ACETYLCHOLINE STIMULATES GROWTH-HORMONE SECRETION IN THE NEONATAL RATPITUITARY

The direct effect of acetylcholine on pituitary growth hormone secreti on during the postnatal period of the rat was studied using a superfus ion system, Acetylcholine elicited a dose related stimulatory effect o n growth hormone (GH) secretion in the pituitaries from 2-day old rats , M(1) muscarinic agonist McN A343 mimicked the GH releasing effect of acetylcholine, nicotine was ineffective, The GH release elicited by a cetylcholine diminished with postnatal development, it was small by th e end of the third postnatal week and was not demonstrable in the adul t pituitaries. The effect of acetylcholine was potently antagonized by pirenzepine (M(1) antagonist) and 4-DAMP (M(3) and M(1) antagonist) b ut not by methoctramine (M(2) antagonist). It is concluded that unlike in the adult, in the newborn rat the cholinergic regulation of pituit ary GH secretion plays a prominent role directly at pituitary level mo st likely via M(1) receptors.