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: In doses of 3 - 10 mg / kg , the serotonin receptor agonist MK - 212 caused a dose - dependent blockade of the response of muscimol .
### Output Text:
In doses of 3 - 10 mg / kg , the <span class="chemical">serotonin</span> receptor agonist <span class="chemical">MK - 212</span> caused a dose - dependent blockade of the response of <span class="chemical">muscimol</span> . 