Does dopamine still have a leading role in advanced Parkinson's disease after subthalamic stimulation?

Genetic deletion of vesicular glutamate transporter in dopamine neurons increases vulnerability to MPTP-induced neurotoxicity in mice

A subset of midbrain dopamine (DA) neurons express vesicular glutamate transporter 2 (VgluT2), which facilitates synaptic vesicle loading of glutamate. Recent studies indicate that such expression can modulate DA-dependent reward behaviors, but little is known about functional consequences of DA neuron VgluT2 expression in neurodegenerative diseases like Parkinson's disease (PD). Here, we report that selective deletion of VgluT2 in DA neurons in conditional VgluT2-KO (VgluT2-cKO) mice abolished glutamate release from DA neurons, reduced their expression of brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB), and exacerbated the pathological effects of exposure to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Furthermore, viral rescue of VgluT2 expression in DA neurons of VglutT2-cKO mice restored BDNF/TrkB expression and attenuated MPTP-induced DA neuron loss and locomotor impairment. Together, these findings indicate that VgluT2 expression in DA neurons is neuroprotective. Genetic or environmental factors causing reduced expression or function of VgluT2 in DA neurons may place some individuals at increased risk for DA neuron degeneration. Therefore, maintaining physiological expression and function of VgluT2 in DA neurons may represent a valid molecular target for the development of preventive therapeutic interventions for PD.

Toward feedback controlled deep brain stimulation: dynamics of glutamate release in the subthalamic nucleus in rats

Deep brain stimulation (DBS) is an effective symptomatic treatment in Parkinson's disease. High frequency stimulation (HFS) of the subthalamic nucleus elicits neurotransmitter release in multiple nuclei. Therefore, we tested the hypothesis that neurotransmitter release during HFS may be used to provide feedback control of the intensity and pattern of HFS. We studied the dynamic relationship between extracellular glutamate levels and HFS in and around the STN in anesthetized rats. We used a pseudorandom binary sequence (PRBS) of stimulation in the STN, the independent forcing function, while measuring extracellular glutamate in the same nucleus, the dependent variable. The PRBS consisted of 90 s periods during which stimulation (100 microA, 150Hz, 10% duty cycle) was either off or on. The stimulation and extracellular glutamate levels were fitted using an autoregressive exogenous model (ARX) to determine the transfer function between HFS and the extracellular glutamate concentration in the STN. The ARX model fit the dynamics of extracellular glutamate levels well (correlation coefficients ranged from 0.74 to 0.99; n=11). The transfer function accurately predicted extracellular glutamate levels in the STN even when the pattern of HFS was modified. We used the transfer function to develop a feedback controlled stimulation algorithm. Feedback controlled HFS maintained extracellular glutamate concentrations at any predefined level, but only intermittent HFS was required. We conclude that the transfer function between HFS and neurotransmitter levels in the brain can be used to design DBS protocols that generate specific temporal patterns of glutamate release in the STN.