Evidence for a striatal NMDA receptor modulation of nigral glutamate release. A dual probe microdialysis study in the awake freely moving rat

Dual probe microdialysis was employed to characterize dialysate glutamate levels from the substantia nigra pars reticulata of awake freely moving rats, and to test its sensitivity to alterations in striatal neurotransmission including striatal N-methyl-d -aspartic acid (NMDA) receptor stimulation and blockade. Intranigral perfusion with low (0.1 mm ) Ca²⁺ medium (60 min) did not affect nigral glutamate levels, whereas intranigral perfusion with tetrodotoxin (10 μm , 60 min) increased nigral glutamate levels. Perfusion of KCl (100 mm , 10 min) in the dorsolateral striatum transiently stimulated nigral glutamate levels (maximal increase + 60%), whereas intrastriatal perfusion (60 min) with low Ca²⁺ medium and tetrodotoxin gradually increased nigral glutamate levels. Intrastriatal perfusion with NMDA (0.1–100 μm , 10 min) dose-dependently stimulated glutamate levels in the substantia nigra pars reticulata. The NMDA (1 μm )-induced increase in nigral glutamate release was transient and maximal (+60% within 20 min), whereas that for NMDA (10 μm ) had a slow onset but was long lasting (+35% after 60 min). Lower (0.1 μm ) and higher (100 μm ) NMDA concentrations were ineffective. The effect of intrastriatal NMDA (1 μm ) was prevented by coperfusion with MK-801 (1 μm ). Intrastriatal MK-801 (10 μm ) alone gradually increased glutamate levels up to +50% after 60 min of perfusion. The present results suggest that glutamate levels in the substantia nigra pars reticulata are sensitive to changes in neuronal transmission in the dorsolateral striatum, and that striatal NMDA receptors regulate nigral glutamate release in both a tonic and phasic fashion.     

Increase in striatal acetylcholine levels by GABAergic agents: dependence on corticostriatal neurons

Surgical lesion of the corticostriatal projections almost totally reduced the ability of GABA agonist agents but not apomorphine to increase rat striatal ACh concentrations. This suggests that in intact rats, the GABAergic inhibition of striatal cholinergic neurons depends, at least in part, upon the corticostriatal (possibly glutamatergic) tract whereas the action of the dopaminergic pathway on cholinergic cells is independent of corticostriatal neurons.     

GABAergic modulation of ventral pallidal dopamine release studied by in vivo microdialysis in the freely moving rat

The mesopallidal dopamine system, which originates from the ventral tegmental area and projects to the ventral pallidum (VP), has been recently shown to play an important role in self-stimulation reward and cocaine reward. VP also receives a GABAergic projection from nucleus accumbens (NAS). The aim of the present study was to examine the involvement of this GABAergic projection in the modulation of VP dopamine release. Both the GABA_A antagonist picrotoxin (2–200 μM) and the GABA_B antagonist phaclofen (20–2,000 μM), perfused locally, dose-responsively increased VP extracellular dopamine 2–2.5-fold. Cocaine (10 μM) produced a 6.5-fold increase of VP dopamine. Neither picrotoxin (200 μM), phaclofen (2,000 μM), nor GABA (20–2,000 μM) altered the response of VP dopamine to locally applied cocaine. GBR 12909 (0.5 μM), a selective dopamine uptake blocker, induced a 3.5-fold increase of VP dopamine. The increase of VP dopamine in response to GBR 12909 was further augmented to 8.5-fold of baseline when picrotoxin (200 μM) was added to the perfusate. The data from the present study demonstrate that the GABAergic NAS-VP projection can modulate ventral pallidal dopamine release. However, the effect of GABA on the mesopallidal dopamine system's response to locally applied cocaine may be complicated by actions of cocaine other than dopamine uptake inhibition. Synapse 29:406–412, 1998. © 1998 Wiley-Liss, Inc.     

Local GABAergic modulation of acetylcholine release from the cortex of freely moving rats

Cortical perfusion with GABA agonists and antagonists modulates the spontaneous release of cortical acetylcholine and GABA in freely moving rats. Twenty-four hours after implantation of a dialysis fibre, cerebral cortex spontaneously released acetylcholine (3.8 +/- 0.2 pmol/10 min) and GABA (6.6 +/- 0.4 pmol/10 min) at a stable rate. Local administration of GABA (1 or 5 mM) or the GABAA agonist muscimol (25 or 50 microM) had no effect on the spontaneous release of acetylcholine. However, bicuculline (1-25 microM), a GABAA antagonist, added to the dialysis perfusate, elicited a concentration-dependent increase of acetylcholine release to approximately double that of control. This effect of bicuculline (25 microM) was completely prevented by coperfusion with muscimol (50 microM). Local administration of the GABAB receptor agonist baclofen (10 or 50 microM) elicited a concentration-dependent increase in spontaneous acetylcholine release with a maximal increase of about 60%. Intracortical administration of baclofen also decreased the spontaneous release of GABA. The GABAB receptor antagonist CGP 35348 (1 mM), administered alone for 20 min through the dialysis fibre, was without effect on spontaneous acetylcholine release; however, it completely blocked both the baclofen-induced increase in acetylcholine release and the decrease in GABA release. These results suggest that cortically released GABA exerts a tonic influence on cholinergic activity.     

Involvement of GABAergic and cholinergic medial septal neurons in hippocampal theta rhythm

Hippocampal theta rhythm (HPCtheta) may be important for various phenomena, including attention and acquisition of sensory information. Two types of HPCtheta (types I and II) exist based on pharmacological, behavioral, and electrophysiological characteristics. Both types occur during locomotion, whereas only type II (atropine-sensitive) is present under urethane anesthesia. The circuit of HPCtheta synchronization includes the medial septum-diagonal band of Broca (MSDB), with cholinergic and gamma-aminobutyric acid (GABA)ergic neurons comprising the two main projections from MSDB to HPC. The primary aim of the present study was to assess the effects of GABAergic MSDB lesions on urethane- and locomotion-related HPCtheta, and compare these effects to those of cholinergic MSDB lesions. Saline, kainic acid (KA), or 192 IgG-saporin (SAP) was injected into MSDB before recording. KA preferentially destroys GABAergic MSDB neurons, whereas SAP selectively eliminates cholinergic MSDB neurons. A fixed recording electrode was placed in the dentate mid-molecular layer, and stimulating electrodes were placed in the posterior hypothalamus (PH), and medial perforant path (PP). Under urethane anesthesia, HPCtheta was induced by tail pinch, PH stimulation, and systemic physostigmine; none of the rats with KA or SAP showed HPCtheta in any of these conditions. During locomotion, HPCtheta was attenuated, but not eliminated, in rats with KA or SAP lesions. Intraseptal KA in combination with either intraseptal SAP or PP lesions reduced locomotion-related HPCtheta beyond that observed with each lesion alone, virtually eliminating HPCtheta. In contrast, intraseptal SAP combined with PP lesions did not reduce HPCtheta beyond the effect of each lesion alone. We conclude that both GABAergic and cholinergic MSDB neurons are necessary for HPCtheta under urethane, and that each of these septohippocampal projections contributes to HPCtheta during locomotion.     

Modulation of striatal acetylcholine concentrations by NMDA and the competitive NMDA receptor-antagonist AP-5: an in vivo microdialysis study

Summary. The effects of local perfusion with the competitive NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP-5) and the glutamate receptor agonist N-methyl-D-aspartate (NMDA) on release of extracellular acetylcholine (ACh) and choline (Ch) in the dorsolateral striatum were studied using in vivo microdialysis in freely moving rats. AP-5 caused a dose-dependent decrease in ACh release that was counteracted by the addition of NMDA. Perfusion with AP-5 also decreased Ch levels. Local perfusion with NMDA induced an elevation of ACh release in low (10⁻⁵ M), but not high (10⁻² M and 10⁻³ M) concentrations, that were associated with massive cellular death. These inhibitory effects of AP-5 and the stimulatory effect of NMDA in non-neurotoxic dosages on ACh release provide further evidence for a tonic stimulation of striatal cholinergic interneurons by glutamatergic neurons via NMDA receptors. Received May 21, 1998; accepted July 8, 1998     

Endotoxin administration stimulates cerebral catecholamine release in freely moving rats as assessed by microdialysis

In vivo microdialysis was used to measure changes in extracellular concentrations of catecholamines and indolamines in freely moving rats in response to administration of endotoxin (lipopolysaccharide, LPS). Dialysis probes were placed stereotaxically in either the medial hypothalamus or the medial prefrontal cortex. We used a repeated-measures design in which each rat received LPS or saline, and each subject was retested with the other treatment one week later. With the dialysis probes in the medial hypothalamus, intraperitoneal (ip) administration of LPS (5 μg) increased dialysate concentrations of norepinephrine (NE, 187%), dopamine (DA, 119%), and all their measured catabolites, except normetanephrine. Dialysate concentrations of NE and DA were elevated significantly in the fourth or fifth (20 min) collection period with a peak response at around 2 hr. They returned to baseline by about 4 hr. When the dialysis probes were placed in the medial prefrontal cortex, the same dose of LPS also elevated dialysate concentrations of NE and DA, but the increases were much smaller (ca. 20%). However, a dose of 100 μg LPS increased dialysate concentrations of NE and DA from the medial prefrontal cortex to an extent comparable to that of the 5 μg dose in the hypothalamus, and the response was more prolonged. Dialysate concentrations of serotonin could not be measured reliably, but those of its catabolite, 5-hydroxyindoleacetic acid (5-HIAA), were also elevated in both regions. The peak of 5-HIAA occurred at around 4 hr. Pretreatment of the rats with indomethacin (10 mg/kg ip) completely prevented the changes due to 100 μg LPS in the medial prefrontal cortex. These results support earlier neurochemical data suggesting that LPS stimulates the release of both DA and NE in the brain, and probably also release of serotonin. © 1995 Wiley-Liss, Inc.     

Characterisation of extracellular amino acids in striatum of freely moving rats by in vivo microdialysis

Summary To investigate the characteristics of extracellular amino acids released from the striatum, we performedin vivo microdialysis in non-anaesthetised, freely moving rats. Amino acids were determined after precolumn derivatisation witho-phthalaldehyde by high-performance liquid chromatography and fluorescence detection. The omission of Ca²⁺ in the perfusion medium partially decreased the basal concentration of aspartate and glutamate. This shows that a small fraction of basal concentration of aspartate and glutamate is of neuronal origin. The effect of high K⁺ and veratrine stimulation was evaluated in the presence or absence of Ca²⁺ or tetrodotoxin (1 M). High K⁺ and veratrine caused a remarkable increase in the aspartate and glutamate efflux. The omission of Ca²⁺ only partially decreased K⁺-stimulated aspartate and glutamate efflux. Tetrodotoxin completely antagonised veratrine-stimulated aspartate and glutamate efflux. Although glycine and taurine releases were stimulated by high K⁺ and veratrine, their release was not always antagonised with Ca²⁺ omission or tetrodotoxin inclusion. Thus, the neuronal origin of stimulated release of glycine and taurine is unclear. Although tetrodotoxin sensitivity and Ca²⁺-dependency are regarded as a basic criterion for classical neurotransmitters in microdialysis experiments, they should not be adapted to the physiological characteristics of the release of amino acids.     

Differential effects of striatal injections of dopaminergic, cholinergic and GABAergic drugs upon swimming behavior of rats

The present study provides a detailed report about similarities and dissimilarities between the effects of neostriatally applied dopaminergic (apomorphine, 250-300 ng; haloperidol, 250-500 ng), cholinergic (carbachol, 50-100 ng; scopolamine, 200-500 ng), and GABAergic (muscimol, 1-2 ng; bicuculline, 5-35 ng) drugs upon swimming of rats. The used swimming test consisted of 4 parts: (a) open-field test for analyzing drug-induced changes in normal behavior; (b) 'swimming without escape' test for analyzing drug-induced changes in the ability to switch from one type of behavior to another; (c) 'swimming with escape' test for analyzing drug-induced changes in the ability to switch from ongoing swimming behavior to climbing behavior by allowing the rats to escape via a rope; and (d) 'rope' test for analyzing drug-induced changes in the kind of contact behaviors needed to switch to the latter climbing behavior. In the open-field test the drugs produced neither abnormal behavior nor motor disturbances, which prevented the display of normal behavior in the remaining tests. Both apomorphine and carbachol produced identical effects in all tests. Muscimol produced overall effects which were not only opposite to those of apomorphine and carbachol, but also comparable to those of scopolamine. All effects elicited by apomorphine, carbachol and muscimol were antagonized by their corresponding antagonists: haloperidol, scopolamine and bicuculline respectively, whereas the effects of the latter were suppressed by their corresponding agonists. These data globally show that dopamine and acetylcholine act in the same direction but opposite to that of GABA as far as it concerns the regions investigated. The finding that haloperidol injected into the GABA target area produced effects which were not only similar to those of haloperidol injected into the dopamine target area, but also dissimilar to those of muscimol and bicuculline injected into the GABA target area, shows that the effects were drug-specific rather than region-specific. Though 3 distinct cholinergic regions were investigated, cholinergic-specific effects could only be elicited from one region, suggesting that the neostriatum is heterogeneous in this respect. Finally, well-delineated dissimilarities between haloperidol-, scopolamine-, and muscimol-treated rats were found in the rope test. These data show that behavior-relevant information transmitted by GABAergic drugs surmounted that transmitted by cholinergic drugs which, in turn, surmounted behavior-relevant information transmitted by dopaminergic drugs.     

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward‐dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ‐aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non‐GABAergic) sources of symmetric synapses. Double pre‐embedding immunoelectron microscopy using substance P and Met‐/Leu‐enkephalin antibodies to label GABAergic terminals from collaterals of “direct” and “indirect” striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P‐ and enkephalin‐positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons. J. Comp. Neurol. 521:2502–2522, 2013. © 2013 Wiley Periodicals, Inc.     

Differential regional and kinetics effects of piribedil and bromocriptine on dopamine metabolites: a brain microdialysis study in freely moving rats

Summary Brain microdialysis coupled to HPLC was applied to freely moving rats to investigate the regional kinetics of piribedil and bromocriptine on the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in striatum, nucleus accumbens, and frontal cortex. Both D2 agonists (20 mg/kg i.p.) decreased DOPAC and HVA in the three brain regions. The responsiveness of frontal cortex to both compounds was greater than those previously reported with other dopaminergic drugs. Regional and temporal differences were observed under piribedil: DOPAC and HVA levels decreased more in the nucleus accumbens than in striatum or frontal cortex but increased over basal values from the 5th hour in the frontal cortex suggesting a late stimulatory effect of piribedil on dopamine synthesis in this area. Such regional effects differentiate piribedil from most other D2 agonists and could explain some behavioural and therapeutic actions possibly related to involvement of nucleus accumbens or/and frontal cortex.     

Effects of peripheral and central catechol-O-methyltransferase inhibition on striatal extracellular levels of dopamine: a microdialysis study in freely moving rats

Tolcapone is a mixed (peripheral and central) catechol-O-methyltransferase (COMT) inhibitor, whereas entacapone is a preferential peripheral COMT inhibitor. Both drugs are able to decrease the peripheral conversion of L-DOPA into 3-O-methyl-DOPA and thereby increase plasma and cerebral levels of L-DOPA, the precursor of dopamine (DA). Tolcapone may also impair the extraneuronal catabolism of DA by inhibiting COMT activity in the brain. To evaluate the role played by peripheral and central COMT inhibition, we compared the effects of tolcapone and entacapone on COMT activity in peripheral tissues, and on striatal extracellular levels of L-DOPA and DA in rats. Tolcapone and entacapone, at the dose of 15 mg/kg p.o., were almost equally effective in inhibiting COMT activity in duodenum and liver. Tolcapone decreased striatal extracellular levels of homovanillic acid (HVA), thus confirming its central COMT inhibitory effect, whereas entacapone did not alter HVA efflux. Following L-DOPA/benserazide administration (50/15 mg/kg p.o.), both COMT inhibitors significantly increased striatal levels of L-DOPA and DA compared with saline. The levels of L-DOPA were similar after treatment with either COMT inhibitors, whereas the increase in DA output was significantly greater in rats given tolcapone compared to those given entacapone. We conclude that the blockade of central DA catabolism by tolcapone contributes to the greater increase in striatal DA levels achieved with this drug.     

Inhibitory GABAergic influence on striatal serotonergic transmission exerted in the dorsal raphe as revealed by in vivo voltammetry

In vivo differential pulse voltammetry with electrochemically treated carbon fiber microelectrodes has been used to investigate the anatomical nature of the GABAergic influence on striatal serotonergic transmission in the rat. Lesion studies and pharmacological treatments demonstrated that the electrochemical signal recorded at 300 mV in the striatum probably corresponds to the oxidation of extracellularly released 5-hydroxyindoleacetic acid. Thus, dorsal raphé lesions or systemic administration of alpha-propyldopacetamide, NSD 1015, pargyline and MK212 decreased, whereas reserpine injection increased the amplitude of the signal. Moreover, L-5-hydroxytryptophan administration caused an increase in the signal which was almost completely prevented by pargyline pretreatment. Acute administration of dipropylacetamide (150 mg/kg i.p.) reduced the amplitude of the signal from the striatum, while injection of gamma-acetylenic GABA (200 mg/kg i.p.) was without effect. Repeated (but not acute) treatment with the GABA receptor agonist, progabide (400 mg/kg i.p.b.i.d. for 14 days), led to a pronounced decrease in the amplitude of the signal from the striatum. A similar effect was observed after intradorsal raphé infusion of GABA (10 and 100 micrograms), gamma-vinyl GABA (100 micrograms) and SL 75102 (10 micrograms), a principal metabolite of progabide. In contrast, local injection of the GABA receptor antagonists, bicuculline (1 and 10 micrograms) or R5135 (0.05 microgram), failed to affect the peak amplitude in the striatum. When infused into the dorsal raphé, R5135 (0.05-0.1 microgram) antagonized the diminution of the signal induced by intradorsal raphé infusion of GABA (100 micrograms) or SL 75102 (10 micrograms). Finally, electrolytic lesion of the habenular nuclei completely blocked the diminution of the signal from striatum induced by an intradorsal raphé infusion of GABA (100 micrograms). These results indicate that the inhibitory GABAergic control of striatal serotonergic transmission is exerted at the level of the dorsal raphé cells and depends upon the integrity of the habenulo-dorsal pathway.     

Characterization of dopamine release in the substantia nigra by in vivo microdialysis in freely moving rats.

Dopamine (DA) is released not only from the terminals of the nigrostriatal projection, but also from the dendrites of these neurons, which arborize in the substantia nigra pars reticulata (SNR). Although striatal DA release has been extensively studied by in vivo microdialysis, dendritic DA release in the SNR has not been characterized by this technique. Extracellular DA was monitored simultaneously in the ipsilateral striatum and SNR. The nigral probe was implanted at a 50 degree angle, permitting 2.5 mm of SNR to be dialyzed. Delivery of the tracer Fluoro-Gold into the striatal probe retrogradely labeled tyrosine hydroxylase-positive cell bodies and dendrites in the vicinity of the nigral probe. Hence, it could be demonstrated that dopaminergic neurons near the nigral probe projected to the vicinity of the striatal probe. Addition of 50 mM KCl to the SNR perfusion solution produced a 3.5-fold increase in DA and a 50% reduction in dihydroxyphenylacetic acid (DOPAC) in the SNR; in contrast, this manipulation in the SNR caused DA release in the striatum to be decreased by 20%, while striatal DOPAC was increased by 50%. Local administration of nomifensine (10 microM) in the SNR produced a sevenfold increase in SNR DA but had no effect on striatal DA. Systemic injection of d-amphetamine (2 mg/kg, s.c.) elevated DA in the SNR and striatum five- to sevenfold, while DOPAC was decreased in both structures by at least 40%. To determine the effect of tetrodotoxin (TTX), basal concentrations of DA in the SNR were first elevated threefold by including nomifensine (1 microM) in the nigral perfusion solution.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo neurochemical evidence that stimulation of accumbal GABAA and GABAB receptors each reduce acetylcholine efflux without affecting dopamine efflux in the nucleus accumbens of freely moving rats

Cholinergic neurons in the nucleus accumbens contain GABA_A and GABA_B receptors that are thought to inhibit neural activity. We analyzed the roles of GABA_A and GABA_B receptors in regulating accumbal acetylcholine efflux of freely moving rats using in vivo microdialysis. The effects of GABA receptor ligands on the accumbal dopamine efflux were also analyzed because accumbal cholinergic and dopaminergic neurons could mutually interact. Drugs were applied intracerebrally through the dialysis probe. Doses of compounds indicate total amount administered (mol) during 30–60 min infusions. To monitor basal acetylcholine, a low concentration of physostigmine (50 nM) was added to the perfusate. GABA_A receptor agonist muscimol (3 and 30 pmol) induced a dose‐related decrease in accumbal acetylcholine. GABA_B receptor agonist baclofen (30 and 300 pmol) also produced a dose‐related decrease in acetylcholine. GABA_A receptor antagonist bicuculline (60 pmol) which failed to alter baseline acetylcholine counteracted the muscimol (30 pmol)‐induced decrease in acetylcholine. GABA_B receptor antagonist 2‐hydroxysaclofen (12 nmol) which failed to change baseline acetylcholine, counteracted the baclofen (300 pmol)‐induced decrease in acetylcholine. Neither muscimol (30 pmol) nor baclofen (300 pmol) which reduced accumbal acetylcholine altered baseline accumbal dopamine. Neither bicuculline (60 pmol) nor 2‐hydroxysaclofen (12 nmol) also affected the baseline dopamine. These results show that GABA_A and GABA_B receptors each exert inhibitory roles in the regulation of accumbal cholinergic neural activity. The present results also provides in vivo neurochemical evidence that stimulation of GABA_A and GABA_B receptors each reduce acetylcholine efflux without affecting dopamine efflux in the nucleus accumbens of freely moving rats.     

Apomorphine and haloperidol effects on striatal H-dopamine release in anesthetized, awake restrained and freely moving rats

The ability of apomorphine (APO) and haloperidol (HAL) to affect the spontaneous release of newly synthesized ³H-DA in the striatum was studied in halothane anesthetized, gallamine paralyzed, awake restrained and freely moving rats. The striatum was continuously superfused through a push-pull cannula with a physiological medium enriched in ³H-tyrosine. Basal levels of ³H-DA release were different in the four experimental models: highest in halothane anesthetized rats, intermediate in awake restrained and gallamine treated rats and lowest in freely moving rats. In all experimental models IV or SC injection of APO (1 mg/kg) inhibited the release of ³H-DA (30–50%) from 15 to 90 min following its administration. In awake restrained and freely moving rats, stereotyped behaviour was observed for one hour following the APO injection. In halothane anesthetized rats the inhibitory effect of APO on ³H-DA release was prevented by pretreatment with HAL (2 mg/kg IV). Injection of HAL (2 mg/kg IV or SC) failed to enhance the release of ³H-DA in anesthetized and awake restrained rats, whilst a long-lasting increase in ³H-DA release was observed in gallamine treated and freely moving animals (55% and 120% respectively). However, catalepsy was observed in both restrained and freely moving rats. It is concluded that the modifications of ³H-DA release produced by HAL but not those produced by APO are dependent on the experimental model used, a fact possibly related to the different sites of action of these two drugs.     

Differential effects of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism in freely moving rats

A dialysis loop cannula was implanted into rat striatum under anesthetized condition, and the area was perfused with Ringer's solution under freely moving condition after 3 days for surgical recovery. Dopamine (DA) and 3,4-dihydroxyphenylacetic acid recovered in the dialysate were measured by high-performance liquid chromatography with electrochemical detection. The effects of M1- and M2-muscarinic receptor agents, which were perfused continuously into the striatum through the dialysis membrane, were investigated. Continuous perfusion of AF102B, an M1-selective agonist, and oxotremorine, a non-selective agonist, resulted in a dose-dependent increase in the striatal DA release. Pirenzepine (10(-5) and 10(-7) M), an M1-selective antagonist, decreased the release of DA, and the stimulatory effect of AF102B (10(-5) M) was completely inhibited by 10(-5) and 10(-7) M pirenzepine, while the stimulatory effect of oxotremorine (10(-4) M) was only partly inhibited by 10(-5) M pirenzepine. AF-DX116 (10(-5) M), an M2-selective antagonist, increased the DA release, and showed an additive effect on the DA release evoked by AF102B (10(-5) M), whereas it produced no significant effect on oxotremorine (10(-5) M)-evoked DA release. These results suggest that in vivo DA release in the rat striatum is modulated by different subtypes of muscarinic receptors; i.e., the stimulatory effect is mainly mediated by M1-sites and inhibitory effect is mainly mediated by M2-sites. The changes in the DA release induced by the various drugs were prevented by pretreatment with tetrodotoxin (TTX). Since action potential-dependent DA release (exocytosis) is blocked by the pretreatment with TTX, those drugs affect DA release by means of action potential-dependent processes.     

Endogenous GABA potentiates the potassium-induced release of dopamine in striatum of the freely moving rat: a microdialysis study

Using microdialysis, a study was made of the effects of an increase of endogenous GABA on basal and potassium-stimulated release of dopamine in striatum of the awake rat. The dopamine metabolites DOPAC and HVA were also measured. Extracellular concentrations of GABA were increased by inhibiting its uptake with nipecotic acid. TTX (10 μM) reduced basal extracellular concentrations of dopamine, and dopamine metabolites, but not GABA. Nipecotic acid (200, 500, and 1000 μM) produced a dose-related increase in basal extracellular concentrations of GABA, but did not change basal extracellular concentrations of dopamine and dopamine metabolites. However, nipecotic acid significantly enhanced the dopamine release produced by perfusion of potassium (50 mM) and also enhanced the extracellular increase of GABA produced by high potassium. These results suggest that an increase of endogenous GABA is facilitating the stimulated release, but not the basal release, of dopamine in the striatum of the awake rat.     

Involvement of cholinergic neurons in the release of dopamine elicited by stimulation of μ-opioid receptors in striatum

The involvement of striatal cholinergic neurons in the release of dopamine (DA) elicited by the (μ-opioid receptor agonist DAGO [d-Ala², NMePhe⁴-Gly⁵(ol)]enkephalin) was explored. The striatal release of DA was measured by microdialysis in rats anesthetized with chloral hydrate. When infused in the striatum, through the microdialysis probe, DAGO increased the extracellular levels of DA. The previous injection in striatum of AF 64-A, a toxin for cholinergic neurons, or the concomitant infusion of the M₂-muscarinic antagonist methoctramine abolished the effect of DAGO on the DA release. It is concluded that stimulation of μ-opioid receptors, by inhibiting the acetylcholine release which stimulates tonically M₂-muscarinic receptors likely associated with dopaminergic nerve endings, indirectly increases the striatal DA release.