Melatonin as the Hormone Which Mediates the Effects of the Pineal Gland on Neuroendocrine-Reproductive Axis of the Syrian Hamster

Occlusion of the Melatonin‐Free Interval Blocks the Short Day Gonadal Response of the Male Syrian Hamster to Programmed Melatonin Infusions of Necessary Duration and Amplitude

“…This contrasts with the neuroendocrine responses to oestrogen where the systems which control the LH surge are sensitive to the duration over which oestrogen concentrations are elevated but not their absolute value (19,20). The requirement for exposure to a hormone-free interval between successive melatonin signals is consistent with numerous studies in which exogenous melatonin, delivered as chronic subcutaneous implants, has been used to manipulate scasonal fertility (2,21,22,(28)(29)(30). In many cases, the high dosages used probably eliminated any detectable cyclic variation in melatonin levels, although in a limited number of cases, serial serum samples have revealed some degree of nocturnal elevation in melatonin concentrations (28)(29)(30).…”

“…This procedure would determine whethcr or not the presence of a melatonin-free interval influenced photoperiodic induction. In other endocrine and neural systems, it is well established that continuous exposure to a ligand may lead to changes in receptor density and/or distribution and indeed, it has been hypothesized that one mode of action of melatonin is to down-regulate its own receptors (21). Differences in the gonadal responses of animals to phasic or continuous exposure to melatonin could potentially be a consequence of reduced availability of the melatonin binding sites, resulting from occlusion of the melatonin-free interval.…”

“…The specific aim was to determine whether occlusion of the melatonin-free interval between successive melatonin peaks influenced the interpretation of the signal. Photoperiodic responses can be blocked by the continuous administration of doses of melatonin which are probably sufficiently large to obscure the endogenous signal completely (1-3, 21,22). This study sought to exploit the use of chronic programmed infusions to determine whether, if the phasic signal persisted with appropriate duration and amplitude, it could still be read when the interval between signals was occluded.…”

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

“…In tetrapods, there is both a diumal rhythm of plasma melatonin secretion and a diumal variation in the biological response to melatonin [9], and it has been proposed that the physiological response to melatonin re-quires the concurrence of the circadian rhythms of melatonin secretion and target tis sue receptivity [9,10]. In fishes, although there is a reasonable understanding of the environmental factors that influence melato nin secretion by the pineal gland (see refer ences above), there is relatively little known of the physiological role of the hormone or of the characteristics of melatonin binding to tissues and organs.…”

Abnormalities in Behavior, Learning Ability, and the Cholinergic System Induced by Long-Term Ultraviolet A Irradiation of Mice

“…It is well known that photic stimula tion of the retina is responsible for entrainment of physio logical circadian rhythms [5] and regulation of various neuroendocrine parameters [6], Most research on the reg ulation of circadian and circadian rhythms, and the neu roendocrine changes has employed visible light sources for entrainment [7,8]. It is generally accepted that the crystalline lens of mammals acts as a selective wavelength filter which determines the wavelength cut-off for energy reaching the retina and UVA has been considered to be nonvisible to mammals [9,10].…”