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: BACKGROUND : The septo - hippocampal cholinergic pathway has been implicated in epileptogenesis , and genetic factors influence the response to cholinergic agents , but limited data are available on cholinergic involvement in alcohol withdrawal severity .
### Output Text:
BACKGROUND : The septo - hippocampal cholinergic pathway has been implicated in epileptogenesis , and genetic factors influence the response to cholinergic agents , but limited data are available on cholinergic involvement in <span class="chemical">alcohol</span> withdrawal severity . 