morphine ( mu opioid receptor agonist , 40 micrograms ) , d-ala2-d-leu5-enkephalin ( delta opioid receptor agonist , 80 micrograms ) and u-50488h ( kappa opioid receptor agonist , 160 micrograms ) injected i.v.t . were unable to cause any release of met-enkephalin .
C0014298
enkephalin
C0014295
met-enkephalin