Neuroendocrine regulation of seasonal gonadotrophin and prolactin rhythms: lessons from the Soay ram model.

Marked seasonality, responsiveness to photoperiod, diurnal behaviour, large body size, long lifespan and adaptability in captivity are characteristics that make the Soay ram a useful model for neuroendocrine research. Adult rams are routinely housed indoors under artificial lighting of alternating 16 week periods of long and short days to entrain the seasonal cycles in reproduction, growth and metabolism. The long-term cycles in individuals are monitored directly (measurements of testis diameter, androgen-dependent skin coloration, food intake, pelage moult, locomotor activity) and retrospectively (measurements of reproductive and metabolic hormone concentrations in peripheral blood). A wide spectrum of experimental procedures, including serial blood and cerebrospinal fluid (CSF) sampling with hormone or drug treatments, tissue biopsy, stereotaxic cerebral implantation and surgical lesions, not feasible in smaller species, are used to investigate the multiple interactive neuroendocrine systems regulating seasonality. The results from a recent experiment in which rams received a lesion of the caudal arcuate nucleus (caudal ARCX) or hypothalamo-pituitary disconnection (HPD) are presented to demonstrate the fidelity of long-term data derived from the Soay ram model. The results support the view that the melatonin signal that encodes photoperiod acts within the mediobasal hypothalamus to time the gonadotrophin/gonadal cycle, but acts directly within the pituitary gland to time the prolactin/pelage cycle.