Differential effects of sodium-free media on gamma-aminobutyrate and muscimol-evoked conductance increases recorded from lobster muscle fibres

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.

Potentiation of GABA(A) receptor agonists by GABA uptake inhibitors in the rat ventral midbrain

Whole-cell patch recordings were made from dopamine-containing neurons in the ventral tegmental area (VTA) and substantia nigra zona compacta (SNC). Isoguvacine evoked an outward current (at -60 mV) in a concentration-dependent manner with an EC50 of 62+/-8 microM. The gamma-aminobutyric acid (GABA) uptake inhibitor 1-(2(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO 711) (3 microM) shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 22+/-4 microM. L-Arginine (3 mM) also shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 29+/-5 microM. L-Arginine (3 mM) increased the currents evoked by GABA (100 microM) and muscimol (1 microM) by 208% and 261%, respectively. The GABA uptake inhibitor 4,5,6,7,-tetrahydroisoxazolo[4,5-c]-pyridin-3-ol hydrobromide (THPO) (300 microM) not only mimicked but also occluded the ability of L-arginine (3 mM) to potentiate currents evoked by isoguvacine. Equimolar replacement of Na+ with choline increased GABA-evoked currents, suggesting that a low Na+ concentration has an inhibitory effect on GABA transport. Low Na+ concentration (25 mM) inhibited isoguvacine currents but still occluded the potentiating effects of L-arginine. We conclude that GABA uptake inhibitors potentiate the actions of the GABA(A) receptor agonists, isoguvacine and muscimol, probably because they are effective substrates for GABA transporters in the ventral midbrain.

Effect of central noradrenaline depletion on corticosterone levels and gastric ulcerations in rats

Effects of central noradrenergic depletion on the stress responses of rats were explored using the new selective neurotoxin (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4)). Noradrenergic depletion using DSP-4 was followed by a reduction in basal corticosterone levels after 7 days. Three weeks after DSP-4 treatment, animals exhibited less severe and fewer gastric ulcerations than control animals following 23 h immobilization stress, but stress levels of corticosterone were similar for the two groups. No differences could be found in the peripheral gastric levels of noradrenaline between experimental and control animals, while central noradrenaline was reduced to approximately 30% of control levels. The data support previous findings using other methods that central noradrenaline is an important factor in stress-induced gastric ulceration. The peripheral mechanisms for the protective effects of DSP-4 remain to be elucidated, and studies of these may cast light on the efferent pathways between the central nervous system and gastric mucosa which are involved in stress-induced gastric pathology.

Inhibition of GABA uptake potentiates the conductance increase produced by GABA-mimetic compounds on single neurones in isolated olfactory cortex slices of the guinea-pig

Membrane potential and input conductance were recorded in single neurones in slices of guinea-pig olfactory cortex in vitro. gamma-Aminobutyric acid (GABA) and GABA-mimetic compounds were applied by bath-perfusion. Potency was measured as the concentration required to double the input conductance. The potency of GABA was increased (i.e. the equi-effective concentrations were reduced) by 15.5 +/- 2.3 times (mean +/- s.e. mean) on reducing external [Na+] from 144 to 20 mmol l-1, by replacement with Mg2+. Corresponding potency changes for other agonists were + 10.8 +/- 2.5 for 3-aminopropanesulphonic acid (3-APS); 3.25 +/- 1.06 for isoguvacine and 2.43 +/- 0.69 for muscimol. Nipecotic acid (0.5 mM) produced the following increases in potency: GABA 2.68 +/- 0.02; 3-aminopropanesulphonic acid, 3.11 +/- 0.07; isoguvacine, 1.92 +/- 0.34; muscimol, 2.24 +/- 0.17. The concentration of GABA in the bathing fluid necessary to double input conductance increased with increasing depth of the recording site from the cut surface. The apparent potency fell 10 times for each 60 micron depth increment up to 150 micron. The recording depth also affected the apparent potency of muscimol and 3-APS but to a lesser extent. Reduction of external [Na+] reduced the depth-dependence of both GABA and 3-APS potency. No clear change in the duration of the recurrent inhibitory postsynaptic conductance could be detected in the presence of 0.5 mmol l-1 nipecotic acid. 6 It is suggested that agonist uptake by a Na+-dependent, nipecotic acid-sensitive mechanism severely attenuates the responses of olfactory neurones to exogenous GABA and to its analogues 3-APS, muscimol and isoguvacine, but has little immediate influence on the duration of the GABA-mediated inhibitory postsynaptic conductance.

Diazepam stimulates the binding of GABA and muscimol but not THIP to rat brain membranes

Diazepam (10-1000 nM) enhanced the binding of [3H]GABA and of the monocyclic GABA agonist [3H]muscimol, but failed to alter binding of the bicyclic GABA agonist [3H]THIP to fresh, well washed rat brain membranes incubated at 2 degrees C. Although stimulation of [3H]diazepam binding by THIP was observed at higher incubation temperatures and in the presence of chloride ions, these measures did not induce a corresponding enhancement of [3H]THIP binding by diazepam. These results extend earlier observations of the unusual behavior of THIP as a selective GABA agonist, and emphasize that enhancement of benzodiazepine binding by GABA agonists is not necessarily reflected in a complementary manner by any action of benzodiazepines on the binding of GABA agonists.

Effects of various cations on the slow K+ conductance increases induced by carbachol, histamine and dopamine in Aplysia neurones

A study was made of the effects of various cations other than K+ on three K+ conductance increases induced by carbachol, histamine and dopamine in an identified group of Aplysia neurones: the 'A' neurones of the cerebral ganglion. The 3 responses were sensitive to alterations of both the extracellular and the intracellular concentrations of Na+ and Ca2+. In particular, they could be reduced markedly by: (a) lowering [Na]0 (replacing NaCl by either Tris-HCl, glucosamine chloride, MgCl2 or sucrose); (b) increasing [Na]i (by intracellular injection of Na+, or by blockade of the Na+-K+ pump); (c) increasing the extracellular divalent cation concentration; or (d) increasing [Ca]i4. Some of the effects of Na+ and divalent cations appear to occur on reaction steps common to the three K+ responses, while others probably imply reaction steps specific to one of the systems, since they differ according to the agonist used. The sensitivity to Na+ and Ca2+ of slow inhibitory responses due entirely to an increase in K+ conductance must be taken into account in the interpretation of some slow hyperpolarizing responses previously assumed to involve changes in Na+ conductance.

Na+-dependent and Na+-independent binding of GABA in CNS subcellular particles--a search for GABA receptors

Studies of the Na+-dependent and Na+-independent binding of [3H] gamma-aminobutyric acid ([3H]GABA) and [3H]muscimol to central nervous system (CNS) subcellular preparations have been briefly reviewed and analyzed. Although some of the binding process that have been detected in vitro, in the presence or absence of Na+, appear to represent GABA-receptor interactions, further studies conducted with physiologic media and with highly purified membrane preparations are necessary in order to define the properties of those neuronal membrane receptors that mediate the in vivo actions of GABA.