Regulation of [3H] dopamine release from mesolimbic and mesocortical areas of guinea pig brain by sigma receptors.

The role of sigma (sigma) receptors in brain function is poorly defined. They are located in limbic areas, including nucleus accumbens (NAC) and prefrontal cortex (PFC), both of which are thought to be involved in schizophrenia. Many antipsychotics (APs), including haloperidol, bind with high affinity to sigma receptors. Dopaminergic hyperactivity in NAC is thought to underlie positive symptoms of schizophrenia, while dopaminergic hypoactivity in PFC is thought to underlie negative symptoms. Sigma receptors regulate N-methyl-D-aspartate (NMDA)-stimulated [3H] dopamine ([3H]DA) release in caudate-putamen (CP), the neuroanatomical substrate for extrapyramidal side effects resulting from chronic AP treatment. In the current study, we investigated whether sigma receptors could similarly regulate DA release in mesolimbic and mesocortical tissue, and the relative participation of different sigma receptor subtypes in this process. We found that, in NAC, regulation of DA release by the prototypical sigma agonist (+)pentazocine was mediated predominantly by the sigma 1 receptor, whereas in the PFC a portion of the (+)pentazocine effect was likely mediated by the sigma 2 receptor. We also observed, in both the NAC and PFC, that regulation of DA release by the sigma agonist BD737 was mediated primarily by the sigma 1 receptor. In addition, we determined that (+)pentazocine or BD737 effects on DA release were not mediated via opioid receptors, nor the phencyclidine (PCP) binding site within the NMDA receptor-operated cation channel, nor by sigma receptor effects upon [3H]DA accumulated by noradrenergic terminals in PFC.

Sigma receptor regulation of norepinephrine release from rat hippocampal slices.

Multiple sigma receptor subtypes have been identified in the hippocampus, yet their physiological role remains largely undefined. In the current study, we examined the role of sigma receptors in the regulation of N-methyl-D-aspartate (NMDA)-stimulated [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. Both sigma agonists (+)pentazocine and BD737 inhibited stimulated norepinephrine release in a concentration-dependent manner. The sigma1 antagonist DuP 734 completely antagonized the inhibition of release by all concentrations of BD737 tested. However, DuP 734 only partially reversed inhibition of release by (+)pentazocine concentrations above 100 nM. 1,3 Di-o-tolylguanidine (DTG), but not haloperidol, antagonized BD737-mediated inhibition of release. DTG also completely antagonized inhibition of release by 100 nM (+)pentazocine yet haloperidol produced only a partial reversal. A combination of DuP 734 and haloperidol produced complete reversal of (+)pentazocine-mediated inhibition, suggesting potential involvement of multiple sigma receptor subtypes in the regulation of norepinephrine release. Both (+)pentazocine and BD737 failed to inhibit stimulated release in the presence of tetrodotoxin, suggesting that sigma receptors regulating NE release are not located on noradrenergic nerve terminals. These results suggest that sigma receptors may be a therapeutic target for disorders resulting from noradrenergic imbalance in hippocampus.

sigma1 Receptors in rat striatum regulate NMDA-stimulated [3H]dopamine release via a presynaptic mechanism.

The role of the sigma1 receptor in the regulation of N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine release from rat striatal slices was examined. The sigma receptor agonist 1S,2R-(--)-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)cy clohexylamine (BD737) inhibited stimulated release in a concentration-dependent manner. The sigma1 receptor antagonist, 1-(cyclopropylmethyl)-4-(2'-(4"-fluorophenyl)-2'-oxoethyl)piperidi ne HBr (DuP 734), reversed inhibition of release by BD737. Haloperidol, di-o-tolylguanidine (DTG) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD1008) reversed the BD737-mediated inhibition of release. Haloperidol and DTG also antagonized inhibition of stimulated release by (+)-pentazocine. Furthermore, BD737 and (+)-pentazocine inhibited stimulated release in the presence of tetrodotoxin, suggesting that sigma1 receptors regulating dopamine release are located on dopaminergic nerve terminals. These data suggest that sigma1 receptors may be important in the regulation of glutamate-stimulated dopamine release.

Regulation of [3H]norepinephrine release from guinea pig hippocampus by sigma2 receptors.

The binding profile of the sigma2 receptor ligand endo-N-(8-methyl-8-azabicyclo[3.2.1.]oct-3-yl)-2,3-dihydro-(1-methyl)eth yl-2-oxo-1H-benzimidazole-1-carboxamidehydrochloride (BIMU-8) had previously been determined, but its agonist/antagonist status at sigma2 receptors had not been identified. We therefore investigated the effects of BIMU-8 for its ability to regulate the stimulated release of [3H]norepinephrine from slices of guinea pig hippocampus. BIMU-8 alone, at a concentration chosen to occupy 50% of sigma2 receptors, had no significant effect on N-methyl-D-aspartate (NMDA)-stimulated release of [3H]norepinephrine. We have shown previously that the sigma receptor agonist (+)-pentazocine inhibits NMDA-stimulated release in a concentration-dependent manner, producing a biphasic inhibition curve. Similarly, the sigma receptor agonist 1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl )cyclohexylamine (BD737) produced a broad inhibition curve. The inhibition by low concentrations of (+)-pentazocine or BD737 that selectively activated sigma1 receptors was reversed by the sigma1-selective receptor antagonist (1-(cyclopropylmethyl)-4-2'-oxoethyl)piperidine HBr (DuP 734). In the current study, when the sigma1 component of inhibition by (+)-pentazocine was blocked by DuP 734, the remaining component of inhibition mediated by sigma2 receptors was reversed by BIMU-8. Our results suggest that (1) BIMU-8 is an antagonist at sigma2 receptors and that (2) sigma2 receptors contribute to regulation of norepinephrine release in guinea pig hippocampus.

Regulation of [3H]dopamine release from rat striatal slices by sigma receptor ligands.

Sigma receptors have been located in several areas of the brain that control motor function, including on the dopaminergic projections from substantia nigra to striatum. In the current study, the regulation of N-methyl-D-aspartate-stimulated [3H]dopamine release from slices of rat striatum by several sigma ligands has been tested. Both isomers of the benzomorphans SKF10,047 and pentazocine inhibited the stimulated release of dopamine in a concentration-related manner. All these compounds probably activate sigma and non-sigma receptors, including phencyclidine receptors, over the broad concentration ranges tested. However, concentrations of (+)pentazocine below about 100 nM appear to act solely through sigma receptors. This phase of inhibition was reversed by the sigma antagonist N-[-2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-[1- pyrimidinyl-1-piperazine butanol and by the sigma1-selective antagonist (1-(cyclopropylmethyl)-4-2'4"-fluorophenyl)-(2'-oxoethyl)piperi din e HBr. Neither of these antagonists affected stimulated release in the absence of (+)pentazocine. The synthetic sigma ligands 2-(4-morpholino)ethyl 1-phenylcyclohexane-1-carboxylate hydrochloride, 6-[6-(4-hydroxypiperidinyl)-hexoxy]-3-methylflavone hydrochloride and alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1- piperazine butanol enhanced NMDA-stimulated DA release significantly in the presence of (+)pentazocine. These drugs have affinity at non-sigma receptors as well, and their stimulatory effects may be mediated through these receptors along with nonreceptor mechanisms. Our findings on the regulation of dopamine support earlier assertions that sigma receptors may be important in the regulation of motor function.

Functional and binding properties of sigma receptors in rat cerebellum.

Autoradiographic studies have shown that sigma receptors are enriched in the locus coeruleus, the origin of noradrenergic projections to the cerebellum, as well as in the Purkinje, molecular, and granular layers and the interpositus cerebellar nucleus of the cerebellum itself. In contrast, the cerebellum is relatively poor in phencyclidine (PCP) binding sites, which have been historically confused with sigma sites. The high ratio of sigma to PCP receptors in cerebellum is advantageous for discriminating sigma-mediated physiological effects. sigma agonists and antagonists have been shown to regulate N-methyl-D-aspartate (NMDA)-stimulated norepinephrine release in hippocampus, which is innervated by locus coeruleus projections. We now report that sigma drugs also regulate norepinephrine release from cerebellum. In contrast to findings in the hippocampus, where regulation is via sigma 1 and sigma 2 receptors, sigma-mediated regulation in cerebellum seems to be primarily via sigma 1 receptors. In radioligand binding studies, we find that sigma receptors primarily of the sigma 1 type are present in the cerebellum. We further report that binding to sigma receptors in cerebellum is not affected by the addition of NMDA or glycine or by the presence of NMDA antagonists, suggesting that sigma receptors are not located within the NMDA-operated cation channel in this brain region.