Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Abstract

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11–P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1+ and VGluT2+ transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT+ transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, peri-somatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to—and to some extent may enable— the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Laminar specific gene expression reveals differences in postnatal laminar maturation in mouse auditory, visual, and somatosensory cortex

“…At later ages, we observed two Cux1 + horizontal bands with differing intensities with higher intensity at the bottom band in layers 2–4. To test if and how the high intensity band overlaps with thalamocortical inputs during development, we colabeled with vGlut2, which labels lemniscal thalamocortical axon terminals (Hackett, Clause, Takahata, Hackett, & Polley, ).…”

A Corticothalamic Circuit for Dynamic Switching between Feature Detection and Discrimination

“…5A). The auditory sub-region of TRN is composed of GABAergic neurons that project to MGBv, whereas the mouse MGBv contains no GABAergic interneurons, but rather only glutamatergic neurons that project both to TRN and to the middle layers of A1 (Hackett et al, 2015; Ito et al, 2011; Jones, 2007) (Fig. 5B).…”

Presynaptic Neuronal Nicotinic Receptors Differentially Shape Select Inputs to Auditory Thalamus and Are Negatively Impacted by Aging

“…We took advantage of known differential expression of VGluTs in the auditory pathway to better address our prediction that nAChRs exist on descending excitatory terminals. Although VGluT1 is dominant in corticothalamic projections, tectothalamic terminals have been shown to express VGluT2 (Barroso-Chinea et al, 2007;Ito and Oliver, 2010;Hackett et al, 2011Hackett et al, , 2016. Figure 4 shows colocalization of the nAChR ␤2 subunit with VGluT1 labeling of presumptive corticothalamic terminals in the area of a distal dendrite of the ventral MGB (vMGB) neuron studied.…”

“…One may compensate for decline in ascending auditory signal quality through increased use of cognitive resources by focusing their attention on important acoustic stimuli (Bertoli et al, 2001, Alain et al, 2004Bidelman et al, 2014). Unfortunately, even after adjusting for age-related temporal deficits, older adults are less able to compensate for degraded auditory input by using attentional strategies to help them encode the stimulus (Harris et al, 2012). The overall deficit in hearing seen in presbycusis is then explained best, not only by a loss of ascending auditory signal fidelity, but also by loss of higher-order cognitive abilities, including attention (Wingfield et al, 2005).…”