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 show an inhibitory interaction of early stress , caused by maternal deprivation , combined with ' adolescent ' stress , simulated by corticosterone treatment , on dopaminergic regulation of PPI .
### Output Text:
These results show an inhibitory interaction of early stress , caused by maternal deprivation , combined with ' adolescent ' stress , simulated by <span class="chemical">corticosterone</span> treatment , on dopaminergic regulation of PPI . 