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: In the present study , the effects of manipulating the activity of the gamma - aminobutyric acid ( GABA ) - mediated synaptic inhibition within the substantia nigra on seizures produced by pilocarpine in rats , were investigated .
### Output Text:
In the present study , the effects of manipulating the activity of the <span class="chemical">gamma - aminobutyric acid</span> ( <span class="chemical">GABA</span> ) - mediated synaptic inhibition within the substantia nigra on seizures produced by <span class="chemical">pilocarpine</span> in rats , were investigated . 