Excitatory and inhibitory responses mediated by GABAA and GABAB receptors in guinea pig distal colon

Components of the GABAergic signaling in the peripheral cholinergic synapses of vertebrates: a review

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system. Since the 1970s, many studies have focused on the role of GABA in the mammalian peripheral nervous system, and particularly in the cholinergic synapses. In this review, we present current findings for the cholinergic neurons of vegetative ganglia as well as for the neurons innervating smooth and striated muscles. Synaptic contacts formed by these neurons contain GABA and the enzyme, glutamic acid decarboxylase, which catalyzes the synthesis of GABA from glutamate. Newly formed GABA is released in the cholinergic synapses and mostly all the peripheral cholinergic synaptic contacts contain iono- and metabotropic GABA receptors. Although the underlying molecular mechanism of the release is not well understood, still, it is speculated that GABA is released by a vesicular and/or non-vesicular way via reversal of the GABA transporter. We also review the signaling role of GABA in the peripheral cholinergic synapses by modulating acetylcholine release, but its exact physiological function remains to be elucidated.

Peripheral GABA receptors regulate colonic afferent excitability and visceral nociception

KEY POINTS

While the presence of GABA receptors on primary afferents has been well described, most functional analyses have focused on the regulation of transmitter release from central terminals and/or signalling in the sensory neuron cell body. Evidence that GABA receptors are transported to peripheral terminals and that there are several sources of GABA in the colon raise the possibility that GABA signalling in the periphery may influence colonic afferent excitability. GABA_A and GABA_B are present and functional in the colon, where exogenous agonists decrease the excitability of colonic afferents and suppress visceral nociception. Endogenous GABA release within the colon is sufficient to establish the resting excitability of colonic afferents as well as the behavioural response to noxious stimulation of the colon, primarily via GABA_A receptors. Peripheral GABA receptors may serve as a viable target for the treatment of visceral pain.

ABSTRACT

It is well established that GABA receptors at the central terminals of primary afferent fibres regulate afferent input to the superficial dorsal horn. However, the extent to which peripheral GABA signalling may also regulate afferent input remains to be determined. The colon was used to explore this issue because of the numerous endogenous sources of GABA that have been described in this tissue. The influence of GABA signalling on colonic afferent excitability was assessed in an ex vivo mouse colorectum pelvic nerve preparation where test compounds were applied to the receptive field. The visceromotor response (VMR) evoked by noxious colorectal distension was used to assess the impact of GABA signalling on visceral nociception, where test compounds were applied directly to the colon. Application of either GABA_A or GABA_B receptor agonists attenuated the colonic afferent response to colon stretch. Conversely, GABA_A and GABA_B receptor antagonists increased the stretch response. However, while the noxious distension-induced VMR was attenuated in the presence of GABA_A and GABA_B receptor agonists, the VMR was only consistently increased by GABA_A receptor antagonists. These results suggest that GABA receptors are present and functional in the peripheral terminals of colonic afferents and activation of these receptors via endogenous GABA release contributes to the establishment of colonic afferent excitability and visceral nociception. These results suggest that increasing peripheral GABA receptor signalling could be used to treat visceral pain.

Gamma-aminobutyric acid receptor type B agonist baclofen inhibits acid-induced excitation of secondary peristalsis but not heartburn sensation

BACKGROUND AND AIM

Acute esophageal acid infusion promotes distension-induced secondary peristalsis. The gamma-aminobutyric acid receptor type B (GABA-B) receptors activation inhibits secondary peristalsis. This study aimed to test the hypothesis whether acid excitation of secondary peristalsis can be influenced by baclofen.

METHODS

Secondary peristalsis was performed with intra-esophageal slow and rapid air injections in 13 healthy subjects. Direct esophageal infusion of 0.1 N HCl following pretreatment with placebo or baclofen was randomly performed at least 1 week apart. Symptom intensity, distension thresholds, and peristaltic parameters were determined and compared between each study protocol.

RESULTS

The intensity of heartburn symptom in response to esophageal acid infusion was significantly greater with baclofen than the placebo (P = 0.002). The threshold volume of secondary peristalsis during slow air injections in response to acid infusion was significantly greater with baclofen than the placebo (P = 0.001). Baclofen significantly increased the threshold volume of secondary peristalsis during rapid air injections in response to acid infusion (P = 0.001). The frequency of secondary peristalsis in response to acid infusion was significantly decreased by baclofen as compared with the placebo (P = 0.001). Baclofen significantly decreased peristaltic amplitudes in response to acid infusion during rapid air injections (P = 0.007).

CONCLUSIONS

Gamma-aminobutyric acid receptor type B agonist baclofen inhibits acid excitation of secondary peristalsis in human esophagus, which is probably mediated by both muscular and mucosal mechanoreceptors. This work supports the evidence of potential involvement of GABA-B receptors in negative modulation of acid excitation of esophageal perception as well as secondary peristalsis.

Ammonia modifies enteric neuromuscular transmission through glial γ-aminobutyric acid signaling

Impaired gut motility may contribute, at least in part, to the development of systemic hyperammonemia and systemic neurological disorders in inherited metabolic disorders, or in severe liver and renal disease. It is not known whether enteric neurotransmission regulates intestinal luminal and hence systemic ammonia levels by induced changes in motility. Here, we propose and test the hypothesis that ammonia acts through specific enteric circuits to influence gut motility. We tested our hypothesis by recording the effects of ammonia on neuromuscular transmission in tissue samples from mice, pigs, and humans and investigated specific mechanisms using novel mutant mice, selective drugs, cellular imaging, and enzyme-linked immunosorbent assays. Exogenous ammonia increased neurogenic contractions and decreased neurogenic relaxations in segments of mouse, pig, and human intestine. Enteric glial cells responded to ammonia with intracellular Ca²⁺ responses. Inhibition of glutamine synthetase and the deletion of glial connexin-43 channels in hGFAP::CreER^T2+/-/connexin43^f/f mice potentiated the effects of ammonia on neuromuscular transmission. The effects of ammonia on neuromuscular transmission were blocked by GABA_A receptor antagonists, and ammonia drove substantive GABA release as did the selective pharmacological activation of enteric glia in GFAP::hM3Dq transgenic mice. We propose a novel mechanism whereby local ammonia is operational through GABAergic glial signaling to influence enteric neuromuscular circuits that regulate intestinal motility. Therapeutic manipulation of these mechanisms may benefit a number of neurological, hepatic, and renal disorders manifesting hyperammonemia.NEW & NOTEWORTHY We propose that local circuits in the enteric nervous system sense and regulate intestinal ammonia. We show that ammonia modifies enteric neuromuscular transmission to increase motility in human, pig, and mouse intestine model systems. The mechanisms underlying the effects of ammonia on enteric neurotransmission include GABAergic pathways that are regulated by enteric glial cells. Our new data suggest that myenteric glial cells sense local ammonia and directly modify neurotransmission by releasing GABA.

Influence of GABA-B Agonist Baclofen on Capsaicin-Induced Excitation of Secondary Peristalsis in Humans

OBJECTIVES

Esophageal instillation of capsaicin enhances secondary peristalsis, but the γ-aminobutyric acid receptor type B (GABA-B) agonist baclofen inhibits secondary peristalsis. This study aimed to investigate whether baclofen could influence heartburn perception and secondary peristalsis subsequent to capsaicin infusion in healthy adults.

METHODS

Secondary peristalsis was performed by slow and rapid mid-esophagus air injections in 15 healthy subjects. Two different sessions including esophageal infusion of capsaicin-containing red pepper sauce (0.84 mg) following pre-treatment with placebo or baclofen were randomly performed to test the effects on heartburn perception and secondary peristalsis.

RESULTS

The intensity of heartburn symptom subsequent to capsaicin infusion was significantly greater after pre-treatment of baclofen as compared with the placebo (P=0.03). Baclofen significantly increased the threshold volume of secondary peristalsis to slow air injections subsequent to esophageal capsaicin infusion (P<0.001). Baclofen significantly increased the threshold volume of secondary peristalsis to rapid air injections subsequent to esophageal capsaicin infusion (P<0.01). The frequency of secondary peristalsis subsequent to capsaicin infusion was significantly decreased with baclofen as compared with the placebo (P<0.002). Baclofen had no effect on any of the peristaltic parameters of secondary peristalsis subsequent to capsaicin infusion.

CONCLUSIONS

The GABA-B agonist baclofen appears to attenuate the esophagus to capsaicin-induced excitation of secondary peristalsis in healthy adults. Our study suggests the inhibitory modulation for GABA-B receptors on capsaicin-sensitive afferents mediating secondary peristalsis in human esophagus.

The Regulatory Effects of Lateral Hypothalamus Area GABAB Receptor on Gastric Ischemia-Reperfusion Injury in Rats

HIGHLIGHTS

The aim of the research was to determine the functional effects and molecular mechanisms of GABA_B receptor on ischemia reperfusion-induced gastric injury in rats.

The lateral hypothalamus area GABA_B receptor attenuated the ischemia reperfusion-induced gastric injury by up-regulating the production of GABA, GABA_BR, and down-regulating P-GABA_BR in the brain.

This work would provide a new therapeutic strategy for acute gastric injury.

Gastric ischemia-reperfusion (GI-R) injury progression is largely associated with excessive activation of the greater splanchnic nerve (GSN). This study aims to investigate the protective effects of GABA_B receptor (GABA_BR) in the lateral hypothalamic area (LHA) on GI-R injury. A model of GI-R injury was established by clamping the celiac artery for 30 min and then reperfusion for 1 h. The coordinate of FN and LHA was identified in Stereotaxic Coordinates and then the L-Glu was microinjected into FN, GABA_B receptor agonist baclofen, or GABA_B receptor antagonist CGP35348 was microinjected into the LHA, finally the GI-R model was prepared. The expression of GABA_BR, P-GABA_BR, NOX2, NOX4, and SOD in the LHA was detected by western blot, PCR, and RT-PCR. The expression of IL-1β, NOX2, and NXO4 in gastric mucosa was detected by western blot. We found that microinjection of L-Glu into the FN or GABA_B receptor agonist (baclofen) into the LHA attenuated GI-R injury. Pretreatment with GABA_B receptor antagonist CGP35348 reversed the protective effects of FN stimulation or baclofen into the LHA. Microinjection of baclofen into the LHA obviously reduced the expression of inflammatory factor IL-1β, NOX2, and NOX4 in the gastric mucosa.

Conclusion: The protective effects of microinjection of GABA_BR agonist into LHA on GI-R injury in rats could be mediated by up-regulating the production of GABA, GABA_BR, and down-regulating P-GABA_BR in the LHA.

GABA and GABA receptors in the gastrointestinal tract: from motility to inflammation

Although an extensive body of literature confirmed γ-aminobutyric acid (GABA) as mediator within the enteric nervous system (ENS) controlling gastrointestinal (GI) function, the true significance of GABAergic signalling in the gut is still a matter of debate. GABAergic cells in the bowel include neuronal and endocrine-like cells, suggesting GABA as modulator of both motor and secretory GI activity. GABA effects in the GI tract depend on the activation of ionotropic GABAA and GABAC receptors and metabotropic GABAB receptors, resulting in a potential noteworthy regulation of both the excitatory and inhibitory signalling in the ENS. However, the preservation of GABAergic signalling in the gut could not be limited to the maintenance of physiologic intestinal activity. Indeed, a series of interesting studies have suggested a potential key role of GABA in the promising field of neuroimmune interaction, being involved in the modulation of immune cell activity associated with different systemic and enteric inflammatory conditions. Given the urgency of novel therapeutic strategies against chronic immunity-related pathologies, i.e. multiple sclerosis and Inflammatory Bowel Disease, an in-depth comprehension of the enteric GABAergic system in health and disease could provide the basis for new clinical application of nerve-driven immunity. Hence, in the attempt to drive novel researches addressing both the physiological and pathological importance of the GABAergic signalling in the gut, we summarized current evidence on GABA and GABA receptor function in the different parts of the GI tract, with particular focus on the potential involvement in the modulation of GI motility and inflammation.

Opposite role played by GABAA and GABAB receptors in the modulation of peristaltic activity in mouse distal colon

We investigated the role of GABA on intestinal motility using as model the murine distal colon. Effects induced by GABA receptors recruitment were examined in whole colonic segments and isolated circular muscle preparations to analyze their influence on peristaltic reflex and on spontaneous and neurally-evoked contractions. Using a modified Trendelenburg set-up, rhythmic peristaltic contractions were evoked by gradual distension of the colonic segments. Spontaneous and neurally-evoked mechanical activity of circular muscle strips were recorded in vitro as changes in isometric tension. GABA, at low concentrations (10-50 µM), potentiated peristaltic activity and the neural cholinergic contractions, whilst it, at higher concentrations (500 µM-1mM), had inhibitory effects. GABA excitatory effects were mimicked by muscimol, GABAA-receptor agonist, and prevented by bicuculline, GABAA-receptor antagonist, which per se reduced peristaltic activity and the cholinergic contractile responses. Inhibitory effects were mimicked by baclofen, GABAB-receptor agonist, and antagonized by phaclofen, GABAB-receptor antagonist and by hexamethonium, neural nicotinic receptor antagonist. Guanethidine was ineffective on GABA effects. Non-cholinergic responses were not affected by GABA agents. All drugs failed to affect the response to carbachol. Lastly, GABAC receptor agonist/antagonist had any effect on colonic motility. In conclusion, GABA in mouse distal colon is a modulator of peristaltic activity via the regulation of acetylcholine release from cholinergic neurons through interaction with GABAA or GABAB receptors. GABAA receptors are recruited at low GABA concentrations, increasing acetylcholine release and propulsive activity. At high GABA concentrations the activation of GABAB receptors overrides GABAA receptor effects, decreasing acetylcholine release and peristaltic activity.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

The analgesic effects of the GABAB receptor agonist, baclofen, in a rodent model of functional dyspepsia

BACKGROUND

The amino acid γ-aminobutyric acid (GABA) is an important modulator of pain but its role in visceral pain syndromes is just beginning to be studied. Our aims were to investigate the effect and mechanism of action of the GABA(B) receptor agonist, baclofen, on gastric hypersensitivity in a validated rat model of functional dyspepsia (FD).

METHODS

10-day-old male rats received 0.2 mL of 0.1% iodoacetamide in 2% sucrose daily by oral gavages for 6 days. Control group received 2% sucrose. At 8-10 weeks rats treated with baclofen (0.3, 1, and 3 mg kg(-1) bw) or saline were tested for behavioral and electromyographic (EMG) visceromotor responses; gastric spinal afferent nerve activity to graded gastric distention and Fos protein expression in dorsal horn of spinal cord segments T8-T10 to noxious gastric distention.

KEY RESULTS

Baclofen administration was associated with a significant attenuation of the behavioral and EMG responses (at 1 and 3 mg kg(-1)) and expression of Fos in T8 and T9 segments in neonatal iodoacetamide sensitized rats. However, baclofen administration did not significantly affect splanchnic nerve activity to gastric distention. Baclofen (3 mg kg(-1)) also significantly reduced the expression of spinal Fos in response to gastric distention in control rats to a lesser extent than sensitized rats.

CONCLUSIONS & INFERENCES

Baclofen is effective in attenuating pain associated responses in an experimental model of FD and appears to act by central mechanisms. These results provide a basis for clinical trials of this drug in FD patients.

A Gut Feeling about GABA: Focus on GABA(B) Receptors

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the body and hence GABA-mediated neurotransmission regulates many physiological functions, including those in the gastrointestinal (GI) tract. GABA is located throughout the GI tract and is found in enteric nerves as well as in endocrine-like cells, implicating GABA as both a neurotransmitter and an endocrine mediator influencing GI function. GABA mediates its effects via GABA receptors which are either ionotropic GABA(A) or metabotropic GABA(B). The latter which respond to the agonist baclofen have been least characterized, however accumulating data suggest that they play a key role in GI function in health and disease. Like GABA, GABA(B) receptors have been detected throughout the gut of several species in the enteric nervous system, muscle, epithelial layers as well as on endocrine-like cells. Such widespread distribution of this metabotropic GABA receptor is consistent with its significant modulatory role over intestinal motility, gastric emptying, gastric acid secretion, transient lower esophageal sphincter relaxation and visceral sensation of painful colonic stimuli. More intriguing findings, the mechanisms underlying which have yet to be determined, suggest GABA(B) receptors inhibit GI carcinogenesis and tumor growth. Therefore, the diversity of GI functions regulated by GABA(B) receptors makes it a potentially useful target in the treatment of several GI disorders. In light of the development of novel compounds such as peripherally acting GABA(B) receptor agonists, positive allosteric modulators of the GABA(B) receptor and GABA producing enteric bacteria, we review and summarize current knowledge on the function of GABA(B) receptors within the GI tract.

Antisecretory factor as a potential health-promoting molecule in man and animals

Antisecretory factor (AF) is a protein secreted in plasma and other tissue fluids in mammals with proven antisecretory and anti-inflammatory activity; its immunohistological distribution suggests a role in the immune system. The expression level and the distribution of AF protein are altered during an immunological response. Exposure to bacterial toxins induces secretion of AF in plasma, probably reflecting a natural defence mechanism to agents causing diarrhoea, thereby contributing to a favourable clinical outcome and disease termination. An increase of AF levels in plasma by dietary means, such as specially processed cereals (SPC), has been demonstrated in human subjects and animals. Administration of SPC to patients affected by inflammatory bowel disease, gastroenteritis and Ménière's disease relieved symptoms and improved quality of life. A recent study showed the positive effect of SPC diet supplementation on prevention of the effects of exposure to low levels of blast overpressure in rats, reducing the extent of intracranial pressure increase and cognitive function impairment. AF-rich egg yolk powder improved health status in children suffering acute and chronic diarrhoea, reducing the frequency and increasing the consistency of stools. This kind of functional food could be used for prophylaxis in populations exposed to a high risk of morbidity and mortality caused by diarrhoea and as a complementary therapy in patients affected by chronic intestinal inflammatory disease to improve well-being. In pig husbandry AF-inducing diets, owing to their antisecretory activity and anti-inflammatory action, are a suitable option as an alternative to antibiotic growth promoters to counteract post-weaning diarrhoea.

Luminal administration ex vivo of a live Lactobacillus species moderates mouse jejunal motility within minutes

Gut commensals modulate host immune, endocrine, and metabolic functions. They also affect peripheral and central neural reflexes and function. We have previously shown that daily ingestion of Lactobacillus reuteri (LR) for 9 d inhibits the pseudoaffective cardiac response and spinal single-fiber discharge evoked by visceral distension, and decreases intestinal motility and myenteric AH cell slow afterhyperpolarization (sAHP) by inhibiting a Ca-activated K (IK(Ca)) channel. We tested whether luminal LR could acutely decrease motility in an ex vivo perfusion model of naive Balb/c jejunum. Live LR dose dependently decreased motor complex pressure wave amplitudes with 9- to 16-min onset latency and an IC(50) of 5 × 10(7) cells/ml Krebs. Heat-killed LR or another live commensal, Lactobacillus salivarius, were without effect. The IK(Ca) channel blocker TRAM-34, but neither the opener (DCEBIO) nor the hyperpolarization-activated cationic channel inhibitor ZD7288 (5 μM) (or TTX 1 μM), mimicked the LR effect on motility acutely ex vivo. We provide evidence for a rapid, strain-specific, dose-dependent action of a live Lactobacillus on small intestinal motility reflexes that recapitulates the long-term effects of LR ingestion. These observations may be useful as a first step to unraveling the pathways involved in bacteria to the nervous system communication.

Functional evidence for different roles of GABAA and GABAB receptors in modulating mouse gastric tone

The aims of the present study were to investigate, using mouse whole stomach in vitro, the effects of gamma-aminobutyric acid (GABA) and GABA receptor agonists on the spontaneous gastric tone, to examine the subtypes of GABA receptors involved in the responses and to determine the possible site(s) of action. GABA induced gastric relaxation, which was antagonized by the GABA(A)-receptor antagonist, bicuculline, potentiated by phaclofen, GABA(B)-receptor antagonist, but not affected by 1,2,5,6-Tetrahydropyridin-4-yl methylphosphinic acid hydrate (TPMPA), GABA(C)-receptor antagonist. Muscimol, GABA(A)-receptor agonist, mimicked GABA effects inducing relaxation, which was significantly reduced by bicuculline, N omega-nitro-L-arginine methyl ester (L-NAME), inhibitor of NO synthase or apamin, inhibitor of small conductance Ca(2+)-dependent K(+) channels, which blocks the purinergic transmission in this preparation. It was abolished by tetrodotoxin (TTX) or l-NAME plus apamin. Baclofen, a specific GABA(B)-receptor agonist, induced an increase in the gastric tone, which was antagonized by phaclofen and abolished by TTX or atropine. Bicuculline, but not phaclofen or TPMPA, per se induced an increase in gastric tone, which was prevented by L-NAME. In conclusion, our results suggest that GABA is involved in the regulation of mouse gastric tone, through modulation of intrinsic neurons. Activation of GABA(A)-receptors mediates relaxation through neural release of NO and neurotransmitters, activating Ca(2+)-dependent K(+) channels, likely purines, while activation of GABA(B)-receptors leads to contraction through acetylcholine release.

Presynaptic GABA-A receptors prevent depression of nicotinic transmission in rabbit coeliac ganglion neurones

We investigated the involvement of GABA-A receptors in the modulation of the nicotinic transmission of central origin in isolated rabbit coeliac ganglia. Our study was performed in vitro and the electrical activity of the ganglionic neurones was recorded using intracellular recording techniques. During iterative stimulation of the splanchnic nerves, the synaptic action potential probability decreased gradually, indicating a depression of the nicotinic activation. Pharmacological agents acting at GABA-A receptors modulated the action potential probability during the train of pulses. Muscimol (a GABA-A receptor agonist), diazepam (a benzodiazepine site agonist) and 1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (a GABA uptake blocker) increased this probability. Conversely, gabazine or bicuculline (two GABA-A receptor antagonists), picrotoxin (a picrotoxin site agonist) and flumazenil (a benzodiazepine site antagonist) reduced it. These results demonstrate that endogenous GABA, released during the train of pulses, facilitates the central nicotinic activation of the ganglionic neurones by acting on GABA-A receptors. Muscimol also reduced the amplitude ratio of excitatory postsynaptic potentials triggered during the paired-pulse protocol without any change in postsynaptic properties. This result is consistent with a presynaptic action of GABA-A receptors. Our study shows that presynaptic GABA-A receptors facilitate the central nicotinic activation of prevertebral ganglionic neurones and thus play a novel role in the integrative properties of the sympathetic prevertebral ganglia.

Functional evidence for GABA as modulator of the contractility of the longitudinal muscle in mouse duodenum: Role of GABAA and GABAC receptors

We investigated, in vitro, the effects of gamma-aminobutyric acid (GABA) on the spontaneous mechanical activity of the longitudinal smooth muscle in mouse duodenum. GABA induced an excitatory effect, consisting in an increase in the basal tone, which was antagonized by the GABA(A)-receptor antagonist, bicuculline, potentiated by (1,2,5,6-Tetrahydropyridin-4-yl)methylphosphinic acid hydrate (TPMPA), a GABA(C)-receptor antagonist and it was not affected by phaclofen, a GABA(B)-receptor antagonist. Muscimol, GABA(A) receptor agonist, induced a contractile effect markedly reduced by bicuculline, tetrodotoxin (TTX), hexamethonium and atropine. Cis-4-aminocrotonic acid (CACA), a specific GABA(C) receptor agonist, induced an inhibitory effect, consisting in the reduction of the amplitude of the spontaneous contractions and muscular relaxation, which was antagonised by TPMPA, GABA(C)-receptor antagonist, TTX or N(omega)-nitro-l-arginine methyl ester (L-NAME), nitric oxide (NO) synthase inhibitor, but not affected by hexamethonium. In conclusion, our study indicates that GABA is a modulator of mechanical activity of longitudinal muscle in mouse duodenum. GABA may act through neuronal presynaptic receptors, namely GABA(A) receptors, leading to the release of ACh from excitatory cholinergic neurons, and GABA(C) receptors increasing the release of NO from non-adrenergic, non-cholinergic inhibitory neurons.

GABA-induced calcium signaling in cultured enteric neurons is reinforced by activation of cholinergic pathways

GABA is an important inhibitory transmitter in the CNS. In the enteric nervous system, however, both excitatory and inhibitory actions have been reported. Here, we investigated the effects of GABA on the intracellular Ca2+ concentration of guinea-pig myenteric neurons (at 35 degrees C) using Fura-2-AM. Neurons were identified by 75 mM K+ depolarization (5 s), which evoked a transient intracellular Ca2+ concentration increase. GABA (10 s) induced a dose dependent (5 nM-1 microM) transient intracellular Ca2+ concentration rise in the majority of neurons (500 nM GABA: 251+/-17 nM, n=232/289). Interestingly, the response to 5 microM GABA (n=18) lasted several minutes and did not fully recover. GABA response amplitudes were significantly (P<0.001) reduced by GABAA and GABAB receptor antagonists (10 microM) bicuculline and phaclofen. The GABAA agonist isoguvacine (10 microM) and GABAB agonist baclofen (10 microM) induced similar responses as 50 nM GABA, while the GABAC agonist cis-4-aminocrotonic acid (CACA) (10 microM) only elicited small responses in a minority of neurons. Removal of extracellular Ca2+ abolished all responses while depletion of intracellular Ca2+ stores by thapsigargin (5 microM) did not alter the responses to 500 nM GABA (n=13), but reduction of Ca2+ influx through voltage-dependent Ca2+ channels did. The nicotinic antagonist hexamethonium (100 microM) also reduced GABA responses by almost 70% suggesting that GABA stimulates cholinergic pathways, while the purinergic receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and the 5-HT3 receptor blocker ondansetron only had minor effects.

CONCLUSION

GABA elicits transient intracellular Ca2+ concentration responses in the majority of myenteric neurons through activation of GABAA and GABAB receptors and much of the response can be attributed to facilitation of ACh release. Thus GABA may act mainly as a modulator that sets the state of excitability of the enteric nerve network. A concentration of 5 microM GABA, although frequently used in pharmacological experiments, seems to cause a detrimental response reminiscent of the neurotoxic effects glutamate has in the CNS.

Effect of γ-aminobutyric acid on spontaneous contraction of ileum smooth muscle in mice

AIM: To investigate the effects of γ-aminobutyric acid (GABA) on the spontaneous contraction of mouse ileac smooth muscle and the interactions between GABA and β-receptor and nitric oxide (NO).

METHODS: The contraction changes of the isolated smooth muscle stripswere recorded by tension transducer, and the change of the tension was used as the marker to evaluate the effects of GABA. The effects of ODQ, L-NNA, propranolol on the action of GABA were also analyzed. .

RESULTS: GABA inhibited the spontaneous contraction of mouse ileum at the concentrations ranging from 1×10^-6 to 1×10^-3 mol/L. The contraction amplitudes were decreased by (34.71±7.35)% and (22.23±4.69)%, respectively, when 1×10^-6 and 1×10^-3 mol/L GABA were used. Picrotoxin showed no significant effect on the inhibitory actions caused by GABA. The effects of GABA on the spontaneous contraction were decreased in the presence of L-NNA or ODQ. L-Arg (5×10^-7 mol/L) decreased the effect of GABA at the concentration of 1×10^-6 mol/L, but not at the concentration of 1×10^-3 mol/L. Propranolol (3×10^-6 mol/L) reduced the effect of GABA on the contraction of mouse ileac smooth muscle.

CONCLUSION: GABA inhibits the spontaneous contraction of mouse ileac smooth muscle. This effect, which is influenced by the excitation of β-receptor may need the participation of cGMP and NO.

Characterization of GABA(B) receptors involved in inhibition of motility associated with acetylcholine release in the dog small intestine: possible existence of a heterodimer of GABA(B1) and GABA(B2) subunits

Characterization of the gamma-aminobutyric acid (GABA)(B) receptor involved in the motility of dog small intestine was analyzed by application of the microdialysis method to the small intestine of the whole body of the dog. The reverse transcription-polymerase chain reaction (RT-PCR) was used. Intraarterial administration of muscimol induced acceleration of motility associated with acetylcholine (ACh) release, these responses being antagonized by bicuculline. Intraarterial administration of baclofen induced inhibition of motility associated with ACh release, these responses being antagonized by CGP62349. GABA induced inhibition of motility associated with decrease in ACh release. CGP62349 alone induced acceleration of motility associated with increase in ACh release. RT-PCR revealed the presence of mRNAs for both subunits of GABA(B) receptor, GABA(B1) and GABA(B2), in the dog small intestine, although GABA(B1) subunits were 6 isoforms of GABA(B1) (GABA(B1(a)) - GABA(B1(g))), except GABA(B1(d)). Thus, the GABA(B) receptor located at cholinergic neurons as a heterodimer with subunits of GABA(B1) and GABA(B2) in the dog small intestine operates predominantly relative to the GABA(A) receptor in physiological motility.

Effects of antisecretory factor-derived peptides on contractions in guinea pig colon

Two antisecretory factor (AF)-derived peptides have been studied in relation to effects on motility of guinea pig colon. Colon segments were isolated from adult guinea pigs and incubated in Tyrode Ringer. Motility was measured as the force and frequency of contractions upon addition of the derived peptides AF 1 (8 amino acids (aa)) and AF 3 (10 amino acids). At the lowest concentration (5 pM), peptide AF 1 induced a negative effect on the force of contraction in colon segments; an effect that was abolished by the cholinergic agonist carbachol. Peptide AF 3 induced a significant increase in the force of colon contractions at all concentrations (5-180 pM), with carbachol only reducing the effect of peptide AF 3 at a concentration of 15 pM. Both peptides increased contractile frequency, although the overall response was lower for peptide AF 3 than for peptide AF 1. It is concluded that antisecretory factor-derived peptides may play a role in regulating colon motility such that under pathophysiological conditions, they may serve to hasten the evacuation of noxious agents from the large intestine.

Functional evidence for a role of GABA receptors in modulating nerve activities of circular smooth muscle from rat colon in vitro

In the enteric nervous system, activation of neuronal GABA(A)- and GABA(B)-receptors has been shown to modulate neuronal activity. The consequences of this modulation depend on the location in the gastrointestinal tract or the animal species studied. These data illustrate the complexity of GABA-induced effects. Furthermore, the GABA(C)-receptor has been identified in a neuroendocrine cell line suggesting a modulating role of this third type of GABA receptor in intestinal functions. Therefore, the modulating role of GABA-receptor agonists was determined in circular preparations of rat distal colon during electrical nerve stimulation (NS) in vitro. Mechanical response to NS was characterized by a relaxation followed at the end of the stimulation by an off-contraction. In normal Krebs solution (basal conditions), muscimol and baclofen, respectively GABA(A)- and GABA(B)-agonists, induced a significant increase of the electrically induced off-contraction. The GABA(C) agonist, CACA, showed no significant effect on the response to NS. Excitatory effects of muscimol on the off-contraction were abolished in the presence of atropine. Furthermore, in the presence of atropine, muscimol increased the amplitude of the electrically induced relaxation; similarly the baclofen-induced increase of off-contraction amplitude was significantly lower than that observed in control conditions. Baclofen and muscimol effects on the off-contraction were abolished in the presence of hexamethonium or guanethidine. Furthermore, muscimol and baclofen did not induce any significant change on the response to NS in the presence of L-NAME and apamin together. Thus, it seems that in rat distal colon, GABA regulates significantly both excitatory (through GABA(A)- and GABA(B)-receptors) and inhibitory (through GABA(A)-receptors) neuronal activities. We also gave evidence for a possible interplay between GABAergic intrinsic neurons and adrenergic nerve terminals. Finally, it is shown for the first time the presence of the GABA vesicular transporter (VIAAT) around myenteric ganglia of rat colon.

Effects of GABA on circular smooth muscle spontaneous activities of rat distal colon

GABAergic regulation of intestinal motility through the modulation of non-adrenergic non-cholinergic (NANC) neurons remains poorly understood especially in rat colon where very few studies have been undertaken. Therefore, the effects of GABA on circular preparations of rat distal colon were investigated using classical organ bath chambers to record spontaneous mechanical activities (SMA). SMA was characterized by the occurrence of rhythmic phasic contractions (type-I) or by spontaneously occurring large contractions superimposed on small rhythmic contractions (type-II). In the presence of atropine and guanethidine (NANC conditions), these large contractions were inhibited by bicuculline, a GABA(A)-receptor antagonist as well as by TTX, L-NAME and apamin together, or L 732-138, a NK1-receptor antagonist. In NANC conditions, GABA induced a transient monophasic relaxation or a biphasic effect characterized by a relaxation followed by a tonic contraction in both type-I and -II preparations. Both the inhibitory and excitatory effects of GABA were blocked by TTX and L-NAME + apamin; the GABA-induced contraction was also sensitive to L 732-138. The responses to GABA were mimicked by the GABA(A)-receptor agonist, muscimol, whereas baclofen and CACA, respectively GABA(B) and GABA(C)-receptors agonists showed no effect. These results demonstrated that only GABA(A)-receptors seem to be involved in the regulation of SMA in rat distal colon in NANC conditions. Release of NANC inhibitory transmitter (NO and probably ATP) and NANC excitatory transmitter (maybe substance P) might be involved.

Distribution and effects of PACAP, VIP, nitric oxide and GABA in the gut of the African clawed frog Xenopus laevis

The distribution and possible effects on gastrointestinal motility of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide and γ-amino-butyric acid(GABA) were investigated in the African clawed frog (Xenopus laevis)using immunohistochemistry and in vitro strip preparations. PACAP-and VIP-immunoreactive nerve fibres were common in the myenteric plexus as well as in the longitudinal and circular muscle layers all along the gastrointestinal tract. Double labelling demonstrated a close correlation between PACAP and VIP immunoreactivities, indicating that the two neurotransmitters are colocalised within the enteric nervous system. Occasionally, PACAP- and VIP-positive nerve cell bodies were seen in the myenteric or submucous plexa. In addition, VIP immunoreactivity coexisted with helospectin immunoreactivity. Nitric oxide synthase (NOS)-immunoreactive nerve cells were found in the myenteric plexus at an average density for the whole gastrointestinal tract of 4584±540 cells cm-2. The NOS-immunoreactive nerve cells were usually multipolar with an average size of 11.3±3.7 × 23.2±6.6 μm. Some NOS-immunoreactive nerve fibres were VIP-immunoreactive but not all VIP-positive fibres showed NOS immunoreactivity. GABA immunoreactivity was found in nerve fibres and nerve cells in the myenteric plexus of all regions of the gut. Few GABA-immunoreactive nerve fibres were VIP-immunoreactive. PACAP 27, VIP,sodium nitroprusside (a nitric oxide donor; NaNP) and GABA caused similar responses on spontaneously contracting circular preparations of the cardiac stomach of X. laevis. The mean force developed was decreased, mainly by a reduction in resting tension, while the amplitude of contractions was not necessarily affected. The NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) increased the mean force developed, indicating a nitrergic tone in the preparations. In contrast, PACAP 27, VIP, NaNP, GABA and L-NAME had no significant effect on longitudinal strip preparations from the duodenum. These results indicate that PACAP, VIP, nitric oxide and GABA, which are known to be important inhibitory neurotransmitters in other vertebrates, are widely spread in the enteric nervous system of Xenopus laevis and may be involved in the inhibitory control of gastric motility. Although no effect of PACAP,VIP, nitric oxide or GABA on the longitudinal strips of the duodenum was seen in this study, this does not rule out the possibility that they might play an important role in controlling intestinal motility as well.

The pharmacology of gamma-aminobutyric acid and acetylcholine receptors at the echinoderm neuromuscular junction

This review describes the various subtypes of gamma-aminobutyric acid (GABA) receptors found at the echinoderm neuromuscular junction (NMJ), based on pharmacological and physiological studies. The review focuses mainly on holothurian GABA receptors at the NMJ located between the radial nerve and longitudinal muscle of the body wall (LMBW) and compares them to GABA receptors described at other echinoderm NMJs. Since a primary action of GABA on the holothurian LMBW is to modulate contractile responses to the excitatory neurotransmitter, acetylcholine (ACh), the pharmacology of echinoderm nicotinic ACh receptors (nAChRs) and muscarinic ACh receptors (mAChRs) is also addressed. GABA responses have been described in the asteroids, echinoids and holothuroids but not in the other echinoderm classes. Some actions of GABA on echinoderm muscle include regulation of basal tone and spontaneous rhythmic contractions and modulation of cholinergic responses. Both GABA A and B receptor subtypes are present at the echinoderm NMJ, a feature also common to the arthropods, molluscs and chordates. Echinoderm GABA A receptors may mediate the excitatory responses to GABA. The GABA A receptor antagonist bicuculline has a paradoxical effect on contractility, stimulating large protracted contractions of the LMBW. The GABA A agonist muscimol potentiates cholinergic contractions of the holothurian LMBW. Another population of GABA receptors is inhibitory and is sensitive to the GABA B agonist baclofen and GABA B antagonists phaclofen and 2-OH-saclofen. The pre- and/or postsynaptic location of the GABA A and B receptors is not currently known. The folded GABA analogue 4-cis-aminocrotonic acid has no effect on the contractility of the holothurian LMBW so GABA C receptors are probably lacking in this preparation. Pharmacological studies have shown that distinct nAChRs and mAChRs are colocalized in numerous echinoderm muscle preparations. Most recently, nAChR agonists were used to characterize pharmacologically receptors at the holothurian LMBW that bind ACh. Nicotinic AChRs with unique pharmacological profiles are localized both pre- and postsynaptically at this NMJ, where their physiological action is to enhance muscle tone. Muscarinic agonists also have excitatory actions on the LMBW but their action is to stimulate phasic, rhythmic contractions of the muscle. The location of mAChRs at the echinoderm NMJ, however, is unknown.Since most of the studies described in the present review have used whole-mount preparations consisting largely of a combination of muscle fibers, neurons and connective tissue, it is extremely difficult to determine pharmacologically the exact location of the various receptor subtypes. Additional electrophysiological studies on isolated neurons and muscle fibers are therefore required to clearly define extra-, pre- and/or postsynaptic sites for the receptor subtypes at the echinoderm NMJ.

GABA(B) receptors inhibit mechanosensitivity of primary afferent endings

The modulatory effects of baclofen on the sensitivity of peripheral afferent endings to mechanical stimulation were investigated using an in vitro ferret gastroesophageal vagal afferent preparation. Changes in sensitivity of three types of gastroesophageal vagal afferent endings previously categorized as mucosal, tension, and tension-mucosal (TM) receptors according to their mechanoreceptive field characteristics were investigated. Baclofen (30-200 microM) dose dependently reduced responses of mucosal afferents to mucosal stroking with calibrated von Frey hairs (10-1000 mg). This was reversed by the GABA(B) receptor antagonist SCH50911 (1 microM). TM afferent responses to mucosal stroking (10-1000 mg) were unaffected by baclofen (30-200 microM). However, baclofen (30-200 microM) significantly inhibited the response of 11 of 18 TM afferents to circumferential tension. This was reversed by SCH50911 (1 microM). Baclofen (100 and 200 microM) significantly inhibited the response of all tension receptor afferents to circumferential tension in the lower range (1-3 gm) but not in the higher range (4-7 gm). This inhibition was reversed by SCH50911 (1 microM; n = 3). This study provides the first direct evidence for the inhibitory modulation of peripheral mechanosensory endings by the G-protein-coupled GABA(B) receptor. Inhibition was dose-dependent, pharmacologically reversible, and selective to certain aspects of mechanosensitivity. These findings have important relevance to strategies for selective reduction of sensory input to the CNS at a peripheral site.

Numerical simulation of motility patterns of the small bowel. II. Comparative pharmacological validation of a mathematical model

A model of a locus of the small bowel, described earlier by the authors (Miftakhov et al., 1999) was validated in a comparison of the results of numerical simulations of pharmacological compounds to their effects in biological studies. The actions of the following four classes of drugs were simulated, those: (i) acting on the sarcoplasmic reticulum, (ii) altering the permeability of L- and T-type Ca(2+)channels on the smooth muscle membrane, (iii) motilides, and (iv) benzodiazepines. The strong qualitative resemblance between the theoretical and experimental results supports the robustness of the model.

Transcription and translation of two glutamate decarboxylase genes in the ileum of rat, mouse and guinea pig

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter, synthesised from glutamate by glutamate decarboxylase (GAD), in the central nervous system. Two forms of GAD, designated GAD 65 and GAD 67, are encoded by distinct genes and have been demonstrated in the mammalian brain. GABA has been postulated to be synthesised in neurons of the enteric nervous system (ENS), but evidence for its role as an enteric neurotransmitter is equivocal. We therefore aimed to determine whether GAD 65 and GAD 67 messenger RNAs (mRNAs) and proteins were expressed in the ileum of mice, rats and guinea pigs. Using an RNase protection assay, both GAD 65 and GAD 67 mRNAs were detected in the rodent small intestine. Antisera specific for GAD 65 or GAD 67, used in immunoblot analyses, revealed GAD 65-like and GAD 67-like immunoreactivity in rat and guinea pig ileum. Anti-GAD 65 antisera detected a major band of 65 kDa. Anti-GAD 67 antisera detected a major band of 55 kDa, which probably represented a breakdown product, and a minor band of 67 kDa. Analysis of immunoblot extracts of rat and guinea pig ileum revealed more GAD 67-like than GAD 65-like immunoreactivity. GAD enzymatic activity was high in the rat and guinea-pig brain, and low in the whole and dissected ileum. These results demonstrate that both GAD 65 and GAD 67 genes are transcribed and translated in the ileum of three rodent species and lend indirect support to the postulate that GABA is synthesised by neurons of the ENS and intestinal endocrine cells.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.