Dopaminergic regulation of striatal cholinergic interneurons: an in vivo microdialysis study

Summary In vivo microdialysis was used to study the putative inhibitory effects of dopamine on cholinergic interneurons in the striatum of conscious rats. The dopamine receptor agonists apomorphine (0.3 and 3 mg/kg, s.c.) and (±)N-0437 (1.4 mg/kg, s.c.) decreased interstitial concentrations of acetylcholine while increasing those of choline. In contrast, the dopamine receptor antagonists haloperidol (0.1 and 1 mg/kg, i.p.) and (±)sulpiride (20 mg/kg, i.p.) enhanced striatal acetylcholine output but had little effect on choline. Previously, a lack of effect of these drugs on striatal acetylcholine was reported. The main methodological difference between these studies was that the calcium concentration of the microdialysis perfusion solution was 3.4 mM in the former study versus 1.2 mM in the present experiments. The results of this study reemphasize the importance of the calcium concentration in determining the effects of drugs on central neurotransmitter release, and confirm a role of dopamine in the regulation of striatal cholinergic interneurons.Send offprint requests to G. Damsma at the present address     

Norepinephrine release in amygdala of rats during chronic nicotine self-administration: an in vivo microdialysis study

The essential role of the amygdala in learning and memory, including cue-associated learning, is influenced by local release of norepinephrine (NE). The current study investigated changes in amygdaloid NE secretion in rats learning to self-administer nicotine in an unlimited access model (23 h/day). In vivo microdialysis of NE was performed for 9 h intervals during three phases of nicotine self-administration: acquisition (day 1); early maintenance, when self-administration rates first stabilized (day 8.4+/-0.7); and later, during fully stable maintenance (day 17.6+/-1.0). On day 1, a greater number of self-administration episodes (SAEs) were associated with elevated NE levels in rats bar-pressing for nicotine (88% vs. 39% with saline). By early maintenance, such episodes increased threefold and overall NE levels were greater. During later maintenance, however, bar-pressing behavior was similar and NE was elevated by the first SAE of the day, but total daily NE levels were no longer elevated. In all the three phases, the enhanced NE release during the first daily SAE did not occur in the last SAE 9 h later. Thus, in an animal model of unlimited nicotine self-administration that approximates the human pattern of nicotine consumption via smoking, the amygdaloid NE response to nicotine diminishes over each day and with the stabilization of self-administration. The decline of amygdaloid NE secretion after long-term nicotine self-administration likely reflects desensitization to the pharmacological effects of nicotine. In addition, amygdaloid NE release, which enhances the consolidation of amygdala-dependent memory, may no longer be necessary once self-administration behavior has been established.     

Chronic morphine induces long-lasting changes in acetylcholine release in rat nucleus accumbens core and shell: an in vivo microdialysis study

 Previously, only in vitro studies have shown that chronic administration of morphine provokes long-lasting enhanced activity of accumbal cholinergic neurons, which may contribute to the behavioural sensitization, positive reinforcement and aversive effects associated with enhanced drug-seeking. The present study was aimed at clarifying whether these adaptive changes would also be supported by in vivo microdialysis measurements in freely moving rats, distinguishing between the accumbal substructures shell and core, and observing behavioural changes simultaneously. Acute administration of morphine dose-dependently decreased acetylcholine (ACh) release in the nucleus accumbens (NAc), with 10 mg/kg SC being most effective, 5 mg/kg ineffective. On day 5 of spontaneous abstinence from chronic morphine treatment (10–40 mg/kg morphine dose once daily for 5 days), when withdrawal symptoms were still present, even a lower morphine dose (5 mg/kg) was effective in decreasing ACh release in the NAc. During the later phase of abstinence, when no withdrawal symptoms were detectable, the opposite effect, i.e. an increase of ACh release was found. This later effect may represent a long-lasting neuroadaptive effect of morphine. These adaptive effects seemed to be more prominent in the NAc shell. Concurrent with these changes in ACh release, morphine challenges produced marked behavioural stereotypes, possibly indicating behavioural sensitization. Received: 7 April 1998 / Final version: 23 July 1998     

Lack of effect of morphine in reducing the release of labelled acetylcholine from brain slices stimulated electrically

Morphine in low concentration (0.3–30 μM) failed to reduce the evoked release of [³H]-ACh from slices of cortex, hippocampus or striatum stimulated at low or high frequencies. Reduced ACh release observed in vivo following morphine administration is probably an indirect action of morphine on the activity of cholinergic neurons.     

Effects of acute ethanol on opioid peptide release in the central amygdala: an in vivo microdialysis study

Rationale  There is experimental evidence that indicates that the endogenous opioid system of the central nucleus of the amygdala (CeA) may mediate some of the reinforcing effects of ethanol. However, the precise interactions of ethanol with the endogenous opioid system at the level of the CeA have not been investigated. Objectives  The aim of the current study was to investigate the hypothesis that acute systemic ethanol administration will increase the release of endogenous opioid peptides at the level of the CeA in a time- and dose-dependent manner. Materials and methods  Rats were implanted with a unilateral guide cannula to aim microdialysis probes at the CeA. Intraperitoneal injections of saline and various doses of ethanol (0.8, 1.6, 2.0, 2.4, and 2.8 g ethanol/kg body weight) were administered to the rats. Dialysate samples were collected at 30-min intervals at distinct time points prior to and following treatment. Radioimmunoassays specific for β-endorphin, met-enkephalin, and dynorphin A1–8 were used to determine the effect of ethanol on the content of the opioid peptides in the dialysate. Results  We report that the 2.8-g/kg dose of ethanol induced a long-lasting increase in β-endorphin release from 60 min onwards following administration and, later, an ongoing increase in dynorphin A1–8 release. None of the ethanol doses tested elicited significant changes in dialysate met-enkephalin content compared to the saline treatment. Conclusions  Acute systemic ethanol administration induced a dose- and time-dependent increase in β-endorphin and dynorphin A1–8 release at the level of the CeA, which may be involved in ethanol consumption.     

Functional regulation by dopamine receptors of serotonin release from the rat hippocampus: in vivo microdialysis study

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat hippocampus was investigated by use of in vivo microdialysis. Dialysate 5-HT levels were reduced by co-perfusion of 10 M tetrodotoxin (TTX) and were elicited by K⁺ (60 and 120 mM) stimulation in a concentration-dependent manner. Local perfusion (10 M) and peripheral administration (20 mg/kg, i.p.) of fluoxetine produced increases in 5-HT levels. These results indicate that the spontaneous 5-HT levels in the rat hippocampus can be used as indices of neuronal origin from the serotonergic nerve terminals. The nonselective dopamine (DA) receptor agonist apomorphine (1, 10 and 100 M), when perfused through the probe over a period of 40 min, increased 5-HT release in a concentration-dependent manner. Apomorphine-induced (100 M) increases in 5-HT release was abolished by pretreatment with the selective D₂ receptor antagonist, S(–)-sulphide (1 and 10 M), but not prevented by pretreatment with the selective D₁ receptor antagonist, R(+)-SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) (1 M). S(–)-Sulpiride and R(+)-SCH-23390 by themselves did not alter the spontaneous 5-HT levels. The 5-HT release was elevated by perfusion of the selective DA reuptake inhibitor GBR 12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine)(1, 10 and 100 M), indicating the possibility of not only exogenous but also endogenous DA-mediated facilitatory effects on 5-HT release in vivo. The 5-HT release was also elevated by perfused (±)-PPHT ((±)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin)(1, 10 and 100 M), the selective D₂ receptor agonist, in a concentration-dependent manner. On the other hand, (±)-PPHT (100 M) failed to increase 5-HT release in catecholamine (CA)-lesioned rats pretreated with 6-hydroxydopamine (6-OHDA)(200 g/rat, i.c.v.). The (±)-PPHT-induced (100 M) increase in 5-HT release was prevented not only by pretreatment with 10 M S(–)-sulphide but also by pretreatment with the ₂-adrenoceptor antagonist idazoxan (10 M). These findings suggest that the functional regulation of 5-HT release via D₂ receptors exists in the rat hippocampus. Furthermore our results indicate that the facilitatory effect of 5-HT release via D₂ receptors may be mediated indirectly by noradrenergic neurons, but not mediated directly through D₂ receptors located on serotonergic nerve terminals.     

Olanzapine increases in vivo dopamine and norepinephrine release in rat prefrontal cortex, nucleus accumbens and striatum

The in vivo effects of olanzapine on the extracellular monoamine levels in rat prefrontal cortex (Pfc), nucleus accumbens (Acb) and striatum (Cpu) were investigated by means of microdialysis. Sequential doses of olanzapine at 0.5, 3 and 10 mg/kg (SC) dose-dependently increased the extracellular dopamine (DA) and norepinephrine (NE) levels in all three brain areas. The increases appeared 30 min after olanzapine administration, reached peaks around 60–90 min and lasted for at least 2 h. The highest DA increases in the Acb and Cpu were induced by olanzapine at 3 mg/kg but at 10 mg/kg in the Pfc. The peak DA increase in the Pfc (421% ± 46 of the baseline) was significantly larger than those in the Acb (287% ± 24) and Cpu (278% ± 28). Similarly, the highest NE increase in the Pfc (414%±40) induced by 10 mg/kg olanzapine was larger than those in the Acb (233% ± 39) and Cpu (223% ± 24). The DA and NE increases in the Pfc induced by olanzapine at 3 and 10 mg/kg (SC) were slightly larger than those induced by clozapine at the same doses. In contrast, haloperidol (0.5 and 2 mg/kg, SC) did not change Pfc DA and NE levels. Extracellular levels of a DA metabolite, DOPAC, and tissue concentrations of a released DA metabolite, 3-methoxytyramine, were also increased by olanzapine, consistent with enhanced DA release. However, olanzapine at the three sequential doses did not alter the extracellular levels of either 5-HT or its metabolite, 5-HIAA, in any of the three brain areas. In conclusion, the present studies demonstrate that in the case of sequential dosing olanzapine more effectively enhances DA and NE release in the Pfc than in the subcortical areas, which may have an impact on its atypical antipsychotic actions. Received: 7 May 1997/Final version: 15 September 1997     

Local modulation of the 5-HT release in the dorsal striatum of the rat: an in vivo microdialysis study

Using in vivo microdialysis in freely moving rats, we examined the involvement of major striatal transmitters on the local modulation of the 5-HT release. Tetrodotoxin reduced the striatal 5-HT output to 15–20% of baseline. The selective 5-HT_1B receptor agonist CP 93129 (50 μM) reduced (50%) and the 5-HT_2A/2C receptor agonist DOI (1–100 μM) increased (220%) the 5-HT output. Neither GABA nor baclofen (100 nM–100 μM) altered the 5-HT output. The glutamate reuptake inhibitor -trans-PDC (1–4 mM) raised 5-HT to 280% of baseline. This effect was not antagonized by the NMDA receptor antagonist MK-801 (0.5 mg/kg i.p.). Local MK-801 (10–100 μM) did not significantly alter the 5-HT output. Finally, neither carbachol (10–100 μM) nor quipirole (10 μM–1 mM) affected 5-HT. These data suggest that the striatal 5-HT release is influenced by local serotonergic and glutamatergic (but not GABAergic) inputs.     

Acute effects of morphine on dopamine synthesis and release and tyrosine metabolism in the rat striatum

The effect of the acute administration of morphine (60 mg/kg) on the metabolism of dopamine (DA) and tyrosine in the rat striatum were investigated using in vivo and in vitro methods. 30 min after morphine injection, the initial accumulation of ³H-DA in tissues seen after the i.v. injection of L-3,5-H-tyrosine was enhanced as well as the accumulation of ³HH₂O and ³H-DA in tissues and medium of striatal slices incubated with the ³H precursor. These results and the estimation of the conversion index of tyrosine into DA (³H-DA/tyrosine specific activity) indicated that morphine stimulated DA synthesis. This effect was not seen 2 hr later. The enhanced DA synthesis was associated with a parallel increase in the release of newly synthesized ³H-DA as indicated by the greater accumulation of ³H-DA in incubating medium of slices of morphine-pretreated rats. Therefore the increased endogenous levels of DA seen after morphine are related to the increased synthesis of the transmitter. Apart from its effect on dopaminergic neuron activity, morphine also induced marked changes in tyrosine metabolism, particularly at 2 or 2.5 hr after its injection. Tyrosine levels in plasma and in striatum were about 160% of the control levels at this time: these changes in the size of the precursor pool may explain the enhanced accumulation of ³H-tyrosine seen in plasma and striatum 10 min after the ³H-amino acid injection. Marked changes in endogenous levels of DA and tyrosine occurred in the striatum of control animals from 9.00 to 12.00 am. Acute treatment with morphine significantly affected these diurnal variations.     

Nitric oxide modulates the release of acetylcholine in the ventral striatum of the freely moving rat

The influence of nitric oxide on acetylcholine release in the ventral striatum was investigated by the push-pull superfusion technique in the conscious, freely moving rat. Superfusion with the nitric oxide donors S-nitroso-N-acetylpenicillamine or with 3-morpholino-sydnonimine caused a pronounced increase in striatal acetylcholine release. This effect was prevented by superfusion with tetrodotoxin. Pre-superfusion with the guanylyl cyclase inhibitor methylene blue abolished the effect of 3-morpholino-sydnonimine. Superfusion of the ventral striatum with the guanylyl cyclase inhibitor LY83583 decreased acetylcholine release by 60% of basal release, whereas the less specific guanylyl cyclase inhibitor methylene blue was ineffective in this respect. Superfusion of the ventral striatum with inhibitors of nitric oxide synthase also led to different effects on basal acetylcholine release. Superfusion with L-N^G-methylarginine did not influence basal acetylcholine release, whereas superfusion with L-N^G-nitroarginine or with L-N^G-nitroarginine methyl ester led to a substantial decrease in acetylcholine output, the latter compound being more effective. The effect of L-N^G-nitroarginine was abolished by simultaneous superfusion with L-arginine.The effects of NO donors and of LY83583 suggest that NO increases acetylcholine release, probably by a cGMP-dependent mechanism. The effectiveness of nitric oxide synthase inhibitors shows that the activity of striatal neurons is under the permanent influence of nitric oxide, that leads, via a direct or indirect mechanism, to continuous enhancement of acetylcholine release.In conclusion, our findings suggest that NO synthesized in the ventral striatum acts as an intercellular messenger which modulates acetylcholine release.     

A partial noradrenergic lesion induced by DSP-4 increases extracellular noradrenaline concentration in rat frontal cortex: a microdialysis study in vivo

The effect of systemic administration of the selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) on noradrenaline efflux in the frontal cortex was studied in freely-moving rats using microdialysis in vivo. Five days after treatment with DSP-4 (40 mg/kg IP), the noradrenaline content of the frontal cortex was reduced by 75%. Yet, noradrenaline efflux in the frontal cortex was nearly two-fold greater in DSP-4 treated rats than in saline-injected controls. Local infusion of the noradrenaline-selective uptake blocker, desipramine (5 μM), via the microdialysis probe, increased noradrenaline efflux in rats from both groups. Perfusion of Ringer’s solution, containing 80 mM K⁺, also increased noradrenaline efflux in both groups, but the increase after DSP-4 pretreatment was greater than in the controls. In contrast, removal of Ca²⁺ from the infusion medium reduced noradrenaline efflux in both treatment groups. These results indicate that, at this dose, DSP-4 increases the extracellular concentration of noradrenaline in rat frontal cortex despite causing a partial lesion of noradrenergic neurones. This is due to an increase in the release of noradrenaline, although reduced clearance is also likely. These data challenge the assumption that depletion of noradrenaline content after treatment with DSP-4 invariably translates into diminished noradrenergic transmission. Received: 4 September 1997/Final version: 7 September 1997     

Effect of l-tetrahydropalmatine on dopamine release and metabolism in the rat striatum

In vivo voltammetric measurements of striatal extracellular DOPAC concentrations and of striatal DA release in combination with post-mortem analysis of striatal catechols were performed in the rat to study the effects on the nigro-striatal neurons of a Chinese neuroleptic, l-tetrahydropalmatine (l-THP), which is known to block post-synaptic dopaminergic receptors. l-THP injected at doses that cause sedation in rats and mice (5–10 mg/kg) induced a marked increase in post-mortem DOPAC levels (+250%), while no significant effect was observed on post-mortem DA levels. The extracellular DOPAC concentration was increased to 155±9% of the control value after l-THP administration (1 mg/kg, IP). Further, an injection of l-THP (1 mg/kg, IP) induced an increase in extracellular DA concentration (220% of the basal value), reversed the decrease in extracellular DA concentration induced by apomorphine (0.05 mg/kg, SC) and enhanced the effects of the electrical stimulation of the nigro-striatal pathway. These results confirm that l-THP acts on the nigro-striatal neurons as a dopaminergic antagonist that can block both post-and presynaptic receptors.Visiting scholar from Shanghai, Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 200031, China     

Activation of somatostatin receptors in the globus pallidus increases rat locomotor activity and dopamine release in the striatum

Rationale  Somatostatin and its receptors have been localized in brain nuclei implicated in motor control, such as the striatum, nucleus accumbens, ventral pallidum, and globus pallidus (GP). Objectives  The objective of this study was to investigate the role of somatostatin receptors (sst_1,2,4) in the GP on dopamine (DA)-mediated behaviors, such as locomotor activity, and to examine the GP–striatum circuitry by correlating the effect of somatostatin in the GP with the release of DA in the striatum. Materials and methods  Animals received saline, somatostatin (60, 120, 240 ng/0.5 μl per side) or the following selective ligands: L-797,591 (sst₁ analog, 60, 120, 240 ng/0.5 μl per side), L-779,976 (sst₂ analog, 120, 240, 480 ng/0.5 μl per side), L-803,087 (sst₄ analog; 120, 240, 480 ng/0.5 μl per side), L-796,778 (sst₃ analog, 240 ng/0.5 μl per side), SRA-880 (sst₁ selective antagonist + somatostatin, 120 ng/0.5 μl per side), CYN154806 (sst₂ selective antagonist + somatostatin, 120 ng/0.5 μl per side) bilaterally in the GP of the rat. Locomotor activity was measured for 60 min. The effect of somatostatin, administered intrapallidally, on the extracellular concentrations of DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid in the striatum was also studied in the behaving rat using in vivo microdialysis methodology. Results  Somatostatin increased the locomotor activity of the rat in a dose-dependent manner. This effect was mediated by activation of the sst₁, sst₂, and sst₄ receptors. Selective sst agonists increased locomotor activity in a statistical significant manner, while selective sst₁ and sst₂ antagonists reversed the somatostatin-mediated locomotor activity to control levels. DA levels increased in the striatum after intrapallidal infusion of somatostatin (240 ng/side). Conclusions  These data provide behavioral and neurochemical evidence of the functional role of somatostatin receptors in the GP–striatum circuitry. We announce with great sorrow that our colleague and friend Prof. Christina Spyraki passed away on September 2, 2006.     

Effects of acute chlorpyrifos exposure on in vivo acetylcholine accumulation in rat striatum

This study examined the acute effects of chlorpyrifos (CPF) on cholinesterase inhibition and acetylcholine levels in the striatum of freely moving rats using in vivo microdialysis. Adult, male Sprague-Dawley rats were treated with vehicle (peanut oil, 2 ml/kg) or CPF (84, 156 or 279 mg/kg, sc) and functional signs of toxicity, body weight and motor activity recorded. Microdialysis was conducted at 1, 4 and 7 days after CPF exposure for measurement of acetylcholine levels in striatum. Rats were then sacrificed and the contralateral striatum and diaphragm were collected for biochemical measurements. Few overt signs of cholinergic toxicity were noted in any rats. Body weight gain was significantly affected in the high-dose (279 mg/kg) group only, while motor activity (nocturnal rearing) was significantly reduced in all CPF-treated groups at one day (84 mg/kg) or from 1-4 days (156 and 279 mg/kg) after dosing. Cholinesterase activities in both diaphragm and striatum were markedly inhibited (50-92%) in a time-dependent manner, but there were relatively minimal dose-related changes. In contrast, time- and dose-dependent changes in striatal acetylcholine levels were noted, with significantly higher levels noted in the high-dose group compared to other groups. Maximal increases in striatal acetylcholine levels were observed at 4-7 days after dosing (84 mg/kg, 7-9-fold; 156 mg/kg, 10-13-fold; 279 mg/kg, 35-57-fold). Substantially higher acetylcholine levels were noted when an exogenous cholinesterase inhibitor was included in the perfusion buffer, but CPF treatment-related differences were substantially lower in magnitude under those conditions. The results suggest that marked differences in acetylcholine accumulation can occur with dosages of CPF eliciting relatively similar degrees of cholinesterase inhibition. Furthermore, the minimal expression of classic signs of cholinergic toxicity in the presence of extensive brain acetylcholine accumulation suggests that some compensatory process(es) downstream from synaptic neurotransmitter accumulation limits the expression of toxicity following acute CPF exposure.     

Effects of morphine on dopamine metabolism in rat striatum and limbic structures in relation to the activity of dopaminergic neurones

Summary Administration of morphine results in efflux of dopamine provided that the nerve impulse flow of the dopaminergic neurones is impaired. In the present study we investigated whether the morphine-induced increase in dopamine metabolite levels is related to impulse flow in a similar way. Pretreatment with gamma-butyrolactone to impair nerve impulse flow, abolished the effect of morphine on the concentrations of dihydroxyphenylacetic acid and homovanillic acid. Pretreatment with apomorphine had a similar effect, as well as combined pretreatment with gamma-butyrolactone and apomorphine. Since gamma-butyrolactone and apomorphine both reduce nerve impulse flow, but gamma-butyrolactone increases while apomorphine decreases dopamine biosynthesis, it would appear that the antagonism of morphine-induced increases in dopamine metabolites is due to the common property of impulse flow reduction. It was also shown, however, that pretreatment with -methyl-paratyrosine, which inhibits dopamine biosynthesis, resulted in antagonism of morphine's effect on dopamine metabolite levels. It is concluded, therefore, that morphine-induced dopamine efflux is observed under conditions when no effect on dopamine metabolism is observed, and vice versa. Three effects of morphine on dopaminergic neurones can be distinguished: an increase in impulse flow in nigrostriatal dopaminergic neurones, increased dopamine biosynthesis and catabolism, and efflux of dopamine. The first effect probably is effected in the cell body areas, while the latter two effects may be produced at the level of the nerve terminals. It is discussed whether a single cellular effect of morphine on dopaminergic nerve terminals may underly the effect on dopamine efflux and on dopamine metabolism.     

吗啡对小鼠纹状体细胞外尿嘧啶释放的影响及机制初探

目的:研究吗啡对小鼠纹状体细胞外尿嘧啶释放的影响并探讨相关机制。方法:采用脑内微透析技术结合高效液相紫外检测器检测尿嘧啶含量,紫外分光光度法检测纹状体总RNA含量。结果:与空白对照组相比,吗啡(10,20mg.kg-1,ip)可显著降低小鼠纹状体尿嘧啶含量且呈现剂量依赖性;盐酸纳洛酮(4mg.kg-1,ip)自身对小鼠纹状体尿嘧啶的释放无影响,却可以显著回升尿嘧啶含量至基础水平。吗啡(10,20mg.kg-1,ip)对小鼠纹状体细胞内RNA含量呈下调作用,与对尿嘧啶含量的下调结果一致,呈现剂量依赖性;盐酸纳洛酮(4mg.kg-1,ip)对小鼠纹状体总RNA含量无明显影响但能拮抗吗啡对总RNA含量的下调作用。结论:吗啡引起小鼠纹状体尿嘧啶释放降低的作用可能与μ受体相关且与细胞内总RNA的含量变化存在相关性。     

Alterations in striatal acetylcholine overflow by cocaine,morphine, and MK-801: relationship to locomotor output

The activity of cholinergic interneurons in the striatum appears to be modulated by a variety of different systems including dopamine, opiate, and glutamate. The purpose of this study was to characterize the effects of drugs known to act on these three systems (i.e., cocaine, morphine, and MK-801) on striatal ACh overflow with microdialysis procedures, and to determine if alterations in ACh function induced by these agents are related to changes in locomotor activity. Cocaine was found to increase striatal ACh following intraperitoneal injections of 20 and 40 mg/kg, but not 10 mg/kg. The increases in locomotor activity induced by cocaine appeared to be dose dependent, while the effects on striatal ACh were not. Injections of 0.1 mg/kg MK-801 (a non-competitive NMDA receptor antagonist) produced dramatic increases in locomotor activity while decreasing striatal ACh overflow. A lower dose (0.03 mg/kg) of MK-801 failed to alter locomotor activity or striatal ACh. Morphine produced an apparent dose-dependent elevation in striatal ACh while only the lowest dose (5 mg/kg) increased locomotor activity. These appears to be no relationship between alterations in striatal ACh and locomotor output following systemic administration of these psychoactive agents.     

Dopaminergic-cholinergic interactions in the striatum: the critical significance of calcium concentrations in brain microdialysis

Summary Brain microdialysis experiments were performed to assess the effects of calcium (1.2 mmol/l and 3.4 mmol/l) in the perfusio solution on a variety of pharmacological treatments known to affect the release of dopamine (DA) and/or acetylcholine (ACh). Intrastriatal infusion of the muscarinic receptor agonist oxotremorine (100 M), the selective dopamine D-2 receptor agonist (–)-N-0437 (1 M), and the indirect DA agonists (+)amphetamine (10 M) and nomifensine (1 M) via the dialysis probe did not affect the overflow of ACh when the perfusion fluid contained 3.4 mmol/l calcium. In contrast, these compounds produced pronounced decreases in the overflow of ACh at 1.2 mmol/l calcium. Intrastriatal infusion of the muscarinic receptor antagonist atropine (1 M) increased the output of ACh both at 1.2 mmol/l and 3.4 mmol/1 calcium. The selective DA D-2 receptor antagonist (–)-sulpiride (1 M) did not affect the overflow of ACh at either calcium concentration. Infusion of oxotremorine and atropine had no effect on the overflow of DA at either 1.2 mmol/l or 3.4 mmol/l calcium. (–)-N-0437 decreased and (–)-sulpirde increased DA overflow, both effects being independent of the calcium concentration in the perfusion fluid. Nomifensine and (–)amphetamine caused relatively (but not absolutely) larger increases in the overflow of DA at 1.2 mmol/1 calcium. These findings emphasize the critical importance of the calcium concentration of the perfusion fluid in determining the nature of pharmacological responses in microdialysis experiments, and demonstrate that locally applied dopaminergic drugs can modulate striatal cholinergic function.Part of these results have been presented in Nottingham at the fourth meeting for Electrochemical detection, HPLC and in vivo monitoring in the biosciencesSend offprint requests to G. Damsma at the present address     

Adenosine A2A receptor stimulation increases release of acetylcholine from rat hippocampus but not striatum,and does not affect catecholamine release

Rat striatal and hippocampal slices, preincubated with [³H] dopamine (DA) {or [³H] noradrenaline (NA)} and [¹⁴C] choline, were superfused continuously and stimulated electrically. 2-chloroadenosine (2-CADO 0.001–100 μM), a non-selective adenosine receptor agonist, produced a concentration-dependent inhibition of the electrically evoked DA and acetylcholine (ACh) release from the striatal slices and of the electrically evoked NA and ACh release from the hippocampal slices. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 3, 30 and 200 nM), a selective adenosine A₁ receptor antagonist, caused a concentration-dependent, parallel, rightward shift of the 2-CADO concentration-response curve, consistent with competitive antagonism. The pA₂ values ranged between 8.4 and 8.8. In the case of ACh release from the hippocampus, but in no other case, was there an increase in release of radioactivity at low concentrations of 2-CADO in the presence of DPCPX. The stimulation in the hippocampus could be blocked by a selective adenosine A_2A receptor antagonist KF 17837. By itself KF 17837 (0.1–100 μM) had no effect on electrically evoked NA release from hippocampal slices, but decreased electrically evoked ACh release. This inhibition was counteracted by DPCPX (1 μM). These results show that, under the conditions used, DA release in the striatum, and NA release in the hippocampus, as well as ACh release from the striatum are regulated by adenosine A₁ but not by adenosine A_2A receptors. By contrast, ACh release from the hippocampus is tonically regulated both by adenosine A₁ receptors, which inhibit release, and by adenosine A_2A receptors which stimulate release. Received: 25 April 1996 / Accepted: 30 August 1996     

The effect of morphine on purine and acetylcholine release from rat cerebral cortex: Evidence for a purinergic component in morphine's action

Morphine enhances the release of adenosine and its metabolites from the rat cerebral cortex and inhibits the release of acetylcholine. Naloxone antagonizes the effects of morphine on both purine and acetylcholine release. The adenosine antagonists, caffeine and theophylline, reduce morphine's effects on acetylcholine release, and at the same time increase the spontaneous release of acetylcholine. It is suggested that morphine, acting at a naloxone-sensitive site, enhances the level of extracellular adenosine, which in turn inhibits the release of acetylcholine, and that some of morphine's actions are mediated by a purinergic step.