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: In a second experiment , rats received tricyclic antidepressant desipramine IP followed in 15 min by 4 - aminopyridine or saline .
### Output Text:
In a second experiment , rats received tricyclic antidepressant <span class="chemical">desipramine</span> IP followed in 15 min by <span class="chemical">4 - aminopyridine</span> or saline . 