Pineal Gland, Melatonin, and timekeeping in nonmammalian vertebrates: Avian perspective

The pineal gland, shaped somewhat like a pinecone, is a small organ that lies deep in the brain of mammals, but is superficially placed in the brain of nonmammal vertebrates. In birds, its anatomical location is very distinct, lying superficially in the triangular space formed behind the junction of cerebral hemispheres and the anterior aspects of cerebellum. The known product of the pineal gland is melatonin, which is produced at night regardless of the nature of the species, diurnal, nocturnal, or crepuscular. Melatonin, an indoleamine hormone, is almost universally present, albeit with varied functions, and is generally called as evolutionary connecting molecule. In birds as in other vertebrates, melatonin is involved in the circadian organization behavioral and physiological functions as well as in photoperiodic measurement. Unlike in mammals in which the duration of melatonin is a critical determinant of the seasonal reproductive response, daily changes in melatonin profile seem not critical for photoperiodic induction of gonadal growth and development. Melatonin also plays significant roles in the other daily and seasonal functions, like the immune responses. As part of the multioscillatory circadian timekeeping in nonmammal species, melatonin perhaps is involved both as the pacemaking and coupling agent. As latter, it contributes to the flexibility of the clock system in concurrence with changes in the external environment (e.g., photoperiod). The degree to which these roles are assigned could be linked to the interdependence of the self-sustainment of participating oscillators of the circadian oscillations, the photoperiod environment, as well as the annual life-history state of the species. Finally, whether results from laboratory studies would hold true for free-living populations remains largely unknown at this time.