Some Dummied Out lines of dialogue suggests that Wheatley was the one who released the neurotoxin on "Bring your Daughter to Work Day", rather than GLaDOS.
Wheatley: Over there is where they used to keep the old neurotoxin release button. BIG responsibility, the guy in charge of the neurotoxin release button. And guess who he WAS? He wasn't me. But I was his assistant, and I did a lot of his admin. You know, in the end, yes, they let me go. It's all politics, to be honest. It's a big popularity contest, it's all about who you know, and whose back you're willing to scratch, who doesn't touch — or, in my case, who did accidentally touch — the neurotoxin button. But not entirely my fault! You shoulda seen the SIZE of that thing, it was huge! I should have gotten a raise for all the times I DIDN'T bump into it."