THE PINEAL ORGAN OF TELEOST FISHES

The pineal organ of teleost fish is a directly photosensory organ that contains photoreceptor cells similar to those of the retina. It conve ys photoperiod information to the brain via neural pathways and by rel ease of indoleamines, primarily melatonin into the circulation. The ph otoreceptor cells respond to changes in ambient illumination with a gr adual modulation of neurotransmission to second-order neurons that inn ervate various brain centres, and by modulation of indoleamine synthes is. Melatonin is produced rhythmically, and melatonin synthesis may be regulated either directly by ambient photoperiod, or by an endogenous circadian oscillator that is entrained by the photoperiod. During nat ural conditions? melatonin is produced at highest levels during the ni ght. Although the pineal organ undoubtedly influences a variety of phy siological parameters, as assessed by experimental removal of the pine al organ and/or administration of exogenous indoleamines, its role in any physiological situation is not clear cut. The effects of any inter ference with pineal functions appear to vary with the time of year and experimental photothermal regimes. There are strong indications that the pineal organ is one component in a central neural system that cons titutes the photoperiod-responding system of the animal, i.e. the syst em that is responsible for correct timing of daily and seasonal physio logical rhythms. It is important to envisage the pineal organ as a par t of this system; it interacts with other photosensory structures (the retina, possibly extraretinal non-pineal photoreceptors) and circadia n rhythm generators.