Evidence of a direct projection from the cardiovascular-reactive dorsal medulla to the intermediolateral cell column of the spinal cord in cats as revealed by light and electron microscopy

Abstract

To ascertain whether the dorsal medulla, a well-established vasopressor structure, would project directly to the sympathetic intermediolateral cell column of the spinal cord, we have carried out both anterograde and retrograde tracing studies in cats. For anterograde tracing, biotin–dextran was iontophoretically delivered into the cardiovascular-reactive dorsal medulla following its functional identification by electrical stimulation. The anterogradely transported biotin–dextran was then visualized using the avidin–biotin–horseradish peroxidase complex method. By light microscopy, dextran-labelled varicose axons were observed bilaterally in the intermediolateral nucleus extending from segments T1 to L3, but concentrated in segments T1–T5, notably at levels T2–T4. Electron microscopic examination revealed the localization of biotin–dextran reaction product in some small myelinated axons and axon terminals in the intermediolateral cell column. The majority of tracer-labelled axonal boutons contained spherical synaptic vesicles and made asymmetric synaptic contacts primarily with small dendrites. A few boutons contained polymorphic synaptic vesicles and tended to form symmetric axodendritic synapses. Spinally projecting neurons of the dorsal medulla were identified using the retrograde transport of horseradish peroxidase injected into the electrically cardiovascular-reactive intermediolateral nucleus. The labelled neurons were localized in the medullary dorsomedial reticular formation ventromedial to the nucleus of the solitary tract, approximately 0.5–5 mm rostral to the obex. The projection was bilateral, but was relatively denser in the rostral portion of the contralateral dorsal medulla. The present findings support the hypothesis that the dorsal medulla, through its direct pathway innervating the intermediolateral nucleus, may serve as a sympathetic premotor structure for regulation of arterial pressure.