GABAergic neurons of the cat dorsal raphe nucleus express c-fos during carbachol-induced active sleep

Serotonergic neurons of the dorsal raphe nucleus (DRN) cease firing during active sleep (AS, also called rapid-eye-movement sleep). This cessation of electrical activity is believed to play a 'permissive' role in the generati on of AS. In the present study we explored the possibility that GABAegic ce lls in the DRN are involved in the suppression of serotonergic activity dur ing AS. Accordingly, we examined whether immunocytochemically identified GA BAergic neurons in the DRN were activated, as indicated by their expression of c-fos, during carbachol-induced AS (AS-carbachol). Three chronically-pr epared cats were euthanized after prolonged episodes of AS that was induced by microinjections of carbachol into the nucleus pontis oralis. Another fo ur cats (controls) were maintained 2 h in quiet wakefulness before being eu thanized. Thereafter, immunocytochemical studies were performed on brainste m sections utilizing antibodies against Fos, GABA and serotonin. When compa red with identically prepared tissue from awake cats, the number of Fos+ ne urons was larger in the DRN during AS-carbachol (35.9 +/- 5.6 vs. 13.9 +/- 4.4, P < 0.05). Furthermore, a larger number of GABA+ Fos+ neurons were obs erved during AS-carbachol than during wakefulness (24.8 +/- 3.3 vs. 4.0 +/- 1.0, P < 0.001). These GABA+ Fos+ neurons were distributed asymmetrically with a larger number located ipsilaterally to the site of injection. There was no significant difference between control and experimental animals in t he number of non-GABAergic neurons that expressed c-fos in the DRN. We ther efore suggest that activated GABAegic neurons of the DRN are responsible fo r the inhibition of serotonergic neurons that occurs during natural AS. (C) 2000 Elsevier Science B.V. All rights reserved.