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: In contrast to the protection provided by the putative antagonists , the well - characterized sigma receptor agonist di - o - tolylguanidine ( DTG ) and the novel sigma receptor agonist BD1031 ( 3R - 1 - [ 2 - ( 3 , 4 - dichlorophenyl ) ethyl ] - 1 , 4 - diazabicyclo [ 4 . 3 . 0 ] nonane ) each worsened the behavioral toxicity of cocaine .
### Output Text:
In contrast to the protection provided by the putative antagonists , the well - characterized sigma receptor agonist <span class="chemical">di - o - tolylguanidine</span> ( <span class="chemical">DTG</span> ) and the novel sigma receptor agonist <span class="chemical">BD1031</span> ( <span class="chemical">3R - 1 - [ 2 - ( 3 , 4 - dichlorophenyl ) ethyl ] - 1 , 4 - diazabicyclo [ 4 . 3 . 0 ] nonane</span> ) each worsened the behavioral toxicity of <span class="chemical">cocaine</span> . 