BRAIN-STEM SITES FOR THE CARBACHOL ELICITATION OF THE HIPPOCAMPAL THETA-RHYTHM IN THE RAT

The effects of brainstem microinjections of carbachol on the hippocamp al theta rhythm were examined in urethane anesthetized rats. The two m ost effective theta-eliciting sites with carbachol were the nucleus po ntis oralis (RPO) and the acetylcholine-containing pedunculopontine te gmental nucleus (PPT) of the dorsolateral pontine tegmentum. RPO injec tions generated theta at mean latencies of 38.5 +/- 70.8 s and for mea n durations of 12.9 +/- 5.1 min. Five of seven RPO injections gave ris e to theta virtually instantaneously, i.e., before the completion of t he injection. PPT injections generated theta at mean latencies of 1.7 +/- 1.1 min and for mean durations of 11.9 +/- 6.0 min. Injections ros tral or caudal to RPO in the caudal midbrain reticular formation (RF) or the caudal pontine RF (nucleus pontis caudalis) generated theta at considerably longer latencies (generally greater than 5 min) or were w ithout effect. Medullary RF injections essentially failed to alter the hippocampal EEG. The finding that theta was produced at very short la tencies at RPO suggests that RPO, the putative brainstem source for th e generation of theta, is modulated by a cholinergic input. The furthe r demonstration that theta was also very effectively elicited with PPT injections suggests this acetylcholine-containing nucleus of the dors olateral pens may be a primary source of cholinergic input to RPO in t he generation of theta. The hippocampal theta rhythm is a major event of REM sleep. The present results are consistent with earlier work sho wing that each of the other major events of REM sleep, as well as the REM state, are cholinergically activated at the level of the pontine t egmentum.