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Changes in Pineal Indoleamines in Rats after Single Melatonin Injections: Evidence for a Diurnal Sensitivity to Melatonin
Author(s) -
Míguez Jesús M.,
Simonneaux Valérie,
Pévet Paul
Publication year - 1996
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1111/j.1365-2826.1996.tb00696.x
Subject(s) - melatonin , pinealocyte , medicine , endocrinology , pineal gland , serotonin , circadian rhythm , 5 hydroxytryptophan , chemistry , biology , receptor
We recently determined that melatonin stimulated serotonin (5‐HT) secretion from rat pineal glands by increasing 5‐HT release from the pinealocytes (μM melatonin concentrations) and by inhibiting 5‐HT uptake in the pineal sympathetic nerve endings (mM melatonin concentrations). The present study investigated whether a single melatonin injection could alter the content of indoleamines in the rat pineal gland, as well as its possible dependence on the daytime of administration. Melatonin (150 μg/kg) was i.p. injected at 8 time points (11.00 h, 14.00 h, 17.00 h, 20.00 h, 23.00 h, 02.00 h, 05.00 h and 08.00 h) to rats kept in 12:12 h light:dark cycle (lights on at 07.00 h). Melatonin injections in the afternoon (17:00 h) and late in the nighttime (02.00 h and 05.00 h) decreased pineal 5‐HT content 90 min later. The levels of 5‐hydroxyindoleacetic acid (5‐HIAA) were also decreased 90 min after the melatonin treatment at 14.00 h, 17.00 h and 02.00 h. The effect of melatonin on 5‐HT content was a long‐lasting effect (still evident after 180 min) only when injected at 02.00 h, whereas 5‐HIAA levels were found to be decreased 180 min after melatonin treatment at 14.00 h and 23.00 h. No changes in these compounds were detected 240 min after melatonin treatment. Moreover, melatonin did not change 5‐hydroxytryptophan levels at any of the daytime points studied. By contrast, 90 min after the injection of melatonin at 20.00 h, an increased content of pineal N‐acetylserotonin was observed. This effect of melatonin could be mediated through a phase alteration of the pineal N‐acetyltransferase activity rhythm by acting on the suprachiasmatic clock, althought a direct melatonin effect on the pineal rhythmic function cannot be excluded. The effects of the hormone on 5‐HT and 5‐HIAA contents agree with previous findings on the inhibitory effect of pharmacological doses of melatonin on pineal 5‐HT uptake, which presumably would result in a decreased intraneuronal content of 5‐HT and its acid metabolite. These data point to an acute regulatory action of exogenous melatonin on the pineal melatonin synthesis pathway which seems to be limited to two daytime phases: the afternoon‐early evening period and the second half of the night.