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: Motor impairment , dopamine ( DA ) neuronal activity and proenkephalin ( PENK ) gene expression in the caudate - putamen ( CPu ) were measured in 6 - OHDA - lesioned and treated ( L - DOPA + benserazide ) CB1 KO and WT mice .
### Output Text:
Motor impairment , <span class="chemical">dopamine</span> ( <span class="chemical">DA</span> ) neuronal activity and <span class="chemical">proenkephalin</span> ( <span class="chemical">PENK</span> ) gene expression in the caudate - putamen ( CPu ) were measured in <span class="chemical">6 - OHDA</span> - lesioned and treated ( <span class="chemical">L - DOPA + benserazide</span> ) CB1 KO and WT mice . 