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: Chronic T treatment significantly decreased 5 - HT and 5 - HIAA in certain brain areas , but to a much lesser extent than PCPA .
### Output Text:
Chronic <span class="chemical">T</span> treatment significantly decreased <span class="chemical">5 - HT</span> and <span class="chemical">5 - HIAA</span> in certain brain areas , but to a much lesser extent than <span class="chemical">PCPA</span> . 