EPSPs in rat neocortical neurons in vitro. I. Electrophysiological evidence for two distinct EPSPs

1. To investigate excitatory postsynaptic potentials (EPSPs),
intracellular recordings were performed in layer II/III neurons of
the rat medial frontal cortex. The average resting membrane
potential of the neurons was more than -75 mV and their average
input resistance was greater than 20 M omega. The amplitudes of the
action potentials evoked by injection of depolarizing current
pulses were greater than 100 mV. The electrophysiological
properties of the neurons recorded were similar to those of
regular-spiking pyramidal cells. 2. Current-voltage relationships,
determined by injecting inward and outward current pulses,
displayed considerable inward rectification in both the
depolarizing and hyperpolarizing directions. The steady-state input
resistance increased with depolarization and decreased with
hyperpolarization, concomitant with increases and decreases,
respectively, in the membrane time constant. 3. Postsynaptic
potentials were evoked by electrical stimulation via a bipolar
electrode positioned in layer IV of the neocortex.
Stimulus-response relationships, determined by gradually increasing
the stimulus intensity, were consistent among the population of
neurons examined. A short-latency EPSP [early EPSP (eEPSP)] was the
response with the lowest threshold. Amplitudes of the eEPSP ranged
from 4 to 8 mV. Following a hyperpolarization of the membrane
potential, the amplitude of the eEPSP decreased. Upon
depolarization, a slight increase in amplitude and duration was
observed, accompanied by a significant increase in time to peak. 4.
The membrane current underlying the eEPSP (eEPSC) was measured
using the single-electrode voltage-clamp method. The amplitude of
the eEPSC was apparently independent of the membrane potential in 8
of 12 neurons tested. In the other 4 neurons, the amplitude of the
eEPSC increased with hyperpolarization and decreased with
depolarization. 5. Higher stimulus intensities evoked, in addition
to the eEPSP, a delayed EPSP [late EPSP (lEPSP)] in greater than
90% of the neurons tested. The amplitude of the lEPSP ranged from
12 to 20 mV, and the latency varied between 20 and 60 ms. The
amplitude of the lEPSP varied with membrane potential, decreasing
with depolarization and increasing following hyperpolarization. The
membrane current underlying the lEPSP (lEPSC) displayed a similar
voltage dependence. 6. At stimulus intensities that led to the
activation of inhibitory postsynaptic potentials (IPSPs), the lEPSP
was no longer observed.