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: Cholecystokinin - octapeptide restored morphine - induced hippocampal long - term potentiation impairment in rats .
### Output Text:
<span class="chemical">Cholecystokinin - octapeptide</span> restored <span class="chemical">morphine</span> - induced hippocampal long - term potentiation impairment in rats . 