Activation of 5-HT2B receptors in the medial amygdala causes anxiolysis in the social interaction test in the rat

In a recent study, we reported the presence of neurones expressing 5-HT2B receptor protein in the medial amygdaloid nucleus of the adult rat brain. In the present study, bilateral micro-injection of the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride (BW 723C86, 0.09 and 0.93 nmol, 5 min pretest) into the medial amygdaloid nuclei increased the total interaction time of a pair of male rats in the social interaction test, to a comparable extent to chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) without altering locomotor activity; indicative of anxiolytic activity. The increase in social interaction was prevented by pretreatment with the 5-HT2C/2B receptor antagonist N-(1-methyl-5-indoyl)-N'-(3-pyridyl) urea hydrochloride (SE 200646A, at 2 but not 1 mg/kg p.o., 1 hr pretest), which did not alter behaviour when given alone. Intra-amygdala BW 723686 (0.09, 0.31 and 0.93 nmol, 5 min pretest) did not significantly alter the number of punished responses made when the same rats were examined seven days later in a Vogel punished drinking test, although chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) produced the expected anxiolytic profile. The results are consistent with the proposal that activation of 5-HT2B receptors in the medial amygdala induces anxiolysis in the social interaction model but has little effect on behaviour in a punished conflict model of anxiety. These data suggest that serotonergic neurotransmission in this nucleus may selectively affect specific kinds of anxiety generated by different animal models. (C) 1997 Elsevier Science Ltd.