However, if the sound is less salient, prediction from top-down inputs becomes important ( Kommajosyula et al., 2019). In the auditory information processing, such as in language interpretation, high timing resolution is important ( Dagnino-Subiabre et al., 2009), and bottom-up inputs from the inferior colliculus (IC) preserve temporal information of sound. Thus, the MGB relays neural information of the sound to the auditory cortex (AC) and segregates speech signals into different patterns for comprehension of the conveyed messages, interpretation of emotions associated with speech, and identification of the individual speaking ( Dagnino-Subiabre et al., 2009 Bartlett, 2013 Chen et al., 2019 Kommajosyula et al., 2019 Mihai et al., 2019). The thalamus receives sensory information from the lower brainstem centers and modulates and sends this information to the cortex ( Sherman and Guillery, 2002).
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The medial geniculate body (MGB) is the predominant auditory sector in the thalamus. Our findings imply that time course of synaptic current and spike waveforms elicited by IC inputs is modified in the Kv4.2 domains. In summary, our study demonstrated that the Kv4.2-immunopositive domains of the MGV dendrites received excitatory and inhibitory ascending auditory inputs preferentially from the IC, and not from the RTN or cortex. In inhibitory axons either from the IC or from the RTN, the frequency of terminals that were in contact with Kv4.2-positive puncta was higher in IC than in RTN. Furthermore, EM showed that the terminals forming asymmetric synapses with Kv4.2-immunopositive MGV dendritic domains were significantly larger than those forming synapses with Kv4.2-negative MGV dendritic domains. VGluT2-immunopositive terminals were significantly larger than VGluT1-immunopositive terminals.
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The frequency of contact with Kv4.2-immunopositive puncta was higher in vesicular glutamate transporter 2 (VGluT2)-positive excitatory axon terminals, which are supposed to be extending from the IC, than in VGluT1-immunopositive terminals, which are expected to be originating from the AC. The postsynaptic distribution of Kv4.2 protein was confirmed using electron microscopy (EM). At the protein level, Kv4.2-immunopositive patches were sparsely distributed in both the dendrites and the soma of neurons. We found that Kv4.2 mRNA was expressed in most MGV neurons. Here, we aimed to examine the detailed distribution of Kv4.2, in MGV neurons to understand its specific role in auditory attention. Since potassium channel is important for shaping synaptic current and spike waveforms, subcellular distribution of Kv4.2 is likely important for integration of various inputs. Kv4.2 is one of the isoforms of the Shal-related subfamily of potassium voltage-gated channels that are expressed in MGB. However, detailed mechanisms of the integration of different inputs in a single MGV neuron remain unclear. MGV is involved in auditory attention by processing descending excitatory and inhibitory inputs from the auditory cortex (AC) and reticular thalamic nucleus (RTN), respectively. The ventral division of MGB (MGV) receives excitatory and inhibitory inputs from the inferior colliculus (IC). The medial geniculate body (MGB) is the thalamic center of the auditory lemniscal pathway. 8Department of Systems Function and Morphology, Graduate School of Innovative Life Science, University of Toyama, Toyama, Japan.7Department of Anatomy, Kanazawa Medical University, Uchinada, Japan.6Research and Education Program for Life Science, University of Fukui, Fukui, Japan.
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