Anatomy and Structural Biology

The striatum is a site of integration of neural pathways involved in reinforcement learning. Traditionally, inputs from cerebral cortex are thought to be reinforced by dopaminergic afferents signaling the occurrence of biologically salient sensory events. Here, we detail an alternative route for short-latency sensory-evoked input to the striatum requiring neither dopamine nor the cortex. Using intracellular recording techniques, we measured subthreshold inputs to spiny projection neurons (SPNs) in urethane-anesthetized rats. Contralateral whole-field light flashes evoked weak membrane potential responses in approximately two-thirds of neurons. However, after local disinhibitory injections of the GABA A antagonist bicuculline into the deep layers of the superior colliculus (SC), but not the overlying visual cortex, strong, light-evoked, depolarizations to the up state emerged at short latency (115 ± 14 ms) in all neurons tested. Dopamine depletion using αmethyl-para-tyrosine had no detectable effect on striatal visual responses during SC disinhibition. In contrast, local inhibitory injections of GABA agonists, muscimol and baclofen, into the parafascicular nucleus of the thalamus blocked the early, visual-evoked up-state transitions in SPNs. Comparable muscimol-induced inhibition of the visual cortex failed to suppress the visual responsiveness of SPNs induced by SC disinhibition. Together, these results suggest that shortlatency, preattentive visual input can reach the striatum not only via the tecto-nigro-striatal route but also through tecto-thalamo-striatal projections. Thus, after the onset of a biologically significant visual event, closely timed short-latency thalamostriatal (glutamate) and nigrostriatal (dopamine) inputs are likely to converge on striatal SPNs, providing depolarizing and neuromodulator signals necessary for synaptic plasticity mechanisms.

Striatal cholinergic interneurons, also known as tonically active neurons (TANs), acquire a pause in firing during learning of stimulusreward associations. This pause response to a sensory stimulus emerges after repeated pairing with a reward. The conditioned pause is dependent on dopamine from the substantia nigra, but its underlying cellular mechanism is unknown. Using in vivo intracellular recording, we found that both subthreshold and suprathreshold depolarizations in cholinergic interneurons induced a prolonged afterhyperpolarization (AHP) associated with a pause in their tonic firing. The AHP duration was dependent on the level of depolarization, whether elicited by intracellular current injection or by activation of excitatory inputs from the cortex. High-frequency stimulation of the substantia nigra induced potentiation of the cortically evoked excitation and increased the prolonged AHP after the stimulus. These findings from anesthetized animals suggest that a substantia nigra-induced AHP produces stimulus-associated firing pauses in cholinergic interneurons. This mechanism may underlie the acquisition of the pause response in TANs recorded from behaving animals during learning.

Pauses in the tonic firing of striatal cholinergic interneurons emerge during reward-related learning and are triggered by neutral cues which develop behavioural significance. In a previous in vivo study we have proposed that these pauses in firing may be due to intrinsically generated afterhyperpolarisations (AHPs) evoked by excitatory synaptic inputs, including those below the threshold for action potential firing. The aim of this study was to investigate the mechanism of the AHPs using a brain slice preparation which preserved both cerebral hemispheres. Augmenting cortically evoked postsynaptic potentials (PSPs) by repetitive stimulation of cortical afferents evoked AHPs that were unaffected by blocking either GABAA receptors with bicuculline, or GABAB receptors with saclofen or CGP55845. Apamin (a blocker of small conductance Ca2+-activated K+ channels) had minimal effects, while chelation of intracellular Ca2+ with BAPTA reduced the AHP by about 30%. In contrast, blocking hyperpolarisation and cyclic nucleotide activated (HCN) cation current (IH) with ZD7288 or Cs+ diminished the size of the AHPs by 60% and reduced the proportion of episodes that contained this hyperpolarisation. The reversal potential (−20 mV) and voltage dependence of the AHPs were consistent with the hypothesis that a transient deactivation of IH caused most of the AHP at hyperpolarised potentials, while the slow AHP-type Ca2+-activated K+ channels increasingly contributed at more depolarised membrane potentials. Subthreshold somatic current injections yielded similar AHPs with a median duration of ∼700 ms that were not affected by firing of a single action potential. These results indicate that transient deactivation of HCN channels evokes pauses in tonic firing of cholinergic interneurons, an event likely to be elicited by augmentation of afferent synaptic inputs during learning.

Tonically active neurons in the primate striatum, believed to be cholinergic interneurons (CINs), respond to sensory stimuli with a pronounced pause in firing. Although inhibitory and neuromodulatory mechanisms have been implicated, it is not known how sensory stimuli induce firing pauses in CINs in vivo. Here, we used intracellular recordings in anesthetized rats to investigate the effectiveness of a visual stimulus at modulating spike activity in CINs. Initially, no neuron was visually responsive. However, following pharmacological activation of tecto-thalamic pathways, the firing pattern of most CINs was significantly modulated by a light flashed into the contralateral eye. Typically, this induced an excitation followed by a pause in spike firing, via an underlying depolarization-hyperpolarization membrane sequence. Stimulation of thalamic afferents in vitro evoked similar responses that were independent of synaptic inhibition. Thus, visual stimulation likely induces an initial depolarization via a subcortical tecto-thalamo-striatal pathway, pausing CIN firing through an intrinsic afterhyperpolarization.

The analysis of the neural mechanisms responsible for rewardrelated learning has benefited from recent studies of the effects of dopamine on synaptic plasticity. Dopamine-dependent synaptic plasticity may lead to strengthening of selected inputs on the basis of an activity-dependent conjunction of sensory afferent activity, motor output activity, and temporally related firing of dopamine cells. Such plasticity may provide a link between the reward-related firing of dopamine cells and the acquisition of changes in striatal cell activity during learning. This learning mechanism may play a special role in the translation of reward signals into context-dependent response probability or directional bias in movement responses. Abbreviations HFS high-frequency stimulation ICSS intracranial self-stimulation LTP long-term potentiation www.current-opinion.com Current Opinion in Neurobiology 2003, 13:685-690 30. Tremblay L, Hollerman JR, Schultz W: Modifications of reward expectation-related neuronal activity during learning in primate striatum.

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy. q : S 0 8 9 3 -6 0 8 0 ( 0 2 ) 0 0 0 4 5 -X Neural Networks 15 www.elsevier.com/locate/neunet

Cortico-striatal spike-timing dependent plasticity (STDP) is modulated by dopamine in vitro. The present study investigated STDP in vivo using alternative procedures for modulating dopaminergic inputs. Postsynaptic potentials (PSP) were evoked in intracellularly recorded spiny neurons by electrical stimulation of the contralateral motor cortex. PSPs often consisted of up to three distinct components, likely representing distinct cortico-striatal pathways. After baseline recording, bicuculline (BIC) was ejected into the superior colliculus (SC) to disinhibit visual pathways to the dopamine cells and striatum. Repetitive cortical stimulation (~60; 0.2Hz) was then paired with postsynaptic spike discharge induced by an intracellular current pulse, with each pairing followed 250ms later by a light flash to the contralateral eye (n=13). Changes in PSPs, measured as the maximal slope normalized to 5-min pre, ranged from potentiation (~120%) to depression (~80%). The determining factor was the relative timing between PSP components and spike: PSP components coinciding or closely following the spike tended towards potentiation, whereas PSP components preceding the spike were depressed. Importantly, STDP was only seen in experiments with successful BIC-mediated disinhibition (n=10). Cortico-striatal high-frequency stimulation (50 pulses at 100Hz) followed 100ms later by a light flash did not induce more robust synaptic plasticity (n=9). However, an elevated post-light spike rate correlated with depression across plasticity protocols (R2=0.55, p=0.009, n=11 active neurons). These results confirm that the direction of cortico-striatal plasticity is determined by the timing of pre- and postsynaptic activity and that synaptic modification is dependent on the activation of additional subcortical inputs.

Keywords: fCRP ERP Novelty P3 Stimulus-response Ideomotor a b s t r a c t Performance of voluntary behavior requires the selection of appropriate movements to attain a desired goal. We propose that the selection of voluntary movements is often contingent on the formation of a movement heuristic or set of internal rules governing movement selection. We used event-related potentials (ERPs) to identify the electrophysiological correlates of the formation of movement heuristics during movement-outcome learning. In two experiments, ERPs from non-learning control tasks were compared to a movement-learning task in which a movement heuristic was formed. We found that novelty P3 amplitude was negatively correlated with improved performance in the movement-learning task. Additionally, enhancement of novelty P3 amplitude was observed during learning even after controlling for memory, attentional and inter-stimulus interval parameters. The feedback correct-related positivity (fCRP) was only elicited by sensory effects following intentional movements. These findings extend previous studies demonstrating the role of the fCRP in performance monitoring and the role of the P3 in learning. In particular, the present study highlights an integrative role of the fCRP and the novelty P3 for the acquisition of movement heuristics. While the fCRP indicates that the goal of intentional movements has been attained, the novelty P3 engages stimulus-driven attentional mechanisms to determine the primary aspects of movement and context required to elicit the sensory effect.

The tonically active neurons (TANs) are a population of neurons scattered sparsely throughout the striatum that show intriguing patterns of firing activity during reinforcement learning. Following repeated pairings of a neutral stimulus with a primary reward, TANs develop a transient cessation of firing activity in response to the stimulus, termed the "conditioned pause response." In tasks where specific cues are arranged to signal the probability of particular outcomes, the pause response to both cue and outcome may differ in ways that suggest the involvement of different inputs to the same neuron. Here we review the cellular properties of cholinergic interneurons and describe the response to their afferents in terms of inducing TAN-like pauses in tonic firing. Recent work has shown that thalamostriatal inputs to cholinergic neurons transiently suppress firing activity via dopamine release. Because these pauses are initiated by subcortical pathways with limited sensory processing abilities, we propose that they are an ideal correlate for the pauses observed in TANs in response to cues signaling trial initiation. On the other hand, pauses that accompany outcome presentation contain higher-level information, including an apparent sensitivity to reward prediction error. Thus, these pauses may be mediated by cortical inputs to cholinergic interneurons. Although there is evidence linking cholinergic pauses to synaptic plasticity, much remains to be discovered about the effect of this relatively sparse but influential population on the striatal learning system.

The basal ganglia have been associated with processes of reinforcement learning. A strong line of supporting evidence comes from the recording of dopamine (DA) neurones in behaving monkeys. Unpredicted, biologically salient events, including rewards cause a stereotypic short-latency (70-100 ms), short-duration (100-200 ms) burst of DA activitythe phasic response. This response is widely considered to represent reward prediction errors used as teaching signals in appetitive learning to promote actions that will maximise future reward acquisition. For DA signalling to perform this function, sensory processing afferent to DA neurones should discriminate unpredicted reward-related events. However, the comparative response latencies of DA neurones and orienting gaze-shifts indicate that phasic DA responses are triggered by pre-attentive sensory processing. Consequently, in circumstances where biologically salient events are both spatially and temporally unpredictable, it is unlikely their identity will be known at the time of DA signalling. The limited quality of afferent sensory processing and the precise timing of phasic DA signals, suggests that they may play a less direct role in 'Law of Effect' appetitive learning. Rather, the 'time-stamp' nature of the phasic response, in conjunction with the other signals likely to be present in the basal ganglia at the time of phasic DA input, suggests it may reinforce the discovery of unpredicted sensory events for which the organism is responsible. Furthermore, DA-promoted repetition of preceding actions/movements should enable the system to converge on those aspects of context and behavioural output that lead to the discovery of novel actions.

Multifunctional agents with limited motor resources must decide what actions will best ensure their survival. Moreover, given that in an unpredictable world things don't always work out, considerable advantage is to be gained by learning from experience -instrumental behaviour that maximises reward and minimises punishment. In this review we will argue that the re-entrant looped architecture of the basal ganglia represents biological solutions to these fundamental behavioural problems of selection and reinforcement. A potential solution to the selection problem is provided for by selective disinhibition within the parallel loop architecture that connects the basal ganglia with external neural structures. The relay points within these loops permit the signals of a particular channel to be modified by external influences. In part, these influences have the capacity to modify overall selections so that the probability of re-selecting reinforced behaviours in the future is altered. This is the basic process of instrumental learning, which we suggest decomposes into two sub-problems for the agent: (i) learning which external events it causes to happen and learning precisely what it is doing that is causal; and (ii) having determined agency and discovered novel action-outcome routines, how best to exploit this knowledge to maximise future reward acquisitions. Considerations of connectional architecture and signal timing suggest that the short-latency, sensory-evoked dopamine response, which can modulate the re-entrant loop structure within the basal ganglia, is ideally suited to reinforce the determination of agency and the discovery of novel actions. Alternatively, recent studies showing that presence or absence of reward can selectively modulate the magnitude of signals in structures providing input signals to the basal ganglia, offer an alternative mechanism for biasing selection within the re-entrant loop architecture. We suggest that this mechanism may be better suited to ensure the prioritisation of inputs associated with reward.

We present evidence for the hypothesis that transitions between the low-and high-firing states of the cortical slow oscillation correspond to neuronal phase transitions. By analyzing intracellular recordings of the membrane potential during the cortical slow oscillation in rats, we quantify the temporal fluctuations in power and the frequency centroid of the power spectrum in the period of time before "down" to "up" transitions. By taking appropriate averages over such events, we present these statistics as a function of time before transition. The results demonstrate an increase in fluctuation power and time scale broadly consistent with the slowing of systems close to phase transitions. The analysis is complicated and limited by the difficulty in identifying when transitions begin, and removing dc trends in membrane potential.

The spiny projection neurons of the neostriatum are a site at which dopamine inputs from the substantia nigra converge with excitatory inputs from the cerebral cortex. These two systems interact in certain learning and motor control mechanisms of the brain. We investigated these interactions using intracellular recording from spiny striatal neurons in urethane-anaesthetized rats. We found that acute dopamine depletion was associated with long-term depression of corticostriatal synaptic input. Electrical stimulation of the cortex which mimicked synchronous cortical input to striatal neurons also induced long-term depression of corticostriatal inputs. In intact control animals, but not in dopaminedepleted animals, this depression was prevented or reversed by concomitant stimulation of the substantia nigra. In agreement with previous in vitro studies, our in vivo findings show that long-term depression occurs in the corticostriatal pathway, and in addition show that it is regulated by dopaminergic inputs from the substantia nigra. This form of synaptic plasticity may therefore be important for understanding disturbances of the motor system seen in humans with Parkinson's disease. ᭧

AbstractöThere is a large body of data on the ¢ring properties of dopamine cells in anaesthetised rats or rat brain slices. However, the extent to which these data relate to more natural conditions is uncertain, as there is little quantitative information available on the ¢ring properties of these cells in freely moving rats. We examined this by recording from the midbrain dopamine cell ¢elds using chronically implanted microwire electrodes. (1) In most cases, slowly ¢ring cells with broad action potentials were profoundly inhibited by the dopamine agonist apomorphine, consistent with previously accepted criteria. However, a small group of cells was found that were di⁄cult to classify because of ambiguous combinations of properties.

The spontaneously hypertensive rat (SHR) and the Wistar-Kyoto (WKY) inbred rat strains display behavioral differences characterized by relative increases and decreases in levels of activity. Both strains have subsequently been utilized as animal models of hyperactive and hypoactive behavioral traits. The etiology of these behavioral characteristics is poorly understood, but may stem from alterations in the physiology of selected neural circuits or catecholamine systems. This study investigated the cellular properties of neurons from three genetically related strains: the SHR; WKY; and Wistar (WI). In vivo intracellular recordings were made under urethane anesthesia from spiny projection neurons in the striatum, a brain area involved in behavioral activation. Results obtained from 71 spiny projection neurons indicate that most cellular properties of these neurons were very similar across the three strains. However, the amplitude and half-duration of both spontaneously occurring and current-evoked action potentials were found to be significantly different between the SHR and WKY strains with neurons from the SHR firing action potentials of relatively greater amplitude and shorter duration. Action potential parameters measured from the WI rats were intermediate between the two other strains. These differences in action potentials between two behaviorally distinct strains may reflect altered functioning of particular membrane conductances.

This study aimed to use a learning inventory (the Approaches and Study Skills Inventory for Students, ASSIST) to measure the impact of a curriculum change on students' approaches to learning in two large courses in a health sciences first year programme. The two new Human Body Systems (HUBS) courses were designed to encourage students to take a deep approach to learning. ASSIST was completed by 599 students enrolled in a biology class in 2006 that was part of the old curriculum, and by 705 students at the beginning and end of the new HUBS courses in 2007. Changes in students' approaches to learning over time were examined. The ASSIST scores for both HUBS courses reflected the dominance of a surface approach, followed by a strategic and then a deep approach. However, by the end of the year, students were taking a deep and strategic approach to their studies to a greater extent, and a surface approach to a lesser extent. Moreover, students enrolled in the new course adopted a deep approach to their studies to a significantly greater degree than those studying the old curriculum. Despite the predominance of a surface approach, the results suggest that it is possible to bring about small but significant positive changes in students' learning behaviour in a very large class through curriculum change. The proportion of students preferring a surface approach, and results showing that high performance on the final exam was significantly correlated with a surface approach, probably reflected contextual factors, including assessment, and is the focus of ongoing curriculum development.