DORSAL BORDER PERIAQUEDUCTAL GRAY NEURONS PROJECT TO THE AREA DIRECTLY ADJACENT TO THE CENTRAL CANAL EPENDYMA OF THE C4-T8 SPINAL-CORD IN THE CAT

In a previous study horseradish peroxidase (HRP) injections in the upp er thoracic and cervical spinal cord revealed some faintly labeled sma ll neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study descri bes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WG A-HRP) injections into cervical and upper thoracic spinal segments res ulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not f ound after injections in more caudal segments. WGA-HRP injections in t he dorsal border PAG region surprisingly resulted in anterogradely lab eled fibers terminating in the area dorsally and laterally adjoining t he central canal ependyma of the C4-T8 spinal cord. No anterogradely l abeled fibers were found more caudal in the spinal cord. The labeled f ibers found in the upper cervical cord were not located in the area im mediately adjoining the ependymal layer of the central canal, but in t he lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border P AG-spinal neurons terminate in the neuropil of the subependymal area a nd in the vicinity of the basal membranes of capillaries located later ally to the central canal. The terminal profiles contain electron-luce nt and dense-cored vesicles, suggesting a heterogeneity of possible tr ansmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the te rmination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypot halamohypophysial system is discussed.