Alexey Semyanov

Tonically active GABAA receptors: modulating gain and maintaining the tone

GABAA receptors not only respond to the local release of GABA from presynaptic terminals, but can also mediate a persistent tonic current. This reflects the activation of high-affinity GABAA receptors by ambient GABA concentrations. Tonic GABAA-receptor-mediated signalling occurs in different brain regions, shows cell-type-specific differences in magnitude and pharmacology, and changes during brain development. Some clues to the adaptive significance of this phenomenon are beginning to emerge: in cerebellar granule cells, it alters the gain of transmission of rate-coded sensory information; in the hippocampus, it acts in a cell-type-specific manner to regulate the excitability of the network. Because tonic conductances can be modulated by changes in GABA release and uptake, and by modulators of high-affinity GABAA receptors including neurosteroids, this phenomenon provides a potentially important new window onto neuronal information processing and pathological states such as epilepsy.

GABA uptake regulates cortical excitability via cell type-specific tonic inhibition.

GABAA receptors can mediate both phasic synaptic inhibition and a persistent tonic form of signaling. We show that, in the presence of intact GABA uptake, guinea pig hippocampal interneurons, but not pyramidal cells, express a tonic GABAA receptor–mediated conductance. This conductance was pharmacologically distinct from spontaneous inhibitory postsynaptic currents (IPSCs). Inhibiting GABA uptake resulted in the expression of a comparable GABAA receptor–mediated tonic conductance in pyramidal cells. Reducing the tonic conductance in interneurons enhanced their excitability and the inhibitory drive to pyramidal cells. These results point to a role for cell type–dependent tonic inhibition in regulating cortical excitability.

Multiple and Plastic Receptors Mediate Tonic GABAA Receptor Currents in the Hippocampus

Persistent activation of GABAA receptors by extracellular GABA (tonic inhibition) plays a critical role in signal processing and network excitability in the brain. In hippocampal principal cells, tonic inhibition has been reported to be mediated by α5-subunit-containing GABAA receptors (α5GABAARs). Pharmacological or genetic disruption of these receptors improves cognitive performance, suggesting that tonic inhibition has an adverse effect on information processing. Here, we show that α5GABAARs contribute to tonic currents in pyramidal cells only when ambient GABA concentrations increase (as may occur during increased brain activity). At low ambient GABA concentrations, activation of δ-subunit-containing GABAA receptors predominates. In epileptic tissue, α5GABAARs are downregulated and no longer contribute to tonic currents under conditions of raised extracellular GABA concentrations. Under these conditions, however, the tonic current is greater in pyramidal cells from epileptic tissue than in pyramidal cells from nonepileptic tissue, implying substitution of α5GABAARs by other GABAA receptor subtypes. These results reveal multiple components of tonic GABAA receptor-mediated conductance that are activated by low GABA concentrations. The relative contribution of these components changes after the induction of epilepsy, implying an adaptive plasticity of the tonic current in the presence of spontaneous seizures.

Modulation of GABAergic Signaling among Interneurons by Metabotropic Glutamate Receptors

Synapses between hippocampal interneurons are an important potential target for modulatory influences that could affect overall network behavior. We report that the selective group III metabotropic receptor agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depresses GABAergic transmission to interneurons more than to pyramidal neurons. The L-AP4-induced depression is accompanied by changes in trial-to-trial variability and paired-pulse depression that imply a presynaptic site of action. Brief trains of stimuli in Schaffer collaterals also depress GABAergic transmission to interneurons. This depression persists when GABA(B) receptors are blocked, is enhanced by blocking glutamate uptake, and is abolished by the group III metabotropic receptor antagonist (alpha-methylserine-O-phosphate (MSOP). The results imply that GABAergic transmission among interneurons is modulated by glutamate spillover from excitatory afferent terminals.

Kainate receptor-dependent axonal depolarization and action potential initiation in interneurons

Kainate receptor agonists are powerful chemoconvulsants and excitotoxins. These properties are in part explained by depolarization of hippocampal principal neurons. However, kainate also depresses evoked inhibitory signals in pyramidal neurons, and promotes spontaneous GABA release from interneurons. The mechanisms underlying these phenomena are not fully understood, nor are the consequences for the inhibitory traffic among interneurons. We report that both the amplitude and the frequency of spontaneous IPSCs recorded in interneurons were enhanced by low concentrations of kainate, but action potential-independent IPSCs were unaffected. In the presence of GABAA receptor antagonists, kainate lowered the threshold for antidromic action potential generation, suggesting that interneuron axons are directly depolarized; this effect was mimicked by synaptically released glutamate. Kainate application also induced spontaneous antidromic action potentials. Axonal receptors are thus important in initiating the intense interneuronal activity triggered by kainate, which in turn influences inhibitory signaling to principal cells.

Outwardly Rectifying Tonically Active GABAA Receptors in Pyramidal Cells Modulate Neuronal Offset, Not Gain

Hippocampal pyramidal cell excitability is regulated both by fast synaptic inhibition and by tonically active high-affinity extrasynaptic GABAA receptors. The impact of tonic inhibition on neuronal gain and offset, and thus on information processing, is unclear. Offset is altered by shunting inhibition, and the gain of a neuronal response to an excitatory input can be modified by changing the level of “background” synaptic noise. Therefore, tonic activation of GABAA receptors would be expected to modulate offset and, in addition, to alter gain through a shunting effect on synaptic noise. Here we show that tonically active GABAA receptors in CA1 pyramidal cells show marked outward rectification, while the peaks of IPSCs exhibit a linear current–voltage relationship. As a result, tonic GABAA receptor-mediated currents have a minimal effect upon subthreshold membrane potential variation due to synaptic noise, but predominantly affect neurons at spiking threshold. Consistent with this, tonic GABAA receptor-mediated currents in pyramidal cells exclusively affect offset and not gain. Modulation of tonically active GABAA receptors by fluctuations in extracellular GABA concentrations or neuromodulators acting on high-affinity receptors potentially provides a powerful mechanism to alter neuronal offset independently of neuronal gain.

Neural Cell Adhesion Molecule-Associated Polysialic Acid Regulates Synaptic Plasticity and Learning by Restraining the Signaling through GluN2B-Containing NMDA Receptors

The neural cell adhesion molecule (NCAM) is the predominant carrier of α2,8 polysialic acid (PSA) in the mammalian brain. Abnormalities in PSA and NCAM expression are associated with schizophrenia in humans and cause deficits in hippocampal synaptic plasticity and contextual fear conditioning in mice. Here, we show that PSA inhibits opening of recombinant NMDA receptors composed of GluN1/2B (NR1/NR2B) or GluN1/2A/2B (NR1/NR2A/NR2B) but not of GluN1/2A (NR1/NR2A) subunits. Deficits in NCAM/PSA increase GluN2B-mediated transmission and Ca2+ transients in the CA1 region of the hippocampus. In line with elevation of GluN2B-mediated transmission, defects in long-term potentiation in the CA1 region and contextual fear memory in NCAM/PSA-deficient mice are abrogated by application of a GluN2B-selective antagonist. Furthermore, treatment with the glutamate scavenger glutamic-pyruvic transaminase, ablation of Ras-GRF1 (a mediator of GluN2B signaling to p38 MAPK), or direct inhibition of hyperactive p38 MAPK can restore impaired synaptic plasticity in brain slices lacking PSA/NCAM. Thus, PSA carried by NCAM regulates plasticity and learning by inhibition of the GluN2B-Ras-GRF1-p38 MAPK signaling pathway. These findings implicate carbohydrates carried by adhesion molecules in modulating NMDA receptor signaling in the brain and demonstrate reversibility of cognitive deficits associated with ablation of a schizophrenia-related adhesion molecule.

Cholinergic Axons Modulate GABAergic Signaling among Hippocampal Interneurons via Postsynaptic α7 Nicotinic Receptors

Homopentameric α7 nicotinic receptors have a high affinity for acetylcholine (ACh), are permeable to Ca2+ ions, and are abundant in hippocampal interneurons. Although nicotinic agonists evoke inward currents and Ca2+ transients in stratum radiatum interneurons, the role of endogenous ACh in modulating synaptic integration by interneurons is incompletely understood. Many cholinergic axonal varicosities do not have postsynaptic specializations, but α7 receptors frequently occur close to synaptic GABAA receptors. These observations raise the possibility that α7 nicotinic receptors activated by ACh released from cholinergic axons modulate GABAergic transmission in interneurons. We show that agonists of α7 receptors profoundly depress GABAergic IPSCs recorded in stratum radiatum interneurons in the CA1 region of the hippocampus. This depression is accompanied by a small increase in GABA release. α7 nicotinic receptor agonists also depress GABA- or muscimol-evoked currents in interneurons, indicating that the major effect is a postsynaptic modulation of GABAA receptors. The depression of GABA-evoked currents is abolished by chelating Ca2+ in the recorded interneuron and attenuated by inhibitors of PKC. We also show that stimuli designed to release endogenous ACh from cholinergic axons evoke an α7 receptor-dependent heterosynaptic depression of GABAergic IPSCs in interneurons. This heterosynaptic modulation is amplified by blocking cholinesterases. These results reveal a novel mechanism by which cholinergic neurons modulate information processing in the hippocampus.

Glutamatergic Modulation of GABAergic Signaling Among Hippocampal Interneurons: Novel Mechanisms Regulating Hippocampal Excitability

Summary: u2002Purpose: Because interneurons play a central role in regulating the excitability of the hippocampal formation, it is important to understand the mechanisms that modulate γ‐aminobutyric acid (GABA)ergic signaling among them. This study addresses the modulation of GABA release from interneuron terminals by presynaptic glutamate receptors.

Relative picrotoxin insensitivity distinguishes ionotropic GABA receptor-mediated IPSCs in hippocampal interneurons.

Inhibitory GABAergic signalling in the hippocampus plays an important role in synchronizing principal cells and regulating the excitability of this seizure-prone structure. Distinct mechanisms modulate release from GABAergic terminals in the hippocampus, depending on whether the postsynaptic partner is an interneuron or a principal cell. Here, we report that postsynaptic ionotropic GABA receptors in principal cells and interneurons also show a striking pharmacological difference. The broad-spectrum antagonist picrotoxin (PTX) was less potent at blocking IPSCs evoked in stratum radiatum interneurons than in pyramidal neurons in the CA1 region. GABA-evoked currents in membrane patches from interneurons showed a smaller mean unitary conductance than in patches from pyramidal neurons. Because retinal GABA(C) receptors show decreased picrotoxin sensitivity and conductance, we examined the effect of the GABA(C) receptor agonist cis-aminocrotonic acid (CACA). Although this agent evoked picrotoxin-resistant currents in interneurons, these were enhanced by the GABA(A) allosteric modulator pentobarbital. Moreover, both picrotoxin-resistant IPSCs and CACA-evoked currents were blocked by the GABA(A) receptor-selective antagonist bicuculline. The presence of relatively picrotoxin-resistant GABA(A) receptors in interneurons provides a potential target for agents to modulate the activity of sub-populations of hippocampal neurons.