[Effects of pentobarbital on hippocampal dentate field potentials in unrestrained rats]

The field potential technique was used to examine the effects of pentobarbital (40 mg.kg-1, ip) on the excitatory synaptic transmission and synchronous discharge of granule cells in the hippocampal dentate area of unrestrained rats. Two components of the dentate field potential evoked by perforant path stimulation were analyzed; the population EPSP and the population spike representing an extracellular excitatory postsynaptic potential and synchronous discharge, respectively. The rate of rise (slope) of the population EPSP decreased within 5 min after an injection of pentobarbital. This depression of excitatory synaptic transmission lasted for about 2 hrs and then recovered to near the pre-drug control level within 3 hrs. The ratio of population spike amplitude to population EPSP slope (S/E ratio) was calculated to estimate the readiness of granule cells for synchronous discharge. After an injection of pentobarbital the S/E ratio increased for 2 hrs, indicating that the synchronous discharge of granule cells was facilitated by the drug in spite of the depression of excitatory synaptic transmission. Under pentobarbital anesthesia, the onset and peak latencies of population spikes were prolonged for 0.2-0.4 msec.