O-Endorphin, an opiate-like peptide, has potent antinociceptive properties when it is administered directly into the brain and assayed in the tail-flick, hot-plate, and writhing tests in mice and in the wet shake test in rats. On a molar basis, O-endorphin is 18 to 33 times more potent than morphine and its actions are blocked by the specific opiate antagonist, naloxone hydrochloride. The activity of f-endorphin in vivo is also compared to other peptides that show opiate-like activity in assays in vitro. The existence of endogenous ligands for the opiate receptor in the brain has been suggested by Collier (1) and Goldstein (2). In searching for such ligands, Terenius and Wahlstrom (3) and Hughes (4), using the receptor binding assay and bioassays with mouse vas deferens and guinea pig ileum, independently found opiate-like substances in the brain. Subsequently, Hughes et al. (5) purified and characterized two opiate-like peptides, termed enkephalins, which have the following amino acid sequences: H-Tyr-Gly-Gly-Phe-Met-OH and H-Tyr-Gly-GlyPhe-Leu-OH. Other workers (6, 7) have confirmed the results of Hughes et al. Concurrently, Goldstein and his associates (8,9) found opiate-like materials in a crude preparation of corticotropin (ACTH). Synthetic corticotropin and a-melanotropin lacked similar activity. Upon purification and characterization, the corticotropin contaminant was shown to be a peptide with a molecular weight of approximately 1750. Recently, Li and Chung (10) isolated and determined the sequence of an untriakontapeptide from camel pituitary gland. This peptide, named fl-endorphin, has been synthesized (11) and shown to possess opiate-like activity in receptor binding assays and in the guinea pig ileum bioassay (10-12).In the isolation and identification of opiate-like peptides from brain and pituitary tissue, the criteria for specific activity has been based on bioassays in vitro. Information on the pharmacological properties of these peptides in vivo is lacking, partly due to the scarcity of purified or synthetic materials. In this article, we report on the relative analgesic properties of fiendorphin and related peptides when assayed in the mouse and rat in uivo.
MATERIALS AND METHODSMale ICR mice weighing 25-30 g (Simonsen Laboratories, Gilroy, Calif.) and male Sprague-Dawley rats weighing 250-350 g were used in all the experiments.Methionine-enkephalin was purchased from Bachem Laboratories (Marina del Reyes, Calif). f,-Endorphin was synthesized as previously described (11). fl-Lipotropin (LPH) was isolated from sheep pituitary glands by the procedure described by Li et a!. (13 With 2-fold increase in latency of reaction time as a quantal index of inhibition, the median antinociceptive dose (AD50) and 95% confidence limits were calculated according to the method of Litchfield and Wilcoxon (20). At least eight animals were tested at each dose with three to five dose levels used for determining the ADs,0.Writhing Test in Mice. When "analgesia" was measured by the writhing method (17), acetic...
