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 contrast , serotonin release from platelets does not provoke migraine , it may even counteract the headache and the concomitant CGRP release in this model .
### Output Text:
In contrast , <span class="chemical">serotonin</span> release from platelets does not provoke migraine , it may even counteract the headache and the concomitant <span class="chemical">CGRP</span> release in this model . 