Supraspinal Nitric Oxide Synthesis Inhibition Enhanced Antinociception of Morphine in Morphine Tolerant Rats

Song, Ho Kyung; Jang, Yeon

Abstract

BACKGROUND: Opioids such as morphine are widely used in the treatment for pain, but chronic treatment with morphine can be complicated by the development of tolerance. The mechnisms of tolerance were still not completely understood, but recently it has been reported that NOS inhibitors can prevent development of morphine tolerance in animals. The present study accessed the possible role of supraspinal NO on antinociceptive effect of morphine in morphine tolerance using a highly specific inhibitor of the neuronal isoform of NOS, 1-(2-trifluoromethylphenyl) imidazole (TRIM).
METHODS
Thirty two male SD rats (300 g) were prepared with intracerebroventricular (icv) and IV cannulae. We administrated IV morphine, 3 mg/kg, daily for 4 days, resulting in tolerance. On the fifth day, a challenge dose of morphine, 3 mg/kg, was administered following pretreatment with icv TRIM, 10ng. We also evaluated the antinociceptive effect of icv TRIM alone and the effect on a single dose of morphine (3 mg/kg) in morphine nave rats. Antinociception from morphine was determined by response to intraplantar injection of 5% formalin 100nl was qualified as the number of flinches in the first 0 10 min (first phase), 10 40 min Phase IIa, and 40 60 min (Phase IIb).
RESULTS
Pretreatment with icv TRIM significantly enhanced the antinociceptive effects of systemically administered morphine in morphine tolerant rats. The antinociceptive effect of morphine in opioid nave rats was also significantly increased by pretreatment with icv TRIM.
CONCLUSIONS
Our results further support the hypothesis that supraspinal NO modulates morphine-sensitive nociceptive process in morphine tolerance due to chronic intravenous administration.