Long-range Input Circuits Of The Interneurons In The Mouse Forelimb Cortical Region

The caudal forelimb area(CFA)of the mouse cortex is essential in forelimb movement and motor regulation.The excitatory neurons of CFA can control different muscle groups of the forelimb,while the inhibitory interneurons receive direct input from the cortex and brainstem to maintain the excitatory-inhibitory balance of the cortex.According to the molecular markers,interneurons in the forelimb motor cortex can be classified into three main groups:parvalbumin-positive(PV~+)neuron,somatostatin-positive(SOM~+)neuron,and vasoactive intestinal peptide-positive(VIP~+)neuron.Previous research has elucidated the logic of local connections of distinct types of interneurons in the cortex.However,their long-range inputs are still unclear.In this study,retrograde rabies virus tracer and fluorescence microscopic imaging techniques were applied to analyze the long-range inputs of interneurons in forelimb motor cortex and explore the degenerative changes of the thalamic-cortical circuits in Parkinson’s disease.The main results were as follows:(1)Recombinant rabies virus was used to label the long-range inputs of interneurons in CFA motor/sensory region.The distribution of these input neurons in the cortex and thalamus was different.Quantitative analysis showed that the inputs of CFA motor region were mainly from the frontal/motor/sensory cortex,anterior lateral thalamus and intralaminar thalamic nucleus;while the inputs of CFA sensory region were mainly from the secondary sensory cortex,lateral and posterior thalamic nucleus.These results indicated that interneurons in different sub-regions of forelimb motor cortex had different input patterns in the whole brain,and the spatial distribution characteristics of thalamic input indicated that the thalamic-cortical neurons in forelimb motor circuits could be divided into two groups according to the projection-preference.(2)Comparing the long-range inputs of PV~+,SOM~+and VIP~+neurons in the forelimb motor cortex,the inputs of different types of interneurons were found to come from the same brain regions.There was no significant difference in the proportion of inputs in the most brain regions(56/62),and the whole brain input patterns of different types of interneurons were highly similar(r>0.9).Cluster analysis and covariance analysis were used to further classify the input patterns:the PV~+neurons received more orbitofrontal cortex input,the VIP~+neurons receive more ventrolateral thalamic input,and the SOM~+neurons received more inputs from the globus pallidus.These results indicated that the PV~+,SOM~+and VIP~+neurons in the CFA had similar whole-brain input patterns.However,different types of interneurons received inputs from different combinations of regions.(3)Parkinson’s disease mouse model with depletion of dopamine and motor ability injury was established by injecting dopamine toxin.Combining monosynaptic rabies virus tracing system,the whole brain input pattern of SOM~+neurons was similar in healthy mice and Parkinson’s disease mice,but the proportion of input from ventral posterolateral thalamus was reduced in Parkinson’s disease mice and the response of thalamic neurons for movement was also reduced.These results indicated that the depletion of dopamine in mice could affect the thalamic input of SOM~+interneurons in forelimb cortex,which means the levels of dopamine in the substantia nigra and striatum may relate to the loop connection between thalamus and cortical interneurons.In summary,the monosynaptic rabies virus tracing system and fluorescence imaging technique were applied to map the long-term input connections of three types of interneurons in different sub-regions of mouse forelimb cortex.These results presented an anatomical basis for the abnormal activity of cortical SOM~+neurons in Parkinson’s disease,and provided important guides to further understanding cortical intricate circuits and inhibitory signal regulation in forelimb motor function.