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: Furthermore , selective destruction of the spinal 5 - HT nerves , produced by bilateral spinal injection of 5 , 7 - dihydroxytryptamine , reduced the magnitude of the vasodepressor or the bradycardiac responses to clonidine microinjected into the area near the ventrolateral surface of the medulla oblongata in rats .
### Output Text:
Furthermore , selective destruction of the spinal <span class="chemical">5 - HT</span> nerves , produced by bilateral spinal injection of <span class="chemical">5 , 7 - dihydroxytryptamine</span> , reduced the magnitude of the vasodepressor or the bradycardiac responses to <span class="chemical">clonidine</span> microinjected into the area near the ventrolateral surface of the medulla oblongata in rats . 