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: In conclusion , although Ro4368554 did not improve a time - related retention deficit , it reversed a cholinergic and a serotonergic memory deficit , suggesting that both mechanisms may be involved in the facilitation of object memory by Ro4368554 and , possibly , other 5 - HT ( 6 ) receptor antagonists .
### Output Text:
In conclusion , although <span class="chemical">Ro4368554</span> did not improve a time - related retention deficit , it reversed a cholinergic and a serotonergic memory deficit , suggesting that both mechanisms may be involved in the facilitation of object memory by <span class="chemical">Ro4368554</span> and , possibly , other <span class="chemical">5 - HT</span> ( 6 ) receptor antagonists . 