Research reportModulation of GABAA receptor-mediated inhibition by postsynaptic calcium in epileptic hippocampal neurons
Introduction
It has been suggested that GABAA receptor/channel function can be modulated by intracellular calcium (see Ref. [1]for review). Low concentration of intracellular calcium is reported to increase GABAA receptor function 17, 27, while an appreciable increase in the intracellular calcium (>1 μM) was found to decrease GABAA receptor-channel function via phosphorylation-dephosphorylation mechanisms 13, 21, 22. Proposed sources of intracellular calcium increase for this type of modulation are: (1) the voltage-dependent calcium channels [30]and (2) the NMDA subtype glutamate receptor [6]. In epilepsy, there is ample evidence to show that NMDA receptor-mediated synaptic responses are pathologically increased in the hippocampus in temporal lobe epilepsy patients 2, 15, 23and in animal models of chronic epilepsy 16, 26, 40. Increased activities of the NMDA receptor appear to work in favor of inducing glutamate-mediated modulation of GABAA receptor-mediated inhibition in epileptic neurons. Indeed, systemic administration of a NMDA receptor antagonist was reported to restore GABAA receptor-mediated inhibition in epileptic animals [18]. In acute hippocampal slices, epileptogenic responses induced by a bath-application of an NMDA receptor agonist were accompanied by a concurrent reduction of GABAA receptor-mediated inhibition [35].
In the present study, it is hypothesized that epileptic activation of the NMDA receptor triggers an epileptogenic increase of intracellular free calcium, and this calcium-increase is crucial for down-regulating GABAA receptor-mediated inhibition in the epileptic hippocampus. In order to experimentally regulate the increase of intracellular calcium and clamp its concentration, a calcium chelator, BAPTA, was used in an intrapipette solution. Dentate granule cells (DGCs) were visualized prior to the patch clamp recording in slice preparations. Although this visualized patch clamp recording technique is used widely in immature brains, it has not been fully utilized in aged/adult and diseased brains. There is no report to date that the technique has been applied to human epileptic hippocampal specimens in slices.
Section snippets
The pilocarpine model
Sprague–Dawley male rats (100–150 g; N=84) were injected with 320–350 mg/kg pilocarpine (i.p.) according to methods previously described [16]. Sixty-three of 84 rats developed Stage 1 to Stage 5 seizures within 30 min after the injection of pilocarpine, and subsequently experienced status epilepticus. The status epilepticus was controlled by diazepam (4 mg/kg) 3 h after the onset. In the remaining 21 rats, pilocarpine induced no status epilepticus. In the rats that experienced status
Preparation of thin slices from human epileptic hippocampus and from the rat pilocarpine model
In the present study, hippocampal slices were prepared with a thickness of 250 μm in order to visualize individual dentate granule cells (DGCs) prior to the whole-cell patch clamp recording of GABAA receptor-mediated inhibitory postsynaptic currents (GABAA IPSCs). This thickness is 1/2 that of the hippocampal slices which were prepared from the same types of epileptic specimen with the use of the `blind method' for the patch clamp recording from thick slices 14, 15.
With a thickness of 250 μm,
Discussion
The present results with an intracellular BAPTA application suggest that the increase in intracellular calcium concentration, induced by the activation of the NMDA receptor, is directly involved in and responsible for the induction of the response-decrement of GABAA IPSCs in the DGCs of human epileptic hippocampus and the pilocarpine-treated and seizure-experienced rat hippocampus. The present study also demonstrated that the mechanisms responsible for this transient decrement of GABAA
Acknowledgements
I thank Dr. I. Fried for providing human hippocampal specimens, and the members of the UCLA Epilepsy Surgery Program for their clinical expertise. Human study was conducted under the guidance of Declaration of Helsinki and UCLA Human Subject Protection Committee and supported by an NINDS program project grant P01-NS02808. The pilocarpine project was supported by an NINDS FIRST Award R29-NS31180.
References (41)
- et al.
Bursting in human epileptogenic neocortex is depressed by an N-methyl-d-aspartate antagonist
Neurosci. Lett.
(1987) - et al.
NMDA receptor-mediated excitability in dendritically deformed dentate granule cells in pilocarpine-treated rats
Neurosci. Lett.
(1991) - et al.
NMDA receptor activation mediates the loss of GABAergic inhibition induced by recurrent seizures
Epilepsy Res.
(1990) Patch-clamping in slices of mammalian CNS
Trends in Neuroscience
(1990)- et al.
Activity-dependent depression of monosynaptic fast IPSCs in hippocampus: contributions from reductions in chloride driving force and conductance
Brain Res.
(1995) - et al.
Calcium entry increases the sensitivity of cerebellar Purkinje cells to applied GABA and decreases inhibitory synaptic currents
Neuron
(1991) Mechanism of modulation of GABA-activated current by internal calcium in rat central neurons
Brain Res.
(1991)- et al.
Epileptiform discharges evoked in hippocampal brain slices from epileptic patients
Brain Res.
(1989) - et al.
Loss of Calbindin-D28K immunoreactivity from dentate granule cells in human temporal lobe epilepsy
Neuroscience
(1997) - et al.
Effects of rapid buffers on CA2+ diffusion and Ca2+ oscillations
Biophys. J.
(1994)
Function of synapses in the CA1 region of the hippocampus: their contribution to the generation or control of eileptiform activity
Comp. Biochem. Physiol.
Gating of GABAergic inhibition in hippocampal pyramidal cells
Ann. N.Y. Acad. Sci.
Reduction of rat hippocampal calcium-binding protein following commissural, amygdala, septal, perforant path, and olfactory bulb kindling
Epilepsia
Simultaneous expression of long-term depression of NMDA and long-term potentiation of AMPA receptor-mediated synaptic responses in the CA1 area of the kainic acid-lesioned hippocampus
Eur. J. Neurosci.
Neuron loss, granule cell axon reorganization, and functional changes in the dentate gyrus of epileptic kainate-treated rats
J. Comp. Neurol.
Suppression of GABAA receptor responses by NMDA application in hippocampal neurones acutely isolated from the adult guinea-pig
J. Physiol.
Noise analysis of miniature IPSCs in adult rat brain slices: properties and modulation of synaptic GABAA receptor channels
J. Neurophysiol.
Correlation of criteria used for localizing the epileptic focus in patients considered for surgical treatment of epilepsy
Ann. Neurol.
Pattern of neuronal cell death in the rat hippocampus after status epilepticus. Relationship to calcium-binding protein content and ischemic vulnerability
Brain Res. Bull.
Cited by (25)
Altered functional efficacy of hippocampal interneuron during epileptogenesis following febrile seizures
2017, Brain Research BulletinCitation Excerpt :In addition, Macdonald and their colleague (Macdonald and Rogawski, 2007) reported that the main therapeutic action of diverse antiepileptic drugs, such as benzodiazepines and barbiturates, is to enhance GABAA receptor currents. Although hyperthermic seizure suppressed GABAergic transmission in hippocampal neurons of immature rats for shifting the excitation and inhibition balance, the remaining interneurons can become hyperactive to compensate for the loss of GABAergic inhibition (Rowley et al., 1995; Beau and Alger, 1998; Isokawa, 1998; Cossart et al., 2001) and the newborn DG granule cells following FS are a cause of long-lasting increased GABAA receptor expression (Swijsen et al., 2012). Thus, least in part, it is likely that the increased immunoreactivity of GABAA-α1 receptor may be a common phenomenon in the recurrent seizure models, such as FS model.
Modulation of abnormal synaptic transmission in hippocampal CA3 neurons of spontaneously epileptic rats (SERs) by levetiracetam
2011, Brain Research BulletinCitation Excerpt :SER hippocampus has a lower density of benzodiazepine receptors with normal affinity [29]. GABAA receptor-mediated inhibition is reduced in the epileptogenic hippocampus [11]. Tremor rats, one of the parent strains of SER, exhibit a higher sensitivity to GABA in the hippocampus [16], thus indicating that the SER hippocampus also expresses a higher sensitivity to GABA.
Enhanced pyridoxal 5′-phosphate synthetic enzyme immunoreactivities do not contribute to GABAergic inhibition in the rat hippocampus following pilocarpine-induced status epilepticus
2009, NeuroscienceCitation Excerpt :In particular, F-actin depolymerization in dendritic spines increases neuronal excitability (Steward and Schuman, 2001; Malinow and Malenka, 2002; Ostroff et al., 2002; Ouyang et al., 2005; Sierra-Paredes et al., 2006). In the hippocampi of human epilepsy patients and animal models, indeed, the level of F-actin is decreased in dendritic spines, which can respond to changes in the local cellular environment, including newly formed afferents, in a plastic manner (Furukawa et al., 1995, 1997; Isokawa, 1998, 2000; Sierra-Paredes et al., 2006; Swann et al., 2000; Kim et al., 2008). Therefore, the upregulation of PLK and PNPO expression may be involved in dendritic spine reorganization, which in turn, promotes the reverberation of recurrent excitatory networks in the hippocampus following SE.
Acquired Epilepsy: Cellular and Molecular Mechanisms
2007, Molecular NeurologyPhysiological studies of human dentate granule cells
2007, Progress in Brain ResearchCitation Excerpt :Many of these changes were recently reviewed by Magloczky and Freund (2005). Physiologically, several groups have reported a decrease in evoked GABAA receptor-mediated inhibition in MTS hippocampi relative to comparison material (Isokawa, 1998; Williamson et al., 1999; Fig. 3). The degree of reduction is similar to that seen in the pilocarpine rat model (Kobayashi and Buckmaster, 2003).