Remodeling of cholinergic input to the hippocampus after noise exposure and tinnitus induction in Guinea pigs

Abstract

Here, we investigate remodeling of hippocampal cholinergic inputs after noise exposure and determine the relevance of these changes to tinnitus. To assess the effects of noise exposure on the hippocampus, guinea pigs were exposed to unilateral noise for 2 hr and 2 weeks later, immunohistochemistry was performed on hippocampal sections to examine vesicular acetylcholine transporter (VAChT) expression. To evaluate whether the changes in VAChT were relevant to tinnitus, another group of animals was exposed to the same noise band twice to induce tinnitus, which was assessed using gap‐prepulse Inhibition of the acoustic startle (GPIAS) 12 weeks after the first noise exposure, followed by immunohistochemistry. Acoustic Brainstem Response (ABR) thresholds were elevated immediately after noise exposure for all experimental animals but returned to baseline levels several days after noise exposure. ABR wave I amplitude‐intensity functions did not show any changes after 2 or 12 weeks of recovery compared to baseline levels. In animals assessed 2‐weeks following noise‐exposure, hippocampal VAChT puncta density decreased on both sides of the brain by 20–60% in exposed animals. By 12 weeks following the initial noise exposure, changes in VAChT puncta density largely recovered to baseline levels in exposed animals that did not develop tinnitus, but remained diminished in animals that developed tinnitus. These tinnitus‐specific changes were particularly prominent in hippocampal synapse‐rich layers of the dentate gyrus and areas CA3 and CA1, and VAChT density in these regions negatively correlated with tinnitus severity. The robust changes in VAChT labeling in the hippocampus 2 weeks after noise exposure suggest involvement of this circuitry in auditory processing. After chronic tinnitus induction, tinnitus‐specific changes occurred in synapse‐rich layers of the hippocampus, suggesting that synaptic processing in the hippocampus may play an important role in the pathophysiology of tinnitus.

Noise Exposure Alters Glutamatergic and GABAergic Synaptic Connectivity in the Hippocampus and Its Relevance to Tinnitus

“…To determine whether altered glutamatergic and GABAergic innervation in the hippocampus accompanies noise exposure, we used immunohistohemical detection of VGLUT-1, VGLUT-2, and VGAT to identify glutamatergic and GABAergic terminals in hippocampal subregions. Immediately after noise exposure, animals exhibited elevated ABR thresholds on the ipsilateral side at 8, 12, 16, and 20 kHz and recovered to baseline levels at 8 and 20 kHz within one week [ 45 ]. Control animals and contralateral sides of exposed animals did not exhibit any changes in ABR thresholds using this noise exposure paradigm in previous studies [ 11 , 44 ].…”

“…The same animals were used as in a previously published work from our lab [ 45 ]. Pigmented guinea pigs of either sex ( n = 19) were obtained from Elm Hill Labs at 2-3 weeks of age.…”

“…Guinea pigs were anesthetized with ketamine/xylazine (40 mg/kg ketamine; 10 mg/kg xylazine), and were then unilaterally exposed to a 7 kHz centered noise band at 97 dB SPL for 2 hours via a closed speaker system inserted in the left ear canal ( Figure 1(a) ). Auditory brainstem responses(ABRs) were recorded in response to tone bursts (0.5 ms cosine squared gating, 5 ms duration) at 8, 12, 16, and 20 kHz, before and after each noise exposure to assess shifts in hearing thresholds and wave I amplitude-intensity functions as previously described [ 45 ]. ABRs were also assessed two weeks after the noise exposure to determine hearing threshold recovery.…”

“…The procedures of brain collection, tissue preparation, and immunohistochemistry were as described before [ 45 ]. Hippocampus was sectioned into five series of coronal sections (30 μ m) with a cryostat (Leica, CM 3050S), mounted on glass slides, air dried for 24 hours, and stored at -20°.…”

“…Image processing was performed as previously described [ 45 , 49 ]. The control and exposed animal images were acquired, processed, and analyzed using identical methods.…”