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: A naloxone ( 5 mg / kg ) challenge was also ineffective in altering the seizure thresholds of either control of MSG - treated mice .
### Output Text:
A <span class="chemical">naloxone</span> ( 5 mg / kg ) challenge was also ineffective in altering the seizure thresholds of either control of <span class="chemical">MSG</span> - treated mice . 