PELVIC VISCERAL INPUT INTO THE NUCLEUS GRACILIS IS LARGELY MEDIATED BY THE POSTSYNAPTIC DORSAL COLUMN PATHWAY. Al-Chaer, Elie D., Nada B. Lawand, Karin N. Westlund and William D. Willis. Department of Anatomy and Neurosciences and Marine Biomedical Institute, University of Texas Medical Branch, Galveston, Texas 77555-1069.
APStracts 3:0128N, 1996.
1. The purpose of this study was to investigate a proposed role for the postsynaptic dorsal column (PSDC) pathway in mediating visceral nociceptive input into the dorsal column nuclei. 2. In one group of animals, the hypogastric nerves were sectioned, thereby restricting colorectal input into the cord to pelvic afferent pathways known to converge on lower lumbar and sacral segments. Extracellular recordings were made from 41 nucleus gracilis (NG) cells that responded to colorectal distension (CRD). Results reported are from 15 NG cells that were tested before and after the administration of morphine into the sacral cord by microdialysis. 3. The responses of eleven NG cells to CRD were dramatically reduced by morphine infused into the sacral cord through a microdialysis fiber. This reduction was reversed by an intravenous injection of naloxone. Microdialysis administration of CNQX or a lesion of the dorsal column (DC) also abolished the responses of the NG cells to CRD. 4. Four NG cells that responded to CRD showed an increase in their background activity around 25 minutes after an injection of mustard oil (MO). This increase in activity was counteracted by morphine or by a lesion of the DC. 5. In a second group of animals, recordings were made from 28 PSDC cells in the L6-S1 segments of the cord. These units were antidromically activated by stimulation of the upper cervical fasciculus gracilis (FG). The projections of 5 PSDC neurons into the NG were traced using antidromic mapping. Results are reported of the responses of 12 PSDC cells to CRD and to cutaneous stimuli before and after morphine administration into the sacral cord by microdialysis. 6. Morphine given spinally reduced the responses of 12 PSDC cells to CRD. This reduction was reversed by an intravenous injection of naloxone. CNQX administered spinally also abolished the responses of the PSDC cells tested to CRD. 7. Four other PSDC cells were studied before and after an injection of mustard oil (MO) into the colon. Their background activity started to increase within 25 minutes after the injection. Morphine suppressed this increase in background activity and this effect of morphine was reversed by naloxone. 8. The responses of NG cells to cutaneous stimuli were not significantly affected by morphine in the dose used. On the other hand, morphine significantly reduced the responses of PSDC cells to noxious cutaneous stimuli although this effect was not as dramatic as that on responses to visceral stimuli. 9. From the results of the studies described in this and the companion paper, we conclude that there is an important pelvic visceral nociceptive pathway involving PSDC neurons that synapse in the NG. The NG in turn activates neurons in the ventral posterolateral (VPL) nucleus of the thalamus. We presume that activation of VPL neurons by noxious visceral stimulation contributes to visceral pain sensation and hence that pelvic visceral pain depends largely on activity in the dorsal column - medial lemniscus system (DC-ML).

Received 6 March 1996; accepted in final form 20 May 1996.
APS Manuscript Number J178-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 17 June 96