Circannual prolactin rhythms: calendar-like timer revealed in the pituitary gland.

Although photoperiodic regulation of annual rhythms in reproduction and other functions has been well characterized, the basis for the endogenous generation of circannual rhythms during exposure to constant conditions has not been elucidated. Lincoln and colleagues have recently reported that circannual prolactin rhythms in rams persist after hypothalamo-pituitary disconnection, but not after pinealectomy. Does the pars tuberalis, the site of pituitary gland melatonin receptors, generate circannual rhythms and integrate photoperiodic signals mediated by melatonin?