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The activity of the nitropropane analog of baclofen, 3-amino-2-(4-chlorophenyl)-nitropropane (N-BAC), has been examined at central and peripheral GABAB receptors. N-BAC was less potent than baclofen as a GABAB receptor agonist in depressing repetitive twitch contractions in the guinea-pig isolated ileum (IC50s for baclofen = 4.1 +/- 1.3 microM; N-BAC = 9.2 +/- 0.3 microM) and vas deferens (IC50s for baclofen = 30 microM; N-BAC = 100 microM), competitively antagonised by phaclofen, 2-hydroxysaclofen and CGP 35348 (3-aminopropyl-P-di-ethoxymethylphosphinic acid). In the ileum, the pA2 values for CGP 35348 with baclofen (4.7 +/- 0.2) and N-BAC (4.6 +/- 0.3) were not significantly different (P > 0.05), indicating that both agonists activate the same receptor type. By contrast, in rat neocortical slices, N-BAC was 20 times weaker than baclofen in attenuating spontaneous discharges, sensitive to CGP 35348, whilst it was 100 times less potent than baclofen in depressing evoked CA1 population spikes in the hippocampus. This new GABAB receptor agonist, N-BAC, is thus more active at peripheral than central GABAB receptors.

R-(-)-beta-phenyl-GABA has been compared at GABAB receptors using cortical and ileal preparations. R-(-)-beta-phenyl-GABA (EC50 = 25 microM) was a less potent full agonist than R,S-(+/-)-baclofen (EC50 = 2.5 microM), in depressing CA1 population spikes of rat hippocampal slices, and 5 times less potent in attenuating the spontaneous discharges of rat neocortex. However, R-(-)-beta-phenyl-GABA (100-400 microM) was only a weak partial agonist in the ileum. All these actions were sensitive to CGP 35348 (3-aminopropyl-(P-diethoxymethyl)-phosphinic acid) and therefore mediated by GABAB receptors.

In the guinea-pig isolated ileum, beta-(2-thienyl)-gamma-aminobutyric acid (BTG; 100-500 microM) reversibly and competitively (pA2 = 4.3 +/- 0.1) antagonised the baclofen-induced (5-100 microM) depression of cholinergic twitch contractions, but not that to adenosine or morphine. By contrast, in rat neocortical slice preparations, BTG (100-500 microM) acted as an agonist, abolishing the frequency and amplitude of spontaneous discharges, sensitive to 2-hydroxysaclofen (100-500 microM). BTG exhibits differential actions at GABAB receptors in brain and periphery.

The actions of 2-hydroxy-saclofen (2-OH-S), a recently developed analog of baclofen, were studied at presynaptic GABAB receptors in the rat hippocampal slice. Baclofen (0.5-20 microM) reduces the amplitude of excitatory postsynaptic potentials (EPSPs) recorded from hippocampal CA1 pyramidal neurons. In the presence of 200-500 microM 2-OH-S, the synaptic depressant action of baclofen is significantly reduced. These data show that 2-OH-S is an effective antagonist at presynaptic GABAB receptors on excitatory terminals in the hippocampus.

The actions of the GABA analog 3-aminopropanephosphinic acid (3-APA) were studied in the guinea-pig isolated ileal preparation and at synapses between cultured rat hippocampal neurons. Like the GABAB receptor agonist, baclofen, 3-APA inhibited the electrically evoked ileal twitch. The EC50 for 3-APA was 0.8 microM; the EC50 for baclofen was 9 microM. In addition, the depressant responses to 3-APA and baclofen were blocked by the GABAB receptor antagonists phaclofen, saclofen, 2-hydroxy-saclofen and delta-aminovaleric acid. 3-APA also mimicked the presynaptic action of baclofen at GABAergic synapses between embryonic rat hippocampal neurons in culture. 3-APA reduced the amplitude of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) by greater than 50% at a concentration of 1 microM, while baclofen reduced synaptic transmission to a similar degree at 10 microM. 3-APA did not alter membrane conductance, nor did the drug alter postsynaptic responses to GABA. These data show that 3-APA is a potent agonist at presynaptic GABAB receptors in the periphery and on GABAergic neurons from the central nervous system. The activity of 3-APA at central postsynaptic GABAB receptors remains to be studied.

1. The phosphono-analogues of gamma-aminobutyric acid (GABA), 4-amino-butylphosphonic acid (4-ABPA), 3-amino-2-(4-chlorophenyl)-propylphosphonic acid (phaclofen) and 3-amino-2-cyclohexylpropyl-phosphonic acid, each antagonized the GABA- and baclofen-induced GABAB-receptor-mediated depression of twitch responses to transmural stimulation in the guinea-pig isolated ileum, in a concentration-dependent, reversible and surmountable manner (apparent pA2 = 4.0 +/- 0.1, 4 +/- 0.2 and 3.7 +/- 0.2 respectively, compared with 3.9 +/- 0.1 for delta-aminovaleric acid). No such activity was found in a variety of related analogues. 2. By contrast, 3-amino-propylphosphonic acid (3-APPA) behaved as a partial agonist, itself partly depressing ileal twitch contractions in a manner sensitive to 4-ABPA and phaclofen, as well as antagonizing the depression of the ileal twitch by GABA and baclofen (apparent pA2 = 4.0 +/- 0.2). 3. Both 4-ABPA and phaclofen also antagonized the baclofen-induced depression of the twitch in the guinea-pig isolated vas deferens (apparent pA2 = 4.0 +/- 0.1 for each), whilst 3-APPA behaved as a partial agonist, slightly depressing the vas twitch, and antagonised the baclofen-induced depression of the twitch (apparent pA2 = 3.9 +/- 0.1). 4. None of these phosphono-analogues exhibited any action at ileal GABAA-receptors, nor influenced the ileal twitch depression with morphine, adenosine or noradrenaline, suggesting their selectivity as antagonists at GABAB-receptors.

Gamma-aminobutyric acid (GABA) and its receptors are found in a wide range of peripheral tissues, including parts of the peripheral nervous system, endocrines, and non-neural tissues such as smooth muscle and the female reproductive system. In all these, both GABAA- and GABAB-receptor types are found, with good evidence for a physiological role in the gut, pancreatic islets and the urinary bladder. In some tissues, the pharmacology of GABA-induced actions is quite atypical and should be further explored with the newer ligands and modulators for GABAA- and GABAB-receptors.

In the isolated ileum of the guinea-pig, gamma-aminobutyric acid (GABA) elicited both GABAA- and GABAB-receptor-mediated responses. Barbiturates significantly potentiated the GABAA receptor-induced contractions over the lower dose range of applied GABA (less than 50 microM), without affecting the GABAB-receptor-induced 'after-relaxation', their relative potencies being thiopentone (ThP) greater than pentobarbitone (PB) greater than barbitone (Bb) greater than phenobarbitone (PhB). Also, higher doses of ThP, PB and Bb elicited bicuculline- and picrotoxinin-sensitive GABA-like contractions. PB shifted the dose-response curve for GABA in the presence of bicuculline methochloride to the left without altering the slope or the dose ratio, and not only displaced the dose-response curve for GABA in the presence of picrotoxinin but also restored the slope and the maximum response towards that of the control GABA-induced response. These results further characterise the GABAA-receptor sites of the guinea-pig enteric nervous system, where GABA is evidently a neurotransmitter.

Phaclofen, the phosphonic acid derivative of baclofen, reversibly antagonized the depression of the cholinergic twitch response of the guinea pig ileum and distal colon by either baclofen or GABA. When administered microelectrophoretically, phaclofen reversibly blocked the presumed presynaptic reduction by baclofen of the monosynaptic excitation of spinal interneurones by impulses in primary afferent fibres of the cat but did not block the postsynaptic depressant action of baclofen on these neurones. Phaclofen may thus be useful in determining the physiological significance of central and peripheral bicuculline-insensitive receptors with which GABA and (-)-baclofen interact.

In preloaded, isolated segments of duodenum, jejunum, ileum and colon, ethylenediamine (EDA) caused a 3-mercaptopropionic acid (3-MPA)-sensitive release of [3H]GABA. Both EDA and GABA caused a contraction and a delayed 'after-relaxation' in the ileum, but only a delta-aminovaleric acid (DAVA)-sensitive relaxation in other intestinal segments; these actions of EDA were reduced by 3-MPA and therefore due to release of endogenous GABA. Baclofen induced a DAVA-sensitive relaxation in all tissues. Evidently, GABA modulates cholinergic excitation of the smooth muscle at all levels of the intestine.

When administered microelectrophoretically, a sulphonic acid derivative of baclofen, 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid, reversibly reduced the presynaptic reduction by (-)-baclofen of the monosynaptic excitation of spinal interneurones by impulses in low threshold primary afferent fibres of the cat as well as the postsynaptic depression by (-)-baclofen of the firing of these neurones. This compound, 2-hydroxy-saclofen, may be useful in assessing the physiological significance of central baclofen receptors.

Cortisol had a biphasic action on GABAA-receptor-mediated contractile responses, enhancing at picomolar concentrations (1-10 pM) and inhibiting at higher concentrations (10-1000 nM). There was a sinistral shift of the GABA dose-response curve in the presence of 10 pM cortisol, with a significant potentiation of the GABA-induced contractions over the lower dose range of GABA (3-30 microM), whereas 100 nM cortisol caused a non-parallel dextral shift of the GABA dose-response curve, with a depression of the maximum GABA response indicative of non-competitive antagonism. Cortisol at various concentrations did not affect GABAB-receptor-mediated ileal relaxations, or the baclofen-induced depression of twitch contractions to transmural stimulation. Such concentrations of cortisol also did not affect ileal responses to exogenously applied acetylcholine or cholinergic twitch contractions themselves. These results suggest that cortisol is a specific, and very potent modulator at GABAA-receptor complexes in the guinea pig ileum.

4,6,6-Trimethylcaprolactam antagonised GABAA receptor-mediated contractile responses in guinea-pig isolated ileum, displacing the GABA dose-response curve to the right in a non-parallel manner, and causing a depression of the maximum response. Pentobarbitone not only potentiated the GABAA receptor-mediated contractions but also reversed this non-competitive antagonism by 4,6,6-trimethylcaprolactam, shifting the dose-response curve for GABA to the left and restoring the maximum response. It is conclude that this caprolactam acts at the picrotoxin-barbiturate site on the Cl(-)-ionophore complex.

The effects of the GABAB receptor antagonist 2-hydroxy-saclofen were studied using intracellular recording of synaptic potential from CA1 hippocampal neurons. 2-Hydroxy-saclofen (50-200 microM) reversibly blocked the late, GABAB receptor-mediated inhibitory postsynaptic potential (IPSP) but not the early, GABAA receptor-mediated IPSP. In addition, the hyperpolarizing response to baclofen was reduced by similar concentrations of 2-hydroxy-saclofen. This suggests that 2-hydroxy-saclofen is a potent antagonist at postsynaptic GABAB receptors on hippocampal neurons.

Saclofen (the direct sulphonic analogue of baclofen) is a competitive antagonist of baclofen at GABAB receptors in guinea pig ileum and rat cortical slices (estimated pA2 = 5.3), at least twice as potent as 2-hydroxy-saclofen (pA2 = 5). A series of related sulphonic analogues also antagonised baclofen in the guinea pig ileum, including 2-hydroxy-saclofen amide (pA2 = 3.3), 3-amino-2-hydroxy-2-phenyl-propylsulphonic acid (pA2 = 3.5), 3-amino-2-(benzo-(b)-furan-2-yl)-propylsulphonic acid (pA2 = 4.3), and 3-amino-2-(4-chlorophenyl)-prop-1-enesulphonic acid (pA2 = 2.5-3), but none were more active than saclofen which is the most potent specific GABAB antagonist yet found.

2-hydroxy-saclofen (2-OH-S), a sulphonic analogue of baclofen, slightly increased the twitch height and reversibly antagonised the GABA- and baclofen-induced depression of twitch contractions in the guinea pig vas deferens and isolated ileum, causing a parallel dextral shift in the baclofen dose-response curve in a competitive manner (pA2 = 5.0) in the latter tissue. 2-OH-S (10-50 microM) reversibly elevated the spike height and antagonised the baclofen (8-20 microM)-induced suppression of ictal discharges in rat cortical slices superfused in Mg2+-free Krebs solution, the spike height declining to control level within 15 min of washout. The antagonism by 2-OH-S on GABAB receptor-mediated actions is selective, as 2-OH-S did not affect depressive responses to adenosine or morphine, or contractile responses to GABA (GABAA receptor-mediated), acetylcholine and carbachol in the ileum. Compared to phaclofen, 2-OH-S is a more potent competitive antagonist of GABAB receptor-mediated actions in the central and peripheral nervous system.

Pregnenolone and its sulphate had concentration-dependent dual actions on gamma-aminobutyric acid (GABAA)-receptor-mediated ileal contractile responses, enhancing at 0.001-1 microM but inhibiting at 10 microM. There was a sinistral shift of the GABA dose-response curve in the presence of 0.01 microM pregnenolone, with a significant potentiation over the lower concentration range of GABA (3-30 microM), whilst 10 microM pregnenolone depressed the curve by 50% without a significant change in the EC50, in a manner resembling non-competitive inhibition with picrotoxin. Pregnenolone and its sulphate evidently have modulatory actions at ileal GABAA-receptor complexes in keeping with those seen using neurochemical studies in the central nervous system.

Baclofen, 3-amino-propylphosphonic acid (3-APPA), and beta-phenyl-GABA (BPG), each reduced the frequency of spontaneous paroxysmal discharges in rat neocortical slices maintained in Mg2+-free medium, reversibly antagonised by phaclofen and 4-amino-butylphosphonic acid (4-ABPA). At lower concentrations, not influencing the discharges, both 3-APPA and BPG also reversibly antagonised this action of baclofen. However, des-chloro-phaclofen was inactive. Thus, phaclofen and 4-ABPA are GABAB-receptor antagonists in neocortex, whereas both 3-APPA and BPG have partial agonist/antagonist activity at cortical GABAB-receptors.