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: We found that ( i ) gabapentin reduced the activations in the bilateral operculoinsular cortex , independently of the presence of central sensitization ; ( ii ) gabapentin reduced the activation in the brainstem , only during central sensitization ; ( iii ) gabapentin suppressed stimulus - induced deactivations , only during central sensitization ; this effect was more robust than the effect on brain activation .
### Output Text:
We found that ( i ) <span class="chemical">gabapentin</span> reduced the activations in the bilateral operculoinsular cortex , independently of the presence of central sensitization ; ( ii ) <span class="chemical">gabapentin</span> reduced the activation in the brainstem , only during central sensitization ; ( iii ) <span class="chemical">gabapentin</span> suppressed stimulus - induced deactivations , only during central sensitization ; this effect was more robust than the effect on brain activation . 