Previous studies suggested that aminoglycoside antibiotics induced ototoxicity involves excitotoxic activation of cochlear NMDA receptors. Our results suggest that memantine, when co-administered with amikacin, may reduce the cochleotoxic effects caused by amikacin. A smaller but not negligible reduction in efficacy is observed. The same functional results were observed using DPOAE. These results provide compelling evidence that activation of NMDA receptors on hair cells or their afferent synapses is a necessary step in aminoglycoside-induced hearing loss (Fetoni et al. 2012).
NMDA receptors are located at each synapse at the bottom of the mammalian auditory pathway. Their developmental regulation and unique composition of inner ear subunits promote protective and neurotrophic roles after acute injury by regulating AMPA-R expression and helping restore synaptic inputs. This is in contrast to chronic injury associated with overactivation of NMDA-R. with nerve death. In our case, they reside at synaptic sites between cochlear hair cells and radical dendrites of spiral ganglion afferents. Aminoglycosides can mimic the effects of polyamines on NMDA receptors (Bonnington and McNaughton, 2003). The association between aminoglycosides and polyamines may explain the glutamate-like excitotoxicity induced by aminoglycosides. Overstimulation of NMDA receptors (NMDARs) increases nitric oxide (NO) formation, causing oxidative stress in hair cells. In addition, several studies have shown that gentamicin treatment can increase the expression of nNOS and iNOS and induce hair cell damage (Bonnington and McNaughton, 2003; Ramma et al. 2019).
There is evidence that aminoglycoside-induced cochleotoxicity results from excitotoxic processes mediated by the polyamine-like effects of aminoglycosides at cochlear NMDA receptors (Goforth et al., 2011). As the polyamine agonist properties of aminoglycosides should induce neuronal excitotoxicity, the present study tests this hypothesis within his CNS. Here we report that intraperitoneal injection of amikacin results in significant damage to cochlear structures (Hasegawa et al.2007; Homayoun and Moghaddam, 2007). Neuronal damage caused by various injuries, including excitotoxicity, is associated with reactive astrocyte and microglia proliferation and macrophage infiltration at the site of injury (Clause et al. 2014; Felix and Ehrenberger,1990; Ikonomidou et al,1999; Jackson et al,2004).
Memantine, a drug recently approved for treating moderate-to-severe Alzheimer's disease [28, 29], reduces excitability between hair cells and afferent fibers of the auditory nerve by blocking NMDA receptors. It is thought to inhibit neurotransmission (Olivares et al. 2012; Ralli et al. 2014). It likely also affects the central auditory pathway (Oestreicher et al,1998). Memantine has been previously studied for its ability to suppress tinnitus in animals ([31, 32] and humans and has been found to have little or no effect (Marcotti et al., 2005; Tikhonravov et al.2010). Dose-related effects of memantine on a mismatch negativity-like response in anesthetized rats. [Lobarinas et al. 2006; Xie et al. 2011). Using an aminoglycoside toxicity model to damage hair cells and spiral ganglion neurons, we show that memantine protects cochlear morphology and physiology. Previous studies have suggested that NMDARs may also contribute to signaling in the mature cochlea (Zheng et al. 2012). Immunostaining for the GluN1 subunit is robust around the base of the IHC in adult rats (Zheng et al. 2012; Zhang et al. 2017; Piva et al. 2021). Basel et al. proposed that aminoglycosides cause a polyamine-like enhancement of glutamate-NMDA receptors, leading to excitotoxicity and eventual hair cell death (Basile et al, 1996). These authors showed that NMDA antagonists could prevent hair cell loss and hyperacusis. However, they did not analyze neurons in the spiral ganglion, a key relay station between peripheral auditory organs and the central auditory system. This is particularly important because NMDA receptors are predominantly expressed by spiral ganglion neurons rather than hair cells (Bienkowski et al. 2000).
The results in this report demonstrate that NMDA antagonists attenuate damage to spiral ganglion neurons, thereby minimizing hair cell loss and positively affecting functional responses. Although some features of damage caused by noise and aminoglycosides differ, our results suggest that both involve common mechanisms. It is currently unclear whether the protective properties of NMDA antagonists act directly on hair cells or indirectly through neurons in spiral ganglia.
The ability of neurotrophic factors to block spiral ganglion neurons from aminoglycoside toxicity has been demonstrated (Martin et al. 1998). Neurotrophic factors have been shown to effectively regulate intracellular calcium levels and reduce oxidative damage (Domarecka et al. 2000). These observations suggest that oxidative stress may be a common factor in multiple auditory pathologies, and additional chemicals aimed at reducing the activation of NMDA or its downstream effectors have also been previously reported. It reinforces the idea that it is beneficial in protecting the peripheral auditory system from various pathologies, as suggested (Schacht, 1999; Fritzsch et al. 1997).
Aminoglycoside antibiotics are widely used in clinical practice and have long been known to cause both cochlear and vestibular ototoxicity. The loss of outer hair cells of the organ of Corti at the base of the cochlea is the first evidence of aminoglycoside ototoxicity (Zhang et al. 2017; Piva et al. 2021).
The results of this study were limited by the frequency spectrum of the commercially available His DPOAE instrument designed for human use. A major strength of the present study is the combined use of DPOAE with microscopy and immunochemistry. Future studies should focus on further (long-term) reversibility of cochlear morphological changes after administering NMDA antagonists. This can also be extended by looking at issues such as dosing duration and schedule (more than once a day).
The results of this study were limited by the frequency spectrum of the commercially available His DPOAE instrument designed for human use. These results support further studies on dose-dependent changes in DPOAE in animals treated with amikacin and memantine, the combined pharmacokinetics of these two drugs to investigate the mechanisms of enhanced ototoxicity, and complementary electron microscopy. It should be complemented by research.