Methylphenidate-induced increases in vesicular dopamine sequestration and dopamine release in the striatum: the role of muscarinic and dopamine D2 receptors

Methylphenidate (MPD) administration alters the subcellular distribution of vesicular monoamine transporter-2 (VMAT-2)-containing vesicles in rat striatum. This report reveals previously undescribed pharmacological features of MPD by elucidating its receptor-mediated effects on VMAT-2-containing vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles) and on striatal dopamine (DA) release. MPD administration increased DA transport into, and decreased the VMAT-2 immunoreactivity of, the membrane-associated vesicle subcellular fraction. These effects were mimicked by the D2 receptor agonist quinpirole and blocked by the D2 receptor antagonist eticlopride. Both MPD and quinpirole increased vesicular DA content. However, MPD increased, whereas quinpirole decreased, K(+)-stimulated DA release from striatal suspensions. Like MPD, the muscarinic receptor agonist, oxotremorine, increased K(+)-stimulated DA release. Both eticlopride and the muscarinic receptor antagonist scopolamine blocked MPD-induced increases in K(+)-stimulated DA release, whereas the N-methyl-d-aspartate receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) was without effect. This suggests that D2 receptors mediate both the MPD-induced redistribution of vesicles away from synaptosomal membranes and the MPD-induced up-regulation of vesicles remaining at the membrane. This results in a redistribution of DA within the striatum from the cytoplasm into vesicles, leading to increased DA release. However, D2 receptor activation alone is not sufficient to mediate the MPD-induced increases in striatal DA release because muscarinic receptor activation is also required. These novel findings provide insight into the mechanism of action of MPD, regulation of DA sequestration/release, and treatment of disorders affecting DA disposition, including attention-deficit hyperactivity disorder, substance abuse, and Parkinson's disease.     

In vivo dopamine clearance rate in rat striatum: regulation by extracellular dopamine concentration and dopamine transporter inhibitors

Dopamine transporter (DAT) inhibitors are expected to decrease dopamine (DA) clearance from the extracellular space of the brain. However, mazindol and cocaine have been reported to "anomalously" increase DA clearance rate. To better understand in vivo DAT activity both in the absence and presence of DAT inhibitors, clearance of exogenously applied DA was measured in dorsal striata of urethane-anesthetized rats using high-speed chronoamperometry. As higher amounts of DA were ejected, DA signal amplitudes, but not time courses, increased. Clearance rates increased until near maximal rates of 0.3 to 0.5 microM/s were attained. Provided baseline clearance rates were relatively low (< 0.1 microM/s), local application of either nomifensine or cocaine markedly increased exogenous DA signal amplitudes and time courses. Relative to the low baseline group, locally applied nomifensine decreased clearance rate when baseline clearance was high ( approximately 0.4 microM/s). However, even when baseline clearance rates were high, systemic injection of nomifensine, mazindol, GBR 12909, or benztropine increased DA signal amplitudes to a greater extent than time courses, consistent with the observed increases in clearance rates. In contrast, despite low baseline clearance rates, systemic injection of cocaine, WIN 35,428, or d-amphetamine preferentially increased DA signal time course, consistent with the observed decreases in clearance rates. Our results emphasize that as extracellular DA concentrations increase, DAT velocity increases to a maximum, partially explaining the ability of DAT inhibitors to increase DA clearance rates. However, by itself, kinetic activation is not sufficient to explain the ability of certain systemically administered DAT inhibitors to anomalously increase DA clearance.     

Increased dopamine receptor signaling and dopamine receptor-G protein coupling in denervated striatum

Chronic interruption of the nigrostriatal dopaminergic pathway leads to sensitized dopaminergic responses in striatum. We attempted to explore the mechanism(s) underlying this dopaminergic supersensitivity by assessing dopamine receptor signaling and receptor-G protein coupling in unilateral 6-hydroxydopamine-lesioned rats. Dopamine-stimulated adenylyl cyclase activity as well as dopamine-activated guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and [(3)H]palmitate incorporation by Galpha proteins were enhanced in tissues obtained from denervated striata without apparent changes in Galpha protein levels. Moreover, high-affinity binding sites of the D(1) dopamine receptor increased in lesioned compared with control striata without altering the expression level of the receptor. These denervation-mediated changes appear to correlate with the increase in D(1) dopamine receptor binding sites that co-immunoprecipitated with Galphas(olf)/q(11) proteins. In contrast, the total number of D(2) receptor binding sites was increased, yielding an increase in absolute number of high-affinity sites without significant changes in the proportion of high-affinity sites. Stimulation of the D(2) dopamine receptor enhanced coupling to Galphai protein; this was increased in the striata lesioned. The results provide an important molecular mechanism by which dopamine receptor-regulated signaling is enhanced following denervation of dopaminergic input to striatum. Although D(1) dopamine receptor supersensitivity appears to be mediated by enhanced coupling of the receptor to its G proteins, sensitization in the D(2) dopamine receptor system is mediated by increased D(2) receptor density and enhanced D(2) receptor-Gi protein coupling.     

Paraquat exposure reduces nicotinic receptor-evoked dopamine release in monkey striatum

Paraquat, an herbicide widely used in the agricultural industry, has been associated with lung, liver, and kidney toxicity in humans. In addition, it is linked to an increased risk of Parkinson's disease. For this reason, we had previously investigated the effects of paraquat in mice and showed that it influenced striatal nicotinic receptor (nAChR) expression but not nAChR-mediated dopaminergic function. Because nonhuman primates are evolutionarily closer to humans and may better model the effects of pesticide exposure in man, we examined the effects of paraquat on striatal nAChR function and expression in monkeys. Monkeys were administered saline or paraquat once weekly for 6 weeks, after which nAChR levels and receptor-evoked [(3)H]dopamine ([(3)H]DA) release were measured in the striatum. The functional studies showed that paraquat exposure attenuated dopamine (DA) release evoked by alpha3/alpha6beta2(*) (nAChR that is composed of the alpha3 or alpha6 subunits, and beta2; the asterisk indicates the possible presence of additional subunits) nAChRs, a subtype present only on striatal dopaminergic terminals, with no decline in release mediated by alpha4beta2(*) (nAChR containing alpha4 and beta2 subunits, but not alpha3 or alpha6) nAChRs, present on both DA terminals and striatal neurons. Paraquat treatment decreased alpha4beta2(*) but not alpha3/alpha6beta2(*) nAChR expression. The differential effects of paraquat on nAChR expression and receptor-evoked [(3)H]DA release emphasize the importance of evaluating changes in functional measures. The finding that paraquat treatment has a negative impact on striatal nAChR-mediated dopaminergic activity in monkeys but not mice indicates the need for determining the effects of pesticides in higher species.     

Strain differences in basal and cocaine-evoked dopamine dynamics in mouse striatum

In vivo microdialysis was used to characterize basal dopamine (DA) dynamics and cocaine-evoked DA levels in the striatum of 129/Sv-ter, C57BL/6J, DBA/2J, and Swiss-Webster mice. Basal dialysate levels of DA did not differ in the four strains tested. Similarly, the no net flux method of quantitative microdialysis revealed no difference in extracellular levels between strains. However, the in vivo extraction fraction of DA was significantly less in 129/Sv-ter (53%) mice compared with C57BL/6J (68%), DBA/2J (69%), and Swiss-Webster (67%) mice, indicating a lower rate of basal DA uptake in the 129/Sv-ter strain. Perfusion of K(+) (60 and 100 mM) through the microdialysis probe significantly increased dialysate DA levels and there was no difference between strains in the magnitude of this effect. The acute administration of cocaine (5-20 mg/kg i.p.) increased DA levels in the four strains tested. Cocaine-evoked DA levels (in nanomoles) were significantly greater in 129/Sv-ter compared with C57BL/6J, DBA/2J, or Swiss-Webster mice after administration of either 5, 10, or 20 mg/kg cocaine. However, the percentage increase in DA did not differ across strains. These data demonstrate that there are strain-related differences in basal DA dynamics in the striatum of the mouse. Basal DA uptake was reduced in striatum of 129/Sv-ter mice compared with C57BL/6J, DBA/2J, or Swiss-Webster mice. In addition, the response of DA levels to cocaine may be enhanced in 129/Sv-ter compared with C57BL/6J, DBA/2J, or Swiss-Webster mice.     

Withdrawal from repeated cocaine alters dopamine transporter protein turnover in the rat striatum

Several studies have shown that repeated cocaine administration, followed by withdrawal, alters dopamine transporter (DAT) levels in the rat. These changes must arise from changes in either transporter protein production or degradation, or both. Previously, our laboratory developed an approach to measure the synthesis rate, degradation rate constant, and half-life of DAT in the rat striatum and nucleus accumbens after administration of the irreversible dopamine transporter ligand, RTI-76 [3beta-(3-p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylethyl ester hydrochloride]. Transporter binding was measured with [(3)H]GBR12935 [1-(2-[diphenylmethoxy]ethyl)-4-[3-phenylpropyl]piperazine]. These initial studies showed that: 1) the half-life of the transporter was between 2 and 3 days in these two brain regions; 2) pretreatment with dopamine D1 and D2 receptor agonists and antagonists over several days differentially altered DAT half-lives in the striatum and nucleus accumbens; and 3) pretreatment with cocaine for several days increased the half-life of DAT by decreasing the degradation rate constant in both brain regions. In the present study, we determined that repeated pretreatment (10 days) with 20 mg/kg cocaine (i.p.) and a subsequent withdrawal period (10 days) alters the dopamine transporter turnover in the rat striatum, but not in the nucleus accumbens. Cocaine pretreatment and withdrawal reduced the half-life of the transporter protein from 2.1 days to 0.94 day in the striatum, but did not alter the half-life of 2.2 days in the nucleus accumbens. The results indicate the complex and long-lasting effects of cocaine administration on cellular processes. The mechanism(s) of these effects remains to be elucidated.     

Phenylephrine and norepinephrine increase dopamine transporter ligand binding in striatum.

PURPOSE: 8-(2-[18F]fluoroethyl)-2 beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane ([18F]FECNT) is a radiotracer for positron emission tomography (PET) used to trace the location and binding potential (BP) of the dopamine transporter (DAT). We tested the hypothesis that adrenergic receptor agonists increase the BP of [18F]FECNT for the DAT. PROCEDURES: Anesthetized monkeys were scanned twice to determine the ligand BP at baseline and during administration of phenylephrine (3-6 microg/kg/min) or norepinephrine (0.5-1 microg/kg/min). Standard immunohistochemistry and immunoblot analyses were performed in rats to determine if the quantity of DAT changed with phenylephrine. [18F]FECNT uptake in human embryonic kidney (HEK) cells, stably transfected with DAT cDNA, was measured by gamma scintillation counting during phenylephrine. RESULTS: The PET measured BP of [18F]FECNT increased by 50% and 45% during the phenylephrine and norepinephrine infusion, respectively. The immunohistochemistry and immunoblot analyses did not show a difference in total DAT. CONCLUSION: Adrenergic agonists increase the BP of [18F]FECNT in monkey striatum.     

alpha2C adrenoceptors inhibit adenylyl cyclase in mouse striatum: potential activation by dopamine.

alpha2C adrenoceptors occur in high density in the striatum, but the functional role of these receptors is uncertain. Mice with targeted inactivation of the alpha2C adrenoceptor gene (Adra2c-/-) and genetically related control mice expressing the wild-type alpha2C adrenoceptor (Adra2c+/+) were used to determine whether striatal alpha2C adrenoceptors modulate adenylyl cyclase activation. In striatal slices from Adra2c+/+ mice, the alpha2 adrenoceptor antagonist RX821002 facilitated forskolin-stimulated cyclic AMP accumulation in a concentration-dependent manner. In contrast, RX821002 had no effect on forskolin-stimulated cAMP accumulation in striatal slices from Adra2c-/- mice or in striatal slices from Adra2c+/+ mice treated with reserpine and alpha-methyl-rho-tyrosine to deplete monoamine neurotransmitters. Given the sparse innervation of the striatum by noradrenergic neurons, the possibility that dopamine can activate the mouse alpha2C adrenoceptor at physiologically relevant concentrations was investigated using normal rat kidney (NRK) cells transfected with the mouse alpha2A or alpha2C adrenoceptor cDNA (NRK-alpha2A or NRK-alpha2C cells). Inhibition of [3H]RX821002 binding by agonists in homogenates of transfected cells revealed an affinity of dopamine for alpha2C adrenoceptors that was higher than the affinity of norepinephrine for its cognate receptor, the alpha2A adrenoceptor. Both norepinephrine and dopamine inhibited forskolin-stimulated cAMP accumulation in intact NRK-alpha2C cells. In NRK-alpha2A cells, norepinephrine facilitated forskolin-stimulated cAMP accumulation, an effect not observed for dopamine. Together, these data demonstrate that the alpha2C adrenoceptor is negatively coupled to adenylyl cyclase and is tonically activated in mouse striatal slices. The endogenous activator of the striatal alpha2C adrenoceptor may be dopamine, as well as norepinephrine.     

Differences in pharmacological properties of dopamine release between the substantia nigra and striatum: an in vivo electrochemical study

The properties of dopamine (DA) release in the rat substantia nigra (SN) and striatum were investigated using high-speed chronoamperometric recordings in brain slices. In both brain regions, a 2-min bath superfusion with 30 mM KCl produced robust DA-like electrochemical signals, with the mean amplitude of the signal being >10-fold greater in the striatum than the SN. The reproducibility of the response was confirmed by a second stimulus (S2)/first-stimulus (S1) ratio of >0.8 in both regions. The bath application of tetrodotoxin significantly reduced the S2/S1 ratio in both the striatum and SN, implicating the requirement for voltage-sensitive sodium channels in the DA-release process. However, the application of cadmium chloride, a nonselective blocker of voltage-sensitive calcium channels, reduced the S2/S1 ratio only in the striatum and not within the SN. Moreover, removal of Ca2+ from the buffer did not significantly affect release within the SN, despite a >85% reduction in release within the striatum. In addition, although the D2 receptor antagonist sulpiride enhanced the S2/S1 ratio in the striatum, no effect of this agent was seen in the SN. Finally, the application of d-amphetamine produced DA-like electrochemical signals in both the striatum and SN. However, the amplitude of the d-amphetamine-evoked response, relative to the KCl-evoked release, was much smaller in the striatum than in the SN. Taken together, these data support the hypothesis that differences in the mechanism or mechanisms of release exist between somatodendritic and axonal elements within the nigrostriatal pathway.     

The influence of environmental temperature on the transient effects of methamphetamine on dopamine levels and dopamine release in rat striatum.

When male rats were injected four times (once every 2 hr) with 5 mg/kg methamphetamine (METH) at an environmental temperature of 23 degrees C, transient changes occurred in the levels of striatal dopamine (DA) and the regulation of striatal DA release. Striatal DA levels were minimally affected 1 day after METH treatment, but 3 days after METH treatment, striatal DA levels decreased to approximately 40% of control. DA levels returned to 70% of control 2 weeks after METH. Similarly, striatal tyrosine hydroxylase (TH) activity decreased to approximately 50% of control activity 3 days after METH treatment at 23 degrees C, but did not differ from controls at 1 or 14 days after METH treatment. No changes in striatal DA levels were observed in rats treated with four doses of 5 mg/kg METH at an environmental temperature of 4 degrees C. Striatal DA levels decreased modestly to approximately 70% of controls 3 days after treatment with four doses of 10 mg/kg METH at 4 degrees C, but DA levels returned to control levels 14 days after METH treatment. Furthermore, striatal TH activity was not affected by 10 mg/kg METH at 4 degrees C. Thus, a cold environmental temperature (4 degrees C) reduced the effects of METH on striatal DA levels and striatal TH activity. Changes in the presynaptic regulation of DA release after either 5 mg/kg (23 degrees C) or 10 mg/kg (4 degrees C) METH treatment were determined in vitro using striatal slices.(ABSTRACT TRUNCATED AT 250 WORDS)     

D-2 dopamine receptors in aging mouse striatum: determination of high- and low-affinity agonist binding sites

Striatal D-2 dopamine receptors in homogenates from aged male C57BL/6J mice were examined for high and low-affinity agonist binding. High-affinity dopamine binding requires the ternary complex of the D-2 receptor and a guanine nucleotide binding regulatory protein (N). Thus, changes in the interaction of D-2 and N could contribute to previously reported changes in agonist binding in aged rodents and humans. Qualitative experiments indicated no age-change in the ability of guanine nucleotides to reduce the apparent potency of dopamine at D-2 receptors. Also, no age differences were observed in the ability of guanine nucleotides to decrease N-[3H]propylnorapomorphine binding, suggesting that the ability of guanine nucleotides to dissociate D-2 and N was similar with age. Quantitative determination of the high- (RH) and low-affinity (RL) agonist binding components of striatal D-2 dopamine receptors in the absence of guanine nucleotides indicated differences in the density of RH, and the percentage of D-2 receptors measured as RH, between the ages of 3 and 12 months. No changes in RH or percentage of RH occurred after midlife. In contrast, the total D-2 receptor population, [3H]spiperone maximum binding, declined progressively from 3 to 24 months. Age-changes were restricted to D-2 receptor density; the dissociation constants for agonist and antagonist binding were similar across age. The data suggest that age-changes in striatal D-2 dopamine receptors can occur in the density of the D-2 receptor and in the mechanism that confers the property of high-affinity agonist binding upon the D-2 receptor.     

Effect of alpha-methyldopa on dopamine synthesis and release in rat striatum in vitro.

The effect of alpha-methyldopa and alpha-methyldopamine (alpha-MDA) on the rate of hydroxylation of radioactive tyrosine was studied in striatal slices from rat brain. This was done by measuring the formation of 3-H-H2O as well as the accumulation of 3-H-dopamine (3-H-DA) from L-3, 5-3-H-tyrosine. alpha-Methyldopa, at tissue concentrations similar to those found in vivo after systemic administration, produced a decrease in both 3-H-H2O and 3-H-DA. The marked decrease (91thyldopa injection, also inhibited 3-H-H2O formation. The inhibitory effect of alpha-methyldopa on 3-H-H2O formation was not reduced by the addition of brocresine, which prevents the formation of alpha-MDA. The effects of alpha-methyldopa and alpha-MDA on the release of 3-H-DA that had been taken up into brain slices, was also studied. Although alpha-methyldopa, 1000 muM, did not increase the release of 3H-DA from tissue, alpha-MDA did. However, the latter was more potent in inhibiting 3-H-H2O formation from 3-H-tyrosine than in releasing 3-H-DA. These results, as well as the close similarity between the percent reduction of 3-H-H2O formation and tissue 3-H-DA levels, suggest that alpha-methyldopa decreases tissue levels of dopamine by inhibiting tyrosine hydroxylase activity in DA neurons.     

运动疲劳对大鼠纹状体单胺类递质含量及多巴胺受体表达的影响

目的:通过测定纹状体单胺类神经递质及多巴胺受体表达量在运动疲劳前后的变化,探讨纹状体多巴胺系统信号通路在运动疲劳后中枢调控中的作用。方法:雄性SD大鼠随机分为对照组和实验组,每组12只大鼠。以10天递增负荷游泳运动建立大鼠运动疲劳模型。采用高效液相色谱法及实时荧光定量PCR技术,测定纹状体区5-羟色胺、去甲肾上腺素、5-羟基吲哚乙酸、肾上腺素、多巴胺、3,4-二羟基苯乙酸、高香草酸及多巴胺受体的表达量。结果:实验组纹状体5-羟色胺含量较对照组增加14.23%(P0.05),多巴胺含量增加27.35%(P0.01);运动疲劳后纹状体多巴胺Ⅰ型受体没有出现显著浓度变化,而多巴胺Ⅱ型受体浓度显著升高(P0.01)。结论:运动疲劳使纹状体区5-羟色胺、多巴胺与多巴胺Ⅱ型受体含量增高,多巴胺可能与Ⅱ型受体结合激活间接通路影响锥体外系对运动功能的调节。     

Effects of delta and mu opiopeptides on the turnover and release of dopamine in rat striatum

Two mu and two delta opiopeptides were administered intracisternally and morphine was administered systemically to rats. The level of dopamine (DA) and its catabolites, homovanillic acid, dihydroxyphenylacetic acid and 3-methoxytramine were measured by high-pressure liquid chromatography with electrical detector in rat striatum to determine: 1) whether opioids alter the release of DA from striatal neuron (which would be indicated by changes in the level of 3-methoxytramine, the extraneuronal catabolite) and 2) whether delta or mu ligands have a greater effect on DA turnover. We found that the levels of 3-methoxytramine did not rise in response to the administration of any opiopeptide or morphine. However, mu opiopeptides produced a small but significant decrease in these levels, indicating that there was no increase, but instead a slight decrease in DA release. The delta opiopeptides produced larger increases in homovanillic acid and dihydroxyphenylacetic acid than the mu ligands, indicating that delta ligands are more effective on an equidose basis in increasing the turnover of striatal DA. The opiopeptides were also tested for pharmacological activity at the same dose (3 micrograms/rat). All four peptides were effective in reducing locomotor activity and producing analgesia. One peptide, Tyr-d-Ala-Gly-N-Mephe-Met-O-ol, also produced catalepsy. There was no segregation of these two behavioral responses according to ligand specificity. Morphine acted like a delta ligand in affecting DA turnover.     

Effect of chronic haloperidol treatment on stimulated synaptic overflow of dopamine in the rat striatum.

In vivo voltammetry was used to assess the change in stimulated striatal dopamine overflow in response to various treatments with the dopamine receptor antagonist haloperidol. Dopamine overflow was induced with stimulating electrodes implanted in the medial forebrain bundle of anesthetized rats while dopamine concentrations were monitored with Nafion-coated, carbon-fiber microelectrodes implanted in the striatum. An acute challenge of haloperidol (0.5 mg kg-1, i.p.) given to naive animals caused stimulated overflow to increase at all stimulation frequencies (10-60 Hz), with the greatest change, 5-fold, occurring at 30 Hz. These results have been compared to those obtained in a different group of rats given daily injections of haloperidol (0.5 mg kg-1, s.c.) for 30 consecutive days. On the 30th day, dopamine striatal tissue levels and uptake kinetics were not altered by this treatment, but 3,4-dihydroxyphenylacetic acid tissue levels were elevated almost 2-fold. A challenge dose of haloperidol (0.5 mg kg-1, i.p.) administered to the animals treated with chronic haloperidol did not elicit a change in stimulated dopamine overflow. In two other groups, rats were withdrawn from 30-day haloperidol treatment for 3 days or 14 days before experimentation. Stimulated dopamine overflow concentrations in both groups were not significantly different from naive animals. When the withdrawn animals were given a haloperidol challenge (0.5 mg kg-1, i.p.), 15- and 12-fold increases in overflow for 3-day and 14-day withdrawal groups, respectively, were observed at a stimulation frequency of 30 Hz. Thus, chronic treatment with haloperidol induces long-lasting effects on the capacity of dopamine receptors to modulate dopamine release.     

Frequency-dependent muscarinic receptor modulation of acetylcholine and dopamine release from rabbit striatum

The effects of muscarinic receptor activation on the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) or [3H]ACh were investigated in rabbit striatal slices. Release was measured in the presence of 10 microM hemicholinium and 1 microM sulpiride to block choline uptake and prevent the effects of released DA on DA receptors modulating release. Stimulation (120 pulses, 20 mA, 2 msec) at 0.3, 3 and 10 Hz produced (3H or 14C) ACh release that sharply declined with increasing stimulation frequency. A flat frequency-release curve was obtained for DA. Oxotremorine (OXO), a direct muscarinic agonist (1-100 microM), produced a concentration-dependent inhibition of ACh release, inversely related to stimulation frequency, at a fixed number of pulses (120). When the number of pulses was modified to produce similar amounts of ACh release (20 pulses at 0.1 Hz, 39 pulses at 0.3 Hz, 120 pulses at 3 Hz and 350 pulses at 10 Hz), much greater inhibition of ACh release by OXO (0.3 and 3 microM) was obtained with lower frequencies and lower number of pulses. Physostigmine, an acetylcholinesterase inhibitor, decreased ACh release with an inverse relationship to stimulation frequency. Atropine (1 microM), a selective muscarinic antagonist, enhanced the release of ACh more at 10 Hz than at 0.3 and 3 Hz and completely antagonized the effects of OXO (10 microM) and physostigmine (1 microM) at all three stimulation frequencies. OXO (3 and 10 microM) enhanced DA release at 3 Hz. Physostigmine (1 microM) and atropine (1 microM) had no effect on DA release.(ABSTRACT TRUNCATED AT 250 WORDS)