Pontine carbachol elicits multiple rapid eye movement sleep-like neural events in urethane-anaesthetized rats

Microinjection of a cholinergic agonist, carbachol, into the pontine reticu lar formation of chronically instrumented intact or acutely decerebrate rat s and cats has been used extensively to study rapid eye movement sleep mech anisms. In this study, we sought to develop a reduced carbachol model of ra pid eye movement sleep-like neural events exhibiting multiple physiological markers of this state, and allowing for the use of invasive electrophysiol ogical techniques. Accordingly, we investigated whether pontine carbachol c ould produce rapid eye movement sleep-like motor atonia and electrocortical changes in urethane-anaesthetized rats. We recorded cortical and hippocamp al electroencephalograms and genioglossus and inspiratory intercostal muscl e activities in 13 urethane-anaesthetized, spontaneously breathing, tracheo tomized and vagotomized rats. In steady-state periods with high-voltage/low -frequency electroencephalogram activity, carbachol microinjections (15-40 nl, 10 mM) were placed in the medial pontine reticular formation. In 12 rat s, carbachol elicited episodes of stereotyped hypotonia of genioglossus but not intercostal muscle activity, typical of rapid eye movement sleep, with a latency and duration of 2.2+/-0.3 min (mean+/-S.E.M.) and 11.0+/-2.9 min , respectively. In four of these rats, also similar to rapid eye movement s leep, the major suppression of genioglossus activity (-74+/-9%) was accompa nied by electroencephalogram desynchronization, appearance of hippocampal t heta rhythm, and a respiratory rate increase ( +14+/-3%). In the remaining eight rats, the stereotyped suppression of genioglossus activity (-48+/-3%) occurred without electroencephalogram desynchronization and hippocampal th eta, and was accompanied by a respiratory rate decrease (-6+/-2%); a patter n of response typical of decerebrate animals. Within a rat, similar pattern s of response to repeated carbachol injections at the same anatomical site were obtained. Pontine atropine prevented responses to subsequent carbachol injections. Thus, in urethane-anaesthetized rats, pontine carbachol consis tently produced a differential suppression of pharyngeal versus respiratory pump muscle activity, and in a subset of animals, this was also accompanie d by cortical and hippocampal electrographic changes typical of rapid eye m ovement sleep. This shows that complex and stereotyped neuronal events unde rlying both ascending and descending signs of rapid eye movement sleep can be pharmacologically activated under general anaesthesia. Such a reduced pr eparation may be useful for studies into the central neuronal mechanisms un derlying generation of rapid eye movement sleep; particularly for studies r equiring techniques that are difficult to implement in intact, naturally sl eeping animals. The acceleration of the respiratory rate observed only when carbachol induced electroencephalogram desynchronization suggests that neu ral events associated with electrocortical changes contribute to the respir atory rate increases observed in natural rapid eye movement sleep. (C) 1999 IBRO. Published by Elsevier Science Ltd.