Rapid desensitization of alpha 1 beta 1 GABA A receptors expressed in Sf9 cells under optimized conditions

Exocytosis from pancreatic β-cells: mathematical modelling of the exit of low-molecular-weight granule content

Pancreatic β-cells use Ca(2+)-dependent exocytosis of large dense core vesicles to release insulin. Exocytosis in β-cells has been studied biochemically, biophysically and optically. We have previously developed a biophysical method to monitor release of endogenous intragranular constituents that are co-released with insulin. This technique involves the expression of ionotropic membrane receptors in the β-cell plasma membrane and enables measurements of exocytosis of individual vesicles with sub-millisecond resolution. Like carbon fibre amperometry, this method allows fine details of the release process, like the expansion of the fusion pore (the narrow connection between the granule lumen and the extracellular space), to be monitored. Here, we discuss experimental data obtained with this method within the framework of a simple mathematical model that describes the release of low-molecular constituents during exocytosis of the insulin granules. Our findings suggest that the fusion pore functions as a molecular sieve, allowing differential release of low- and high-molecular-weight granule constituents.

Corelease and differential exit via the fusion pore of GABA, serotonin, and ATP from LDCV in rat pancreatic beta cells

The release of γ-aminobutyric acid (GABA) and ATP from rat β cells was monitored using an electrophysiological assay based on overexpression GABA_A or P₂X₂ receptor ion channels. Exocytosis of LDCVs, detected by carbon fiber amperometry of serotonin, correlated strongly (∼80%) with ATP release. The increase in membrane capacitance per ATP release event was 3.4 fF, close to the expected capacitance of an individual LDCV with a diameter of 0.3 μm. ATP and GABA were coreleased with serotonin with the same probability. Immunogold electron microscopy revealed that ∼15% of the LDCVs contain GABA. Prespike “pedestals,” reflecting exit of granule constituents via the fusion pore, were less frequently observed for ATP than for serotonin or GABA and the relative amplitude (amplitude of foot compared to spike) was smaller: in some cases the ATP-dependent pedestal was missing entirely. An inward tonic current, not dependent on glucose and inhibited by the GABA_A receptor antagonist SR95531, was observed in β cells in clusters of islet cells. Noise analysis indicated that it was due to the activity of individual channels with a conductance of 30 pS, the same as expected for individual GABA_A Cl⁻ channels with the ionic gradients used. We conclude that (a) LDCVs accumulate ATP and serotonin; (b) regulated release of GABA can be accounted for by exocytosis of a subset of insulin-containing LDCVs; (c) the fusion pore of LDCVs exhibits selectivity and compounds are differentially released depending on their chemical properties (including size); and (d) a glucose-independent nonvesicular form of GABA release exists in β cells.

The mechanism of SR95531 inhibition at GABAA receptors examined in human α1β1 and α1β1γ2S receptors

We examined the interaction of GABA and the competitive inhibitor SR95531 at human alpha1beta1gamma2S and alpha1beta1 GABA(A) receptors expressed in Sf9 cells. The efficacy and potency of inhibition depended on the relative timing of the GABA and SR95531 applications. In saturating (10 mM) GABA, the half-inhibitory concentrations of SR95531 (IC50) when coapplied with GABA to alpha1beta1gamma2S or alpha1beta1 receptors were 49 and 210 microM for the peak and 18 and 130 microM for the plateau current, respectively. Our data are explained by an inhibition mechanism in which SR95531 and GABA bind to two sites on the receptor where the binding of GABA allows channel opening but SR95531 does not. The SR95531 affinity for both receptor types was approximately 200 nM and the binding rate was found to be 10-fold faster than that for GABA. The dual binding-site model gives insights into the differential effects of GABA and SR95531 on the peak and plateau currents. The model predicts the effect of SR95531 on GABA currents in the synapse (GABA concentration approximately mM) and at extrasynaptic (GABA concentration < or = microM) sites. The IC50 (50-100 nM) for the synaptic response to SR95531 was insensitive to the GABA affinity of the receptors whereas the IC50 (50-800 nM) for extrasynaptic inhibition correlated with the GABA affinity.

Penicillin blocks human α1β1 and α1β1γ2S GABAA channels that open spontaneously

We used the open-channel blocker, penicillin (10 mM), as a tool to investigate if the human alpha1beta1 or alpha1beta1gamma2S gamma-aminobutyric acid type A (GABAA) receptor channels opened in the absence of GABA. Application of penicillin to cells expressing the receptors resulted in a transient inward whole-cell current, the off-current, upon penicillin removal. The amplitude of the off-current was dependent on the duration of the penicillin application, it reversed in polarity at depolarized potentials and exhibited "run-down" similar to the GABA-activated currents. Bicuculline (100 microM) blocked the off-current response. Pentobarbital (50 microM) enhanced the peak off-current amplitude by 2.8 and 3.4 in alpha1beta1 and alpha1beta1gamma2S receptors, respectively. Diazepam (1 microM) only enhanced the off-current peak response in alpha1beta1gamma2S receptors (1.6) and induced the development of an inward current when applied alone. The results are consistent with that the alpha1beta1 or alpha1beta1gamma2) GABAA receptors can open in the absence of GABA and raise the question of what role spontaneous channel openings have in the function of GABAA receptors.

Glucose inhibition of glucagon secretion from rat alpha-cells is mediated by GABA released from neighboring beta-cells

gamma-Aminobutyric acid (GABA) has been proposed to function as a paracrine signaling molecule in islets of Langerhans. We have shown that rat beta-cells release GABA by Ca(2+)-dependent exocytosis of synaptic-like microvesicles. Here we demonstrate that GABA thus released can diffuse over sufficient distances within the islet interstitium to activate GABA(A) receptors in neighboring cells. Confocal immunocytochemistry revealed the presence of GABA(A) receptors in glucagon-secreting alpha-cells but not in beta- and delta-cells. RT-PCR analysis detected transcripts of alpha(1) and alpha(4) as well as beta(1-3) GABA(A) receptor subunits in purified alpha-cells but not in beta-cells. In whole-cell voltage-clamp recordings, exogenous application of GABA activated Cl(-) currents in alpha-cells. The GABA(A) receptor antagonist SR95531 was used to investigate the effects of endogenous GABA (released from beta-cells) on pancreatic islet hormone secretion. The antagonist increased glucagon secretion at 1 mmol/l glucose twofold and completely abolished the inhibitory action of 20 mmol/l glucose on glucagon release. Basal and glucose-stimulated secretion of insulin and somatostatin were unaffected by SR95531. The L-type Ca(2+) channel blocker isradipine evoked a paradoxical stimulation of glucagon secretion. This effect was not observed in the presence of SR95531, and we therefore conclude that isradipine stimulates glucagon secretion by inhibition of GABA release.

Regulated exocytosis of GABA-containing synaptic-like microvesicles in pancreatic beta-cells

We have explored whether γ-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic β-cells. To this end, β-cells were engineered to express GABA_A-receptor Cl⁻-channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every β-cell contains ∼3,500 such vesicles, and that insulin-containing large dense core vesicles exclude GABA. Quantal release of GABA, seen as rapidly activating and deactivating Cl⁻-currents, was observed during membrane depolarizations from −70 mV to voltages beyond −40 mV or when Ca²⁺ was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca²⁺ entry was inhibited using Cd²⁺. Analysis of the kinetics of GABA release revealed that GABA-containing vesicles can be divided into a readily releasable pool and a reserve pool. Simultaneous measurements of GABA release and cell capacitance indicated that exocytosis of SLMVs contributes ∼1% of the capacitance signal. Mathematical analysis of the release events suggests that every SLMV contains 0.36 amol of GABA. We conclude that there are two parallel pathways of exocytosis in pancreatic β-cells and that release of GABA may accordingly be temporally and spatially separated from insulin secretion. This provides a basis for paracrine GABAergic signaling within the islet.

Evidence for inhibitory effect of the agonist gaboxadol at human α1β2γ2S GABAA receptors

Gaboxadol (THIP; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) is an agonist at GABA(A) receptors. THIP concentrations (0.01-50 mM) were applied rapidly to Sf9 cells expressing the human alpha(1)beta(2)gamma(2S) GABA(A) receptors. The EC(50) values for the peak current in THIP alone or THIP plus 1 microM diazepam were 154 and 53 microM, respectively. In supersaturating THIP (10-50 mM) the rate of current decay increased and an off-current developed when THIP was rapidly removed. The mean currents measured over the first 4 s in 10 mM and higher THIP concentrations were 0.6 or less of the 1 mM THIP mean current. Diazepam (1 microM) increased the 4 s mean current when evoked by 10 to 20 mM THIP but not 50 mM THIP. No similar effects on the current time-course were recorded in supersaturating gamma-aminobutyric acid (GABA) concentrations (50 and 80 mM). The results demonstrate an inhibitory as well as agonist effect of THIP at alpha(1)beta(2)gamma(2S) GABA(A) receptors.

Heterogeneity of functional GABA(A) receptors in rat dentate gyrus neurons revealed by a change in response to drugs during the whole-cell current time-course

We examined if the drug sensitivity of GABA(A) receptors in dentate gyrus granule neurons changed during the whole-cell current time-course. Effects of drugs on currents evoked immediately (the peak current) upon drug application and currents remaining about two seconds later (semi-plateau current) were compared. The apparent affinity for GABA (EC(50)) of the peak and the semi-plateau current were 14 and 4 microM, respectively. Bicuculline inhibited 50% of the peak and the semi-plateau current (IC(50)) at 7 and 36 microM, respectively, while 100 microM was required for full inhibition of the 100 microM GABA-evoked current. Zinc inhibited about 50% of the peak current with an IC(50) value of 94 microM whereas biphasic, but complete inhibition of the semi-plateau current was recorded with IC(50) values of 3 and 558 microM. The decay phase of the 100 microM GABA-evoked current was fitted by a fast (tau(1), 100-300 ms) and a slow (tau(2), 1-2 s) time-constants in all cells. The relative current amplitude associated with the fast (A1) and the slow (A2) component varied. The A1 current amplitude appeared more sensitive to bicuculline than the A2 current while the opposite was true for zinc. The results are consistent with heterogenous population of functional GABA(A) receptors in the dentate gyrus granule neurons.

Role of human GABA(A) receptor beta3 subunit in insecticide toxicity

The gamma-aminobutyric acid type A (GABA(A)) receptor is the target for the major insecticides alpha-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [(3)H]ethynylbicycloorthobenzoate ([(3)H]EBOB). This study tests the hypothesis that GABA(A) receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamma2) and in combination in insect Sf9 cells. Binding parameters were similar for [(3)H]EBOB in the beta3 homooligomer, alpha1beta3gamma2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, alpha-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1beta3gamma2 subtype (IC50 values of 16-33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1beta3gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or alpha-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > alpha-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1beta3gamma2 receptors. Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.

Bicuculline, pentobarbital and diazepam modulate spontaneous GABA(A) channels in rat hippocampal neurons

Spontaneously opening, chloride-selective channels that showed outward rectification were recorded in ripped-off patches from rat cultured hippocampal neurons and in cell-attached patches from rat hippocampal CA1 pyramidal neurons in slices. In both preparations, channels had multiple conductance states and the most common single-channel conductance varied. In the outside-out patches it ranged from 12 to 70 pS (Vp=40 mV) whereas in the cell-attached patches it ranged from 56 to 85 pS (-Vp=80 mV). Application of GABA to a patch showing spontaneous channel activity evoked a rapid, synchronous activation of channels. During prolonged exposure to either 5 or 100 microM GABA, the open probability of channels decreased. Application of GABA appeared to have no immediate effect on single-channel conductance. Exposure of the patches to 100 microM bicuculline caused a gradual decrease on the single-channel conductance of the spontaneous channels. The time for complete inhibition to take place was slower in the outside-out than in the cell-attached patches. Application of 100 microM pentobarbital or 1 microM diazepam caused 2 - 4 fold increase in the maximum channel conductance of low conductance (<40 pS) spontaneously active channels. The observation of spontaneously opening GABA(A) channels in cell-attached patches on neurons in slices suggests that they may have a role in neurons in vivo and could be an important site of action for some drugs such as benzodiazepines, barbiturates and general anaesthetics.

Mutant human alpha(1)beta(1)(T262Q) GABA(A) receptors are directly activated but not modulated by pentobarbital

Pentobarbital activates GABA(A) receptors and enhances GABA-activated currents. A threonine residue (262) in the second membrane spanning region at the 12' position in the beta(1) subunit, alpha(1)beta(1)(T12'Q), is necessary for the potentiating action of pentobarbital. We examined whether T12'Q-mutated receptors expressed in Spodoptera frugipedra (Sf 9) cells responded to direct activation by pentobarbital. In both mutant and wild type receptors, pentobarbital (100 microM to 1 mM) evoked a current response. The pentobarbital EC(50) values were similar; 119 and 158 microM for alpha(1)beta(1) and alpha(1)beta(1)(T12'Q) receptors, respectively. The results show it is possible to discriminate between agonistic and potentiating effects of pentobarbital, suggesting these actions involve separate mechanisms.

Effects of isoflurane and hexafluorodiethyl ether on human recombinant GABA(A) receptors expressed in Sf9 cells

Effects of volatile anesthetics and a volatile convulsant on human recombinant gamma-aminobutyric acid (GABA) type A receptor responses were studied using the whole cell configuration of the patch clamp technique. Sf9 cells were transfected with bacuroviruses carrying cDNAs of alpha1beta2, alpha1beta2gamma2s, alpha3beta2 and alpha3beta2gamma2s subunit combinations of the human GABA(A) receptor. Clinical concentrations of isoflurane (a volatile anesthetic) enhanced the GABA-induced current of the alpha1beta2gamma2s and alpha3beta2gamma2s GABA(A) subunit combinations. On the other hand, isoflurane suppressed the current of the alpha1beta2 and alpha3beta2 subunit combinations, indicating that the anesthetic effects depended upon the presence of gamma2s subunit. A high concentration (2 mM) of isoflurane generated a surge current following the washout of GABA and the anesthetic. Hexafluorodiethyl ether (a volatile convulsant) decreased the GABA-response of the both alpha3beta2gamma2s and alpha3beta2 constructs without generating a surge current. The results suggest that volatile agents affect the receptor-ionophore complex via direct interaction with proteins but not through a perturbation of the membrane lipid environment. A hypothetical sequential model for the anesthetic action is presented.

A threonine residue in the M2 region of the beta1 subunit is needed for expression of functional alpha1beta1 GABA(A) receptors

Although there is a high degree of homology in the M2 transmembrane segments of alpha1 and beta1 subunits, subunit-specific effects were observed in alpha1beta1 GABA(A) receptors expressed in Spodoptera frugipedra (Sf9) cells when the conserved 13' threonine residue in the M2 transmembrane region was mutated to alanine. When threonine 263 (13') was mutated to alanine in the beta1 subunit, high-affinity muscimol binding and the response to GABA were abolished. This did not occur when the threonine 263 (13') was mutated to alanine in the alpha1 subunit, but the rate of desensitisation increased and the effect of bicuculline, a competitive inhibitor, was reduced. The results show differential effects of subunits on receptor function and support a role for M2 in desensitisation.

Volatile anesthetics and a volatile convulsant differentially affect GABA(A) receptor-chloride channel complex

1. General anesthetics at clinical concentrations are known to affect neurotransmitter-gated ion channels in postsynaptic membranes. 2. Volatile anesthetics suppress excitatory transmissions, whereas they potentiate inhibitory chloride currents evoked by GABA or glycine. Hexafluorodiethyl ether (a volatile convulsant) markedly depresses the GABA(A) response but not the glycine-evoked chloride current or the glutamate-induced excitatory response. 3. Molecular biology has revealed that GABA(A) receptor is a heteromeric pentamer composed of alpha, beta, gamma, delta, and/or rho subunits. In baculovirus-Sf9 expression system, the gamma subunit was crucial for potentiation of the GABA-induced chloride current by volatile anesthetics. 4. Following sustained presence of GABA and high concentrations of isoflurane, simultaneous washout of both agents evoked a slowly decaying surge current, whose nature is controversial.

Signal transmission in ligand-gated receptors

At synapses, a transmitter released from a pre-synaptic terminal binds to specific, ligand-gated receptors in the post-synaptic membrane to open up ion channels through the receptor molecules. The flow of ions through these channels generates electrical signals. Electrophysiological techniques have been used over the past 50 years to understand transmission of these signals at synapses. The most recent of these, the patch-clamp technique, allows very small picoamp currents through single-channel molecules to be recorded but gives little information about receptor structure or how drugs influence their function. Now, the subunits of most ligand-gated ion channels have been cloned and sequenced. Cryo-electronmicroscopy has revealed the structure of the ion channel activated by nicotinic agonists. It is pentameric and only a small part of it is in the membrane. In spite of this simple structure, the conductance of chloride channels activated by gamma-aminobutyric acid (GABA(A) channels) is very variable and can be increased markedly by drugs such as diazepam. Site-directed mutagenesis and labelling of cysteine residues in the open and the closed states are being used to define the residues that line the ion channel. Similar methods are being used to find the way in which drugs such as general anaesthetics modulate the function of GABA(A) receptors.

Nature of the 5' residue in the M2 domain affects function of the human alpha 1 beta 1 GABAA receptor

The effects on the functional properties of the alpha 1 beta 1 GABAA receptor when the 5' (alpha 1 Val260; beta 1 Ile255) hydrophobic amino acids in the second transmembrane (M2) region were changed to threonine were examined. In response to a saturating concentration of GABA, the current evoked in mutant receptors showed a decreased rate of desensitization and at equilibrium was a greater fraction of the peak current than in wild-type receptors. The half-saturation concentration of the peak current response to GABA in mutant receptors was comparable to that in wild-type receptors, but the Hill coefficient was reduced to less than one. It was concluded that the 5' amino acids in the M2 region have a role in the conformational changes that occur within the alpha 1 beta 1 GABAA receptor in response to GABA.