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: Compared with sham - operated controls , lesions significantly ( p < 0 . 25 ) blunted the early ( < 60 min ) free - field locomotor hypoactivity caused by nicotine ( 0 . 5 mg kg ( - 1 ) , i . m . ) , enhanced the later ( 60 - 120 min ) nicotine - induced hyperactivity , and raised spontaneous nocturnal activity .
### Output Text:
Compared with sham - operated controls , lesions significantly ( p < 0 . 25 ) blunted the early ( < 60 min ) free - field locomotor hypoactivity caused by <span class="chemical">nicotine</span> ( 0 . 5 mg kg ( - 1 ) , i . m . ) , enhanced the later ( 60 - 120 min ) <span class="chemical">nicotine</span> - induced hyperactivity , and raised spontaneous nocturnal activity . 