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: Mediation of enhanced reflex vagal bradycardia by L - dopa via central dopamine formation in dogs .
### Output Text:
Mediation of enhanced reflex vagal bradycardia by <span class="chemical">L - dopa</span> via central <span class="chemical">dopamine</span> formation in dogs . 