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: The aim of the present study was to assess the contribution of the glycine site of NMDA receptors in the striatum to the regulation of muscle tone .
### Output Text:
The aim of the present study was to assess the contribution of the <span class="chemical">glycine</span> site of <span class="chemical">NMDA</span> receptors in the striatum to the regulation of muscle tone . 