1. Introduction
In 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.
Methods
Genetically engineered rabies pseudotype lentivirus
expressing the human IL2-receptor α was used for
selective ablation of thalamacortical tract. By fusing a
pseudotype HIV-I Lentiviral vector with glycoprotein
C 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 -β actin promoter and encodes for
FugG-C (Inoue et al., 2012). Transfer plasmid encodes
for Interleukin-2 receptor -α subunit (Il-2R )α as well
as green fluorescent protein (GFP). The completed
NeuRet-Il2R -α GFP vector is transferred into M1
region for retrograde infection of thethalamocortical
tract neurons.
Acknowledgments
Supported 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 animal’s 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, CA
Investigating the functional significance of corticospinal and thalamocortical
neurons in learning and performance of complex behavioral tasks in the adult
mammalian motor system
C 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 animals
Figure 2: Eighteen rats were trained one
week post injection in distal forelimb reach task to obtain a
sugar pellet by extending the forelimb through panel opening. Performance is quantified by recording
successful attempts; extend forelimb and retract sugar pellet, and unsuccessful attempt; reach but
miss pellet or inability to retract pellet.
Figure 3: A camera records the rat’s stride
and the software is able to produce a multitude
of stride assays. Animals that received the
thalamocortical ablation (green) diverged from the rest of the group in both
forelimb stride contact and intensity. This group showed greater stride contact with diminished stride
intensity. Both control and corticospinal rats showed the opposite; greater stride intensity than stride
contact .
Animals with both CST and TCT ablations
showed a decline in forelimb grip strength
Figure 4: Post lesion rats are habituated to grip
strength meter for 3 minutes prior to session. Each
session consisted of 4 trials to obtain maximum grip
strength for each animal.
Figure 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 selectivelyα
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 thalamus
M1
Frontal
Striatum
GP
SNr
Dentate
Red Nucleus
Reticular
NucleusPontine
Thalamus
Va/Vl
Intermediate lobe of
cerebellum
References
Cruikshank SJ., Urabe H., Nurmikko Av., and Connors BW.(2009) Pathway-Specific Feedforwardcircuits between Thalamus and Neocortex Revealed by
Selective OpticalStimulation of Axons. Neuron 65.2: 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.