Effects of Prenatal Ethanol Exposure on the Postnatal Development of the Brainstem Auditory Evoked Potential in the Rat

Fetal alcohol syndrome. Hearing, speech, language, and vestibular disorders

Fetal alcohol syndrome (FAS) refers to a pattern of anomalies that include craniofacial, CNS, growth, and various sensory anomalies. We have observed that FAS is associated with four kinds of hearing disorders: (1) developmentally delayed auditory function, (2) sensorineural hearing loss, (3) intermittent conductive hearing loss owing to recurrent serous otitis media, and (4) central hearing loss. As is the case with other syndromes associated with craniofacial anomalies and hearing impairments, speech and language pathologies also are common in FAS patients. Although auditory and vestibular systems arise from similar embryological tissue, vestibular dysfunction is variable in FAS. Early identification and intervention to treat hearing, language, and speech problems should improve the functional level of FAS in children.

Hearing, speech, language, and vestibular disorders in the fetal alcohol syndrome: a literature review

Fetal alcohol syndrome (FAS) is characterized in part by mental impairment, as well as craniofacial and ocular anomalies. These conditions are traditionally associated with childhood hearing disorders, because they all have a common embryonic origin in malformations of the first and second branchial arches, and have similar critical periods of vulnerability to toxic insult. A review of human and animal research indicates that there are four types of hearing disorders associated with FAS. These are: (1) a developmental delay in auditory maturation, (2) sensorineural hearing loss, (3) intermittent conductive hearing loss due to recurrent serous otitis media, and (4) central hearing loss. The auditory and vestibular systems share the same peripheral apparatuses (the inner ear and eighth cranial nerve) and are embryologically and structurally similar. Consequently, vestibular disorders in FAS children might be expected. The evidence for vestibular dysfunction in FAS is ambiguous, however. Like other syndromes associated with craniofacial anomalies, hearing disorders, and mental impairment, FAS is also characterized by a high prevalence of speech and language pathology. Hearing disorders are a form of sensory deprivation. If present during early childhood, they can result in permanent hearing, language, and mental impairment. Early identification and intervention to treat hearing, language, and speech disorders could therefore result in improved outcome for the FAS child. Specific recommendations are made for intervention and future research.

Alcohol, auditory functioning and deafness

This paper reviews studies of alcohol and auditory impairment and the treatment of deaf people with alcohol problems and concludes that: (a) alcohol affects auditory functioning, (b) prenatal exposure to alcohol is a risk factor for hearing impairment in FAS children,

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.

Prenatal ethanol exposure in rats: long-lasting effects on learning

Pregnant Sprague-Dawley rats were fed a liquid diet containing either 0% (group C), 18% (group L), or 36% (group H) ethanol-derived calories (EDC) from gestational day 1 to 20. Male offspring were assessed under a conditioned taste aversion paradigm (PND 35-45), in a complex maze (PND 68-80), and for operant behavior (temporal response differentiation and motivation to work for food, PND 140-198). Although conditioned taste aversion was fully acquired by all groups, retention of the conditioned taste aversion response was impaired in group H animals. Importantly, deficits in the acquisition of timing behavior were found in group H (group L not tested), confirming that this operant task is quite sensitive in detecting prenatal drug effects and demonstrating that neurological effects of prenatal ethanol exposure persist into late adulthood.

Electrophysiological and behavioral findings in rats prenatally exposed to alcohol

Clinical studies have shown that behavioral and electrophysiological abnormalities occur in some human infants born to alcoholic mothers. The present study used rats to evaluate the effects of prenatal ethanol exposure on locomotor behavior and two paradigms for the generation of auditory event-related potentials (ERPs). Pregnant rats were fed liquid diets, one group received alcohol and the other was a nutritional control. The offspring were allowed to mature prior to the electrophysiological and behavioral testing. Prenatal alcohol exposed rats showed significantly more locomotor behavior in the first 4 hours of their activity cycle than nonethanol exposed rats, although not over a subsequent 8-hour period. Evaluation of the electrophysiological data revealed that the prenatal alcohol exposed group had significantly longer latencies of their P1 and N1 ERP components, in the hippocampus, than the control group. These results support anatomical data suggesting that the hippocampus may be an important area in which to direct further study of what brain mechanisms may be altered by prenatal ethanol exposure in the rat.

Prenatal alcohol exposure attenuates pentylenetetrazol-induced convulsions in rats

Female rats prenatally exposed to alcohol were tested at 35 days of age for susceptibility to pentylenetetrazol (PTZ)-induced convulsions. There was a significant dose-response effect of PTZ. Contrary to expectations, alcohol-exposed females were slightly but still significantly less convulsion-prone than pair-fed controls. These results suggest prenatal alcohol exposure reduces the susceptibility to convulsions in rats.

Audiogenic seizure susceptibility and auditory brainstem responses in rats prenatally exposed to alcohol

Rats prenatally exposed to alcohol (0%, 17.5%, or 35% ethanol-derived calories) were tested for audiogenic seizure susceptibility on one of postnatal days 18 to 23, then retested 5 days later. Prenatal alcohol exposure did not influence audiogenic seizure susceptibility or severity. There was, however, a significant increase in seizure incidence on the retest day for all groups, suggesting a priming effect. Auditory brainstem response (ABR) data suggested that prenatal alcohol exposure and acoustic trauma (i.e., exposure to an alarm bell used for eliciting audiogenic seizures) induced measurable sensorineural hearing loss, and that the combined exposure to alcohol and acoustic trauma interacted additively to produce greater hearing loss than either alone.

Chronic exposure to alcohol during development alters the membrane properties of cerebellar Purkinje neurons in culture

The active and passive membrane properties of developing Purkinje neurons in control cultures and cultures chronically treated with 20 or 40 mM ethanol for 1 or 2 weeks were examined using whole-cell current-clamp techniques. The membrane properties were characterized by the features of the voltage responses evoked by intracellular current injection of a series of depolarizing and hyperpolarizing current pulses. Analysis of these responses and background spontaneous activity showed several differences between the control and ethanol-treated Purkinje neurons: (1) membrane input resistance was significantly larger in the ethanol-treated neurons; (2) the percentage of neurons exhibiting immature firing patterns was significantly higher in the ethanol-treated neurons; (3) the afterhyperpolarization following a current-evoked train of action potentials was significantly larger in the ethanol-treated neurons; (4) spontaneous activity (synaptic potentials and synaptically evoked spike events) was significantly reduced in neurons treated with 40 mM ethanol for 1 week; spontaneous activity in neurons treated with 20 mM ethanol for 1 or 2 weeks was similar to that observed in the control group. These differences indicate that ethanol exposure during development directly alters the physiological properties of this CNS neuronal type. These neuronal actions of ethanol may contribute to the behavioral deficits observed in animals models of fetal alcohol syndrome. Similar target sites of ethanol action are likely to be present in the human CNS neurons and may be involved in human fetal alcohol syndrome.

Sensorineural hearing loss as evidenced by the auditory brainstem response following prenatal cocaine exposure in the Long-Evans rat

Prenatal cocaine exposure has been associated with a variety of adverse neurological effects. Three recent studies found evidence that prenatal cocaine exposure is associated with abnormal auditory electrophysiology, suggesting abnormal processing of auditory information. The present study used the auditory brainstem response to evaluate the effects of prenatal cocaine exposure on hearing in an animal model (Long-Evans rat). We report that prenatal cocaine exposure can cause elevated ABR thresholds and latency-intensity curves consistent with a recruitment-type sensorineural hearing loss.

New approaches to research on the long-term consequences of prenatal exposure to alcohol

As the summary presentation of a symposium on prenatal alcohol-induced brain damage and long-term postnatal consequences, this paper proposes the establishment of two main research priorities--to begin to correlate long-term behavioral effects with alterations in underlying neural substrates, and to explore the mechanisms of neuroteratogenicity. To reach these goals, three objectives are described. First, animal and human research must become more interrelated. Second, experimental observations should be integrated into formal models that incorporate both neural structure and function. Third, researchers should choose well-defined dependent measures that are derived from models of brain function based on modern concepts of cognitive neuroscience. Examples of neuropsychological tests that may serve as the bases for structure/function relationships are presented. Incorporating these objectives into future research will facilitate understanding of the fundamental issues concerning prenatal alcohol exposure and will begin to provide the bases for rational intervention or treatment.

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.

Workshop on the qualitative and quantitative comparability of human and animal developmental neurotoxicity, Work Group I report: comparability of measures of developmental neurotoxicity in humans and laboratory animals

Assessment measures used in developmental neurotoxicology are reviewed for their comparability in humans and laboratory animals, and their ability to detect comparable adverse effects across species. Compounds used for these comparisons include: substances of abuse, anticonvulsant drugs, ethanol, methylmercury, lead, PCBs, and ionizing radiation. At the level of functional category (sensory, motivational, cognitive and motor function, and social behavior), close agreement was found across species for all neurotoxic agents reviewed, particularly at high exposure levels. This was true even though the specific end points used to assess these functions often varied substantially across species. In addition, it was found that: 1) the U.S. EPA Developmental Neurotoxicology Test Battery presented at the Workshop would have identified the hazard to humans of exposure to the above compounds, although it may have underestimated human risk in some cases; 2) assessment of developmental neurotoxicity should involve evaluation of all categories of function; 3) for most compounds reviewed, the neurotoxic effects of prenatal exposure cannot be attributed to maternal toxicity, and exposure at or just below the threshold for such toxicity is an appropriate upper level for developmental neurotoxicity testing; 4) maternal exposure during the postnatal period poses a number of serious methodological problems; and 5) animal studies would better parallel human studies if more emphasis was placed on evaluation during development.

Thiamine deficiency in utero alters response to ethanol in adulthood

To determine whether prenatal thiamine deficiency, a frequent concomitant of alcoholism, reduces the response to ethanol during adulthood in the rat as does ethanol exposure in utero (Abel et al. 1981), pregnant Sprague-Dawley rats received either control or thiamine deficient diets together with daily injections of the thiamine antagonist pyrithiamine. At 7 months of age, male offspring were exposed to precisely regulated ethanol vapor concentrations in an inhalation chamber for 24 h and blood ethanol concentrations (BECs) and ethanol-induced intoxication were determined. Prenatally thiamine deficient rats and controls were indistinguishable in terms of appearance, body and liver weights, and the ratios of liver to body weight and brain to liver weight. However, total body water was significantly greater, and BECs and behavioral impairment were decreased, in the experimental rats. These findings indicate that prenatal thiamine deprivation is associated with reduced pharmacologic effect of ethanol as a result of increases in its volume of distribution and rate of metabolism.

Neuron development in the superior colliculus of the fetal mouse following maternal alcohol exposure

Pregnant Swiss Webster mice were given a liquid diet with ethanol (EtOH) or isocaloric amounts of maltose dextrin on gestation day (GD) 0 through 18. On GD 18, maternal blood samples were obtained. Fetuses were then removed and fetal brains were prepared for light microscopy. Fetal weight was reduced in the EtOH-exposed group. The ratio of midbrain cross sectional area to cerebral aqueduct was reduced in the ethanol group, while the density of neuronal nuclear population in both the dense outer layer (DS) and sparse inner layer (SS) of the developing superior colliculus was increased. Mean nuclear volume was decreased in the SS.

Chronic in utero alcohol exposure affects auditory function in rats and in humans

In one experiment, brainstem auditory evoked potentials (BAEPs) were used to study the postnatal development of the peripheral and brainstem auditory pathways in rat pups prenatally exposed to alcohol. The results indicated that prenatal alcohol exposure retards development of the peripheral and brainstem auditory pathways and that prolonged auditory transmission times, despite a "catch-up" trend, persist in maturity. BAEP latency-intensity profiles demonstrated that a significant proportion (19%) of the alcoholized rats had recruitment-type sensorineural hearing loss. In a second study, a group of 12 fetal alcohol syndrome (FAS) children were evaluated for auditory function. These children showed unusually high incidence rates of sensorineural hearing loss (33%) and conductive hearing loss secondary to recurrent serous otitis media (92%). A final study used the cortical auditory evoked potential (CAEP) and found evidence of dysfunctional processing of auditory information at the cortical level in rats prenatally exposed to alcohol. The implications of these findings for the evaluation and treatment of FAS children and for evoked potential studies on children of alcoholics and learning disabled children are discussed.

Changes in the brainstem auditory evoked response of the rabbit during the first postnatal month

Brainstem auditory evoked responses (BAERs) were recorded in 73 albino rabbits during the first postnatal month. Responses could not be evoked before the ninth day post-term using free field click stimulation at 60 dBHL. The onset of BAERs to these stimuli on or after day 9 was coincident with the onset of behavioural responses to sounds and, in the majority of animals, with eye opening. The onset of BAERs was delayed in animals with low body weight. The intensity required to evoke detectable BAERs in normally grown animals decreased rapidly after day 9 post-term. The most significant changes in the form of the BAER in the first postnatal month were an increase in the amplitude of peak III and the separation of peaks IV and V. Peak I and the negative dip after peak V (Vn) were consistent features of the BAER during development. The latencies of these deflections and the interval between them decreased by approximately 1.5 and 4 ms respectively up to the end of the first month post-term. On days 9 and 10 post-term, stimulation at a higher rate (40 Hz) failed to evoke a BAER in some animals. In other animals the change in stimulation rate from 10 to 40 Hz produced a large increase in the latency of peak V. The unusually large changes in the latencies of peaks and the interpeak intervals during the development of the rabbit indicate that this animal may be particularly suitable for studies of perinatal complications on development of the brainstem when the BAER is to be used as non-invasive measure of neural function.

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.

Postnatal development of the brainstem auditory evoked potential and far-field cochlear microphonic in non-sedated rat pups

Normal postnatal development of the scalp-recorded cochlear microphonic (CM) response and brainstem auditory evoked potential (BAEP) were studied in rat pups. BAEP latencies decreased and amplitudes increased as a function of maturation. These changes occurred rapidly between 14 and 23 days of age with gradual change occurring thereafter. The observed latency changes indicated that the auditory pathway matures in a sequential manner from the most peripheral to the most rostral structures. For example, CM latency stabilized (matured) by day 17 while BAEP wave I and II latencies stabilized by days 23 and 35, respectively. BAEP waves III and IV still showed significant latency decreases between days 42 and 70. The between-litter variability for CM and BAEP wave latencies also decreased with age. In contrast to peak latency measures, CM and BAEP amplitudes followed an independent time course of postnatal development. CM amplitude did not increase significantly after day 14. Amplitudes of all 4 BAEP components increased steadily from day 14 to 29, then stabilized. Unlike latency variability, amplitude variability was independent of age. There were no significant gender-dependent differences in amplitudes or latencies between the ages of 14 and 70 days.