Membrane properties of rat subicular neurons in vitro

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 MΩ (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 after hyperpolarization 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 Ca²⁺-channel blockers Co²⁺ (2 mM) and Cd²⁺ (1 mM) (n = 5 cells) but disappeared during application of TTX (1 μM; 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.