tokens: The intradermal injection of mu ( morphine , Tyr - D - Ala - Gly - NMe - Phe - Gly - ol and morphiceptin ) , kappa ( trans - 3 , 4 - dichloro - N - methyl - N [ 2 - ( 1 - pyrrolidinyl ) cyclohexyl ] benzeneactemide ) and delta ( [ D - Pen2 . 5 ] - enkephalin and [ D - Ser2 ] - [ Leu ] enkephalin - Thr ) selective opioid - agonists , by themselves , did not significantly affect the mechanical nociceptive threshold in the hindpaw of the rat .