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Electrocochleography is the best electrophysiological measurement for cochlear condition investigation in guinea pig models. In this article, we will review new issues in recording electrocochleography in guinea pigs in two parts including measurement parameters and clinical applications.
Keywords: Electrocochleography; Guinea Pig; The cochlea
Abbrevations: EcohG: Electrocochleography; CAP: Compound Action Potential; CM: Cochlear Microphonics; SP: Summating Potential; AP: Action Potential; SGNs: Spiral Ganglion Neurons; FFT: Fast Fourier transformation; DC: Direct Current
Due to tremendous difficulties associated with human experiments on auditory system, such as dissection hardness due to stiff temporal bone, animal models are the best choice for studies on the cochlear physiology. Researchers have shown that range of small mammals like cat, chinchilla, guinea pig, rat, and mouse, are appropriate options for providing insight into the human cochlear physiology. Guinea pigs are one of the classic models in auditory research due to extensive similarities in hearing range and the cochlea’s structure with the humans. Many years ago, Georg von Bekesy described the mechanics of cochlea in guinea pigs. On the other hand in 2003, the successful efforts in regenerating the hair cells were done in guinea pigs .
One of the best ways of studying the auditory system and specially the cochlea in these animals is recording auditory evoked responses. They represent activity within the auditory system that is stimulated or evoked by sounds. These recordings play a vital role in the identification and diagnosis of auditory system pathologies. Electrocochleography (EcochG) is the earliest auditory evoked response and its components arise from the inner ear and auditory (8th cranial) nerve fibers near the inner ear (distal end of 8th nerve). It is considered as the most advantageous electrophysiological potentials in documentation of cochlear status .
Researchers all over the world have investigated the cochlear condition in multiple guinea pig’s peripheral auditory
system disorders models via EcochG. This article will review
the most important measurement parameters as well as clinical applications of EcochG in guinea pigs.
One of the main applications of recording EcochG in guinea pigs is better understanding physiology of the auditory system, specially the cochlea. EcochG has several beneficial applications in cochlear physiology studies including studying the role of different channels in mammalian cochlea [3-5], cochlear micromechanics such as nonlinearity , possible roles of nitric oxide  and effects of changing perilymphatic K+ in the cochlea .
Another goal of recording EcochG in guinea pigs is determining the underlying mechanism of cochlear diseases, such as endolymphatic hydrops [9-12], hyperbilirubinemia [13,14], noise-induced hearing loss [15,16], cochlear ischemia , perilymphatic fistula  and hidden hearing loss . Studying effects of various drugs on cochlear function is the next goal for recording EcochG in guinea pigs. For instance, investigating influence of drug-induced ototoxicity on cochlear function such as cisplatin [20-24], Quinine  and ethyl benzene , the effect of blood flow promoting drugs , the effect of anesthetics such as isoflurane , histamine and its antagonists , and the effect of a dopaminergic agonist in cochlear physiology and physiopathology .
Three different types of stimuli have been used in articles for
recording the EcochG response. Two most dominant stimuli are
Clicks and Tone burst. Clicks consisted of biphasic alternating
acoustic pulses (100 μs/phase)[18,28] and 100ms electrical
pulse . Tone bursts included Trains of 8 [10,20,21,26,28]
or 10 [9,13,14,29] ms probes, with frequencies of 2-32KHz. The
probes had cosine-shaped rise and fall times of 4 ms at 0.5 kHz,
2ms at 1kHz, 1.5ms at 2kHz and 1ms at the higher frequencies
[9,10,13,14,17, 18,20,21,26,28,29]. In one research with the aim
of study of CM latency, audiometric tones of 250, 500, 1000,
2000 and 4000 Hz were used .
Two main electrode arrays can be used in recording EcochG
parameters in guinea pigs. One of them, which is called “round
window approach”, is the most widespread method for yielding
optimal and high quality recordings of the CM, SP, and AP.
Animals are anesthetized by an intraperitoneal injection of
anesthetics and are placed in a head holder. Body temperature
is maintained by a heating pad at 37 °C . Cochlea is exposed
through a dorsal approach . Once the skin and muscles are
incised behind the ear, bulla is opened and the round window of
the cochlea is exposed . An Ag-Cl-electrode is placed at the
round window niche of the ear. Reference and ground electrodes
are usually placed on the skull [34,35] or neck musculature .
In studies which recording the EcochG response is repeated in
multiple time intervals, the electrode is chronically implanted at
round window .
To measure cochlear responses from either the scala vestibuli
or the scala tympani, after using appropriate anesthetic agent,
the animal’s head is guarded dorsally in stereotactic machinery.
The bony frame of bulla is opened and a 0.2mm hole is made
into the cochlea. The electrode is made of a Teflon coated Ag-
Cl recording wire which is placed on scala vestibuli or the scala
It is possible also to record the EcochG components by
placing the active electrode on posterior superior wall of the
external ear canal near the tympanic membrane, and reference
and ground electrodes on the vertex and the frontal region
There are plenty of procedures for analyzing the latency,
threshold and amplitude of EcochG components. CAP waveforms
are analyzed by determining the amplitude, latency and threshold
of the first and second negative peaks (N1 and N2). The CAPs
threshold is usually defined as the lowest intensity stimulus
which evokes a specific magnitude of CAP (0.5mV) [10,26]. There
is also an alternative method which uses a software algorithm
and is based on adjusting the level of stimulus until the response
is just visually noticeable above the noise floor of the recording
. Most researchers use peak to peak amplitude method for
analyzing CAP amplitude, as the voltage difference between the
first negative peak after stimulus onset (N1) and the following
positive peak [9,10,13]. While the others believe that as the CAP
is principally superimposed on the SP, the amplitude of the CAP
(N1) must be measured relative to the SP and not relative to the
base line of the recording or the next positive peak [20,26,28].
Using the FFT in a window from stimulus onset to 2ms
after offset , applying a first-order Boltzmann equation
to the CM waveform in the second half of the CM waveform
, or measuring peak-to-peak amplitude in the middle of the
sinusoidal response  are various techniques of assessing CM
amplitude. Measuring the response a few milliseconds after the
onset of stimuli is to avoid contamination from CAP .
SP can be observed as the DC shift in round window potential
occurring both at the onset and offset of the tone and there are
arguments for and against using either as the SP measure [39,40].
The onset SP could be under-estimated because of the start of the
negative-going N1 wave of the CAP, whereas the slower slope of
the offset SP is probably the result of contamination by changes
in asynchronous neural firing . To overcome these issues,
SP amplitude is measured as the difference between the preresponse
baseline potential and the DC level from approximately
5.5-6.5ms following stimulus onset , to be concurrent with
the relatively stable plateau after the CAP .
Endolymphatic Hydrops (EH): Injecting artificial
endolymph into scala media in anaesthetized guinea pigs is as
an acute model of endolymphatic hydrops . With injecting
volumes up to 1-2μl endolymph, results in fundamental changed
in EcochG recording parameters. These changes includes an
increase in CAP threshold specially at low frequencies , an
increase in SP amplitude, and also a change in the asymmetric
distortion of the CM, resulting from a shift in the nonlinear
electro-mechanical transduction [44,45]. Researchers have
suggested a mechanism underlying these changes. That is,
Reissner’s membrane is swelled into scala vestibuli and the organ
of Corti is dislocated toward the scala tympani due to increase
in hydrostatic pressure of the endolymph . These cases
modify cochlear sensitivity and nonlinear mechano-electrical
transduction . But when the injected volume increases to
3μl, a fast and sudden recovery of the changes is observed, which
is highly similar to the clinical findings observed in Meniere’s
Disease . Receiving dexamethasone can prevent the reported
EcochG findings in experimentally-induced endolymphatic
Guinea pigs are considered as worthy animals in modeling
human cases with hyperbilirubinemia, as their auditory system
is immature at birth, and this is a good characteristic in matching
these models with preterm neonates with hyperbilirubinemia
. To establish the hyperbilirubinemia model, animals received an intraperitoneal injection of bilirubin at 100mg/kg
The EcochG shows normal CM, elevated CAP threshold,
and significantly prolonged peak latencies and duration. These
results suggest that hyperbilirubinemia in neonatal guinea
pigs impaired auditory peripheral neuromechanisms that
targeted mainly the IHC synapses, the myelin sheath of SGNs
and their fibers, and there is a potential relationship between
hyperbilirubinemia and auditory neuropathy . Receiving
taurine, a 2-amino-ethanesulfonic acid, which is an abundant
sulfur containing amino acid present in the inner ear in
mammals , limits bilirubin-induced neural damage in the
auditory system, which is revealed by significant attenuation of
EcochG abnormalities 
Because of high similarities in hearing range between guinea
pigs and humans, these small mammals have considered as
NIHL model in several researches. The pathogenic mechanisms
of noise-induced cochlear damage could be analyzed via
EcochG response. The temporary threshold shift as well as
permanent threshold shift is reflected in EcochG response as
an increase in CAP Threshed and latency , and decrease in
CM amplitude . The protective effects of antioxidants such
as N acetylcysteine on noise induced hearing impairment, is
monitored effectively by investigating the changes in EcochG
Noise exposures that result in reversible changes in
cochlear neural threshold can cause a reduced neural output
at supra-threshold acoustic stimuli. This so-called “hidden
hearing loss” , is associated with a selective loss of synapses
between IHCs and the high threshold and low spontaneous
rate population of primary afferent neurons . EcochG is the
best electrophysiological measure for either OHC or IHC and
auditory nerve output. Persistent depression of the amplitudes
of both CAP and SP in response to supra-threshold sounds is the
prominent occurrence which happens in hidden hearing loss,
which is representative of IHC-afferent synapse as well as hair
cell malfunction .
Making a crossed incision on the round window membrane
of guinea pig is a known way for modeling perilymphatic fistula.
This procedure makes several changes in EcochG components,
including SP and AP amplitude reduction and latency increase,
and increases in the SP/AP ratios. The proposed mechanism for
the changes which are observed by this intervention is related
to attenuation in the afferent nerve fibers activity, as well as
anatomic and functional chance in hair cells behavior, specially
their active cochlear mechanism .
An experimental local ischemia model of the guinea pig
cochlea is reported frequently in literature. In this method,
mechanically compressing anterior inferior cerebellar artery
results in reduction of cochlear blood flow. The degree of
induced cochlear ischemia is correlated with the alternations
in N1 and N2 parameters of EcochG. As CAP of the cochlear
nerve are sensitive to anoxia or ischemia, lower rate of cochlear
blood flow is related to shorter survival time of N1 and N2 or
prolongation of their latencies .