Rapid desensitization of acetylcholine receptors of eel electroplaques following iontophoretic application of agonist compounds.

1. The electrical potential difference across the innervated membrane of the electroplaque of Electrophorus electricus was measured with an intracellular micro-electrode while an extracellular double-barrelled micropipette was used to apply acetylcholine and carbamylcholine iontophoretically very close to the point of insertion of the recording electrode. 2. The average depolarizing response to brief (several msec) pulses of carbamylcholine decayed 22 times more slowly than the response to acetylcholine. Treatment of the electroplaque with eserine or neostigmine prolonged the acetylcholine responses. 3. When a steady current of acetylcholine was applied for several seconds, the membrane first depolarized, then partially repolarized. Usually no repolarization was seen during long pulses of carbamylcholine or long pulses of acetylcholine in the presence of eserine or neostigmine. 4. During long conditioning pulses of acetylcholine or carbamylcholine, the responses to brief test pulses of acetylcholine showed a progressive decline in amplitude, but recovered after termination of the conditioning pulse. Desensitization half-times as short as 0-6 sec were observed, making these results similar to those obtained in the frog motor end-plate.