A minimal model to account for the response of N-Methyl-D-aspartate receptors expressed in Xenopus oocyte injected with rat brain mRNA.

N-Methyl-D-aspartate (NMDA) receptors were expressed in Xenopus oocytes by injecting rat brain mRNA. NMDA-elicited responses in the oocytes were measured by the voltage-clamping method. The following measurements were made in the presence of 50 microM glycine (Gly) to establish the relationship between the NMDA concentration and the current: (1) the NMDA-induced membrane current before desensitization; (2) the NMDA-induced membrane current after desensitization equilibrium; (3) the fraction of the active form of the receptor after desensitization equilibrium in the presence and absence of 50 microM Gly; (4) the rate of the recovery of desensitized receptors upon removal of NMDA. Gly was essential for not only the activation of NMDA receptors but also their desensitization. These results were analyzed on the basis of a minimal model where one agonist and one Gly binding site were assumed. The equilibrium and rate constants of the model were evaluated for NMDA in the presence of saturating amounts of Gly. This model will be useful for systematically explaining the complicated responses of NMDA receptors.     

Acetylcholine receptor-mediated membrane current in oocytes injected with Electrophorus electricus mRNA: analyses of nicotine, succinylcholine, and decamethonium responses on the basis of the minimal model

Nicotinic acetylcholine receptor was synthesized in Xenopus oocytes after injection of the mRNA purified from Electrophorus electricus electroplax. Nicotine, succinylcholine, and decamethonium (agonist)-elicited membrane currents in the injected oocytes were measured electrophysiologically by the voltage-clamping method. The following four different measurements were made to establish the relationship between the agonist concentration and the membrane current: 1) the agonist-induced membrane current before desensitization, 2) the agonist-induced membrane current after desensitization equilibrium, 3) the fraction of the active form of the receptors after desensitization equilibrium, 4) the rate of recovery of desensitized receptors upon removal of the agonist. These results were analyzed on the basis of the minimal model proposed from receptor-mediated ion translocation measurements. The equilibrium and rate constants of the model were evaluated for nicotine, succinylcholine, and decamethonium, and could explain the observed electrical responses in the injected oocyte, i.e. the characteristics of the receptor response caused by these agonists.     

Multiphasic desensitization of the GABAA receptor in outside-out patches.

GABAA receptor function was studied in outside-out patches from guinea pig hippocampal neurons using a drug application system with an exchange time of under 1.5 ms. Application of GABA to these patches induced a Cl- conductance that desensitized with prolonged exposure. Increasing GABA concentrations induced larger conductance increases that were associated with more complex patterns of desensitization. Smaller GABA responses desensitized with monophasic kinetics, whereas large responses displayed bi- and triphasic kinetics. Desensitization of the response to 1 mM GABA was triphasic in about 70% of the patches (tau = 15.4, 207, and 1370 ms) and biphasic in about 30% of the patches (tau = 44 and 725 ms). All phases of desensitization reversed at the Cl- equilibrium potential. Over the concentration range from 3 microM to 3 mM, both the rate and the extent of desensitization increased; however, complete desensitization was rarely observed. The increase in desensitization rate was due to an increase in the relative contribution of the faster phases with increasing GABA. The time constants of the three phases were independent of concentration. The different phases are not mediated by separate receptor populations, because double pulse experiments demonstrated interconversion among the fastest phase and the two slower phases. We demonstrate the plausibility of a model in which multiphasic desensitization is a consequence of the faster association rate at higher GABA concentrations.     

Minimal model analyzing response of glycine receptors expressed in Xenopus oocyte: inhibition by a lipid hydroperoxide.

Glycine receptor (GlyR) was expressed in Xenopus oocytes by injecting rat brain mRNA. Glycine (Gly)-elicited responses in the oocyte were measured by the voltage-clamping method. The following measurements were made to establish the relationship between Gly concentration and the current: 1) Gly-induced membrane current before desensitization, 2) Gly-induced membrane current after desensitization equilibrium, 3) fraction of the active form of the receptor after desensitization equilibrium, 4) rate of recovery of the desensitized receptors upon removal of Gly. These results were analyzed on the basis of the minimal model proposed for nicotinic acetylcholine and gamma-aminobutyric acid A receptor. The equilibrium and rate constants of the model were evaluated for GlyR. The effects of procaine and 13-L-hydroperoxylinoleic acid (LOOH) on GlyR were examined electrophysiologically. LOOH noncompetitively inhibited the receptor with the inhibition constant of 27 microM, while 1 mM procaine, a local anesthetic, did not inhibit GlyR at all.     

Different effects of pentobarbital on two gamma-aminobutyrate receptors from rat brain: channel opening, desensitization, and an additional conformational change

The effect of pentobarbital on the responses of the gamma-aminobutyric acid (GABA) receptor from rat brain was studied in quantitative measurements of GABA-mediated chloride-exchange rates (reflecting channel-opening equilibrium) and receptor desensitization rates by using 36Cl- tracer ion with native membrane vesicles. Pentobarbital effected the two phases of 36Cl- influx in different ways, supporting previous evidence that these are mediated by two different receptors [Cash, D. J., & Subbarao, K. (1987) Biochemistry 26, 7556; Cash, D. J., & Subbarao, K. (1987) Biochemistry 26, 7562]. Both the chloride-exchange rate and the desensitization rate of the faster desensitizing receptor were increased by pentobarbital at concentrations above 20 microM by an allosteric effect shifting the response curve to lower GABA concentrations. A similar enhancement of the responses of the slower desensitizing receptor occurred up to 200 microM pentobarbital. Two pentobarbital effector sites were involved in the allosteric mechanism. Above 500 microM pentobarbital, both the initial chloride-exchange rate and the desensitization rate of the slower desensitizing receptor were decreased. This inhibition, which was immediate, occurred with saturating as well as low GABA concentrations and therefore was not attributed to decreased GABA binding but to inhibitory sites for pentobarbital, different from the allosteric activating sites and the GABA binding sites. The chloride ion exchange activity was seen to recover with time, at concentrations above 1000 microM pentobarbital, in a process with a very steep dependence on pentobarbital concentration. This reactivation was attributed to the conversion of an initial form of the receptor to a final form that was less inhibited by pentobarbital. The similarity of the effects of pentobarbital on the chloride ion exchange with its effects on electrophysiological measurements supports the fact that these different techniques study the same phenomena. Comparisons of the effects of pentobarbital on desensitization and on high-affinity ligand binding measurements suggest that increased GABA binding at equilibrium reflects an increased conversion to the desensitized state.     

Effects of Bisphenol A and its Derivatives on the Response of GABAA Receptors Expressed in Xenopus Oocytes

To study the effects of bisphenol-A (BPA) known to have estrogenic actions, and its derivatives, 3,5-dimethylphenol (DMP) and p-t-butylphenol (TBP), on ionotropic γ-aminobutyric acid (GABA) receptors, GABA_A receptors were expressed in Xenopus oocytes by injecting both poly(A)⁺RNA prepared from rat whole brain and cRNAs synthesized from cloned cDNAs of α₁ and β₁ subunit of the bovine receptors, and their electrical responses were measured by the voltage clamping method. BPA caused the potentiation and inhibition of the former receptor-responses, while it caused only inhibition of the latter ones. In the presence of low concentrations of GABA, DMP and TBP potentiated the responses of both receptors. DMP and TBP also increased the rate of decay of the response, possibly by desensitization of the receptors when GABA solution was continuously bath-applied. Diethyl terephthalate (DTP), which is also known to have estrogenic actions, had little effect on both the responses and the decay of both receptors.     

Effects of bisphenol A and its derivatives on the response of GABA(A) receptors expressed in Xenopus oocytes.

To study the effects of bisphenol-A (BPA) known to have estrogenic actions, and its derivatives, 3,5-dimethylphenol (DMP) and p-t-butylphenol (TBP), on ionotropic gamma-aminobutyric acid (GABA) receptors, GABA(A) receptors were expressed in Xenopus oocytes by injecting both poly(A)+ RNA prepared from rat whole brain and cRNAs synthesized from cloned cDNAs of alpha1 and beta1 subunit of the bovine receptors, and their electrical responses were measured by the voltage clamping method. BPA caused the potentiation and inhibition of the former receptor-responses, while it caused only inhibition of the latter ones. In the presence of low concentrations of GABA, DMP and TBP potentiated the responses of both receptors. DMP and TBP also increased the rate of decay of the response, possibly by desensitization of the receptors when GABA solution was continuously bath-applied. Diethyl terephthalate (DTP), which is also known to have estrogenic actions, had little effect on both the responses and the decay of both receptors.     

Potentiation and Inhibition of Ionotropic Neurotransmitter Receptors Expressed in Xenopus Oocyte by Linoleic Acid and Its Hydroperoxide

Abstract: To study the effects of lipid hydroperoxide on ionotropic neurotransmitter receptors, γ-aminobutyric acid (GABA), N -methyl- d -aspartate (NMDA), and non-NMDA receptors (GABARs, NMDARs, and non-NMDARs, respectively) were expressed in Xenopus oocytes that received an injection of mRNA prepared from rat whole brain. Linoleic acid (LA) and its hydroperoxide 13- l -hydroperoxylinoleic acid (LOOH) prepared with soybean lipoxygenase inhibited the response of GABARs in the presence of GABA at high concentrations. The inhibition was stronger when the inhibitors were perfused 1 min before a mixture of GABA and the inhibitors than when they were perfused simultaneously with GABA. On the other hand, only LOOH potentiated the response of GABARs in the presence of GABA at low concentrations, possibly increasing the affinity of GABA to the receptors. Both LA and LOOH accelerated the rate of desensitization of GABARs, but LOOH did not affect their equilibrium between the active and desensitized form of the receptors. They also inhibited the response of NMDARs in a noncompetitive manner but barely inhibited the response of non-NMDARs in the presence of kainate at various concentrations. These results suggest the possibility that production of lipid hydroperoxide modulates the neural transmission in the brain, especially through GABARs.     

Kinetic properties of GABA rho1 homomeric receptors expressed in HEK293 cells

The rho1 subunit of the ionotropic GABA receptors is thought to contribute to the formation of the GABA(C) receptors with pharmacological and physiological properties distinct from those of GABA(A) receptors. Previous characterization of this subunit expressed in the Xenopus oocytes revealed an ion channel with slow activation and deactivation and no desensitization, quite different from the properties of GABA(C) receptors observed in native cells. We expressed the human rho1 subunit in human embryonic kidney (HEK) 293 cells and quantitatively characterized the kinetic properties of these receptors using a rapid drug application device. The rho1 subunit expressed in HEK293 cells exhibited pharmacological and kinetic properties qualitatively identical to those described when rho1 was expressed in the oocytes. An apparent desensitizing current observed during a constant GABA application was determined to be secondary to an E(Cl) shift. Detailed kinetic analyses and parameter estimation for a five-state kinetic model revealed that the channel is best described by a set of rate constants with a notably faster GABA unbinding K(off) rate compared to the parameters proposed for the same subunit expressed in the oocytes. The same subunit expressed in hippocampal neurons showed activation and deactivation kinetics identical to the current characterized in HEK293 cells. The kinetic properties of rho1 subunit expressed in a nonoocyte model system may be better described quantitatively by the rate constants presented here.     

Constitutive signaling by Kaposi's sarcoma-associated herpesvirus G-protein-coupled receptor desensitizes calcium mobilization by other receptors

We coexpressed Kaposi's sarcoma-associated herpesvirus G protein-coupled receptors (KSHV-GPCRs) with thyrotropin-releasing hormone (TRH) receptors or m1-muscarinic-cholinergic receptors in Xenopus oocytes and in mammalian cells. In oocytes, KSHV-GPCR expression resulted in pronounced (81%) inhibition (heterologous desensitization) of Ca(2+)-activated chloride current responses to TRH and acetylcholine. Similar inhibitions of cytoplasmic free Ca(2+) responses to TRH were observed in human embryonic kidney HEK 293 EM cells and in mouse pituitary AtT20 cells. Further study of oocytes showed that this inhibition was partially reversed by interferon-gamma-inducible protein 10 (IP-10), an inverse agonist of KSHV-GPCR. The basal rate of (45)Ca(2+) efflux in oocytes expressing KSHV-GPCRs was 4.4 times greater than in control oocytes, and IP-10 rapidly inhibited increased (45)Ca(2+) efflux. In the absence of IP-10, growth-related oncogene alpha caused a further 2-fold increase in (45)Ca(2+) efflux. In KSHV-GPCR-expressing oocytes, responses to microinjected inositol 1,4,5-trisphosphate were inhibited by 74%, and this effect was partially reversed by interferon-gamma-inducible protein 10. Treatment with thapsigargin suggested that the pool of calcium available for mobilization by TRH was decreased in oocytes coexpressing KSHV-GPCRs. These results suggest that constitutive signaling by KSHV-GPCR causes heterologous desensitization of responses mediated by other receptors, which signal via the phosphoinositide/calcium pathway, which is caused by depletion of intracellular calcium pools.     

Effects of concanavalin A on desensitization kinetics of GABA responses inAchatina fulica neurons

The effects of the lectin concanavalin A (Con A), on the kinetics of desensitization of the responses of voltage clampedAchatina fulica LP5 neuron to microperfused acetylcholine (ACh) and GABA were compared. Both ACh and GABA elicited increases in chloride conductance which decayed biphasically during prolonged applications of these agonists; an initial rapid decay was followed by a later slow decay. Con A (5 g/ml) accelerated both the fast and the slow decays of responses to ACh. Con A (5 g/ml) also accelerated the fast decay of responses to GABA, but the slow decay was unaffected, even by 20 g/ml or more of the lectin. It is suggested that, at least in the case of GABA receptor, the fast and slow decays involve distinct desensitization kinetics. The effects of Con A on the desensitization of the ACh and GABA responses were reversed byd-mannose, a competitive and specific inhibitor of Con A binding to membrane sugar residues. These results provide further evidence that receptor desensitization can be influenced by perturbing the sugar moieties associated with the subunits comprising these signalling macromolecules. The carbohydrate residues may play an important role in regulating desensitization of transmitter receptors.Abbreviations ACh acetylcholine - Con A concanavalin A     

Go G-proteins mediate rapid heterologous desensitization of G-protein coupled receptors in Xenopus oocytes

We have shown previously that responses to lysophosphatidic acid (LPA) in Xenopus oocytes exhibit pronounced rapid homologous desensitization mediated by Go family of G-proteins (Itzhaki-Van Ham et al., 2004, J Cell Physiol, 200: 125-133). The present study was aimed at examining the involvement of Go G-proteins in rapid heterologous desensitization of native and expressed G-protein-coupled receptors in Xenopus oocytes. Threshold stimulation of the native lysophosphatidic acid receptors (LPA-Rs) induced about 50% rapid desensitization of responses evoked by stimulation of either native trypsin or expressed M1-muscarinic cholinergic receptors (M1-Rs). Similarly, threshold stimulation of expressed M1-Rs or thyrotropin-releasing hormone receptors induced 40% rapid desensitization of responses to LPA. Inactivation of all Gi/o G-proteins with pertussis toxin (PTX) completely abolished rapid heterologous desensitization in all protocols. Depletion of either Galphao or Galphao1 by antisense oligodeoxynucleotides targeted at either member of the Galphao family decreased or completely abolished rapid heterologous desensitization. Expression of two dominant negative mutants of the human Galphao family, highly homologous to oocyte Galphao species, either decreased or virtually abolished rapid desensitization. Homologous and heterologous desensitizations of the LPA response were non-additive and proceeded, apparently, via the same pathway. We conclude that Go G-proteins mediate both homologous and heterologous rapid desensitization of responses mediated by G-protein-coupled receptors (GPCRs) coupled to the phosphoinositide phospholipase C-inositol 1,4,5-trisphosphate-Ca(2+) (PI-PLC-InsP(3)-Ca(2+)) pathway in Xenopus oocytes.     

Multiple mechanisms of block by the anesthetic isoflurane of a γ-aminobutyric acid activated chloride channel in crayfish

Outside-out patches were excised from the membrane of the deep extensor abdominal muscle (DEAM), containing γ-aminobutyric acid (GABA)- activated chloride channels, in the crayfish Astacus astacus. GABA and isoflurane (iso) were applied in pulses by a liquid filament switch, and their effects on the GABA-elicited chloride currents were investigated. Application of iso alone elicited no current responses and pre-application of iso prior to GABA had no effects on the GABA-elicited current. Co-application of GABA and iso resulted in a reduction of the initial chloride current and subsequent decline of the current to a steady state, indicating that iso binds to the receptor after GABA has bound. Recovery currents at the end of the co-application pulse, their amplitudes decreasing with pulse duration, confirmed this suggestion. Open-time distributions of the blocked channel showed a shift of the long open-time towards a new time constant, indicating a second block mechanism via the long open state A₅O_s of the channel. Removal of GABA and iso after reaching the equilibrium state of the block resulted in recovery currents containing exclusively openings from the long open state A₅O_s, confirming the suggestion of an open channel block only at one of the open states. Accepted: 24 June 1997     

Protein kinase and phosphatase modulation of quail brain GABA(A) and non-NMDA receptors co-expressed in Xenopus oocytes

The GABA(A) receptor and the non-NMDA subtype of the ionotropic glutamate receptor were co-expressed in Xenopus oocytes by injection of quail brain mRNA. The oocytes were treated with various protein kinase (PK) and protein phosphatase (PP) activators and inhibitors and the effects on receptor functioning were monitored. Two phorbol esters, 4-beta-phorbol 12-myristate-13-acetate (PMA) and 4-beta-phorbol 12,13-dibutyrate (PDBu); the cGMP-dependent PK activators sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG); and the PP inhibitor okadaic acid (OA) reduced the amplitude of the GABA-induced currents, whilst the PK inhibitor staurosporine potentiated it. In addition, PMA, PDBu, SNP, and OA reduced the desensitization of the GABA-induced response. Identical treatments generally had similar but less pronounced effects on responses generated by kainate (KA) but the desensitization characteristic of the non-NMDA receptor was not affected. None of the treatments had any effect on the reversal potentials of the induced currents. Immunoblots revealed that the oocytes express endogenous PKG and guanylate cyclase. The results are discussed in terms of the molecular structures of GABA(A) and non-NMDA receptors and the potential functional consequences of phosphorylation/dephosphorylation.     

GABA increases both the conductance and mean open time of recombinant GABAA channels co-expressed with GABARAP

The single channel properties of recombinant gamma-aminobutyric acid type A (GABA(A))alphabetagamma receptors co-expressed with the trafficking protein GABARAP were investigated using membrane patches in the outside-out patch clamp configuration from transiently transfected L929 cells. In control cells expressing alphabetagamma receptors alone, GABA activated single channels whose main conductance was 30 picosiemens (pS) with a subconductance state of 20 pS, and increasing the GABA concentration did not alter their conductance. In contrast, when GABA(A) receptors were co-expressed with GABARAP, the GABA-activated single channels displayed multiple, high conductances (> or =40 pS), and GABA (> or =10 microM) was able to increase their conductance, up to a maximum of 60 pS. The mean open time of GABA-activated channels in control cells expressing alphabetagamma receptors alone was 2.3 +/- 0.1 ms for the main 30-pS channel and shorter for the subconductance state (20 pS, 0.8 +/- 0.1 ms). Similar values were measured for the 30- and 20-pS channels active in patches from cells co-expressing GABARAP. However higher conductance channels (> or =40 pS) remained open longer, irrespective of whether GABA or GABA plus diazepam activated them. Plotting mean open times against mean conductances revealed a linear relationship between these two parameters. Since high GABA concentrations increase both the maximum single channel conductance and mean open time of GABA(A) channels co-expressed with GABARAP, trafficking processes must influence ion channel properties. This suggests that the organization of extrasynaptic GABA(A) receptors may provide a range of distinct inhibitory currents in the brain and, further, provide differential drug responses.     

Functional consequences of the loss of high affinity agonist binding to gamma-aminobutyric acid type A receptors. Implications for receptor desensitization

We reported previously that tyrosine 62 of the beta2 subunit of the gamma-aminobutyric acid, type A (GABA(A)) receptor is an important determinant of high affinity agonist binding and that recombinant alpha1beta2gamma2(L) receptors carrying the Y62S mutation lack measurable high affinity sites for [3H]muscimol. We have now examined the effects of disrupting these sites on the macroscopic desensitization properties of receptors expressed in Xenopus oocytes. Desensitization was measured by the ability of low concentrations of bath-perfused agonist to reduce the current responses elicited by subsequent challenges with saturating concentrations of GABA. Wild-type receptors were desensitized by pre-perfused muscimol with an IC50 approximately 0.7 microm, which correlates well with the lower affinity sites for this agonist that are measured in direct binding studies. Receptors carrying the beta2 Y62S and Y62F mutations desensitized at slightly higher (2-7-fold) agonist concentrations. However, at low perfusate concentrations, the Y62S-containing receptor recovered from the desensitized state even in the continued presence of agonist. The characteristics of desensitization in the wild-type and mutant receptors lead us to suggest that the major role of the high affinity agonist-binding site(s) of the GABA(A) receptor is not to induce desensitization but rather to stabilize the desensitized state once it has been formed.     

Responses to GABA, glycine and beta-alanine induced in Xenopus oocytes by messenger RNA from chick and rat brain

Poly (A)+ messenger RNA (mRNA) was extracted from rat and chick brains, and injected into oocytes of Xenopus laevis. This led to the expression of receptors that evoked membrane currents in response to gamma-aminobutyric acid (GABA), glycine and beta-alanine. These currents all inverted at about the chloride equilibrium potential in the oocyte, and showed a marked rectification at negative potentials. Oocytes injected with mRNA from chick optic lobe gave large responses to GABA and beta-alanine, but small responses to glycine. In contrast, one fraction of mRNA from rat cerebral cortex (obtained by sucrose density gradient centrifugation) caused oocytes to develop sensitivity to GABA, glycine and beta-alanine, but very little to GABA. The pharmacological properties of the three amino acid responses also differed. Barbiturate and benzodiazepines potentiated the responses to GABA and beta-alanine, but not to glycine. Strychnine reduced the responses to glycine and beta-alanine, but not to GABA, whereas bicuculline reduced the responses to GABA and beta-alanine, but not to glycine. We conclude that different species of mRNA code for receptors to GABA and glycine, and possibly also for separate beta-alanine receptors.     

Channel opening of gamma-aminobutyric acid receptor from rat brain: molecular mechanisms of the receptor responses

The function of gamma-aminobutyric acid (GABA) receptors, which mediate transmembrane chloride flux, can be studied by use of 36Cl- isotope tracer with membrane from mammalian brain by quench-flow technique, with reaction times that allow resolution of the receptor desensitization rates from the ion flux rates. The rates of chloride exchange into the vesicles in the absence and presence of GABA were characterized with membrane from rat cerebral cortex. Unspecific 36Cl- influx was completed in three phases of ca. 3% (t 1/2 = 0.6 s), 56% (t 1/2 = 82 s), and 41% (t 1/2 = 23 min). GABA-mediated, specific chloride exchange occurred with 6.5% of the total vesicular internal volume. The GABA-dependent 36Cl- influx proceeded in two phases, each progressively slowed by desensitization. The measurements supported the presence of two distinguishable active GABA receptors on the same membrane mediating chloride exchange into the vesicles with initial first-order rate constants of 9.5 s-1 and 2.3 s-1 and desensitizing with first-order rate constants of 21 s-1 and 1.4 s-1, respectively, at saturation. The half-response concentrations were similar for both receptors, 150 microM and 114 microM GABA for desensitization and 105 microM and 82 microM for chloride exchange, for the faster and slower desensitizing receptors, respectively. The two receptors were present in the activity ratio of ca. 4/1, similar to the ratio of "low-affinity" to "high-affinity" GABA sites found in ligand binding experiments. The desensitization rates have a different dependence on GABA concentration than the channel-opening equilibria.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic analyses of alcohol-induced potentiation of the response of GABA(A) receptors composed of alpha(1) and beta(1) subunits.

To investigate the kinetics of both the potentiation and desensitization of the response of ionotropic GABA receptors (GABA(A) receptors) in the presence of various compounds, we expressed receptors composed of alpha(1) and beta(1) subunits by injecting cells with the cRNAs synthesized from cloned bovine GABA(A) receptor cDNAs and measured the electrical responses of the cells electrophysiologically with or without the compounds. The potentiation of the GABA(A) receptor-mediated response was quantitatively analyzed using a simple model with the assumption that the receptors have two identical binding sites for GABA molecules with a dissociation constant of K(1), and one potentiation site for the compound with a dissociation constant of K(p), and that the binding of the compound to the potentiation site only increases the affinity of the GABA binding sites, changing K(1) to K(1p). The estimated K(p) and K(1p) were dependent on the functional groups and the chain length of the compounds. These results could be satisfactorily analyzed using this simple model. The potentiation of the GABA(A) receptor-mediated response by the components of essential oils used for aromatherapy was also examined. These compounds accelerated the decay of the response, possibly due to desensitization of the receptors, which was also analyzed on the basis of the model.     

gamma-Amino butyric-N-acid sensitivity of mouse and human oocytes

gamma-Aminobutyric acid (GABA)-sensitivity was studied in mouse and human oocytes using electrophysiological techniques. Isolated mouse oocytes at the germinal vesicle (GV) or metaphase II stage, and human oocytes at the GV stage or following resumption of meiosis in culture, were sensitive to GABA. The transmitter usually hyperpolarized the membrane, with a concomitant decrease followed by an increase in membrane conductance, at threshold concentrations as low as 10(-10) M. GABA response was not evoked in Cl-free medium. Bicuculline (10(-5)-10(-6) M) reversibly blocked GABA (10(-9)-10(-4) M) responses. In contrast mouse fertilized eggs were insensitive to GABA at concentrations of 10(-5) M or lower. A possible biological role of the neurotransmitter GABA is discussed.