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 shift to the left in the dose - response relation and fall in threshold suggested increased sensitivity to norepinephrine after treatment .
### Output Text:
A shift to the left in the dose - response relation and fall in threshold suggested increased sensitivity to <span class="chemical">norepinephrine</span> after treatment . 