VOLTAGE DEPENDENCE OF EXCITATORY POSTSYNAPTIC POTENTIALS OF RAT NEOCORTICAL NEURONS

1. The properties of excitatory postsynaptic potentials (EPSPs) of rat neocortical neurons were investigated with a fast single-electrode current-voltage clamp in vitro. Typically, apparently pure EPSPs were obtained by selection of electric stimuli of low intensity. 2. The amplitude and time integral of the EPSP increased when the neuron was depolarized. At threshold for generation of action potentials, the amplitude of EPSPs was increased by approximately 30% [from 5.0 +/- 2.1 to 6.3 +/- 1.0 (SD) mV, n = 12]. The integral of EPSPs was maximally about fourfold (3.7 +/- 1.5, n = 16) larger than at resting membrane potential (E(m)). The mechanisms involved in this augmentation of EPSPs were further investigated. 3. The amplitude and the time integral of excitatory postsynaptic currents (EPSCs) decreased linearly with shifts in command potential from -100 to -60 mV. The decrease of the EPSC integral with depolarization indicates that the enhancement of the EPSP may be brought about by recruitment of a voltage-dependent inward current. 4. Evoking EPSPs at various delays after the onset of small depolarizing current pulses (0.3-0.6 nA, 600 ms) revealed that augmentation decays with time. The integral of EPSPs evoked approximately 80 ms after the onset of the current pulse was 3.7 (+/- 1.5, n = 16) times larger than at E(m). The integral of EPSPs evoked at 480 ms, however, were only twofold (+/- 0.7, n = 16) larger. Hence EPSPs evoked after a delay of 80 ms were 1.7-fold (+/- 0.4, n = 24) larger than EPSPs evoked after 480 ms. EPSCs were independent of the delay of stimulation at all potentials. 5. Intracellular application of the lidocaine derivative N-(2,6-dimethyl-phenylcarbamoylmethyl) triethylammonium bromide (QX 314) at 100 mM from pipettes rapidly abolished fast action potentials and inward rectification. During comparable depolarizations the increase in EPSP integrals was much smaller in QX 314-treated neurons than in controls. On average, the integral of EPSPs evoked at 70-90 ms was 1.7 times (+/- 1.0) larger than at E(m), and the integral of EPSPs evoked with larger delays was close to the value obtained at resting E(m) (0.9 +/- 0.3, n = 8). The ratio of EPSP integrals early versus late (1.8 +/- 0.5) is comparable to controls suggesting that QX 314-sensitive currents are unlikely to be involved in the time-dependent enhancement. 6. Mimicking EPSPs by brief depolarizations atop long depolarizations revealed a time- and voltage-dependent enhancement comparable to that of EPSPs. The integral of early (80-ms delay) pulses was 1.7-fold larger than the integral of late (480-ms delay) extrapulses. This augmentation of early extrapulses is in close agreement with the enhancement of the EPSP and lends support to the hypothesis of a recruitment of intrinsic inward currents. Application of 2 mM Co2+ abolished the time dependence of the extra depolarizations, implying a Ca2+ current in the time dependent enhancement of such small depolarizations. 7. EPSPs evoked 480 ms after the onset of a depolarization were progressively increased by increasing durations of preceding hyperpolarizing steps back to E(m). The time constant of recovery of augmentation was closely matched by the time constant of deinactivation of a transient inward current. 8. Evoking EPSPs by paired stimuli of variable intervals (10-100 ms) indicated that the second EPSP is preferentially enhanced by depolarization, when the stimulus interval is decreased to approximately 15 ms. Below the threshold for action-potential generation, the second EPSP was on the average twofold larger than when evoked with a delay of 100 ms. Enhancement of EPSPs evoked as the second of a pair exhibits a similar time dependence as do single stimulus EPSPs, decaying with increasing delays of the pair. 9. Application of the N-methyl-D-aspartate (NMDA) antagonist D-2-amino-5-phosphono-valeric acid (D-APV) affected neither the augmentation of early evoked EPSPs nor the time dependence in neurons without late EPSP (l-EPSP) components. In neurons exhibiting l-EPSP components, the EPSPs evoked early and late during prolonged depolarizations were enhanced alike. Application of D-APV to these neurons reduced the integral of EPSPs evoked late, thus unmasked the usual time dependence. The contribution of NMDA receptor-mediated components to the EPSP enhancement therefore appears to be negligible. 10. The opposite voltage dependence of EPSPs and EPSCs suggests that "boosting" of EPSPs is brought about by recruitment of an additional inward current. The similarity in time and voltage dependence of EPSPs and small depolarizations generated by extra pulses, together with the Co2+ sensitivity of the latter, suggests the recruitment of a transient, low-threshold Ca2+ current. This recruitment represents an AND-gate with a discrete time window.