Rectospinal neurons: cell bodies, pathways, immunocytochemistry and ultrastructure.

A novel class of enteric neurons projecting directly from the rectal wall to the spinal cord, "rectospinal neurons", was investigated in rats by combined retrograde neuronal tracing, immunocytochemistry and electron microscopy. Rectospinal neurons were almost confined to myenteric ganglia of the distal rectum below the pelvic diaphragm and were labeled preferentially by injections into spinal cord segments L6/S1. Injections into more rostral spinal cord segments resulted in hardly any labeled enteric neurons. Dorsal and ventral rhizotomy experiments indicated an almost exclusive projection of rectospinal neurons through dorsal roots L6/S1 to the respective spinal cord segments. Among various peptides immunostained, vasoactive intestinal polypeptide and calcitonin gene-related peptide were selectively found in rectospinal neurons, which were also shown to contain calbindin, neurofilament protein- and peripherin-immunoreactivity. Vasoactive intestinal polypeptide- and calbindin-immunostaining were frequently co-localized in the same perikarya, while calcitonin gene-related peptide-immunoreactive rectospinal neurons probably represented a separate population. Neonatal capsaicin treatment did not significantly reduce the number of rectospinal neurons. Electron microscopy revealed synaptic contacts on the surface of rectospinal neurons. Taken together, these results establish rectospinal neurons as an anatomically and neurochemically distinct class of enteric neurons. Synaptic contacts on rectospinal neurons suggest that these neurons may function as a direct link from the enteric to the central nervous system, thus indicating that connections between these two networks are reciprocal.