Behavioural effects in gerbils of the 5-HT3 receptor antagonists, BRL 43694 and ICS 205-930, under circumstances of high and low light intensity.

The 5-HT3 receptor antagonists, BRL 43694 and ICS 205-930, were each given for 21 days in the drinking fluid at 1.3 mg/l (120 micrograms/kg daily), to Mongolian gerbils, while the controls received tap water to drink. Effects of the treatments in reducing aversion to a brightly lit environment were assessed on behaviour during social encounters with an unfamiliar untreated resident, under bright white light and in a two-compartment black and white test box, after 12-16 days of treatment. Effects on behaviour under dim red illumination, when encountering unfamiliar untreated residents, were examined after 17-19 days. Behaviour during social encounters was recorded by ethological procedures. During encounters under bright white light, the frequency and duration of the social element "attend" were increased by BRL 43694 and ICS 205-930 and the frequency of "nose" was increased by BRL 43694. In the light-dark box, BRL 43694, though not ICS 205-930, reduced the time spent in the dark compartment. Under dim red light, BRL 43694 and ICS 205-930 increased the occurrence of the social elements, "sniff", "follow" and "sniff chin", suggesting increased sensitivity to olfactory stimuli. Increases of social investigation were associated with compensatory changes to non-social behaviour. It is suggested that 5-HT3 receptor antagonists may, on the one hand, increase sensitivity to social stimuli under dim red illumination and, on the other hand, show an apparent anxiolytic potential, associated with increase of other elements of social investigation under the more aversive test conditions of bright white light.