Two-fold elevation of endogenous GDNF levels in mice improves motor coordination without causing side-effects.

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of dopaminergic neurons in vitro and in vivo. For this reason, GDNF is currently in clinical trials for the treatment of Parkinson's disease (PD). However, how endogenous GDNF influences dopamine system function and animal behavior is not fully understood. We recently generated GDNF hypermorphic mice that express increased levels of endogenous GDNF from the native locus, resulting in augmented function of the nigrostriatal dopamine system. Specifically, Gdnf  mice have a mild increase in striatal and midbrain dopamine levels, increased dopamine transporter activity, and 15% increased numbers of midbrain dopamine neurons and striatal dopaminergic varicosities. Since changes in the dopamine system are implicated in several neuropsychiatric diseases, including schizophrenia, attention deficit hyperactivity disorder (ADHD) and depression, and ectopic GDNF delivery associates with side-effects in PD models and clinical trials, we further investigated Gdnf  mice using 20 behavioral tests. Despite increased dopamine levels, dopamine release and dopamine transporter activity, there were no differences in psychiatric disease related phenotypes. However, compared to controls, male Gdnf  mice performed better in tests measuring motor function. Therefore, a modest elevation of endogenous GDNF levels improves motor function but does not induce adverse behavioral outcomes.