Adrenergic and Cholinergic Regulation of in vitro Melatonin Release during Ontogeny in the Pineal Gland of Long Evans Rats

Melatonin, produced by the pineal gland, plays an important role in a great variety of neuroendocrine functions. The rhythmic release of melatonin by the mammalian pineal gland is regulated by norepinephrine (NE) acting via α- and β-adrenergic receptors utilizing distinct signal transduction pathways. Acetylcholine has been demonstrated to exert various effects in the mammalian pineal gland, including an inhibitory action on the NE-induced stimulation of melatonin production. However, data obtained by different laboratories on the interaction of adrenergic receptors are not consistent and whether muscarinic and/or nicotinic receptors participate in the various effects of acetylcholine is still contradictory. To investigate noradrenergic as well as cholinergic mechanisms during ontogeny, we have investigated in vitro melatonin release from isolated pineal glands of Long Evans rats of different ages. NE as well as the β-adrenergic receptor agonist isoproterenol (ISO) significantly elevated the melatonin release in pineal glands from postnatal week 2 on. In pineal glands originating from 2- to 4-week-old rats, simultaneous activation of α- and β-adrenergic receptors by ISO and the α-adrenergic receptor agonist methoxamine (MET) or NE resulted in significantly weaker stimulation of melatonin production than β-receptor activation alone. Acetylcholine evoked a significant increase in melatonin release in pineal glands from 2- to 4-week-old rats. In pineal glands from 8- to 20-week-old animals, ISO, ISO + MET or NE stimulated pineal melatonin release to comparable maxima, whereas acetylcholine was without effect. Our data indicate (1) that the adrenergic stimulation of pineal melatonin production in Long Evans rats is dominated by a β-adrenergic mechanism, (2) that additional α-adrenergic receptor activation is inhibitory and (3) dependent on the developmental status of the animal, and (4) that acetylcholine acting via muscarinic receptors has the capacity to stimulate melatonin release during early ontogeny. These data suggest that the melatonin-generating system of the pineal gland of Long Evans rats undergoes substantial functional changes during early postnatal development, including adrenergic as well as cholinergic mechanisms.