Effects of 8-OH-DPAT on ultrasonic vocalization and audiogenic immobility reaction in pre-weanling rats.

Emotional reactivity in preweanling rats was assessed by observations of two reactions characteristic for two different age periods, respectively. One reaction, ultrasonic vocalization in rat pups separated from mother and littermates, was observed during early postnatal age (10 days). The other reaction, rigid immobility elicited by a sudden sound, was observed at the age of weaning (20 days). Previous studies from this laboratory indicated that the serotonergic system was involved in the control of both of these reactions. In the present study this notion was further tested by investigating the effects on these reactions of the congener of ergot 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), characterized as a centrally-acting, potent and selective agonist of 5-hydroxytryptamine (5-HT). Rat pups, 10 days of age, were treated subcutaneously with 7.5, 15 and 30 micrograms/kg of 8-OH-DPAT and subsequently tested for ultrasonic vocalization. Animals, 20 days old, were treated with 15, 30 and 60 micrograms/kg of 8-OH-DPAT before testing of the immobility reaction. The results showed a dose-dependent decrease of the amount of ultrasonic vocalization and of the duration of the immobility reaction, indicating an anxiolytic-like action of 8-OH-DPAT in both behavioural patterns. A possible explanation for the antagonistic effect of 8-OH-DPAT is that this drug exerts a preferential agonistic effect on presynaptic (auto-) 5-HT receptors.