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: Gamma - hexachlorocyclohexane ( gamma - HCH ) , the active ingredient of the insecticide lindane , has been shown to decrease seizure threshold to pentylenetrazol ( PTZ ) 3 h after exposure to gamma - HCH and conversely increase threshold to PTZ - induced seizures 24 h after exposure to gamma - HCH ( Vohland et al . 1981 ) .
### Output Text:
<span class="chemical">Gamma - hexachlorocyclohexane</span> ( <span class="chemical">gamma - HCH</span> ) , the active ingredient of the insecticide <span class="chemical">lindane</span> , has been shown to decrease seizure threshold to pentylenetrazol ( <span class="chemical">PTZ</span> ) 3 h after exposure to <span class="chemical">gamma - HCH</span> and conversely increase threshold to <span class="chemical">PTZ</span> - induced seizures 24 h after exposure to <span class="chemical">gamma - HCH</span> ( Vohland et al . 1981 ) . 