Direct Comparison of the Response of Voltammetry and Microdialysis to Electrically Evoked Release of Striatal Dopamine

Abstract: Carbon fiber microelectrodes either were implanted directly into striatal tissue or were mounted into the outlet of microdialysis probes that were implanted into striatal tissue. This allowed voltammetry and microdialysis to be used under identical in vivo experimental conditions to monitor extracellular dopamine levels during electrical stimulation of the medial forebrain bundle both before and after uptake inhibition with nomifensine. The marked differences between the results obtained with each technique cannot be explained on the basis of their inherent analytical attributes (sensitivity, temporal response, etc.). The results demonstrate that the microdialysis recovery factor for endogenous dopamine increases after uptake inhibition, an observation that stands in contradiction to the existing theory and practice of the microdialysis technique. The observations led to the development of a numerical model that rationalizes the observations reported herein and that allows in vivo voltammetry and in vivo microdialysis results to be interpreted within a single theoretical framework.     

Real-Time Measurement of Electrically Evoked Extracellular Dopamine in the Striatum of Freely Moving Rats

Abstract: The real-time measurement of electrically evoked dopamine was established in brain extracellular fluid of freely moving rats. Dopamine was monitored by fast-scan cyclic voltammetry at carbon fiber microelectrodes lowered into the striatum by means of a detachable micromanipulator. A stimulating electrode, previously implanted in the substantia nigra, was used to evoke striatal dopamine efflux. Evoked extracellular dopamine was both current and frequency dependent. When low current intensities (±125 µA) and frequencies (10–20 Hz) were applied, detectable levels of dopamine were elicited without a perceptible behavioral response. Reproducible concentrations of extracellular dopamine could be evoked in the same rat for at least 2 months. These concentrations, moreover, were significantly higher in freely moving rats compared with rats anesthetized with Equithesin. Analysis of measured curves for dopamine uptake and release rates revealed that anesthesia inhibits release but does not affect uptake. It is concluded that (a) fast-scan cyclic voltammetry at carbon fiber microelectrodes is a viable technique for the measurement of electrically evoked dopamine in brain extracellular fluid of freely moving rats, (b) it is possible to determine in situ rate constants for dopamine release and uptake from these temporally and spatially resolved measurements of levels of dopamine, and (c) transient changes in extracellular dopamine levels elicited by electrical stimulation are affected by anesthesia.     

Theory for the impact of basal turnover on dopamine clearance kinetics in the rat striatum after medial forebrain bundle stimulation and pressure ejection

Although microdialysis measurements suggest that extracellular dopamine concentrations in the rat striatum are in the low nanomolar range, some recent voltammetry studies suggest that the concentration may be considerably higher, perhaps in the micromolar range. The presence of such high dopamine levels in the extracellular space has to be rationalized with the rapid, linear clearance of extracellular dopamine observed after electrical stimulation of the medial forebrain bundle. Kinetic analysis of dopamine clearance after evoked release suggests that the basal extracellular dopamine concentration is below the K(M) of dopamine uptake, which is near 0.2 microm. However, dopamine clearance after pressure ejection of dopamine into the rat striatum is slow and non-linear, which may alternatively be a sign that basal dopamine release is only slightly slower than the maximal velocity of dopamine uptake, Vmax. A high basal extracellular dopamine concentration would exist if basal dopamine release were only slightly slower than the Vmax of uptake. This report introduces a new kinetic analysis of dopamine uptake that sheds light on the possible source of the different clearance rates observed following evoked dopamine release and dopamine pressure ejection. Furthermore, the analysis rationalizes the rapid dopamine clearance after evoked release with the possibility that basal extracellular dopamine levels are above the K(M) of the transporter.     

Cortical Regulation of Subcortical Dopamine Release: Mediation via the Ventral Tegmental Area

Abstract: In vivo microdialysis was used to determine the extent to which ionotropic glutamate receptors in the ventral tegmental area (VTA) regulate dopamine release in the nucleus accumbens. Coapplication of 2-amino-5-phosphonopentanoic acid (AP5; 200 µ M ) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 µ M ) to the VTA via reverse dialysis decreased extracellular concentrations of dopamine in the nucleus accumbens by ∼30%. In accordance with previous results, electrical stimulation of the prefrontal cortex increased dopamine release by 60%. Application of AP5 and CNQX to the VTA during cortical stimulation blocked the effect of stimulation on dopamine release. These results indicate that ionotropic glutamate receptors in the VTA are critically involved in basal and evoked dopamine release in the nucleus accumbens and suggest that a glutamatergic projection from the prefrontal cortex regulates the activity of dopaminergic neurons in the VTA.     

Evidence on extracellular dopamine level in rat striatum: implications for the validity of quantitative microdialysis

Microdialysis zero-net-flux (ZNF) method is commonly used to monitor drug-induced changes in neurotransmitter baseline and release/uptake processes. Recent studies in this field suggest that microdialysis ZNF method seriously underestimates the resting concentration of extracellular dopamine in the rat neostriatum because probe implantation preferentially damages nearby dopamine release sites and that dopamine uptake inhibition increases the relative recovery of dopamine by microdialysis. This study assessed the validity of these claims by examining current data on extracellular dopamine levels at rest and after drug application obtained by voltammetry, a technique thought to induce less tissue disruption than microdialysis. To obtain the extracellular baseline value for dopamine from the evoked overflow data, we modified the existing dopamine kinetic model to suit both the resting and stimulated circumstances. It was found that dopamine uptake inhibition did in fact decrease the microdialysis relative recovery of dopamine, implying that the average basal extracellular dopamine level is within the range of 7-20 nm in rat striatum. This study concludes that the microdialysis ZNF method indeed underestimates the extracellular dopamine concentration, although not by as much as had been thought. Chronic microdialysis damages both neurotransmitter release and uptake, but it does so in a somewhat relative and proportional way for both processes. Thus the validity of the microdialysis ZNF method is not seriously undermined.     

"Passive stabilization" of striatal extracellular dopamine across the lesion spectrum encompassing the presymptomatic phase of Parkinson's disease: a voltammetric study in the 6-OHDA-lesioned rat

Symptoms of Parkinson's disease do not present until the degeneration of nigrostriatal dopaminergic neurons is nearly complete. Maintenance of dopaminergic tone governing striatal efferents is postulated to preserve motor control during the presymptomatic phase, but the neuroadaptation responsible for normalization is not completely understood. In particular, the prevailing view that surviving dopaminergic neurons compensate by up-regulating release has been difficult to demonstrate directly. Here we investigate dopaminergic neurotransmission in the hemiparkinsonian rat using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. Electrical stimulation was used to elicit extracellular dopamine levels mimicking the steady-state dynamics of tonic dopaminergic signaling. In agreement with microdialysis studies, evoked steady-state dopamine levels remained constant over the entire lesion spectrum (0 to approximately 85%) observed during the presymptomatic stage. Kinetic analysis of the voltammetric recordings demonstrated that evoked dopamine concentrations were normalized without plasticity of dopamine release and uptake, suggesting that the primary mechanisms controlling ambient levels of extracellular dopamine were not actively altered. In the present study, we formalize this neuroadaptation as "passive stabilization" . We further propose that passive stabilization is mediated by the simple physical principles of diffusion and steady state, is predicated on extrasynaptic transmission, and forms the basis for a new compensation model of preclinical parkinsonism.     

Evoked Extracellular Dopamine In Vivo in the Medial Prefrontal Cortex

Abstract: The measurement of evoked extracellular dopamine in the medial prefrontal cortex by using fast-scan cyclic voltammetry with carbon-fiber microelectrodes was established and release characteristics of mesoprefrontal dopamine neurons were examined in vivo in anesthetized rats. Despite the sparse dopaminergic innervation and the presence of more dense noradrenergic and serotonergic innervations overall in the medial prefrontal cortex, the measurement of extracellular dopamine was achieved by selective recording in dopamine-rich terminal fields and selective activation of ascending dopamine neurons. This was confirmed by electrochemical, pharmacological, and anatomical evidence. An increased release capacity for mesoprefrontal dopamine neurons was also demonstrated by the slower decay of the evoked dopamine response after inhibition of catecholamine synthesis and the maintenance of the evoked dopamine response at higher levels in the medial prefrontal cortex compared with the striatum during supraphysiological stimulation.     

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.     

Stimulation of prefrontal cortex at physiologically relevant frequencies inhibits dopamine release in the nucleus accumbens

The prefrontal cortex (PFC) is thought to provide an excitatory influence on the output of mesoaccumbens dopamine neurons. The evidence for this influence primarily arises from findings in the rat that chemical or high-intensity and high-frequency (60-200 Hz) electrical stimulations of PFC increase burst activity of midbrain dopamine neurons, and augment terminal release of dopamine in the nucleus accumbens. However, PFC neurons in animals that are engaged in PFC-dependent cognitive tasks increase their firing frequency from a baseline of 1-3 Hz to 7-10 Hz, suggesting that the commonly used high-frequency stimulation parameters of the PFC may not be relevant to the behavioral states that are associated with PFC activation. We investigated the influence of PFC activation at lower physiologically relevant frequencies on the release of dopamine in the nucleus accumbens. Using rapid (5-min) microdialysis measures of extracellular dopamine in the nucleus accumbens, we found that although PFC stimulation at 60 Hz produces the expected increases in accumbal dopamine release, the same amplitude of PFC stimulation at 10 Hz significantly decreased these levels. These results indicate that activation of PFC, at frequencies that are associated with increased cognitive demand on this region, inhibits the mesoaccumbens dopamine system.     

Endogenous opioids inhibit the in vitro release of endogenous dopamine preferentially in the neural lobe of the rat neurointermediate lobe

The release of endogenous dopamine (DA) from the in vitro incubated combined neurointermediate lobe (NIL) or isolated neural lobe (NL) was studied. In the presence of the DA uptake inhibitor GBR 12921 (200 nM), electrical stimulation of the pituitary stalk caused an increase of the outflow of DA from the NIL in a frequency-dependent manner. Naloxone (1 microM) enhanced the DA release from the NIL evoked by electrical stimulation at 7 or 15 Hz by about 40%, but had no effect on DA release evoked by stimulation at 3 Hz. When the electrical stimulation was carried out at 15 Hz, the evoked DA release (expressed as fraction of the DA tissue content) from the NL amounted to only 15% of that from the combined NIL. Naloxone (1 microM) increased the evoked DA release from the isolated NL by 242%. Thus, the effect of naloxone on DA release from the combined NIL may be confined mainly to the NL. In conclusion, DA release from the NL is under inhibitory control of endogenous opioids released from the NL during stimulation at 7 or 15 Hz. Beta-Endorphin, known to be released spontaneously at a high rate from in vitro incubated NILs, appears to lack inhibitory effects on DA release from the NIL.     

Mice transgenic for exon 1 of Huntington's disease: properties of cholinergic and dopaminergic pre-synaptic function in the striatum

In Huntington's disease (HD), neuronal loss is most prominent in the striatum leading to emotional, cognitive and progressive motor dysfunction. The R6/2 mice, transgenic for exon 1 of the HD gene, develop a neurological phenotype with similarities to these features of HD. In striatal tissue, electrically evoked release of tritiated acetylcholine (ACh) and dopamine (DA) were compared in wild-type (WT) and R6/2 mice. In R6/2 mice, the evoked release of ACh, its M2 autoreceptor-mediated maximum inhibition and its dopamine D2 heteroreceptor-mediated maximum inhibition was diminished to 51%, 74% and 87% of controls, respectively. Also, the activities of choline acetyltransferase and of synaptosomal high-affinity choline uptake decreased progressively with age in these mice. In the DA release model, however, electrical stimulation elicited equal amounts of [3H]-DA both in WT and R6/2 mice. Moreover, high-affinity DA uptake into striatal slices was similar in WT and R6/2 mice. In order to confirm these findings in vivo, intrastriatal levels of extracellular DA were measured by intracerebral microdialysis in freely moving mice: striatal DA levels were found to be equal in WT and R6/2 mice. In conclusion, in the transgenic R6/2 mice changes occur mainly in striatal cholinergic neurones and their pre-synaptic modulation, but not in the dopaminergic afferent terminals. Whether similar events also contribute to the pathogenesis of HD in humans has to be established.     

Effect of acute administration of L-tryptophan on the release of 5-HT in rat hippocampus in relation to serotoninergic neuronal activity: an in vivo microdialysis study.

Here we have used the brain microdialysis method to test the effect of the 5-HT precursor L-tryptophan on 5-HT release. The release of endogenous 5-HT was measured in ventral hippocampus of the anesthetized rat both under basal conditions and when serotoninergic neuronal activity was raised by electrical stimulation of the dorsal raphe nucleus (DRN). Low frequency electrical stimulation of the DRN evoked a frequency-dependent (2-10 Hz) release of hippocampal 5-HT. The electrically evoked release of 5-HT was markedly enhanced by pretreatment with L-tryptophan (50 and 100 mg/kg i.p.). The effect of L-tryptophan on evoked release of 5-HT was dose-related, detectable at low (2 Hz) stimulation frequencies, and became stronger as the stimulation frequency increased. L-Tryptophan (10, 50 and 100 mg/kg i.p.) had no effect on basal output of 5-HT. We conclude from these findings that elevation of 5-HT precursor availability increases 5-HT release in hippocampus in vivo under conditions of increased serotoninergic neuronal activity.     

Frequency-Dependent Modulation of Dopamine Release by Nicotine and Dopamine D<Subscript>1</Subscript> Receptor Ligands: An In Vitro Fast Cyclic Voltammetry Study in Rat Striatum

Nicotine is a highly addictive drug and exerts this effect partially through the modulation of dopamine release and increasing extracellular dopamine in regions such as the brain reward systems. Nicotine acts in these regions on nicotinic acetylcholine receptors. The effect of nicotine on the frequency dependent modulation of dopamine release is well established and the purpose of this study was to investigate whether dopamine D₁ receptor (D₁R) ligands have an influence on this. Using fast cyclic voltammetry and rat corticostriatal slices, we show that D₁R ligands are able to modulate the effect of nicotine on dopamine release. Nicotine (500 nM) induced a decrease in dopamine efflux at low frequency (single pulse or five pulses at 10 Hz) and an increase at high frequency (100 Hz) electrical field stimulation. The D₁R agonist SKF-38393, whilst having no effect on dopamine release on its own or on the effect of nicotine upon multiple pulse evoked dopamine release, did significantly prevent and reverse the effect of nicotine on single pulse dopamine release. Interestingly similar results were obtained with the D₁R antagonist SCH-23390. In this study we have demonstrated that the modulation of dopamine release by nicotine can be altered by D₁R ligands, but only when evoked by single pulse stimulation, and are likely working via cholinergic interneuron driven dopamine release.     

Biochemistry of Somatodendritic Dopamine Release in Substantia Nigra: An In Vivo Comparison with Striatal Dopamine Release

Abstract: The somatodendritic release of dopamine in substantia nigra previously has been suggested to be nonvesicular in nature and thus to differ from the classical, exocytotic release of dopamine described for the dopaminergic nerve terminal in striatum. We have compared the effects of reserpine, a compound that disrupts vesicular sequestration of monoamines, on the storage and release of dopamine in substantia nigra and striatum of rats. Reserpine administration (5 mg/kg, i.p.) significantly decreased the tissue level of dopamine in substantia nigra pars reticulata, substantia nigra pars compacta, and striatum. In these brain areas, reserpine-induced reductions in tissue dopamine level occurred within 2 h and persisted at 24 h postdrug. In vivo measurements using microdialysis revealed that reserpine administration rapidly decreased the extracellular dopamine concentration to nondetectable levels in substantia nigra as well as in striatum. In both structures, it was observed that reserpine treatment significantly attenuated the release of dopamine evoked by a high dose of amphetamine (10 mg/kg, i.p.) given 2 h later. In contrast, dopamine efflux in response to a low dose of amphetamine (2 mg/kg, i.p.) was not altered by reserpine pretreatment either in substantia nigra or in striatum. The present data suggest the existence, both at the somatodendritic and at the nerve terminal level, of a vesicular pool of dopamine that is the primary site of transmitter storage and that can be displaced by high but not low doses of amphetamine. The physiological release of dopamine in substantia nigra and in striatum is dependent on the integrity of this vesicular store.     

Stimulation of the locus coeruleus elicits noradrenaline and dopamine release in the medial prefrontal and parietal cortex

Our previous studies have suggested that dopamine and noradrenaline may be coreleased from noradrenergic nerve terminals in the cerebral cortex. To further clarify this issue, the effect of electrical stimulation of the locus coeruleus on extracellular noradrenaline, dopamine and DOPAC in the medial prefrontal cortex, parietal cortex and caudate nucleus was analysed by microdialysis in freely moving rats. Stimulation of the locus coeruleus for 20 min with evenly spaced pulses at 1 Hz failed to modify cortical catecholamines and DOPAC levels. Stimulation with bursts of pulses at 12 and 24 Hz increased, in a frequency-related manner, not only noradrenaline but also dopamine and DOPAC in the two cortices. In both cortices noradrenaline returned to baseline within 20 min of stimulation, irrespective of the stimulation frequency, whereas dopamine returned to normal within 20 and 60 min in the medial prefrontal cortex and within 60 and 80 min in the parietal cortex after 12 and 24 Hz stimulation, respectively. DOPAC remained elevated throughout the experimental period. Phasic stimulation of the locus coeruleus at 12 Hz increased noradrenaline in the caudate nucleus as in the cerebral cortices but was totally ineffective on dopamine and DOPAC. Tetrodotoxin perfusion into the medial prefrontal cortex dramatically reduced noradrenaline and dopamine levels and suppressed the effect of electrical stimulation. These results indicate that electrical stimulation-induced increase of dopamine is a nerve impulse exocytotic process and suggest that cortical dopamine and noradrenaline may be coreleased from noradrenergic terminals.     

NMDA-mediated release of glutamate and GABA in the subthalamic nucleus is mediated by dopamine: an in vivo microdialysis study in rats

The present study investigated the effects of N-methyl-D-aspartic acid.H2O (NMDA) on the dopamine, glutamate and GABA release in the subthalamic nucleus (STN) by using in vivo microdialysis in rats. NMDA (100 micromol/L) perfused through the microdialysis probe evoked an increase in extracellular dopamine in the STN of the intact rat of about 170%. This coincided with significant increases in both extracellular glutamate (350%) and GABA (250%). The effect of NMDA perfusion on neurotransmitter release at the level of the STN was completely abolished by co-perfusion of the selective NMDA-receptor antagonist MK-801 (10 micromol/L), whereas subthalamic perfusion of MK-801 alone had no effect on extracellular neurotransmitter concentrations. Furthermore, NMDA induced increases in glutamate were abolished by both SCH23390 (8 micromol/L), a selective D1 antagonist, and remoxipride (4 micromol/L), a selective D2 antagonist. The NMDA induced increase in GABA was abolished by remoxipride but not by SCH23390. Perfusion of the STN with SCH23390 or remoxipride alone had no effect on extracellular neurotransmitter concentrations. The observed effects in intact animals depend on the nigral dopaminergic innervation, as dopamine denervation, by means of 6-hydroxydopamine lesioning of the substantia nigra, clearly abolished the effects of NMDA on neurotransmitter release at the level of the STN. Our work points to a complex interaction between dopamine, glutamate and GABA with a crucial role for dopamine at the level of the STN.     

Effects of Frequency and Pattern of Medial Forebrain Bundle Stimulation on Caudate Dialysate Dopamine and Serotonin

In vivo microdialysis was employed to detect changes in extracellular dopamine and serotonin in the rat caudate in response to electrical stimulation of the medial forebrain bundle. Extracellular dopamine concentrations increased linearly as a function of the frequency (4-33 Hz) of evenly spaced stimuli in both the presence and absence of cocaine added to the dialysate. Because dopamine neurons are known to fire in single-spike and burst patterns, stimulation pulses were also delivered in a bursting pattern. The response of extracellular dopamine was augmented in both the presence and absence of cocaine when the same number of stimuli were delivered in bursts as compared to an evenly spaced pattern. Serotonin, which was only assessed in the presence of cocaine, similarly increased linearly with frequency, but, in contrast to the dopamine response, levels of serotonin were not augmented by stimuli presented in bursts. These results suggest that microdialysis can be used to detect physiological changes in synaptic transmitter concentrations.     

Aberrant striatal dopamine transmitter dynamics in brain-derived neurotrophic factor-deficient mice

Brain-derived neurotrophic factor (BDNF) modulates the synaptic transmission of several monoaminergic neuronal systems, including forebrain dopamine-containing neurons. Recent evidence shows a strong correlation between neuropsychiatric disorders and BDNF hypofunction. The aim of the present study was to characterize the effect of low endogenous levels of BDNF on dopamine system function in the caudate-putamen using heterozygous BDNF (BDNF(+/-) ) mice. Apparent extracellular dopamine levels in the caudate-putamen, determined by quantitative microdialysis, were significantly elevated in BDNF(+/-) mice compared with wildtype controls (12 vs. 5 nM, respectively). BDNF(+/-) mice also had a potentiated increase in dopamine levels following potassium (120 mM)-stimulation (10-fold) relative to wildtype controls (6-fold). Slice fast-scan cyclic voltammetry revealed that BDNF(+/-) mice had reductions in both electrically evoked dopamine release and dopamine uptake rates in the caudate-putamen. Superfusion of BDNF led to partial recovery of the electrically stimulated dopamine release response in BDNF(+/-) mice. Conversely, tissue accumulation of L-3,4-dihydroxyphenylalanine, extracellular levels of dopamine metabolites, and spontaneous locomotor activity were unaltered. Together, this study indicates that endogenous BDNF influences dopamine system homeostasis by regulating the release and uptake dynamics of pre-synaptic dopamine transmission.     

Effects of (−)-Sulpiride on Dopamine Release in Striatum of Developing Rats: Degree of Depolarization Influences Responsiveness

Abstract: The purpose of this study was to determine the effects of localized delivery of the D₂ antagonist (−)-sulpiride (via microdialysis) on spontaneous and evoked dopamine release in the neostriatum of urethane-anesthetized rats 5, 10, 15, 21, and 70 days of age. Sulpiride increased spontaneous dopamine release approximately threefold relative to baseline measures, and this effect decreased with maturation. The relationship between sulpiride- and potassium-evoked release was complex; sulpiride increased evoked dopamine outflow at 5, 10, and 15 days of age. At 21 and 70 days of age, however, the effects of sulpiride were inversely related to the degree of stimulation with potassium. Furthermore, the D₂ agonist quinpirole (100 µ M ) reversed the effects of sulpiride (10 µ M ), suggesting receptor mediation. These experiments demonstrate that the maturational decline in the efficacy and potency of D₂ antagonism appears to be related to the degree of stimulation at the nerve terminal.     

Inhibition of the Electrically Evoked Release of [3H]Acetylcholine in Rat Striatal Slices: An Experimental Model for Drugs that Enhance Dopaminergic Neurotransmission

The activation by endogenous dopamine of the inhibitory 3,4-dihydroxyphenylethylamine (dopamine) receptors modulating the electrically evoked release of [3H]acetylcholine [( 3H]ACh) and [3H]dopamine in rat striatal slices is a function of the concentration of dopamine accumulated in the synaptic cleft during electrical stimulation. When the release of 3H-neurotransmitters was elicited with a 2-min period of stimulation at a frequency of 1 Hz, neither dopamine autoreceptors nor dopamine receptors modulating [3H]ACh were activated by endogenously released dopamine. On the other hand, exposure to (S)-sulpiride facilitated the release of [3H]dopamine and [3H]ACh elicited when the 2-min stimulation was carried out at a frequency of 3 Hz but this effect was not observed at a lower frequency of stimulation (1 Hz). In the presence of amphetamine the dopamine receptors modulating the electrically evoked release of [3H]ACh can be activated by endogenous dopamine even at the lower frequency of stimulation (1 Hz). Similar effects can be obtained if the neuronal uptake of dopamine is inhibited by cocaine or nomifensine. The inhibition by amphetamine of the release of [3H]ACh elicited by electrical stimulation at 1 Hz involves dopamine receptors and can be fully antagonized by clozapine, haloperidol, chlorpromazine, or pimozide. The stereoselectivity of this antagonism can be demonstrated with the optical enantiomers of sulpiride and butaclamol. This inhibitory effect of amphetamine on cholinergic neurotransmission appears to be the result of the stimulation of dopamine receptors of the D2 subtype, as they were resistant to blockade by the preferential D1 receptor antagonist SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)