Yawning: A Central Cholinergic Response under Monoaminergic Modulation

482 Yawning: A Central Cholinergic Response under Monoaminergic Modulation B. HOLMGREN and R. URBA-HOLMGREN Department of Neurophysiology, Centro Nacional d e Investigaciones Cietificas, La Habana (Cuba) Yawning is a cholinergic response in the sense that it occurs when the balance created by the release of a number of neurotransmitters is overcome by an excess of cholinergic activity. Intraperitoneal injections of physostigmine or pilocarpine in neonatal rats induce stereotyped yawning; the effect declines towards the middle of the second week of life. In infant rats yawning is blocked by scopolamine, thus suggesting that muscarinic receptors are involved. Phenoxybenzamine, a blocker of a-adrenergic receptor activity, increases significantly physotigmine-induced yawning thus suggesting that noradrenergic influences are inhibitory. A strong facilitatory influence of serotoninergic mechanisms on yawning is indicated by a 3t o 4-fold potentiation of physostigmine-induced yawning obtained with Lu 10-171, a potent and selective serotonin uptake inhibitor. In summary, cholinergically-induced yawning in infant rats is presented as a simple experimental model of a central cholinergic response which is subject t o negative and positive modulating influences by noradrenergic and serotoninergic mechanisms.

Control of Transmitter Release during Repetitive Activation of Motor Nerve Terminals M.A. KAMENSKAYA

Department o f Human and Animal Physiology, Moscow State University, 1 1 7234 Moscow (U.S.S.R.) During repetitive activation of motor nerve terminals transmitter release at neurornuscular junctions is facilitated in amphibia but depressed in mammals. Experiments have been 1.6

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Fig. 1, The ratios of the quantal content of the 2nd (mz) end-plate potential (EPP) t o the 1st ( m l ) and of the 40th (m40) t o the 1st in a train of 40 stimuli at 50 Hz. Open columns, frog sartorius msucle; hatched columns, mouse diaphragm; filled columns, human intercostal muscle. A and B, synapses with a mean quantal content o f , respectively, 60 and 150 for a single EPP.