Enkephalin-containing sympathetic preganglionic neurons projecting to the inferior mesenteric ganglion: evidence from combined retrograde tracing and immunohistochemistry.

The origin of fibers containing enkephalin immunoreactivity in the inferior mesenteric ganglion of the guinea-pig was studied by combining retrograde axonal tracing and indirect immunofluorescence techniques. Fast Blue was applied into the inferior mesenteric ganglion. Three days later colchicine was administered into the subarachnoid space in order to increase the peptide content of the spinal cord cell bodies. The drug was injected through a catheter which was inserted into the cisterna magna and moved to the appropriate spinal cord levels. After the colchicine injection the animals were perfused with formalin and the L2-L3 spinal cord segments were dissected. Cryostat sections of the spinal cord were analyzed in a fluorescence microscope and subsequently processed for indirect immunohistochemistry using antiserum against enkephalin. Several sympathetic preganglionic neurons containing both Fast Blue and enkephalin-like immunoreactivity were seem mainly in the intermediolateral cell column of the cord. The observations strongly support the view that at least some of the enkephalin-containing fibers in the inferior mesenteric ganglion originate in the sympathetic preganglionic nuclei of the spinal cord. These findings are discussed in view of recent physiological studies which have shown that enkephalin may have a presynaptic inhibitory action on preganglionic neurons as well as on substance P containing primary afferent neurons in the inferior mesenteric ganglion.