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: Seizure activity due to PTZ and picrotoxin ( PTX ) was significantly decreased ; however , seizure activity due to 3 - mercaptopropionic acid ( MPA ) , bicuculline ( BCC ) , methyl 6 , 7 - dimethoxy - 4 - ethyl - B - carboline - 3 - carboxylate ( DMCM ) , or strychnine ( STR ) was not different from control .
### Output Text:
Seizure activity due to <span class="chemical">PTZ</span> and <span class="chemical">picrotoxin</span> ( <span class="chemical">PTX</span> ) was significantly decreased ; however , seizure activity due to <span class="chemical">3 - mercaptopropionic acid</span> ( <span class="chemical">MPA</span> ) , <span class="chemical">bicuculline</span> ( <span class="chemical">BCC</span> ) , <span class="chemical">methyl 6 , 7 - dimethoxy - 4 - ethyl - B - carboline - 3 - carboxylate</span> ( <span class="chemical">DMCM</span> ) , or <span class="chemical">strychnine</span> ( <span class="chemical">STR</span> ) was not different from control . 