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: Since cortical spreading depression has been shown to liberate NO in animals , this finding may help our understanding of the coupling between cortical spreading depression and headache in migraine with aura .
### Output Text:
Since cortical spreading depression has been shown to liberate <span class="chemical">NO</span> in animals , this finding may help our understanding of the coupling between cortical spreading depression and headache in migraine with aura . 