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: Norepinephrine signaling through beta - adrenergic receptors is critical for expression of cocaine - induced anxiety .
### Output Text:
<span class="chemical">Norepinephrine</span> signaling through beta - adrenergic receptors is critical for expression of <span class="chemical">cocaine</span> - induced anxiety . 