Characterizing subcutaneous cortical auditory evoked potentials in mice

Highlights

• Robust auditory-related waves can be recorded non-invasively and subcutaneously in anaesthetised mice.

• The generators of these waveforms have been localised to the contralateral auditory cortex.

Abstract

Auditory Brainstem Responses (ABRs) are a reliably robust measure of auditory thresholds in the mammalian hearing system and can be used to determine deficits in the auditory periphery. However, because these measures are limited to the lower stages of the auditory pathway, they are insensitive to changes or deficits that occur in the thalamic and cortical regions. Cortical Auditory Evoked Potentials (CAEPs), as longer latency responses, capture information from these regions. However they are less frequently used as a diagnostic tool, particularly in rodent models, due to their inherent variability and subsequent difficult interpretation.

The purpose of this study was to develop a consistent measure of subcutaneous CAEPs to auditory stimuli in mice and to determine their origin. To this end, we investigated the effect on the CAEPs recorded in response to different stimuli (noise, click, and tone (16 kHz) bursts), stimulus presentation rates (2/s, 6/s, 10/s) and electrode placements. Recordings were examined for robust CAEP components to determine the optimal experimental paradigm. We argue that CAEPs can measure robust and replicable cortical responses. Furthermore, by deactivating the auditory cortex with lidocaine we demonstrated that the contralateral cortex is the main contributor to the CAEP. Thus CAEP measurements could prove to be of value diagnostically in future for deficits in higher auditory areas.

Introduction

Auditory Brainstem Responses (ABRs) recorded near the ears are frequently used to measure auditory thresholds in mammals and can be used to identify peripheral hearing deficits. ABRs occur within the first 10 ms after sound presentation and are thought to emanate from successive activation of the ascending auditory pathway (Felix et al., 2018; Winkler et al., 2013). However, ABRs do not capture cortical activity. To circumvent this limitation, human studies have long made use of scalp electroencephalography (EEG) to focus on higher order auditory areas. In particular, click stimulation induces two main middle-latency waves in the EEG, Na/Pa (∼15–35 ms) and Nb/Pb (35–60 ms) the origins of which remain controversial with most likely generators being the auditory thalamus and primary auditory cortex, while the midbrain or secondary auditory cortex are less likely (Musiek and Nagle, 2018; Picton et al., 1974). So called longer latency waves N1/P2/N2 occur at around 100/180/300 ms, respectively, and are thought to reflect higher order processing involving primary, secondary and associative auditory cortices (Godey et al., 2001; Tremblay and Burkard, 2012). These Cortical Auditory Evoked Potentials (CAEPs) have already been widely used in human studies to assess pharmacodynamics, cortical lesions, sensory processing and deficits and clinical outcomes on auditory cortical function (Davies et al., 2010; Ibañez et al., 1989; Johnson, 2009; Litscher, 1995; Supp et al., 2018; Winkler et al., 2013).

In mice, an important genetic animal model, there have been only a few studies using middle or long latency CAEPs and the majority of these were obtained epidurally (Farley et al., 2019; Metzger et al., 2007; O'Reilly and Conway, 2021; Siegel et al., 2003). Subcutaneous CAEPs are surprisingly infrequently used diagnostically despite being less invasive and still providing relatively good spatial information.

To assess the usability of subcutaneous CAEP recordings in mice, and to determine which stimulation and recording parameters produce the most robust results, we recorded CAEPs using multiple stimulus types, presentation rates and electrode placements. We also investigated their generators using auditory cortical deactivation. Our findings suggest that CAEPs in the latency range of 14–80 ms provide robust, reliable and minimally invasive measures of the contralateral thalamo-cortical response, allowing for assessment of the cortical auditory function in mice under different conditions.