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 present study aimed to investigate the anticonvulsant activity as well as the effects on the level of hippocampal amino acid neurotransmitters ( glutamate , aspartate , glycine and GABA ) of N - ( 2 - propylpentanoyl ) urea ( VPU ) in comparison to its parent compound , valproic acid ( VPA ) .
### Output Text:
The present study aimed to investigate the anticonvulsant activity as well as the effects on the level of hippocampal <span class="chemical">amino acid</span> neurotransmitters ( <span class="chemical">glutamate</span> , <span class="chemical">aspartate</span> , <span class="chemical">glycine</span> and <span class="chemical">GABA</span> ) of <span class="chemical">N - ( 2 - propylpentanoyl ) urea</span> ( <span class="chemical">VPU</span> ) in comparison to its parent compound , <span class="chemical">valproic acid</span> ( <span class="chemical">VPA</span> ) . 