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: However , inhibition of postsynaptic 5 - HT receptors neither inhibited nor potentiated reversal of catalepsy and leaves open the question as to the site or mechanism for this effect .
### Output Text:
However , inhibition of postsynaptic <span class="chemical">5 - HT</span> receptors neither inhibited nor potentiated reversal of catalepsy and leaves open the question as to the site or mechanism for this effect . 