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: CONCLUSIONS : Results of these studies suggest that B1 and B2 receptors are engaged in transmission of nociceptive stimuli in both diabetic and toxic neuropathy .
### Output Text:
CONCLUSIONS : Results of these studies suggest that B1 and B2 receptors are engaged in transmission of nociceptive stimuli in both diabetic and toxic neuropathy . 