Enhancement of CNS activity by thiopentone: comparison of nocifensive reflexes with hippocampal EEG.

1. This study tested the hypothesis that the nociceptive effects of thiopentone, are pharmacodynamically distinct from EEG effects. 2. We concurrently examined the effects of thiopentone on nocifensive reflex thresholds and on the power spectrum of the hippocampal electroencephalogram (hEEG) in chronically instrumented rats. 3. Pharmacodynamic descriptors were derived to characterize the biphasic (enhancement followed by depression) relationship between plasma thiopentone concentrations and the CNS effects. 4. Peak facilitation of nocifensive reflexes occurred at 13 (10-16) microg/ml whereas maximal enhancement of hEEG was observed at 16 (12-20) microg/ml. 5. The enhancement produced by low concentrations of thiopentone on nocifensive reflexes appear to pharmacodynamically distinct from the hEEG excitation produced within a similar range of thiopentone concentrations.