Showing papers by "Shiu-Fun Pang published in 1991"

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.

Melatonin in the Gastrointestinal Tract

Abstract: Melatonin was initially speculated to be solely synthesized and secreted by the pineal gland. However, such postulation was subsequently impugned by the discovery of hydroxyindole-O-methyltransferase (HIOMT; EC 2.1.2.4), the enzyme responsible for the methylation of N-acetylserotonin to melatonin, in the retinas and guts of vertebrates (Baker et al. 1965, Cardinali and Rosner, 1972; Quay and Ma, 1976). Thereafter the presence of melatonin was progressively detected in extra-pineal tissues including the gastrointestinal tracts of mammals (Bubenik et al., 77; Reiter et al., 83; Pang and Allen, 86; Mhatre et al., 88). The levels of melatonin detected within the digestive tracts were postulated to be synthesized and secreted locally by intestinal tissues as pinealectomy did not elicit any significant alterations in the levels of melatonin among the alimentary canals (Bubenik 80). Despite the detection of melatonin within gastrointestinal tracts, the function of this indoleamine among the digestive systems remain minimally investigated. In our present study, we shall synopsize some of the previous findings pertaining to melatonin in the gastrointestinal tracts together with the recent detection of iodomelatonin binding sites among jejunal membrane preparations and N-acetyltransferase (NAT; EC 2.3.1.5) activities within the duodenum of quails.

Melatonin Binding Sites in the Nervous and Immune Systems

Abstract: Melatonin (N-acetylserotonin) was isolated from the bovine pineal gland in the late fifties. Following this discovery, studies in birds and mammals have established that melatonin is synthesized and secreted by the pineal gland in a diurnal fashion with high levels in the dark period. This indole mediates many of the endocrine effects of the pineal gland such as entrainment of locomotor activity and seasonal reproduction in animals (Reiter, 1987; Reiter and Pang, 1989). A major site of pineal melatonin action appears to be the brain (Fraschini, 1969; Quay, 1970; Anton-Tay, 1971; Trentini, 1979).

Rhythmic release pattern of pineal melatonin in rodents.

Abstract: The plasma melatonin in the confluens sinuum of anesthetized rabbits and rats exhibited an episodic release pattern, with pulses superimposed on a basal level. The rhythmicity of pineal melatonin release revealed by our experimental paradigm was dependent on the blood sampling interval. In rabbits, it was found that the plasma melatonin level in the confluens sinuum was 7-15 times higher than that obtained from the plasma collected at the same time from the peripheral artery where the melatonin level also showed a pulsatile pattern. Diurnal variation in pineal melatonin level in the confluens sinuum was observed only in acutely blinded rabbits and rats. Electrical stimulation of the unilateral cervical sympathetic trunk of rabbits resulted in elevation of the level of plasma melatonin in the confluens sinuum, while the frequency of pulse peaks of plasma melatonin revealed at 4-min intervals was not affected. Similar results were obtained in rabbits with systemic administration of beta-adrenergic agents. These findings suggest that the release of pineal melatonin into the confluens sinuum is under the regulation of the cervical sympathetic system. The sympathetic influence on pineal melatonin secretion seems to vary with age. Furthermore, age-related fluctuation of systemic blood melatonin appeared to be the results of changes in the rate of pineal melatonin secretion, body growth and ageing.