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: Control experiments indicated that neither an increase in pain sensitivity ( pre - training naloxone ) nor an induced aversive state ( post - training naloxone ) appear to be responsible for the influence of naloxone on the scopolamine - induced retention deficit .
### Output Text:
Control experiments indicated that neither an increase in pain sensitivity ( pre - training <span class="chemical">naloxone</span> ) nor an induced aversive state ( post - training <span class="chemical">naloxone</span> ) appear to be responsible for the influence of <span class="chemical">naloxone</span> on the <span class="chemical">scopolamine</span> - induced retention deficit . 