Cytochrome P450 2E1 gene polymorphisms/haplotypes and Parkinson's disease in a Swedish population.

Cytochrome P450 2E1 (CYP2E1), which inter alia is located in dopamine containing neurons in the substantia nigra, has been hypothesized to be of importance for the pathophysiology of Parkinson's disease (PD), either by its production of reactive oxygen species (ROS) or by its capability to detoxify putative neurotoxins. Numerous polymorphisms in the coding and non-coding regions of the gene for this enzyme have been reported. Different variants may account for inter-individual differences in the activity of the enzyme or production of ROS. In this study, the CYP2E1 gene was examined in a control population (n = 272) and a population with PD (n = 347), using a tag-single nucleotide polymorphism (tSNP) approach founded on HapMap Data. Six tSNPs were used in the analysis and haplotype block data were obtained. In case of significance, the SNP was further examined regarding early/late age of disease onset and presence of relatives with PD. We found an association between allele and genotype frequencies of the C/G polymorphism at intron 7 (rs2070676) of this gene and PD (P value of 0.026 and 0.027, respectively). Furthermore, analysis of the rs2070676 polymorphism in subgroups of patients with age of disease onset higher than 50 years and those not having a relative with PD also demonstrated a significant difference with controls. This was seen in both genotype (corresponding to P value = 0.039 and 0.032) and allele (P = 0.027 and 0.017 respectively) frequency. As a representative of many polymorphisms or in possible linkage disequilibrium with other functional variants, it is possible that rs2070676 could influence the regulation of the enzyme. In conclusion, our results display an association between the rs2070676 polymorphism and PD. Additional investigations are needed to elucidate the importance of this polymorphism for the activity of CYP2E1 and PD susceptibility.

Cytochrome P450 2E1 in the substantia nigra: relevance for dopaminergic neurotransmission and free radical production.

Cytochrome P450 2E1 (CYP2E1) has been detected in brain regions which are of relevance for the pathophysiology of Parkinson's disease, such as the substantia nigra (SN). Furthermore, CYP2E1 is known to generate reactive oxygen species (ROS), toxic molecules which have been implicated in the pathogenesis of the disease. We have previously reported that CYP2E1 inhibition increases extracellular dopamine (DA) in the SN. The aims of the present study were by using in vivo microdialysis in rat, to elucidate the mechanisms responsible for the increase in extracellular DA induced by CYP2E1 inhibition and to explore whether ROS is produced in the SN, both with and without the presence of an exogenous CYP2E1 substrate. The effect of inhibition of CYP2E1 by phenylethyl isothiocyanate (100 mg/kg) on extracellular DA in the SN was unaltered following pretreatment with gamma-butyrolactone and GBR-12909, drugs that inhibit firing of DA neurons and DA re-uptake, respectively. Preadministration of tetrodotoxin or reserpine, however, abolished the effect of CYP2E1 inhibition. Administration of isoflurane, an anesthetic which is metabolized by CYP2E1, increased the production of *OH in the SN, as measured by the transformation of 4-hydroxybenzoic acid to 3,4-dihydroxybenzoic acid during local perfusion compared with animals given other anesthetics. The results support the notion that CYP2E1 is located near or in the same compartment in the SN as stored DA, tentatively the endoplasmatic reticulum, and that the enzyme activity might modulate the amount of DA that is available for release. Furthermore, our findings indicate that the production of ROS can be stimulated by CYP2E1 substrates.

Association between the estrogen receptor beta gene and age of onset of Parkinson's disease.

The purpose of this study was to investigate the potential contribution of genetic variants in the estrogen receptor beta gene to the aetiology of Parkinson's disease (PD). Several lines of evidence from human and animal studies suggest a protective role for estrogen in PD. Recently the estrogen receptor beta subtype was reported to be an important mediator of estrogen actions in the nigrostriatal dopamine system. Two single nucleotide polymorphisms at position 1730 and 1082 in the ER beta gene were genotyped, using pyrosequencing, in 260 patients with PD and 308 controls recruited from the Swedish population. Neither of the two estrogen receptor beta polymorphisms was associated with an increased risk for PD. However, the G allele of the A1730G polymorphism was more frequent in patients with an early age of onset than in patients with a late age of onset of PD (P = 0.006). Patients carrying the GG genotype had an odds ratio of 2.2 for having an early onset of PD compared to non-carriers. In conclusion, our results indicate that genetic variation in the estrogen receptor beta gene may influence the age of onset of PD.

Influence of R-type (Cav2.3) and t-type (Cav3.1-3.3) antagonists on nigral somatodendritic dopamine release measured by microdialysis.

The release of dopamine from soma and dendrites of dopaminergic neurons in substantia nigra has been reported to be calcium-dependent, but it remains to be determined which calcium channels mediate this effect. We have used in vivo microdialysis in rat substantia nigra and striatum to investigate the effect of Ca(v)3.1-3.3 (T-type) and Ca(v)2.3 (R-type) calcium channel antagonists on somatodendritic and terminal dopamine release. Local reverse dialysis administration of 0.1-10 microM of the Ca(v)2.3 inhibitor SNX-482, or 100 microM of mibefradil, decreased the concentrations of dopamine and its metabolites in dialysate from substantia nigra, whereas 1 microM mibefradil or 40-80 microM nickel(II) induced an increase in nigral dialysate dopamine concentrations. Dopamine concentrations in striatal dialysates were decreased only by 10 microM of SNX-482 or 100 microM of mibefradil. Nickel(II) induced an increase in striatal dialysate dopamine concentration similar to that in substantia nigra. The results indicate a role for Ca(v)2.3 (R-type) voltage sensitive calcium channels in the calcium dependency of somatodendritic dopamine release, but argue against a calcium dependency mediated substantially by Ca(v)3.1-3.3 (T-type) channels.

An investigation of dopaminergic metabolites in the striatum and in the substantia nigra in vivo utilising radiolabelled L-DOPA and high performance liquid chromatography: a new approach in the search for transmitter metabolites.

Although the major routes of dopamine metabolism seem to be established, at least in terminal regions such as the striatum, it is important to search for previously unknown metabolites and to investigate the relevance of previously suggested minor alternative pathways. An urgent issue is to verify and quantify the transformation of dopamine to putative toxic species, another is to further explore metabolism of dopamine located in cell bodies/dendrites, e.g. in the substantia nigra. We have developed a new method in order to widen the search for alternative metabolites of dopamine. The method is based on systemic injection of tritiated L-DOPA to rats in vivo. Brain tissue was homogenised and centrifuged and the resulting supernatant fractioned following passage through a liquid chromatography system. The radioactivity of each fraction was measured using a scintillation system. By identifying fractions containing major catecholamines and metabolites, according to a standard solution, novel metabolites can be searched for in the remaining fractions. It was possible to obtain sufficient radioactivity in separate fractions of supernatant of homogenised tissue, even from such a small brain nucleus as substantia nigra. Radioactivity was obtained in those fractions that contained the major catecholamines and their metabolites, as well as in other fractions where it may represent previously unknown metabolites of L-DOPA/dopamine. The method was used to evaluate the possibility that cytochrome P450 2E1 is involved in the metabolism of dopamine in the substantia nigra. Significant changes in the radioactivity pattern were induced by inhibition of the enzyme but conclusions about whether cytochrome P450 2E1 is involved in the metabolism of dopamine or not requires further studies. The method can be used to study the metabolism of dopamine and can be extended, by using other radiolabelled precursors, also to evaluate metabolism of other transmitters, e.g. serotonin.

Inhibition of cytochrome P450 2E1 induces an increase in extracellular dopamine in rat substantia nigra: a new metabolic pathway?

We presented data previously on dopamine (DA) synthesis and catabolism in the rat substantia nigra (SN) suggesting that a substantial part of the synthesized DA in this brain part is metabolized by unknown nonclassical metabolic pathways. On the basis of that a relatively high density of cytochrome P450 2E1 (CYP 2E1) has been detected in rat SN the aim of the present study was to investigate the possibility that this enzyme is involved in the metabolism of DA. Systemic administration of either phenylethyl isothiocyanate (100 mg/kg ip), diethyldithiocarbamate (500 mg/kg, ip) or diallyl sulfide (200 mg/kg, sc or ip), three different inhibitors of cytochrome P450 2E1, induced an increase of the extracellular DA concentration in the SN, measured with microdialysis in awake rats, by 130%, 90%, and 35%, respectively. A tendency to increased concentrations of the classical DA metabolites in the dialysate from the SN was also observed in some experiments. In the striatum, no profound effects were induced by the drugs on the concentrations of DA or its metabolites. The results show that CYP 2E1 activity affects dopaminergic neurotransmission in the SN, possibly by participating in DA metabolism. Other mechanisms, such as the influence on the DA transporter or the release process cannot, however, be ruled out.

Local 5,7-dihydroxytryptamine lesions of rat amygdala: release of punished drinking, unaffected plus-maze behavior and ethanol consumption.

Several serotonergic drugs are effective for anxiety disorders, but underlying mechanisms are unclear, and findings in experimental animals are difficult to reconcile with human data. It has been proposed that differential effects of serotonin within specific anatomical systems may account for these difficulties, and the amygdala has been suggested as one of the structures involved. To examine this hypothesis, the neurotoxin 5,7-dihydroxytryptamine was administered locally in rat amygdala. Within the amygdala, serotonin was depleted by approximately 80%, with other transmitters unaffected, and serotonin transporter labelling was decreased by approximately 85%. Cortical areas near the lesion site were also affected, although to a lesser degree. Other forebrain areas were unaffected. Lesions resulted in a specific anti-conflict effect in a punished drinking test, but did not influence elevated plus-maze behavior (under baseline conditions and after restraint stress), locomotor activity or ethanol intake. These data suggest that the punished drinking test and the elevated plus-maze may activate different components of fear circuitry, and that the serotonergic input to the amygdala specifically participates in fear-related behavioral suppression mediated by this structure.

Activation of nigral dopamine neurons by the selective GABA(B)-receptor antagonist SCH 50911.

Previous studies have shown that systemic as well as local administration of the GABA(B)-receptor agonist baclofen is associated with a decrease in firing rate, a regularisation of firing rhythm and a decrease in burst firing activity of dopamine (DA) containing midbrain neurons. In the present electrophysiological study we have utilised the novel, selective and potent GABA(B)-receptor antagonist SCH 50911 in order to further analyse the importance of GABA(B)-receptors for the overall activity of rat nigral DA neurons. SCH 50911 given intravenously (1-64 mg/kg) or locally, by microiontophoretic techniques, was found to increase firing rate and to increase the burst firing activity of DA neurons. The present data suggest that the GABA(B)-receptor antagonist blocks somatodendritic receptors on nigral DA neurons. This GABA-receptor input appears to be of a tonic nature. It is proposed that the activation of nigral DA neurons may underlie the beneficial effects of GABA(B)-receptor antagonists in the modulation of cognition and that GABA(B)-receptor antagonists may be of therapeutic value in the treatment of Parkinson's disease.

Effects of mCPP on the extracellular concentrations of serotonin and dopamine in rat brain.

Intravenous administration of m-chloro-phenylpiperazine (mCPP) (0.25 or 2.5 mg/kg) induced a marked and dose-related increase in extracellular concentrations of serotonin in hippocampus (300-1,400% of baseline) as measured using in vivo microdialysis in awake male Wistar rats of the spontaneously hypertensive (SH) strain. Indicating that the effect of mCPP was caused by a reversal of the serotonin transporter, it was antagonized by pretreatment with the serotonin re-uptake inhibitor citalopram (10 mg/kg) but was unaffected by local administration of the sodium channel blocker tetrodotoxin (TTX; 1 microns). mCPP was also shown to induce an increase in extracellular concentrations of dopamine in the nucleus accumbens and the striatum of SH rats and in the nucleus accumbens of rats of the Sprague-Dawley (SD) strain; this effect of mCPP was, however, much weaker (125-170% of baseline) than the effect on serotonin; moreover, it seems to be TTX-sensitive. In anesthetized SD rats, mCPP induced a moderate reduction of nigral dopamine cell firing rate; supporting the assumption that this effect is secondary to the observed increase in dopamine release, it was blocked by pretreatment either with the dopamine synthesis inhibitor alpha-methyl-para-tyrosine or with the dopamine D2 receptor antagonist haloperidol. In conclusion, the results suggest that mCPP induces a marked, TTX-insensitive increase in serotonin release in rat brain, but only a modest and TTX-sensitive increase in the extracellular levels of dopamine.

Central administration of dopamine D3 receptor antisense to rat: effects on locomotion, dopamine release and [3H]spiperone binding.

A 15-mer, all-phosphorothioate-modified antisense oligodeoxynucleotide (ASO) targeted against rat dopamine D3 receptor mRNA (4 microM, 5 days) significantly reduced (28%) the amount of binding sites labelled with [3H]spiperone in monolayer cultured Chinese hamster ovary (CHO) cells transfected with the complementary desoxyribonucleic acid (cDNA) for the rat D3 receptor. In contrast, D3-ASO treatment did not reduce the amount of bound [3H]spiperone in CHO cells transfected with D2 receptor cDNA. Intracerebroventricular infusion of D3-ASO (osmotic minipump, 10 microg/microl/h, 7 days) influenced dopamine receptor density in the limbic forebrain such that the upper part of the dopamine/[3H]spiperone displacement curve--tentatively representing the D3 receptor--was altered significantly. Spontaneous locomotor activity of non-habituated rats was increased significantly in D3-ASO-treated animals; in addition, in vivo microdialysis revealed a moderate increase in dopamine release in the nucleus accumbens in these animals. In all experiments, an oligodeoxynucleotide comprising the same nucleotides as the antisense sequence, but in random order, was used as control. It is concluded that the antisense strategy is useful for investigating the functional role of dopamine D3 receptors and that the dopamine D3 receptor is involved in rat locomotor behaviour.

Inhibition of firing rate and changes in the firing pattern of nigral dopamine neurons by gamma-hydroxybutyric acid (GHBA) are specifically induced by activation of GABA(B) receptors.

Previous studies have shown that administration of gamma-hydroxybutyric acid (GHBA) or the GABA(B) receptor agonist baclofen are associated with a decrease in firing rate, a regularisation of firing pattern and a decrease in burst activity of midbrain dopamine (DA) neurons in the substantia nigra (SN). In the present study we compared the ability of the novel GABA(B) receptor antagonist SCH 50911 and the selective antagonist of GHBA binding sites, NCS-382, to antagonise the effects of baclofen or GHBA, respectively, on the neuronal activity of DA neurons in anaesthetised rats. SCH 50911 (75 mg/kg, i.v.) was found to antagonise the decrease in firing rate, the regularisation of firing rhythm and the decrease of burst activity in DA cells, induced by baclofen (1-32 mg/kg, i.v.) or GHBA (12.5-1600 mg/kg, i.v.). NCS-382 (100 mg/kg, i.v.) did not affect the baclofen-induced changes in neuronal activity. Neither was the drug able to influence the GHBA-induced alterations in firing rate or in burst activity, although NCS-382 to some extent antagonised the regularisation of the firing pattern observed following low doses of GHBA (< or =100 mg/kg). The results of the present study give further support for the notion that the GHBA-induced changes in neuronal activity of nigral dopamine neurons are mediated by stimulation of GABA(B) receptors.

Effects of local administration of L-, N-, and P/Q-type calcium channel blockers on spontaneous dopamine release in the striatum and the substantia nigra: a microdialysis study in rat.

The pivotal role for voltage-sensitive calcium channels in initiating synaptic transmitter release is undisputed, but it is only partly known to what extent the different subtypes contribute in vivo. Their importance for the dendritic release of dopamine has not been investigated in vivo previously. To evaluate comprehensively the relative importance of different voltage-sensitive calcium channel subtypes for striatal dopamine release, and to further investigate the mechanism of dendritic dopamine release in the reticulate part of substantia nigra, dopamine was measured by in vivo microdialysis in the striatum or substantia nigra of awake rats. The calcium channel blockers nimodipine, omega-conotoxin-GVIA, omega-agatoxin-IVA, and neomycin were administered locally through the dialysis probes and compared with calcium-free perfusion. Results indicate that dopamine release in the striatum is mainly dependent on N- and P/Q-type channels, but the dendritic dopamine release in the substantia nigra is mediated mainly by some other calcium-dependent mechanism, for example, calcium mobilization through T-, O-, or R-type calcium channels. A portion of the dendritic release is calcium independent but can be inhibited partially by neomycin, which might suggest a role for inositol 4,5-bisphosphate breakdown products.

3-Methoxytyramine formation following monoamine oxidase inhibition is a poor index of dendritic dopamine release in the substantia nigra.

This study was undertaken, using microdialysis, to compare the extracellular concentration of 3-methoxytyramine and dopamine in dialysate from the striatum and substantia nigra, after pargyline (75 mg/kg), after pargyline plus amphetamine (3 mg/kg), and after pargyline plus reserpine (5 mg/kg) administration. Treatment with pargyline alone increased the extracellular dopamine concentration by 70% in the striatum and by 140% in the substantia nigra and induced in both regions a time-dependent accumulation of 3-methoxytyramine. The addition of d-amphetamine to pargyline increased the extracellular dopamine concentration, compared with pargyline-treated controls, to the same extent in both the substantia nigra (maximally by 360%) and the striatum (maximally by 400%), but the concomitant increase of 3-methoxytyramine accumulation in the dialysate was relatively smaller in the substantia nigra compared with the striatum. Reserpine treatment decreased the extracellular dopamine concentration in both regions below the detection level (<10% of basal value). When pargyline was added to reserpine, the striatal extracellular dopamine concentration increased to 50% of pargyline-treated controls and the striatal 3-methoxytyramine accumulation was less than in pargyline-treated controls. However, in the substantia nigra, the addition of pargyline to reserpine resulted in dopamine concentrations as high as after pargyline only and the 3-methoxytyramine accumulation was not changed compared with pargyline-treated controls. In summary, our results indicate that dopamine in the substantia nigra is released from reserpine-sensitive storage sites and that pargyline-induced 3-methoxytyramine accumulation is a poor indicator of the local dopamine release. The latter observation may be explained by the fact that the dopamine-metabolizing enzyme, catechol-O-methyltransferase, is located inter alia in the dopamine-containing cell bodies/dendrites in the substantia nigra, in contrast to the situation in the terminals in the striatum where catechol-O-methyltransferase is located only in nondopaminergic cells.

Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices.

The importance of voltage-dependent sodium channels and different types of voltage-sensitive calcium channels for depolarisation-induced release of endogenous dopamine from dendrites and cell bodies in superfused guinea pig substantia nigra slices was investigated. The stimulatory effect of veratridine (10 microM) on dopamine release was only marginally attenuated in Ca(2+)-free medium but was completely blocked by tetrodotoxin (1 microM) and by the dopamine reuptake inhibitor GBR 12909 (10 microM). Low extracellular concentration of Na+ stimulated the dopamine release. Potassium-evoked dopamine release was completely Ca(2+)-dependent, not blocked by GBR 12909 and partially blocked by tetrodotoxin. Nifedipine (20 microM), omega-conotoxin GVIA (0.5 microM), penfluridol (5 microM), and Ni2+ (20 microM) had no effect, amiloride (1 mM) attenuated and neomycin (350 microM), and omega-agatoxin IVA (1 microM) almost totally blocked the potassium-induced dopamine release. The results suggest that veratridine released dopamine mostly by reversing the dopamine transporter. High concentrations of potassium induced release of nigral dopamine by opening of voltage-sensitive calcium channels of P/Q type but not L-type, N-type and probably not T-type. The depolarisation evoked by high concentrations of potassium seems to open voltage-sensitive calcium channels both by the depolarisation induced by potassium per se and by the secondary depolarisation induced by opening of voltage-dependent sodium channels.

Inhibition of dopamine re-uptake: significance for nigral dopamine neuron activity.

In the present study the effect of inhibition of the re-uptake of dopamine (DA) was analysed with respect to DA release and to the firing pattern of DA neurons in the substantia nigra (SN). Intravenous administration of GBR 12909 (0.5-8 mg/kg), a specific and potent inhibitor of DA re-uptake, was found to dose-dependently increase the DA concentration both in the SN and in the striatum, as measured by microdialysis. However, the drug failed to significantly affect the firing rate of the nigral DA neurons. In contrast, GBR 12909 dose-dependently induced a regularisation of the firing pattern, concomitant with a reduction in burst activity. An acute hemisection of the brain, which by itself produced a slight regularisation of the firing pattern of the nigral DA neurons without changing the firing rate of the ability of the DA neurons to fire in bursts, was found to prevent the regulatory action of GBR 12909. Pretreatment with the selective GABAB-receptor antagonist CGP 35348 (200 mg/kg, i.v., 5 min) did not significantly affect the firing rate, the regularity of the DA neurons, or their ability to fire in bursts. However, CGP 35348 markedly antagonised the ability of GBR 12909 to induce pacemaker-like firing or a decrease in burst activity of the nigral DA neurons. The results of the present study suggest that a striatonigral feedback projection may serve to control the activity of nigral DA neurons not primarily by regulating the firing rate, but, preferably, by modulating the firing pattern of the neurons. In this regard, activation of somatodendritic GABAB-receptors may form the final link in this feedback inhibitory control system.

The GABAB-receptor antagonist, CGP 35348, antagonises gamma-hydroxybutyrate- and baclofen-induced alterations in locomotor activity and forebrain dopamine levels in mice.

Previous studies have shown that administration of gamma-hydroxybutyric acid (GHBA) or baclofen is associated with a decrease in locomotor activity as well as an increase of dopamine (DA) in brain. In the present study we analyse whether these actions are related to activation of GABAB-receptors utilising a GABAB-receptor antagonist, CGP 35348. Administration of GHBA (200 or 800 mg/kg, i.p.) or baclofen (4 or 16 mg/kg, i.p.) induced a marked and dose-dependent decrease in locomotor activity in mice, that was antagonised by pretreatment with CGP 35348 (400 mg/kg, i.p.). Treatment with the highest doses of GHBA and baclofen produced clear-cut increases in forebrain DA concentration. Also these effects were effectively antagonised by pretreatment with CGP 35348. Treatment with the GABAB-receptor antagonist alone did not influence the locomotor activity or brain DA concentration. These results indicate that the behaviourally depressive and DA increasing effects of GHBA and baclofen are mediated by activation of GABAB-receptors.

Dopamine D3 receptor antisense influences dopamine synthesis in rat brain.

Intracerebroventricular infusion of an all-phosphorothioate antisense oligodeoxynucleotide targeted at the rat dopamine D3 receptor mRNA (10 micrograms h-1, 5 days) resulted in a significant reduction (19%) of the binding of the [D2 + D3] ligand [3H]spiperone in the limbic forebrain, where D3 receptors are relatively abundant, but not in caudate-putamen, where D3 receptors are sparse. In nucleus accumbens antisense treatment also caused an increase in dopamine synthesis; in contrast, antisense administration did not counteract the effect of apomorphine on dopamine formation. No effect of antisense administration on dopamine synthesis was observed in caudate-putamen. The results support the usefulness of the antisense strategy for the study of D3 receptors and suggest that D3 receptors may influence dopamine synthesis.

Pharmacologically induced cessation of burst activity in nigral dopamine neurons: significance for the terminal dopamine efflux.

Results obtained previously indicate that the firing pattern of midbrain dopamine (DA) neurons is of importance for the terminal DA release. In the present combined electrophysiological and microdialysis study, the influence of the firing pattern on striatal DA release was studied by using the previously observed ability of low doses of gamma-hydroxybutyrate (GHBA) to profoundly regularize the firing pattern of rat DA neurons in the substantia nigra (SN). Administration of GHBA (200 mg/kg, i.v.) did not significantly reduce the firing rate of any of the DA neurons recorded from, but rather caused a slight transient excitation. However, this dose of the drug caused a profound regularization of the firing pattern and abolished burst activity of the DA neurons. The DA concentration in the dialysate obtained from the striatum (10 min sampling periods) decreased with the lowest value (67% of predrug value) observed at the sampling period 20-30 min after the GHBA administration. As a complement to microdialysis, the 3-methoxytyramine (3-MT) accumulation in striatal tissue following monoamine oxidase inhibition was determined as an indirect measure of DA release in vivo. The 3-MT concentrations in the striatum decreased to 84% of controls following 200 mg/kg of GHBA. To exclude an effect on DA release conceivably mediated by GHBA locally in the striatum, GHBA (10(-7)-10(-3) M) was given locally in the dialysis probe and was found to increase DA in the dialysate (maximally to 140% of controls).(ABSTRACT TRUNCATED AT 250 WORDS)

The mesolimbic dopamine-activating properties of ethanol are antagonized by mecamylamine.

It has been suggested that ethanol may interact with the central nicotinic acetylcholine receptor, thus providing a basis for the often observed high consumption of both ethanol and nicotine. In the present in vivo microdialysis study, ethanol (2.5 g/kg) moderately increased dopamine overflow in the rat nucleus accumbens. The central nicotinic acetylcholine receptor antagonist mecamylamine totally counteracted this effect in a dose (1.0 mg/kg) that did not alter dopamine overflow per se. Ethanol also increased the overflow of dihydroxyphenylacetic acid and homovanillic acid, but this effect was not altered by mecamylamine (1.0 mg/kg). Furthermore, the ethanol-induced enhancement of 3,4-dihydroxyphenylalanine accumulation in the mesolimbic dopamine terminal area after NSD 1015 (an inhibitor of l-aromatic amino acid decarboxylase) was completely antagonized by mecamylamine in doses (3.0 and 6.0 mg/kg) that exerted no effects per se. Neither ethanol nor mecamylamine changed the catecholamine synthesis rate in the striatum or the cerebral cortex. These results provide further evidence that ethanol-induced activation of the mesolimbic dopamine system (increased dopamine synthesis and release) may be mediated via stimulation of central nicotinic acetylcholine receptors. It is suggested that antagonists of central nicotinic acetylcholine receptors may be useful in the treatment of alcoholism.

GABAB-receptor activation alters the firing pattern of dopamine neurons in the rat substantia nigra.

Previous electrophysiological experiments have emphasized the importance of the firing pattern for the functioning of midbrain dopamine (DA) neurons. In this regard, excitatory amino acid receptors appear to constitute an important modulatory control mechanism. In the present study, extracellular recording techniques were used to investigate the significance of GABAB-receptor activation for the firing properties of DA neurons in the substantia nigra (SN) in the rat. Intravenous administration of the GABAB-receptor agonist baclofen (1-16 mg/kg) was associated with a dose-dependent regularization of the firing pattern, concomitant with a reduction in burst firing. At higher doses (16-32 mg/kg), the firing rate of the DA neurons was dose-dependently decreased. Also, microiontophoretic application of baclofen regularized the firing pattern of nigral DA neurons, including a reduction of burst firing. Both the regularization of the firing pattern and inhibition of firing rate produced by systemic baclofen administration was antagonized by the GABAB-receptor antagonist CGP 35348 (200 mg/kg, i.v.). The GABAA-receptor agonist muscimol produced effects on the firing properties of DA neurons that were opposite to those observed following baclofen, i.e., an increase in firing rate accompanied by a decreased regularity. The NMDA receptor antagonist MK 801 (0.4-3.2 mg/kg, i.v.) produced a moderate, dose-dependent increase in the firing rate of the nigral DA neurons as well as a slightly regularized firing pattern. Pretreatment with MK 801 (3.2 mg/kg, i.v., 3-10 min) did neither promote nor prevent the regularization of the firing pattern or inhibition of firing rate on the nigral DA neurons produced by baclofen. The present results clearly show that GABAB-receptors can alter the firing pattern of nigral DA neurons, hereby counterbalancing the previously described ability of glutamate to induce burst firing activity on these neurons.