The structure-activity relationship for GABA and related compounds in the crayfish muscle

Studies on the mechanism of action of picrotoxinin and other convulsants at the crustacean muscle GABA receptor

The actions of picrotoxinin, bicuculline and penicillin-G were investigated on the GABA-receptor system of lobster muscle by using intracellular recording. The highly potent antagonist, picrotoxinin, produced a lateral shift and depression in the maximum of the GABA dose--conductance curve (designated as mixed antagonism); bicuculline, a weak antagonist, caused only a depression in the maximum with little or no lateral shift, whereas penicillin-G, an even weaker antagonist, produced a greater depression at the top of the dose--response curve. The possible sites of antagonist action were examined, with a critical re-evaluation of a drug-receptor model previously proposed to account for the antagonistic behaviour of picrotoxinin (the mixed antagonistic model); this model was extended to include the actions of bicuculline and penicillin-G. Antagonism was examined (i) towards different GABA receptor agonists; (ii) in various external anion media; (iii) at varying external pH; and (iv) when two different antagonists were combined. The GABA agonists were differentially antagonized by picrotoxinin and bicuculline, but external pH and substituent anions caused only minor perturbations to the inhibition. Combination experiments suggested at least three sites for GABA antagonists binding on crustacean muscle: (i) the GABA recognition site or sites; (ii) the ionic selectivity site in the ionophore; and (iii) a highly lipophilic site which may be part of the GABA receptor or ionophore. The mixed antagonism model accounted for the pH and external anion data but required modification to a cyclic scheme to explain the antagonism of a partial agonist. A model based on two-state receptor theory could only account for the antagonism of GABA if picrotoxinin was assumed not only to perturb L (the R rightleftharpoons T conformation constant) but also to affect the agonist binding affinity. It is suggested that picrotoxinin and bicuculline may antagonize GABA responses by stabilizing the closed form of the activated channel, whereas penicillin-G may block the channel in the open state.

The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored. 2. All the neurotransmitters except glycine had a measurable effect on this system. 3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated. 4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors. 5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated. 6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect. 7. Finally, L-Dopa was stimulatory, but only at a high concentration.

The action of GABA receptor agonists and antagonists on muscle membrane conductance in Schistocerca gregaria

1. The properties of postsynaptic gamma-aminobutyric acid (GABA) receptors in the extensor tibiae muscle of Schistocerca gregaria were studied by conventional electrophysiological recording techniques. 2. GABA and other active GABA receptor agonists produced rapid, dose-dependent, reversible increases in membrane conductance. 3. In two microelectrode experiments the ED50 for GABA was approximately 1 mM. In three microelectrode experiments (assuming short cable theory conditions) the ED50 for GABA was 2.3 mM. The Hill coefficient for GABA estimated from the latter experiments was 1.4. 4. The relative potency of muscimol/GABA at the ED50 for GABA was 1.36. 3-Aminopropane sulphonic acid (3-APS) and isonipecotic acid were weakly active, baclofen and piperidine-4-sulphonic acid (P4S) were inactive. Isoguvacine produced depolarizations and increases in conductance in preparations which hyperpolarized in response to GABA. These depolarizations were enhanced by both picrotoxin and pitrazepin although the increases in input conductance were depressed. 5. Picrotoxin (20 microM), (+)-bicuculline (20-100 microM) and pitrazepin (1-10 microM) all reversibly antagonized GABA-induced responses. Such antagonism was not competitive in the case of picrotoxin and (+)-bicuculline but was competitive for pitrazepin. Schild plot analysis gave an average pA2 value of 5.5 for pitrazepin. 6. The significance of these results is briefly discussed.

Effects of gamma-aminobutyric acid on isolated cone photoreceptors of the turtle retina

Isolated cones dissociated from the retina of the freshwater turtle were voltage clamped using a single 'patch' pipette electrode. gamma-Aminobutyric acid (GABA) applied ionophoretically to the axon terminal evoked an inward current in cells held at -66 mV when they were recorded with patch pipettes filled with the 'control' pipette solution containing 120 mM-Cl-. Polarity of the GABA-induced current reversed near 0 mV when examined with the pipette filled with the control pipette solution. The reversal potential depended strongly on both external and intrapipette Cl- concentrations ([Cl-]o, and [Cl-]p). The reversal potential agreed with the equilibrium potential for Cl- calculated by the Nernst equation with given [Cl-]o and [Cl-]p. The reversal potential was not affected by concentrations of either external Na or K ions. Voltage responses evoked by GABA were hyperpolarizing from its resting level of about -50 mV immediately after the rupture of the patch membrane. The response polarity reversed into depolarization in a few seconds when [Cl-]p was greater than 24 mM, while hyperpolarizing responses persisted when [Cl-]p was less than 12 mM. Thus, the intracellular Cl- concentration of undisturbed isolated cones was estimated to be between 12 and 24 mM. Cones were desensitized to GABA in the presence of GABA (greater than 100 nM) in the medium, or by a prolonged ionophoretic application. The maximum reduction in response amplitude was about 70% in both experiments. Muscimol was as potent as GABA, while beta-p-chlorophenyl-GABA (baclofen) was ineffective even at 100 microM. GABA was antagonized by bicuculline competitively, and by picrotoxin non-competitively. These observations suggest that turtle cones have GABAA receptors which associate with chloride channels. The present results suggest that GABA, presumably released continuously from monophasic horizontal cells in the dark, would exert a tonic hyperpolarization in red-sensitive and green-sensitive cones. Suppression by light of tonic GABA release would depolarize these types of cones by disinhibition. Disinhibitory depolarization in cones may contribute to the centre surround antagonism in retinal neurones, and to the biphasic colour responses recorded in a subtype of horizontal cells.

Some properties of solubilized GABA receptor

gamma-Aminobutyric acid (GABA) receptor was solubilized from synaptic membrane of the rat brain by various detergents. Nonidet P-40, a non-ionic detergent, was found to be an effective solubilizing agent, since it caused no interference on the receptor binding assay, yielded a [3H]muscimol binding protein with a high specific activity and no aggregation, and preserved good stability of the solubilized fraction. Ammonium sulfate precipitation of the solubilized supernatant significantly increased the binding of [3H]muscimol to GABA receptor, possibly by removing heat-stable and small molecular inhibitory substances. The specific [3H]muscimol binding to the soluble fraction obtained by Nonidet P-40 treatment and subsequent ammonium sulfate precipitation, was saturable with KD 13 and 64 nM, and Bmax 3.4 and 1.8 pmol/mg protein, respectively. The enhancement of the [3H]muscimol binding by diazepam as found in synaptic membrane was also detected in the soluble fraction. Molecular weight of the [3H]muscimol binding site was determined by gel filtration on Sephadex G-200 and was calculated to be 270,000 daltons. This value was identical with that of the [3H]flunitrazepam binding site which appeared in the same solubilized fraction. These results indicate that the properties of solubilized GABA receptor are identical to those of membrane-bound GABA receptor. Furthermore, the present results suggest that both GABA and benzodiazepine receptors may reside on the same macromolecule in synaptic membrane.

Dose-conductance relationships for GABA agonists and the effect of uptake inhibitors in crayfish stretch receptor neurons

The interaction of GABA (gamma-aminobutyric acid) and structurally-related compounds with postsynaptic GABA receptors was studied quantitatively by measuring receptor-mediated increases in membrane input conductance in isolated crayfish stretch receptor neurons (SRN). The following compounds, in order of decreasing potency, were effective agonists: muscimol greater than GABA greater than isoguvacine greater than (-)gamma-amino-beta-hydroxybutyric acid greater than beta-gu anidinopropionic acid greater than 3-aminopropanesulfonic acid greater than (+)gamma-amino-beta-hydroxybutyric acid greater than isonipecotic acid greater than THIP. A highly significant correlation was found between the log potencies for GABA agonists that were obtained in the SRN and those obtained in our laboratory using mammalian GABA receptor binding assays. Hill plot analyses of the log concentration-conductance data from the SRN indicated a Hill slope (nH) of approximately 2 for all agonists except GABA and guanidinopropionic acid (nH greater than 2), two compounds known to be actively accumulated by cellular GABA uptake processes. Nipecotic acid, guvacine, and L-alpha, beta-diaminopropionic acid, blockers of GABA uptake processes, had essentially no effect by themselves on the SRN membrane input conductance at concentrations up to 5 mM, however, they potentiated the effects of sub-maximal concentrations of GABA and decreased the steepness of the log concentration-conductance curve, and consequently nH, for GABA. The effects of muscimol, however, were not affected. When the influence of uptake processes was considered, it appeared that all agonists tested acted by the same cooperative mechanism which required at least two molecules of agonist to activate a receptor-ionophore unit.

Pharmacological characterization of GABA receptors mediating vasodilation of verebral arteries in vitro

GABA (gamma-aminobutyric acid) produced a dose-dependent dilation of isolated cat and dog cerebral artery segments which had been given an active, tonic contraction by either prostaglandin F2 alpha or serotonin. No effect of GABA on extracranial blood vessels was observed. The GABA-induced dilation could be blocked in a dose-dependent manner by either bicuculline or picrotoxin. The latter agent appeared to act as a competitive antagonist. GABA agonists muscimol, imidazoleacetic acid, delta-aminovaleric acid, (+/-)gamma-amino-beta-hydroxybutyric acid, and beta-alanine also relaxed actively contracted cerebral arteries dose-dependently. The relative potency of these agonists was consistent with that established for GABA receptors on neurons and invertebrate striated muscle. GABA was also tested on two human cerebral arteries and found to cause a small dilation. The results support the existence of a cerebrovascular GABA receptor which may mediate an interaction between GABA and the cerebral circulatory system.

gamma-Aminobutyric acid-stimulated chloride permeability in crayfish muscle

gamma-Aminobutyric acid selectively increased Cl- permeability in isolated strips of crayfish abdominal muscle. Muscle fibers incubated in Van Harrevald's solution at room temperature took up 36Cl- to the extent of 700 ml/kg wet weight with a halftime of 2.5 min. During 15-S incubations, the control 36Cl- uptake space was 131 +/- 4 ml/kg (n = 60) and this was significantly increased by gamma-aminobutyric acid at 200 muM or higher concentrations to 177 +/- 4 ml/kg (n = 48, P less than 0.05). This effect was specific for chloride since gamma-aminobutyric acid did not increase the uptake by crayfish muscle of radioactive sucrose, inositol, or propionate. gamma-Aminobutyric acid stimulation of 36Cl- uptake is mediated by receptor-ionophore function since the process shows pharmacological properties virtually identical to those observed by electrophysiological techniques. The gamma-aminobutyric acid stimulation of Cl- permeability is dose dependent with 50% of the maximal effect at 40 muM gamma-aminobutyric acid and the dose vs. response curve is somewhat sigmoid. The gamma-aminobutyric acid agonist muscimol causes the same maximal effect on Cl- uptake as gamma-aminobutyric acid, but acts at 5-fold lower concentrations, i.e. is more potent. However, the partial agonist gamma-amino, beta-hydroxybutyric acid produced little or no stimulation of 36Cl- flux. The response to gamma-aminobutyric acid was blocked by 2 mM beta-guanidinopropionate or gamma-guanidinobutyrate, 0.5 mM bicuculline, and 10 muM picrotoxinin. Picrotoxinin inhibition was dose dependent with 50% inhibition occurring at 4 muM. Antagonists did not affect control 36Cl- uptake. These results confirm electrophysiological observations that the postsynaptic response to the inhibitory neurotransmitter gamma-aminobutyric acid involves a rapid increase in membrane permeability to Cl-.