In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

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