Photoperiodic effects on endocrine and immune function in cattle.

Photoperiod is the most common environmental factor monitored by animals to alter long-term physiological processes, particularly reproduction. As cattle are not strict seasonal breeders, the influence of photoperiod on cattle has been studied less extensively than in other large mammals, but the lack of effect of daylength on reproduction enhances the utility of the bovine model in examining the effect of this factor on growth, lactation and immune function. In cattle, as in other species, increasing exposure to light reduces the duration of melatonin secretion. A long day pattern of melatonin secretion increases circulating prolactin and insulin-like growth factor I (IGF-I) concentrations and these endocrine shifts are consistent with observed effects on lactation, and body growth and composition in cattle. In addition, we have observed that long day photoperiod decreases the metabolic perturbations encountered in response to the immune challenge with lipopolysaccharide. Although circulating hormones, such as prolactin and IGF-I, are certainly involved in these responses, receptor-mediated effects of photoperiod are also active. Currently, our focus is on photoperiod-induced shifts in receptor expression in the mammary gland, liver and leucocytes, as target tissues for the photoneuroendocrine response.