the puffer eaten in japan is a saltwater fish. saltwater puffers secrete toxin from various glands that can be removed by well-trained chef. there are trace amounts left after this removal which create a sense of euphoria in the eater, hence the continued popularity of the dish.
however, in freshwater puffers, the skin and musculature of the fish are poisonous. since the poison is fatal in doses of even less than a milliliter... you might as well just regard eating freshwater puffers as attempted suicide.
--EDIT--
also, these poisons cannot be broken down through cooking nor is there any known antidote. so don't eat fugu unless you really trust your chef.