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 .