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: To investigate this hypothesis , the present study in rats examined the memory - altering abilities of structural analogs of PREGS , which differ in their modulation of NMDAR and / or GABA ( A ) R function .
### Output Text:
To investigate this hypothesis , the present study in rats examined the memory - altering abilities of structural analogs of <span class="chemical">PREGS</span> , which differ in their modulation of NMDAR and / or <span class="chemical">GABA</span> ( A ) R function . 