Opioid receptors have been pharmacologically classified as µ, δ, κ and ε. We have recently reported that the antinociceptive effect of morphine (a µ-opioid receptor agonist), but not that of β-endorphin (a novel µ/ε-opioid receptor agonist), is attenuated by whole body irradiation (WBI). It is unclear at present whether WBI has differential effects on the antinociceptive effects of µ-, δ-, κ- and ε-opioid receptor agonists. In our current experiments, male ICR mice were exposed to WBI (5Gy) from a 60 Co gamma-source and the antinociceptive effects of opioid receptor agonists were assessed two hours later using the hot water (52℃) tail-immersion test. Morphine and D-Ala2,N-Me-Phe4,Gly-ol-enkephalin (DAMGO), [D-Pen2-D-Pen5]enkephalin (DPDPE), trans-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl]¬benzeneacetamide (U50,488H), and β-endorphin were tested as agonists for µ, δ, κ, and ε-opioid receptors, respectively. WBI significantly attenuated the antinociceptive effects of morphine and DAMGO, but increased those of β-endorphin. The antinociceptive effects of DPDPE and U50,488H were not affected by WBI. In addition, to more preciously understand the differential effects of WBI on µ- and ε¬opioid receptor agonists, we assessed pretreatment effects of β-funaltrexamine (β-FNA, a µ-opioid receptor antagonist) or β-endorphin1-27 (β-EP1-27, an ε-opioid receptor antagonist), and found that pretreatment with β-FNA significantly attenuated the antinociceptive effects of morphine and β endorphin by WBI. significantly reversed the β-EP1-27 attenuation of morphine by WBI and significantly attenuated the increased effects of β-endorphin by WBI. The results demonstrate differential sensitivities of opioid receptors to WBI, especially for µ- and ε-opioid receptors.

Whole-body y-irradiation(WBI), which produces an oxidative stress, is reported to attenuate the acute antinociceptive action of morphine (aμ-opioid receptor agonist), but not DPLPE (að-opioid receptor agonist), in mice. Recently, we also reported that antinociceptive effect of morphine, but not β-endorphin (a novel ε-opioid receptor agonist), was attenuated by oxidative stress. These findings prompted us to investigate the effect of WBI on the antinociception of morphine and β-endorphin in mice. Mice were exposed to WBI (5 Gy) from a 60Co gamma-source and tested 2 hours later for antinociception produced by intracerebroventricular administration of morphine or β-endorphin using the hot water tail-immersion and the writhing tests. WBI significantly attenuated the antinociception produced by morphine only in the hot water tail-immersion test, whereas the antinociception of -endorphin was significantly potentiated by WBI in both tests. These results demonstrate a differential sensitivity of μ- and ε-opioid receptors to WBI, and support the hypothesis that morphine and β-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.

The purpose of this study was to determine the effect of varying levels of photobiostimulation treatment dosage on plasma -endorphin concentration in humans. The subjects of this study were 21 healthy men and women, who were students of the Department of Physical Therapy, College of Health Science, Seonam University. This study was performed from October 26, 1998 to November 5, 1998. All subjects were assigned to one of three groups: a 2.0 laser group, a 4.0 laser group, an 6.0 laser group. He-Ne laser (632.8 nm wave length) and infrared laser (820 nm wave length) of three different energy densities (2.0, 4.0, and 6.0 ) were applied on the Su-Sam-Ri (L I 10) and Hab-Gog (L I 4) of acupuncture points. Blood samples were taken at pre-treatment, 30 min's post-treatment and 60 min's post-treatment. The level of endorphin was measured by radio immuno assay. The data were analyzed by descriptive statistics and repeated measure two-way ANOVA. The results of this study were as follows: 1) The human plasma -endorphin concentrations were noted to significantly increase due to the energy densities of laser photobiostimulation (p<0.05). 2) The human plasma -endorphin concentrations were noted to significantly increase during the period after laser photobiostimulation (p<0.05).