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Changes in rostral ventromedial medulla (RVM) neurons after the selective loss of mu-opioid receptor expressing cells.

Meng ID, Harasawa I, Lai J, Porreca F, Fields HL (2002) Changes in rostral ventromedial medulla (RVM) neurons after the selective loss of mu-opioid receptor expressing cells. Neuroscience 2002 Abstracts 351.9. Society for Neuroscience, Orlando, FL.

Summary: Different subpopulations of RVM neurons inhibit or facilitate dorsal horn nociceptive transmission. Microinjection of saporin conjugated to the mu-opioid receptor (MOR) agonist dermorphin (derm-sap) into the RVM selectively ablates MOR expressing neurons and diminishes neuropathic pain symptoms (Porreca et al., 2001). We examined the properties of neurons surviving a single RVM injection of derm-sap or sap control. Three classes of RVM neurons (On, Off, and Neutral) have been described with distinct responses to noxious stimuli and MOR agonists. On-cells increase and Off-cells cease firing just prior to a tail flick; MOR agonists inhibit On-cells and disinhibit Off-cells. Neutral cells are unaffected by either noxious stimulation or MOR agonists. Using single unit recording in lightly anesthetized rats a total of 10 electrode tracks were made per rat and each unit encountered was characterized according to its tail flick related activity. Injection of derm-sap (n=8) resulted in fewer On- and Off-cells when compared to saporin controls (n=8). The number of Neutral cells remained unchanged. In separate experiments, after derm-sap pretreatment RVM injections of the MOR agonist DAMGO were ineffective whereas injections of the glutamate receptor agonist homocysteic acid into the same sites increased tail flick latencies. The decrease in number of On-cells after derm-sap is consistent with evidence that these neurons express MOR and facilitate nociceptive transmission. The decrease in number of Off-cells indicates that inhibitory neurons responsible for producing the Off-cell tail flick related pause also express MOR.

Related Products: Dermorphin-SAP / MOR-SAP (Cat. #IT-12)

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