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: The rapid alternations of rigidity and the signs of dopaminergic activation observed in the animals of the AS / KS group might be due to rapid shifts in the predominance of various DA - innervated structures .
### Output Text:
The rapid alternations of rigidity and the signs of dopaminergic activation observed in the animals of the AS / KS group might be due to rapid shifts in the predominance of various DA - innervated structures . 