This thesis investigated the contribution of descending fibers from the rostral ventromedial medulla (RVM) to nociception, and to opioid and non-opioid analgesia in rats. Inactivation of descending fibers was induced by microinjection of the GAGA-A agonist muscimol in the RVM, and nociception was evaluated using the tail immersion, hot plate and formalin tests. In all three tests, microinjection of muscimol (6.25--400 ng) in the RVM increased nociceptive responses of animals, suggesting that descending fibers tonically inhibit dorsal horn neurons of the spinal cord. In the formalin test, microinjection of muscimol (50 ng) in the RVM reduced the slope of the formalin concentration-response relation, so that responses were increased at low, but not at high concentrations of formalin. Conversely, microinjection of the GABA-A antagonist bicuculline in the RVM decreased the slope of the formalin concentration-response relation, so that responses were decreased at high but not at low concentrations of formalin. Microinjection of muscimol in the RVM abolished analgesia induced by systemic morphine in the tail immersion and hot plate tests, but only decreased analgesia by 60% in the formalin test. Microinjection of muscimol (50 ng) in the RVM abolished morphine analgesia elicited intracerebroventricularly, suggesting that the supraspinal effects of morphine are mediated via descending fibers from the RVM. Peripheral effects may account for the residual analgesic effect of systemic morphine after inactivation of descending fibers in the formalin test. Microinjection of muscimol (50 ng) in the RVM abolished buprenorphine analgesia in all tests. The dopamine uptake inhibitor nomifensine was devoid of analgesic activity in the tail immersion and hot plate tests. In the formalin test, nomifensine produced a dose-dependent analgesia, which was not affected by the microinjection of muscimol (50 ng) in the RVM. It is concluded that, descending fibers from the RVM tonically inhib
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.36938 |
| Date | January 2000 |
| Creators | Gilbert, Annie-Kim. |
| Contributors | Franklin, Keith (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Psychology.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001808294, proquestno: NQ70030, Theses scanned by UMI/ProQuest. |
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