Muscarinic receptors differentially modulate the persistent potassium current in striatal spiny projection neurons

A spiking computational model for striatal cholinergic interneurons

Cholinergic interneurons in the striatum, also known as tonically active interneurons or TANs, are thought to have a strong effect on corticostriatal plasticity and on striatal activity and outputs, which in turn play a critical role in modulating downstream basal ganglia activity and movement. Striatal TANs can exhibit a variety of firing patterns and responses to synaptic inputs; furthermore, they have been found to display various surges and pauses in activity associated with sensory cues and reward delivery in learning as well as with motor tic production. To help explain the factors that contribute to TAN activity patterns and to provide a resource for future studies, we present a novel conductance-based computational model of a striatal TAN. We show that this model produces the various characteristic firing patterns observed in recordings of TANs. With a single baseline tuning associated with tonic firing, the model also captures a wide range of TAN behaviors found in previous experiments involving a variety of manipulations. In addition to demonstrating these results, we explain how various ionic currents in the model contribute to them. Finally, we use this model to explore the contributions of the acetylcholine released by TANs to the production of surges and pauses in TAN activity in response to strong excitatory inputs. These results provide predictions for future experimental testing that may help with efforts to advance our understanding of the role of TANs in reinforcement learning and in motor disorders such as Tourette's syndrome.

A tonic nicotinic brake controls spike timing in striatal spiny projection neurons

Striatal spiny projection neurons (SPNs) transform convergent excitatory corticostriatal inputs into an inhibitory signal that shapes basal ganglia output. This process is fine-tuned by striatal GABAergic interneurons (GINs), which receive overlapping cortical inputs and mediate rapid corticostriatal feedforward inhibition of SPNs. Adding another level of control, cholinergic interneurons (CINs), which are also vigorously activated by corticostriatal excitation, can disynaptically inhibit SPNs by activating α4β2 nicotinic acetylcholine receptors (nAChRs) on various GINs. Measurements of this disynaptic inhibitory pathway, however, indicate that it is too slow to compete with direct GIN-mediated feedforward inhibition. Moreover, functional nAChRs are also present on populations of GINs that respond only weakly to phasic activation of CINs, such as parvalbumin-positive fast-spiking interneurons (PV-FSIs), making the overall role of nAChRs in shaping striatal synaptic integration unclear. Using acute striatal slices from mice we show that upon synchronous optogenetic activation of corticostriatal projections blockade of α4β2 nAChRs shortened SPN spike latencies and increased postsynaptic depolarizations. The nAChR-dependent inhibition was mediated by downstream GABA release, and data suggest that the GABA source was not limited to GINs that respond strongly to phasic CIN activation. In particular, the observed decrease in spike latency caused by nAChR blockade was associated with a diminished frequency of spontaneous inhibitory postsynaptic currents in SPNs, a parallel hyperpolarization of PV-FSIs, and was occluded by pharmacologically preventing cortical activation of PV-FSIs. Taken together, we describe a role for tonic (as opposed to phasic) activation of nAChRs in striatal function. We conclude that tonic activation of nAChRs by CINs maintains a GABAergic brake on cortically-driven striatal output by ‘priming’ feedforward inhibition, a process that may shape SPN spike timing, striatal processing, and synaptic plasticity.

Pauses in cholinergic interneuron firing exert an inhibitory control on striatal output in vivo

The cholinergic interneurons (CINs) of the striatum are crucial for normal motor and behavioral functions of the basal ganglia. Striatal CINs exhibit tonic firing punctuated by distinct pauses. Pauses occur in response to motivationally significant events, but their function is unknown. Here we investigated the effects of pauses in CIN firing on spiny projection neurons (SPNs) – the output neurons of the striatum – using in vivo whole cell and juxtacellular recordings in mice. We found that optogenetically-induced pauses in CIN firing inhibited subthreshold membrane potential activity and decreased firing of SPNs. During pauses, SPN membrane potential fluctuations became more hyperpolarized and UP state durations became shorter. In addition, short-term plasticity of corticostriatal inputs was decreased during pauses. Our results indicate that, in vivo, the net effect of the pause in CIN firing on SPNs activity is inhibition and provide a novel mechanism for cholinergic control of striatal output.

Nerve cells or neurons communicate with one another using electrical impulses and chemical messengers called neurotransmitters. Additional molecules known as neuromodulators regulate the communication process. In contrast to neurotransmitters, neuromodulators do not send messages directly from one neuron to the next. Instead they change the way that neurons respond to neurotransmitters.

For example, the neuromodulator acetylcholine is most abundant in a region called the striatum. Located deep within the brain, the striatum contributes to learning and memory, motivation, and movement. Studies in rodents show that neurons within the striatum called cholinergic interneurons are almost continuously active. Each time these cells fire, they release acetylcholine. But whenever an animal experiences something unusual or important, the interneurons temporarily stop firing. Zucca et al. wanted to know whether these pauses in firing also act as a signal within the striatum.

To find out, Zucca et al. inserted a light-sensitive ion channel into cholinergic interneurons in the mouse striatum. Activating the ion channels with a laser beam stopped the interneurons from firing. Zucca et al. showed that these pauses in firing reduced the activity of another group of neurons, the spiny projection neurons. These are the major output neurons of the striatum. They send messages from the striatum to other parts of the brain. The results thus suggest that cholinergic interneurons signal notable events by temporarily blocking output from the striatum.

Understanding how cholinergic interneurons work will help reveal how the striatum drives behavior. It may also lead to treatments for diseases caused by cholinergic system dysfunction. Many patients with Parkinson’s disease or schizophrenia take medicines to block the effects of acetylcholine. Understanding how acetylcholine affects the striatum may help clarify how these treatments work.

TRPC3 channel mediates excitation of striatal cholinergic interneurons

Striatal cholinergic interneurons exhibit tonic firing and more positive membrane potentials, however, the mechanism is unclear. In the present study, we found that intracellular perfusion of TRPC3 antibody induced outward current in striatal cholinergic interneurons identified by electrophysiological characteristics. The TRPC3 channel blocker flufenamic acid induced hyperpolarization, and reduced firing rate and outward current which was similar to the effect of TRPC3 channel antibody. Furthermore, by using single-cell RT-PCR we confirmed the co-existence of TRPC3 channel and D5 receptor mRNA in striatal cholinergic interneurons identified by electrophysiological characteristics and expression of choline acetyltransferase (Chat) mRNA. These results implied that the TRPC3 channel is involved in modulating the depolarization of cholinergic interneurons.

Global actions of nicotine on the striatal microcircuit

The question to solve in the present work is: what is the predominant action induced by the activation of cholinergic-nicotinic receptors (nAChrs) in the striatal network given that nAChrs are expressed by several elements of the circuit: cortical terminals, dopamine terminals, and various striatal GABAergic interneurons. To answer this question some type of multicellular recording has to be used without losing single cell resolution. Here, we used calcium imaging and nicotine. It is known that in the presence of low micromolar N-Methyl-D-aspartate (NMDA), the striatal microcircuit exhibits neuronal activity consisting in the spontaneous synchronization of different neuron pools that interchange their activity following determined sequences. The striatal circuit also exhibits profuse spontaneous activity in pathological states (without NMDA) such as dopamine depletion. However, in this case, most pathological activity is mostly generated by the same neuron pool. Here, we show that both types of activity are inhibited during the application of nicotine. Nicotine actions were blocked by mecamylamine, a non-specific antagonist of nAChrs. Interestingly, inhibitory actions of nicotine were also blocked by the GABA_A-receptor antagonist bicuculline, in which case, the actions of nicotine on the circuit became excitatory and facilitated neuronal synchronization. We conclude that the predominant action of nicotine in the striatal microcircuit is indirect, via the activation of networks of inhibitory interneurons. This action inhibits striatal pathological activity in early Parkinsonian animals almost as potently as L-DOPA.

Cholinergic interneurons suppress action potential initiation of medium spiny neurons in rat nucleus accumbens shell

Acetylcholine plays a crucial role in the regulation of neural functions, including dopamine release, synaptic activity, and intrinsic electrophysiological properties of the nucleus accumbens (NAc) shell. Although the effects of acetylcholine on the action potential properties of NAc medium spiny (MS) neurons have been reported, how intrinsic acetylcholine released from NAc cholinergic interneurons regulates the neural activity of MS neurons is still an open issue. To explore the cholinergic effects on the subthreshold responses and action potential properties of MS neurons in the NAc shell, we first tested the effects of carbachol, a non-selective cholinergic agonist, on MS neuronal activity. Then, we tested the effects of the activation of cholinergic interneurons on the electrophysiological properties of MS neurons via multiple whole-cell patch-clamp recordings. Bath application of carbachol induced resting membrane potential depolarization accompanied by an increase in the voltage response to negative current injection. These increases were blocked by the pre-application of pirenzepine, an M1 muscarinic receptor antagonist. In spite of the facilitative effect on voltage responses of negative current injection, carbachol diminished the characteristic slowly-depolarizing ramp potentials, which respond to positive current pulse injection. Thus, carbachol increased the rheobase and shifted the frequency-current curve toward the right. Repetitive spike firing of a cholinergic interneuron following positive current injection induced a similar increase in the rheobase, which delayed the action potential initiation in 38.9% MS neurons. In contrast to the bath application of carbachol, cholinergic interneuronal stimulation had little effect on the resting membrane potential in MS neurons. These results suggest that the acetylcholine released from a cholinergic interneuron is sufficient to suppress the repetitive spike firing of the adjacent MS neurons, although the depolarization of the resting membrane potential may require simultaneous activation of multiple cholinergic interneurons.

Complex autonomous firing patterns of striatal low-threshold spike interneurons

During sensorimotor learning, tonically active neurons (TANs) in the striatum acquire bursts and pauses in their firing based on the salience of the stimulus. Striatal cholinergic interneurons display tonic intrinsic firing, even in the absence of synaptic input, that resembles TAN activity seen in vivo. However, whether there are other striatal neurons among the group identified as TANs is unknown. We used transgenic mice expressing green fluorescent protein under control of neuronal nitric oxide synthase or neuropeptide-Y promoters to aid in identifying low-threshold spike (LTS) interneurons in brain slices. We found that these neurons exhibit autonomous firing consisting of spontaneous transitions between regular, irregular, and burst firing, similar to cholinergic interneurons. As in cholinergic interneurons, these firing patterns arise from interactions between multiple intrinsic oscillatory mechanisms, but the mechanisms responsible differ. Both neurons maintain tonic firing because of persistent sodium currents, but the mechanisms of the subthreshold oscillations responsible for irregular firing are different. In LTS interneurons they rely on depolarization-activated noninactivating calcium currents, whereas those in cholinergic interneurons arise from a hyperpolarization-activated potassium conductance. Sustained membrane hyperpolarizations induce a bursting pattern in LTS interneurons, probably by recruiting a low-threshold, inactivating calcium conductance and by moving the membrane potential out of the activation range of the oscillatory mechanisms responsible for single spiking, in contrast to the bursting driven by noninactivating currents in cholinergic interneurons. The complex intrinsic firing patterns of LTS interneurons may subserve differential release of classic and peptide neurotransmitters as well as nitric oxide.

Muscarinic modulation of striatal function and circuitry

Striatal cholinergic interneurons are pivotal modulators of the striatal circuitry involved in action selection and decision making. Although nicotinic receptors are important transducers of acetylcholine release in the striatum, muscarinic receptors are more pervasive and have been more thoroughly studied. In this review, the effects of muscarinic receptor signaling on the principal cell types in the striatum and its canonical circuits will be discussed, highlighting new insights into their role in synaptic integration and plasticity. These studies, and those that have identified new circuit elements driven by activation of nicotinic receptors, make it clear that temporally patterned activity in cholinergic interneurons must play an important role in determining the effects on striatal circuitry. These effects could be critical to the response to salient environmental stimuli that serve to direct behavior.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

The Neurodynamics of Cognition: A Tutorial on Computational Cognitive Neuroscience

Computational Cognitive Neuroscience (CCN) is a new field that lies at the intersection of computational neuroscience, machine learning, and neural network theory (i.e., connectionism). The ideal CCN model should not make any assumptions that are known to contradict the current neuroscience literature and at the same time provide good accounts of behavior and at least some neuroscience data (e.g., single-neuron activity, fMRI data). Furthermore, once set, the architecture of the CCN network and the models of each individual unit should remain fixed throughout all applications. Because of the greater weight they place on biological accuracy, CCN models differ substantially from traditional neural network models in how each individual unit is modeled, how learning is modeled, and how behavior is generated from the network. A variety of CCN solutions to these three problems are described. A real example of this approach is described, and some advantages and limitations of the CCN approach are discussed.

A computational model of how cholinergic interneurons protect striatal-dependent learning

An essential component of skill acquisition is learning the environmental conditions in which that skill is relevant. This article proposes and tests a neurobiologically detailed theory of how such learning is mediated. The theory assumes that a key component of this learning is provided by the cholinergic interneurons in the striatum known as tonically active neurons (TANs). The TANs are assumed to exert a tonic inhibitory influence over cortical inputs to the striatum that prevents the execution of any striatal-dependent actions. The TANs learn to pause in rewarding environments, and this pause releases the striatal output neurons from this inhibitory effect, thereby facilitating the learning and expression of striatal-dependent behaviors. When rewards are no longer available, the TANs cease to pause, which protects striatal learning from decay. A computational version of this theory accounts for a variety of single-cell recording data and some classic behavioral phenomena, including fast reacquisition after extinction.

Computational perspectives on forebrain microcircuits implicated in reinforcement learning, action selection, and cognitive control

Abundant new information about signaling pathways in forebrain microcircuits presents many challenges, and opportunities for discovery, to computational neuroscientists who strive to bridge from microcircuits to flexible cognition and action. Accurate treatment of microcircuit pathways is especially critical for creating models that correctly predict the outcomes of candidate neurological therapies. Recent models are trying to specify how cortical circuits that enable planning and voluntary actions interact with adaptive subcortical microcircuits in the basal ganglia. The basal ganglia are strongly implicated in reinforcement learning, and in all behavior and cognition over which the frontal lobes exert flexible control. The persisting role of the basal ganglia shows that ancient vertebrate designs for motivated action selection proved adaptable enough to support many "modern" behavioral innovations, including fluent generation of language and speech. This paper summarizes how recent models have incorporated realistic representations of microcircuit features, and have begun to trace their computational implications. Also summarized are recent empirical discoveries that provide guidance regarding how to formulate the rules for synaptic modification that govern learning in cortico-striatal pathways. Such efforts are contributing to an emerging synthesis based on an interlocking set of computational hypotheses regarding cortical interactions with basal ganglia and thalamic nuclei. These hypotheses specify how specialized microcircuits solve learning and control problems inherent to the brain's parallel design.

A dopamine-acetylcholine cascade: simulating learned and lesion-induced behavior of striatal cholinergic interneurons

The giant cholinergic interneurons of the striatum are tonically active neurons (TANs) that respond with pauses to appetitive and aversive cues and to novel events. Whereas tonic activity emerges from intrinsic properties of these neurons, glutamatergic inputs from intralaminar thalamic nuclei and dopaminergic inputs from midbrain are required for genesis of pause responses. No prior computational models encompass both intrinsic and synaptically gated dynamics. We present a mathematical model that robustly accounts for behavior-related electrophysiological properties of TANs in terms of their intrinsic physiological properties and known afferents. In the model, balanced intrinsic hyperpolarizing and depolarizing currents engender tonic firing and glutamatergic inputs from thalamus (and cortex) both directly excite and indirectly inhibit TANs. If this inhibition, probably mediated by GABAergic nitric oxide synthase interneurons, exceeds a threshold, a persistent K+ conductance current amplifies its effect to generate a prolonged pause. Dopamine (DA) signals modulate both the intrinsic mechanisms and the external inputs of TANs. Simulations revealed that many learning-dependent behaviors of TANs, including acquired pauses to task-relevant cues, are explicable without recourse to learning-dependent changes in synapses onto TANs, due to a tight coupling between DA bursts and TAN pauses. These interactions imply that reward-predicting cues often cause striatal projection neurons to receive a cascade of signals: an adaptively scaled DA burst, a brief acetylcholine (ACh) burst, and an ACh pause. A sensitivity analysis revealed a unique TAN response surface, which shows that DA inputs robustly cooperate with thalamic inputs to control cue-dependent pauses of ACh release, which strongly affects performance- and learning-related dynamics in the striatum.

Excitatory effects of serotonin on rat striatal cholinergic interneurones

We investigated the effects of 5-hydroxytryptamine (5-HT, serotonin) in striatal cholinergic interneurones with gramicidin-perforated whole-cell patch recordings. Bath-application of serotonin (30 microm) significantly and reversibly increased the spontaneous firing rate of 37/45 cholinergic interneurones tested. On average, in the presence of serotonin, firing rate was 273 +/- 193% of control. Selective agonists of 5-HT(1A), 5-HT3, 5-HT4 and 5-HT7 receptors did not affect cholinergic interneurone firing, while the 5-HT2 receptor agonist alpha-methyl-5-HT (30 microm) mimicked the excitatory effects of serotonin. Consistently, the 5-HT2 receptor antagonist ketanserin (10 microm) fully blocked the excitatory effects of serotonin. Two prominent after-hyperpolarizations (AHPs), one of medium duration that was apamin-sensitive and followed individual spikes, and one that was slower and followed trains of spikes, were both strongly and reversibly reduced by serotonin; these effects were fully blocked by ketanserin. Conversely, the depolarizing sags observed during negative current injections and mediated by hyperpolarization-activated cationic currents were not affected. In the presence of apamin and tetrodotoxin, the slow AHP was strongly reduced by 5-HT, and fully abolished by the calcium channel blocker nickel. These results show that 5-HT exerts a powerful excitatory control on cholinergic interneurones via 5-HT2 receptors, by suppressing the AHPs associated with two distinct calcium-activated potassium currents.

Origin of the slow afterhyperpolarization and slow rhythmic bursting in striatal cholinergic interneurons

Striatal cholinergic interneurons recorded in slices exhibit three different firing patterns: rhythmic single spiking, irregular bursting, and rhythmic bursting. The rhythmic single-spiking pattern is governed mainly by a prominent brief afterhyperpolarization (mAHP) that follows single spikes. The mAHP arises from an apamin-sensitive calcium-dependent potassium current. A slower AHP (sAHP), also present in these neurons, becomes prominent during rhythmic bursting or driven firing. Although not apamin sensitive, the sAHP is caused by a calcium-dependent potassium conductance. It is not present after blockade of calcium current with cadmium or after calcium is removed from the media or when intracellular calcium is buffered with bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. It reverses at the potassium equilibrium potential. It can be generated by subthreshold depolarizations and persists after blockade of sodium currents by tetrodotoxin. It is slow, being maximal approximately 1 s after depolarization onset, and takes several seconds to decay. It requires >300-ms depolarizations to become maximally activated. Its voltage sensitivity is sigmoidal, with a half activation voltage of -40 mV. We conclude the sAHP is a high-affinity apamin-insensitive calcium-dependent potassium conductance, triggered by calcium currents partly activated at subthreshold levels. In combination with those calcium currents, it accounts for the slow oscillations seen in a subset of cholinergic interneurons exhibiting rhythmic bursting. In all cholinergic interneurons, it contributes to the slowdown or pause in firing that follows driven activity or prolonged subthreshold depolarizations and is therefore a candidate mechanism for the pause response observed in cholinergic neurons in vivo.

The Mechanism of Intrinsic Amplification of Hyperpolarizations and Spontaneous Bursting in Striatal Cholinergic Interneurons

Striatal cholinergic interneurons pause their ongoing firing in response to sensory stimuli that have acquired meaning as a signal for learned behavior. In slices, these cells exhibit both spontaneous activity patterns and spontaneous pauses very similar to those seen in vivo. The mechanisms responsible for ongoing firing and spontaneous pauses were studied in striatal slices using perforated patch recordings. All hyperpolarizations, whether spontaneous or generated by current injection, were amplified and shaped by two hyperpolarization-activated currents. Hyperpolarization onsets were regeneratively amplified by a potassium current (KIR) whose activation promoted further hyperpolarization. The termination of hyperpolarizations was controlled by a time-dependent nonspecific cation current (HCN). The duration and even the sizes of spontaneous and driven hyperpolarizations and pauses in spontaneous activity in cholinergic interneurons are largely autonomous properties of the neuron, rather than reflections of characteristics of the input eliciting the response.

Delayed rectifier potassium currents and Kv2.1 mRNA increase in hippocampal neurons of scopolamine-induced memory-deficient rats

To explore the ionic mechanisms of memory deficits induced by cholinergic lesion, whole-cell patch clamp recording techniques in combination with single-cell RT-PCR were used to characterize delayed rectifier potassium currents (IK) in acutely isolated hippocampal pyramidal neurons of scopolamine-induced cognitive impairment rats. Scopolamine could induce deficits in spatial memory of rats. The peak amplitude and current density of IK measured in hippocampal pyramidal neurons were increased from 1.2+/-0.6 nA and 38+/-19 pA/pF of the control group (n=12) to 1.8+/-0.5 nA and 62+/-24 pA/pF (n=48, P<0.01) of the scopolamine-treated group. The steady-state activation curve of IK was shifted about 8 mV (P<0.01) in the direction of hyperpolarization in scopolamine-treated rats. The mRNA level of Kv2.1 was increased (P<0.01) in the scopolamine-treated group, but there was no significant change of Kv1.5 mRNA level. The present study demonstrated for the first time that IK was enhanced significantly in hippocampal pyramidal neurons of scopolamine-induced cognitive impairment rats. The increase of Kv2.1 mRNA expression in hippocampal pyramidal cells might be responsible for the enhancement of IK and could be the ionic basis of the memory deficits induced by scopolamine.

Muscarinic receptors involved in the subthreshold cholinergic actions of neostriatal spiny neurons

Administration of the peptide MT-1 (48 nM), a selective agonist of muscarinic M(1)-type receptors, mimicked the subthreshold actions of muscarine (1 microM) on neostriatal neurons, i.e., it produced a reduction in subthreshold inward rectification leading to an enhancement in input resistance (R(N)) and evoked discharge. In all recorded cells, MT-1 effects remained in the presence of the specific peptidergic antagonist of the M(4)-type receptor, MT-3 (10 nM), but were blocked by the specific M(1)-type receptor antagonist MT-7 (5 nM). These results suggest that most muscarinic facilitatory actions in the subthreshold voltage range occur through M(1)-type receptors. However, in a fraction of cells (40%) muscarine produced an excitability enhancement not blocked by MT-7. This additional facilitatory action, not present when using MT-1, was blocked by MT-3, suggesting it was mediated by M(4)-type receptor activation. This facilitation could not be blocked by Cs(+), TTX, or Cd(2+), but only by a reduction in extracellular sodium. This result is the first evidence that M(4)-type receptor activation enhances a cationic inward current in a fraction of neostriatal projection neurons.

Cholinergic interneuron characteristics and nicotinic properties in the striatum

The neostriatum (dorsal striatum) is composed of the caudate and putamen. The ventral striatum is the ventral conjunction of the caudate and putamen that merges into and includes the nucleus accumbens and striatal portions of the olfactory tubercle. About 2% of the striatal neurons are cholinergic. Most cholinergic neurons in the central nervous system make diffuse projections that sparsely innervate relatively broad areas. In the striatum, however, the cholinergic neurons are interneurons that provide very dense local innervation. The cholinergic interneurons provide an ongoing acetylcholine (ACh) signal by firing action potentials tonically at about 5 Hz. A high concentration of acetylcholinesterase in the striatum rapidly terminates the ACh signal, and thereby minimizes desensitization of nicotinic acetylcholine receptors. Among the many muscarinic and nicotinic striatal mechanisms, the ongoing nicotinic activity potently enhances dopamine release. This process is among those in the striatum that link the two extensive and dense local arbors of the cholinergic interneurons and dopaminergic afferent fibers. During a conditioned motor task, cholinergic interneurons respond with a pause in their tonic firing. It is reasonable to hypothesize that this pause in the cholinergic activity alters action potential dependent dopamine release. The correlated response of these two broad and dense neurotransmitter systems helps to coordinate the output of the striatum, and is likely to be an important process in sensorimotor planning and learning.

Muscarinic and nicotinic presynaptic modulation of EPSCs in the nucleus accumbens during postnatal development

We have studied the modulatory effects of cholinergic agonists on excitatory postsynaptic currents (EPSCs) in nucleus accumbens (nAcb) neurons during postnatal development. Recordings were obtained in slices from postnatal day 1 (P1) to P27 rats using the whole cell patch-clamp technique. EPSCs were evoked by local electrical stimulation, and all experiments were conducted in the presence of bicuculline methchloride in the bathing medium and with QX-314 in the recording pipette. Under these conditions, postsynaptic currents consisted of glutamatergic EPSCs typically consisting of two components mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors. The addition of acetylcholine (ACh) or carbachol (CCh) to the superfusing medium resulted in a decrease of 30-60% of both AMPA/KA- and NMDA-mediated EPSCs. In contrast, ACh produced an increase ( approximately 35%) in both AMPA/KA and NMDA receptor-mediated EPSCs when administered in the presence of the muscarinic antagonist atropine. These excitatory effects were mimicked by the nicotinic receptor agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and blocked by the nicotinic receptor antagonist mecamylamine, showing the presence of a cholinergic modulation mediated by nicotinic receptors in the nAcb. The antagonistic effects of atropine were mimicked by pirenzepine, suggesting that the muscarinic depression of the EPSCs was mediated by M(1)/M(4) receptors. In addition, the inhibitory effects of ACh on NMDA but not on AMPA/KA receptor-mediated EPSC significantly increased during the first two postnatal weeks. We found that, under our experimental conditions, cholinergic agonists produced no changes on membrane holding currents, on the decay time of the AMPA/KA EPSC, or on responses evoked by exogenous application of glutamate in the presence of tetrodotoxin, but they produced significant changes in paired pulse ratio, suggesting that their action was mediated by presynaptic mechanisms. In contrast, CCh produced consistent changes in the membrane and firing properties of medium spiny (MS) neurons when QX-314 was omitted from the recording pipette solution, suggesting that this substance actually blocked postsynaptic cholinergic modulation. Together, these results suggest that ACh can decrease or increase glutamatergic neurotransmission in the nAcb by, respectively, acting on muscarinic and nicotinic receptors located on excitatory terminals. The cholinergic modulation of AMPA/KA and NMDA receptor-mediated neurotransmission in the nAcb during postnatal development could play an important role in activity-dependent developmental processes in refining the excitatory drive on MS neurons by gating specific inputs.