c-fos Expression in brainstem premotor interneurons during cholinergicallyinduced active sleep in the cat

The present study was undertaken to identify trigeminal premotor interneuro ns that become activated during carbachol-induced active sleep (c-AS). Thei r identification is a critical step in determining the neural circuits resp onsible for the atonia of active sleep. Accordingly, the retrograde tracer cholera toxin subunit B (CTb) was injected into the trigeminal motor nuclei complex to label trigeminal interneurons. To identify retrograde-labeled a ctivated neurons, immunocytochemical techniques, designed to label the Fos protein, were used. Double-labeled (i.e., CTb+, Fos(+)) neurons were found exclusively in the ventral portion of the medullary reticular formation, me dial to the facial motor nucleus and lateral to the inferior olive. This re gion, which encompasses the ventral portion of the nucleus reticularis giga ntocellularis and the nucleus magnocellularis, corresponds to the rostral p ortion of the classic inhibitory region of Magoun and Rhines (1946). This r egion contained a mean of 606 +/- 41.5 ipsilateral and 90 +/- 32.0 contrala teral, CTb-labeled neurons. These cells were of medium-size with an average soma diameter of 20-35 mu m. Approximately 55% of the retrogradely labeled cells expressed c-fos during a prolonged episode of c-AS. We propose that these neurons are the interneurons responsible for the nonreciprocal postsy naptic inhibition of trigeminal motoneurons that occurs during active sleep .