Abstract
Intracerebroventricular (i. c. v.) injection of melatonin (2, 5, 10, 20μg/mouse) produced hyperalgesiadose-dependently in the hot plate and 0.3% acetic acid-writhing tests. Melatonin antagonized theanalgesic effect of μ-endorphin.β-MSH, which was thought to be controlled by the auto regulatory systembetween melatonin and α-MSH, caused analgesic action. Melatonin also antagonized the β-MSH-inducedanalgesia. Serotonin, structual analogue of melatonin, had significant analgesic effect and melatonincaused a parallel shift to the right of the dose-response curve of serotonin. Thus, it is suggested that the hyperalgesic effect of melatonin may involve the opioidergic systemdirectly or indirertly. Since there is competitive antagonism between serotonin and melatonin, it maybe that the melatonin-induced hyperalgesia also interacts with the serotoninergic system directly.
