Acetylcholine-mediated modulation of striatal function

Tonic enhancement of endocannabinoid-mediated retrograde suppression of inhibition by cholinergic interneuron activity in the striatum

Tonically active cholinergic interneurons in the striatum modulate activities of striatal outputs from medium spiny (MS) neurons and significantly influence overall functions of the basal ganglia. Cellular mechanisms of this modulation are not fully understood. Here we show that ambient acetylcholine (ACh) derived from tonically active cholinergic interneurons constitutively upregulates depolarization-induced release of endocannabinoids from MS neurons. The released endocannabinoids cause transient suppression of inhibitory synaptic inputs to MS neurons through acting retrogradely onto presynaptic CB1 cannabinoid receptors. The effects were mediated by postsynaptic M(1) subtype of muscarinic ACh receptors, because the action of a muscarinic agonist to release endocannabinoids and the enhancement of depolarization-induced endocannabinoid release by ambient ACh were both deficient in M1 knock-out mice and were blocked by postsynaptic infusion of guanosine-5'-O-(2-thiodiphosphate). Suppression of spontaneous firings of cholinergic interneurons by inhibiting Ih current reduced the depolarization-induced release of endocannabinoids. Conversely, elevation of ambient ACh concentration by inhibiting choline esterase significantly enhanced the endocannabinoid release. Paired recording from a cholinergic interneuron and an MS neuron revealed that the activity of single cholinergic neuron could influence endocannabinoid-mediated signaling in neighboring MS neurons. These results clearly indicate that striatal endocannabinoid-mediated modulation is under the control of cholinergic interneuron activity. By immunofluorescent and immunoelectron microscopic examinations, we demonstrated that M1 receptor was densely distributed in perikarya and dendrites of dopamine D1 or D2 receptor-positive MS neurons. Thus, we have disclosed a novel mechanism by which the muscarinic system regulates striatal output and may contribute to motor control.

Cholinergic interneurons control the excitatory input to the striatum

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

Cholinergic circuits and signaling in the pathophysiology of schizophrenia

Central cholinergic signaling has long been associated with aspects of memory, motivation, and mood, each affected functions in neuropsychiatric disorders such as schizophrenia. In this chapter, we review evidence related to the core hypothesis that dysregulation of central cholinergic signaling contributes to the pathophysiology of schizophrenia. Although central cholinergic circuits are resistant to simplification-particularly when one tries to parse the contributions of various classes of cholinergic receptors to disease related phenomena--the potential role of ACh signaling in Schizophrenia pathophysiology deserves careful consideration for prospective therapeutics. The established role of cholinergic circuits in attentional tuning is considered along with recent work on how the patterning of cholinergic activity may modulate corticostriatal circuits affected in schizophrenia.

Role of acetylcholine transmission in nucleus accumbens and ventral tegmental area in heroin-seeking induced by conditioned cues

The involvement of cholinergic transmission in heroin self-administration and the reinstatement of heroin-seeking was examined in rats trained to nose-poke for i.v. heroin. Systemic treatment with physostigmine, an inhibitor of acetylcholinesterase, modestly reduced the acquisition and rate of heroin self-administration, and this suppression of heroin intake was reversed by pretreatment with scopolamine but not by mecamylamine. Following 10-14 days of self-administration, rats were left in the home environment for 14 days. Subsequently, rats were evaluated for extinction of nose-pokes during the first hour after being returned to the self-administration apparatus. One hour later a conditioned stimulus (house light, light in the nose-poke hole, sound of the infusion pump) was presented to initiate cue-induced reinstatement. Physostigmine produced a dose-dependent inhibition of cue-induced reinstatement, but only the dose of 0.5 mg/kg significantly decreased nose-poke responding in the extinction test. Chronic treatment with physostigmine (0.1 mg/kg) did not impair performance during acquisition of heroin self-administration. However, during a subsequent reinstatement test conducted in the absence of physostigmine pretreatment, heroin seeking was significantly below that of rats chronically pretreated with saline. To evaluate brain regions mediating the effects of systemic drug treatment on reinstatement, physostigmine was microinjected into the nucleus accumbens (NAc) or ventral tegmental area (VTA). Microinjection of physostigmine into the NAc prior to presenting conditioned cues inhibited the reinstatement of heroin-seeking, without affecting extinction responding. In contrast, microinjection of physostigmine into the VTA augmented the reinstatement induced by conditioned cues and extinction responding. Inactivation of either NAc or VTA by microinjecting tetrodotoxin blocked both extinction responding and cue-induced reinstatement. These data demonstrate that cholinergic transmission influences heroin self-administration and reinstatement. Moreover, cue-induced reinstatement was inhibited by physostigmine in the NAc and potentiated by cholinergic stimulation in the VTA.

Striatal cholinergic interneurons express a receptor-insensitive homomeric TASK-3-like background K+ current

Large aspiny cholinergic interneurons provide the sole source of striatal acetylcholine, a neurotransmitter essential for normal basal ganglia function. Cholinergic interneurons engage in multiple firing patterns that depend on interactions among various voltage-dependent ion channels active at different membrane potentials. Leak conductances, particularly leak K(+) channels, are of primary importance in establishing the prevailing membrane potential. We have combined molecular neuroanatomy with whole cell electrophysiology to demonstrate that TASK-3 (K(2P)9.1, Kcnk9) subunits contribute to leak K(+) currents in striatal cholinergic interneurons. Immunostaining for choline acetyltransferase was combined with TASK-3 labeling, using nonradioactive cRNA probes or antisera selective for TASK-3, to demonstrate that striatal cholinergic neurons universally express TASK-3. Consistent with this, we isolated a pH-, anesthetic-, and Zn(2+)-sensitive current with properties expected of TASK-3 homodimeric channels. Surprisingly, activation of Galphaq-linked receptors (metabotropic glutamate mGluR1/5 or histamine H1) did not appear to modulate native interneuron TASK-3-like currents. Together, our data indicate that homomeric TASK-3-like background K(+) currents contribute to establishing membrane potential in striatal cholinergic interneurons and they suggest that receptor modulation of TASK channels is dependent on cell context.

A convergent model for cognitive dysfunctions in Parkinson's disease: the critical dopamine-acetylcholine synaptic balance

Parkinson's disease is classically characterised as a motor neurodegenerative disorder. Motor symptoms in the disorder are secondary to an altered dopamine-acetylcholine balance due to reduced striatal dopaminergic tone and subsequent cholinergic overactivity. In the past, anticholinergic drugs were given to improve motor aspects of the disease. There is now an increasing interest in the cognitive and non-motor symptoms of Parkinson's disease and in cholinesterase-inhibitor therapy for dementia associated with Parkinson's disease. In this Personal View, we reconsider the dopamine-acetylcholine balance theory and look at recent clinical findings and the possible cooperative role of dopamine and acetylcholine in the induction and maintenance of the long-lasting changes of striatal and cortical synaptic plasticity. We also discuss a convergent versus parallel model to explain cognitive dysfunctions in Parkinson's disease according to dopamine-acetylcholine dependent alterations in synaptic plasticity.

Altered responses to dopaminergic D2 receptor activation and N-type calcium currents in striatal cholinergic interneurons in a mouse model of DYT1 dystonia

Early-onset torsion dystonia (DYT1) is an autosomal dominant disease caused by a deletion in the gene encoding the protein torsinA. Recently, a transgenic mouse model of DYT1 has been described, expressing either the human wild-type torsinA (hWT) or mutant torsinA (hMT). We recorded the activity of striatal cholinergic interneurons of hWT, hMT, and control mice. In slice preparations, no significant differences were observed in resting membrane potential (RMP), firing activity, action potential duration or Ih current. Quinpirole, a D2-like dopamine receptor agonist, did not produce detectable effects on RMP of cholinergic interneurons in control mice and hWT mice, but in the hMT mice caused membrane depolarization and an increase in the firing rate. D2 receptor activation inhibits N-type high-voltage-activated calcium currents. We found that, in isolated interneurons from hMT mice, the quinpirole-mediated inhibition of N-type currents was significantly larger than in hWT and controls. Moreover, the N-type component was significantly over-represented in hMT mice. The altered sensitivity of N-type channels in hMT mice could account for the paradoxical excitatory effect of D2 stimulation. Our data support the existence of an imbalance between striatal dopaminergic and cholinergic signaling in DYT1 dystonia.

Acetylcholinesterase modulates stress-induced motor responses through catalytic and noncatalytic properties

BACKGROUND

Cholinergic neurotransmission notably participates in stress-induced motor responses. Here we report the contribution of alternative splicing of acetylcholinesterase (AChE) pre-mRNA to modulate these responses. More specifically, we induced stress-associated hypofunction of dopaminergic, mainly D2 dopamine receptor-mediated neurotransmission by haloperidol and explored stress induced hyperlocomotion and catalepsy, an extreme form of immobility, induced in mice with AChE deficiencies.

METHODS

Conditional transgenic (Tet/AS) mice were created with tetracycline-induced antisense suppression of AChE gene expression. Locomotion and catalepsy times were measured in Tet/AS and strain-matched control mice, under open-field exposure threat and under home-cage safety.

RESULTS

In vitro, NGF-treated PC12 cells failed to extend neurites upon Tet/AS suppression. In vivo, Tet/AS but not control mice showed stress-associated hippocampal deposits of heat-shock protein 70 and GRP78 (BiP), predicting posttranscriptional changes in neuronal reactions. Supporting this notion, their striatal cholinergic neurons demonstrated facilitated capacity for neurite extension, attributing these in vivo changes in neurite extension to network interactions. Tet/AS mice presented stress-induced hyperlocomotion. Moreover, the dopamine antagonist haloperidol induced longer catalepsy in threatened Tet/AS than in control mice. When returned to home-cage safety, Tet/AS mice showed retarded release from catalepsy.

CONCLUSIONS

Acetylcholinesterase modulates stress-induced motor responses and facilitates resumption of normal motor behavior following stress through both catalytic and noncatalytic features.

Neuroadaptations of Cdk5 in cholinergic interneurons of the nucleus accumbens and prefrontal cortex of inbred alcohol-preferring rats following voluntary alcohol drinking

BACKGROUND

Neurobiological studies have identified brain areas and related molecular mechanisms involved in alcohol abuse and dependence. Specific cell types in these brain areas and their role in alcohol-related behaviors, however, have not yet been identified. This study examined the involvement of cholinergic cells in inbred alcohol-preferring rats following 1 month of alcohol drinking. Cyclin-dependent kinase 5 (Cdk5) immunoreactivity (IR), a marker of neuronal plasticity, was examined in cholinergic neurons of the nucleus accumbens (NuAcc) and prefrontal cortex (PFC) and other brain areas implicated in alcohol drinking, using dual immunocytochemical (ICC) procedures. Single Cdk5 IR was also examined in several brain areas implicated in alcohol drinking.

METHODS

The experimental group self-administered alcohol using a 2-bottle-choice test paradigm with unlimited access to 10% (v/v) alcohol and water for 23 h/d for 1 month. An average of 6 g/kg alcohol was consumed daily. Control animals received identical treatment, except that both bottles contained water. Rats were perfused and brain sections were processed for ICC procedures.

RESULTS

Alcohol drinking resulted in a 51% increase in Cdk5 IR cholinergic interneurons in the shell NuAcc, while in the PFC there was a 51% decrease in the percent of Cdk5 IR cholinergic interneurons in the infralimbic region and a 46% decrease in Cdk5 IR cholinergic interneurons in the prelimbic region. Additionally, single Cdk5 IR revealed a 42% increase in the central nucleus of the amygdala (CNA).

CONCLUSIONS

This study identified Cdk5 neuroadaptation in cholinergic interneurons of the NuAcc and PFC and in other neurons of the CNA following 1 month of alcohol drinking. These findings contribute to our understanding of the cellular and molecular basis of alcohol drinking and toward the development of improved region and cell-specific pharmacotherapeutic and behavioral treatment programs for alcohol abuse and alcoholism.

Activation of muscarinic and nicotinic acetylcholine receptors in the nucleus accumbens core is necessary for the acquisition of drug reinforcement

Neurotransmitter release in the nucleus accumbens core (NACore) during the acquisition of remifentanil or cocaine reinforcement was determined in an operant runway procedure by simultaneous tandem mass spectrometric analysis of dopamine, acetylcholine, and remifentanil or cocaine itself. Run times for remifentanil or cocaine continually decreased over the five consecutive runs of the experiment. Intra-NACore dopamine, acetylcholine, and drug peaked with each intravenous remifentanil or cocaine self-administration and decreased to pre-run baseline with half-lives of approximately 10 min. As expected, remifentanil or cocaine peaks did not vary between the five runs. Surprisingly, however, drug-contingent dopamine peaks also did not change over the five runs, whereas acetylcholine peaks did. Thus, the acquisition of drug reinforcement was paralleled by a continuous increase in acetylcholine overflow in the NACore, whereas the overflow of dopamine, the expected prime neurotransmitter candidate for conditioning in drug reinforcement, did not increase. Local intra-accumbens administration by reverse microdialysis of either atropine or mecamylamine completely and reversibly blocked the acquisition of remifentanil reinforcement. Our findings suggest that activation of muscarinic and nicotinic acetylcholine receptors in the NACore by acetylcholine volume transmission is necessary during the acquisition phase of drug reinforcement conditioning.

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

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

Adaptive acetylcholinesterase splicing patterns attenuate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice

Balanced dopaminergic cholinergic interactions are crucial for proper basal ganglia function. This is dramatically demonstrated by the worsening of Parkinson's disease symptoms following acetylcholinesterase (AChE) inhibition. Typically, in the brain, the synapse-anchored synaptic AChE (AChE-S) variant is prevalent whereas the soluble readthrough AChE (AChE-R) variant is induced in response to cholinesterase inhibition or stress. Because of the known functional differences between these variants and the fact that AChE-R expression is triggered by various stimuli that themselves are often associated with Parkinson's disease risk, we hypothesized that the splice shift to AChE-R plays a functional role in Parkinsonian progression. After establishing that Paraoxon-induced AChE inhibition indeed aggravates experimental Parkinsonism triggered by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice, we tested the roles of individual AChE variants by exposing transgenic mice overexpressing either the AChE-S or AChE-R variant to MPTP. Differential reductions of tyrosine hydroxylase levels in the striatum and substantia nigra indicated that transgenic AChE-R expression confers resistance as compared with the parent FVB/N strain. In contrast, AChE-S overexpression accelerated the MPTP-induced damage. Survival, behavioral measures and plasma corticosterone levels were also compatible with the extent of the dopaminergic damage. Our findings highlight the functional differences between individual AChE variants and indicate that a naturally occurring stress or AChE inhibitor-induced splicing shift can act to minimize dopaminergic cholinergic imbalances. We propose that inherited or acquired alternative splicing deficits could accelerate Parkinsonism and that, correspondingly, adaptive alternative splicing events may attenuate disease progression.

Acetyl-l-carnitine protects striatal neurons against in vitro ischemia: The role of endogenous acetylcholine

The neuronal death after ischemia is closely linked to the essential role of mitochondrial metabolism. Inhibition of mitochondrial respiratory chain reduces ATP generation leading to a dysregulation of ion metabolism. Acetyl-L-carnitine (ALC) influences the maintenance of key mitochondrial proteins for maximum energy production and it may play a neuroprotective role in some pathological conditions. In this study we have analyzed ALC-mediated neuroprotection on an in vitro model of brain ischemia. Field potential recordings were obtained from a rat corticostriatal slice preparation. In vitro ischemia (oxygen and glucose deprivation) was delivered by switching to a solution in which glucose was omitted and oxygen was replaced with N2. Ten minutes of in vitro ischemia caused an irreversible loss of the field potential amplitude. Pretreatment with ALC produced a progressive and dose-dependent recovery of the field potential amplitude following in vitro ischemia. The neuroprotective effect of ALC was stereospecific since the pretreatment with two different carnitine-related compounds did not cause neuroprotection. The choline transporter inhibitor hemicholinium-3 blocked the neuroprotective effect of ALC. ALC-mediated neuroprotection was also prevented either by the non-selective muscarinic antagonist scopolamine, or by the putative M2-like receptor antagonist methoctramine. Conversely, the effect of ALC was not altered by the M1-like receptor antagonist pirenzepine. These findings show that ALC exert a neuroprotective action against in vitro ischemia. This neuroprotective effect requires the activity of choline uptake system and the activation of M2 muscarinic receptors.

Reduced expression and capacity of the striatal high-affinity choline transporter in hyperdopaminergic mice

Behavioral and neuronal abnormalities observed in mice exhibiting a reduced expression of the dopamine transporter model important aspects of schizophrenia, addiction, and attentional disorders. As the consequences of a chronic hyperdopaminergic tone for striatal output regulation have remained poorly understood, the present experiments were designed to determine the status of striatal interneuronal cholinergic neurotransmission in dopamine transporter knockdown animals. The high-affinity choline transporter represents the rate-limiting step of acetylcholine synthesis and release. Compared with wild type mice, striatal high-affinity choline transporter expression in dopamine transporter knockdown mice was significantly decreased. As in vivo basal striatal acetylcholine release did not differ between the strains, reduced high-affinity choline transporter expression in dopamine transporter knockdown mice was not due to reduced basal cholinergic activity. Furthermore, the proportion of high-affinity choline transporters expressed in plasma membrane-enriched versus vesicular membrane-enriched fractions did not differ from wild type animals, suggesting that changes in intracellular high-affinity choline transporter trafficking were not associated with lower overall levels of striatal high-affinity choline transporters. Synaptosomal choline uptake assays indicated a reduced capacity of striatal high-affinity choline transporters in dopamine transporter knockdown mice, and thus the functional significance of the reduced level of high-affinity choline transporter expression. Likewise, in vivo measures of the capacity of striatal high-affinity choline transporters to clear increases in extracellular choline concentrations, using choline-sensitive microelectrodes, revealed a 37-41% reduction in hemicholinium-sensitive clearance of exogenous choline in dopamine transporter knockdown mice. Furthermore, clearance of potassium-evoked choline signals was reduced in dopamine transporter knockdown mice (1.63+/-0.15 microM/s) compared with wild type animals (2.29+/-0.21 microM/s). Dysregulated striatal cholinergic neurotransmission is hypothesized to disrupt the integration of thalamic and cortical information at spiny projection neurons and thus to contribute to abnormal striatal information processing in dopamine transporter knockdown mice.

Scopolamine induces impairments in the recognition of human facial expressions of anger and disgust

RATIONALE

Recent psychopharmacological studies lend support to the notion of partially dissociable neuronal systems dedicated to processing specific emotions. For example, GABA-ergic enhancement after an acute dose of the benzodiazepine, diazepam, produces specific impairments in anger and fear recognition. However, it is unclear if these impairments are a general property of benzodiazepines and other drugs that produce a similar profile of neurocognitive impairment to benzodiazepines, such as the anticholinergic, scopolamine.

OBJECTIVE

We investigated the effects of scopolamine and the benzodiazepine, lorazepam, on emotion-recognition accuracy.

METHODS

A double-blind independent group design was used with 48 healthy volunteers to compare the effects of scopolamine and lorazepam with an inactive placebo on a commonly used emotion-recognition task. Control measures included an episodic memory task and subjective mood ratings.

RESULTS

Anger and disgust recognition accuracy was impaired after scopolamine. In contrast, lorazepam produced no impairment in emotion-recognition despite producing similar levels of sedation and anterograde amnesia to scopolamine.

CONCLUSIONS

Scopolamine-induced cholinergic hypofunction selectively impaired the recognition accuracy of disgust and anger facial expressions. The effects of scopolamine on emotion-recognition are similar to those found in Huntington's disease patients. Furthermore, the impairments in anger and fear recognition previously observed with diazepam do not appear to be a general property of benzodiazepines. This suggests that alterations in emotional processing involving changes in the ability to recognize threat-related emotions (particularly, fear and anger) may not be a principal mechanism underlying anxiolysis or paradoxical aggression seen with benzodiazepines.

Plastic and behavioral abnormalities in experimental Huntington's disease: A crucial role for cholinergic interneurons

Huntington's disease (HD) is a fatal hereditary neurodegenerative disease causing degeneration of striatal spiny neurons, whereas cholinergic interneurons are spared. This cell-type specific pathology produces an array of abnormalities including involuntary movements, cognitive impairments, and psychiatric disorders. Although the genetic mutation responsible for HD has been identified, little is known about the early synaptic changes occurring within the striatal circuitry at the onset of clinical symptoms. We therefore studied the synaptic plasticity of spiny neurons and cholinergic interneurons in two animal models of early HD. As a pathogenetic model, we used the chronic subcutaneous infusion of the mitochondrial toxin 3-nitropropionic acid (3-NP) in rats. This treatment caused striatal damage and impaired response flexibility in the cross-maze task as well as defective extinction of conditioned fear suggesting a perseverative behavior. In these animals, we observed a loss of depotentiation in striatal spiny neurons and a lack of long-term potentiation (LTP) in cholinergic interneurons. These abnormalities of striatal synaptic plasticity were also observed in R6/2 transgenic mice, a genetic model of HD, indicating that both genetic and phenotypic models of HD show cell-type specific alterations of LTP. We also found that in control rats, as well as in wild-type (WT) mice, depotentiation of spiny neurons was blocked by either scopolamine or hemicholinium, indicating that reversal of LTP requires activation of muscarinic receptors by endogenous acetylcholine. Our findings suggest that the defective plasticity of cholinergic interneurons could be the primary event mediating abnormal functioning of striatal circuits, and the loss of behavioral flexibility typical of early HD might largely depend on cell-type specific plastic abnormalities.

Meaningful silences: how dopamine listens to the ACh pause

Mesostriatal dopaminergic neurons (DANs) and striatal cholinergic neurons (tonically active neurons, TANs) participate in signalling the behavioural or reward-related significance of stimuli in the environment. An antagonistic balance between dopamine (DA) and ACh is well known to regulate postsynaptic signal integration in the striatum. Recent findings have revealed additional presynaptic ACh-DA interactions of previously unappreciated sophistication. Striatal ACh acts presynaptically to polarize powerfully how opposing DAN activities are transduced into DA release. Furthermore, characteristic reward-related activities of TANs and DANs are temporally coincident but differently variant with reward probability. Reward-related DA signals could therefore be governed by the concomitant activity in TANs. This article discusses the dynamic implications for DA signalling when these phenomena act in concert. TAN pauses might powerfully enhance the contrast, or salience, of DA signals offered by reward-related bursts, and even by reward omission-related pauses, in DANs. Through such mechanisms, TAN-DAN interactions would be functionally cooperative.

Activity of tonically active neurons in the monkey putamen during initiation and withholding of movement

Tonically active neurons (TANs) of the primate striatum are putative interneurons that respond to events of motivational significance, such as primary rewards, and to sensory stimuli that predict such events. Because TANs influence striatal projection neurons, TANs may play a role in the initiation and control of movement. To examine this issue, we recorded from putaminal TANs in macaque monkeys trained to make the same arm movement in two ways--in reaction to an external cue and also after a variable delay without an explicit instruction to move (self-timed movements). On other trials, the animals had to withhold movement following an external cue. The task design ensured that the three types of trials were effectively randomly interleaved, equally frequent, and similar in overall timing. Separately, we presented "playback" trials in which the same sequence of visual stimulation and reward was presented while the animals fixated without making the arm movement. We found that TAN responses were strongly affected by behavioral context. In particular, TAN responses were strikingly stronger when the animals actively withheld movements than on the corresponding playback trials, even though the stimulus sequence and reward timing were identical and no movement was made in either case. Many TANs also became active in the absence of a proximate sensory cue on self-timed movements, suggesting that TANs may reflect internal processes that are specific to self-timed movements. These results suggest that TANs may directly participate in, or monitor the motivational significance of, an animal's actions as well as external events.

Vitamin A deficiency induces motor impairments and striatal cholinergic dysfunction in rats

Vitamin A and its derivatives, retinoids, are involved in the regulation of gene expression by binding two nuclear receptor families, retinoic acid receptors and retinoid X receptors. Retinoid receptors are highly expressed in the striatum, revealing an involvement of this system in the control of movement as demonstrated by previous observations in knockout mice. To further assess the role of retinoids in adult striatal function, the present study investigated the effect of vitamin A deprivation on rat motor activity and coordination, the rate of synthesis and release of dopamine, the functioning of D1 and D2 receptors and their expression in the striatum. Moreover, the content of acetylcholine in the striatum was measured. Results show that 24 weeks of postnatal vitamin A deprivation induced severe locomotor deficits and impaired motor coordination. Vitamin A deprivation rats showed a significant hyperactivity following D1 receptor stimulation by R(+)-6-chloro-7,8-dihydroxy-1-phenyil-2,3,4,5-tetrahydro-1H-3-benzazepine or amphetamine and reduced catalepsy in response to haloperidol treatment. This different response to the above drugs is not due to a change in striatal DA release or synthesis between vitamin A deprivation and control animals. In situ hybridization experiments showed identical level of expression for the D1 and D2 receptor transcripts. On the other hand, the striatal tissue content of acetylcholine was reduced significantly by about 30% starting from the initial manifestation of motor deficits. We suggest that the locomotor impairment could be imputable to the dysfunction in striatal cholinergic interneurons. Our results stress the basic role of vitamin A in the maintenance of basal ganglia motor function in the adult rat brain.

Activation of protein kinase C and inositol 1,4,5-triphosphate receptors antagonistically modulate voltage-gated sodium channels in striatal neurons

Regulation of voltage-gated sodium channels is crucial to firing patterns that constitute the output of medium spiny neurons (MSN), projecting neurons of the striatum. This modulation is thus critical for the final integration of information processed within the striatum. It has been shown that the adenylate cyclase pathway reduces sodium currents in MSN through channel phosphorylation by cAMP-dependent protein kinase. However, it is unknown whether a phospholipase C (PLC)-mediated signaling cascade could also modulate voltage-gated sodium channels within MSN. Using the whole-cell patch clamp technique, we investigated the effects of activation of two key components in PLC-mediated signaling cascades: protein kinase C (PKC) and inositol-1,4,5-triphosphate (IP(3)) receptors on voltage-dependent sodium current. Cellular dialysis with phorbol 12-myristate 13-acetate, an activator of PKC, significantly reduced peak sodium current amplitude, while adenophostin A, an activator of IP(3) receptors, significantly increased peak sodium current amplitude. This effect of adenophostin was abolished by calcium chelation or by FK506, an inhibitor of calcineurin. These results suggest an antagonistic role of PKC and IP(3) in the modulation of striatal voltage-gated sodium channels, peak current amplitude being decreased through phosphorylation by PKC and increased through dephosphorylation by calcineurin.

The neurochemical and behavioral effects of the novel cholinesterase-monoamine oxidase inhibitor, ladostigil, in response to L-dopa and L-tryptophan, in rats

The novel drugs, ladostigil (TV3326) and TV3279, are R and S isomers, respectively, derived from a combination of the carbamate cholinesterase (ChE) inhibitor, rivastigmine, and the pharmacophore of the monoamine oxidase (MAO) B inhibitor, rasagiline. They were developed for the treatment of comorbidity of dementia with Parkinsonism. In the present study, we determined the effects of these drugs on both aminergic neurotransmitter levels and motor behavioral activity in naïve and in L-dopa- or L-tryptophan-induced rats. Chronic treatment of rats with ladostigil (52 mg kg(-1) for 21 days) inhibited hippocampal and striatal MAO A and B activities by >90%, increased striatal levels of dopamine and serotonin, and inhibited striatal ChE activity by approximately 50%. Chronic TV3279 (26 mg kg(-1) for 21 days) similarly inhibited approximately 50% of striatal ChE activity, but did not affect MAO activity or amine levels. In sharp contrast to the inductive effect of the MAO A/B inhibitor, tranylcypromine (TCP), on stereotyped hyperactivity in response to L-dopa (50 mg kg(-1)) or L-tryptophan (100 mg kg(-1)), ladostigil completely inhibited these behavioral hyperactivity syndromes. Accordingly, acute rivastigmine (2 mg kg(-1)) and chronic TV3279 abolished the ability of TCP to initiate L-dopa-induced hyperactivity, while scopolamine (0.5 mg kg(-1)) reversed the inhibitory effect of chronic ladostigil on L-dopa-induced hyperactivity, suggesting that ladostigil may attenuate successive locomotion by activating central cholinergic muscarinic receptors.Finally, while chronic ladostigil administration to naïve rats resulted in preserved spontaneous motor behavior, acute treatment with ladostigil decreased motor performance, compared to control animals. In contrast, chronic as well as acute treatments with TV3279 reduced spontaneous motor activity. Thus, the aminergic potentiation by ladostigil may counteract its cholinergic inhibitory effect on spontaneous motor behavior. Our results suggest that potentiation of both aminergic and cholinergic transmission systems by ladostigil contributes equally to motor behavior performance, which is substantially impaired in comorbidity of dementia with Parkinsonism including dementia with Lewy bodies (DLB).

Direct detection of brain acetylcholine synthesis by magnetic resonance spectroscopy

The cholinergic system is an important modulatory neurotransmitter system in the brain. Changes in acetylcholine concentration have been previously determined directly in animal models and human brain biopsy specimens, and indirectly, by the effects of drugs, in living humans. Here, we developed a method for direct determination of acetylcholine synthesis in living brain tissue. The method is based on administration of choline, enriched with carbon-13 (stable isotope) in the two methylene positions, and detection of labeled acetylcholine and all other metabolic fates of choline, by carbon-13 magnetic resonance spectroscopy. We tested this method in rat brain slices and found it to be specific for acetylcholine synthesis in both the cortex and hippocampus. This method is potentially useful as a research tool for exploring the cholinergic system role in cognitive processes and memory storage as well as in diseases in which the malfunction of the cholinergic system has been implicated.

Modification of motor cortical excitability by an acetylcholinesterase inhibitor

Acetylcholine powerfully modulates the excitability of neocortical neurones and networks. This study applied focal transcranial magnetic stimulation (TMS) to eight healthy subjects to test the effects of a single oral dose of 40 mg tacrine, an acetylcholinesterase inhibitor, on motor cortical excitability. It was found that tacrine decreased short-interval intracortical inhibition, and increased intracortical facilitation and short-interval intracortical facilitation. Motor thresholds, motor evoked potential amplitude, cortical silent period (CSP) duration, and measures of spinal and neuromuscular excitability remained unchanged. The effects peaked at 2-6 h and fully reversed after 24 h. All effects can be explained by a reduction of motor cortical GABAergic inhibitory neurotransmission via activation of presynaptic muscarinic M2 receptors, but other more complex mechanisms may also have contributed and are discussed. The findings predict that acetylcholine has the potential to promote plasticity and learning in human motor cortex.

Nanomolar concentrations of kynurenic acid reduce extracellular dopamine levels in the striatum

Precise regulation of dopaminergic activity is of obvious importance for the physiology and pathology of basal ganglia. We report here that nanomolar concentrations of the astrocyte-derived neuroinhibitory metabolite kynurenic acid (KYNA) potently reduce the extracellular levels of striatal dopamine in unanesthetized rats in vivo. This effect, which is initiated by the KYNA-induced blockade of alpha7 nicotinic acetylcholine receptors, highlights the functional relevance of glia-neuron interactions in the striatum and indicates that even modest increases in the brain levels of endogenous KYNA are capable of interfering with dopaminergic neurotransmission.

Striatal glutamate release evoked in vivo by NMDA is dependent upon ongoing neuronal activity in the substantia nigra, endogenous striatal substance P and dopamine

The aim of the present microdialysis study was to investigate whether the increase in striatal glutamate levels induced by intrastriatal perfusion with NMDA was dependent on the activation of extrastriatal loops and/or endogenous striatal substance P and dopamine. The NMDA-evoked striatal glutamate release was mediated by selective activation of the NMDA receptor-channel complex and action potential propagation, as it was prevented by local perfusion with dizocilpine and tetrodotoxin, respectively. Tetrodotoxin and bicuculline, perfused distally in the substantia nigra reticulata, prevented the NMDA-evoked striatal glutamate release, suggesting its dependence on ongoing neuronal activity and GABA(A) receptor activation, respectively, in the substantia nigra. The NMDA-evoked glutamate release was also dependent on striatal substance P and dopamine, as it was antagonized by intrastriatal perfusion with selective NK(1) (SR140333), D(1)-like (SCH23390) and D(2)-like (raclopride) receptor antagonists, as well as by striatal dopamine depletion. Furthermore, impairment of dopaminergic transmission unmasked a glutamatergic stimulation by submicromolar NMDA concentrations. We conclude that in vivo the NMDA-evoked striatal glutamate release is mediated by activation of striatofugal GABAergic neurons and requires activation of striatal NK(1) and dopamine receptors. Endogenous striatal dopamine inhibits or potentiates the NMDA action depending on the strength of the excitatory stimulus (i.e. the NMDA concentration).

Analysis of acetylcholine and choline in microdialysis samples by liquid chromatography/tandem mass spectrometry

A sensitive liquid chromatography/electrospray ionisation tandem mass spectrometric (LC/ESI-MS/MS) method was developed for the analysis of acetylcholine and choline in microdialysis samples. A Ringer's solution that contains high (150 mM) concentrations of inorganic salts was used to extract acetylcholine and choline from a rat or mouse brain. The separation of acetylcholine, choline, an internal standard acetyl-beta-methylcholine, endogenous compounds and inorganic cations was achieved with hydrophilic interaction chromatography using a diol column. The eluent consisted of 20 mM ammonium formate (pH 3.3) and acetonitrile (20:80) which is favourable for the ESI process. Limits of detection (signal-to-noise (S/N) ratio = 3) of 0.02 nM (0.2 fmol) for acetylcholine and 1 nM (10 fmol) for choline were observed using standards diluted in Ringer's solution. A good linearity was obtained from the limit of quantitation: 0.1 nM (S/N ratio = 10) to 50 nM (r = 0.999) for acetylcholine and within the concentration range of 100-3500 nM (r = 0.998) for choline. The between-day repeatability of the method was good; RSD was 3.1% at 1 nM level of acetylcholine and 3.5% at 1000 nM level of choline. The recoveries for addition of 1 or 2.5 nM acetylcholine and 0.2 or 1 microM choline in microdialysis balancing samples were between 93 and 101% indicating that no suppressing endogenous compounds were co-eluting with acetylcholine or choline. The developed method was applied to the analysis of microdialysis balancing samples collected from rat and mouse brains.

Modulatory action of metabotropic glutamate receptor (mGluR) 5 on mGluR1 function in striatal cholinergic interneurons

Within basal ganglia, group I metabotropic glutamate receptor subtypes (mGluR1 and 5) frequently co-localize in the same neuron. However, little is known about how these receptors functionally interact. We addressed this issue by means of electrophysiological recordings of striatal cholinergic interneurons, a neuronal subtype that co-express both group I mGluRs. The group I non-selective agonist 3,5-DHPG induced a membrane depolarization/inward current that was prevented by co-application of LY 367385, a selective mGluR1 antagonist, and SIB 1757 or MPEP, blockers of mGluR5 subtype. The reversal potential for the response to 3,5-DHPG was close to the equilibrium potential for potassium channels. Repeated bath or focal applications of 3,5-DHPG induced a progressive decline in the amplitude of the membrane depolarization, suggesting that group I mGluRs undergo receptor desensitization. Interestingly, in the presence of the mGluR5 blocker, SIB 1757, this event was not observed, whereas it occurred in LY 367385. PKC blockers chelerythrine and calphostin C mimicked the inhibitory effect of SIB 1757. In a subset of interneurons, in MPEP or SIB 1757, 3,5-DHPG induced a 0.5-1 Hz oscillatory response, that was prevented by L-type Ca2+ channel blockers, and by the tyrosine kinase inhibitors genistein and lavendustin. Together, these data suggest that mGluR5 modulates mGluR1 activity to shape cell excitability.

Rationale for and use of NMDA receptor antagonists in Parkinson's disease

N-Methyl-d-aspartate (NMDA) glutamate receptors are a class of excitatory amino acid receptors, which have several important functions in the motor circuits of the basal ganglia, and are viewed as important targets for the development of new drugs to prevent or treat Parkinson's disease (PD). NMDA receptors are ligand-gated ion channels composed of multiple subunits, each of which has distinct cellular and regional patterns of expression. They have complex regulatory properties, with both agonist and co-agonist binding sites and regulation by phosphorylation and protein-protein interactions. They are found in all of the structures of the basal ganglia, although the subunit composition in the various structures is different. NMDA receptors present in the striatum are crucial for dopamine-glutamate interactions. The abundance, structure, and function of striatal receptors are altered by the dopamine depletion and further modified by the pharmacological treatments used in PD. In animal models, NMDA receptor antagonists are effective antiparkinsonian agents and can reduce the complications of chronic dopaminergic therapy (wearing off and dyskinesias). Use of these agents in humans has been limited because of the adverse effects associated with nonselective blockade of NMDA receptor function, but the development of more potent and selective pharmaceuticals holds the promise of an important new therapeutic approach for PD.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole-cell patch-clamp recordings in brain slices of postnatal rats. Under voltage-clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at -85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 microm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike-triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine-induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid, suggesting that the current was associated with PLC in a protein kinase C- and Ca2+ -independent manner. The phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5-bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura-2 demonstrated a muscarine-induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC-dependent pathway.

Cholinergic drugs for Alzheimer's disease enhance in vitro dopamine release

Alzheimer's disease is a neurodegenerative disorder associated with a decline in cognitive abilities. Patients also frequently have noncognitive symptoms, such as anxiety, depression, apathy, and psychosis, that impair daily living. The most commonly prescribed treatments for Alzheimer's disease are acetylcholinesterase inhibitors, such as donepezil and galantamine. Enhanced cholinergic functions caused by these compounds are believed to underlie improvements in learning, memory, and attention. The noncognitive aspects of dementia, however, are usually linked to serotonin and dopamine rather than acetylcholine because those neurotransmitter systems most directly influence mood, emotional balance, and psychosis. Fast-scan cyclic voltammetry applied to mouse striatal brain slices was used to measure the real-time release of dopamine arising from spontaneous activity or from single electrical stimulations. At concentrations that include their prescribed dosage ranges, donepezil (1-1000 nM) and galantamine (50-1000 nM) increase action potential-dependent dopamine release. Consistent with previous literature, the data support slightly different modes of action for donepezil and galantamine. The ability of these commonly prescribed drugs to alter catecholamine release may underlie their influence over noncognitive symptoms of dementia. Furthermore, these results suggest that acting via nicotinic receptors, these drugs may serve presently untapped therapeutic roles by altering dopamine release in other disorders involving dopaminergic systems.

Delta9-tetrahydrocannabinol decreases somatic and motivational manifestations of nicotine withdrawal in mice

The possible interactions between Delta9-tetrahydrocannabinol (Delta9-THC) and nicotine remain unclear in spite of the current association of cannabis and tobacco in humans. The aim of the present study was to explore the interactions between these two drugs of abuse by evaluating the consequences of Delta9-THC administration on the somatic manifestations and the aversive motivational state associated with nicotine withdrawal in mice. Acute Delta9-THC administration significantly decreased the incidence of several nicotine withdrawal signs precipitated by mecamylamine or naloxone, such as wet-dog-shakes, paw tremor and scratches. In both experimental conditions, the global withdrawal score was also significantly attenuated by acute Delta9-THC administration. This effect of Delta9-THC was not due to possible adaptive changes induced by chronic nicotine on CB1 cannabinoid receptors, as the density and functional activity of these receptors were not modified by chronic nicotine administration in the different brain structures investigated. We also evaluated the consequences of Delta9-THC administration on c-Fos expression in several brain structures after chronic nicotine administration and withdrawal. c-Fos was decreased in the caudate putamen and the dentate gyrus after mecamylamine precipitated nicotine withdrawal. However, acute Delta9-THC administration did not modify c-Fos expression under these experimental conditions. Finally, Delta9-THC also reversed conditioned place aversion associated to naloxone precipitated nicotine withdrawal. Taken together, these results indicate that Delta9-THC administration attenuated somatic signs of nicotine withdrawal and this effect was not associated with compensatory changes on CB1 cannabinoid receptors during chronic nicotine administration. In addition, Delta9-THC also ameliorated the aversive motivational consequences of nicotine withdrawal.

Differential effects of M1 muscarinic receptor blockade and nicotinic receptor blockade in the dorsomedial striatum on response reversal learning

The present studies determined whether blockade of M(1)-like muscarinic or nicotinic cholinergic receptors in the dorsomedial striatum affects acquisition or reversal learning of a response discrimination. Testing occurred in a modified cross-maze across two consecutive sessions. In the acquisition phase, a rat learned to turn to the left or to the right. In the reversal learning phase, a rat learned to turn in the opposite direction as required during acquisition. Experiment 1 investigated the effects of the M(1)-like muscarinic receptor antagonist, pirenzepine infused into the dorsomedial striatum on acquisition and reversal learning. Experiment 2 examined the effects of the nicotinic cholinergic antagonist, mecamylamine injected into the dorsomedial striatum on acquisition and reversal learning. Bilateral injections of pirenzepine at 10 microg, but not 1 microg, selectively impaired reversal learning. Analysis of the errors indicated that pirenzepine treatment did not impair the initial shift, but increased reversions back to the original response choice following the initial shift. Bilateral injections of mecamylamine, 6 or 18 microg, did not affect acquisition or reversal learning. The results suggest that activation of M(1) muscarinic cholinergic receptors, but not nicotinic cholinergic receptors, in the dorsomedial striatum is important for facilitating the flexible shifting of response patterns.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinson's disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

Anatomical funneling, sparse connectivity and redundancy reduction in the neural networks of the basal ganglia

The major anatomical characteristics of the main axis of the basal ganglia are: (1) Numerical reduction in the number of neurons across layers of the feed-forward network, (2) lateral inhibitory connections within the layers, and (3) neuro-modulatory effects of dopamine and acetylcholine, both on the basal ganglia neurons and on the efficacy of information transmission along the basal ganglia axis. We recorded the simultaneous activity of neurons in the output stages of the basal ganglia as well as the activity of dopaminergic and cholinergic neurons during the performance of a probability decision-making task. We found that the functional messages of the cholinergic and dopaminergic neurons differ, and that the cholinergic message is less specific than that of the dopaminergic neurons. The output stage of the basal ganglia showed uncorrelated neuronal activity. We conclude that despite the huge numerical reduction from the cortex to the output nuclei of the basal ganglia, the activity of these nuclei represents an optimally compressed (uncorrelated) version of distinctive features of cortical information.

Tonically active neurons in the primate caudate nucleus and putamen differentially encode instructed motivational outcomes of action

To achieve a goal, animals procure immediately available rewards, escape from aversive events, or endure the absence of rewards. The neuronal substrates for these goal-directed actions include the limbic system and the basal ganglia. In the striatum, tonically active neurons (TANs), presumed cholinergic interneurons, were originally shown to respond to reward-associated stimuli and to evolve their activity through learning. Subsequent studies revealed that they also respond to aversive event-associated stimuli such as an airpuff on the face and that they are less selective to whether the stimuli instruct reward or no reward. To address this paradox, we designed a set of experiments in which macaque monkeys performed a set of visual reaction time tasks while expecting a reward, during escape from an aversive event, and in the absence of a reward. We found that TANs respond to instruction stimuli associated with motivational outcomes (312 of 390; 80%) but not to unassociated ones (51 of 390; 13%), and that they mostly differentiate associated instructions (217 of 312; 70%). We also found that a higher percentage of TANs in the caudate nucleus respond to stimuli associated with motivational outcomes (118 of 128; 92%) than in the putamen (194 of 262; 74%), whereas a higher percentage of TANs in the putamen respond to go signals for the lever release (112 of 262; 43%) than in the caudate nucleus (27 of 128; 21%), especially for an action expecting a reward. These findings suggest a distinct, pivotal role of TANs in the caudate nucleus and putamen in encoding instructed motivational contexts for goal-directed action planning and learning.

Ethanol-induced Fos immunoreactivity in the extended amygdala and hypothalamus of the rat brain: focus on cholinergic interneurons of the nucleus accumbens

BACKGROUND

The primary goal of this study was to investigate the effects of varying doses of ethanol on cellular activation, as measured by Fos immunoreactivity, in brain areas that have been implicated in the reinforcing and anxiolytic effects of substance abuse and dependence, namely, the extended amygdala and hypothalamus. Specific regions examined included the central nucleus of the amygdala, bed nucleus of the stria terminalis, substantia innominata, and nucleus accumbens of the extended amygdala, as well as the paraventricular nucleus of the hypothalamus. The cholinergic interneurons of the nucleus accumbens were of particular interest, because these cells have recently been reported to play a pivotal role in substance abuse.

METHODS

Adult Sprague-Dawley rats underwent 10 days of handling and 5 days of habituation. Animals then received an injection of saline or 0.5, 1, or 2 g/kg of ethanol. Rats were perfused 2 hr after the injections, and brain sections were processed for single Fos or dual Fos/choline acetyltransferase immunolabeling procedures. The number of Fos-positive neurons was calculated from a 0.45-mm sample area from each of the brain regions examined.

RESULTS

A dose of 2 g/kg of ethanol significantly increased the number of Fos-immunoreactive neurons in the central nucleus of the amygdala by 149%, in the shell nucleus accumbens by 80%, and in the paraventricular nucleus of the hypothalamus by 321%. Additionally, 1 g/kg of ethanol significantly increased the percentage of Fos-immunoreactive cholinergic neurons in the nucleus accumbens by 59%.

CONCLUSIONS

The findings reported in this study reveal region-specific and dose-dependent changes in Fos immunoreactivity in the extended amygdala and hypothalamus and, more specifically, an increase in neuronal activation of cholinergic cells in the shell nucleus accumbens. These findings contribute to our current knowledge of the brain areas and cellular microcircuits involved in the underlying basis of substance abuse and dependence.

Engagement of rat striatal neurons by cortical epileptiform activity investigated with paired recordings

The striatum is thought to play an important role in the spreading of epilepsy from cortical areas to deeper brain structures, but this issue has not been addressed with intracellular techniques. Paired recordings were used to assess the impact of cortical epileptiform activity on striatal neurons in brain slices. Bath-application of 4-amynopyridine (100 microM) and bicuculline (20 microM) induced synchronized bursts in all pairs of cortical neurons (< or = 5 mm apart) in coronal, sagittal, and oblique slices (which preserve connections from the medial agranular cortex to the striatum). Under these conditions, striatal medium spiny neurons (MSs) displayed a strong increased spontaneous glutamatergic activity. This activity was not correlated to the cortical bursts and was asynchronous in pairs of MSs. Sporadic, large-amplitude synchronous depolarizations also occurred in MSs. These events were simultaneously detected in glial cells, suggesting that they were accompanied by considerable increases in extracellular potassium. In oblique slices, cortically driven bursts were also observed in MSs. These events were synchronized to cortical epileptiform bursts, depended on non-N-methyl-D-aspartate (NMDA) glutamate receptors, and persisted in the cortex, but not in the striatum, after disconnection of the two structures. During these bursts, MS membrane potential shifted to a depolarized value (59 +/- 4 mV) on which an irregular waveform, occasionally eliciting spikes, was superimposed. Thus synchronous activation of a limited set of corticostriatal afferents can powerfully control MSs. Cholinergic interneurons located < 120 microm from simultaneously recorded MSs, did not display cortically driven bursts, suggesting that these cells are much less easily engaged by cortical epileptiform activity.

Coincident but Distinct Messages of Midbrain Dopamine and Striatal Tonically Active Neurons

Midbrain dopamine and striatal tonically active neurons (TANs, presumed acetylcholine interneurons) signal behavioral significance of environmental events. Since striatal dopamine and acetylcholine affect plasticity of cortico-striatal transmission and are both crucial to learning, they may serve as teachers in the basal ganglia circuits. We recorded from both neuronal populations in monkeys performing a probabilistic instrumental conditioning task. Both neuronal types respond robustly to reward-related events. Although different events yielded responses with different latencies, the responses of the two populations coincided, indicating integration at the target level. Yet, while the dopamine neurons' response reflects mismatch between expectation and outcome in the positive domain, the TANs are invariant to reward predictability. Finally, TAN pairs are synchronized, compared to a minority of dopamine neuron pairs. We conclude that the striatal cholinergic and dopaminergic systems carry distinct messages by different means, which can be integrated differently to shape the basal ganglia responses to reward-related events.

Simultaneous monitoring of acetylcholine and catecholamine release in the in vivo rat adrenal medulla

To simultaneously monitor acetylcholine release from pre-ganglionic adrenal sympathetic nerve endings and catecholamine release from post-ganglionic adrenal chromaffin cells in the in vivo state, we applied microdialysis technique to anesthetized rats. Dialysis probe was implanted in the left adrenal medulla and perfused with Ringer's solution containing neostigmine (a cholinesterase inhibitor). After transection of splanchnic nerves, we electrically stimulated splanchnic nerves or locally administered acetylcholine through dialysis probes for 2 min and investigated dialysate acetylcholine, choline, norepinephrine and epinephrine responses. Acetylcholine was not detected in dialysate before nerve stimulation, but substantial acetylcholine was detected by nerve stimulation. In contrast, choline was detected in dialysate before stimulation, and dialysate choline concentration did not change with repetitive nerve stimulation. The estimated interstitial acetylcholine levels and dialysate catecholamine responses were almost identical between exogenous acetylcholine (10 microM) and nerve stimulation (2 Hz). Dialysate acetylcholine, norepinephrine and epinephrine responses were correlated with the frequencies of electrical nerve stimulation, and dialysate norepinephrine and epinephrine responses were quantitatively correlated with dialysate acetylcholine responses. Neither hexamethonium (a nicotinic receptor antagonist) nor atropine (a muscarinic receptor antagonist) affected the dialysate acetylcholine response to nerve stimulation. Microdialysis technique made it possible to simultaneously assess activities of pre-ganglionic adrenal sympathetic nerves and post-ganglionic adrenal chromaffin cells in the in vivo state and provided quantitative information about input-output relationship in the adrenal medulla.

Neuronal vulnerability following inhibition of mitochondrial complex II: a possible ionic mechanism for Huntington's disease

An impaired complex II (succinate dehydrogenase, SD) striatal mitochondrial activity is one of the prominent metabolic alterations in Huntington's disease (HD), and intoxication with 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, mimics the motor abnormalities and the pathology of HD. We found that striatal spiny neurons responded to this toxin with an irreversible membrane depolarization/inward current, while cholinergic interneurons showed a hyperpolarization/outward current. Both these currents were sensitive to intracellular concentration of ATP. The 3-NP-induced depolarization was associated with an increased release of endogenous GABA, while acetylcholine levels were reduced. Moreover, 3-NP induced a higher depolarization in presymptomatic R6/2 HD transgenic mice compared to wild-type (WT) mice, showing an increased susceptibility to SD inhibition. Conversely, the hyperpolarization did not significantly differ from the one recorded in WT mice. The diverse membrane changes induced by SD inhibition may contribute to the cell-type-specific neuronal death in HD.

Contributions of memory circuits to language: the declarative/procedural model

The structure of the brain and the nature of evolution suggest that, despite its uniqueness, language likely depends on brain systems that also subserve other functions. The declarative/procedural (DP) model claims that the mental lexicon of memorized word-specific knowledge depends on the largely temporal-lobe substrates of declarative memory, which underlies the storage and use of knowledge of facts and events. The mental grammar, which subserves the rule-governed combination of lexical items into complex representations, depends on a distinct neural system. This system, which is composed of a network of specific frontal, basal-ganglia, parietal and cerebellar structures, underlies procedural memory, which supports the learning and execution of motor and cognitive skills, especially those involving sequences. The functions of the two brain systems, together with their anatomical, physiological and biochemical substrates, lead to specific claims and predictions regarding their roles in language. These predictions are compared with those of other neurocognitive models of language. Empirical evidence is presented from neuroimaging studies of normal language processing, and from developmental and adult-onset disorders. It is argued that this evidence supports the DP model. It is additionally proposed that "language" disorders, such as specific language impairment and non-fluent and fluent aphasia, may be profitably viewed as impairments primarily affecting one or the other brain system. Overall, the data suggest a new neurocognitive framework for the study of lexicon and grammar.

Antipsychotic-induced rabbit syndrome: epidemiology, management and pathophysiology

Rabbit syndrome is an antipsychotic-induced rhythmic motion of the mouth/lips, resembling the chewing movements of a rabbit. The movement consists of a vertical-only motion, at about 5Hz, with no involvement of the tongue. Usually, the involuntary movements associated with rabbit syndrome appear after a long period (in most cases months or years) of antipsychotic treatment; however, a few patients with the syndrome have had treatment histories with no antipsychotic involvement. The reported prevalence of rabbit syndrome ranges from 2.3 to 4.4% of patients treated with typical antipsychotics. There have been isolated reports of rabbit syndrome in patients treated with the atypical agents risperidone and clozapine. Patients with rabbit syndrome are most often misdiagnosed as having oral tardive dyskinesia. In such cases the key for correct diagnosis is the involvement of tardive tongue movements, which does not occur in rabbit syndrome. The treatment of rabbit syndrome is empirical, reflecting poor understanding of its neuropathology. The first step is to reduce the amount of antipsychotic treatment as much as possible. However, since, in most cases, full withdrawal of antipsychotic treatment is impossible, the syndrome cannot be completely abolished without additional measures. The next stage of treatment involves specific drugs that aim to control the syndrome. Anticholinergic drugs are the best known treatment. Rabbit syndrome does not respond to treatment with levodopa or dopamine agonists. The most striking aspect of this syndrome is its specificity. Rabbit syndrome affects only the buccal region, and within this area it involves a highly stereotyped involuntary movement. This immediately focuses attention on the basal ganglia, in particular the substantia nigra pars reticulata, which is also implicated in oral dyskinesia. Continuing neurophysiological and pharmacological research of the basal ganglia holds the key to better understanding and treatment of this syndrome in the coming years.

Behavioural parameters in aged rats are related to LTP and gene expression of ChAT and NMDA-NR2 subunits in the striatum

Striatal parameters were assessed for their relevance to age-related behavioural decline. Forty aged rats (28-30 months) were tested in the water maze and open field. Of these, seven superior and seven inferior learners were compared with each other in terms of levels of in vitro short- and long-term potentiation (STP and LTP), and gene expression of choline acetyltransferase (ChAT) as well as of the NMDA-NR2A-C subunits assessed by quantitative RT-PCR. Results revealed that the superior as compared with the inferior learners had higher levels of ChAT mRNA in the striatum. For the superior group, ChAT mRNA was correlated with escape on to the cued platform in the water maze, whereas level of LTP was predictive of place learning in the water maze and rearing activity in the open field. For the inferior group, expression of NR2A and NR2B was positively correlated with place learning and probe trial performance in the water maze. The results show that individual differences in various behaviours of aged rats were accounted for by variability in striatal parameters, i.e. LTP, ChAT and NMDA-NR2 subunit mRNA. Notably, the correlations found were heterogeneous amid the groups, e.g. variability in place learning was explained by variability in levels of LTP in the superior learners, but in levels of NR2A-B mRNA in the inferior group.

Tonically active neurons in the striatum encode motivational contexts of action

In order to achieve a goal, one procures immediately available rewards, escape from aversive events or endures absence of rewards. The neuronal substrate for these goal-directed actions includes the limbic system and the basal ganglia. In the basal ganglia, classes of projection neurons in the striatum show activity with motivational as well as sensorimotor properties, such as expectation of reward and task schedule for obtaining reward. Tonically active neurons (TANs), presumed cholinergic interneurons in the striatum, respond to reward-associated stimuli, evolve their activity through learning and respond also to aversive event-associated stimuli such as airpuff on the face. A recent study showed that responses to visual cues are less selective to whether the cue instructs reward or no reward. To address this paradox, we asked macaque monkeys to perform a set of visual reaction time tasks while expecting the reward, aversive event or absence of reward. We found that TANs respond to instruction stimuli associated with motivational outcomes but not to unassociated ones, and that they mostly differentiate associated instructions. We also found that the higher percentage of TANs in the caudate nucleus respond to stimuli associated with motivational outcomes than in the putamen, whereas the higher percentage of TANs in the putamen respond to GO signals than in the caudate nucleus especially for an action anticipating a reward. These findings suggest a distinct, pivotal role played by TANs in the caudate nucleus and putamen in encoding instructed motivational contexts for goal-directed action selection and learning in the striatum.

Cholinergic interneurons of the nucleus accumbens and dorsal striatum are activated by the self-administration of cocaine

The nucleus accumbens, a major component of the ventral striatum, and the dorsal striatum are primary targets of the mesolimbic dopamine pathway, which is a pathway that plays a critical role in reward and addiction. The shell compartment of the nucleus accumbens and the ventromedial striatum, in particular, receive extensive afferent projections from the ventral tegmental area, which is the major afferent source of the mesolimbic pathway [Prog Brain Res 99 (1993) 209; J Neurosci 7 (1987) 3915]. The present study focused on striatal cholinergic interneurons as potential key neurons involved in the neural basis of drug reinforcement. The main finding of this study is that cholinergic interneurons located in the shell compartment of the nucleus accumbens and the ventromedial striatum were activated, as measured by Fos labeling, following a 1 h session of the self-administration of cocaine in rats. A direct correlation existed between the percent of cholinergic interneurons that were activated and the amount of cocaine that was self-administered. The greatest amount of administered cocaine (approximately 10 mg/kg) resulted in the activation of approximately 80% of the cholinergic neurons. No such correlation existed in the group of animals that self-administered saline. In addition, activation was not found in the core compartment of the nucleus accumbens or the dorsolateral striatum, which receive extensive innervation from the substantia nigra and thus are more closely tied to the motor effects of the drug. In conclusion, cocaine-driven neuronal activation was specific to the shell compartment of the nucleus accumbens (R(2)=0.9365) and the ventromedial striatum (R(2)=0.9059). These findings demonstrate that cholinergic interneurons are involved in the initial stage of cocaine intake and that these neurons are located in areas of the nucleus accumbens and dorsal striatum that are more closely tied to the rewarding and hedonic effects rather than the motor effects of cocaine intake.

The Kir6.1-protein, a pore-forming subunit of ATP-sensitive potassium channels, is prominently expressed by giant cholinergic interneurons in the striatum of the rat brain

ATP-sensitive potassium channels comprise a complex of two structurally different proteins: a member of the inwardly rectifying Kir6 family (Kir6.1 or Kir6.2) and a sulfonylurea receptor (SUR1 or SUR2). Their regulation by intracellular ADP/ATP-concentrations and through various pharmacological agents has profound implications for the excitability of cells and, in the case of neurons, for neurotransmitter release. We previously showed that in rat brain, the Kir6.1 subunit is predominantly expressed in astrocytes in contrast to the Kir6.2 subunit, which is exclusively expressed in neurons. In this report we show, that in addition to the astrocytic expression, the Kir6.1 protein is also found in a small subset of neurons in distinct areas of the brain, like the hypothalamic supraoptic and paraventricular nuclei and the striatum. The Kir6.1-positive neurons in the striatum could be characterized as cholinergic interneurones, verified by immunofluorescence double staining. This complete colocalization of the Kir6.1 subunit in cholinergic interneurons is interesting with respect to the pharmacological potential of these channels. A selective modulation of the Kir6.1 subunit in the cholinergic striatal interneurons may eventually be of therapeutic value for the treatment of Parkinson's disease.