Effects of the central analgesic tramadol on the uptake and release of noradrenaline and dopamine in vitro.

1. The centrally acting analgesic, tramadol, has low affinity for opioid receptors and therefore presumably other mechanisms of analgesic action. Neurotransmitter release and uptake experiments were used to characterize the effects of tramadol on the central noradrenergic and dopaminergic systems. 2. Tramadol inhibited the uptake of [3H]-noradrenaline into purified rat hypothalamic synaptosomes with an IC50 of 2.8 microM; the (-)-enantiomer was about ten times more potent than the (+)-enantiomer. Results with the principal metabolite O-desmethyltramadol were very similar. Inhibition of dopamine uptake into purified rabbit caudate nucleus synaptosomes was very weak with 62% inhibition of 100 microM. 3. Rat occipital cortex slices were preincubated with [3H]-noradrenaline and rabbit caudate nucleus slices with [3H]-dopamine, then superfused and stimulated electrically. Tramadol, 1 and 10 microM, enhanced the stimulation-evoked [3H]-noradrenaline overflow by 25 and 69%, respectively; the (-)-enantiomer was more potent than the racemate or the (+)-enantiomer. Tramadol, 10 microM, had no effect on dopamine release. 4. The effects of tramadol on the stimulation-evoked [3H]-noradrenaline release were abolished when uptake sites were already blocked by a high concentration of cocaine. 5. The metabolite O-desmethyltramadol showed a slight facilitation of the stimulation-evoked noradrenaline release; the effect was more pronounced in the presence of a high concentration of naloxone. In the presence of cocaine, inhibition of the release was observed similar to the effect of morphine but less potent. 6. The results show that tramadol blocks noradrenaline uptake with selectivity as compared to dopamine uptake. The interaction with the noradrenaline transporter is stereoselective.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential labelling of dopamine receptors in rat brain in vivo: Comparison of [3H]piribedil, [3H]S 3608 and [3H]N,n-propylnorapomorphine

The in vivo distribution of radioactivity in brain and labelling of cerebral dopamine receptors in rats derived from the administration of the atypical dopamine agonists [³H]piribedil and [³H]S 3608 has been compared to that of the classical dopamine agonist [³H]N,n-propylnorapomorphine (NPA). Radioactivity derived from [³H]piribedil accumulated in the substantia nigra, nucleus accumbens and cervical spinal cord, and this accumulation was prevented by administration of (+)-butaclamol and apomorphine only in substantia nigra and the nucleus accumbens. Radioactivity derived from [³H]S 3608 accumulated in the same areas and, additionally, in the frontal cortex and tuberculum olfactorium; this accumulation was prevented by administration of (+)-butaclamol and apomorphine only in substantia nigra, nucleus accumbens and tuberculum olfactorium. Neither ligand caused accumulation of radioactivity in the striatum. In contrast, radioactivity derived from [³H]NPA accumulated in the substantia nigra, nucleus accumbens, striatum, and tuberculum olfactorium. Radioactivity derived from [³H]NPA was prevented from accumulating in all these areas by (+)-butaclamol and by apomorphine, but piribedil only prevented accumulation in the substantia nigra and nucleus accumbens and S 3608 only prevented accumulation in substantia nigra, tuberculum olfactorium and nucleus accumbens. Neither piribedil nor S 3608 prevented accumulation of radioactivity derived from [³H]NPA in the striatum. Piribedil and S 3608 showed equal capacity in vitro to displace [³H]spiperone and [³H]NPA from striatal or nucleus accumbens tissue preparations. These results suggest that, in vivo piribedil and S 3608 selectively interact with dopamine receptors in the substantia nigra and nucleus accumbens, but not with those in striatum, perhaps due to differential distribution within brain.     

Dopamine receptor binding sites in the rat superior colliculus

Following intravenous administration of [3H]spiperone or [3H]N,n-propylnorapomorphine (NPA) to rats, radioactivity derived from the ligands accumulated in the striatum and superior colliculus when compared with cerebellar levels. The accumulation of [3H]spiperone in both areas was prevented by intraperitoneal administration of (+)-butaclamol, haloperidol and sulpiride but not by (-)-butaclamol, cinanserin, propranolol or prazosin. The accumulation of [3H]NPA was prevented by administration of (+)-butaclamol, haloperidol and apomorphine but not by (-)-butaclamol. In in-vitro experiments, membrane preparations from the superior colliculus showed a small number of specific binding sites for both [3H]spiperone and [3H]NPA. The dissociation constant (KD) for [3H]NPA was not different from that for striatal preparations but that for [3H]spiperone was 10-fold higher. We conclude that dopamine receptors may be present within the superior colliculus.     

Phenoxybenzamine blocks dopamine autoreceptors irreversibly: Implications for multiple dopamine receptor hypotheses

Phenoxybenzamine in μM concentrations increased the electrically evoked overflow of recently taken up [³H]dopamine from superfused slices of cat caudate nucleus, an effect which is also observed for dopamine receptor antagonists. The magnitude of the increase in electrically evoked [³H]dopamine release caused by 1 μM phenoxybenzamine was equal to that elicited by maximally effective concentrations of the specific dopamine receptor antagonist, S-sulpiride. Phenoxybenzamine (1 μM) completely antagonized the inhibition of [³H]dopamine release caused by the dopamine receptor agonist pergolide (10 nM). The α-adrenoceptor antagonist phentolamine (1 μM) had no effect on the electrically evoked overflow of [³H]dopamine, ruling out the possibility that the effect of phenoxybenzamine could be attributed to α-adrenoceptor blockade. A 20 min exposure to 1 μM phenoxybenzamine increased the electrically evoked [³H]dopamine overflow even after the tissue had been washed for two and a half hours. When the caudate slices were exposed for 30 min to the reversible dopamine receptor antagonist S-sulpiride (1 μM) and washed for two and a half hours, no similar increase in stimulation-evoked [³H]dopamine overflow was observed. When sulpiride (1 μM) was present during the exposure to phenoxybenzamine (1 μM), no increase in electrically evoked [³H]dopamine overflow was observed after the washout period. Thus phenoxybenzamine at 1 μM appears to block irreversibly the dopamine autoreceptor in the caudate nucleus. Phenoxybenzamine has been previously reported to block irreversibly dopamine-stimulated adenylate cyclase (D₁) and neuroleptic receptor binding (D₂). The present demonstration that phenoxybenzamine also blocks the dopamine autoreceptor irreversibly thus supports the view that all currently well-established dopamine receptors are sensitive to phenoxybenzamine.     

Influence of antipsychotics on presynaptic receptors modulating the release of dopamine in synaptosomes of the nucleus accumbens of rats

The release of preloaded [3H]dopamine (DA) from superfused synaptosomes stimulated by 30 mM K+ was investigated in the nucleus accumbens of rats. Under conditions preventing the uptake of DA (presence of 40 microM cocaine) release of [3H]DA was inhibited by DA and apomorphine in a concentration-dependent manner (IC50s 0.65 and 0.3 microM, respectively). The maximal inhibitory effects of DA, as well as of apomorphine, were about 50% of the controls. The DA-induced inhibition was antagonized by antipsychotics completely; the rank order of antagonistic potencies was haloperidol greater than clozapine greater than sulpiride; methiothepine was ineffective. Furthermore, the K+-stimulated release of [3H]DA was inhibited by serotonin in a concentration-dependent manner (IC50 = 0.9 microM). This inhibitory effect was antagonized by methiothepine with a high efficiency, by clozapine and methysergide with moderate efficiencies; haloperidol and sulpiride were ineffective. The experimental system demonstrated appears to be suitable for characterizing the DA- and serotonin-antagonistic potencies of antipsychotics and other drugs on presynaptic autoreceptors as well as receptors modulating release of DA in the nucleus accumbens.     

Modulation of [3H]-dopamine released by different frequencies of stimulation from rabbit retina.

Rabbit retinal pieces were incubated with [3H]-dopamine and superfused with Krebs solution. Calcium-dependent tritium release was elicited twice within each experiment by electrical field stimulation (360 pulses, 2 ms, 20 mA) at frequencies of 1 Hz, 3 Hz, and 6 Hz. The evoked release of [3H]-dopamine for the first period of stimulation (S1) was 2.09 +/- 0.23% (n = 5) at 1 Hz and was of similar magnitude at all other frequencies of stimulation employed. The D2-dopamine receptor agonist, LY 171555 (quinpirole HCl; 0.01-1 microM) added to the superfusion medium before the second period of stimulation, inhibited the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, and was more potent the lower the stimulation frequency. The isomer of quinpirole, LY 181990 (0.01-1 microM) did not inhibit the stimulation-evoked overflow of tritium, regardless of concentration or stimulation frequency. The stereoisomers of the D2-dopamine receptor antagonist sulpiride (0.01-3 microM) increased the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, being more potent the higher the frequency of stimulation. S-sulpiride was more potent than R-sulpiride at all stimulation frequencies. The inhibitory effect of quinpirole was stereoselectively antagonized by sulpiride, but not by LY 181990. The alpha-adrenoceptor antagonist phentolamine (1 microM) did not modify the quinpirole-induced inhibition of [3H]-dopamine release. When the synaptic concentration of dopamine was increased by the presence of the dopamine uptake inhibitor nomifensine (3 microM), the potency of the agonist quinpirole in inhibiting the release of [3H]-dopamine elicited by field stimulation at 1 Hz (180 pulses) was decreased. In contrast, the potency of S-sulpiride in enhancing the evoked release of [3H]-dopamine was increased when nomifensine was present in the superfusion medium. Picomolar concentrations of the hormone melatonin (0.1-10 nM) inhibited the calcium-dependent release of [3H]-dopamine from rabbit retina with the same potency regardless of the frequency of stimulation applied (1 Hz, 3 Hz or 6 Hz). The potency of D2-dopamine receptor agonists and antagonists in modifying [3H]-dopamine release from rabbit retina appears to depend on the synaptic concentration of dopamine which is altered by the frequency of stimulation, and by the dopamine uptake inhibitor nomifensine.(ABSTRACT TRUNCATED AT 400 WORDS)     

False labelling of dopaminergic terminals in the rabbit caudate nucleus: uptake and release of [3H]-5-hydroxytryptamine.

The effect of the catecholamine uptake inhibitor nomifensine and of the 5-hydroxytryptamine (5-HT) uptake blocker 6-nitroquipazine on the accumulation of [3H]-5-HT (0.1 microM, 60 min incubation) and [3H]-dopamine (0.1 microM, 30 min incubation) into slices of hippocampus and caudate nucleus of the rabbit was investigated. In addition, the influence of nomifensine on the electrically evoked [3H]-5-HT release from caudate nucleus slices and of nomifensine and 6-nitroquipazine on [3H]-5-HT released from caudate nucleus slices was analysed. In hippocampal slices, which contain practically no dopaminergic but densely distributed 5-hydroxytryptaminergic and noradrenergic nerve terminals (ratio of dopamine:5-HT:noradrenaline about 1:30:25), nomifensine (1, 10 microM) did not affect the accumulation of [3H]-5-HT; 6-nitroquipazine (1 microM) reduced [3H]-5-HT uptake to about 35% of controls. In the caudate nucleus, however, where dopamine is the predominant monoamine (ratio of dopamine:5-HT:noradrenaline about 400:25:15) nomifensine (1, 10 microM) reduced the tritium accumulation to 65% whereas 6-nitroquipazine (1 microM) was ineffective. The combination of both drugs (1 microM each) led to a further decrease to about 15%. The uptake of [3H]-dopamine into hippocampal slices was blocked by both nomifensine (1 microM) and 6-nitroquipazine (1 microM) whereas in caudate nucleus slices only nomifensine (1, 10 microM) reduced the accumulation of [3H]-dopamine. The combination of both drugs was not more effective than nomifensine alone. The different effects of both uptake inhibitors in the hippocampus and caudate nucleus suggest a neurone specific rather than a substrate specific mode of action.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stereoisomers of apomorphine differ in affinity and intrinsic activity at presynaptic dopamine receptors modulating [3H]dopamine and [3H]acetylcholine release in slices of cat caudate

We investigated the effects of R(?)-apomorphine and S(+)-apomorphine on dopamine receptors modulating electrically evoked [³H]dopamine and [³H]acetylcholine release from slices of cat caudate nucleus. R(?)-Apomorphine inhibited the release of both [³H]dopamine and [³H]acetylcholine with an IC₅₀ of 20 nM, while S(+)-apomorphine was without inhibitory action on the electrically evoked release of either neurotransmitter at concentrations up to 1 μM. At a concentration of 1 μM, however, S(+)-apomorphine antagonized the inhibition by R(?)-apomorphine, producing a parallel five-fold shift to the right in the concentration-response curve to R(?)-apomorphine. These results indicate that S(+)-apomorphine is devoid of intrinsic activity to stimulate presynaptic dopamine receptors modulating the electrically evoked release of dopamine and acetylcholine. In addition, S(+)-apomorphine has an approximately ten-fold lower affinity for presynaptic dopamine receptors compared to R(?)-apomorphine.     

Influence of antipsychotics and serotonin antagonists on presynaptic receptors modulating the release of serotonin in synaptosomes of the nucleus accumbens of rats

In the nucleus accumbens of rats the release of [3H]serotonin (5-HT) from superfused synaptosomes stimulated by 30 mM K+ was investigated. In the presence of 40 microM of the uptake inhibitor cocaine the release of [3H]5-HT was inhibited by 5-HT in a concentration-dependent manner (IC50 = 0.45 microM). The maximum inhibitory effect of 5-HT was 54% of controls. The inhibition of K+-stimulated release of [3H]5-HT induced by 5-HT was antagonized completely by methiothepine and clozapine, respectively, whereas methysergide had only a weak antagonizing effect in a concentration of 20 microM or less, haloperidol was ineffective. Furthermore, the synaptosomal K+-stimulated release of [3H]5-HT was also inhibited by dopamine (DA) in a concentration-dependent manner (IC50 = 0.1 microM). This inhibitory effect was antagonized by antipsychotic drugs, the rank order of antagonistic potencies was sulpiride greater than haloperidol greater than clozapine; methiothepine was ineffective. The experimental system (the K+-stimulated synaptosomal release of [3H]5-HT seems to be a suitable model for differentiating dopaminergic and/or serotonergic components of antipsychotics or other drugs on presynaptic receptors.     

Anti-ischemic and cognition-enhancing properties of NNC-711, a gamma-aminobutyric acid reuptake inhibitor

The objective of this study was to examine the interaction of dopamine D2 receptor antagonists and dopamine uptake inhibitors on the regulation of extracellular dopamine release in the nucleus accumbens of Wistar rats employing in vivo microdialysis and in vitro dopamine uptake studies. Application of the D2 receptor antagonists raclopride (100 microm) or sulpiride (100 microm) alone through the microdialysis probe in the nucleus accumbens for 60 min increased the extracellular levels of dopamine in the nucleus accumbens to 150% and 200% of basal, respectively. Perfusion of the nucleus accumbens for 60 min with the dopamine uptake inhibitors, 1-[2-[bis(4-Fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine dihydrochloride (GBR 12909; 100 microm) or 1-[2-(Diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine dihydrochloride (GBR 12935; 100 microm) alone, increased the extracellular levels of dopamine in the nucleus accumbens to 400% and 350% of basal, respectively. Co-perfusion of 100 microM GBR 12909 or GBR 12935 with either 100 microM sulpiride or raclopride produced a significant reduction in the GBR 12909 or GBR 12935 induced increase in the extracellular levels of dopamine to basal levels. In vitro, GBR 12909 (1-9 nM) dose-dependently inhibited active uptake of [3H]dopamine in homogenates of the nucleus accumbens. Addition of 100 microm sulpiride had little effect on GBR 12909 inhibition of [3H] dopamine uptake, suggesting that dopamine D2 receptor antagonists are not blocking the actions of the GBR-type dopamine uptake inhibitors at the dopamine transporter. Overall, the data suggest that complex interactions occur in vivo between D2 antagonists and GBR-type dopamine uptake inhibitors, which negate their effects on elevating the extracellular levels of dopamine in the nucleus accumbens.     

Histamine H(3) receptor-mediated inhibition of depolarization-induced, dopamine D(1) receptor-dependent release of [(3)H]-gamma-aminobutryic acid from rat striatal slices

1. A study was made of the regulation of [(3)H]-gamma-aminobutyric acid ([(3)H]-GABA) release from slices of rat striatum by endogenous dopamine and exogenous histamine and a histamine H(3)-agonist. Depolarization-induced release of [(3)H]-GABA was Ca(2+)-dependent and was increased in the presence of the dopamine D(2) receptor family antagonist, sulpiride (10 microM). The sulpiride-potentiated release of [(3)H]-GABA was strongly inhibited by the dopamine D(1) receptor family antagonist, SCH 23390 (1 microM). Neither antagonist altered basal release. 2. The 15 mM K(+)-induced release of [(3)H]-GABA in the presence of sulpiride was inhibited by 100 microM histamine (mean inhibition 78+/-3%) and by the histamine H(3) receptor-selective agonist, immepip, 1 microM (mean inhibition 81+/-5%). The IC(50) values for histamine and immepip were 1.3+/-0.2 microM and 16+/-2 nM, respectively. The inhibitory effects of histamine and immepip were reversed by the H(3) receptor antagonist, thioperamide, 1 microM. 3. The inhibition of 15 mM K(+)-induced [(3)H]-GABA release by immepip was reversed by the H(3) receptor antagonist, clobenpropit, K(d) 0.11+/-0.04 nM. Clobenpropit alone had no effect on basal or stimulated release of [(3)H]-GABA. 4. Elevated K(+) caused little release of [(3)H]-GABA from striatal slices from reserpinized rats, unless the D(1) partial agonist, R(+)-SKF 38393, 1 microM, was also present. The stimulated release in the presence of SKF 38393 was reduced by 1 microM immepip to the level obtained in the absence of SKF 38393. 5. These observations demonstrate that histamine H(3) receptor activation strongly inhibits the dopamine D(1) receptor-dependent release of [(3)H]-GABA from rat striatum; primarily through an interaction at the terminals of GABA neurones.     

Neuroleptic receptors: stereoselectivity for neuroleptic enantiomers.

In order to identify a pair of neuroleptic enantiomers with the highest stereoselective interaction with neuroleptic/dopamine receptors, the effects of eight pairs of neuroleptic enantiomers were tested on the specific binding of 3H-spiperone to crude homogenates of calf caudate nucleus. The ratios of the Ki values were: (+)-butaclamol/(-)-butaclamol = 3000; dexclamol/(-)-analogue = 151; (+)-isobutaclamol/(-)-isobutaclamol = 146; (-)-CTC/(+)-CTC= 109; (-)-centbutindole/(+)-centbutindole = 20; S(+)-octoclothepin/R(-)-octoclothepin = 11. Thus, the neuroleptic receptor is highly stereoselective for the rigid butaclamol derivatives, but much less so for the flexible neuroleptics. The 3H-apomorphine binding site, however, had a stereoselectivity ratio of only 7 for isobutaclamol, further suggesting that the high affinity sites (i.e. nM) for 3H-neuroleptic binding and for 3H-apomorphine binding are different.     

Evidence for multiple receptors mediating fluid secretion in salivary glands of ticks

Using isolated salivary glands of the ixodid tick Amblyomma hebraeum Koch, we tested the effectiveness of butaclamol and sulpiride in blocking fluid secretion stimulated by a number of agonists. (+)-Butaclamol was a potent inhibitor of dopamine, N-methyldopamine and noradrenaline (Ki congruent to 30-60 nM), but was less effective on ergometrine (Ki congruent to 310 nM). Tranylcypromine-stimulated fluid secretion in the absence and presence of (+)-butaclamol and (+/-)-sulpiride suggested that tranylcypromine's action is mediated through two receptors. (+/-)-Sulpiride, though a rather weak antagonist of ergometrine (Ki congruent to 6150 nM), was ineffectual as a dopamine blocker, indicating distinct receptor sites on this epithelium for dopamine and ergometrine. Both (+)-butaclamol and sulpiride reversed the autoinhibition associated with supramaximal levels of dopamine. Sulpiride also abolished spiperone's potentiation of dopamine. Butaclamol, on the other hand, had no such effect on spiperone's potentiation of dopamine. Finally, although the CNS of ticks contains both dopamine and noradrenaline in quantity (congruent to 650 and congruent to 370 ng . g-1 res respectively), the salivary glands contain far more dopamine than noradrenaline (congruent to 85 and congruent to 6 ng . g-1 respectively). The data support the hypothesis that dopamine is a natural transmitter substance in the tick salivary gland, and that there are distinct receptor sites in the epithelium mediating the actions of catecholamines, ergot alkaloids and butyrophenones. The physiological significance of the ergot alkaloid and butyrophenone sites is not clear.     

Effect of SCH 39166, a novel dopamine D1 receptor antagonist, on [3H]acetylcholine release in rat striatal slices.

SCH 39166 is a novel and selective dopamine D1 receptor antagonist. It has been reported to have potential antipsychotic properties and reduced extrapyramidal side-effect liabilities (EPS). The current studies investigated the pharmacological effects of SCH 39166 on striatal cholinergic function in order to further characterize its dopamine D1 receptor selectivity and to address its EPS liability. Electrically stimulated [3H]acetylcholine (ACh) release from rat striatal slices was measured and comparisons were made between SCH 39166, SCH 23390, (-)-sulpiride, haloperidol or apomorphine on their effect on [3H]ACh release. Results indicated that apomorphine inhibited [3H]ACh release from striatal slices (IC50 = 0.31 microM). (-)-Sulpiride and haloperidol completely reversed the inhibition of [3H]ACh release seen with apomorphine. In contrast, SCH 39166, as well as, SCH 23390 did not reverse the inhibition of [3H]ACh release induced by apomorphine. These findings indicate that dopamine D2 receptors are primarily involved in modulation of [3H]ACh release. Furthermore, selective dopamine D1 receptor antagonists, such as SCH 39166, are ineffective in modulating striatal [3H]ACh release, suggesting that striatal cholinergic hyperactivity and possibly EPS will not be a consequence of dopamine D1 receptor blockade.     

In vitro characterisation of dopamine receptors in the superior colliculus of the rat

In membrane preparations of superior colliculus of the rat, the binding of [3H]spiperone (0.15 nM) was displaced by the incorporation of (+)-butaclamol, haloperidol, apomorphine and (+/-)-sulpiride, but not by (-)-butaclamol, prazosin, propranolol, ketanserin or cinanserin. The Ki values for the displacement of [3H]spiperone by (+/-)-sulpiride, (+)-butaclamol and haloperidol were similar in tissue preparations from superior colliculus and striatum. Equilibrium analysis of the specific binding of [3H]spiperone (0.03-1.0 nM), defined by 10(-5) M (+/-)-sulpiride, to membrane preparations of the superior colliculus, showed the interaction to be saturable and of high affinity. However, the Bmax was only approximately 10% of that found in preparations of striatum; the apparent dissociation constant (KD) was the same in both preparations of the superior colliculus and striatum. Uptake of [3H]dopamine into synaptosomal preparations of the superior colliculus was approximately 20% of that found in synaptosomes from the striatum. In preparations of striatum nomifensine, but not desipramine or fluoxetine, inhibited the uptake of [3H]dopamine. However, in preparations from the superior colliculus, nomifensine, desipramine and fluoxetine were without effect on the uptake of [3H]dopamine. Dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were present in small concentrations in the superior colliculus. Homovanillic acid (HVA) was present in larger concentrations and the HVA plus DOPAC/dopamine ratios were greater in the superior colliculus than in the striatum. The superior colliculus contained only small amounts of noradrenaline but 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were present in larger amounts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of methylenedioxymethamphetamine on the release of monoamines from rat brain slices

The effects of 3,4-methylenedioxymethamphetamine (MDMA) on monoamine release were investigated in superfused slices of rat striatum and hippocampus. MDMA (10 microM) increased the resting release of radioactivity from slices incubated in [3H]dopamine, [3H]5-hydroxytryptamine or [3H]noradrenaline. These effects of MDMA (10 microM) were blocked by the neuronal uptake inhibitors, cocaine (10 microM), fluoxetine (1 microM) and desmethylimipramine (1 microM), respectively. MDMA (10 microM) enhanced the stimulation-induced efflux of radioactivity from slices incubated in [3H]noradrenaline but not from slices incubated in [3H]5-hydroxytryptamine or [3H]dopamine. These results demonstrate for the first time a direct noradrenaline-releasing action of MDMA and differential effects of MDMA on the stimulation-induced release of noradrenaline, dopamine and 5-hydroxytryptamine from rat superfused brain slices.     

Contributory role for nornicotine in nicotine neuropharmacology: nornicotine-evoked [3H]dopamine overflow from rat nucleus accumbens slices.

Nornicotine is a tobacco alkaloid and an active nicotine metabolite, which accumulates in brain to pharmacologically relevant concentrations following repeated nicotine administration to rats. Furthermore, nornicotine is self-administered by rats, indicating that it has reinforcing efficacy and may contribute to nicotine dependence. Since drugs of abuse activate the mesolimbic dopamine (DA) system to produce rewarding effects, the present study tested the hypothesis that nornicotine evokes DA release from nucleus accumbens in a nicotinic receptor-mediated manner. Rat nucleus accumbens slices were preloaded with [3H]DA and superfused for 60 min in the absence and presence of a range of alkaloid concentrations. Superfusate samples were collected and alkaloid-evoked [3H]overflow was determined. S(-)-Nornicotine (EC(50) value = 3.0 microM), R(+)-nornicotine (EC(50) value = 0.48 microM), and S(-)-nicotine (EC(50) value = 70 nM) evoked [3H]overflow in a concentration-dependent manner. For each nornicotine enantiomer, 0.3 microM was the lowest concentration to evoke significant [3H]overflow. Dihydro-beta-erythroidine (DHbetaE, 10 microM), a classical nicotinic receptor antagonist, inhibited the S(-)-nornicotine-evoked [3H]overflow, indicating the involvement of nicotinic receptors. Furthermore, the effect of S(-)-nornicotine was calcium-dependent, consistent with a nicotinic receptor-mediated mechanism. Whereas S(-)-nornicotine was found previously to be more potent in the striatum, R(+)-nornicotine was more potent than its enantiomer in nucleus accumbens, suggesting the involvement of different nicotinic receptor subtypes in these brain regions. Thus, the results of the current study indicate that nornicotine stimulated DA release from nucleus accumbens in a nicotinic receptor-mediated manner, further supporting the hypothesis that nornicotine contributes to tobacco dependence.     

Agonist and antagonist effects of 3-PPP enantiomers on functional dopamine autoreceptors and postsynaptic dopamine receptors in vitro

In contrast to racemic 3-PPP (3-(3-hydroxyphenyl)-N-n-propylpiperidine), (+)-3-PPP appeared to inhibit the electrically evoked release of both [3H]dopamine (DA) and [14C]acetylcholine (ACh) from superfused rat neostriatal slices, although it was considerably less potent in this respect that the DA receptor agonists apomorphine, TL-99 (6,7-dihydroxy-N,N-dimethyl-2-aminotetralin) and LY 141865. At concentrations higher than 1 microM both of the 3-PPP enantiomers increased the spontaneous efflux of 3H but not that of 14C. (+)3-PPP also inhibited the cholera toxin-stimulated release of immunoreactive alpha-MSH from dispersed intermediate lobe cells of the rat pituitary gland. The inhibitory effects of (+)3-PPP on both transmitter and alpha-MSH release were antagonized by the selective D-2 receptor antagonist (-)-sulpiride. Neither [3H]DA nor [14C]ACh release were inhibited by (-)3-PPP but, in contrast, the release-inhibiting effect of the selective D-2 receptor agonist LY 141865 as well as that of (+)3-PPP were antagonized by (-)3-PPP, although less effectively than by (-)sulpiride. The inhibitory effect of LY 141865 on alpha-MSH release from intermediate lobe cells was also antagonized by (-)3-PPP. The data indicate that (+)3-PPP is a weak agonist and (-)3-PPP a weak antagonist at D-2 receptors and that neither of the 3-PPP enantiomers interacts selectively with DA autoreceptors mediating presynaptic modulation of striatal DA release.     

The actions of (-)N-n-propylnorapomorphine and selective dopamine D1 and D2 receptor agonists to modify the release of [3H]dopamine from the rat nucleus accumbens

The ability of (-)N-n-propylnorapomorphine and selective D1 and D2 dopamine receptor agonists and antagonists to modify the release of [3H]dopamine, induced by potassium from the nucleus accumbens, was studied using an in vitro superfusion technique. (-)N-n-Propylnorapomorphine, in picomolar concentrations, inhibited the release of [3H]dopamine, the inhibition being antagonised by fluphenazine and the selective D2 receptor antagonist sulpiride; the selective D1 receptor antagonist SCH 23390 was ineffective. The selective D1 receptor agonist SKF 38393 and the selective D2 agonist quinpirole, both inhibited the potassium-induced release of [3H]dopamine; no synergistic effect was observed to a combined treatment with SKF 38393 and quinpirole. The effects of SKF 38393 and quinpirole were selectively antagonised by SCH 23390 and sulpiride, respectively, although both antagonists failed to modify the release of [3H]dopamine when administered alone. Receptor antagonists for other transmitter sites, e.g. noradrenaline, 5-hydroxytryptamine and acetylcholine, failed to modify potassium-induced release of [3H]dopamine, when administered alone or to prevent the inhibition of the release caused by (-)N-n-propylnorapomorphine. It is concluded that the action of dopamine agonists on both dopamine D1 and D2 receptors in the nucleus accumbens can reduce the release of [3H]dopamine in the in vitro system. Comparable actions in vivo may contribute to the ability of dopamine agonists to moderate locomotor responding.     

No evidence for presynaptic opioid receptors on cholinergic,but presence of κ-receptors on dopaminergic neurons in the rabbit caudate nucleus: involvement of endogeneous opioids

Summary The effects of various opioid receptor agonists and antagonists were studied in rabbit caudate nucleus slices preincubated with either [³H]dopamine or [³H] choline, superfused with medium (containing in most experiments the D₂ receptor antagonist domperidone) and subjected to electrical field stimulation. The stimulation-evoked [³H]overflow from slices prelabeled with [³H]dopamine (evoked [³H]dopamine release) was significantly reduced by preferential -opioid receptor agonists, like U-50,488 H, but not by µ- or -opioid receptor selective drugs. Opioid receptor antagonists shifted the concentration/response curve of U-50,488 H to the right (apparent pA₂-value of the -selective antagonist nor-binaltorphimine: 10.1) and enhanced the evoked dopamine release in the presence of a mixture of peptidase inhibitors.On the other hand, the [³H]overflow from rabbit caudate nucleus slices prelabeled with [³H]choline (evoked acetylcholine release) remained almost unaffected by any opioid receptor agonist, as long as the presynaptic D₂ heteroreceptor was blocked with domperidone: in the absence of domperidone, U-50,488 H exhibited facilitatory effects. For comparison, the effects of the preferential -opioid receptor agonist DPDPE was also studied in slices of the rat striatum, where it clearly inhibited the evoked acetylcholine release.From our data we conlude that in the rabbit caudate nucleus the evoked dopamine release is inhibited by both exogenous and endogenous opioids via presynaptic -opioid receptors, whereas the evoked release of acetylcholine is not, or only indirectly (via released dopamine) affected by opioids. Correspondence to R. Jackisch at the above address