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 the present study , we investigated the changes occurring at the protein level in striatal samples obtained from the unilaterally 6 - hydroxydopamine - lesion rat model of PD treated with saline , L - DOPA or bromocriptine using two - dimensional difference gel electrophoresis and mass spectrometry ( MS ) .
### Output Text:
In the present study , we investigated the changes occurring at the protein level in striatal samples obtained from the unilaterally <span class="chemical">6 - hydroxydopamine</span> - lesion rat model of PD treated with saline , <span class="chemical">L - DOPA</span> or <span class="chemical">bromocriptine</span> using two - dimensional difference gel electrophoresis and mass spectrometry ( MS ) . 