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: Infusion of the central nervous system alpha2 - adrenergic receptor agonist clonidine also resulted in a greater reduction in MAP in both groups of SHR that were fed the high ( compared with the basal ) sodium chloride diet .
### Output Text:
Infusion of the central nervous system <span class="chemical">alpha2 - adrenergic receptor agonist</span> <span class="chemical">clonidine</span> also resulted in a greater reduction in MAP in both groups of SHR that were fed the high ( compared with the basal ) <span class="chemical">sodium chloride</span> diet . 