PHOTOPERIODIC MEDIATION OF SEASONAL BREEDING AND IMMUNE FUNCTION IN RODENTS - A MULTIFACTORIAL APPROACH

Winter is energetically-demanding; thermoregulatory demands increase w hen food availability usually decreases. Physiological and behavioral adaptations, including termination of breeding, have evolved among non tropical animals to cope with winter energy shortages. Presumably, sel ection for mechanisms that permit physiological and behavioral anticip ation of seasonal ambient changes have led to current seasonal breedin g patterns for many populations. Energetically-challenging winter cond itions can directly induce death via hypothermia, starvation, or shock ; surviving these demanding conditions likely evokes significant stres s responses. The stress of coping with energetically-demanding conditi ons may increase adrenocortical steroid levels to the extent that immu ne function is compromised, Individuals would enjoy a survival advanta ge if seasonally-recurring stressors could be anticipated and countere d by shunting energy reserves to bolster immune function. The primary environmental cue that permits physiological anticipation of season is daily photoperiod, a cue that is mediated by melatonin. However, othe r environmental factors, such as low food availability and ambient tem peratures, may interact with photoperiod to affect immune function and disease processes. Laboratory studies of seasonal changes in mammalia n immune function consistently report that immune function is enhanced in short day lengths. Prolonged melatonin treatment mimics short clay s, and also enhances immune function in rodents. In sum, melatonin may be past of an integrative system to coordinate reproductive, immunolo gic, and other physiological processes to cope successfully with energ etic stressors during winter. Social factors influence immune function and changes in social interactions may also contribute to seasonal ch anges in immune function. The mechanisms by which social factors are t ransduced into immune responses are largely unspecified. In order to u nderstand the optimization of immune function it is necessary to under stand the interaction of factors, on both mechanistic and functional l evels, that affect immunity.