The corticostriatal input to giant aspiny interneurons in the rat: a candidate pathway for synchronising the response to reward-related cues

The Integration of Top-down and Bottom-up Inputs to the Striatal Cholinergic Interneurons

BACKGROUND

Cholinergic interneurons (ChIs) are important for learning and memory. They exhibit a multiphasic excitation-pause-rebound response to reward or sensory cues indicating a reward, believed to gate dopamine-dependent learning. Although ChIs receive extensive top-down inputs from the cortex and bottom-up inputs from the thalamus and midbrain, it is unclear which inputs are involved in the development of ChI multiphasic activity.

METHODS

We used a single-unit recording of putative ChIs (pChIs) in response to cortical and visual stimulation to investigate how top-down and bottom-up inputs regulate the firing pattern of ChIs.

RESULTS

We demonstrated that cortical stimulation strongly regulates pChIs, with the maximum firing rate occurring at the peak of the inverted local field potential (iLFP), reflecting maximum cortical stimulation. Pauses in pChIs occurred during the descending phase of iLFP, indicating withdrawal of excitatory cortical input. Visual stimulation induced long pauses in pChIs, but it is unlikely that bottom- up inputs alone induce pauses in behaving animals. Also, the firing pattern of ChIs triggered by visual stimulation did not correlate with the iLFP as it did after cortical stimulation. Top-down and bottom-up inputs independently regulate the firing pattern of ChIs with similar efficacy but notably produce a well-defined pause in ChI firing.

CONCLUSION

This study provides in vivo evidence that the multiphasic ChI response may require both top-down and bottom-up inputs. The findings suggest that the firing pattern of ChIs correlated to the iLFP might be a useful tool for estimating the degree of contribution of top-down and bottom-up inputs in regulating the firing activity of ChIs.

Coincidence of cholinergic pauses, dopaminergic activation and depolarisation of spiny projection neurons drives synaptic plasticity in the striatum

Dopamine-dependent long-term plasticity is believed to be a cellular mechanism underlying reinforcement learning. In response to reward and reward-predicting cues, phasic dopamine activity potentiates the efficacy of corticostriatal synapses on spiny projection neurons (SPNs). Since phasic dopamine activity also encodes other behavioural variables, it is unclear how postsynaptic neurons identify which dopamine event is to induce long-term plasticity. Additionally, it is unknown how phasic dopamine released from arborised axons can potentiate targeted striatal synapses through volume transmission. To examine these questions we manipulated striatal cholinergic interneurons (ChIs) and dopamine neurons independently in two distinct in vivo paradigms. We report that long-term potentiation (LTP) at corticostriatal synapses with SPNs is dependent on the coincidence of pauses in ChIs and phasic dopamine activation, critically accompanied by SPN depolarisation. Thus, the ChI pause defines the time window for phasic dopamine to induce plasticity, while depolarisation of SPNs constrains the synapses eligible for plasticity.

Recurrent Implication of Striatal Cholinergic Interneurons in a Range of Neurodevelopmental, Neurodegenerative, and Neuropsychiatric Disorders

Cholinergic interneurons are "gatekeepers" for striatal circuitry and play pivotal roles in attention, goal-directed actions, habit formation, and behavioral flexibility. Accordingly, perturbations to striatal cholinergic interneurons have been associated with many neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. The role of acetylcholine in many of these disorders is well known, but the use of drugs targeting cholinergic systems fell out of favor due to adverse side effects and the introduction of other broadly acting compounds. However, in response to recent findings, re-examining the mechanisms of cholinergic interneuron dysfunction may reveal key insights into underlying pathogeneses. Here, we provide an update on striatal cholinergic interneuron function, connectivity, and their putative involvement in several disorders. In doing so, we aim to spotlight recurring physiological themes, circuits, and mechanisms that can be investigated in future studies using new tools and approaches.

Lateral ventral tegmental area GABAergic and glutamatergic modulation of conditioned learning

The firing activity of dorso-medial-striatal-cholinergic interneurons (dmCINs) is a neural correlate of classical conditioning. Tonically active, they pause in response to salient stimuli, mediating acquisition of predictive cues/outcome associations. Cortical and thalamic inputs are typical of the rather limited knowledge about underlying circuitry contributing to this function. Here, we dissect the midbrain GABA and glutamate-to-dmCIN pathways and evaluate how they influence conditioned behavior. We report that midbrain neurons discriminate auditory cues and encode the association of a predictive stimulus with a footshock. Furthermore, GABA and glutamate cells form selective monosynaptic contacts onto dmCINs and di-synaptic ones via the parafascicular thalamus. Pathway-specific inhibition of each sub-circuit produces differential impairments of fear-conditioned learning. Finally, Vglut2-expressing cells discriminate between CSs although Vgat-positive neurons associate the predictive cue with the outcome. Overall, these data suggest that each component of the network carries information pertinent to sub-domains of the behavioral strategy.

Cholinergic Transmission at Muscarinic Synapses in the Striatum Is Driven Equally by Cortical and Thalamic Inputs

The release of acetylcholine from cholinergic interneurons (ChIs) directly modulates striatal output via muscarinic receptors on medium spiny neurons (MSNs). While thalamic inputs provide strong excitatory input to ChIs, cortical inputs primarily regulate MSN firing. Here, we found that, while thalamic inputs do drive ChI firing, a subset of ChIs responds robustly to stimulation of cortical inputs as well. To examine how input-evoked changes in ChI firing patterns drive acetylcholine release at cholinergic synapses onto MSNs, muscarinic M4-receptor-mediated synaptic events were measured in MSNs overexpressing G-protein gated potassium channels (GlRK2). Stimulation of both cortical and thalamic inputs was sufficient to equally drive muscarinic synaptic events in MSNs, resulting from the broad synaptic innervation of the stimulus-activated ChI population across many MSNs. Taken together, this indicates an underappreciated role for the extensive cholinergic network, in which small populations of ChIs can drive substantial changes in post-synaptic receptor activity across the striatum.

Mamaligas et al. find that, while cortical inputs were previously thought to provide weak input to striatal cholinergic interneurons, they can drive firing in a subset of cells. As a result of the broad connectivity of cholinergic cells, cortical and thalamic inputs equally drive synaptic acetylcholine release onto MSNs.

Selective remodeling of glutamatergic transmission to striatal cholinergic interneurons after dopamine depletion

The widely held view that the pathophysiology of Parkinson's disease arises from an under-activation of the direct pathway striatal spiny neurons (dSPNs) has gained support from a recently described weakening of the glutamatergic projection from the parafascicular nucleus (PfN) to dSPNs in experimental parkinsonism. However, the impact of the remodeling of the thalamostriatal projection cannot be fully appreciated without considering its impact on cholinergic interneurons (ChIs) that themselves preferentially activate indirect pathway spiny neurons (iSPNs). To study this thalamostriatal projection, we virally transfected with Cre-dependent channelrhodopsin-2 (ChR2) the PfN of Vglut2-Cre mice that were dopamine-depleted with 6-hydroxydopamine (6-OHDA). In parallel, we studied the corticostriatal projection to ChIs in 6-OHDA-treated transgenic mice expressing ChR2 under the Thy1 promoter. We found the 6-OHDA lesions failed to affect short-term synaptic plasticity or the size of unitary responses evoked optogenetically in either of these projections. However, we found that NMDA-to-AMPA ratios at PfN synapses-that were significantly larger than NMDA-to-AMPA ratios at cortical synapses-were reduced by 6-OHDA treatment, thereby impairing synaptic integration at PfN synapses onto ChIs. Finally, we found that application of an agonist of the D5 dopamine receptors on ChIs potentiated NMDA currents without affecting AMPA currents or short-term plasticity selectively at PfN synapses. We propose that dopamine depletion leads to an effective de-potentiation of NMDA currents at PfN synapses onto ChIs which degrades synaptic integration. This selective remodeling of NMDA currents at PfN synapses may counter the selective weakening of PfN synapses onto dSPNs in parkinsonism.

Contribution of cholinergic interneurons to striatal pathophysiology in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by the loss of nigral dopaminergic neurons innervating the striatum, the main input structure of the basal ganglia. This creates an imbalance between dopaminergic inputs and cholinergic interneurons (ChIs) within the striatum. The efficacy of anticholinergic drugs, one of the earliest therapy for PD before the discovery of L-3,4-dihydroxyphenylalanine (L-DOPA) suggests an increased cholinergic tone in this disease. The dopamine (DA)-acetylcholine (ACh) balance hypothesis is now revisited with the use of novel cutting-edge techniques (optogenetics, pharmacogenetics, new electrophysiological recordings). This review will provide the background of the specific contribution of ChIs to striatal microcircuit organization in physiological and pathological conditions. The second goal of this review is to delve into the respective contributions of nicotinic and muscarinic receptor cholinergic subunits to the control of striatal afferent and efferent neuronal systems. Special attention will be given to the role played by muscarinic acetylcholine receptors (mAChRs) in the regulation of striatal network which may have important implications in the development of novel therapeutic strategies for motor and cognitive impairment in PD.

Pauses in Cholinergic Interneuron Activity Are Driven by Excitatory Input and Delayed Rectification, with Dopamine Modulation

Cholinergic interneurons (ChIs) of the striatum pause their firing in response to salient stimuli and conditioned stimuli after learning. Several different mechanisms for pause generation have been proposed, but a unifying basis has not previously emerged. Here, using in vivo and ex vivo recordings in rat and mouse brain and a computational model, we show that ChI pauses are driven by withdrawal of excitatory inputs to striatum and result from a delayed rectifier potassium current (IKr) in concert with local neuromodulation. The IKr is sensitive to Kv7.2/7.3 blocker XE-991 and enables ChIs to report changes in input, to pause on excitatory input recession, and to scale pauses with input strength, in keeping with pause acquisition during learning. We also show that although dopamine can hyperpolarize ChIs directly, its augmentation of pauses is best explained by strengthening excitatory inputs. These findings provide a basis to understand pause generation in striatal ChIs. VIDEO ABSTRACT.

Dopamine and Acetylcholine, a Circuit Point of View in Parkinson's Disease

Data from the World Health Organization (National Institute on Aging, 2011) and the National Institutes of Health (He et al., 2016) predicts that while today the worldwide population over 65 years of age is estimated around 8.5%, this number will reach an astounding 17% by 2050. In this framework, solving current neurodegenerative diseases primarily associated with aging becomes more pressing than ever. In 2017, we celebrate a grim 200th anniversary since the very first description of Parkinson’s disease (PD) and its related symptomatology. Two centuries after this debilitating disease was first identified, finding a cure remains a hopeful goal rather than an attainable objective on the horizon. Tireless work has provided insight into the characterization and progression of the disease down to a molecular level. We now know that the main motor deficits associated with PD arise from the almost total loss of dopaminergic cells in the substantia nigra pars compacta. A concomitant loss of cholinergic cells entails a cognitive decline in these patients, and current therapies are only partially effective, often inducing side-effects after a prolonged treatment. This review covers some of the recent developments in the field of Basal Ganglia (BG) function in physiology and pathology, with a particular focus on the two main neuromodulatory systems known to be severely affected in PD, highlighting some of the remaining open question from three main stand points:

- Heterogeneity of midbrain dopamine neurons.

- Pairing of dopamine (DA) sub-circuits.

- Dopamine-Acetylcholine (ACh) interaction.

A vast amount of knowledge has been accumulated over the years from experimental conditions, but very little of it is reflected or used at a translational or clinical level. An initiative to implement the knowledge that is emerging from circuit-based approaches to tackle neurodegenerative disorders like PD will certainly be tremendously beneficial.

Cortical Control of Striatal Dopamine Transmission via Striatal Cholinergic Interneurons

Corticostriatal regulation of striatal dopamine (DA) transmission has long been postulated, but ionotropic glutamate receptors have not been localized directly to DA axons. Striatal cholinergic interneurons (ChIs) are emerging as major players in striatal function, and can govern DA transmission by activating nicotinic receptors (nAChRs) on DA axons. Cortical inputs to ChIs have historically been perceived as sparse, but recent evidence indicates that they strongly activate ChIs. We explored whether activation of M1/M2 corticostriatal inputs can consequently gate DA transmission, via ChIs. We reveal that optogenetic activation of channelrhodopsin-expressing corticostriatal axons can drive striatal DA release detected with fast-scan cyclic voltammetry and requires activation of nAChRs on DA axons and AMPA receptors on ChIs that promote short-latency action potentials. By contrast, DA release driven by optogenetic activation of intralaminar thalamostriatal inputs involves additional activation of NMDA receptors on ChIs and action potential generation over longer timescales. Therefore, cortical and thalamic glutamate inputs can modulate DA transmission by regulating ChIs as gatekeepers, through ionotropic glutamate receptors. The different use of AMPA and NMDA receptors by cortical versus thalamic inputs might lead to distinct input integration strategies by ChIs and distinct modulation of the function of DA and striatum.

Dopaminergic Regulation of Striatal Interneurons in Reward and Addiction: Focus on Alcohol

Corticobasal ganglia networks coursing through the striatum are key structures for reward-guided behaviors. The ventral striatum (nucleus accumbens (nAc)) and its reciprocal connection with the ventral tegmental area (VTA) represent a primary component of the reward system, but reward-guided learning also involves the dorsal striatum and dopaminergic inputs from the substantia nigra. The majority of neurons in the striatum (>90%) are GABAergic medium spiny neurons (MSNs), but both the input to and the output from these neurons are dynamically controlled by striatal interneurons. Dopamine is a key neurotransmitter in reward and reward-guided learning, and the physiological activity of GABAergic and cholinergic interneurons is regulated by dopaminergic transmission in a complex manner. Here we review the role of striatal interneurons in modulating striatal output during drug reward, with special emphasis on alcohol.

Striatal cholinergic interneurons and cortico-striatal synaptic plasticity in health and disease

Basal ganglia disorders such as Parkinson's disease, dystonia, and Huntington's disease are characterized by a dysregulation of the basal ganglia neuromodulators (dopamine, acetylcholine, and others), which impacts cortico-striatal transmission. Basal ganglia disorders are often associated with an imbalance between the midbrain dopaminergic and striatal cholinergic systems. In contrast to the extensive research and literature on the consequences of a malfunction of midbrain dopaminergic signaling on the plasticity of the cortico-striatal synapse, very little is known about the role of striatal cholinergic interneurons in normal and pathological control of cortico-striatal transmission. In this review, we address the functional role of striatal cholinergic interneurons, also known as tonically active neurons and attempt to understand how the alteration of their functional properties in basal ganglia disorders leads to abnormal cortico-striatal synaptic plasticity. Specifically, we suggest that striatal cholinergic interneurons provide a permissive signal, which enables long-term changes in the efficacy of the cortico-striatal synapse. We further discuss how modifications in the striatal cholinergic activity pattern alter or prohibit plastic changes of the cortico-striatal synapse. Long-term cortico-striatal synaptic plasticity is the cellular substrate of procedural learning and adaptive control behavior. Hence, abnormal changes in this plasticity may underlie the inability of patients with basal ganglia disorders to adjust their behavior to situational demands. Normalization of the cholinergic modulation of cortico-striatal synaptic plasticity may be considered as a critical feature in future treatments of basal ganglia disorders.

SOX2 reprograms resident astrocytes into neural progenitors in the adult brain

Glial cells can be in vivo reprogrammed into functional neurons in the adult CNS; however, the process by which this reprogramming occurs is unclear. Here, we show that a distinct cellular sequence is involved in SOX2-driven in situ conversion of adult astrocytes to neurons. This includes ASCL1⁺ neural progenitors and DCX⁺ adult neuroblasts (iANBs) as intermediates. Importantly, ASCL1 is required, but not sufficient, for the robust generation of iANBs in the adult striatum. These progenitor-derived iANBs predominantly give rise to calretinin⁺ interneurons when supplied with neurotrophic factors or the small-molecule valproic acid. Patch-clamp recordings from the induced neurons reveal subtype heterogeneity, though all are functionally mature, fire repetitive action potentials, and receive synaptic inputs. Together, these results show that SOX2-mediated in vivo reprogramming of astrocytes to neurons passes through proliferative intermediate progenitors, which may be exploited for regenerative medicine.

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SOX2 induces ASCL1-positive neural progenitors in the adult mouse brain

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Ascl1 in resident astrocytes is required for SOX2-mediated in vivo reprogramming

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Induced ASCL1-positive neural progenitors generate mature calretinin neurons

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Whole-brain mapping of inputs to projection neurons and cholinergic interneurons in the dorsal striatum

The dorsal striatum integrates inputs from multiple brain areas to coordinate voluntary movements, associative plasticity, and reinforcement learning. Its projection neurons consist of the GABAergic medium spiny neurons (MSNs) that express dopamine receptor type 1 (D1) or dopamine receptor type 2 (D2). Cholinergic interneurons account for a small portion of striatal neuron populations, but they play important roles in striatal functions by synapsing onto the MSNs and other local interneurons. By combining the modified rabies virus with specific Cre- mouse lines, a recent study mapped the monosynaptic input patterns to MSNs. Because only a small number of extrastriatal neurons were labeled in the prior study, it is important to reexamine the input patterns of MSNs with higher labeling efficiency. Additionally, the whole-brain innervation pattern of cholinergic interneurons remains unknown. Using the rabies virus-based transsynaptic tracing method in this study, we comprehensively charted the brain areas that provide direct inputs to D1-MSNs, D2-MSNs, and cholinergic interneurons in the dorsal striatum. We found that both types of projection neurons and the cholinergic interneurons receive extensive inputs from discrete brain areas in the cortex, thalamus, amygdala, and other subcortical areas, several of which were not reported in the previous study. The MSNs and cholinergic interneurons share largely common inputs from areas outside the striatum. However, innervations within the dorsal striatum represent a significantly larger proportion of total inputs for cholinergic interneurons than for the MSNs. The comprehensive maps of direct inputs to striatal MSNs and cholinergic interneurons shall assist future functional dissection of the striatal circuits.

Severe drug-induced repetitive behaviors and striatal overexpression of VAChT in ChAT-ChR2-EYFP BAC transgenic mice

In drug users, drug-related cues alone can induce dopamine release in the dorsal striatum. Instructive cues activate inputs to the striatum from both dopaminergic and cholinergic neurons, which are thought to work together to support motor learning and motivated behaviors. Imbalances in these neuromodulatory influences can impair normal action selection and might thus contribute to pathologically repetitive and compulsive behaviors such as drug addiction. Dopamine and acetylcholine can have either antagonistic or synergistic effects on behavior, depending on the state of the animal and the receptor signaling systems at play. Semi-synchronized activation of cholinergic interneurons in the dorsal striatum drives dopamine release via presynaptic nicotinic acetylcholine receptors located on dopamine terminals. Nicotinic receptor blockade is known to diminish abnormal repetitive behaviors (stereotypies) induced by psychomotor stimulants. By contrast, blockade of postsynaptic acetylcholine muscarinic receptors in the dorsomedial striatum exacerbates drug-induced stereotypy, exemplifying how different acetylcholine receptors can also have opposing effects. Although acetylcholine release is known to be altered in animal models of drug addiction, predicting whether these changes will augment or diminish drug-induced behaviors thus remains a challenge. Here, we measured amphetamine-induced stereotypy in BAC transgenic mice that have been shown to overexpress the vesicular acetylcholine transporter (VAChT) with consequent increased acetylcholine release. We found that drug-induced stereotypies, consisting of confined sniffing and licking behaviors, were greatly increased in the transgenic mice relative to sibling controls, as was striatal VAChT protein. These findings suggest that VAChT-mediated increases in acetylcholine could be critical in exacerbating drug-induced stereotypic behaviors and promoting exaggerated behavioral fixity.

The intralaminar thalamus-an expressway linking visual stimuli to circuits determining agency and action selection

Anatomical investigations have revealed connections between the intralaminar thalamic nuclei and areas such as the superior colliculus (SC) that receive short latency input from visual and auditory primary sensory areas. The intralaminar nuclei in turn project to the major input nucleus of the basal ganglia, the striatum, providing this nucleus with a source of subcortical excitatory input. Together with a converging input from the cerebral cortex, and a neuromodulatory dopaminergic input from the midbrain, the components previously found necessary for reinforcement learning in the basal ganglia are present. With this intralaminar sensory input, the basal ganglia are thought to play a primary role in determining what aspect of an organism's own behavior has caused salient environmental changes. Additionally, subcortical loops through thalamic and basal ganglia nuclei are proposed to play a critical role in action selection. In this mini review we will consider the anatomical and physiological evidence underlying the existence of these circuits. We will propose how the circuits interact to modulate basal ganglia output and solve common behavioral learning problems of agency determination and action selection.

Cortical and thalamic excitation mediate the multiphasic responses of striatal cholinergic interneurons to motivationally salient stimuli

Cholinergic interneurons are key components of striatal microcircuits. In primates, tonically active neurons (putative cholinergic interneurons) exhibit multiphasic responses to motivationally salient stimuli that mirror those of midbrain dopamine neurons and together these two systems mediate reward-related learning in basal ganglia circuits. Here, we addressed the potential contribution of cortical and thalamic excitatory inputs to the characteristic multiphasic responses of cholinergic interneurons in vivo. We first recorded and labeled individual cholinergic interneurons in anesthetized rats. Electron microscopic analyses of these labeled neurons demonstrated that an individual interneuron could form synapses with cortical and, more commonly, thalamic afferents. Single-pulse electrical stimulation of ipsilateral frontal cortex led to robust short-latency (<20 ms) interneuron spiking, indicating monosynaptic connectivity, but firing probability progressively decreased during high-frequency pulse trains. In contrast, single-pulse thalamic stimulation led to weak short-latency spiking, but firing probability increased during pulse trains. After initial excitation from cortex or thalamus, interneurons displayed a "pause" in firing, followed by a "rebound" increase in firing rate. Across all stimulation protocols, the number of spikes in the initial excitation correlated positively with pause duration and negatively with rebound magnitude. The magnitude of the initial excitation, therefore, partly determined the profile of later components of multiphasic responses. Upon examining the responses of tonically active neurons in behaving primates, we found that these correlations held true for unit responses to a reward-predicting stimulus, but not to the reward alone, delivered outside of any task. We conclude that excitatory inputs determine, at least in part, the multiphasic responses of cholinergic interneurons under specific behavioral conditions.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

Pause and rebound: sensory control of cholinergic signaling in the striatum

Cholinergic interneurons have emerged as one of the key players controlling network functions in the striatum. Extracellularly recorded cholinergic interneurons acquire characteristic responses to sensory stimuli during reward-related learning, including a pause and subsequent rebound in spiking. However, the precise underlying cellular mechanisms have remained elusive. Here, we review recent advances in our understanding of the regulation of cholinergic interneuron activity. We discuss evidence of mechanisms that have been proposed to underlie sensory responses, including antagonistic actions by dopamine, recurrent inhibition via local interneurons, and an intrinsically generated membrane hyperpolarization in response to excitatory inputs. The review highlights outstanding questions and concludes with a model of the sensory responses and their downstream effects through dynamic acetylcholine receptor activation.

Relationships between the firing of identified striatal interneurons and spontaneous and driven cortical activities in vivo

The striatum is comprised of medium-sized spiny projection neurons (MSNs) and several types of interneuron, and receives massive glutamatergic input from the cerebral cortex. Understanding of striatal function requires definition of the electrophysiological properties of neurochemically identified interneurons sampled in the same context of ongoing cortical activity in vivo. To address this, we recorded the firing of cholinergic interneurons (expressing choline acetyltransferase; ChAT) and GABAergic interneurons expressing parvalbumin (PV) or nitric oxide synthase (NOS), as well as MSNs, in anesthetized rats during cortically defined brain states. Depending on the cortical state, these interneurons were partly distinguished from each other, and MSNs, on the basis of firing rate and/or pattern. During slow-wave activity (SWA), ChAT+ interneurons, and some PV+ and NOS+ interneurons, were tonically active; NOS+ interneurons fired prominent bursts but, contrary to investigations in vitro, these were not typical low-threshold spike bursts. Identified MSNs, and other PV+ and NOS+ interneurons, were phasically active. Contrasting with ChAT+ interneurons, whose firing showed poor brain state dependency, PV+ and NOS+ interneurons displayed robust firing increases and decreases, respectively, upon spontaneous or driven transitions from SWA to cortical activation. The firing of most neurons was phase locked to cortical slow oscillations, but only PV+ and ChAT+ interneurons also fired in time with cortical spindle and gamma oscillations. Complementing this diverse temporal coupling, each interneuron type exhibited distinct responses to cortical stimulation. Thus, these striatal interneuron types have distinct temporal signatures in vivo, including relationships to spontaneous and driven cortical activities, which likely underpin their specialized contributions to striatal microcircuit function.

Acetylcholine-based entropy in response selection: a model of how striatal interneurons modulate exploration, exploitation, and response variability in decision-making

The basal ganglia play a fundamental role in decision-making. Their contribution is typically modeled within a reinforcement learning framework, with the basal ganglia learning to select the options associated with highest value and their dopamine inputs conveying performance feedback. This basic framework, however, does not account for the role of cholinergic interneurons in the striatum, and does not easily explain certain dynamic aspects of decision-making and skill acquisition like the generation of exploratory actions. This paper describes basal ganglia acetylcholine-based entropy (BABE), a model of the acetylcholine system in the striatum that provides a unified explanation for these phenomena. According to this model, cholinergic interneurons in the striatum control the level of variability in behavior by modulating the number of possible responses that are considered by the basal ganglia, as well as the level of competition between them. This mechanism provides a natural way to account for the role of basal ganglia in generating behavioral variability during the acquisition of certain cognitive skills, as well as for modulating exploration and exploitation in decision-making. Compared to a typical reinforcement learning model, BABE showed a greater modulation of response variability in the face of changes in the reward contingences, allowing for faster learning (and re-learning) of option values. Finally, the paper discusses the possible applications of the model to other domains.

Cholinergic modulation of synaptic integration and dendritic excitability in the striatum

Modulatory interneurons such as, the cholinergic interneuron, are always a perplexing subject to study. Far from clear-cut distinctions such as excitatory or inhibitory, modulating interneurons can have many, often contradictory effects. The striatum is one of the most densely expressing brain areas for cholinergic markers, and actylcholine (ACh) plays an important role in regulating synaptic transmission and cellular excitability. Every cell type in the striatum has receptors for ACh. Yet even for a given cell type, ACh affecting different receptors can have seemingly opposing roles. This review highlights relevant effects of ACh on medium spiny neurons (MSNs) of the striatum and suggests how its many effects may work in concert to modulate MSN firing properties.

Visual-induced excitation leads to firing pauses in striatal cholinergic interneurons

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.

Spike-timing dependent plasticity in striatal interneurons

Basal ganglia, an ensemble of interconnected subcortical nuclei, are involved in adaptive motor planning and procedural learning. Striatum, the primary input nucleus of basal ganglia, extracts the pertinent cortical and thalamic information from background noise in relation with the environmental stimuli and motivation. The striatum comprises different neuronal populations: the GABAergic striatal output neurons, three classes of GABAergic interneurons and the cholinergic cells. Striatal interneurons exert a powerful control of striatal output neuron excitability and therefore shape the cortico-basal ganglia information processing. Besides output neurons, striatal interneurons also receive directly cortical information and are able to adapt their behavior depending on the level of cortical and striatal activation. In this review, we focus on the corticostriatal long-term synaptic efficacy changes occurring in interneurons, and especially the spike-timing dependent plasticity (STDP), as a Hebbian synaptic learning rule. Combined with the striatal local interactions between interneurons and output neurons, we will consider the functional consequences of the interneuron plasticity on the striatal output. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Neuroanatomical and neurochemical substrates of timing

We all have a sense of time. Yet, there are no sensory receptors specifically dedicated for perceiving time. It is an almost uniquely intangible sensation: we cannot see time in the way that we see color, shape, or even location. So how is time represented in the brain? We explore the neural substrates of metrical representations of time such as duration estimation (explicit timing) or temporal expectation (implicit timing). Basal ganglia (BG), supplementary motor area, cerebellum, and prefrontal cortex have all been linked to the explicit estimation of duration. However, each region may have a functionally discrete role and will be differentially implicated depending upon task context. Among these, the dorsal striatum of the BG and, more specifically, its ascending nigrostriatal dopaminergic pathway seems to be the most crucial of these regions, as shown by converging functional neuroimaging, neuropsychological, and psychopharmacological investigations in humans, as well as lesion and pharmacological studies in animals. Moreover, neuronal firing rates in both striatal and interconnected frontal areas vary as a function of duration, suggesting a neurophysiological mechanism for the representation of time in the brain, with the excitatory-inhibitory balance of interactions among distinct subtypes of striatal neuron serving to fine-tune temporal accuracy and precision.

The cholinergic system and neostriatal memory functions

The striatum is one of the major forebrain regions that strongly expresses muscarinic and nicotinic cholinergic receptors. This article reviews the current knowledge and our new findings about the striatal cholinoceptive organization and its role in a variety of cognitive functions. Pharmacological and genetic manipulations have indicated that the cholinergic and dopaminergic system in the striatum modulate each other's function. In addition to modulating the dopaminergic system, nicotinic cholinergic receptors facilitate GABA release, whereas muscarinic receptors attenuate GABA release. The striatal cholinergic system has also been implicated in various cognitive functions including procedural learning and intradimensional set shifting. Together, these data indicate that the cholinergic system in the striatum is involved in a diverse set of cognitive functions through interactions with other neurotransmitter systems including the dopaminergic and GABAergic systems.

Cortico-basal ganglia circuitry: a review of key research and implications for functional connectivity studies of mood and anxiety disorders

There is considerable evidence that dysfunction of the cortico-basal ganglia circuits may be associated with several mood and anxiety disorders. However, it is unclear whether circuit abnormalities contribute directly either to the neurobiology of these conditions or to the manifestation of symptoms. Understanding the role of these pathways in psychiatric illness has been limited by an incomplete characterization of normal function. In recent years, studies using animal models and human functional imaging have greatly expanded the literature describing normal cortico-basal ganglia circuit function. In this paper, recent key studies of circuit function using human and animal models are reviewed and integrated with findings from other studies conducted over the previous decades. The literature suggests several hypotheses of cortico-basal ganglia circuitry function in mood and anxiety disorders that warrant further exploration. Hypotheses are proposed herein based upon the cortico-basal ganglia mechanisms of: (1) feedforward and feedback control, (2) circuit integration and (3) emotional control. These are presented as models of circuit function, which may be particularly relevant to future investigations using neuroimaging and functional connectivity analyses.

Number and type of synapses on the distal dendrite of a rat striatal cholinergic interneuron: a quantitative, ultrastructural study

Knowledge of the innervation of interneurons within the striatum is critical to determining their role in the functioning of the striatal network. To this end, the synaptic innervation of a distal dendrite of a rat striatal cholinergic interneuron was quantified for the first time. These synaptic data were compared to three other dendrites from rat striatal interneurons and to published data from dendrites in the mammalian cerebral cortex. To label the cholinergic interneurons and their distal dendrites, a male Wistar rat was perfused and the striatum was double-immunolabelled with an antibody to choline acetyltransferase (ChAT) and an antibody to m2 muscarinic receptor. After processing for transmission electron microscopy, a cholinergic interneuron was located and an m2-labelled distal dendrite identified by tracing it through serial ultrathin sections to this double-immunolabelled soma. Two interneuronal distal dendrites in the same tissue, and another from a second rat, were used for comparison. The widths and lengths of the four distal dendrites, the total number and type of synapses, and the number of synapses per mum for each distal dendrite were measured. Symmetric synapses were the most common type on all four dendrites. There were 0.73 synapses per mum on the distal dendrite of the identified striatal cholinergic interneuron. Two other interneuronal dendrites that were positive for the m2 muscarinic receptor antibody showed similar synaptic densities of 0.62 and 0.83 synapses per microm of distal dendrite, respectively. On a third unlabelled interneuronal distal dendrite located in the lateral striatum, there were 2.17 synapses per microm. This interneuron was thought to be a parvalbumin interneuron rather than a calretinin interneuron, which would more likely be medially located. These data suggest that the number of synapses per microm on the distal dendrite of the cholinergic interneuron, and possibly two other cholinergic interneurons, is three times lower than that of a likely parvalbumin interneuron in the rat striatum. The number of synapses per microm of distal dendrite for a striatal cholinergic interneuron is also lower than the published 1.22-3.3 synapses per microm of dendrite for neurons in the mammalian cerebral cortex. Such anatomical data are important for the construction of new generation computer models that are better able to emulate the operation of striatal cholinergic interneurons.

Spike-timing dependent plasticity in the striatum

The striatum is the major input nucleus of basal ganglia, an ensemble of interconnected sub-cortical nuclei associated with fundamental processes of action-selection and procedural learning and memory. The striatum receives afferents from the cerebral cortex and the thalamus. In turn, it relays the integrated information towards the basal ganglia output nuclei through which it operates a selected activation of behavioral effectors. The striatal output neurons, the GABAergic medium-sized spiny neurons (MSNs), are in charge of the detection and integration of behaviorally relevant information. This property confers to the striatum the ability to extract relevant information from the background noise and select cognitive-motor sequences adapted to environmental stimuli. As long-term synaptic efficacy changes are believed to underlie learning and memory, the corticostriatal long-term plasticity provides a fundamental mechanism for the function of the basal ganglia in procedural learning. Here, we reviewed the different forms of spike-timing dependent plasticity (STDP) occurring at corticostriatal synapses. Most of the studies have focused on MSNs and their ability to develop long-term plasticity. Nevertheless, the striatal interneurons (the fast-spiking GABAergic, NO-synthase and cholinergic interneurons) also receive monosynaptic afferents from the cortex and tightly regulated corticostriatal information processing. Therefore, it is important to take into account the variety of striatal neurons to fully understand the ability of striatum to develop long-term plasticity. Corticostriatal STDP with various spike-timing dependence have been observed depending on the neuronal sub-populations and experimental conditions. This complexity highlights the extraordinary potentiality in term of plasticity of the corticostriatal pathway.

Conditional routing of information to the cortex: a model of the basal ganglia's role in cognitive coordination

The basal ganglia play a central role in cognition and are involved in such general functions as action selection and reinforcement learning. Here, we present a model exploring the hypothesis that the basal ganglia implement a conditional information-routing system. The system directs the transmission of cortical signals between pairs of regions by manipulating separately the selection of sources and destinations of information transfers. We suggest that such a mechanism provides an account for several cognitive functions of the basal ganglia. The model also incorporates a possible mechanism by which subsequent transfers of information control the release of dopamine. This signal is used to produce novel stimulus-response associations by internalizing transferred cortical representations in the striatum. We discuss how the model is related to production systems and cognitive architectures. A series of simulations is presented to illustrate how the model can perform simple stimulus-response tasks, develop automatic behaviors, and provide an account of impairments in Parkinson's and Huntington's diseases.

IH current generates the afterhyperpolarisation following activation of subthreshold cortical synaptic inputs to striatal cholinergic interneurons

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 GABA(A) receptors with bicuculline, or GABA(B) receptors with saclofen or CGP55845. Apamin (a blocker of small conductance Ca(2+)-activated K(+) channels) had minimal effects, while chelation of intracellular Ca(2+) with BAPTA reduced the AHP by about 30%. In contrast, blocking hyperpolarisation and cyclic nucleotide activated (HCN) cation current (I(H)) 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 I(H) caused most of the AHP at hyperpolarised potentials, while the slow AHP-type Ca(2+)-activated K(+) channels increasingly contributed at more depolarised membrane potentials. Subthreshold somatic current injections yielded similar AHPs with a median duration of approximately 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.

Recurrent inhibitory network among striatal cholinergic interneurons

The striatum plays a central role in sensorimotor learning and action selection. Tonically active cholinergic interneurons in the striatum give rise to dense axonal arborizations and significantly shape striatal output. However, it is not clear how the activity of these neurons is regulated within the striatal microcircuitry. In this study, using rat brain slices, we find that stimulation of intrastriatal cholinergic fibers evokes polysynaptic GABA(A) IPSCs in cholinergic interneurons. These polysynaptic GABA(A) IPSCs were abolished by general nicotinic acetylcholine receptor antagonists and also by a specific antagonist of nicotinic receptors containing beta2 subunits. Dopamine receptor antagonists or dopamine depletion failed to block polysynaptic IPSCs, indicating that phasic dopamine release does not directly mediate the polysynaptic transmission. Dual recording from pairs of cholinergic interneurons revealed that activation of a single cholinergic interneuron is capable of eliciting polysynaptic GABA(A) IPSCs both in itself and in nearby cholinergic interneurons. Although polysynaptic transmission arising from a single cholinergic interneuron was depressed during repetitive 2 Hz firing, intrastriatal stimulation reliably evoked large polysynaptic IPSCs by recruiting many cholinergic fibers. We also show that polysynaptic GABAergic inhibition leads to a transient suppression of tonic cholinergic interneuron firing. We propose a novel microcircuit in the striatum, in which cholinergic interneurons are connected to one another through GABAergic interneurons. This may provide a mechanism to convert activation of cholinergic interneurons into widespread recurrent inhibition of these neurons via nicotinic excitation of striatal GABAergic neurons.

Cell-specific spike-timing-dependent plasticity in GABAergic and cholinergic interneurons in corticostriatal rat brain slices

Striatum, the main input nucleus of basal ganglia, is involved in the learning of cognitive and motor sequences in response to environmental stimuli. Striatal output neurons (medium spiny neurons, MSNs) integrate cortical activity and the two main classes of interneurons (GABAergic and cholinergic interneurons) tightly regulate the corticostriatal information transfer. We have explored the transmission between cortex and striatal interneurons and their capability to develop activity-dependent long-term plasticity based on the quasi-coincident cortical and striatal activities (spike-timing-dependent plasticity, STDP). We have observed glutamatergic monosynaptic connections between cortical cells and both striatal interneurons. Excitatory postsynaptic current latencies and rise times revealed that a cortical stimulation activates GABAergic interneurons before cholinergic, and both interneurons before MSNs. In addition, we have observed that striatal interneurons are able to develop bidirectional long-term plasticity and that there is a cell-specificity of STDP among striatal interneurons. Indeed, in GABAergic interneurons, long-term depression (LTD) and long-term potentiation (LTP) are induced by post-pre and pre-post STDP protocols, respectively. Cholinergic interneurons displayed a partially reversed STDP when compared to GABAergic interneurons: post-pre protocols induced LTP as well as LTD (the induction of either LTP or LTD is correlated with rheobase) and pre-post protocols induced LTD. The cell-specificity of STDP also concerned the receptors activated for the induction of LTP and LTD in GABAergic and cholinergic interneurons: in GABAergic interneurons LTP and LTD required NMDA receptor-activation whereas, in cholinergic interneurons, LTP was underlain by NMDA receptor-activation and LTD by metabotropic glutamate receptors.

Single unit and population responses during inhibitory gating of striatal activity in freely moving rats

The striatum is thought to be an essential region for integrating diverse information in the brain. Rapid inhibitory gating (IG) of sensory input is most likely an early factor necessary for appropriate integration to be completed. Gating is currently evaluated in clinical settings and is dramatically altered in a variety of psychiatric illnesses. Basic neuroscience research using animals has revealed specific neural sites involved in IG including the hippocampus, thalamus, brainstem, amygdala and medial prefrontal cortex. The present study investigated local IG in the basal ganglia structure of the striatum using chronic recording microwires. We obtained both single unit activations and local field potentials (LFPs) in awake behaving rats from each wire during the standard two-tone paradigm. Single units responded with different types of activations including a phasic and sustained excitation, an inhibitory response and a combination response that contained both excitatory and inhibitory components. IG was observed in all the response types; however, non-gating was observed in a large proportion of responses as well. Positive wave field potentials at 50-60 ms post-stimulus (P60) showed consistent gating across the wire arrays. No significant correlations were found between single unit and LFP measures of gating during the initial baseline session. Gating was strengthened (Tamp/Camp ratios approaching 0) following acute stress (saline injection) at both the single unit and LFP level due to the reduction in the response to the second tone. Alterations in sensory responding reflected by changes in the neural response to the initial tone were primarily observed following long-term internal state deviation (food deprivation) and during general locomotion. Overall, our results support local IG by single neurons in striatum but also suggest that rapid inhibition is not the dominant activation profile observed in other brain regions.

Differences in striatal spiny neuron action potentials between the spontaneously hypertensive and Wistar-Kyoto rat strains

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.

Cholinergic interneurons control the excitatory input to the striatum

How the extent and time course of presynaptic inhibition depend on the action potentials of the neuron controlling the terminals is unknown. We investigated this issue in the striatum using paired recordings from cholinergic interneurons and projection neurons. Glutamatergic EPSCs were evoked in projection neurons and cholinergic interneurons by stimulation of afferent fibers in the cortex and the striatum, respectively. A single spike in a cholinergic interneuron caused significant depression of the evoked glutamatergic EPSC in 34% of projection neurons located within 100 microm and 41% of cholinergic interneurons located within 200 microm. The time course of these effects was similar in the two cases, with EPSC inhibition peaking 20-30 ms after the spike and disappearing after 40-80 ms. Maximal depression of EPSC amplitude was up to 27% in projection neurons and to 19% in cholinergic interneurons. These effects were reversibly blocked by muscarinic receptor antagonists (atropine or methoctramine), which also significantly increased baseline EPSC (evoked without a preceding spike in the cholinergic interneuron), suggesting that some tonic cholinergic presynaptic inhibition was present. This was confirmed by the fact that lowering extracellular potassium, which silenced spontaneously active cholinergic interneurons, also increased baseline EPSC amplitude, and these effects were occluded by previous application of muscarinic receptor antagonists. Collectively, these results show that a single spike in a cholinergic interneuron exerts a fast and powerful inhibitory control over the glutamatergic input to striatal neurons.

Acetylcholine receptor binding and cholinergic interneuron density are unaltered in a genetic animal model of primary paroxysmal dystonia

The underlying pathophysiological mechanisms of hereditary types of paroxysmal dyskinesias are still unknown, but basal ganglia dysfunctions seem to play a critical role. In fact, numerous pharmacological, neurochemical, immunohistochemical and electrophysiological investigations in the dt(sz) hamsters, a unique rodent model of age-dependent primary paroxysmal dystonia, revealed alterations within the basal ganglia, particularly of the GABAergic and dopaminergic neurotransmitter systems. A deficit in several types of striatal GABAergic interneurons in dt(sz) mutant hamsters seems to play a crucial pathophysiological role, but deficits in other types of striatal interneurons cannot be excluded by previous studies. In view of ameliorating effects of anti-cholinergic drugs in dystonic patients, we therefore investigated the density of striatal cholinergic interneurons in the present study. These interneurons were marked specifically by the enzyme choline acetyltransferase and counted by using a stereological counting method in a blinded fashion. Additionally, acetylcholine receptor binding was determined in mutant and nondystonic control hamsters by autoradiographic analyses with the nonselective muscarinic ligand [(3)H]-quinuclidinyl benzilate (QNB) in 11 brain (sub)regions. There were no significant differences in the density of striatal cholinergic interneurons between dt(sz) mutant hamsters (789 +/- 39 interneurons/mm(3)) and nondystonic controls (807 +/- 36 interneurons/mm(3)). [(3)H]QNB binding was also comparable between mutant and control hamsters. These results point to an unaltered striatal cholinergic neurotransmitter system in dt(sz) hamsters under basal conditions.

Control of spontaneous firing patterns by the selective coupling of calcium currents to calcium-activated potassium currents in striatal cholinergic interneurons

The spontaneous firing patterns of striatal cholinergic interneurons are sculpted by potassium currents that give rise to prominent afterhyperpolarizations (AHPs). Large-conductance calcium-activated potassium (BK) channel currents contribute to action potential (AP) repolarization; small-conductance calcium-activated potassium channel currents generate an apamin-sensitive medium AHP (mAHP) after each AP; and bursts of APs generate long-lasting slow AHPs (sAHPs) attributable to apamin-insensitive currents. Because all these currents are calcium dependent, we conducted voltage- and current-clamp whole-cell recordings while pharmacologically manipulating calcium channels of the plasma membrane and intracellular stores to determine what sources of calcium activate the currents underlying AP repolarization and the AHPs. The Cav2.2 (N-type) blocker omega-conotoxin GVIA (1 microM) was the only blocker that significantly reduced the mAHP, and it induced a transition to rhythmic bursting in one-third of the cells tested. Cav1 (L-type) blockers (10 microM dihydropyridines) were the only ones that significantly reduced the sAHP. When applied to cells induced to burst with apamin, dihydropyridines reduced the sAHPs and abolished bursting. Depletion of intracellular stores with 10 mM caffeine also significantly reduced the sAHP current and reversibly regularized firing. Application of 1 microM omega-conotoxin MVIIC (a Cav2.1/2.2 blocker) broadened APs but had a negligible effect on APs in cells in which BK channels were already blocked by submillimolar tetraethylammonium chloride, indicating that Cav2.1 (Q-type) channels provide the calcium to activate BK channels that repolarize the AP. Thus, calcium currents are selectively coupled to the calcium-dependent potassium currents underlying the AHPs, thereby creating mechanisms for control of the spontaneous firing patterns of these neurons.

Modulation of an afterhyperpolarization by the substantia nigra induces pauses in the tonic firing of striatal cholinergic interneurons

Striatal cholinergic interneurons, also known as tonically active neurons (TANs), acquire a pause in firing during learning of stimulus-reward 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 after-hyperpolarization (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.