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: Among the eight mGlu receptor subtypes , mGlu7 receptor is prominently expressed in the basal ganglia , but its role in restoring motor function in animal models of PD is not known .
### Output Text:
Among the eight mGlu receptor subtypes , mGlu7 receptor is prominently expressed in the basal ganglia , but its role in restoring motor function in animal models of PD is not known . 