KAPPA-OPIOID EFFECTS ON FETAL BEHAVIOR - CENTRAL ADMINISTRATION OF U50,488

The kappa opioid agonist U50,488 was administered to E21 rat fetuses v ia intracisternal (IC), intrahemispheric (IH), or intrathecal (IT) inj ection. The IC administration of U50,488 promoted a threefold increase in motor activity, which was predominated by movements of caudal regi ons of the body (rearlimbs, body trunk, and tail). The agonist effect was reversed by IC administration of the selective kappa opioid antago nist nor-binaltorphimine. The IH injection of U50,488 had little effec t on fetal behavior, but IT injection resulted in pronounced increases in fetal activity, including rearlimb, trunk and tail movements. The IT administration of nor-binaltorphimine blocked U50,488 effects, wher eas IH injection of the antagonist had little influence on fetal behav ior. These findings suggest that kappa opioid receptors located in the spinal cord may play a role in the regulation of fetal motor behavior .