Circadian Information and Messages in the Modified Photoreceptor Cells of the Avian Pineal Organ: Retrospect and Prospect

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Abstract
The modified photoreceptor cell (MP), homologous to the cone-like photoreceptor, is the principal cell type of the avian pineal organ. Two types of secretions (indoles and specific proteins – pinealins) are produced in this polarized cell. The synthesis of indoles undergoes striking circadian and circannual changes. MP appear to be receptoneuroendocrine cells which transduce circadian information into 5-methoxyindolic (melatonin, 5-methoxytryptophol, 5-methoxytryptamine) and possibly peptidic neurohormones. Taking into account different stages of differentiation and/or different physiological states (due to cyclic changes) for MP, several mechanisms of transduction of photoperiodic inputs into circadian hormonal outputs have been hypothesized.

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