Acetylcholine-mediated modulation of striatal function

Juxtacellular labeling of tonically active neurons and phasically active neurons in the rat striatum

Tonically active neurons (TANs) and phasically active neurons (PANs) are widely believed to be the cholinergic interneurons and GABAergic projection neurons, respectively, in the striatum based on in vivo intracellular recordings coupled with morphological examinations of anesthetized rats, and on histochemical, electrophysiological, and labeling studies of in vitro slice preparations. TANs of alert behaving animals exhibit prolonged pause responses to behaviorally significant events. PANs, on the other hand, are mostly inactive when subjects are quiet and not performing any actions, but exhibit burst discharges in response to external stimuli and/or voluntary actions. Several other types of interneurons have also been identified in the striatum, such as parvalbumin-containing GABAergic interneurons (fast-spiking cells), somatostatin-containing interneurons, and calretinin-containing interneurons. To identify the neurochemical and morphological characteristics of TANs and PANs in a more direct manner, we conducted juxtacellular labeling, combining electrophysiology with immunohistochemistry and morphology in anesthetized rats. All of the juxtacellularly labeled TANs (n=3) among those recorded (n=10) were ChAT-positive and had large cell somata with aspiny dendrites. Thus, although our observations are based on a limited number of neurons, our findings provide the most convincing evidence to date that TANs in the striatum are cholinergic neurons. We also found that the majority of PANs are GABA-immunoreactive (46 of 48 tested) and approximately two-thirds had spiny dendrites (30 of 48 tested), indicating that the majority are medium-sized, spiny, GABAergic projection neurons, consistent with general beliefs. Conversely, the remaining one-third of PANs had aspiny dendrites (n=18), indicating that they were interneurons. Therefore, the present study reveals that TANs are cholinergic neurons and that the majority of PANs are medium-sized, spiny, GABAergic projection neurons, while a smaller number are GABAergic interneurons.

Impaired plasticity at specific subset of striatal synapses in the Ts65Dn mouse model of Down syndrome

BACKGROUND

Trisomy 21 or Down syndrome (DS) is the most frequent genetic cause of mental retardation. There is limited insight into the biological basis for the cognitive and motor deficits in DS. Because the striatum plays a key role in the regulation and learning of voluntary movements and in cognitive processes, our study was aimed at investigating striatal synaptic transmission and plasticity in a well-accepted genetic model of DS.

METHODS

Electrophysiological recordings were performed in a corticostriatal slice preparation from trisomic (Ts65Dn) and wild-type mice. Synaptic properties and plasticity, long-term potentiation (LTP) and long-term depression (LTD), were investigated.

RESULTS

The basal electrophysiological properties of striatal principal spiny neurons and cholinergic interneurons were spared in the Ts65Dn mouse model of DS. Striatal principal spiny neurons from Ts65Dn mice maintained their ability to undergo LTP and LTD. Conversely, LTP was lost in striatal cholinergic interneurons of Ts65Dn mice. The loss of LTP in striatal cholinergic interneurons of Ts65Dn mice was accompanied by a severe impairment of endogenous cholinergic signaling within the striatum.

CONCLUSIONS

The intrastriatal cholinergic system that was thought to be spared in DS is functionally altered in the Ts65Dn genetic model of DS. Altered cholinergic transmission might play a critical role in the pathophysiology of motor and cognitive deficits in DS, leading to an abnormal processing of neuronal inputs within the basal ganglia. Targeting striatal cholinergic transmission might represent a new therapeutic strategy in DS.

Convergent evidence for abnormal striatal synaptic plasticity in dystonia

Dystonia is a functionally disabling movement disorder characterized by abnormal movements and postures. Although substantial recent progress has been made in identifying genetic factors, the pathophysiology of the disease remains a mystery. A provocative suggestion gaining broader acceptance is that some aspect of neural plasticity may be abnormal. There is also evidence that, at least in some forms of dystonia, sensorimotor "use" may be a contributing factor. Most empirical evidence of abnormal plasticity in dystonia comes from measures of sensorimotor cortical organization and physiology. However, the basal ganglia also play a critical role in sensorimotor function. Furthermore, the basal ganglia are prominently implicated in traditional models of dystonia, are the primary targets of stereotactic neurosurgical interventions, and provide a neural substrate for sensorimotor learning influenced by neuromodulators. Our working hypothesis is that abnormal plasticity in the basal ganglia is a critical link between the etiology and pathophysiology of dystonia. In this review we set up the background for this hypothesis by integrating a large body of disparate indirect evidence that dystonia may involve abnormalities in synaptic plasticity in the striatum. After reviewing evidence implicating the striatum in dystonia, we focus on the influence of two neuromodulatory systems: dopamine and acetylcholine. For both of these neuromodulators, we first describe the evidence for abnormalities in dystonia and then the means by which it may influence striatal synaptic plasticity. Collectively, the evidence suggests that many different forms of dystonia may involve abnormal plasticity in the striatum. An improved understanding of these altered plastic processes would help inform our understanding of the pathophysiology of dystonia, and, given the role of the striatum in sensorimotor learning, provide a principled basis for designing therapies aimed at the dynamic processes linking etiology to pathophysiology of the disease.

GABA concentration in schizophrenia patients and the effects of antipsychotic medication: a proton magnetic resonance spectroscopy study

Gamma-amino butyric acid (GABA) is thought to play a role in the pathophysiology of schizophrenia. High magnetic field proton magnetic resonance spectroscopy ((1)H-MRS) provides a reliable measurement of GABA in specific regions of the brain. This study measured GABA concentration in the anterior cingulate cortex (ACC) and in the left basal ganglia (ltBG) in 38 patients with chronic schizophrenia and 29 healthy control subjects. There was no significant difference in GABA concentration between the schizophrenia patients and the healthy controls in either the ACC (1.36+/-0.45 mmol/l in schizophrenia patients and 1.52+/-0.54 mmol/l in control subjects) or the ltBG (1.13+/-0.26 mmol/l in schizophrenia patients and 1.18+/-0.20 mmol/l in control subjects). Among the right handed schizophrenia patients, the GABA concentration in the ltBG was significantly higher in patients taking typical antipsychotics (1.25+/-0.24 mmol/l) than in those taking atypical antipsychotics (1.03+/-0.24 mmol/l, p=0.026). In the ACC, the GABA concentration was negatively correlated with the dose of the antipsychotics (rs=-0.347, p=0.035). In the ltBG, the GABA concentration was positively correlated with the dose of the anticholinergics (rs=0.403, p=0.015). To the best of our knowledge, this is the first study to have directly measured GABA concentrations in schizophrenia patients using (1)H-MRS. Our results suggest that there are no differences in GABA concentrations in the ACC or the ltBG of schizophrenia patients compared to healthy controls. Antipsychotic medication may cause changes in GABA concentration, and atypical and typical antipsychotics may have differing effects. It is possible that medication effects conceal inherent differences in GABA concentrations between schizophrenia patients and healthy controls.

Transient receptor potential vanilloid 1 channels control acetylcholine/2-arachidonoylglicerol coupling in the striatum

The neurotransmitter acetylcholine (Ach) controls both excitatory and inhibitory synaptic transmission in the striatum. Here, we investigated the involvement of the endocannabinoid system in Ach-mediated inhibition of striatal GABA transmission, and the potential role of transient receptor potential vanilloid 1 (TRPV1) channels in the control of Ach-endocannabinoid coupling. We found that inhibition of Ach degradation and direct pharmacological stimulation of muscarinic M1 receptors reduced striatal inhibitory postsynaptic currents (IPSCs) through the stimulation of 2-arachidonoylglicerol (2AG) synthesis and the activation of cannabinoid CB1 receptors. The effects of M1 receptor activation on IPSCs were occlusive with those of metabotropic glutamate receptor 5 stimulation, and were prevented in the presence of capsaicin, agonist of TRPV1 channels. Elevation of anandamide (AEA) tone with URB597, a blocker of fatty acid amide hydrolase, mimicked the effects of capsaicin, indicating that endogenous AEA acts as an endovanilloid substance in the control of M1-dependent 2AG-mediated synaptic effects in the striatum. Accordingly, both capsaicin and URB597 effects were absent in mice lacking TRPV1 channels. Pharmacological interventions targeting AEA metabolism and TRPV1 channels might be considered alternative therapeutic routes in disorders of striatal cholinergic or endocannabinoid neurotransmission.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Ongoing expression of Nkx2.1 in the postnatal mouse forebrain: potential for understanding NKX2.1 haploinsufficiency in humans?

Coordinated movements require the caudate-putamen and the globus pallidus, two nuclei belonging to the basal ganglia, to be intact and functioning properly. Many neurons populating these regions derive from the medial ganglionic eminence, a transient structure that expresses the transcription factor Nkx2.1 during prenatal development. Accordingly, the basal ganglia of Nkx2.1(-/-) mice are heavily affected and a substantial loss of several types of GABAergic interneurons has been observed. Interestingly, heterozygous mutation of the NKX2.1 gene in humans has been described as causing an unusual disorder from the second year of life onwards, which is mainly characterized by disturbances of motor abilities and delayed speech development. In the present study, we therefore investigated whether Nkx2.1 is still expressed in the young adult and aged mouse forebrain. After birth, the most intense immunolabeling for Nkx2.1 was detected in several components of the hypothalamic region, in the subventricular zone of the ventral tips lining the lateral ventricles, and in neighboring structures including the striatum, the globus pallidus and the various nuclei of the septal complex. Surprisingly, this staining pattern was substantially maintained into adulthood. Double immunocytochemistry for Nkx2.1 and various neuronal markers revealed that mainly parvalbumin-containing GABAergic neurons, but also cholinergic neurons, of the ventral forebrain express this protein. Moreover, in situ hybridization confirmed that these neurons maintain synthesis of Nkx2.1 throughout life. The robust expression of Nkx2.1 by these neurons points to a broad functional spectrum within the adult forebrain.

Cholinergic modulation of GABAergic and glutamatergic transmission in the dorsal subcoeruleus: mechanisms for REM sleep control

STUDY OBJECTIVES

Dorsal subcoeruleus (SubCD) neurons are thought to promote PGO waves and to be modulated by cholinergic afferents during REM sleep. We examined the differential effect of the cholinergic agonist carbachol (CAR) on excitatory and inhibitory postsynaptic currents (PSCs), and investigated the effects of CAR on SubCD neurons during the developmental decrease in REM sleep.

DESIGN

Whole-cell patch clamp recordings were conducted on brainstem slices of 7- to 20-day-old rats.

MEASUREMENTS AND RESULTS

CAR acted directly on 50% of SubCD neurons by inducing an inward current, via both nicotinic and muscarinic M1 receptors. CAR induced a potassium mediated outward current via activation of M2 muscarinic receptors in 43% of SubCD cells. Evoked stimulation established the presence of NMDA, AMPA, GABA, and glycinergic PSCs in the SubCD. CAR was found to decrease the amplitude of evoked EPSCs in 31 of 34 SubCD cells, but decreased the amplitude of evoked IPSCs in only 1 of 13 SubCD cells tested. Spontaneous EPSCs were decreased by CAR in 55% of cells recorded, while spontaneous IPSCs were increased in 27% of SubCD cells. These findings indicate that CAR exerts a predominantly inhibitory role on fast synaptic glutamatergic activity and a predominantly excitatory role on fast synaptic GABAergic/glycinergic activity in the SubCD.

CONCLUSION

We hypothesize that during REM sleep, cholinergic "REM-on" neurons that project to the SubCD induce an excitation of inhibitory interneurons and inhibition of excitatory events leading to the production of coordinated activity in SubCD projection neurons. The coordination of these projection neurons may be essential for the production of REM sleep signs such as PGO waves.

Cholinergic modulation of multivesicular release regulates striatal synaptic potency and integration

The pleiotropic actions of neuromodulators on pre- and postsynaptic targets make disentangling the mechanisms underlying regulation of synaptic transmission challenging. In the striatum, acetylcholine modulates glutamate release via activation of muscarinic receptors (mAchRs), although the consequences for postsynaptic signaling are unclear. Using two-photon microscopy and glutamate uncaging to examine individual synapses in the rat striatum, we found that glutamatergic afferents have a high degree of multivesicular release (MVR) in the absence of postsynaptic receptor saturation. We found that mAchR activation decreased both the probability of release and the concentration of glutamate in the synaptic cleft. The corresponding decrease in synaptic potency reduced the duration of synaptic potentials and limited temporal summation of afferent inputs. These findings reveal a mechanism by which a combination of basal MVR and low receptor saturation allow the presynaptic actions of a neuromodulator to control the engagement of postsynaptic nonlinearities and regulate synaptic integration.

Dopamine D2-receptor activation elicits akinesia, rigidity, catalepsy, and tremor in mice expressing hypersensitive {alpha}4 nicotinic receptors via a cholinergic-dependent mechanism

Recent studies suggest that high-affinity neuronal nicotinic acetylcholine receptors (nAChRs) containing alpha4 and beta2 subunits (alpha4beta2*) functionally interact with G-protein-coupled dopamine (DA) D(2) receptors in basal ganglia. We hypothesized that if a functional interaction between these receptors exists, then mice expressing an M2 point mutation (Leu9'Ala) rendering alpha4 nAChRs hypersensitive to ACh may exhibit altered sensitivity to a D(2)-receptor agonist. When challenged with the D(2)R agonist, quinpirole (0.5-10 mg/kg), Leu9'Ala mice, but not wild-type (WT) littermates, developed severe, reversible motor impairment characterized by rigidity, catalepsy, akinesia, and tremor. While striatal DA tissue content, baseline release, and quinpirole-induced DA depletion did not differ between Leu9'Ala and WT mice, quinpirole dramatically increased activity of cholinergic striatal interneurons only in mutant animals, as measured by increased c-Fos expression in choline acetyltransferase (ChAT)-positive interneurons. Highlighting the importance of the cholinergic system in this mouse model, inhibiting the effects of ACh by blocking muscarinic receptors, or by selectively activating hypersensitive nAChRs with nicotine, rescued motor symptoms. This novel mouse model mimics the imbalance between striatal DA/ACh function associated with severe motor impairment in disorders such as Parkinson's disease, and the data suggest that a D(2)R-alpha4*-nAChR functional interaction regulates cholinergic interneuron activity.

Choline transporter gene variation is associated with attention-deficit hyperactivity disorder

The neurotransmitter acetylcholine (ACh) plays a critical role in brain circuits mediating motor control, attention, learning and memory. Cholinergic dysfunction is associated with multiple brain disorders including Alzheimer’s Disease, addiction, schizophrenia and Attention-Deficit Hyperactivity Disorder (ADHD). The presynaptic choline transporter (CHT, SLC5A7) is the major, rate-limiting determinant of ACh production in the brain and periphery and is consequently upregulated during tasks that require sustained attention. Given the contribution of central cholinergic circuits to the control of movement and attention, we hypothesized that functional CHT gene variants might impact risk for ADHD. We performed a case-control study, followed by family-based association tests on a separate cohort, of two purportedly functional CHT polymorphisms (coding variant Ile89Val (rs1013940) and a genomic SNP 3’ of the CHT gene (rs333229), affording both a replication sample and opportunities to reduce potential population stratification biases. Initial genotyping of pediatric ADHD subjects for two purportedly functional CHT alleles revealed a 2–3 fold elevation of the Val89 allele (n = 100; P = 0.02) relative to healthy controls, as well as a significant decrease of the 3’SNP minor allele in Caucasian male subjects (n = 60; P = 0.004). In family based association tests, we found significant overtransmission of the Val89 variant to children with a Combined subtype diagnosis (OR = 3.16; P = 0.01), with an increased Odds Ratio for a haplotype comprising both minor alleles. These studies show evidence of cholinergic deficits in ADHD, particularly for subjects with the Combined subtype, and, if replicated, may encourage further consideration of cholinergic agonist therapy in the disorder.

Impairment of bidirectional synaptic plasticity in the striatum of a mouse model of DYT1 dystonia: role of endogenous acetylcholine

DYT1 dystonia is a severe form of inherited dystonia, characterized by involuntary twisting movements and abnormal postures. It is linked to a deletion in the dyt1 gene, resulting in a mutated form of the protein torsinA. The penetrance for dystonia is incomplete, but both clinically affected and non-manifesting carriers of the DYT1 mutation exhibit impaired motor learning and evidence of altered motor plasticity. Here, we characterized striatal glutamatergic synaptic plasticity in transgenic mice expressing either the normal human torsinA or its mutant form, in comparison to non-transgenic (NT) control mice. Medium spiny neurons recorded from both NT and normal human torsinA mice exhibited normal long-term depression (LTD), whereas in mutant human torsinA littermates LTD could not be elicited. In addition, although long-term potentiation (LTP) could be induced in all the mice, it was greater in magnitude in mutant human torsinA mice. Low-frequency stimulation (LFS) can revert potentiated synapses to resting levels, a phenomenon termed synaptic depotentiation. LFS induced synaptic depotentiation (SD) both in NT and normal human torsinA mice, but not in mutant human torsinA mice. Since anti-cholinergic drugs are an effective medical therapeutic option for the treatment of human dystonia, we reasoned that an excess in endogenous acetylcholine could underlie the synaptic plasticity impairment. Indeed, both LTD and SD were rescued in mutant human torsinA mice either by lowering endogenous acetylcholine levels or by antagonizing muscarinic M1 receptors. The presence of an enhanced acetylcholine tone was confirmed by the observation that acetylcholinesterase activity was significantly increased in the striatum of mutant human torsinA mice, as compared with both normal human torsinA and NT littermates. Moreover, we found similar alterations of synaptic plasticity in muscarinic M2/M4 receptor knockout mice, in which an increased striatal acetylcholine level has been documented. The loss of LTD and SD on one hand, and the increase in LTP on the other, demonstrate that a 'loss of inhibition' characterizes the impairment of synaptic plasticity in this model of DYT1 dystonia. More importantly, our results indicate that an unbalanced cholinergic transmission plays a pivotal role in these alterations, providing a clue to understand the ability of anticholinergic agents to restore motor deficits in dystonia.

Muscarinic enhancement of persistent sodium current synchronizes striatal medium spiny neurons

Network dynamics denoted by synchronous firing of neuronal pools rely on synaptic interactions and intrinsic properties. In striatal medium spiny neurons, N-methyl-d-aspartate (NMDA) receptor activation endows neurons with nonlinear capabilities by inducing a negative-slope conductance region (NSCR) in the current-voltage relationship. Nonlinearities underlie associative learning, procedural memory, and the sequential organization of behavior in basal ganglia nuclei. The cholinergic system modulates the function of medium spiny projection neurons through the activation of muscarinic receptors, increasing the NMDA-induced NSCR. This enhancement is reflected as a change in the NMDA-induced network dynamics, making it more synchronous. Nevertheless, little is known about the contribution of intrinsic properties that promote this activity. To investigate the mechanisms underlying the cholinergic modulation of bistable behavior in the striatum, we used whole cell and calcium-imaging techniques. A persistent sodium current modulated by muscarinic receptor activation participated in the enhancement of the NSCR and the increased network synchrony. These experiments provide evidence that persistent sodium current generates bistable behavior in striatal neurons and contributes to the regulation of synchronous network activity. The neuromodulation of bistable properties could represent a cellular and network mechanism for cholinergic actions in the striatum.

Physiology and pharmacology of striatal neurons

The basal ganglia occupy the core of the forebrain and consist of evolutionarily conserved motor nuclei that form recurrent circuits critical for motivation and motor planning. The striatum is the main input nucleus of the basal ganglia and a key neural substrate for procedural learning and memory. The vast majority of striatal neurons are spiny GABAergic projection neurons, which exhibit slow but temporally precise spiking in vivo. Contributing to this precision are several different types of interneurons that constitute only a small fraction of total neuron number but play a critical role in regulating striatal output. This review examines the cellular physiology and modulation of striatal neurons that give rise to their unique properties and function.

Changes to interneuron-driven striatal microcircuits in a rat model of Parkinson's disease

Striatal interneurons play key roles in basal ganglia function and related disorders by modulating the activity of striatal projection neurons. Here we have injected rabies virus (RV) into either the rat substantia nigra pars reticulata or the globus pallidus and took advantage of the trans-synaptic spread of RV to unequivocally identify the interneurons connected to striatonigral- or striatopallidal-projecting neurons, respectively. Large numbers of RV-infected parvalbumin (PV+/RV+) and cholinergic (ChAT+/RV+) interneurons were detected in control conditions, and they showed marked changes following intranigral 6-hydroxydopamine injection. The number of ChAT+/RV+ interneurons innervating striatopallidal neurons increased concomitant with a reduction in the number of PV+/RV+ interneurons, while the two interneuron populations connected to striatonigral neurons were clearly reduced. These data provide the first evidence of synaptic reorganization between striatal interneurons and projection neurons, notably a switch of cholinergic innervation onto striatopallidal neurons, which could contribute to imbalanced striatal outflow in parkinsonian state.

Functional organization of the basal ganglia: therapeutic implications for Parkinson's disease

The basal ganglia (BG) are a highly organized network, where different parts are activated for specific functions and circumstances. The BG are involved in movement control, as well as associative learning, planning, working memory, and emotion. We concentrate on the "motor circuit" because it is the best understood anatomically and physiologically, and because Parkinson's disease is mainly thought to be a movement disorder. Normal function of the BG requires fine tuning of neuronal excitability within each nucleus to determine the exact degree of movement facilitation or inhibition at any given moment. This is mediated by the complex organization of the striatum, where the excitability of medium spiny neurons is controlled by several pre- and postsynaptic mechanisms as well as interneuron activity, and secured by several recurrent or internal BG circuits. The motor circuit of the BG has two entry points, the striatum and the subthalamic nucleus (STN), and an output, the globus pallidus pars interna (GPi), which connects to the cortex via the motor thalamus. Neuronal afferents coding for a given movement or task project to the BG by two different systems: (1) Direct disynaptic projections to the GPi via the striatum and STN. (2) Indirect trisynaptic projections to the GPi via the globus pallidus pars externa (GPe). Corticostriatal afferents primarily act to inhibit medium spiny neurons in the "indirect circuit" and facilitate neurons in the "direct circuit." The GPe is in a pivotal position to regulate the motor output of the BG. Dopamine finely tunes striatal input as well as neuronal striatal activity, and modulates GPe, GPi, and STN activity. Dopaminergic depletion in Parkinson's disease disrupts the corticostriatal balance leading to increased activity the indirect circuit and reduced activity in the direct circuit. The precise chain of events leading to increased STN activity is not completely understood, but impaired dopaminergic regulation of the GPe, GPi, and STN may be involved. The parkinsonian state is characterized by disruption of the internal balance of the BG leading to hyperactivity in the two main entry points of the network (striatum and STN) and excessive inhibitory output from the GPi. Replacement therapy with standard levodopa creates a further imbalance, producing an abnormal pattern of neuronal discharge and synchronization of neuronal firing that sustain the "off" and "on with dyskinesia" states. The effect of levodopa is robust but short-lasting and converts the parkinsonian BG into a highly unstable system, where pharmacological and compensatory effects act in opposing directions. This creates a scenario that substantially departs from the normal physiological state of the BG.

Molecular profiling of striatonigral and striatopallidal medium spiny neurons past, present, and future

Defining distinct molecular properties of the two striatal medium spiny neurons (MSNs) has been a challenging task for basal ganglia (BG) neuroscientists. Identifying differential molecular components in each MSN subtype is crucial for BG researchers to understand functional properties of these two neurons. The two MSN populations are morphologically identical except in their projections through the direct verses indirect BG pathways and they are heterogeneously dispersed throughout the dorsal striatum (dStr) and nucleus accumbens (NAc). These characteristics have made it difficult for researchers to distinguish and isolate these two neuronal populations thereby hindering progress toward a more comprehensive understanding of their differential molecular properties. Researchers began to investigate molecular differences in the striatonigral and striatopallidal neurons using in situ hybridization (ISH) techniques and single cell reverse transcription-polymerase chain reaction (scRT-PCR). Currently the field is utilizing more advanced techniques for large-scale gene expression studies including fluorescence activated cell sorting (FACS) of MSNs, from which RNA is purified, from fluorescent reporter transgenic mice or use of transgenic mice in which ribosomes from each MSN are tagged and can be immunoprecipitated followed by RNA isolation, a technique termed translating ribosomal affinity purification (TRAP). Additionally, the availability of fluorescent reporter mice for each MSN subtype is allowing, scientists to perform more accurate histology studies evaluating differential protein expression and signaling changes in each cell subtype. Finally, researchers are able to evaluate the role of specific genes in vivo by utilizing cell type-specific mouse models including Cre driver lines that can be crossed with conditional overexpression or knockout systems. This is a very exciting time in the BG field because researchers are well equipped with the most progressive tools to comprehensively evaluate molecular components in the two MSNs and their consequence on BG functional output in the normal, diseased, and developing brain.

Midbrain dopaminergic neurons and striatal cholinergic interneurons encode the difference between reward and aversive events at different epochs of probabilistic classical conditioning trials

Midbrain dopaminergic neurons (DANs) typically increase their discharge rate in response to appetitive predictive cues and outcomes, whereas striatal cholinergic tonically active interneurons (TANs) decrease their rate. This may indicate that the activity of TANs and DANs is negatively correlated and that TANs can broaden the basal ganglia reinforcement teaching signal, for instance by encoding worse than predicted events. We studied the activity of 106 DANs and 180 TANs of two monkeys recorded during the performance of a classical conditioning task with cues predicting the probability of food, neutral, and air puff outcomes. DANs responded to all cues with elevations of discharge rate, whereas TANs depressed their discharge rate. Nevertheless, although dopaminergic responses to appetitive cues were larger than their responses to neutral or aversive cues, the TAN responses were more similar. Both TANs and DANs responded faster to an air puff than to a food outcome; however, DANs responded with a discharge elevation, whereas the TAN responses included major negative and positive deflections. Finally, food versus air puff omission was better encoded by TANs. In terms of the activity of single neurons with distinct responses to the different behavioral events, both DANs and TANs were more strongly modulated by reward than by aversive related events and better reflected the probability of reward than aversive outcome. Thus, TANs and DANs encode the task episodes differentially. The DANs encode mainly the cue and outcome delivery, whereas the TANs mainly encode outcome delivery and omission at termination of the behavioral trial episode.

Encoding of probabilistic rewarding and aversive events by pallidal and nigral neurons

Previous studies have rarely tested whether the activity of high-frequency discharge (HFD) neurons of the basal ganglia (BG) is modulated by expectation, delivery, and omission of aversive events. Therefore the full value domain encoded by the BG network is still unknown. We studied the activity of HFD neurons of the globus pallidus external segment (GPe, n=310), internal segment (GPi, n=149), and substantia nigra pars reticulata (SNr, n=145) in two monkeys during a classical conditioning task with cues predicting the probability of food, neutral, or airpuff outcomes. The responses of BG HFD neurons were long-lasting and diverse with coincident increases and decreases in discharge rate. The population responses to reward-related events were larger than the responses to aversive and neutral-related events. The latter responses were similar, except for the responses to actual airpuff delivery. The fraction of responding cells was larger for reward-related events, with better discrimination between rewarding and aversive trials in the responses with an increase rather than a decrease in discharge rate. GPe and GPi single units were more strongly modulated and better reflected the probability of reward- than aversive-related events. SNr neurons were less biased toward the encoding of the rewarding events, especially during the outcome epoch. Finally, the latency of SNr responses to all predictive cues was shorter than the latency of pallidal responses. These results suggest preferential activation of the BG HFD neurons by rewarding compared with aversive events.

Aging of the striatum: mechanisms and interventions

Motor function declines with increasing adult age. Proper regulation of the balance between dopamine (DA) and acetylcholine (ACh) in the striatum has been shown to be fundamentally important for motor control. Although other factors can also contribute to this age-associated decline, a decrease in the concentration and binding potential of the DA D(2) receptor subtype in the striatum, especially in the cholinergic interneurons, are involved in the mechanism. Our studies have shown that gene transfer of the DA D(2) receptor subtype with adenoviral vectors is effective in ameliorating age-associated functional decline of the striatal cholinergic interneurons. These achievements confirm that an age-associated decrease of D(2)R contributes functional alteration of the interaction of DA and ACh in the striatum and demonstrate that these age-associated changes indeed are modifiable.

Nucleus accumbens core acetylcholine is preferentially activated during acquisition of drug- vs food-reinforced behavior

Acquisition of drug-reinforced behavior is accompanied by a systematic increase of release of the neurotransmitter acetylcholine (ACh) rather than dopamine, the expected prime reward neurotransmitter candidate, in the nucleus accumbens core (AcbC), with activation of both muscarinic and nicotinic ACh receptors in the AcbC by ACh volume transmission being necessary for the drug conditioning. The present findings suggest that the AcbC ACh system is preferentially activated by drug reinforcers, because (1) acquisition of food-reinforced behavior was not paralleled by activation of ACh release in the AcbC whereas acquisition of morphine-reinforced behavior, like that of cocaine or remifentanil (tested previously), was, and because (2) local intra-AcbC administration of muscarinic or nicotinic ACh receptor antagonists (atropine or mecamylamine, respectively) did not block the acquisition of food-reinforced behavior whereas acquisition of drug-reinforced behavior had been blocked. Interestingly, the speed with which a drug of abuse distributed into the AcbC and was eliminated from the AcbC determined the size of the AcbC ACh signal, with the temporally more sharply delineated drug stimulus producing a more pronounced AcbC ACh signal. The present findings suggest that muscarinic and nicotinic ACh receptors in the AcbC are preferentially involved during reward conditioning for drugs of abuse vs sweetened condensed milk as a food reinforcer.

Nicotinic and muscarinic cholinergic receptors coexist on GABAergic nerve endings in the mouse striatum and interact in modulating GABA release

Muscarinic cholinergic receptors (mAChRs) and nicotinic cholinergic receptors (nAChRs) regulating GABA release from striatal nerve endings were studied by monitoring release of previously accumulated [(3)H]GABA or endogenous GABA from superfused mouse striatal synaptosomes. Oxotremorine inhibited the release of [(3)H]GABA elicited by depolarization with 4-aminopyridine (4-AP), an effect antagonized by atropine. Agonists at nAChRs, including the alpha(4)beta(2)( *) subunit-selective RJR2403, provoked the release of [(3)H]GABA as well as of the endogenous transmitter; these effects also were prevented by oxotremorine and pilocarpine suggesting coexpression of functional mAChRs and alpha(4)beta(2)( *) nAChRs on GABAergic nerve endings. The inhibitory effects of oxotremorine on the release of [(3)H]GABA evoked by 4-AP or by RJR2403 were: (i) prevented by the M(2)/M(4) mAChR antagonist himbacine; (ii) insensitive to the M2 antagonist AFDX116; (iii) blocked by the selective M(4) mAChR antagonists MT3, thus indicating the involvement of receptors of the M(4) subtype. In conclusion, in the corpus striatum, acetylcholine released from cholinergic interneurons can activate alpha(4)beta(2)( *) nAChRs mediating release of GABA; this evoked release can be negatively modulated by M(4) mAChRs coexpressed on the same GABAergic terminals.

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.

Cholinergic innervation and thalamic input in rat nucleus accumbens

Cholinergic interneurons are the only known source of acetylcholine in the rat nucleus accumbens (nAcb); yet there is little anatomical data about their mode of innervation and the origin of their excitatory drive. We characterized the cholinergic and thalamic innervations of nAcb with choline acetyltransferase (ChAT) immunocytochemistry and anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L) from the midline/intralaminar/paraventricular thalamic nuclei. The use of a monoclonal ChAT antiserum against whole rat ChAT protein allowed for an optimal visualization of the small dendritic branches and fine varicose axons of cholinergic interneurons. PHA-L-labeled thalamic afferents were heterogeneously distributed throughout the core and shell regions of nAcb, overlapping regionally with cholinergic somata and dendrites. At the ultrastructural level, several hundred single-section profiles of PHA-L and ChAT-labeled axon terminals were analyzed for morphology, synaptic frequency, and the nature of their synaptic targets. The cholinergic profiles were small and apposed to various neuronal elements, but rarely exhibited a synaptic membrane specialization (5% in single ultrathin sections). Stereological extrapolation indicated that less than 15% of these cholinergic varicosities were synaptic. The PHA-L-labeled profiles were comparatively large and often synaptic (37% in single ultrathin sections), making asymmetrical contacts primarily with dendritic spines (>90%). Stereological extrapolation indicated that all PHA-L-labeled terminals were synaptic. In double-labeled material, some PHA-L-labeled terminals were directly apposed to ChAT-labeled somata or dendrites, but synapses were never seen between the two types of elements. These observations demonstrate that the cholinergic innervation of rat nAcb is largely asynaptic. They confirm that the afferents from midline/intralaminar/paraventricular thalamic nuclei to rat nAcb synapse mostly on dendritic spines, presumably of medium spiny neurons, and suggest that the excitatory drive of nAcb cholinergic interneurons from thalamus is indirect, either via substance P release from recurrent collaterals of medium spiny neurons and/or by extrasynaptic diffusion of glutamate.

Double dissociation of pharmacologically induced deficits in visual recognition and visual discrimination learning

Monkeys trained in either one-trial recognition at 8- to 10-min delays or multi-trial discrimination habits with 24-h intertrial intervals received systemic cholinergic and dopaminergic antagonists, scopolamine and haloperidol, respectively, in separate sessions. Recognition memory was impaired markedly by scopolamine but not at all by haloperidol, whereas habit formation was impaired markedly by haloperidol but only minimally by scopolamine. These differential drug effects point to differences in synaptic modification induced by the two neuromodulators that parallel the contrasting properties of the two types of learning, namely, fast acquisition but weak retention of memories versus slow acquisition but durable retention of habits.

Dysregulated information processing by medium spiny neurons in striatum of freely behaving mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant condition that compromises behavioral output. Dysfunction of medium spiny neurons (MSNs), which are the sole output system of the striatum, is thought to underlie HD pathophysiology. What is not known is how HD alters MSN information processing during behavior, which likely drives the HD behavioral phenotype. We recorded from populations of MSNs in two freely behaving and symptomatic HD mouse models: R6/2 transgenics are based on a C57BL/6J*CBA/J background and show robust behavioral symptoms, whereas knock-in (KI) mice have a 129sv background and express relatively mild behavioral signs. At the single-unit level, we found that the MSN firing rate was elevated in R6/2 but not in KI mice compared with their respective wild-type (WT) controls. In contrast, burst activity, which corresponds to periods of high-frequency firing, was altered in both HD models compared with WT. At the population level, we found that correlated firing between pairs of MSNs was a prominent feature in WT that was reduced in both HD models. Similarly, coincident bursts, which are bursts between pairs of neurons that overlap in time and occur more often in pairs of MSNs that exhibit correlated firing, were decreased in HD mice. Our results indicate an important role in both bursting and correlated burst firing for information processing in MSNs. Dysregulation of this processing scheme, moreover, is a key component of HD pathophysiology regardless of the severity of HD symptoms, genetic construct, and background strain of the mouse models.

Muscarinic M(1) modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

In some neurons, muscarinic M(1)-class receptors control L-type (Ca(V)1) Ca(2+)-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca(V)2.1, 2.2, respectively) Ca(2+)-channels are controlled by M(2)-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca(2+) channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M(4)-type receptors in neostriatal neurons, and leave in isolation the M(1)-type receptors to study them separately. We then asked what Ca(2+) channels are modulated by M(1)-type receptors only. We found that M(1)-receptors do modulate L, N and P/Q-types Ca(2+) channels. This modulation is blocked by the M(1)-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M(1)-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca(2+) entry via N- and P/Q-type Ca(2+) channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca(2+) channels; while the modulation of P/Q-type Ca(2+) channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca(2+) channels. Finally, we found no evidence of calcineurin modulating these Ca(2+) channels during M(1)-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D(2)-receptor modulation.

Cellular and subcellular distribution of rat brain prolyl oligopeptidase and its association with specific neuronal neurotransmitters

Prolyl oligopeptidase (POP) is a serine endopeptidase that hydrolyzes proline-containing peptides shorter than 30-mer. It has been suggested that POP is associated with cognitive functions and inositol 1,4,5-triphosphate (IP(3)) signaling. However, little is known about the distribution and physiological role of POP in the brain. We used immunohistochemistry to determine the cellular and subcellular distribution of POP in the rat brain. POP was specifically expressed in the glutamatergic pyramidal neurons of the cerebral cortex, particularly in the primary motor and somatosensory cortices, and also in the CA1 field of hippocampus. Purkinje cells of the cerebellum were also intensively immunostained for POP. Double immunofluorescence indicated that POP was present in the gamma-aminobutyric acid (GABA)ergic and cholinergic interneurons of the thalamus and cortex but not in the nigrostriatal dopaminergic neurons. POP did not colocalize with astrocytic markers in any part of the rat brain. We used postembedding immunoelectron microscopy to determine the distribution of POP at the subcellular level. POP was mainly present in neuronal cytosol and membranes, hardly at all in neuronal plasma membrane, but more extensively in intracellular membranes such as the rough endoplasmic reticulum and Golgi apparatus. Our findings point to a role for POP--evidently modifying neuropeptide levels--in excitatory and inhibitory neurotransmission in the central nervous system via glutamatergic, GABAergic, and cholinergic neurotransmission systems. Furthermore, according to our results, POP may be involved in thalamocortical neurotransmission, memory and learning functions of the hippocampal formation, and GABAergic regulation of voluntary movements. Subcellular distribution of POP points to a role in protein processing and secretion.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Influence of central nicotinic receptors on arachidonylcyclopropylamide (ACPA)-induced antinociception in mice

The nicotinic cholinergic receptors have been reported to be involved in several actions of cannabinoids (e.g., bradycardia, hypothermia). However, the influence of central cholinergic system on cannabinoids antinociceptive effect has not been reported. This study investigated the possible part played by nicotinic cholinergic modulator drugs on the antinociceptive effect of central administration of arachidonylcyclopropylamide (ACPA) in mice. The antinociceptive effects of intracerebroventricular (i.c.v.) administration of ACPA using the formalin test have been studied in mice. The effects of nicotine or mecamylamine (a nicotinic cholinergic antagonist) on ACPA analgesia are also studied. i.c.v. administration of ACPA (0.004-1 microg/mice) induced antinociceptive effect in mice. i.c.v. administration of nicotine (0.1 or 0.5 microg/mice) or mecamylamine (2 microg/mice) potentiated or antagonized ACPA antinociceptive effects, respectively. It is concluded that ACPA-induced analgesia is influenced by central nicotinic cholinergic activity.

Loss of muscarinic autoreceptor function impairs long-term depression but not long-term potentiation in the striatum

Muscarinic autoreceptors regulate cholinergic tone in the striatum. We investigated the functional consequences of genetic deletion of striatal muscarinic autoreceptors by means of electrophysiological recordings from either medium spiny neurons (MSNs) or cholinergic interneurons (ChIs) in slices from single M(4) or double M(2)/M(4) muscarinic acetylcholine receptor (mAChR) knock-out (-/-) mice. In control ChIs, the muscarinic agonist oxotremorine (300 nM) produced a self-inhibitory outward current that was mostly reduced in M(4)(-/-) and abolished in M(2)/M(4)(-/-) mice, suggesting an involvement of both M(2) and M(4) autoreceptors. In MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, muscarine caused a membrane depolarization that was prevented by the M(1) receptor-preferring antagonist pirenzepine (100 nM), suggesting that M(1) receptor function was unaltered. Acetylcholine has been involved in striatal long-term potentiation (LTP) or long-term depression (LTD) induction. Loss of muscarinic autoreceptor function is predicted to affect synaptic plasticity by modifying striatal cholinergic tone. Indeed, high-frequency stimulation of glutamatergic afferents failed to induce LTD in MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, as well as in wild-type mice pretreated with the M(2)/M(4) antagonist AF-DX384 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,1 1-dihydro-6H-pyrido[2,3b][1,4] benzodiazepin-6-one). Interestingly, LTD could be restored by either pirenzepine (100 nM) or hemicholinium-3 (10 microM), a depletor of endogenous ACh. Conversely, LTP induction did not show any difference among the three mouse strains and was prevented by pirenzepine. These results demonstrate that M(2)/M(4) muscarinic autoreceptors regulate ACh release from striatal ChIs. As a consequence, endogenous ACh drives the polarity of bidirectional synaptic plasticity.

Repeated exposure to methamphetamine causes long-lasting presynaptic corticostriatal depression that is renormalized with drug readministration

Addiction-associated behaviors such as drug craving and relapse are hypothesized to result from synaptic changes that persist long after withdrawal and are renormalized by drug reinstatement, although such chronic synaptic effects have not been identified. We report that exposure to the dopamine releaser methamphetamine for 10 days elicits a long-lasting (>4 month) depression at corticostriatal terminals that is reversed by methamphetamine readministration. Both methamphetamine-induced chronic presynaptic depression and the drug's selective renormalization in drug-experienced animals are independent of corresponding long-term changes in synaptic dopamine release but are due to alterations in D1 dopamine and cholinergic receptor systems. These mechanisms might provide a synaptic basis that underlies addiction and habit learning and their long-term maintenance.

Effects of Striatal GABAA-Receptor Blockade on Striatal and Cortical Activity in Monkeys

To elucidate the role of ambient striatal γ-aminobutyric acid (GABA) in the regulation of neuronal activity in the basal ganglia–thalamocortical circuits, we studied the effects of blocking striatal GABAA receptors on the electrical activities of single striatal neurons, on local field potentials (LFPs) in the striatum, and on motor cortical electroencephalograms (EEGs) in two monkeys. Striatal LFPs were recorded with a device that allowed us to simultaneously record field potentials and apply drugs by reverse microdialysis at the same site. Administration of the GABAA-receptor antagonist gabazine (SR95531, 10 and 500 μM) induced large-amplitude LFP fluctuations at the infusion site, occurring every 2–5 s for about 2 h after the start of the 20-min drug administration. These events were prevented by cotreatment with a GABAA-receptor agonist (muscimol, 100 μM) or a combination of ionotropic glutamate receptor antagonists (CNQX and MK-801, each given at 100 μM). Gabazine (10 μM) also increased the firing of single neurons recorded close to the injection site, but in most cases there was no correlation between single-neuron activity and the concomitantly recorded LFP signals from the same striatal region. In contrast, intrastriatal application of gabazine increased the correlation between striatal LFPs and EEG, and resulted in the appearance of recurrent EEG events that were temporally related to the striatal LFP events. These data provide evidence that a GABAergic “tone” in the monkey striatum controls the spontaneous activity of striatal neurons, as well as the level of striatal and cortical synchrony.

The vesicular glutamate transporter VGLUT3 synergizes striatal acetylcholine tone

Three subtypes of vesicular transporters accumulate glutamate into synaptic vesicles to promote its vesicular release. One of the subtypes, VGLUT3, is expressed in neurons, including cholinergic striatal interneurons, that are known to release other classical transmitters. Here we showed that disruption of the Slc17a8 gene (also known as Vglut3) caused an unexpected hypocholinergic striatal phenotype. Vglut3(-/-) mice were more responsive to cocaine and less prone to haloperidol-induced catalepsy than wild-type littermates, and acetylcholine release was decreased in striatum slices lacking VGLUT3. These phenotypes were associated with a colocalization of VGLUT3 and the vesicular acetylcholine transporter (VAChT) in striatal synaptic vesicles and the loss of a synergistic effect of glutamate on vesicular acetylcholine uptake. We propose that this vesicular synergy between two transmitters is the result of the unbalanced bioenergetics of VAChT, which requires anion co-entry for continuing vesicular filling. Our study reveals a previously unknown effect of glutamate on cholinergic synapses with potential functional and pharmacological implications.

Presynaptic nicotinic receptors: a dynamic and diverse cholinergic filter of striatal dopamine neurotransmission

The effects of nicotine on dopamine transmission from mesostriatal dopamine neurons are central to its reinforcing properties. Only recently however, has the influence of presynaptic nicotinic receptors (nAChRs) on dopaminergic axon terminals within striatum begun to be understood. Here, rather than simply enhancing (or inhibiting) dopamine release, nAChRs perform the role of a presynaptic filter, whose influence on dopamine release probability depends on presynaptic activity in dopaminergic as well as cholinergic neurons. Both mesostriatal dopaminergic neurons and striatal cholinergic interneurons play key roles in motivational and sensorimotor processing by the basal ganglia. Moreover, it appears that the striatal influence of dopamine and ACh cannot be fully appreciated without an understanding of their reciprocal interactions. We will review the powerful filtering by nAChRs of striatal dopamine release and discuss its dependence on activity in dopaminergic and cholinergic neurons. We will also review how nicotine, acting via nAChR desensitization, promotes the sensitivity of dopamine synapses to activity. This filtering action might provide a mechanism through which nicotine promotes how burst activity in dopamine neurons facilitates goal-directed behaviour and reinforcement processing. More generally, it indicates that we should not restrict our view of presynaptic nAChRs to simply enhancing neurotransmitter release. We will also summarize current understanding of the forms and functions of the diverse nAChRs purported to exist on dopaminergic axons. A greater understanding of nAChR form and function is imperative to guide the design of ligands with subtype-selective efficacy for improved therapeutic interventions in nicotine addiction as well as Parkinson's disease.

Cholinergic interneurons are differentially distributed in the human striatum

Background

The striatum (caudate nucleus, CN, and putamen, Put) is a group of subcortical nuclei involved in planning and executing voluntary movements as well as in cognitive processes. Its neuronal composition includes projection neurons, which connect the striatum with other structures, and interneurons, whose main roles are maintaining the striatal organization and the regulation of the projection neurons. The unique electrophysiological and functional properties of the cholinergic interneurons give them a crucial modulating function on the overall striatal response.

Methodology/Principle Findings

This study was carried out using stereological methods to examine the volume and density (cells/mm³) of these interneurons, as visualized by choline acetyltransferase (ChAT) immunoreactivity, in the following territories of the CN and Put of nine normal human brains: 1) precommissural head; 2) postcommissural head; 3) body; 4) gyrus and 5) tail of the CN; 6) precommissural and 7) postcommissural Put. The distribution of ChAT interneurons was analyzed with respect to the topographical, functional and chemical territories of the dorsal striatum. The CN was more densely populated by cholinergic neurons than the Put, and their density increased along the anteroposterior axis of the striatum with the CN body having the highest neuronal density. The associative territory of the dorsal striatum was by far the most densely populated. The striosomes of the CN precommissural head and the postcommissural Put contained the greatest number of ChAT-ir interneurons. The intrastriosomal ChAT-ir neurons were abundant on the periphery of the striosomes throughout the striatum.

Conclusions/Significance

All these data reveal that cholinergic interneurons are differentially distributed in the distinct topographical and functional territories of the human dorsal striatum, as well as in its chemical compartments. This heterogeneity may indicate that the posterior aspects of the CN require a special integration of information by interneurons. Interestingly, these striatal regions have been very much left out in functional studies.

Re-emergence of striatal cholinergic interneurons in movement disorders

Twenty years ago, striatal cholinergic neurons were central figures in models of basal ganglia function. But since then, they have receded in importance. Recent studies are likely to lead to their re-emergence in our thinking. Cholinergic interneurons have been implicated as key players in the induction of synaptic plasticity and motor learning, as well as in motor dysfunction. In Parkinson's disease and dystonia, diminished striatal dopaminergic signalling leads to increased release of acetylcholine by interneurons, distorting network function and inducing structural changes that undoubtedly contribute to the symptoms. By contrast, in Huntington's disease and progressive supranuclear palsy, there is a fall in striatal cholinergic markers. This review gives an overview of these recent experimental and clinical studies, placing them within the context of the pathogenesis of movement disorders.

Cholinergic neurons of the adult rat striatum are immunoreactive for glutamatergic N-methyl-d-aspartate 2D but not N-methyl-d-aspartate 2C receptor subunits

Cholinergic neurons of the striatum play a crucial role in controlling output from this region. Their firing is under the control of a relatively limited glutamatergic input, deriving principally from the thalamus. Glutamate transmission is effected via three major subtypes of receptors, including those with affinity for N-methyl-d-aspartate (NMDA) and the properties of individual receptors reflect their precise subunit composition. We examined the distribution of NMDA2C and NMDA2D subunits in the rat striatum using immunocytochemistry and show that a population of large neurons is strongly immunoreactive for NMDA2D subunits. From their morphology and ultrastructure, these neurons were presumed to be cholinergic and this was confirmed with double immunofluorescence. We also show that NMDA2C is present in a small number of septal and olfactory cortical neurons but absent from the striatum. Receptors that include NMDA2D subunits are relatively insensitive to magnesium ion block making neurons more likely to fire at more negative membrane potentials. Their localization to cholinergic neurons may enable very precise regulation of firing of these neurons by relatively small glutamatergic inputs.

Motor deficits and altered striatal gene expression in aphakia (ak) mice

Like humans with Parkinson's disease (PD), the ak mouse lacks the majority of the substantia nigra pars compacta (SNc) and experiences striatal denervation. The purpose of this study was to test whether motor abnormalities in the ak mouse progress over time, and whether motor function could be associated with temporal alterations in the striatal transcriptome. Ak and wt mice (28 to 180 days old) were tested using paradigms sensitive to nigrostriatal dysfunction. Results were analyzed using a linear mixed model. Ak mice significantly underperformed wt controls in rotarod, balance beam, string test, pole test and cotton shred tests at all ages examined. Motor performance in ak mice remained constant over the first 6 months of life, with the exception of the cotton shred test, in which ak mice exhibited marginal decline in performance. Dorsal striatal semi-quantitative RT-PCR for 19 dopaminergic, cholinergic, glutaminergic and catabolic genes was performed in 1- and 6-month-old groups of ak and wt mice. Preproenkephalin levels in ak mice were elevated in both age groups. Drd1, 3 and 4 levels declined over time, in contrast to increasing Drd2 expression. Additional findings included decreased Chrnalpha6 expression and elevated VGluT1 expression at both time points in ak mice and elevated AchE expression in young ak mice only. Results confirm that motor ability does not decline significantly for the first 6 months of life in ak mice. Their striatal gene expression patterns are consistent with dopaminergic denervation, and change over time, despite relatively unaltered motor performance.

TrpC3/C7 and Slo2.1 are molecular targets for metabotropic glutamate receptor signaling in rat striatal cholinergic interneurons

Large aspiny cholinergic interneurons provide the sole source of striatal acetylcholine, a neurotransmitter critical for basal ganglia function; these tonically active interneurons receive excitatory inputs from corticostriatal glutamatergic afferents that act, in part, via metabotropic glutamate receptors (mGluRs). We combined electrophysiological recordings in brain slices with molecular neuroanatomy to identify distinct ion channel targets for mGluR1/5 receptors in striatal cholinergic interneurons: transient receptor potential channel 3/7 (TrpC3/C7) and Slo2.1. In recordings obtained with methanesulfonate-based internal solutions, we found an mGluR-activated current with voltage-dependent and pharmacological properties reminiscent of TrpC3 and TrpC7; expression of these TrpC subunits in cholinergic interneurons was verified by combined immunohistochemistry and in situ hybridization, and modulation of both TrpC channels was reconstituted in HEK293 (human embryonic kidney 293) cells cotransfected with mGluR1 or mGluR5. With a chloride-based internal solution, mGluR agonists did not activate interneuron TrpC-like currents. Instead, a time-dependent, outwardly rectifying K(+) current developed after whole-cell access, and this Cl(-)-activated K(+) current was strongly inhibited by volatile anesthetics and mGluR activation. This modulation was recapitulated in cells transfected with Slo2.1, a Na(+)- and Cl(-)-activated K(+) channel, and Slo2.1 expression was confirmed histochemically in striatal cholinergic interneurons. By using gramicidin perforated-patch recordings, we established that the predominant agonist-activated current was TrpC-like when ambient intracellular chloride was preserved, although a small K(+) current contribution was observed in some cells. Together, our data indicate that mGluR1/5-mediated glutamatergic excitation of cholinergic interneurons is primarily a result of activation of TrpC3/TrpC7-like cationic channels; under conditions when intracellular NaCl is elevated, a Slo2.1 background K(+) channel may also contribute.

Impact of 6-hydroxydopamine lesions and cocaine exposure on mu-opioid receptor expression and regulation of cholinergic transmission in the limbic-prefrontal territory of the rat dorsal striatum

Information processing within the striatum is regulated by local circuits involving dopamine, cholinergic interneurons and neuropeptides released by recurrent collaterals of striatal output neurons. In the limbic-prefrontal territory of the dorsal striatum, enkephalin inhibits the NMDA-evoked release of acetylcholine directly through micro-opioid receptors (MORs) located on cholinergic interneurons and indirectly through MORs of output neurons of striosomes. In this territory, we investigated the consequence of changes in dopamine transmission, bilateral 6-hydroxydopamine-induced degeneration of striatal dopaminergic innervation or cocaine (acute and chronic) exposure on (i) MOR expression in both cholinergic interneurons and output neurons of striosomes, and (ii) the direct and indirect enkephalin-MOR regulations of the NMDA-evoked release of acetylcholine. Expression of MORs in cholinergic interneurons was preserved after 6-hydroxydopamine and down-regulated after cocaine treatments. Accordingly, the direct enkephalin-MOR control of acetylcholine release was preserved after 6-hydroxydopamine treatment and lost after cocaine exposure. Expression of MORs in output neurons of striosomes was down-regulated in the 6-hydroxydopamine situation and either preserved or up-regulated after acute or chronic cocaine exposure, respectively. Accordingly, the indirect enkephalin-MOR control of acetylcholine release disappeared in the 6-hydroxydopamine situation but surprisingly, despite preservation of MORs in striosomes, disappeared after cocaine treatment. Showing that MORs of striosomes are still functional in this situation, the MOR agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin inhibited the NMDA-evoked release of acetylcholine after cocaine exposure. Therefore, alteration in the regulation of cholinergic transmission by the enkephalin-MOR system might play a major role in the motivational and cognitive disorders associated with dopamine dysfunctions in fronto-cortico-basal ganglia circuits.

Dopamine-mediated regulation of corticostriatal synaptic plasticity

The striatum represents the main input into the basal ganglia. Neurons projecting from the striatum receive a large convergence of afferents from all areas of the cortex and transmit neural information to the basal ganglia output structures. Corticostriatal transmission is essential in the regulation of voluntary movement, in addition to behavioural control, cognitive function and reward mechanisms. Long-term potentiation (LTP) and long-term depression (LTD), the two main forms of synaptic plasticity, are both represented at corticostriatal synapses and strongly depend on the activation of dopamine receptors. Here, we discuss possible feedforward and feedback mechanisms by which striatal interneurons, in association with striatal spiny neurons and endogenous dopamine, influence the formation and maintenance of both LTP and LTD. We also propose a model in which the spontaneous membrane oscillations of neurons projecting from the striatum (named 'up' and 'down' states), in addition to the pattern of release of endogenous dopamine, bias the synapse towards preferential induction of LTP or LTD. Finally, we discuss how endogenous dopamine crucially influences changes in synaptic plasticity induced by pathological stimuli, such as energy deprivation.