Aberrant neuronal responsiveness in the genetically epilepsy-prone rat: acoustic responses and influences of the central nucleus upon the external nucleus of inferior colliculus

Rodent Brain Pathology, Audiogenic Epilepsy

The review presents data which provides evidence for the internal relationship between the stages of rodent audiogenic seizures and post-ictal catalepsy with the general pattern of animal reaction to the dangerous stimuli and/or situation. The wild run stage of audiogenic seizure fit could be regarded as an intense panic reaction, and this view found support in numerous experimental data. The phenomenon of audiogenic epilepsy probably attracted the attention of physiologists as rodents are extremely sensitive to dangerous sound stimuli. The seizure proneness in this group shares common physiological characteristics and depends on animal genotype. This concept could be the new platform for the study of epileptogenesis mechanisms.

The Inferior Colliculus in Alcoholism and Beyond

Post-mortem neuropathological and in vivo neuroimaging methods have demonstrated the vulnerability of the inferior colliculus to the sequelae of thiamine deficiency as occurs in Wernicke-Korsakoff Syndrome (WKS). A rich literature in animal models ranging from mice to monkeys-including our neuroimaging studies in rats-has shown involvement of the inferior colliculi in the neural response to thiamine depletion, frequently accomplished with pyrithiamine, an inhibitor of thiamine metabolism. In uncomplicated alcoholism (i.e., absent diagnosable neurological concomitants), the literature citing involvement of the inferior colliculus is scarce, has nearly all been accomplished in preclinical models, and is predominately discussed in the context of ethanol withdrawal. Our recent work using novel, voxel-based analysis of structural Magnetic Resonance Imaging (MRI) has demonstrated significant, persistent shrinkage of the inferior colliculus using acute and chronic ethanol exposure paradigms in two strains of rats. We speculate that these consistent findings should be considered from the perspective of the inferior colliculi having a relatively high CNS metabolic rate. As such, they are especially vulnerable to hypoxic injury and may be provide a common anatomical link among a variety of disparate insults. An argument will be made that the inferior colliculi have functions, possibly related to auditory gating, necessary for awareness of the external environment. Multimodal imaging including diffusion methods to provide more accurate in vivo visualization and quantification of the inferior colliculi may clarify the roles of brain stem nuclei such as the inferior colliculi in alcoholism and other neuropathologies marked by altered metabolism.

The genetic audiogenic seizure hamster from Salamanca: The GASH:Sal

The hamster has been previously described as a paroxysmal dystonia model, but our strain is currently recognized as a model of audiogenic seizures (AGS). The original first epileptic hamster appeared spontaneously at the University of Valladolid, where it was known as the GPG:Vall line, and was transferred to the University of Salamanca where a new strain was developed, named GASH:Sal. By testing auditory brainstem responses, the GASH:Sal exhibits elevated auditory thresholds that indicate a hearing impairment. Moreover, amplified fragment length polymorphism analysis distinguished genetic differences between the susceptible GASH:Sal hamster strain and the control Syrian hamsters. The GASH:Sal constitutes an experimental model of reflex epilepsy of audiogenic origin derived from an autosomal recessive disorder. Thus, the GASH:Sal exhibits generalized tonic-clonic seizures, characterized by a short latency period after auditory stimulation, followed by wild running, a convulsive phase, and finally stupor, with origin in the brainstem. The seizure profile of the GASH:Sal is similar to those exhibited by other models of inherited AGS susceptibility, which decreases after six months of age, but the proneness across generations is maintained. The GASH:Sal can be considered a reliable model of audiogenic seizures, suitable to investigate current antiepileptic pharmaceutical treatments as well as novel therapeutic drugs. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Commissural functional topography of the inferior colliculus assessed in vitro

The inferior colliculus (IC) receives ascending and descending information from several convergent neural sources. As such, exploring the neural pathways that converge in the IC is crucial to uncovering their multi-varied roles in the integration of auditory and other sensory information. Among these convergent pathways, the IC commissural connections represent an important route for the integration of bilateral information in the auditory system. Here, we describe the preparation and validation of a novel in vitro slice preparation for examining the functional topography and synaptic properties of the commissural and intrinsic projections in the IC of the mouse. This preparation, in combination with modern genetic approaches in the mouse, enables the specific examination of these pathways, which potentially can reveal cell-type specific processing channels in the auditory midbrain.

Projections from the inferior colliculus to the tectal longitudinal column in the rat

We have used the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) to study with albino rats the projections from the inferior colliculus (IC) to the tectal longitudinal column (TLC), a newly discovered nucleus that spans the midbrain tectum longitudinally, on each side of the midbrain, immediately above the periaqueductal gray matter. We studied the projections of the medial IC, which includes the classical central nucleus (CNIC) and the dorsal cortex (DCIC), and those of the lateral IC, equivalent to the classical external cortex (ECIC). Following unilateral injections of PHA-L into the medial IC, numerous terminal fibers are labeled bilaterally in the TLC. The ipsilateral projection is denser and targets the entire nucleus, whereas the contralateral projection targets significantly only the caudal half or two-thirds of the TLC. Fibers from the medial IC reach the TLC by two routes: as collaterals of axons that travel in the commissure of the IC and as collaterals of thick ipsilateral colliculogeniculate axons; the latter travel through the deep superior colliculus on their way to the TLC. Within the TLC, individual IC fibers tend to run longitudinally. The injection of PHA-L into the lateral IC indicates that this subdivision sends a weak, bilateral projection to the TLC whose trajectory, morphology and distribution are similar to those of the projection from the medial IC. These results demonstrate that all subdivisions of the IC send projections to the TLC, suggesting that the IC may be one of the main sources of auditory input to this tectal nucleus.

GABA-immunoreactive neurons and terminals in the cat periaqueductal gray matter: a light and electron microscopic study

Immunocytochemical and electron microscopic methods were used to study the GABAergic innervation in adult cat periaqueductal gray matter (PAG). A mouse monoclonal antibody against gamma -aminobutyric acid (GABA) was used to visualize the inhibitory neuronal system of PAG. At light microscopy, GABA-immunopositive (GABA(IP)) neurons formed two longitudinally oriented columns in the dorsolateral and ventrolateral PAG that accounted for 36% of the neuronal population of both PAG columns; their perikaryal cross-sectional area was smaller than that of unlabeled (UNL) neurons found in the same PAG subdivisions. At electron microscopic level, patches of GABA immunoreactivity were readily detected in neuronal cell bodies, proximal and distal dendrites, axons and axon terminals. Approximately 35-36% of all terminals were GABA(IP); they established symmetric synapses with dendrites (84.72% of the sample in the dorsolateral PAG and 86.09% of the sample in the ventrolateral PAG) or with cell bodies (7-10% of the sample). Moreover, 49.15% of GABA(IP) axon terminals in the dorsolateral and 52.16% in the ventrolateral PAG established symmetric synapses with GABA(IP) dendrites. Immunopositive axon terminals and unlabeled terminals were also involved in the formation of a complex synaptic arrangment, i.e. clusters of synaptic terminals in close contact between them that were often observed in the PAG neuropil. Moreover, a fair number of axo-axonic synapses between GABA(IP) and/or UNL axon terminals were present in both PAG subdivisions. Several dendro-dendritic synapses between labeled and unlabeled dendrites were also observed in both PAG subdivisions. These results suggest that in the cat PAG there exist at least two classes of GABArgic neurons. The first class could exert a tonic control on PAG projecting neurons, the second could act on those GABAergic neurons that in turn keep PAG projecting neurons under tonic inhibition. The functional implications of this type of GABAergic synapse organization are discussed in relation to the dishinibitory processes that take place in the PAG.

Aversive effects elicited by electrical stimulation of the inferior colliculus in normal and audiogenic seizure susceptible rats

Trains of electrical stimulations were applied to the dorsal or ventral part of the inferior colliculus (IC) of audiogenic seizure susceptible rats from the AGSR strain. Threshold and duration of wild running (WR), were evaluated in the first experiment. All stimulation sites elicited WR, even in normal control rats. Stimulation of the IC of AGSR rats required a lower quantity of current, i.e., such brain sites were more sensitive to the current, than normal controls. The duration of post-stimulus WR was shorter in AGSR rats. Lower quantities of current applied to the ventral IC were needed to elicit WR than to the dorsal IC in AGSR rats. In a second experiment, using the same stimulations sites in the same rats, the emotional effect of the stimulation was tested through an instrumental learning procedure (switch-off paradigm) in which the rat was trained to press a bar to put an end to the stimulation. Both dorsal and ventral IC stimulation sites sustained switch-off behavior in AGSR rats, but only ventral IC stimulation sites sustained switch-off learning in control rats.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Role of GABA abnormalities in the inferior colliculus pathophysiology - audiogenic seizures

gamma-Aminobutyric acid (GABA), acting at GABA(A) receptors, mediates inhibition in inferior colliculus (IC) central nucleus (ICc) neurons and plays a prominent role in mediating acoustically evoked non-monotonicity, offset inhibition, and binaural inhibition, and is also important in tonic inhibition. The IC plays an important role in a number of pathophysiological conditions that involve hearing, including tinnitus, age-related hearing loss, and audiogenic seizures (AGS). AGS are a major form of rodent neurological disorder that can be genetically mediated and can also be readily induced in both young and mature animals. A deficit in GABA-mediated inhibition in IC neurons has been shown to be a critical mechanism in genetic and induced forms of AGS. Thus, both endogenously evoked GABA-mediated inhibition and exogenously applied GABA are reduced in efficacy in IC neurons of rats that are susceptible to AGS. GABA-mediated inhibition in IC neurons is significantly more easily blocked by a GABA(A) antagonist in genetic and induced forms of AGS in vivo and in vitro. AGS can be induced in normal animals by treatments that reduce the effectiveness of GABA in the IC. Glutamate-mediated excitation is a critical element of neurotransmission in IC neurons, and excessive activation of glutamate receptors in the IC is also strongly implicated as the other major mechanism in the pathophysiology of AGS. These neurotransmitter abnormalities result in excessive firing of ICc neurons that acts as the critical initiation mechanism for triggering seizures in response to intense acoustic stimuli.

Neurons in the deep layers of superior colliculus are a requisite component of the neuronal network for seizures during ethanol withdrawal

Ethanol withdrawal (ETX) in ethanol-dependent animals and humans often results in seizure susceptibility. The deep layers of superior colliculus (DLSC) are proposed to be involved in the neuronal networks of several types of seizures. In rodents, ETX results in susceptibility to audiogenic seizures (AGS), and the DLSC are implicated as a critical component of the seizure network in a genetic form of AGS. Ethanol inhibits NMDA receptors, and the binding at these receptors is increased during ETX in certain brain regions. Therefore, the effect of focal microinjection into DLSC of a competitive NMDA receptor antagonist, DL-2-amino-7-phosphonoheptanoic acid (AP7) on ETX seizures was examined. AP7 (2 and 5 nmol/side) microinjected bilaterally into DLSC suppressed AGS, supporting a critical role of the DLSC in the AGS network during ETX. DLSC neuronal firing changes in behaving rats were subsequently examined, using chronically implanted microwire electrodes. Acoustically-evoked DLSC firing was significantly suppressed during ethanol intoxication and during ETX. However, DLSC neurons began firing tonically 1-2 s before the onset of the wild running behavior of AGS. Acoustically-evoked DLSC firing was suppressed during post-ictal depression with recovery beginning as the righting reflex returned. These data support a requisite role of the DLSC in AGS during ETX. These neuronal firing changes suggest an important role of DLSC neurons in generation of the wild running phase of AGS during ETX, which may be a general pathophysiological mechanism and a critical event in the initiation of wild running, since a similar pattern was seen previously in a genetic form of AGS.

Involvement of cAMP- and Ca(2+)/calmodulin-dependent neuronal protein phosphorylation in mechanisms underlying genetic predisposition to audiogenic seizures in rats

It was shown that increased excitability in neurons underlying epilepsies would be maintained by abnormalities in protein phosphorylation systems. This study was initiated to compare the functioning of Ca(2+)/calmodulin- and cAMP-dependent systems of protein phosphorylation in homogenates of neocortex and hippocampus in three animal groups: genetically prone to audiogenic seizures (GPAS) rats, GPAS rats exposed to daily repeated audiogenic seizures (AGPAS rats) and nonepileptic Wistar ones. We found significant differences in phosphorylation of 270, 58, 54 and 42 kDa proteins in neocortex and hippocampus of GPAS rats in comparison with Wistar ones. Daily repeated seizures induced further modifications of phosphorylation of these proteins in only hippocampus of AGPAS rats as compared with GPAS ones. Ca(2+)-independent, functional CAMKII activity was considerably increased in hippocampus but decreased in neocortex of GPAS rats in comparison with Wistar ones. The activity of PKA was increased both in neocortex and hippocampus of GPAS rats. Daily repeated audiogenic seizures induced the decrease of Ca(2+)-independent CAMKII activity in hippocampus and the increase of PKA activity in neocortex of AGPAS rats in comparison with GPAS ones. The present results indicate that modification of 270, 58, 54, and 42 kDa proteins phosphorylation as well as altered CAMKII and PKA activities might be involved in mechanisms of genetic predisposition to audiogenic seizures.

gamma-Aminobutyric acid transporters in the cat periaqueductal gray: a light and electron microscopic immunocytochemical study

The gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) mediate GABA uptake into presynaptic axon terminals and glial processes, thus contributing to the regulation of the magnitude and duration of the action of GABA at the synaptic cleft. The aim of the present study was to investigate the expression of three high-affinity GABA transporters (GAT-1, GAT-2, and GAT-3) in the periaqueductal gray matter (PAG) of adult cats by using immunocytochemistry with affinity-purified antibodies. Light microscopic observations revealed GAT-1 immunoreactivity in punctate structures, particularly dense in the lateral portion of the dorsolateral PAG column. Weak GAT-2-immunopositive puncta were homogeneously distributed in the PAG. GAT-3 immunoreactivity was detected in each column of the PAG but was more intense in the dorsolateral PAG column and around the aqueduct. Electron microscopic studies showed GAT-1 immunoreactivity in distal astroglial processes, in unmyelinated and small myelinated axons, and in axon terminals making symmetric synapses on both PAG neurons and dendrites. GAT-2 immunoreactivity was present mostly in the form of patches of different sizes in the cytoplasm of neuronal elements like the perikarya and dendrites of PAG neurons, in myelinated and unmyelinated axons, and in the axon terminals forming both symmetric and asymmetric synapses. Labeling was also observed in nonneuronal elements. Astrocytic cell bodies and their distal processes as well as the ependymal cells lining the wall of the aqueduct showed patches of GAT-2 immunoreactivity. Electron microscopic observation revealed GAT-3 immunoreactivity exclusively in distal astrocytic processes adjacent to the somata of PAG neurons and in axon terminals making both symmetric and asymmetric synapses. The present results suggest that three types of termination systems of GABAergic transmission are present in the cat periaqueductal gray matter.

Developmental and genetic audiogenic seizure models: behavior and biological substrates

Audiogenic seizure (AGS) models of developmental or genetic origin manifest characteristic indices of generalized seizures such as clonus or tonus in rodents. Studies of seizure-resistant strains in which AGS is induced by intense sound exposure during postnatal development provide models in which other neural abnormalities are not introduced along with AGS susceptibility. A critical feature of all AGS models is the reduction of neural activity in the auditory pathways from deafness during development. The initiation and propagation of AGS activity relies upon hyperexcitability in the auditory system, particularly the inferior colliculus (IC) where bilateral lesions abolish AGS. GABAergic and glutaminergic mechanisms play crucial roles in AGS, as in temporal lobe models of epilepsy, and participate in AGS modulatory and efferent systems including the superior colliculus, substantia nigra, basal ganglia and structures of the reticular formation. Catecholamine and indolamine systems also influence AGS severity. AGS models are useful for elucidating the underlying mechanisms for formation and expression of generalized epileptic behaviors, and evaluating the efficacy of modern treatment strategies such as anticonvulsant medication and neural grafting.

Audiogenic kindling in the Wistar rat: a potential model for recruitment of limbic structures

Repetitive high intensity (110 dB) sound stimulation induces a forebrain-kindling phenomenon in animals predisposed to sound induced seizures. Wistar audiogenic rats (WARs) have been reported to develop a mixed brainstem-limbic seizure pattern, after more than five to ten stimuli. Besides the original brainstem wild running and tonic-clonic seizures, new behavioral patterns appear resembling those of electrical amygdala kindling. Although audiogenic kindling is a well-known phenomenon, electrographic limbic recruitment during the kindling has never been reported. Our objective was to use electrophysiology to test the hypothesis of gradual and sequential involvement of the amygdala and then cortex during audiogenic kindling. We used video-EEG recordings with cortical and deep electrode implants (inferior colliculus and basolateral amygdaloid nuclei) during audiogenic kindling on eight WARs, and their respective controls, submitted to a protocol of three acoustic stimuli per day. A new design for 'on site' source follower circuits was used in order to minimize noise during the recording of EEG data from the wild running episode and the subsequent tonic-clonic or motor limbic seizures. The video-EEG equipment assembled allowed synchronous recordings of both behavior and EEG. WARs first recordings showed electrodecremental responses after seizure onset and a probable epileptiform activity, particularly in the inferior colliculus, during the tonic phase of seizure. All animals showed very similar polyspike-wave activity in the amygdala, after behavioral seizure patterns (Racine's scale) occurred. The morphology of such epileptiform EEG activity is very similar to that reported for electrical amygdala kindling. Also, when audiogenic kindling continued, both inferior colliculus and cortical electrodes presented high amplitude and synchronized epileptiform polyspike activity.

Synaptic response patterns of neurons in the cortex of rat inferior colliculus

The present study examined synaptic potentials of neurons in inferior colliculus (IC) cortex slice and the roles of GABA and glutamate receptors in generating these potentials. Multipolar (82%) and elongated (18%) cells were observed with intracellular biocytin staining. Electrical stimulation of the IC commissure (CoIC) elicited only inhibitory postsynaptic potentials (IPSPs) (10% of cells), only excitatory postsynaptic potentials (EPSPs) (51%), or both (38%). IPSPs were elicited at lower thresholds and shorter latencies than EPSPs (mean: 1.6+/-1.2 ms) and IPSPs were observed in all neurons following membrane depolarization. Short-latency EPSPs were blocked by non-NMDA receptor antagonists, and longer-latency EPSPs were blocked by NMDA antagonists. CoIC stimulation evoked short-latency IPSPs (mean: 0.55+/-0.33 ms) in 48% of neurons, and the IPSPs persisted despite glutamate receptor blockade, which implies monosynaptic inhibitory input. A GABA(A) antagonist blocked IPSPs and paired pulse inhibition of EPSPs, suggesting GABA(A) receptor mediation. A GABA(B) antagonist reduced paired pulse inhibition of IPSPs, suggesting GABA(B) receptor modulation. Thus, GABA-mediated inhibition plays a critical role in shaping synaptic responses of IC cortex neurons. Normal GABAergic function in IC has been shown to be important in acoustic coding, and reduced efficacy of GABA function in IC neurons is critical in IC pathophysiology in presbycusis, tinnitus and audiogenic seizures.

Differential roles in the neuronal network for audiogenic seizures are observed among the inferior colliculus subnuclei and the amygdala

The inferior colliculus (IC) is established as the initiation site within the neuronal network for audiogenic seizures (AGS), but the relative importance of the IC subnuclei in AGS is controversial. The lateral and basolateral subdivisions of the amygdala are implicated in the expansion of the AGS network that occurs during AGS kindling. However, the role of the amygdala in the AGS network in nonkindled AGS is unknown. NMDA receptors are implicated in modulation of AGS and in neurotransmission in both the IC and amygdala. Therefore, changes in AGS severity in genetically epilepsy-prone rats (GEPR-9s) were examined after bilateral focal microinjection into IC subnuclei or lateral/basolateral subdivisions of the amygdala of a competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP). Blockade of AGS in IC central nucleus (ICc) and external cortex (ICx) was observed at identical doses of CPP, but these doses were ineffective in IC dorsal cortex (ICd). Microinjection of CPP into the amygdala did not produce significant changes in AGS severity except at doses 20 times those effective in IC. The latter data contrast with the anticonvulsant effects of amygdala microinjections on seizure severity in kindled AGS reported previously. The present data in concord with neuronal recording studies of these nuclei suggest that the ICc is the most critical site in AGS initiation, the ICx in propagation, and that the ICd plays a lesser role in the AGS network. The amygdala does not appear to play a requisite role in the neuronal network for AGS in animals that have not been subjected to AGS kindling.

The periaqueductal grey is a critical site in the neuronal network for audiogenic seizures: modulation by GABA(A), NMDA and opioid receptors

The nuclei comprising the neuronal network for audiogenic seizures (AGS) are located primarily in the brainstem. Previous studies suggested a role for the periaqueductal grey (PAG) in the AGS network. The present study evaluated this possibility in genetically-epilepsy prone rats (GEPR-9s) by examining the effects of bilateral focal microinjection of a competitive NMDA receptor antagonist (DL-2-amino-7-phosphonoheptanoic acid (AP7), 1 and 5 nmol/side), a GABA(A) agonist (gaboxedol (THIP), 10 and 15 nmol) or an opioid peptide receptor antagonist (naloxone, 5 nmol) into PAG, based on the proposed role of these receptors in PAG neurotransmission. Blockade of NMDA receptors by AP7 (both doses) or activation of GABA(A) receptors with THIP (15 nmol/side) in the PAG suppressed AGS susceptibility. Naloxone displayed a seizure-suppressant effect that was delayed and incomplete. The seizure suppressant effect of AP7 or naloxone, unlike THIP, was observed at doses that did not produce motor quiescence. These data suggest that the PAG is a requisite nucleus in the neuronal network for AGS in GEPR-9s and that GABA(A), opioid peptide and NMDA receptors in the PAG modulate AGS propagation.

Spatial and temporal relationships between C-Fos expression and kindling of audiogenic seizures in Wistar rats

In a strain of Wistar rats selected in our laboratory, audiogenic seizures (AS), characterized by a wild running phase followed by a tonic seizure, can be elicited by exposure to sound. In these animals repeated daily stimulations induce permanent changes which reflect the extension of seizure activity from the brainstem to the forebrain. C-Fos immunoreactivity was used to further characterize the sound-susceptibility of the strain and to specify the spatiotemporal relationships between c-Fos expression and development of AS kindling. AS susceptible rats appeared to be more sensitive to a subthreshold sound as compared to controls. Sound-evoked wild running induced a similar pattern of c-Fos as a full AS in naive rats, confirming the epileptic nature of this early component. AS-induced c-Fos labeling in the auditory pathways of the brainstem extended to the forebrain with repetition of AS and marked increases in c-Fos expression sequentially occurred in the amygdala and perirhinal cortex, followed by the frontoparietal cortex, the piriform cortex, and finally the hippocampus and entorhinal cortex. These results show that the kindled AS preferentially propagate from the brainstem, through the amygdala and the perirhinal cortex, to the motor cortex, with the piriform cortex and hippocampus as secondary targets. No more c-Fos expression was detected 24 h after an AS. A down-regulation of cortical c-Fos induction was observed 1 and 2 days after daily exposure to kindled AS, with full recovery of c-Fos expression after a 5-day seizure-free period. This suggests a regulatory function of c-Fos expression in development of kindling.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

In vitro electrophysiology of neurons in subnuclei of rat inferior colliculus

We compared membrane and synaptic properties of neurons in the three major subdivisions of inferior colliculus (IC), central nucleus (ICc, N=18), external cortex (ICx, N=38), and dorsal cortex (ICd, N=31) of slices from rat IC, using intracellular neuronal recording. Three types of responses occurred in each IC subdivision in response to depolarizing currents: on-type (N=20), rapidly-adapting (N=11), and sustained firing (N=56), which was most common. The on-type neurons have lower input resistances and shorter time constants, with wider and lower amplitude action potentials (APs) than sustained neurons. A calcium-mediated 'hump' was often evoked by depolarizing current pulses in ICd neurons (11 of 28), was infrequent in ICx, but was absent in ICc. ICx and ICc neurons often exhibited spontaneous repetitive spike firing, lower repetitive AP firing thresholds, and faster repetitive spike firing than ICd neurons. Calcium-mediated fast after-hyperpolarizations and spike frequency adaptation were regularly seen in IC. Neurons in ICx and ICd, but not ICc, had synaptic responses to stimulation of the collicular commissure (CoIC). In ICx, large epileptiform depolarizing events were often elicited by strong electrical stimulation of CoIC, which was not normally seen in ICd. These results indicate that ICx neurons exhibit a greater degree of synaptic excitability than neurons in ICc or ICd, which may contribute to the proposed role of ICx in pathological IC hyperexcitability.

Comparison of neuronal response patterns in the external and central nuclei of inferior colliculus during ethanol administration and ethanol withdrawal

Convulsive seizures during ethanol withdrawal (ETX) in rodents can be precipitated by acoustic stimulation. The inferior colliculus (IC) is strongly implicated in the neuronal network for these audiogenic seizures (AGS) in animals undergoing ETX. Previous evidence indicates that the central nucleus of IC (ICc) is important in AGS initiation in ETX, but the ICc does not project directly to motor pathways. The external nucleus of IC (ICx) receives convergent output from a broad range of ICc neurons, which is not tonotopically organized, and projects to several nuclei with major motor connections. Lesion, neuroanatomical, and stimulation experiments suggest the involvement of the ICx in the AGS network in several forms of AGS, including ETX. The present study examined ICx neuronal firing patterns in awake behaving rats during ethanol administration and during ETX to examine the role of this structure directly. ICx neuronal responses during both ethanol intoxication and ETX were significantly suppressed as compared to pre-ethanol responses. ICx neuronal responsiveness was reduced (habituated) at faster (>0.25 Hz) rates of stimulus presentation. However, immediately prior to the onset of AGS, there was an increase in ICx neuronal responses that continued into the wild running phase of AGS. This increase in neuronal responses temporally corresponded to the sustained ICc neuronal responses during ETX just prior to AGS. The enhanced ICx neuronal responsiveness may be mediated, in part, by changes in GABA and glutamate receptor regulation previously observed during ETX. The net result of these changes involves a functional reversal of response habituation normally observed in ICx neurons. These data illuminate the nature of the changes in ICx neuronal function that serves to transmit the sensory input that originates in the ICc and propagates seizure to the brainstem AGS network nuclei responsible for the convulsive motor behaviors of ETX seizures.