Effect of 4-aminopyridine on synaptic transmission in rat hippocampal slices

Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in area CA1 of rat hippocampal slices in vitro. The responses evoked by spontaneously released glutamate and GABA were recorded from area CA1 pyramidal neurons in rat hippocampal slices in whole-cell mode. The glutamate and GABA receptor-associated ligand-gated currents were obtained from dissociated single hippocampal pyramidal cells. The results showed that 4-aminopyridine (4-AP) had obvious effects on both presynaptic and postsynaptic events. Applications of 4-AP in micromolar concentration resulted in persistent enhancement of the initial slope of fEPSPs with the half-maximal enhancement concentration (EC₅₀) of 46.7±2.68 μM. At the concentration of 200 μM, 4-AP increased the initial slopes of the total fEPSPs, NMDA- and AMPA-mediated fEPSPs components to 225.6±23.8%, 177.4±20.1% and 142.3±18.9%, respectively, but had no effect on the fiber volley. The half-maximal stimulus intensity to induce responses was reduced from 5.14±0.27 to 3.58±0.23 V. The frequencies of mEPSCs and mIPSCs were increased to 324.2±25.4% and 287.3±36.3% by 200 μM 4-AP. The amplitude histograms of mEPSCs and mIPSCs were fitted with Gaussian distributions. After 200 μM 4-AP application, the first and second peaks in Gaussian distributions of mEPSCs were shifted from 8.73±0.94 and 17.78±2.13pA to 10.48±0.82 and 21.14±2.45 pA, while those of mIPSCs were shifted from 13.65±0.96 and 25.51±2.95 pA to 11.21±1.04 and 23.08±2.37 pA. At 200 μM, 4-AP reduced paired-pulse facilitation and accelerated synaptic fatigue induced by stimulation at 10 Hz (for 1 s) and the ratio of fEPSPs₁₀/fEPSPs ₁ was decreased from 1.62±0.16 to 0.61±0.15. At 200 μM, 4-AP inhibited postsynaptic GABA currents induced by 5 μM GABA to 68.2±15.5%: by countering the effect of enhanced release of GABA from presynaptic terminals, this could depress the inhibitory pathway. Also at 200 μM, 4-AP increased NMDA currents to 155.3±17.8%, but had no significant effect on AMPA currents (94.2±15.6%). Our experimental results thus show that 4-AP-induced changes of synaptic transmission in area CA1 of rat hippocampus may be attributed to 4-AP's effects on both presynaptic terminals and postsynaptic receptors.