The ultrastructure of the central nucleus of the inferior colliculus of the genetically epilepsy-prone rat

An abnormal GABAergic system in the inferior colliculus provides a basis for audiogenic seizures in genetically epilepsy-prone rats

In this review of neuroanatomical studies of the genetically epilepsy-prone rat (GEPR), three main topics will be covered. First, the number of GABAergic neurons and total neurons in the inferior colliculus of GEPRs will be compared to those of the nonepileptic Sprague-Dawley rat. Next, the number of small neurons in the inferior colliculus will be described in both developmental and genetic analyses of GEPRs and their backcrosses. Last, results from two types of studies on the propagation pathways for audiogenic seizures in GEPRs will be shown. Together, these studies demonstrate a unique GABAergic, small neuron defect in the inferior colliculus of GEPRs that may play a vital role in the initiation and spread of seizure activity during audiogenic seizures. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

The Wistar Audiogenic Rat (WAR) strain and its contributions to epileptology and related comorbidities: History and perspectives

In the context of modeling epilepsy and neuropsychiatric comorbidities, we review the Wistar Audiogenic Rat (WAR), first introduced to the neuroscience international community more than 25years ago. The WAR strain is a genetically selected reflex model susceptible to audiogenic seizures (AS), acutely mimicking brainstem-dependent tonic-clonic seizures and chronically (by audiogenic kindling), temporal lobe epilepsy (TLE). Seminal neuroethological, electrophysiological, cellular, and molecular protocols support the WAR strain as a suitable and reliable animal model to study the complexity and emergent functions typical of epileptogenic networks. Furthermore, since epilepsy comorbidities have emerged as a hot topic in epilepsy research, we discuss the use of WARs in fields such as neuropsychiatry, memory and learning, neuroplasticity, neuroendocrinology, and cardio-respiratory autonomic regulation. Last, but not least, we propose that this strain be used in "omics" studies, as well as with the most advanced molecular and computational modeling techniques. Collectively, pioneering and recent findings reinforce the complexity associated with WAR alterations, consequent to the combination of their genetically-dependent background and seizure profile. To add to previous studies, we are currently developing more powerful behavioral, EEG, and molecular methods, combined with computational neuroscience/network modeling tools, to further increase the WAR strain's contributions to contemporary neuroscience in addition to increasing knowledge in a wide array of neuropsychiatric and other comorbidities, given shared neural networks. During the many years that the WAR strain has been studied, a constantly expanding network of multidisciplinary collaborators has generated a growing research and knowledge network. Our current and major wish is to make the WARs available internationally to share our knowledge and to facilitate the planning and execution of multi-institutional projects, eagerly needed to contribute to paradigm shifts in epileptology. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

AGS-induced expression of Narp is concomitant with expression of AMPA receptor subunits GluR1 and GluR2 in hippocampus but not inferior colliculus of P77PMC rats

To explore mechanisms of epileptogenesis in audiogenic seizures (AGS), we examined the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleopropionic acid (AMPA) receptor subunits GluR1 and GluR2 and of the GluR-associated protein Narp in the hippocampus and the inferior colliculus (IC) from AGS-susceptible P77PMC rats after a single AGS and audiogenic kindling. Western blotting and immunohistochemistry showed that Narp was rapidly induced in both the hippocampus and the IC by AGS. In the hippocampus, up-regulation of Narp was concomitant with GluR1 and GluR2 under both conditions of a single AGS and AGS kindling. In the IC, however, Narp was up-regulated, GluR2 down-regulated, and GluR1 unchanged after kindling. In comparison with kindling, neither GluR1 nor GluR2 was changed, while Narp significantly increased in the IC following a single AGS. These findings suggest that down-regulation of AMPA receptor GluR2 subunit in the IC may contribute to AGS-mediated epileptogenesis, and up-regulation of Narp in the IC may be involved in audiogenic seizures.

Elevated levels of group-III metabotropic glutamate receptors in the inferior colliculus of genetically epilepsy-prone rats following intracollicular administration of L-serine-O-phosphate

The selective group-III metabotropic glutamate receptor agonist, L-serine-O-phosphate (L-SOP), when injected bilaterally into the inferior colliculus of the sound sensitive genetically epilepsy-prone (GEP) rats produces a short proconvulsant excitation followed by a long phase of protection against sound-induced seizures lasting for 2-4 days. We have studied this prolonged suppression of audiogenic seizures using pharmacological and molecular biological approaches including semiquantitative RT-PCR and western blotting. The intracerebroventricular injection of the protein synthesis inhibitor cycloheximide (120 microg) 30 min beforehand significantly reduces the proconvulsant seizure activity and the prolonged anticonvulsant effect of intracollicular L-SOP (500 nmol/side). The sensitive semiquantitative RT-PCR revealed a significant up-regulation in mGlu(4) and mGlu(7) mRNA levels in the inferior colliculus at 2 days (maximum suppression of audiogenic seizures) after intracollicular L-SOP injection compared with the non-injected, 2-day post-vehicle treated and 7-day (return to expressing audiogenic seizures) post-drug or vehicle-treated groups. No significant changes were observed in mGlu(6) or mGlu(8) mRNA expression levels in drug-treated compared with control groups. Examination of mGlu(4a) and mGlu(7a) protein levels using western blotting showed a significant increase in mGlu(7a) but no significant change in mGlu(4a) protein levels 2 days after L-SOP treatment compared with the control groups (non-injected and 2-day vehicle-injected group). These results suggest that up-regulation of mGlu(7) receptors is involved in the prolonged anticonvulsant effect of L-SOP against sound-induced seizures in GEP rats. The potential use of mGlu(7) agonists as novel anti-epileptic agents merits investigation.

Seizures and proto-oncogene expression of fos in the brain of adult genetically epilepsy-prone rats

The mechanisms and brain circuitry that render genetically epilepsy-prone rats (GEPRs) susceptible to acoustically induced seizures are not completely known. The present study explores the neuroanatomy of acoustically induced seizures by immunohistochemical analysis of the proto-oncoprotein fos after intense acoustic stimulation (AS) with and without seizures. Acoustic stimulation induced tonic convulsions in GEPR-9s, but not in control rats. Locations of brain nuclei showing fos-like immunoreactive (FLI) neurons following AS with and without seizures were mapped. Semiquantitative methods were used to compare FLI neuron numerical densities in AS control rats and GEPRs. Many brain areas exhibited profound FLI in AS control rats and GEPRs. Unexpectedly, the cochlear nuclei and the central nucleus of the inferior colliculi (ICc), both of which are requisite for AGS initiation, exhibited a diminished fos expression in animals having seizures compared to AS controls. In contrast, GEPRs displayed a significant increase in FLI neurons within the dorsal cortex of the IC (ICd) compared to AS controls. This finding may suggest a seizure-related amplification of the auditory signal between the ICc and the ICd. Other nuclei, known to be involved in auditory transmission (i.e., superior olivary complex; trapezoid nucleus; dorsal nucleus of the lateral lemniscus, DNLL), did not show differential FLI densities between seizure and AS control animals. In contrast, seizure-induced FLI was observed in many nonauditory brain nuclei. Of particular interest was the identification of an intensely labeled nucleus in the GEPR. This nucleus resides in the most posterior and dorsal-lateral part of the pedunculopontine tegmental nucleus-pars compacta (PPTn-pc) immediately adjacent to the DNLL and extends posteriorly into the superior lateral subnucleus of the lateral parabrachial area (SLPBn). Therefore, we have tentatively termed this nucleus the PPSLPBn. The PPSLPBn lies in a region previously described as a mesencephalic locomotor region and a suspected functional involvement of this nucleus in display of seizure activity is under investigation. Other brain stem nuclei showing differential fos expression between GEPRs and AS control rats are also described.

The role of the inferior colliculus in a genetic model of audiogenic seizures

Previous studies have shown the functional importance of the inferior colliculus (IC) for the propagation and initiation of audiogenic seizures in several models of epilepsy in rats. A review of the cell types and cytoarchitecture of the IC, including its three major subdivisions, is presented. Significant increases in GABA levels and the number of GABAergic neurons are found in the central nucleus of the IC (ICCN) of genetically epilepsy-prone rats (GEPR-9s) as compared to Sprague-Dawley rats that do not display audiogenic seizures. Two independent anatomical methods were used to determine the number of GABAergic neurons, immunocytochemistry and in situ hybridization. In both types of preparation, the labeled cells in the ICCN appeared to be of different sizes but the number of small cells with diameters less than 15 microns showed the greatest increase. Nissl-stained sections showed that the total number of neurons in the ICCN was increased in GEPR-9s and indicated that the increase in GABAergic neurons was not due to a change in the phenotype of collicular neurons from non-GABAergic to GABAergic. The number of small neurons in Nissl-stained sections of the ICCN was shown to correlate with seizure severity in the offspring of crosses made between Sprague-Dawley rats and GEPR-9s. Furthermore, the GEPR-3s that display moderate seizures showed a significant increase in the number of small neurons in the ICCN, and the magnitude of this increase was predicted from this correlation. Finally, the use of knife cuts through the midbrain indicated that the ICCN sends an important projection to the external nucleus and that this projection plays a vital role in the propagation of seizure activity from the site of seizure initiation in the ICCN. It remains to be resolved how the increase in small GABAergic neurons in the ICCN is responsible for the known pharmacological defects observed at GABAergic synapses.

Inferior colliculus neuronal membrane and synaptic properties in genetically epilepsy-prone rats

Previous studies using single-unit recording techniques have shown that the inferior colliculus is critical for audiogenic seizure initiation in genetically epilepsy-prone rats (GEPR). In order to investigate cellular abnormalities that may be important in causing audiogenic seizure susceptibility, intracellular recordings were made from neurons of inferior colliculus dorsal cortex (ICd) in a GEPR variety that exhibits severe audiogenic seizures (GEPR-9). GEPR neuronal membrane and synaptic properties were compared to those of normal Sprague-Dawley rats (SD), the strain from which GEPR were derived. We found six electrophysiological differences between GEPR and normal SD ICd neurons, all of which could promote seizures in GEPR. (1) Input resistance was higher in GEPR than in normal ICd neurons. (2) Threshold for repetitive action potential firing was closer to resting membrane potential in GEPR ICd neurons. (3) GEPR neurons showed faster repetitive spike firing than normal SD neurons. (4) Anode break spikes occurred at the offset of a hyperpolarizing pulse more often in GEPR than in normal SD neurons. (5) Stimulation of the commissure of the inferior colliculus caused synaptic paired pulse inhibition in normal ICd neurons, but paired pulse facilitation was always observed in GEPR neurons. (6) In GEPR, a large epileptiform depolarizing event could be elicited by strong electrical stimulation of the commissure of the inferior colliculus. In normal SD rats, similar epileptiform activity was seen only after application of bicuculline or NMDA. Our results suggest that both abnormal neuronal membrane properties and altered synaptic transmission are likely to contribute to seizure predisposition and audiogenic seizure initiation in GEPR.

Noradrenergic abnormalities in the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) has central nervous system noradrenergic deficits as compared to normal rats. It is possible that these deficits contribute to seizure predisposition because they are exhibited by seizure-naive as well as by seizure-experienced GEPRs. On the basis of pharmacological studies, it is hypothesized that there is an inverse relation between seizure predisposition and levels of noradrenergic activity in brain. Neurochemical studies indicate that deficits exist in areas innervated by both the locus ceruleus and the lateral tegmental noradrenergic systems. These deficits exist in GEPRs without seizure experience and are more pronounced in the severe seizure strain as compared to the moderate seizure strain. We review eight experimental steps undertaken to identify more precisely the anatomical location of noradrenergic determinants of seizure predisposition. These steps illustrate the theoretical bases for the studies and describe the specific experiments completed. Evidence supports the hypothesis that noradrenergic deficits in the superior colliculus and/or ventrally adjacent regions are determinants of seizure predisposition.

The inferior colliculus of GEPRs contains greater numbers of cells that express glutamate decarboxylase (GAD67) mRNA

Previous studies have shown significantly greater GABA levels and numbers of GABAergic neurons in the central nucleus of the inferior colliculus (ICCN) of genetically epilepsy-prone rats (GEPR-9s). In the present study, in situ hybridization and emulsion autoradiographic techniques were used to determine whether there are also elevated numbers of ICCN cells that contain the 67-kD form of mRNA for the GABA synthesizing enzyme, glutamate decarboxylase (GAD), in GEPR-9s as compared to normal Sprague-Dawley (SD) rats. Hybridization with a 35S-labeled RNA probe complementary to a span of monkey GAD mRNA labeled cells throughout the brain including the ICCN. Labeled cells in the ICCN appeared to be of different sizes that corresponded with previous descriptions of GABAergic neurons from immunocytochemical studies. In the GEPR-9s, a larger number of GAD67 cRNA labeled neurons was observed in the ICCN as compared to SD rats. The external nucleus of the inferior colliculus was also found to contain significantly greater numbers of GAD67 cRNA labeled neurons whereas in the frontal cortex, a region of the brain that is not required for audiogenic seizure activity in GEPR-9s, there were no significant differences in hybridization between GEPR-9s and SD rats. Interestingly, within the superficial layers of the superior colliculus there was a higher density of hybridization in GEPR-9s than in SD rats indicating higher levels of GAD expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sustained pyridoxine treatment on seizure susceptibility and regional brain amino acid levels in genetically epilepsy-prone BALB/c mice

Epilepsy-prone and epilepsy-resistant substrains were selectively bred from a strain of BALB/c mice; audiogenic-sensitive epilepsy-prone animals showed enhanced sensitivity to chemical convulsants. Treatment with pyridoxine (100 mg/L in drinking water) initiated at mating and continued throughout pregnancy and the life of the offspring abolished the enhanced sensitivity to chemical convulsants and reduced the severity of audiogenic seizures. Withdrawal of pyridoxine restored the enhanced seizure sensitivity. [1H] Nuclear magnetic resonance (NMR) spectroscopy of perchloric acid extracts of tissue was used to determine the concentrations of several compounds [N-acetylaspartate (NAA), GABA, glutamate, aspartate, alanine, taurine, creatine, cholines, inositol] in the hippocampus, neocortex, brainstem, and cerebellum of untreated and pyridoxine-treated 6-week-old female animals. The ratios of the concentrations of excitatory to inhibitory putative neurotransmitter amino acids tended to be higher in epilepsy-prone animals, with the most pronounced difference being a significantly elevated glutamate/GABA ratio in every brain region examined. Pyridoxine treatment abolished this imbalance in the hippocampus, brainstem, and cerebellum, but not in the neocortex. Treatment of epilepsy-resistant animals with pyridoxine using the same protocol decreased the glutamate/GABA concentration ratio in the hippocampus, brainstem, and neocortex and resulted in impaired development of the animals. The amino acid imbalance and the accompanying seizure susceptibility in these genetically epilepsy prone mice may originate from an inborn error in pyridoxine metabolism or in a pyridoxine-dependent enzyme system.

Glutamate in the inferior colliculus plays a critical role in audiogenic seizure initiation

Alterations of excitant amino acid (EAA) action are implicated in seizure susceptibility in the genetically epilepsy-prone rat (GEPR). The inferior colliculus (IC) is critical for audiogenic seizure (AGS) initiation in the GEPR. The present study observed that bilateral microinjection into the IC of L-canaline, a glutamate synthesis inhibitor, decreased AGS severity in the GEPR and also decreased potassium-evoked release of glutamate from IC slices. Bilateral microinjection of NMDA receptor antagonists, 2-amino-7-phosphonoheptanoate (AP7) or 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP) into IC blocked AGS, and an antagonist at non-NMDA EAA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), also blocked AGS. NMDA receptor antagonists were 5-200 times more effective than CNQX. Microinjection of a non-competitive NMDA receptor antagonist, dizocilpine (MK-801), into IC had little effect except with very high doses. Microinjection of CPP or AP7 into the IC blocked AGS at considerably lower doses as compared to pontine reticular formation (PRF). However, MK-801 attenuated AGS when microinjected into PRF at doses that were ineffective in IC. Systemically administered CPP blocked AGS and significantly reduced IC neuronal firing in the behaving GEPR, suggesting an important action of systemically administered NMDA receptor antagonists on brainstem auditory nuclei critical to AGS. The present results support a critical role for glutamate acting, in part, through NMDA receptors in IC in initiation of AGS.

GABA neurons in seizure disorders: a review of immunocytochemical studies

Immunocytochemical studies have identified alterations in GABA neurons in several models of seizure disorders. However, the changes have varied among different epilepsy models, and these variations presumably reflect the diversity of mechanisms that can lead to seizure disorders. In models of cortical focal epilepsy, there is strong evidence for decreases in the number of GABAergic elements, and the changes closely parallel the time course of seizure development. By contrast, in some genetic models of epilepsy, increases in the number of immunocytochemically-detectable neurons have been observed in selected brain regions. In several models of temporal lobe epilepsy, there presently is little immunocytochemical evidence for alterations of GABA neurons within the hippocampal formation despite physiological demonstrations of decreased GABA-mediated inhibition in this region. However, it remains possible that certain types of GABA neurons could be differentially affected in some seizure disorders while other types are preserved. Thus, distinguishing between different classes of GABA neurons and determining their functional roles represent major challenges for future studies of GABA neurons in seizure disorders.

Differences in dopamine beta-hydroxylase immunoreactivity between the brains of genetically epilepsy-prone and Sprague-Dawley rats

Biochemical studies have indicated that norepinephrine is present in lower levels in certain brain regions of genetically epilepsy-prone rats (GEPR-9s) as compared to non-epileptic Sprague-Dawley (SD) rats. In this study, the immunocytochemical localization of dopamine beta-hydroxylase (DBH), the synthesizing enzyme for norepinephrine, was compared between GEPR-9s and SD rats. Brain regions caudal to the inferior colliculus, such as the cerebellum and locus coeruleus, showed no differences in the distribution of DBH-like immunoreactive (DBH-I) neurons and fibers. In contrast, differences in the distribution of DBH-I fibers were observed in more rostral brain regions including the central nucleus of the inferior colliculus, thalamus, piriform, orbital and somatosensory cortices and hippocampus. In these areas, the number, and often the staining intensity, of DBH-I processes was lower in GEPR-9s as compared to SD rats. It was interesting to note that other cortical regions displayed no differences in DBH immunoreactivity between GEPR-9s and SD rats. These results provide anatomical data that support previously described biochemical results. Furthermore, the reduced number of fibers and their decreased staining intensity in specific brain regions provide greater details to resolve the localization of deficiencies in the noradrenergic fiber plexus of GEPR-9s.

The genetically epilepsy-prone rat. A valuable model for the study of the epilepsies

In order to develop a rational clinical treatment for any pathological state, the molecular bases for that state must be understood. As simple and logical as that statement appears, it remains the major obstacle to effective treatment of the family of neurological disorders collectively called the epilepsies. Under the term, the epilepsies are grouped as several types of seizure processes that undoubtedly have multiple pathophysiological causes. Thus, the search to elucidate the molecular bases for the epilepsies has as one of its fundamental components the careful selection of an appropriate model system. The search for an "ideal" seizure model has essentially followed two paths. In the first, animals are rendered "epileptic" by artificial methods and then the pathophysiological, electrophysiological, and pharmacological changes are evaluated. In the second, animals are developed with a genetic predisposition to seizures and used to evaluate the molecular bases for the seizure-prone state. Work using both types of models have provided valuable information about the epileptic state. This review describes an epilepsy model developed using the second approach, namely, the Genetically Epilepsy-Prone Rat (GEPR). These animals represent a valuable model for the study of the inborn neurological defect that predisposes these animals to seizures. A brief description of the work done in several laboratories characterizing the model is presented. Finally, the value of the GEPR as a model for studying the pathophysiology of the epilepsies will be described.

Effects of excitant amino acids on acoustic responses of inferior colliculus neurons

Iontophoretic application of the excitant amino acids (EAAs), glutamate, aspartate and N-methyl-D-aspartate (NMDA) resulted in increased acoustically evoked and spontaneous firing of most neurons in the central nucleus of inferior colliculus (ICC). The excitatory effects of these EAAs were blocked by simultaneous application of EAA antagonists which selectively block the NMDA receptor subtype, 2-amino-5-phosphonovalerate or D-alpha-aminoadipate and to a lesser extent with non-selective EAA antagonists, such as glutamic acid diethylester. Application of NMDA receptor-selective EAA antagonists alone greatly reduced the firing of most ICC neurons examined, but non-selective EAA antagonists either increased or produced little change in firing of most ICC neurons examined. In this and previous studies cholinergic agonists were found to increase the firing of ICC neurons, but the cholinergic agonists were less effective in exciting ICC neurons than EAA agonists. Cholinergic antagonists in a previous study were considerably less effective in inhibiting the discharge of ICC neurons than were the EAA antagonists in the present study. These results, in conjunction with previous neurochemical and anatomical localization studies, support a possible role of an EAA as a candidate for afferent excitatory transmitter in neurons of the inferior colliculus.

Induction of audiogenic seizures in normal and genetically epilepsy-prone rats following focal microinjection of an excitant amino acid into reticular formation and auditory nuclei

An excitant amino acid (EAA), N-methyl-D-aspartate (NMDA), induces susceptibility to seizures when bilaterally microinjected into subcortical auditory nuclei of normal rats. Thirty-five percent of animals exhibit only audiogenic seizures (AGS) after infusions of NMDA into inferior colliculus (IC). Infusions into cochlear nucleus and medial geniculate body never produce susceptibility to AGS without non-audiogenic seizures (N-AGS). The overall seizure incidence (AGS and N-AGS) with IC infusions is 100%, but the incidence is less than 50% with infusions into cochlear nucleus or medial geniculate body. Although AGS susceptibility is induced by NMDA infusions in normal animals, the seizures are submaximal in severity and lack tonic components. Bilateral infusions of NMDA into IC or reticular formation of the substrain of genetically epilepsy-prone rats (GEPRs) that exhibits submaximal AGS (GEPR-3s) do not increase seizure severity. These data along with studies showing increased EAA levels and excitotoxic-like damage in the IC of the GEPR and blockade of AGS with an EAA receptor antagonist or synthesis inhibitor suggest that an EAA in the IC is involved in initiation of AGS in the GEPR. However, EAA action in the GEPR IC is not sufficient to induce the complete spectrum of seizure behaviors, and additional mechanisms may be required for induction of maximal severity audiogenic seizures.