Nucleus raphe obscurus (nRO) regulation of anorectal motility in rats

Previous research has demonstrated that anorectal contractions in the rat are modulated by activation of spinal autonomic circuits. In the present study, anterograde tracing of descending pathways originating from the caudal nucleus raphe obscurus (nRO) revealed that this nucleus projects to cells within the intermediolateral (IML) cell column of the thoracic cord and the sacral parasympathetic nucleus (SPN). These anatomical studies suggested that the nRO may influence the regulation of spinal reflexes of the pelvic floor. In a second set of experiments, acute rat preparations were used to investigate changes in anorectal motility during electrical stimulation of the nRO. Anorectal contractions were measured by a fluid-filled manometer. Electrical stimulation of the nRO significantly reduced spontaneous anorectal activity when compared to baseline contractions recorded for 1 min prior to stimulation. Stimulation sites outside the nRO did not affect anorectal contractions when compared to either (a) the 1-min pre-stimulation baseline for that site or (b) the 1-min stimulation period for sites within the nRO. Stimulation of caudal portions of the nRO were more likely than the rostral nRO to reduce anorectal contractions. Given that the SPN contains preganglionic neurons which may be involved in control of anorectal contractions (mediated via the pelvic nerve), the studies presented here suggest a functional role for nRO regulation of preganglionic motoneurons innervating the distal gut of the rat.