PELVIC VISCERAL INPUT INTO THE NUCLEUS GRACILIS IS LARGELY MEDIATED BY THE POSTSYNAPTIC DORSAL COLUMN PATHWAY

1. The purpose of this study was to investigate a proposed role for th e postsynaptic dorsal column (PSDC) pathway in mediating visceral noci ceptive input into the dorsal column (DC) nuclei. 2. In one group of a nimals, the hypogastric nerves were sectioned, thereby restricting col orectal input into the cord to pelvic afferent pathways known to conve rge 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 tes ted before and after the administration of morphine into the sacral co rd by microdialysis. 3. The responses of 11 NG cells to CRD were drama tically reduced by morphine infused into the sacral cord through a mic rodialysis fiber. This reduction was reversed by an intravenous inject ion of naloxone. Microdialysis administration of 6-cyano-7-nitroquinox aline-2,3-dione (CNQX) or a lesion of the DC also abolished the respon ses of the NG cells to CRD. 4. Four NG cells that responded to CRD sho wed an increase in their background activity similar to 25 min after a n injection of mustard oil (MO). This increase in activity was counter acted by morphine or by a lesion of the DC. 5. in a second group of an imals, recordings were made from 28 PSDC cells in the L(6)-S-1 segment s of the cord. These units were antidromically activated by stimulatio n of the upper cervical fasciculus gracilis. The projections of five P SDC neurons into the NG were traced with the use of antidromic mapping . Results are reported for the responses of 12 PSDC cells to CRD and t o cutaneous stimuli before and after morphine administration into the sacral cord by microdialysis. 6. Morphine given spinally reduced the r esponses of 12 PSDC cells to CRD. This reduction was reversed by an in travenous injection of naloxone. CNQX administered spinally also aboli shed the responses to CRD of the PSDC cells tested. 7. Four other PSDC cells were studied before and after an injection of MO into the colon . Their background activity started to increase within 25 min after th e 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 m orphine in the dose used. On the other hand, morphine significantly re duced the responses of PSDC cells to noxious cutaneous stimuli althoug h this effect was not as dramatic as that on responses to visceral sti muli. 9. From the results of the studies described in this and the com panion paper, we conclude that there is an important pelvic visceral n ociceptive pathway involving PSDC neurons that synapse in the NG. The NG in turn activates neurons in the ventral posterolateral (VPL) nucle us of the thalamus. We presume that activation of VPL neurons by noxio us visceral stimulation contributes to visceral pain sensation and thu s that pelvic visceral pain depends largely on activity in the DC-medi al lemniscus system.