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Melatonin Receptors in Discrete Brain Areas of the Male Rat

The distribution of melatonin receptors in six discrete brain areas of mature (3-4 months old) and aged (greater than 24 months old) male rats was recorded every 4 h during a 24-hour light: dark cycle (L:D 14:10h). 125I-melatonin was used as a melatonin receptor probe. In the mature animals, specific binding of 125I-melatonin was found in all brain areas investigated, i.e. hypothalamus, medulla pons, hippocampus, cerebellum, parietal cortex and striatum. The density of 125I-melatonin-binding sites in the hypothalamus, medulla pons and hippocampus exhibited clear diurnal rhythms with different patterns and phases. No such rhythm was evident in the cerebellum, parietal cortex and striatum. The apparent affinity of the binding sites was similar in all the brain regions and did not change at any of the times recorded. In the old male rats, the density of 125I-melatonin binding sites in the hypothalamus was only 10% of that in the mature animals at 13 h after the onset of light and was vanishingly small throughout the 24-hour period. The 24-hour mean of the binding site density in the parietal cortex, hippocampus and medulla pons was significantly lower than in mature rats with no apparent diurnal variations. The age-related decrease in the density of melatonin-binding sites was less pronounced in the cerebellum and striatum. In all brain areas tested, apart from the hypothalamus, the decrease in receptor densities was not accompanied by changes in the apparent affinity towards the ligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian variations in melatonin-binding sites in discrete areas of the male rat brain

The binding of 125I‐melatonin to synaptosomes prepared from whole brains of male rats of the CD strain and from the brain, hypothalamus and striatum of male rats of the Sabra‐Wistar strain was assessed throughout a 24 h period. The animals were maintained under a daily schedule of 14 h light (05:00–19:00 h) and 10 h darkness. In whole brain preparations the density of binding sites at 18:00 h was higher by about 70% than at 02:00 h with no variations in apparent affinity of the binding sites throughout the daily period. Specific binding of 125I‐melatonin was found in both hypothalamus and striatum of the male rat with a distinct diurnal variation in binding site density in the hypothalamus only. The density of 125I‐melatonin‐binding sites in the hypothalamus was maximal between 10:00 and 18:00 h and dropped sharply after the lights went off. The apparent 125I‐melatonin‐binding affinities in these regions were constant and very similar to those in whole brain preparations. The daily variations in densities of 125I‐melatonin‐binding sites in discrete brain areas may represent a diurnal rhythmicity in the responsiveness of the neuroendocrine axis to melatonin.

Effects of long-term administration of melatonin and a putative antagonist on the ageing rat

Adult rats were treated with either melatonin, the putative melatonin antagonist N-(2,4 dinitrophenyl)-5-methoxytryptamine (ML-23), their combination, or a vehicle for 16 months via the drinking water. The survival rates, serum testosterone and densities of 125I-melatonin binding sites in the medulla-pons and hypothalamus of the animals at the age of 27-29 months were significantly higher in the melatonin than vehicle-treated group. Surprisingly, ML-23 without or with melatonin, also prolonged the life-span of the aged animals. ML-23 treatment greatly increased 125I-melatonin binding in the medulla-pons whereas this increase was prevented by melatonin supplementation. Thus melatonin can attenuate age-related decrease in survival rates, testosterone and brain 125I-melatonin binding sites, while chronic blockade by the putative antagonist also elicits melatonin-mimetic responses, perhaps by effecting supersensitivity.

Diurnal Rhythms of Pineal Nucleic Acids and Protein

A diurnal rhythm in pineal protein was found to occur in mature rats. In the male, a peak level is reached about 4 p.m., preceded by a gradual increase during the period of light and followed by a dec

Diurnal variations in melatonin binding sites in the hamster brain: impact of melatonin☆

The distribution of 125I-melatonin binding sites in the male Syrian hamster brain was recorded at 3 times over a 24 h period. The binding in the hypothalamus, hippocampus, medulla-pons and midbrain of the hamsters varied significantly over the 24 h period with different patterns and phases. No such variations were observed in the parietal cortex. Daily morning (10.00 h) or late afternoon (18.00 h) injections of melatonin for 28 days markedly increased the serum concentrations of melatonin at all times recorded. Serum concentrations of testosterone were significantly lower in animals injected with melatonin in the late afternoon than in the untreated controls; no such decrease was observed in animals injected in the morning despite the continuously elevated levels of circulating melatonin. The daily melatonin injections did not significantly affect 125I-melatonin binding in the hypothalamus, parietal cortex and medulla-pons. In the midbrain, 125I-melatonin binding decreased regardless of the time of injection. In the hippocampus, morning melatonin injections caused a marked decrease in 125I-melatonin binding at all times recorded whereas melatonin injected in the late afternoon led to a decrease in 125I-melatonin binding at 10.00 h only. These results indicate diurnal variations in 125I-melatonin binding sites in discrete brain areas of the golden hamster, persisting despite prolonged duration of elevated levels of circulating melatonin. The differential effects of timed melatonin injections on the hippocampal 125I-melatonin binding sites are positively correlated with the counter-antigonadal response produced by morning melatonin injections.

The effect of heat on rat pineal hydroxyindole-O-methyl transferase activity

Exposure of adult male rats to continuously elevated temperature of 32–34°C caused a significant decrease of HIOMT activity involved in the specific metabolic process of production of melatonin, considered an active pineal hormone. The effect was already evident after 24 h exposure and increased further during the next 48 h. The results obtained substantiate previous data that the pineal gland may be involved in the system regulating adaptation to extreme temperature changes.

The effect of 17 β-estradiol on pineal metabolism

Abstract A single injection of 10 μg 17β-estradiol was administered subcutaneously to immature female rats aged 21 and 31 days and the pineal RNA, DNA and protein levels were measured 15, 18 and 24 hours after injection. An increase in the protein content of the pineal occurred 24 hours after injection, accompanied by a prior elevation in pineal RNA and DNA, indicating accelerated protein metabolism. No change were found in either the pineal nucleic acids or protein of immature female rats that received seven daily injections of 5μg 17β-estradiol/rat. The suggestion is put forward that the intensified pineal protein synthesis is caused by increased LH secretion.

The effect of light and darkness on nucleic acids and proteiin metabolism of the pineal gland

Abstract Exposure of female rats to continuous light from the day of weaning — 21st day of age — for ten, twenty and thirty days caused a significant inhibition in their pineal RNA and protein synthesis, amounting to 20 per cent compared with control animals kept in conditions of alternating light and darkness. The main inhibition occurred within the first ten days of exposure, coinciding with the period of most active physiological production of nucleotides and proteins. No change was found in the pineal DNA levels of the rats exposed to continuous light, indicating that the reduced RNA and protein results from a decreased cellular metabolism unrelated to physiological cell multiplication. This would imply an inhibition of parenchymal hypertrophy of the pineal under conditions of continuous light. The three parameters tested were the same in rats exposed to continuous darkness and animals kept under an alternating diurnal light-dark cycle.

The effect of pinealectomy on rat plasma corticosterone levels under various conditions of light

Abstract The interrelationship between the pineal gland and adrenal cortex and possible role light may be playing in this connection has been studied by subjecting pinealectomized and control maturing female rats to constant light, constant darkness or an alternate light-dark schedule and then measuring their blood corticosterone levels. Elevated plasma corticosterone levels could be seen in pinealectomized animals maintained for 10 days in alternating light and constant darkness. However, only in the light-deprived rats did the increase reach significant proportions. Thirty days after pinealectomy no disparity existed between the various groups. These data indicate that the pineal gland exerts an inhibitory effect on adrenocortical function especially in rats kept in darkness. Light was found to affect the pituitary-adrenal axis by stimulating corticosterone secretion, but this was not dependent upon presence of the pineal. Pinealectomy combined with continuous light had no additive effect, the same degree of increase in plasma corticosterone being produced by each factor alone or the two together.

Pineal N-acetyltransferase depression in rats exposed to heat.

Exposure of adult male rats to increased temperature of 33±1°C for 3 and 10 days brought about decreases in pineal N-acetyltransferase activity. These and previous findings of pineal HIOMT inhibition under similar conditions support the postulation of a possible thermoregulatory role for the pineal gland.