AGE-DEPENDENT MUSCARINIC STIMULATION OF BETA-ENDORPHIN SECRETION FROMRAT NEUROINTERMEDIATE LOBE IN-VITRO

The effect of acetylcholine on the neurointermediate lobe beta-endorph in secretion was studied in the neonatal and in the adult rat in vitro . Acetylcholine stimulated beta-endorphin secretion from the 2-day-and 5-day-old neurointermediate lobe, the effect was dose dependent and m ore pronounced in the presence of the cholinesterase inhibitor eserine . The 10-day-, the 21-day-old and the adult rat neurointermediate lobe s did not respond to acetylcholine, even in the presence of eserine. B asal beta-endorphin secretion was elevated by the D-2 receptor antagon ist sulpiride, but acetylcholine was without effect in the 10-day-old and in the adult neurointermediate lobe even after dopamine receptor b lockade. The beta-endorphin stimulatory response to acetylcholine was diminished by the M-1 muscarinic receptor antagonist pirenzepine and b locked by the M-3 > M-1 antagonist 4-diamino-phenyl-piperidine (4-DAMP ). The selective M-2 antagonist methoctramine and nicotine had no effe ct. These data indicate that the neurointermediate robe beta-endorphin secretion is under special muscarinic cholinergic regulation for a re latively short time after birth. The disappearance of this stimulatory cholinergic effect in later life might be due to changes in the intra cellular secretory machinery in the IL and/or to the uncoupling of the cholinergic receptors from the intracellular signal transduction syst em(s) responsible for the stimulated secretion in the rat melanotrope cells. (C) 1997 Elsevier Science Inc.