Acetylcholine Stimulates Growth Hormone Secretion in the Neonatal Rat Pituitary

The direct effect of acetylcholine on pituitary growth hormone secretion during the postnatal period of the rat was studied using a superfusion system. Acetylcholine elicited a dose related stimulatory effect on growth hormone (GH) secretion in the pituitaries from 2-day old rats. M, muscarinic agonist McN A343 mimicked the GH releasing effect of acetylcholine, nicotine was ineffective. The GH release elicited by acetylcholine diminished with postnatal development, it was small by the end of the third postnatal week and was not demonstrable in the adult pituitaries. The effect of acetylcholine was potently antagonized by pirenzepine (M₁ antagonist) and 4-DAMP (M₃ and M₁ antagonist) but not by methoctramine (M₂ antagonist). It is concluded that unlike in the adult, in the newborn rat the cholinergic regulation of pituitary GH secretion plays a prominent role directly at pituitary level most likely via M₁ receptors.