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 the present study we aimed to investigate plasma levels of CGRP during headache induced by the NO donor glyceryl trinitrate ( GTN ) in 16 patients with chronic tension - type headache and 16 healthy controls .
### Output Text:
In the present study we aimed to investigate plasma levels of CGRP during headache induced by the <span class="chemical">NO</span> donor <span class="chemical">glyceryl trinitrate</span> ( <span class="chemical">GTN</span> ) in 16 patients with chronic tension - type headache and 16 healthy controls . 