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: Administered to healthy volunteers , a subanesthetic dose of the non - competitive NMDA receptor antagonist ketamine leads to psychopathological symptoms similar to those observed in schizophrenia .
### Output Text:
Administered to healthy volunteers , a subanesthetic dose of the non - competitive <span class="chemical">NMDA</span> receptor antagonist <span class="chemical">ketamine</span> leads to psychopathological symptoms similar to those observed in schizophrenia . 