Photoperiodic Mediation of Seasonal Breeding and Immune Function In Rodents: A Multi-Factorial Approach

Abstract

SYNOPSIS. Winter is energetically-demanding; thermoregulatory demands increase when food availability usually decreases. Physiological and behavioral adaptations, including termination of breeding, have evolved among nontropical animals to cope with winter energy shortages. Presumably, selection for mechanisms that permit physiological and behavioral anticipation of seasonal ambient changes have led to current seasonal breeding patterns for many populations. Energetically—challenging winter conditions can directly induce death via hypothermia, starvation, or shock; surviving these demanding conditions likely evokes significant stress responses. The stress of coping with energetically-demanding conditions may increase adrenocortical steroid levels to the extent that immune function is compromised. Individuals would enjoy a survival advantage if seasonally-recurring stressors could be anticipated and countered 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, other environmental factors, such as low food availability and ambient temperatures, may interact with photoperiod to affect immune function and disease processes. Laboratory studies of seasonal changes in mammalian immune function consistently report that immune function is enhanced in short day lengths. Prolonged melatonin treatment mimics short days, and also enhances immune function in rodents. In sum, melatonin may be part of an integrative system to coordinate reproductive, immunologic, and other physiological processes to cope successfully with energetic stressors during winter. Social factors influence immune function and changes in social interactions may also contribute to seasonal changes in immune function. The mechanisms by which social factors are transduced into immune responses are largely unspecified. In order to understand the optimization of immune function it is necessary to understand the interaction of factors, on both mechanistic and functional levels, that affect immunity.