Pudendal motor and premotor neurons in the male rat: a WGA transneuronal study

Anatomical studies using retrograde neuronal tracers were carried out to identify pudendal motoneurons and putative lumbosacral interneurons involved in the generation of penile reflexes in the adult male rat (Sprague-Dawley). In agreement with previous studies, injection of the direct neuronal tracers, horseradish peroxidase or fluoro-gold, into the left M. bulbospongiosus (dorsal division) resulted in direct retrograde labeling of motoneurons only in the ipsilateral dorsomedial (DM) nucleus. In contrast, similar injections of the transneuronal tracer, wheat germ agglutinin (WGA), resulted in direct retrograde labeling of ipsilateral DM motoneurons as well as transneuronal labeling of contralateral DM motoneurons and of bilaterally located putative spinal interneurons. The WGA-labeling was determined to be transneuronal by a series of nerve cut experiments. The direct retrograde WGA-labeling of ipsilateral DM motoneurons and the transneuronal WGA-labeling of contralateral DM motoneurons and bilateral putative spinal interneurons occurred with different time courses. Direct retrograde labeling of ipsilateral DM motoneurons was seen at 20 hours survival and persisted up to 7 days survival. The shortest survival period for detecting transneuronal labeling of contralateral DM motoneurons and putative interneurons was 2 days. Transneuronal WGA-labeling of contralateral DM motoneurons was transient in that the intensity of label increased from 2 to 3 days survival but was markedly reduced at 7 days survival. At the same time, the number of WGA-labeled putative interneurons increased with longer survival times up to 7 days. WGA-labeled putative interneurons were located primarily in regions receiving dendritic projections from WGA-labeled DM motoneurons and, in particular, 1) the ventral gray matter between the DM and dorsolateral nuclei and 2) near the central canal and extending dorsally to the dorsal gray commissure. The rapid and extensive transneuronal transport between DM nuclei suggests that direct synaptic coupling may mediate coordinated, bilateral activation of DM motoneurons.