Abolition of the neocortically monitored theta rhythm after ibotenic acid lesion of the parafascicular nucleus in behaving rats.

Seven adult Sprague-Dawley rats, chronically implanted with standard electrodes to monitor frontoparietal electroencephalographic (EEG) and nuchal electromyographic (EMG) activity, received, under deep anesthesia, unilateral or bilateral microinjections of ibotenic acid in the lateral part of the parafascicular nucleus of the thalamus. Four days after the injections (corresponding to the period of neuronal destruction), obliteration of the oscillatory activity in the theta range was found on the side ipsilateral to the injection, while on the intact hemisphere the rhythm was well developed. The asymmetry between the two hemispheres was particularly evident during REM sleep but was also seen during attentive but immobile alertness. In bilaterally injected rats, the neocortical theta rhythm was abolished on both hemispheres. These results suggest that in freely-moving rats the lateral parafasciculus neurons are part of the network on which the emergence of the theta rhythm relies.