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: At hippocampal Schaeffer collateral - CA1 synapses , long - term potentiation was preserved in BMC - transplanted rats compared to epileptic controls .
### Output Text:
At hippocampal Schaeffer collateral - CA1 synapses , long - term potentiation was preserved in BMC - transplanted rats compared to epileptic controls . 