Brain stem auditory-evoked potentials in different strains of rodents T.-J. C H E N and S.-S. C H E N Department of Neurology and Physiology, Graduate Institute of Medicine, Kaohsiung Medical College, Taiwan, Republic of China CHEN,T.-J. & CHEN,S.-S. 1990. Brain stem auditory-evoked potentials in different strains of rodents. Acta Physiol Scand 138, 529-538. Received 16 May 1989, accepted 30 November 1989. ISSN 00014772. Department of Neurology and Physiology, Graduate Institute of Medicine, Kaohsiung Medical College, Taiwan, Republic of China. This study was conducted to evaluate variations in brain stem auditory-evoked potentials (BAEPs) among different strains of rodents. BAEPs were recorded by routine procedures from rodents of different strains or species. These included 22 Long-Evans, 28 Wistar and 28 Sprague-Dawley rats, and six hamsters. Within the first 10 ms, there were five consistent and reproducible positive waves of BAEPs in each rodent, named I, 11, 111, IV and V in correspondence with the nomenclature ofwaves I-VII in human BAEPs. These BAEPs were also similar to those observed in other vertebrates and in human controls. However, there were variations in waveforms and peak latencies among rodents, even in the rats of the same strain that came from different laboratory centres. At optimal stimulation intensity, usually around 90 dB, the mean latencies of the waves varied as follows: I, 1.23-1.53 ms; 11, 1.88-2.28 ms; 111, 2.62-2.94 ms; IV, 3.49-3.97 ms; and V, 4.47-5.14 ms. They were significantly different between species, but not in different strains of rats if they came from the same animal centre. The conduction time in the central portion illustrated by interpeak latencies between I and 111, 111 and V, and I and V was dependent on the species (P < 0.05).When recorded in a soundproof incubator, the minimal hearing threshold showed a significant species difference. The animal BAEP model can be employed for evaluating the physiological function or the pathological conditions of the brain stem. The confirmation of BAEP variations among different species or strains will be helpful in deciding which kind of rodents will be appropriate to serve as animal models for the various purposes of BAEP studies Kry words : brain stem auditory-evoked potential, rodents, strain differences.
Brain stem auditory-evoked potentials (BAEPs) arc recorded by a computer-averaging system from the scalp in response to sounds such as clicks or tone bursts. They consist of 5-8 distinct peaks in periods of 10-ms latency recordings. They are assumed to arise from the nerve tracts and nuclei of the ascending auditory pathways (Buchwald 1983). These potentials were first Correspondence: Dr Shun-Sheng Chen MD, PhD, Department of Neurology and Physiology, Graduate Institute of Medicine, Kaohsiung Medical College, Kaohsiung City, 80708, Taiwan, Republic of China.
recognized in cats (Jewett 1970) and soon after reported in human beings (Jewett et al. 1970). As clinical and research tools in human and animal studies, BAEPs have proved to be extremely useful in detecting brain stem dysfunctions or lesions in neurological diseases, in determining the acoustic pathways, and in the study of a drug effect or for supplementing methods of evaluating brain death (Starr 1976, Bhargave et af. 1978, Starr & Achor 1978, Achor & Starr 1980b, Chen et af. 1988). T o explore various neurological problems in human beings, many animal experiments con-
529 21-2
-5 30
T.-J. Chen a r d S.-S. Chen
cerning BAEPs have been performed, since from physiological and neuropathological aspects there are many difficulties in performing human studies that are not present when studying animals. T h e experimental conditions are more controllable in animals, facilitating research into the generators of these el-oked potentials and the morphological changes or the effect of drugs on BAEPs. Man! kinds of animals h a w been emplo!-cd for studying BAEPs, including monkeys (Allen & Starr 1978, Cazals et 01. 1978), baboons (Branston et 01. 1976), dogs (llorgan et al. 1980), cats (Berry rt al. 1976, Huang 8i Buchwald 1978, Achor & Starr 1980b), rats (Borbely 1970, Amochaev et al. 1979, Osako et al. 1979) and mice (Henry & Haythorn 1978); in particular cats h a w been used extensively. Rats or othcr rodents are preferred, because the! are less espensive, the experimental conditions are much more easily controlled and the rat has a similar acoustic pathway to that of the cat and man. In the past, various species or strains of rats and other rodents including Sprague-Dawley (Church rt al. 1984, Shapiro rt al. 19841, 1.ong-Evans (Church & Gritzke 1987), Sabra (Gold et al. 1985), mice (McGinn rt al. 1973, Henry 1979)~hamsters (Schweitzer 1987) and guinea-pigs (Eggermont 1976, Schwent et a!. 1980, Gardi & Berlin 1981) have been used for the study of BAEPs. Most studies have focused on lesions in the brain stem, generators of B.4EPs, brain stem development, effects of drugs or sedation, or temperature effect, etc. (Jewett bi Romano 1972, Henry 1979, Osako et al. 1979, Gl;o & E'anagihara 1980, Gardi 8i Berlin 1981, Shapiro r t al. 1984, Gold r t al. 1985). In spite of the frequent use of rats as animal models in Br\EP studies. there are no comparisons of one strain uith another. So far the identification and nomenclature of evoked \t a\ e5, waveforms or peak latencies of H.\EPs in different rodents or even in the same strain of rats are not uniformly recognized (Henry ei Haythorn 1978, Amochaev et al. 1979, Osalio et al. 1979, Schwent et at. 1980). T h e method of recording BAEPs and the experimental conditions, such as the sites of the recording electrodes and the stimulation parameters, including intensity, frequency and interval, and physiological factors such as temperature and age, were not controlled in most studies. This study was conducted to evaluate vari-
ations of BAEPs in the different rodents and to compare them with human BAEPs. T h e study also attempted to determine which species of rodents were the most appropriate to serve as an animal model for the different purposes of the BL4EPsstudies.
MrZTERIALS AND M E T H O D S Four types of rodents and 14 normal human controls were used for this experiment. Normal human controls were all adults with ages ranging from 20 to 2; years. The rodents included six hamsters and three strains of rats: 22 Long-Evans, 28 Wistar and 28 Sprague-Dawley. Both the Wistar and Sprague-Dawley rats came from two separate animal laboratory centres in Taiwan and were studied as two different groups. All the rodents were mature males with ages ranging from 3 to j months. Before the experiment, they were fed normally with laboratory chow and water. In the animal study, to permit electrodc placement and recording, all animals were anaesthetized with sodium pentobarbital 50 mg kg-' intraperitoneally. Since thermoregulatory mechanisms were impaired by anaesthesia, the deep rectal temperature was controlled between 36.5 and 37.5 "C with an electric heating pad and a temperature-controlled incubator. .4fter anaesthesia, two stainless-steel screws were inserted into the skull, overlying the right mastoid bone and the left frontal bone, to serve as the permanent active and reference electrodes respectivell-. In addition, one needle electrode was attached to the left mastoid to serve as the earth electrode. In our human study, the subjects were lying in a quiet laboratory room. Two surface recording electrodes fixed with collodion-soaked gauze were placed, one at F, and one over the mastoid ipsilateral to the stimulated ear. The contralateral mastoid was used as an earth. BAEPs were recorded using a signal processor (7S11.4,San-Ei Company, Japan). The sound stimuli were clicks, delivered at a repetitive rate of 13.3 s-' via an earphone 2 cm from the animal's right ear or directl!. onto the human right ear. The stimulation intensity was always fixed at 90 dB for the human controls, and for the rodents from a maximum of I 10 dB down until the minimal hearing threshold was reached, usually at around 30 dB. The evoked potentials were then amplified and filtered with a bandpass between 80 and 300oHz. Each RAEP recording was averaged from a total of 1024 clickevoked responses for the first Io-ms period following the stimulation. The brain stem potentials were compiled by a computer-averaging system and then plotted on graph paper by an X-Y recorder. .4fter recording BAEP waves at various stimulation intensities, the consistency of waveforms, peak latencies and interpeak latencies (IPLs) in each rodent was
BAEPs in rodent model
531
Table I . Comparison of peak latencies of BAEPs among human controls and rodents: stimulation at 90 dB (mean fSD) Wave (ms)
n
Human controls Long-Evans (I) Sprague-Dawley (I) Sprague-Dawley (11) Wistar (I) Wistar (11) Hamster (I)
I
14
1.44f0.08 1.4540.09 16 1 . 4 7 f o . 1 1 12 1 . 4 5 5 0 . 1 2 I7 1 . 5 3 f 0 . 1 2 11 1.4450.10 6 1.23&0.11
22
I1
111
IV
V
2.47 fo.24 1.9350.11 1.88f0.09 2.21f0.16 I.9IfO.IO 2.28+0.09 2.09f0.12
3.67k0.17 2.6240.13 2.64f0.10 2.88f0.19 2.64fo.11 2.9450.13 2.94fo.1~
4.88 f o . 1 5 3.49k0.13 3.51k0.12 3.785023 3.50f0.17 3.97k0.18 3.94fo.17
5.47 0.19 6.94 f0.25 4.52f0.22 5.21f0.48 4.56k0.27 4.69&0.31 4.47f0.20 5.03k0.22 6.16fo.17 5.14f0.26 -
VI
(I), (11) : rodents from two different animal laboratory centres. Human controls; Long-Evans (I); Sprague-Dawley (I); Wistar (I); hamster (I): F = 9.71; 45.88; 184.43; 241.02; and 52.58 for waves I-V. P < 0.01for waves I-V. Long-Evans (I); Sprague-Dawley (I); Wistar (I): F = 2.650; 0.958; 0.310; 0.133; and 0.470 for waves I-V. P > 0.05 for waves I-V.
ANOVA:
Student's t-test: Sprague-Dawley (I) us (11) : t = -0.46; 6.94; 4.33; 4.04; and 0.995 for waves I-V. P < 0.05 for waves 11, I11 and IV; P > 0.05 for waves I and V.
Wistar (I) z's (11): t = -2.04; 9.93; 6.57; 6.98; and 6.20 for waves I-V. P < 0.05 for waves 11-V; P > 0.05 for wave I.
evaluated. Comparisons were made of BAEPs from human controls and each species, strain or different supply centre of the rodents. Latency was measured as the time from stimulus onset to a wave's peak. Amplitude was measured as the distance from the positive peak to the succeeding negative peak. The rate of rise of each peak was also calculated in each rodent. The optimal stimulation intensities were defined when the waveforms of the BEAPs were at their best, that is when the artefact of sinusoid waves disappeared. Also the evoked waves at optimal stimulation intensity were drawn and overlapped altogether for determining the pattern of definite BAEPs in each rodent.
and V (Church et al. 1984,Shapiro et al. 1984), I-V (Gyo & Yanagihara 1980), PI-PV (Henry 1979)or P I - P ~(Wada & Starr 1983). Before wave I, at a duration of 0.5-0.8 ms, there were cochlear microphonic potentials, which were variable in the different rodents. Usually in rats, wave I was split into two waves named I A and I B, but I A was inconsistent both among different species and rats of the same strain (Fig. I). When peak I was the single peak, its latency closed to I B , so the latency of peak I was calculated from I B in all of the rodent BAEP recordings. T h e waveforms of waves I-V in each rodent RESULTS altered while responding to various stimulation T h e initial five waves of BAEPs in the hamsters intensities (Figs. z and 3). At low intensity level, and the three strains of rats were rather the latencies increased and the waveforms consistent, however there were variations in changed slightly. At high intensity, the responses waveforms and peak latencies, which are illus- no longer changed proportionally in all their trated in Table I and Fig. I. These positive peaks with increasing intensity, but instead waves were named waves I, 11, 111, I V and V changed in form (Figs. 2 and 3). T h e sinusoid respectively, in correspondence with the no- waves were superimposed on waves I-V if the menclature of waves I-VII in the BAEPs of intensity was increased u p to IOO or I 10 dB. T h e human beings and hamsters (Jewett 1970, optimal stimulation intensity was defined by the Buchwald 1983). They were also compatible maximal intensity until the sinusoid artefacts with the terminology of waves Ia, I b, 11,111,IV disappeared, which was usually reached at 90 dB,
532
T.-J. Chen mi S . 4 . Chrn
Human
1 0.25 Y V
II
I 1 Ins
Long-Evans
\ I
I
I
I I
Sprague Dawley
L-
0 . 5 ~
1mr
Comparison of B.4EPs in different species and strains of rodents and human controls; stimulation intensit! was set at 90 dB Fig.
I.
except in hamsters, when the intensity was a little lower, at around 70 dR (Fig. 2). While at optimal stimulation intensity, the mean latcncies of the was-es varied as follo\~ed : I, 1.23--1.j3 ms; 11, 1.88--2.28 ms; 111, 2.622.94 m s ; IV, 3.49-3.97 m s ; I-, 4.47-5.14 rns; and 1-1, j.21-6.16 ms (Table I ) . .is for the normal human controls, the peak latencics of naves 1Ll-I were : 1.44 0.08 ms, 2.4j & ~ . + m s , 3.67f0.17 ms, 4 . 8 8 f o . I j ms, -j..+9* 0.19 ms and h.94+0.25 rns. There q e r e significant differences bet\l;een the various rat species (P< 0.01, ANOVA). Compared with the human controls, on average, the latency of each peak was 0.j-1.0 rns faster than the corresponding peaks of human B.4E:Ps. I n different strains of rats, the waveforms and latencies of ELMPs were not significantly different pro\-ided
they came from the same animal laboratory centre (Table I). However with rats from different centres, even in the same strain, they unexpectedly showed different peak latencies, cspccially evident in those wavcs beyond wave I1 (Table I ) . For presenting conduction time in the central portion, intcrpeak latencies (IPLs) were calculated between I and 111, I11 and V, and I and V at the stimulation intensity of 90 dB (Table 2). I n rodents, T h e mean IPLs ranged from 1.1I to 1.71 ms, 1.83 to 2.26 m s and 2.95 to 3.94 rns respectively. I n human controls, they z.23+_0.17ms, 1.80k0.15 m s and were 4.03 2 0.21 ms. Generally speaking, IPL is highly dependent on the species, that is there are significant differences in the central conduction time of various rodents and human beings ( P
0.05, ANOVA). Each strain from the different animal centres showed different IPLs (Table 2 ) . The stimulation intensity also affected central conduction time : the IPL decreased with increasing intensities of stimuli. The present percentages of waves I-VI were also apparently dependent on stimulus intensity. Between I 10 and 90 dB, the rates of rise of waves I-IV were almost I O O ~in ~all rodents. However on decreasing stimulation below 60 dB, waves I and V disappeared gradually, to be followed by the disappearance of all other waves except waves I1 and 111. The persistence of wave I1 was a typical characteristic in all rodents even during minimal hearing stimulus. The presence of wave V or VI was quite inconsistent and their rates of rise were lower even at high-intensity stimulation. The minimal hearing thresholds in various kinds of rodents were significantly different (P < 0.05). At 30 dB stimulation, the presence of wave I1 was in Long-Evans rats 68.2%; in S p r a g u e Dawley 56.3 yo;in Wistar 29.4%; and in hamster 0.01,ANOVA).
100.OyO.
As for the amplitude of BAEPs, there were variations among the four types of rodents, and even within each strain of rats. Table 3 illustrates relative amplitudes of II/I, I I I / I and IV/I in each rodent. As shown in the table, the highest amplitude at optimal stimulation intensity was wave 11, except in hamsters. However, in Long-Evans and Sprague-Dawley rats, the highest amplitude shifted from wave I1 to wave I11 when stimulation intensity was increased, while in hamsters wave 111 was usually the highest. DISCUSSION The response patterns of the BAEPs were remarkably consistent in different individuals within each strain of rats and hamsters, and were similar to those studied in other mammals and in normal human controls. BAEPs apparently arise in connection with centres in the auditory pathways, from the eighth cranial nerve to the midbrain, although specific cellular generators have still not been clearly identified (Henry 1979, Achor & Starr 1980b, Gardi & Berlin 1981). The centres involved are found in all mammals, and BAEPs have been recorded in at
534
T.--?. Chen and S.-5'. Chen
110
I
50 '
30
I
1 uv I
lmr'
Fig. 3. Example of serial BAEPs from a Long-Evans rat. The stimulation intensity level was decreased from IIodB down to the minimal hearing threshold.
least five orders (Kevanishvili & Kajaia 1973, to be consistent and their major components Henry 1979, '4chor & Starr 1980b, Gardi & similar in the different species and strains of Berlin 1981, Wada & Starr 1983, Legatt et a!. rodents. However, some properties have been 1986, Lumenta et al. 1986). Compared with proved to have different latencies, IPLs, rates those in human beings, the waveforms of waves and amplitudes of the peaks, as well as differences 1-1; in rodents are similar to human waves I-\, in the number and turning of initial microphonic though the variation is greater from wave VI on. waves in this far-field recording. From the literature, different species have BAEPs in various vertebrates and mammals have been compared, and they demonstrate been adopted for studies of BAEPs, and there essentially identical electrophpsiology within the are variations in the many parameters. I n first 10 ms (Kevanishvili & Kajaia 1973, Huang examining the causes of these differences, 1980, Corwin et al. 1982, Wada & Starr 1983, variations in recording techniques, recording Legatt et al. 1986, Lumenta et al. 1986). Yet sites, stimulation and physiological factors played virtually nothing is known about differences a major role (Henry & Haythorn 1978, between various strains of rodents. From the Amochaev et al. 1979, Osako et al. 1979, Church present study, the R.'\EP wave patterns appeared et al. 1984, Shapiro et a / . 1984). I n our study of
BAEPs in rodent model
535
Table 2. Comparison of IPLs of BAEPs among human controls and rodents: stimulation at 90 dB (mean k SD) Wave (ms) Human controls Long-Evans (I) Sprague-Dawley (I) Sprague-Dawley (11) tliistar (I) Wistar (11) Hamster (I)
12
1-111
14 z.z3&0.17 1.17fo.11 16 1.17fo.11 12 I 4 3 f0. I 5 17 1.11k0.13 I1 1.51f0.14 6 1.71fo.14
22
111-v
I-v
1.80+0.15 1.90&0.26 1.90&0.29 I .83 0.23 1.83fo.18
4.03 k 0 . 2 1 3.0750.21 3.08f0.31 3.26 k 0.29 2.95 f0.23 3.60 & 0 . 2 5 3.945 0 . 2 5
+
2.IIkO.15
2.26+0.25
(I), (11): rodents from two different animal laboratory centres. ANOVA: Human controls; Long-Evans (I); Sprague-Dawley (I); Wistar (I); hamster (I): F = 201.38; 3.33; and 54.08 for IPL 1-111; 111-V; I-V. P < 0.01for IPL 1-111 and I-V; P < 0.05 for IPL 111-V. Long-Evans (I); Sprague-Dawley (I); Wistar (I) : F = 1.50; 0.37; and 1.13 for IPL 1-111; 111-V; I-V. P > 0.05 for IPL 1-111, 111-V and I-V. Student’s t-test: Sprague-Dawley (I) us (11): t = 5.30; -0.57; and 1.30 for IPL 1-111; 111-V; and I-V. P < 0.05 for IPL 1-111; P > 0.05 for IPL 111-V and I-V. Wistar (I) us (11): t = 7.62; 3.83; and 6.18 for IPL 1-111; 111-V; and I-V. P < 0.05 for IPL 1-111, 111-V and I-V.
Table 3. Relative amplitude of BAEPs in different strains of rodents: stimulation at 70 dB (mean fSD)
I I O dB,
90 dB and
Amplitude (pV) dB
Rodents
n
II/I
III/I
IV/I
IIO
(a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster (a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster (a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster
22
6.04f 4.16 2.41 & 1.54 2.38 0.84
6.77 5.28 2.89 & 2.02 2.30f 1.25 1.74f o . 7 5 5.07 f2.46 2.35 f0.73 2.33 & 1.50 2.22 f 1.46 2.61 f 1.00 3.05 & 2.63 I .76 _+ 0.86 2.82 f2.13
f2.84 0.91 f0.53 0.63 k0.47 0.91 $0.53 1.98+1.09 0.81 50.45 0.76f0.47 I .07 f0.45 0.85 f0.53 0.93 0.84 0.66f0.51 1.40f0.56
90
70
I2 I1
6
0.92f0.25
6.88 5 3.55 2.68 0.71 I1 3.64f 1 . 2 1 6 I . I 3 0.43 3.81 f 1.71 22 I2 3.05 & 1.30 I1 3.44f 1.73 6 1.36f0.26 22
I2
+
2.52
(I), (11): rodents from two separate animal laboratory centres. F-value, ANOVA: A t I I O dB: II/I, 7.34; III/I, 5.17; IV/I, 3.18 (P < 0.01). A t 90 dB; II/I, 13.19; III/I, 8.96; IV/I, 8.51 (P < 0.01). At 70 dB: II/I, 3.74 (P< 0.01);III/I, 1.03; IV/I, 1.65 (P > 0.05). At 90 dB: II/I: (a) us (b), (c), (d); (b) 2’s (c), (d); (c) 2’s (d) (P< 0.05). III/I: (a) us (b), (c), (4 (P< 0 . 0 5 ) .
536
T.-J. Chen and S.-S. Chvn
R.4F.Ps among different species and strains of rodents, these factors were strictl!- controlled. I n this study and in contrast to other animal studies cited in the literature, hamsters and all strains of rats tested shared certain properties that u-ere relati\ el! constant : ( I ) There were sinusoid waves that sj-nchronized with the peaks of the RAEPs at high intensities. ( 2 ) There was a peak, usually wa\e 11, \\ith the highest amplitude consistent at all intensities. (3) When decreasing stimulation intensity, wave I disappeared first. (4) Peak latencies varied in a linear fashion with changes in intensity. ( j ) Variations in the amplitude of the peaks were independent of stimulus changes. These characteristics differed from human BAEPs, especially with regard to the highest peak and the first-disappearing waw. In human Hi\EPs, waves I, 111 and 1- u-ere the most apparent and nave I' disappeared last with decreasing intensity. Howevcr. there wcre still fignificant differences in the peak latencies and IPLs amongst each type of rodent, that is both the peripheral and central conduction I clocities were \ariable in different rodents. Unezpecti\ely, rats of the same strains but coming from separate animal centres showed significantly different peak latencies and IPLs. T h e control of strain and species is very important, and it is most important that all of the animals should ha\ e comc from the same animal centre \+hen presented for biological stud!-. T h e minimal hearing thresholds indicated by the rate of rise of each peak were significantly different in the different strains of rat and hamsters. There wcrc variations in relative amplitudes among the four types of rodents. It could be that differences in anatomical factors in different species or strains may affect latencics, u a \ eforms, minimal hearing threshold and even amplitudes of RL4EPs.These anatomical factors include the number of neurons activated, the length and the geometry of the fibre tracts, the spatial alignment of the neurons and the conductivity of the brain tissues (Huang 1980, (;orwin t't crl. 1 9 8 ~ ) . Ph! siologicall!-, there are many interacting Factors that can affect both the latency and amplitude of BAEPs in various species. L a t e n q would bc shortened bv a faster rate of rise to threshold at the spike-initiating loci, by faster
conduction in axons of larger myelinated fibres, and by quick synaptic transmission (Jewett & Romano 1972). Though much of the brain stem physiology is quite similar, anatomical differences may also exist in the brain stems of different species or strains, e.g. sizes of the brain stem nuclei and the lengths of the acoustic fibre tracts (Huang 1980, Corwin et a / . 1982, Hill P t a/. 1985). T h e level of consciousness has been shown by several investigators to have no effect on the latencj- of BAEPs, but the changes in amplitude could be reasonably explained by the level of anaesthesia ( M e n & Starr 1978). Jcwett & Romano (1972) have proved that the amplitude of RAEPs in both rats and cats depends on the depth of anaesthesia. T h a t means the supplementary injection of pentobarbital may have been associated with the variation of the amplitude of all waves in the course of our experiment. T h c influence of non-pathological factors on rat B.4EPs has been well surveyed in our laboratory and was published in another stud>- (Chen & Chen 1988). T h e effects of the influencing factors on rat BAEPs are concluded to be similar to human beings (Chen & Chen I 988). -411 other non-pathological factors wcre controlled in this study. I n conclusion, the auditory evoked responses in hamsters and the three strains of rats were similar to those of other vertebrates, including the normal human controls. T h e response pattern appears to be consistent within each species. There were some differences in waveforms, latencies and the minimal hearing threshold between different rodent species, or rats of the same strain from different animal centres, which helped decide which rodents were appropriate as animal models for the various purposes of the BAEP studies.
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BrlEPs in rodent model AMOCHAEV, A,, JOHNSON,R.C., SAI.AMY, A. & SHAN, S.N. 1979. Brainstem auditory evoked potentials and myelin changes in triethyltin-induced edema in young adult rats. Exp Neurol 66, 629-635. BERRY,H., BLAIR,R.L., BILBAO,J. & BRIANT,T.D. 1976. Click evoked eighth nerve and brainstem responses : experimental observations in the cat. 3 Otolaryngol 5, 64-73. BHARGAVE, V.K., SALAMY, A. & MCKEAN,C.M. 1978. Effect of cholinergic drugs on auditory evoked responses of rat cortex. Neuropharmacology 17, 1009-10 13. BORBELY,L.A. 1970. Changes in click-evoked responses as a function of depth in auditory cortex of the rat. Brain Res 21, 217-247. BRANSTON, N.M., SYMON, L. & CROCKARD, A. 1976. Recovery of the cortical evoked response following temporary middle cerebral artery occlusion in baboons : relation to local blood flow and Po,. Stroke 7. 151-157. BUCHWALD, J.S. 1983. Generators. I n : E.J. Moroe (ed.) Bases of Auditory Brain-Stem Evoked Responses, pp. 157-196. Grune & Stratton, New York. CAZALS,Y., BORIES,V. & ARAN, J.M. 1978. Some uncertainties in scalp recordings of brainstem auditory potentials in monkey and man. Rev Laryngo Oto Rhino1 99, 663-679. CHEN, T.J. & CHEN, S.S. 1988. Effects of nonpathological factors on brain stem auditory evoked potentials in rats. Kaohs 3 Med Sci 4, 553-564. CHEN,T.J., Hsu, H.K. & CHEN, S.S. 1988. Brain stem auditory evoked potentials in brain stem dysfunction. Kaohs 3 Med Sci 4, 238-247. CHURCH,M.W. & GRITZKE,R. 1987. Effects of ketamine anesthesia on the rat brain-stem auditory evoked potential as a function of dose and stimulus intensity. Electroenceph Clin Neurophysiol 67, 570583. CHURCH,M.W., WILLIAMS,H.L. & HOLLOWAY, J.A. 1984. Brain-stem auditory evoked potentials in the rat: effects of gender, stimulus characteristics and ethanol sedation. Electroenceph Clin Neurophysiol 59, 328-339. CORWIN,J.T., BULLOCK,T.H. & SCHWEITZER, J. 1982. The auditory brain stem response in five vertebrate classes. Electroenceph Clin Neurophystol 54, 629-64'. EGGERMONT, J.J. 1976. Analysis of compound action potential responses to tone bursts in the human and guinea pig cochlea.3Acoust Soc Am 60, 1132-1139. GARDI,J.N. & BERLIN,C.I. 1981. Binaural interaction components. Their possible origins in guinea pig auditory brainstem response. Arch Otolarpgol107, 164-1 68. GOLD,S., CAHANI, M., SOHMER, H., HOROWITZ, M. & SHAHAR, A. 1985. Effects of body temperature elevation on auditory nerve-brain-stem evoked
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responses and EEGs in rats. Electroenceph Clin Neuroph.ysiol 60, 146-1 53. GYO, K. & YANAGIHARA, N. 1980. Electrically and acoustically evoked brainstem responses in guinea pig. Acta Otolaryngol 90, 25-3 I . HENRY,K.R. 1979. Differential changes of auditory nerve and brainstem short latency evoked potentials in the laboratory mouse. Electroenceph Clin Neurophysiol 46, 452-459. HENRY, K.R. & HAYTHORN, M.M. 1978. Effects of age and stimulus intensity of the far field auditory brain stem potentials in laboratory mouse. Develop Psychobiol 1 1 , 161-168. HILL,M.W., HEAVENS, R.P. & BALDWIN, B.A. 1985. Auditory evoked potentials recorded from conscious sheep. Brain Res Bull 15, 453-458. HUANG,C.M. 1980. A comparative study of the brain stem auditory response in mammals. Brain Rrs 184, 2 1 5-2 19. HUANG, C.M. & BUCHWALD, J.S. 1978. Factors that affect the amplitudes and latencies of the vertex short latency acoustic responses in the cat. Electroenceph Clin Neuroph.ysio1 4, 179-186. JEWETT, D.L. 1970. Volume-conducted potentials in response to auditory stimuli as detected by averaging in the cat. Electroenccph Clin Neuroph.ysio1 28, 609618. JEWETT, D.L., ROMANO,M.N. & WILLISON,J.S. 1970. Human auditory evoked potentials : possible brainstem components detected on the scalp. Science 167, I 517-1518. JEWETT, D . & ROMANO,M. 1972. Neonatal development of auditory system potentials averaged from the scalp of the rat and cat. Brain Res 36, 101-11 5 . KEVANISHVILI, Z.S. & KAJAIA,O.A. 1973. On the origin of the auditory averaged evoked responses recorded from the scalp in the anesthetized cat. Acta Otolaryngol 76, 98-108. LEGATT, A.D., AREZZO, J.C. & VAUGHAN, H.G. 1986. Short-latency auditory evoked potentials in the monkey. 11. Intracranial generators. Electroenceph Clin Neurophysiol 64, 53-73. LUMENTA, C.B., SCHOBER, R., SPRICK,C. & BOCK, W.J. 1986. Findings confirming brain stem generations of auditory evoked potentials by stereotactic lesions in the rabbit. Neurol Res 8, 114116. MCGINN,M.D., WILLOTT,J.F. & HENRY K.R. 1973. Effects of conductive hearing loss on auditory evoked potentials and audiogenic seizures in mice. Nature 244, 2 55-256. MORGAN,J.L., COULTER,D.B., MARSHALL, A.E. & GOETSCH,D.D. 1980. Effects of neomycin on the waveform of auditory-evoked brainstem potentials in dogs. Am 3 Vet Res 41, 1077-1081. OSAKO, S., TOKIMOTO, T. & MATSUURA, S. 1979. Effects of kanamycin on the auditory evoked
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high-dose pentobarbital. Electroenceph CIin responses during postnatal development of the .Veurophystol 58, 266-276. hearing of the rat. Acra Otola~yngol88, 359-368. L. 1987. Development of brainstem STARR,A. 1976. Auditory brain stem responses in SCHWEITZER, brain death. Brain 99, 543-554. auditory evoked responses in the hamster. Heear Res STARR, A. & ACHOR,L.J. 1978. Auditory brain stem Zj, 249-2jj. SCHWEST, Y.L., \~ILLlS?.OS, J.S. &. JEWETT, D.L. responses in neurological diseases. A r c h Neural 32, 76 I -768. 1980. The effects of ototosicity on the auditory A. 1983. Generation of auditory brain stem response and the scalp recorded cochlear WADA, S.I. & STARR, brain stem responses (ABRs). 111. Effects of lesions microphonic in guinea pigs. Luryzgosrope 110, of the superior olive, lateral lemniscus and inferior 1350-1359. colliculus on the ABR in guinea pig. Electroenceph SHAPIRO, S.Al., AIOLLER, -4.R. & SHIU.G.K. 1984. CIm Neurophystol 56, 352-366. Brain-stem auditory evoked potentials in rats with
Brain stem auditory-evoked potentials (BAEPs) arc recorded by a computer-averaging system from the scalp in response to sounds such as clicks or tone bursts. They consist of 5-8 distinct peaks in periods of 10-ms latency recordings. They are assumed to arise from the nerve tracts and nuclei of the ascending auditory pathways (Buchwald 1983). These potentials were first Correspondence: Dr Shun-Sheng Chen MD, PhD, Department of Neurology and Physiology, Graduate Institute of Medicine, Kaohsiung Medical College, Kaohsiung City, 80708, Taiwan, Republic of China.
recognized in cats (Jewett 1970) and soon after reported in human beings (Jewett et al. 1970). As clinical and research tools in human and animal studies, BAEPs have proved to be extremely useful in detecting brain stem dysfunctions or lesions in neurological diseases, in determining the acoustic pathways, and in the study of a drug effect or for supplementing methods of evaluating brain death (Starr 1976, Bhargave et af. 1978, Starr & Achor 1978, Achor & Starr 1980b, Chen et af. 1988). T o explore various neurological problems in human beings, many animal experiments con-
529 21-2
-5 30
T.-J. Chen a r d S.-S. Chen
cerning BAEPs have been performed, since from physiological and neuropathological aspects there are many difficulties in performing human studies that are not present when studying animals. T h e experimental conditions are more controllable in animals, facilitating research into the generators of these el-oked potentials and the morphological changes or the effect of drugs on BAEPs. Man! kinds of animals h a w been emplo!-cd for studying BAEPs, including monkeys (Allen & Starr 1978, Cazals et 01. 1978), baboons (Branston et 01. 1976), dogs (llorgan et al. 1980), cats (Berry rt al. 1976, Huang 8i Buchwald 1978, Achor & Starr 1980b), rats (Borbely 1970, Amochaev et al. 1979, Osako et al. 1979) and mice (Henry & Haythorn 1978); in particular cats h a w been used extensively. Rats or othcr rodents are preferred, because the! are less espensive, the experimental conditions are much more easily controlled and the rat has a similar acoustic pathway to that of the cat and man. In the past, various species or strains of rats and other rodents including Sprague-Dawley (Church rt al. 1984, Shapiro rt al. 19841, 1.ong-Evans (Church & Gritzke 1987), Sabra (Gold et al. 1985), mice (McGinn rt al. 1973, Henry 1979)~hamsters (Schweitzer 1987) and guinea-pigs (Eggermont 1976, Schwent et a!. 1980, Gardi & Berlin 1981) have been used for the study of BAEPs. Most studies have focused on lesions in the brain stem, generators of B.4EPs, brain stem development, effects of drugs or sedation, or temperature effect, etc. (Jewett bi Romano 1972, Henry 1979, Osako et al. 1979, Gl;o & E'anagihara 1980, Gardi 8i Berlin 1981, Shapiro r t al. 1984, Gold r t al. 1985). In spite of the frequent use of rats as animal models in Br\EP studies. there are no comparisons of one strain uith another. So far the identification and nomenclature of evoked \t a\ e5, waveforms or peak latencies of H.\EPs in different rodents or even in the same strain of rats are not uniformly recognized (Henry ei Haythorn 1978, Amochaev et al. 1979, Osalio et al. 1979, Schwent et at. 1980). T h e method of recording BAEPs and the experimental conditions, such as the sites of the recording electrodes and the stimulation parameters, including intensity, frequency and interval, and physiological factors such as temperature and age, were not controlled in most studies. This study was conducted to evaluate vari-
ations of BAEPs in the different rodents and to compare them with human BAEPs. T h e study also attempted to determine which species of rodents were the most appropriate to serve as an animal model for the different purposes of the BL4EPsstudies.
MrZTERIALS AND M E T H O D S Four types of rodents and 14 normal human controls were used for this experiment. Normal human controls were all adults with ages ranging from 20 to 2; years. The rodents included six hamsters and three strains of rats: 22 Long-Evans, 28 Wistar and 28 Sprague-Dawley. Both the Wistar and Sprague-Dawley rats came from two separate animal laboratory centres in Taiwan and were studied as two different groups. All the rodents were mature males with ages ranging from 3 to j months. Before the experiment, they were fed normally with laboratory chow and water. In the animal study, to permit electrodc placement and recording, all animals were anaesthetized with sodium pentobarbital 50 mg kg-' intraperitoneally. Since thermoregulatory mechanisms were impaired by anaesthesia, the deep rectal temperature was controlled between 36.5 and 37.5 "C with an electric heating pad and a temperature-controlled incubator. .4fter anaesthesia, two stainless-steel screws were inserted into the skull, overlying the right mastoid bone and the left frontal bone, to serve as the permanent active and reference electrodes respectivell-. In addition, one needle electrode was attached to the left mastoid to serve as the earth electrode. In our human study, the subjects were lying in a quiet laboratory room. Two surface recording electrodes fixed with collodion-soaked gauze were placed, one at F, and one over the mastoid ipsilateral to the stimulated ear. The contralateral mastoid was used as an earth. BAEPs were recorded using a signal processor (7S11.4,San-Ei Company, Japan). The sound stimuli were clicks, delivered at a repetitive rate of 13.3 s-' via an earphone 2 cm from the animal's right ear or directl!. onto the human right ear. The stimulation intensity was always fixed at 90 dB for the human controls, and for the rodents from a maximum of I 10 dB down until the minimal hearing threshold was reached, usually at around 30 dB. The evoked potentials were then amplified and filtered with a bandpass between 80 and 300oHz. Each RAEP recording was averaged from a total of 1024 clickevoked responses for the first Io-ms period following the stimulation. The brain stem potentials were compiled by a computer-averaging system and then plotted on graph paper by an X-Y recorder. .4fter recording BAEP waves at various stimulation intensities, the consistency of waveforms, peak latencies and interpeak latencies (IPLs) in each rodent was
BAEPs in rodent model
531
Table I . Comparison of peak latencies of BAEPs among human controls and rodents: stimulation at 90 dB (mean fSD) Wave (ms)
n
Human controls Long-Evans (I) Sprague-Dawley (I) Sprague-Dawley (11) Wistar (I) Wistar (11) Hamster (I)
I
14
1.44f0.08 1.4540.09 16 1 . 4 7 f o . 1 1 12 1 . 4 5 5 0 . 1 2 I7 1 . 5 3 f 0 . 1 2 11 1.4450.10 6 1.23&0.11
22
I1
111
IV
V
2.47 fo.24 1.9350.11 1.88f0.09 2.21f0.16 I.9IfO.IO 2.28+0.09 2.09f0.12
3.67k0.17 2.6240.13 2.64f0.10 2.88f0.19 2.64fo.11 2.9450.13 2.94fo.1~
4.88 f o . 1 5 3.49k0.13 3.51k0.12 3.785023 3.50f0.17 3.97k0.18 3.94fo.17
5.47 0.19 6.94 f0.25 4.52f0.22 5.21f0.48 4.56k0.27 4.69&0.31 4.47f0.20 5.03k0.22 6.16fo.17 5.14f0.26 -
VI
(I), (11) : rodents from two different animal laboratory centres. Human controls; Long-Evans (I); Sprague-Dawley (I); Wistar (I); hamster (I): F = 9.71; 45.88; 184.43; 241.02; and 52.58 for waves I-V. P < 0.01for waves I-V. Long-Evans (I); Sprague-Dawley (I); Wistar (I): F = 2.650; 0.958; 0.310; 0.133; and 0.470 for waves I-V. P > 0.05 for waves I-V.
ANOVA:
Student's t-test: Sprague-Dawley (I) us (11) : t = -0.46; 6.94; 4.33; 4.04; and 0.995 for waves I-V. P < 0.05 for waves 11, I11 and IV; P > 0.05 for waves I and V.
Wistar (I) z's (11): t = -2.04; 9.93; 6.57; 6.98; and 6.20 for waves I-V. P < 0.05 for waves 11-V; P > 0.05 for wave I.
evaluated. Comparisons were made of BAEPs from human controls and each species, strain or different supply centre of the rodents. Latency was measured as the time from stimulus onset to a wave's peak. Amplitude was measured as the distance from the positive peak to the succeeding negative peak. The rate of rise of each peak was also calculated in each rodent. The optimal stimulation intensities were defined when the waveforms of the BEAPs were at their best, that is when the artefact of sinusoid waves disappeared. Also the evoked waves at optimal stimulation intensity were drawn and overlapped altogether for determining the pattern of definite BAEPs in each rodent.
and V (Church et al. 1984,Shapiro et al. 1984), I-V (Gyo & Yanagihara 1980), PI-PV (Henry 1979)or P I - P ~(Wada & Starr 1983). Before wave I, at a duration of 0.5-0.8 ms, there were cochlear microphonic potentials, which were variable in the different rodents. Usually in rats, wave I was split into two waves named I A and I B, but I A was inconsistent both among different species and rats of the same strain (Fig. I). When peak I was the single peak, its latency closed to I B , so the latency of peak I was calculated from I B in all of the rodent BAEP recordings. T h e waveforms of waves I-V in each rodent RESULTS altered while responding to various stimulation T h e initial five waves of BAEPs in the hamsters intensities (Figs. z and 3). At low intensity level, and the three strains of rats were rather the latencies increased and the waveforms consistent, however there were variations in changed slightly. At high intensity, the responses waveforms and peak latencies, which are illus- no longer changed proportionally in all their trated in Table I and Fig. I. These positive peaks with increasing intensity, but instead waves were named waves I, 11, 111, I V and V changed in form (Figs. 2 and 3). T h e sinusoid respectively, in correspondence with the no- waves were superimposed on waves I-V if the menclature of waves I-VII in the BAEPs of intensity was increased u p to IOO or I 10 dB. T h e human beings and hamsters (Jewett 1970, optimal stimulation intensity was defined by the Buchwald 1983). They were also compatible maximal intensity until the sinusoid artefacts with the terminology of waves Ia, I b, 11,111,IV disappeared, which was usually reached at 90 dB,
532
T.-J. Chen mi S . 4 . Chrn
Human
1 0.25 Y V
II
I 1 Ins
Long-Evans
\ I
I
I
I I
Sprague Dawley
L-
0 . 5 ~
1mr
Comparison of B.4EPs in different species and strains of rodents and human controls; stimulation intensit! was set at 90 dB Fig.
I.
except in hamsters, when the intensity was a little lower, at around 70 dR (Fig. 2). While at optimal stimulation intensity, the mean latcncies of the was-es varied as follo\~ed : I, 1.23--1.j3 ms; 11, 1.88--2.28 ms; 111, 2.622.94 m s ; IV, 3.49-3.97 m s ; I-, 4.47-5.14 rns; and 1-1, j.21-6.16 ms (Table I ) . .is for the normal human controls, the peak latencics of naves 1Ll-I were : 1.44 0.08 ms, 2.4j & ~ . + m s , 3.67f0.17 ms, 4 . 8 8 f o . I j ms, -j..+9* 0.19 ms and h.94+0.25 rns. There q e r e significant differences bet\l;een the various rat species (P< 0.01, ANOVA). Compared with the human controls, on average, the latency of each peak was 0.j-1.0 rns faster than the corresponding peaks of human B.4E:Ps. I n different strains of rats, the waveforms and latencies of ELMPs were not significantly different pro\-ided
they came from the same animal laboratory centre (Table I). However with rats from different centres, even in the same strain, they unexpectedly showed different peak latencies, cspccially evident in those wavcs beyond wave I1 (Table I ) . For presenting conduction time in the central portion, intcrpeak latencies (IPLs) were calculated between I and 111, I11 and V, and I and V at the stimulation intensity of 90 dB (Table 2). I n rodents, T h e mean IPLs ranged from 1.1I to 1.71 ms, 1.83 to 2.26 m s and 2.95 to 3.94 rns respectively. I n human controls, they z.23+_0.17ms, 1.80k0.15 m s and were 4.03 2 0.21 ms. Generally speaking, IPL is highly dependent on the species, that is there are significant differences in the central conduction time of various rodents and human beings ( P
0.05, ANOVA). Each strain from the different animal centres showed different IPLs (Table 2 ) . The stimulation intensity also affected central conduction time : the IPL decreased with increasing intensities of stimuli. The present percentages of waves I-VI were also apparently dependent on stimulus intensity. Between I 10 and 90 dB, the rates of rise of waves I-IV were almost I O O ~in ~all rodents. However on decreasing stimulation below 60 dB, waves I and V disappeared gradually, to be followed by the disappearance of all other waves except waves I1 and 111. The persistence of wave I1 was a typical characteristic in all rodents even during minimal hearing stimulus. The presence of wave V or VI was quite inconsistent and their rates of rise were lower even at high-intensity stimulation. The minimal hearing thresholds in various kinds of rodents were significantly different (P < 0.05). At 30 dB stimulation, the presence of wave I1 was in Long-Evans rats 68.2%; in S p r a g u e Dawley 56.3 yo;in Wistar 29.4%; and in hamster 0.01,ANOVA).
100.OyO.
As for the amplitude of BAEPs, there were variations among the four types of rodents, and even within each strain of rats. Table 3 illustrates relative amplitudes of II/I, I I I / I and IV/I in each rodent. As shown in the table, the highest amplitude at optimal stimulation intensity was wave 11, except in hamsters. However, in Long-Evans and Sprague-Dawley rats, the highest amplitude shifted from wave I1 to wave I11 when stimulation intensity was increased, while in hamsters wave 111 was usually the highest. DISCUSSION The response patterns of the BAEPs were remarkably consistent in different individuals within each strain of rats and hamsters, and were similar to those studied in other mammals and in normal human controls. BAEPs apparently arise in connection with centres in the auditory pathways, from the eighth cranial nerve to the midbrain, although specific cellular generators have still not been clearly identified (Henry 1979, Achor & Starr 1980b, Gardi & Berlin 1981). The centres involved are found in all mammals, and BAEPs have been recorded in at
534
T.--?. Chen and S.-5'. Chen
110
I
50 '
30
I
1 uv I
lmr'
Fig. 3. Example of serial BAEPs from a Long-Evans rat. The stimulation intensity level was decreased from IIodB down to the minimal hearing threshold.
least five orders (Kevanishvili & Kajaia 1973, to be consistent and their major components Henry 1979, '4chor & Starr 1980b, Gardi & similar in the different species and strains of Berlin 1981, Wada & Starr 1983, Legatt et a!. rodents. However, some properties have been 1986, Lumenta et al. 1986). Compared with proved to have different latencies, IPLs, rates those in human beings, the waveforms of waves and amplitudes of the peaks, as well as differences 1-1; in rodents are similar to human waves I-\, in the number and turning of initial microphonic though the variation is greater from wave VI on. waves in this far-field recording. From the literature, different species have BAEPs in various vertebrates and mammals have been compared, and they demonstrate been adopted for studies of BAEPs, and there essentially identical electrophpsiology within the are variations in the many parameters. I n first 10 ms (Kevanishvili & Kajaia 1973, Huang examining the causes of these differences, 1980, Corwin et al. 1982, Wada & Starr 1983, variations in recording techniques, recording Legatt et al. 1986, Lumenta et al. 1986). Yet sites, stimulation and physiological factors played virtually nothing is known about differences a major role (Henry & Haythorn 1978, between various strains of rodents. From the Amochaev et al. 1979, Osako et al. 1979, Church present study, the R.'\EP wave patterns appeared et al. 1984, Shapiro et a / . 1984). I n our study of
BAEPs in rodent model
535
Table 2. Comparison of IPLs of BAEPs among human controls and rodents: stimulation at 90 dB (mean k SD) Wave (ms) Human controls Long-Evans (I) Sprague-Dawley (I) Sprague-Dawley (11) tliistar (I) Wistar (11) Hamster (I)
12
1-111
14 z.z3&0.17 1.17fo.11 16 1.17fo.11 12 I 4 3 f0. I 5 17 1.11k0.13 I1 1.51f0.14 6 1.71fo.14
22
111-v
I-v
1.80+0.15 1.90&0.26 1.90&0.29 I .83 0.23 1.83fo.18
4.03 k 0 . 2 1 3.0750.21 3.08f0.31 3.26 k 0.29 2.95 f0.23 3.60 & 0 . 2 5 3.945 0 . 2 5
+
2.IIkO.15
2.26+0.25
(I), (11): rodents from two different animal laboratory centres. ANOVA: Human controls; Long-Evans (I); Sprague-Dawley (I); Wistar (I); hamster (I): F = 201.38; 3.33; and 54.08 for IPL 1-111; 111-V; I-V. P < 0.01for IPL 1-111 and I-V; P < 0.05 for IPL 111-V. Long-Evans (I); Sprague-Dawley (I); Wistar (I) : F = 1.50; 0.37; and 1.13 for IPL 1-111; 111-V; I-V. P > 0.05 for IPL 1-111, 111-V and I-V. Student’s t-test: Sprague-Dawley (I) us (11): t = 5.30; -0.57; and 1.30 for IPL 1-111; 111-V; and I-V. P < 0.05 for IPL 1-111; P > 0.05 for IPL 111-V and I-V. Wistar (I) us (11): t = 7.62; 3.83; and 6.18 for IPL 1-111; 111-V; and I-V. P < 0.05 for IPL 1-111, 111-V and I-V.
Table 3. Relative amplitude of BAEPs in different strains of rodents: stimulation at 70 dB (mean fSD)
I I O dB,
90 dB and
Amplitude (pV) dB
Rodents
n
II/I
III/I
IV/I
IIO
(a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster (a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster (a) Long-Evans (I) (b) Sprague-Dawley (11) (c) Wistar (11) (d) Hamster
22
6.04f 4.16 2.41 & 1.54 2.38 0.84
6.77 5.28 2.89 & 2.02 2.30f 1.25 1.74f o . 7 5 5.07 f2.46 2.35 f0.73 2.33 & 1.50 2.22 f 1.46 2.61 f 1.00 3.05 & 2.63 I .76 _+ 0.86 2.82 f2.13
f2.84 0.91 f0.53 0.63 k0.47 0.91 $0.53 1.98+1.09 0.81 50.45 0.76f0.47 I .07 f0.45 0.85 f0.53 0.93 0.84 0.66f0.51 1.40f0.56
90
70
I2 I1
6
0.92f0.25
6.88 5 3.55 2.68 0.71 I1 3.64f 1 . 2 1 6 I . I 3 0.43 3.81 f 1.71 22 I2 3.05 & 1.30 I1 3.44f 1.73 6 1.36f0.26 22
I2
+
2.52
(I), (11): rodents from two separate animal laboratory centres. F-value, ANOVA: A t I I O dB: II/I, 7.34; III/I, 5.17; IV/I, 3.18 (P < 0.01). A t 90 dB; II/I, 13.19; III/I, 8.96; IV/I, 8.51 (P < 0.01). At 70 dB: II/I, 3.74 (P< 0.01);III/I, 1.03; IV/I, 1.65 (P > 0.05). At 90 dB: II/I: (a) us (b), (c), (d); (b) 2’s (c), (d); (c) 2’s (d) (P< 0.05). III/I: (a) us (b), (c), (4 (P< 0 . 0 5 ) .
536
T.-J. Chen and S.-S. Chvn
R.4F.Ps among different species and strains of rodents, these factors were strictl!- controlled. I n this study and in contrast to other animal studies cited in the literature, hamsters and all strains of rats tested shared certain properties that u-ere relati\ el! constant : ( I ) There were sinusoid waves that sj-nchronized with the peaks of the RAEPs at high intensities. ( 2 ) There was a peak, usually wa\e 11, \\ith the highest amplitude consistent at all intensities. (3) When decreasing stimulation intensity, wave I disappeared first. (4) Peak latencies varied in a linear fashion with changes in intensity. ( j ) Variations in the amplitude of the peaks were independent of stimulus changes. These characteristics differed from human BAEPs, especially with regard to the highest peak and the first-disappearing waw. In human Hi\EPs, waves I, 111 and 1- u-ere the most apparent and nave I' disappeared last with decreasing intensity. Howevcr. there wcre still fignificant differences in the peak latencies and IPLs amongst each type of rodent, that is both the peripheral and central conduction I clocities were \ariable in different rodents. Unezpecti\ely, rats of the same strains but coming from separate animal centres showed significantly different peak latencies and IPLs. T h e control of strain and species is very important, and it is most important that all of the animals should ha\ e comc from the same animal centre \+hen presented for biological stud!-. T h e minimal hearing thresholds indicated by the rate of rise of each peak were significantly different in the different strains of rat and hamsters. There wcrc variations in relative amplitudes among the four types of rodents. It could be that differences in anatomical factors in different species or strains may affect latencics, u a \ eforms, minimal hearing threshold and even amplitudes of RL4EPs.These anatomical factors include the number of neurons activated, the length and the geometry of the fibre tracts, the spatial alignment of the neurons and the conductivity of the brain tissues (Huang 1980, (;orwin t't crl. 1 9 8 ~ ) . Ph! siologicall!-, there are many interacting Factors that can affect both the latency and amplitude of BAEPs in various species. L a t e n q would bc shortened bv a faster rate of rise to threshold at the spike-initiating loci, by faster
conduction in axons of larger myelinated fibres, and by quick synaptic transmission (Jewett & Romano 1972). Though much of the brain stem physiology is quite similar, anatomical differences may also exist in the brain stems of different species or strains, e.g. sizes of the brain stem nuclei and the lengths of the acoustic fibre tracts (Huang 1980, Corwin et a / . 1982, Hill P t a/. 1985). T h e level of consciousness has been shown by several investigators to have no effect on the latencj- of BAEPs, but the changes in amplitude could be reasonably explained by the level of anaesthesia ( M e n & Starr 1978). Jcwett & Romano (1972) have proved that the amplitude of RAEPs in both rats and cats depends on the depth of anaesthesia. T h a t means the supplementary injection of pentobarbital may have been associated with the variation of the amplitude of all waves in the course of our experiment. T h c influence of non-pathological factors on rat B.4EPs has been well surveyed in our laboratory and was published in another stud>- (Chen & Chen 1988). T h e effects of the influencing factors on rat BAEPs are concluded to be similar to human beings (Chen & Chen I 988). -411 other non-pathological factors wcre controlled in this study. I n conclusion, the auditory evoked responses in hamsters and the three strains of rats were similar to those of other vertebrates, including the normal human controls. T h e response pattern appears to be consistent within each species. There were some differences in waveforms, latencies and the minimal hearing threshold between different rodent species, or rats of the same strain from different animal centres, which helped decide which rodents were appropriate as animal models for the various purposes of the BAEP studies.
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