Direct brain projections onto the spinal generator of ejaculation in the rat

A spinal generator for ejaculation (SGE) has been identified in the rat that orchestrates peripheral events leading to ejaculation. Despite physiological evidence of cerebral influences exerted on the SGE, brain-descending pathways to the SGE have not been fully delineated. A tracing study combining retrograde and anterograde approaches was undertaken in adult male rats in order to identify brain sites containing neurons that directly project onto SGE neurons. Fluorogold (FG) was microinjected as a retrograde tracer into the SGE area in the central medial gray of the third lumbar (L3) spinal segment. FG-positive neurons were found in various structures in medulla oblongata, pons, and forebrain. Among the brain structures already known as participating in the brain control of ejaculation and harboring retrogradelly-labeled neurons, the ventrolateral part of the gigantocellular nucleus and the raphe pallidus/magnus in medulla oblongata as well as the lateral hypothalamus were targeted with the anterograde tracer dextran amine (DA). Galanin and substance P receptor (NK1) were used as markers of SGE neurons. DA-positive fibers and varicosities originating in the targeted brain sites were found to make close appositions with neurons expressing galanin or NK1 receptors in central medial gray of L3-L4 spinal segments. This study provides new insights regarding the anatomical support for the brain control of ejaculation via direct influences onto the SGE.