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: The effects of two unselective potassium ( K ( + ) - ) channel blockers , quinine ( 12 . 5 , 25 and 50 mg / kg ) and 4 - aminopyridine ( 1 and 2 mg / kg ) , on conditioned place preference and biphasic changes in motor activity induced by morphine ( 10 mg / kg ) were tested in Wistar rats .
### Output Text:
The effects of two unselective <span class="chemical">potassium</span> ( <span class="chemical">K</span> ( + ) - ) channel blockers , <span class="chemical">quinine</span> ( 12 . 5 , 25 and 50 mg / kg ) and <span class="chemical">4 - aminopyridine</span> ( 1 and 2 mg / kg ) , on conditioned place preference and biphasic changes in motor activity induced by <span class="chemical">morphine</span> ( 10 mg / kg ) were tested in Wistar rats . 