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: As naloxone and clonidine do not appear to interact with the same receptor site , the observed functional antagonism suggests the release of an endogenous opiate by clonidine or alpha - methyldopa and the possible role of the opiate in the central control of sympathetic tone .
### Output Text:
As <span class="chemical">naloxone</span> and <span class="chemical">clonidine</span> do not appear to interact with the same receptor site , the observed functional antagonism suggests the release of an endogenous opiate by <span class="chemical">clonidine</span> or <span class="chemical">alpha - methyldopa</span> and the possible role of the opiate in the central control of sympathetic tone . 