Effects of a 4‐Aminopyridine in Calcium Movements and Changes of Membrane Potential in Pinched‐Off Nerve Terminals from Rat Cerebral Cortex

Abstract: A study was conducted on the influence of 4‐aminopyridine (4‐AP) on the radiocalcium uptake and membrane potential of rat cortical synaptosomes previously depolarized by biochemical procedures. The initial calcium entry into isolated nerve terminals was substantally enhanced in the presence of 10‐⁻⁴M 4‐AP in potassium‐rich media (60 mM). The fast initial phase of potassium‐stimulated calcium entry involves different kinetic characteristics in the presence and the absence of 4‐AP. In the presence of 4‐AP, the fast component of calcium entry reached a peak during the first 15 s immediately following depolarization. The potassium‐stimulated synaptosomal calcium influx at 15 s was 3.5 ± 0.17 times higher in the presence of 4‐AP (10‐⁻⁴M) and 2.6 ± 0.11 times higher in the absence of 4‐AP as compared with the respective unstimulated uptake values. In the absence of 4‐AP, however, the calcium entry did not show a peak until 45 s after depolarization. The total amount of calcium accumulated in the synaptosome treated and untreated by 4‐AP is equal at the end of this initial uptake period. The effect of 4‐AP on membrane potential during depolarization of synaptosomes evoked by potassium‐rich medium was also determined by means of a potential sensitive fluorescent dye. It was found that 4‐AP had no effect on the final level of membrane potential, induced by high [K⁺]_o; however, the kinetics showed significant differences. The initial phase of synaptosomal membrane depolarization was slower in the presence of 4‐AP: τ‐4‐AP: 42 ± 4.0 s, τ‐control: 18 ± 2.5 s. The initial calcium entry into the nerve terminals was increased during the 4‐AP treatment in potassium‐rich media—as resulted from our calcium uptake studies—and thus, during this higher calcium influx, the degree of membrane depolarization was decreased. This faster initial calcium influx constitutes a feasible explanation for the unique effect of 4‐AP termed “chemical potentiation” on transmitter release at chemical synapses.