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: These results suggest the involvement of quinine - sensitive but not 4 - aminopyridine - sensitive K ( + ) - channels in morphine reward .
### Output Text:
These results suggest the involvement of <span class="chemical">quinine</span> - sensitive but not <span class="chemical">4 - aminopyridine</span> - sensitive <span class="chemical">K</span> ( + ) - channels in <span class="chemical">morphine</span> reward . 