AGE-DEPENDENT APPEARANCE OF SYNAPTIC CURRENTS IN RAT NEOCORTICAL NEURONS IN CULTURE

Rat neocortical neurons grown in dissociated cell culture were recorde d with the whole-cell patch-clamp technique. Spontaneous inward curren ts were observed in cells that were held at a membrane potential of -8 0 mV in medium containing tetrodotoxin and Cd2+. These currents displa yed amplitudes up to 140 pA and rise time of 1.8 +/- 0.2 ms (mean +/- SD, n = 15). They reversed near 0 mV and showed no voltage-dependent f requency of occurrence. Hence, they were presumably due to spontaneous release of transmitter. The inward currents appeared around day 10 in culture and were detected up to 4 weeks. When cells of different ages were compared, the maximal probability of recording these inward even ts occurred at around 3 weeks in culture. The inward currents were not reduced by application of bicuculline methiodide which is a competiti ve antagonist of the GABA(A) receptor, but were blocked by the broad-s pectrum glutamate receptor antagonist kynurenic acid. Moreover, sponta neous inward events were not affected by DL-2-aminophosphano-valerate (NMDA receptor antagonist) but disappeared following application of th e non-NMDA receptors antagonist 6-cyano-7-nitroquinoxaline-2,3-dione ( CNQX). Our observations indicate that the inward currents represent mi niature synaptic events that are primarily mediated by non-NMDA excita tory amino acid receptor subtypes. Furthermore, our findings indicate that they develop over time and are not present in neurons that are gr own in culture for less than 10 days. (C) 1994 Wiley-Liss, Inc.