Presynaptic effects of 4-aminopyridine and streptomycin on the neuromuscular junction

Studies were done to assess the effects of 4-aminopyridine (4AP) and streptomycin (SM) on transmitter release parameters and extracellularly recorded presynaptic action potential. The application of 5 micrometer 4AP resulted in a marked increase in the mean quantal content (m1) associated with an increase in the total number of the store of available quanta (n) but had no effect on the probability of release (p) and the fractional release (P). Focal recording showed that 50 micrometer 4AP modified the shape of the presynaptic action potential from a triphasic configuration to a diphasic one. In contrast, prolongation of the muscle action potential was found only at higher concentrations (greater than 1 mM) of of 4AP. Thus, the increase in n with 4 AP was associated with prolongation of the presynaptic action potential evoked by blocking the K current. SM (172 micrometer mM) had no effect on p and P. Reduction of n by SM was completely reversed by 4AP. As the presynaptic action potential change induced by 4AP was not antagonised by SM, it may be that the decrease of n by SM followed a modification of the voltage-dependent Ca channel.