POTENCY DIFFERENCES OF MORPHINE-TYPE AGENTS BY RADIANT HEAT AND "CRAMPING" ANALGESIC ASSAYS PROVIDING EVIDENCE FOR A POTENTIATING SUBSTANCE FROM THE POSTERIOR PITUITARY GLAND

¹ Department of Pharmacology and Toxicology, The University of Wisconsin, Service Memorial Institutes, Madison, Wisconsin

Prior administration of the morphine-type compounds prevented the "cramping" response produced by oxytocin and thereby provided the basis for a quantal assay procedure. When an oxytocin-containing posterior pituitary extract (oxytocin concentrate) was employed to produce "Cramping," the ED50 values of the analgesic agents studied were markedly reduced relative to those obtained by a radiant heat procedure or when synthetic oxytocin was used to produce "cramping." The decrease in the ED50 values was proportional to the known analgesic potencies of the agents studied. The oxytocin-containing extract did not possess analgesic activity but potentiated that of morphine by the radiant heat assay. The presence of the pituitary gland is necessary for potentiation to occur. The potentiation could not be accounted for by vasopressin and did not occur with synthetic oxytocin. The findings provide evidence for the presence of a substance in oxytocin-containing posterior pituitary extracts which potentiates the analgesic activity of the morphine-type agents in proportion to their relative potencies.