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: Neuroleptic drugs such as haloperidol , which block dopamine receptors , also cause catalepsy in rodents .
### Output Text:
Neuroleptic drugs such as <span class="chemical">haloperidol</span> , which block <span class="chemical">dopamine</span> receptors , also cause catalepsy in rodents . 