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 have previously reported that CCK - 8 significantly alleviated morphine - induced amnesia and reversed spine density decreases in the CA1 region of the hippocampus in morphine - treated animals .
### Output Text:
We have previously reported that <span class="chemical">CCK - 8</span> significantly alleviated <span class="chemical">morphine</span> - induced amnesia and reversed spine density decreases in the CA1 region of the hippocampus in <span class="chemical">morphine</span> - treated animals . 