Properties of mEPSCs recorded in layer II neurones of rat barrel cortex

Voltage-clamp recordings from layer II neurones in somatosensory cortex of rats aged between 12 and 17 days showed a high frequency of spontaneous postsynaptic currents (sPSCs), which on average was 33 +/- 13 Hz (s.d.). sPSCs were mediated largely by glutamatergic AMPA receptors. Their rates and amplitudes were independent of blocking sodium channels with 1 microM tetrodotoxin (TTX). Most of them, therefore, represent genuine miniature excitatory postsynaptic currents (mEPSCs). The rise time of the fastest (10 %) mEPSCs was 288 +/- 86 micros (10-90 %) and the half-width was 1073 +/- 532 micros. The amplitude was -5.9 +/- 1.1 pA with a coefficient of variation (CV) of 0.44 +/- 0.14. The rate of mEPSCs was very temperature sensitive with a Q(10) (33-37 degrees C) of 8.9 +/- 0.9. Due to this temperature sensitivity, we estimated that the microscope lamp contributed an increase in temperature of about 4 degrees C to the tissue in the focal volume of the condenser. Cell-type differences in the rate of mEPSCs were found between pyramidal/multipolar and bipolar cells. The latter had a frequency of about a third of that seen in the other cell groups. Recordings in layer II are ideally suited to investigate mechanisms of spontaneous transmitter release.