Effect of serotonergic drugs on footshock-induced ultrasonic vocalization in adult male rats.

Modulation of ultrasonic vocalization (20-30kHz) emitted by adult rats under stressful conditions such as unavoidable foot-shock has been evaluated as a model of anxiety. The effects of 5-HT(1A) receptor agonists with different intrinsic activities and the role of other 5-HT(1) receptor subtypes, and of 5-HT(2) and 5-HT(3) receptors, in mediation of ultrasonic vocalization were studied, as were the effects of increasing serotonergic activity by administration of the 5-HT releaser fenfluramine or the 5-HT precursor 1-5 HTP. The time spent vocalizing 1-6min after four increascapable (1.0mA) footshocks was recorded. Drugs with affinity for 5-HT(1A) receptors (i.e. 8-OHDPAT, flesinoxan, ipsapirone, buspirone, gepirone, NAN-190) abolished the vocalization irrespective of their efficacy. The mixed 5-HT(1) receptor and beta-adrenoceptor antagonists (-)-alprenolol and pindolol inhibited foot-shock-induced ultrasonic vocalization, whereas (-) penbutolol was ineffective. The beta(1)-adrenoceptor antagonist metoprolol and the beta(2)-adrenoceptor antagonist ICI 118.551 were without effect. This suggests that (-)-alprenolol and pindolol act as partial 5-HT(1) agonists in the test model. The non-selective 5-HT(1) receptor agonists eltoprazine, m-CPP and 5-MeODMT and the 5-HT(2) receptor agonists DO1 and d-LSD also abolished the vocalization, whereas the 5-HT(2) receptor antagonist ritanserin and the 5-HT(3) receptor antagonists ondansetron, ICS 205-930 and zacopride were without effect. (-)-Penbutolol reversed 8-OHDPAT-induced inhibition. Ritanserin reversed DOI-induced inhibition of ultrasonic vocalization, but not 8-OHDPAT-induced inhibition. This suggests that there is no functional interaction between 5-HT(1A) and 5-HT(2) receptors in this model. Fenfluramine and 1-5-HTP dose-dependently inhibited footshock-induced ultrasonic vocalization. These findings indicate that the effect most likely is mediated by postsynaptic 5-HT receptors, although contribution by presynaptic 5-HT receptors cannot be excluded. In conclusion, this study indicates that 5-HT(1A) receptors and 5-HT(2) receptors are involved in mediation of ultrasonic vocalization.