c‐Fos and Arc/Arg3.1 expression in auditory and visual cortices after hearing loss: Evidence of sensory crossmodal reorganization in adult rats

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Reorganization of sensory cortical areas after deprivation is a key component of plastic mechanisms underlying adaptations to the loss of sensory input. Plastic reorganization of auditory and visual cortices after auditory or visual loss has been extensively studied during postnatal development (Barone, Lacassagne, & Kral, 2013; Bavelier & Neville, 2002; King, 2015; Kral, 2002; Kral et al., 2003; Kral & Eggermont, 2007; Lomber, Meredith, & Kral, 2010; Meredith et al., 2011; Merzenich, 2000). However, cross‐modal reorganization after deafness in adults has been relatively overlooked (Barone et al., 2013; Meredith et al., 2011; Allman, Keniston, & Meredith, 2009; Kok, Chabot, & Lomber, 2014), thereby limiting the understanding of functional recovery from or adaptation to deafness, once the developmental window of intervention is closed.
Sensory deprivation induces changes in the activity of mature cortical neurons (Bavelier & Neville 2002; Allman et al., 2009; Petrus et al., 2014). Also intermodal somatosensory conversion of the receptive field of neurons of the auditory cortex (AC) in the adult ferret has been shown by single‐unit recordings after deafness (Bizley, Nodal, Bajo, Nelken, & King, 2007; Allman et al., 2009; Meredith & Allman, 2015). However, electrophysiological recordings provide limited information on spatial domains. This may be partly overcome by anatomical mapping, specifically using neuronal activity markers. This immunolabeling method is suitable for a more global, spatial analysis of possible changes in neuronal activity patterns among cytoarchitectural areas of the cerebral cortex.
c‐Fos is a powerful neuronal functional marker previously used by us to analyze time‐dependent changes in activity in the inferior colliculus after ablation of the AC (Clarkson, Juíz, & Merchán, 2010). The immediate early gene Arc/Arg3.1 is a key molecular tool for cortical plastic adaptations, which is involved in the turnover of fast excitatory AMPA glutamate receptor subunits, among other functions (Bramham, Worley, Moore, & Guzowski, 2008; Byers et al., 2015; Chowdhury et al., 2006; Derkach et al., 2007; Ju et al., 2004). Thus, after performing an immunocytochemical analysis of the activity‐dependent immediate early genes c‐Fos and Arc/Arg3.1, we suggest that spatiotemporal changes in the expression of their protein products in adult rat cortical neurons predict novel, long‐term cellular mechanisms of cross‐modal adaptation to deafness.

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