leu [ enkephalin ( 400.0 micrograms/kg ) and the enkephalin analog [ dala , dleu ] enkephalin ( 0.4 , 4.0 , and 40.0 micrograms/kg ) were intraperitoneally administered to rats 5 min before they were trained on aversively motivated tasks .
C0014294
leu [ enkephalin
C0014298
enkephalin