Molecular and Cellular Regulation of Pineal Organ Responses

TLDR: This chapter focuses on the structural and functional properties of the fish pineal organ, and on the molecular mechanisms, that contribute to the synthesis of rhythmic output signals, including melatonin, the timekeeping molecule of the organism.

Abstract: Publisher Summary This chapter focuses on the structural and functional properties of the fish pineal organ, and on the molecular mechanisms, that contribute to the synthesis of rhythmic output signals, including melatonin, the timekeeping molecule of the organism. The pineal gland of fish is an evagination of the roof of the diencephalon, which locates in a window below the skull. The structural characteristics and functional properties of the pineal photoreceptors classify them into the cone family of photoreceptor cells, as found in the retina. The pineal epithelium is organized like a retina that would contain cone‐like photoreceptor cells, connecting to second‐order neurons. Supporting “interstitial cells” would be the homologous of the retinal Muller cells. The fish pineal gland is a nonvisual photosensitive organ that transduces the light information and elaborates nervous and neurohormonal messages in response to changes in illumination—the nervous message is an excitatory neurotransmitter and the hormonal message is melatonin. The main function of the fish pineal organ is to integrate light information and elaborate messages that will affect the animal's physiology. The photoreceptor cells occupy a key position because they are at the interface between the environment and the organism providing rapid (time scale of seconds) and less rapid (time scale of several hours) responses to environmental light.