The effects of decentralization on substance P-immunoreactivity in the anterior major pelvic ganglion of the male guinea pig.

The anterior major pelvic ganglion (AMPG) of the male guinea pig possesses a substance P (SP)-immunoreactive peri-neuronal plexus. Selective nerve transections involving the principal inputs of the AMPG, the hypogastric and pelvic nerves, indicate that the SP-immunoreactive peri-neuronal plexus is derived from multiple sources: an extrinsic source involving both the hypogastric and pelvic nerves, and another source (possibly the projections of small intensely fluorescent cells). SP-immunoreactivity (IR) is not normally present in the neuronal perikarya of the AMPG. Evidence is presented that suggests the absence of SP-IR is due to an active suppression of SP-synthesis. This seems to be achieved by a trans-synaptic mechanism involving the hypogastric nerve which, after transection, leads to the appearance of neuronal perikarya exhibiting SP-IR (less than 1% of the total neuronal population of the AMPG). Up to 65% of the neuronal perikarya of the AMPG have the ability to synthesize SP, as demonstrated by SP-IR after 24 h in vitro. A more potent factor in the down-regulation of SP synthesis seems to be exerted by the pelvic genito-urinary organs, especially the prostate.