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: Although preclinical and clinical findings suggest pulsatile stimulation of striatal postsynaptic receptors as a key mechanism underlying levodopa - induced dyskinesias , their pathogenesis is still unclear .
### Output Text:
Although preclinical and clinical findings suggest pulsatile stimulation of striatal postsynaptic receptors as a key mechanism underlying <span class="chemical">levodopa</span> - induced dyskinesias , their pathogenesis is still unclear . 