Once you have the nerve toxin, the alarm will sound. Zap both lab rats and then enter the lab to grab the toxin. From here, Steve will find another lab rat and this will encourage the locked lab rat to open the door. The lab rat with the nerve toxin is locked inside his lab and won't open the door.
Zap them when they exit (use Michael's special ability to get them both, otherwise your colleagues will shoot one of them down) and then enter the next corridor towards the door with the security lock. Take cover and wait for the two guards to walk out. Continue ascending the lab halls and corridors until you reach the security doors.
