Ethanol withdrawal induces increased firing in inferior colliculus neurons associated with audiogenic seizure susceptibility

Ethanol withdrawal (ETX) in ethanol-dependent rats results in susceptibility to seizures, including generalized tonic-clonic audiogenic seizures (AGS). The inferior colliculus (IC) is strongly implicated in AGS initiation during ETX, but IC neuronal mechanisms subserving AGS are unclear. The present study examined IC (central nucleus) single neuronal firing during repeated (4 day) intragastric ethanol administration and during ETX. This involved microwire electrodes implanted chronically into freely moving rats and acoustic stimulation in intensities up to 105 dB SPL. During initial ethanol administration the animals were stuporous, and IC spontaneous neuronal firing and acoustically evoked firing at high stimulus intensities were significantly reduced. This firing reduction is consistent with the action of ethanol to enhance gamma-aminobutyric acid (GABA)-mediated inhibition, which is prominent in IC neurons at high stimulus intensities. During ETX the animals were agitated, and spontaneous IC neuronal firing and acoustically evoked firing at all stimulus intensities were significantly increased during the period of AGS susceptibility. Previous studies indicate that IC neuronal responses are tightly regulated by GABA and glutamate. The IC firing increases during ETX in the present study may involve the down-regulation of GABAA receptors and supersensitivity of glutamate receptors reported to occur during ETX. Previous studies also indicate that focal blockade of GABAA receptors or activation of glutamate receptors produces AGS susceptibility in normal rats. Therefore, the IC neuronal firing increases observed in the present study may play a critical role in initiation of AGS during ethanol withdrawal.

Seizures during ethanol withdrawal are blocked by focal microinjection of excitant amino acid antagonists into the inferior colliculus and pontine reticular formation

Physical dependence on ethanol can result in seizure susceptibility during ethanol withdrawal. In rats, generalized tonic-clonic seizures are precipitated by auditory stimulation during the ethanol withdrawal syndrome. Excitant amino acids (EAAs) are implicated as neurotransmitters in the inferior colliculus and the brain stem reticular formation, which play important roles in the neuronal network for genetic models of audiogenic seizures (AGSs). Ethanol blocks the actions of EAAs in various brain regions, including the inferior colliculus. In this study, dependence was produced by intragastric administration of ethanol for 4 days. During ethanol withdrawal, AGSs were blocked by systemic administration of competitive or noncompetitive NMDA antagonists 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or dizocilpine (MK-801). Focal microinjections of NMDA or non-NMDA antagonists into the inferior colliculus or the pontine reticular formation also inhibited AGSs. MK-801 was the most potent anticonvulsant systemically. When injected into the inferior colliculus, CPP had a more potent anticonvulsant effect than either MK-801 or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The inferior colliculus was more sensitive than the pontine reticular formation to the anticonvulsant effects of both competitive NMDA and non-NMDA antagonists. The results of the present support the idea that continued ethanol administration may lead to development of supersensitivity to the action of EAAs in inferior colliculus and pontine reticular formation neurons. This may be a critical mechanism subserving AGS susceptibility during ethanol withdrawal.

Increased responsiveness of pontine reticular formation neurons associated with audiogenic seizure susceptibility during ethanol withdrawal

Susceptibility to audiogenic seizures (AGS) is observed during ethanol withdrawal (ETX). The pontine reticular formation (PRF) is implicated in the propagation pathway for AGS during ETX. The present study examined the changes in single PRF neuronal firing patterns produced by ethanol and during ETX following repeated intragastrically administered ethanol. Microwire electrode bundles were implanted into PRF and single neuronal responses in freely moving rats were examined. During initial ethanol administration the animals were stuporous, and spontaneous and acoustically-evoked PRF neuronal firing were reduced significantly. During ETX the animals were susceptible to AGS and displayed agitated and irritable behavior. At this time a significant increase in spontaneous and acoustically-evoked PRF neuronal firing was observed. Repetition-induced response attenuation (habituation) of PRF neuronal responses was significantly diminished during ETX, leading to an exaggerated acoustic startle response, which may be a physiological basis for AGS. Previous reports indicate that ethanol enhances the effects of GABA and decreases the effects of glutamate. The PRF neuronal firing increases during EXT in the present study may involve the down-regulation of GABAA receptors and supesensitivity of glutamate receptors reported to occur during ETX, which could contribute to AGS susceptibility. The PRF neuronal firing increases observed in the present study in concord with previous observation of AGS blockade by PRF microinjections during ETX further support an important role of this brain region in the propagation of AGS during ethanol withdrawal.

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.

Loss of synaptic inhibition during repetitive stimulation in genetically epilepsy-prone rats (GEPR)

Genetically epilepsy-prone rats (GEPR) are an animal model of generalized motor seizures. The underlying causes of the predisposition to seizures in GEPR have not been fully determined. The brainstem auditory system is critical for audiogenic seizures in GEPR, and neurophysiological abnormalities have been observed in these areas, but recent evidence suggests that non-auditory brain areas may also be abnormal. This may account for the lowered threshold in GEPR for various non-audiogenic seizures. Because the normal responses of the hippocampal Schaffer collateral/CA1 synapse are relatively well understood, we studied single and repetitive synaptic responses in hippocampal slices of GEPR in vitro. Our hypothesis was that altered excitatory or inhibitory synaptic transmission may contribute to GEPR non-audiogenic seizure predisposition. We recorded extracellular EPSPs, population spikes, and afferent volleys in hippocampal area CA1, and compared GEPR responses to those of Sprague-Dawley (SD) rats, the strain from which GEPR were derived. GEPR responses to single synaptic stimuli were not significantly different from SD. The second of a pair of closely spaced EPSPs or population spikes was larger in both GEPR and SD (paired pulse facilitation), but the magnitude of population spike facilitation was significantly increased in GEPR. Short trains of four stimuli caused inhibition of population spike firing in SD, an effect that was much reduced in GEPR. When SD slices were treated with bicuculline, a GABAA receptor antagonist, enhanced paired pulse facilitation and loss of inhibition during trains of stimuli were seen, similar to the patterns seen in GEPR.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of GABA uptake with tiagabine inhibits audiogenic seizures and reduces neuronal firing in the inferior colliculus of the genetically epilepsy-prone rat

Tiagabine is a new anticonvulsant drug that blocks the uptake of GABA, prolonging the action of this inhibitory transmitter. In the present study the effects of systemically administered tiagabine [30 mg/kg, ip (ED50)] were examined on audiogenic seizure (AGS) severity and neuronal firing in the inferior colliculus (IC) in the freely moving genetically epilepsy-prone rat (GEPR-9). The IC is known to be critical to AGS initiation. The effects of focal microinjection of tiagabine into the IC were also examined. Bilateral focal microinjection of tiagabine into the IC significantly reduced seizure severity in the GEPR-9. Systemically administered tiagabine also produced a significant reduction in seizure severity in the GEPR-9. Tiagabine produced a reduction in IC (central nucleus) neuronal firing, which was significant only at high acoustic intensities (90-105 dB), concomitant with the considerable reduction in seizure severity. These data are consistent with enhancement by tiagabine of gamma-aminobutyric acid (GABA)-mediated inhibition in IC, which is most prominent at high acoustic intensities. The time course of the reduction in neuronal firing of IC neurons paralleled the reduction in seizure severity. Previous studies have shown that two forms of GABA-mediated inhibition (intensity-induced and offset inhibition) in IC neurons are most prominent at high stimulus intensities, which are required to induce AGS. The blockade of GABA uptake by tiagabine may act to inhibit audiogenic seizures, in part, by intensifying these naturally occurring forms of acoustically evoked inhibition in inferior colliculus neurons.

GABA in the inferior colliculus plays a critical role in control of audiogenic seizures

Previous studies have implicated a decreased efficacy of GABA as an important defect subserving the audiogenic seizures of the genetically epilepsy-prone rat (GEPR-9). The inferior colliculus (IC) is a critical site for audiogenic seizure (AGS) initiation, and the pontine reticular formation (PRF) is implicated in the propagation of AGS and in other generalized seizure models. The present study observed that microinjection of baclofen, a GABA-B receptor agonist, into IC protects against AGS, and blockade of the breakdown of endogenous GABA by gabaculine, a GABA transaminase inhibitor, increased GABA levels and blocked AGS susceptibility in the GEPR-9. Microinjection of baclofen or gabaculine into the PRF reduced AGS severity, but the doses required were considerably greater and the degree of anticonvulsant effect was less. Uptake of [3H]GABA into GEPR-9 synaptosomes from the IC is significantly increased as compared to normal, which could contribute to the diminished effectiveness of GABA in the GEPR-9. Previous studies indicate that GABA-A receptor agonists block AGS with IC microinjection, and recent data indicate that blockade of GABA uptake in this nucleus significantly reduced AGS severity. These data taken together strongly support the critical importance of the defect in GABA function in the IC in modulating susceptibility to audiogenic seizure initiation in the GEPR-9.

Stimulation or blockade of the dorsal nucleus of the lateral lemniscus alters binaural and tonic inhibition in contralateral inferior colliculus neurons

Recent studies have demonstrated that several specific types of acoustically-evoked GABA-mediated inhibition occur in neurons of the central nucleus of inferior colliculus (ICc). The dorsal nucleus of the lateral lemniscus (DNLL) provides a major GABAergic projection to ICc. The present study examined the effects of electrical or chemical stimulation or reversible blockade within the DNLL on the discharge characteristics of ICc neurons in anesthetized rats. Microinjection of a local anesthetic (lidocaine) or a GABA-A agonist (THIP) via a cannula placed into DNLL reversibly blocked acoustically-evoked binaural inhibition and increased spontaneous firing in most contralateral ICc neurons. Trains of electrical pulses or microinjection of the excitant amino acid, kainate, into DNLL resulted in reduced acoustically-evoked firing, which was similar to binaural inhibition, in most contralateral ICc neurons examined. The effects of DNLL electrical stimulation were reversibly blocked by microinjection of THIP into the stimulation site, suggesting that the effect of the electrical stimulation is mediated by direct effects on cell bodies of DNLL neurons. These data support the idea that contralateral GABAergic input from the DNLL is inhibitory to ICc neurons. Thus, binaural inhibition and tonic inhibition in ICc neurons may be mediated, in part, by the GABAergic projection from the contralateral DNLL.

Noncompetitive and competitive NMDA antagonists exert anticonvulsant effects by actions on different sites within the neuronal network for audiogenic seizures

Excitant amino acids are implicated in audiogenic seizure (AGS) susceptibility in the genetically epilepsy-prone rat (GEPR). In the present study systemic administration of NMDA receptor antagonists significantly decreased AGS severity in the GEPR. Systemic administration of the competitive NMDA antagonists 3-((+-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP) and 2-amino-7-phosphonoheptanoic acid and the non-competitive antagonist dizocilpine (MK-801) were effectively anticonvulsant in the GEPR. The inferior colliculus is the most critical nucleus for AGS initiation in the GEPR and an excitant amino acid is implicated as an important excitatory transmitter in inferior colliculus neurons. Systemically administered CPP significantly reduced inferior colliculus neuronal firing in the normal behaving rat and the GEPR concurrently with blockade of AGS and this effect occurred at nearly all sound intensities tested. Systemic administration of MK-801, while effective in blocking AGS, produced no consistent change in inferior colliculus neuronal firing, which is consistent with its very low potency in blocking AGS with bilateral microinjection into the inferior colliculus. These findings suggest that an important action of competitive, but not noncompetitive, NMDA antagonists is on brain stem auditory nuclei, especially the inferior colliculus, that are critical to AGS. MK-801 appears to exert its anticonvulsant effects in AGS network sites beyond the inferior colliculus. These findings and recent inferior colliculus slice studies suggest that NMDA receptors in inferior colliculus may have quantitatively different properties from those in other brain regions. These differences in NMDA receptor function in inferior colliculus may reflect NMDA receptor heterogeneity observed in binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Repetition of audiogenic seizures in genetically epilepsy-prone rats induces cortical epileptiform activity and additional seizure behaviors

Repetition of seizures appears to increase severity in a number of seizure models, but the nature of these severity increases has not been elucidated in naturally occurring genetic epilepsy models. The genetically epilepsy-prone rat (GEPR) is highly susceptible to many seizure provoking stimuli, and high intensity acoustic stimuli induce audiogenic seizures (AGS). The role of forebrain structures in AGS in the GEPR has not been clear, and the presence of cortical epileptiform EEG activity in the GEPR is controversial. The present study examined the effects of 21 daily AGS repetitions on behavior and EEG activity recorded from the cortex of two GEPR substrains that exhibit moderate (GEPR-3) or severe AGS (GEPR-9). The results indicated that AGS repetition induced seizure severity increases in both GEPR substrains and resulted in prominent cortical epileptiform EEG activity. The AGS behavioral patterns remained distinctly different in the two substrains throughout seizure repetition. In each substrain a different additional behavioral phase was expressed; the GEPR-9 exhibited post-tonic clonus, and the GEPR-3 exhibited facial and forelimb clonus. These findings indicate that seizure repetition results in expansion of the neuronal network subserving AGS to involve forebrain structures. The medial geniculate body and amygdala appear to be part of this expanded network, and long-term potentiation, which was reported in the pathway between the latter brain structures, may be involved. These data suggest that recruitment of forebrain structures into the AGS neuronal network appears to be essential for production of the additional ictal behaviors evoked by AGS repetition.

Loss of intensity-induced inhibition in inferior colliculus neurons leads to audiogenic seizure susceptibility in behaving genetically epilepsy-prone rats

The genetically epilepsy-prone rat (GEPR) exhibits elevated seizure sensitivity and audiogenic seizures (AGS). The inferior colliculus (IC) is the most critical brain region for AGS initiation. The present study evaluated IC neuronal firing and convulsive behavior simultaneously in freely moving GEPRs. High intensity acoustic stimulation produces neuronal firing reductions (intensity-induced inhibition) in about 50% of IC neurons in normal rats. However, in GEPR IC neurons, intensity-induced inhibition is significantly less effective than normal. Offset inhibition is also reduced in GEPR IC neurons, which leads to a greater than normal incidence of offset (afterdischarge) responses at high stimulus intensities. At AGS onset most IC neurons exhibit burst firing and reductions of acoustically evoked neuronal responses. Responsiveness to acoustic stimuli returns following AGS. This change in IC neuronal firing pattern suggests that the network that governs IC neuronal firing has temporarily changed from the auditory system to the network that mediates seizure propagation. GABA is strongly implicated in intensity-induced, binaural, and offset inhibition in IC neurons. The diminished efficacy of these forms of GABA-mediated acoustically evoked inhibition in the GEPR IC extends previous results, showing reduced effectiveness of exogenously applied GABA and benzodiazepine in GEPR IC neurons. This reduced effectiveness of GABA-mediated inhibition along with excess excitant amino acids in GEPR IC, previously reported, appear to be vital neurotransmitter mechanisms, subserving the exaggerated output of IC neurons at high acoustic intensities. This exaggerated IC firing may be instrumental in seizure initiation in this epilepsy model.

Effect of norepinephrine depletion on audiogenic-like seizures elicited by microinfusion of an excitant amino acid into the inferior colliculus of normal rats

Infusions of an excitant amino acid, N-methyl-D-aspartate (NMDA) into the inferior colliculus (IC) render normal rats susceptible to audiogenic seizures (AGS) and/or spontaneous audiogenic-like seizures without tonic components. The excess excitant amino acid in the IC and the anticonvulsant effects of NMDA antagonists in genetically epilepsy-prone rats (GEPRs), along with innate norepinephrine (NE) deficits and anticonvulsant effects of NE agonists in these animals suggest a mutual role of excitant amino acids and NE in regulating AGS in GEPRs. Saline or 6-hydroxydopamine (6-OHDA, 4 micrograms/side in 2 microliters) was infused bilaterally into the locus coeruleus (LC) of normal male rats and guide cannulas were implanted into the IC. Two weeks later, NMDA was infused bilaterally into the IC (0.5 microliters; 10 nmol/side) and 10 min later the rats were subjected to an electric bell (110 db, 60 s) unless preceded by spontaneous tonic seizures. Tonic seizures were not observed in male rats following NMDA infusions in rats with LC infusions of saline. However, a marked increase in the incidence of tonic seizures was observed in the 6-OHDA-treated rats which were markedly depleted of brain NE as determined by HPLC. These findings indicate that a NE deficit greatly enhances the incidence of tonic convulsions and support the hypothesis that an excitant amino acid excess in the GEPR IC may act to initiate AGS, whereas the NE deficit may allow expression of the tonic components of AGS seen in some GEPRs.

Involvement of GABA in acoustically-evoked inhibition in inferior colliculus neurons

Most criteria for establishing GABA as an inhibitory neurotransmitter in the central nucleus of inferior colliculus (ICc) have been satisfied, but the role of GABA in acoustic coding in ICc is not established. The present study examined this issue by evaluating the effects of iontophoretic application of agents that alter activity at GABA receptors on potential forms of acoustically-evoked inhibition in ICc neurons. Application of the GABAA antagonist, bicuculline, selectively blocked the firing reduction at high intensities observed during non-monotonic rate-intensity functions in ICc neurons. Binaural inhibition was selectively blocked by bicuculline and increased by nipecotic acid. Application of GABA, nipecotic acid (GABA uptake inhibitor) and a benzodiazepine (flurazepam), which enhances the action of GABA, increased the duration and intensity of ipsilateral inhibition and response pause, while bicuculline blocked these acoustically-evoked inhibitory events. Offset inhibition was increased by nipecotic acid application and reduced by bicuculline with the appearance of an offset peak. The present data support an important role for GABA as a neurotransmitter, mediating, in part, non-monotonicity, binaural inhibition, response pause and offset inhibition in ICc neurons. Alterations of these GABA-mediated inhibitory phenomena may occur in auditory dysfunctions observed with aging and audiogenic seizures.

Audiogenic seizure severity and hearing deficits in the genetically epilepsy-prone rat

Hearing deficits have been observed in rodents that are susceptible to audiogenic seizures (AGS), including the genetically epilepsy-prone rat (GEPR). AGS susceptibility can be induced in normal animals by treatments that damage the cochlea. In this study, we measured the relative degree of hearing loss in animals from the GEPR substrains that exhibit different degrees of AGS severity and examined the relationship between the deficit and the AGS severity. Auditory brain stem response (ABR) thresholds to clicks in the GEPR substrain that exhibits exclusively maximal AGS severity (GEPR-9) were significantly elevated, and latencies for ABR peaks I, III, and IV were significantly increased as compared to normal Sprague-Dawley rats. ABR thresholds for the substrain of GEPRs were even higher than those in the GEPR-9, and ABR waveforms were distorted. ABR peak IV was significantly longer than normal in the GEPR-3 substrain, as were mean interpeak intervals and central conduction times. These data indicate that significant hearing deficits occur in the GEPR-3 substrain. In non-AGS-susceptible progeny of the GEPR-9 [GEPR-0(9)], ABR thresholds were not significantly different from normal. These data along with studies of ABR thresholds in thyroid-deficient rats suggest that an inverted U-shaped relationship exists between hearing deficit and AGS severity. That is, moderate threshold elevations are associated with increasing AGS severity, but when the hearing deficit exceeds a certain level, a decrement in AGS severity occurs.

Non-N-methyl-D-aspartate receptors may mediate ipsilateral excitation at lateral superior olivary synapses

Principal cells of the lateral superior olivary nucleus (LSO) are thought to receive a direct excitatory input from spherical bushy cells located in the ipsilateral ventral cochlear nucleus (VCN) and an indirect input from the contralateral VCN globular bushy cells via a secure synapse in the medial nucleus of the trapezoid body (MNTB). MNTB bushy cells project to the somata and proximal dendrites of LSO principal cells. LSO neurons display phasic 'chopper' temporal response patterns to ipsilateral tone-burst stimuli at characteristic frequency (CF), while binaural stimuli suppress this ipsilaterally evoked activity. This suppression is sensitive to interaural differences in intensity, phase and time, suggesting a role for these neurons in the localization of sound in space. In the present study, the nature of the neurotransmitter mediating fast ipsilateral excitation of LSO neurons was examined using iontophoretic application of excitant amino acid (EAA) agonists and antagonists. N-methyl-D-aspartate (NMDA) and quisqualate (QUIS) were used as agonists, while the selective NMDA receptor antagonist D. L-2-amino-5-phosphonovaleric acid (APV), and the non-selective receptor EAA antagonist cis-2,3-piperidine-dicarboxylic acid (PDA) were used to study ipsilaterally evoked neuronal responses. In 3 additional experiments the selective non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) replaced PDA. Ipsilateral, tone-evoked and spontaneous activities were generally enhanced by EAA agonists while partial blockade of tone-evoked, ipsilateral excitation was observed with EAA antagonists. Both PDA and DNQX more effectively blocked ipsilateral tone-evoked excitations and spontaneous activity than did the NMDA-receptor antagonist, APV.(ABSTRACT TRUNCATED AT 250 WORDS)

On the role of GABA as an inhibitory neurotransmitter in inferior colliculus neurons: iontophoretic studies

Significant neurochemical, immunocytochemical, and ligand binding studies support a role for GABA as an inhibitory neurotransmitter in the inferior colliculus (IC). The present study attempted to satisfy some of the remaining criteria for establishing transmitter identity by utilizing iontophoretic application onto IC neurons of agents affecting the action of gamma-aminobutyric acid (GABA). The agents examined include GABA, a GABAB agonist (baclofen), a GABAA antagonist (bicuculline), a GABA uptake inhibitor (nipecotic acid), and a benzodiazepine (flurazepam), thought to exert its actions on the GABA receptor complex. Application of GABA results in inhibition of the spontaneous firing and acoustically evoked responses of inferior colliculus neurons. The inhibitory effect of GABA is enhanced by the simultaneous application of nipecotic acid or flurazepam. These agents as well as baclofen produce firing reductions when applied alone in higher doses. The effect of GABA can be blocked by application of bicuculline, and acoustically evoked (binaural) inhibition can also be selectively blocked by low doses of this GABAA antagonist. These data along with previous studies utilizing different techniques fulfill many of the criteria for establishment of GABA as an important inhibitory transmitter in the inferior colliculus.

Injections of noradrenergic and GABAergic agonists into the inferior colliculus: effects on audiogenic seizures in genetically epilepsy-prone rats

Genetically epilepsy-prone rats (GEPRs) which display tonic seizures (GEPR-9s) in response to acoustic stimulation were used in these studies. Other laboratories have shown that GEPR-9s have a reduced concentration of brain norepinephrine (NE). Previous reports have also indicated that audiogenic seizures (AGS) in these animals are inhibited by treatments that enhance noradrenergic (NA) neurotransmission. AGS in GEPRs are believed to be initiated in the inferior colliculus (IC) where GABA has been shown to exert inhibitory influences in GEPRs that display submaximal AGS. The present study examined whether the IC is a crucial site for NA suppression of tonic seizures by examining the effect of microinfusing NA agonists into the IC. The intracollicular effect of a GABA agonist, muscimol, on sound-induced tonic convulsions in GEPR-9s was also examined. Bilateral microinfusion of NE, phenylephrine, clonidine or isoproterenol failed to alter the AGS. In contrast, muscimol (30 or 60 ng/side) infused into the IC abolished the tonic and clonic components of the AGS in GEPR-9s. These findings suggest that enhancement of GABAergic neurotransmission in the IC markedly attenuates AGS in the GEPR, while augmentation of NA neurotransmission has little effect in this brain region.

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.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway

The afferent pathway involved in initiation of audiogenic seizures in the genetically epilepsy-prone rat was investigated by bilateral microinfusion of the excitant amino acid antagonist 2-amino-7-phosphonoheptanoate into the major brain stem and subcortical nuclei of the auditory system. This antagonist has been shown to possess anticonvulsant properties in other seizure models, and an excitant amino acid has been implicated as a putative neurotransmitter in several of these nuclei. Seizure severity was significantly reduced following infusion of this agent into the cochlear nucleus, superior olivary complex, inferior colliculus, and medial geniculate body. Many of these animals exhibited a complete blockade of seizures. The smallest effective dose in the cochlear nucleus and the medial geniculate body was 5 nmol per side. The smallest effective dose in the olive was 1 nmol, and in the inferior colliculus 0.1 nmol per side was protective. The onset of anticonvulsant effectiveness was earliest in the inferior colliculus. These findings showed that the inferior colliculus was the most sensitive auditory center to the anticonvulsant action of 2-amino-7-phosphonoheptanoate and that imbalance between inhibitory and excitatory transmission within this brain structure may be crucial in the initiation of audiogenic seizures in the genetically epilepsy-prone rat.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. II. Efferent seizure propagating pathway

Previous studies indicate that the inferior colliculus is the brain stem auditory nucleus most sensitive to the chemical blockade of audiogenic seizures in the genetically epilepsy-prone rat. Other auditory structures do not appear to be as important. This study attempted to define the efferent pathways involved in propagation of the seizure from the colliculus to the spinal cord where the motor components of the convulsion are generated. This study also determined whether certain nuclei which have been implicated in the propagation of seizures in other epilepsy models are involved in audiogenic seizures. The excitant amino acid antagonist, 2-amino-7-phosphonoheptanoate, was infused bilaterally into several of those sites. The drug was effective in significantly reducing seizure severity with infusion of 5 nmol bilaterally into the midbrain and the pontine reticular formation or the substantia nigra. However, similar drug doses were not effective when infused into the entopeduncular nucleus even though prominent behavioral effects were observed with this infusion. Infusion of 2-amino-7-phosphonoheptanoate into the prepiriform cortex resulted in a small but significant reduction in seizure severity. These results suggest that inhibition of excitatory transmission within the substantia nigra and the reticular formation effectively blocks the output pathway for the audiogenic seizures, whereas the role of the prepiriform cortex in this process is relatively minor.

The genetically epilepsy-prone rat

1. The genetically epilepsy-prone rat (GEPR) is a valuable model for investigating mechanisms involved in epilepsy because of the controllable nature of the convulsions and their genetic origin. 2. The GEPR exhibits audiogenic seizures (AGS) and also displays higher than normal sensitivity to convulsant drugs, kindling, electroshock and hyperthermic seizures. 3. An abnormal electroencephalographic pattern and increased thresholds for auditory evoked potentials from the cochlea and brainstem are observed in the GEPR. 4. Afterdischarge-like responses and decreased sound-induced inhibition are observed in neurophysiological recordings from neurons of the inferior colliculus (IC) in the GEPR. 5. Significant deficits of norepinephrine and serotonin are observed in many regions of the GEPR brain. 6. Increases in the number of GABAergic neurons and a reduced effectiveness of iontophoretically-applied GABA are observed in the IC of this animal. 7. GABA agonists or an excitant amino acid (EAA) antagonist block AGS susceptibility when microinjected into brainstem auditory nuclei of the GEPR up to the level of IC. 8. A GABA antagonist or an EAA agonist induces susceptibility to AGS in normal rats following microinjection into IC. An increase in EAA release in IC during AGS in the GEPR is also observed. 9. This increased release of EAA and the reduced effectiveness of GABA in IC may be important seizure initiation mechanisms in the GEPR. 10. The AGS pathway in the GEPR appears to involve the auditory nuclei up to the IC as well as the brainstem reticular formation and substantia nigra but not the entopenduncular nucleus or hippocampus.

Strychnine alters the fusiform cell output from the dorsal cochlear nucleus

Anatomical and physiological evidence suggests that fusiform cells, the major output neurons of the dorsal cochlear nucleus (DCN), receive significant inhibitory input. Fusiform cells often display strongly non-monotonic rate-intensity functions and pauser-buildup or buildup tone-evoked temporal responses, patterns which may be mediated by inhibitory neurotransmitters. Other neurons located within the fusiform cell layer or in the more superficial molecular layer display varied rate-intensity functions and temporal responses. Neurons displaying response properties characteristic of fusiform cells are sensitive to iontophoretic application of the inhibitory amino acid neurotransmitter, glycine. Application of the glycine receptor antagonist, strychnine, alters the non-monotonic portion of the rate-intensity function at doses which do not alter spontaneous activity or near-threshold tone-evoked responses. These neurons are also sensitive to GABA and the GABAB agonist, (-)-baclofen, but are insensitive to the GABAA antagonist, bicuculline. DCN neurons which display monotonic rate-intensity functions and temporal response properties different than those associated with fusiform cells are sensitive to bicuculline, (-)-baclofen, and GABA. These data suggest that a glycinergic input onto fusiform cells may control the non-monotonic nature of the response of these neurons near characteristic frequency and therefore may contribute significantly to the nature of the output of the DCN.