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: Immunocytochemical techniques were used to examine alterations in the expression of neuronal nitric oxide synthase ( NOS ) in bladder pathways following acute and chronic irritation of the urinary tract of the rat .
### Output Text:
Immunocytochemical techniques were used to examine alterations in the expression of neuronal <span class="chemical">nitric oxide</span> synthase ( NOS ) in bladder pathways following acute and chronic irritation of the urinary tract of the rat . 