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: Thus it appears that the difference between seizure sensitive and insensitive animals may be due to a difference in hippocampal nicotinic receptor concentration as measured with alpha - bungarotoxin binding .
### Output Text:
Thus it appears that the difference between seizure sensitive and insensitive animals may be due to a difference in hippocampal nicotinic receptor concentration as measured with alpha - bungarotoxin binding . 