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: We conclude that the analgesic action of opioids on the peripheral terminals of primary afferents is via a binding site with characteristics of the mu - opioid receptor and that this action is mediated by inhibition of the cyclic adenosine monophosphate second messenger system .
### Output Text:
We conclude that the analgesic action of opioids on the peripheral terminals of primary afferents is via a binding site with characteristics of the mu - opioid receptor and that this action is mediated by inhibition of the <span class="chemical">cyclic adenosine monophosphate</span> second messenger system . 