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: The aim of the present study was to investigate changes in the plasma calcitonin gene - related peptide ( CGRP ) concentration and platelet serotonin ( 5 - hydroxytriptamine , 5 - HT ) content during the immediate headache and the delayed genuine migraine attack provoked by nitroglycerin .
### Output Text:
The aim of the present study was to investigate changes in the plasma <span class="chemical">calcitonin gene - related peptide</span> ( <span class="chemical">CGRP</span> ) concentration and platelet <span class="chemical">serotonin</span> ( <span class="chemical">5 - hydroxytriptamine</span> , <span class="chemical">5 - HT</span> ) content during the immediate headache and the delayed genuine migraine attack provoked by <span class="chemical">nitroglycerin</span> . 