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: Hypothalamic prolactin receptor messenger ribonucleic acid levels , prolactin signaling , and hyperprolactinemic inhibition of pulsatile luteinizing hormone secretion are dependent on estradiol .
### Output Text:
Hypothalamic prolactin receptor messenger <span class="chemical">ribonucleic acid</span> levels , prolactin signaling , and hyperprolactinemic inhibition of pulsatile luteinizing hormone secretion are dependent on <span class="chemical">estradiol</span> . 