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 study suggests that the activity - increasing effects of morphine are mediated by the release of catecholamines from adrenergic neurons in the brain .
### Output Text:
The study suggests that the activity - increasing effects of <span class="chemical">morphine</span> are mediated by the release of <span class="chemical">catecholamines</span> from adrenergic neurons in the brain . 