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 males , the non - competitive NMDA antagonist dextromethorphan enhanced the antihyperalgesic effect of low to moderate doses of morphine in a dose - and time - dependent manner .
### Output Text:
In males , the non - competitive <span class="chemical">NMDA</span> antagonist <span class="chemical">dextromethorphan</span> enhanced the antihyperalgesic effect of low to moderate doses of <span class="chemical">morphine</span> in a dose - and time - dependent manner . 