Chronic halothane modification of EEG-like activity recorded from somatosensory cortex and deep nuclei in freely behaving rats

Chronic exposure to the anesthetic agent halothane has been implicated in morphological and biochemical alterations of central nervous system tissue. In the present experiments, analysis of electroencephalographic (EEG) recordings has been used to examine effects on brain electrical acivity. EEGs were recorded from freely behaving rats with stereotaxically implanted permanent semimicroelectrodes. Recordings were taken from the somatosensory cortex (SC), nucleus parafasciculus thalami (PF), mesencephalic central gray (CG), and the ventromedial hypothalamus (VMH) before (control) and after 28 and 56 days of chronic intermittent halothane administration (0.5%, 3 hr/day, 5 days/week). On each recording day (0, 28 and 56), EEGs were obtained prior to haltothane exposure and following exposure to 0.25%, 0.5% and 1.5% halothane. In halothane-naive rats (day 0), the EEG dominant frequency (DF) showed a dose-response pattern consisting of an initial increase with 0.25% (significant only for the PF) followed by suppression at 0.5% and a marked significant decrease in all regions at 1.5%. On day 28, the pre-drug DF recorded from three of four regions showed a slowing trend. Additionally, with 1.5% halothane, only the SC DF was signficantly decreased. Following 56 days of intermittent exposure, the predrug EEG frequencies were significantly decreased in all regions as compared to naive values. Subsequent administration of 0.25% halothane produced a significant increase in all regional DFs which was also obtained with 0.5% and with 1.5% for the CG and VMH. The high DF values from the PF, CG and VMH at 0.5% and from the CG and VMH at 1.5% represent statistically significant increases over naive 1.5% values. Chronic halothane exposure is thus shown to progressively alter EEG activity and the EEG pattern of dose-responsiveness in four brain regions.