Regulation of epileptiform activity in hippocampus by nicotinic acetylcholine receptor activation.
ABSTRACT Nicotinic acetylcholine receptors (nAChRs) regulate neuronal excitability within the CNS. To assess the possible modulatory influence of nAChRs on epileptiform activity, a range of nAChR ligands were applied during experimentally induced epileptiform activity in rat hippocampal slices. Bath application of the potassium channel blocker 4-aminopyridine (4AP; 10-50 microM) resulted in the development of spontaneous epileptiform bursting activity in area CA3 that consisted of short duration (257+/-15 ms) field events occurring regularly at a frequency of 0.4+/-0.02 Hz. Subsequent co-application of the selective nAChR agonists 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 0.3-300 microM), choline (0.01-3mM) and lobeline (3-30 microM) produced sustained and concentration-dependent increases in burst frequency with maximal frequency potentiation of 37+/-5%, 27+/-5% and 24+/-11%, respectively. DMPP (10-30 microM; n=31) also potentiated epileptiform bursting induced by reducing GABA(A) receptor-mediated synaptic transmission using 20 microM bicuculline or enhancing NMDA receptor-mediated excitation by lowering extracellular Mg(2+). Irrespective of the epileptiform model studied all nAChR agonist induced frequency potentiation was reversed upon washout of the agonist or co-application of one of the selective nAChR antagonists dihydro-beta-erythroidine (10-30 microM), mecamylamine (50-200 microM) or alpha-bungarotoxin (100 nM). These results provide compelling evidence that activation of nAChRs exacerbate epileptiform activity in the rat hippocampus.
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ABSTRACT: Objective(s) Nicotinic acetylcholine receptors (nAChRs) regulate epileptiform activity and produce a sustained pro- epileptogenic action within the hippocampal slices. In the present study, we investigated the effect of nAChRs on evoked glutamatergic synaptic transmission in area CA3 and CA1 of rat hippocampal slices to identify possible excitatory circuits through which activation of nAChRs produce their pro-epileptogenic effects. Materials and Methods Hippocampal slices (400 µm thick) prepared in vitro from male Wistar rats (3-5 weeks), using standard procedures. Following 1 hr equilibration in artificial cerebrospinal fluid (ACSF), slices transferred to an interface recording chamber. Stimulatory electrodes placed within the hilus or Schaffer-collateral pathways and extracellular field recordings made in the stratum radiatum of the CA1 and CA3 regions to investigate evoked synaptic responses. Results Bath application of the selective nAChR agonist dimethylphenyl-piperanzinium (DMPP, 30 µM) resulted in a sustained and reversible enhancement of glutamate afferent evoked fEPSP amplitude by 15.7±5.1% (mean±SEM; n=8 of 12) in the CA3 region of the hippocampus but not in the CA1 (-5.25±8.3%, mean±SEM; n=5). Conclusion
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ABSTRACT: The involvement of brain nicotinic acetylcholine receptors (nAChRs) in the neurotoxicological effects of soman, a potent acetylcholinesterase (AChE) inhibitor and a chemical warfare agent, is not clear. This is partly due to a poor understanding of the role of AChE in brain nAChR-mediated functions. To test the hypothesis that AChE inhibition builds sufficient acetylcholine (ACh) in the brain and facilitates nAChR-dependent glutamate transmission, we used whole-cell patch-clamp technique to record spontaneous glutamate excitatory postsynaptic currents (EPSCs) from CA1 stratum radiatum interneurons (SRI) in hippocampal slices. First, the frequency, amplitude and kinetics of EPSCs recorded from slices of control guinea pigs were compared to those recorded from slices of guinea pigs after a single injection of the irreversible AChE inhibitor soman (25.2 μg/kg, s.c). Second, EPSCs were recorded from rat hippocampal slices before and after their superfusion with the reversible AChE inhibitor donepezil (100 nM). The frequency of EPSCs was significantly higher in slices taken from guinea pigs 24 h but not 7 days after the soman injection than in slices from control animals. In 52% of the rat hippocampal slices tested, bath application of donepezil increased the frequency of EPSCs. Further, exposure to donepezil increased both burst-like and large-amplitude EPSCs, and increased the proportion of short (20-100 ms) inter-event intervals. Donepezil's effects were suppressed significantly in presence of 10 μM mecamylamine or 10 nM methyllycaconitine. These results support the concept that AChE inhibition is able to recruit nAChR-dependent glutamate transmission in the hippocampus and such a mechanism can contribute to the acute neurotoxicological actions of soman.NeuroToxicology 03/2013; DOI:10.1016/j.neuro.2013.02.005 · 3.05 Impact Factor
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ABSTRACT: Rett syndrome (RTT) is a disorder with a pronounced neurological phenotype and is caused mainly by mutations in the X-linked gene MECP2. A common feature of RTT is an abnormal EEG and a propensity for seizures. In the current study we aimed to assess brain network excitability and seizure propensity in a mouse model of RTT. Mice in which Mecp2 expression was silenced (Mecp2(stop/y)) showed a higher seizure score (mean = 6 ± 0.8 compared to 4 ± 0.2 in wild-type, WT) and more rapid seizure onset (median onset = 10 mins in Mecp2(stop/y) and 32 mins in WT) when challenged with the convulsant drug kainic acid (25mg/Kg). Hippocampal slices from Mecp2(stop/y) brain displayed no spontaneous field potential activities under control conditions but showed higher power gamma frequency field potential oscillations compared to WT in response to kainic acid (400 nM) in vitro. Brain slices challenged with the GABA(A) receptor antagonist bicuculline (0.1-10μM) and the potassium channel blocker 4-aminopyridine (1-50μM) also revealed differences between genotypes with hippocampal circuits from Mecp2(stop/y) mouse slices showing enhanced epileptiform burst duration and frequency. In contrast to these network level findings, single cell analysis of pyramidal cells by whole-cell patch clamp recording revealed no detectable differences in synaptic or biophysical properties between MeCP2-containing and MeCP2-deficient neurons. These data support the proposal that loss of MeCP2 alters network level excitability in the brain to promote epileptogenesis.Neuroscience 12/2012; 231. DOI:10.1016/j.neuroscience.2012.11.058 · 3.33 Impact Factor