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Spinal µ-opiate receptor (MOR)-expressing dorsal horn neurons: Role in modulating pain and opiate analgesia.

Kline IV RH, Wiley RG (2009) Spinal µ-opiate receptor (MOR)-expressing dorsal horn neurons: Role in modulating pain and opiate analgesia. Neuroscience 2009 Abstracts 560.13/CC72. Society for Neuroscience, Chicago, IL.

Summary: Selective destruction of MOR-expressing interneurons in lamina II of the dorsal horn of the spinal cord increases reflex nocifensive responses to formalin and decreases the anti-nociceptive effects of morphine on the hotplate and in the formalin test. The interpretation of these studies is limited because reflex-based assays may not accurately reflect the cerebral component of nociception. Therefore, we sought to determine the effects of selectively destroying MOR-expressing dorsal horn neurons on baseline operant responses to aversive thermal and mechanical stimuli in a shuttle box task and effects of systemic morphine and naloxone in the same task. The preference apparatus consisted of a 15 X 15 X 30 cm smoked Plexiglas vented chamber placed upon two adjoining temperature-controlled smooth aluminum floor plates (thermal preference task) or one smooth temperature-controlled floor plate adjoined to a room temperature surface covered with 40 grit sandpaper (mechanical preference task). For both preference tasks, response functions were obtained by pairing a 44°C plate or the sandpaper surface with either 11°, 16°, 25°, 38° or 46°C. Rats were intrathecally injected over the lumbar cord with either 625ng of derm-sap (n=7) or blank-sap (n=6) followed by daily thermal or mechanical preference testing on a randomized schedule. Derm-sap treated rats showed enhanced avoidance of aversive thermal stimuli and the aversive mechanical stimulus. Morphine and naloxone significantly altered responses of control rats (blank-sap), but not derm-sap rats, in both thermal and mechanical preference tasks. We interpret these results as showing that the derm-sap lesion produces hyperalgesia/allodynia, impairs the anti-nociceptive and analgesic effects of morphine and therefore indicating that postsynaptic dorsal horn MOR-expressing neurons play a key role in modulating nociception, pain and opiate analgesia. Dysfunction of these neurons may also play a role in pathological pain states.

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