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: It is widely believed that dopamine ( DA ) mediates methamphetamine ( METH ) - induced toxicity to brain dopaminergic neurons , because drugs that interfere with DA neurotransmission decrease toxicity , whereas drugs that increase DA neurotransmission enhance toxicity .
### Output Text:
It is widely believed that <span class="chemical">dopamine</span> ( <span class="chemical">DA</span> ) mediates <span class="chemical">methamphetamine</span> ( <span class="chemical">METH</span> ) - induced toxicity to brain dopaminergic neurons , because drugs that interfere with <span class="chemical">DA</span> neurotransmission decrease toxicity , whereas drugs that increase <span class="chemical">DA</span> neurotransmission enhance toxicity . 