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IntroductionIn this study, the primary goal is to evaluate how distinct populations of neurons in the thalamus and primary motor cortex contribute to learning and performance of skilled motor tasks in rodents. Thalamocortical inputs to the motor cortex are postulated to relay information from a variety of nuclei that mediate and coordinate motor performance, including the basal ganglia and cerebellum. Prior studies attempting to define the function of these neuronal systems in skilled motor behavior have involved ablation strategies that resulted in damage to non-target cell populations, thus confounding the interpretation of experimental findings. The goal of the present study is to use a novel viral approach to selectively eliminate distinct populations of neurons implicated in motor function and examine subsequent changes in motor performance. MethodsGenetically engineered rabies pseudotype lentivirusexpressing the human IL2-receptor a was used for selective ablation of thalamacortical tract. By fusing apseudotype HIV-I Lentiviral vector with glycoproteinC type (Fug-C) we are able to produce the Neuret vector. The Lentivirus genome consists of envelope, transfer, and packaging plasmid. Envelope plasmid is controlled by cytomeg-alovirus enhancer/ chicken -actin promoter and encodes forFugG-C (Inoue et al., 2012). Transfer plasmid encodesfor Interleukin-2 receptor -subunit (Il-2R) as well as green fluorescent protein (GFP). The completed NeuRet-Il2R -GFP vector is transferred into M1 region for retrograde infection of thethalamocortical tract neurons.

AcknowledgmentsSupported by the NIH in part by a MARC U-STAR Award (T34GM087193), the Veterans Administration, and Academic enrichment program. Conclusions Thalamacortical tract neurons dispersed within the Va/Vl nuclei necessitated adequate performance of complex motor task. Preceding disruption of TCT reveals an inability to initiate learnt motor behavior.

Apart from appropriate motor task initiation, inadequacies in the animals stride and forelimb strength were observed indicative of motor control deficits.

The basal ganglia and cerebellum are two significant brain regions fundamental for learning and motor performance whose input relies on TCT neurons for proper execution- the lack there of indicated by a decline in motor performance.

In an attempt to back tract through the complex circuitry involved in distal forelimb reach, succeeding basalthalamic and dentatethalimc ablations will be satisfied.

Department of Neurosciences, UC San Diego; Veterans Affairs Med Ctr, San Diego, CAInvestigating the functional significance of corticospinal and thalamocortical neurons in learning and performance of complex behavioral tasks in the adult mammalian motor systemC Hissom, JM Conner, MH Tuszynski

Pre and post ablation forelimb reach test exemplifies functional significance of TCT akin to motor performance Stride contact and stride intensity deviate from both control and CST for TCT lesion animalsFigure 1: Rats are injected into primary motor cortex (M1) with Neuret viral vector, which retrogradely infects neurons projecting from thalamus to M1. Succeeding a one week period, the cells are infected and expressing Il2R receptor that will selectively bind the selected immunotoxin. A second injection delivers the immunotoxin into the ventral anterior/ventral thalamic nuclei of thalamus insuring a selective thalamocotical track ablation. Figure 5: Primary motor cortex (M1) transmits several electrical impulses intracranially by way of the Basal ganglia and Cerebellum that are pivotal for motor execution and sensory processing. These signals coalesce onto the above mentioned thalamic nuclei. Both the Basal-ganglia thalamocortical circuit (back arrows) and corticopontine cerebellar loop (blue arrows) terminate on Va/Vl nuclei of thalamus (Cruikshank et al 2010). Basal ganglia facilitates initiation of motor commands by disinhibition of thalamic neurons and cerebellum coordinates and governs appropriate motor performance. Basal-ganglia thalamocortical circuit and corticopontine cerebellar loop impart motor command information by way of the Va/Vl nuclei of thalamusReferencesCruikshank SJ., Urabe H., Nurmikko Av., and Connors BW.(2009) Pathway-Specific Feedforwardcircuits between Thalamus and Neocortex Revealed by Selective OpticalStimulation of Axons.Neuron65.2: 230-45.

Inoue KI., Koketsu D., Kato S., Kobayashi K., Nambu A., Takada M. (2012) Immunotoxin mediated tract targeting in the primate brain: selective elimination of the cortico subthalamic hyperdirect pathway. PLoS ONE 7.6: e39149.