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: High doses of isoniazid increase hypotension induced by vasodilators and change the accompanying reflex tachycardia to bradycardia , an interaction attributed to decreased synthesis of brain gamma - aminobutyric acid ( GABA ) .
### Output Text:
High doses of <span class="chemical">isoniazid</span> increase hypotension induced by vasodilators and change the accompanying reflex tachycardia to bradycardia , an interaction attributed to decreased synthesis of brain <span class="chemical">gamma - aminobutyric acid</span> ( <span class="chemical">GABA</span> ) . 