Electrical properties of neurons in the mediolateral part of the lateral septum: intracellular recordings from guinea-pig brain slices.

Membrane properties of 174 neurons were studied in the mediolateral part of the lateral septum (LSml) using an in vitro slice preparation of guinea-pig brain. Intracellular recordings were correlated with morphological data obtained from 34 neurons intracellularly stained with horseradish peroxidase (HRP). Neurons were divided into three classes according to their electrical responses. Class A and B neurons displayed the common property of an overshooting of spikes in response to the direct application of weak depolarizing current pulses. Class A neurons (59.2% of the total) generated tetrodotoxin-insensitive, high-threshold Ca2+ spikes in a control medium. Class B neurons (20.7% of the total) generated high-threshold Ca2+ spikes only if tetraethylammonium was used to block delayed rectifying K+ current. Features common to class A and B neurons included the inactivation of Na+ conductance, the participation of high-threshold Ca2+ conductance in the generation of spikes--when repetitive discharges were elicited by strong depolarizing current pulses--and Cs(+)-sensitive, Ba(2+)-insensitive anomalous rectification. Class C neurons (20.1% of the total) displayed discharges comprising small-amplitude Na+ spikes followed by slow and large Ca2+ spikes, suggesting a locus of impalement which was not the soma. HRP-filled class B neurons (n = 5) were characterized by small to medium perikarya with spindly dendrites. The majority of HRP-filled class A (15/21) and all class C (n = 8) neurons showed large perikarya with thick primary dendrites and spiny dendritic branches. Thus, class A and C neurons typify the guinea-pig LSml in their morphological characteristics and in their ability to generate high-threshold Ca2+ spikes in a control medium.