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: Sex differences in NMDA antagonist enhancement of morphine antihyperalgesia in a capsaicin model of persistent pain : comparisons to two models of acute pain .
### Output Text:
Sex differences in <span class="chemical">NMDA</span> antagonist enhancement of <span class="chemical">morphine</span> antihyperalgesia in a <span class="chemical">capsaicin</span> model of persistent pain : comparisons to two models of acute pain . 