Acetylcholine receptors: topographic distribution and pharmacological properties of two receptor types on a single molluscan neurone.

1. The iontophoretic application of acetylcholine (ACh) on to identified neurones in the buccal ganglion of the mollusc Navanax produced a biphasic or monophasic membrane potential change which was a function of the current intensity and site of ACh application.2. Low iontophoretic currents, 200 msec in duration, applied to the somatic surface facing the neuropile, caused a monophasic potential change of 6-10 sec duration, which had a reversal potential of about - 50 mV, varied with changes in the [Cl](o) of the bathing medium, and was not blocked by the cholinolytics tested.3. ACh applied more distal to the soma, in the neuropile, produced a 1-3 sec monophasic response whose reversal potential was more positive than - 30 mV, varied in amplitude with changes in the [Na](o) of the medium, and was blocked by cholinolytics such as tubocurarine, hexamethonium and atropine.4. With larger iontophoretic currents a biphasic response could be obtained, depolarization followed by hyperpolarization, which represented a superposition of the above monophasic potentials.5. The cholinomimetics propionylcholine and butyrylcholine caused a biphasic response like that to ACh. Carbamylcholine and tetramethylammonium also produced a biphasic response but with a more prominent Cl component than that to ACh. Acetyl-beta-methylcholine, oxytremorine and pilocarpine only produced a response comparable to the chloride phase of the ACh response.6. Anticholinesterases prolonged both phases of the ACh response.7. It was concluded that each of the identified neurones possess two types of cholinoceptive sites, which are pharmacologically distinct, produced different changes in membrane permeability and are distributed differently over the axo-somatic membrane complex.