Evidence that short photoperiod-induced gonadal regression in the Mongolian gerbil is mediated by the action of melatonin in the medial hypothalamus

Effect of different photoperiods on gonadal maintenance and development in Mongolian gerbils (Meriones unguiculatus)

The role of photoperiod in adult testicular maintenance and body weight and juvenile development was assessed in male Mongolian gerbils (Meriones unguiculatus). Gerbils were raised on a 14L (14 hr of light) photoperiod. In the first study, adult gerbils with functional testes were transferred to thirteen different photoperiods (0L, 2L, 4L, 6L, 8L, 10L, 12L, 14L, 16L, 18L, 20L, 22L, or 24L) and body weights and testicular size were measured every week for 10 weeks. Body weights were similar in all groups. Testicular regression had occurred in animals housed on 0L, 2L, 4L, 6L, 8L, and 24L by week 10. In the second study, 14L-born prepubertal gerbils were transferred to thirteen different photoperiods as in the first study. Body weights and testicular development were examined for 10 weeks. At the end of 10 weeks the body weights of animals in all groups except 24L were similar to those of adults. Animals in 24L had a lower body weight gain. Exposure to 0L, 2L, and 24L inhibited testicular development and testes weights were significantly different from those of the other groups. These results demonstrate that maintenance of body weight in adult gerbils appears to be independent of photoperiodic signal. Exposure to very long (24L) and short photoperiods (< 10 hr) causes testicular regression in adult gerbils. Moreover, different photoperiods experienced in early life can influence prepubertal testis growth and body weight gain.

Biology of mammalian photoperiodism and the critical role of the pineal gland and melatonin

In mammals, photoperiodic information is transformed into a melatonin secretory rhythm in the pineal gland (high levels at night, low levels during the day). Melatonin exerts its effects in discrete hypothalamic areas, most likely through MT1 melatonin receptors. Whether melatonin is brought to the hypothalamus from the cerebrospinal fluid or the blood is still unclear. The final action of this indoleamine at the level of the central nervous system is a modulation of GnRH secretion but it does not act directly on GnRH neurones; rather, its action involves a complex neural circuit of interneurones that includes at least dopaminergic, serotoninergic and aminoacidergic neurones. In addition, this network appears to undergo morphological changes between seasons.

Acute and chronic effects of melatonin as an anticonvulsant in male gerbils

Melatonin, a hormone produced in the pineal gland and released into the general circulation on a diurnal basis, has been implicated in many behavioral processes, where it has been shown to have anxiolytic, sedative, and anticonvulsant effects. Male gerbils (Meriones unguiculatus) injected daily with melatonin (25 micrograms, s.c.) exhibited a reduced seizure response to pentylenetetrazol (PTZ, 60 mg/kg, s.c.). The present studies determined 1) whether melatonin's effect was related to the time of day that it was administered and 2) whether a single acute injection of melatonin at various doses could produce anticonvulsant activity. Gerbils provided with 13 weeks of daily melatonin injections (25 micrograms, s.c.) exhibited fewer convulsions after PTZ treatment irrespective of the time of day melatonin was injected. In addition, the melatonin-treated gerbils had lower mortality rates (1/12) than the untreated or vehicle-injected gerbils (5/12). On the other hand, single acute injections of melatonin (0.1-10 mg/kg, i.p.) produced no anticonvulsant activity. It appears that the anticonvulsant effects of melatonin occur only after the animals are chronically exposed to the indole. In addition, melatonin's anticonvulsant ability may utilize a different mechanism than those involved in its endocrine effects, since no diurnal difference in melatonin's anticonvulsant activity was observed.

The ovine pars tuberalis does not appear to be targeted by melatonin to modulate luteinizing hormone secretion, but may be important for prolactin release

The pineal hormone, melatonin, transduces photoperiodic information to the neuroendocrine axis of seasonally breeding mammals to regulate reproduction. It is not known where or how melatonin achieves this effect, but the recent identification of the pars tuberalis (PT) as the area with the highest density of melatonin binding sites suggests that this pituitary subdivision may be an important target for the actions of this indoleamine on luteinizing hormone (LH) and prolactin release. The present study was designed to test this hypothesis. Ovariectomized oestradiol-implanted ewes were exposed to inhibitory long days for 85 days and then received melatonin micro-implants (Day 0) in the mediobasal hypothalamus (MBH; n = 7) or PT (Melatonin-PT; n = 5). The effect of these micro-implants was compared to ewes receiving empty micro-implants in the PT (Sham-PT; n = 5). For LH, bi-weekly jugular blood samples were collected and for prolactin, samples were collected every 20 min for 5 h, with the first hour discarded, on Days -4, 26 and 69. Melatonin implanted in the MBH stimulated LH secretion in 3 ewes by Day 46 +/- 0 after implantation, and one ewe by Day 67 after implantation. In contrast, no Melatonin-PT or Sham-PT ewes exhibited an increase in LH secretion by the end of the study (Day 70). A subsequent experiment, in which the Sham-PT ewes were implanted with melatonin both subcutaneously and in the PT showed that the micro-implants did not impair the ability of the ovine reproductive neuroendocrine axis to respond to melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)

An attempt to correlate brain areas containing melatonin-binding sites with rhythmic functions: a study in five hibernator species

High affinity melatonin-binding sites have been described, by means of autoradiography with 2-125I-melatonin as the ligand, in more than 60 brain areas of about 20 mammalian species, with dramatic variations in the nature and number of labelled structures among the different species studied. As melatonin is involved in the synchronization of biological rhythms, we have tried to correlate the brain areas containing melatonin-binding sites with some rhythmic functions typical of given species. Therefore, we have studied the location of melatonin-binding sites in the complete brain of five long-day breeders with hibernation cycles, viz. one insectivore and four rodents. With the exception of the suprachiasmatic nuclei and the pars tuberalis of the pituitary, both of which contain binding sites in all five species, few reactive structures are common, even among species from the same family, e.g. the edible dormouse and the garden dormouse.

Melatonin receptors: localization, molecular pharmacology and physiological significance

A pre-requisite to understanding the physiological mechanisms of action of melatonin is the identification of the target sites where the hormone acts. The radioligand 2-[125I]iodo-melatonin has been used extensively to localize binding sites in both the brain and peripheral tissues. In general these binding sites have been found to be high affinity, with Kd in the low picomolar range, and selective for structural analogues of melatonin. Also the affinity of these sites can generally be modulated by guanine nucleotides, consistent with the notion that they are putative G-protein coupled receptors. However, only a few studies have demonstrated that these putative receptors mediate biochemical and cellular responses. In the pars tuberalis (PT) and pars distalis (PD) of the pituitary, the amphibian melanophore and vertebrate retina, evidence indicates that melatonin acts to inhibit intracellular cyclic AMP through a G-protein coupled mechanism, demonstrating that this is a common signal transduction pathway for many melatonin receptors. However in the pars distalis the inhibition of calcium influx and membrane potential are also important mediators of melatonin effects. How many different forms or states of the melatonin receptor exist is unknown, but clearly the identification of the structure of the melatonin receptor(s) and its ability to interact with different G-proteins and signal transduction pathways are quintessential to our understanding of the physiological mechanisms of action of melatonin. In parallel the recent development of new melatonin analogues will greatly aid our understanding of the pharmacology of the melatonin receptor both in terms of the development of potent melatonin receptor antagonists and for the definition of receptor sub-types. The wide species and phylogenic diversity of melatonin binding sites in the brain has probably generated more questions than answers. Nevertheless the localization of melatonin receptors to the suprachiasmatic nucleus of the hypothalamus is at least consistent with circadian effects within the foetus and the adult. In contrast the PT of the pituitary presents an enigma in relation to the seasonal effects of melatonin. A model of how melatonin might mediate the timing of the circannual events through the PT is proposed.