Melatonin and the brain in photoperiodic mammals.

The reproductive cycle of photoperiodic species is driven by seasonal changes in daylength. The pineal gland transduces photic information into an endocrine signal. The duration of the nocturnal bout of melatonin secretion is a direct indicator of night-length. The circadian rhythm of melatonin production is driven by a multisynaptic pathway from the suprachiasmatic nuclei (SCN), via the parvocellular portion of the paraventricular nucleus to the preganglionic sympathetic neurons of the thoracic spinal cord. The melatonin signal acts as an interval timer. The cellular basis of the detection of the signal is unknown. The site of detection is possibly within the anterior hypothalamus. The SCN are not essential components of the system that responds to the pineal interval timer. Photoperiod and the pineal melatonin signal have pronounced effects on the function of endogenous opioids, which are probably related to changes in the neuroendocrine mechanisms that regulate gonadotropin release.