BEHAVIOR OF RAPHE CELLS PROJECTING TO THE DORSOMEDIAL MEDULLA DURING CARBACHOL-INDUCED ATONIA IN THE CAT

1. The activity of most brainstem serotonergic cells is suppressed dur ing sleep, particularly the rapid eye movement (REM) phase. Thus, they may play a major role in state-dependent changes in CNS functioning. Our main goal was to search for medullary raphe cells having axonal br anches in the region of the hypoglossal (XII) motor nucleus and assess their behaviour during the atonia produced by microinjections of a ch olinergic agonist, carbachol, into the dorsal pontine tegmentum. In ch ronic animals, such microinjections evoke a desynchronized sleep-like state similar to natural REM sleep; in decerebrate animals, they produ ce eye movements and a motor suppression similar to the postural atoni a of REM sleep. 2. In decerebrate, paralysed, vagotomized and artifici ally ventilated cats, we recorded extracellularly from medullary raphe cells antidromically activated from the XII nucleus region. Forty-fiv e cells recorded in the raphe obscurus and pallidus nuclei were antidr omically activated with latencies characteristic of non-myelinated fib res (4.4-42.0 ms). For thirty-three of the forty-five cells, we found one or more axonal branches within or just below the XII nucleus. The remaining twelve cells, in addition to the XII nucleus, had axonal ram ifications in the medial nucleus of the solitary tract (NTS) and/or th e dorsal motor nucleus of the vagus (DMV). 3. A subset of fourteen spo ntaneously active cells with identified axonal projections were held l ong enough to be recorded during the carbachol-induced atonia, and eig ht of these also during the subsequent recovery and a systemic adminis tration of the serotonergic 1A receptor agonist (+/-)8-hydroxy-2-(di-N -propylamino)tetrealin hydrobromide (8-OH-DPAT). All but one mere supp ressed during the atonia in parallel to the suppression of XII, phreni c and postural nerve activities (firing rate, 1.3 +/- 0.7 Hz before an d 0.1 +/- 0.2 Hz after carbachol (means +/- S.D.)). Following the reco very from the atonia, the firing rates of the eight cells increased to the pre-carbachol level (1.6 +/- 1.0 Hz). Subsequently, all were sile nced by 8-OH-DPAT. 4. These cells fulfil most physiological criteria f or serotonergic cells and have the potential to modulate, in a state-d ependent manner, activities in the motor XII nucleus, visceral sensory NTS, and DMV. The decrements in serotonergic neuronal activity that o ccur during the carbachol-induced atonia suggest that a similar withdr awal of serotonergic input may occur during REM sleep and contribute t o the characteristic reductions in upper airway motor tone.