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Vavers E, Zvejniece B, Stelfa G, Svalbe B, Vilks K, Kupats E, Mezapuke R, Lauberte L, Dambrova M, Zvejniece L. Genetic inactivation of the sigma-1 chaperone protein results in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures. Neurobiol Dis 2021; 150:105244. [PMID: 33385516 DOI: 10.1016/j.nbd.2020.105244] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2020] [Revised: 12/07/2020] [Accepted: 12/27/2020] [Indexed: 02/02/2023] Open
Abstract
There is a growing body of evidence demonstrating the significant involvement of the sigma-1 chaperone protein in the modulation of seizures. Several sigma-1 receptor (Sig1R) ligands have been demonstrated to regulate the seizure threshold in acute and chronic seizure models. However, the mechanism by which Sig1R modulates the excitatory and inhibitory pathways in the brain has not been elucidated. The aim of this study was to compare the susceptibility to seizures of wild type (WT) and Sig1R knockout (Sig1R-/-) mice in intravenous pentylenetetrazol (PTZ) and (+)-bicuculline (BIC) infusion-induced acute seizure and Sig1R antagonist NE-100-induced seizure models. To determine possible molecular mechanisms, we used quantitative PCR, Western blotting and immunohistochemistry to assess the possible involvement of several seizure-related genes and proteins. Peripheral tissue contractile response of WT and Sig1R-/- mice was studied in an isolated vasa deferentia model. The most important finding was the significantly decreased expression of the R2 subunit of the GABA-B receptor in the hippocampus and habenula of Sig1R-/- mice. Our results demonstrated that Sig1R-/- mice have decreased thresholds for PTZ- and BIC-induced tonic seizures. In the NE-100-induced seizure model, Sig1R-/- animals demonstrated lower seizure scores, shorter durations and increased latency times of seizures compared to WT mice. Sig1R-independent activities of NE-100 included downregulation of the gene expression of iNOS and GABA-A γ2 and inhibition of KCl-induced depolarization in both WT and Sig1R-/- animals. In conclusion, the results of this study indicate that the lack of Sig1R resulted in decreased expression of the R2 subunit of the GABA-B receptor and increased susceptibility to seizures. Our results confirm that Sig1R is a significant molecular target for seizure modulation and warrants further investigation for the development of novel anti-seizure drugs.
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Affiliation(s)
- Edijs Vavers
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia.
| | - Baiba Zvejniece
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; University of Latvia, Riga LV-1586, Latvia
| | - Gundega Stelfa
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; Latvia University of Life Sciences and Technologies, Jelgava LV-3001, Latvia
| | - Baiba Svalbe
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia
| | - Karlis Vilks
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; University of Latvia, Riga LV-1586, Latvia
| | - Einars Kupats
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; Riga Stradins University, Riga LV-1007, Latvia
| | | | - Lasma Lauberte
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia
| | - Maija Dambrova
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; Riga Stradins University, Riga LV-1007, Latvia
| | - Liga Zvejniece
- Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia
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Lee Y, Rodriguez OC, Albanese C, Santos VR, Cortes de Oliveira JA, Donatti ALF, Fernandes A, Garcia-Cairasco N, N'Gouemo P, Forcelli PA. Divergent brain changes in two audiogenic rat strains: A voxel-based morphometry and diffusion tensor imaging comparison of the genetically epilepsy prone rat (GEPR-3) and the Wistar Audiogenic Rat (WAR). Neurobiol Dis 2017; 111:80-90. [PMID: 29274430 DOI: 10.1016/j.nbd.2017.12.014] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2017] [Revised: 11/24/2017] [Accepted: 12/19/2017] [Indexed: 02/06/2023] Open
Abstract
Acoustically evoked seizures (e.g., audiogenic seizures or AGS) are common in models of inherited epilepsy and occur in a variety of species including rat, mouse, and hamster. Two models that have been particularly well studied are the genetically epilepsy prone rat (GEPR-3) and the Wistar Audiogenic Rat (WAR) strains. Acute and repeated AGS, as well as comorbid conditions, displays a close phenotypic overlap in these models. Whether these similarities arise from convergent or divergent structural changes in the brain remains unknown. Here, we examined the brain structure of Sprague Dawley (SD) and Wistar (WIS) rats, and quantified changes in the GEPR-3 and WAR, respectively. Brains from adult, male rats of each strain (n=8-10 per group) were collected, fixed, and embedded in agar and imaged using a 7 tesla Bruker MRI. Post-acquisition analysis included voxel-based morphometry (VBM), diffusion tensor imaging (DTI), and manual volumetric tracing. In the VBM analysis, GEPR-3 displayed volumetric changes in brainstem structures known to be engaged by AGS (e.g., superior and inferior colliculus, periaqueductal grey) and in forebrain structures (e.g., striatum, septum, nucleus accumbens). WAR displayed volumetric changes in superior colliculus, and a broader set of limbic regions (e.g., hippocampus, amygdala/piriform cortex). The only area of significant overlap in the two strains was the midline cerebellum: both GEPR-3 and WAR showed decreased volume compared to their control strains. In the DTI analysis, GEPR-3 displayed decreased fractional anisotropy (FA) in the corpus callosum, posterior commissure and commissure of the inferior colliculus (IC). WAR displayed increased FA only in the commissure of IC. These data provide a biological basis for further comparative and mechanistic studies in the GEPR-3 and WAR models, as well as provide additional insight into commonalities in the pathways underlying AGS susceptibility and behavioral comorbidity.
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Affiliation(s)
- Yichien Lee
- Preclinical Research Imaging Laboratory, Georgetown University, Washington, DC, USA; Lombardi Comprehensive Cancer Center, Georgetown University, Washington, DC, USA; Department of Pathology, Georgetown University, Washington, DC, USA
| | - Olga C Rodriguez
- Preclinical Research Imaging Laboratory, Georgetown University, Washington, DC, USA; Lombardi Comprehensive Cancer Center, Georgetown University, Washington, DC, USA; Department of Pathology, Georgetown University, Washington, DC, USA
| | - Chris Albanese
- Preclinical Research Imaging Laboratory, Georgetown University, Washington, DC, USA; Lombardi Comprehensive Cancer Center, Georgetown University, Washington, DC, USA; Department of Pathology, Georgetown University, Washington, DC, USA; Department of Oncology, Georgetown University, Washington, DC, USA
| | | | - José Antônio Cortes de Oliveira
- Neurophysiology and Experimental Neuroethology Laboratory (LNNE), Department of Physiology, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto, Brazil
| | - Ana Luiza Ferreira Donatti
- Neurophysiology and Experimental Neuroethology Laboratory (LNNE), Department of Physiology, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto, Brazil
| | - Artur Fernandes
- Neurophysiology and Experimental Neuroethology Laboratory (LNNE), Department of Physiology, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto, Brazil; Laboratory of Epigenetics and Reproduction, Department of Genetics, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto, Brazil
| | - Norberto Garcia-Cairasco
- Neurophysiology and Experimental Neuroethology Laboratory (LNNE), Department of Physiology, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto, Brazil
| | - Prosper N'Gouemo
- Department of Pediatrics, Georgetown University, Washington, DC, USA; Interdisciplinary Program in Neuroscience, Georgetown University, Washington, DC, USA
| | - Patrick A Forcelli
- Department of Pharmacology & Physiology, Georgetown University, Washington, DC, USA; Interdisciplinary Program in Neuroscience, Georgetown University, Washington, DC, USA; Department of Neuroscience, Georgetown University, Washington, DC, USA.
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Yudkoff M, Daikhin Y, Melø TM, Nissim I, Sonnewald U, Nissim I. The ketogenic diet and brain metabolism of amino acids: relationship to the anticonvulsant effect. Annu Rev Nutr 2007; 27:415-30. [PMID: 17444813 PMCID: PMC4237068 DOI: 10.1146/annurev.nutr.27.061406.093722] [Citation(s) in RCA: 110] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
In many epileptic patients, anticonvulsant drugs either fail adequately to control seizures or they cause serious side effects. An important adjunct to pharmacologic therapy is the ketogenic diet, which often improves seizure control, even in patients who respond poorly to medications. The mechanisms that explain the therapeutic effect are incompletely understood. Evidence points to an effect on brain handling of amino acids, especially glutamic acid, the major excitatory neurotransmitter of the central nervous system. The diet may limit the availability of oxaloacetate to the aspartate aminotransferase reaction, an important route of brain glutamate handling. As a result, more glutamate becomes accessible to the glutamate decarboxylase reaction to yield gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter and an important antiseizure agent. In addition, the ketogenic diet appears to favor the synthesis of glutamine, an essential precursor to GABA. This occurs both because ketone body carbon is metabolized to glutamine and because in ketosis there is increased consumption of acetate, which astrocytes in the brain quickly convert to glutamine. The ketogenic diet also may facilitate mechanisms by which the brain exports to blood compounds such as glutamine and alanine, in the process favoring the removal of glutamate carbon and nitrogen.
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Affiliation(s)
- Marc Yudkoff
- Children's Hospital of Philadelphia and Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
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Yudkoff M, Daikhin Y, Nissim I, Lazarow A, Nissim I. Ketogenic diet, brain glutamate metabolism and seizure control. Prostaglandins Leukot Essent Fatty Acids 2004; 70:277-85. [PMID: 14769486 DOI: 10.1016/j.plefa.2003.07.005] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/06/2003] [Accepted: 07/01/2003] [Indexed: 11/23/2022]
Abstract
We do not know the mode of action of the ketogenic diet in controlling epilepsy. One possibility is that the diet alters brain handling of glutamate, the major excitatory neurotransmitter and a probable factor in evoking and perpetuating a convulsion. We have found that brain metabolism of ketone bodies can furnish as much as 30% of glutamate and glutamine carbon. Ketone body metabolism also provides acetyl-CoA to the citrate synthetase reaction, in the process consuming oxaloacetate and thereby diminishing the transamination of glutamate to aspartate, a pathway in which oxaloacetate is a reactant. Relatively more glutamate then is available to the glutamate decarboxylase reaction, which increases brain [GABA]. Ketosis also increases brain [GABA] by increasing brain metabolism of acetate, which glia convert to glutamine. GABA-ergic neurons readily take up the latter amino acid and use it as a precursor to GABA. Ketosis also may be associated with altered amino acid transport at the blood-brain barrier. Specifically, ketosis may favor the release from brain of glutamine, which transporters at the blood-brain barrier exchange for blood leucine. Since brain glutamine is formed in astrocytes from glutamate, the overall effect will be to favor the release of glutamate from the nervous system.
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Affiliation(s)
- Marc Yudkoff
- Department of Pediatrics, University of Pennsylvania School of Medicine, Children's Hospital of Philadelphia, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104, USA.
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Abstract
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.
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Affiliation(s)
- Carl L Faingold
- Department of Pharmacology, Southern Illinois University School of Medicine, P.O. Box 19629, Springfield, IL 62794-9629, USA.
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Feng HJ, Faingold CL. Modulation of audiogenic seizures by histamine and adenosine receptors in the inferior colliculus. Exp Neurol 2000; 163:264-70. [PMID: 10785466 DOI: 10.1006/exnr.2000.7382] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Susceptibility to behaviorally similar audiogenic seizures (AGS) occurs genetically and is inducible during ethanol withdrawal (ETX). Comparisons between AGS mechanisms of genetically epilepsy-prone rats (GEPR-9s) and ethanol-withdrawn rats (ETX-Rs) are yielding information about general pathophysiological mechanisms of epileptogenesis. The inferior colliculus (IC) is the AGS initiation site. Excitatory amino acid (EAA) abnormalities in the IC are implicated in AGS, and histamine and adenosine receptor activation each reduce EAA release and inhibit several seizure types. Previous studies indicate that focal infusion of an adenosine receptor agonist into the IC blocked AGS in GEPR-9s, but the effects of adenosine receptor activation in the IC on AGS in ETX-Rs are unknown. The effects of histamine receptor activation on either form of AGS are also unexamined. The present study evaluated effects of histamine or a nonselective adenosine A(1) agonist, 2-chloroadenosine, on AGS by focal microinjection into the IC. Ethanol dependence and AGS susceptibility were induced in normal rats by intragastric ethanol. Histamine (40 or 60 nmol/side) significantly reduced AGS in GEPR-9s, but histamine in doses up to 120 nmol/side did not affect AGS in ETX-Rs. 2-Chloroadenosine (5 or 10 nmol/side) did not affect AGS in ETX-Rs, despite the effectiveness of lower doses of this agent in GEPR-9s reported previously. Thus, histamine and adenosine receptors in the IC modulate AGS of GEPR-9s, but do not modulate ETX-induced AGS. The reasons for this difference may involve the chronicity of AGS susceptibility in GEPR-9s, which may lead to more extensive neuromodulation as compensatory mechanisms to limit the seizures compared to the acute AGS of ETX-Rs.
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Affiliation(s)
- H J Feng
- Department of Pharmacology, Southern Illinois University School of Medicine, Springfield, Illinois 62794-9629, USA
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Molnar LR, Thayne KA, Fleming WW, Taylor DA. The role of the sodium pump in the developmental regulation of membrane electrical properties of cerebellar Purkinje neurons of the rat. BRAIN RESEARCH. DEVELOPMENTAL BRAIN RESEARCH 1999; 112:287-91. [PMID: 9878791 DOI: 10.1016/s0165-3806(98)00181-3] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
The postnatal development of the rat cerebellum has been well described morphologically and functionally. However, information regarding the electrical characteristics of Purkinje neurons during development is sparse. Using standard intracellular recording, the basic electrical properties of Purkinje neurons in cerebellar slices were compared at different postnatal ages. There was a significant, progressive increase in the resting membrane potential (RMP) of Purkinje neurons with age as well as a small but significant decrease in the cellular input resistance (Rin). The cardiac glycoside, ouabain (1 mM), an inhibitor of the sodium pump, depolarized Purkinje neurons significantly more with age. The magnitude of the increase in depolarizing activity of ouabain was equivalent to the magnitude of the increase in membrane potential. The number of ouabain binding sites was also found to increase with age suggesting an age related increase in the number of sodium pump sites. These results suggest that the predominant cellular mechanism which underlies the increase in membrane potential of Purkinje neurons during development is an increase in the density of Na+, K+ pump sites and in the contribution of the electrogenic sodium pump.
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Affiliation(s)
- L R Molnar
- Department of Pharmacology and Toxicology, R.C. Byrd Health Sciences Center, West Virginia University School of Medicine, P.O. Box 9223, Morgantown, WV 26506-9223, USA
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Faingold CL, Randall ME. Neurons in the deep layers of superior colliculus play a critical role in the neuronal network for audiogenic seizures: mechanisms for production of wild running behavior. Brain Res 1999; 815:250-8. [PMID: 9878768 DOI: 10.1016/s0006-8993(98)01136-6] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Recent investigations suggest that the deep layers of superior colliculus (DLSC) play a role in the neuronal network for audiogenic seizures (AGS). The present study examined DLSC neuronal firing and convulsive behavior simultaneously in freely-moving genetically epilepsy-prone rats (GEPR-9s) using chronically implanted microwire electrodes. An abrupt onset of acoustically-evoked firing at approximately 80-90 dB was observed in DLSC neurons of GEPR-9s, which was significantly above the normal threshold. DLSC neurons began to exhibit rapid tonic burst firing 1-2 s prior to the onset of the wild running behavior at the beginning of AGS. As the tonic phase of the seizure began, DLSC firing ceased, and only returned towards normal following post-ictal depression. These neuronal mechanisms may be relevant to other seizure models in which the DLSC is implicated. The temporal pattern of neuronal firing during AGS is specific to DLSC and differs markedly from those observed elsewhere in the AGS neuronal network. The temporal firing pattern suggests that the DLSC plays a primary role in the generation of the wild running phase of AGS. Previous studies indicate that the inferior colliculus is dominant during AGS initiation, and the pontine reticular formation is dominant during the tonic extension phase of AGS. Taken together these data suggest that the neurons in the neuronal network undergo a dominance shift as each specific convulsive behavior of AGS is elaborated.
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Affiliation(s)
- C L Faingold
- Department of Pharmacology, P.O. Box 19629, Southern Illinois University School of Medicine, Springfield, IL 62794-9629, USA.
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Faingold CL. Locomotor behaviors in generalized convulsions are hierarchically driven from specific brain-stem nuclei in the network subserving audiogenic seizure. Ann N Y Acad Sci 1998; 860:566-9. [PMID: 9928365 DOI: 10.1111/j.1749-6632.1998.tb09106.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Affiliation(s)
- C L Faingold
- Department of Pharmacology, Southern Illinois University School of Medicine, Springfield 62794-9629, USA.
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Faingold CL, N'Gouemo P, Riaz A. Ethanol and neurotransmitter interactions--from molecular to integrative effects. Prog Neurobiol 1998; 55:509-35. [PMID: 9670216 DOI: 10.1016/s0301-0082(98)00027-6] [Citation(s) in RCA: 201] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
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.
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Affiliation(s)
- C L Faingold
- Department of Pharmacology, Southern Illinois University School of Medicine, Springfield 62794-1222, USA
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Abstract
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.
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Affiliation(s)
- Y Li
- Department of Pharmacology, Southern Illinois University School of Medicine, Springfield 62794-1222, USA
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Chakravarty DN, Faingold CL. Aberrant neuronal responsiveness in the genetically epilepsy-prone rat: acoustic responses and influences of the central nucleus upon the external nucleus of inferior colliculus. Brain Res 1997; 761:263-70. [PMID: 9252025 DOI: 10.1016/s0006-8993(97)00331-4] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The inferior colliculus (IC) central nucleus (ICc), is critical for audiogenic seizure (AGS) initiation in the genetically epilepsy-prone rat (GEPR). The ICc lacks direct motor outputs but sends a major projection to the external nucleus of IC (ICx), which does project to the sensorimotor integration nuclei within the AGS neuronal network. The present study compared acoustic responses of ICx neurons in the GEPR and normal anesthetized rat and evaluated whether the GEPR exhibits functional abnormalities in the pathway from ICc to ICx. There is a significantly greater incidence of sustained repetitive response patterns to the acoustic stimulus in GEPR ICx neurons (75%) than in normal ICx neurons (24%). Following unilateral microinjection of N-methyl-D-aspartate (NMDA) into the contralateral ICc, acoustically-evoked ICx excitation and inhibition were each increased in normal animals, which is consistent with the mixed projections previously reported in this pathway and observed with electrical stimulation in the present study. The NMDA-induced ICx firing increase may be relevant to AGS, since, in previous studies, bilateral focal microinjection of NMDA into the ICc induced AGS susceptibility in normal rats [23]. However, the incidence and degree of the ICx neuronal response changes after NMDA microinjection was not abnormal in the GEPR. These data suggest that the hyperresponsiveness of ICx neurons may not involve abnormal transmission between the ICc and ICx, despite the elevated ICx neuronal responses to acoustic stimuli. However, the ICx hyperresponsivess of the GEPR, which is likely due to the known decrease in effectiveness of GABA-mediated inhibition in GEPR neurons, may be a major mechanism subserving the critical role that this structure plays in the AGS network.
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Affiliation(s)
- D N Chakravarty
- Department of Pharmacology, Southern Illinois University School of Medicine, Springfield 62794-1222, USA
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