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: It was found that three drugs stimulating central NA receptors failed to affect the analgesic ED50 of all antinociceptive agents and they enhanced catalepsy induced by morphine and fentanyl .
### Output Text:
It was found that three drugs stimulating central <span class="chemical">NA</span> receptors failed to affect the analgesic ED50 of all antinociceptive agents and they enhanced catalepsy induced by <span class="chemical">morphine</span> and <span class="chemical">fentanyl</span> . 