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: The electrophysiological recording in vitro showed that Rg1 restored the LTP in slices from the rats treated with morphine , but not changed LTP in the slices from normal saline - or morphine / Rg1 - treated rats ; this restoration could be inhibited by N - methyl - D - aspartate ( NMDA ) receptor antagonist MK801 .
### Output Text:
The electrophysiological recording in vitro showed that <span class="chemical">Rg1</span> restored the LTP in slices from the rats treated with <span class="chemical">morphine</span> , but not changed LTP in the slices from normal saline - or <span class="chemical">morphine</span> / <span class="chemical">Rg1</span> - treated rats ; this restoration could be inhibited by <span class="chemical">N - methyl - D - aspartate</span> ( <span class="chemical">NMDA</span> ) receptor antagonist <span class="chemical">MK801</span> . 