Differential Effects of Whole-body γ-Irradiation on Antinociception Induced by Morphine and β-Endorphin Administered Intracerebroventricularly in the Mouse

Two separate lines of evidence suggested the present study. First, intracerebroventricularly (i.c.v.) administered morphine (a μ-opioid receptor agonist) and β-endorphin (an ε-opioid receptor agonist) produce antinociception by activating different descending pain inhibitory systems. Second, γ-irradiation attenuates the acute antinociceptive action of i.c.v. injected morphine, but not DPLPE (a δ-opioid receptor agonist), in mice. These findings prompted us to investigate the effect of γ-irradiation on the antinociception produced by i.c.v. injected morphine and β-endorphin in male ICR mice. In one group, mice were exposed to whole-body irradiation at a dose of 5 Gy from a Co γ-source and the antinociceptive effects were tested 5, 30, 60, 90 and 180 min after irradiation using the 1 % acetic acid-induced writhing test (10 ml/kg). The antinociceptive effect was produced time-dependently and reached its maximum at 90 min after irradiation. Thus, time was fixed in the following studies. In another group, mice were irradiated with 5 Gy and tested 90 minutes later for antinociception produced by i.c.v. administration of morphine (50 and 100 ng/mouse) or β-endorphin (31 ng/mouse). Irradiation significantly potentiated the antinociception produced by β-endorphin. However, the antinociception produced by morphine was not affected by irradiation. These results demonstrate a differential sensitivity of μand ε-opioid receptors to γ-irradiation, in addition, support the hypothesis that morphine and β-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.