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: In contrast to controls , methoctramine increased - - instead of decreased - - the tonic responses at high frequencies .
### Output Text:
In contrast to controls , <span class="chemical">methoctramine</span> increased - - instead of decreased - - the tonic responses at high frequencies . 