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Contractions of the guinea-pig ileum elicited by GABA were reversibly antagonised by various concentrations of picrotoxinin and bicuculline. This antagonism was specific for GABA-induced responses, responses to ACh, BK, and HA being unaffected. The chloride ion channel blockers, furosemide, 2 x 10-4 mol/l, and piretanide, 5.5 x 10-5 mol/l, substantially reduced GABA-induced contractions of the ileum but not those elicited by ACh or electrical stimulation of the cholinergic nerves. The action of GABA in stimulating the intrinsic cholinergic nerves appears to be Cl- dependent.

1. Alfaxalone (1-100 nM) potentiated gamma-aminobutyric acidA (GABAA)-receptor-mediated contractile responses in the guinea-pig isolated ileum, with a leftward shift of the GABA concentration-response curve, and a significant potentiation of the GABA-induced contractions over the lower concentration-range for GABA (3-30 microM). Alfadalone on the other hand, did not affect contractile responses to GABA. 2. Picrotoxinin (10 microM) induced a non-parallel rightward shift of the GABA concentration-response curve, with a 50% depression of the maximum response to GABA. Alfaxalone (100 nM) potentiated the responses to GABA in the presence of picrotoxinin (10 microM) over the GABA concentration-range of 10-100 microM, causing a leftward shift of the concentration-response curve, but without affecting the depression of the maximum response by picrotoxinin. 3. Bicuculline methochloride (10 microM) caused a parallel rightward shift of the GABA concentration-response-curve; the ratio of this shift was unchanged in the presence of alfaxalone (100 microM), although the latter itself displaced the curve leftwards. 4. Alfaxalone (1-100 mM) also induced a similar potentiation of contractile responses to 3-amino-1-propanesulphonic acid (3-APS), a GABA agonist not subject to uptake. Such concentrations of alfaxalone were ineffective against contractile responses to exogenous acetylcholine. 5. Higher concentrations of alfaxalone (1 microM and above), however, elicited a GABA-like ileal contraction, sensitive to both picrotoxinin (10 microM) and bicuculline (10 microM). 6. In conclusion, alfaxalone potentiated GABAA-receptor-mediated contractile responses in the guinea-pig isolated ileum by acting at a modulatory site on GABAA-receptor-chloride-ionophore complexes of GABA-sensitive myenteric neurones, whilst high concentrations of alfaxalone exhibited a GABA-mimetic action at GABAA-receptors in the ileum. It is suggested that more than one site may exist where steroids interact with the GABAA-receptor-ionophore complexes.

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.

Lack of effect of tetrodotoxin and of an extract from the posterior salivary gland of the blue-ringed octopus following injection into the octopus and following application to its brachial nerve. Toxicon 23, 997-999, 1985. Injections of the blue-ringed octopus salivary gland extract and tetrodotoxin into the blue-ringed octopus have no ill-effect on the animals. Similarly, in vitro nerve preparations from the animal were not affected by these materials although they are both extremely potent on bioelectrically excitable preparations from other species.

In isolated segments of the guinea-pig ileum, there was: (a) an early, short-lived (less than 20 s) depression by gamma-aminobutyric acid (GABA) of contractile responses to 5-hydroxytryptamine (5-HT), acetylcholine(ACh), or nicotine, also seen with 3-amino-1-propanesulphonic acid (3APS) or muscimol in place of GABA, and sensitive to bicuculline, picrotoxinin or piretanide, and (b) a delayed, longer-lasting (30 s-1 min) depression of responses to 5-HT and nicotine, but not exogenously applied ACh, also seen with baclofen and only antagonised by delta-aminovaleric acid (DAVA). At 25 degrees C, all these effects were still observed but slowed, whilst at 37 degrees C after cold storage (6 degrees C) overnight, the early, short-lived depression was reduced or eliminated, yet the delayed depression was enhanced. It is concluded that the early, short-lived depression is mediated through GABAA-receptor sites, and the delayed, longer-lasting depression through GABAB-receptor sites on neurones of the myenteric plexus; effects consistent with GABA being a neurotransmitter in the enteric nervous system.

The actions of gamma-aminobutyric acid (GABA) and its analogues, 3-amino-1-propanesulphonic acid (3APS) and baclofen, have been investigated using isolated segments of the guinea-pig ileum and distal colon. GABA and 3APS, but not baclofen, induced GABAA-receptor mediated effects; prompt, dose-dependent contractions of the ileum which were antagonised by bicuculline, picrotoxinin, piretanide, tetramethylenedisulphotetramine, atropine and tetrodotoxin. Baclofen and GABA, but not 3APS, induced a dose-dependent GABAB-receptor mediated depression of electrically elicited twitch contractions of the ileum, unaffected by the GABAA-receptor antagonists or by antagonism of adenosine, adrenergic, opiate or nicotinic receptors. In the distal colon, baclofen and GABA caused a bicuculline- and picrotoxinin-insensitive depression of spontaneous cholinergic contractions. Desensitization to GABA and baclofen, and cross-desensitization to both agonists was observed. Combined antagonism of GABAA-receptors and desensitization to baclofen slowed pellet expulsion to the same extent as GABA desensitization alone, indicating that both GABAA- and GABAB-receptor sites are involved in this modification of peristalsis by GABA.

Two novel beta-(benzo[b]furan) analogues of baclofen (4-amino-3-benzo[b]furan-2-ylbutanoic acid, 9G, and 4-amino-3-(5-methoxybenzo[b]furan-2-yl)butanoic acid, 9H) antagonised the baclofen-induced depression of twitch contractions in the guinea-pig isolated ileum (estimated apparent pA2 3.9 and 4.1 respectively); both 9G and 9H also antagonised in a dose-dependent manner the baclofen-induced reduction of repetitive paroxysmal discharges in rat neo-cortical slice preparations maintained in Mg2+-free Krebs solution. These benzofurans evidently represents a new class of GABAB-receptor antagonist.

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.

Localization of [3H]GABA in the guinea-pig myenteric plexus has been studied using light microscopic autoradiography. In the presence of beta-alanine, 10(-3) M, an inhibitor of glial cell high affinity GABA transport, [3H]GABA was transported by a high affinity uptake system into neuronal elements of the plexus. Scattered neurones accumulating [3H]GABA showed localization of silver grains over the soma and axonal processes. In addition a large population of uptake sites for [3H]GABA was found within the secondary and tertiary meshworks of the plexus so that dense accumulations of silver grains were observed localized over distinct 'tracts' within all three meshworks of the plexus. These results are considered to provide strong evidence for GABAergic neurons in the enteric nervous system.

Possible involvement of gamma-aminobutyric acid (GABA) in intestinal motility of the guinea-pig has been investigated using the speed of propulsion of faecal pellets along isolated segments of guinea-pig distal colon, and the rate of pellet expulsion from freshly excised colon. GABA antagonism, by bicuculline or by tachyphylaxis to GABA, substantially reduced intestinal motility and generally reduced the amplitude of the reflex contraction associated with pellet propulsion. These results support the view that GABAergic mechanisms may be involved in the propulsive activity of the guinea-pig distal colon.

Skeletal components and the exudate of the sponge Neofibularia mordens, as well as four fractions of the extract were investigated. Possible causes, and management, of the contact dermatitis resulting from application of extract fractions were studied. Antimicrobial activity of the sponge extract fractions, together with their effect on small crustaceans, was determined. Two fractions exhibited strong neuroactivity on guinea-pig isolated intestine, octopus-arm nerve-muscle and crustacean cheliped nerve-muscle preparations.

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.

Axons of the rat isolated cervical vagus nerve responded to gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) with a readily reversible dose-dependent depolarization. Prior application of 5-HT at 2 or 30 microM equally depressed the depolarizing responses to GABA, shifting the GABA dose-response curve to the right and depressing the maximum; by contrast, responses to 5-HT were affected only by large doses of GABA (greater than 300 microM), and to a lesser degree. In addition, the fade in responses to high doses of GABA was also reduced in the presence of 5-HT. It is suggested that such all-or-none depressive actions may explain the blockade of GABA-induced responses by 5-HT observed in other tissues such as the guinea-pig ileum.

In the isolated ileum of the guinea pig, neurally mediated rhythmic longitudinal mechanical activity was induced by avermectin, a macrolide anthelmintic that releases gamma-aminobutyric acid (GABA) and modulates the GABAA-receptor-ionophore complex. This avermectin-induced activity was dependent on GABA, being reduced or abolished by bicuculline, a GABAA-receptor antagonist, and by 3-mercaptopropionic acid which prevents neural GABA release. These results provide additional direct evidence that GABA is a functional neurotransmitter in the myenteric plexus of the guinea pig intestine, evidently involved in the regulation of intestinal motility.

High affinity uptake, and the distribution of 3H-radiolabelled gamma-aminobutyrate (GABA), cis-3-aminocyclohexanecarboxylic acid, beta-alanine, proline, and leucine have been examined autoradiographically in laminar preparations of the myenteric plexus from the guinea-pig intestine. Following labelling with [3H]proline and [3H]leucine, which are incorporated into neurons, silver grains were concentrated over recognisable perikarya in the ganglia and meshworks of the plexus, whilst [3H]GABA labelled a smaller proportion of neurons and their processes. Specificity of labelling in the sites of [3H]GABA-uptake was established using combinations of labelled and unlabelled GABA, beta-alanine, and cis-3-aminocyclohexanecarboxylic acid, substrates for glial or neuronal high affinity GABA uptake systems. Only myenteric neurons and their processes were labelled significantly by [3H]GABA and its analogue cis-3-[3H]aminocyclohexanecarboxylic acid. Using autoradiographs of laminar preparations and paraffin sections, [3H]GABA labelling was found over nerve fibre bundles that could be traced from their ganglionic origins through the interconnecting meshworks of the myenteric plexus into the innervation of the deep muscular plexus of the circular muscle layer where GABA is evidently concerned with prejunctional modulation of transmitter release. The extensive but selective distribution of [3H]GABA high affinity uptake sites in neural elements of the guinea-pig myenteric plexus is consistent with GABA being an enteric neurotransmitter.

The lateral olfactory tract (LOT) of the ferret has been shown to project to most of the pyriform lobe, as in the cat. Only a small medio-posterior region of the pyriform cortex (the angular cortex), which has a distinctly different appearance in Nissl stained sections, proves to be devoid of olfactory connections. Despite the fact that sub-areas can be recognized within olfactory cortex, there is an underlying constancy in design throughout. Latency measurements indicate that fine collateral branches of theLOT reach the posterior olfactory cortex, whereas mainly larger diameter (faster conducting) fibres comprise the contribution to anterior olfactory cortex. Thus in the present context it is sufficient to recognise just an anterior and posterior subdivisionof olfactory cortex, contingent on the above criterion and correlated with cytoarchitectural features, chiefly variations in cortical layers II and III of the regions concerned. Pyriform cortex directly in receipt of LOT fibres, and associated deeper cortical zones, generated potentials in the olfactory bulbs which can only be attributed to centrifugal input, thus further substantiating an olfactory role for this cortex. In addition, non-cortical regions in receipt of LOT fibres, namely the anterior olfactory nucleus, olfactory tubercle, cortical amygdaloid nucleus and nucleus of the lateral olfactory tract, also contribute centrifugal input to the bulbs. All these regions are thereby capable of providing a rather direct feedback on olfactory bulb activity. The exact pathways concerned have yet to be determined.

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.

In the guinea-pig isolated ileum, both gamma-aminobutyric acid (GABA) and baclofen induced a dose-dependent depression of cholinergic twitch contractions to transmural stimulation, sensitive to delta-aminovaleric acid (DAVA) and phosphonobaclofen (phaclofen). beta-Phenyl-GABA (BPG) antagonised this depressant action of baclofen and GABA, whilst itself weakly depressing ileal twitch contractions, an effect insensitive to DAVA or phaclofen, and thus unrelated to any GABAB-receptor-mediated effects. These results suggest that the baclofen receptors in the ileum that are antagonised by BPG differ from either of baclofen receptors in the spinal cord, where the presynaptic receptors are blocked by phaclofen and the postsynaptic receptors are insensitive to phaclofen, with BPG having baclofen-like actions at both sites. Interaction of BPG and baclofen with different receptor populations may explain the differing therapeutic actions of these compounds.

Both baclofen and gamma-aminobutyric acid (GABA) induced a dose-dependent depression of cholinergic twitch contractions in the transmurally stimulated guinea-pig isolated ileum. Over a range of 0.6-2.4 mM Ca2+, the degree of depression was inversely related to the Ca2+ concentration, with an increased sensitivity and sinistral shift of the dose-response curve at lower Ca2+ concentrations. Partial occupation of Ca2+ channels by Ruthenium Red (0.1 microM) also potentiated the depressive responses to baclofen and GABA. It is concluded that these agonists, acting through GABAB receptors, limit the availability of Ca2+ required for neurotransmitter release in myenteric motor nerves.

gamma-Aminobutyric-acid (GABA)-mimetic responses were induced by ethylenediamine (EDA) in the isolated ileum of the guinea-pig maintained in bicarbonate buffered Krebs-Henseleit (KBC) solution, pH 7.4, 37 degrees C, the responses consisting of a contraction followed by a relaxation. There were no such responses to EDA in bicarbonate-free phosphate buffered (KPO) or HEPES buffered (KHO) Krebs solution, gassed with 100% O2, pH 7.4, 37 degrees C, yet the ileum responded to GABA in bicarbonate-free Krebs solution. Similar GABA-mimetic responses were induced by EDA in the isolated ileum maintained in bicarbonate-free KPO or KHO modified Krebs solution, gassed with O2, if HCO3- (5mM) was first added immediately before the test dose of EDA (0.1-1 mM), the threshold [HCO3-]being 2 mM for EDA-induced responses in these preparations. However, ileal GABA-mimetic responses were induced in bicarbonate-free KPO or KHO solutions by EDA that had been pretreated with carbon dioxide, where the final [HCO3-]in the bath did not exceed 25 microM. Ethylenediamine monocarbamate (synthetic EDAC) released [3H]-GABA from preloaded segments of ileum maintained in bicarbonate-free KPO or KHO solution containing amino-oxyacetic acid and beta-alanine, the release being sensitive to 3-mercaptopropionic acid which prevents GABA release. EDA itself did not evoke any such release in the absence of bicarbonate, but released [3H]-GABA from segments maintained in KBC solution. 4 GABA-mimetic responses were induced by EDAC in the isolated ileum maintained in bicarbonate-free KPO solution, as was a delta-aminovalerate-sensitive depression of ileal twitch responses elicited by transmural stimulation, all of which were also sensitive to 3-mercaptopropionic acid. 5 It is concluded that GABA-mimetic responses to EDA in the isolated ileum of the guinea-pig, maintained in normal Krebs bicarbonate medium, result from the release of endogenous GABA by ethylenediamine monocarbamate formed through the rapid reaction of EDA with the carbon dioxide of bicarbonate buffered Krebs solution. Furthermore, in the ileum, HCO3 ions per se are not necessary for this GABA-releasing property of EDA if the latter is first converted to the monocarbamate, since syntheticethylenediamine monocarbamate elicits ileal GABA-mimetic responses in the total absence of bicarbonate.

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.

Ethylenediamine (EDA) released [3H]-gamma-aminobutyric acid ([3H]-GABA) in a dose-dependent manner from the isolated preloaded ileum of the guinea-pig maintained in Krebs-bicarbonate solution (pH 7.4, 37 degrees C), in the presence of beta-alanine and amino-oxyacetic acid (AOAA) to prevent GABA uptake into glial cells and catabolism. This release was reversibly prevented by 3-mercaptopropionic acid (3-MPA), also in a dose-dependent manner. In the isolated ileal preparations of the guinea-pig maintained in Krebs-bicarbonate solution, EDA induced a dose-dependent transient, cholinergic contractile response (GABAA-receptor-mediated effect), followed by an 'after-relaxation' (GABAB-receptor-mediated effect). EDA also induced a transient contraction superimposed on repetitive twitch responses to electrical transmural stimulation of the cholinergic neurones, followed by a depression of the twitch contractions. This GABAA-receptor-mediated contraction was antagonized by bicuculline methochloride and picrotoxinin, whilst the GABAB-receptor-mediated 'after-relaxation', and depression of cholinergic twitch contractions, was susceptible to antagonism by delta-aminovaleric acid. The pA2 value for bicuculline methochloride antagonism of EDA was estimated to be 5.8, identical with that for GABA. 3-Mercaptopropionic acid also prevented these pharmacological actions induced by EDA without affecting responses to GABA, 3-aminopropranesulphonic acid, muscimol, baclofen or the twitch responses to transmural stimulation. It is concluded that EDA releases both [3H]-GABA and endogenous GABA in the guinea-pig ileum, thus providing further evidence that GABA is a transmitter in the enteric nervous system.

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.

In isolated cervical segments of rat vagus nerve, over the temperature range 5-45 degrees C, reversible depolarizations to 5-hydroxytryptamine (5-HT) were maximal at 10 degrees C and negligible at 45 degrees C. Dose-response curves for this depolarization were compared at 20 degrees C and 37 degrees C, with a similar sensitivity but marked proportionate reduction at 37 degrees C vs 20 degrees C, whilst the 5-HT-induced depression of vagal compound action potentials seen at 20 degrees C was abolished at 37 degrees C; such changes at 20 degrees C or 37 degrees C were insensitive to inhibitors of 5-HT-uptake (zimelidine) or MAO (pargyline). 5-HT may participate in presynaptic inhibition of C-fibre afferents.

A device is described for holding the head of a rabbit in a stereotaxic apparatus, employing the principle of anchorage by counter thrust applied between orbital clamps and a plate inserted against the upper molars, which bears the weight of the head. Rotation in the sagittal plane is accomplished by pivoting the head-holder assembly on a U-frame which bolts to a conventional stereotaxic machine. Rigid support of the head by this device has been proven in many experiments, and accurate reproducibility of stereotaxic placement of probes with reference to bregma has been confirmed in animals within limited ranges of size.

1. Commonly used loop diuretics produce side effects in man which are similar to chemically induced myotonia. These diuretics have structural affinity with known myotonic agents. 2. We have observed EMG myotonia in vivo in leg muscles of rats treated with intravenous frusemide. 3. In the presence of several different diuretics, rat isolated diaphragm, soleus and extensor digitorum longus muscles as well as frog sartorius muscles produce typically myotonic contractions with relaxation times up to several seconds. 4. Intracellular recording of action potentials from diuretic-treated muscles reveals long lasting after-discharges following a brief electrical stimulus, again typical of chemically induced myotonia. 5. Having demonstrated a myotonic action of several diuretics we suggest a need for caution in using these drugs in persons with hereditary myotonia and a need to be aware of possible provocation of myotonia in subclinical cases. Myopathies and neuropathies which are known to result from chronic exposure to myotonic agents also need to be considered. 6. In our study, the diuretic, acetazolamide, unmasked subthreshold myotonia. This seems to be at variance with reports of its usefulness in the treatment of myotonia. 7. Diuretics should probably not be employed in the treatment of herbicide intoxication where their myotonic activity would be expected to add to the known myotonic activity of the herbicide.

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.

Rhythmic neurally mediated spontaneous relaxations of the longitudinal muscle in the isolated ileum of the guinea pig are modified by GABA antagonism. Bicuculline methochloride, a GABAA-receptor antagonist, and picrotoxinin, a Cl--ionophore blocker, as well as GABA desensitization reduced or most often abolished these naturally occurring periodic spontaneous relaxations, indicating a physiological involvement of GABA in the control of intestinal motility. The possible mechanism whereby GABA may induce such relaxations is discussed in relation to GABA actions in the enteric nervous system where GABA is a neurotransmitter.

Rhythmic neurally mediated spontaneous contractions of the longitudinal muscle in the isolated ileum of the guinea pig, sensitive to tetrodotoxin and atropine, were depressed and most often abolished by the GABAA receptor antagonists, bicuculline methiodide, RU 5135, and picrotoxin, a Cl- -ionophore blocker, as well as by GABA desensitization. 3-Mercaptopropionic acid, known to prevent GABA release, also reduced these naturally occurring spontaneous contractions. All these strongly indicate a physiological involvement of endogenous GABA in the control of spontaneous rhythmic activity in the intestine.

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.