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 later enhancement was attenuated after intra - accumbens shell treatment with GR 55562 ( 1 microg / side ) .
### Output Text:
The later enhancement was attenuated after intra - accumbens shell treatment with <span class="chemical">GR 55562</span> ( 1 microg / side ) . 