Reversible Functional Changes Evoked by Anodal Epidural Direct Current Electrical Stimulation of the Rat Auditory Cortex

Abstract

Rat auditory cortex was subjected to 0.1 mA anodal direct current in seven 10-min sessions on alternate days. Based on the well-known auditory cortex control of olivocochlear regulation through corticofugal projections, auditory brainstem responses (ABRs) were recorded as an indirect test of the effectiveness and reversibility of the multisession protocol of epidural stimulation. Increases of 20–30 dB ABR auditory thresholds shown after epidural stimulation reverted back to control levels 10 min after a single session. However, increases in thresholds revert 4 days after multisession stimulation. Less changes in wave amplitudes and threshold shifts were shown in ABR recorded contralaterally to the electrically stimulated side of the brain. To assess tissue effects of epidural electric stimulation on the brain cortex, well characterized functional anatomical markers of glial cells (GFAP/astrocytes and Iba1/microglial cells) and neurons (c-Fos) were analyzed in alternate serial sections by quantitative immunocytochemistry. Restricted astroglial and microglial reactivity was observed within the cytoarchitectural limits of the auditory cortex. However, interstitial GFAP overstaining was also observed in the ventricular surface and around blood vessels, thus supporting a potential global electrolytic stimulation of the brain. These results correlate with extensive changes in the distribution of c-Fos immunoreactive neurons among layers along sensory cortices after multisession stimulation. Quantitative immunocytochemical analysis supported this idea by showing a significant increase in the number of positive neurons in supragranular layers and a decrease in layer 6 with no quantitative changes detected in layer 5. Our data indicate that epidural stimulation of the auditory cortex induces a reversible decrease in hearing sensitivity due to local, restricted epidural stimulation. A global plastic response of the sensory cortices, also reported here, may be related to electrolytic effects of electric currents.

Effects of Multisession Anodal Electrical Stimulation of the Auditory Cortex on Temporary Noise-Induced Hearing Loss in the Rat

“…The electrical stimulation protocol was applied in awake animals for seven sessions on alternating days (days 0–12 of the protocol) ( Figure 1 ). For more details, please see Colmenárez-Raga et al (2019) . To assess if the cortical damage after ES enables AC to drive corticofugal responses, serial sections of brain AC were immunostained for GAD 67 in rats from the ES group (please see below).…”

“…ABR recordings of both sides were analyzed separately. The final signal was filtered with a 500-Hz high-pass filter and a 3,000-Hz low-pass filter (for more details on the ABR recording method, see Colmenárez-Raga et al, 2019 ). Wave II was first recorded in ABRs and then used to calculate thresholds (©MatLab R-2017 a).…”

“…To locate the area of stimulation in the cortex, brains were serially sectioned in the coronal plane into 40-μm sections and immunostained for IBA1 and GFAP, according to our previously published method ( Colmenárez-Raga et al, 2019 ). Glial reaction, both for IBA-1 and GFAP, was delimited on the auditory cortices (data not shown) as previously described by our group.…”

“…In our previous studies, we have recently communicated that chronic anodal epidural stimulation in rats promotes AC activation with transient hearing threshold elevation, as demonstrated by auditory brainstem recordings (ABRs) ( Colmenárez-Raga et al, 2019 ). Based on these results, we hypothesized that a multisession stimulation protocol of the AC may induce a sustained and reversible decrease in hearing sensitivity.…”

“…More specifically, we analyze the effects of sound overactivation in combination with multisession AC epidural activation in the inner ear of the rat. For this purpose, we applied our previously tested protocol of AC anodal epidural activation ( Colmenárez-Raga et al, 2019 ), followed by a sound stimulation protocol designed for TTS induction [through a single session of 107 dB at a restricted frequency band (8–16 kHz)]. In our experimental approach, the protocol for acoustic stimulation was quite similar to those previously applied by other authors for TTS induction ( Kujawa and Liberman, 2009 , in mice or Lobarinas et al, 2017 , in rats).…”