For PSD-95, variance analysis revealed a significant group effect on PSD-95 expression (F (4,10) = 15.82, p < 0.001, partial η2 = 0.86). Compared with control, PSD-95 levels were reduced significantly in the CA-21 (p = 0.040), CA-28 (p = 0.013), and CA-60 (p < 0.001) groups, while CA-14 showed no difference (p = 0.896). Post-hoc testing indicated that CA-60 animals exhibited lower PSD-95 expression than CA-14 (p < 0.001) and CA-21 (p = 0.027), whereas the contrast between CA-60 and CA-28 did not reach significance (p = 0.082; Fig. 4A and D).
The current study showed that auditory sensory deprivation induced by cochlear ablation impaired spatial learning and memory, as evaluated by the MWM test. The severity of the impairment varied depending on the developmental stage at which deprivation occurred. Specifically, rats that underwent cochlear ablation at PND 28 and PND 60 (CA-28, CA-60 groups) exhibited more pronounced deficits in spatial learning and memory compared with those ablated at PND 14 and PND 21 (CA-14, CA-21 groups). These findings suggest that the impact of auditory deprivation on spatial learning and memory is age-dependent, with greater impairments observed when HL occurs at later developmental stages.Previous studies have applied several methods to establish auditory sensory deprivation in vivo, including prolonged noise exposure, administration of ototoxic agents (e.g., aminoglycosides and cisplatin), and surgical cochlear ablation. Noise exposure can model progressive, frequency-specific hearing loss; however, the extent of cochlear damage varies depending on exposure parameters and species-specific susceptibility, and may partially recover.Ototoxic pharmaceuticals have the capacity to target selectively the cochlear hair cells and the spiral ganglion neurons; nevertheless, the efficacy of these drugs is influenced by systemic metabolism, the permeability of the blood-labyrinth barrier, and potential off-target toxicity . In contrast, cochlear ablation produces an immediate, complete, and irreversible cessation of peripheral auditory input. This enables precise control over the timing of deprivation and reduces variability between subjects. In the current study, bilateral cochlear ablation was chosen to ensure definitive elimination of peripheral auditory activity on a pre-determined experimental day. ABR confirmed complete auditory deprivation after surgery. To eliminate the possibility of vestibular dysfunction influencing spatial learning and memory performance, we evaluated vestibular function using a semi-quantitative behavioral test battery comprising postural asymmetry scoring, head roll tilt observation, and the tail elevation reflex test-a validated, objective method for detecting vestibular deficits in rodents. This methodological choice enabled us to isolate the effects of loss of auditory input from other sensory or motor influences, thereby strengthening the interpretability of our findings.
Recent human neuroimaging studies have demonstrated that HL is associated with structural brain changes, particularly in the hippocampus. Individuals with HL tend to exhibit reduced hippocampal volumes, and more severe HL has been linked to greater degrees of hippocampal atrophy. These findings suggest that hearing deprivation may affect hippocampal structure as well as function, providing further foundation for interpreting behavioral results in animal models of HL. Given the known anatomical and functional connections between the auditory cortex and the hippocampus, cortical changes may contribute to the observed hippocampal deficits. While our study did not directly assess the auditory cortex, previous research has shown its connection to the hippocampus
Guo et al. studied the link between HL and cognitive function in mice. They found that mice with HL had impaired social memory but no significant differences in other cognitive tests. This supports our research indicating minimal effects on spatial learning and memory in adulthood when auditory input is removed early in life. Another study by Qian et al. provides evidence of cochlear hair cell ablation effects on spatial learning and memory. Qian et al. investigated sensorineural hearing loss (SNHL) induced by diphtheria toxin injection at postnatal day 30 (PND30) or 60 (PND60) and assessed spatial learning and memory using a radial 8-arm maze at PND90-120. Their results showed that mice with SNHL exhibited more working memory errors and reference memory errors than wild-type controls, confirming impaired cognitive function. The severity of impairment persisted beyond two months post-auditory insult, with older-onset hearing loss (P60) associated with more pronounced spatial memory deficits compared to younger-onset SNHL (PND30), suggesting the age at which HL occurs influences the severity of central deficits.
The development and organization of the auditory cortex depend heavily on stimulus-driven learning and have limited developmental windows, referred to as "sensitive periods," during which the capacity for plasticity is maximal. In rodent models, these windows coincide with early postnatal stages, such as PND14, when auditory pathways and auditory-hippocampal connections are still maturing and synaptic plasticity is high. At this stage reduction or elimination of auditory input can be partially compensated for by other sensory modalities, such as visual or olfactory input. with aging (e.g., from PND28 to PND60), these pathways become stabilized. Sudden deprivation of auditory input leads to reduced sensory stimulation of the hippocampus, disruption of place cell activity, and reduced synaptic plasticity. Behavioral evidence suggests that auditory deprivation at a younger age is associated with adaptive reorganization and partial recovery of function. By contrast, deprivation at a later age leads to permanent impairments in functions. These findings indicate that the closure of the sensitive period is accompanied by reduced flexibility and limited compensatory capacity. The results demonstrated that age plays a crucial role in the effects of auditory input deprivation. Deprivation at an older age (PND60) exerts a more pronounced impairment of spatial learning and memory than deprivation at a younger age (PND14). This difference can be attributable to the greater plasticity present at earlier developmental stages, which may facilitate compensatory mechanisms for reduced auditory sensory input. Synaptic plasticity is a multifaceted process involving numerous molecular and functional components. In the present study, we focused on synaptophysin, PSD-95, and BDNF because of their well-established and complementary roles in hippocampal synaptic architecture. Synaptophysin is a presynaptic vesicle membrane protein essential for neurotransmitter release and synaptic vesicle cycling, serving as a reliable marker of presynaptic integrity. PSD-95 (postsynaptic density protein 95) is a major scaffolding protein that anchors NMDA and AMPA receptors at excitatory synapses, thereby stabilizing postsynaptic signaling complexes and regulating synaptic strength. BDNF (brain-derived neurotrophic factor) is a neurotrophin critical for neuronal survival, dendritic growth, and activity-dependent synaptic modulation, and is a key mediator of experience-dependent plasticity. By assessing these markers, we captured both pre- and postsynaptic elements as well as neurotrophin-mediated signaling, providing a mechanistic framework for interpreting the observed age-dependent behavioral deficits. In a previous experimental model, temporary conductive hearing loss (TCHL) induced at PND 14 and assessed at PND 42 was associated with pronounced deficits in hippocampal long-term potentiation and attenuated NMDA receptor-mediated synaptic transmission.The observed discrepancies between their findings and those presented in this study can be due to the nature and extent of HL as in the present study, ablation of the cochlea on PND 14 resulted in complete and irreversible sensorineural hearing loss, causing immediate and complete elimination of auditory input. In contrast, the study by Zhao et al. performed tympanic membrane perforation, which causes partial conductive hearing loss. Another possible explanation is that incomplete hearing loss may result in distorted auditory input, which can be more damaging to neural development than complete silence.Thus, the brain may struggle to interpret the distorted signals, resulting in maladaptive plasticity. Distorted auditory input during development can lead to maladaptive neural reorganization, even more than complete deprivation. Duration of auditory deprivation may also have been influential; previous research has shown that auditory deprivation, can induce molecular and cellular changes in the hippocampus within 7 to 14 days. These changes include altered expression of genes associated with neuroplasticity (e.g., BDNF, Arc), changes in synaptic receptor composition (AMPA/NMDA ratios), and increased microglial activation and oxidative stress. However, structural remodeling in the hippocampus -- characterized by changes in dendritic architecture, volume reduction, and persistent impairments in neurogenesis -- usually appears over a period of one to three months. In the study performed by Zhao et al., TCHL effects were assessed at PND 42 following 28 days of auditory deprivation. In contrast, in the present study, outcomes were evaluated at four months of age, after a more extended deprivation period of 2 to 3.5 months (depending on the experimental groups), indicating a much longer period of deprivation. Therefore, in our study, the extended duration of auditory deprivation likely provided a broader window for structural remodeling and compensatory neuroplasticity.
Beckmann et al. investigated the effects of gradual sensory hearing loss on synaptic plasticity in the hippocampus. They used the C57BL/6 mice model, which exhibits progressive, hereditary hearing loss beginning in early adulthood, to investigate how cumulative deafness affects the function of the hippocampus. Immunohistochemistry and electrophysiology evaluations were conducted to determine alterations in synaptic plasticity and the expression levels of associated receptors in two- and four-month-old mice. They found that four-month-old mice exhibited more significant synaptic plasticity deterioration compared to two-month-old mice, resulting in severe memory and learning deficits. Their results also showed profound changes in hippocampal synaptic plasticity, spatial memory, and neuroreceptor expression, including alterations in NMDA receptor subunit composition (increased GluN2A and decreased GluN2B levels), and changes in metabotropic glutamate receptors. These results suggest that progressive age-related hearing loss leads to significant spatial memory impairment in later life, possibly mediated by receptor-level changes and disrupted synaptic plasticity within the hippocampus. Studies have shown that the mammalian brain exhibits remarkable adaptability to sensory deprivation. After sensory loss, cortical and subcortical reorganization initially impairs cognitive processes. On the other hand, studies have shown that cognitive recovery can occur after prolonged vision loss. This raises the possibility that a similar recovery may occur over a prolonged period after HL.
Additionally, research indicates that decreased auditory stimulation can lead to a decline in neurogenesis in the hippocampus. Kurioka et al. examined how unilateral or bilateral conductive hearing loss, induced by occluding the external ear canal, affects hippocampal neurogenesis in a mouse model. They found that the number of doublecortin-positive (DCX) and Ki-67-positive cells in the hippocampus decreased significantly in mice with unilateral or bilateral hearing loss. Neurogenesis in the hippocampus plays a crucial role in spatial learning and memory. In fact, several studies have shown that reduced adult hippocampal neurogenesis negatively affects the spatial learning and memory abilities of rodents. The results of the current study revealed that decreased neurogenesis is associated with significant impairments in spatial learning and memory. Additionally, the results showed that eliminating auditory input at an early age had a minimal effect on spatial learning and memory. This can be attributed to neural networks in the brain, especially during the critical early developmental period and at younger ages, that exhibit a high degree of neuroplasticity, a fascinating aspect of brain function that allows it to reorganize itself with age. The hippocampus of younger rats seems to be less dependent on auditory perception and, thus, more easily adapts to HL. In addition, the hippocampus of younger rats may inherently have greater neuroplasticity than that of adult rats, increasing their ability to adapt to reduced sensory input. A combination of these factors may be responsible for the differences observed. Further investigations and studies are needed in this area.
The current study has some limitations. First, although the primary aim of this study was to investigate the age-dependent effects of peripheral auditory deprivation on spatial learning and memory in adult rats, the underlying biological mechanisms were not examined in detail. In particular, we did not assess changes in the auditory cortex that might contribute to hippocampal changes following HL. This limits our ability to determine whether cortical reorganization mediates the observed cognitive deficits. Second, molecular analyses were limited to hippocampal markers of neurogenesis and plasticity. A broader investigation including synaptic and neurotransmitter-related molecules -- such as NMDA receptor subunits, GABAergic components, and metabotropic glutamate receptors -- could provide deeper insight into the cellular basis of cognitive impairment. Another limitation of the present study is that only male rats were used. This choice was made because spatial learning and memory are strongly influenced by sex hormones and estrous cycle-related variability, and restricting the study to males could reduce hormonal confounding. Nevertheless, this limits the generalizability of the findings across sexes.
In conclusion, peripheral auditory deprivation induced by transcanal cochlear ablation impaired spatial learning and memory, disrupted hippocampal neurogenesis and synaptic plasticity in rats, and had effects that were strongly dependent on the developmental stage. Animals deprived at later stages showed the most pronounced spatial memory deficits and concomitant reductions in hippocampal plasticity, whereas deprivation early in life resulted in minimal long-term impairment. These findings reveal the mechanistic link between auditory input and hippocampal function, showing that the timing of auditory deprivation critically shapes cognitive outcomes. Further research is needed to clarify the role of cortical changes, unravel the molecular mechanisms underlying developmental stage-specific sensitivity, and evaluate potential interventions to mitigate the long-term cognitive consequences of auditory deprivation.