ABSTRACT
In common with reproduction, immune function exhibits strong seasonal patterns, which are driven by annual changes in day length (photoperiod) and melatonin secretion. Whereas changes in melatonin communicate seasonal time into the reproductive axis via subcortical receptors, the relevant melatonin targets for communicating seasonal time into the immune system remain unspecified. The authors report that melatonin implants targeting the hypothalamic suprachiasmatic nuclei (SCN) induced a winter phenotype in the immune system. SCN melatonin implants attenuated infection-induced anorexia and cachexia, indicating that the SCN mediate the effects of melatonin on these behavioral and metabolic symptoms of infection. However, SCN melatonin implants failed to induce winter-like peripheral leukocyte concentrations or behavioral thermoregulatory responses to infection. In contrast, subcutaneous melatonin implants induced winter-like changes in all behavioral and immunological parameters. Melatonin acts directly at the SCN to induce seasonal changes in neural-immune systems that regulate behavior. The data identify anatomical overlap between neural substrates mediating the effects of melatonin on the reproductive and immune systems but also suggest that the SCN are not the sole mediator of photoperiodic effects of melatonin on immunity.