byetta received approval april, 2005, so is a relatively new drug on the market. It mimics a glucagon hormone in its action for diabetes. It also acts like a neurotropic factor (like GDNF, BDNF)in experiments with both cultured cells and adult mice --in mice, new neuron growth appeared, the # of dopamine producing cells increased in the substantia nigra, and dopamine balance was "normalized" ("increased the number of both tyrosine hydroxylase- and vesicular monoamine transporter 2-positive neurons in the substantia nigra").