The acetylcholine receptors of the semicircular canal in the frog (Rana pipiens).

We present preliminary evidence that acetylcholine has three distinct modes of action on the discharge rate of afferent fibers from the frog isolated semicircular canal. These are: (I) a facilitatory effect which is of mixed muscarinic-nicotinic type and is mediated by a receptor which we call "atropine-preferring'; (II) a suppressive effect unmasked by atropine and antagonized by strychnine whose action is mediated by a "strychnine-preferring" receptor; (III) a suppressive effect produced by the cholinesterase inhibitors eserine and echothiophate, which being antagonized by curare, is mediated by a "curare-preferring" receptor.