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: When injected into the accumbens shell ( but not the core ) before cocaine , CP 93129 ( 0 . 1 - 10 microg / side ) enhanced the locomotor response to cocaine ; the maximum effect being observed after 10 microg / side of the agonist .
### Output Text:
When injected into the accumbens shell ( but not the core ) before <span class="chemical">cocaine</span> , <span class="chemical">CP 93129</span> ( 0 . 1 - 10 microg / side ) enhanced the locomotor response to <span class="chemical">cocaine</span> ; the maximum effect being observed after 10 microg / side of the agonist . 