Melatonin modulates the neural activity in photosensory pineal organ of the trout: evidence for endocrine-neuronal interactions.

Hormonal and neural signals transmitted from the pineal organ to the brain in cold-blooded vertebrates presumably convert information about the ambient illumination into signals which may be used to mediate photoperiodic and circadian responses. The possible intrapineal function of melatonin was investigated by recording intra- and extracellularly from photoreceptors and second-order neurons in the isolated superfused pineal organ of the trout (Salmo gairdneri). Melatonin added through the perfusion bath to the explanted pineal organ caused a dose-related and reversible inhibition of ganglion cells of the luminance type whereas the hormone did not significantly affect the membrane potential of photoreceptors and their light-evoked response. The observed effects seem to be independent from photoperiod and adaptation conditions. These results suggest that melatonin provides a feedforward signal to intrapineal neurons regulating the neural output of the organ.