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: The three compounds vary in their affinities for sigma2 receptors and exhibit negligible affinities for dopamine , opioid , GABA ( A ) and NMDA receptors .
### Output Text:
The three compounds vary in their affinities for sigma2 receptors and exhibit negligible affinities for <span class="chemical">dopamine</span> , opioid , <span class="chemical">GABA</span> ( A ) and <span class="chemical">NMDA</span> receptors . 