Gosh, I sure hate to hear that is your experience with speutering bunnies.i think it depends on the rats personality and drivers before neutering. neutering vastly reduces the hormones. This can take an edge of there previous behaviour and tends to return them to the rat they were before the hormones kicked in albeit a bit older. I've found most rats get playful again before slowing and ageing normally, definitely more relaxed. In does they can become a little more stable and less flighty or excitable which can make them seem less interesting but i don't think changes thete personality, just allows them to show it more freely.
The only time I've found any evidence of personality changes from neutering is in rabbits. This is thought to be because rabbits don't deal with anesthetic well so often breath only lightly when under. This can cause the brain to be partially starved of oxygen leading to minor brain damage and so potentially more dopey afterwards. Even this isn't common, though it is possible in a heavy surgery (more likley in a spay than castration due to surgery complexity) out could happen to rats too
Ah, I see.This wasnt my personal expereince with rabbit neuters, however my vet was a local rabbit expert and had spent some time researching it. We spoke about it at length when my girl needed spaying (she was a complicated case, her first spay had to be cancelled as she kept holding her breath whenver the vet turned her onto her back, thankfully she was less stubborn the second time).