Regulation of pineal melatonin secretion: comparison between mammals and birds.

The melatonin secretory pattern of the perifused rat and chicken pineal was compared in response to different lighting conditions and norepinephrine administration. The following main differences were observed. 1. Explanted (perifused) rat pineal showed, after a rapid initial surge, a steady continuous basal secretion of melatonin. This was independent from the day-night (light-dark) periods. Chicken pineal, however, showed a characteristic daily rhythm of melatonin production with peak in the dark and nadir in the light phase. 2. This daily rhythm could not be extinguished by keeping the pineal donor birds in constant light- or constant dark environment for at least 2 weeks immediately before sacrifice. 3. Short light impulse (5 min), applied in the middle of the dark phase, was ineffective in birds. Keeping the perifusion chambers in continuous light or darkness, however, depressed the amplitude of the circadian rhythm even in the next cycle. 4. Rat pineal responds to norepinephrine stimulation with a dose-related increase of melatonin release, independently from the phase of the day, while in the chicken, norepinephrine slightly inhibits both the diurnal and the nocturnal level of melatonin secretion. 5. It can be inferred that melatonin secretion of the mammalian pineal gland is primarily regulated by a peripheral (sympathetic) innervation. This is modulated, under in vivo circumstances, by different environmental factors, mainly by light conditions transmitted by neural mechanisms. In contrast, the primary secretory process of the avian pineal is based on an intrinsic circadian rhythm. This might be genetically coded or maintained by yet unknown neurohormonal mechanisms and/or external factors (e.g. magnetic fields). This fairly stable circadian rhythm is only modulated by environmental lighting conditions.