Rapid effects of melatonin on hormonal and behavioral stressful responses in ewes

Sheep are gregarious mammals with complex social interactions. As such, they are very sensitive to social isolationand constitute a relevant animal model to study specifically the biological consequences of social stress. We examined previously the behavioral and endocrine responses in ewes isolated socially in the familiar conspecific withdrawal model (FCW) and showed that stressful responses increased and maintenance behaviors decreased, confirming that social isolationis a strong stressor in sheep. Melatoninsynchronizes seasonal and circadian rhythms; and several studies reported its implication in cognitive processes as emotion. Here we investigated its role in the modulation of social stressfulresponses. Firstly, we studied ewes in the FCW model during the day (characterized by low melatoninlevels) and the night (characterized by high melatoninlevels). We found lower stressful responses (significant lower levels of cortisol plasma, number of foot pawings, of circling attempts) during the night as compared to the day. To investigate whether these effects were due to melatoninor to darkness, we submitted ewes to FCW during the night with lights on, a condition that suppresses melatoninsecretion. Ewes infused with melatoninunder these conditions showed decreased stressful responses (significant lower levels cortisol plasma, number of vocalizations, time spent with the head out of the cage) as compared to ewes infused with saline. These findings demonstrate that melatonindiminishes the endocrine and behavioral impact of social isolationin ewes and support the idea that melatoninhas a calming effect in socially stressfulsituations.