Rat suddenly attacks

Question
I have a rat, about five or six months old, who up until now has always been fairly sweet and affectionate. He was quite neurotic when I got him, but as he's grown up he's become bolder, although he gets scared easily, especially as he's blind in one eye. Usually he's allowed to run around in my living room (appropriately rat proofed) when he's out, and tends to stay under the coffee table next to me, coming out for cuddles and a little massage and whatnot. Earlier on tonight he was acting perfectly normal, being actually slightly more affectionate than normal if I'm honest. He went off for a wander around the floor, and when he came back jumped onto my arm, sniffed my thumb (still acting fairly normal,) got to just below my elbow and suddenly clamped his teeth into my arm, hissing and clamping his arms around me. It was utterly unprovoked as far as I can tell, and the resulting wound was about an inch long and went straight to the muscle. I immediately returned him to the cage, but while I took him back he was once more being affectionate and, well, normal. It seems like he just flipped out - when I walk up to the cage now he's begging to be let out and licking at my fingers like he normally does. What sort of thing could have provoked this? I'll admit to being totally shocked - no rat I've had has ever just gone for me like that - it was definitely an attack rather than a simple reaction to fear.

Answer
 Sometimes this happens for no apparent reason, it's really difficult to say exactly what caused it.  He may have thought you were a predator, or was protecting his 'territory'.  If this was an isolated incident and he is acting normal now hopefully it will be the last time something like this happens.  If he shows any further signs of aggression it may simply be his personality changing as a result of genetics.  Sometimes at around 6-8 months male rats can go through a behaviour change, which can sometimes result in them becoming aggressive for no reason.  I would recommend getting him neutered, as this can reduce or even eliminate aggression, as well as reduce his smell and frequency of urine marking.  I hope everything turns out all right, good luck!