Brainstem carbachol injections in the urethane anesthetized rat produce hippocampal theta rhythm and cortical desynchronization: a comparison of pedunculopontine tegmental versus nucleus pontis oralis injections

Previous research has demonstrated that brainstem injections of acetylcholine agonists (e.g., carbachol) produced electrophysiological indicators of rapid-eye-movement (REM) sleep in the cat. Recent reports now indicate that this phenomenon may hold true for rats as well. Relatively few reports, however, have examined the effect of these injections on REM indicators in the anesthetized rat, a preparation useful for elucidating underlying neurobiological mechanisms controlling REM sleep processes. The present study compared the effect of injections of carbachol (5 μg in 250 nl) into the pedunculopontine tegmental nucleus (PPTg) or the nucleus pontis oralis (NPO) on two tonic indicators of REM sleep in the urethane-anesthetized rat. Namely, changes in the hippocampal EEG and in the cortical EEG. Carbachol injections into either site produced a change in both the hippocampal EEG and cortical EEG to a REM-like state at short latencies. The length of these changes (duration of effect), however, was site-dependent. Thus, PPTg carbachol injections induced significantly longer lasting effects in both the hippocampal and cortical EEG than did NPO injections. The results suggest that brainstem carbachol injections in rats, as in cats, may provide a useful model for investigating tonic REM sleep processes.