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: Under controlled conditions , infant rats emit ultrasonic vocalizations during extreme cold exposure and after administration of the alpha ( 2 ) adrenoceptor agonist , clonidine .
### Output Text:
Under controlled conditions , infant rats emit ultrasonic vocalizations during extreme cold exposure and after administration of the alpha ( 2 ) adrenoceptor agonist , <span class="chemical">clonidine</span> . 