5-Hydroxytryptamine_2C receptors on pudendal motoneurons innervating the external anal sphincter

The aim of this study was to determine the localization of 5-hydroxytryptamine_2C (5-HT_2C) receptors on the motoneurons innervating the external anal sphincter (EAS) of male rats. Motoneurons were retrogradely labeled after percutaneous intramuscular injection of Fluorogold (FG) into the EAS. Using fluorescent immunohistochemistry, FG-positive EAS motoneurons that were immunoreactive for the 5-HT_2C receptor (5-HT_2C-IR) were targeted for specific examination with widefield microscopy or confocal laser scanning microscopy with spectral separation. Widefield microscopy revealed distributions of FG-positive EAS motoneurons in the L5-S1 gray matter corresponding to the dorsomedial cell group. 5-HT_2C-IR positive cells were distributed in the intermediolateral cell column and the ventral horn. Ventral horn 5-HT _2C-IR labeling included the dorsomedial cell group as well as the dorsolateral, ventromedial and ventrolateral areas. Confocal analysis of FG-positive EAS motoneurons and 5-HT_2C-IR positive motoneuron profiles adjacent to EAS motoneurons that were not labeled with FG but presumably innervate the bulbospongiosus muscle confirmed that EAS motoneurons were immunopositive for the 5-HT_2C receptor. These data suggest that previously identified descending serotonergic immunopositive fibers observed terminating on EAS motoneurons might mediate their input through the activation of 5-HT_2C receptors.