Effects of tetrodotoxin on changes in extracellular free calcium induced by repetitive electrical stimulation and iontophoretic application of excitatory amino acids in the sensorimotor cortex of cats.

Extracellular free calcium ([Ca2+]o) was measured with double barreled ion-sensitive reference electrodes in the sensorimotor cortex of cats before and after application of tetrodotoxin (TTX). Electrical stimulation of the cortical surface or of the thalamic ventrobasal complex resulted in reductions of [Ca2+]o (delta Ca) by up to 0.45 mM (baseline 1.2--1.3 mM). Iontophoretic applications of the excitatory amino acids glutamate, aspartate and DL-homocysteate evoked delta Ca by up to 1.2 mM. delta Ca were largest at a depth of 100-300 micron below cortical surface. After application of 10(-5) M TTX to the cortical surface, the delta Ca evoked by electrical stimulation disappeared and the accompanying slow negative potentials were reduced in amplitude. In contrast, delta Ca evoked by excitatory amino acids were only slightly affected. It is suggested that excitatory amino acids activate voltage-dependent postsynaptic Ca2+ conductances in neocortical neurones.