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: These results suggest that chronic nicotine - treated rats develop locomotor hyperactivity in response to nicotine initially due to increases of both the density of nicotinic receptors and DA concentration , followed by inducing DA receptor supersensitivity in the striatum .
### Output Text:
These results suggest that chronic <span class="chemical">nicotine</span> - treated rats develop locomotor hyperactivity in response to <span class="chemical">nicotine</span> initially due to increases of both the density of nicotinic receptors and <span class="chemical">DA</span> concentration , followed by inducing <span class="chemical">DA</span> receptor supersensitivity in the striatum . 