Abstract

The effect of the neuronal acetylcholine-gated ion channel receptor agonist (+/-)-epibatidine was studied on neurotransmitter release in vitro and motor behavior in vivo. (+/-)-Epibatidine (3-300 nM) caused a concentration- and calcium-dependent release of [3H]-dopamine from striatal slices and [3H]-norepinephrine release from hippocampal and thalamic slices. (+/-)-Epibatidine-induced neurotransmitter release was inhibited in all three regions by mecamylamine (3 microM). In contrast, D-tubocurarine (10-100 microM) inhibited only (+/-)-epibatidine-induced [3H]-norepinephrine release from the hippocampus and the thalamus. Conversely, dihydro beta-erythroidine (3-100 microM) inhibited (+/-)-epibatidine-induced [3H]-dopamine release in the striatum without significantly altering [3H]-norepinephrine release from either the hippocampus or the thalamus. This is consistent with the observation that different nAChRs modulate dopamine release as compared with norepinephrine release. The effect of (+/-)-epibatidine on both [3H]-dopamine and [3H]-norepinephrine release was tetrodotoxin-sensitive, suggesting the involvement of sodium channels. (+/-)-Epibatidine (1-3 micrograms/kg s.c.) produced ipsilateral turning in the unilaterally [6(OH)-DA]-lesioned rat. This effect was mimicked by (-)-nicotine (0.35 mg/kg s.c.). Both (+/-)-epibatidine- and (-)-nicotine-induced turning were significantly inhibited by mecamylamine (3 mg/kg s.c.), indicating that the turning response was mediated by nAChRs. (+/-)-Epibatidine also increased locomotor activity in a dose-dependent manner. (+/-)-Epibatidine-induced hyperactivity was blocked by D1 and D2 receptor antagonists, SCH 23390 and eticlopride, respectively, suggesting that both dopamine receptor subtypes might be required for the locomotor effect of (+/-)-epibatidine. These results demonstrate that (+/-)-epibatidine displays nAChR agonist activity in the rat CNS and that certain effects are mediated via nAChR-stimulated catecholamine release and subsequent activation of corresponding receptors.