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: Such an induced muscular rigidity by the narcotic agent was significantly antagonized or even reduced by prior electrolytic lesions of the locus coeruleus or pretreatment with the alpha - adrenoceptor blocker , prazosin .
### Output Text:
Such an induced muscular rigidity by the narcotic agent was significantly antagonized or even reduced by prior electrolytic lesions of the locus coeruleus or pretreatment with the alpha - adrenoceptor blocker , <span class="chemical">prazosin</span> . 