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-, 2- and 4- weeks 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.

A positive correlation exists between increasing age and the incidence of hippocampal-associated dysfunction and disease. Normal L-glutamate neurotransmission is absolutely critical for hippocampal function, while abnormal glutamate neurotransmission has been implicated in many neurodegenerative diseases. Previous studies, overwhelmingly utilizing ex vivo methods, have filled the literature with contradicting reports about hippocampal glutamate regulation during aging. For our studies, enzyme-based ceramic microelectrode arrays (MEA) were used for rapid (2 Hz) measurements of extracellular glutamate in the hippocampal trisynaptic pathway of young (3-6 months), late-middle aged (18 mo.) and aged (24 mo.) urethane-anesthetized Fischer 344 rats. Compared to young animals, glutamate terminals in cornu ammonis 3 (CA3) showed diminished potassium-evoked glutamate release in aged rats. In late-middle aged animals, terminals in the dentate gyrus (DG) showed increased evoked release compared to young rats. The aged DG demonstrated an increased glutamate clearance capacity, indicating a possible age-related compensation to deal with the increased glutamate release that occurred in late-middle age. To investigate the impact of changes in glutamate regulation on the expression of a disease process, we modified the MEA technology to allow recordings in unanesthetized rats. These studies permitted us to measure glutamate regulation in the hippocampal formation without anesthetic effects, which showed a significant increase in basal glutamatergic tone during aging. Status epilepticus was induced by local application of 4-aminopyridine. Realtime glutamate measurements allowed us to capture never-before-seen spontaneous and highly rhythmic glutamate release events during status epilepticus. A significant correlation between pre-status tonic glutamate and the quantity of status epilepticus-associated convulsions and glutamate release events was determined. Taken together, this body of work identifies the DG and CA3 as the loci of age-associated glutamate dysregulation in the hippocampus, and establishes elevated levels of glutamate as a key factor controlling severity of status epilepticus in aged animals. Based upon the potential ability to discriminate brain areas experiencing seizure (i.e. synchronized spontaneous glutamate release) from areas not, we have initiated the development of a MEA for human use during temporal lobe resection surgery. The final studies presented here document the development and testing of a human microelectrode array prototype in non-human primates.