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: These findings indicate that in spontaneously hypertensive rats the effects of central alpha - adrenoceptor stimulation involve activation of opiate receptors .
### Output Text:
These findings indicate that in spontaneously hypertensive rats the effects of central alpha - adrenoceptor stimulation involve activation of opiate receptors . 