Abstract
Visual cortex (VC) over-activation analysed by evoked responses has been demonstrated in congenital deafness and after long-term acquired hearing loss in humans. However, permanent hearing deprivation has not yet been explored in animal models. Thus, the present study aimed to examine functional and molecular changes underlying the visual and auditory cross-modal reaction. For such purpose, we analysed cortical visual evoked potentials (VEPs) and the gene expression (RT-qPCR) of a set of markers for neuronal activation (c-Fos) and activity-dependent homeostatic compensation (Arc/Arg3.1). To determine the state of excitation and inhibition, we performed RT-qPCR and quantitative immunocytochemistry for excitatory (receptor subunits GluA2/3) and inhibitory (GABAA-α1, GABAB-R2, GAD65/67 and parvalbumin-PV) markers. VC over-activation was demonstrated by a significant increase in VEPs wave N1 and by up-regulation of the activity-dependent early genes c-Fos and Arc/Arg3.1 (thus confirming, by RT-qPCR, our previously published immunocytochemical results). GluA2 gene and protein expression were significantly increased in the auditory cortex (AC), particularly in layers 2/3 pyramidal neurons, but inhibitory markers (GAD65/67 and PV-GABA interneurons) were also significantly upregulated in the AC, indicating a concurrent increase in inhibition. Therefore, after permanent hearing loss in the rat, the VC is not only over-activated but also potentially balanced by homeostatic regulation, while excitatory and inhibitory markers remain imbalanced in the AC, most likely resulting from changes in horizontal intermodal regulation.
Highlights
To build the perceptual scene, the mammalian brain has developed neural circuits, specialised in analysing and mixing different sources of sensory information (Teichert and Bolz 2018)
For the whole population (n = 6), a significant increase of 156.39% was shown after comparing mean amplitudes between control and bilateral deaf animal groups (Fig. 3b)
These results are compatible with a long-term increase in visual cortex (VC) activation in deaf animals
Summary
To build the perceptual scene, the mammalian brain has developed neural circuits, specialised in analysing and mixing different sources of sensory information (Teichert and Bolz 2018). This ability requires a dynamic multimodal exchange of information along all stations of the sensory pathways, from the brainstem to the cerebral cortex. After optogenetic stimulation of auditory axons in mice brain slices, recordings from visual cortex (VC) pyramidal neurons and interneurons show increased EPSC responses, demonstrating the combined effect of excitation and inhibition underlying intermodal horizontal interactions (Ibrahim et al 2016). Since receptive fields result from inhibitory GABA interactions, demonstrated by iontophoresis (Firzlaff and Schuller 2001; Tremere et al 2001), sensory conversion may reflect imbalanced multimodal neurotransmission between cortices
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