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 SE survivors , similar stimulation resulted in a population spike followed , at a variable latency , by negative DC shifts and repetitive afterdischarges of 3 - 60 s duration , which were blocked by ionotropic glutamate receptor antagonists .
### Output Text:
In SE survivors , similar stimulation resulted in a population spike followed , at a variable latency , by negative DC shifts and repetitive afterdischarges of 3 - 60 s duration , which were blocked by ionotropic <span class="chemical">glutamate</span> receptor antagonists . 