Intracellular recordings in thalamic neurones during spontaneous spike and wave discharges in rats with absence epilepsy.

1. In vivo extracellular and intracellular recordings were performed from thalamocortical (TC) neurones in a genetic model of absence epilepsy (genetic absence epilepsy rats from Strasbourg) during spontaneous spike and wave discharges (SWDs). 2. Extracellularly recorded single units (n = 14) fired either a single action potential or a high frequency burst of up to three action potentials, concomitantly with the spike component of the spike-wave complex. 3. Three main events characterized the intracellular activity of twenty-six out of twenty-eight TC neurones during SWDs: a small amplitude tonic hyperpolarization that was present throughout the SWD, rhythmic sequences of EPSP/IPSPs occurring concomitantly with the spike-wave complexes, and a small tonic depolarization at the end of the SWD. The rhythmic IPSPs, but not the tonic hyperpolarization, were mediated by activation of GABAA receptors since they reversed in polarity at -68 mV and appeared as depolarizing events when recording with KCl-filled electrodes. 4. The intracellular activity of the remaining two TC neurones consisted of rhythmic low threshold Ca2+ potentials, with a few EPSP/IPSP sequences present at the start of the SWD. 5. These results obtained in a well-established genetic model of absence epilepsy do not support the hypothesis that the intracellular activity of TC neurones during SWDs involves rhythmic sequences of GABAB IPSPs and low threshold Ca2+ potentials.