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Pentameric GABA A receptors are composed from 19 possible subunits. The GABA A β subunit is unique because the β 1 and β 3 subunits can assemble and traffic to the cell surface as homomers, whereas most of the other subunits, including β 2 , are heteromers. The intracellular domain (ICD) of the GABA A subunits has been implicated in targeting and clustering GABA A receptors at the plasma membrane. Here, we sought to test whether and how the ICD is involved in functional expression of the β 3 subunit. Since θ is the most homologous to β but does not form homomers, we created two reciprocal chimeric subunits, swapping the ICD between the β 3 and θ subunits, and expressed them in HEK293 cells. Surface expression was detected with immunofluorescence and functional expression was quantified using whole-cell patch-clamp recording with fast perfusion. Results indicate that, unlike β 3 , neither the β 3 /θ IC nor the θ/β 3IC chimera can traffic to the plasma membrane when expressed alone; however, when expressed in combination with either wild-type α 3 or β 3 , the β 3 /θ IC chimera was functionally expressed. This suggests that the ICD of α 3 and β 3 each contain essential anterograde trafficking signals that are required to overcome ER retention of assembled GABA A homo- or heteropentamers.

Histamine is an important neurotransmitter that exerts its physiological actions through H1-4 metabotropic receptors in mammals. It also directly activates ionotropic GABAA receptor (GABAAR) β3 homooligomers and potentiates GABA responses in αβ heterooligomers in vitro, but the respective histamine binding sites in GABAARs are unknown. We hypothesized that histamine binds at the extracellular β+β- interface at a position homologous to the GABA binding site of heterooligomeric GABAARs. To test this, we individually mutated several residues at the putative ligand binding minus side of a rat GABAAR β3 wild type subunit and of a β3 subunit that was made insensitive to trace Zn(2+) inhibition [β3(H267A); called (Z)β3]. (Z)β3, (Z)β3(Y62L), (Z)β3(Q64A), (Z)β3(Q64E), α1(Z)β3, or α1(Z)β3(Y62L) receptors were studied in HEK293T cells using whole cell voltage clamp recording. β3, β3(Y62C), β3(Q64C), β3(N41C), β3(D43C), β3(A45C) or β3(M115C) receptors were examined in Xenopus oocytes using two-electrode voltage clamp. Histamine directly activated (Z)β3 and β3 homooligomers and potentiated GABA actions in α1(Z)β3 heterooligomers. Receptors containing (Z)β3(Y62L), β3(Y62C) and β3(D43C) showed markedly reduced histamine potency, but homo- and heterooligomers with (Z)β3(Q64E) exhibited increased potency. The GABAAR αβ(γ) competitive antagonist bicuculline elicited sub-maximal agonist currents through (Z)β3 homooligomers, the potency of which was strongly decreased by (Z)β3(Y62L). Mutations β3(N41C), β3(A45C) and β3(M115C) disturbed receptor expression or assembly. Computational docking into the crystal structure of homooligomeric β3 receptors resulted in a histamine pose highly consistent with the experimental findings, suggesting that histamine activates β3 receptors via a site homologous to the GABA site in αβγ receptors., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Dopamine is a critical neuromodulator that activates GPCRs in mammals or ligand-gated ion channels in invertebrates. The present study demonstrates that dopamine (0.1-10 mm) exerts novel, opposing effects on different populations of mammalian (rat) GABAA receptors. Using whole-cell patch-clamp electrophysiology, we observed direct dopamine-mediated inhibition of tonic-level (1 μm) GABA-evoked currents in untransfected striatal neurons that could be recapitulated in HEK293 cells containing α1β3 or α1β2γ2 subunits. Surprisingly, direct activation by dopamine was seen in the absence of GABA with α1β2γ2, α5β3γ2, or α1β3γ2 transfections. This activity was also present in α1β3γ2 receptors containing a mutant β3 subunit (H267A [(Z)β3]) insensitive to trace levels of inhibitory Zn(2+). Dopamine activation required β and γ subunits but not α subunits ((Z)β3γ2 EC50 value, 660 μm). Dopamine activity was fully blocked by picrotoxin but not GABAA competitive antagonists, and was strongly correlated with spontaneous receptor activity. We also report opposing effects of bicuculline and gabazine, such that bicuculline surprisingly activated non-α-containing (β3γ2) GABAA receptors, whereas gabazine suppressed spontaneous activity in these receptors. Our results suggest that dopamine may directly inhibit GABAA receptors that are both immediately adjacent to dopamine release sites in the striatum and activated by tonic GABA. Furthermore, synaptic/phasic release of dopamine may directly enhance signaling at some spontaneously active noncanonical GABAA receptors that lack α subunits., (Copyright © 2015 the authors 0270-6474/15/353525-12$15.00/0.)

There is ample pharmacological and physiological evidence for yet unidentified histamine receptors in mammalian brain that are linked to a Cl(-) conductance. In invertebrates, two histamine-gated chloride channels (HisCl α1 and α2) are already well known. HisCl channels are members of the Cys-loop receptor superfamily of ligand-gated ion channels and are closely related to the mammalian GABA(A) and glycine receptors (GlyR). Indeed, they share particularly strong homology within the ligand binding and ion channel domains. Here we discuss the possibility that mammalian HisCl channels might exist among the known GABA(A) or GlyR subunits. Studies published to date support this hypothesis, including evidence for direct histamine gating of GABA(A) β homomers, histamine potentiation of GABA(A) αβ and αβγ heteromeric receptors, and GABA(A) receptor blockade by some antihistamines. We explore what is known about the binding-site structure, function and pharmacology of invertebrate HisCl channels and other histamine binding sites to support and inform a broader search for HisCl channels among the mammalian GABA(A) and GlyR subunits. The discovery and identification of HisCl-like channels in mammals would not only enhance understanding of inhibitory signaling and histamine function in the mammalian brain, but also provide new avenues for development of therapeutic compounds targeting this novel histamine site. This commentary is therefore intended to foster consideration of a novel and potentially important target of histamine and histaminergic drugs in the CNS., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Calcium (Ca2+) channels are sensitive to ethanol and Ca2+ signaling is a critical regulator of axonal growth and guidance. Effects of acute and chronic exposure to ethanol (22, 43, or 87 mM) on voltage-gated Ca2+ channels (VGCCs) in whole cells, and KCl-induced Ca2+ transients in axonal growth cones, were examined using dissociated hippocampal cultures. Whole-cell patch-clamp analysis in neurons with newly-formed axons (Stage 3) revealed that rapidly inactivating, low-voltage activated (LVA) and non-inactivating, high-voltage activated (HVA) currents were both inhibited in a dose-dependent manner by acute ethanol, with relatively greater inhibition of HVA currents. When assessed by Fluo-4-AM imaging, baseline fluorescence and Ca2+ response to ethanol in Stage 3 neurons was similar compared to neurons without axons, but peak Ca2+ transient amplitudes in response to bath-applied KCl were greater in Stage 3 neurons and were decreased by acute ethanol. The amplitude of Ca2+ transients elicited specifically in axonal growth cones by focal application of KCl was also inhibited by acute exposure to moderate-to-high concentrations of ethanol (43 or 87 mM), whereas a lower concentration (22 mM) had no effect. When 43 or 87 mM ethanol was present continuously in the medium, KCl-evoked Ca2+ transient amplitudes were also reduced in growth cones. In contrast, Ca2+ transients were increased by continuous exposure to 22 mM ethanol. Visualization using a fluorescent dihydropyridine analog revealed that neurons continuously exposed to ethanol expressed increased amounts of L-type Ca2+ channels, with greater increases in axonal growth cones than cell bodies. Thus, acute ethanol reduces Ca2+ current and KCl-induced Ca2+ responses in whole cells and axonal growth cones, respectively, and chronic exposure is also generally inhibitory despite apparent up-regulation of L-type channel expression. These results are consistent with a role for altered growth cone Ca2+ signaling in abnormal neuromorphogenesis associated with fetal alcohol spectrum disorders., (Copyright © 2011 IBRO. Published by Elsevier Ltd. All rights reserved.)

Differences in binding-site residues of GluR2 (AMPAR) and GluR6 (KAR) subunits have been identified that might account for their functional and pharmacological differences. Specifically, residues A518, A689 and N721 in GluR6 replace highly conserved threonine and serine residues found in other ionotropic glutamate receptor (iGluR) subunits. To define how these natural substitutions impact GluR6 function, we used patch clamp recording with ultrafast perfusion to characterize the effects of A518T, A689S and N721T on agonist potency, efficacy and response kinetics. We find these natural substitutions impact GluR6 function less than would be expected from reverse mutations in other iGluRs. There was little effect of individual or combined mutations on glutamate potency, deactivation or desensitization kinetics. Altered recovery kinetics were seen that were greatest after combined mutations. Kainate potency and response kinetics were also unchanged in the mutants, whereas kainate efficacy was reduced in A518T and increased the T/S/T mutant relative glutamate. Notably, A518T and A689S mutation permitted AMPA to bind as a weak competitive antagonist and the effects of these mutations were additive. N721T mutation further enhanced AMPA binding, allowing AMPA to activate and fully desensitize the receptors. Alternative mutations altering side chain length at position 518 produced far greater changes in glutamate affinity and response kinetics than did the natural mutations. We conclude that these nonconserved residues in GluR6 define the size of the agonist-binding pocket, exerting a steric influence on the bound agonist and the extent of binding-domain closure that can influence agonist potency, deactivation, desensitization and recovery kinetics.

Allosteric modulators and mutations that slow AMPAR desensitization have additional effects on deactivation and agonist potency. We investigated whether these are independent actions or the natural consequence of slowing desensitization. Effects of cyclothiazide (CTZ), trichlormethiazide (TCM), and CX614 were compared at wild-type GluR1 and "nondesensitizing" GluR1-L497Y mutant receptors by patch-clamp recording with ultrafast perfusion. CTZ, TCM, or L/Y mutation all essentially blocked GluR1 desensitization; however, the effects of L/Y mutation on deactivation and glutamate EC50 were three to five times greater than for modulators. CTZ and TCM further slowed desensitization of L/Y mutant receptors but paradoxically accelerated deactivation and increased agonist EC50. Results indicate that CTZ and TCM target deactivation and agonist potency independently of desensitization, most likely by modifying agonist dissociation (koff). Conversely, CX614 slowed desensitization and deactivation without affecting EC50 in both wild-type and L/Y receptors. The S750Q or combined L497Y-S750Q mutations abolished all CTZ and TCM actions without disrupting CX614 activity. Notably, the S/Q mutation also restored L/Y deactivation and EC50 to wild-type levels without restoring desensitization, further demonstrating that desensitization can be modulated independently of deactivation and EC50 by mutagenesis and possibly by allosteric modulators.

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. The cellular regulation of glutamate receptor (GluR) ion channel function and expression is important for maintaining or adjusting target cell excitability to meet ever-changing demands, for example, in relation to developmental or use-dependent synaptic plasticity. Dysregulation of GluR function or expression may be a contributing factor in certain forms of epilepsy, stroke/ischemia, head trauma, cognitive impairments, and neurodegenerative disease. Recent years have seen substantial progress in understanding how GluRs operate in terms of their structural and functional properties, their synaptic targeting and membrane anchoring by PDZ-domain proteins, and their activity-dependent cycling at the plasma membrane. Yet precious little is known about the earliest events in GluR biogenesis or the mechanisms in place to ensure the GluRs that reach the cell surface are processed, folded, and oligomerized in an appropriate manner. Indeed, only a minor fraction of the GluR content of cells is expressed at any given time on the cell surface, whereas most of the remaining receptors exist in the endoplasmic reticulum (ER). The functional competence and significance of the ER fraction of receptors are presently unknown, but they are generally thought to represent immature, unassembled, or improperly assembled subunits. Some are ultimately destined for insertion in the plasma membrane. Others may be targeted for proteosomal degradation. Still others might provide a latent pool of fully functional receptors that can be recruited to enhance cell excitability in response to specific signals or under pathological conditions. This review will explore the structural and functional elements that regulate GluR assembly and export from the ER.

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 domains in the N-terminal and M3-M4 loop regions, respectively. In the present study, we sought to confirm that the conserved ligand-binding residues identified in the AMPA receptor S1S2 domains also participate in ligand binding of GluR6 kainate receptors. Amino acid substitutions were made in the GluR6 parent at R523, T690, and E738 to alter their potential interactions with ligand. Mutant receptors were expressed in human embryonic kidney 293 cells, confirmed by Western blot analysis, and tested by [3H]kainate binding and patch-clamp recording. Each of the binding site mutations was sufficient to reduce [3H]kainate binding to undetectable levels and eliminate functional responses to glutamate or kainate. As with our studies of other nonfunctional mutants (Fleck et al., 2003), immunocytochemical staining and cell-surface biotinylation studies showed that the mutant receptors were retained intracellularly and did not traffic to the cell surface. Endoglycosidase-H digests and colocalization with endoplasmic reticulum (ER) markers demonstrated that the mutant receptors are immaturely glycosylated and retained in the ER. Immunoprecipitation, native PAGE, and functional studies confirmed that the GluR6-binding site mutants are capable of multimeric assembly, indicating their retention in the ER does not result from a gross protein folding error. Together, these results confirm the role of R523, T690, and E738 directly in ligand binding to GluR6 and further support our previous report that nonfunctional GluRs are retained intracellularly by a functional checkpoint in ER quality control.

Even though conventional systemic doses of cimetidine and other histamine H(2) antagonists display minimal brain penetration, central nervous system (CNS) effects (including seizures and analgesia) have been reported after administration of these drugs in animals and man. To test the hypothesis that cimetidine-like drugs produce these CNS effects via inhibition of GABA(A) receptors, the actions of these drugs were studied on seven different, precisely-defined rat recombinant GABA(A) receptors using whole-cell patch clamp recordings. The H(2) antagonists famotidine and tiotidine produced competitive and reversible inhibition of GABA-evoked currents in HEK293 cells transfected with various GABA(A) receptor subunits (IC(50) values were between 10-50 microM). In contrast, the H(2) antagonist ranitidine and the cimetidine congener improgan had very weak (if any) effects (IC(50) > 50 microM). Since the concentrations of cimetidine-like drugs required to inhibit GABA(A) receptors in vitro (greater than 50 microM) are considerably higher than those found during analgesia and/or seizures (1-2 microM), the present results suggest that cimetidine-like drugs do not appear to produce seizures or analgesia by directly inhibiting GABA(A) receptors.

18-Methoxycoronaridine, a novel iboga alkaloid congener, reduces drug self-administration in animal models of addiction. Previously, we proposed that these effects are mediated by the ability of 18-methoxycoronaridine to inhibit nicotinic alpha3beta4 acetylcholine receptors. In an attempt to identify more potent 18-methoxycoronaridine analogs, we have tested a series of 18-methoxycoronaridine congeners by whole-cell patch clamp recording of HEK 293 cells expressing recombinant nicotinic alpha3beta4 receptors or glutamate NR1/NR2B N-methyl-d-aspartate (NMDA) receptors. The congeners exhibited a range of inhibitory potencies at alpha3beta4 receptors. Five congeners had IC(50) values similar to 18-methoxycoronaridine, and all of these were ineffective at NMDA receptors. The congeners also retained their ability to reduce morphine and methamphetamine self-administration. These data are consistent with the importance of nicotinic alpha3beta4 receptors as a therapeutic target to modulate drug seeking. These compounds may constitute a new class of synthetic agents that act via the nicotinic alpha3beta4 mechanism to combat addiction.

Variation of the methoxycarbonyl and C-18 substituents of the antiaddictive compound 18-methoxycoronaridine, and contraction of its isoquinuclidine ring segment, provided 15 congeners for SAR evaluation at opioid and alpha3beta4 nicotinic acetylcholine receptors. The opioid activities were relatively low, and the alpha3beta4 nicotinic acetylcholine receptor activities were found to correlate with in vivo antiaddictive activities.

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 segments in the N-terminal and M3-M4 loop regions, respectively. Molecular and atomic level structural analyses have identified specific S1 and S2 residues that interact directly with ligands, interact with one another in a dimeric configuration, and influence channel gating and desensitization properties of AMPA receptors. Other studies suggest that KA receptor gating and desensitization may differ mechanistically. In particular, a leucine (L) to tyrosine (Y) mutation in the S1 segment of AMPA receptors is sufficient to block desensitization, whereas KA receptors naturally contain a tyrosine residue at the equivalent position (Y751 in GluR6) but retain the fast-desensitizing phenotype. We hypothesized that KA receptor desensitization is preserved by a compensatory substitution in the S2 segment. We generated a series of GluR6 mutants that converted individual S2 domain residues to their AMPA receptor equivalents. Various S2 mutations had effects on the kinetics of desensitization and recovery from desensitization, but no single amino acid substitution was found to block desensitization, as in the L/Y mutant AMPA receptors, or to prevent desensitization to KA. Other mutations designed to neutralize residues thought to interact across the dimer interface had dramatic effects on channel gating and desensitization. These results are consistent with a close but imperfect structural homology between AMPA and KA receptors and support the role of conserved S1S2 domain interactions at the dimer interface in GluR channel function.

Although the adult brain contains nicotinic acetylcholine (ACh) receptors vital to cortical function, little is known about the assembly of embryonic receptor subunits into functional receptors or their role in fetal brain development. We now report the first evidence of functional nicotinic ACh receptors on stem and progenitor cells of fetal mouse cerebral cortex as early as embryonic day 10.

This study set out to profile the activity of (S)-desmethylzopiclone (SEP-174559) at subtypes of the gamma-aminobutyric acid type-A (GABA(A)) receptor and other neurotransmitter receptor ion channels. Recombinant receptors were expressed in human embryonic kidney 293 cells and examined functionally by patch-clamp recording with fast perfusion of agonist and drug solutions. Micromolar concentrations of SEP-174559 potentiated GABA(A) receptor currents evoked by subsaturating concentrations of GABA. The potentiation was related to a leftward shift in the GABA dose-response curves, suggesting the drug acts to increase GABA binding affinity. The potentiation strictly required the presence of the gamma2 subunit; no enhancement was seen for receptors containing instead the gamma1 subunit or lacking a gamma subunit altogether. SEP-174559 and its parent compound, racemic zopiclone, were not selective between alpha1-, alpha2-, or alpha3-bearing GABA(A) receptors. Within the nicotinic receptor superfamily, SEP-174559 did not affect serotonin type-3 receptor function but was found to inhibit nicotinic acetylcholine (nACh) receptors. The inhibition of nACh receptors was noncompetitive and was mimicked by zopiclone, alprazolam, and diazepam. In the glutamate receptor superfamily, SEP-174559 inhibited N-methyl-D-aspartate (NMDA) receptor currents but did not affect non-NMDA receptors. These data confirm that SEP-174559 has benzodiazepine-like actions at gamma2-bearing subtypes of the GABA(A) receptor and suggest additional actions of benzodiazepine-site ligands at nACh and NMDA receptors.

The iboga alkaloid ibogaine and the novel iboga alkaloid congener 18-methoxycoronaridine are putative anti-addictive agents. Using patch-clamp methodology, the actions of ibogaine and 18-methoxycoronaridine at various neurotransmitter receptor ion-channel subtypes were determined. Both ibogaine and 18-methoxycoronaridine were antagonists at alpha 3 beta 4 nicotinic receptors and both agents were more potent at this site than at alpha 4 beta 2 nicotinic receptors or at NMDA or 5-HT(3) receptors; 18-methoxycoronaridine was more selective in this regard than ibogaine. In studies of morphine and methamphetamine self-administration, the effects of low dose combinations of 18-methoxycoronaridine with mecamylamine or dextromethorphan and of mecamylamine with dextromethorphan were assessed. Mecamylamine and dextromethorphan have also been shown to be antagonists at alpha 3 beta 4 nicotinic receptors. All three drug combinations decreased both morphine and methamphetamine self-administration at doses that were ineffective if administered alone. The data are consistent with the hypothesis that antagonism at alpha 3 beta 4 receptors is a potential mechanism to modulate drug seeking behavior. 18-Methoxycoronaridine apparently has greater selectivity for this site than other agents and may be the first of a new class of synthetic agents acting via this novel mechanism to produce a broad spectrum of anti-addictive activity.

The adult cerebral cortex contains nicotinic acetylcholine (ACh) receptors vital to cortical function. However, little is known about the assembly of embryonic nicotinic receptor subunits into functional receptors or whether they play an active role in cortical development. We now report evidence of functional nicotinic acetylcholine receptor channels in fetal mouse cerebral cortex as early as embryonic day 10 (E10), when the cortex consists of dividing stem and progenitor cells. Patch-clamp electrophysiological measurements indicate that nicotine and ACh evoke sizable inward currents characteristic of nicotinic receptors, that are strongly rectifying with a reversal potential near 0 mV. Three different nicotinic agonists, ACh, nicotine, and dimethylphenylpiperazinium, evoked cytosolic Ca(2+) signals. Agonist-evoked Ca(2+) signals and electrophysiological responses were found in greater than 70% of all E10-E11 cells tested and were blocked by nicotinic receptor antagonists. The Ca(2+) response to nicotinic agonists was markedly prolonged in cells from early embryonic stages relative to later stages of development. alpha3, alpha4, and alpha7 receptor subunit proteins were detected immunocytochemically in cortical cells from E10 to birth. The incidence of each subunit declined with embryonic age, suggesting a role in early development. We discuss the possible function of nicotinic receptors in early cortical development and their role as a target for nicotine in the developmental pathologies associated with the fetal tobacco syndrome., ((c) 2001 Elsevier Science.)

We examined the vesicular accumulation of the excitatory amino-acid (EAA) neurotransmitters, L-glutamate and L-aspartate, together with the non-metabolisable EAA analogue D-aspartate. Synaptosomes derived from whole brain were incubated in various concentrations of [3H]-amino acids under conditions to facilitate vesicular turnover. Synaptosomes were then lysed in hypotonic medium and vesicles immunoprecipitated with monoclonal anti-synaptophysin antibodies coupled to sepharose beads. Using this method, saturable vesicular accumulation was observed for [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate but not for the excitatory amino acid receptor ligands [3H]-AMPA or [3H]-kainate. Vesicular accumulation (t(1/2)=7.45 min) was markedly slower than synaptosomal accumulation (t(1/2)=1.03 min) and was substantially reduced at 4 degrees C. Maximal accumulation of [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate was estimated to be 98, 68, and 112 pmol/mg of synaptosomal protein, respectively, and uptake affinities 1.6, 3.4, and 2.1 mM, respectively. Maximal accumulation of [3H]-L-glutamate was non-competitively inhibited by both 100 microM unlabeled L-aspartate and 100 microM D-aspartate, suggesting that all are accumulated into a common vesicular pool by different transporters.

This study examined whether preaccumulated D,L-threo-beta-hydroxyaspartate (tHA), a competitive substrate for the high-affinity excitatory amino acid (EAA) transporter, is released as a false transmitter from EAA-releasing nerve terminals. Potassium-stimulation (50 mM for 1 min) evoked significant release of the endogenous EAAs (aspartate and glutamate) from superfused neocortical minislices. Endogenous EAA release was largely calcium-dependent and was inhibited by tetanus toxin, a neurotoxin which specifically blocks vesicular exocytosis. In parallel experiments, minislices were pre-incubated with 500 microM tHA. Potassium (50 mM) evoked significant release of tHA and this release was also calcium-dependent and reduced by tetanus toxin. Pre-accumulation of tHA did not affect the release of endogenous glutamate whereas the release of endogenous aspartate was significantly attenuated. These data suggest that tHA selectively accumulates in a vesicular aspartate pool and is released upon depolarization as a false transmitter from EAA nerve terminals.

Human cortical heterotopia and neuronal migration disorders result in epilepsy; however, the precise mechanisms remain elusive. Here we demonstrate severe neuronal dysplasia and heterotopia throughout the granule cell and pyramidal cell layers of mice containing a heterozygous deletion of Lis1, a mouse model of human 17p13.3-linked lissencephaly. Birth-dating analysis using bromodeoxyuridine revealed that neurons in Lis1+/- murine hippocampus are born at the appropriate time but fail in migration to form a defined cell layer. Heterotopic pyramidal neurons in Lis1+/- mice were stunted and possessed fewer dendritic branches, whereas dentate granule cells were hypertrophic and formed spiny basilar dendrites from which the principal axon emerged. Both somatostatin- and parvalbumin-containing inhibitory neurons were heterotopic and displaced into both stratum radiatum and stratum lacunosum-moleculare. Mechanisms of synaptic transmission were severely disrupted, revealing hyperexcitability at Schaffer collateral-CA1 synapses and depression of mossy fiber-CA3 transmission. In addition, the dynamic range of frequency-dependent facilitation of Lis1+/- mossy fiber transmission was less than that of wild type. Consequently, Lis1+/- hippocampi are prone to interictal electrographic seizure activity in an elevated [K(+)](o) model of epilepsy. In Lis1+/- hippocampus, intense interictal bursting was observed on elevation of extracellular potassium to 6.5 mM, a condition that resulted in only minimal bursting in wild type. These anatomical and physiological hippocampal defects may provide a neuronal basis for seizures associated with lissencephaly.

Heterozygous mutation or deletion of the beta subunit of platelet-activating factor acetylhydrolase (PAFAH1B1, also known as LIS1) in humans is associated with type I lissencephaly, a severe developmental brain disorder thought to result from abnormal neuronal migration. To further understand the function of PAFAH1B1, we produced three different mutant alleles in mouse Pafah1b1. Homozygous null mice die early in embryogenesis soon after implantation. Mice with one inactive allele display cortical, hippocampal and olfactory bulb disorganization resulting from delayed neuronal migration by a cell-autonomous neuronal pathway. Mice with further reduction of Pafah1b1 activity display more severe brain disorganization as well as cerebellar defects. Our results demonstrate an essential, dosage-sensitive neuronal-specific role for Pafah1b1 in neuronal migration throughout the brain, and an essential role in early embryonic development. The phenotypes observed are distinct from those of other mouse mutants with neuronal migration defects, suggesting that Pafah1b1 participates in a novel pathway for neuronal migration.

We report that a novel sulfonylamino compound, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetam ide (PEPA), selectively potentiates glutamate receptors of the AMPA subtype. PEPA (1-200 microM) dose dependently potentiated glutamate-evoked currents in Xenopus oocytes expressing AMPA (GluRA-GluRD), but not kainate (GluR6 and GluR6+KA2) or NMDA (zeta1 + epsilon1-epsilon4), receptor subunits. PEPA was effective at micromolar concentrations and, in contrast to the action of cyclothiazide, preferentially modulated AMPA receptor flop isoforms. At 200 microM, PEPA potentiated glutamate responses by 50-fold in oocytes expressing GluRCflop (EC50 approximately 50 microM) versus only threefold for GluRCflip; a similar preference for flop isoforms was observed for other AMPA receptor subunits. Dose-response analysis for GluRCflop revealed that 100 microM PEPA produced a sevenfold increase in AMPA receptor affinity for glutamate. PEPA produced considerably weaker potentiation of kainate-evoked than glutamate-evoked currents, suggesting modulation of the process of receptor desensitization. In human embryonic kidney 293 cells transfected with AMPA receptor subunits, PEPA either abolished or markedly slowed the rate of onset of desensitization and potentiated steady-state equilibrium currents evoked by glutamate with subunit (GluRC >/= GluRD > GluRA) and splice-variant (flop > flip) selectivity similar to that observed in oocytes. Our results show that PEPA is a novel, flop-preferring allosteric modulator of AMPA receptor desensitization at least 100 times more potent than aniracetam.

We analyzed the effects of two growth factors that regulate oligodendrocyte progenitor (O-2A) development on the expression of glutamate receptor (GluR) subunits in cortical O-2A cells. In the absence of growth factors, GluR1 was the AMPA subunit mRNA expressed at the lowest relative level. Basic fibroblast growth factor (bFGF) caused an increase in GluR1 and GluR3 steady-state mRNA levels. Platelet-derived growth factor (PDGF) did not modify the mRNA levels for any of the AMPA subunits but selectively potentiated the effects of bFGF on GluR1 mRNA (4.5-fold increase). The kainate-preferring subunits GluR7, KA1, and KA2 mRNAs were increased by bFGF, but these effects were not modified by cotreatment with PDGF. Nuclear run-on assays demonstrated that PDGF+bFGF selectively increased the rate of GluR1 gene transcription (2.5-fold over control). Western blot analysis showed that GluR1 protein levels were increased selectively (sixfold over control) by PDGF+bFGF. Functional expression was assessed by rapid application of AMPA to cultured cells. AMPA receptor current densities (pA/pF) were increased nearly fivefold in cells treated with PDGF+bFGF, as compared with untreated cells. Further, AMPA receptor channels in cells treated with PDGF+bFGF were more sensitive to voltage-dependent block by intracellular polyamines, as expected from the robust and selective enhancement of GluR1 expression. Our combined molecular and electrophysiological findings indicate that AMPA receptor function can be regulated by growth factor-induced changes in the rate of gene transcription.

AMPA receptor GluRA subunits with mutations at position 750, a residue shown previously to control allosteric regulation by cyclothiazide, were analyzed for modulation of deactivation and desensitization by cyclothiazide, aniracetam, and thiocyanate. Point mutations from Ser to Asn, Ala, Asp, Gly, Gln, Met, Cys, Thr, Leu, Val, and Tyr were constructed in GluRAflip. The last four of these mutants were not functional; S750D was active only in the presence of cyclothiazide, and the remaining mutants exhibited altered rates of deactivation and desensitization for control responses to glutamate, and showed differential modulation by cyclothiazide and aniracetam. Results from kinetic analysis are consistent with aniracetam and cyclothiazide acting via distinct mechanisms. Our experiments demonstrate for the first time the functional importance of residue 750 in regulating intrinsic channel-gating kinetics and emphasize the biological significance of alternative splicing in the M3-M4 extracellular loop.

1. The kinetics of onset of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor desensitization by glutamate, and the extent of attenuation of AMPA receptor desensitization by cyclothiazide, showed pronounced cell-to-cell variation in cultures of rat hippocampal neurons. Cultures prepared from area CA1 stratum radiatum tended to show weaker modulation by cyclothiazide than cultures prepared from the whole hippocampus. 2. Kinetic analysis of concentration jump responses to glutamate revealed multiple populations of receptors with fast (approximately 400 ms), intermediate (approximately 2-4 s), and slow (> 20 s) time constants for recovery from modulation by cyclothiazide. The amplitudes of these components varied widely between cells, suggesting the existence of at least three populations of AMPA receptor subtypes, the relative density of which varied from cell to cell. 3. The complex patterns of sensitivity to cyclothiazide seen in hippocampal neurons could be reconstituted by assembly of recombinant AMPA receptor subunits generated from cDNAs encoding the flip (i) and flop (o) splice variants of the GluR-A and GluR-B subunits. Recovery from modulation by cyclothiazide was slower for GluR-AiBi and GluR-AoBi than for GluR-AiBo and GluR-AoBo. 4. Coexpression of the flip and flop splice variants of GluR-A, in the absence of GluR-B, revealed that heteromeric AMPA receptors with intermediate sensitivity to cyclothiazide, similar to responses observed for the combinations GluR-AoBi or GluR-AiBo, could be generated independently of the presence of the GluR-B subunit. However, recovery from modulation by cyclothiazide was twofold slower for GluR-AiBi than for homomeric GluR-Ai, indicating that the GluR-A and GluR-B subunits are not functionally equivalent in controlling sensitivity to cyclothiazide. 5. These results demonstrate that AMPA receptors expressed in hippocampal neurons are assembled in a variety of subunit and splice variant combinations that might serve as a mechanism to fine-tune the kinetics of synaptic transmission.

We observed that a transient increase in extracellular potassium concentration (50 mM for 40 s) was sufficient to induce long-term potentiation (LTP) of synaptic transmission in area CA1 of the hippocampal slice. Potassium-induced potentiation of the Schaffer collateral/commissural synapses demonstrated several features characteristic of tetanus-induced LTP: (1) population excitatory post-synaptic potential (EPSP) amplitudes were enhanced to a similar magnitude (on average 70% above baseline) which (2) lasted for more than 20 min; (3) induction was blocked by bath application of the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (D-APV), and (4) was attenuated by reduction of the concentration of calcium in the extracellular medium. Induction of either potassium-induced LTP or tetanus-induced LTP occluded the subsequent expression of the other. Finally, exposure to high potassium in the absence of electrical stimulation was sufficient to induce LTP. Taken together, these data indicate that brief depolarizing stimuli other than tetanus can induce LTP. Because potassium-induced LTP is not restricted to the subset of afferents examined electrophysiologically, such a method could facilitate analyses of the biochemical events underlying both the induction and expression of LTP.