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 animal models were considered to reflect the positive symptoms of schizophrenia , and the above evidence suggests that altered 5 - HT6 receptors are involved in the pathophysiology of psychotic disorders .
### Output Text:
These animal models were considered to reflect the positive symptoms of schizophrenia , and the above evidence suggests that altered <span class="chemical">5 - HT6</span> receptors are involved in the pathophysiology of psychotic disorders . 