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 the counterbalanced method , quinine attenuated morphine - induced place preference , whereas 4 - aminopyridine was ineffective .
### Output Text:
In the counterbalanced method , <span class="chemical">quinine</span> attenuated <span class="chemical">morphine</span> - induced place preference , whereas <span class="chemical">4 - aminopyridine</span> was ineffective . 