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 results showed that intracollicular microinjection of MK - 801 and AP7 previous to systemic injections of haloperidol significantly attenuated the catalepsy , as indicated by a reduced latency to step down from a horizontal bar .
### Output Text:
The results showed that intracollicular microinjection of <span class="chemical">MK - 801</span> and <span class="chemical">AP7</span> previous to systemic injections of <span class="chemical">haloperidol</span> significantly attenuated the catalepsy , as indicated by a reduced latency to step down from a horizontal bar . 