Regulation of parkinsonian motor behaviors by optogenetic control of Basal Ganglia circuitry.

D eep brain stimulation (DBS) of the subthalamic nucleus (STN) is an FDA approved, widely applied therapeutic option for patients with Parkinson’s Disease (PD) who suffer from on/off motor fluctuations and dopaminergic medication induced dyskinesias. While the efficacy of DBS is well established, the mechanisms responsible for its effects have been widely debated, with little scientific evidence to support one mechanism over another. In PD, selective loss of dopaminergic pars compacta neurons in the substantia nigra results in dopaminergic deficit in the basal ganglia that ultimately leads to aberrant hyperactivity of the output nuclei of the basal ganglia, the STN and the internal segment of the globus pallidus. High frequency stimulation of the STN in PD has been hypothesized to exert its therapeutic effect by one or a combination of several possible mechanisms: 1) Inhibition of the hyperactive STN glutamatergic cells by high frequency stimulation; 2) Indirect modulation of STN neurons via a glial based effect; 3) Modulation of basal ganglionic-cortical circuitry via orthodromic or antidromic stimulation of axonal pathways abutting the STN. Dissecting these mechanisms has been very challenging because of the lack of molecular tools that specifically impact one pathway without co-altering the others. The optogenetic tools developed in Karl Deisseroth’s laboratory at Stanford have allowed unprecedented access to pathologic circuits in disease states. ‘‘Optogenetics’’ is the term Deisseroth coined to describe the approach he pioneered to study how neural circuits drive behavior. Using viral vectors and transgenic mice, light-activated ion channels derived from algae (including Channelrhodopsin-2 [ChR2]) are transduced into specific neuronal subpopulations. Small fiber-optic probes are placed and the appropriate wavelength of light is shined on the neurons, opening the transduced ion channels and depolarizing the neurons. The depolarizations can be used to inhibit or drive neural activity with millisecond precision, depending on the channel type being opened. Recognizing that our knowledge of PD pathogenesis is incomplete, investigators have used optogenetic techniques to tease apart the complex nervous system pathways impacted in a mouse model of PD. Recent work published in 2010 in Nature by Kravitz et al used these techniques to provide evidence that therapeutic efficacy in PD is based on augmentation of direct pathway circuits. A previous paper published in 2009 in Science by Gradinaru and colleagues from the Deisseroth lab at Stanford used optogenetics to show that DBS in PD acts significantly through white matter axonal stimulation of cortical motor-STN pathways. In the more recent study out of the University of California San Francisco, Kravitz and colleagues transduced the ChR2 gene into D1 and D2-expressing subpopulations of medium spiny neurons (MSNs) in the rodent striatum. According to the classical PD model, D1 receptors are excitatory and D2 receptors are inhibitory, and the symptoms of PD arise from understimulation of D1-expressing neurons (the direct pathway) and release of D2-expressing neurons (indirect pathway). Transduction of D1and D2-expressing MSNs with ChR2 provided a unique opportunity to test this mechanistic hypothesis in PD. Expression of ChR2 depolarizes neurons when exposed to certain wavelengths of light, allowing for either inhibition or tonic activation of neural activity. Driving indirect pathway activity with optogenetics reproduced the deficits of PD, resulting in fewer and slower movements and gait freezing. Using optogenetics to drive direct pathway activity completely rescued deficits in the 6-OHDAmouse model of PD. This finding lends support to classical models of basal ganglia function, and it suggests that augmenting direct pathway function is the likely therapeutic mechanism of levodopa, and that supporting its function need not involve dopamine supplementation. Given the large projection of D1expressing MSNs to the globus pallidus pars interna (GPi), augmenting the function of this GABA-ergic population likely dampens GPi activity, which is posited to be a ‘‘final common pathway’’ for surgical approaches to PD. On first glance, the Kravitz study appears to contradict prior optogenetic studies by Gradinaru and colleagues, who elegantly demonstrated that inhibition of primary motor cortex afferents to the subthalamic nucleus, rather than direct inhibition of subthalamic neurons, rescued deficits in 5-OHDA-lesioned mice in a manner similar to DBS. However, this finding does not exclude a model in which dopamine depletion leads to a hyperactive GPi, which interferes with interactions between the primary motor cortex and thalamus. Subthalamic DBS could still rescue function by ablating afferents to the primary motor cortex that carry inhibitory impulses. In this case,