SPINAL-CORD LAMINA-V AND LAMINA-VII INTERNEURONAL PROJECTIONS TO SYMPATHETIC PREGANGLIONIC NEURONS

This light and electron microscopic study sought to localize spinal co rd interneurons that contribute to the normal and abnormal physiologic al regulation of spinal sympathetic preganglionic function. Sympatheti c preganglionic neurons in caudal C8 through T4 of rat spinal cord wer e retrogradely labeled with wheat germ agglutinin (WGA) and/or cholera beta subunit (CT beta) following injections into the superior cervica l ganglion (SCG). With two exceptions, the observed locations of retro gradely WGA- and CT beta-labeled sympathetic preganglionic neurons wer e as expected from previous studies. The exceptions were restricted po pulations of cells in caudal C8 and rostral T1 spinal segments. These neurons were classified as ventrolateral (vlSPN) and ventromedial (vmS PN) sympathetic preganglionic neurons; their somata and dendrites enci rcled dorsolateral lamina M motoneurons. Only WGA was transported tran sneuronally following the retrograde labeling of sympathetic pregangli onic neurons. Transneuronally WGA-labeled spinal interneurons were loc ated principally in the reticulated division of lamina V and dorsolate ral lamina VII. A strict segmental organization was observed. All tran sneuronally labeled interneurons were ipsilateral to, and coextensive with, retrogradely WGA-labeled sympathetic preganglionic neurons. Elec tron microscopic observations suggested that retrograde transsynaptic passage of WGA occurred within the sympathetic preganglionic neuropil and showed further that similar classes of organelles were WGA immunor eactive in retrogradely labeled sympathetic preganglionic neurons and in transneuronally labeled lamina V and lamina VII neurons: 1) cistern ae and vesicles at the trans face of the Golgi apparatus, 2) large end osomes/dense bodies, and 3) multivesicular bodies. The data are consis tent with two hypotheses: 1) Somatic and visceral primary afferent inp uts to thoracic spinal cord modify segmental sympathetic preganglionic function through activation of a disynaptic pathway involving lamina V and/or lamina VII interneurons, and 2) long-loop propriospinal pathw ays access sympathetic preganglionic neurons through symmetrical, segm ental interneuronal circuitry. (C) 1994 Wiley-Liss, Inc.