Alterations of physostigmine-induced yawning by chronic lithium administration in rats. 
The effect of chronic lithium pretreatment on physostigmine-induced yawning was investigated in male rats.  Intraperitoneal administration of physostigmine to rats induced yawning in a biphasic manner.  However the maximum response was obtained by 0.2 mg/kg of the drug.  Intracerebroventricular administrations of a putative M1 and M2 muscarinic receptor antagonists, pirenzepine and methoctramine decreased physostigmine-induced yawning.  Intraperitoneal administration of a non-selective muscarinic receptor antagonist, atropine, also decreased the physostigmine-induced yawning significantly.  Chronic lithium pretreatment (30 days) reduced yawning induced by physostigmine.  The inhibitory effect of pirenzepine, methoctramine and atropine on physostigmine-induced yawning increased in rats pretreated with chronic lithium.  These findings indicate that yawning is induced by a central cholinergic mechanism and that chronic pretreatment of lithium may interact with the cholinergic-induced behaviour.