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 research towards the development of new atypical antipsychotic agents , one strategy is that the dopaminergic system can be modulated through manipulation of the serotonergic system .
### Output Text:
In research towards the development of new atypical antipsychotic agents , one strategy is that the dopaminergic system can be modulated through manipulation of the serotonergic system . 