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: We conclude that Rg1 may significantly improve the spatial learning capacity impaired by chonic morphine administration and restore the morphine - inhibited LTP .
### Output Text:
We conclude that <span class="chemical">Rg1</span> may significantly improve the spatial learning capacity impaired by chonic <span class="chemical">morphine</span> administration and restore the <span class="chemical">morphine</span> - inhibited LTP . 