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.
SUMMARY AND CONCLUSIONS
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