Antinociception induced by stimulation of the habenular complex of the rat

The changes in the tail-flick latency to noxious heat stimulation and in the threshold for defensive/affective reactions to noxious pressure of the skin were studied following electrical stimulation of the habenular complex (HbC) and adjacent brain structures in the male rat. Single brief (15 s), low intensity (53 microA r.m.s.) stimulation of the HbC caused no significant increase in the locomotor activity or motor deficit but induced a potent and short-lasting antinociception as revealed by both algesimetric tests. Animals stimulated in the HbC also displayed poor avoidance learning in a conditioned place preference paradigm, thus suggesting that aversion is unlikely to determine antinociception. Rats daily stimulated in the HbC became tolerant to the antinociception induced by HbC stimulation or to a high systemic dose of morphine (6 mg/kg i.p.). These results indicate that stimulation of the HbC may increase the thresholds of spinally and supraspinally integrated reflexes, thus supporting the hypothesis that this nucleus may play a role in pain control, probably involving an opioid-dependent mechanism.