Trans-4-aminocrotonic Acid Research Articles

Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid (TACA)

Investigations into the pharmacology of different types of cys-loop GABA receptor have relied for years on the chemical modification of GABA-like compounds. The GABA metabolite GABOB is an attractive molecule to modify due to its convenient chemical structure. In the process of developing new GABA-mimic compounds from GABOB as a starting compound three small molecule GABA derivatives were synthesized using a variety of chemical transformations. Amongst these, a new and reliable method to synthesize TACA (trans-4-aminocrotonic acid) is reported.

Sodium and water intake are not affected by GABAC receptor activation in the lateral parabrachial nucleus of sodium-depleted rats

The activation of GABAergic receptors, GABAA and GABAB, in the lateral parabrachial nucleus (LPBN) increases water and sodium intake in satiated and fluid-depleted rats. The present study investigated the presence of the GABAC receptor in the LPBN, its involvement in water and sodium intake, and its effects on cardiovascular parameters during the acute fluid depletion induced by furosemide combined with captopril (Furo/Cap). One group of male Wistar rats (290–300g) with bilateral stainless steel LPBN cannulas was used to test the effects of a GABAC receptor agonist and antagonist on the fluid intake and cardiovascular parameters. We investigated the effects of bilateral LPBN injections of trans-4-aminocrotonic acid (TACA) on the intake of water and 0.3M NaCl induced by acute fluid depletion (subcutaneous injection of Furo/Cap). c-Fos expression increased (P<0.05), suggesting LPBN neuronal activation. The injection of different doses of TACA (0.5, 2.0 and 160 nmol) in the LPBN did not change the sodium or water intake in Furo/Cap-treated rats (P>0.05). Treatment with the GABAC receptor antagonist (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid sulfate (ZAPA, 10nmol) or with ZAPA (10nmol) plus TACA (160nmol) did not change the sodium or water intake compared with that for vehicle (saline) (P>0.05). Bilateral injections of the GABAC agonist in the LPBN of Furo/Cap-treated rats did not affect the mean arterial pressure (MAP) or heart rate (HR). The GABAC receptor expression in the LPBN was confirmed by the presence of a 50kDa band. Although LPBN neurons might express GABAC receptors, their activation produced no change in water and sodium intake or in the cardiovascular parameters in the acute fluid depletion rats. Therefore, the GABAC receptors in the LPBN might not interfere with fluid and blood pressure regulation.

Structural basis for selective GABA binding in bacterial pathogens

GABA acts as an intercellular signal in eukaryotes and as an interspecies signal in host-microbe interactions. Structural characteristics of selective eukaryotic GABA receptors and bacterial GABA sensors are unknown. Here, we identified the selective GABA-binding protein, called Atu4243, in the plant pathogen Agrobacterium tumefaciens. A constructed atu4243 mutant was affected in GABA transport and in expression of the GABA-regulated functions, including aggressiveness on two plant hosts and degradation of the quorum-sensing signal. The GABA-bound Atu4243 structure at 1.28 Å reveals that GABA adopts a conformation never observed so far and interacts with two key residues, Arg(203) and Asp(226) of which the role in GABA binding and GABA signalling in Agrobacterium has been validated using appropriate mutants. The conformational GABA-analogue trans-4-aminocrotonic acid (TACA) antagonizes GABA activity, suggesting structural similarities between the binding sites of the bacterial sensor Atu4243 and mammalian GABA(C) receptors. Exploration of genomic databases reveals Atu4243 orthologues in several pathogenic and symbiotic proteobacteria, such as Rhizobium, Azospirillum, Burkholderia and Pseudomonas. Thus, this study establishes a structural basis for selective GABA sensors and offers opportunities for deciphering the role of the GABA-mediated communication in several host-pathogen interactions.

Muscimol and related GABA receptor agonists: the potency of GABAergic drugs in vivo determined after intranigral injection.

Contralateral turning behaviour following unilateral intranigral injection of a large series of GABA analogues was investigated. The results indicated that the turning behaviour was induced stereospecifically and was selectively antagonized by the GABA antagonist bicuculline methochloride. The comparative potencies of a series of GABA agonists related to muscimol in general corresponded well to the affinity for 3H-GABA receptor sites and to the depressant action on single neurons using microelectrophoretic administration. However, the GABA agonists trans-aminocrotonic acid and 3-aminopropanesulphonic acid were much weaker than expected from in vitro studies. The GABA-uptake inhibitors nipecotic acid and guvacine showed only weak and short-lasting effects. The GABA-transaminase inhibitor gamma-acetylenic GABA showed delayed effects compared with the agonists which acted immediately. It is proposed that this behavioural effect may be a sensitive and quantitative method for evaluation of GABA agonists in vivo.

Glutamatergic and GABAergic effects of fipronil on olfactory learning and memory in the honeybee

We investigated here the role of transmissions mediated by GABA and glutamate-gated chloride channels (GluCls) in olfactory learning and memory in honeybees, both of these channels being a target for fipronil. To do so, we combined olfactory conditioning with injections of either the GABA- and glutamate-interfering fipronil alone, or in combination with the blocker of glutamate transporter L-trans-Pyrrolidine-2,4-Dicarboxylicacid (L-trans-PDC), or the GABA analog Trans-4-Aminocrotonic Acid (TACA). Our results show that a low dose of fipronil (0.1 ng/bee) impaired olfactory memory, while a higher dose (0.5 ng/bee) had no effect. The detrimental effect induced by the low dose of fipronil was rescued by the coinjection of L-trans-PDC but was rather increased by the coinjection of TACA. Moreover, using whole-cell patch-clamp recordings, we observed that L-trans-PDC reduced glutamate-induced chloride currents in antennal lobe cells. We interpret these results as reflecting the involvement of both GluCl and GABA receptors in the impairment of olfactory memory induced by fipronil.

Vigabatrin transport across the human intestinal epithelial (Caco‐2) brush‐border membrane is via the H+‐coupled amino‐acid transporter hPAT1

The aim of this investigation was to determine if the human proton-coupled amino-acid transporter 1 (hPAT1 or SLC36A1) is responsible for the intestinal uptake of the orally-administered antiepileptic agent 4-amino-5-hexanoic acid (vigabatrin). The Caco-2 cell line was used as a model of the human small intestinal epithelium. Competition experiments demonstrate that [3H]GABA uptake across the apical membrane was inhibited by vigabatrin and the GABA analogues trans-4-aminocrotonic acid (TACA) and guvacine, whereas 1-(aminomethyl)cyclohexaneacetic acid (gabapentin) had no affect. Experiments with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)-loaded Caco-2 cells demonstrate that apical exposure to vigabatrin and TACA induce comparable levels of intracellular acidification (due to H+/amino-acid symport) to that generated by GABA, suggesting that they are substrates for a H+ -coupled absorptive transporter such as hPAT1. In hPAT1 and mPAT1-expressing Xenopus laevis oocytes [3H]GABA uptake was inhibited by vigabatrin, TACA and guvacine, whereas gabapentin failed to inhibit [3H]GABA uptake. In Na+ -free conditions, vigabatrin and TACA evoked similar current responses (due to H+/amino-acid symport) in hPAT1-expressing oocytes under voltage-clamp conditions to that induced by GABA (whereas no current was observed in water-injected oocytes) consistent with the ability of these GABA analogues to inhibit [3H]GABA uptake. This study demonstrates that hPAT1 is the carrier responsible for the uptake of vigabatrin across the brush-border membrane of the small intestine and emphasises the therapeutic potential of hPAT1 as a delivery route for orally administered, clinically significant GABA-related compounds.

Correlation of the apparent affinities and efficacies of gamma-aminobutyric acid(C) receptor agonists.

gamma-Aminobutyric acid (GABA), trans-4-aminocrotonic acid (TACA), muscimol, imidazole-4-acetic acid (I4AA), cis-4-aminocrotonic acid (CACA), and isoguvacine are all GABA(C) receptor agonists. These compounds have different apparent sensitivities (EC(50)) and efficacies (I(max)) on exogenously expressed human rho1 homomeric GABA(C) receptors. It is not clear if these differences are due to distinct binding affinities and/or distinct gating kinetics. In this study, using a recently developed single oocyte binding technique, we determined the apparent dissociation constants (K(i) values) of these compounds from their IC(50) values for [(3)H]GABA displacement. The apparent K(i) values fell into two distinct groups. The high affinity group was comprised of agonists with longer distances between the nitrogen atom of the amino or imidazole group and the carbon atom of the carboxyl or isoxazole group. The single oocyte binding technique, in conjunction with two-electrode voltage clamp, has allowed a direct correlation of the apparent affinity, efficacy, and potency of agonists on intact functional GABA(C) receptors. The correlation and coupling of these parameters are discussed in terms of a simple proposed activation mechanism.

Taurine release modified by GABAergic agents in hippocampal slices from adult and developing mice

In order to characterize the possible regulation of taurine release by GABAergic terminals, the effects of several agonists and antagonists of GABA receptors on the basal and K+-stimulated release of [3H]taurine were investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice using a superfusion system. Taurine release was concentration-dependently potentiated by GABA, which effect was reduced by phaclofen, saclofen and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) at both ages, suggesting regulation by both GABA(B) and GABA(C) receptors. The involvement of GABA(A) receptors could not be excluded since the antagonist bicuculline was able to affect both basal and K+-evoked taurine release. Furthermore, several GABA(B) receptor effectors were able to inhibit K+-stimulated taurine release in the adults, while the GABA(C) receptor agonists trans-4-aminocrotonic acid (TACA) and cis-4-aminocrotonic acid (CACA) potentiated this release. The potentiation of taurine release by agents acting on the three types of GABA receptors in both adult and developing hippocampus further indicates the involvement of transporters operating in an outward direction. This inference is corroborated by the moderate but significant inhibition of taurine uptake by the same compounds.

Analogues of gamma-aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) substituted in the 2 position as GABAC receptor antagonists.

1. gamma-Aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) have been shown to activate GABAC receptors. In this study, a range of C2, C3, C4 and N-substituted GABA and TACA analogues were examined for activity at GABAC receptors. 2. The effects of these compounds were examined by use of electrophysiological recording from Xenopus oocytes expressing the human rho 1 subunit of GABAC receptors with the two-electrode voltage-clamp method. 3. trans-4-Amino-2-fluorobut-2-enoic acid was found to be a potent agonist (KD = 2.43 microM). In contrast, trans-4-amino-2-methylbut-2-enoic acid was found to be a moderately potent antagonist (IC50 = 31.0 microM and KB = 45.5 microM). These observations highlight the possibility that subtle structural substitutions may change an agonist into an antagonist. 4. 4-Amino-2-methylbutanoic acid (KD = 189 microM), 4-amino-2-methylenebutanoic acid (KD = 182 microM) and 4-amino-2-chlorobutanoic acid (KD = 285 microM) were weak partial agonists. The intrinsic activities of these compounds were 12.1%, 4.4% and 5.2% of the maximal response of GABA, respectively. These compounds more effectively blocked the effects of the agonist, GABA, giving rise to KB values of 53 microM and 101 microM, respectively. 5. The sulphinic acid analogue of GABA, homohypotaurine, was found to be a potent partial agonist (KD = 4.59 microM, intrinsic activity 69%). 6. It was concluded that substitution of a methyl or a halo group in the C2 position of GABA or TACA is tolerated at GABAC receptors. However, there was dramatic loss of activity when these groups were substituted at the C3, C4 and nitrogen positions of GABA and TACA. 7. Molecular modelling studies on a range of active and inactive compounds indicated that the agonist/competitive antagonist binding site of the GABAC receptor may be smaller than that of the GABAA and GABAB receptors. It is suggested that only compounds that can attain relatively flat conformations may bind to the GABAC receptor agonist/competitive antagonist binding site.

Ionotropic GABA receptor from lobster olfactory projection neurons.

This study reports an ionotropic GABA (gamma-aminobutyric acid) receptor in projection neurons acutely dissociated from the olfactory lobe of the brain of the spiny lobster and analyzed by whole cell and cell-free patch-clamp recording. GABA evokes a macroscopic current in the cells that is linear from -100 to + 100 mV, reverses at the imposed chloride equilibrium potential, has a permeability sequence of Cl- > acetate > bicarbonate > phosphate > propionate and SCN- > Br- > I- > Cl- > F-, and is reversibly blocked by the Cl channel blocker picrotoxin but not tert-butylbicyclophosphorothionate (TBPS). The current is bicuculline insensitive and activated by muscimol, isoguvacine, cis-4-aminocrotonic acid (CACA), and trans-aminocrotonic acid (TACA), as well as by the GABA(C)-receptor antagonists 4,5,6,7-tetrahydroisoxazolo [5,4,-c]pyridin-3-ol (THIP), 3-amino-1-propanesulfonic acid (3-APS), and imidazole-4-acetic acid (I-4AA), but not the GABA(B)-receptor agonists baclofen and 3-aminopropylphosphonic acid (3-APA). Agonist potency for the receptor is TACA > muscimol > GABA > I-4AA > isoguvacine > 3-APS > CACA > THIP. Unitary chloride currents in cell-free, outside-out patches from the cells share enough of these pharmacological properties to indicate that the channel underlies the macroscopic current. The receptor mediates an inhibitory current in the cells in vivo. The receptor is similar, if not identical, to one from neurons cultured from the thoracic ganglia of the clawed lobster. The more extensive pharmacological characterization of the receptor reported here indicates that this lobster CNS receptor is pharmacologically distinct from previously characterized ionotropic GABA receptors.

Agonist pharmacology of two Drosophila GABA receptor splice variants.

1. The Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunits, RDLac and DRC 17-1-2, form functional homo-oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N-terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo-oligomers and their agonists pharmacology compared with that of insect and vertebrate iontropic GABA receptors. 2. The actions of GABA, isoguvacine and isonipecotic acid on RDLac and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists of both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDLac. However, the relative potencies of these agonists on each receptor were very similar. 3. A more complete agonist profile was established for RDLac homo-oligomers. The most potent agonists of these receptors were GABA, muscimol and trans-aminocrotonic acid (TACA), which were approximately equipotent. RDLac homo-oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA > ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid) > isoguvacine > imidazole-4-acetic acid > or = isonipecotic acid > or = cis-aminocrotonic acid (CACA) > beta-alanine. 3-Aminopropane sulphonic acid (3-APS), a partial agonist of RDLac homo-oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4. SR95531, an antagonist of vertebrate GABAA receptors, competitively inhibited the GABA responses of RDLac homo-oligomers, which have previously been found to insensitive to bicuculline. However, its potency (IC50 500 microM) was much reduced when compared to GABAA receptors. 5. The agonist pharmacology of Drosophila RDLac homo-oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDLac homo-oligomers from bicuculline-insensitive vertebrate GABAC receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDLac and DRC 17-1-2 homo-oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.

Preferential suppression of the ON pathway by GABAC receptors in the amphibian retina

1. Electrophysiological recordings were obtained from neurons in the amphibian intact retina and retinal slice preparations. The effects of gamma-aminobutyric acid (GABA) were evaluated in the presence of bicuculline or SR95531, which block the GABAA receptor, and baclofen, which saturates the GABAB receptor. 2. Under these conditions, GABA preferentially reduced ON light responses in amacrine and ganglion cells, apparently through a presynaptic mechanism that reduced bipolar cell input. GABA also produced a small hyperpolarization in the resting membrane potential of ganglion cells. 3. Picrotoxin blocked these effects of GABA. The action of GABA was duplicated by muscimol and by trans-aminocrotonic acid. Cis-aminocrotonic acid was neither a potent nor selective agonist. This pharmacology is indicative of the GABAC receptor. 4. In voltage-clamp recordings of ganglion cells in the slice preparation, GABA produced a large chloride conductance that was blocked by bicuculline or SR95531, and a smaller chloride conductance that was not blocked by these GABAA receptor antagonists, but was blocked by picrotoxin. This indicates that ganglion cells possess both GABAA and GABAC receptors. 5. The GABAC receptor current was relatively nondesensitized. Consequently, whereas the peak GABAA receptor current was more than fivefold larger than the GABAC receptor current, after desensitization the latter current was larger. Both currents reversed near the chloride equilibrium potential.

GABA- and glycine-activated currents in the rod bipolar cell of the rabbit retina.

1. Voltage- and ligand-gated currents were recorded from solitary rabbit rod bipolar cells using the whole cell patch-clamp technique. The rod bipolar cell forms a single, stereotypical physiological and morphological class of cells that was easily identified from other neurons and support cells after enzymatic and mechanical dissociation from isolated retina. Protein kinase C immunoreactivity confirmed the validity of using a purely morphological identification of this cell type. 2. Voltage steps in 15-mV increments from a holding potential of -45 mV elicited a large outward current activated near -30 mV. These voltage-gated currents were eliminated by using equimolar substitutions of Cs+ and tetraethylammonium+ for K+ in the pipette, indicating that they represent a mixture of K+ currents. 3. The putative inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated inward Cl- currents when pressure-applied from pipettes placed near the axon terminals of rod bipolar cells, which were voltage-clamped at -45 mV. With changes in intracellular or extracellular Cl- concentration, the reversal potential of these ligand-gated currents changed as predicted by the Nernst equation for Cl- activity. The dose-response curves for GABA and glycine were sigmoidal with saturating concentrations of 100 and 300 microM, respectively. 4. GABA-activated currents were 1) reversibly reduced by the allosteric inhibitor picrotoxin and the competitive antagonist bicuculline; 2) potentiated by the benzodiazepine diazepam and the barbiturate barbital sodium; and 3) indistinguishable from muscimol-activated currents. There was no response to the GABAB agonist baclofen. Collectively, these data strongly suggest that the GABA-activated currents in rabbit rod bipolar cells are mediated by the GABAA receptor. This is similar to the GABA-activated currents in other mammalian rod bipolar cells. 5. Application of the conformationally restricted GABA analogue cis-4-aminocrotonic acid (CACA) failed to elicit a response, whereas the conformationally extended GABA analogue trans-4-aminocrotonic acid (TACA) elicited a response similar to that of GABA. Although bicuculline appeared to suppress the GABA-activated current slightly more than the TACA-activated current (not significant using Student's t-distribution), GABA- and TACA-activated currents were equally suppressed by picrotoxin and equally enhanced by diazepam and barbital sodium. These data, coupled with the inefficacy of CACA, argue against the existence of a GABAC-type channel in the rod bipolar cell of the rabbit and suggest that GABA and TACA were activating the same GABAA receptor-channel complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotransmitter receptors of starburst amacrine cells in rabbit retinal slices

The receptor pharmacology of cholinergic ("starburst") amacrine cells was studied in a newly developed rabbit retinal slice preparation with whole-cell patch clamp. Displaced starburst cells were labeled with the fluorescent dye 4,6-diamidino-2-phenylindole (DAPI), and their dendritic morphology was identified in the slice with Lucifer yellow. Under conditions in which synaptic transmission was blocked by Cd2+, starburst cells responded vigorously to the bath-applied neurotransmitters GABA, glycine, and glutamate. The response to GABA consisted of an inward current and an increase in noise, which could be mimicked by the GABAA agonists muscimol and trans-4-aminocrotonic acid (TACA), but not by the GABAB agonist baclofen or the GABAC agonist cis-4-aminocrotonic acid (CACA). The GABA-evoked currents were reversibly inhibited by bicuculline and picrotoxin and had a reversal potential close to the chloride equilibrium potential. Noise analysis of GABA-activated whole-cell currents yielded elementary conductance estimates of 12.5 pS. Glycine (30-200 microM) also activated a Cl- conductance in starburst cells, which could be completely blocked by strychnine. The non-NMDA agonists kainate (KA, 30-100 microM) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 60 microM) evoked robust responses, which were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and which reversed near the equilibrium potential for cations. NMDA coapplied with glycine in salines free of Cd2+ and Mg2+ elicited small but detectable responses. The I/V relation of the NMDA-evoked response showed a characteristic "J"-shaped region in a saline containing 1 mM Mg2+ and 0 Cd2+, indicating that NMDA receptors were present directly on starburst cells. This was consistent with our finding that whole-cell currents evoked by KA and NMDA had different noise characteristics. These results place new constraints on models of starburst cell function and suggest that GABA-mediated inhibition of the starburst cell itself may play an important role in directional selectivity in the retina.

A novel GABA receptor on bipolar cell terminals in the tiger salamander retina

We studied the pharmacology of the GABA receptors on bipolar cell terminals in the retinal slice preparation. Whole-cell patch-clamp recordings were made from the somas of bipolar cells and GABA was puffed near their terminals, after synaptic transmission was blocked. GABA puffs evoked a large chloride current that was reduced by picrotoxin, but in many cells this current was insensitive to blockade by the competitive GABAA receptor antagonists bicuculline and SR95531. Pentobarbital, an enhancer of GABAA receptor-mediated responses, did not significantly increase the magnitude of the current responses to GABA puffed at the bipolar cell terminals. To confirm the effectiveness of GABAA antagonists and pentobarbital in the slice preparation, we measured GABA currents in ganglion cells. In contrast to bipolar cells, the ganglion cell GABA responses were strongly reduced by both bicuculline and SR95531. In addition, pentobarbital strongly enhanced the action of GABA at the ganglion cells. The isomeric GABA agonists cis- and transaminocrotonic acid (CACA and TACA), elicited picrotoxin-sensitive currents in both bipolar and ganglion cells. TACA was more effective than CACA at both cell types. In bipolar cells, TACA and CACA currents were relatively resistant to bicuculline blockade, but in ganglion cells both currents were reduced by bicuculline. GABA receptors on bipolar terminals appear to be pharmacologically different from the GABA receptors found on ganglion cell dendrites. The bipolar cell terminal GABA receptor pharmacology is similar to the pharmacology reported for the rho 1 GABA receptor subunit that was isolated from retina and expressed in Xenopus oocytes (Cutting et al., 1991; Polenzani et al., 1991; Shimada et al., 1992). This receptor, which is both bicuculline and pentobarbital insensitive, has been called the GABAC receptor (Johnston, 1986; Shimada et al., 1992). However, some bipolar cells were somewhat sensitive to blockade by bicuculline, suggesting that these cells had both GABAA and GABAC receptors on their bipolar terminals.

Pharmacology of GABA rho 1 and GABA alpha/beta receptors expressed in Xenopus oocytes and COS cells.

1. The rho 1 protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of gamma-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists. 2. cDNAs for the rho 1 and the alpha 5 beta 1 receptors for GABA were expressed as homo- and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration-response experiments in voltage clamped cells. 3. The most potent agonists at the rho 1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the alpha 5 beta 1 receptor the rank order was muscimol > GABA > 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP). The most specific agonists were cis-(2-(aminomethyl)-cyclopropyl-carboxylic acid (CAMP) and THIP for the rho 1 and the alpha 5 beta 1 receptors, respectively. 4. Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at rho 1 receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of rho 1 seems independent of the expression system. 5. Agonists THIP, piperidine-4-sulphonic acid (P4S), and isoguvacine, whose C-C-C-N chains are constrained by rings into a folded conformation and were potent at the alpha 5 beta 1 receptor, were among the weakest at the rho 1 receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the alpha 5 beta 1 receptor but moderately potent at the pl receptor. These findings suggest that the rho l receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6. In contrast to the alpha 5 beta 1 receptor, gradations in maximum responses were apparent in the rho l receptor,suggesting various degrees of partial agonism. In particular, imidazole-4-acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent Kd for activating the rho l receptor of 16 microM; but it had an apparent Kd for competitively blocking the receptor of 0.64 microM. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor.7. Hill coefficients approached 2 at the rho l receptor, but were closer to unity at the alpha 5 beta 1 receptor. Thus,the rho l receptor displayed higher cooperativity.8. Unlike typical GABAA receptors, the rho l receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.

Pharmacological characteristics of two different types of inhibitory GABA receptors on Achatina fulica neurones

GABA (γ-aminobutyric acid) receptors of Achatina fulica neurones have been classified into two types associated with neuronal inhibition and one type with excitation. The pharmacological features of muscimol I and baclofen types associated with inhibition were investigated in this study. Activation of muscimol I type receptors on TAN (tonically autoactive neurone) by GABA, muscimol and trans-4-aminocrotonic acid (TACA) produced a transient outward current (I out) with an increase in membrane conductance (g). Their relative potencies at GABA ED 50 (approximately 10 −4 M) were : GABA : muscimol : TACA = 1:0.6:0.3. The relation between I out and g increase (Δ g) induced by various concentrations of these compounds was linear. The Hill coefficients for GABA were close to 1.0. The GABA effects were potentiated by pentobarbitone, antagonized competitively by pitrazepin and non-competitively by picrotoxin and diazepam, and unaffected by bicuculline. The reversal potentials of the effects of GABA, muscimol and TACA on TAN changed under various [Cl −] 0 according to the Nernst equation for E c1, but not under various [K +] 0 and [Na +] 0. Activation of baclofen type GABA receptors on RPeNLN (right pedal nerve large neurone) by GABA and (±)-baclofen produced a slow I out with an increase in g. The two compounds were almost equipotent (ED 50: approximately 3 × 10 −4 M). The relation between I out and Δ g produced by various concentrations was linear. The Hill coefficients for GABA were also close to 1.0. The reversal potentials of GABA and (±)-baclofen on RPeNLN changed under various [K +] 0 according to the Nernst equation for E K, but not under various [Cl −] 0 and [Na +] 0. The two compounds hardly affected the voltage-gated and slowly inactivating calcium current. The I out produced by GABA and (±)-baclofen was reduced by tetraethylammonium chloride, but was unaffected by 4-aminopyridine, bicuculline, pitrazepin and picrotoxin. In conclusion, the pharmacological features of muscimol I type GABA receptors are partly comparable to those of mammalian GABA A receptors, except for the influences of bicuculline and diazepam: the features of the baclofen type GABA receptor, which did not occur with muscimol I type receptors in the same neurone, were similar to those GABA B.

Pharmacology of a novel effect of γ-aminobutyric acid on the frog optic tectum in vitro

Excitatory postsynaptic field potentials (termed U 1 and U 2 waves) were extracellularly recorded from superficial layers of the frog optic tectum in vitro following stimulation of the contralateral optic nerve. γ-Aminobutyric acid (GABA) largely enhanced the U 1 and U 2 waves, an effect which was progressively diminished by lowering the extracellular Cl − content. Muscimol or 3-aminopropanesulphonic acid (3-APS) was more potent than GABA in enhancing the U 1 and U 2 waveforms, whereas trans-4-aminocrotonic acid (TACA) was equipotent, its cis isomer (CACA) being less active. The action of GABA, TACA and CACA was relatively insensitive to bicuculline (100 μM). Pentobarbitone increased the U 1 and U 2 waves and antagonized the action of GABA. Midazolam did not change the GABA-induced responses. Picrotoxin antagonized both pentobarbitone and GABA effects. It is suggested that in the frog optic tectum GABA can activate an atypical receptor mechanism blocked by picrotoxin but comparatively insensitive to bicuculline.

3- and 5-Isoxazolol zwitterions: an ab initio molecular orbital study relating to GABA agonism and antagonism

The Hartree-Fock ab initio molecular orbital method has been applied to eight compounds: GABA (gamma-amino butyric acid) (1), its partially rigidified analog, TACA (trans-4-aminocrotonic acid) (2), six isoxazolol analogs; muscimol (5-aminomethylisoxazol-3-ol (3), THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) (4), THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) (5), isomuscimol (3-aminomethylisoxazol-5-ol) (6), iso-THIP (4,5,6,7-tetrahydroisoxazolo[3,4-c] pyridin-5-ol) (7), and iso-THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-5-ol) (8). GABA is an endogenous inhibitory transmitter. The four following molecules (2), (3), (4) and (5) are agonist: they bind themselves to the GABA receptors and induce approximately the same effect as GABA. (6) is lightly agonist, presenting a lower affinity. Compounds (7) and (8) are antagonists, giving rise to convulsion. Optimized molecular conformations of GABA (1), muscimol (3) and isomuscimol (6) are discussed. Geometric and electronic parameters showing the presence of intramolecular hydrogen bonds are presented. The permutation of the heteroatoms in the isoxazole ring has no effect on the side-chain orientation explaining maybe the agonist character of isomuscimol, being able to adopt easily and exactly the active conformation. Atomic charge distributions and electronic overlap populations for all compounds have been computed in order to try to understand why their GABAergic activities can be so different. The computed values show that the 3-isoxazolol ring mimics in a good way the carboxylic function of GABA. They also illustrate the larger electronic delocalization within the 5-isoxazolol ring and therefore the resulting antagonist character, except for isomuscimol.