The avian pinealocytes have an intrinsic circadian clock function that controls rhythmic synthesis of melatonin, and an environmental light signal can reset the phase of the clock. In addition to the photoendocrine function, the melatonin synthesis of the pinealocytes is regulated by neural signals from sympathetic nerves. Thus the avian pinealocytes show diagnostic characters which seem to represent an evolutionary transition from photosensory cells of lower vertebrates to the neuroendocrinal cells of mammals. To understand the evolutionary background of the regulatory mechanism for the melatonin synthesis in this organ, we screened the chicken pineal cDNA library to find α-subunits of heterotrimeric G-proteins involved in the photic and neural regulations. In addition to the transducin-like α- subunit (G(t)α) supposed to mediate the photic pathway, we isolated cDNA clones encoding G(i2)α, G(i3)α, and G(o1)α and its splicing variant G(o2)α. The deduced amino acid sequence of each Gα had a potential site for pertussis toxin-catalyzed ADP-ribosylation. As it is known that adrenergic receptor-mediated inhibition of melatonin synthesis is blocked by pertussis toxin, the G-proteins identified in the present study are likely to contribute to this neuroendocrine function of the chicken pineal cells.
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