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 serotonin 6 ( 5 - HT6 ) receptor is therapeutically targeted by several second generation antipsychotics , such as clozapine and olanzapine , and d - amphetamine - induced hyperactivity in rats is corrected with the use of a selective 5 - HT6 receptor antagonist .
### Output Text:
The <span class="chemical">serotonin</span> 6 ( <span class="chemical">5 - HT6</span> ) receptor is therapeutically targeted by several second generation antipsychotics , such as <span class="chemical">clozapine</span> and <span class="chemical">olanzapine</span> , and <span class="chemical">d - amphetamine</span> - induced hyperactivity in rats is corrected with the use of a selective <span class="chemical">5 - HT6</span> receptor antagonist . 