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 is suggested that in rats the brain NA plays a less important function than the other monoamines in the behavioural activity of potent analgesics .
### Output Text:
It is suggested that in rats the brain <span class="chemical">NA</span> plays a less important function than the other <span class="chemical">monoamines</span> in the behavioural activity of potent analgesics . 