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: These findings demonstrate a sexually - dimorphic interaction between NMDA antagonists and morphine in a persistent pain model that can be distinguished from those observed in acute pain models .
### Output Text:
These findings demonstrate a sexually - dimorphic interaction between <span class="chemical">NMDA</span> antagonists and <span class="chemical">morphine</span> in a persistent pain model that can be distinguished from those observed in acute pain models . 