The role of rhythms in the response to melatonin

Rhythmicity of melatonin secretion is critical to the regulation of mammalian reproduction by daylength. In the ewe, photoperiod determines ovarian function by modulating the ability of oestradiol to suppress pituitary secretion of luteinizing hormone (LH). This influence of daylength depends in turn upon the pineal gland, which mediates photic control of the frequency at which the brain stimulates the pituitary to secrete gonadotropin. Photoperiod determines the pattern of melatonin secretion, most notably setting the duration of the nightly elevation in serum concentrations. Replacement of melatonin in pinealectomized ewes drives responsiveness to oestradiol negative feedback; LH levels are determined by the duration of the nightly melatonin infusion rather than by the photoperiod to which ovariectomized, oestradiol-implanted pinealectomized ewes are exposed. Refractoriness to stimulatory and inhibitory photoperiods may reflect circannual modulation of the responsiveness of neuroendocrine mechanisms to melatonin signals of a given duration.