Sensory Receptors as a Special Class of Hormonal Cells

Abstract

Classically, sensory receptors are specialized cells which detect specific environmental disturbances and send out neural signals for the integration, control and/or regulation of effector organs. Recently, a special class of sensory receptors called sensori-hormonal cells which employ hormones as their means of flux of biological information has been proposed. These sensori-hormonal cells are capable of detecting and transducing environmental signals directly into the secretion of hormones within the same cells. Theoretically, all sensory receptors may have examples capable of direct sensori-hormonal transduction. However, only one group of sensori-hormonal cells, the photoendocrine cells, have so far been studied. The photoendocrine cells including the photoreceptors of fish retinas and pinealocytes of bird pineals are capable of detecting light and/or darkness and transducing the electromagnetic radiation energy into a hormonal output. Generally, light suppresses and darkness stimulates the biosynthesis and secretion of melatonin by these photoendocrine cells. Contrary to many hormonal systems which employ principally the feedback mechanism for regulation, the sensori-hormonal cells are predominantly controlled by the feedforward mechanism. However, other factors may serve as additional means of regulation by influencing the system and affecting the transduction processes and/or the synthesis and secretion of the hormone. The ability of sensori-hormonal transduction is suggested to be important for the survival of the organism itself and/or its species and sensori-hormonal cells or their equivalent should appear early in the course of animal evolution. It is further suggested that the sequence of appearance of melatonin functions in the course of evolution would be: hormone – humoral factor – neuromodulator – neurotransmitter.

Effect of melatonin on the retinal glutamate/glutamine cycle in the golden hamster retina

“…melatonin levels significantly increase after pinealectomy, supporting the proposal that in the golden hamster, retinal melatonin is generated within the tissue itself (6). In contrast to pineal melatonin which is secreted to the body fluids, retinal melatonin is thought to act locally within the eye (7). Melatonin selectively inhibits the calcium-dependent release of dopamine (8), and it has been implicated in the regulation of photoreceptor disc shedding and phagocytosis (1), melanosome aggregation in pigment epithelium, and cone photoreceptor retinomotor movements (9).…”

2-[¹²⁵l]lodomelatonin Binding Sites in the Lung and Heart: A Link between the Photoperiodic Signal, Melatonin, and the Cardiopulmonary System

“…The brain, retina and pituitary are consid ered by many to be the major sites of pineal melatonin action [8-13] but a direct melato nin action on other peripheral tissues has also been proposed [14,15] [25], rat adrenal gland [26] and hamster spleen [27]. The above findings are in line with the notion that the end organ responses to melatonin that would increase the chance of survival of the organ ism would be naturally selected [28,29], In light of the above, we would like to hypothe size that animals exhibiting notable seasonal changes in their energy requirements should have lungs and hearts sensitive to the photoperiodic signal, melatonin, and that melato nin receptors should be present in these end organs. In our laboratory, we tested the above hypothesis by investigating the 2-[125I]iodomelatonin (a labeled melatonin agonist) bind ing sites in the heart and lung.…”

2-[¹²⁵I]Iodomelatonin Binding in the Brains of Four Salmonids

“…An alternate expla nation might be that fish brain melatonin receptors do not necessarily 'see' a diurnal rhythm in melatonin levels. In salmonid fish, much of the melatonin that appears in the general circulation is derived from the retina, as well as from the pineal [7,[30][31][32][33]. The vas cular system of fish is such that the blood sup ply to the brain is derived from arterial blood originating from the dorsal aorta; blood enter ing the heart via the vena cava has passed through the liver and may well have been depleted of melatonin by the association of the hepatocytes.…”