Abstract: | Effect of β-endorphin and morphine injected intraventricularly on the release of immunoreactive Met-enkephalin, Leu-enkephalin and dynorphin1–13 from the spinal cord was studied in anesthetized rats. Intraventricular β-endorphin, 16 μg, caused a marked spinal release of immunoreactive Met-enkephalin and to a much lesser extent, of immunoreactive Leu-enkephalin while intraventricular morphine, 40 μg, did not cause any significant release of immunoreactive enkephalins. The release of immunoreactive Met-enkephalin was not blocked by the pretreatment with 5 mg/kg naloxone, i.p. Immunoreactive dynorphin1–13 was not released by either β-endorphin or morphine. High performance liquid chromatographic analysis indicated that immunoreactive Met-enkephalin released by β-endorphin had a retention time identical to [3H]Met-enkephalin. These findings in conjunction with previous pharmacological studies suggest different modes of pharmacological action for β-endorphin and morphine. |