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: Antinociceptive effect of morphine was reduced in chronically treated rats ( 39 + / - 10 vs . 18 + / - 5 au ) while the combination - induced antinociception was remained similar as an acute treatment ( 298 + / - 7 vs . 280 + / - 17 au ) .
### Output Text:
Antinociceptive effect of <span class="chemical">morphine</span> was reduced in chronically treated rats ( 39 + / - 10 vs . 18 + / - 5 au ) while the combination - induced antinociception was remained similar as an acute treatment ( 298 + / - 7 vs . 280 + / - 17 au ) . 