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: Although ORL1 receptors which display a high degree of sequence homology with classical opioid receptors are abundant in the hippocampus , little is known regarding their role in learning and memory .
### Output Text:
Although ORL1 receptors which display a high degree of sequence homology with classical opioid receptors are abundant in the hippocampus , little is known regarding their role in learning and memory . 