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: It was concluded that morphine catalepsy can be elicited by opioid and cholinergic receptors , and the potentiation of morphine induced by nicotine may also be mediated through cholinergic receptor mechanisms .
### Output Text:
It was concluded that <span class="chemical">morphine</span> catalepsy can be elicited by opioid and cholinergic receptors , and the potentiation of <span class="chemical">morphine</span> induced by <span class="chemical">nicotine</span> may also be mediated through cholinergic receptor mechanisms . 