Morphine-induced Modulation of Nociceptive Spinal Dorsal Horn Neuronal Activities after Formalin-induced Inflammatory Pain

• Published in: The Korean journal of physiology & pharmacology Vol. 9; no. 2; pp. 77 - 86
• Main Authors: Park, Joo-Min, Li, Kang-Wu, Jung, Sung-Jin, Kim, Jun, Kim, Sang-Jeong
• Format: Journal Article
• Language: Korean
• Published: 대한생리학회-대한약리학회 01.01.2005; The Korean Journal of Physiology & Pharmacology Editorial Office; 대한약리학회
• Subjects: 약리학; Morphine; Nociceptive dorsal horn neurons; spinal cord; GABA A basal tone; Subacute inflam matory state; Subacute inflammatory state; GABA_A$ basal tone
• ISSN: 1226-4512; 2093-3827

In this study, we examined the morphine-induced modulation of the nociceptive spinal dorsal horn neuronal activities before and after formalin-induced inflammatory pain. Intradermal injection of formalin induced time-dependent changes in the spontaneous activity of nociceptive dorsal horn neurons. In naive cats before the injection of formalin, iontophoretically applied morphine attenuated the naturally and electrically evoked neuronal responses of dorsal horn neurons. However, neuronal responses after the formalin-induced inflammation were significantly increased by morphine. Bicuculline, $GABA_A$ antagonist, increased the naturally and electrically evoked neuronal responses of dorsal horn neurons. This increase in neuronal responses due to bicuculline after the formalin-induced inflammation was larger than that in the naive state, suggesting that basal $GABA_A$ tone increased after the formalin injection. Muscimol, $GABA_A$ agonist, reduced the neuronal responses before the treatment with formalin, but not after formalin treatment, again indicating an increase in the GABAergic basal tone after the formalin injection which saturated the neuronal responses to GABA agonist. Morphine-induced increase in the spinal nociceptive responses after formalin treatment was inhibited by co-application of muscimol. These data suggest that formalin-induced inflammation increases $GABA_A$ basal tone and the inhibition of this augmented $GABA_A$ basal tone by morphine results in a paradoxical morphineinduced increase in the spinal nociceptive neuronal responses after the formalin-induced inflammation.