Neural Elements in Pineal Organs of Vertebrates

Abstract

Phylogenetically, the pineal organ changes from a photoreceptor organ endowed with true photoreceptor cells to an endocrine gland influenced by light stimuli perceived in the retina. In poikilothermic vertebrates, the pineal complex contains photoreceptor cells which are synaptically connected to intrapineal second-order neurons. The axons of these second-order neurons constitute the central pinealofugal innervation which connects the pineal complex with various nuclei located in the diencephalon and mesencephalon. Changes in the structure and function of pineal photoreceptors are accompanied by the reduction of intrapineal neurons in the course of evolution. In mammals, pinealocytes receive information from a biological clock system—present inside the suprachiasmatic nucleus of the hypothalamus—entrained by the photoperiod and have lost structural and functional characteristics of photoreceptor cells. Nevertheless, mammalian pinealocytes are considered as derivatives of the pineal photoreceptor cells of anamniotic vertebrates (Oksche, 1971; Collin, 1971). This functional shift of the pinealocyte has been confirmed by immunohistochemical and biochemical studies showing that neuroendocrine pinealocytes of mammals express photoreceptor-specific molecules which otherwise are synthesized only by true (retinal and pineal) photoreceptor cells (Korf et al, 1986; Kramm et al., 1993). Concomitantly with the loss of the direct photosensitivity of pinealocytes, the central pinealofugal innervation is reduced in mammals. Alternatively, the pineal gland of mammals receives a conspicuous autonomic innervation (Cajal, 1904; Kappers, 1960), which is essential for the regulation of melatonin metabolism in these species. The autonomic nervous system also innervates the pineal organ of reptiles and birds, but in these species it appears to be less important than in mammals. Autonomic nerve fibers are scarce or even absent in the pineal organ of poikilothermic vertebrates such as fishes and amphibians (see Vollrath, 1981 for details).