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: To further validate the hypothesis that the anti - cocaine effects of the novel ligands involved antagonism of sigma receptors , an antisense oligodeoxynucleotide against sigma1 receptors was also shown to significantly attenuate the convulsive and locomotor stimulatory effects of cocaine .
### Output Text:
To further validate the hypothesis that the anti - <span class="chemical">cocaine</span> effects of the novel ligands involved antagonism of sigma receptors , an antisense <span class="chemical">oligodeoxynucleotide</span> against sigma1 receptors was also shown to significantly attenuate the convulsive and locomotor stimulatory effects of <span class="chemical">cocaine</span> . 