Differential impact of hypothermia and pentobarbital on brain-stem auditory evoked responses

Altered brainstem auditory evoked potentials in a rat central sensitization model are similar to those in migraine

Migraine symptoms often include auditory discomfort. Nitroglycerin (NTG)-triggered central sensitization (CS) provides a rodent model of migraine, but auditory brainstem pathways have not yet been studied in this example. Our objective was to examine brainstem auditory evoked potentials (BAEPs) in rat CS as a measure of possible auditory abnormalities. We used four subdermal electrodes to record horizontal (h) and vertical (v) dipole channel BAEPs before and after injection of NTG or saline. We measured the peak latencies (PLs), interpeak latencies (IPLs), and amplitudes for detectable waveforms evoked by 8, 16, or 32 kHz auditory stimulation. At 8 kHz stimulation, vertical channel positive PLs of waves 4, 5, and 6 (vP4, vP5, and vP6), and related IPLs from earlier negative or positive peaks (vN1-vP4, vN1-vP5, vN1-vP6; vP3-vP4, vP3-vP6) increased significantly 2h after NTG injection compared to the saline group. However, BAEP peak amplitudes at all frequencies, PLs and IPLs from the horizontal channel at all frequencies, and the vertical channel stimulated at 16 and 32 kHz showed no significant/consistent change. For the first time in the rat CS model, we show that BAEP PLs and IPLs ranging from putative bilateral medial superior olivary nuclei (P4) to the more rostral structures such as the medial geniculate body (P6) were prolonged 2h after NTG administration. These BAEP alterations could reflect changes in neurotransmitters and/or hypoperfusion in the midbrain. The similarity of our results with previous human studies further validates the rodent CS model for future migraine research.

Induction of therapeutic hypothermia requires modulation of thermoregulatory defenses

Hypothermia has been linked to beneficial neurologic outcomes in different clinical situations and its therapeutic value is considered important. For example, in asphyctic neonates and in patients with out-of-hospital cardiac arrest (with ventricular fibrillation as the initial cardiac rhythm), rapid installation of hypothermia has been reported to add substantial therapeutic benefits over nonthermal standard treatments. Yet, in other groups of patients in which the application of therapeutic hypothermia may be applied with clinical benefits, the optimization of therapy remains less straightforward, as the body possesses vigorous defense mechanisms to protect it from inducing hypothermia, that is, especially in conscious patients and/or in those in which the hypothalamus remains intact, such as stroke patients or patients who suffer a myocardial infarction or spinal cord injury. This overview summarizes the body's primary reactions to hypothermia and the defense mechanisms available or evoked. Then, clinically applicable ways to overcome these forceful cold defenses of the body are described to ensure both an optimal induction process for therapeutic hypothermia and maximal subjective comfort for these conscious patients.

Auditory brainstem response findings in hypothyroid and hyperthyroid disease

OBJECTIVE

Retrocochlear sensorineural hearing loss, associated with hypothyroidism, has been reported. This study investigated that claim.

METHODS

Audiometric and auditory brainstem response (ABR) measures of patients with hyper- and hypothyroidism.

RESULTS

The hyperthyroid group (14 patients) gave no audiometric or ABR results that differed significantly from normal controls. The audiometric findings for the hypothyroid group (21 patients) showed that 36% of this group had a four frequency average threshold greater than 25dB. The statistically significant findings for ABR were a reduction in the amplitude of waves III and V and an abnormal increase in the I-V interval, appearing to confirm a retrocochlear disorder. However, as these patients have a low body temperature, the data were analysed by temperature and the results showed that the ABR abnormalities could be explained by body temperature.

CONCLUSIONS

Hyperthyroid patients did not differ significantly from the controls. The four frequency average threshold and ABR results in hypothyroid patients did differ from the controls. However there is no clear evidence that the retrocochlear involvement is a direct result of the hypothyroidism. The abnormalities in the ABR can be explained by the low body temperature seen in these patients.

SIGNIFICANCE

This corrects the impression that hypothyroid patients have retrocochlear problems (indicated by ABR); a conclusion confirmed by data from 7 patients after thyroxin treatment when body temperatures were close to normal values and ABR I-V intervals within or near normal limits.

General anesthesia changes gap-evoked auditory responses in guinea pigs

CONCLUSION

General anesthesia induced by sodium pentobarbital reduces temporal resolution as represented by an increase in the threshold of gap-evoked auditory responses in guinea pigs.

OBJECTIVES

To explore the potential impact of general anesthesia by sodium pentobarbital (PB), a common anesthetic used in animal research, on gap-evoked responses.

MATERIALS AND METHODS

The evoked potentials in response to gaps formed by bursts of broadband noise were recorded from electrodes implanted in the inferior colliculus (IC) and the auditory cortex (AC) of guinea pigs. The gap responses were compared in three conditions: unanesthetized and anesthetized with two doses of sodium pentobarbital (40 mg/kg and 20 mg/kg).

RESULTS

PB increased the gap response thresholds, especially when applied at the higher dose. The threshold shift induced by PB was greater in the AC than in the IC. In addition, the higher dose of PB significantly increased the gap response latency in both IC and AC, and decreased response amplitude in IC only.

Changes in respiratory-modulated neural activities, consistent with obstructive and central apnea, during fictive seizures in an in situ anaesthetized rat preparation

Sudden unexplained death in epilepsy (SUDEP) has been proposed to result from seizure-induced changes in respiratory and cardiac function. Our purpose was to characterize changes in respiration during seizures. We used a preparation of the anaesthetized, perfused in situ rat. This preparation has the advantage over in vivo preparations in that delivery of oxygen to the brain does not depend upon the lungs or cardiovascular system. Electroencephalographic activity was recorded as were activities of the hypoglossal, vagus and phrenic nerves. The hypoglossal and vagus nerves innervate muscles of the upper airway and larynx while the phrenic nerve innervates the diaphragm. Fictive seizures were elicited by injections of penicillin into the parietal cortex or the carotid artery. Following elicitation of the fictive seizures, activities of the hypoglossal and vagal nerves declined greatly while phrenic activity was little altered. Such a differential depression of activities of nerves to the upper airway and larynx, compared to that to the diaphragm, would predispose to obstructive apnea in intact preparations. With more time, activity of the phrenic nerve also declined or ceased. These changes characterize central apnea. The major conclusion is that seizures may result in recurrent periods of obstructive and central apnea. Thus, seizures can adversely alter respiratory function in a profound manner.

From neuroscience to application in neuropharmacology: A generation of progress in electrophysiology

A continuum from neuronal cellular/subcellular properties to system processes appears to exist in many instances and to allow privileged approaches in neuroscience and neuropharmacology research. Brain signals and the cholinergic and GABAergic systems, in vivo and in vitro evidence from studies on the retina, or the "gamma band" oscillations in neuron membrane potential/spiking rate and neuronal assemblies are examples in this respect. However, spontaneous and stimulus-event-related signals at any location and time point reflect brain state conditions that depend on neuromodulation, neurotransmitter interaction, hormones (e.g., glucocorticois, ACTH, estrogens) and neuroendocrine interaction at different levels of complexity, as well as on the spontaneous or experimentally-induced changes in metabolism (e.g., glucose, ammonia), blood flow, pO2, pCO2, acid/base balance, K activity, etc., that occur locally or systemically. Any of these factors can account for individual differences and/or changes over time that often are (or need to be) neglected in pharmaco-EEG studies or are dealt with statistically and by controlling the experimental conditions. As a result, the electrophysiological effects of neuroactive drugs are to an extent non-specific and require adequate modeling and precise correlation with independent parameters (e.g., drug kinetics, vigilance, hormonal profile or metabolic status, etc.) to avoid biased results in otherwise controlled studies.

Neurotoxicity of carbonyl sulfide in F344 rats following inhalation exposure for up to 12 weeks

Carbonyl sulfide (COS), a high-priority Clean Air Act chemical, was evaluated for neurotoxicity in short-term studies. F344 rats were exposed to 75-600 ppm COS 6 h per day, 5 days per week for up to 12 weeks. In rats exposed to 500 or 600 ppm for up to 4 days, malacia and microgliosis were detected in numerous neuroanatomical regions of the brain by conventional optical microscopy and magnetic resonance microscopy (MRM). After a 2-week exposure to 400 ppm, rats were evaluated using a functional observational battery. Slight gait abnormality was detected in 50% of the rats and hypotonia was present in all rats exposed to COS. Decreases in motor activity, and forelimb and hindlimb grip strength were also detected. In rats exposed to 400 ppm for 12 weeks, predominant lesions were in the parietal cortex area 1 (necrosis) and posterior colliculus (neuronal loss, microgliosis, hemorrhage), and occasional necrosis was present in the putamen, thalamus, and anterior olivary nucleus. Carbonyl sulfide specifically targeted the auditory system including the olivary nucleus, nucleus of the lateral lemniscus, and posterior colliculus. Consistent with these findings were alterations in the amplitude of the brainstem auditory evoked responses (BAER) for peaks N3, P4, N4, and N5 that represented changes in auditory transmission between the anterior olivary nucleus to the medial geniculate nucleus in animals after exposure for 2 weeks to 400 ppm COS. A concentration-related decrease in cytochrome oxidase activity was detected in the posterior colliculus and parietal cortex of exposed rats as early as 3 weeks. Cytochrome oxidase activity was significantly decreased at COS concentrations that did not cause detectable lesions, suggesting that disruption of the mitochondrial respiratory chain may precede these brain lesions. Our studies demonstrate that this environmental air contaminant has the potential to cause a wide spectrum of brain lesions that are dependent on the degree and duration of exposure.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Spindle activity in children during cardiac surgery and hypothermic cardiopulmonary bypass

Hypothermia has marked effects on the electrical activity of the brain, which has been shown in animals as well as in humans. The aim of this study was to investigate EEG spindle activity in children during cardiac surgery and hypothermic cardiopulmonary bypass. The authors obtained intraoperative 21-channel EEG recordings in 36 children (mean age, 22 months; range, 6 days to 69 months) with congenital heart disease. Bipolar EEG derivations were analyzed visually for rhythmic spindle activity based on morphology, frequency, duration, and amplitude. Linear regression analysis for duration, frequency, and amplitude versus rectal temperature was performed in each individual. Spindle activity was observed in 17 children (16 children < 12 months of age). Progressive slowing of spindle frequency with decreasing rectal temperature was found (mean decrease, 0.54 +/- 0.31 Hz/ degrees C). Spindle duration increased on average by 0.69 +/- 0.39 second/ degrees C during cooling procedures. Spindle amplitude did not show any correlation to changes in rectal temperature. The current study demonstrates spindle activity during hypothermic cardiopulmonary bypass with temperature-dependent spindle modifications of frequency and duration. Although the temperature-dependent changes in this study confirm temperature coefficients of other EEG studies, the reasons for the clear age relationship and the "nature" of these spindles remain unknown.

Mechanisms of intraoperative brainstem auditory evoked potential changes

Brainstem auditory evoked potential (BAEP) changes during intraoperative monitoring may reflect damage to or potentially reversible dysfunction of the ear, the eighth nerve, or the brainstem auditory pathways up to the level of the mesencephalon. They may also be caused by other physiologic mechanisms such as anesthesia, hypothermia, and acoustic masking from drilling noise, or they may result from technical factors that prevent proper stimulus delivery or recording of an evoked potential that is actually present. Cochlear ischemia or infarction resulting from compromise of the internal auditory artery and inner ear damage during temporal bone drilling will affect all BAEP components, including wave I. Direct mechanical or thermal trauma to the eighth nerve will delay, attenuate, and possibly eliminate waves III and V, but wave I, which is generated at the cochlear end of the eighth nerve, may be preserved. During scraping of tumor off the eighth nerve, force applied in an ear-toward-brainstem direction can avulse the fragile fibers of the distal eighth nerve at the area cribrosa. Prolonging the I-to-III interpeak interval during retraction of the cerebellum and brainstem reflects stretching of the eighth nerve, and is often reversible. Vasospasm within the eighth nerve can cause similar, potentially reversible BAEP changes. Damage to the brainstem auditory pathways at or below the level of the mesencephalon will delay and attenuate or eliminate wave V. Wave III is affected similarly if the damage is at or caudal to the region of the superior olivary complex. These BAEP changes may reflect direct mechanical or thermal damage to the brainstem, brainstem compression, or ischemia or infarction resulting from vascular compromise. During BAEP monitoring, examination of the pattern of BAEP changes, analysis of their correlation with surgical maneuvers, and investigation for possible contributory technical factors can help to determine the cause of the BAEP changes and provide the appropriate information to the rest of the surgical team.

Flash-, somatosensory-, and peripheral nerve-evoked potentials in rats perinatally exposed to Aroclor 1254

Pregnant Long-Evans rats were exposed to 0, 1 or 6 mg/kg/day of Aroclor 1254 (A1254; Lot no. 124-191), a commercial mixture of polychlorinated biphenyls (PCBs), from gestation day (GD) 6 through postnatal day (PND) 21. At 128-140 days of age, male and female offspring were tested for visual-, somatosensory- and peripheral nerve-evoked potentials. The evoked responses increased in amplitude with larger stimulus intensities, and gender differences were detected for some endpoints. In contrast, developmental exposure to A1254 failed to significantly affect the electrophysiological measures. A subset of the animals were tested for low-frequency hearing dysfunction using reflex modification audiometry (RMA). An elevated threshold for a 1-kHz tone was observed, replicating previous findings of A1254-induced auditory deficits [Hear. Res. 144 (2000) 196; Toxicol. Sci. 45(1) (1998) 94; Toxicol. Appl. Pharmacol. 135(1) (1995) 77.]. These findings indicate no statistically significant changes in visual-, somatosensory- or peripheral nerve-evoked potentials following developmental exposure to doses of A1254 that produce behavioral hearing deficits. However, subtle changes in the function of the visual or somatosensory systems cannot be disproved.

Robust moving averages, with Hopfield neural network implementation, for monitoring evoked potential signals

This technical note describes a robust version of moving averages, that enables reliable monitoring of the evoked potential (EP) signals. A cluster analysis (CA) procedure is introduced to robustify the signal averaging (SA). It is implemented via a Hopfield neural network (HNN), which performs selection of the trials forming a cluster around the current state of the EP signal. The core of this cluster serves as an estimate of the instantaneous EP. The effectiveness of the method, indicated by application to real data, and its computation efficiency, due to the use of simple matrix operations, makes it very promising for clinical observations.

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.

Hearing in the marsupial Monodelphis domestica as determined by auditory-evoked brainstem responses

The audiogram of the marsupial Monodelphis domestica was determined by measuring brainstem auditory-evoked potentials in response to pure-tone stimuli of 2-90 kHz. The frequency range at 60 dB SPL comprises frequencies between about 2.3 and 63 kHz. The best frequency range with average thresholds of 3 dB SPL in young adults (3 months old) is between 12 and 16 kHz. In older animals, thresholds are higher. The overall shape and frequency range of the audiogram and the absolute thresholds indicate that hearing in this small marsupial is typically mammalian and cannot be considered primitive.

The comparative effects of sodium thiosulfate, diethyldithiocarbamate, fosfomycin and WR-2721 on ameliorating cisplatin-induced ototoxicity

The efficacies of four agents in ameliorating cisplatin-induced ototoxicity were investigated. Hamsters were given a series of 5 cisplatin injections (3 mg/kg/injection once every other day, i.p.) either alone or in combination with 1600 mg/kg/injection sodium thiosulfate (STS), 300 mg/kg/injection diethyldithiocarbamate (DDTC), 18 mg/kg/injection WR-2721, or 300 mg/kg/injection fosfomycin (n = 10/group). Ototoxicity was assessed electrophysiologically by auditory brainstem responses (ABRs) and anatomically by cochlear histology. The greatest auditory protection was given by STS, followed by DDTC. WR-2721 and fosfomycin did not provide any protection. All of the animals in the STS and DDTC groups survived, while some fatalities occurred in the fosfomycin, WR-2721, and cisplatin-only groups. Thus, the agents that were protective against ototoxicity were also protective against mortality. The ABRs also provided evidence of cisplatin-induced neuropathy. In summary, STS and DDTC hold promise for ameliorating the ototoxic effects of cisplatin chemotherapy and the hamster proved to be an excellent model of cisplatin ototoxicity.

The ototoxicity of 3,3'-iminodipropionitrile: functional and morphological evidence of cochlear damage

Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.

High-rate sequential sampling of auditory brain-stem and somatosensory evoked responses in hypoxia

We developed a high-rate sequential recording technique that allowed simultaneous measurements of both auditory brain-stem response (ABR) and somatosensory evoked potential (SEP) every 10 sec. Using this method, a transient increase in amplitude of all the ABR and SEP components in response to hypoxia in dogs could be detected. The increase in amplitude preceded the prolongation of latency. Our study showed that there were successive changes of evoked potentials in response to hypoxia. A transient increase in amplitude is the first to occur, followed by a latency prolongation and an amplitude decrease for both ABRs and SEPs.

EEG and prognosis of neurologic recovery of dogs under profound hypothermic circulatory arrest

Deep hypothermia (18-20 degrees C) (DH) during prolonged circulatory arrest and cardiopulmonary bypass is used to repair complex intracardiac lesions and vascular neurosurgical lesions. DH diminishes the risk of ischemic damage and multiorgan failure after circulatory arrest. Profound hypothermia (PH) to 6-7 degrees C has recently been reported to improve the neurological outcome of dogs after 2 h of circulatory arrest. There are no reports of the possible utility of EEG activity to predict the neurological outcome. As a part of a controlled study of cardiopulmonary bypass and 2 h of circulatory arrest we compared EEG recovery to the neurological outcome in 2 groups of dogs: 4 under DH and 4 under PH. All of the dogs under PH had a good outcome: mean neurodeficit score was 6.25/500 in PH and 139.25/500 in DH dogs (P < 0.03); mean histopathological score was 19.25/100 for DH and 47.75/100 in PH dogs (P < 0.03). EEG activity 2 h after reperfusion and starting of rewarming correlated with eventual neurological outcome. EEG variables associated with good outcome were: main final frequency and degree of rhythmicity of the activity. We conclude that PH exerted a protective effect for animals undergoing 2 h of circulatory arrest. EEG was a useful tool for predicting neurological outcome under the studied conditions.

Glutamate neurotoxicity in the developing rat cochlea is antagonized by kynurenic acid and MK-801

Glutamate (Glu) is neurotoxic in the neonatal rat cochlea, producing hearing impairment which is largely due to the death of spiral ganglion cells, whereas the receptor hair cells are spared. Dendritic processes of the spiral ganglion are postsynaptic to the primary afferent synapse of the auditory system. The experiments reported here were designed to test whether this apparent excitotoxicity can be blocked by Glu antagonists. The broad-spectrum antagonist kynurenic acid (KYNA) was coadministered with Glu initially to determine whether the high-frequency hearing deficit caused by Glu may be mediated by excitatory amino acid receptors. Subsequently, the N-methyl-D-aspartate (NMDA)-specific receptor blocker MK-801 was used to test whether NMDA receptors may be involved in the effect. Both antagonists partially blocked the high-frequency hearing impairment caused by Glu. The blocker-alone control groups exhibited mid-frequency effects of unknown origin. The significant antagonism of Glu-induced impairment is consistent with the hypothesis that Glu or a similar excitatory amino acid is an important afferent transmitter in the cochlea.

Thermal influences on nervous system function

The various effects of temperature change are only partially predictable. Temporal measures relevant to membrane activity, action potentials, synaptic transmission, and evoked potentials are all consistently increased with cooling and decreased by warming. However, the various measures of amplitude at different levels, and even within similar preparations, are contradictory: Some laboratories report increased amplitudes with cooling and others report decreased amplitudes under similar conditions. Emphasis is given to identifying factors that may resolve the differences. These include: (a) the rate of temperature change, (b) sites of cooling, stimulation and recording, (c) stimulus characteristics, and (d) fundamental differences in temperature sensitivities of different neural tissue. Other factors that may affect the ability to predict thermal influences on neural function from existing formulations are: relative ion permeabilities, metabolic ion pumps, the resting potential at the onset of cooling, and an animal's acclimated temperature at sacrifice.