Age-related hearing loss (ARHL, presbycusis), is the most prevalent age-linked neurodegenerative and communications disorders affecting over one-third of the world's population, including about 40 million in the US (Jayakody et al., 2018). ARHL is a complex sensory disorder displaying reduced sensitivity to sound starting at high frequencies, speech perception deficits - particularly in noisy backgrounds, and slowed central processing of acoustic feature information (Gates and Mills, 2005, Wang and Puel, 2020). Extrinsic and intrinsic factors including genetics, aging, drugs, and loud noise contribute to the development of presbycusis (Fetoni et al., 2011, Tavanai and Mohammadkhani, 2017). While there have been advances in the understanding of the etiologies of presbycusis, interventions are still limited to hearing aids and counseling, since there are no FDA-approved drugs to prevent or treat this predominant communication disorder (Frisina et al., 2016).

Three types of changes in the cochlea with age include: sensory, neural, and metabolic presbycusis (Ohlemiller, October 2004, Cheslock and De Jesus, 2022). Sensory presbycusis involves the loss of hair cells; neural presbycusis consists of the decline of the number of spiral ganglion neurons (SGNs); and metabolic presbycusis is the degeneration of the stria vascularis (SV). The latter also causes declines of the endocochlear potential (EP) (Mills and Schmiedt, 2004). Cells in the organ of Corti (OC) and SV are extensively coupled by gap junctions, which are crucial in maintaining ionic balances and hearing function. These gap junctions provide intercellular communication; thus, helping regulate and move ions, electrolytes, secondary messengers, and metabolites between cochlear cells (Kelly et al., 2019, Lefebvre and Van De Water, 2000). Gap junctions are also hypothesized to be involved in regulating the EP via K+ recycling (Kikuchi et al., 2000).

Connexins (Cxs) are gap junction proteins that join to form gap junction ion channels and have been found in various types of cochlear tissue such as the SV, OC, supporting cells and SGNs (Liu and Yang, 2015, Qu et al., 2007, Tang et al., 2006, Verselis, 2019). Mutations in genes encoding Cx26 are responsible for up to 50% of all nonsyndromic sensorineural hearing loss, specifically the 35delG mutation (Erbe et al., 2004, Park et al., 2000). Cx30 and Cx43, which are encoded by gap junction genes beta-6 and alpha-1 (GJB6 and GJA1, respectively) are two of the seven connexin proteins detected thus far in the cochlea whose mutations cause hearing loss or deafness (Martínez et al., 2009, Wu et al., 2019). We hypothesize that connexins are associated with ARHL since mutations in these proteins are known to cause hearing loss and connexin knockout mice display an absence of the EP, increases in reactive oxygen species (ROS), and cell degeneration (Chen and Zhao, 2014, Lin et al., 2013). Moreover, it has previously been reported that Cx26 and Cx30 expression decreased with age in the C57BL/5J mouse cochela (Tajima et al., 2020). Additionally, aging in other areas of the body – including the retina, heart, bladder, and bones – alters connexin expression (Watanabe et al., 2004, Jones et al., 2004, Davis et al., 2018, Mansour et al., 2013, Jones et al., 2004).

Aldosterone, a steroid hormone that helps regulate sodium (Na+) and potassium (K+) ion levels, has been shown to have protective effects against ARHL (Frisina et al., 2016, Ding et al., 2014, Halonen et al., 2016, Tadros et al., 2005, Trune and Kempton, 2001). These pre-clinical studies have shown that aldosterone is effective in slowing down the progression of ARHL, lowering/improving hearing thresholds, and preventing apoptosis in spiral ganglion neurons. Correlations between aldosterone and connexin expression changes have also been observed. (Suzuki et al., 2009) For example, Suzuki et al. (2009) noticed an upregulation of Cx43 mRNA gene expression after rat ventricular myocytes were treated with aldosterone. In light of these previous findings, we tested the hypothesis that aldosterone is beneficial for the aging cochlea in maintaining Cx30 and Cx43 expression and improving hearing thresholds. The present study provides novel insights into cochlear connexin protein expression changes with age and new treatment actions of aldosterone in delaying or preventing key aspects of ARHL.