Nigrostriatal Dopamine-neuron Function from Neurotrophic-like Peptide Treatment and Neurotrophic Factor Depletion

OF DISSERTATION NIGROSTRIATAL DOPAMINE-NEURON FUNCTION FROM NEUROTROPHIC-LIKE PEPTIDE TREATMENT AND NEUROTROPHIC FACTOR DEPLETION Trophic factors have shown great promise in their potential to treat neurological disease. In particular, glial cell line-derived neurotrophic factor (GDNF) has been identified as a potent neurotrophic factor for midbrain dopamine (DA) neurons in the substantia nigra (SN), which lose function in Parkinson’s disease (PD). GDNF progressed to phase II clinical trials, which did not meet proposed endpoints. The large size and binding characteristics of GDNF have been suspected to contribute to some of the shortcomings of GDNF related to delivery to target brain regions. Smaller peptides derived from GDNF (Dopamine-Neuron Stimulating Peptides – DNSPs) have been recently investigated and appear to demonstrate trophic-like effects comparable to GDNF. In the described studies, a time course study was conducted to determine in vivo DA-release characteristics 1-, 2and 4weeks after peptide treatment. These studies determined the effects on DA terminals within striatal sub-regions using microelectrodes. A heterogeneous effect on striatal sub-regions was apparent with the maximum effect in the dorsal striatum – corresponding to terminals originating from the SN. Dysregulation of GDNF or GDNF signaling is believed to contribute to motor dysfunction in aging and PD. Thus, it is hypothesized that GDNF is necessary for the maintenance and function of neurons. To extend this line of investigation, in vivo functional measures (DA-release and -uptake) and behavioral and cellular alterations were investigated in a transgenic mouse model (Gdnf) with reduced GDNF protein levels. The described studies determined that both DA-uptake and -release properties were altered in middle-aged Gdnf mice with only modest reductions in DA neurochemical levels. GDNF levels in Gdnf mice were restored to levels comparable to wild-type (WT) counterparts by treatment with GDNF. GDNF protein supplementation led to enhanced motor behavior and increased markers for DA neurons in the SN of Gdnf mice. Gdnf mice appeared to show a heightened sensitivity to GDNF treatment compared to WT counterparts. Overall, this body of work examines novel synthetic peptides with potential to enhance DA-neuron function and expands upon the current understanding of GDNF’s role in the nigrostriatal pathway.