Reduced Thalamic Excitation to Motor Cortical Pyramidal Tract Neurons in a Mouse Model of Parkinsonism

Degeneration of midbrain dopaminergic (DA) neurons causes a reduced motor output from the primary motor cortex (M1), underlying the motor symptoms of Parkinson’s disease (PD). However, cellular and circuitry mechanisms of M1 dysfunction in PD remain undefined. Using multidisciplinary approaches, we found that DA degeneration induces cell-subtype- and inputs-specific reduction of thalamic excitation to M1 pyramidal tract (PT) neurons. Physiological and anatomical analyses suggest that DA degeneration induces a loss of thalamocortical synapses to M1 PT neurons, resulting in an impaired thalamic driving of their activities. Moreover, we showed that the decreased thalamocortical connectivity are mediated by an excessive activation of NMDA receptors of M1 PT neurons. Further, the decreased thalamocortical transmission in parkinsonism can be rescued by chemogenetically suppressing basal ganglia outputs. Together, our data suggest that the reduced motor cortical outputs in parkinsonism are not only an immediate consequence of basal ganglia inhibition but also involves specific local circuitry adaptations within M1. This study reveals novel insight in the pathophysiology of parkinsonian motor deficits.