Muscarinic receptors depress GABAergic synaptic transmission in rat midbrain dopamine neurons

Varenicline for the Treatment of Cocaine Dependence

OBJECTIVES

Varenicline is a partial agonist at the α2β4 and α6β2 nAChR receptors and a full agonist at α7 receptors. Both α7 and α6β2 receptors are implicated in the neural reward circuitry activated by cocaine use. A preliminary clinical trial suggested that varenicline treatment reduced cocaine use. This trial was intended to replicate and extend the findings of the previous trial.

METHODS

This was a 12-week, double-blind, placebo-controlled clinical trial involving 156 subjects with DSM IV cocaine dependence. Subjects received up to 2 mg of varenicline or identical placebo daily along with weekly relapse prevention psychotherapy. The primary outcome measure was cocaine use measured by thrice-weekly urine drug screens. Additional outcome measures included end of study cocaine abstinence, cocaine craving, cocaine withdrawal symptom severity, cigarette use, and global improvement measure by the Clinical Global Improvement Scale.

RESULTS

End of study cocaine abstinence, measured by urine drug screens during the last 3 weeks of the trial, was not different between groups (8% in the varenicline treated subjects and versus 9% in placebo-treated subjects). Generalized estimating equations analysis of urine drug screen results showed no significant difference between groups in cocaine abstinence over the 12 weeks of the trial. There were no significant differences between the 2 groups in cocaine craving or cocaine withdrawal symptom severity. Varenicline was well-tolerated. There were no medication-associated serious adverse events.

CONCLUSIONS

Varenicline plus cognitive-behavioral therapy does not seem to be an efficacious treatment for cocaine dependence.

Cell-type-specific synaptic modulation of mAChR on SST and PV interneurons

The muscarinic acetylcholine receptor (mAChR) antagonist, scopolamine, has been shown to have a rapid antidepressant effect. And it is believed that GABAergic interneurons play a crucial role in this action. Therefore, characterizing the modulation effects of mAChR on GABAergic interneurons is crucial for understanding the mechanisms underlying scopolamine's antidepressant effects. In this study, we examined the effect of mAChR activation on the excitatory synaptic transmissions in two major subtypes of GABAergic interneurons, somatostatin (SST)- and parvalbumin (PV)-expressing interneurons, in the anterior cingulate cortex (ACC). We found that muscarine, a mAChR agonist, non-specifically facilitated the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in both SST and PV interneurons. Scopolamine completely blocked the effects of muscarine, as demonstrated by recovery of sESPCs and mEPSCs in these two types of interneurons. Additionally, individual application of scopolamine did not affect the EPSCs of these interneurons. In inhibitory transmission, we further observed that muscarine suppressed the frequency of both spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in SST interneurons, but not PV interneurons. Interestingly, scopolamine directly enhanced the frequency of both sIPSCs and mIPSCs mainly in SST interneurons, but not PV interneurons. Overall, our results indicate that mAChR modulates excitatory and inhibitory synaptic transmission to SST and PV interneurons within the ACC in a cell-type-specific manner, which may contribute to its role in the antidepressant effects of scopolamine.

The role of dopaminergic midbrain in Alzheimer's disease: Translating basic science into clinical practice

Mammalian brain cortical functions, from executive and motor functioning to memory and emotional regulation, are strictly regulated by subcortical projections. These projections terminate in cortical areas that are continuously influenced by released neurotransmitters and neuromodulators. Among the subcortical structures, the dopaminergic midbrain plays a pivotal role in tuning cortical functions that commonly result altered in many neurological and psychiatric disorders. Incidentally, extensive neuropathological observations support a strong link between structural alterations of the dopaminergic midbrain and significant behavioural symptomatology observed in patients suffering from Alzheimer 's disease(AD). Here, we will review recent progress on the involvement of the dopaminergic system in the pathophysiology of AD as well as the current therapeutic strategies targeting this system.

Pontomesencephalic Tegmental Afferents to VTA Non-dopamine Neurons Are Necessary for Appetitive Pavlovian Learning

The ventral tegmental area (VTA) receives phenotypically distinct innervations from the pedunculopontine tegmental nucleus (PPTg). While PPTg-to-VTA inputs are thought to play a critical role in stimulus-reward learning, direct evidence linking PPTg-to-VTA phenotypically distinct inputs in the learning process remains lacking. Here, we used optogenetic approaches to investigate the functional contribution of PPTg excitatory and inhibitory inputs to the VTA in appetitive Pavlovian conditioning. We show that photoinhibition of PPTg-to-VTA cholinergic or glutamatergic inputs during cue presentation dampens the development of anticipatory approach responding to the food receptacle during the cue. Furthermore, we employed in vivo optetrode recordings to show that photoinhibition of PPTg cholinergic or glutamatergic inputs significantly decreases VTA non-dopamine (non-DA) neural activity. Consistently, photoinhibition of VTA non-DA neurons disrupts the development of cue-elicited anticipatory approach responding. Taken together, our study reveals a crucial regulatory mechanism by PPTg excitatory inputs onto VTA non-DA neurons during appetitive Pavlovian conditioning.

GABA modulates baroreflex in the ventral tegmental area in rat

There are some reports demonstrating the cardiovascular functions of the ventral tegmental area (VTA). About 20-30% of the VTA neurons are GABAergic, which might play a role in baroreflex modulation. This study was performed to find the effects of GABA(A), GABA(B) receptors and reversible synaptic blockade of the VTA on baroreflex. Drugs were microinjected into the VTA of urethane anesthetized rats, and the maximum change of blood pressure and the gain of the reflex bradycardia in response to intravenous phenylephrine (Phe) injection were compared with the preinjection and the control values. Microinjection of bicuculline methiodide (BMI, 100 pmol/100 nl), a GABA(A) antagonist, into the VTA strongly decreased the Phe-induced hypertension, indicating that GABA itself attenuated the baroreflex. Muscimol, a GABA(A) agonist (30 mM, 100 nl), produced no significant changes. Baclofen, a GABA(B) receptor agonist (1000 pmole/100 nl), moderately attenuated the baroreflex, however phaclofen, a GABA(B) receptor antagonist (1000 pmole/100 nl), had no significant effect. In conclusion, for the first time, we demonstrated that GABA(A) receptors of the VTA strongly attenuate and GABA(B) receptors of the VTA moderately attenuate baroreflex in rat.

Changes in sensitivity of reward and motor behavior to dopaminergic, glutamatergic, and cholinergic drugs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a leading cause of intellectual disability. FXS is caused by loss of function of the FMR1 gene, and mice in which Fmr1 has been inactivated have been used extensively as a preclinical model for FXS. We investigated the behavioral pharmacology of drugs acting through dopaminergic, glutamatergic, and cholinergic systems in fragile X (Fmr1 (-/Y)) mice with intracranial self-stimulation (ICSS) and locomotor activity measurements. We also measured brain expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis. Fmr1 (-/Y) mice were more sensitive than wild type mice to the rewarding effects of cocaine, but less sensitive to its locomotor stimulating effects. Anhedonic but not motor depressant effects of the atypical neuroleptic, aripiprazole, were reduced in Fmr1 (-/Y) mice. The mGluR5-selective antagonist, 6-methyl-2-(phenylethynyl)pyridine (MPEP), was more rewarding and the preferential M1 antagonist, trihexyphenidyl, was less rewarding in Fmr1 (-/Y) than wild type mice. Motor stimulation by MPEP was unchanged, but stimulation by trihexyphenidyl was markedly increased, in Fmr1 (-/Y) mice. Numbers of midbrain TH+ neurons in the ventral tegmental area were unchanged, but were lower in the substantia nigra of Fmr1 (-/Y) mice, although no changes in TH levels were found in their forebrain targets. The data are discussed in the context of known changes in the synaptic physiology and pharmacology of limbic motor systems in the Fmr1 (-/Y) mouse model. Preclinical findings suggest that drugs acting through multiple neurotransmitter systems may be necessary to fully address abnormal behaviors in individuals with FXS.

Somatodendritic targeting of M5 muscarinic receptor in the rat ventral tegmental area: implications for mesolimbic dopamine transmission

Muscarinic modulation of mesolimbic dopaminergic neurons in the ventral tegmental area (VTA) plays an important role in reward, potentially mediated through the M5 muscarinic acetylcholine receptor (M5R). However, the key sites for M5R-mediated control of dopamine neurons within this region are still unknown. To address this question we examined the electron microscopic immunocytochemical localization of antipeptide antisera against M5R and the plasmalemmal dopamine transporter (DAT) in single sections through the rat VTA. M5R was located mainly to VTA somatodendritic profiles (71%; n = 627), at least one-third (33.2%; n = 208) of which also contained DAT. The M5R immunoreactivity was distributed along cytoplasmic tubulovesicular endomembrane systems in somata and large dendrites, but was more often located at plasmalemmal sites in small dendrites, the majority of which did not express DAT. The M5R-immunoreactive dendrites received a balanced input from unlabeled terminals forming either asymmetric or symmetric synapses. Compared with dendrites, M5R was less often seen in axon terminals, comprising only 10.8% (n = 102) of the total M5R-labeled profiles. These terminals were usually presynaptic to unlabeled dendrites, suggesting that M5R activation can indirectly modulate non-DAT-containing dendrites through presynaptic mechanisms. Our results provide the first ultrastructural evidence that in the VTA, M5R has a subcellular location conducive to major involvement in postsynaptic signaling in many dendrites, only some of which express DAT. These findings suggest that cognitive and rewarding effects ascribed to muscarinic activation in the VTA can primarily be credited to M5R activation at postsynaptic plasma membranes distinct from dopamine transport.

Striatal input- and rate-dependent effects of muscarinic receptors on pallidal firing

The globus pallidus (GP) plays a key role in the overall basal ganglia (BG) activity. Despite evidence of cholinergic inputs to GP, their role in the spiking activity of GP neurons has not received attention. We examine the effect of local activation and blockade of muscarinic receptors (MRs) in the spontaneous firing of GP neurons both in normal and ipsilateral striatum-lesioned rats. We found that activation of MRs produces heterogeneous responses in both normal and ipsilateral striatum-lesioned rats: in normal rats the response evoked by MRs depends on the predrug basal firing rate; the inhibition evoked by MRs is higher in normal rats than in striatum-lesioned rats; the number of neurons that undergo inhibition is lower in striatum-lesioned rats than in normal rats. Our data suggest that modulation of MRs in the GP depends on the firing rate before their activation and on the integrity of the striato-pallidal pathway.

Results of an initial clinical trial of varenicline for the treatment of cocaine dependence

BACKGROUND

Cocaine use, abuse and dependence remains a pressing public health problem. Based on its mechanism of action, varenicline, an alpha4beta2 partial agonist seemed to be a likely candidate for treating cocaine dependence.

METHODS

Cocaine dependent participants (n=37) were enrolled in a 9-week double-blind placebo controlled clinical trial. Varenicline was titrated up to a target dose of 1mg BID during the first week of medication.

RESULTS

Varenicline was associated with lower odds of cocaine use than placebo (OR=2.02, p=0.08), as measured by thrice-weekly urinalysis results. Compared to placebo-treated participants, varenicline treated participants had significantly decreased rates of cocaine reward, as measured by the Multiple Choice Procedure (MCP) (p=0.02).

CONCLUSIONS

Varenicline appears to decrease cocaine use and reward, suggesting that further investigation of varenicline may be warranted.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Muscarinic receptor blockade in the ventral tegmental area attenuates cocaine enhancement of laterodorsal tegmentum stimulation-evoked accumbens dopamine efflux in the mouse

The reinforcing properties of cocaine have been related to increased extracellular concentrations of dopamine in the nucleus accumbens (NAc). M5 muscarinic acetylcholine receptors (mAChRs) on dopamine cells in the ventral tegmental area (VTA) facilitate mesoaccumbens dopamine transmission and are critically involved in mediating natural and drug reinforcement. We investigated the effects of pharmacological blockade of mAChRs in the VTA on cocaine's ability to enhance electrically evoked NAc dopamine efflux. Using fixed potential amperometry together with carbon fiber recording microelectrodes positioned in the NAc core, we quantified dopamine oxidation currents (dopamine efflux) evoked by brief stimulation (15 monophasic pulses at 50 Hz every 30 s) of the laterodorsal tegmentum (LDT) in urethane (1.5 g/kg, i.p.) anesthetized mice. Compared to predrug baseline responses, cocaine (5 or 10 mg/kg, i.p.) dose-dependently enhanced LDT stimulation-evoked NAc dopamine efflux, whereas the nonsubtype selective mAChR antagonist scopolamine (10 microg/0.5 microl) microinfused into the VTA diminished LDT-evoked NAc dopamine efflux. Preinfusion of scopolamine into the VTA diminished the facilitatory actions of cocaine on LDT stimulation-evoked NAc dopamine efflux, and when infused at the peak effect of cocaine attenuated LDT-evoked dopamine efflux to below predrug baseline levels. These findings suggest that LDT cholinergic inputs to dopamine neurons in the VTA, via activation of mAChRs (probably of the M5 subtype), are involved in modulating the facilitatory effects of cocaine on NAc dopamine neurotransmission. They also suggest that the development of antagonists aimed at selectively disrupting M5 receptor function may be valuable in reducing abuse liability of psychostimulants.

Ethanol action on dopaminergic neurons in the ventral tegmental area: interaction with intrinsic ion channels and neurotransmitter inputs

The dopaminergic system originating in the midbrain ventral tegmental area (VTA) has been extensively studied over the past decades as a critical neural substrate involved in the development of alcoholism and addiction to other drugs of abuse. Accumulating evidence indicates that ethanol modulates the functional output of this system by directly affecting the firing activity of VTA dopamine neurons, whereas withdrawal from chronic ethanol exposure leads to a reduction in the functional output of these neurons. This chapter will provide an update on the mechanistic investigations of the acute ethanol action on dopamine neuron activity and the neuroadaptations/plasticities in the VTA produced by previous ethanol experience.

Synaptotagmin-1 functions as a Ca2+ sensor for spontaneous release

Spontaneous ‘mini’ release occurs at all synapses, but its nature remains enigmatic. Here, we show that in murine cortical neurons, >95% of spontaneous release is induced by Ca²⁺-binding to synaptotagmin-1, the Ca²⁺-sensor for fast synchronous neurotransmitter release. Thus, spontaneous and evoked release use the same Ca²⁺-dependent release mechanism. As a consequence, synaptotagmin-1 mutations that alter its Ca²⁺-affinity alter spontaneous and evoked release correspondingly. Paradoxically, synaptotagmin-1 deletions (as opposed to point mutations) massively increase spontaneous release. This increased spontaneous release remains Ca²⁺-dependent, but is activated at lower Ca²⁺-concentrations, and with a lower Ca²⁺-cooperativity, than synaptotagmin-driven spontaneous release. Thus, in addition to serving as Ca²⁺-sensor for spontaneous and evoked release, synaptotagmin-1 clamps a second, more sensitive Ca²⁺-sensor for spontaneous release which resembles the Ca²⁺-sensor for evoked asynchronous release. Viewed together, these data suggest that synaptotagmin-1 controls both evoked and spontaneous release at a synapse as a simultaneous Ca²⁺-dependent activator and clamp of exocytosis.

Targets, receptors and effects of muscarinic neuromodulation on giant neurones of the rat dorsal cochlear nucleus

Although cholinergic modulation of the cochlear nucleus (CN) is functionally important, neither its cellular consequences nor the types of receptors conveying it are precisely known. The aim of this work was to characterise the cholinergic effects on giant cells of the CN, using electrophysiology and quantitative polymerase chain reaction. Application of the cholinergic agonist carbachol increased the spontaneous activity of the giant cells; which was partly the consequence of the reduction in a K(+) conductance. This effect was mediated via M4 and M3 receptors. Cholinergic modulation also affected the synaptic transmission targeting the giant cells. Excitatory synaptic currents evoked by the stimulation of the superficial and deep regions of the CN were sensitive to cholinergic modulation: the amplitude of the first postsynaptic current was reduced, and the short-term depression was also altered. These changes were mediated via M3 receptors alone and via the combination of M4, M2 and M3 receptors, when the superficial and deep layers, respectively, were activated. Inhibitory synaptic currents evoked from the superficial layer showed short-term depression, but they were unaffected by carbachol. In contrast, inhibitory currents triggered by the activation of the deep parts exhibited no significant short-term depression, but they were highly sensitive to cholinergic activation, which was mediated via M3 receptors. Our results indicate that pre- and postsynaptic muscarinic receptors mediate cholinergic modulation on giant cells. The present findings shed light on the cellular mechanisms of a tonic cholinergic modulation in the CN, which may become particularly important in evoking contralateral excitatory responses under certain pathological conditions.

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.

Differential modulation by nicotine of substantia nigra versus ventral tegmental area dopamine neurons

Midbrain dopamine (DA) neurons are found in two nuclei, the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). The SNc dopaminergic projections to the dorsal striatum are involved in voluntary movement and habit learning, whereas the VTA projections to the ventral striatum contribute to reward and motivation. Nicotine induces profound DA release from VTA dopamine neurons but substantially less from the SNc. Nicotinic acetylcholine receptor (nAChR) expression differs between these nuclei, but it is unknown whether there are differences in nAChR expression on the afferent projections to these nuclei. Here we have compared the nicotinic modulation of excitatory and inhibitory synaptic inputs to VTA and SNc dopamine neurons. Although nicotine enhances both the excitatory and inhibitory drive to SNc DA cells with response magnitudes similar to those seen in the VTA, the prevalence of these responses in SNc is much lower. We also found that a mixture of nAChR subtypes underlies the synaptic modulation in SNc, further distinguishing this nucleus from the VTA, where alpha7 nAChRs enhance glutamate inputs and non-alpha7 receptors enhance GABA inputs. Finally, we compared the nicotine sensitivity of DA neurons in these two nuclei and found larger response magnitudes in VTA relative to SNc. Thus the observed differences in nicotine-induced DA release from VTA and SNc are likely due to differences in nAChR expression on the afferent inputs as well as on the DA neurons themselves. This may explain why nicotine has a greater effect on behaviors associated with the VTA than the SNc.

The role of dopamine for the pathophysiology of schizophrenia

Since decades, experimental approaches and clinical experience have suggested a dopaminergic system's dysregulation playing an important role within the pathophysiology of schizophrenia. This paper summarizes the actual standard of knowledge of the physiological fundamentals and hypothesized dysbalances of the dopamine (DA) system with respect to schizophrenia including interaction with other neurotransmitter systems (glutamate, GABA). The assumed functional role of DA with respect to physiological and illness-associated cognitive performance, especially working memory, reward, and motivation, as it was assessed by fMRI studies, is presented. A third focus concentrates on giving a short survey of SPECT and PET studies measuring the amount of the striatal and extrastriatal DA, the striatal and extrastriatal dopamine D2 receptor, and the dopamine transporter (DAT) comparing first-episode, drug-naïve, treated, and relapsing schizophrenic patients and healthy control persons.

Presynaptic GABAA receptors facilitate GABAergic transmission to dopaminergic neurons in the ventral tegmental area of young rats

Gamma-aminobutyric acid A receptor (GABA(A)R)-mediated postsynaptic currents (IPSCs) were recorded from dopaminergic neurons of the ventral tegmental area of young rats in acute brain slices and from mechanically dissociated neurons. Low concentrations (0.1-0.3 microm) of muscimol, a selective GABA(A)R agonist, increased the amplitude, and reduced the paired pulse ratio of evoked IPSCs. Moreover, muscimol increased the frequency but not the amplitude of spontaneous IPSCs (sIPSCs). These data point to a presynaptic locus of muscimol action. It is interesting that 1 microm muscimol caused an inhibition of sIPSCs, which was reversed to potentiation by the GABA(B) receptor antagonist CGP52432. Isoguvacine, a selective GABA(A)R agonist that belongs to a different class, mimicked the effects of muscimol on sIPSCs: it increased them at low ( Collapse

Key Words Collapse

MESH Headings

• Animals
• Cadmium/pharmacology
• Dopamine/metabolism
• GABA Agonists/pharmacology
• In Vitro Techniques
• Inhibitory Postsynaptic Potentials/drug effects
• Muscimol/pharmacology
• Neurons/metabolism
• Neurons/physiology
• Presynaptic Terminals/metabolism
• Protein Isoforms/physiology
• Rats
• Rats, Sprague-Dawley
• Receptors, GABA-A/physiology
• Synaptic Transmission/physiology
• Tetrodotoxin/pharmacology
• Ventral Tegmental Area/cytology
• Ventral Tegmental Area/metabolism
• Ventral Tegmental Area/physiology
• gamma-Aminobutyric Acid/physiology

Collapse

Grants

• R01 AA011989 NIAAA NIH HHS
• AT 001182 NCCIH NIH HHS
• R21 AT001182 NCCIH NIH HHS
• AA015925 NIAAA NIH HHS
• AA11989 NIAAA NIH HHS
• R21 AA015925 NIAAA NIH HHS

Collapse

Affiliation(s)

Cheng Xiao Department of Anesthesiology, Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, 185 South Orange Avenue, Newark, NJ 07103, USA
Chunyi Zhou
Keyong Li
Jiang-Hong Ye

Collapse

Presynaptic modulation of GABA release in the basal ganglia

Presynaptic receptors provide plasticity to GABAergic synapses in the basal ganglia network, in which GABA neurons outnumber all other neurons. Presynaptic receptors, mostly of the metabotropic type, enhance or reduce the strength of synaptic inhibition and are activated by ligands being released from the GABA terminals themselves (autoreceptors) or by ligands coming from other sources (heteroreceptors), including the target neurons innervated by the GABA terminals. The latter mechanism, termed retrograde signaling, is given particular emphasis as far as it occurs in substantia nigra.

Cholinergic axons in the rat ventral tegmental area synapse preferentially onto mesoaccumbens dopamine neurons

Cholinergic afferents to the ventral tegmental area (VTA) contribute substantially to the regulation of motivated behaviors and the rewarding properties of nicotine. These actions are believed to involve connections with dopamine (DA) neurons projecting to the nucleus accumbens (NAc). However, this direct synaptic link has never been investigated, nor is it known whether cholinergic inputs innervate other populations of DA and gamma-aminobutyric acid (GABA) neurons, including those projecting to the prefrontal cortex (PFC). We addressed these questions by using electron microscopic analysis of retrograde tract-tracing and immunocytochemistry for the vesicular acetylcholine transporter (VAChT) and for tyrosine hydroxylase (TH) and GABA. In tissue labeled for TH, VAChT(+) terminals frequently synapsed onto DA mesoaccumbens neurons but only seldom contacted DA mesoprefrontal cells. In tissue labeled for GABA, one-third of VAChT(+) terminals innervated GABA-labeled dendrites, including both mesoaccumbens and mesoprefrontal populations. VAChT(+) synapses onto DA and mesoaccumbens neurons were more commonly of the asymmetric (presumed excitatory) morphological type, whereas VAChT(+) synapses onto GABA cells were more frequently symmetric (presumed inhibitory or modulatory). These findings suggest that cholinergic inputs to the VTA mediate complex synaptic actions, with a major portion of this effect likely to involve an excitatory influence on DA mesoaccumbens neurons. As such, the results suggest that natural and drug rewards operating through cholinergic afferents to the VTA have a direct synaptic link to the mesoaccumbens DA neurons that modulate approach behaviors.

Roles of pedunculopontine tegmental cholinergic receptors in brain stimulation reward in the rat

RATIONALE

The brainstem pedunculopontine tegmental nucleus (PPTg) is proposed to mediate hypothalamic self-stimulation reward via cholinergic activation of the ventral tegmental area (VTA). However, to date there is little direct evidence to support this hypothesis.

OBJECTIVES

To further study the role of PPTg in hypothalamic self-stimulation reward.

METHODS

By using in vivo microdialysis, the levels of extracellular acetylcholine (ACh) in the PPTg and VTA were detected during lateral hypothalamic (LH) self-stimulation in rats. Rate-frequency curve shift procedure was used to evaluate the effects of nonselective muscarinic antagonist scopolamine (1 approximately 100 microg/microl) and nicotinic antagonist mecamylamine (5 approximately 100 microg/microl) microinjected into the PPTg on the rewarding efficacy of LH self-stimulation. Subsequently, the drugs were injected into the PPTg, and the extracellular ACh in the VTA was measured.

RESULTS

LH self-stimulation produced a concurrent ACh release in the PPTg and VTA. Intra-PPTg injection of scopolamine (100 microg/microl) significantly reduced the frequency threshold for LH self-stimulation reward, but nicotinic antagonist mecamylamine did not shift the threshold. However, mecamylamine (10, 25 microg/microl) injected into the PPTg robustly diminished the nicotine-potentiated LH self-stimulation reward. The extracellular ACh in the VTA was dramatically increased by intra-PPTg scopolamine (10, 100 microg/microl), but not by mecamylamine.

CONCLUSIONS

Results confirm that PPTg plays an important role in brain stimulation reward by modulating the cholinergic activity of the VTA. The PPTg muscarinic receptors contribute to an inhibitory modulation of reward effects by self-stimulation, whereas nicotinic receptors seem to be more involved in nicotine potentiation of brain stimulation reward.

The role of dopamine in reward and pleasure behaviour--review of data from preclinical research

OBJECTIVE

The purpose of this article is to review some of the basic aspects of the dopaminergic system and its role in reward and pleasure behaviour. We also discuss the association between dopamine and unpleasant symptoms that are commonly found in neuropsychiatric disorders and may also be side-effects of neuroleptic drugs.

METHOD

A computer-based search of the literature, augmented by extensive bibliography-guided article reviews, were used to find basic information on the dopamine and the reward systems, and symptoms such as dysphoria, anhedonia and depression.

RESULTS

Central dopaminergic neurotransmission is complex, having multiple actions at each level of the mesocorticolimbic reward pathway. The role of dopamine in the reward process was classically associated with the ability to experience pleasure; recent data suggest a more motivational role. Dysfunction of the dopamine transmission in the reward circuit is associated with symptoms such as anhedonia, apathy and dysphoria found in several neuropsychiatric disorders, including Parkinson's disease, depression, drug addiction, and neuroleptic-induced dysphoria.

CONCLUSION

Viewing the dysfunctions of the reward pathways within a broader spectrum and exploring its complex relations with the dopaminergic transmission may help understand the pathophysiology of these neuropsychiatric disorders and lead to a rational development of novel treatments.

Carbachol induces burst firing of dopamine cells in the ventral tegmental area by promoting calcium entry through L-type channels in the rat

Enhanced activity of the central dopamine system has been implicated in many psychiatric disorders including schizophrenia and addiction. Besides terminal mechanisms that boost dopamine levels at the synapse, the cell body of dopamine cells enhances terminal dopamine concentration through encoding action potentials in bursts. This paper presents evidence that burst firing of dopamine cells in the ventral tegmental area was under cholinergic control using nystatin-perforated patch clamp recording from slice preparations. The non-selective cholinergic agonist carbachol excited the majority of recorded neurones, an action that was not affected by blocking glutamate and GABA ionotropic receptors. Twenty per cent of dopamine cells responded to carbachol with robust bursting, an effect mediated by both muscarinic and nicotinic cholinoceptors postsynaptically. Burst firing induced as such was completely dependent on calcium entry as it could be blocked by cadmium and more specifically the L-type blocker nifedipine. In the presence of the sodium channel blocker tetrodotoxin, carbachol induced membrane potential oscillation that had similar kinetics and frequency as burst firing cycles and could also be blocked by cadmium and nifedipine. Direct activation of the L-type channel with Bay K8644 induced strong bursting which could be blocked by nifedipine but not by depleting internal calcium stores. These results indicate that carbachol increases calcium entry into the postsynaptic cell through L-type channels to generate calcium-dependent membrane potential oscillation and burst firing. This could establish the L-type channel as a target for modulating the function of the central dopamine system in disease conditions.

Midbrain muscarinic receptor mechanisms underlying regulation of mesoaccumbens and nigrostriatal dopaminergic transmission in the rat

Laterodorsal (LDT) and pedunculopontine (PPT) tegmental nuclei in the mesopontine project cholinergic inputs to the midbrain ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), respectively, to directly and indirectly influence the activity of dopamine neuronal cells via actions on muscarinic and nicotinic receptors. The present study investigated the role of midbrain muscarinic receptors in the functional modulation of VTA and SNc dopamine cell activity as reflected by alterations in, respectively, nucleus accumbens (NAc) and striataldopamine efflux. In vivo chronoamperometry was used to measure changes in basal dopamine efflux via stearate-graphite paste electrodes implanted unilaterally in the NAc or striatum of urethane-anaesthetized rats, following blockade or activation of, respectively, VTA or SNc muscarinic receptors. Intra-VTA or -SNc infusion of the muscarinic antagonist scopolamine (200 microg/microL) reduced, respectively, NAc and striatal dopamine efflux while infusion of the muscarinic and nicotinic agonist carbachol (0.5 microg/microL) or the prototypical muscarinic agonist muscarine (0.5 microg/microL) increased NAc and striatal dopamine efflux. Transient decreases in dopamine efflux preceded these increases selectively in the striatum, suggesting a reduction in excitatory or increase in inhibitory drive to the SNc by preferential activation of M3 muscarinic receptors on GABA interneurons and glutamatergic inputs. This was confirmed by showing that selective blockade of M3 receptors with p-F-HHSiD (0.5 microg/microL) increased striatal, but not NAc, dopamine efflux. Together, these findings suggest that midbrain muscarinic receptors, probably M5 subtypes on VTA and SNc dopamine neurons, contribute to the tonic excitatory regulation of forebrain basal dopamine transmission whereas presynaptic M3 receptors serve to counter excessive excitation of nigral dopamine cell activity.

M3-like muscarinic receptors mediate Ca2+ influx in rat mesencephalic GABAergic neurones through a protein kinase C-dependent mechanism

GABAergic neurones in the mesencephalon are important regulators of dopamine neurones. Cholinergic projections from mesopontine nuclei preferentially synapse onto these GABAergic neurones, thus suggesting that ACh can regulate dopamine neurones indirectly by modulating GABAergic interneurones. Muscarinic receptors mediate excitation of these interneurones through a Ca(2+)-dependent mechanism. Using a mesencephalic primary culture model, we show here that muscarine (10 microM) increases intracellular Ca2+ concentrations ([Ca2+]i) in GABAergic interneurones. Compatible with previous anatomical data, our pharmacological studies further suggest that the M3 receptor is the primary mediator of this increase. The rise in [Ca2+]i induced by muscarine was not activity-dependent but required influx of Ca2+ from the extracellular medium. Consistent with the known coupling of the M3 receptor to PKC, the effect of muscarine was blocked by bisindolylmaleimide, a selective PKC antagonist. The effect of muscarine was inhibited by SKF 96365 and verapamil, drugs known to block non-selective cationic channels such as those formed by transient receptor potential (TRPC) proteins. Finally, GABAergic neurones were found to be immunopositive for TRPC1, 3, 5 and 6. Taken together, these results suggest that the Ca(2+)-dependent regulation of mesencephalic GABAergic neurones by muscarinic receptors requires activation of some receptor-operated Ca2+ channels through a PKC-dependent mechanism.

Scaling of prediction error does not confirm chaotic dynamics underlying irregular firing using interspike intervals from midbrain dopamine neurons

Dopamine neurons in the substantia nigra pars compacta often fire in an irregular, single spike mode in vivo, and a similar firing pattern can be observed in vitro when small conductance calcium-activated potassium channel blockers are applied to the bath. It is not clear whether the irregular firing is due to stochastic processes or nonlinear deterministic processes. A previous study [Neuroscience 104 (2001) 829] used nonlinear forecasting methods applied to a continuous function derived from the interspike interval (ISI) data from irregularly firing dopamine neurons to show that the predictability scaled exponentially with forecast horizon and was consistent with nonlinear deterministic chaos. However, we show here that the observed exponential scaling is also consistent with a stochastic process, because it did not differ significantly from that of shuffled surrogate data. On the other hand, nonlinear forecasting directly from the ISI data using the package TISEAN provided some evidence for nonlinear deterministic structure in four of five records obtained in vitro and in two of nine records obtained in vivo. Although we cannot rule out a role for nonlinear chaotic dynamics in structuring the firing pattern, we suggest an alternate hypothesis that includes a mechanism by which the firing pattern can become more variable in the presence of a constant level of background noise.

Midbrain muscarinic receptors modulate morphine-induced accumbal and striatal dopamine efflux in the rat

Midbrain dopamine neurons are critical in mediating the rewarding effects of opiates in dependent rats, as well as modulating some manifestations of opiate withdrawal. Morphine is known to excite dopamine neurons and thereby facilitate forebrain dopamine transmission through inhibition of GABA neurons. Cholinergic neurons in the mesopontine laterodorsal and pedunculopontine tegmental nuclei provide the principal source of excitatory cholinergic input to ventral tegmental area and substantia nigra pars compacta dopamine-containing neurons, via actions on midbrain muscarinic and nicotinic acetylcholine receptors. The present study hypothesized that a reduction in tonic cholinergic input via blockade of midbrain muscarinic receptors would reduce the pharmacological effects of morphine on forebrain dopamine release. Using in vivo chronoamperometry, alterations in morphine-evoked dopamine efflux were monitored at stearate-graphite paste electrodes implanted unilaterally in the nucleus accumbens and striatum of urethane (1.5 g/kg) anesthetized rats, following the pharmacological inhibition of ventral tegmental area/substantia nigra pars compacta muscarinic receptors. The facilitatory effects of morphine (2.0 mg/kg, i.v.) on accumbens and striatal dopamine efflux were markedly reduced by prior infusion of the non-selective muscarinic receptor antagonist scopolamine (200 microg/microl) into the ventral tegmental area or substantia nigra pars compacta, respectively. These findings demonstrate that decreased activation of midbrain muscarinic receptors attenuates the excitatory effects of morphine on mesoaccumbens and nigrostriatal dopaminergic transmission.

Presynaptic muscarinic acetylcholine receptors suppress GABAergic synaptic transmission in the intermediate grey layer of mouse superior colliculus

The intermediate grey layer (the stratum griseum intermediale; SGI) of the superior colliculus (SC) receives cholinergic inputs from the parabrachial region of the brainstem. It has been shown that cholinergic inputs activate nicotinic acetylcholine (nACh) receptors on projection neurons in the SGI. Therefore, it has been suggested that they facilitate the initiation of orienting behaviours. In this study, we investigated the effect of muscarinic acetylcholine (mACh) receptor activation on GABAergic synaptic transmission to SGI neurons using the whole-cell patch-clamp recording technique in slice preparations from mice. The GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked in SGI neurons by focal electrical stimulation were suppressed by bath application of 10 microm muscarine chloride. During muscarine application, both the paired-pulse facilitation index and the coefficient of variation of IPSCs increased; however, the current responses induced by a transient pressure application of 1 mm GABA were not affected by muscarine. Muscarine reduced frequencies of miniature IPSCs (mIPSCs) while the amplitudes of mIPSCs remained unchanged. These results suggested that mAChR-mediated inhibition of IPSCs was of presynaptic origin. The suppressant effect of muscarine was antagonized by an M1 receptor antagonist, pirenzepine dihydrochloride (1 microM), and a relatively specific M3 receptor antagonist, 4-DAMP methiodide (50 nM). By contrast, an M2 receptor antagonist, methoctramine tetrahydrochloride (10 microM), was ineffective. These results suggest that the cholinergic inputs suppress GABAergic synaptic transmission to the SGI neurons at the presynaptic site via activation of M1 and, possibly, M3 receptors. This may be an additional mechanism by which cholinergic inputs can facilitate tectofugal command generation.

Circulating neurotransmitters during the different wake-sleep stages in normal subjects

We investigated the changes of circulating neurotransmitters during the wake-sleep cycle in order to find possible correlations with the activity of central neurocircuitry functioning. Noradrenaline (NA), adrenaline (Ad), dopamine (DA), platelet serotonin (p-5HT), plasma serotonin (f-5HT) and plasma tryptophan (TRP) were assessed during the morning (supine resting + 1-min orthostasis + 5-min exercise) and at night (supine resting + slow wave sleep (SWS) + REM sleep). Only NA increased in the plasma during short-lasting (1-min) orthostasis morning waking period. Both NA and Ad rose during moderate exercise. The nocturnal results demonstrated that whereas Ad dropped during the supine resting, NA did not fall until SWS period. Although DA did not show significant changes during the nocturnal test, the NA/DA ratio showed significant reduction. The analysis of correlations supports the postulation that this finding reflects the DA modulatory role on neural sympathetic activity. Both f-5HT and p-5HT values were lower during sleep cycle than wake periods. However, they showed progressive rises during sleep stages. Conversely, the f-5HT/p-5HT ratio showed significantly greater values during the SWS period than during supine resting and REM periods. These findings are consistent with the postulation that f-5HT/p-5HT ratio is positively associated with parasympathetic activity during the sleep-cycle. We concluded that the profile of sleep-cycle circulating neurotransmitters differs from that obtained during waking periods. According to the above, we attempted to correlate the profile of circulating neurotransmitters with the very well-known central neurocircuitry functioning during wake-sleep cycle, in experimental mammals.

Regulation of rat mesencephalic GABAergic neurones through muscarinic receptors

Central dopamine neurones are involved in regulating cognitive and motor processes. Most of these neurones are located in the ventral mesencephalon where they receive abundant intrinsic and extrinsic GABAergic input. Cholinergic neurones, originating from mesopontine nuclei, project profusely in the mesencephalon where they preferentially synapse onto local GABAergic neurones. The physiological role of this cholinergic innervation of GABAergic neurones remains to be determined, but these observations raise the hypothesis that ACh may regulate dopamine neurones indirectly through GABAergic interneurones. Using a mesencephalic primary culture model, we studied the impact of cholinergic agonists on mesencephalic GABAergic neurones. ACh increased the frequency of spontaneous IPSCs (151 +/- 49%). Selective activation of muscarinic receptors increased the firing rate of isolated GABAergic neurones by 67 +/- 13%. The enhancement in firing rate was Ca(2+) dependent since inclusion of BAPTA in the pipette blocked it, actually revealing a decrease in firing rate accompanied by membrane hyperpolarization. This inhibitory action was prevented by tertiapin, a blocker of GIRK-type K(+) channels. In addition to its excitatory somatodendritic effect, activation of muscarinic receptors also acted presynaptically, inhibiting the amplitude of unitary GABAergic synaptic currents. Both the enhancement in spontaneous IPSC frequency and presynaptic inhibition were abolished by 4-DAMP (100 nm), a preferential M3 muscarinic receptor antagonist. The presence of M3-like receptors on mesencephalic GABAergic neurones was confirmed by immunocytochemistry. Taken together, these results demonstrate that mesencephalic GABAergic neurones can be regulated directly through muscarinic receptors. Our findings provide new data that should be helpful in better understanding the influence of local GABAergic neurones during cholinergic activation of mesencephalic circuits.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Dendritic projections and dye-coupling in dopaminergic neurons of the substantia nigra examined in horizontal brain slices from young rats

We examined the rostro-caudal dendritic spread of striatally projecting dopaminergic neurons of the Substantia Nigra pars compacta (SNc) and investigated the presence of dye-coupling after labeling these cells with a mixture of lucifer yellow (LY) and neurobiotin (NB) or with LY alone. Whole cell recordings were made from horizontal brain slices (400 microm) obtained from P5-P20 rats. SNc neurons retrogradely labeled with Fluoro-Gold and located in the region containing tyrosine hydroxylase-immunoreactive cells displayed Ih current and other properties characteristic of SNc neurons. To prevent extracellular leakage, dyes were introduced into patch pipettes after the establishment of whole cell configuration, and cells were filled under visual control. In contrast to previous studies conducted in coronal sections that identified dendritic projections of SNc neurons mainly in the medio-lateral and ventral directions, almost all neurons labeled in our study (53/54) additionally displayed a large rostro-caudal dendritic span (649 +/- 219 microm). Dye-coupling between SNc neurons was not observed under basal conditions, in the presence of gap junction "openers" (forskolin, trimethylamine), or after neurons were filled with LY using sharp intracellular microelectrodes. As a "positive control," dye-coupling was demonstrated in four hippocampal dentate gyrus neurons that were filled using the same patch pipette technique. In addition, none of the tested SNc cells (n = 12) showed expression of connexin 36 (the "neuronal" connexin) when tested with single-cell RT-PCR. In conclusion, this study revealed extensive rostro-caudal dendritic projections of SNc neurons. Under our in vitro conditions, no evidence was found for dye-coupling among these neurons.

Actions of the sodium channel inhibitor 202W92 on rat midbrain dopaminergic neurons

Excessive glutamatergic activity is implicated in Parkinson's disease (PD) and sodium channel blockade, resulting in inhibition of glutamate release, is a potential therapeutic approach to PD therapy. Beneficial effects of riluzole and lamotrigine have been reported in animal models of PD, but these compounds have relatively low potency as sodium channel inhibitors and also inhibit N and P/Q-type calcium channels. 202W92, a structural analog of lamotrigine, is a potent sodium channel inhibitor, with no effect on N, P/Q-type channels. Here we present the effects of 202W92 on single patch-clamped dopaminergic neurons. 202W92 (> or =10 microM) inhibited spontaneous action potential firing and reduced amplitude and frequency of evoked action potentials. It also inhibited the frequency of 4-aminopyridine (4-AP)- and electrically evoked excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs), with >80% inhibition at 10 microM (IC(50) 1.5 microM). EPSC and IPSC amplitudes were partially inhibited. 202W92 did not affect postsynaptic responses to locally applied glutamate and GABA, nor spontaneously occurring mini-IPSCs. These actions of 202W92 are compatible with sodium channel inhibition and depression of transmitter release.

Muscarinic and nicotinic cholinergic mechanisms in the mesostriatal dopamine systems

The striatum and its dense dopaminergic innervation originating in the midbrain, primarily from the substantia nigra pars compacta and the ventral tegmental area, compose the mesostriatal dopamine (DA) systems. The nigrostriatal system is involved mainly in motor coordination and in disorders such as Tourette's syndrome, Huntington's disease, and Parkinson's disease. The dopaminergic projections from the ventral tegmental area to the striatum participate more in the processes that shape behaviors leading to reward, and addictive drugs act upon this mesolimbic system. The midbrain DA areas receive cholinergic innervation from the pedunculopontine tegmentum and the laterodorsal pontine tegmentum, whereas the striatum receives dense cholinergic innervation from local interneurons. The various neurons of the mesostriatal systems express multiple types of muscarinic and nicotinic acetylcholine receptors as well as DA receptors. Especially in the striatum, the dense mingling of dopaminergic and cholinergic constituents enables potent interactions. Evidence indicates that cholinergic and dopaminergic systems work together to produce the coordinated functioning of the striatum. Loss of that cooperative activity contributes to the dysfunction underlying Parkinson's disease.

Rewarding effects of the cholinergic agents carbachol and neostigmine in the posterior ventral tegmental area

Rats learned to lever-press for microinjections of the cholinergic agonist carbachol (30-500 pmol per infusion) or the acetylcholinesterase inhibitor neostigmine (7.5-75 pmol per infusion) into the posterior ventral tegmental area (VTA) of the brain. Intracranial carbachol self-administration was site-specific. Carbachol was not reliably self-administered into a site just dorsal to the VTA or into the adjacent substantia nigra and was self-administered only weakly into the adjacent anterior VTA or interpeduncular nucleus. Carbachol produced conditioned place preferences when injected into the posterior but not into the anterior VTA or sites dorsal to the posterior VTA. Rats self-administered carbachol less when it was co-infused with the muscarinic cholinergic receptor antagonist scopolamine or the nicotinic cholinergic receptor antagonist dihydro-beta-erythroidine, and also when the rats were pretreated with the D1 dopamine antagonist SCH 23390. These findings implicate both nicotinic and muscarinic cholinergic neurotransmission in ventral tegmental reward function and suggest special involvement of the posterior portion of the VTA in cholinergic reward function.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Acetylcholine attenuates synaptic GABA release to supraoptic neurons through presynaptic nicotinic receptors

Both inhibitory GABAergic and excitatory glutamatergic inputs to supraoptic nucleus (SON) neurons can influence the release of vasopressin and oxytocin. Acetylcholine is known to excite SON neurons and to increase vasopressin release. The functional significance of cholinergic receptors, located at the presynaptic nerve terminals, in the regulation of the excitability of SON neurons is not fully known. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the inhibitory GABAergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slice were identified microscopically, and the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the whole-cell voltage-clamp technique. The mIPSCs were abolished by the GABA(A) receptor antagonist, bicuculline (10 microM). Acetylcholine (100 microM) significantly reduced the frequency of mIPSCs of SON neurons from 3.59+/-0.36 to 1.62+/-0.20 Hz (n=37), but did not alter the amplitude and the decay time constant of mIPSCs. Furthermore, the nicotinic receptor antagonist, mecamylamine (10 microM, n=13), eliminated the inhibitory effect of acetylcholine on mIPSCs of SON neurons. The muscarinic receptor antagonist, atropine (100 microM), did not alter significantly the effect of acetylcholine on mIPSCs in most of the 17 SON neurons studied. These results suggest that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by inhibition of presynaptic GABA release. Activation of presynaptic nicotinic receptors located in the GABAergic terminals plays a major role in the cholinergic regulation of the inhibitory GABAergic input to SON neurons.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area

Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the mu-opioid receptor (muOR) in the ventral tegmental area (VTA). We examined the functional sites for muOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against muOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of muOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. MuOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. MuOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The muOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), muOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that muOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that muOR agonists directly affect a small population of dopaminergic neurons expressing muOR on their dendrites in VTA and/or terminals in target regions.

Distinct muscarinic receptors enhance spontaneous GABA release and inhibit electrically evoked GABAergic synaptic transmission in the chick lateral spiriform nucleus

The effects of muscarinic agonists on GABAergic synaptic transmission were examined using whole-cell patch-clamp recording in chick brain slices containing the lateral spiriform nucleus. Bath application of muscarine (10 microM) both increased the frequency of spontaneous GABAergic postsynaptic currents and reduced the amplitude of evoked GABAergic polysynaptic postsynaptic currents elicited by focal afferent fiber electrical stimulation. Both of these muscarinic actions were reversible and dose-dependent. Two M(1) antagonists, telenzepine and pirenzipine, and to a lesser extent the M(2) antagonist methoctramine, protected against muscarine's inhibition of the evoked polysynaptic currents. Other M(2) antagonists (tripitramine and gallamine) as well as the M(3) antagonist 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride) and an M(4) antagonist (tropicamide) provided little or no protection against muscarine in this assay. In contrast, 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride, tropicamide and telenzepine, but not pirenzepine, methoctramine, tripitramine and gallamine, blocked muscarine's enhancement of spontaneous GABAergic currents. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride] and CDD-0097 (5-propargyloxycarbonyl-1,4,5,6-tetrahydropyrimidine hydrochloride), two M(1) agonists, mimicked muscarine's inhibition of the evoked polysynaptic GABAergic currents but did not mimic muscarine's enhancement of spontaneous GABAergic currents. Both actions of muscarine persisted when slices were pretreated with pertussis toxin or N-ethylmaleimide, which inactivate G-proteins coupled to M(2) and M(4) receptors while leaving G-proteins coupled to M(1), M(3) and M(5) receptors intact. Muscarine had no significant effect on the amplitude of the direct postsynaptic current elicited by exogenous GABA in the presence of tetrodotoxin. The results demonstrate that distinct muscarinic receptors oppositely modulate GABAergic transmission in the lateral spiriform nucleus. The receptor mediating the inhibition of evoked GABAergic polysynaptic currents is pharmacologically similar to an M(1) receptor, while the enhancement of spontaneous GABAergic currents appears to be mediated by an M(3) receptor.

Glutamate receptor stimulation induces a persistent rhythmicity of the GABAergic inputs to rat midbrain dopaminergic neurons

The substantia nigra pars compacta and the ventral tegmental area are part of a complex network in the basal ganglia involved in behaviours as diverse as motor planning, generation of pleasure and drug addiction. Here we report that in the dopaminergic neurons of the rat ventral midbrain a brief coactivation of group I metabotropic and NMDA glutamate receptors may transform a temporally dispersed synaptic GABAergic input into a rhythmic pattern (range 4.5-22.5 Hz), probably through a mechanism involving electrotonic couplings. The plastic and long-lasting modification in the temporal code of the inhibitory synaptic activity induced by glutamate may be a key element in determining the function of midbrain dopaminergic neurons in both normal and pathological behaviour.

Vesicular acetylcholine transporter in the rat nucleus accumbens shell: subcellular distribution and association with mu-opioid receptors

Cholinergic interneurons in the nucleus accumbens shell (AcbSh) are implicated in the reinforcing behaviors that develop in response to opiates active at mu-opioid receptors (MOR). We examined the electron microscopic immunocytochemical localization of the vesicular acetylcholine transporter (VAChT) and MOR to determine the functional sites for storage and release of acetylcholine (ACh), and potential interactions involving MOR in this region of rat brain. VAChT was primarily localized to membranes of small synaptic vesicles in axon terminals. Less than 10% of the VAChT-labeled terminals were MOR-immunoreactive. In contrast, 35% of the cholinergic terminals formed symmetric or punctate synapses with dendrites showing an extrasynaptic plasmalemmal distribution of MOR. Membranes of tubulovesicles in other selective dendrites were also VAChT-labeled, and almost half of these dendrites displayed plasmalemmal MOR immunoreactivity. The VAChT-labeled dendritic tubulovesicles often apposed unlabeled axon terminals that formed symmetric synapses. Our results indicate that in the AcbSh MOR agonists can modulate the release of ACh from vesicular storage sites in axon terminals as well as in dendrites where the released ACh may serve an autoregulatory function involving inhibitory afferents. These results also suggest, however, that many of the dendrites of spiny projection neurons in the AcbSh are dually influenced by ACh and opiates active at MOR, thus providing a cellular substrate for ACh in the reinforcement of opiates.