Morphologic and Neurochemical Abnormalities in the Auditory Brainstem of the Genetically Epilepsy‐prone Hamster (GPG/Vall)

Abstract

Summary:Purpose: This study was performed to evaluate whether audiogenic seizures, in a strain of genetically epilepsyprone hamsters (GPG/Vall), might be associated with morphologic alterations in the cochlea and auditory brainstem. In addition, we used parvalbumin as a marker of neurons with high levels of activity to examine changes within neurons.Methods: Cochlear histology as well as parvalbumin immunohistochemistry were performed to assess possible abnormalities in the GPG/Vall hamster. Densitometry also was used to quantify levels of parvalbumin immunostaining within neurons and fibers in auditory nuclei.Results: In the present study, missing outer hair cells and spiral ganglion cells were observed in the GPG/Vall hamster. In addition, an increase was noted in the size of spiral ganglion cells as well as a decrease in the volume and cell size of the cochlear nucleus (CN), the superior olivary complex nuclei (SOC), and the nuclei of the lateral lemniscus (LL) and the inferior colliculus (IC). These alterations were accompanied by an increase in levels of parvalbumin immunostaining within CN, SOC, and LL neurons, as well as within parvalbumin-immunostained fibers in the CN and IC.Conclusions: These data are consistent with a cascade of atrophic changes starting in the cochlea and extending along the auditory brainstem in an animal model of inherited epilepsy. Our data also show an upregulation in parvalbumin immunostaining in the neuropil of the IC that may reflect a protective mechanism to prevent cell death in the afferent sources to this nucleus. Key Words: Calcium-binding protein-AGS-Animal model-Hamster.Animal models of inherited epilepsy are essential to understand the origin of epileptic seizures because they represent an abnormal nervous system and reproduce the mechanisms of hyperexcitability than occur in nature (1). In numerous species, including rat (2,3), mouse (4,5), gerbil (6), and baboon (7), genetic models of epilepsy are available. In these models, several steps have been identified in the cascade leading from seizure-activity episodes to long-lasting and quasi-permanent modifications of the neuronal circuitry organization. These include neuron atrophy (8), cell death (9), neuronal degeneration (10), synaptic rearrangement (11), abnormalities in the expression of neurotransmitters and/or their receptors (12)(13)(14), and changes in intracellular signaling pathways in which calcium-related mechanisms are involved (15-17).

Morphologic and neurochemical alterations in the superior colliculus of the genetically epilepsy-prone hamster (GPG/Vall)

“…Although the functional significance of this hypertrophy is not clear, it has been hypothesized that it may represent an increased metabolic support necessary for the new growth of synaptic endings that might be replacing degenerating axons (Bothwell et al, 2001) due to brain damage in epileptic patients (Houser, 1992;Du et al, 1993). Consistent with this hypothesis, the atrophic changes reported in the ascending auditory pathway in the GPG/Vall hamster (Fuentes-Santamaria et al, 2005a) could lead to a reduction in synapses in the deep layers of the SC. Such atrophy would induce sprouting of new terminals that consequently, may increase the excitatory flux into SC neurons and as a result, the likelihood of epileptic seizures (Bothwell et al, 2001).…”

“…These seizures involve several stages including (1) a short latency period from the initiation of sound stimulation to the onset of an explosive burst of wild running, (2) a wild running phase where the hamster falls over on its side, (3) tonic-clonic convulsions and (4) postictal period (Macías Fernandez et al, 1992). Similar to other animal models of audiogenic seizures (AGS) (Penny et al, 1983(Penny et al, , 1986Roberts et al, 1985;Roberts and Ribak, 1986), several morphologic and neurochemical abnormalities have been observed in the auditory pathway of the GPG/Vall hamster (Fuentes-Santamaria et al, 2005a). These include atrophic changes starting in the cochlea and extending along the auditory brainstem as well as upregulation of calciumbuffering mechanisms in the inferior colliculus (IC) and in its afferent sources (Fuentes-Santamaria et al, 2005a).…”

“…In particular, a reduction in the volume of the SC of the epileptic hamster has been observed compared to control animals. Decreases in volume have been associated with structural damage not only in animal models of AGS (Fuentes-Santamaria et al, 2005a), but also in different brain regions of patients having different types of epilepsies (Barr et al, 1997;Lawson et al, 2000;Bernasconi et al, 2001). The present findings also report a significant increase in the mean cross-sectional area of neurons located in the deep layers of the SC of the epileptic hamster compared to control animals which is consistent with alterations in presumptive output neurons projecting to Values expressed as nmol/mg wwt, are means and standard error.…”

“…Neuronal hypertrophy also has been reported for spiral ganglion cells in the GPG/Vall hamster (Fuentes-Santamaria et al, 2005a) and also for neurons in the neocortex of patients with temporal lobe epilepsy (Bothwell et al, 2001). Although the functional significance of this hypertrophy is not clear, it has been hypothesized that it may represent an increased metabolic support necessary for the new growth of synaptic endings that might be replacing degenerating axons (Bothwell et al, 2001) due to brain damage in epileptic patients (Houser, 1992;Du et al, 1993).…”

“…A profile was defined as immunostained if the mean gray value was above that threshold for detection that was set as two standard deviations above the mean optical density of the whole image (Fuentes-Santamaria et al, 2003, 2005aAlvarado et al, 2004Alvarado et al, , 2005Alvarado et al, , 2007. Since it has been reported that immunostaining can be used as a relative measure of antigen levels (Huang et al, 1996;Yao and Godfrey, 1997), the mean gray level within the SC was used as an indirect indicator of the amount of either calbindin or parvalbumin in superficial and deep layers, respectively.…”