Abstract
The electrophysiological properties of neurons in the rat perirhinal cortex were analyzed with intracellular recordings in an in vitro slice preparation. Cells included in this study (n = 59) had resting membrane potential (RMP) = โ73.9 ยฑ 8.5 mV (mean ยฑ SD), action potential amplitude = 95.5 ยฑ 10.4 mV, input resistance = 36.1 ยฑv 15.7 Mฮฉ, and time constant = 13.9 ยฑ 3.4 ms. When filled with neurobiotin (n = 27) they displayed a pyramidal shape with an apical dendrite and extensive basal dendritic tree. Injection of intracellular current pulses revealed: 1) a tetrodotoxin (TTX, 1 ฮผM)โsensitive, inward rectification in the depolarizing direction (n = 6), and 2) a timeโ and voltageโdependent hyperpolarizing sag that was blocked by extracellular Cs^+^ (3 mM, n = 5) application. Prolonged (up to 3 s) depolarizing pulses made perirhinal cells discharge regular firing of fast action potentials that diminished over time in frequency and reached a steady level (i.e., adapted). Repetitive firing was followed by an afterhyperpolarization that was decreased, along with firing adaptation, by the Ca^2+^โchannel blocker Co^2+^ (2 mM, n = 6). Action potential broadening became evident during repetitive firing. This behavior, which was more pronounced when larger pulses of depolarizing current were injected (and thus when repetitive firing attained higher rates), was markedly decreased by Co^2+^ application. Subthreshold membrane oscillations at 5โ12 Hz became apparent when cells were depolarized by 10โ20 mV from RMP, and action potential clusters appeared with further depolarization. Application of glutamatergic and GABA~A~ receptor antagonists (n = 4), Co^2+^ (n = 6), or Cs^+^ (n = 5) did not prevent the occurrence of these oscillations that were abolished by TTX (n = 6). Our results show that pyramidalโlike neurons in the perirhinal cortex are regular firing cells with electrophysiological features resembling those of other cortical pyramidal elements. The ability to generate subthreshold membrane oscillations may play a role in synaptic plasticity and thus in the mnemonic processes typical of this limbic structure. Hippocampus 2001;11:662โ672. ยฉ 2001 WileyโLiss, Inc.