Journal of Neurophysiology, Vol 70, Issue 3 1244-1248, Copyright © 1993 by APS

ARTICLES

Membrane properties of rat subicular neurons in vitro

D. Mattia, G. G. Hwa and M. Avoli
Montreal Neurological Institute, McGill University, Quebec, Canada.

1. Conventional intracellular recordings were performed in rat hippocampal slices to investigate the electrophysiological properties of subicular neurons. These cells had a resting membrane potential (RMP) of -66 +/- 7.2 mV (mean +/- SD; n = 50), input resistance of 23.6 +/- 8.2 M omega (n = 51), time constant of 7.1 +/- 1.9 ms (n = 51), action potential amplitude of 85.8 +/- 13.8 mV (n = 50), and duration of 2.9 +/- 1.2 ms (n = 48). Analysis of the current-voltage relationship revealed membrane inward rectification in both depolarizing and hyperpolarizing direction. The latter type was readily abolished by Cs+ (3 mM; n = 6 cells). 2. Injection of depolarizing current pulses of threshold intensity induced in all subicular neurons (n = 51) recorded at RMP a burst of two to three fast action potentials (frequency = 212.7 +/- 90 Hz, n = 13 cells). This burst rode on a slow depolarizing envelope and was followed by an afterhyperpolarization and later by regular spiking mode once the pulse was prolonged. Similar bursts were also generated upon termination of a hyperpolarizing current pulse. 3. The slow depolarization underlying the burst resembled a low-threshold response, which in thalamic cells is caused by a Ca2+ conductance and is contributed by the Cs(+)-sensitive inward rectifier. However, bursts in subicular cells persisted in medium containing the Ca(2+)-channel blockers Co2+ (2 mM) and Cd2+ (1 mM) (n = 5 cells) but disappeared during application of TTX (1 microM; n = 3 cells). Hence they were mediated by Na+. Blockade of the hyperpolarizing inward rectification by Cs+ did not prevent the rebound response (n = 3 cells). 4. Our findings demonstrate that intrinsic bursts, presumably related to a "low-threshold" Na+ conductance are present in rat subicular neurons. Similar intrinsic characteristics have been suggested to underlie the rhythmic activity described in other neuronal networks, although in most cases the low-threshold electrogenesis was caused by Ca2+. We propose that the bursting mechanism might play a role in modulating incoming signals from the classical hippocampal circuit within the limbic system.

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