Adenosine A2A receptor-induced inhibition of NMDA and GABAA receptor-mediated synaptic currents in a subpopulation of rat striatal neurons

The function of adenosine A(2A) receptors, localized at the enkephalin-containing GABAergic medium spiny neurons of the striatum, has been discussed controversially. Here we show that, in the absence of external Mg(2+), the adenosine A(2A) receptor agonist CGS 21680 postsynaptically depressed the NMDA, but not the non-NMDA (AMPA/kainate) receptor-mediated fraction of the electrically evoked EPSCs in a subpopulation of striatal neurons. Current responses to locally applied NMDA but not AMPA were also inhibited by CGS 21680. However, in the presence of external Mg(2+), the inhibition by CGS 21680 of the GABA(A) receptor-mediated IPSCs led to a depression of the EPSC/IPSC complexes. The current response to the locally applied GABA(A) receptor agonist muscimol was unaltered by CGS 21680. Whereas, the frequency of spontaneous (s)IPSCs was inhibited by CGS 21680, their amplitude was not changed. Hence, it is suggested that under these conditions the release rather than the postsynaptic effect of GABA was affected by CGS 21680. In conclusion, under Mg(2+)-free conditions, CGS 21680 appeared to postsynaptically inhibit the NMDA receptor-mediated component of the EPSC, while in the presence of external Mg(2+) this effect turned into a presynaptic inhibition of the GABA(A) receptor-mediated IPSC.     

Modulation of GABAA and glycine receptors by chlormethiazole

The influence of chlormethiazole, on currents evoked by γ-aminobutyric acid (GABA) and glycine, was investigated under voltage-clamp conditions, in bovine chromaffin cells and murine spinal neurones, respectively. Chlormethiazole (30 and 100 μM) dose dependently potentiated currents activated by either inhibitory neurotransmitter. The potentiation of the GABA-evoked response occurred without altering the reversal potential and was not influenced by the benzodiazepine receptor antagonist Ro 15-1788 (300 nM). GABA-gated channels, recorded from outside-out membrane patches, showed increased probability of being in the conducting state in the presence of chlormethiazole. High concentrations of chlormethiazole (3 mM) activated bicuculline (1 μM)-sensitive whole-cell currents with a reversal potential similar to the chloride equilibrium potential. Chlormethiazole potentiates GABA- and glycine-activated currents and at higher doses, directly activates the GABA_A receptor.     

Mechanisms of potentiation of the mammalian GABAA receptor by the marine cembranoid eupalmerin acetate

BACKGROUND AND PURPOSE: Eupalmerin acetate (EPA) is a marine diterpene compound isolated from the gorgonian octocorals Eunicea succinea and Eunicea mammosa. The compound has been previously shown to modulate muscle-type and neuronal nicotinic acetylcholine receptors, which are inhibited in the presence of low micromolar concentrations of EPA. In this study, we examined the effect of EPA on another transmitter-gated ion channel, the GABA(A) receptor. EXPERIMENTAL APPROACH: Whole-cell and single-channel recordings were made from HEK 293 cells transiently expressing rat wild-type and mutant alpha1beta2gamma2L GABA(A) receptors. KEY RESULTS: Our findings demonstrate that, at micromolar concentrations, EPA potentiates the rat alpha1beta2gamma2L GABA(A) receptor. The analysis of single-channel currents recorded in the presence of EPA showed that the kinetic mode of action of EPA is similar to that of neuroactive steroids. Mutations to residues alpha1Q241 and alpha1N407/Y410, previously shown to affect receptor modulation by neurosteroids, also diminished potentiation by EPA. Exposure to a steroid antagonist, (3alpha,5alpha)-17-phenylandrost-16-en-3-ol, reduced potentiation by EPA. Additionally, exposure to EPA led to potentiation of GABA(A) receptors activated by very high concentrations (1-10 microM) of allopregnanolone. In tadpole behavioural assays, EPA caused loss of righting reflex and loss of swimming reflex. CONCLUSIONS AND IMPLICATIONS: We conclude that EPA either interacts with the putative neurosteroid binding site on the GABA(A) receptor or shares with neurosteroids the key transduction elements involved in channel potentiation by steroids. The results indicate that cembranoids represent a novel class of GABA(A) receptor modulators.     

PKC modulation of GABAA receptor endocytosis and function is inhibited by mutation of a dileucine motif within the receptor beta 2 subunit

The modulation of GABAA receptors by protein kinase C is complex and involves effects on both ion channel function and receptor trafficking. Although PKC regulates receptor cell surface expression the mechanism is not well understood. Using immunofluorescence studies in HEK 293 cells, we demonstrate that activation of PKC by the phorbol ester PMA promotes receptor endocytosis and is dependent on the presence of a gamma subunit. This endocytosis is blocked by the dominant negative dynamin mutant K44A indicating that PKC-induced receptor endocytosis involves the dynamin endocytic pathway. Mutation of a dileucine motif within the receptor beta2 subunit inhibits the effect of PKC activation on receptor endocytosis. Using patch clamp analysis, we show that PKC activation produces a robust inhibition of GABA-gated chloride currents in cells expressing wildtype GABAA receptors, but it is ineffective in modulating receptors lacking the dileucine motif. Furthermore, the introduction into the patch pipette of a 10-amino acid peptide corresponding to the dileucine motif present in the receptor beta2 subunit prevents PKC modulation of wildtype recombinant receptors. Furthermore, in cerebral cortical neuronal slices inclusion of this peptide in the patch pipette prevents PKC modulation of native GABAA receptors. Using limited chymotrypsin digestion assays, we also show that PKC increases receptor internalization in primary cultures of cerebral cortical neurons. Lastly, PKC inhibitors do not block constitutive receptor endocytosis or affect GABA-gated chloride currents suggesting that PKC-dependent phosphorylation is not required for GABAA receptor endocytosis but plays a modulatory role in the process.     

Modulation of GABAA receptor gene expression by allopregnanolone and ethanol

Expression of specific γ-aminobutyric acid type A (GABA_A) receptor subunit genes in neurons is affected by endogenous modulators of receptor function such as neuroactive steroids. This effect of steroids appears to be mediated through modulation of GABA_A receptor signalling mechanisms that control the expression of specific receptor subunit genes. Furthermore, the specific outcomes of such signalling appear to differ among neurons in different regions of the brain. Neuroactive steroids such as the progesterone metabolite allopregnanolone might thus exert differential effects on GABA_A receptor plasticity in distinct neuronal cell populations, likely accounting for some of the physiological actions of these compounds. Here we summarise experimental data obtained both in vivo and in vitro that show how fluctuations in the concentration of allopregnanolone regulate both the expression and function of GABA_A receptors and consequently affect behaviour. Such regulation is operative both during physiological conditions such as pregnancy and lactation as well as in pharmacologically induced states such as pseudopregnancy and long-term treatment with steroid derivatives or anxiolytic-hypnotic drugs. Accordingly, long-lasting exposure of GABA_A receptors to ethanol, as well as its withdrawal, induces marked effects on receptor structure and function. These results suggest the possible synergic action between endogenous steroids and ethanol in modulating the functional activity of specific neuronal populations.     

Furosemide interactions with brain GABAA receptors

1. The loop diuretic furosemide is known to antagonize the function of γ-aminobutyric acid type A (GABA_A) receptors. The purpose of the present study was to examine the direct interaction of furosemide with the GABA_A receptors by autoradiography and ligand binding studies with native rat and human receptors and with recombinant receptors composed of rat subunits.
2. Autoradiography with [³⁵S]-t-butylbicyclophosphorothionate ([³⁵S]-TBPS) as a ligand indicated that furosemide (0.1–1 mM) reversed the 5 μM GABA-induced inhibition of binding only in the cerebellar granule cell layer of rat brain sections. In all other regions studied, notably also in the hippocampal and thalamic areas, furosemide failed to antagonize GABA. Furosemide 1 mM decreased [³⁵S]-TBPS binding only in a limited number of brain regions, but facilitation of the GABA-inhibition of the binding was much more widespread.
3. In well-washed rat cerebellar, but not cerebrocortical, membranes, furosemide enhanced the [³⁵S]-TBPS binding over basal level in the absence of added GABA. The GABA_A antagonist, SR 95531, and the convulsant, Ro 5-4864, blocked this furosemide-induced increase. Both interactions with the furosemide enhancement are likely to be allosteric, since furosemide affected the binding of [³H]-SR 95531 and [³H]-Ro 5-4864 identically in the cerebellar and cerebrocortical membranes. Maximal GABA-antagonism induced by furosemide in cerebellar membranes was further increased by SR 95531 but not by Ro 5-4864, indicating additive antagonism only for SR 95531. In human cerebellar receptors, only GABA antagonism by furosemide, but not the enhancement without added GABA, was observed.
4. In recombinant GABA_A receptors, furosemide antagonism of GABA-inhibition of [³⁵S]-TBPS binding depended only on the presence of α6 and β2/3 subunits, irrespective of the presence or absence of γ2 or δ subunits.
5. In α6β3γ2 receptors, clozapine reversed the enhancement of [³⁵S]-TBPS binding by furosemide in the absence of GABA. However, it failed to affect the GABA-antagonism of furosemide, suggesting that the enhancement of basal binding and the GABA antagonism might represent two different allosteric actions of furosemide.
6. In conclusion, the present results indicate that furosemide is a subtype-selective GABA_A antagonist with a mode of action not shared by several other antagonists, which makes furosemide a unique compound for development of potential GABA_A receptor subtype-specific and -selective ligands.

     

The in vitro and in vivo enantioselectivity of etomidate implicates the GABAA receptor in general anaesthesia

General anaesthetics exhibiting enantioselectivity afford valuable tools to assess the fundamental mechanisms underlying anaesthesia. Here, we characterised the actions of the R-(+)- and S-(-)-enantiomers of etomidate. In mice and tadpoles, R-(+)-etomidate was more potent (approximately 10-fold) than S-(-)-etomidate in producing loss of the righting reflex. In electrophysiological and radioligand binding assays, the enantiomers of etomidate positively regulated GABAA receptor function at anaesthetic concentrations and with an enantioselectivity paralleling their in vivo activity. GABA-evoked currents mediated by human recombinant GABAA receptors were potentiated by either R-(+)- or S-(-)-etomidate in a manner dependent upon receptor subunit composition. A direct, GABA-mimetic, effect was similarly subunit dependent. Modulation of GABA receptor activity was selective; R-(+)-etomidate inhibited nicotinic acetylcholine, or 5-hydroxytryptamine3 receptor subtypes only at supra-clinical concentrations and ionotropic glutamate receptor isoforms were essentially unaffected. Acting upon reticulothalamic neurones in rat brain slices, R-(+)-etomidate prolonged the duration of miniature IPSCs and modestly enhanced their peak amplitude. S-(-)-etomidate exerted qualitatively similar, but weaker, actions. In a model of locomotor activity, fictive swimming in Xenopus laevis tadpoles, R-(+)- but not S-(-)-etomidate exerted a depressant influence via enhancement of GABAergic neurotransmission. Collectively, these observations strongly implicate the GABAA receptor as a molecular target relevant to the anaesthetic action of etomidate.     

Menthol shares general anesthetic activity and sites of action on the GABA(A) receptor with the intravenous agent, propofol

Menthol and related compounds were investigated for modulation of recombinant human gamma-aminobutyric acid type A (GABA(A), alpha(1)beta(2)gamma(2s)) receptor currents expressed in Xenopus oocytes. Sub-maximal (EC(20)) GABA currents were typically enhanced by co-applications of 3-300 microM (+)-menthol (e.g. by approximately 2-fold at 50 microM) > isopulegol > isomenthol> alpha-terpineol > cyclohexanol. We studied menthol's actions on GABA(A) receptors compared to sedatives (benzodiazepines) and intravenous anesthetics (barbiturates, steroids, etomidate and propofol). Flumazenil (a benzodiazepine antagonist) did not inhibit menthol enhancements while currents directly activated by 50 microM propofol were significantly inhibited (by 26+/-3%) by 50 microM (+)-menthol. GABA(A) receptors containing beta(2) subunits with either a point mutation in a methionine residue to a tryptophan at the 286 position (in transmembrane domain 3, TM-3) or a tyrosine to a tryptophan at the 444 position (TM-4) are insensitive to modulation by propofol. Enhancements of GABA EC(20) currents by menthol were equally abolished in GABA(A) alpha(1)beta(2)(M286W)gamma(2s) and alpha(1)beta(2)(Y444W)gamma(2s) receptors while positive modulations by benzodiazepines, barbiturates and steroids were unaffected. Menthol may therefore exert its actions on GABA(A) receptors via sites distinct from benzodiazepines, steroids and barbiturates, and via sites important for modulation by propofol. Finally, using an in vivo tadpole assay, addition of (+)-menthol resulted in a loss of righting reflex with an EC(50) of 23.5+/-4.7 microM (approximately10-fold less potent anesthesia than propofol). Thus, menthol and analogs share general anesthetic action with propofol, possibly via action at similar sites on the GABA(A) receptor.     

Measurement of GABAA receptor function in rat cultured cerebellar granule cells by the Cytosensor microphysiometer

1. γ-Aminobutyric acid (GABA), acting via the GABA_A receptor, increased the extracellular acidification rate of rat primary cultured cerebellar granule cells, measured by the Cytosensor microphysiometer.
2. The optimal conditions for the measurement of GABA_A receptor function in cerebellar granule cells by microphysiometry were: cells seeded at 9–12×10⁵ cells/transwell cup and maintained in vitro for 8 days, GABA stimulation performed at 25°C, with a stimulation time of 33 s.
3. GABA stimulated a concentration-dependent increase in the extracellular acidification rate with an EC₅₀ of 2.0±0.2 μM (mean±s.e.mean, n=7 experiments) and maximal increase (E_max) over basal response of 15.4±1.2%.
4. The sub-maximal GABA-stimulated increase in acidification rate could be potentiated by the 1,4-benzodiazepine, flunitrazepam (100 nM). The 10 nM GABA response showed the maximal benzodiazepine facilitation (GABA alone, 1.4 μV s⁻¹, GABA+flunitrazepam, 3.8 μV s⁻¹, mean increment over basal, n=7).
5. The GABA-stimulated increase in acidification rate was inhibited by the GABA_A antagonist, bicuculline (100 μM) (90% inhibition at 1 mM GABA).
6. The results of this study show that activation of GABA_A receptors in rat cerebellar granule cells caused an increase in the extracellular acidification rate; an effect which was potentiated by benzodiazepines and inhibited by a GABA_A receptor antagonist. This paper defines the conditions and confirms the feasibility of using microphysiometry to investigate GABA_A receptor function in primary cultured CNS neurones. The microphysiometer provides a rapid and sensitive technique to investigate the regulation of the GABA_A receptor in populations of neurones.

     

Hippocampal Extracellular Signal-Regulated Kinase Signaling has a Role in Passive Avoidance Memory Retrieval Induced by GABAA Receptor Modulation in Mice

Available evidence strongly suggests that the γ-aminobutyric acid type A (GABA_A) receptor has a crucial role in memory retrieval. However, the signaling mechanisms underlying the role of GABA_A receptor modulation in memory retrieval are unclear. We conducted one-trial passive avoidance task with pre-retention trial drug administration in the hippocampus to test the effects of GABA_A receptor modulation on memory retrieval. We further tested the co-involvement of signaling molecules: extracellular signal-regulated kinase (ERK), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and cAMP responsive element-binding protein (CREB). First, we observed that the phosphorylation of hippocampal ERK was required for memory retrieval during the task. Accordingly, to investigate whether GABA_A receptor activation or inhibition induces ERK phosphorylation during memory retrieval, drugs that target the GABA_A receptor were administered into the hippocampus before the retention trial. Muscimol, a GABA_A receptor agonist, and diazepam, an agonist to benzodiazepine-binding site of GABA_A receptor, blocked retention trial-induced ERK phosphorylation and impaired memory retrieval. Furthermore, co-treatment with sub-effective dose of U0126, a mitogen-activated protein kinase inhibitor, blocked the upregulation of ERK phosphorylation and impaired memory retrieval, and bicuculline methiodide (BMI), a GABA_A receptor antagonist, increased ERK phosphorylation induced by the retention trial and facilitated memory retrieval. Finally, the effects of BMI were blocked by the co-application of a sub-effective dose of U0126. These results suggest that GABA_A receptor-mediated memory retrieval is closely related to ERK activity.     

Selective antagonism of the GABAA receptor by ciprofloxacin and biphenylacetic acid

1. Previous studies have shown that ciprofloxacin and biphenylacetic acid (BPAA) synergistically inhibit γ-aminobutyric acid (GABA)_A receptors. In the present study, we have investigated the actions of these two drugs on other neuronal ligand-gated ion channels.
2. Agonist-evoked depolarizations were recorded from rat vagus and optic nerves in vitro by use of an extracellular recording technique.
3. GABA (50 μM)-evoked responses, in the vagus nerve in vitro, were inhibited by bicuculline (0.3–10 μM) and picrotoxin (0.3–10 μM), with IC₅₀ values and 95% confidence intervals (CI) of 1.2 μM (1.1–1.4) and 3.6 μM (3.0–4.3), respectively, and were potentiated by sodium pentobarbitone (30 μM) and diazepam (1 μM) to (mean±s.e.mean) 168±18% and 117±4% of control, respectively. 5-Hydroxytryptamine (5-HT; 0.5 μM)-evoked responses were inhibited by MDL 72222 (1 μM) to 10±4% of control; DMPP (10 μM)-evoked responses were inhibited by hexamethonium (100 μM) to 12±5% of control, and αbMeATP (30 μM)-evoked responses were inhibited by PPADS (10 μM) to 21±5% of control. Together, these data are consistent with activation of GABA_A, 5-HT₃, nicotinic ACh and P2_X receptors, respectively.
4. Ciprofloxacin (10–3000 μM) inhibited GABA_A-mediated responses in the vagus nerve with an IC₅₀ (and 95% CI) of 202 μM (148–275). BPAA (1–1000 μM) had little or no effect on the GABA_A-mediated response but concentration-dependently potentiated the effects of ciprofloxacin by up to 33,000 times.
5. Responses mediated by 5-HT₃, nicotinic ACh and P2_X receptors in the vagus nerve and strychnine-sensitive glycine receptors in the optic nerve were little or unaffected by ciprofloxacin (100 μM), BPAA (100 μM) or the combination of these drugs (both at 100 μM).
6. GABA (1 mM)-evoked responses in the optic nerve were inhibited by bicuculline with an IC₅₀ of 3.6 μM (2.8–4.5), a value not significantly different from that determined in the vagus nerve. Ciprofloxacin also inhibited the GABA-evoked response with an IC₅₀ of 334 μM (256–437) and BPAA (100 μM) potentiated these antagonist effects. However, the magnitude of the synergy was 48 times less than that seen in the vagus nerve.
7. These data indicate that ciprofloxacin and BPAA are selective antagonists of GABA_A receptors, an action that may contribute to their excitatory effects in vivo. Additionally, our data suggest that the molecular properties of GABA_A receptors in different regions of the CNS influence the extent to which these drugs synergistically inhibit the GABA_A receptor.

     

6,2'-Dihydroxyflavone, a subtype-selective partial inverse agonist of GABAA receptor benzodiazepine site

Neuroactivity of a number of flavonoids is mediated by modulation of type A gamma-aminobutyric acid (GABA(A)) receptor function via benzodiazepine sites, mostly as partial agonists. In the present study, 6,2'-dihydroxyflavone (DHF) was characterized for potential inverse agonistic activity, and its mechanism of action was explored for receptor subtype selectivity. In whole-cell patch clamp studies on neuroblastoma IMR-32 cells expressing native GABA(A) receptors, DHF decreased GABA-induced currents, to an extent similar to that induced by the partial inverse agonist FG-7142, which could be blocked by flumazenil, a BZ site antagonist. In mouse behavioral models, DHF elicited significant anxiogenic-like effects in the elevated plus-maze test, and enhanced cognitive performance in the step-through passive avoidance test, as expected for an inverse agonist. However, DHF did not exhibit any proconvulsant effects, a typical action of inverse agonists. In electrophysiological studies on subtypes of recombinant GABA(A) receptors expressed in HEK 293T cells, DHF decreased GABA-induced currents in alpha(1)beta(3)gamma(2), alpha(2)beta(3)gamma(2), or alpha(5)beta(3)gamma(2), but not alpha(3)beta(3)gamma(2) receptors. The results demonstrated DHF as a partial inverse agonist-like modulator of GABA(A) receptors with selectivity in receptor subtypes as well as behavioral effects. The DHF subtype-selectivity suggested that alpha(3)-containing subtypes could be a mediator of the convulsion activities of GABA(A) receptor inverse agonists. Moreover, the pharmacological profile displayed in mouse behavioral models supported DHF as a useful lead compound for the development of cognition-enhancing agents devoid of convulsion side effects.     

Differences in kinetics of structurally related competitive GABA(A) receptor antagonists

Previously, 4-alkyl and 4-aryl substituted analogues of the low-efficacy partial GABA(A) receptor agonist 5-(4-piperidyl)-3-isothiazole (4-PIOL) have been identified as competitive GABA(A) receptor antagonists. These structurally related competitive antagonists show marked differences in their kinetic properties. The kinetics of 20 4-alkyl and 4-aryl substituted analogues of 4-PIOL, two 4-arylalkyl substituted 3-isothiazolol analogues and the classical GABA(A) receptor antagonist SR95531 was studied in cultured cerebral cortical neurons using whole-cell patch-clamp techniques. The kinetics of the antagonists was studied indirectly by measuring the changes in the response of the full GABA(A) receptor agonist isoguvacine (IGU) induced by concurrent application of an antagonist. When added, the majority of the antagonists did not affect the rate of deactivation of the IGU-induced responses. When removed, however, the majority of the antagonists slowed the reactivation phase of IGU implying that the dissociation of the antagonist from the GABA(A) receptor is the rate-limiting step. Surprisingly, the functional off-rates of the antagonists seemed to correlate better with the lipophilicity of the compounds than with the affinity and potency. This suggests that the dissociation of the tested antagonists from the GABA(A) receptor is restricted by lipophilic interactions, perhaps with the aromatic amino acids surrounding the GABA binding site.     

Modulation of GABA(A) receptor-mediated currents by benzophenone derivatives in isolated rat Purkinje neurones

We investigated modulation of GABA(A) receptor-mediated whole-cell currents in cerebellar Purkinje neurones by several derivatives of benzophenone. A metabolite of phenazepam, 5-bromo-2'-chloro-2-aminobenzophenone (I), caused dual modification of peak amplitudes of GABA-gated currents that depended upon the concentration of applied GABA and incubation time. Following short 10 s pre-incubations, 1-30 microM I facilitated activation and delayed deactivation of currents evoked by 500 ms pulses of 20 microM GABA. In addition, 10 microM I prominently enhanced desensitisation of currents during applications of 500 microM GABA mainly by decreasing the value of the fast time constant of the desensitisation. Continuous 6 min incubation with 10 microM I during GABA stimulation or its administration between but not during 1 s pulses of 500 microM GABA led to a gradual, partly reversible attenuation of GABA-activated currents. This inhibition was not observed when I was applied only during pulses of GABA, indicating that the blockade was not use-dependent. One of the possible mechanisms of this down-modulation could be an intracellular effect of I, because when applied intracellularly it caused slow inhibition of responses to consecutive GABA pulses. When 3-30 microM I was applied on the background of small 'plateau'-like current 5-7 s after application of 500 microM GABA, it was able to block open channels with on and off rates similar to those observed with 30 microM picrotoxin but much slower than in the case of 500 microM benzylpenicillin.At a concentration of 10 microM, 5-substituted benzophenones, but not 2-aminobenzophenone or benzophenone itself, exhibited modulatory properties similar to I and distinct from those of picrotoxin and benzylpenicillin. Therefore, we conclude that derivatives of benzophenone are a novel class of GABA(A) receptor modulators with a unique pharmacological profile.     

Allosteric Modulation of GABAA Receptor Subtypes: Effects on Visual Recognition and Visuospatial Working Memory in Rhesus Monkeys

Non-selective positive allosteric modulators (PAMs) of GABA_A receptors (GABA_ARs) are known to impair anterograde memory. The role of the various GABA_AR subtypes in the memory-impairing effects of non-selective GABA_AR PAMs has not been fully elucidated. The current study assessed, in rhesus monkeys, effects of modulation of α1, α2/3, and α5GABA_ARs on visual recognition and spatial working memory using delayed matching-to-sample (DMTS) and self-ordered spatial search (SOSS) procedures, respectively. The DMTS procedure (n=8) involved selecting a previously presented ‘sample'' image from a set of multiple images presented after a delay. The SOSS procedure (n=6) involved touching a number of boxes without repeats. The non-selective GABA_AR PAM triazolam and the α1GABA_A preferential PAMS zolpidem and zaleplon reduced accuracy in both procedures, whereas the α5GABA_A preferential PAMs SH-053-2′F-R-CH₃ and SH-053-2′F-S-CH₃, and the α2/3GABA_A preferential PAM TPA023B were without effects on accuracy or trial completion. The low-efficacy α5GABA_AR negative allosteric modulator (NAM) PWZ-029 slightly increased only DMTS accuracy, whereas the high-efficacy α5GABA_AR NAMs RY-23 and RY-24 did not affect accuracy under either procedure. Finally, the slopes of the accuracy dose-effect curves for triazolam, zolpidem, and zaleplon increased with box number in the SOSS procedure, but were equivalent across DMTS delays. The present results suggest that (1) α1GABA_ARs, compared with α2/3 and α5GABA_ARs, are primarily involved in the impairment, by non-selective GABA_AR PAMs, of visual recognition and visuospatial working memory in nonhuman primates; and (2) relative cognitive impairment produced by positive modulation of GABA_ARs increases with number of locations to be remembered, but not with the delay for remembering.     

Block and allosteric modulation of GABAergic currents by oenanthotoxin in rat cultured hippocampal neurons

Background and purpose:

Oenanthotoxin (OETX), a polyacetylenic alcohol from plants of the genus Oenanthe, has recently been identified as potent inhibitor of GABA-evoked currents. However, the effects of OETX on the inhibitory postsynaptic currents (IPSCs), as well as the pharmacological mechanism(s) underlying its effects on GABA_A receptors, remain unknown. The purpose of this study was to elucidate the mechanism underlying the inhibition of GABAergic currents by OETX.

Experimental approach:

Effects of OETX on GABAergic currents were studied using the patch clamp technique on rat cultured hippocampal neurons. Miniature IPSCs (mIPSCs) were recorded in the whole-cell configuration, while the current responses were elicited by ultrafast GABA applications onto the excised patches.

Key results:

OETX potently inhibited both mIPSCs and current responses, but its effect was much stronger on synaptic currents. Analysis of the effects of OETX on mIPSCs and evoked currents disclosed a complex mechanism: allosteric modulation of both GABA_A receptor binding and gating properties and a non-competitive, probably open channel block mechanism. In particular, OETX reduced the binding rate and nearly abolished receptor desensitization. A combination of rapid clearance of synaptic GABA and OETX-induced slowing of binding kinetics is proposed to underlie the potent action of OETX on mIPSCs.

Conclusions and implications:

OETX shows a complex blocking mechanism of GABA_A receptors, and the impact of this toxin is more potent on mIPSCs than on currents evoked by exogenous GABA. Such effects on GABAergic currents are compatible with the convulsions and epileptic-like activity reported for OETX.     

Neurosteroid Modulation of GABAA Receptor Function by Independent Action at Multiple Specific Binding Sites

Neurosteroids are endogenous modulators of GABA_A receptors that mediate anxiety, pain, mood and arousal. The 3-hydroxyl epimers, allopregnanolone (3α-OH) and epi-allopregnanolone (3β-OH) are both prevalent in the mammalian brain and produce opposite effects on GABA_A receptor function, acting as positive and negative allosteric modulators, respectively. This Perspective provides a model to explain the actions of 3α-OH and 3β-OH neurosteroids. The model is based on evidence that the neurosteroid epimers bind to an overlapping subset of specific sites on GABA_A receptors, with their net functional effect on channel gating being the sum of their independent effects at each site.     

Metabolism,pharmacokinetics and excretion of the GABAA receptor partial agonist [14C]CP-409,092 in rats

1. The metabolism and excretion of a GABA_A partial agonist developed for the treatment of anxiety, CP-409,092; 4-oxo-4,5,6,7-tetrahydro-1H-indole-3-carboxylic acid (4-methylaminomethyl-phenyl)-amide, were studied in rats following intravenous and oral administration of a single doses of [¹⁴C]CP-409,092.

2. The pharmacokinetics of CP-409,092 following single intravenous and oral doses of 4 and 15?mg kg^?1, respectively, were characterized by high clearance of 169?±?18?ml min^?1 kg^?1, a volume of distribution of 8.99?±?1.46 l kg^?1, and an oral bioavailability of 2.9% ± 3%.

3. Following oral administration of 100?mg kg^?1 [¹⁴C]CP-409,092, the total recovery was 89.1% ± 3.2% for male rats and 89.3% ± 0.58% for female rats. Approximately 87% of the radioactivity recovered in urine and faeces were excreted in the first 48?h. A substantial portion of the radioactivity was measured in the faeces as unchanged drug, suggesting poor absorption and/or biliary excretion. There were no significant gender-related quantitative/qualitative differences in the excretion of metabolites in urine or faeces.

4. The major metabolic pathways of CP-409,092 were hydroxylation(s) at the oxo-tetrahydro-indole moiety and oxidative deamination to form an aldehyde intermediate and subsequent oxidation to form the benzoic acid. The minor metabolic pathways included N-demethylation and subsequent N-acetylation and oxidation.

5. The present work demonstrates that oxidative deamination at the benzylic amine of CP-409,092 and subsequent oxidation to form the acid metabolite seem to play an important role in the metabolism of the drug, and they contribute to its oral clearance and low exposure.

     

Hypothalamic GABAA receptor blockade modulates cerebral cortical systems sensitive to acute stressors

Pharmacologic blockade of GABA binding sites in the hypothalamus elicits a pattern of physiological and behavioral arousal. The latter outcome implicates a perturbation in the neural functioning of higher brain centers. The effect that hypothalamic GABA_A receptor modulation has on the function of cerebral cortical neural substrates linked with responses to stressors was assessed using microinfusion of bicuculline methiodide (BMI) into the medial hypothalamus of freely moving, handling habituated rats. BMI led to rapid increases in frontal cortical dopamine (DA) utilization (calculated from the sum of the levels of the DA metabolites, homovanilic and dihydroxyphenylacetic acids, divided by DA levels) resembling that identified following restraint-induced stress. Also, cortical GABA_A receptor function [using chloride (Cl^–) enhancement of³H-flunitrazepam (Flu) binding as an index] was disrupted; i.e. there was a loss of typical Cl^– enhancement of³H-Flu binding in animals after BMI infusions. However, placing animals in restraint after BMI infusion reversed the effects of BMI, with both DA utilization and Cl^– facilitated³H-Flu binding similar to control basal values. Muscimol infusions in separately prepared animals did not alter either frontal cortical DA utilization or GABA_A receptor function. The present results implicate GABA in the hypothalamus as gating activity of cortical systems involved in sensation of and/or responses to stressors. These findings may have important implications for effects of autonomic arousal on neural substrates involved in mediating stress responses.     

Changes in GABAA receptor gamma 2 subunit gene expression induced by long-term administration of oral contraceptives in rats

The effects of oral contraceptives (OCs) on neurosteroid concentrations were evaluated in female rats and women. In rats, ethynylestradiol and levonorgestrel (0.030 and 0.125 mg, respectively, subcutaneously) administered daily for 6 weeks reduced the concentrations of pregnenolone (-41%) progesterone (-74%) and allopregnanolone (-79%) in the cerebral cortex; the plasma concentrations of these steroids were also reduced but by smaller extents. OC administration for 3 months also reduced the serum concentrations of pregnenolone, progesterone and allopregnanolone in women. Chronic administration of OCs in rats increased the abundance of gamma-aminobutyric acid type A (GABA(A)) receptor gamma 2L and gamma 2S subunit mRNAs and the relative protein in the cerebral cortex, while the amounts of various alpha and beta subunit mRNAs were unaffected. Ovariectomy did not modify the effect of OCs administration on the concentrations of neurosteroids in the rat cerebral cortex (but not in the plasma) as well as on the GABA(A) receptor gene expression, suggesting a direct effect of OCs in brain. Finally, rats treated with OCs exhibited an anxiety-like behavior in the elevated plus-maze test. These results indicate that long-term treatment with OCs induced a persistent reduction in the concentrations of pregnenolone, progesterone and its GABA(A) receptor-active metabolite, allopregnanolone, both in rats and women. In rats this effect was associated with a plastic adaptation of GABA(A) receptor gene expression in the rat cerebral cortex.