A comparison of short-latency auditory-evoked potentials in two strains of mice: possible neurophysiological correlates of susceptibility to audiogenic seizures?

Pathology of the cochlea following a spontaneous mutation in DBA/2 mice

The DBA/2 strain of mice usually presents with noise-induced epileptic seizures and hearing disorders. After a spontaneous mutation a strain with early hearing loss and circling behaviour was produced. This strain presents with clinical symptoms found in diseases connected to inner ear disorders. These animals do not suffer from periodical disorders, however, but have functional disturbances continuously and can therefore serve as an animal model for diseases originating from both parts of the inner ear. The genetic inheritance appears to be autosomal recessive. Offspring showed circling behaviour and severe pathology in the vestibular part of the inner ear. In the present study pathology of the cochlear part of the inner ear was visualized using conventional microscopical techniques. The content of actin and fodrin was labelled immunohistochemically, and hearing was assessed with auditory brainstem recordings. After 1 month the animals showed deterioration of the cochlear part of the inner ear. At 6 months no organ of Corti remained and the animals were deaf. Transmission and scanning electron microscopy revealed severe apical hair cell changes. The content of alpha-actinin and fodrin in the DBA/2 mouse was already fainter than that in age-matched CBA control mice at the age of 1 month. Labelling of antibodies against fodrin increased in the supporting cells of the older animals, probably owing to the replacement of hair cells.

Flash and pattern reversal visual evoked potentials in C57BL/6J and B6CBAF1/J mice

Visual system responses (visual evoked potentials) to flash (FVEP) and pattern reversal (PRVEP) stimuli were recorded in mice. Two strains were used: black C57BL/6J mice and agouti B6CBAF1/J mice (first generation offspring of C57BL/6J females and CBA/J males.) Subjects were sedated with ketamine and xylazine. Flash rate (FVEP) and stimulus spatial frequency and pattern reversal rate (PRVEP) were varied to determine optimum stimulus parameters. Normative FVEP and PRVEP data were collected from mice of both strains after determination of optimum parameters. Five positive and four negative alternating peaks were routinely observed in the FVEP, while three positive and three negative alternating peaks were seen with the PRVEP. Varying the flash rate, the pattern reversal rate, and spatial frequency significantly affected nearly all amplitude and latency measures in the responses. Significant differences between strains were seen on some, but not all, latency and amplitude measures when the stimulus parameters were varied.

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.

Kindling of audiogenic seizures in the rat

A strain of Wistar rats was inbred in our laboratory for its susceptibility to sound. The seizures are characterized by one or two wild running fits which terminate in a tonic dorsiflexion with open mouth, followed by a catatonic state. During the tonic phase of the seizure, the cortical EEG is flattened for 2 to 3 s. Then, a slow and regular low-voltage (9-12 c/s) activity is observed during 40 to 60 s. When these animals are submitted to daily sound-stimulations, the behavioral as well as the EEG manifestations of the audiogenic seizures change progressively. After 5 to 30 exposures, the wild running becomes disorganized by occurrence of myoclonic jerks of the limbs and the body. In some animals, the tonic extension disappears and a myoclonic seizure develops progressively with facial and forelimb clonus, rearing and falling. In other animals, the tonic phase still occurs and is followed by a generalized clonic phase. During both the myoclonic and the tonicoclonic seizures, rhythmic spikes, polyspikes and spike and waves of high amplitude (1-10 c/s) during 40 to 120 s are observed on EEG recordings. These EEG modifications often outlast the sound stimulation. The pharmacological reactivity in rats exposed to single or repeated audiogenic seizures is similar: phenytoin and carbamazepine suppress both kinds of seizures at low doses whereas ethosuximide is efficacious only at high doses. In order to know whether the repeated exposure to sound or the repetition of seizures are responsible of the observed changes in audiogenic seizures, animals susceptible to sound were exposed daily to the seizure-inducing sound after previous injection of Diazepam, which prevented them from convulsing. On the other hand, sound susceptible animals were injected daily with a dose of PTZ inducing one or several convulsions without exposure to sound. None of these treatments ever facilitated the development of kindled audiogenic seizures. The progressive modification of behavioral and EEG modifications occurring when audiogenic seizures are repeated suggests that kindling has developed, the seizure extending from the brainstem to forebrain structures.

Lifelong susceptibility to acoustic trauma: changing patterns of cochlear damage over the life span of the mouse

Age-related differences in susceptibility to noise-induced threshold shift (NITS) were examined over the entire life span of the CBA/J mouse. Mice of varying ages were given a single 5-min exposure to a 124-dB octave-band (12-24 kHz) noise. Susceptibility began at 15-16 days postpartum and increased rapidly until approximately 20 days of age. During this phase, NITS (as measured by increased action potential threshold) was greatest at 16 kHz. Overall susceptibility was consistently high from 20 to 90 days. During this phase, NITS became most severe at 32 kHz. From 120 days until beyond the end of its actuarial life span (527 days), NITS no longer occurred at 2-16 kHz, but the 64-kHz response retained its susceptibility to acoustic trauma. Mice at 20 and 60 days of age showed the same pattern of decreasing susceptibility as the intensity of the noise exposure was reduced to 114 and 104 dB, indicating that the absence of a tightly restricted critical period is not peculiar to a particular sound pressure level.