Effects of monaural and binaural auditory deprivation on audiogenic seizure susceptibility in BALB-c mice

Mice of the $\text{BALB}\text{c}$ strain were divided into three groups at 20 ± 1 days of age. One group was deprived binaurally of auditory input by destroying the tympanic membrane bilaterally. A second group was monaurally deprived of auditory input by unilateral tympanic membrane destruction. The third group of control animals was sham-operated and the membranes were left intact. At 27 ± 1 days of age, all three groups of mice were tested behaviorally for audiogenic seizures. On exposure to the loud sound, a majority of the auditory-deprived animals of both operative groups exhibited audiogenic seizure reactions, but none of the sham-operated controls showed seizure behavior. These findings lend support to the hypothesis that induction of seizure susceptibility is a result of auditory deprivation.     

Effect of priming and tympanic membrane destruction on development of audiogenic seizure susceptibility in BALB-c mice

Induction of audiogenic seizure susceptibility was studied in $\text{rmBALB}\text{c}$ mice by bell priming or by destruction of their tympanic membranes. Bell priming was not so effective at 14 days of age as at 21 days. However, a high incidence of seizure was induced in the 14- and 21-day-old mice by rupturing their tympanic membranes; the incidence and severity of seizures thus induced was higher in the 14-day-old group. The rate of development of seizure susceptibility was also faster in the younger mouse. The results provide further support for the hypothesis that auditory deprivation resulting from priming is a critical factor for development of the priming effect. They also indicate that susceptibility of the cochlea to stimulation damage and the maturational stage of the mouse may influence the effect of priming for audiogenic seizures.     

Developmental changes in auditory evoked potentials in the inferior colliculi of mice during periods of susceptibility to priming

In mice, which are not normally susceptible to audiogenic seizures, a single exposure to an intense acoustic stimulus between the ages of 12 and 26 days renders the animals susceptible thereafter to audiogenic seizures. This phenomenon is known as “acoustic priming”. In the present study, auditory evoked potentials were recorded from the inferior colliculi of 116 unanaesthetized mice bearing chronically implanted electrodes. The amplitude of the evoked potentials increased sharply between approximately 15 and 22 days of age, suggesting that the “critical period” for priming coincides with a period of rapid development in the auditory system. Since there is substantial evidence from the work of other investigators that priming is due to disuse of the auditory pathways resulting from sound-induced cochlear damage (and can be mimicked by puncturing the eardrum or stoping up the auditory meatus). A plausible hypothesis is that the development of the auditory pathways is modulated by input, as if damage to the receptors during the “critical period” of development leads to proliferation of functional connections.     

Glial Contribution to Seizure: Carbonic Anhydrase Activity in Epileptic Mammalian Brain

The activity of carbonic anhydrase (CA), a glial enzyme, was measured in the epileptic cortex of audiogenic DBA/2 mice and of cats with a freeze lesion. In mice, the activity increased with age from birth to 24 days, but were always higher in audiogenic mice than in normal C57/BL mice, reflecting species differences. The difference between the two strains increased sharply from 25 to 40 days of age, after the period of maximal audiogenic susceptibility. Acetazolamide, a CA-specific inhibitor, greatly decreased the seizure severity score of DBA/2 mice after a single intraperitoneal (i.p.) administration (150 mg/kg). After 24 days of age, when CA activities were high, the effect of acetazolamide was less important, suggesting that the increased cortical CA activity might reflect a protective mechanism. In cats with a freeze lesion, no significant changes in CA activities were observed in the actively discharging primary and secondary foci as compared with the nonepileptogenic perifocal cortex and the control cortex of sham-operated animals. The results indicate that the cortex of genetically susceptible audiogenic mice has an increased CA activity. The hypothesis of an adaptive glial mechanism, relating to the age-dependent decrease of seizure susceptibility in DBA/2 mice, is postulated.     

Role of the superior colliculus in the expression of acute and kindled audiogenic seizures in Wistar audiogenic rats

Purpose: The role of the superior colliculus (SC) in seizure expression is controversial and appears to be dependent upon the epilepsy model. This study shows the effect of disconnection between SC deep layers and adjacent tissues in the expression of acute and kindling seizures. Methods: Subcollicular transections, ablation of SC superficial and deep layers, and ablation of only the cerebral cortex were evaluated in the Wistar audiogenic rat (WAR) strain during acute and kindled audiogenic seizures. The audiogenic seizure kindling protocol started 4 days after surgeries, with two acoustic stimuli per day for 10 days. Acute audiogenic seizures were evaluated by a categorized seizure severity midbrain index (cSI) and kindled seizures by a severity limbic index (LI). Results: All subcollicular transections reaching the deep layers of the SC abolished audiogenic seizures or significantly decreased cSI. In the unlesioned kindled group, a reciprocal relationship between limbic and brainstem pattern of seizures was seen. The increased number of stimuli provoked an audiogenic kindling phenomenon. Ablation of the entire SC (ablation group) or of the cerebral cortex only (ctx‐operated group) hampered the acquisition of limbic behaviors. There was no difference in cSI and LI between the ctx‐operated and ablation groups, but there was a difference between ctx‐operated and the unlesioned kindled group. There was also no difference in cSI between SC deep layer transection and ablation groups. Results of histologic analyses were similar for acute and kindled audiogenic seizure groups. Conclusions: SC deep layers are involved in the expression of acute and kindled audiogenic seizure, and the cerebral cortex is essential for audiogenic kindling development.     

Noise-induced hearing loss, auditory evoked potentials, and protection from audiogenic seizures in mice

Protection from audiogenic seizures by exposing susceptible mice to intense acoustic stimulation prior to seizure testing was investigated in three experiments. Auditory evoked potentials were measured from the inferior colliculi of two strains of mouse, the and , and were related to parallel measures of the acoustic startle (Preyer) response and audiogenic seizures. Exposed groups were subjected to intense sound while they were anesthetized with ether prior to testing. Control groups were treated identically but not exposed to sound before testing. As a result of this prior exposure to sound, audiogenic seizures were reduced, Preyer thresholds were elevated, and the absolute threshold for auditory evoked potentials was elevated. However, at intensities sufficient to induce audiogenic seizures, the amplitudes of the auditory evoked potentials were not affected. These results suggest that whatever noise-induced hearing loss occurred, it was not observed at high intensities. Interference with patterns of neural activity was suggested as the mechanism of protection from seizures.     

Decreased GABA effectiveness in the inferior colliculus neurons during ethanol withdrawal in rats susceptible to audiogenic seizures

The inferior colliculus (IC) is the initiation site in the neuronal network for audiogenic seizure (AGS) in rats undergoing ethanol withdrawal (ETX). Considerable evidence supports a role of gamma-aminobutyric acid (GABA)-mediated inhibition in normal acoustic processing in the IC. Altered GABA-mediated inhibition in the IC is suggested to be important in the control of AGS initiation. The present study used microiontophoresis to examine the effectiveness of GABA on acoustically-evoked neuronal responses in the central nucleus of the IC (ICc). GABA effectiveness was compared in normal controls and a group of animals displaying high audiogenic seizure susceptibility (100% AGS) (HAGS), and a group exhibiting a low (mean, 33%) incidence of AGS (LAGS). Ethanol was administered for 4 days in three daily doses (9–15 g/kg/day) sufficient to maintain a moderate degree of intoxication. Tonic-clonic seizures were observed in HAGS animals, while LAGS rats exhibited less severe seizures, consisting primarily of wild running. Iontophoretic application of GABA consistently inhibited We neuronal firing in controls and in animals undergoing ETX. However, the mean dose (current) of GABA required to produce a 50% reduction of the ICc neuronal firing in the HAGS group was nearly twice that of the control animals. The mean dose of GABA for 50% inhibition in the LAGS group was about one-half that of the control group. Both of these differences were statistically significant. These data suggest that decreased GABA effectiveness in the IC neurons of HAGS susceptible animals is an important mechanism contributing to the propagation of severe AGS seen during ETX in these animals.     

Effects of Age on Seizure Susceptibility in Genetically Epilepsy-Prone Rats (GEPR-9s)

To study the effect of age on seizure latency, intensity, reproducibility, and mortality in genetically epilepsy-prone rats of the severe colony (GEPR-9s), 472 seizure-naive rats, ranging in age from 14 to 65 days, received a series of three audiogenic stimulations. Both the percentage of rats having one or more seizures and the percentage of seizures that were stage 9 generally increased with advancing age of the animal at the time of the first stimulation. Mean latency to seizure onset decreased while seizure intensity increased with increasing age of the animal. Reproducibility of seizure stage also increased with advancing age of the animal. The effects of senescence on seizure susceptibility were also investigated in an additional 18 prepubescent rats (25-35 days) who received three audiogenic stimulations and were tested again between the ages of 480 and 540 days with identical testing procedures. No significant changes occurred with either latency to seizure onset or seizure intensity in rats tested during prepubescence and again at senescence. Although GEPR-9s provide an excellent model of inherited seizures, latency to seizure onset, seizure intensity, and seizure reproducibility is dependent on age of the animal. Once established, however, audiogenic-induced seizures persist throughout life.     

Different effects of unilateral and bilateral lesions of the dorsal raphe nucleus on puberty and first ovulation

The effects of unilateral and bilateral electrolytic lesions of the dorsal raphe nuclei (DRN) of 30-day-old rats, on the spontaneous and induced ovulation, were analyzed. The bilateral lesion and the lesion on the right side of the DRN delayed the age of first vaginal estrous. None of the animals with bilateral lesion on the DRN ovulated on the day of first estrous (0/8 vs. ,p < 0.05). The ovulation rate in animals with unilateral lesion on the DRN was similar to sham-operated animals, but the number of ova shed by ovulating animals increased in the ovary ipsilateral to the lesion (lesion on right DRN, right ovary: 6.5 ± 0.5 vs. 4.5 ± 0.4; lesion on left DRN, left ovary: 6.4 ± 0.3 vs. 4.2 ± 0.4, p < 0.05). By the injection of human chorionic gonadotropin, ovulation was restored in rats with a bilateral lesion on the DRN (3/5 vs. 0/8, p < 0.05). The present results suggest that serotoninergic input to the hypothalamus, arising from the DRN, exerts a facilitatory influences on the control of luteinizing hormone release. To explain the increase in the number of ova shed by the left and right ovary, observed in rats with an ipsilateral lesion, we suggest the existence of a neural connection between the DRN and the ovary.     

Priming for audiogenic seizures in BALB/c mice as a function of stimulus exposure duration and age.

$\text{BALB}\text{c}$ mice which are normally resistant to audiogenic seizure were made susceptible by priming them with a fixed-intensity acoustic stimulus of varying duration at 14, 21, or 35 days of age. Seven days after priming the mice were tested for susceptibility to audiogenic seizure. The data show that the duration of the priming stimulus must be within certain limits for induction of seizure susceptibility and that there is an optimum duration(s) for achieving maximum seizure incidence for each age. These data support the hypothesis that the cochleas of mice of different ages are somehow differentially susceptible to the effects of noise damage which is the mechanism by which priming induces seizure susceptibility.     

Developmental exposure to an environmental PCB mixture delays the propagation of electrical kindling from the amygdala

Developmental PCB exposure impairs hearing and induces brainstem audiogenic seizures in adult offspring. The degree to which this enhanced susceptibility to seizure is manifest in other brain regions has not been examined. Thus, electrical kindling of the amygdala was used to evaluate the effect of developmental exposure to an environmentally relevant PCB mixture on seizure susceptibility in the rat. Female Long-Evans rats were dosed orally with 0 or 6 mg/kg/day of the PCB mixture dissolved in corn oil vehicle 4 weeks prior to mating and continued through gestation and up until postnatal day (PND) 21. On PND 21, pups were weaned, and two males from each litter were randomly selected for the kindling study. As adults, the male rats were implanted bilaterally with electrodes in the basolateral amygdala. For each animal, afterdischarge (AD) thresholds in the amygdala were determined on the first day of testing followed by once daily stimulation at a standard 200 μA stimulus intensity until three stage 5 generalized seizures (GS) ensued. Developmental PCB exposure did not affect the AD threshold or total cumulative AD duration, but PCB exposure did increase the latency to behavioral manifestations of seizure propagation. PCB exposed animals required significantly more stimulations to reach stage 2 seizures compared to control animals, indicating attenuated focal (amygdala) excitability. A delay in kindling progression in the amygdala stands in contrast to our previous finding of increased susceptibility to brainstem-mediated audiogenic seizures in PCB-exposed animals in response to a an intense auditory stimulus. These seemingly divergent results are not unexpected given the distinct source, type, and mechanistic underpinnings of these different seizure models. A delay in epileptogenesis following focal amygdala stimulation may reflect a decrease in neuroplasticity following developmental PCB exposure consistent with reductions in use-dependent synaptic plasticity that have been reported in the hippocampus of developmentally PCB exposed animals.     

Effects of neural transplantation on seizures in the immature genetically epilepsy-prone rat.

To study the hypothesis that neural transplantations can alter seizure susceptibility in a genetic animal model of epilepsy, 93 pubescent genetically epilepsy-prone rats with stage 9 seizures received either bilateral inferior colliculi (N = 21) or lateral ventricle (N = 42) transplants or sham transplants (N = 30). The grafts consisted of embryonic locus ceruleus, neocortical, or cerebellar tissue. Starting 2 days after the transplantation the rats were subjected to audiogenic stimulations every other day for 61 days. Latency to the running and tonic phase, seizure severity score, and duration of the tonic and clonic phase were compared in the neural transplant and sham-operated controls. Rats that received transplants had a longer latency to the tonic phase and a shorter duration of the clonic phase than the controls. At age 110 days the rats had electrodes implanted bilaterally into the angular bundle and were kindled. No difference in kindling rate was found between the rats that received neural grafts and the sham-operated controls. Cerebrospinal fluid concentration of norepinephrine was not altered by the transplants. This study demonstrates that the anticonvulsant effects of neural transplants, using the genetically epilepsy-prone model of epilepsy, are mild.     

Audiogenic seizures susceptibility in transgenic mice with fragile X syndrome

PURPOSE: To evaluate their susceptibility to audiogenic seizures, five groups of knockout mice with various forms of fragile X genetic involvement [hemizygous males (n = 46), and homozygous (n = 38) and heterozygous females (n = 45), and their normal male (n = 45) and female (n = 52) littermates] were studied. METHODS: All mouse groups were tested at ages 17, 22, 35, and 45 days. Audiogenic seizure susceptibility was scored, and the analysis of variance was used for the evaluation of the effects of age and genetic condition on seizure severity score (SSS). RESULTS: All groups of knockout fragile X mice exhibited SSSs significantly higher than those observed in their wild-type littermates; among knockout mice, hemizygous males and homozygous females showed the highest SSSs. Hemizygous males showed higher SSSs with increasing age, from 17 to 45 days; homozygous females showed a peak at age 22 days, followed by a decrease; finally, heterozygous females had their highest SSSs at age 17 days. CONCLUSIONS: This study demonstrates that an increased susceptibility to audiogenic seizures is present in fragile X knockout mice at all the ages tested. These results support the validity of this animal model also for epilepsy and seizures in the human fragile X syndrome.     

Priming for audiogenic seizures in mice as a function of stimulus exposure duration and age

mice which are normally resistant to audiogenic seizure were made susceptible by priming them with a fixed-intensity acoustic stimulus of varying duration at 14, 21, or 35 days of age. Seven days after priming the mice were tested for susceptibility to audiogenic seizure. The data show that the duration of the priming stimulus must be within certain limits for induction of seizure susceptibility and that there is an optimum duration(s) for achieving maximum seizure incidence for each age. These data support the hypothesis that the cochleas of mice of different ages are somehow differentially susceptible to the effects of noise damage which is the mechanism by which priming induces seizure susceptibility.     

Positive transfer of audiogenic kindling to electrical hippocampal kindling in rats.

Audiogenic seizures in genetically susceptible rodents are provoked by intense acoustic stimulations which result in a tonic seizure associated with a short flattening of the EEG. These seizures have been shown to involve primarily brainstem structures. Daily exposure to sound for 30-40 days produced a permanent change in the evoked seizure with development of facial myoclonias, rearing and falling, or of tonic-clonic seizures accompanied by high amplitude cortical spike-and-wave discharges. Kindled audiogenic seizures appear similar to seizures kindled from amygdala or hippocampus, suggesting that repeated auditory stimulations cause a progressive propagation of the epileptic discharge toward limbic structures. To verify this hypothesis, the behavioral and EEG development of electrical hippocampal kindling has been studied in 7 non epileptic controls (NE), 8 acoustic susceptible (AS), and 8 audiogenic kindled rats (KAS). The behavioral and EEG development of the electrical hippocampal kindling was similar in the AS and the NE rats. However, 2 animals in the AS group but no controls exhibited behavioral running and bouncing during the course of hippocampal kindling. In the KAS group, the hippocampal kindling was clearly facilitated as compared to NE and AS: behavioral stage greater than or equal to 5 was reached in a mean of 4 stimulations in KAS versus 30 and 22 stimulations respectively in NE and AS groups. This positive transfer phenomenon suggests that during kindling of audiogenic seizures, epileptic discharge spreads from the brainstem to the forebrain and progressively involves the hippocampus.     

Long-lasting behavioral and dendritic spine deficits in the monocularly deprived albino rat

Monocular deprivation in the rat from the time of normal eye opening until 45 days of age significantly reduces the density of dendritic spines in visual cortex contralateral to the deprived eye. In addition, these animals are severely impaired in their ability to discriminate complex visual pattern using the deprived eye. In the present experiment we tested the ability of monocularly deprived rats to recover from these deficits if given 30 days of visual experience with the deprived eye beginning at 45 days of age. Animals to be tested behaviorally were trained to discriminate between columns and rows of 5-mm squares. We found that rats cross-sutured at 45 days of age and given 30 days of visual experience with the deprived eye were not significantly different from animals who began training within 2 days of cross suture. Both groups were significantly impaired in the acquisition of this discrimination relative to animals trained using their nondeprived eye. Two groups of animals were examined to assess the effects of 30 days of experience on dendritic spine density in the deprived hemisphere. One group was killed immediately at 45 days of age, the other group had the deprived eye opened at that time and then had 30 days of visual experience prior to killing. No significant recovery in spine density was found. These results have implications for a possible structure-function relationship between dendritic spines and visual behavior in the rat.     

Expression of vitamin D receptor mRNA in the hippocampal formation of rats submitted to a model of temporal lobe epilepsy induced by pilocarpine

Vitamin D (VD), is a steroid hormone with multiple functions in the central nervous system (CNS), producing numerous physiological effects mediated by its receptor (VDR). Clinical and experimental studies have shown a link between VD dysfunction and epilepsy. Along these lines, the purpose of our work was to analyze the relative expression of VDR mRNA in the hippocampal formation of rats during the three periods of pilocarpine-induced epilepsy. Male Wistar rats were divided into five groups: (1) control group; rats that received saline 0.9%, i.p. and were killed 7 days after its administration (CTRL, n = 8), (2) SE group; rats that received pilocarpine and were killed 4 h after SE (SE, n = 8), (3) Silent group—7 days; rats that received pilocarpine and were killed 7 days after SE (SIL 7d, n = 8), (4) Silent group—14 days; rats that received pilocarpine and were killed 14 days after SE (SIL 14d, n = 8), (5) Chronic group; rats that received pilocarpine and were killed 60 days after the first spontaneous seizure, (chronic, n = 8). The relative expression of VDR mRNA was determined by real-time PCR. Our results showed an increase of the relative expression of VDR mRNA in the SIL 7 days, SIL 14 days and Chronic groups, respectively (0.060 ± 0.024; 0.052 ± 0.035; 0.085 ± 0.055) when compared with the CTRL and SE groups (0.019 ± 0.017; 0.019 ± 0.025). These data suggest the VDR as a possible candidate participating in the epileptogenesis process of the pilocarpine model of epilepsy.