Comparison of the effects of yohimbine and clonidine on the behaviour of female mice during social encounters in an "approach-avoidance" situation.

Effects of yohimbine (2 and 5 mg/kg, i.p.) and clonidine (10 and 50 micrograms/kg, i.p.) on the behaviour of adult female CD1 mice during 5 min encounters in a neutral cage with unfamiliar male partners have been examined by ethological procedures at 30 min after injection. Yohimbine induced dose-related increases in the frequency, bout length and duration of the immobile postures, "sit" and "social crouch", while decreasing the frequency of "explore", "scan", "attend" and "investigate", and increasing their bout lengths in a dose-related manner. These results suggest that yohimbine decreased the rate of switching from one behavioural act to another. Pausing between acts was increased by yohimbine to a similar extent at both of the tested dose levels. The act "wash" was increased in duration by yohimbine, whereas the strenuous activity of "digging" showed a dose-related decrease in frequency, duration and bout length. It is proposed that these effects are induced by the known interactions of yohimbine with receptors for dopamine as well as with alpha 2-adrenoceptors. Clonidine reduced motor activity, evident as a dose-related increase in the frequency and duration of "sitting" coupled with decreased frequency and increased bout length of the act, "explore" (significant at 50 micrograms/kg). Clonidine also dose-dependently reduced the frequency and duration of substrate "sniffing". Clonidine decreased occurrence of the specific social acts, "attend" and "investigate", as well as reducing frequency although not duration of overall social investigation. These findings have parallels with reported clinical effects of clonidine, such as sedation and impairments of attention, which must limit its clinical usefulness.