EFFECTS OF DAY-LENGTH VARIATIONS ON EMOTIONAL RESPONSES TOWARDS UNFAMILIARITY IN SWISS MICE

Pineal melatonin secretion occurs at night in all vertebrates and the duration of its secretion is negatively correlated with day length. As an anxiolytic activity of melatonin has been shown in rats and mice, this study examined possible changes of emotional reactivity in respon se to day length variations in Swiss mice. Three groups of mice were o bserved in a free-exploratory test: a group submitted to a short-day e xposure (6:18 h light-dark cycle) for 2 weeks, a group submitted to a long-day exposure (18:6 h light-dark cycle) for 2 weeks and a control group which was maintained in housing 12:12 h light-dark cycle. The sh ort-day exposed group of mice exhibited significantly fewer attempts t o enter into the unfamiliar enclosure, spent significantly more time i n it and presented significantly more rears than controls whereas the long-day exposed group of mice made more attempts than controls. These results suggest a decreased emotional level in short-day exposed mice and an increased level in long-day exposed mice. This could be interp reted as confirming the idea of anxiolytic-like properties of melatoni n; however, the specific role of this hormone in the changes of anxiet y related to day length must be assessed by further measures of potent ial variations of circulating melatonin. (C) 1997 Elsevier Science B.V .