Excitatory Repetitive Transcranial Magnetic Stimulation Over Prefrontal Cortex in a Guinea Pig Model Ameliorates Tinnitus

Abstract

Tinnitus, a phantom auditory perception that can seriously affect quality of life, is generally triggered by cochlear trauma and associated with aberrant activity throughout the auditory pathways, often referred to as hyperactivity. Studies suggest that non-auditory structures, such as prefrontal cortex (PFC), may be involved in tinnitus generation, by affecting sensory gating in auditory thalamus, allowing hyperactivity to reach the cortex and lead to perception. Indeed, human studies have shown that repetitive transcranial magnetic stimulation (rTMS) of PFC can alleviate tinnitus. The current study investigated whether this therapeutic effect is achieved through inhibition of thalamic hyperactivity, comparing effects of two common clinical rTMS protocols with sham treatment, in a guinea pig tinnitus model. Animals underwent acoustic trauma and once tinnitus developed were treated with either intermittent theta burst stimulation (iTBS), 20 Hz rTMS, or sham rTMS (10 days, 10 min/day; weekdays only). Tinnitus was reassessed and extracellular recordings of spontaneous tonic and burst firing rates in auditory thalamus made. To verify effects in PFC, densities of neurons positive for calcium-binding proteins, calbindin and parvalbumin, were investigated using immunohistochemistry. Both rTMS protocols significantly reduced tinnitus compared to sham. However, spontaneous tonic firing decreased following 20 Hz stimulation and increased following iTBS in auditory thalamus. Burst rate was significantly different between 20 Hz and iTBS stimulation, and burst duration was increased only after 20 Hz treatment. Density of calbindin, but not parvalbumin positive neurons, was significantly increased in the most dorsal region of PFC indicating that rTMS directly affected PFC. Our results support the involvement of PFC in tinnitus modulation, and the therapeutic benefit of rTMS on PFC in treating tinnitus, but indicate this is not achieved solely by suppression of thalamic hyperactivity.

Hearing Loss Increases Inhibitory Effects of Prefrontal Cortex Stimulation on Sound Evoked Activity in Medial Geniculate Nucleus

“…A tungsten in a glass recording electrode was used to record single neuron activity in the MGN of the thalamus ( Merrill and Ainsworth, 1972 ). Entry into the right (contralateral to noise stimulus) MGN was verified by the presence of a robust cluster response to broadband noise stimulus (50 ms duration, 1 ms rise and fall times, sample rate of 96 kHz) ( Cook et al, 2021 ; Zimdahl et al, 2021 ). Electrophysiological recordings of single neuron activity were recorded in MGN in response to stimulation of the right (ipsilateral to MGN) PFC.…”

Long-Term Effects of Repetitive Transcranial Magnetic Stimulation on Tinnitus in a Guinea Pig Model

“…Detailed descriptions of behavioral testing for tinnitus [ 33 , 34 , 35 , 36 , 37 ], AT surgery [ 10 , 38 , 39 , 40 ] and high-intensity rTMS and immunohistochemistry [ 33 ] have been described in previous papers from our laboratory and will only be described briefly in the following sections.…”

“…Images and photomicrographs were captured using a Nikon Eclipse 80i (×20 objective lens) and an integrated Digital Sight Camera (NIS-elements Basic Elements software, Nikon Australia, Rhodes, NSW, Australia). Since we showed in our previous study [ 33 ] that rTMS only altered the densities of calbindin in the dorsal areas (close to skull) of the prefrontal cortex, neuronal densities of calbindin were analyzed in two dorsal regions of interest of the prefrontal cortex. The soma and two dendrites of an immunopositive cell had to be visible in order to be counted.…”

“…The soma and two dendrites of an immunopositive cell had to be visible in order to be counted. For details on the regions of interest and calculations of densities see [ 33 ].…”

“…This animal model allows for more invasive measurements of brain activity and structure and eliminates the placebo effect that is common amongst perceptual disorders such as pain or tinnitus [ 31 , 32 ]. Recently, we have shown therapeutic effects of an excitatory, high-frequency, high-intensity rTMS protocol over the PFC on the behavioral symptoms of tinnitus in our animal model [ 33 ]. However, only acute effects were investigated.…”