Effects of 4-aminopyridine on the cat superior cervical ganglion.

These experiments tested the effect of 4-aminopyridine (4-AP) on acetylcholine (ACh) release, 45Ca++ accumulation and transmission in the perfused superior cervical ganglion of the cat. The 4-AP increased the amount of ACh released during preganglionic nerve stimulation, but it did not alter spontaneous ACh release. The 4-AP-induced increase of ACh release was compensated for by increased ACh synthesis because stimulation in the presence of the drug did not deplete tissue ACh content. When ACh release was suppressed by Mg++ or by low Ca++, 4-AP restored release to normal, but it did not do so when Ca++ was absent. This is interpreted as consistent with the idea that 4-AP increases Ca++ influx into nerve terminals and this was supported by measures of 45Ca++ accumulation by ganglia. Thus, preganglionic nerve stimulation increased 45Ca++ accumulation by ganglia and 4-AP increased this measure; Mg++ decreased the stimulation-induced change in 45Ca++ accumulation and 4-AP reversed this effect of Mg++. Depression of ganglionic transmission caused by Mg++ was readily antagonized by 4-AP, but the compound did not as readily augment transmission depressed by tubocurarine or by trimethaphan.