Patterns of Unilateral and Bilateral Projections From Layers 5 and 6 of the Auditory Cortex to the Inferior Colliculus in Mouse

Abstract

The auditory cortex sends massive projections to the inferior colliculus, but the organization of this pathway is not yet well understood. Previous work has shown that the corticocollicular projection emanates from both layers 5 and 6 of the auditory cortex and that neurons in these layers have different morphological and physiological properties. It is not yet known in the mouse if both layer 5 and layer 6 project bilaterally, nor is it known if the projection patterns differ based on projection location. Using targeted injections of Fluorogold into either the lateral cortex or dorsal cortex of the inferior colliculus, we quantified retrogradely labeled neurons in both the left and right lemniscal regions of the auditory cortex, as delineated using parvalbumin immunostaining. After dorsal cortex injections, we observed that approximately 18–20% of labeled cells were in layer 6 and that this proportion was similar bilaterally. After lateral cortex injections, only ipsilateral cells were observed in the auditory cortex, and they were found in both layer 5 and layer 6. The ratio of layer 5:layer 6 cells after lateral cortex injection was similar to that seen after dorsal cortex injection. Finally, injections of different tracers were made into the two inferior colliculi, and an average of 15–17% of cells in the auditory cortex were double-labeled, and these proportions were similar in layers 5 and 6. These data suggest that (1) only the dorsal cortex of the inferior colliculus receives bilateral projections from the auditory cortex, (2) both the dorsal and lateral cortex of the inferior colliculus receive similar layer 5 and layer 6 auditory cortical input, and (3) a subpopulation of individual neurons in both layers 5 and 6 branch to innervate both dorsal cortices of the inferior colliculus.

Age-related changes in the primary auditory cortex of newborn, adults and aging bottlenose dolphins (Tursiops truncatus) are located in the upper cortical layers

“…Cell surface density seemed appreciably lower in this layer, as well as in layer VI, which could be due to the ascending myelinated fiber bundles reaching for higher cortical target layers, or alternatively to efferent axons leaving A1. Here we also emphasize the potential role of layers V and VI in the descending projections to the inferior colliculus, a pathway yet to be mapped and clarified in dolphins (Vaithiyalingam Chandra Sekaran et al, 2021). Cells of layer VI were arranged rather sparsely with multiform, middle-sized neurons.…”

Developmental exposure to polychlorinated biphenyls prevents recovery from noise-induced hearing loss and disrupts the functional organization of the inferior colliculus

“…For instance, the presence of tonotopic disruption at the level of the midbrain suggests that disruption in the AC may be inherited from lower centers. Alternatively, given the massive feedback to the DCIC from the AC (Asilador and Llano, 2020;Vaithiyalingam Chandra Sekaran et al, 2021;Yudintsev et al, 2021), the tonotopic disruption observed in the current study could be related to disrupted top-down corticocollicular inputs. Future work will be needed to sort out these possibilities.…”

“…Therefore, the IC is a good candidate to examine if developmental exposure to PCBs could exacerbate the long-term negative central effects of noise exposure later in adulthood. The IC comprises a major lemniscal division, the central nucleus (ICC), that receives primarily ascending auditory projections (Willard FH and DK, 1983;JF, 2001;DL, 2005), and two non-lemniscal divisions known as the dorsal (DCIC) and lateral (LCIC) cortices that receive massive descending auditory as well as multisensory inputs (Coleman and Clerici, 1987;Saldaña et al, 1996;Winer et al, 1998;Bajo and Moore, 2005;Schreiner and Winer, 2005;Loftus et al, 2008;Lesicko et al, 2016;Vaithiyalingam Chandra Sekaran et al, 2021). To examine the differential impact of PCBs on excitatory vs. inhibitory neurons in the IC, and because the IC is superficially accessible in mice (Fig.…”

“…The virus drives the expression of the jRGECO1a in non-GABAergic and GABAergic cells via synapsin ( Syn ) promoter. The detailed surgical procedures were described before (Vaithiyalingam Chandra Sekaran et al, 2021). In brief, the surgery was done under aseptic techniques.…”

Microprism-based two-photon imaging of the mouse inferior colliculus reveals novel organizational principles of the auditory midbrain

“…The brain was then fixed by a subsequent perfusion of 4% PFA. Selected 40 μm coronal sections at the level of the IC were immunostained for NeuN using (NeuN (D4G40) XP® Rabbit mAB, Cell Signaling, Cat# 62994, 1:100) primary antibody and (goat anti-rabbit IgG conjugated to Alexa 405, 1:200) secondary antibody following the immunostaining procedures that were previously described (Issa, Sekaran, & Llano, 2023; Vaithiyalingam Chandra Sekaran et al, 2021). Confocal images were obtained using a Confocal Zeiss LSM 710 Microscope using 10 and 40x objectives and tile scan functions.…”