Selective depression of dopamine-induced depolarization by morphine and enkephalins in snail neurons

Abstract

Morphine, met-enkephalin, and leu-enkephalin in a concentration of 1×10−5 M depress rapidly and reversibly the amplitude of depolarization induced by dopamine application toHelix pomatia neurons; the effect is naloxone-dependent. The amplitudes of dopamine-induced hyperpolarization and also of the depolarization and hyperpolarization responses to acetylcholine application are unchanged under these circumstances. The hypothesis of blocking of chemosensitive sodium channels by enkephalins is discussed. It is suggested that this hypothesis is true for high concentrations of morphine and enkephalins (1×10−4 to 1×10−3 M). In lower concentrations (1×10−5 M) morphine and enkephalins lead to modulation of the reponses to the action of neurotransmitters, evidently through their influence on the cyclic nucleotide system.