Barbiturates and transmitter release at the frog neuromuscular junction

The effects of the convulsant and anticonvulsant barbiturates, 5(2-cyclohexylidine ethyl)-5-ethyl barbituric acid (CHEB) an phenobarbital (PHB) were studied at the frog neuromuscular junction. Spontaneous transmitter liberation was augmented by both drugs irrespective of the external Ca2+ concentration, but CHEB was considerably more active than PHB. Miniature end plate potential amplitude was equally reduced by both drugs. The net effect of these agents on end plate potential (e.p.p.) amplitude varied depending on the external Ca2+ concentration. In Ca2+-deficient Ringer's solution both drugs increased the quantal content and the e.p.p. amplitude (CHEB more than PHB). In Ringer's solution containing a normal concentration of Ca2+ ions, both drugs depressed the evoked response and failed to alter the quantal content significantly.