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: Susceptibility to seizures produced by pilocarpine in rats after microinjection of isoniazid or gamma - vinyl - GABA into the substantia nigra .
### Output Text:
Susceptibility to seizures produced by <span class="chemical">pilocarpine</span> in rats after microinjection of <span class="chemical">isoniazid</span> or <span class="chemical">gamma - vinyl - GABA</span> into the substantia nigra . 