GABA(A) signalling is involved in N/OFQ anxiolytic-like effects but not in nocistatin anxiogenic-like action as evaluated in the mouse elevated plus maze

Nociceptin/orphanin FQ (N/OFQ) and nocistatin are two neuropeptides originated from the same precursor prepronociceptin/orphanin FQ (ppN/OFQ). N/OFQ is the endogenous ligand of the NOP receptor, while the target of action of nocistatin is still unknown. N/OFQ modulates various biological functions, including anxiety. Conversely, nocistatin either behaves as a functional N/OFQ antagonist or evokes per se effects opposite to those of N/OFQ. Here we investigated the interaction between the anxiolytic-like effects of N/OFQ and the anxiogenic-like action of nocistatin with those evoked by GABA_A receptor ligands in the mouse elevated plus maze. The anxiogenic-like effects of the GABA_A receptor antagonist pentylenetetrazol (20 mg/kg; intraperitoneal, i.p.) were abolished by the co-treatment with N/OFQ (10 pmol; intracerebroventricular, i.c.v.) while potentiated by the administration of nocistatin (0.01 pmol; i.c.v.). The anxiolytic-like effects of the benzodiazepine receptor agonist diazepam (0.75 mg/kg, i.p.) were reversed by nocistatin (0.1 pmol; i.c.v.), whereas signs of sedation were observed when mice were co-treated with diazepam and N/OFQ (3 pmol). Interesting enough, the i.p. treatment with flumazenil (1 mg/kg) blocked the anxiolytic-like effects of N/OFQ (10 pmol; i.c.v.), but not the anxiogenic effect elicited by nocistatin. Collectively, our findings suggest that the effects on anxiety elicited by pentylenetetrazol and diazepam can be counteracted or potentiated in the presence of N/OFQ and nocistatin. In addition, the effects on anxiety of N/OFQ, but not nocistatin, appear to be dependent on the benzodiazepine site of the GABA_A receptor.     

Dipeptide Tyr-Leu (YL) exhibits anxiolytic-like activity after oral administration via activating serotonin 5-HT1A, dopamine D1 and GABAA receptors in mice

We found that Tyr-Leu (YL) dose-dependently exhibits potent anxiolytic-like activity (0.1-1 mg/kg, i.p.) comparable to diazepam in the elevated plus-maze test in mice. YL was orally active (0.3-3 mg/kg). A retro-sequence peptide or a mixture of Tyr and Leu was inactive. The anxiolytic-like activity of YL was inhibited by antagonists for serotonin 5-HT_1A, dopamine D₁ and GABA_A receptors; however, YL had no affinity for them. We also determined the order of their activation is 5-HT_1A, D₁ and GABA_A receptors using selective agonists and antagonists. Taken together, YL may exhibit anxiolytic-like activity via activation of 5-HT_1A, D₁ and GABA_A receptors.     

Central nervous system activities of two diterpenes isolated from Aloysia virgata

Using the guide of a competitive assay for the benzodiazepine binding site in the γ-aminobutyric acid type A receptor (GABA_A), two active diterpenes were isolated from the aerial parts of Aloysia virgata (Ruíz & Pavón) A.L. Jussieu var. platyphylla (Briquet) Moldenke. These compounds, identified as (16R)-16,17,18-trihydroxyphyllocladan-3-one (1) and (16R)-16,17-dihydroxyphyllocladan-3-one (2) on the basis of spectral data, competitively inhibited the binding of [³H]-FNZ to the benzodiazepine binding site with K_i ± S.E.M. values of 56 ± 19 μM and 111 ± 13 μM, respectively. The behavioral actions of these diterpenes, intraperitoneally (i.p.) administered in mice, were examined in the plus-maze, holeboard, locomotor activity and light/dark tests. Compound 1 exhibited anxiolytic-like effects in mice evidenced by a significant increase of the parameters measured in the holeboard test (the number of head dips at 0.3 mg/kg and 3 mg/kg, the rears at 1 mg/kg and the time spent head-dipping at 3 mg/kg), in the plus-maze assay (the percentage of open arm entries at 1 mg/kg) and in the light/dark test (the time in light and the number of transitions at 1 mg/kg). Compound 2 augmented the number of rearings in the holeboard apparatus (at 0.3 mg/kg and 1 mg/kg) and the locomotor activity (at 1 mg/kg). These results reveal the presence of neuroactive compounds in Aloysia virgata.     

Kaempferol from the leaves of Apocynum venetum possesses anxiolytic activities in the elevated plus maze test in mice

The present work evaluated the anxiolytic activity of an aqueous extract of Apocynum venetum L. (Apocynaceae) and bioguided its fractionation using the elevated plus maze (EPM) in mice as a model of anxiety. A single treatment of AV extract markedly increased the percentage time spent on the open arms of the EPM in two distinct concentration ranges of 22.5–30 and 100–125 mg/kg p.o., respectively, indicating a putative anxiolytic-like activity. Fractions showing anxiolytic effects in concentrations equal to 30 or 125 mg/kg of whole extract were antagonized using the benzodiazepine antagonist flumazenil (3 mg/kg i.p.) or the 5-HT_1A receptor antagonist WAY-100635 (0.5 mg/kg i.p.). All active fractions in a concentration equal to 125 mg/kg were effectively blocked by the benzodiazepine antagonist flumazenil, while the anxiolytic activities of fractions in the lower dose equivalent to 30 mg/kg of whole extract were inhibited by the 5-HT_1A receptor antagonist WAY-100635. Through further separation of AV fractions it was possible to isolate and characterize the flavonol kaempferol which showed an anxiolytic-like activity in concentrations from 0.02 to 1.0 mg/kg p.o. The anxiolytic activity of kaempferol was partially antagonized by concomitant administration of flumazenil, but not by WAY-100635. In conclusion, our study clearly demonstrates that AV extract possesses anxiolytic-like activity and that at least one of its flavonoids, kaempferol, can elicit the same kind of neuropharmacological activity.     

Development of ligands at γ-aminobutyrric acid type A (GABAA) receptor subtype as new agents for pain relief

The identification of compounds with selective anxiolytic-like effects, exerted through the benzodiazepine site on γ-aminobutyric acid type A (GABA_A) receptors, and that show pronounced antihyperalgesia in several pain models, has oriented research towards the development of new agents for the relief of pain. Starting from our previously reported ligands at the benzodiazepine site on GABA_A receptors showing selective anxiolytic-like effects, we have designed new compounds with the aim of identifying those devoid of the typical side effects of the classical benzodiazepines. Our preliminary results indicate that compounds 4, 10(±) and 11 have a very promising antihyperalgesic profile in different animal pain models (peripheral mono-neuropathy, STZ-induced hyperalgesia). In particular 11 exhibits high potency since it exerted its protective effect starting from the dose of 3 mg/kg po, after single injection.     

Rubimetide,humanin, and MMK1 exert anxiolytic-like activities via the formyl peptide receptor 2 in mice followed by the successive activation of DP1, A2A,and GABAA receptors

Rubimetide (Met-Arg-Trp), which had been isolated as an antihypertensive peptide from an enzymatic digest of spinach ribulose-bisphosphate carboxylase/oxygenase (Rubisco), showed anxiolytic-like activity prostaglandin (PG) D₂-dependent manner in the elevated plus-maze test after administration at a dose of 0.1 mg/kg (ip.) or 1 mg/kg (p.o.) in male mice of ddY strain. In this study, we found that rubimetide has weak affinities for the FPR1 and FPR2, subtypes of formyl peptide receptor (FPR). The anxiolytic-like activity of rubimetide (0.1 mg/kg, ip.) was blocked by WRW4, an antagonist of FPR2, but not by Boc-FLFLF, an antagonist of FPR1, suggesting that the anxiolytic-like activity was mediated by the FPR2. Humanin, an endogenous agonist peptide of the FPR2, exerted an anxiolytic-like activity after intracerebroventricular (icv) administration, which was also blocked by WRW4. MMK1, a synthetic agonist peptide of the FPR2, also exerted anxiolytic-like activity. Thus, FPR2 proved to mediate anxiolytic-like effect as the first example of central effect exerted by FPR agonists. As well as the anxiolytic-like activity of rubimetide, that of MMK1 was blocked by BW A868C, an antagonist of the DP₁-receptor. Furthermore, anxiolytic-like activity of rubimetide was blocked by SCH58251 and bicuculline, antagonists for adenosine A_2A and GABA_A receptors, respectively. From these results, it is concluded that the anxiolytic-like activities of rubimetide and typical agonist peptides of the FPR2 were mediated successively by the PGD₂-DP₁ receptor, adenosine-A_2A receptor, and GABA-GABA_A receptor systems downstream of the FPR2.     

Triton x-100 inhibits agonist-induced currents and suppresses benzodiazepine modulation of GABAA receptors in Xenopus oocytes

Changes in lipid bilayer elastic properties have been proposed to underlie the modulation of voltage-gated Na⁺ and L-type Ca²⁺ channels and GABA_A receptors by amphiphiles. The amphiphile Triton X-100 increases the elasticity of lipid bilayers at micromolar concentrations, assessed from its effects on gramicidin channel A appearance rate and lifetime in artificial lipid bilayers. In the present study, the pharmacological action of Triton-X 100 on GABA_A receptors expressed in Xenopus laevis oocytes was examined. Triton-X 100 inhibited GABA_A α₁β₃γ_2S receptor currents in a noncompetitive, time- and voltage-dependent manner and increased the apparent rate and extent of desensitization at 10 μM, which is 30 fold below the critical micelle concentration. In addition, Triton X-100 induced picrotoxin-sensitive GABA_A receptor currents and suppressed allosteric modulation by flunitrazepam at α₁β₃γ_2S receptors. All effects were independent of the presence of a γ_2S subunit in the GABA_A receptor complex. The present study suggests that Triton X-100 may stabilize open and desensitized states of the GABA_A receptor through changes in lipid bilayer elasticity.     

The GABAA receptor is an FMRP target with therapeutic potential in fragile X syndrome

Previous research indicates that the GABA_Aergic system is involved in the pathophysiology of the fragile X syndrome, a frequent form of inherited intellectual disability and associated with autism spectrum disorder. However, the molecular mechanism underlying GABA_Aergic deficits has remained largely unknown. Here, we demonstrate reduced mRNA expression of GABA_A receptor subunits in the cortex and cerebellum of young Fmr1 knockout mice. In addition, we show that the previously reported underexpression of specific subunits of the GABA_A receptor can be corrected in YAC transgenic rescue mice, containing the full-length human FMR1 gene in an Fmr1 knockout background. Moreover, we demonstrate that FMRP directly binds several GABA_A receptor mRNAs. Finally, positive allosteric modulation of GABA_A receptors with the neurosteroid ganaxolone can modulate specific behaviors in Fmr1 knockout mice, emphasizing the therapeutic potential of the receptor.     

Obovatol isolated from Magnolia obovata enhances pentobarbital-induced sleeping time: Possible involvement of GABAA receptors/chloride channel activation

This study aimed to investigate the effects of obovatol isolated from Magnolia obovata on pentobarbital-induced sleeping behaviors and to determine whether these effects were mediated by GABA_A receptors/chloride channel activation, using a western blot technique and Cl^? sensitive fluorescence probe. GABA_A receptors subunits expression and chloride influx were investigated in cultured cerebellar granule cells. Obovatol (0.05, 0.1, and 0.2 mg/kg) prolonged the sleeping time induced by pentobarbital (42 mg/kg). In addition, obovatol (20 and 50 μM) significantly increased Cl^? influx in the primary cultured cerebellar granule cells. Moreover, obovatol increased the expression of GABA_A receptor α-, β-, and γ-subunits. However, it had no effect on the abundance of the expression of glutamic acid decarboxylase (GAD), suggesting that obovatol might not activate GAD. These results suggest that obovatol potentiates pentobarbital-induced sleeping time through the GABA_A receptors/chloride channel activation.     

Antidepressant- and Anxiolytic-Like Effects of New Dual 5-HT1A and 5-HT7 Antagonists in Animal Models

The aim of this study was to further characterize pharmacological properties of two phenylpiperazine derivatives: 1-{2-[2-(2,6-dimethlphenoxy)ethoxy]ethyl}-4-(2-methoxyphenyl)piperazynine hydrochloride (HBK-14) and 2-[2-(2-chloro-6-methylphenoxy)ethoxy]ethyl-4-(2- methoxyphenyl)piperazynine dihydrochloride (HBK-15) in radioligand binding and functional in vitro assays as well as in vivo models. Antidepressant-like properties were investigated in the forced swim test (FST) in mice and rats. Anxiolytic-like activity was evaluated in the four-plate test in mice and elevated plus maze test (EPM) in rats. Imipramine and escitalopram were used as reference drugs in the FST, and diazepam was used as a standard anxiolytic drug in animal models of anxiety. Our results indicate that HBK-14 and HBK-15 possess high or moderate affinity for serotonergic 5-HT₂, adrenergic α₁, and dopaminergic D₂ receptors as well as being full 5-HT_1A and 5-HT₇ receptor antagonists. We also present their potent antidepressant-like activity (HBK-14—FST mice: 2.5 and 5 mg/kg; FST rats: 5 mg/kg) and (HBK-15—FST mice: 1.25, 2.5 and 5 mg/kg; FST rats: 1.25 and 2.5 mg/kg). We show that HBK-14 (four-plate test: 2.5 and 5 mg/kg; EPM: 2.5 mg/kg) and HBK-15 (four-plate test: 2.5 and 5 mg/kg; EPM: 5 mg/kg) possess anxiolytic-like properties. Among the two, HBK-15 has stronger antidepressant-like properties, and HBK-14 displays greater anxiolytic-like activity. Lastly, we demonstrate the involvement of serotonergic system, particularly 5-HT_1A receptor, in the antidepressant- and anxiolytic-like actions of investigated compounds.     

Central prostaglandin D2 exhibits anxiolytic-like activity via the DP1 receptor in mice

We found that prostaglandin (PG) D₂, the most abundant PG produced in the central nervous system (CNS), exhibited anxiolytic-like activity at a dose of 10–100 pmol/mouse after intracerebroventricular (i.c.v.) administration in the elevated plus-maze test in mice. A DP₁ receptor-selective agonist, BW245C, mimicked the anxiolytic-like activity of PGD₂, while a DP₂ receptor agonist 13,14-dihydro-15-keto-PGD₂ was inactive. The anxiolytic-like activity of PGD₂ was blocked by a DP₁ antagonist, BWA868C, suggesting that PGD₂-induced anxiolytic-like activity was mediated by the DP₁ receptor. Adenosine A_2A or GABA_A receptor antagonists, SCH58261 or bicuculline, respectively, also blocked its anxiolytic-like activity. Taken together, centrally administered PGD₂ may induce anxiolytic-like activity via the A_2A and GABA_A receptors, downstream of the DP₁ receptor.     

Progestin receptor-mediated reduction of anxiety-like behavior in male rats

Background

It is well known progesterone can have anxiolytic-like effects in animals in a number of different behavioral testing paradigms. Although progesterone is known to influence physiology and behavior by binding to classical intracellular progestin receptors, progesterone''s anxiety reducing effects have solely been attributed to its rapid non-genomic effects at the GABA_A receptor. This modulation occurs following the bioconversion of progesterone to allopregnanolone. Seemingly paradoxical results from some studies suggested that the function of progesterone to reduce anxiety-like behavior may not be entirely clear; therefore, we hypothesized that progesterone might also act upon progestin receptors to regulate anxiety.

Methodology/Principal Findings

To test this, we examined the anxiolytic-like effects of progesterone in male rats using the elevated plus maze, a validated test of anxiety, and the light/dark chamber in the presence or absence of a progestin receptor antagonist, RU 486. Here we present evidence suggesting that the anxiolytic-like effects of progesterone in male rats can be mediated, in part, by progestin receptors, as these effects are blocked by prior treatment with a progestin receptor antagonist.

Conclusion/Significance

This indicates that progesterone can act upon progestin receptors to regulate anxiety-like behavior in the male rat brain.     

Effects of Nigella sativa on apoptosis and GABAA receptor density in cerebral cortical and hippocampal neurons in pentylenetetrazol induced kindling in rats

We investigated the effects of Nigella sativa on apoptosis and gamma-aminobutyric acid (GABA_A) receptor density in cerebral cortical and hippocampal neurons in a pentylenetetrazol (PTZ)-induced kindling model in rats. The PTZ kindling model was produced by injecting PTZ in subconvulsive doses to rats on days 1, 3, 5, 8, 10, 12, 15, 17, 19, 22 and 24 of the study into animals of PTZ treated (PTZ) and PTZ + N. sativa treated (PTZ + NS) groups. Clonic and tonic seizures were induced by injecting a convulsive dose of PTZ on day 26 of the study. Rats in the PTZ + NS group were treated also with a 10 mg/kg methanolic extract of N. sativa 2 h before each PTZ injection. Rats in the control group were treated with 4 ml/kg saline. The number of neurons that expressed GABA_A receptors in the hippocampus and cerebral cortex of rats in the PTZ and PTZ + NS groups increased significantly. There was no significant difference in the number of GABA_A receptors between the PTZ and PTZ + NS groups. GABA_A receptor density of the neurons in the cerebral cortex, but not hippocampus, was increased in PTZ group compared to controls. We observed a significant increase in the number of apoptotic neurons in the cerebral cortex of rats of both the PTZ and PTZ + NS groups compared to controls. We observed a significant decrease in the number of the apoptotic neurons in the cerebral cortex of rats in the PTZ + NS group compared to the PTZ group. N. sativa treatment ameliorated the PTZ induced neurodegeneration in the cerebral cortex as reflected by neuronal apoptosis and neuronal GABA_A receptor frequency.     

Study of GABA<Subscript>A</Subscript> receptors on the sleep-like behavior in <Emphasis Type="Italic">Coturnix japonica</Emphasis> (Temminck Schlegel, 1849) (Galliformes: Aves)

The present study was carried out to investigate the influence of GABA_A signaling on sleep-like behaviors through systemic administration of bicuculline and picrotoxin (GABA_A antagonists) and thiopental (an allosterical modulator). A thiopental (20 mg/kg) injection increased the eye closure frequency compared to the control group. The birds quickly became sleepy with a low frequency of early behavioral stages, such as rapid oral movement (ROM), feather ruffling and blinking. A bicuculline administration (1 and 4 mg/kg) did not modify the frequency of feather ruffling, ROM, eye closure or blinking responses. A lower dose of picrotoxin (2 mg/kg) stimulated an active awakening status, while an intermediate dose (4 mg/kg) elicited a moderate awakening status, which was associated with an increase in the frequency of ROM, blinking and eye closure. At the higher dose (8 mg/kg), the birds exhibited thermoregulatory-like behaviors and convulsions immediately after the injection. Interestingly, picrotoxin (4 mg/kg) intensified the eye closures when given in combination with thiopental (20 mg/kg). Both barbiturate and picrotoxin-induced sleep-like responses have the same behavioral neuropharmacological properties, conceivably because they are correlated with action at an identical site on the GABA_A receptor.     

Withania somnifera root extract prolongs analgesia and suppresses hyperalgesia in mice treated with morphine

Previous studies demonstrated that Withania somnifera Dunal (WS), a safe medicinal plant, prevents the development of tolerance to the analgesic effect of morphine.In the present study, we investigated whether WS extract (WSE) (100 mg/kg, i.p.) may also modulate the analgesic effect induced by acute morphine administration (2.5, 5, 10 mg/kg, s.c.) in the tail-flick and in the hot plate tests, and if it may prevent the development of 2.5 mg/kg morphine-induced rebound hyperalgesia in the low intensity tail-flick test. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for opioid (μ, δ, k), cannabinoid (CB₁, CB₂), glutamatergic (NMDA), GABAergic (GABA_A, GABA_B), serotoninergic (5HT_2A) and adrenergic (α₂) receptors.The results demonstrated that (i) WSE alone failed to alter basal nociceptive threshold in both tests, (ii) WSE pre-treatment significantly protracted the antinociceptive effect induced by 5 and 10 mg/kg of morphine only in tail-flick test, (iii) WSE pre-treatment prevented morphine-induced hyperalgesia in the low intensity tail-flick test, and (iv) WSE exhibited a high affinity for the GABA_A and moderate affinity for GABA_B, NMDA and δ opioid receptors.WSE prolongs morphine-induced analgesia and suppresses the development of morphine-induced rebound hyperalgesia probably through involvement of GABA_A, GABA_B, NMDA and δ opioid receptors. This study suggests the therapeutic potential of WSE as a valuable adjuvant agent in opioid-sparing therapies.     

Anxiolytic-like effect of Griffonia simplicifolia Baill. seed extract in rats

The seeds of Griffonia simplicifolia Baill., a tropical shrub native to West Africa, are rich in 5-hydroxy-l-tryptophan (5-HTP), a direct precursor in the synthesis of serotonin (5-HT). In spite of the modern therapeutic application of Griffonia simplicifolia seed extract in mood disorders, no scientific evidence has been provided till now. For this reason the aim of our study was to investigate the effect of Griffonia simplicifolia seed extract on anxiety behavior. Griffonia simplicifolia seed extract, dosed at 1, 5, 10 and 25 mg/kg, was orally administered in rats which were submitted to the dark-light test and open field test, 60 min after the treatment. In the dark-light test, the administration of the extract at the doses of 10 and 25 mg/kg was able to significantly increase the time spent in the light compartment (P < 0.05). In the open field test, the extract dosed at 5, 10 and 25 mg/kg induced an anti-tigmotactic effect, as indicated by a significant increase of time spent in the central area of the open field (P < 0.01). In conclusion these findings indicate that Griffonia simplicifolia seed extract exerts anxiolytic-like effect in rats and suggest its potential usefulness for the treatment of anxiety in humans.     

Clozapine's Antipsychotic Effects do not Depend on Blockade of 5-HT3 Receptors

Sixteen known 5-HT₃ receptor blockers, including clozapine, fully or partially reverse the inhibitory effect of 1 M GABA on [³⁵S]TBPS binding, indicating that they are also GABA_A antagonists, some of them selective for subsets of GABA_A receptors. The 5-HT₃ receptor blocker, ondansetron, has been reported to produce some antipsychotic and anxiolytic effects. However, no antipsychotic effects have been reported for a large number of highly potent 5-HT₃ receptor blockers. Like clozapine, ondansetron partially reverses the inhibitory effect of GABA on [³⁵S]TBPS binding. Additivity experiments suggest that ten 5-HT₃ receptor blockers tested at low concentrations preferentially block subtypes of GABA_A receptors that are among those blocked by clozapine. Wiley and Porter (29) reported that MDL-72222, the most potent GABA_A antagonist decribed here, partially generalizes (71%) with clozapine in rats trained to discriminate an interoceptive clozapine stimulus, but only at a dose that severly decreases responding. Tropisetron (ICS-205,930) exhibits both GABA-positive and GABA-negative effects. R-(+)-zacopride is 6-fold more potent than S-(–)-zacopride as a GABA_A antagonist. We conclude that the observed antipsychotic and, possibly, anxiolytic effects of some 5-HT₃ receptor blockers are due to selective antagonism of certain GABA_A receptors, and not to blockade of 5-HT₃ receptors. We speculate that the anxiolytic and sedative effects of clozapine and several other antipsychotic drugs may be due to selective blockade of 122 GABA_A receptors which are preferentially located on certain types of GABAergic interneurons (probably parvalbumin positive). Blockade of these receptors will increase the inhibitory output of these interneurons. So far, no highly potent GABA_A antagonists with clozapine-like selectivity have been identified. Such compounds may exhibit improved clozapine-like antipsychotic activity.     

GABAA Receptors in Central Nervous System Disease: Anxiety,Epilepsy, and Insomnia

Brain function is based on an exquisite balance between excitatory and inhibitory neurotransmission. GABAergic neurons provide the major inhibitory control. By controlling spike timing and sculpting neuronal rhythms they play a key role in regulating behavior. GABAergic neurons are highly diverse and operate with a corresponding diversity of GABA_A receptor subtypes. In this article, the contribution of GABA_A receptor deficits to central nervous system disorders, in particular anxiety disorders, epilepsy, schizophrenia and insomnia, is reviewed.     

Modulation of the expression of GABAA receptors in rat cerebellar granule cells by protein tyrosine kinases and protein kinase C

The expression of GABA_A receptors in rat cerebellar granules in culture has been studied by β_2/3 subunit immunocytochemistry and fluorescence confocal microscopy. These cells show labeling all over the cell bodies' plasma membrane and dendrites. Treatment with the protein tyrosine kinase (PTK) inhibitor genistein results in a decrease of the labeling associated with the β_2/3 subunit in both cell bodies and dendrites. No effect was found with an inactive genistein analogue, daidzein. A similar effect was found with a protein kinase C (PKC) activator, phorbol myristate acetate (PMA). The effects of genistein and PMA are additive.The interpretation of the results is that PTK inhibition blocks exocytotic deposit of newly synthesized GABA_A receptors onto the neuronal plasma membrane. On the other hand, PKC activation speeds up endocytotic removal of GABA_A receptors.     

Moderate concentrations of 4-O-methylhonokiol potentiate GABAA receptor currents stronger than honokiol

Background

Magnolia bark preparations from Magnolia officinalis of Asian medicinal systems are known for their muscle relaxant effect and anticonvulsant activity. These CNS related effects are ascribed to the presence of the biphenyl-type neolignans honokiol and magnolol that exert a potentiating effect on GABA_A receptors. 4-O-methylhonokiol isolated from seeds of the North-American M. grandiflora was compared to honokiol for its activity to potentiate GABA_A receptors and its GABA_A receptor subtype-specificity was established.

Methods

Different recombinant GABA_A receptors were functionally expressed in Xenopus oocytes and electrophysiological techniques were used determine to their modulation by 4-O-methylhonokiol.

Results

3 μM 4-O-methylhonokiol is shown here to potentiate responses of the α₁β₂γ₂ GABA_A receptor about 20-fold stronger than the same concentration of honokiol. In the present study potentiation by 4-O-methylhonokiol is also detailed for 12 GABA_A receptor subtypes to assess GABA_A receptor subunits that are responsible for the potentiating effect.

Conclusion

The much higher potentiation of GABA_A receptors at identical concentrations of 4-O-methylhonokiol as compared to honokiol parallels previous observations made in other systems of potentiated pharmacological activity of 4-O-methylhonokiol over honokiol.

General significance

The results point to the use of 4-O-methylhonokiol as a lead for GABA_A receptor potentiation and corroborate the use of M. grandiflora seeds against convulsions in Mexican folk medicine.