Influence of maternal melatonin on melatonin receptors in rat offspring

Abstract

Using quantitative autoradiography, we have studied the influence of maternal plasma melatonin on the expression and density of melatonin receptors in the brain and pituitary of rat offspring. At birth, the same structures displayed melatonin receptors whether the rats were born to and reared by intact or pinealectomized dams. The receptor density was, however, about 20% lower in the group born to pinealectomized dams. At postnatal day 9, when the pups of both groups synthetize rhythmically their own melatonin, this difference was suppressed. These results indicate that melatonin does not appear to be a requirement for the expression of its receptors, but seems to play a stimulatory role in their synthesis.

Generation of the Melatonin Endocrine Message in Mammals: A Review of the Complex Regulation of Melatonin Synthesis by Norepinephrine, Peptides, and Other Pineal Transmitters

“…Notably, a nonsense mutation occurs in the MT2 coding gene in Siberian and Syrian hamsters, which disables the receptor . MEL-Rs are present in the pineal gland of the newborn rat, become rare in 9-day-old rats, and are not detected in adults (Zitouni et al, 1995). MEL-Rs have also been characterized in many peripheral structures such as the Harderian gland, spleen, testis, ovary, vascular system, gut, smooth muscle, and some cells of the immune system (see Vanecek, 1998 for review).…”

“…For example, in neonate but not adult rats, the pineal gland displays MEL-R binding (Zitouni et al, 1995). Exogenous MEL modifies various morphological and biochemical pineal parameters, namely proteic microtubules , enzymatic activities , presynaptic release of neurotransmitters , and pre and postsynaptic release of the MEL precursor serotonin (5-HT; Miguez et al, 1995b).…”

Ontogenesis of the retinohypothalamic tract, vasoactive intestinal polypeptide- and peptide histidine isoleucine-containing neurons and melatonin binding in the hypothalamus of the mink

“…In the present study, film autoradiograms revealed specific I-MEL binding in discrete brain areas of near-term fetus and newborn mink: the olfactory epithelium, the cortex, the thalamus. All these areas were reported to show specific binding in neonate rodents with a large interspecies variation (rat: Zitouni et al 1995; Syrian hamster: Duncan and Davis 1993;Djungarian hamster: Weaver et al 1988;Carlsson et al 1991). The transient expression of specific I-MEL binding in the neonate mink brain corresponds to the decrease of binding sites during ontogenesis in the Syrian hamster (Duncan and Davis 1993) and the rat (Zitouni et al 1995).…”

“…In fact, in the rat, timed melatonin injections are able to entrain the circadian rhythm of N-acetyl transferase activity, the rate-limiting enzyme in melatonin synthesis (Humlova and Illnerova 1990); however, this effect is thought to be mediated by the SCN through melatonin receptors, which are lacking in mink. Furthermore, it is not known whether the pineal of the neonate mink can synthesize melatonin; in the rat, melatonin is present in the pineal by the end of the first week of life, in plasma from day 9, and a daynight difference is observed from day 10 ( Klein et al 1981;Zitouni et al 1995).…”

“…Although pinealectomy during pregnancy does not eliminate the maternal entrainment of the fetal clock (Reppert and Schwartz 1986b), timed prenatal injections of melatonin to pregnant hamsters have been shown to entrain activity rhythms in the newborn hamster (Davis and Mannion 1988) as it does in adult rats (Redman et al 1983). The hypothesis according to which the maternal melatonin conveys signals is supported by the presence of melatonin receptors in the fetal and neonate SCN (Duncan and Davis 1993;Weaver et al 1988;Zitouni et al 1995).…”