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: Erectile function was assessed 6 weeks following uni - and bilateral injections of 6 - hydroxydopamine in the substantia nigra nucleus of the brain .
### Output Text:
Erectile function was assessed 6 weeks following uni - and bilateral injections of <span class="chemical">6 - hydroxydopamine</span> in the substantia nigra nucleus of the brain . 