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: Chronic exposure to PCPA alone significantly decreased locomotor activity and increased irritability but had no effect on sexual behavior , partner preference , or aggression .
### Output Text:
Chronic exposure to <span class="chemical">PCPA</span> alone significantly decreased locomotor activity and increased irritability but had no effect on sexual behavior , partner preference , or aggression . 