Affective and adrenocorticotrophic responses to photoperiod in Wistar rats.

The present study tested the hypothesis that seasonal intervals of exposure to modest changes in photoperiod, typical of those experienced by humans living in temperate latitudes (10-14 h light/day), engage changes in emotional behaviour of Wistar rats, a commonly-used animal model for investigations of affective physiology. Short day lengths (<or= 12 h light/day) induced behavioural despair in a forced-swim test, exploratory anxiety in an open field arena, and anhedonia in a two-bottle sucrose preference task, relative to longer day lengths. Plasma adrenocorticotrophic hormone was lower in short-day relative to long-day rats, but testosterone and corticosterone concentrations were comparable across treatments. In common with animals that engage reproductive responses to day length, reproductively nonresponsive mammals such as Wistar rats exhibit changes in affective state following small changes in day length. Wistar rats may provide an animal model for the study of seasonal mood regulation because the neuroendocrine, depressive, anxious and anhedonic responses of Wistar rats to short days bear similarities to those observed in some human populations. Standard laboratory husbandry practices (exposure to a 12 : 12 h light/dark cycle) may inadvertently deliver a chronic background depressive and anxiogenic stimulus.