THE MEDULLARY RETICULAR-FORMATION IS A SITE OF MUSCLE-RELAXANT ACTIONOF DIAZEPAM ON DEEP BACK AND NECK MUSCLES IN THE FEMALE RAT

We tested the hypothesis that the effect of systemic injections of dia zepam (DZ, 125 mg/kg) to reduce the quality of the reproductive behavi or, lordosis, and to reduce the EMG of lumbar back muscles involved in lordosis (Schwartz-Giblin et al., 1984) is exerted through a reticulo spinal pathway with cells of origin in the nucleus gigantocellularis t hat excites lumbar motoneurons indirectly (Robbins et al., 1990, Robbi ns et al., 1992). In contrast, DZ facilitates lordosis behavior when i nfused into the midbrain central gray (McCarthy et al., 1995). Direct deposits of crystalline mixtures of DZ (20-80 ng) in dextrose were del ivered to the medullary reticular formation (MRF) by diffusion from a cannula inserted through a guide to which a bipolar stimulating electr ode was attached. The multiunit EMG response evoked by 20 (300 ms long ) stimulus trains was recorded in back and neck muscles, lateral longi ssimus and splenius before and 5, 15, 30 and 60 min after local DZ dep osits. There was a significant reduction in EMG response over this tim e period when stimulus intensities were within the range of 1.2-1.5 ti mes threshold (Friedman two-way non-parametric test, P < 0.002). Large amplitude motor units that provide large tensions were the most sensi tive to DZ-induced inhibition. Control deposits of dextrose had no sig nificant effect. Systemic injections of progesterone (1 mg, i.p.) 60 m in after DZ deposits, but not after dextrose deposits, further reduced the MRF-evoked EMG responses over the course of 1 h. As predicted, DZ infusions into the midbrain central gray did not reduce the reticulos pinal-evoked axial muscle response, consistent with the facilitatory e ffect of midbrain central gray infusions of DZ on the lordosis quotien t. The results suggest that benzodiazepine agonists (if endogenous) ac ting at sites in the MRF would be effective muscle relaxants during pr egnancy, prior to the fall in progesterone that precedes labor.