Epilepsy Research, 13 (1992) 3542 0920-121 l/92/$05.00 0 1992 Elsevier Science Publishers
EPIRES
35 B.V. All rights reserved
00504
Noise exposure-induced audiogenic seizure susceptibility in Sprague-Dawley rats
M. Pierson
and S.L. Liebmann
Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, NY 12201-0509, (Received
10 October
1991; revision
received
Key words: Audiogenic
Parameters exposure. neonatal
were evaluated
susceptibility
by day 20, but seizure seizures
induction
on this susceptibility
days 13 and 15 and that susceptibility
adulthood, becomes
for the optimum
The effect of maturation
severity
requires
increasingly
shorter
during
somewhat
of audiogenic
seizure
was also examined. a minimum
is not maximal
the prepubescent
period
1992; accepted
seizures; Epileptogenesis;
susceptibility
It was found
32-36.
(days 1624)
on the age when initial noise exposure
in Sprague-Dawley
(SD) rats by noise
that SD rats are most inducible
Noise exposure
Although
type. Seizure latency
29 April 1992)
Rat
of 2 days to develop.
until days
in older rats revert to the wild-running-only
adult seizures depends
27 April
susceptibility
(from stimulus
between
on day 14 results in universal persists
at high levels into
onset to onset of wild running)
but is stable at older ages. The mean shortness
occurred;
USA
day-14 noise exposures
of latency
in
result in seizures with shortest
latencies. Ontogenetic
comparisons
which are SD substrains)” age in all four groups maximum
were made of susceptibility and noise exposure-induced
of rats but that considerable
seizure severity
among
in these noise exposure-induced Wistar
differences
(WI) rats28. It appears characterize
the absolute
rats, genetically that epileptogenesis
epilepsy
prone
rats (GEPRs,
begins at virtually
the same
severity of seizures and the age dependence
of
the strains.
Introduction Identification of the basis of audiogenic seizure susceptibility is of great interest because this model of epilepsy in rodents is developmental as are many human epilepsies. The issue of causality has been especially studied in one specific rodent substrainl~S-9~15-1~,23,25,26,29-33 , the genetically epilepsy prone rat (GEPR) of which two types3’ may be distinguished, the GEPR-9 and GEPR-3*. Because these GEPR colonies derive from Sprague-
Correspondence to: M. Pierson, Cain Foundation Laboratories, Texas Children’s Hospital, Clinical Care Center #955, 6621 Fannin Street, MC 3-3311, Houston, TX 77030-2399, USA.
Dawley
(SD) ancestors,
one productive
approach
to understanding epileptogenesis in the GEPR would be the comparison of audiogenic seizure susceptibility in GEPRs with that in individuals of the parent strain. However, since SD rats do not normally exhibit audiogenic seizures, it was desirable to experimentally induce the susceptible phe-
* The CEPR-9
and the CEPR-3 colonies received their respective names based on the severity of characteristic seizures. Seizures with a single wild running attack followed by a generalized tonic convulsion involving full extension of the hindlimbs receive a score of 9, while seizures consisting of a single running episode followed by a generalized clonic convulsion receive a score of 3 according to two published audiogenic seizure severity scales’5.27.
36
notype. For this reason we undertook to determine whether susceptibility could be engendered in a
served in these were maintained
seizure-resistant
Both sexes were used in approximately
colony
of SD rats.
Paradigms for experimentally inducing susceptibility have been reported for both mouse and rat strains.
The manipulations
that effect this conver-
sion consist either of cochlear
insult (e.g., exposure
to: ototoxic drugs4*24*27,34,goitrogens35, or intense noise 2,3,‘1,12,21*28)or of direct auditory deprivation (e.g., temporary insertion of earplugs or eardrum rupture3,12). Th e only caveat has been that such treatments must occur during a critical developmental period in order to be effective. The period in each instance coincides with that when synaptogenesis occurs in the central ascending auditory pathway. Since two paradigms have been reported for the induction of audiogenic seizure susceptibility in immature Wistar (WI) rats27,28, a similar outcome in SD rats would seem likely. However, as was recently reported, neonatal administration of the ototoxic drug kanamycin does not engender outright audiogenic seizure susceptibility in SD rats31, although such treatment is 100% successful in WI rats. We have ourselves confirmed the former observation (unpublished results). Thus, it has seemed useful now to examine the effectiveness of intense neonatal noise exposure for engendering susceptibility in SD rats. In the present study, it was found that intense neonatal noise exposure can similarly - and universally - induce susceptibility in the SD. Parameters resulting in maximal susceptibility and the ontogenetic features of such susceptibility are reported here. Because ontogenic changes in seizure susceptibility have been reported for additional strains, the CEPR-3 and the GEPR-95,‘3,29730 and noise exposure-induced WI between noise exposure-inrats2’, comparisons duced SD rats and these other models of audiogenie seizure susceptibility could be presented here. Methods Sprague-Dawley rats used in these experiments were from an in-house breeding colony. The original stock was purchased from Taconic Farms, strain designation: Tac:N(SD)fBR. The spontaneous rate of audiogenic seizure susceptibility ob-
rats was 2/363 on a 12/12-h
(0.6%). Animals light/dark cycle. equal num-
bers; pups were weaned on day (d) 24. Noise exposures and subsequent tests consisted of the ringing
of a lo-inch,
manually
controlled
electric fire alarm bell held at the lip of a 36 x 39 cm metal canister. The intensity of the signal was invariant, being 125-128 dB SPL A (‘A’ refers to a high frequency-weighted scale of sound level meters). Octave band analysis of this signal has been described previously28. Seizure tests lasted a maximum of 60 s and consisted of the presentation of this same signal. Convulsive seizures were always preceded by wild running but nonconvulsive seizures consisting of running-only also occurred. The bell was turned off if, and when, wild running seizures gave way to convulsions (clonic or tonic). Individual rats were tested on only one occasion, since alterations in long-term susceptibility have been reported to occur in WI28 as well as GEPR’3,30 rats if previously tested. All animal use procedures were approved by an institutional animal care and welfare committee. The first experiment addressed the identification of the critical period for induction of susceptibility. Rats were exposed to the bell for 6 or 10 min. Seizure susceptibility tests were conducted on d 36. The age of rats during initial noise exposure was one of the following: 11, 12, 13, 14, 15, 16, 18, 20, 24, or 28 days. For IO-min exposed rats, the exposure ages did not include d 11 or d 28. A second question was to determine the effect of different durations of noise exposure on d 15 on susceptibility being tested on d 28. Exposure durations included 1, 2, 4, 6, 8, 10 and 12 min. A third experiment considered the age dependence of the evolution of susceptibility. Several groups received lo-min noise exposures on d 14, while susceptibility was tested at a variety of ages including: days 15, 16, 18, 20, 24, 28, 32, 36, 44 and 52. Significance of differences in incidence of seizures or convulsions was analyzed by the Fisher exact test (FET). Differences in latency were analyzed by Student’s t test.
37
,y(, , ~~~,~~,~~~f~ 11; AGE AT TIME OF 6-min EXPOSURE
20 (101
24 (~~1
28 (101
AGE AT TIME OF lo-min EXPOSURE
Fig. 1. Sensitive ages when Sprague-Dawley rats can be induced to become prone to audiogenic seizures. Susceptibility was assessed at d 36; each rat was tested one time only. Sample sizes are shown in parentheses below data. Shaded areas represent convulsive seizures. Effects of both 6-min (left panels) and IO-min (right paneIs) exposures are shown. Latencies were measured from stimulus onset to start of wild running attack and are shown with SEMs in lower panels.
Results
The developmental period during which susceptibility could be induced in SD rats by a 6-min noise exposure ranged from d 13 to d 24 as shown in the left panel of Fig. 1. However, by operationally defining as the critical period for inducibility those ages when exposure resulted in susceptibility in at least 50% of rats, the period between d 13 and d 18 emerged as the most useful. On the other hand when severity of seizures is also taken into account, the critical period becomes more limited since 50% of tests resulted in convulsions only when initial noise exposure had occurred at an age between d 13 and d 15. An increase in duration of exposure did not result in a widening of the critical period for induction of susceptibility (to either convulsive or nonconvulsive types of seizures) as shown in the right panel of Fig. 1. It appeared that d 14 was a slightly better age for inducing susceptibility when using a 6-min exposure, whereas d 15 was the best
age when employing IO-min presentations. However, if shortness of latency was added as a further criterion, then day-14 exposures offered a slight advantage over day-15 exposures (lower panels in Fig. l), since for either duration latencies were shortest after day-14 exposures. Severity of seizures (on d 28) increased monotonically with increases in the duration of initial exposure (here on day 15) as shown in Fig. 2. For example, by interpolation it is apparent that the duration of exposure necessary to induce wild running susceptibility in 50% of rats was 2.5 min; for engendering convulsive seizure sus~ptibility, it was 7.0 min. Nonetheless, not even 12-min exposures (on d 15) resulted in universal convulsive seizures at the later age of d 28. Differences between latencies (to wild running onset) were not signitieantly influenced by duration of initial exposure as shown in the lower panel. Onset of susceptibility to wild running-only seizures did not occur in less than 2 days after initial
38
0
2 (15) (13)
4 (12)
6
6
10
(12)
(121
(16)
DURATION EXPOSURE (min) Fig. 2. Influence of duration of exposure on incidence and severity of seizures. Sprague-Dawley rats were noise-exposed at d 15; susceptibility tests were on d 28. Sample sizes are shown along abscissa below data points. Shaded area represents convulsive responses. Latencies (to wild running onset) and SEMs are shown in lower panel.
noise exposure as shown in Fig. 3. While wild running seizures occurred universally by d 20, convulsive seizures did not occur before d 24 and their incidence was not maximal until d 32 to d 36. Convulsions were never universal. There occurred a slight drop in general susceptibility (i.e., sum of incidences of both running-only and convulsive seizures) (P ~0.05, FET) and a significant drop in incidence of convulsive seizures (P ~0.05, FET) between the ages of d 36 and d 52. Latency, too, was somewhat age-dependent as shown in the lower panel of Fig. 3. It decreased monotonically and significantly between each pair of consecutive test ages between d 16 and d 24 (each P ~0.05, Student’s t test). After d 24 latency did not decrease further (P ~0.05, Student’s t test). In addition to the above data there were behavioral differences in seizures at different ages. Centered at the age of d 24, there was a marked tendency for wild running-only seizures to be highly protracted - often lasting 3-6 min after the bell had been turned off. If one physically interrupted these poststimulus running episodes (e.g., by pick-
25 35 45 AGE AT TIME OF TEST (days)
Fig. 3. Ontogenetic features of audiogenic seizure susceptibility in Sprague-Dawley rats. Susceptibility was induced by lo-min exposures on d 14; each rat was tested one time only. Sample sizes were 10 except on day 36 when n was 12. Shaded area represents convulsive responses. Lower panels show latencies to wild running onset and SEMs.
ing the rat up by the tail), the behavior nonetheless persisted and rats would continue frenzied running (as soon as they were put back down). This surprising behavior occurred only in rats with nonconvulsive (wild running-only) seizures and was first observed in d 20 rats (S/IO of running-onIy seizures). It occurred in 7/8 of such seizures at the age of d 24 but in only 5145 of wild running seizures in d 28 rats. Prolonged poststimulus running was never observed in rats younger than d 20 or older than d 28. The sex of rats had no influence on either the likelihood or the severity of seizures. Of 496 seizure tests conducted in the present study, 243 involved female rats and 253 involved males. The ratio of probabilities of convulsions:wiid runningonly seizures:nonresponsive behaviors among females was 0.34:0.37:0.29. The ratio among males in these respective categories was 0.37:0.34:0.29.
39
appear to be inherently more resistant to severe types of audiogenic seizures, even though the two strains have essentially the same likelihood of becoming susceptible due to neonatal noise exposure. The implications of this difference in seizure severity in the absence of a difference in seizure incidence are discussed below. In view of the differences found here in seizures of noise-induced WI and SD rats, it was of interest to compare susceptibility in these strains to that in genetically susceptible GEPR-9 and GEPR-3 colonies. It is possible to do this because a recent ontogenetic study of audiogenic seizures in GEPRs~~ employed age-based parameters similar to. those employed both here and in our previous study of noise exposure-sensitized WI rats28. From such comparison, as demonstrated in Table I, it becomes apparent that the timing of events during the onset of susceptibility is nearly identical among these four models. That is, age for optimum sensitization by intense noise exposure is similar in SD and WI rats (d 14); the earliest age when susceptibility emerges is similar in all four strains (d 15-16); and susceptibility (not seizure severity) is at peak (nearly universal) levels by roughly the same age (d 1921). By contrast, severity of seizures after d 24 and in adult rats was highly dependent on strain (note, seizure severity scores in Table I reflect behaviors of d 36 SD and WI rats or d 45 GEPRs although seizures of d 23-28 GEPR-3
Discussion It is concluded that noise exposure is useful for inducing audiogenic seizure susceptibility in Sprague-Dawley rats as it is in Wistar rats28. Inducibility in the two strains has in common that wild running seizure susceptibility does not emerge in less than 2 days, that d 14 appears to be the best age for induction (of susceptibility), and that susceptibility can be universally engendered. Since in the WI initial exposure paradigms which engender the highest incidence of wild running attacks also engender the highest seizure severity among seizing rats, it was surprising that seizures in noise-sensitized SDS generally were simple wild running attacks even when 100% of rats in a group were susceptible. Not only were convulsive seizures infrequent, but they did not occur prior to d 24 nor in rats d 44 or older. By contrast in noise exposureinduced WIs, convulsive seizures sometimes occur as early as d 16, occur universally by d 24 and are likely to continue to occur at subsequent ages28. A further difference is that increases in the duration of exposure of SDS do not result in a substantial widening of the critical period for inducibility (left vs. right panels, Fig. 1). By contrast the critical period is widened by 4 days in WI rats as the initial exposure duration is increased from 4 to 8 min28. In short, the predominant difference found between SD and WI rats has been that the former TABLE
I
Comparison
of ontogenetic
characteristics
of audiogenic seizure susceptibility
in four rat strains
Most sensitive age
Earliest
age when
Age (days) when
Age (days) when
Typical
audiogenic
(days) for inducibility
running
seizures are
seizure incidence
seizure severity is
response
scorea
by noise exposure
observed
becomes
greatest
(adult)
SD
13-15
16b
20
32-36
1
WI28
14-16
16b
20”
28-32
6
15
19
23-25
3
16
21
z 45
9
GEPR-329 GEPR-929
1
a Severity
scores in this table do not depend
reflect seizures of greater single 8-10-s followed running clonus
running
by clonus attacks
severity. Typical
attacks;
WI seizures
then tonic convulsion
followed
b Following
on whether
initial noise exposure
universal
a scale devised for seizures in GEPRs”
seizures by adults of each strain are described (severity involving
score partial
by 4-limb clonus; and GEPR-9
then tonus with full extension
’ Revised
(days)
= 6) consist extension
of two running
of hindlimbs;
or in WI rats*’ is used. Higher
attacks
GEPR-3
separated
by a quiescent
seizures-(severity
seizures (severity score = 9) consist of a single running
of both fore- and hindlimbs.
on day 14.
value for WI rats is based on recent data.
Previously
this value was reported
scores
as follows: SD seizures (severity score = 1) are
as day 2428.
score attack
pause
and
= 3) are single followed
by brief
40
or WI rats may be more severe than in adults28,29). Differences in severity have the rank order of: SD < GEPR-3 < WI < GEPR-9. Such comparisons suggest susceptibility may arise from a developmental event which is distinct from factors determining seizure severity. Because audiogenic seizure susceptibility is a type of reflex epilepsy, seizures depend not only on the intrinsic propensity of an individual’s brain to sustain and/or propagate epileptic activity, but also on the presence of an afferent seizure-triggering circuit. We hypothesize that it is the former element which is absolutely dependent on genotype. That is, the relative balance between inhibitory and excitatory neural elements is conceived of as the basis of seizure severity in the individual. This notion is consistent with findings that GEPR rats are not only excessively prone to seizures triggered by sound but also to other proconvulsive treatments such as kindling, electroshock, or pentylenetetrazole or morphine infusion3’. By contrast, the similar age dependence of parameters related to onset of audiogenic seizure susceptibility among GEPRs and noise-induced SDS and WIs suggests a common abnormal developmental event underlies audiogenic seizure susceptibility. By this view, GEPR rats would be expected to possess at least two pertinent genotypic abnormalities: one which mimics the susceptibility-sensitizing effect of noise exposure on SD and WI rat pups and one which determines its general seizure proneness. In the following is discussed how these issues might be approached. Common to all experimental protocols which engender audiogenic seizure susceptibility is that treatment produces auditory deprivation during the developmental period of synaptogenesis in central auditory nuclei. The intense noise exposure used here to induce susceptibility causes transient or permanent hearing loss in rats’9’20. Other effective age-dependent treatments such as neonatal consumption of goitrogens (via lactating dams35), administration of ototoxic drugs4,24*27,34,insertion of ear plugs12, or disruption of eardrums3 also involve auditory deprivation. It is also true that genetically susceptible strains experience auditory deprivation. Auditory dysfunction and cochlear dysgenesis is a trait of both GEPR colonies6,19-21.
(Note, in GEPRs observed auditory abnormalities result from inherited congenital may hypothyroidism23, which, like the feeding of goitrogens, is known to produce cochlear malformation35.) In short, developmental acoustic deprivation does appear to be present in all susceptible individuals and strains. We infer that it is a key element in the induction of the audiogenic seizure susceptible phenotype. One’s ability to experimentally induce susceptibility in SD rats should aid research related to issues of the interplay between genotype and seizure severity. That is, the study of differences between brains of noise-induced susceptible SD rats and genetically susceptible GEPRs will now be possible. Nonetheless, a sizable number of differences have already been found between brains of GEPRs and nonsusceptible SDS or those occasional GEPR offspring which fail to exhibit susceptibility. These previous studies have indicated that brains of susceptible GEPRs have: (1) lower levels of monoamines’5~17, (2) regional alterations in GABA and benzodiazepine binding sites33, (3) physiological deficits in GABAergic inhibitory mechanism8, (4) excessive numbers of small presumably GABAergic cells in inferior colliculus32, and, as mentioned, (5) cochlear dysgenesis involving excessive numbers of cochlear sensory cells25,26. Assessing the importance of these anomalies is a daunting task, yet strategies for this undertaking, too, have been developed. Experimental alterations of availabilities of specific neurotransmitters or receptors or administration of receptor agonists have been examined for their influence on audiogenic seizures. GABAergic agonists and antagonists have been found to respectively inhibit’,” and promote’0’20 these seizures. By contrast, excitatory amino acid agonists while antagonists inhibit” audiogenic promote7’22 seizures. In investigations of the role of monoamines on seizures in GEPRs, the effect of depletion of norepinephrine (NE) of the rarely encounnon-seizing tered GEPR offspring was examined17,30. It was found that if depleted, these rats did exhibit seizures. From this it was inferred that they possessed latent susceptibility since normal SDS cannot be rendered susceptible in this manner. Also, as was noted in the Introduction
41
here, if SD rats are treated during development with the ototoxic antibiotic kanamycin they do not (unlike WI rats) appear to become susceptible. However, it was also found after similarly depleting NE, that kanamycin-treated SDS also exhibit sound-triggered seizures, suggesting they, too, have latent susceptibility3i. Such observations emphasize that an important role is played by genotype in the regulation of seizure severity and per-
haps in the suppression of seizures in latently susceptible rats. Co-consideration of experimentally sensitized and genetic models of audiogenic seizure susceptibility may expand the general understan~ng of factors which engender or influence susceptibility, seizure severity and/or adult prognosis. As exemplified in the present study such factors do not appear to be single entities.
References 1 Browning, R.A. and Faingold, CL., Effects on audiogenic seizures (AGS) in the genetically epilepsy prone rat (GEPR) of microinfusions into inferior colliculus (IC) of noradrenergic (NA) and GABAergic agonists, ~~ffr~uco~og~~, 29 (1987) 142. 2 Chen, C.-S., Acoustic trauma-induced development change in the acoustic startle response and audiogenic seizures in mice, Exp. New& 60 (1978) 400-403. 3 Chen, C.-S., Gates, G.R. and Bock, G.R., Effect of priming and tympanic membrane destruction on development of audiogenic seizure su~ptibility in BALBfc mice, Exp. Neurol., 39 (1973) 277-284. 4 Chen, C.-S. and Aberdeen, G.C., The sensitive period for induction of susceptibility to audiogenic seizures by kanamycin in mice, Arch. Otorhinolaryngol., 232 (1981) 215-220. 5 Consroe, P., Picchioni, A. and Chin, L., Audiogenic seizure susceptible rats, Fed. Proc., 38 (1979) 241 l-2416. 6 Faingold, CL., Gehlbach, G., Travis, M.A. and Caspary, D.M., Inferior colhculus neuronal response abnormalities in genetically epilepsy prone rats: evidence for a deficit in inhibition, Life Sci., 39 (1986) 869-787. 7 Faingold, C.L., Millan, M.H., Boersma, C.A. and Meldrum, B.S., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway, Exp. Newel., 99 (1988) 678-686. 8 Faingold, CL., Walsh, E.J. and Maxwell, J.K., The auditory brainstem response in genetically epilepsy-prone rats susceptible to audiogenic seizures, Epilepsiu, 28 (1987) 583. 9 Faingold, C.L., Walsh, E.J., Maxwell, J.K. and Randall, M.E., Audiogenic seizure severity and hearing deficits in the genetically epilepsy-prone rat, Exp. Neural., 108 (1990) 55-60. 10 Frye, G.D., McCown, T.J., Breese, G.R. and Peterson, S.L., GABAergic modulation of inferior colliculus excitability: role in the ethanol withdrawal audiogenic seizures, J. Phurmacoi. Exp. Ther., 237 (1986) 478-485. 11 Henry, K.R., Audiogenic seizure su~ptibility induced in C57BL/6J mice by prior auditory exposure, Science, 158 (1967) 938-940. 12 Henry, K.R. and Haythom, M.M., Auditory similarities associated with genetic and experimental acoustic deprivation, J. Camp. Physiol. Psychol., 84 (1975) 213218. 13 Hjeresen, D.L., Franck, J.E. and Amend, D.L., Ontogeny of
seizure incidence, latency, and severity in genetically epilepsy-prone rats, Dev. Psychobiol., 20 (1987) 355-363. 14 Iturrian, W.B. and Fink, G.B., Effect of age and conditiontest interval (days) on an audio~ondition~ convulsive response in CF no. 1 mice, Dev. Psychob~o~., 1 (1967) 375380. 15 Jobe, P.C., Picchioni, A.L. and Chin, L., Role of brain norepinephrine in audiogenic seizure in the rat, J. Phurmucol. Exp. Ther., 184 (1973) l-10. 16 Jobe, PC., Dailey, J.W. and Reigel, C.E., Noradrenergic and serotonergic determinants of seizure susceptibility and severity in genetically epilepsy prone rats, Lz* Sci., 39 (1986) 775782. 17 Jobe, P.C., Reigel, C.E., Mishra, P.K. and Dailey, J.W., Neurotransmitter abnormalities as determinants of seizure predisposition in the genetically epilepsy-prone rat. In: G. Nistico, P.L. Morselli, K.G. Lloyd, R.G. Fariello and J.E. Engel (Eds.), Ne~ro~r~s~~ffers, Seizures, and Epilepsy, III, Raven Press, New York, NY, 1986, pp. 387-397. 18 Jones, A.W., Croucher, M.J., Meldrum, B.S. and Watkins, J.C., Suppression of audiogenic seizures in DBA/2 mice by two new dipeptide NMDA receptor antagonists, Nearosci. Z.&t., 45 (1984) 157-161. 19 Lenoir, M., Bock, G.R. and Pujol, R., Supra-normal susceptibility to acoustic trauma of the rat pup cochlea, J. Physiol. (Paris), 75 (1973) 521-524. 20 Lenoir, M. and Pujol, R., Sensitive period to acoustic trauma in rat pup cochlea, Aeta Oroluryngol., 89 (1980) 317-322. 21 McGinn, M.D. and Henry, K.R., Acute versus chronic acoustic deprivation: Effects on auditory evoked potentials and seizures in mice, Dev. Psychob~o~., 8 (1975) 223232. 22 Millan, M.H., Meldrum, B.S. and Faingold, C.L., Induction of audiogenic seizure susceptibility by focal infusion of excitant amino acid or bicuculline into inferior colliculus of normal rats, Exp. Neurol., 91 (1986) 634-639. 23 Mills, S.A. and Savage, D.D., Evidence of hypothyroidism in the genetically epilepsy-prone rat, Epilepsy Res., 2 (1988) 102-i 10. 24 Norris, C.H., Cawthorn, T.H. and Carroll, R.C., Kanamy-
tin
priming
for
Neuropharmacology,
audiogenic
seizures
in
mice,
16 (1977) 375-380.
25 Penny, J.E., Brown, R.D., Hodges,
K.B., Kupetz, S.A., Glenn, D.W. and Jobe, P.C., Cochlear morphology of the audiogenie-~i~re susceptible (AGS) or genetically epilepsy
42
Acta Otolaryngol., 95 (1983) l-12.
prone rat (GEPR), 26 Penny, J.E., Brown, Auditory
aspects
R.D., Wallace,
39 (1986) 7633774.
M.S. and Henley, CM.,
of seizure in the genetically
epilepsy prone
rat, Life Sci., 39 (1986) 887-895. 27 Pierson, optimum tibility
M.G.
and Swann,
dosaze
of audiogenic
in the Wistar
period
and
seizure suscep-
rat, Hearing Res., 32
genie
M.G. and Swann, seizure
susceptibility
noise, Epilepsia, 29 Reigel, Ontogeny
C.E.,
J., Ontogenic induced
features
of audio-
in immature
rats
by
Jobe,
P.C.,
Dailey,
J.W.
seizures
and Savage,
in the genetically
D.D., epi-
lepsy prone rat, Epilepsy Rex, 4 (1989) 63371. 30 Reigel,
C.E., Dailey,
epilepsy-prone
J.W. and Jobe,
rat: An overview
istics and responsiveness
requires
(SD) rats,
audio-
monoamine
de-
Neurosci. Abst.,
16
(1990) 781. 32 Roberts,
R.C. and Ribak,
GABAergic 33 Tacke,
colhculus
changes
in the
of the genetical-
rat, Life Sci., 39 (1986) 789-798.
U. and
receptor
C.E., Anatomical
system in the inferior Braestrup,
binding
C., A study
in audiogenic
on benzodiazepine rats, Acta
seizure-susceptible
Pharmacol. Toxicol., 55 (1984) 252-259.
32 (1991) l-9.
of sound-induced
W.M., Kanamycin-induced
susceptibility
in Sprague-Dawley
ly epilepsy-prone
(1988) l-10. 28 Pierson,
genie seizure (AGS) pletion
J.W., The sensitive
for induction
by kanamycin
31 Reigel, C.E. and Aldrich,
to anticonvulsant
drugs,
J.M.
kanamycin
and induced
Schlessinger, cochlear
K., Acoustic damage,
priming
Brain
Res.,
and 187
(1980) 81-95.
P.C., The genetically
of seizure-prone
34 Tepper,
characterLife Sci.,
35 Van Middlesworth, zures and cochlear roid treatment,
L. and damage
Norris,
C.H.,
Audiogenic
in rats after perinatal
Endocrinology, 106 (1980) 16861690.
sei-
antithy-
EPIRES
35 B.V. All rights reserved
00504
Noise exposure-induced audiogenic seizure susceptibility in Sprague-Dawley rats
M. Pierson
and S.L. Liebmann
Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, NY 12201-0509, (Received
10 October
1991; revision
received
Key words: Audiogenic
Parameters exposure. neonatal
were evaluated
susceptibility
by day 20, but seizure seizures
induction
on this susceptibility
days 13 and 15 and that susceptibility
adulthood, becomes
for the optimum
The effect of maturation
severity
requires
increasingly
shorter
during
somewhat
of audiogenic
seizure
was also examined. a minimum
is not maximal
the prepubescent
period
1992; accepted
seizures; Epileptogenesis;
susceptibility
It was found
32-36.
(days 1624)
on the age when initial noise exposure
in Sprague-Dawley
(SD) rats by noise
that SD rats are most inducible
Noise exposure
Although
type. Seizure latency
29 April 1992)
Rat
of 2 days to develop.
until days
in older rats revert to the wild-running-only
adult seizures depends
27 April
susceptibility
(from stimulus
between
on day 14 results in universal persists
at high levels into
onset to onset of wild running)
but is stable at older ages. The mean shortness
occurred;
USA
day-14 noise exposures
of latency
in
result in seizures with shortest
latencies. Ontogenetic
comparisons
which are SD substrains)” age in all four groups maximum
were made of susceptibility and noise exposure-induced
of rats but that considerable
seizure severity
among
in these noise exposure-induced Wistar
differences
(WI) rats28. It appears characterize
the absolute
rats, genetically that epileptogenesis
epilepsy
prone
rats (GEPRs,
begins at virtually
the same
severity of seizures and the age dependence
of
the strains.
Introduction Identification of the basis of audiogenic seizure susceptibility is of great interest because this model of epilepsy in rodents is developmental as are many human epilepsies. The issue of causality has been especially studied in one specific rodent substrainl~S-9~15-1~,23,25,26,29-33 , the genetically epilepsy prone rat (GEPR) of which two types3’ may be distinguished, the GEPR-9 and GEPR-3*. Because these GEPR colonies derive from Sprague-
Correspondence to: M. Pierson, Cain Foundation Laboratories, Texas Children’s Hospital, Clinical Care Center #955, 6621 Fannin Street, MC 3-3311, Houston, TX 77030-2399, USA.
Dawley
(SD) ancestors,
one productive
approach
to understanding epileptogenesis in the GEPR would be the comparison of audiogenic seizure susceptibility in GEPRs with that in individuals of the parent strain. However, since SD rats do not normally exhibit audiogenic seizures, it was desirable to experimentally induce the susceptible phe-
* The CEPR-9
and the CEPR-3 colonies received their respective names based on the severity of characteristic seizures. Seizures with a single wild running attack followed by a generalized tonic convulsion involving full extension of the hindlimbs receive a score of 9, while seizures consisting of a single running episode followed by a generalized clonic convulsion receive a score of 3 according to two published audiogenic seizure severity scales’5.27.
36
notype. For this reason we undertook to determine whether susceptibility could be engendered in a
served in these were maintained
seizure-resistant
Both sexes were used in approximately
colony
of SD rats.
Paradigms for experimentally inducing susceptibility have been reported for both mouse and rat strains.
The manipulations
that effect this conver-
sion consist either of cochlear
insult (e.g., exposure
to: ototoxic drugs4*24*27,34,goitrogens35, or intense noise 2,3,‘1,12,21*28)or of direct auditory deprivation (e.g., temporary insertion of earplugs or eardrum rupture3,12). Th e only caveat has been that such treatments must occur during a critical developmental period in order to be effective. The period in each instance coincides with that when synaptogenesis occurs in the central ascending auditory pathway. Since two paradigms have been reported for the induction of audiogenic seizure susceptibility in immature Wistar (WI) rats27,28, a similar outcome in SD rats would seem likely. However, as was recently reported, neonatal administration of the ototoxic drug kanamycin does not engender outright audiogenic seizure susceptibility in SD rats31, although such treatment is 100% successful in WI rats. We have ourselves confirmed the former observation (unpublished results). Thus, it has seemed useful now to examine the effectiveness of intense neonatal noise exposure for engendering susceptibility in SD rats. In the present study, it was found that intense neonatal noise exposure can similarly - and universally - induce susceptibility in the SD. Parameters resulting in maximal susceptibility and the ontogenetic features of such susceptibility are reported here. Because ontogenic changes in seizure susceptibility have been reported for additional strains, the CEPR-3 and the GEPR-95,‘3,29730 and noise exposure-induced WI between noise exposure-inrats2’, comparisons duced SD rats and these other models of audiogenie seizure susceptibility could be presented here. Methods Sprague-Dawley rats used in these experiments were from an in-house breeding colony. The original stock was purchased from Taconic Farms, strain designation: Tac:N(SD)fBR. The spontaneous rate of audiogenic seizure susceptibility ob-
rats was 2/363 on a 12/12-h
(0.6%). Animals light/dark cycle. equal num-
bers; pups were weaned on day (d) 24. Noise exposures and subsequent tests consisted of the ringing
of a lo-inch,
manually
controlled
electric fire alarm bell held at the lip of a 36 x 39 cm metal canister. The intensity of the signal was invariant, being 125-128 dB SPL A (‘A’ refers to a high frequency-weighted scale of sound level meters). Octave band analysis of this signal has been described previously28. Seizure tests lasted a maximum of 60 s and consisted of the presentation of this same signal. Convulsive seizures were always preceded by wild running but nonconvulsive seizures consisting of running-only also occurred. The bell was turned off if, and when, wild running seizures gave way to convulsions (clonic or tonic). Individual rats were tested on only one occasion, since alterations in long-term susceptibility have been reported to occur in WI28 as well as GEPR’3,30 rats if previously tested. All animal use procedures were approved by an institutional animal care and welfare committee. The first experiment addressed the identification of the critical period for induction of susceptibility. Rats were exposed to the bell for 6 or 10 min. Seizure susceptibility tests were conducted on d 36. The age of rats during initial noise exposure was one of the following: 11, 12, 13, 14, 15, 16, 18, 20, 24, or 28 days. For IO-min exposed rats, the exposure ages did not include d 11 or d 28. A second question was to determine the effect of different durations of noise exposure on d 15 on susceptibility being tested on d 28. Exposure durations included 1, 2, 4, 6, 8, 10 and 12 min. A third experiment considered the age dependence of the evolution of susceptibility. Several groups received lo-min noise exposures on d 14, while susceptibility was tested at a variety of ages including: days 15, 16, 18, 20, 24, 28, 32, 36, 44 and 52. Significance of differences in incidence of seizures or convulsions was analyzed by the Fisher exact test (FET). Differences in latency were analyzed by Student’s t test.
37
,y(, , ~~~,~~,~~~f~ 11; AGE AT TIME OF 6-min EXPOSURE
20 (101
24 (~~1
28 (101
AGE AT TIME OF lo-min EXPOSURE
Fig. 1. Sensitive ages when Sprague-Dawley rats can be induced to become prone to audiogenic seizures. Susceptibility was assessed at d 36; each rat was tested one time only. Sample sizes are shown in parentheses below data. Shaded areas represent convulsive seizures. Effects of both 6-min (left panels) and IO-min (right paneIs) exposures are shown. Latencies were measured from stimulus onset to start of wild running attack and are shown with SEMs in lower panels.
Results
The developmental period during which susceptibility could be induced in SD rats by a 6-min noise exposure ranged from d 13 to d 24 as shown in the left panel of Fig. 1. However, by operationally defining as the critical period for inducibility those ages when exposure resulted in susceptibility in at least 50% of rats, the period between d 13 and d 18 emerged as the most useful. On the other hand when severity of seizures is also taken into account, the critical period becomes more limited since 50% of tests resulted in convulsions only when initial noise exposure had occurred at an age between d 13 and d 15. An increase in duration of exposure did not result in a widening of the critical period for induction of susceptibility (to either convulsive or nonconvulsive types of seizures) as shown in the right panel of Fig. 1. It appeared that d 14 was a slightly better age for inducing susceptibility when using a 6-min exposure, whereas d 15 was the best
age when employing IO-min presentations. However, if shortness of latency was added as a further criterion, then day-14 exposures offered a slight advantage over day-15 exposures (lower panels in Fig. l), since for either duration latencies were shortest after day-14 exposures. Severity of seizures (on d 28) increased monotonically with increases in the duration of initial exposure (here on day 15) as shown in Fig. 2. For example, by interpolation it is apparent that the duration of exposure necessary to induce wild running susceptibility in 50% of rats was 2.5 min; for engendering convulsive seizure sus~ptibility, it was 7.0 min. Nonetheless, not even 12-min exposures (on d 15) resulted in universal convulsive seizures at the later age of d 28. Differences between latencies (to wild running onset) were not signitieantly influenced by duration of initial exposure as shown in the lower panel. Onset of susceptibility to wild running-only seizures did not occur in less than 2 days after initial
38
0
2 (15) (13)
4 (12)
6
6
10
(12)
(121
(16)
DURATION EXPOSURE (min) Fig. 2. Influence of duration of exposure on incidence and severity of seizures. Sprague-Dawley rats were noise-exposed at d 15; susceptibility tests were on d 28. Sample sizes are shown along abscissa below data points. Shaded area represents convulsive responses. Latencies (to wild running onset) and SEMs are shown in lower panel.
noise exposure as shown in Fig. 3. While wild running seizures occurred universally by d 20, convulsive seizures did not occur before d 24 and their incidence was not maximal until d 32 to d 36. Convulsions were never universal. There occurred a slight drop in general susceptibility (i.e., sum of incidences of both running-only and convulsive seizures) (P ~0.05, FET) and a significant drop in incidence of convulsive seizures (P ~0.05, FET) between the ages of d 36 and d 52. Latency, too, was somewhat age-dependent as shown in the lower panel of Fig. 3. It decreased monotonically and significantly between each pair of consecutive test ages between d 16 and d 24 (each P ~0.05, Student’s t test). After d 24 latency did not decrease further (P ~0.05, Student’s t test). In addition to the above data there were behavioral differences in seizures at different ages. Centered at the age of d 24, there was a marked tendency for wild running-only seizures to be highly protracted - often lasting 3-6 min after the bell had been turned off. If one physically interrupted these poststimulus running episodes (e.g., by pick-
25 35 45 AGE AT TIME OF TEST (days)
Fig. 3. Ontogenetic features of audiogenic seizure susceptibility in Sprague-Dawley rats. Susceptibility was induced by lo-min exposures on d 14; each rat was tested one time only. Sample sizes were 10 except on day 36 when n was 12. Shaded area represents convulsive responses. Lower panels show latencies to wild running onset and SEMs.
ing the rat up by the tail), the behavior nonetheless persisted and rats would continue frenzied running (as soon as they were put back down). This surprising behavior occurred only in rats with nonconvulsive (wild running-only) seizures and was first observed in d 20 rats (S/IO of running-onIy seizures). It occurred in 7/8 of such seizures at the age of d 24 but in only 5145 of wild running seizures in d 28 rats. Prolonged poststimulus running was never observed in rats younger than d 20 or older than d 28. The sex of rats had no influence on either the likelihood or the severity of seizures. Of 496 seizure tests conducted in the present study, 243 involved female rats and 253 involved males. The ratio of probabilities of convulsions:wiid runningonly seizures:nonresponsive behaviors among females was 0.34:0.37:0.29. The ratio among males in these respective categories was 0.37:0.34:0.29.
39
appear to be inherently more resistant to severe types of audiogenic seizures, even though the two strains have essentially the same likelihood of becoming susceptible due to neonatal noise exposure. The implications of this difference in seizure severity in the absence of a difference in seizure incidence are discussed below. In view of the differences found here in seizures of noise-induced WI and SD rats, it was of interest to compare susceptibility in these strains to that in genetically susceptible GEPR-9 and GEPR-3 colonies. It is possible to do this because a recent ontogenetic study of audiogenic seizures in GEPRs~~ employed age-based parameters similar to. those employed both here and in our previous study of noise exposure-sensitized WI rats28. From such comparison, as demonstrated in Table I, it becomes apparent that the timing of events during the onset of susceptibility is nearly identical among these four models. That is, age for optimum sensitization by intense noise exposure is similar in SD and WI rats (d 14); the earliest age when susceptibility emerges is similar in all four strains (d 15-16); and susceptibility (not seizure severity) is at peak (nearly universal) levels by roughly the same age (d 1921). By contrast, severity of seizures after d 24 and in adult rats was highly dependent on strain (note, seizure severity scores in Table I reflect behaviors of d 36 SD and WI rats or d 45 GEPRs although seizures of d 23-28 GEPR-3
Discussion It is concluded that noise exposure is useful for inducing audiogenic seizure susceptibility in Sprague-Dawley rats as it is in Wistar rats28. Inducibility in the two strains has in common that wild running seizure susceptibility does not emerge in less than 2 days, that d 14 appears to be the best age for induction (of susceptibility), and that susceptibility can be universally engendered. Since in the WI initial exposure paradigms which engender the highest incidence of wild running attacks also engender the highest seizure severity among seizing rats, it was surprising that seizures in noise-sensitized SDS generally were simple wild running attacks even when 100% of rats in a group were susceptible. Not only were convulsive seizures infrequent, but they did not occur prior to d 24 nor in rats d 44 or older. By contrast in noise exposureinduced WIs, convulsive seizures sometimes occur as early as d 16, occur universally by d 24 and are likely to continue to occur at subsequent ages28. A further difference is that increases in the duration of exposure of SDS do not result in a substantial widening of the critical period for inducibility (left vs. right panels, Fig. 1). By contrast the critical period is widened by 4 days in WI rats as the initial exposure duration is increased from 4 to 8 min28. In short, the predominant difference found between SD and WI rats has been that the former TABLE
I
Comparison
of ontogenetic
characteristics
of audiogenic seizure susceptibility
in four rat strains
Most sensitive age
Earliest
age when
Age (days) when
Age (days) when
Typical
audiogenic
(days) for inducibility
running
seizures are
seizure incidence
seizure severity is
response
scorea
by noise exposure
observed
becomes
greatest
(adult)
SD
13-15
16b
20
32-36
1
WI28
14-16
16b
20”
28-32
6
15
19
23-25
3
16
21
z 45
9
GEPR-329 GEPR-929
1
a Severity
scores in this table do not depend
reflect seizures of greater single 8-10-s followed running clonus
running
by clonus attacks
severity. Typical
attacks;
WI seizures
then tonic convulsion
followed
b Following
on whether
initial noise exposure
universal
a scale devised for seizures in GEPRs”
seizures by adults of each strain are described (severity involving
score partial
by 4-limb clonus; and GEPR-9
then tonus with full extension
’ Revised
(days)
= 6) consist extension
of two running
of hindlimbs;
or in WI rats*’ is used. Higher
attacks
GEPR-3
separated
by a quiescent
seizures-(severity
seizures (severity score = 9) consist of a single running
of both fore- and hindlimbs.
on day 14.
value for WI rats is based on recent data.
Previously
this value was reported
scores
as follows: SD seizures (severity score = 1) are
as day 2428.
score attack
pause
and
= 3) are single followed
by brief
40
or WI rats may be more severe than in adults28,29). Differences in severity have the rank order of: SD < GEPR-3 < WI < GEPR-9. Such comparisons suggest susceptibility may arise from a developmental event which is distinct from factors determining seizure severity. Because audiogenic seizure susceptibility is a type of reflex epilepsy, seizures depend not only on the intrinsic propensity of an individual’s brain to sustain and/or propagate epileptic activity, but also on the presence of an afferent seizure-triggering circuit. We hypothesize that it is the former element which is absolutely dependent on genotype. That is, the relative balance between inhibitory and excitatory neural elements is conceived of as the basis of seizure severity in the individual. This notion is consistent with findings that GEPR rats are not only excessively prone to seizures triggered by sound but also to other proconvulsive treatments such as kindling, electroshock, or pentylenetetrazole or morphine infusion3’. By contrast, the similar age dependence of parameters related to onset of audiogenic seizure susceptibility among GEPRs and noise-induced SDS and WIs suggests a common abnormal developmental event underlies audiogenic seizure susceptibility. By this view, GEPR rats would be expected to possess at least two pertinent genotypic abnormalities: one which mimics the susceptibility-sensitizing effect of noise exposure on SD and WI rat pups and one which determines its general seizure proneness. In the following is discussed how these issues might be approached. Common to all experimental protocols which engender audiogenic seizure susceptibility is that treatment produces auditory deprivation during the developmental period of synaptogenesis in central auditory nuclei. The intense noise exposure used here to induce susceptibility causes transient or permanent hearing loss in rats’9’20. Other effective age-dependent treatments such as neonatal consumption of goitrogens (via lactating dams35), administration of ototoxic drugs4,24*27,34,insertion of ear plugs12, or disruption of eardrums3 also involve auditory deprivation. It is also true that genetically susceptible strains experience auditory deprivation. Auditory dysfunction and cochlear dysgenesis is a trait of both GEPR colonies6,19-21.
(Note, in GEPRs observed auditory abnormalities result from inherited congenital may hypothyroidism23, which, like the feeding of goitrogens, is known to produce cochlear malformation35.) In short, developmental acoustic deprivation does appear to be present in all susceptible individuals and strains. We infer that it is a key element in the induction of the audiogenic seizure susceptible phenotype. One’s ability to experimentally induce susceptibility in SD rats should aid research related to issues of the interplay between genotype and seizure severity. That is, the study of differences between brains of noise-induced susceptible SD rats and genetically susceptible GEPRs will now be possible. Nonetheless, a sizable number of differences have already been found between brains of GEPRs and nonsusceptible SDS or those occasional GEPR offspring which fail to exhibit susceptibility. These previous studies have indicated that brains of susceptible GEPRs have: (1) lower levels of monoamines’5~17, (2) regional alterations in GABA and benzodiazepine binding sites33, (3) physiological deficits in GABAergic inhibitory mechanism8, (4) excessive numbers of small presumably GABAergic cells in inferior colliculus32, and, as mentioned, (5) cochlear dysgenesis involving excessive numbers of cochlear sensory cells25,26. Assessing the importance of these anomalies is a daunting task, yet strategies for this undertaking, too, have been developed. Experimental alterations of availabilities of specific neurotransmitters or receptors or administration of receptor agonists have been examined for their influence on audiogenic seizures. GABAergic agonists and antagonists have been found to respectively inhibit’,” and promote’0’20 these seizures. By contrast, excitatory amino acid agonists while antagonists inhibit” audiogenic promote7’22 seizures. In investigations of the role of monoamines on seizures in GEPRs, the effect of depletion of norepinephrine (NE) of the rarely encounnon-seizing tered GEPR offspring was examined17,30. It was found that if depleted, these rats did exhibit seizures. From this it was inferred that they possessed latent susceptibility since normal SDS cannot be rendered susceptible in this manner. Also, as was noted in the Introduction
41
here, if SD rats are treated during development with the ototoxic antibiotic kanamycin they do not (unlike WI rats) appear to become susceptible. However, it was also found after similarly depleting NE, that kanamycin-treated SDS also exhibit sound-triggered seizures, suggesting they, too, have latent susceptibility3i. Such observations emphasize that an important role is played by genotype in the regulation of seizure severity and per-
haps in the suppression of seizures in latently susceptible rats. Co-consideration of experimentally sensitized and genetic models of audiogenic seizure susceptibility may expand the general understan~ng of factors which engender or influence susceptibility, seizure severity and/or adult prognosis. As exemplified in the present study such factors do not appear to be single entities.
References 1 Browning, R.A. and Faingold, CL., Effects on audiogenic seizures (AGS) in the genetically epilepsy prone rat (GEPR) of microinfusions into inferior colliculus (IC) of noradrenergic (NA) and GABAergic agonists, ~~ffr~uco~og~~, 29 (1987) 142. 2 Chen, C.-S., Acoustic trauma-induced development change in the acoustic startle response and audiogenic seizures in mice, Exp. New& 60 (1978) 400-403. 3 Chen, C.-S., Gates, G.R. and Bock, G.R., Effect of priming and tympanic membrane destruction on development of audiogenic seizure su~ptibility in BALBfc mice, Exp. Neurol., 39 (1973) 277-284. 4 Chen, C.-S. and Aberdeen, G.C., The sensitive period for induction of susceptibility to audiogenic seizures by kanamycin in mice, Arch. Otorhinolaryngol., 232 (1981) 215-220. 5 Consroe, P., Picchioni, A. and Chin, L., Audiogenic seizure susceptible rats, Fed. Proc., 38 (1979) 241 l-2416. 6 Faingold, CL., Gehlbach, G., Travis, M.A. and Caspary, D.M., Inferior colhculus neuronal response abnormalities in genetically epilepsy prone rats: evidence for a deficit in inhibition, Life Sci., 39 (1986) 869-787. 7 Faingold, C.L., Millan, M.H., Boersma, C.A. and Meldrum, B.S., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway, Exp. Newel., 99 (1988) 678-686. 8 Faingold, CL., Walsh, E.J. and Maxwell, J.K., The auditory brainstem response in genetically epilepsy-prone rats susceptible to audiogenic seizures, Epilepsiu, 28 (1987) 583. 9 Faingold, C.L., Walsh, E.J., Maxwell, J.K. and Randall, M.E., Audiogenic seizure severity and hearing deficits in the genetically epilepsy-prone rat, Exp. Neural., 108 (1990) 55-60. 10 Frye, G.D., McCown, T.J., Breese, G.R. and Peterson, S.L., GABAergic modulation of inferior colliculus excitability: role in the ethanol withdrawal audiogenic seizures, J. Phurmacoi. Exp. Ther., 237 (1986) 478-485. 11 Henry, K.R., Audiogenic seizure su~ptibility induced in C57BL/6J mice by prior auditory exposure, Science, 158 (1967) 938-940. 12 Henry, K.R. and Haythom, M.M., Auditory similarities associated with genetic and experimental acoustic deprivation, J. Camp. Physiol. Psychol., 84 (1975) 213218. 13 Hjeresen, D.L., Franck, J.E. and Amend, D.L., Ontogeny of
seizure incidence, latency, and severity in genetically epilepsy-prone rats, Dev. Psychobiol., 20 (1987) 355-363. 14 Iturrian, W.B. and Fink, G.B., Effect of age and conditiontest interval (days) on an audio~ondition~ convulsive response in CF no. 1 mice, Dev. Psychob~o~., 1 (1967) 375380. 15 Jobe, P.C., Picchioni, A.L. and Chin, L., Role of brain norepinephrine in audiogenic seizure in the rat, J. Phurmucol. Exp. Ther., 184 (1973) l-10. 16 Jobe, PC., Dailey, J.W. and Reigel, C.E., Noradrenergic and serotonergic determinants of seizure susceptibility and severity in genetically epilepsy prone rats, Lz* Sci., 39 (1986) 775782. 17 Jobe, P.C., Reigel, C.E., Mishra, P.K. and Dailey, J.W., Neurotransmitter abnormalities as determinants of seizure predisposition in the genetically epilepsy-prone rat. In: G. Nistico, P.L. Morselli, K.G. Lloyd, R.G. Fariello and J.E. Engel (Eds.), Ne~ro~r~s~~ffers, Seizures, and Epilepsy, III, Raven Press, New York, NY, 1986, pp. 387-397. 18 Jones, A.W., Croucher, M.J., Meldrum, B.S. and Watkins, J.C., Suppression of audiogenic seizures in DBA/2 mice by two new dipeptide NMDA receptor antagonists, Nearosci. Z.&t., 45 (1984) 157-161. 19 Lenoir, M., Bock, G.R. and Pujol, R., Supra-normal susceptibility to acoustic trauma of the rat pup cochlea, J. Physiol. (Paris), 75 (1973) 521-524. 20 Lenoir, M. and Pujol, R., Sensitive period to acoustic trauma in rat pup cochlea, Aeta Oroluryngol., 89 (1980) 317-322. 21 McGinn, M.D. and Henry, K.R., Acute versus chronic acoustic deprivation: Effects on auditory evoked potentials and seizures in mice, Dev. Psychob~o~., 8 (1975) 223232. 22 Millan, M.H., Meldrum, B.S. and Faingold, C.L., Induction of audiogenic seizure susceptibility by focal infusion of excitant amino acid or bicuculline into inferior colliculus of normal rats, Exp. Neurol., 91 (1986) 634-639. 23 Mills, S.A. and Savage, D.D., Evidence of hypothyroidism in the genetically epilepsy-prone rat, Epilepsy Res., 2 (1988) 102-i 10. 24 Norris, C.H., Cawthorn, T.H. and Carroll, R.C., Kanamy-
tin
priming
for
Neuropharmacology,
audiogenic
seizures
in
mice,
16 (1977) 375-380.
25 Penny, J.E., Brown, R.D., Hodges,
K.B., Kupetz, S.A., Glenn, D.W. and Jobe, P.C., Cochlear morphology of the audiogenie-~i~re susceptible (AGS) or genetically epilepsy
42
Acta Otolaryngol., 95 (1983) l-12.
prone rat (GEPR), 26 Penny, J.E., Brown, Auditory
aspects
R.D., Wallace,
39 (1986) 7633774.
M.S. and Henley, CM.,
of seizure in the genetically
epilepsy prone
rat, Life Sci., 39 (1986) 887-895. 27 Pierson, optimum tibility
M.G.
and Swann,
dosaze
of audiogenic
in the Wistar
period
and
seizure suscep-
rat, Hearing Res., 32
genie
M.G. and Swann, seizure
susceptibility
noise, Epilepsia, 29 Reigel, Ontogeny
C.E.,
J., Ontogenic induced
features
of audio-
in immature
rats
by
Jobe,
P.C.,
Dailey,
J.W.
seizures
and Savage,
in the genetically
D.D., epi-
lepsy prone rat, Epilepsy Rex, 4 (1989) 63371. 30 Reigel,
C.E., Dailey,
epilepsy-prone
J.W. and Jobe,
rat: An overview
istics and responsiveness
requires
(SD) rats,
audio-
monoamine
de-
Neurosci. Abst.,
16
(1990) 781. 32 Roberts,
R.C. and Ribak,
GABAergic 33 Tacke,
colhculus
changes
in the
of the genetical-
rat, Life Sci., 39 (1986) 789-798.
U. and
receptor
C.E., Anatomical
system in the inferior Braestrup,
binding
C., A study
in audiogenic
on benzodiazepine rats, Acta
seizure-susceptible
Pharmacol. Toxicol., 55 (1984) 252-259.
32 (1991) l-9.
of sound-induced
W.M., Kanamycin-induced
susceptibility
in Sprague-Dawley
ly epilepsy-prone
(1988) l-10. 28 Pierson,
genie seizure (AGS) pletion
J.W., The sensitive
for induction
by kanamycin
31 Reigel, C.E. and Aldrich,
to anticonvulsant
drugs,
J.M.
kanamycin
and induced
Schlessinger, cochlear
K., Acoustic damage,
priming
Brain
Res.,
and 187
(1980) 81-95.
P.C., The genetically
of seizure-prone
34 Tepper,
characterLife Sci.,
35 Van Middlesworth, zures and cochlear roid treatment,
L. and damage
Norris,
C.H.,
Audiogenic
in rats after perinatal
Endocrinology, 106 (1980) 16861690.
sei-
antithy-
