Modification by oxazepam of the diurnal variations in brain 125I-melatonin binding sites in sham-operated and pinealectomized rats

Effects of diazepam and its metabolites on nocturnal melatonin secretion in the rat pineal and Harderian glands. A comparative in vivo and in vitro study

We investigated the effects of diazepam (DZP) and its three metabolites: nordiazepam (NZP), oxazepam (OZP), and temazepam (TZP) on pineal gland nocturnal melatonin secretion. We looked at the effects of benzodiazepines on pineal gland melatonin secretion both in vitro (using organ perifusion) and in vivo in male Wistar rats sacrificed in the middle of the dark phase. We also examined the effects of these benzodiazepines on in vivo melatonin secretion in the Harderian glands. Neither DZP (10(-5)-10(-6)M) nor its metabolites (10(-4)-10(-5)M) affected melatonin secretion by perifused rat pineal glands in vitro. In contrast, a 10(-4)M suprapharmacological concentration of DZP increased melatonin secretion of perifused pineal glands by 70%. In vivo, a single acute subcutaneous administration of DZP (3 mg/kg body weight) significantly affected pineal melatonin synthesis and plasma melatonin levels, while administration of the metabolites under the same conditions did not. DZP reduced pineal melatonin content (-40%), N-acetyltransferase activity (-70%), and plasma melatonin levels (-40%), but had no affects on pineal hydroxyindole-O-methyltransferase activity. Neither DZP nor its metabolites affected Harderian gland melatonin content. Our results indicate that the in vivo inhibitory effect of DZP on melatonin synthesis is not due to the metabolism of DZP. The results also show that the control of melatonin production in the Harderian glands differ from that observed in the pineal gland.

Reciprocal effects of chronic diazepam and melatonin on brain melatonin and benzodiazepine binding sites

Recent reports indicate that benzodiazepines can suppress melatonin levels and that melatonin can increase brain benzodiazepine binding. We have studied the possibility of reciprocal effects of chronic diazepam and melatonin on brain melatonin and benzodiazepine binding sites. Daily injections (3 weeks) of diazepam markedly reduced 125I-melatonin binding site density in the medulla-pons but not cortex of male rats, whereas benzodiazepine binding was not significantly affected. Melatonin, administered via the drinking water, significantly enhanced benzodiazepine (3H-RO 15-1788) binding in the medulla-pons and slightly reduced it in the cortex, but did not affect 125I-melatonin binding. Diazepam and melatonin combination reversed the suppression by diazepam of 125I-melatonin binding in the medulla-pons and the suppression by melatonin of benzodiazepine (3H-RO 15-1788 and 3H-flunitrazepam) binding in the cerebral cortex. These results indicate benzodiazepine-mediated suppression of brain melatonin binding sites that can be abrogated by melatonin administration.

Melatonin effects on behavior: possible mediation by the central GABAergic system

The best described function of the pineal hormone melatonin is to regulate seasonal reproduction, with its daily production and secretion varying throughout the seasons or the photoperiod. Additionally, a number of behavioral effects of the hormone have been found. This review describes the effects of melatonin in rodent behavior. We focus on: (a) inhibitory effects (sedation, hypnotic activity, pain perception threshold elevation, anti-convulsive activity, anti-anxiety effects); and (b) direct effects on circadian rhythmicity (entrainment, resynchronization, alleviation of jet-lag symptoms, phase-shifting). Most of these effects are clearly time-dependent, with a peak of melatonin activity during the night. One of the possible mechanisms of action for melatonin in the brain is the interaction with the GABAergic system, as suggested by neurochemical and behavioral data. Finally, some pineal hormone effects might be candidates as putative therapies for several human disorders.

Gonadal-GABAergic interaction is an important factor involved in photoperiod-induced 2-[125I] iodomelatonin binding changes in the Japanese quail brain

The type of mechanism(s) by which melatonin alone and/or through the intervention of other putative neurotransmitters is able to control circadian rhythms remains unresolved. Comparison of 2-[125I]iodomelatonin binding pattern in the brain of castrated and gonadally intact Japanese quail (Coturnix japonica), using quantitative receptor autoradiography, displayed that the combination of the intact gonadal condition and a long-day (16L:8D) photostimulatory schedule is responsible for major binding changes. In fact, high and low binding levels were obtained in the suprachiasmatic area and nucleus ectomamillaris (p < 0.01) and in the nucleus preopticus anterior and paleostriatum primitivum (p < 0.001), respectively. A gonadal modulatory role was not always evident in all brain areas as revealed by long-day photic cycles producing diminished (p < 0.01) binding levels in the anterior neostriatum and the nucleus rotundus of both castrated and gonadally intact animals, although elevated values were also found in the substantia grisea centralis (p < 0.05) of the same animals. Saturation binding studies revealed that gonadal and/or photic effects induce alterations in the number of binding sites, whereas the affinity constant varied only in some hypothalamic sites. Testing of GABAergic activity on 2-[125I] iodomelatonin binding levels showed that this inhibitory neurotransmitter was responsible for increasing low receptor values. Moreover, GABA-dependent influences were shown to be mediated via a GABAA receptor subtype since bicuculline (specific antagonist of this site) inhibited the elevated GABA-induced melatonin binding levels in the above brain sites of the gonadally intact quail exposed to both photoperiod cycles. Even in this case, melatonin binding changes were due to the variations in the number of binding sites. The apparent GABAergic-gonadal influence resulting in changes of the 2-[125I] iodomelatonin binding values, under the different photic conditions, provides evidences of other probable neural mechanisms that entrain circadian rhythmicity in neuroendocrine activities and in sociosexual behaviors.