Acetylcholine Inhibits the Release of Somatostatin from Rat Hypothalamus in Vitro*

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Abstract
Acetylcholine, at concentrations of 10-10-10-7 M, inhibited the release of immunoreactive somatostatin (SRIF) from rat hypothalamic segments which had been maintained in short term culture for 24 h. Neostigmine (10-6 M), an anticholinesterase, also inhibited the release of SRIF, whereas atropine (10-6 M), a muscarinic anticholinergic, had no effect on basal SRIF release but blocked the inhibition caused by acetylcholine (1-88 M). However, hexamethonium (10-6 M), a nicotinic antagonist, did not abolish the inhibition induced by acetylcholine. Potassium depolarization (56 mM KC1) caused stimulation of SRIF release, which was dependent on the presence of calcium in the incubation medium. SRIF was measured by a RIA sensitive to 1 pg/tube. Authenticity of immunoreactive SRIF released was suggested by immunological parallelism and chromatographic criteria using gel and high pressure liquid systems. These results suggest that muscarinic cholinergic mechanisms may have a regulatory role modulating the secretion of SRIF and, consequently, GH through actions at a hypothalamic level.