A method for chronically recording brain-stem and cortical auditory evoked potentials from unanesthetized mice

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.

Auditory processing disorder (APD)-potential contribution of mouse research

APD is a common, heterogeneous and poorly understood listening impairment that is receiving increasing recognition, especially in children and the elderly. The primary symptom in humans is poor speech perception despite normal pure tone audiometry. Diagnostic practice is patchy, but current proposals are to distinguish APD by reduced ability to detect, discriminate, localize and order non-speech sounds. APD appears to result predominantly from core malfunctioning of the central auditory system, but with strong cognitive influences and possible additional influences from peripheral auditory and crossmodal sources. APD may be acquired (e.g. through middle ear disease), but it is likely that a more common etiology would involve both environmental and inherited contributions. Mice are increasingly being used for studies of central auditory processing. Most work to date has documented the basic response properties of auditory neurons, or used single gene mutations to investigate specific functions. For high throughput, primary screening, reflex and/or event-related potential measures are required. Pre-pulse inhibition (PPI), the regulation of an acoustic startle reflex by a preceding sound having distinct spatial, temporal or spectral properties, is one way in which auditory processing may be screened. For secondary assessment, standard psychoacoustic tests for mice have been developed.

Inhibition of auditory evoked potentials and prepulse inhibition of startle in DBA/2J and DBA/2Hsd inbred mouse substrains

Previous data have shown differences among inbred mouse strains in sensory gating of auditory evoked potentials, prepulse inhibition (PPI) of startle, and startle amplitude. These measures of sensory and sensorimotor gating have both been proposed as models for genetic determinants of sensory processing abnormalities in patients with schizophrenia and their first-degree relatives. Data from our laboratory suggest that auditory evoked potentials of DBA/2J mice differ from those previously described for DBA/2Hsd. Therefore, we compared evoked potentials and PPI in these two closely related substrains based on the hypothesis that any observed endophenotypic differences are more likely to distinguish relevant from incidental genetic heterogeneity than similar approaches using inbred strains that vary across the entire genome. We found that DBA/2Hsd substrain exhibited reduced inhibition of evoked potentials and reduced startle relative to the DBA/2J substrain without alterations in auditory sensitivity, amplitude of evoked potentials or PPI of startle. These results suggest that gating of auditory evoked potentials and PPI of startle measure different aspects of neuronal function. The differences between the substrains might reflect genetic drift. Alternatively, differences could arise from different rearing environments or other non-genetic factors. Future studies will attempt to determine the cause of these differences in sensory and sensorimotor processing between these two closely related inbred mouse strains.

Effects of prenatal alcohol exposure and aging on auditory function in the rat: preliminary results

This study investigated select aspects of peripheral and central auditory dysfunction, as well as the pathological effects of aging, In an animal model of fetal alcohol syndrome (FAS). Pregnant rats consumed liquid alcohol diets containing 0, 17.5, or 35% ethanol-derived calories, from gestation day 7 to parturition. A fourth group was untreated. Offspring of these mothers were tested for auditory and neurological function, using the auditory brainstem response at 6, 12, and 18 months of age. Some animals in the alcohol-exposed groups showed a peripheral auditory disorder in the form of congenital sensorineural hearing loss. This was correlated with punctate lesions and malformed stereocilia on the auditory sensory receptor cells of the inner ear. Alcohol-exposed animals also showed a central auditory processing disorder characterized by prolonged transmission of neural potentials along the brainstem portion of the auditory pathway. Animals in the highest dose group also showed an augmentation in the age-related deterioration of auditory acuity. Thus, increased peripheral and central auditory dysfunctions and pathological deterioration of auditory function in old age may be sequelae of FAS. Such morbidities have important implications for the long-term clinical assessment and management of FAS patients.

Fast habituation of the long-latency auditory evoked potential in the awake albino rat

Fast habituation of the long-latency, vertex-recorded auditory evoked potential (AEP) peaks in humans was first described by Callaway (1973) as a reduction in AEP amplitude that occurs to the second of a pair of acoustic stimuli when both stimuli are presented with an interstimulus interval (ISI) of no more than 10 sec. When acoustic stimuli are presented in pairs with an ISI of 2 sec and an interpair interval (IPI) of approximately 10 sec, reduction in amplitude to the second tone occurs by as much as 30-50%. Fast habituation may depend somewhat on a subject's anticipation of the stimulus and on other factors related to attention and orienting. Studies in our laboratory have demonstrated this amplitude decrement to the second tone of a pair in human infants, children and adults and have explored the implications of this finding with respect to attentional processes and the allocation of cerebral resources. In the present investigation we describe an animal model of fast habituation. Here, vertex-recorded AEPs were obtained to paired tone stimuli delivered to awake adult male Sprague-Dawley rats chronically implanted with skull electrodes. Findings showed: (a) an AEP wave form with 8 distinct peaks, (b) for one component there was a marked decrement in amplitude from tone 1 to tone 2 in recordings obtained from an electrode placed slightly to the right of midline, and (c) that there were no significant differences in peak latencies across tones. This methodology may further our understanding of fast habituation in humans and may prove useful for studies of attention, orienting, and resource allocation using techniques that are not possible for use with human subjects.

Prenatal cocaine exposure in the Long-Evans rat: III. Developmental effects on the brainstem auditory-evoked potential

Prenatal cocaine exposure has been associated with a variety of adverse neurological effects in infants and laboratory animals. Of particular interest, one group of investigators reported that exposed neonates have an abnormality in the brainstem auditory-evoked potential (BAEP). The particular abnormality, a prolongation in the wave I-V interpeak latency, suggested delayed or desynchronized transmission of subcortical auditory information. To further investigate this possible consequence of prenatal cocaine exposure, pregnant Long-Evans rats were injected daily with 60, 80 or 100 mg/kg cocaine HCl (SC, 2% solution) with half the daily dose given in the morning and the other half given in the afternoon. Treatment was given from gestation days 7 to 20 (sperm positive = GD 0). Ad lib-fed and pair-fed control groups were also used. Offspring were evaluated at the age of 35 days (birth = PD 0) and as adults (6-10 months). BAEPs were elicited by click stimuli presented over a broad range of intensities and repetition rates. Prolongation of the interpeak latencies and a reduction in BAEP amplitudes were observed only in the highest dose (C100) group, only at the age of 35 days, and only at the highest stimulus intensity. While these results support those found in exposed neonates, our data suggest a) that the effects are developmental delays which dissipate with aging and b) that the effects require high cocaine exposure.

The effects of chloralose on the brainstem auditory evoked potential of the rat

The effects of alpha-chloralose were examined on the brainstem auditory evoked potential of the rat. Subjects were given one of four different doses of chloralose. These were a sedative dose (50 mg/kg), an anaesthetic dose (100 mg/kg), a usually fatal dose (150 mg/kg) or an invariably fatal dose (200 mg/kg). Recordings were made 60 min later. There was no evidence of any dose-dependent increase or decrease in the amplitudes of the four principal components (waves I, II, III and IV) of the brainstem auditory potential. There were only minor dose-dependent increases in latency. These were less than 0.1 msec for waves I, II and III and just over 0.1 msec for wave IV. Wave IV was the component which was most sensitive to chloralose, although this was apparent only at a large dose. In general, the latencies showed little or no difference from those recorded previously from awake animals. The results are discussed in relation to a series of experiments, where animals were initially immobilized with chloralose in order to study the effects of a variety of pharmacological agents on the brainstem auditory potential.

Effects of cocaine on the brain-stem auditory evoked potential in the Long-Evans rat

The present study examined the effects of an acute psychoactive dose of cocaine hydrochloride (HCl) in the rat, using the brain-stem auditory evoked potential (BAEP) as an objective, quantitative measure of this substance's effects on brain and auditory electrophysiology. The animals were 8 adult Long-Evans rats (4 female, 4 male). BAEPs were recorded from skull screw electrodes during a baseline period as well as 30-90 min after cocaine HCl treatment (10 mg/kg, i.p.). Normothermia was maintained to control for possible temperature-related effects. Cocaine's effects on the BAEP were examined over a broad range of stimulus intensities (intensity profiles) and repetition rates (rate profiles). Cocaine prolonged latencies of several BAEP components at low stimulus intensities and shortened these latencies at high stimulus intensities. The average BAEP threshold was also increased by cocaine treatment. These results were not strong, but were suggestive of a recruitment type change in auditory function. Cocaine treatment had no convincing effects on the BAEP as a function of stimulus repetition rate.

Dose-dependent effects of atropine sulfate on the brainstem and cortical auditory evoked potentials in the rat

Because brainstem auditory evoked potentials (BAEPs) are frequently recorded in anesthetized animals and humans, it is important to become familiar with the effects on the BAEP of drugs used during anesthesia, including pre-anesthetics. The dose-dependent and stimulus intensity-dependent effects on the BAEP of a pre-anesthetic, atropine sulfate, were studied in the unanesthetized rat. The animal subjects were 11 adult female Long-Evans rats. BAEPs in response to 0.1 ms clicks (12.5/s) were recorded from skull screw electrodes during a baseline period, as well as after saline and atropine treatments. Atropine sulfate was given i.p. in doses ranging from 0.250 to 40 mg/kg. Contrary to a prior report, doses in the standard pre-anesthetic range (i.e. 0.250-1.000 mg/kg) did not convincingly influence the BAEP. Only the highest dose (40 mg/kg) produced a significant and noteworthy change in the BAEP. This effect was characterized by significant amplitude increases in the P1, P2 and P3 components, but not in the P4, P5 and P6 components. This selective effect occurred at the highest stimulus intensity of 110 dB peak equivalent sound pressure level, but not at lower intensities. There were no convincing atropine-induced changes in BAEP latencies. Atropine-induced changes in the cortical auditory evoked potential (CAEP) were characterized by amplitude decrements. Thus, atropine seemed to have an excitatory effect on the BAEP and an inhibitory or depressive effect on the CAEP.

Effects of ketamine anesthesia on the rat brain-stem auditory evoked potential as a function of dose and stimulus intensity

Because ketamine is both an abused substance and a commonly used veterinary anesthetic, its effects on brain and sensory functions are of interest. The present study examined the dose-dependent effects of ketamine anesthesia in the rat, using the brain-stem auditory evoked potential (BAEP) as an objective, quantitative measure of this substance's acute effects on brain and sensory electrophysiology. The animal subjects were 11 young adult female Long-Evans rats. BAEPs were recorded from skull screw electrodes during a baseline period as well as after saline and ketamine treatments. Ketamine was administered (i.p.) in 2 serial doses. The first dose (100 mg/kg) was followed 30-40 min later by a second dose (also 100 mg/kg). Equal volumes of normal saline were also injected serially. An interval of 1-2 weeks occurred between the saline and ketamine treatments, with treatment order counterbalanced. Normothermia was maintained to control for possible temperature-related effects. Ketamine produced prolongations in the latencies of all BAEP components (P1 through P6) that were statistically significant relative to baseline values. These latency shifts were progressively greater for waves P1 through P4 (shifts in P5 and P6 approximated the P4 shift). The effect of the second ketamine injection was to nearly double the latency shifts. Ketamine also had significant and complex effects on BAEP amplitudes that were dependent on dose and stimulus intensity. The results of the present study challenge the belief that the BAEP is resistant to the effects of anesthetics and suggest that the BAEP is useful in characterizing the CNS and sensory effects of these pharmacological agents.

Pentobarbital-induced changes in the mouse brainstem auditory evoked potential as a function of click repetition rate and time postdrug

The effects of pentobarbital (80 mg/kg, i.p.) and saline injections on the BAEP were studied in 10 adult female BDF1 mice. Pentobarbital anesthesia induced statistically significant increases in the amplitudes and latencies of various BAEP components relative to preinjection and saline control values. These pentobarbital-induced changes were maximal shortly after drug administration and dissipated over time in a pattern similar to a drug elimination curve. Since pentobarbital and fast stimulus repetition rates are considered to be 'synaptic stressors', it was predicted that pentobarbital anesthesia and click repetition rate would have combined effects on the BAEP. This prediction was partially supported in that pentobarbital-induced changes in P4 amplitude and latency were significantly dependent on click repetition rate. The pentobarbital-induced effects on earlier BAEP components, however, proved to be largely independent of click rate. Pentobarbital-associated changes in the BAEP were not due to such factors as core temperature changes, circadian variations, and stress. The importance of anesthetic-induced changes in the BAEP for clinical and experimental studies is discussed.

Theophylline-induced changes in the mouse brainstem auditory evoked potential

Because theophylline is a widely used analeptic, there is interest in its possible ototoxic and neurotoxic effects. The present study used the brainstem auditory evoked potential (BAEP) to evaluate the acute effects of theophylline on auditory electrophysiology in mice. Adult female C57BL/6 mice were injected intraperitoneally with 150 mg/kg theophylline or saline. The BAEP thresholds and latency-intensity profiles suggested that theophylline induced a rather slight but statistically significant change in auditory function that was suggestive of a temporary recruitment-type sensorineural deficit. This observation raises concerns about the possible ototoxic effects of theophylline, particularly in asthmatic patients and apneic preterm neonates who receive this drug chronically. While there was BAEP evidence of ototoxic effects for theophylline, there was no evidence of neurotoxic effects.

Age-related hearing loss in BDF1 mice as evidenced by the brainstem auditory evoked potential

Age-related hearing loss was studied in unanesthetized female BDF1 mice using brainstem auditory evoked potential (BAEP) latency-intensity (L-I) profiles and BAEP thresholds. The mice were divided into 5 age-related groups: 3-4 months, 5.5-7 months, 9-10 months, 12 months and 18 months. Relative to the youngest group, BAEP threshold data indicated shifts in auditory sensitivity ranging from +6 dB for the 5.5- to 7-month-old group to +71 dB for the 18-month-old group. The BAEP L-I profiles of older mice were consistent with recruitment-type sensorineural (cochlear) hearing losses. No evidence of retrocochlear involvement was observed.