Antinociception elicited by electrical or chemical stimulation of the rat habenular complex and its sensitivity to systemic antagonists.

The effects of intraperitoneal administration of antagonists to morphine, norepinephrine, acetylcholine, dopamine and 5-hydroxytryptamine (5-HT) have been studied on the antinociceptive effect of electrical stimulation of the rat habenular complex (HbC). The antinociceptive effect of agonists microinjected into the HbC was also examined. A 15-s period of 53 microA rms sine-wave stimulation of the HbC significantly increased the latency of the tail-flick reflex to noxious heat for periods of up to 15 min. This effect was significantly attenuated by pretreating rats with naloxone (1 mg/kg) or phenoxybenzamine (5 mg/kg). Methysergide (5 mg/kg), haloperidol (5 mg/kg), atropine (1 mg/kg), and mecamylamine (1 mg/kg) had little effect on the antinociceptive effect of HbC stimulation. L-Glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and carbachol (0.4 and 0.8 micrograms), but not 5-HT (5 micrograms), dopamine (5 micrograms) or norepinephrine (5 micrograms), induced a dose-dependent increase in the tail-flick latency when microinjected into the HbC. The effect of carbachol was significantly attenuated in rats previously treated with intraperitoneal administration of atropine or mecamylamine and fully depressed in rats previously treated with a combination of these two cholinergic antagonists. It is concluded that antagonists of opiate receptors and alpha-adrenoceptors, but not dopamine or cholinergic receptors, reduce the antinociceptive effects of HbC stimulation. These observations differ from the reported effects of these antagonists on the antinociception caused by stimulating the periaqueductal gray, but resemble the antinociception caused by stimulating the ventrolateral medulla and locus coeruleus.(ABSTRACT TRUNCATED AT 250 WORDS)