Endogenous GABA activates small-conductance K+ channels underlying slow IPSCs in rat hippocampal neurons

Interplay of Entorhinal Input and Local Inhibitory Network in the Hippocampus at the Origin of Slow Inhibition in Granule Cells

Neuronal activity from the entorhinal cortex propagates through the perforant path (PP) to the molecular layer of the dentate gyrus (DG) where information is filtered and converted into sparse hippocampal code. Nearly simultaneous signaling to both granule cells (GC) and local interneurons (INs) engages network interactions that will modulate input integration and output generation. When triggered, GABA release from interneurons counteracts the glutamatergic signals of PP terminals, scaling down the overall DG activation. Inhibition occurs at fast or slow timescales depending on the activation of ionotropic GABA_A-R or metabotropic GABA_B-R. Although postsynaptic GABA_A and GABA_B-R differ in their location at the synapse, mixed GABA_A/B-R IPSPs can also occur. Here we describe a slow inhibition mechanism in mouse GCs recorded from either sex, mediated by GABA_A/B-R in combination with metabotropic glutamate receptors. Short burst PP stimulation in the gamma frequency range lead to a long-lasting hyperpolarization (LLH) of the GCs with a duration that exceeds GABA_B-R IPSPs. As a result, LLH alters GC firing patterns and the responses to concomitant excitatory signals are also affected. Synaptic recruitment of feedforward inhibition and subsequent GABA release from interneurons, also successfully trigger mixed GABA responses in GCs. Together these results suggest that slow inhibition through LLH leads to reduced excitability of GCs during entorhinal input integration. The implication of LLH in regulation of neuronal excitability suggests it also contributes to the sparse population coding in DG.SIGNIFICANCE STATEMENT Our study describes a long-lasting hyperpolarization (LLH) in hippocampal granule cells. We used whole-cell patch-clamp recordings and an optogenetic approach to characterize this event. LLH is a slow inhibitory mechanism that occurs following the stimulation of the perforant pathway in the molecular layer of the dentate gyrus. We found that it is mediated via postsynaptic ionotropic and metabotropic GABA and metabotropic glutamate receptors. The duration of LLH exceeds previously described IPSPs mediated by any of these receptors. The activation of LLH requires presynaptic gamma frequency bursts and recruitment of the local feedforward inhibition. LLH defines prolonged periods of low excitability of GCs and a restrained neuronal discharge. Our results suggest that LLH can contribute to sparse activation of GCs.

GABAB receptors modulate Ca2+ but not G protein-gated inwardly rectifying K+ channels in cerebrospinal-fluid contacting neurones of mouse brainstem

KEY POINTS

Medullo-spinal CSF contacting neurones (CSF-cNs) located around the central canal are conserved in all vertebrates and suggested to be a novel sensory system intrinsic to the CNS. CSF-cNs receive GABAergic inhibitory synaptic inputs involving ionotropic GABA_A receptors, but the contribution of metabotropic GABA_B receptors (GABA_B -Rs) has not yet been studied. Here, we indicate that CSF-cNs express functional GABA_B -Rs that inhibit postsynaptic calcium channels but fail to activate inhibitory potassium channel of the Kir3-type. We further show that GABA_B -Rs localise presynaptically on GABAergic and glutamatergic synaptic inputs contacting CSF-cNs, where they inhibit the release of GABA and glutamate. Our data are the first to address the function of GABA_B -Rs in CSF-cNs and show that on the presynaptic side they exert a classical synaptic modulation whereas at the postsynaptic level they have an atypical action by modulating calcium signalling without inducing potassium-dependent inhibition.

ABSTRACT

Medullo-spinal neurones that contact the cerebrospinal fluid (CSF-cNs) are a population of evolutionary conserved cells located around the central canal. CSF-cN activity has been shown to be regulated by inhibitory synaptic inputs involving ionotropic GABA_A receptors, but the contribution of the G-protein coupled GABA_B receptors has not yet been studied. Here, we used a combination of immunofluorescence, electrophysiology and calcium imaging to investigate the expression and function of GABA_B -Rs in CSF-cNs of the mouse brainstem. We found that CSF-cNs express GABA_B -Rs, but their selective activation failed to induce G protein-coupled inwardly rectifying potassium (GIRK) currents. Instead, CSF-cNs express primarily N-type voltage-gated calcium (Ca_V 2.2) channels, and GABA_B -Rs recruit Gβγ subunits to inhibit Ca_V channel activity induced by membrane voltage steps or under physiological conditions by action potentials. Moreover, using electrical stimulation, we indicate that GABAergic inhibitory (IPSCs) and excitatory glutamatergic (EPSCs) synaptic currents can be evoked in CSF-cNs showing that mammalian CSF-cNs are also under excitatory control by glutamatergic synaptic inputs. We further demonstrate that baclofen reversibly reduced the amplitudes of both IPSCs and EPSCs evoked in CSF-cNs through a presynaptic mechanism of regulation. In summary, these results are the first to demonstrate the existence of functional postsynaptic GABA_B -Rs in medullar CSF-cNs, as well as presynaptic GABA_B auto- and heteroreceptors regulating the release of GABA and glutamate. Remarkably, postsynaptic GABA_B -Rs associate with Ca_V but not GIRK channels, indicating that GABA_B -Rs function as a calcium signalling modulator without GIRK-dependent inhibition in CSF-cNs.

Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells

Sparse neural activity in the dentate gyrus is enforced by powerful networks of inhibitory GABAergic interneurons in combination with low intrinsic excitability of the principal neurons, the dentate granule cells (GCs). Although the cellular and circuit properties that dictate synaptic inhibition are well studied, less is known about mechanisms that confer low GC intrinsic excitability. Here we demonstrate that intact G protein-mediated signaling contributes to the characteristic low resting membrane potential that differentiates mature dentate GCs from CA1 pyramidal cells and developing adult-born GCs. In mature GCs from male and female mice, intact G protein signaling robustly reduces intrinsic excitability, whereas deletion of G protein-activated inwardly rectifying potassium channel 2 (GIRK2) increases excitability and blocks the effects of G protein signaling on intrinsic properties. Similarly, pharmacological manipulation of GABA_B receptors (GABA_BRs) or GIRK channels alters intrinsic excitability and GC spiking behavior. However, adult-born new GCs lack functional GIRK activity, with phasic and constitutive GABA_BR-mediated GIRK signaling appearing after several weeks of maturation. Phasic activation is interneuron specific, arising primarily from nNOS-expressing interneurons rather than parvalbumin- or somatostatin-expressing interneurons. Together, these results demonstrate that G protein signaling contributes to the intrinsic excitability that differentiates mature and developing dentate GCs and further suggest that late maturation of GIRK channel activity is poised to convert early developmental functions of GABA_B receptor signaling into GABA_BR-mediated inhibition.SIGNIFICANCE STATEMENT The dentate gyrus exhibits sparse neural activity that is essential for the computational function of pattern separation. Sparse activity is ascribed to strong local inhibitory circuits in combination with low intrinsic excitability of the principal neurons, the granule cells. Here we show that constitutive activity of G protein-coupled inwardly rectifying potassium channels (GIRKs) underlies to the hallmark low resting membrane potential and input resistance of mature dentate neurons. Adult-born neurons initially lack functional GIRK channels, with constitutive and phasic GABA_B receptor-mediated GIRK inhibition developing in tandem after several weeks of maturation. Our results reveal that GABA_B/GIRK activity is an important determinant of low excitability of mature dentate granule cells that may contribute to sparse DG activity in vivo.

KCTD Hetero-oligomers Confer Unique Kinetic Properties on Hippocampal GABAB Receptor-Induced K+ Currents

GABA_B receptors are the G-protein coupled receptors for the main inhibitory neurotransmitter in the brain, GABA. GABA_B receptors were shown to associate with homo-oligomers of auxiliary KCTD8, KCTD12, KCTD12b, and KCTD16 subunits (named after their T1 K⁺-channel tetramerization domain) that regulate G-protein signaling of the receptor. Here we provide evidence that GABA_B receptors also associate with hetero-oligomers of KCTD subunits. Coimmunoprecipitation experiments indicate that two-thirds of the KCTD16 proteins in the hippocampus of adult mice associate with KCTD12. We show that the KCTD proteins hetero-oligomerize through self-interacting T1 and H1 homology domains. Bioluminescence resonance energy transfer measurements in live cells reveal that KCTD12/KCTD16 hetero-oligomers associate with both the receptor and the G-protein. Electrophysiological experiments demonstrate that KCTD12/KCTD16 hetero-oligomers impart unique kinetic properties on G-protein-activated Kir3 currents. During prolonged receptor activation (one min) KCTD12/KCTD16 hetero-oligomers produce moderately desensitizing fast deactivating K⁺ currents, whereas KCTD12 and KCTD16 homo-oligomers produce strongly desensitizing fast deactivating currents and nondesensitizing slowly deactivating currents, respectively. During short activation (2 s) KCTD12/KCTD16 hetero-oligomers produce nondesensitizing slowly deactivating currents. Electrophysiological recordings from hippocampal neurons of KCTD knock-out mice are consistent with these findings and indicate that KCTD12/KCTD16 hetero-oligomers increase the duration of slow IPSCs. In summary, our data demonstrate that simultaneous assembly of distinct KCTDs at the receptor increases the molecular and functional repertoire of native GABA_B receptors and modulates physiologically induced K⁺ current responses in the hippocampus.

SIGNIFICANCE STATEMENT

The KCTD proteins 8, 12, and 16 are auxiliary subunits of GABA_B receptors that differentially regulate G-protein signaling of the receptor. The KCTD proteins are generally assumed to function as homo-oligomers. Here we show that the KCTD proteins also assemble hetero-oligomers in all possible dual combinations. Experiments in live cells demonstrate that KCTD hetero-oligomers form at least tetramers and that these tetramers directly interact with the receptor and the G-protein. KCTD12/KCTD16 hetero-oligomers impart unique kinetic properties to GABA_B receptor-induced Kir3 currents in heterologous cells. KCTD12/KCTD16 hetero-oligomers are abundant in the hippocampus, where they prolong the duration of slow IPSCs in pyramidal cells. Our data therefore support that KCTD hetero-oligomers modulate physiologically induced K⁺ current responses in the brain.

Visual experience prevents dysregulation of GABAB receptor-dependent short-term depression in adult superior colliculus

Progressive loss of plasticity during development prevents refined circuits from regressing to an immature state and is thought to depend on maturation of GABAergic inhibition. For example, a gradual reduction in size of visual receptive fields (RFs) occurs in the superior colliculus (SC) during development. Maintenance of the refined state throughout adulthood requires early light exposure. Here we investigate the potential role of changes in long- or short-term plasticity in experience-dependent maintenance of refined RFs. Using an acute SC slice preparation, we found that long-term plasticity was not affected by visual deprivation, indicating that it does not underlie deprivation-induced RF enlargement. In contrast, visual deprivation altered short-term plasticity in an unexpected way. Specifically, GABAB receptor (GABABR)-mediated paired pulse depression was increased in slices from dark-reared animals. This increase was mimicked by GABAAR blockade in slices from normally reared animals, suggesting that experience-dependent maintenance of GABAAR function prevents an increase in probability of neurotransmitter release. GABABR-mediated short-term depression in response to strong stimulation (such as occurs during vision) was reduced in slices from dark-reared animals. This change was mimicked in slices from normal animals by reducing GABA release. These results are consistent with the hypothesis that early visual experience maintains GABAergic inhibition and prevents later deprivation-induced alterations of short-term depression in SC. Identifying how plasticity is restricted in mature circuits could guide therapies to enhance recovery of function in adults.

A novel GABAergic action mediated by functional coupling between GABAB-like receptor and two different high-conductance K+ channels in cricket Kenyon cells

The γ-aminobutyric acid type B (GABA(B)) receptor has been shown to attenuate high-voltage-activated Ca(2+) currents and enhance voltage-dependent or inwardly rectifying K(+) currents in a variety of neurons. In this study, we report a novel coupling of GABA(B)-like receptor with two different high-conductance K(+) channels, Na(+)-activated K(+) (K(Na)) channel and Ca(2+)-activated K(+) (K(Ca)) channel, in Kenyon cells isolated from the mushroom body of the cricket brain. Single-channel activities of K(Na) and K(Ca) channels in response to bath applications of GABA and the GABA(B)-specific agonist SKF97541 were recorded with the cell-attached patch configuration. The open probability (P(o)) of both K(Na) and K(Ca) channels was found to be increased by bath application of GABA, and this increase in Po was antagonized by coapplication of the GABAB antagonist CGP54626, suggesting that GABA(B)-like receptors mediate these actions. Similarly, GABA(B)-specific agonist SKF97541 increased the Po of both K(Na) and K(Ca) channels. Perforated-patch recordings using β-escin further revealed that SKF97541 increased the amplitude of the outward currents elicited by step depolarizations. Under current-clamp conditions, SKF97541 decreased the firing frequency of spontaneous action potential (AP) and changed the AP waveform. The amplitude and duration of AP were decreased, whereas the afterhyperpolarization of AP was increased. Resting membrane potential, however, was not significantly altered by SKF97541. Taken together, these results suggest that GABA(B)-like receptor is functionally coupled with both K(Na) and K(Ca) channels and this coupling mechanism may serve to prevent AP formation and limit excitatory synaptic input.

Increased efficiency of the GABAA and GABAB receptor-mediated neurotransmission in the Ts65Dn mouse model of Down syndrome

Cognitive impairment in Down syndrome (DS) involves the hippocampus. In the Ts65Dn mouse model of DS, deficits in hippocampus-dependent learning and synaptic plasticity were linked to enhanced inhibition. However, the mechanistic basis of changes in inhibitory efficiency remains largely unexplored, and efficiency of the GABAergic synaptic neurotransmission has not yet been investigated in direct electrophysiological experiments. To investigate this important feature of neurobiology of DS, we examined synaptic and molecular properties of the GABAergic system in the dentate gyrus (DG) of adult Ts65Dn mice. Both GABAA and GABAB receptor-mediated components of evoked inhibitory postsynaptic currents (IPSCs) were significantly increased in Ts65Dn vs. control (2N) DG granule cells. These changes were unaccompanied by alterations in hippocampal levels of GABAA (α1, α2, α3, α5 and γ2) or GABAB (Gbr1a and Gbr1b) receptor subunits. Immunoreactivity for GAD65, a marker for GABAergic terminals, was also unchanged. In contrast, there was a marked change in functional parameters of GABAergic synapses. Paired stimulations showed reduced paired-pulse ratios of both GABAA and GABAB receptor-mediated IPSC components (IPSC2/IPSC1), suggesting an increase in presynaptic release of GABA. Consistent with increased gene dose, the level of the Kir3.2 subunit of potassium channels, effectors for postsynaptic GABAB receptors, was increased. This change was associated with enhanced postsynaptic GABAB/Kir3.2 signaling following application of the GABAB receptor agonist baclofen. Thus, both GABAA and GABAB receptor-mediated synaptic efficiency is increased in the Ts65Dn DG, thus likely contributing to deficient synaptic plasticity and poor learning in DS.

Recurrent collateral connections of striatal medium spiny neurons are disrupted in models of Parkinson's disease

The principal neurons of the striatum, GABAergic medium spiny neurons (MSNs), are interconnected by local recurrent axon collateral synapses. Although critical to many striatal models, it is not clear whether these connections are random or whether they preferentially link functionally related groups of MSNs. To address this issue, dual whole patch-clamp recordings were made from striatal MSNs in brain slices taken from transgenic mice in which D(1) or D(2) dopamine receptor expression was reported with EGFP (enhanced green fluorescent protein). These studies revealed that unidirectional connections were common between both D(1) receptor-expressing MSN (D(1) MSN) pairs (26%) and D(2) receptor-expressing MSN (D(2) MSN) pairs (36%). D(2) MSNs also commonly formed synapses on D(1) MSNs (27% of pairs). Conversely, only 6% of the D(1) MSNs formed detectable connections with D(2) MSNs. Furthermore, synaptic connections formed by D(1) MSNs were weaker than those formed by D(2) MSNs, a difference that was attributable to fewer GABA(A) receptors at D(1) MSN synapses. The strength of detectable recurrent connections was dramatically reduced in Parkinson's disease models. The studies demonstrate that recurrent collateral connections between MSNs are not random but rather differentially couple D(1) and D(2) MSNs. Moreover, this recurrent collateral network appears to be disrupted in Parkinson's disease models, potentially contributing to pathological alterations in MSN activity patterns and psychomotor symptoms.

Isoniazid-induced reduction in GABAergic neurotransmission alters the function of the cerebellar cortical circuit

The cerebellar cortex contributes to the control of movement, coordination, and certain cognitive functions. The cerebellar network is composed of five different types of neurons that are wired together in a repetitive module. Given that four of these five neurons synthesize and release GABA, this inhibitory neurotransmitter plays a central role in regulation of the excitability and correct functioning of the cerebellar cortex. We have now used isoniazid, an inhibitor of glutamic acid decarboxylase, the enzyme responsible for the synthesis of GABA, to evaluate the contribution of GABAergic transmission in different types of cerebellar cortical neurons to the functioning of the cerebellar circuit. Parasagittal cerebellar slices were prepared from 28- to 40-day-old male rats and were subjected to patch-clamp recording in the voltage- or current-clamp mode. Exposure of the tissue slices to isoniazid (10 mM) resulted in a decrease in the level of GABAergic transmission in Purkinje cells and a consequent increase in the firing rate of spontaneous action potentials that was apparent after 40 min. In granule neurons, isoniazid reduced both tonic and phasic GABAergic currents and thereby altered the flow of information across the cerebellar cortex. Our data support the notion that the amount of GABA at the synaptic level is a major determinant of the excitability of the cerebellar cortex, and they suggest that isoniazid may be a useful tool with which to study the function of the cerebellar network.

Optimizing fluctuation analysis of GABAAergic IPSCs for accurate unitary currents

Nonstationary fluctuation analysis of synaptic currents requires division of currents into bins of time, with little agreement on how to select an optimal bin width. We used simulated inhibitory postsynaptic currents (simIPSCs) in an empirical approach to establish the optimal bin width needed for estimation of the unitary current, ie. We found acceptable accuracy (< or = 5%) at bin widths shorter than the length of the stationary segment of simIPSCs that persisted when Gaussian noise was added to the simulated currents. We also studied evoked and spontaneous IPSCs mediated by receptors for gamma-aminobutyrate (GABA) in thalamic neurons. Similar to simIPSCs, analysis of the IPSCs yielded saturating relationships between bin width and accuracy of unitary current estimate. Whereas standard error decreased, the accuracy of ie estimates increased with decreasing bin width, forming a plateau at bins below 2-3 ms in duration. Using this approach, one can reliably determine the optimal bin width for nonstationary noise analysis.

Analysis of the excitatory and inhibitory components of postsynaptic currents recorded in pyramidal neurons and interneurons in the rat hippocampus

Postsynaptic currents recorded from interneurons and pyramidal cells in hippocampal slices by local voltage clamping were found to be the sum of excitatory (EPSC) and inhibitory (IPSC) components. An approach allowing quantitative assessment of the amplitude and time course of EPSC and IPSC without pharmacological blockade of the major postsynaptic receptors involved in generating these currents was developed. The approach is based on the existence of a significant difference between reversion potentials of cationic and anionic currents and the presence of a linear zone in the voltage-current characteristics of responses to excitatory and inhibitory transmitters. Comparison of the results of this calculation-based method with those of classical pharmacological analysis of the excitatory and inhibitory components of postsynaptic currents showed them to be virtually identical, which allows synaptic currents in defined neurons to be studied without altering the state of synaptic connections throughout the brain slice. IPSC was found to make a smaller contribution to the total postsynaptic current recorded in interneurons as compared with pyramidal neurons in rat hippocampal field CA1.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Distribution and properties of functional postsynaptic kainate receptors on neocortical layer V pyramidal neurons

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic transmission and signal processing. In the present study we investigated the location and properties of functional kainate receptors (KARs) on the somatodendritic membrane of rat neocortical layer V pyramidal neurons. Infrared-guided laser stimulation was used to apply glutamate photolytically to the soma and various sites along the apical dendrite. Electrical currents, resulting from the activation of pharmacologically isolated KARs, were measured by whole-cell patch-clamp recording. In addition, KARs on somatic and dendritic outside-out patches were activated while still within the brain tissue. We found that functional KARs are located on the entire somatodendritic membrane that was examined. Fast kinetics, a linear I-V relationship, and a relatively high single-channel conductance are characteristic features of these receptors. We provide evidence that the unitary properties of somatic and dendritic KARs are identical. Regarding the subcellular distribution of KARs, our results indicate that the density of these receptors increases toward the distal dendrite. They are located mainly at extrasynaptic sites but also mediate fast synaptic signaling triggered by afferent stimulation. The differential distribution speaks in favor of a selective targeting of KARs on central neurons and may reflect a mechanism for a location-dependent regulation of synaptic efficacy. Furthermore, it is feasible to assume that extrasynaptic KARs could be activated by a "spillover" of synaptically released glutamate, ambient glutamate in the CSF, or glutamate released from adjacent astrocytes.

GABABR1 receptor protein expression in human mesial temporal cortex: changes in temporal lobe epilepsy

Immunocytochemistry was used to examine gamma-aminobutyric acid beta (GABA)(B)R1a-b protein expression in the human hippocampal formation (including dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex) and perirhinal cortex. Overall, GABA(B)R1a-b immunostaining was intense and widespread but showed differential areal and laminar distributions of labeled cells. GABA(B)R1a-b-immunoreactive (-ir) neurons were found in the three main layers of the dentate gyrus, the most intense labeling being present in the polymorphic layer, whereas the granule cells were moderately immunoreactive. Except for slight variations, similar distribution patterns of GABA(B)R1a-b immunostaining were found along the different subfields of the Ammon's horn (CA1-CA4). The highest density of GABA(B)R1a-b-ir neurons was localized in the stratum pyramidale, where virtually every pyramidal cell was intensely immunoreactive, including the proximal part of the apical dendrites. Within the subicular complex, a more intense GABA(B)R1a-b immunostaining was found in the subiculum than in the presubiculum or parasubiculum, especially in the pyramidal and polymorphic cell layers. In the entorhinal cortex, distribution of GABA(B)R1a-b immunoreactivity was localized mainly in both pyramidal and nonpyramidal cells of layers II, III, and VI and in the superficial part of layer V, with layers I, IV, and deep layer V being less intensely stained. In the perirhinal cortex, the most intense GABA(B)R1a-b immunoreactivity was located in the deep part of layer III and in layer V and was mainly confined to medium-sized and large pyramidal cells. Thus, the differential expression, but widespread distribution, of GABA(B)R1a-b protein found in the present study suggests the involvement of GABA(B) receptors in many circuits of the human hippocampal formation and adjacent cortical structures. Interestingly, the hippocampal formation of epileptic patients (n = 8) with hippocampal sclerosis showed similar intensity of GABA(B)R1a-b immunostaining in the surviving neurons located within or adjacent to those regions presenting neuronal loss than in the controls. However, surviving neurons in the granule cell layer of the dentate gyrus displayed a significant reduction in immunostaining in 7 of 8 patients. Therefore, alterations in inhibitory synaptic transmission through GABA(B) receptors appears to affect differentially certain hippocampal circuits in a population of epileptic patients. This reduction in GABA(B)R1a-b expression could contribute to the pathophysiology of temporal lobe epilepsy.

GABAB-Receptor-mediated currents in interneurons of the dentate-hilus border

GABA(B)-receptor-mediated inhibition was investigated in anatomically identified inhibitory interneurons located at the border between the dentate gyrus granule cell layer and hilus. Biocytin staining was used to visualize the morphology of recorded cells. A molecular layer stimulus evoked a pharmacologically isolated slow inhibitory postsynaptic current (IPSC), recorded with whole cell patch-clamp techniques, in 55 of 63 interneurons. Application of the GABA(B) receptor antagonists, CGP 35348 (400 microM) or CGP 55845 (1 microM) to a subset of 25 interneurons suppressed the slow IPSC by an amount ranging from 10 to 100%. In 56% of these cells, the slow IPSC was entirely GABA(B)-receptor-mediated. However, in the remaining interneurons, a component of the slow IPSC was resistant to GABA(B) antagonists. Subtraction of this antagonist resistant current from the slow IPSC isolated the GABA(B) component (IPSC(B)). This IPSC(B) had a similar onset and peak latency to that recorded from granule cells but a significantly shorter duration. The GABA(B) agonist, baclofen (10 microM), produced a CGP 55845-sensitive outward current in 19 of 27 interneurons. In the eight cells that lacked a baclofen current, strong or repetitive ML stimulation also failed to evoke an IPSC(B), indicating that these cells lacked functional GABA(B) receptor-activated potassium currents. In cells that expressed a baclofen current, the amplitude of this current was approximately 50% smaller in interneurons with axons that projected into the granule cell dendritic layer (22.2 +/- 5.3 pA; mean +/- SE) than in interneurons with axons that projected into or near the granule cell body layer (46.1 +/- 10.0 pA). Similarly, the IPSC(B) amplitude was smaller in interneurons projecting to dendritic (9.4 +/- 2.7 pA) than perisomatic regions (34.3 +/- 5.1 pA). These findings suggest that GABA(B) inhibition more strongly regulates interneurons with axons that project into perisomatic than dendritic regions. To determine the functional role of GABA(B) inhibition, we examined the effect of IPSP(B) on action potential firing and synaptic excitation of these interneurons. IPSP(B) and IPSP(A) both suppressed depolarization-induced neuronal firing. However, unlike IPSP(A), suppression of firing by IPSP(B) could be easily overcome with strong depolarization. IPSP(B) markedly suppressed N-methyl-D-aspartate but not AMPA EPSPs, suggesting that GABA(B) inhibition may play a role in regulating slow synaptic excitation of these interneurons. Heterogeneous expression of GABA(B) currents in hilar border interneurons therefore may provide a mechanism for the differential regulation of excitation of these cells and thereby exert an important role in shaping neuronal activity in the dentate gyrus.

Variance analysis of current fluctuations of adenosine- and baclofen-activated GIRK channels in dissociated neocortical pyramidal cells

Whole cell recordings were obtained from pyramidal cell somata acutely isolated from rat neocortex. In voltage-clamp mode, adenosine (0.3-1000 microM), and the GABA(B) receptor agonist, baclofen (1-300 microM), induced K+ current responses mediated by G protein-activated inwardly rectifying K+ (GIRK) channels. In our preparation, adenosine activated GIRK currents with an average EC(50) of 2 microM. Baclofen had an average EC50 of 26 microM. To estimate and compare unitary conductance and density of GIRK channels activated by either adenosine or baclofen, we performed variance analysis of current fluctuations associated with the application of the two agonists at increasing concentrations. Irrespective of the agonist tested, GIRK channels displayed an average single-channel conductance of 25 pS at our recording conditions ([K+]o: 60 mM). Assuming that GIRK channel conductance increases in proportion to the square root of [K+]o, this would translate into 5-6 pS at physiological ion gradients. GIRK channels activated by adenosine or baclofen were not only identical in terms of unitary conductance, they also displayed the same average density of 0.5 channels micron(-2) for both agonists. Our data strongly suggest that the two compounds recruit the same type of channel and thus most likely share a common transduction and effector system.

Distinct GABAB actions via synaptic and extrasynaptic receptors in rat hippocampus in vitro

Intracellular recordings were obtained from pyramidal cells to examine gamma-aminobutyric acid-B (GABAB)-mediated synaptic mechanisms in the CA1 region of rat hippocampal slices. To investigate if heterogeneous ionic mechanisms linked to GABAB receptors originate from distinct sets of inhibitory fibers, GABAB-mediated monosynaptic late inhibitory postsynaptic potentials (IPSPs) were elicited in the presence of antagonists of ionotropic glutamate and GABAA receptors and of an inhibitor of GABA uptake and were compared after direct stimulation of inhibitory fibers in three different CA1 layers: stratum oriens, radiatum, and lacunosum-moleculare. No significant differences were found in mean amplitude, rise time, or time to decay to half-amplitude of IPSPs evoked from the three layers. Mean equilibrium potential (Erev) of late IPSPs was similar for all groups and close to the equilibrium potential of K+. Bath application of the GABAB antagonist CGP55845A blocked all monosynaptic late IPSPs. During recordings with micropipettes containing guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS), the mean amplitude of all GABAB IPSPs gradually was reduced. Bath application of Ba2+ completely eliminated monosynaptic late IPSPs evoked from any of the stimulation sites. Late IPSPs were blocked completely during Ba2+ applications that reduced the GABAB-mediated hyperpolarizations elicited by local application of exogenous GABA only by approximately 50%. These results indicate that heterogenous K+ conductances activated by GABAB receptors do not originate from separate sets of inhibitory fibers in these layers. To examine if synchronous release of GABA from a larger number of inhibitory fibers could activate heterogeneous GABAB mechanisms, giant GABAB IPSPs were induced by 4-aminopyridine (4-AP) in the presence of antagonists of ionotropic glutamate and GABAA receptors. The amplitude and time course 4-AP-induced late IPSPs were approximately double that of evoked monosynaptic late IPSPs, but their voltage sensitivity, Erev, and antagonism by the GABAB antagonist CGP55845A and intracellular GTPgammaS were similar. Ba2+ completely abolished 4-AP-induced late IPSPs, whereas responses elicited by exogenous GABA were only reduced by approximately 50% in the same cells. These results indicate that synchronous activation of large numbers of inhibitory fibers, as induced by 4-AP, may not activate heterogenous GABAB-mediated conductances. Similarly, Ba2+ almost completely blocked late inhibitory postsynaptic currents evoked by stimulus trains. Overall, our results show that exogenous GABA can activate heterogenous K+ conductances via GABAB receptors, but that GABA released synaptically, either by electrical stimulation or 4-AP application, can only activate K+ conductances homogeneously sensitive to Ba2+. Thus GABAB receptors located at synaptic and extrasynaptic sites on hippocampal pyramidal cells may be linked to distinct K+ conductances.

GABAB receptors: drugs meet clones

Recently, the long-awaited cloning of the GABAB receptors, the last of the major known neurotransmitter receptors to be identified, has been reported. In addition to an emerging molecular understanding, there have been advances in discerning the specific coupling partners of GABAB receptors in the brain.