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: Platelet serotonin content decreased significantly ( P < 0 . 01 ) after nitroglycerin in subjects with no migraine attack but no consistent change was observed in patients with migraine attack .
### Output Text:
Platelet <span class="chemical">serotonin</span> content decreased significantly ( P < 0 . 01 ) after <span class="chemical">nitroglycerin</span> in subjects with no migraine attack but no consistent change was observed in patients with migraine attack . 