Alterations in the rostral ventromedial medulla after the selective ablation of -opioid receptor expressing neurons

Ichiro Harasawa, Joshua P. Johansen, Howard L. Fields, Frank Porreca, Ian D. Meng

Research output: Contribution to journalArticle

12 Scopus citations

Abstract

The rostral ventromedial medulla (RVM) exerts both inhibitory and excitatory controls over nociceptive neurons in the spinal cord and medullary dorsal horn. Selective ablation of mu-opioid receptor (MOR)-expressing neurons in the RVM using saporin conjugated to the MOR agonist dermorphin-saporin (derm-sap) attenuates stress and injury-induced behavioral hypersensitivity, yet the effect of RVM derm-sap on the functional integrity of the descending inhibitory system and the properties of RVM neurons remain unknown. Three classes of RVM neurons (on-cells, off-cells, and neutral cells) have been described with distinct responses to noxious stimuli and MOR agonists. Using single unit recording in lightly anesthetized rats, RVM neurons were characterized after microinjections of derm-sap or saporin. Derm-sap treatment resulted in a reduction in on-cells and off-cells when compared to saporin controls (P < 0.05). The number of neutral cells remained unchanged. After derm-sap treatment, RVM microinjections of the glutamate receptor agonist homocysteic acid increased tail-flick latencies, whereas the MOR agonist DAMGO had no effect. Furthermore, electrical stimulation of the periaqueductal gray produced analgesia in both derm-sap and saporin controls with similar thresholds. Microinjection of kynurenic acid, a glutamate receptor antagonist, into the RVM disrupted periaqueductal gray stimulation-produced analgesia in both saporin-treated and derm-sap-treated rats. These results indicate that MOR-expressing neurons in the RVM are not required for analgesia produced by either direct or indirect activation of neurons in the RVM.

Original languageEnglish (US)
Pages (from-to)166-173
Number of pages8
JournalPain
Volume157
Issue number1
DOIs
StatePublished - Jan 1 2016

Keywords

  • Analgesia
  • Nucleus raphe magnus
  • Opioid
  • Periaqueductal gray

ASJC Scopus subject areas

  • Neurology
  • Clinical Neurology
  • Anesthesiology and Pain Medicine

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