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: Metabotropic glutamate ( mGlu ) receptors modulate synaptic transmission in the central nervous system and represent promising therapeutic targets for symptomatic treatment of Parkinson ' s disease ( PD ) .
### Output Text:
Metabotropic <span class="chemical">glutamate</span> ( mGlu ) receptors modulate synaptic transmission in the central nervous system and represent promising therapeutic targets for symptomatic treatment of Parkinson ' s disease ( PD ) . 