I adopted a wonderful senior cat four years ago. He was friendly,social and was an active member of the family. He would jump up on high places and was a sweet boy. About a year ago he suddenly became withdrawn and scared of everything including his human family. All he does now is to stay inside his carrier all the time. I have spent big bucks at the university animal hospital and all of his tests are normal. They can find absolutely nothing physically wrong with him. Any advice would be greatly appreciated.