Enkephalin-immunoreactive interneurons extensively innervate sympathetic preganglionic neurons regulating the pelvic viscera.
Enkephalin (ENK)-immunoreactive (IR) axons occur in regions containing spinal autonomic neurons and endogenous opiates contribute to spinal regulation of bladder function. To identify possible spinal sites of opiate action, we used immunocytochemistry for ENK with retrograde tracing from the major pelvic ganglion (MPG), a key location for postganglionic neurons controlling pelvic viscera, with cholera toxin B subunit (CTB) or CTB-horseradish peroxidase (CTB-HRP). We compared the relationship of ENK-IR axons with sympathetic preganglionic neurons (SPNs) projecting to the MPG between intact spinal cords and cords with 2- or 11-week complete transections between thoracic segments 4 and 5. By light microscopy, sections of intact cord showed dense networks of ENK-IR axons surrounding CTB-IR SPNs in the intermediolateral cell column (IML), intercalated nucleus, and central autonomic area of lower thoracic and upper lumbar cord. This staining pattern was similar in rats with 2- or 11-week transections. Ultrastructurally, ENK-IR axons formed synapses on SPNs in all three autonomic subnuclei of intact cord. In the IML, ENK-IR varicosities contributed 52% of the synapses on the somata of MPG-projecting SPNs. In 2-week transected cord, synapses from ENK-IR axons persisted on SPNs and the proportion of input to IML SPNs had increased to 67%, probably reflecting loss of supraspinal input. These results suggest that endogenous opioids could play a major role in controlling sympathetic outflow to the bladder through a direct action on SPNs. The persistence of the dense ENK innervation after complete cord transection indicates that the ENK-IR input to SPNs arises predominantly from intraspinal sources.