Calcium-activated hyperpolarizations in neurons of the mediolateral part of the lateral septum: intracellular studies from guinea pig brain slices.

Intracellular recordings were used to study the afterpotentials that followed a single spike and trains of spikes in class A neurons (n = 85) of the mediolateral part of the lateral septum (LSml) of the guinea pig in in vitro slices. Following a single spike, LSml neurons (n = 56) developed a slow afterhyperpolarization (sAHP), called early sAHP. These sAHP did not sum; other LSml neurons (n = 8) showed a depolarizing afterpotential (DAP) that summed. Twenty-one neurons did not exhibit an afterpotential. Following a train of spikes, LSml neurons (n = 79) developed a long-lasting sAHP, called late sAHP; these sAHP summed. Both the DAP and the early and late sAHP were markedly suppressed in amplitude by addition of Co2+ but persisted in the presence of tetrodotoxin (TTX). Increase in external K+ markedly depressed the early and late sAHP. Apamin and D-tubocurarine selectively blocked early sAHP, with no effect on late sAHP. These results indicate that the early and late sAHP are mainly generated by an activation of two types of Ca(2+)-dependent K+ conductances, with different time courses and pharmacological properties. In LSml neurons, late sAHP mediates the long-term adaptation of repetitive firing.