Differential effects of 4-aminopyridine on acetylcholine release triggered by K+ depolarization, veratridine, or A23187 in rat cerebral cortical synaptosomes.

The effect of 4-aminopyridine on [3H]acetylcholine release was studied in rat cerebral cortical synaptosomes in the presence of a several secretagogues that have different mechanisms of action. As found previously, 4-aminopyridine increased [3H]acetylcholine release in a concentration-dependent manner (5-10 mM); a high concentration (10 mM) also elevated [3H]choline efflux. However, the 35 mM K+ induced release of [3H]acetylcholine was attenuated by 4-aminopyridine at concentrations (less than 5 mM) that had no effect on transmitter release. At no concentration of 4-aminopyridine was the release of transmitter additive with 35 mM K+ induced release. Veratridine-induced release was neither attenuated nor additive with low concentrations of 4-aminopyridine, even when a sub-maximal concentration of the sodium ionophore was used (10 microM). In contrast, A23187-induced release was additive with that caused by 4-aminopyridine. These results suggest that: 1) 4-aminopyridine blocks potassium channels involved in regulating membrane potential in isolated cholinergic terminals; and 2) changes in the activity of these 4-aminopyridine sensitive K+ channels are not important in the nerve terminal's response to depolarization caused by sodium influx.