The antioxidant ebselen prevents neurotoxicity and clinical symptoms in a primate model of Parkinson's disease

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), via its major metabolite 1-methyl-4-phenylpyridinium (MPP(+)), produces in primates including humans clinical, biochemical, and neuropathological changes similar to those which occur in idiopathic Parkinson's disease. Ebselen is an antioxidant drug with glutathione peroxidase-like activity and a proven neuroprotective action in stroke patients. Here we show that Ebselen, when administered before, during, and after MPTP injections, prevents both neuronal loss and clinical symptoms in a primate MPTP model of Parkinson's disease. Ebselen also prevents peroxide radical overproduction induced by serum withdrawal in cultured PC12 cells and hydroxyl radical generation induced by the mitochondrial toxin, MPP(+), in vivo in rat brain. Moreover, Ebselen inhibits MPP(+)-induced toxicity in PC12 cells, without interacting with the dopamine uptake system. Our results demonstrate that compounds which prevent mitochondrial dysfunction and free radical production may be useful as preventive treatment of Parkinson's disease.

Enhancement of memory in cannabinoid CB1 receptor knock-out mice

We have used cannabinoid CB knock-out mice in a two-trial object recognition test to assess the role of cannabinoid CB receptors in memory. Cannabinoid CB1 knock-out mice are able to retain memory for at least 48 h after the first trial whereas the wild-type controls lose their capacity to retain memory after 24 h. These results suggest that endogenous cannabinoid CB receptors play a crucial role in the process of memory storage and retrieval.

Reduction of dopamine release and synthesis by repeated amphetamine treatment: role in behavioral sensitization

Changes in extracellular dopamine concentration in the ventral striatum during repeated amphetamine administration and over the first 7 days of withdrawal were studied by transversal microdialysis in freely moving rats. 2 days after fiber implantation rats were treated with either amphetamine (1.5 mg/kg i.p.) or saline every 12 h for 14 days. In amphetamine-treated rats, the baseline extracellular dopamine concentration, preceding the morning treatment, increased from 0.43 +/- 0.01 on day 1 up to 0.59 +/- 0.02 pmol/40 microliters sample on day 3 of treatment. Thereafter, dopamine fell rapidly on day 5(0.16 +/- 0.01 pmol/40 microliters) and remained at approximately the level reached on day 7(0.11 +/- 0.01 pmol/40 microliters) throughout the treatment and also over the 7 days of withdrawal. In contrast, in control rats, the extracellular dopamine concentration (0.40 +/- 0.01 pmol/40 microliters, on day 1) decreased progressively during the first days of treatment to reach a fairly stable value on day 4 (0.25 +/- 0.01 pmol/40 microliters sample). Thereafter, dopamine remained stable at this level throughout the remaining period of experimentation. Challenge with amphetamine (1.5 mg/kg i.p.) of animals treated with amphetamine for 10 days or withdrawn for 7 days produced a potentiated motor response compared to that in control rats but much less marked dopamine releasing effects. Dopamine synthesis in the ventral striatum, measured as L-dihydroxyphenylalanine formation after blockade of dihydroxyphenylalanine decarboxylase, was found to be reduced by approximately 60% after 2 weeks of amphetamine treatment and in animals withdrawn for 1 day or 7 days. These results indicate that repeated amphetamine treatment causes persistent inhibition of dopamine synthesis and release in the ventral striatum. Such inhibition may be a compensatory response to the repeated stimulation of postsynaptic dopamine receptors by the endogenously released dopamine and also the cause of postsynaptic sensitization to dopamine action.

Chronic morphine increases hippocampal acetylcholine release: possible relevance in drug dependence

Previous studies have shown that cocaine and amphetamine stimulate acetylcholine release in the hippocampus via an action of endogenously released dopamine on dopamine D1 and D2 receptors. The present study was aimed at clarifying if the property of stimulating hippocampal acetylcholine release was shared by morphine. The acute administration of morphine (10 mg/kg i.p.) failed to modify acetylcholine release in the hippocampus. However, after repeated administration (10 mg/kg i.p. twice daily) morphine acquired the ability to stimulate hippocampal acetylcholine release. Thus, at days 5 and 7 of chronic morphine treatment, a challenge dose of morphine (10 mg/kg i.p.) increased acetylcholine release by 50 and 100%, respectively. Concomitantly with the development of the stimulant property on acetylcholine release, morphine also acquired that of producing behavioural stimulation and lost that of producing sedation and catalepsy. The morphine-induced increase in acetylcholine output was suppressed by the blockade of dopamine D1 receptors with SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine) (0.1 mg/kg s.c.), which also suppressed the morphine-induced motor stimulation. Moreover, repeated morphine administration markedly potentiated the stimulant effect of the dopamine D1/D2 receptor agonist apomorphine (R(-)-10, 11-dihydroxyaporphine) (0.1 or 0.5 mg/kg s.c.) both on hippocampal acetylcholine release and on behaviour. These results may suggest that the enhancement of hippocampal acetylcholine release as well as the development of behavioural sensitisation after chronic morphine could be related to the development of dopamine receptor supersensitivity. Moreover, increased acetylcholine transmission in the hippocampus may play a role in the 'memory' of the rewarding effects of drugs of abuse.

Strain-dependent effects of dopamine agonists on acetylcholine release in the hippocampus: an in vivo study in mice

The effects of selective D1 or D2 dopamine receptor agonists and the indirect dopamine agonist cocaine on hippocampal acetylcholine release in mice of the C57BL/6 and DBA/2 inbred strains were investigated using intracerebral microdialysis. The D1 SKF 38393 (10, 20, 30 mg/kg, i.p.), the D2 agonist LY 171555 (0.5, 1, 2 mg/kg, i.p.) and cocaine (5, 10, 15 mg/kg, i.p.) all increased, dose-dependently, acetylcholine release in the hippocampus of C57BL/6 mice. Both the D1 agonist and cocaine did not produce any significant effect in DBA/2 mice. In the latter strain, however, LY 171555 produced a decrease in acetylcholine release that was evident after 60 min from injection of the doses of 0.5 and 1 mg/kg, but not at the dose of 2 mg/kg. The effects observed in C57BL/6 mice as well as those produced by low doses of LY 171555 in the DBA/2 strain were consistent with previous results obtained in rats. The present results indicate major strain-dependent differences in the effects of dopamine agonists on hippocampal acetylcholine release in mice. Moreover, they suggest a complex genotype-related neural organization of dopamine-acetylcholine interactions in the mesolimbic system. Finally, the strain differences in the effects of the dopamine agonists on hippocampal acetylcholine release parallel previously reported strain differences in the effects of these substances on memory consolidation.

Neuroleptics cause stimulation of dopamine D1 receptors and their desensitization after chronic treatment

Recent evidence indicates that the neuroleptic-induced increase of in vivo acetylcholine output in the striatum does not depend on the relief of cholinergic neurons from the inhibitory control by dopamine, but on increased dopamine output onto dopamine D1 receptors. The present microdialysis study was aimed at finding if the neuroleptic-induced increase in striatal acetylcholine release persists after chronic treatment, and how it is correlated with dopamine output. Rats were chronically treated with the dopamine D2 receptor antagonists, haloperidol and (-)-sulpiride (0.5 mg/kg and 50 mg/kg i.p., respectively, daily, for 30 days). The stimulant effect of both neuroleptics on striatal dopamine release persisted unaltered throughout the chronic treatment (by about 100% over basal values). In contrast, the enhancing effects of haloperidol and (-)-sulpiride on striatal acetylcholine release remained unchanged up to day 12 of treatment. Thereafter, tolerance developed, so that both neuroleptics became totally ineffective on day 30 of treatment. Both on day 1 and 30, the neuroleptic-induced dopamine release was reversed by gamma-butyrolactone (gamma-hydroxybutyric acid lactone), suggesting that this effect is mediated by enhanced neuronal activity. On day 1 and day 10, the neuroleptic-induced acetylcholine release was antagonized by the blockade of dopamine D1 receptors with SCH 39166 (trans-(-)-(6aS,13bR)-11-chloro-6,6a,7,8,9,13b- hexahydro-7-methyl-5H-benzo[d]napht[2,1-b]azepine-12-ol, hydrochloride) (0.5 mg/kg i.p.). SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3- benzazepine-7,8-diol hydrochloride) (5 mg/kg i.p.) increased acetylcholine release by about 50% in control rats and in rats treated with (-)-sulpiride or haloperidol for up to 7 days.(ABSTRACT TRUNCATED AT 250 WORDS)

The benzodiazepine receptor antagonist flumazenil increases acetylcholine release in rat hippocampus

The benzodiazepine receptor antagonist flumazenil (2.5-20 mg/kg i.p.) increased acetylcholine (ACh) release by up to 85% in the hippocampus of freely moving rats. In contrast, the benzodiazepine receptor full agonist diazepam (2.5-10 mg/kg i.p.) decreased ACh release up to a maximum of 45% in the same brain area. Injection of flumazenil (10 pmol) or diazepam (10 pmol) into the medial septum increased (95%) or reduced (50%), respectively, ACh release in the hippocampus. The maximum effect produced by those drugs was of the same magnitude as that observed after systemic injection. The changes in hippocampal cholinergic function elicited by activation and blockade of benzodiazepine receptors in the medial septum may thus play a crucial role in the alterations of the cognitive processes elicited by benzodiazepine receptor ligands.

Co-dergocrine (Hydergine) regulates striatal and hippocampal acetylcholine release through D2 receptors

The effect of Co-dergocrine (Hydergine) on acetylcholine (ACh) release in the striatum and hippocampus has been studied by means of brain microdialysis and compared to the effect of SKF 38393 and of LY 171555 selective D1 and D2 dopamine (DA) receptor agonists, respectively. Co-dergocrine (1 and 5 mg kg-1 i.p.) as well as LY 171555 (0.2 and 0.5 mg kg-1 i.p.) decreased the extracellular concentration of ACh in the striatum, whereas SKF 38393 (5 and 10 mg kg-1 i.p.) increased it. On the other hand, Co-dergocrine (1 and 5 mg kg-1), LY 171555 (0.2 and 0.5 mg kg-1) and SKF 38393 (5 and 10 mg kg-1) increased ACh release in the hippocampus in a dose-dependent way. These results show that Co-dergocrine, which is widely used in the treatment of senile mental decline, enhances the release of ACh in the hippocampus in a similar manner to both D1 and D2 DA agonists. This effect might be relevant for the amelioration of cognitive processes. Moreover, our results which demonstrate that Co-dergocrine is able to decrease the release of ACh in the striatum, as are selective D2 agonists, suggest that Co-dergocrine may have a potential therapeutic benefit in Parkinsonian dementia.

Effects of subchronic minaprine on dopamine release in the ventral striatum and on immobility in the forced swimming test

Subchronic (5 mg/kg daily for 9 consecutive days) but not acute minaprine treatment enhanced in vivo dopamine release in the limbic part of the striatum of rats as revealed by intracerebral microdialysis. Moreover, the same subchronic treatment with minaprine reduced immobility in the forced swimming test. The anti-immobility effect of minaprine was not evident after a single injection of the antidepressant. Finally, the subchronic treatment with minaprine was devoid of effects in an activity test. These results suggest that enhanced dopaminergic transmission may contribute to the pharmacological and clinical profile of this drug.

Does dopamine exert a tonic inhibitory control on the release of striatal acetylcholine in vivo?

The role of dopamine transmission on striatal acetylcholine release was investigated by using brain microdialysis. Blockade of dopamine D2 receptors with (-)-sulpiride or haloperidol increased acetylcholine release to a maximum of 80% (after 50 and 0.5 mg/kg, respectively). This effect was prevented by blockade of dopamine D1 receptors with 0.5 mg/kg SCH 39166 or 0.1 mg/kg SCH 23390, or by depletion of dopamine stores after 5 mg/kg reserpine + 150 mg/kg alpha-methyltyrosine. Treatment with SCH 39166, SCH 23390 or reserpine + alpha-methyltyrosine reduced acetylcholine release by about a maximum of 30%. Stimulation of dopamine D2 receptors with LY 171555 (quinpirole) at a low, sedative dose (0.05 mg/kg) reduced acetylcholine release by about 30% with no further reduction at higher doses up to 1 mg/kg. Moreover, LY 171555 (0.1 mg/kg) given to SCH 39166 (0.5 mg/kg)- or SKF 38393 (20 mg/kg)-pretreated rats did not decrease acetylcholine release, suggesting that its effect is through a dopamine D1 receptor-mediated mechanism. In contrast, in dopamine-depleted rats, LY 171555 0.1 mg/kg became more effective in decreasing acetylcholine release (about 70%) also after SCH 39166 (0.5 mg/kg) pretreatment (about 80%), thus acting independently of dopamine D1 receptor mechanisms. These results indicate that, in normal circumstances, endogenous dopamine facilitates striatal acetylcholine release through dopamine D1 receptors. The results argue against the commonly accepted view that dopamine D2 receptors exert a tonic inhibitory control on acetylcholine release. Moreover, they suggest that dopamine D2 receptors, in circumstances of dopamine depletion, may exert an inhibitory control on acetylcholine release independent of dopamine D1 receptor mechanisms.

Stimulation of both dopamine D1 and D2 receptors facilitates in vivo acetylcholine release in the hippocampus

The effect of selective D1 and D2 dopamine (DA) receptor agonists and of a mixed D1/D2 agonist on hippocampal acetylcholine (ACh) release was investigated. LY 171555 (0.5 and 1 mg/kg, i.p.), SKF 38393 (1 to 10 mg/kg, i.p.), CY 208-243 (0.2 mg/kg, i.p.) and apomorphine (0.5 to 2 mg/kg, i.p.), at doses stimulating rat behavior, were found to increase the output of ACh in the hippocampus. Maximal increase was observed after LY 171555 1 mg/kg, SKF 38393 10 mg/kg, CY 208-243 2 mg/kg and apomorphine 2 mg/kg (85, 90, 87 and 210%, respectively). The enhancement of ACh release induced by either SKF 38393 (10 mg/kg) or LY 171555 (1 mg/kg) was prevented by the blockade of D1 receptors with SCH 23390 (0.1 mg/kg, s.c.). Co-administration of maximally active doses of LY 171555 (1 mg/kg) and SKF 38393 (10 mg/kg) produced an additive effect (about 200%). In contrast to the findings with high doses, low, presynaptic doses of LY 171555 and apomorphine reduced ACh output. Maximal reduction was observed after 0.05 mg/kg for both drugs, (43 and 52%, respectively). These results show that activation of dopaminergic transmission either at D1 and/or at D2 receptors enhances ACh output in the hippocampus.

Evidence that neuroleptics increase striatal acetylcholine release through stimulation of dopamine D1 receptors

The relative role of D1 and D2 dopamine receptors in the neuroleptic-induced increase of striatal acetylcholine (ACh) release was investigated using brain microdialysis in freely moving rats. Administration of (-)-sulpiride, haloperidol and clozapine, produced a dose-related increase in ACh release in the striatum. Maximal increase by 52, 45 and 73% over basal values was produced by the dose of 20, 0.25 and 10 mg/kg i.p. of (-)-sulpiride, haloperidol and clozapine, respectively. Administration of the D1 receptor antagonist SCH 23390 (0.1 mg/kg s.c.) decreased ACh output by 30%, completely suppressed the stimulant effect of (-)-sulpiride and haloperidol and only modestly reduced that of clozapine. The inhibitory effect of SCH 23390 vs. (-)-sulpiride or haloperidol-induced ACh output was shared by SCH 39166 (1 mg/kg i.p.), another specific D1 receptor antagonist. On the other hand, SCH 23390 (0.1 mg/kg s.c.) was ineffective in reducing atropine-induced increase in ACh release. A combined treatment with reserpine (5 mg/kg i.p.) and alpha-methyltyrosine (150 mg/kg i.p.), 6 h beforehand, prevented the enhancement of ACh release induced by both (-)-sulpiride and haloperidol, whereas only reduced that by clozapine. The results indicate that neuroleptics increase striatal ACh release by enhancing endogenous extracellular dopamine acting on D1 receptors, and suggest that these receptors play a major physiological role in controlling ACh release in the striatum.

Effects of cocaine and amphetamine on acetylcholine release in the hippocampus and caudate nucleus

The role of dopamine in the control of hippocampal acetylcholine release was evaluated by using in vivo microdialysis. The effects of the two psychostimulants, cocaine and d-amphetamine, were studied on acetylcholine release in the hippocampus and compared to effects observed in the caudate nucleus. Administration of cocaine (10 and 20 mg/kg i.p.) increased acetylcholine release by 130 and 190% in the hippocampus, whereas in the caudate nucleus the enhancement was 51 and 80% over basal values, respectively. After the injection of d-amphetamine (1 and 2 mg/kg i.p.) the enhancement of acetylcholine release was 110 and 210% in the hippocampus whereas it was 35 and 54%, respectively, in the caudate nucleus. As observed in the caudate nucleus, pretreatment with the dopamine D1 receptor antagonist, SCH 23390, antagonized the cocaine- and amphetamine-induced increase in hippocampal acetylcholine release. These results show that cocaine and d-amphetamine, by increasing dopaminergic transmission, enhance the extracellular concentrations of acetylcholine in both brain areas. The relative enhancement in the hippocampus was far greater than that in the caudate nucleus, suggesting that dopaminergic control of cholinergic function differs in these two brain areas. The results also suggest that endogenous dopamine, by facilitating the release of acetylcholine in the hippocampus, may participate in the regulation of hippocampal cognitive processes.

Inhibition of hippocampal acetylcholine release by benzodiazepines: antagonism by flumazenil

Diazepam (2.5-10 mg/kg i.p.) and midazolam (2.5-10 mg/kg i.p.) decreased acetylcholine release in the hippocampus of freely moving rats. This effect was antagonized by pretreatment with flumazenil (1 mg/kg i.p.). These results show that activation of benzodiazepine receptors reduces the in vivo release of acetylcholine in the hippocampus, suggesting that the septo-hippocampal cholinergic system, which has a major role in the regulation of cognitive functions, is under inhibitory control exerted by gamma-aminobutyrate (GABA) neurons.

Cocaine releases limbic acetylcholine through endogenous dopamine action on D1 receptors

Cocaine (10 and 20 mg/kg i.p.) enhanced the extracellular concentration of acetylcholine (ACh) in the ventral striatum of freely moving rats. The enhancement was prevented both by dopamine (DA) D1 receptor blockade with SCH 23390 (0.1 mg/kg s.c.) and by depletion of endogenous DA after coadministration of reserpine (5 mg/kg i.p.) and alpha-methyltyrosine (alpha-MT) (150 mg/kg i.p.). In contrast, blockade of DA D2 receptors with (-)-sulpiride (20 mg/kg i.p.) did not prevent the cocaine-induced increase in ACh release. These results indicate that the cocaine-induced stimulation of ACh release is mediated by an action of DA on D1 receptors, and suggest that the enhancement of ACh release might play a functional role in the central effects of cocaine. Moreover, DA depletion after reserpine + alpha-MT or D1 receptor blockade with SCH 23390 led to a comparable decrease of baseline ACh release, suggesting that striatal cholinergic interneurons are under D1 receptor-mediated facilitatory dopaminergic control.