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: We suggest that opioids may be neurotoxic in the setting of spinal cord ischemia via NMDA receptor activation .
### Output Text:
We suggest that opioids may be neurotoxic in the setting of spinal cord ischemia via <span class="chemical">NMDA</span> receptor activation . 