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: Portal plasma concentrations of neither arginine vasopressin nor oxytocin are significantly altered in this paradigm .
### Output Text:
Portal plasma concentrations of neither <span class="chemical">arginine vasopressin</span> nor <span class="chemical">oxytocin</span> are significantly altered in this paradigm . 