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: First , pretreatment with MK - 801 produced an effective and dose - dependent anticonvulsant action with the lithium - pilocarpine model but not with rats treated with pilocarpine alone , suggesting that different biochemical mechanisms control seizures in these two models .
### Output Text:
First , pretreatment with <span class="chemical">MK - 801</span> produced an effective and dose - dependent anticonvulsant action with the <span class="chemical">lithium</span> - <span class="chemical">pilocarpine</span> model but not with rats treated with <span class="chemical">pilocarpine</span> alone , suggesting that different biochemical mechanisms control seizures in these two models . 