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 addition , reflex bradycardia caused by injected norepinephrine was significantly enhanced by L - dopa , DL - Threo - dihydroxyphenylserine had no effect on blood pressure , heart rate or reflex responses to norepinephrine .
### Output Text:
In addition , reflex bradycardia caused by injected <span class="chemical">norepinephrine</span> was significantly enhanced by <span class="chemical">L - dopa</span> , <span class="chemical">DL - Threo - dihydroxyphenylserine</span> had no effect on blood pressure , heart rate or reflex responses to <span class="chemical">norepinephrine</span> . 