M3 muscarinic receptors mediate cholinergic excitation of the spontaneous activity of subthalamic neurons in the rat

Developmental Age and Biological Sex Influence Muscarinic Receptor Function and Neuron Morphology within Layer VI of the Medial Prefrontal Cortex

Acetylcholine (ACh) neurotransmission within the medial prefrontal cortex (mPFC) plays an important modulatory role to support mPFC-dependent cognitive functions. This role is mediated by ACh activation of its nicotinic (nAChR) and muscarinic (mAChR) classes of receptors, which are both present on mPFC layer VI pyramidal neurons. While the expression and function of nAChRs have been characterized thoroughly for rodent mPFC layer VI neurons during postnatal development, mAChRs have not been characterized in detail. We employed whole-cell electrophysiology with biocytin filling to demonstrate that mAChR function is greater during the juvenile period of development than in adulthood for both sexes. Pharmacological experiments suggest that each of the M1, M2, and M3 mAChR subtypes contributes to ACh responses in these neurons in a sex-dependent manner. Analysis of dendrite morphology identified effects of age more often in males, as the amount of dendrite matter was greatest during the juvenile period. Interestingly, a number of positive correlations were identified between the magnitude of ACh/mAChR responses and dendrite morphology in juvenile mice that were not present in adulthood. To our knowledge, this work describes the first detailed characterization of mAChR function and its correlation with neuron morphology within layer VI of the mPFC.

Muscarinic receptor subtype distribution in the central nervous system and relevance to aging and Alzheimer's disease

Muscarinic acetylcholine receptors (mAChRs) are G proteincoupled receptors (GPCRs) that mediate the metabotropic actions of acetylcholine (ACh). There are five subtypes of mAChR, M₁ - M₅, which are expressed throughout the central nervous system (CNS) on numerous cell types and represent promising treatment targets for a number of different diseases, disorders, and conditions of the CNS. Although the present review will focus on Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI), a number of conditions such as Parkinson's disease (PD), schizophrenia, and others represent significant unmet medical needs for which selective muscarinic agents could offer therapeutic benefits. Numerous advances have been made regarding mAChR localization through the use of subtype-selective antibodies and radioligand binding studies and these efforts have helped propel a number of mAChR therapeutics into clinical trials. However, much of what we know about mAChR localization in the healthy and diseased brain has come from studies employing radioligand binding with relatively modest selectivity. The development of subtype-selective small molecule radioligands suitable for in vitro and in vivo use, as well as robust, commercially-available antibodies remains a critical need for the field. Additionally, novel genetic tools should be developed and leveraged to help move the field increasingly towards a systems-level understanding of mAChR subtype action. Finally, functional, proteomic, and genetic data from ongoing human studies hold great promise for optimizing the design and interpretation of studies examining receptor levels by enabling patient stratification. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Functional Characterization of Acetylcholine Receptors Expressed in Human Neurons Differentiated from Hippocampal Neural Stem/Progenitor Cells

Neurotransmission mediated by acetylcholine receptors (AChRs) plays an important role in learning and memory functions in the hippocampus. Impairment of the cholinergic system contributes to Alzheimer's disease (AD), indicating the importance of AChRs as drug targets for AD. To improve the success rates for AD drug development, human cell models that mimic the target brain region are important. Therefore, we characterized the functional expression of nicotinic and muscarinic AChRs (nAChRs and mAChRs, respectively) in human hippocampal neurons differentiated from hippocampal neural stem/progenitor cells (HIP-009 cells). Intracellular calcium flux in 4-week differentiated HIP-009 cells demonstrated that the cells responded to acetylcholine, nicotine, and muscarine in a concentration-dependent manner (EC₅₀ = 13.4 ± 0.5, 6.0 ± 0.4, and 35.0 ± 2.5 µM, respectively). In addition, assays using subtype-selective compounds revealed that major AD therapeutic target AChR subtypes-α7 and α4β2 nAChRs, as well as M₁ and M₃ mAChRs-were expressed in the cells. Furthermore, neuronal network analysis demonstrated that potentiation of M₃ mAChRs inhibits the spontaneous firing of HIP-009 neurons. These results indicate that HIP-009 cells are physiologically relevant for AD drug screening and hence are loadstars for the establishment of in vitro AD models.

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.

Roles of the M1 muscarinic acetylcholine receptor subtype in the regulation of basal ganglia function and implications for the treatment of Parkinson's disease

Antagonists of the muscarinic acetylcholine receptors (mAChRs) were among the first treatments for Parkinson's disease. However, the clinical utility of mAChR antagonists is limited by adverse effects associated with the blockade of multiple mAChR subtypes. Understanding the roles of specific mAChR subtypes in regulating basal ganglia and motor function could lead to the development of novel agents that have antiparkinsonian activity with fewer adverse effects. Using the novel, highly selective M1 antagonist N-[3-oxo-3-[4-(4-pyridinyl)-1-piperazinyl]propyl]-2,1,3-benzothiadiazole-4-sulfonamide (VU0255035) and the M1 positive allosteric modulator benzylquinolone carboxylic acid, we investigated the roles of M1 receptors in cholinergic excitation and regulation of synaptic transmission in striatal medium spiny neurons (MSNs) and neurons in the subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr). Electrophysiological studies demonstrate that M1 activation has excitatory effects on MSNs but plays little or no role in mAChR-mediated increases in firing frequency or the regulation of synaptic transmission in STN and SNr neurons. On the basis of this profile, M1-selective antagonists may have weak antiparkinsonian activity but would not have the full efficacy observed in nonselective mAChR antagonists. Consistent with this, the M1-selective antagonist VU0255035 partially reversed reserpine-induced akinesia and decreased haloperidol-induced catalepsy in rats but did not have the full efficacy observed with the nonselective mAChR antagonist scopolamine. These results suggest that the M1 receptor participates in the overall regulation of basal ganglia function and antiparkinsonian effects of mAChR antagonists but that other mAChR subtype(s) also play important roles at multiple levels of the basal ganglia motor circuit.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Neuronal activity in the subthalamic nucleus modulates the release of dopamine in the monkey striatum

The primate subthalamic nucleus (STN) is commonly seen as a relay nucleus between the external and internal pallidal segments, and as an input station for cortical and thalamic information into the basal ganglia. In rodents, STN activity is also known to influence neuronal activity in the dopaminergic substantia nigra pars compacta (SNc) through inhibitory and excitatory mono- and polysynaptic pathways. Although the anatomical connections between STN and SNc are not entirely the same in primates as in rodents, the electrophysiologic and microdialysis experiments presented here show directly that this functional interaction can also be demonstrated in primates. In three Rhesus monkeys, extracellular recordings from SNc during microinjections into the STN revealed that transient pharmacologic activation of the STN by the acetylcholine receptor agonist carbachol substantially increased burst firing of single nigral neurons. Transient inactivation of the STN with microinjections of the GABA-A receptor agonist muscimol had the opposite effect. While the firing rates of individual SNc neurons changed in response to the activation or inactivation of the STN, these changes were not consistent across the entire population of SNc cells. Permanent lesions of the STN, produced in two animals with the fiber-sparing neurotoxin ibotenic acid, reduced burst firing and firing rates of SNc neurons, and substantially decreased dopamine levels in the primary recipient area of SNc projections, the striatum, as measured with microdialysis. These results suggest that activity in the primate SNc is prominently influenced by neuronal discharge in the STN, which may thus alter dopamine release in the striatum.

Dopaminergic and GABAergic modulation of glutamate release from rat subthalamic nucleus efferents to the substantia nigra

The regulation of the glutamatergic projection from the subthalamic nucleus (STN) to the substantia nigra (SN) was investigated using dual-probe microdialysis in the awake behaving rat. Reverse dialysis of the cholinergic receptor agonist carbachol (1 mM) into the STN caused an increase in the extracellular concentrations of glutamate and dopamine in the SN. The increase in glutamate was transient and returned toward basal values despite the continued perfusions of the STN with carbachol. Carbachol-stimulated glutamate release was prolonged by perfusion of the selective D2 dopamine receptor antagonist raclopride (100 microM) into the SN and was attenuated by the perfusion of the selective D2-like receptor agonist quinpirole (10 microM). In contrast, perfusion of the D1 dopamine receptor antagonist SCH-23390 (100 microM) did not alter the carbachol-stimulated glutamate release even though it increased basal glutamate concentrations. Perfusion of the GABAA receptor antagonist bicuculline (10 microM) into the SN prolonged the carbachol-stimulated glutamate release in similar fashion as raclopride. The present findings suggest that somatodendritically released dopamine in the SN regulates glutamate release from subthalamic axon terminals by differentially activating dopamine D2 and D1 receptors. Activation of D2 heteroreceptors, located on STN axon terminals, provides a negative feedback control on stimulated subthalamic glutamate release, while D1 receptor activation preferentially regulates basal glutamate concentrations. The findings of the present study also indicate that GABA exerts an inhibitory control on glutamate release in the SN through GABAA receptors.

Overwhelmingly asynchronous firing of rat subthalamic nucleus neurones in brain slices provides little evidence for intrinsic interconnectivity

In Parkinson's disease the neurones of the subthalamic nucleus show increased synchrony and oscillatory burst discharge, thought to reflect a breakdown of parallel processing in basal ganglia circuitry. To understand better the mechanisms underlying this transition, we sought to mimic this change in firing pattern within sagittal slices of rat midbrain. The firing patterns of up to four simultaneously extracellularly recorded subthalamic nucleus (STN) neurones were analysed using burst and oscillation detection programs, and correlated activity between pairs of neurones assessed. In control conditions all but 11 of 488 (2%) neurones fired in a predominantly tonic pattern (with mean oscillation frequency >3 Hz), with no significantly cross-correlated activity in any of 393 pairs of neurones. The glutamate antagonists DL-2-amino-phosphonopentanoic acid (APV), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-methyl-2-(phenylethynyl)pyridine (MPEP) did not change the firing rate or pattern of these cells, providing no evidence for a role of glutamatergic collaterals within the STN under these conditions. The GABA(A) receptor antagonist bicuculline and GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl]phenylmethyl phosphinic acid (CGP 55845) were also without effect on firing rate or pattern in these cells, suggesting that there was no active input from other GABAergic basal ganglia nuclei in this slice. The dopamine receptor antagonist haloperidol caused no significant change to firing rate or pattern of firing in these cells, suggesting that there was no active dopaminergic input in this slice. Excitations of STN neurones by muscarine, (+)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD), N-methyl-D-aspartic acid (NMDA) or dopamine were all unaccompanied by a change in firing pattern or any significant correlated activity between STN neurone pairs. Burst firing could be induced in STN neurones with either the potassium channel blocker tetraethylammonium (TEA; 10 mM; in 100/138 [72%] of cells) or with a combination of NMDA and the calcium-activated potassium channel blocker apamin (in 101/216 [47%] of cells). Burst firing in TEA was unchanged by CNOX and APV, MPEP, CGP55845, haloperidol, dopamine, and ACPD, although muscarine produced a significant increase in oscillation frequency. Burst firing in NMDA and apamin was unchanged by CNQX and APV, dopamine, muscarine and ACPD, although bicuculline caused a significant increase in oscillation frequency. Such burst firing was not accompanied by synchrony in any condition, either alone, or during application of excitatory agents or glutamate or GABA antagonists. As the bursting seen here was unaccompanied by the synchronous activity that has often been observed (pathologically) in vivo, it probably reflects solely intrinsic STN neuronal properties, rather than network activity. No functional role was found for glutamatergic collaterals within the STN, either when cells are firing tonically or burst firing. The circuitry needed to produce synchrony in the STN is most likely not intrinsic to the STN itself, but requires connections with other basal ganglia nuclei, and/or the cortex, which are not present in this preparation.

Noradrenaline increases the firing rate of a subpopulation of rat subthalamic neurones through the activation of α1-adrenoceptors

In the rat subthalamic nucleus, which plays a critical role in the control of motor behaviour, specific binding of [3H]-prazosin was detected by radioligand binding to homogenates and by autoradiography in slices. [3H]-Prazosin binding to homogenates (Bmax 71 +/- 5 fmol/mg protein; Kd 0.27 +/- 0.05 nM) was competed for by alpha1-antagonists. In subthalamic nucleus slices and in the presence of 10 mM LiCl, noradrenaline (100 microM) produced a modest, but consistent, stimulation of [3H]-inositol phosphate accumulation (146 +/- 6% of basal), reversed by the alpha1-antagonist prazosin (1 microM). Extracellular single-unit recordings in slices showed that in a subpopulation (61 out of 94 cells) of rat subthalamic neurones with regular, single-spike firing pattern, noradrenaline induced a concentration-dependent increase in the firing rate (EC50 2.5 +/- 0.2 microM, maximum effect 272 +/- 33% of basal). The action of noradrenaline was mimicked by the selective alpha1-agonist phenylephrine but not by selective alpha2- or beta-agonists, and was blocked by the alpha1-antagonist prazosin but not by alpha2- or beta-antagonists. The excitatory effect of noradrenaline was not prevented by perfusion with low Ca2+/high Mg2+ solution. In four out of 11 neurones perfusion with 3 microM noradrenaline resulted in a shift from bursting to regular firing. Taken together, our results indicate that rat subthalamic neurones express alpha1-adrenoceptors responsible for noradrenaline-induced stimulation of the firing rate of a subpopulation of neurones. By modulating the spontaneous activity of STN neurones, noradrenergic pathways might have a significant role in regulating basal ganglia function and thus motor activity.

Presynaptic dopamine D2 and muscarine M3 receptors inhibit excitatory and inhibitory transmission to rat subthalamic neurones in vitro

Whole-cell patch-clamp recordings were made from subthalamic nucleus (STN) neurones in brain slices from rats. Stimulation with bipolar electrodes evoked synaptic currents mediated by glutamate (EPSCs) and GABAA (IPSCs) receptors. Dopamine reversibly reduced the amplitude of GABAA IPSCs by up to 48 % with an IC50 value of 3.4 +/- 0.8 microM. The dopamine D2 receptor agonist quinpirole, but not the D1 receptor agonist SKF 82958, also inhibited GABAA IPSCs. This effect was completely reversed by the D2 receptor antagonist sulpiride but not by SCH 23390, a D1 antagonist. Muscarine reversibly reduced the amplitude of GABAA IPSCs by up to 70 % with an IC50 value of 0.6 +/- 0.1 microM. Inhibition of IPSCs by muscarine was completely blocked by scopolamine (10 microM), a muscarinic receptor antagonist. The M3 muscarinic receptor antagonist 4-DAMP effectively reversed muscarine-induced inhibition of IPSCs with an IC50 of 0.11 +/- 0.03 microM. Although the M1 receptor antagonist pirenzepine also reversed the inhibition of IPSCs by muscarine, this effect was only observed at relatively high concentrations (IC50 = 21.7 +/- 9.4 microM). Dopamine and muscarine both increased the paired-pulse ratio of GABAA IPSCs. Neither agent produced sustained changes in postsynaptic holding current. Glutamate EPSCs were also inhibited reversibly by dopamine (by up to 29%; IC50 = 16 +/- 3 microM) and muscarine (by up to 41%; IC50 = 1.0 +/- 0.4 microM). However, both agents were more potent and efficacious for reducing GABA IPSCs compared with glutamate EPSCs. These results suggest that the most significant effect of dopamine and muscarine in the STN is to reduce inhibitory synaptic input by acting at presynaptic dopamine D2 and muscarinic M3 receptors, respectively.

Muscarinic receptors mediate enhancement of spontaneous GABA release in the chick brain

The functional role of muscarinic acetylcholine receptors in the lateral spiriform nucleus was studied in chick brain slices. Whole-cell patch-clamp recordings of neurons in the lateral spiriform nucleus revealed that carbachol enhanced GABAergic spontaneous inhibitory postsynaptic currents. The duration of the response to carbachol was significantly reduced after blockade of muscarinic receptors with atropine. In the presence of the nicotinic receptor antagonist dihydro-beta-erythroidine, carbachol produced a delayed but prolonged enhancement of spontaneous GABAergic inhibitory postsynaptic currents that was completely blocked by atropine. Muscarine also enhanced the frequency of spontaneous GABAergic inhibitory postsynaptic currents in a dose-dependent manner, but had no effect on inhibitory postsynaptic current amplitude. While 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride, a M3 antagonist, completely blocked muscarine's effect, telenzepine, a M1 antagonist, and tropicamide, a M4 antagonist, only partially decreased the response to muscarine. Pirenzepine, a M1 antagonist, and methoctramine, a M2 antagonist, potentiated muscarine's enhancement of spontaneous GABAergic inhibitory postsynaptic currents. Muscarine's action was blocked by tetrodotoxin, cadmium chloride and omega-conotoxin GVIA, but was not affected by dihydro-beta-erythroidine, 6-cyano-7-nitroquinoxaline-2,3-dione, D(-)-2-amino-5-phosphonopentanoic acid, naloxone or fluphenazine. These results demonstrate that activation of both muscarinic and nicotinic acetylcholine receptors can enhance GABAergic inhibitory postsynaptic currents in the lateral spiriform nucleus. The muscarinic response has a slower onset but lasts longer than the nicotinic effect. The M3 receptor subtype is predominantly involved in enhancing spontaneous GABAergic inhibitory postsynaptic currents. These M3 receptors must be located some distance from GABA release sites, since activation of voltage-dependent sodium channels, and consequent activation of N-type voltage-dependent calcium channels, is required to trigger enhanced GABA release following activation of muscarinic receptors.

Spontaneously hypertensive rats cholinergic hyper-responsiveness: central and peripheral pharmacological mechanisms

1. The mechanisms and the subtypes of muscarinic receptors implicated in the cardiovascular effects of physostigmine were investigated in conscious normotensive and spontaneously hypertensive rats. 2. Intravenous (i.v.) physostigmine (50 microg kg-1) induced in both strains a long pressor response, accompanied by a bradycardia. This pressor response was larger in spontaneously hypertensive (+41+/-6 mmHg) than in Wistar-Kyoto (+25+/-2 mmHg) rats (P<0.05). 3. Pretreatment with atropine sulphate (0.4 mg kg-1 i.v.), completely abolished the physostigmine-induced pressor response in both normotensive and hypertensive rats. In both strains, the physostigmine pressor response was significantly reduced by the systemic administration of either an alpha1-adrenoceptor antagonist (prazosin, 1 mg kg-1) or a V1A-vasopressin receptor antagonist (AVPX, 20 microg kg-1). This physostigmine pressor effect was completely abolished in both strains when both antagonists were administered concomitantly. 4. In WKY rats, the pressor response to physostigmine (50 microg kg-1 i.v.) was inhibited in a dose-dependent manner by i. c.v. administration of atropine (ID50=3.70 nmoles), the M1 receptor antagonist pirenzepine (ID50=10.71 nmoles), the M2 receptor antagonist methoctramine (ID50=4.31 nmoles), the M3 receptor antagonist p-F-HHSiD (ID50=60.52 nmoles) and the M4 receptor antagonist tropicamide (ID50=214.20 nmoles). In the hypertensive strain, the ID50 were found to be significantly higher for atropine (7.34 nmoles), pirenzepine (21.60 nmoles) and p-F-HHSiD (139.50 nmoles) (P<0.05). 5. The present results indicate that physostigmine acts in normotensive and spontaneously hypertensive rats, through stimulation of both central M2 and M1 cholinoceptors to induce a rise in blood pressure mediated by an increase in plasma vasopressin and sympathetic outflow. Moreover, our results suggest that some modifications of the M1 receptor subtypes in terms of expression or affinity could be responsible for the hyper-responsiveness of the hypertensive strain to cholinomimetic agents.

Pressor and bradycardic effects of tacrine and other acetylcholinesterase inhibitors in the rat

The cardiovascular effects of three different acetylcholinesterase inhibitors: physostigmine, tacrine and rivastigmine injected by intravenous (i.v.) route were compared in freely moving Wistar rats. The three drugs significantly increased both systolic and diastolic blood pressure and decreased heart rate. Compared to physostigmine, a 20-fold higher dose of tacrine and a 40-fold higher dose of rivastigmine was necessary to induce a comparable pressor effect. Tacrine was chosen as a model to study the mechanisms underlying the cardiovascular effects of i.v. cholinesterase inhibitors. Atropine totally abolished while methylatropine did not affect tacrine pressor effects. Conversely, both drugs abolished tacrine-induced bradycardia. The alpha1-adrenoceptor antagonist prazosin or the vasopressin V1 receptor antagonist, [beta-mercapto-beta,beta-cyclopenta-methylenepropionyl1, O-Me-Tyr2, Arg8] vasopressin partially but significantly reduced tacrine pressor effect and mostly abolished it when administered concomitantly. The tacrine pressor response was inhibited in a dose-dependent manner by the i.c.v. administration of the non-selective muscarinic receptor antagonist atropine (ID50 = 1.45 microg), the muscarinic M1 receptor antagonist pirenzepine (ID50 = 4.33 microg), the muscarinic M2 receptor antagonist methoctramine (ID50 = 1.39 microg) and the muscarinic M3 receptor antagonist para-fluoro-hexahydro-sila-difenidol (ID50 = 31.19 microg). Central injection of such muscarinic receptor antagonists did not affect tacrine-induced bradycardia. Our results show that acetylcholinesterase inhibitors induce significant cardiovascular effects with a pressor response mediated mainly by the stimulation of central muscarinic M2 receptors inducing a secondary increase in sympathetic outflow and vasopressin release. Conversely, acetylcholinesterase inhibitor-induced bradycardia appears to be mediated by peripheral muscarinic mechanisms.

Reciprocal interaction between glutamate and dopamine in the pars reticulata of the rat substantia nigra: a microdialysis study

We studied the interactions between glutamate and dopamine in the pars reticulata of the substantia nigra by using microdialysis in unanaesthetized rats. Increased extracellular levels of glutamate in the pars reticulata were obtained by microinjecting the muscarinic agonist carbachol into the ipsilateral subthalamic nucleus. The increase of glutamate levels was followed by increments in extracellular levels of dopamine and GABA. Increased levels of the three neurotransmitters were also observed during the administration of N-methyl-D-aspartate through the microdialysis probe. The increase in glutamate and GABA caused by N-methyl-D-aspartate was blocked by SCH 23390, a selective D1 antagonist. However, the D1 antagonist did not prevent the increase in dopamine levels. The selective D1 agonist SKF 38393, added to the microdialysis probe, increased the levels of the three neurotransmitters. However, after the lesion of the subthalamic nucleus with kainic acid, SKF 38393 increased only the level of GABA but not those of glutamate and dopamine. In addition, the lesion of the subthalamic nucleus produced a drastic (80%) fall in the extracellular levels of glutamate. These data suggest that glutamate, through N-methyl-d-aspartate receptors, stimulates the release of dopamine from dopaminergic dendrites present in the substantia nigra pars reticulata, and that dopamine in turn stimulates the release of glutamate and GABA. Both effects are mediated by D1 dopamine receptors present on subthalamonigral and striatonigral axon terminals, respectively.

Excitation of rat subthalamic nucleus neurones in vitro by activation of a group I metabotropic glutamate receptor

The subthalamic nucleus (SThN) provides a glutamate mediated excitatory drive to several other component nuclei of the basal ganglia, thereby significantly influencing locomotion and control of voluntary movement. We have characterised functionally the metabotropic glutamate (mGlu) receptors in the SThN using extracellular single unit recording from rat midbrain slices. SThN neurones fired action potentials spontaneously at a rate of 10 Hz which was increased by the group I/II mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3 R-ACPD; 1-30 microM) and the group I selective agonist (S, R)-dihydroxyphenylglycine (DHPG; 1-30 microM). However, both the group II selective agonist (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 1 microM) and the group III selective agonist (S)-2-amino-4-phosphonobutanoic acid (L-AP4; 10 microM) were without effect, indicating that the excitation was mediated by a group I mGlu receptor. The excitation caused by DHPG (3 microM) was reversed by co-application of the mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 500 microM). Thus a group I mGlu receptor mediates excitation of SThN neurones, and suggests a use for group I mGlu receptor ligands for treatment of both hypo- and hyperkinetic disorders of basal ganglia origin, such as Parkinson's disease and Huntington's disease.