A COMPARISON OF THE GANGLION POTENTIALS AND BLOCK PRODUCED BY ACETYLCHOLINE AND TETRAMETHYLAMMONIUM.

Potentials recorded from the surface of the superior cervical ganglion of cats after an intra-arterial injection of acetylcholine were characterized by a complex waveform which depended on the amount of drug administered. Small doses of acetylcholine evoked a potential consisting of low amplitude hyperpolarization followed by low amplitude depolarization. Somewhat larger doses of acetylcholine caused a triphasic potential containing an initial period of depolarization in addition to the periods of hyperpolarization and delayed depolarization. Still larger doses of acetylcholine produced usually a monophasic wave of depolarization. Small doses of atropine prolonged the initial period of depolarization and prevented the hyperpolarization and delayed depolarization. Hexamethonium reduced or abolished the initial depolarization and enhanced or unmasked the hyperpolarization. The block of transmission occurring during the falling phase of the initial depolarization or during the hyperpolarization was antagonized by atropine. Unlike acetylcholine, tetramethylammonium produced only a prolonged ganglion depolarization which was unaffected by atropine and blocked by hexamethonium. The block of transmission by tetramethylammonium was partially prevented by atropine. These findings support the proposals that three pharmacologically distinctive cholinoceptive sites are present in sympathetic ganglia and, further, that activation of a cholinoceptive site sensitive to atropine may be involved in the block of transmission produced by acetylcholine and related drugs.