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 altered effects of apomorphine and amphetamine could indicate differential changes in dopamine receptor signalling leading to functional desensitisation , or altered modulation of sensory gating in the nucleus accumbens by limbic structures such as the hippocampus .
### Output Text:
The altered effects of <span class="chemical">apomorphine</span> and <span class="chemical">amphetamine</span> could indicate differential changes in <span class="chemical">dopamine</span> receptor signalling leading to functional desensitisation , or altered modulation of sensory gating in the nucleus accumbens by limbic structures such as the hippocampus . 