Peripheral 2-hydroxy-saclofen-sensitive GABA-B receptors mediate both vagal-dependent and vagal-independent acid secretory responses in rats

High frequency repetitive Transcranial Magnetic Stimulation promotes long lasting phrenic motoneuron excitability via GABAergic networks

Repetitive transcranial magnetic stimulation (rTMS) is a promising, innovative, and non-invasive therapy used clinically. Efficacy of rTMS has been demonstrated to ameliorate psychiatric disorders and neuropathic pain through neuromodulation of affected neural circuits. However, little is known about the mechanisms and the specific neural circuits via which rTMS facilitates these functional effects. The aim of this study was to begin revealing the mechanisms by which rTMS may tap into existing neural circuits, by using a well characterized spinal motor circuit - the phrenic circuit. Here we hypothesized that rTMS can be used to enhance phrenic motoneuron excitability in anesthetized Sprague Dawley rats. Multiple acute rTMS protocols were used revealing 10 Hz rTMS protocol induced a robust, long-lasting increase in phrenic motoneuron excitability, functionally evaluated by diaphragm motor evoked potentials (59.1 ± 21.1 % of increase compared to baseline 60 min after 10 Hz protocol against 6.0 ± 5.8 % (p = 0.007) for Time Control, -5.8 ± 7.4 % (p < 0.001) for 3 Hz, and 5.2 ± 12.5 % (p = 0.008) for 30 Hz protocols). A deeper analyze allowed to discriminate "responder" and "non-responder" subgroups among 10 Hz rTMS treated animals. Intravenous injections of GABA_A and GABA_B receptor agonists prior to 10 Hz rTMS treatment, abolished the enhanced phrenic motoneuron excitability, suggesting GABAergic input plays a mechanistic role in rTMS-induced phrenic excitability. These data demonstrate that a single high frequency rTMS protocol at 10 Hz increases phrenic motoneuron excitability, mediated by a local GABAergic "disinhibition". By understanding how rTMS can be used to affect neural circuits non-invasively we can begin to harness the therapeutic potential of this neuromodulatory strategy to promote recovery after disease or injury to the central nervous system.

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.

Effect of sildenafil on neuropathic pain and hemodynamics in rats

PURPOSE

The inhibition of phosphodiesterase 5 produces an antinociception through the increase of cyclic guanosine monophosphate (cGMP), and increasing cGMP levels enhance the release of gamma-aminobutyric acid (GABA). Furthermore, this phosphodiesterase 5 plays a pivotal role in the regulation of the vasodilatation associated to cGMP. In this work, we examined the contribution of GABA receptors to the effect of sildenafil, a phosphodiesterase 5 inhibitor, in a neuropathic pain rat, and assessed the hemodynamic effect of sildenafil in normal rats.

MATERIALS AND METHODS

Neuropathic pain was induced by ligation of L5/6 spinal nerves in Sprague-Dawley male rats. After observing the effect of intravenous sildenafil on neuropathic pain, GABAA receptor antagonist (bicuculline) and GABAB receptor antagonist (saclofen) were administered prior to delivery of sildenafil to determine the role of GABA receptors in the activity of sildenafil. For hemodynamic measurements, catheters were inserted into the tail artery. Mean arterial pressure (MAP) and heart rate (HR) were measured over 60 min following administration of sildenafil.

RESULTS

Intravenous sildenafil dose-dependently increased the withdrawal threshold to the von Frey filament application in the ligated paw. Intravenous bicuculline and saclofen reversed the antinociception of sildenafil. Intravenous sildenafil increased the magnitude of MAP reduction at the maximal dosage, but it did not affect HR response.

CONCLUSION

These results suggest that sildenafil is active in causing neuropathic pain. Both GABAA and GABAB receptors are involved in the antinociceptive effect of sildenafil. Additionally, intravenous sildenafil reduces MAP without affecting HR.

Peripheral GABAB agonists stimulate gastric acid secretion in mice

1 We characterized the effects of intravenous GABA and preferential GABAA (muscimol), GABAB (R-baclofen and SKF-97541) and GABAC agonists (imidazole-4-acetic acid) on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula, and determined the role of vagal cholinergic mechanisms, and gastrin and somatostatin by using peptide immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 2 The selective GABA(B) agonists R-baclofen (0.1-3 mg kg(-1), i.v.) and SKF-97541 (0.01-0.3 mg kg(-1), i.v.) induced a dose-related stimulation of gastric acid secretion. SKF-97541 was about 10 times more potent than R-baclofen stimulating gastric acid secretion. Neither GABA (0.1-100 mg kg(-1), i.v.) nor muscimol (0.1-3 mg kg(-1)) nor imidazole-4-acetic acid (0.1-10 mg kg(-1)) affected basal gastric acid secretion. 3 Stimulatory effects of SKF-97541 (0.1 mg kg(-1), i.v.) were blocked by the selective GABAB antagonist, 2-hydroxysaclofen, cholinergic blockade with atropine, subdiaphragmatic vagotomy or gastrin immunoneutralization. 4 Somatostatin immunoneutralization or SSTR2 blockade with PRL-2903 enhanced the secretory response to SKF-97541 (0.1 mg kg(-1), i.v.) by 78 and 105%, respectively. 5 In SSTR2 knockout mice, SKF-97541 (0.1 mg kg(-1), i.v.) increased basal gastric acid secretion by 48%. Neither GABA nor muscimol nor imidazole-4-acetic acid modified basal gastric acid secretion in SSTR2 knockout mice. 6 These results indicate that, in mice, stimulation of GABAB receptors increases gastric acid secretion through vagal- and gastrin-dependent mechanisms. Somatostatin implication might be secondary to the release of gastrin and the increase in gastric luminal acidity.

Vagal neurotransmission to the ferret lower oesophageal sphincter: inhibition via GABA(B) receptors

GABA(B) receptors modulate the function of the lower oesophageal sphincter (LOS) in vivo by inhibiting neurotransmitter release in the vagal pathway controlling LOS relaxation. We aimed to determine whether this effect was mediated peripherally on vagal motor outflow to the ferret LOS in vitro. The LOS, with intact vagal innervation, was prepared from adult ferrets and LOS tension measured. Vagal stimulation (0.5 - 10 Hz, 30 V) evoked a tetrodotoxin-sensitive, frequency-dependent relaxation. Both GABA (3x10(-4) M) and (+/-)baclofen (2x10(-4) M) inhibited vagally-stimulated LOS relaxation. The potent GABA(B) receptor-selective agonist 3-APPA dose-dependently inhibited vagally-stimulated LOS relaxation, with an EC(50) value of 0.7 microM Decreased responses following vagal stimulation in the presence of (+/-)baclofen or 3-APPA were reversed with the potent GABA(B) receptor antagonist CGP 62349. Neither CGP 62349 nor muscimol (GABA(A) receptor agonist) alone affected LOS responses following vagal stimulation. Agonists of other G protein-coupled receptors (clonidine (alpha(2)-adrenoceptor) (5x10(-6) M), U50488 (kappa opioid) (10(-5) M), neuropeptide Y (10(-6) M)) did not affect vagally-mediated LOS relaxation. The present study supports a discrete presynaptic inhibitory role for GABA(B) receptors on vagal preganglionic fibres serving inhibitory motorneurones in the ferret LOS.

Stimulatory effect of muscimol on gastric acid secretion stimulated by secretagogues in vagotomized rats under anesthesia

The effect of intravenous administration of muscimol, a GABAA receptor agonist, on gastric acid secretion from perfused stomach was studied in vagotomized rats anesthetized with urethane. Muscimol did not stimulate acid secretion by itself. In contrast, muscimol dose dependently potentiated acid secretion induced by pentagastrin, bethanechol and direct vagal stimulation, but not histamine. Muscimol-potentiated acid secretion induced by pentagastrin and bethanechol was not influenced by pretreatment with atropine or cimetidine, respectively. Muscimol-potentiated acid secretion evoked by direct vagal stimulation was prevented by pretreatment with proglumide, a gastrin receptor antagonist. Muscimol-potentiated acid secretion evoked by bethanechol was dose dependently prevented by bicuculline methiodide, suggesting an involvement of peripheral GABAA receptors. These results suggest that muscimol stimulates acid secretion under certain conditions, and that two mechanisms are involved in this effect. The effects of muscimol on acid secretion may be mediated by increasing the release of histamine by pentagastrin, bethanechol and direct vagal stimulation. In addition, muscimol would also be effective if muscarinic agents were already occupying muscarinic acetylcholine receptors on parietal cells.

Central GABA-A receptors exert a tonic inhibitory control on gastric pepsinogen secretion in anaesthetized rats

1. The purpose of the present study was to analyse the role played by central gamma-aminobutyric acid (GABA-A) receptors in the regulation of gastric basal pepsinogen secretion in anaesthetized rats. 2. The central, but not parenteral, administration of the GABA-A receptor antagonist bicuculline or SR-95531 caused a significant and dose-dependent increase in basal pepsinogen secretion without changes in acid output. The stimulant effect exerted by bicuculline was prevented by atropine or pirenzepine, but not by vagotomy. 3. The central, but not parenteral, administration of the GABA-A receptor agonist muscimol or THIP induced a significant and dose-dependent stimulation of both basal pepsinogen and acid secretion. The excitatory effect exerted by muscimol was prevented by atropine, pirenzepine, or pretreatment with omeprazole, but not by vagotomy. 4. These results suggest that central GABA-A receptors mediate a tonic inhibitory control on gastric pepsinogen secretion, while their phasic activation leads to an excitatory effect on acid output. However, the agonist-induced pepsigogue action appears to be generated peripherally as an indirect consequence of the increase in acid secretion. 5. It is also suggested that central GABA-A receptors affect the gastric secretory functions through non-vagal pathways that are sensitive to the blockade of peripheral cholinergic receptors.