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: Chronic hyperprolactinemia induced by the dopamine antagonist sulpiride caused a 40 % reduction LH pulse frequency in ovariectomized rats , but only in the presence of chronic low levels of estradiol .
### Output Text:
Chronic hyperprolactinemia induced by the <span class="chemical">dopamine</span> antagonist <span class="chemical">sulpiride</span> caused a 40 % reduction LH pulse frequency in ovariectomized rats , but only in the presence of chronic low levels of <span class="chemical">estradiol</span> . 