RECTOSPINAL NEURONS - CELL-BODIES, PATHWAYS, IMMUNOCYTOCHEMISTRY AND ULTRASTRUCTURE

A novel class of enteric neurons projecting directly from the rectal w all to the spinal cord, ''rectospinal neurons'', was investigated in r ats by combined retrograde neuronal tracing, immunocytochemistry and e lectron microscopy. Rectospinal neurons were almost confined to myente ric ganglia of the distal rectum below the pelvic diaphragm and were l abeled preferentially by injections into spinal cord segments L6/S1. I njections into more rostral spinal cord segments resulted in hardly an y labeled enteric neurons. Dorsal and ventral rhizotomy experiments in dicated an almost exclusive projection of rectospinal neurons through dorsal roots L6/S1 to the respective spinal cord segments. Among vario us peptides immunostained, vasoactive intestinal polypeptide and calci tonin gene-related peptide were selectively found in rectospinal neuro ns, which were also shown to contain calbindin, neurofilament protein- and peripherin-immunoreactivity. Vasoactive intestinal polypeptide- a nd calbindin-immunostaining were frequently co-localized in the same p erikarya, while calcitonin gene-related peptide-immunoreactive rectosp inal neurons probably represented a separate population. Neonatal caps aicin treatment did not significantly reduce the number of rectospinal neurons. Electron microscopy revealed synaptic contacts on the surfac e of rectospinal neurons. Taken together, these results establish rect ospinal neurons as an anatomically and neurochemically distinct class of enteric neurons. Synaptic contacts on rectospinal neurons suggest t hat these neurons may function as a direct link from the enteric to th e central nervous system, thus indicating that connections between the se two networks are reciprocal.