for GABA synthesis, upregulation of muscimol binding, and downregulation of benzodiazepine binding to GABAA receptors. Studies in the visual cortex of nonhuman primates have demonstrated that gene expression for GAD and for several GABAA receptor subunit polypeptides is under control of neuronal activity, raising the possibility that similar mechanisms in the hypoactive prefrontal cortex of schizophrenics may explain the abnormalities in GAD and in GABAA receptor regulation. In the present study, which is the first of its type on human cerebral cortex, levels of mRNAs for six GABAA receptor subunits (alpha 1, alpha 2, alpha 5, beta 1, beta 2, gamma 2) and their laminar expression patterns were analyzed in the prefrontal cortex of schizophrenics and matched controls, using in situ hybridization histochemistry and densitometry. Three types of laminar expression pattern were observed: mRNAs for the alpha 1, beta 2, and gamma 2 subunits, which are the predominant receptor subunits expressed in the mature cortex, were expressed at comparatively high levels by cells of all six cortical layers, but most intensely by cells in lower layer III and layer IV. mRNAs for the alpha 2, alpha 5, and beta 1 subunits were expressed at lower levels; alpha 2 and beta 1 were expressed predominantly by cells in layers II, III, and IV; alpha 5 was expressed predominantly in layers IV, V, and VI. There were no significant changes in overall mRNA levels for any of the receptor subunits in the prefrontal cortex of schizophrenics, and the laminar expression pattern of all six receptor subunit mRNAs did not differ between schizophrenics and controls. Because gene expression for GABAA receptor subunits is not consistently altered in the prefrontal cortex of schizophrenics, the previously reported upregulation of muscimol binding sites and downregulation of benzodiazepine binding sites in the prefrontal and adjacent cingulate cortex of schizophrenics are possibly due to posttranscriptional modifications of mRNAs and their translated polypeptides.
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