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: In acute pain models , N - methyl - D - aspartate ( NMDA ) antagonists enhance the antinociceptive effects of morphine to a greater extent in males than females .
### Output Text:
In acute pain models , <span class="chemical">N - methyl - D - aspartate</span> ( <span class="chemical">NMDA</span> ) antagonists enhance the antinociceptive effects of <span class="chemical">morphine</span> to a greater extent in males than females . 