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 spontaneously hypertensive , stroke - prone rats , microinjection of methyldopa into the area of the midline B3 serotonin cell group in the ventral medulla caused a potent hypotension of 30 - 40 mm Hg , which was maximal 2 - 3 h after administration and was abolished by the serotonin neurotoxin 5 , 7 - dihydroxytryptamine ( 5 , 7 - DHT ) injected intracerebroventricularly .
### Output Text:
In spontaneously hypertensive , stroke - prone rats , microinjection of <span class="chemical">methyldopa</span> into the area of the midline B3 <span class="chemical">serotonin</span> cell group in the ventral medulla caused a potent hypotension of 30 - 40 mm Hg , which was maximal 2 - 3 h after administration and was abolished by the <span class="chemical">serotonin</span> neurotoxin <span class="chemical">5 , 7 - dihydroxytryptamine</span> ( <span class="chemical">5 , 7 - DHT</span> ) injected intracerebroventricularly . 