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: Earlier studies have demonstrated that antagonism of sigma1 receptors attenuates the convulsive , lethal , locomotor stimulatory and rewarding actions of cocaine in mice .
### Output Text:
Earlier studies have demonstrated that antagonism of sigma1 receptors attenuates the convulsive , lethal , locomotor stimulatory and rewarding actions of <span class="chemical">cocaine</span> in mice . 