THE FASCICULUS RETROFLEXUS CONTROLS THE INTEGRITY OF REM-SLEEP BY SUPPORTING THE GENERATION OF HIPPOCAMPAL THETA-RHYTHM AND RAPID EYE-MOVEMENTS IN RATS

The fasciculus retroflexus (FR) fiber bundle comprises the intense cho linergic projection from the medial division of the habenula nucleus ( Hbn) of the epithalamus to the interpeduncular nucleus (IPN) of the li mbic midbrain. Due to the widespread connections of the Hbn and IPN, i t could be surmised that the FR is integrated in the processings of va rious subsystems that are known to be involved in the sleep-wake mecha nisms; relevant sites include the limbic forebrain and midbrain areas and more caudal pontine structures. Consequently, the present study ad dressed the significance of the FR in the spontaneous sleep-wake stage -associated variations of the different activity patterns of frontal c ortex and hippocampal electroencephalograms (EEGs), the electrooculogr am, and body movements, in freely behaving rats that had been subjecte d to either bilateral electrolytic lesioning of the FR or control oper ations. The evolution of different state combinations was assessed by the combinatory analysis of different activity stages appearing on the 6-h records. As compared to the control-operated group, the FR lesion ing substantially reduced tare time spent in rapid eye movement (REM) sleep by 79%, moderately decreased the duration of the intermediate st ate of sleep by 29%, and quiet waking state by 44%, but had virtually no effects on the durations of different types of non-HEM sleep (i.e., drowsiness that which involved quiet sleep or slow-wave sleep contain ing delta and spindle state components) or on the times of active waki ng behavior that corresponded to the body movements. Quantitative deco mposition analyses revealed marked variations in the frontal cortex an d hippocampal activity as well as REM during the course of the extract ed sleep-wake stages described and there were also some group differen ces. Of those individual features that were used to determine differen t sleep-wake stages, the overall hippocampal theta time (41% decrease) and single REM frequency (71% reduction during the REM sleep) were mo st affected. In contrast, the various properties of desynchronization/ synchronization patterns of frontal cortex EEGs were consistently hard ly influenced by the FR lesioning. Therefore, the present data suggest the involvement of the FR in the REM sleep processes by establishing prominent associations with the limbic and REM control mechanisms that involve the hippocampus and plausibly pontine ocular activity network s. (C) 1998 Elsevier Science Inc.