The temperature dependency of cochlear adaptation and masking in the guinea pig

Changes in CAP adaptation in experimentally induced endolymphatic hydrops

OBJECTIVE

To determine the effect of experimentally induced endolymphatic hydrops (ELHs) on compound action potential (CAP) adaptation.

METHODS

The CAP was recorded 1 month after surgical treatment to produce ELH. The CAP threshold in response to click stimuli showed no significant differences between the hydropic and control groups. The CAP was elicited from the retroauricular-exposed round window using a silver ball electrode. Click trains consisting of 10 clicks with an interclick interval (Deltat) of either 4 or 8 ms were delivered through a loudspeaker placed in front of the animals' ears. The patterns of adaptation to these click trains were evaluated as functions of Deltat and intensity level.

RESULTS

An abnormal pattern of CAP adaptation was observed in hydropic ears.

CONCLUSIONS

These findings suggest that early ELH influences CAP adaptation.

Effects of hypothermia on cochlear micromechanical properties in humans

The aim of the present experiment was to investigate the influence of hypothermia on transient evoked otoacoustic emissions (TEOAEs) in humans. Hypothermic alterations were brought about directly by cooling the cardiovascular system of young children during open heart bypass surgery. This condition caused TEOAE levels to decrease with a significant positive correlation between total and frequency band TEOAE amplitudes during cooling. TEOAEs were totally abolished at tympanic temperatures around 30 degrees C, without it being possible to find differences in TEOAE alteration among the frequency components. During rewarming, the changes reversed and TEOAEs returned to their initial prehypothermia status, although sometimes only partially. Despite large possible metabolic changes caused by this hypothermic condition at various levels, it is concluded that the motile properties of outer hair cells (OHCs), that are related to TEOAEs, are temperature-sensitive. The total suppression found in deep hypothermia (above 30 degrees C) could conceivably be attributable to a process involving a temperature-dependent energy source.

Changes in CM and CAP with sedation and temperature in the guinea pig: facts and interpretation

The influence of xylazine on the amplitude, latency and waveform of VIIIth nerve compound action potential (CAP) and cochlear microphonic (CM) in response to clicks at 95 dB SPL in normal awake preimplanted guinea pigs was investigated. The animals' temperature was monitored but no thermoregulation was exerted, except in one control experiment. Following a 0.2 ml injection of xylazine, CM showed minor variations while CAP audiograms for tone pips between 0.5 and 25 kHz remained normal. However, a progressive decrease in temperature and a strongly correlated increase in CAP amplitude and in N1 and N2 latencies were noticed. For peak N1 the changes were equivalent to linear amplitude and time expansions, and could be reproduced through CAP synthesis with convolution methods using time expanded unit response model and firing density functions. All changes were maximal after 2 h of sedation and recovered within approximately another 2 h. Whereas xylazine is known to induce hypothermia, all the changes disappeared if the animal was thermoregulated. Therefore the changes are interpreted as a result of hypothermia. The mechanism of N1 latency lengthening and increase in amplitude during hypothermia can be understood as a simultaneous increase in spike duration, hair cell/nerve synaptic delay and postsynaptic time constant. This hypothesis yielded a theoretical temperature coefficient for N1 latency (-52 microseconds/degree C) matching that measured experimentally (-55 microseconds/degree C). When compared with peak N1, peak N2 appeared relatively more expanded. Arguments about the origin of N2 are discussed.

Hypothermia differentially affects tuning curves generated by forward and by simultaneous masking

In the gerbil maintained at euthermic (37.5 degrees C) conditions, forward masking produces a compound action potential tuning curve (CAP TC) which is less sensitive but more sharply tuned than that which is generated by simultaneous masking. These differences between forward- and simultaneously-masked CAP TCs are minimized at hypothermic (30 degrees C) conditions. The unmasking effect occurs at both temperatures, suggesting that hypothermia does not exert these changes by eliminating two-tone suppression.

Hypothermia protects the cochlea from noise damage

Thresholds of the cochlear action potential were obtained from rodents at euthermic (38 degrees C) and hypothermic (30 degrees and 25 degrees C) rectal temperatures. In the gerbil, low and middle frequency (1-8 kHz) thresholds increased an average of 2.3 dB per degrees C decrease of body temperature; at 16 kHz, 3.5 dB/degrees C; and at 32 kHz, an increase of 4.4 dB/degrees C. In the mouse, these values were: 2-16 kHz, 1.4 dB increase per degrees C decrease; 32 kHz, 2.7 dB/degrees C; 64 kHz, 3.8 dB/degrees C. When subjects maximally susceptible to permanent threshold shift (PTS) at low and middle frequencies (anesthetized, immature mice) were exposed to 115 dB noise, hypothermia reduced PTS at these most susceptible frequencies (2-16 kHz). When awake adult mice were exposed to this noise, hypothermia protected them from PTS at their most vulnerable frequency (32 kHz).

Effects of hypothermia on the cat brain-stem auditory evoked response

Effects of systemic hypothermia on the brain-stem auditory evoked responses (BAERs) in 4 pentobarbital anesthetized adult cats placed on total cardiopulmonary bypass were investigated. Hypothermia was achieved by slowly cooling the bypass blood through a heat exchanger. Serial BAERs were recorded at 2 min intervals as brain temperature was lowered from 37 to 22 degrees C and then rewarmed over a 1-2 h period. Temperatures were recorded from the brain, esophagus and rectum. Three effects were produced by controlled systemic hypothermia. First, latencies of each component wave (I-V) of the BAER increased exponentially as brain temperature was lowered to 19 degrees C. Latencies of earlier waves (I-III) increased less than those of the later waves (IV-V). Arrhenius plots of inverse latency (rate) versus reciprocal of the absolute temperature generated a family of straight lines of similar slope for each of the 5 component waves of the BAER. The activation energy for each of the 5 BAER waves was derived from the slope of the Arrhenius plot. The mean and standard deviation of the activation energy of all 5 waves was 9.7 +/- 0.5 kcal/mole degree C. The fact that the activation energy was similar for each BAER component wave (I-V) indicated that the increase in latencies for all 5 waves was governed by the same rate-limiting, temperature-dependent process(es). Second, the rise time and duration of each of the component waves of the BAER increased with decreasing temperature. Third, wave amplitudes increased from 37 to 32 degrees C in a quasiparabolic relation and then, decreased at approximately a linear rate. The BAER wave form completely disappeared below 20.3 degrees C. Slow rewarming of the brain to its initial temperature restored the BAER component waves to their original latencies and amplitudes.

Influence of argon laser stapedotomy on inner ear function and temperature

The influence of argon laser stapedotomy on inner ear function was investigated in guinea pigs. The cochlear microphonics (CM) and the compound action potential (CAP) served as parameters for the functional status of the cochlear. Transitory depression of both potentials was found during and after laser stapedotomy. The time course of CM and CAP depression and recovery is compared to endocochlear temperature changes. Possible implications for clinical use are discussed.

Anesthesia and surgical trauma: their influence on the guinea pig compound action potential

A chronic implant consisting of a fine thermocouple placed on the round window permitted measurement of temperature and of the compound action potential (CAP) of the cochlear nerve in guinea pigs. Thresholds and latencies of the CAP, in response to tone bursts (2-40 kHz), were measured when the animal was awake and unrestrained, and again after several hours of anesthesia. The CAP remained unchanged with a variety of common anesthetics when precise control of round window temperature was maintained. However, when anesthesia was accompanied by several hours of slight cochlear cooling, thresholds were elevated for CAPs evoked by frequencies above 24 kHz and latencies were increased for CAPs evoked by all frequencies tested. The effects of surgery on CAP threshold and latency were also examined. Guinea pigs were tested while still anesthetized at the conclusion of the implantation procedure, and then again several days later while awake. Thresholds and latencies were unchanged. In two anesthetized guinea pigs already implanted with thermocouples, ventral and post-auricular surgery to expose the middle ear had no effect on the CAP, when low-speed drilling was used to open the auditory bulla. However, when small portions of the bulla were broken away with forceps, the CAP in response to high-frequency tone bursts immediately showed elevated thresholds. This alteration of the CAP was clearly different from that produced by lowered temperature, since the latencies at threshold were significantly decreased.

Effect of argon laser stapedotomy on cochlear potentials. II. Alteration of the compound action potential (CAP)

The influence of the argon laser on inner-ear function during and after laser stapedotomy was investigated in guinea pigs, using the compound action potential (CAP) as parameter. With the onset of the argon laser impact, the CAP is depressed or even extinguished up to 10 s. The temporal course of subsequent CAP recovery is similar to that of endocochlear temperature recovery after laser stapedotomy, so that a direct influence of temperature increase and CAP decrease is probable. No permanent attenuation of CAP amplitude was found after a single laser perforation of the stapes footplate.

Noise impairment in the guinea pig. I. Changes in electrical evoked activity along the auditory pathway

Changes in the cochlear microphonics (CM), auditory nerve action potential (AP), and evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) of the guinea pig were assessed after exposure to white noise of 115 dB for 30 min. Both continuous and intermittent (200 ms noise and 200 ms pause) exposures were used. In comparison with the pre-exposure level, CM isopotential curves were shifted by 1.1 +/- 0.5 dB (means +/- S.E.) on the average in the range of 0.5-8 kHz (recorded at the round window). The amplitude-intensity function of the click-evoked auditory nerve action potential decreased by 8.4 +/- 1.2 dB, that of the inferior colliculus evoked response by 20.9 +/- 3.7 dB, and the amplitude-intensity function of the auditory cortex evoked potential decreased by 6.2 +/- 4.7 dB. A similar reduction in the amplitude was found after both continuous and intermittent noise exposure. In contrast to the decrease in amplitudes of evoked potentials, the latency-intensity functions of the individual waves of potentials evoked along the auditory pathway did not change when compared at the same click intensity before and after the exposure. The results suggest that individual auditory nuclei are impaired by the noise to different extents and that the impairment does not increase linearly up to the auditory cortex.

Adaptation in auditory-nerve fibers: a revised model

Adaptation of firing rates in auditory-nerve fibers appears to reflect two distinct processes. Rapid adaptation occupies the first few milliseconds of response and is superimposed upon short-term adaptation which has a time constant of about 40 ms. The properties of the two processes are reviewed and compared, and a phenomenological model is developed that successfully accounts for them. The model consists of several stages which have been tentatively associated with underlying physiological processes. In the first stage stimulus intensity is transformed by a static nonlinearity, followed by a low-pass filter. The filtered output may correspond to the hair-cell receptor potential. It modulates the release of a substance that possibly represents synaptic transmitter. Adaptation is produced by the depletion of transmitter which is located in three stores in cascade. A global store with fixed concentration controls the steady-state response and replenishes a local store which is responsible for short-term adaptation. The local store replenishes a rapidly depleted immediate store. Flow between stores is proportional to concentration gradients with the following exceptions. The immediate store is subdivided into independent volumes or sites and there is no flow among sites or back to the local store. A given site becomes activated only when the receptor potential exceeds its particular activation value and the number of activated sites is proportional to the receptor potential. The flow of transmitter from the immediate store is assumed to be proportional to neural firing rate, with some minor modifications described in the text. The properties of the model are determined from the underlying equations and from a computer simulation. The model produces realistic response properties including PST histograms, onset and steady-state rate-intensity functions, incremental and decremental responses, response modulation for amplitude modulated stimuli, and period histograms for low-frequency tones.

The Q10 of auditory brain stem responses in rats under hypothermia

Auditory brain stem responses (ABRs) were recorded from normal Sprague-Dawley rats ranging in age from 15 to 120 days. Increased latencies for all four waves of the ABR were noted for animals of all ages as their body temperatures were reduced from 38 to 28 degrees C. The Q10S of the latency changes were highly variable but were still significantly higher for the first three waves of the ABR in animals less than 20 days of age compared with older animals. The Q10S for the fourth wave in both groups were alike. Wave heights tended to decrease during cooling, but this effect was inconsistent. The cochlear microphonic was recorded in some animals and tended to decrease in size with cooling. As seen in this study, the significant changes in wave latencies due to small temperature differences underscore the need to carefully control this variable when taking evoked responses from animal preparations and humans subject to variations in body temperature. The interanimal variability of the Q10 precludes any simple 'correction' of latency differences due to temperature for research or clinical purposes.

Circadian variation in the latency of brainstem responses and its relation to body temperature

The auditory brainstem response varies in a circadian rhythm that is negatively correlated with the circadian rhythm in oral temperature. The auditory brainstem responses and oral temperature were recorded every 3 hours from three healthy male subjects during a 2-day period. The data indicate that a reduction of 1 degree C in oral temperature is associated with an increase of 200 microseconds in the latency of wave V of the auditory brainstem response, and of 160 microseconds in the interval between waves I and V.

Narrow-band analysis of compound action potentials for several stimulus conditions in the guinea pig

Compound action potentials (AP) were recorded under various stimulus conditions in 31 guinea pigs. Stimulus attenuation, decrease of inter-stimulus interval, increase of the level of a continuous wide-band noise maker, and lowering the animal's temperature all resulted in a drop of the AP amplitude and an increase in latency. A narrow-band analysis of the compound APs makes it possible to describe these AP changes in terms of the response behaviour of small cochlear regions according to their central frequencies. The results show that intensity-dependent changes in the AP parameters can be explained on the basis of the tuning properties of the auditory nerve fibres when the effect of the rise time of the tone-burst stimulus is taken into account. Shortening of the inter-stimulus interval produces a complex interaction in terms of tone-burst frequency and the region along the cochlear partition that contributes dominantly to the AP. It is concluded that response contributions from the narrow bands with a central frequency near the tone-burst frequency show the most adaptation. The change in amplitude for narrow-band responses under increased masking is similar to that for stimulus attenuation. It seems, however, that the underlying masking mechanism is more comparable to the adaptation mechanism. Cooling of the animal did not affect the sharpness of tuning. In all four recording situations there seems to be a decrease in the amount of synchronization of single-fibre responses as reflected in the width of narrow-band action potentials.

Influence of the stimulus repetition rate on brain-stem-evoked responses in man

In order to record brain-stem-evoked responses as fast as possible, the influence of the stimulus repetition rate was investigated. The repetition frequency was varied from 2.5 to 80 Hz. The amplitudes of N2-N4 diminish uniformly with increasing stimulus rate. The repetition rate has little or no influence on the amplitude of N5; however, increasing the repetition frequency about 10 Hz causes an increase in the latencies of N2-N5. It seems that the decrease in the amplitude of N2-N4 and the increase in the latencies of N2-N5 are of cochlear origin, since the amplitude and the latency of the cochlear responses are influenced in the same way by the repetition rate as the above-mentioned brain stem responses.

The influence of cochlear temperature on the electrical travelling wave pattern in the guinea pig cochlea

Cochlear microphonics (CM) were recorded in the guinea pig using differential recording from the first and third cochlear turns as well as by using a 10-electrode array inserted in the scala tympani of the basal turn. The excitation profile as represented by the CM was quantified by amplitude and phase data, and was measured at normal temperature (38 degrees C) and a 10 degrees lower cochlear temperature. It appears that the CM pattern shifts toward the base of the cochlea as a result of cooling. This is explained on the basis of an increased compliance of the basilar membrane with an optional influence of increased viscosity of the perilymph.