Gamma-aminobutyrate as an autonomic neurotransmitter: release and uptake of [3H]gamma-aminobutyrate in guinea pig large intestine and cultured enteric neurons using physiological methods and electron microscopic autoradiography

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.

Elements of molecular machinery of GABAergic signaling in the vertebrate cholinergic neuromuscular junction

It is generally accepted that gamma-aminobutyric acid (GABA) is a signaling molecule abundant in central synapses. In a number of studies though, it has been shown that GABA signaling functions in the peripheral nervous system as well, in particular, in the synapses of sympathetic ganglia. However, there exists no firm evidence on the presence of GABAergic signaling cascade in the intercellular junctions of the somatic nerve system. By the use of immunohistochemistry methods, in the synaptic area of cholinergic neuromuscular contact in rat diaphragm, we have detected glutamate decarboxylase, the enzyme involved in synthesis of GABA, molecules of GABA, and also GAT-2, a protein responsible for transmembrane transport of GABA. Earlier we have also shown that metabotropic GABA_B receptors have overlapping localization in the same compartment. Moreover, activation of GABA_B receptors affects the intensity of acetylcholine release. These data taken together, allows us to suggest that in the mammalian cholinergic neuromuscular junction, GABA is synthesized and performs certain synaptic signaling function.

Neurally Released GABA Acts via GABAC Receptors to Modulate Ca2+ Transients Evoked by Trains of Synaptic Inputs, but Not Responses Evoked by Single Stimuli, in Myenteric Neurons of Mouse Ileum

γ-Aminobutyric Acid (GABA) and its receptors, GABAA,B,C, are expressed in several locations along the gastrointestinal tract. Nevertheless, a role for GABA in enteric synaptic transmission remains elusive. In this study, we characterized the expression and function of GABA in the myenteric plexus of the mouse ileum. About 8% of all myenteric neurons were found to be GABA-immunoreactive (GABA+) including some Calretinin+ and some neuronal nitric oxide synthase (nNOS+) neurons. We used Wnt1-Cre;R26R-GCaMP3 mice, which express a genetically encoded fluorescent calcium indicator in all enteric neurons and glia. Exogenous GABA increased the intracellular calcium concentration, [Ca2+]i of some myenteric neurons including many that did not express GABA or nNOS (the majority), some GABA+, Calretinin+ or Neurofilament-M (NFM)+ but rarely nNOS+ neurons. GABA+ terminals contacted a significantly larger proportion of the cell body surface area of Calretinin+ neurons than of nNOS+ neurons. Numbers of neurons with GABA-induced [Ca2+]i transients were reduced by GABAA,B,C and nicotinic receptor blockade. Electrical stimulation of interganglionic fiber tracts was used to examine possible effects of endogenous GABA release. [Ca2+]i transients evoked by single pulses were unaffected by specific antagonists for each of the 3 GABA receptor subtypes. [Ca2+]i transients evoked by 20 pulse trains were significantly amplified by GABAC receptor blockade. These data suggest that GABAA and GABAB receptors are not involved in synaptic transmission, but suggest a novel role for GABAC receptors in modulating slow synaptic transmission, as indicated by changes in [Ca2+]i transients, within the ENS.

The pathogenesis of proventricular dilatation disease

Bornaviruses cause neurologic diseases in several species of birds, especially parrots, waterfowl and finches. The characteristic lesions observed in these birds include encephalitis and gross dilatation of the anterior stomach — the proventriculus. The disease is thus known as proventricular dilatation disease (PDD). PDD is characterized by extreme proventricular dilatation, blockage of the passage of digesta and consequent death by starvation. There are few clinical resemblances between this and the bornaviral encephalitides observed in mammals. Nevertheless, there are common virus-induced pathogenic pathways shared across this disease spectrum that are explored in this review. Additionally, a review of the literature relating to gastroparesis in humans and the control of gastric mobility in mammals and birds points to several plausible mechanisms by which bornaviral infection may result in extreme proventricular dilatation.

Expression of GABAA receptor β2/3 subunits in the rat major pelvic ganglion

Several pharmacological and physiological studies have suggested that GABA(A) receptors (GABA(A) Rs) may exist in the rat major pelvic ganglion (MPG), a large coalescent pelvic ganglion that contains both sympathetic and parasympathetic components which innervates pelvic organs. However, the presence of GABA(A) R in the MPG has never been demonstrated directly by morphological studies. In the present study, we used immunohistochemistry to demonstrate the existence of GABA(A) R beta2/3 subunits for the first time in the rat MPG. We also analyzed the neurochemical properties of MPG neurons expressing GABA(A) R beta2/3 subunits. GABA(A) R beta2/3-immunoreactive (-IR) neurons occupied 27.4+/-7.0% of the whole neuronal population, and many of these (77.6%) were co-localized with tyrosine hydroxylase (TH). Likewise, most (86.5%) of TH-IR neurons were GABA(A) R beta2/3-positive. GABA(A) R beta2/3 subunits were also expressed in a few VIP- or NOS-IR neurons, the cholinergic or non-adrenergic, non-cholinergic (NANC) neurons. These results suggest that GABA(A) Rs are involved in the modulation of most sympathetic, noradrenergic neurons and also a subset of VIP and NOS neurons of the rat MPG.

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.

Depolarization-evoked GABA release from myenteric plexus is partially coupled to L-, N-, and P/Q-type calcium channels

1. There are many evidences suggesting that gamma-aminobutyrate (GABA) is an important neurotransmitter and/or neuromodulator in the gut. 2. Using the myenteric plexus-longitudinal muscle preparation from the guinea pig ileum, we investigated the evoked release of [3H] GABA from enteric neurons by electrical pulses or high KCl, which occurs in a calcium-dependent and -independent way. In addition, using selective calcium channel blockers, we report the participation of distinct subtypes of calcium channels in the evoked release, showing a minor participation of L- and Q-type calcium channels, while N- and P-type have a participation of approximately 15%, each. However, regardless of the combination of Ca2+ channel blockers, we did not observe an inhibition greater than 50% of the calcium-dependent component of [3H] GABA release. 3. Thus, while the observed Ca2+-independent release mostly probable occur via reversal of the membrane GABA transporter, in our conditions, a considerable portion of the Ca2+-dependent evoked release of [3H] GABA is not coupled to L-, N-, or P/Q-type calcium channels, suggesting the involvement of intracellular calcium stores or other ways of getting calcium across the membrane.

5-aminolevulinic acid, but not 5-aminolevulinic acid esters, is transported into adenocarcinoma cells by system BETA transporters

5-aminolevulinic acid (5-ALA) and its ester derivatives are used in photodynamic therapy as precursors for the formation of photosensitizers. This study relates to the mechanisms by which 5-ALA is transported into cells. The transport of 5-ALA has been studied in a human adenocarcinoma cell line (WiDr) by means of [14C]-labeled 5-ALA. The rate of uptake was saturable following Michaelis-Menten kinetics (K(m) = 8-10 mM and Vmax = 18-20 nmol.(mg protein x h)-1), and Arrhenius plot of the temperature-dependent uptake of 5-ALA was characterized by a single discontinuity at 32 degrees C. The activation energy was 112 kJ.mol-1 in the temperature range 15 degrees-32 degrees C and 26 kJ.mol-1 above 32 degrees C. Transport of 5-ALA was Na+ and partly Cl(-)-dependent. Stoichiometric analysis revealed a Na+:5-ALA coupling ratio of 3:1. With the exception of valine, methionine and threonine, zwitterionic and basic amino acids inhibited the transport of 5-ALA. 5-ALA methyl ester was not an inhibitor of 5-ALA uptake. The transport was most efficiently inhibited, i.e. by 65-75%, by the beta-amino acids, beta-alanine and taurine and by gamma-aminobutyric acid (GABA). Accordingly, 5-ALA, but not 5-ALA methyl ester, was found to inhibit cellular uptake of [3H]-GABA and [14C]-beta-alanine. Protoporphyrin IX (PpIX) accumulation in the presence of 5-ALA (0.3 mM) was attenuated 85% in the presence of 10 mM beta-alanine, while PpIX formation in cells treated with 5-ALA methyl ester (0.3 mM) or 5-ALA hexyl ester (4 microM) was not significantly influenced by beta-alanine. Thus, 5-ALA, but not 5-ALA esters, is transported by beta-amino acid and GABA carriers in this cell line.

GABA(A) receptors on calbindin-immunoreactive myenteric neurons of guinea pig intestine

These studies were carried out to characterize the properties of gamma-aminobutyric acidA (GABA(A)) receptors on guinea pig intestinal myenteric neurons maintained in primary culture. In addition, the type of neuron expressing GABA(A) receptors was identified using immunohistochemical methods. Whole-cell patch clamp recordings of currents elicited by GABA and acetylcholine (ACh) were obtained using pipettes containing Neurobiotin. After electrophysiological studies, neurons were processed for localization of calbindin-D28K-immunoreactivity (calbindin-ir). GABA (1 mM) and ACh (3 mM) caused inward currents in most cells tested. GABA currents were mimicked by muscimol (1-300 microM) and were blocked by bicuculline (10 microM) indicating that GABA was acting at GABA(A) receptors. GABA currents were associated with a conductance increase and a linear current/voltage relationship with a reversal potential of 1 +/- 1 mV (n = 5). Pentobarbital (PB, 3-1000 microM) and diazepam (DZP, 0.01-10 microM) potentiated GABA-induced currents. A maximum concentration of DZP (1 microM) increased GABA-induced currents 3.1 +/- 0.3 times while PB (1000 microM) increased GABA currents by 11 +/- 2 times. In outside-out patches, the amplitude of GABA-activated single-channel currents was linearly related to membrane potential with a single-channel conductance of 28.5 + 0.5 pS (n = 10). PB and DZP increased the open probability of GABA-induced single-channel currents. Neurons containing calbindin-ir were large, were isolated from other neurons and had GABA current amplitudes of -3.4 +/- 0.3 nA (n = 48). Neurons with weak or absent calbindin-ir were smaller, were localized in clusters of cells and had GABA-induced current amplitudes of -0.6 +/- 0.1 nA (n = 20). ACh-induced currents were smaller in calbindin-ir neurons (-0.7 +/- 0.1 nA) compared to weakly calbindin-ir neurons (-1.4 +/- 0.1 nA). These results indicate that myenteric calbindin-ir neurons express a high density of GABA(A) receptors. Cell size and location allow visual identification of neurons likely to contain calbindin-ir permitting targeted studies of the properties of these neurons.

Expression of functional GABAA receptors in cholecystokinin-secreting gut neuroendocrine murine STC-1 cells

1. Gastrointestinal neuroendocrine (NE) cells synthesize, store and secrete gamma-aminobutyric acid (GABA). Recently, an autocrine-paracrine function of GABA has been proposed for secretion from NE cells. 2. To search for functional GABAA receptors in NE gut cells, we performed whole-cell and perforated-patch-clamp studies in the intestinal cholecystokinin (CCK)-secreting NE cell line STC-1. 3. Application of GABA evoked currents in STC-1 cells. These effects were mimicked by muscimol, an agonist of GABAA receptors, and blocked by picrotoxin or bicuculline, antagonists of GABAA receptors. The GABA- or muscimol-activated currents reversed near 0 mV, which under the recording conditions used was consistent with the activation of the GABAA receptor-Cl- channel complex. 4. In contrast to the effect on most neurons, GABA as well as muscimol led to a (reversible) depolarization of the membrane potential of STC-1 cells. Membrane depolarization in turn activated voltage-gated Ca2+ channels and increased intracellular Ca2+ concentrations in STC-1 cells. 5. In accordance with the observed membrane depolarization and activation of voltage-gated Ca2+ channels, both GABA and muscimol stimulated Ca2+-dependent CCK release. In contrast, bicuculline inhibited the GABA-induced secretion of CCK. 6. Using the reverse transcription-polymerase chain reaction (RT-PCR), mRNA of the GABAA receptor subunits alpha2, alpha3, alpha5, beta1, beta3 and delta could be detected in STC-1 cells. 7. In summary, we have shown that the CCK-secreting gut NE cell line STC-1 expresses functional GABAA receptors and that GABA stimulates CCK release. Thus, GABA is involved in the fine tuning of CCK secretion from the gut NE cell line STC-1.

Immunohistochemical demonstration of GABAB receptors in the rat gastrointestinal tract

Immunohistochemical localization of GABA(B)-receptors was demonstrated in the rat gastrointestinal tract using a monoclonal antibody (GB-1) raised against the purified GABA(B)-receptor. Immunoreactive staining for GABA(B)-receptors was found in some populations of endocrine, muscular and neuronal components in the stomach and gut wall. Positive mucosal epithelial, probably endocrine, cells were distributed throughout the stomach and intestine. Double immunostaining indicated that such positive cells for GABA(B)-receptors often co-possessed serotonin in the small intestine but not in the gastric body. In the muscular layer of the digestive canal, positive staining was seen as dotty granules punctuated on the surface of muscle fibers. In the enteric nervous system, positive neuronal somata were found in both submucosal and myenteric ganglia throughout the entire canal extending from the stomach to the rectum. This is the first report to visualize the cellular localization of GABA(B)-receptors in the gastrointestinal system of the rat, and should provide a fundamental basis for future studies on gastrointestinal functions regulated by GABA(B)-receptors.

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.

Two types of functionally different GABAA receptors mediate GABA modulation of cholinergic transmission in cat terminal ileum

1. The effects of GABA (1 microM-2 mM) on longitudinally or circularly oriented organ bath preparations of cat terminal ileum consisted of a relaxation phase with an inhibition of the rhythmic spontaneous phasic contractions, followed by a phase of contractions characterized by an elevation in basal tone and an increase in amplitude of the spontaneous phasic contractions. 2. Muscimol (100 microM), but not baclofen (100 microM), mimicked the relaxation phase of the response to applied GABA (100 microM) in all tissue preparations. In addition, muscimol induced a phase of contractile activity in the circular muscle layer whilst baclofen exerted a 'GABA-like' contractile effect on the longitudinal muscle layer. Bicuculline (30 microM) or picrotoxinin (30 microM) antagonized the GABA- or muscimol-induced relaxations in all preparations and decreased the GABA- but not the baclofen-induced contractions of the longitudinal muscle layer. 3. Tetrodotoxin (0.5 microM) or atropine (0.1 microM) prevented the bicuculline-sensitive phases of the GABA or muscimol effects on both muscle layers but not the contractile effect of baclofen on the longitudinal muscle layer. 4. The bicuculline-sensitive phases of the GABA effect on both muscle layers were almost completely eliminated by 1 nM pirenzepine. At this concentration pirenzepine did not affect the electrically-evoked cholinergic twitch contractions or contractile responses to applied acetylcholine of both muscle layers. 5. During electrically-evoked cholinergic twitch contractions of both muscle layers, GABA (100 microM) had an inhibitory effect. The inhibition occurred in the presence of pirenzepine (1 nM) but not of bicuculline (30 microM). 6. It is suggested that two types of functionally different bicuculline-sensitive GABAA receptors mediate an exitatory presynaptic and an inhibitory prejunctional action of GABA on the cholinergic transmission in cat terminal ileum.

Localization of GABAA receptor immunoreactivity in NO synthase positive myenteric neurones

GABAA receptors were localized within laminar preparations of the rat distal colon myenteric plexus using a monoclonal antibody (mAb 62-3G1) to the affinity purified GABAA receptor/benzodiazepine receptor/Cl- channel complex. The immunofluorescence procedure showed that approximately half of the myenteric ganglion cells displayed extensive GABAA receptor labelling of their soma. This population was further characterised by treating some GABAA-receptor-labelled laminar preparations for the histochemical demonstration of nitric oxide (NO) synthase-related NADPH-dependent diaphorase activity. A subpopulation of the GABAA-receptor-immunoreactive cells (35%) were also found to display intense NO-synthase-related activity. These findings extend our understanding of the GABAA-receptor-related innervation of the rat gut wall herein referred to as 'A-GABAergic' and provides an anatomical basis for the pharmacologically-identified GABA-nitrergic pathway in the mammalian gut.

Immunopositive GABAergic neural sites display nitric oxide synthase-related NADPH diaphorase activity in the human colon

In the enteric nervous system, gamma-aminobutyric acid (GABA) is a transmitter of interneurons which are proposed to innervate excitatory and inhibitory motor neurons. Nitric oxide (NO) is a putative transmitter of enteric inhibitory motor nerves targeted by GABA. In addition, NO is synthesized by a variety of enteric nerves throughout the gut wall indicative of its potential to be a transmitter of other nerve types, including interneurons. We sought to determine if some populations of nitrergic neurons are interneurons in human infant colon. As enteric neural GABA is exclusive to interneurons, colocalization with NO synthase-related NADPH diaphorase was examined. GABA-transaminase (GABA-T) immunohistochemistry was used to identify GABAergic neurons and a histochemical protocol was used as a marker of neuronal NO synthase-related NADPH diaphorase activity in enteric layers. GABA-T immunoreactive neurons were seen in the ganglionated nerve networks of the myenteric and submucosal layers. GABA-T immunoreactive fibres were also present in the longitudinal and circular muscle layers. A subpopulation of GABA-T immunoreactive neurons within both the myenteric and submucosal ganglia express NO synthase-related activity. This colocalization extends further to a subpopulation of fibers within the muscle layers. These findings strongly suggest that in addition to its role in inhibitory motor neurons, NO may also be a transmitter of enteric interneurons.

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.

Interaction of the neurotoxic pesticides ivermectin and lindane with the enteric GABAA receptor-ionophore complex in the guinea-pig

In isolated segments of guinea-pig small intestine, gamma-aminobutyric acid (GABA) (3-300 microM), the GABAA receptor agonist 3-aminopropane sulphonic acid (3-APS) (3-300 microM) and ivermectin (1-300 microM) caused concentration-dependent nerve-mediated cholinergic contractions sensitive to tetrodotoxin (1 microM) and hyoscine (1 microM). The EC50 values were 30.2 +/- 4.3, 24.6 +/- 3.1 and 4.8 +/- 0.6 microM, respectively. Picrotoxinin (10 microM), an allosteric blocker of the Cl- channel associated with GABAA receptors, non-competitively antagonized the contractile response caused by each agonist. Like picrotoxinin, lindane (10, 30 microM) caused a dose-related shift to the right of the concentration-response curve to GABA, 3-APS and ivermectin with depression of the maximum response. SR 95531 (3 microM), a competitive antagonist of GABAA receptors, caused a parallel dextral shift of the concentration-response curve to ivermectin with an apparent single point pA2 value of 6.5. Our results suggest that ivermectin and lindane, two neurotoxic pesticides interfering with central GABAErgic transmission, exert agonist and non-competitive antagonist properties at the enteric GABAA receptor-ionophore complex. This peripheral complex can thus be considered as an additional target for the action of both these compounds.

Protection by muscimol against gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in spontaneously hypertensive rats

The effects of prolonged administration of the gamma-aminobutyric acid receptor agonist muscimol on enhanced induction of gastric carcinogenesis by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in spontaneously hypertensive rats (SHR), and on the norepinephrine concentration in the gastric wall and the labeling index of gastric mucosa were investigated. SHR and normotensive Wistar Kyoto (WKY) rats as controls were given a solution of MNNG (25 micrograms/ml) for 25 weeks and then i.p. injections of 0.5 mg/kg body weight of muscimol every other day. In control WKY rats, gastric cancers were found in 1 (7%) of 14 rats examined at week 52. In SHR treated with NaCl solution only, the incidence of gastric cancers was significantly increased to 50% compared with that in control WKY rats. However, treatment of SHR with muscimol significantly increased its incidence to 12% compared with the value in SHR treated with NaCl solution only. The norepinephrine concentration in the gastric wall and the labeling index of the gastric mucosa were significantly greater in SHR than in WKY rats. Prolonged administration of muscimol to SHR significantly reduced the norepinephrine concentration in the antral portion of the gastric wall or the labeling index of the antral epithelial cells. These findings indicate that long-term treatment of SHR with muscimol attenuated the enhancement of gastric carcinogenesis in SHR.

Light- and electron-microscopic immunochemical analysis of nerve fibre types innervating the taenia of the guinea-pig caecum

The present work was undertaken to determine by immunocytochemical methods which of the putative enteric neurotransmitters are contained in axons supplying the guinea-pig taenia coli and what proportion of axons is accounted for by the presence of these substances. Numerous fibres displayed immunoreactivity for dynorphin (DYN), enkephalin (ENK), gamma-aminobutyric acid (GABA), nitric oxide synthase (NOS), substance P (SP) and vasoactive intestinal peptide (VIP), but, in contrast to other gut regions, fibres showing immunoreactivity for gastrin-releasing peptide, galanin and neuropeptide Y were rare in the taenia. Fibres reactive for calbindin, calcitonin gene-related peptide, cholecystokinin, 5-hydroxytryptamine and somatostatin were also rare. Tyrosine hydroxylase-like immunoreactivity (TH-LI) was present in numerous fibres that disappeared after extrinsic denervation, a procedure that did not detectably affect any of the other major groups of fibres. Simultaneous staining of extrinsically denervated preparations revealed that SP-LI and VIP-LI were located in separate fibres, and ultrastructural studies showed these to be 58% and 33% of intrinsic fibres supplying the muscle. Immunoreactivity for the general marker, neuron-specific enolase, was located in 95-98% of axons. ENK-LI and DYN-LI were in the same axons, and similar proportions of the fibres with either SP-LI or VIP-LI, about 85%, contained immunoreactivity for ENK and DYN. All VIP-LI fibres, but no SP-LI fibres, were reactive for NOS. The results imply that the taenia of the guinea-pig caecum is innervated by two major groups of enteric neurons: (i) excitatory neurons that contain ACh, SP, other tachykinins, and, in most cases, DYN-LI and ENK-LI; and (ii) inhibitory neurons that contain NOS-LI, VIP-LI, in most cases, the two opioids and, quite probably, ATP as a transmitter. GABA-LI is contained in a smaller population of intrinsic axons. Even though the taenia represents one of the simplest tissues for examining transmission from enteric neurons to intestinal muscle, it shares some of the complexity of other regions, in that four major axon types supply the muscle and both the enteric excitatory and enteric inhibitory neurons contain multiple transmitters.

GABAB receptor-mediated mechanisms in human intestine in vitro

The spontaneous motility of longitudinal muscle of human jejunum was recorded and the effect of gamma-aminobutyric acid-ergic (GABAergic) drugs was tested. GABA and (-)-baclofen (10(-6)-10(-4) M) dose dependently reduced the amplitude and frequency of the spontaneous contractions; muscimol and 3-aminopropanesulfonic acid (3 x 10(-5) M) were ineffective. The effect of 3 x 10(-5) M GABA was reduced by 3 x 10(-3) M 5-aminovaleric acid but not by 3 x 10(-5) M picrotoxin. The dose-response curve for GABA was shifted to the right by 3 x 10(-3) M 3-aminopropanesulfonic acid. Tetrodotoxin 3 x 10(-7) M prevented the GABAergic action, whereas various receptor antagonists tested did not affect it. GABAergic drugs did not influence the spontaneous motility of either circular or longitudinal muscles of human colon. It is suggested that GABAB receptor activation induces the inhibition of human jejunum longitudinal muscle motility by a neurogenic mechanism. The possible involvement of postganglionic cholinergic neurons is to be evaluated by other techniques.

A new monoclonal antibody against the GABA-protein conjugate shows immunoreactivity in sensory neurons of the rat

A monoclonal antibody, 115AD5, was raised against GABA coupled to bovine serum albumin. The monoclonal antibody 115AD5 also reacted with other GABA-protein conjugates. The specificity of the monoclonal antibody was corroborated by enzyme-linked immunoassay, dot-immunobinding experiments and immunostaining of rat cerebellum sections. The monoclonal antibody 115AD5 could successfully be applied on Vibratome and cryostat sections using either indirect immunofluorescence or peroxidase techniques. In rat cerebellar cortex the monoclonal antibody 115AD5 gave an intense immunoreaction in stellate cells, in Golgi neurons, and in basket cells and their processes around Purkinje cell bodies. Purkinje cell dendrites showed GABA immunoreactivity while the cell bodies were non-reactive or only weakly reactive. There was labelling in some nuclei of Purkinje cells. GABA immunoreactivity was also found in dot-like structures in the granular layer. A large population of sensory neurons in rat thoracic and lumbar spinal dorsal root ganglia presented an intense immunoreactivity for the monoclonal antibody 115AD5. Nerve bundles immunoreactive for GABA were also seen in these ganglia. In the trigeminal ganglion, a major population of sensory neurons and some of their processes presented immunoreactivity for GABA. In the sensory nodose ganglion of the vagus nerve, many neuronal cell bodies and some fibres were immunoreactive for GABA. Ligation of the vagus nerve caudal to the ganglion resulted in an increased GABA immunoreactivity in neuronal somata of the ganglion, as well as in nerve fibres on the ganglionic side of the ligature. The present results suggest that in the rat, a population of sensory neurons in thoracic and lumbar spinal dorsal root ganglia, as well as in the trigeminal and nodose ganglia contain GABA. The presence of GABA immunoreactivity in these neurons raises the possibility of a neurotransmitter or modulator role.

Localization of [3H]GABA-labelled nerve fibre networks in the rat intestinal mucosa

Laminar preparations of the rat colonic mucosa were treated for [3H]GABA (5 x 10(-8) M) autoradiography. Under conditions specific for high-affinity labelling of neuronal elements, a network of fibres overlying the base of the mucosal crypts and a second network coursing in close association with the crypts, were intensely labelled. Along the course of the overlying network of fibres, were dense accumulations of silver grains reminiscent of junctions for fibres projecting into the mucosa. All labelling could be prevented by the specific neuronal uptake inhibitor, L-DABA (10(-3) M).

High-affinity uptake of [3H]GABA by submucous ganglion cells, nerve fibres and peri- and para-vascular fibres in guinea-pig and rat intestine

Segments of the intestinal wall from the guinea-pig and rat were dissected, and laminae from the submucous layer subjected to light microscopic [3H]GABA autoradiography. The laminae were carefully prepared so that their planar arrangement could be easily viewed. Intense labelling of fine processes by [3H]GABA was found in the different laminae of the submucous including Henle's or Schabadasch's plexus, the vascular nerve plexus, and a network of fibres subjacent to the muscularis mucosae. This labelling was extensive and represented neuronal-specific high-affinity uptake of radiolabelled GABA. The pattern of labelling was different between the laminae; however, within individual laminae, the distribution of labelled fibres was characteristic of the local nerve networks. Ganglia displayed intense labelling of neuropil and ganglion cells. These results provide strong evidence for the presence of GABAergic nerve cells and processes in the mammalian intestinal submucosa.

GABAB receptor-mediated inhibition of the neurogenic vasopressor response in the pithed rat

1. The effects of gamma-aminobutyric acid (GABA) and related drugs on the vasopressor response induced by electrical stimulation (single pulse of 30 V and 1 ms) of the preganglionic sympathetic nerve fibres or by injection of noradrenaline 0.3 nmol kg-1 were studied in the pithed rat. 2. The electrically-induced increase in diastolic blood pressure was inhibited by GABA and the GABAB-receptor agonist R-(--)-baclofen but was not affected by its S-(+)-enantiomer and by the GABAA-receptor agonists muscimol and 3-aminopropane sulphonic acid. 3. The dose-response curve of R-(--)-baclofen for its inhibitory effect on the electrically-induced vasopressor response was shifted to the right by the GABAB-receptor antagonist 2-hydroxysaclofen, but was not affected by the GABAA-receptor antagonist bicuculline. 2-Hydroxysaclofen and bicuculline by themselves did not affect the electrically-induced vasopressor response. 4. The increase in diastolic blood pressure induced by exogenous noradrenaline was not affected by the GABA-related drugs, which also had no (or very slight) effects on the basal diastolic blood pressure. 5. It is concluded that GABA inhibits catecholamine release in the resistance vessels of the rat via GABAB-receptors, probably located presynaptically on the postganglionic sympathetic nerve fibres.

Selective uptake of gamma-[3H]aminobutyric acid by neural elements and vascular nerves of the rat intestinal submucosa

Laminae of the rat intestinal submucosa were examined autoradiographically for gamma-[3H]aminobutyric acid ([3H]GABA) high-affinity uptake sites. In the presence of 10(-3) M beta-alanine, to prevent high-affinity uptake and localization of radiolabelled GABA by glia. [3H]GABA was accumulated into elements of Henle's or Schabadasch's plexus, and vascular nerves. Densely labelled fibres in the nerve plexus could be followed through the ganglia and interconnecting fasciculi, and often formed a dense neuropil in the ganglia. Cell soma were never labelled. Densely labelled fibres of the nerve plexus were sometimes found to be contiguous with fibres coursing with the blood vessels. All labelling could be prevented by the neural specific high-affinity uptake blocker, L-diaminobutyric acid (L-DABA; 10(-3) M).

The motor response to ethylenediamine of the rat isolated duodenum: involvement of GABAergic transmission?

We have studied the motor response to ethylenediamine (EDA), a well known releaser of endogenous GABA, on the longitudinal muscle of the rat isolated proximal duodenum in presence of atropine (3 microM) and guanethidine (3 microM). EDA produced a concentration-(0.03-3 microM)-dependent relaxation which was potentiated when the preparations were exposed to GABA during the equilibration period. The GABA-induced potentiation of the response to EDA was prevented by nipecotic acid, an inhibitor of GABA uptake. The response to EDA was partially inhibited by 3-mercaptopropionic acid, a known inhibitor of GABA release. However, contrary to the relaxant response produced by exogenous GABA, the EDA-induced relaxation was picrotoxin-(0.1 microM)-resistant. In preparations pre-exposed to GABA, the response to EDA was partially tetrodotoxin-(1 microM)-sensitive. By contrast, in preparations not exposed to GABA, the EDA-induced relaxation was totally tetrodotoxin resistant. The response to EDA was abolished or largely inhibited in preparations excised from rats in which the proximal duodenum was chemically denervated by exposure (2 weeks before), to benzalkonium chloride (BZK). Likewise, the indirect relaxations produced by electrical field simulation. DMPP, capsaicin or GABA were abolished by BZK pretreatment while noradrenaline was still effective. These findings indicate that the relaxant response to EDA is neurogenic in origin, while being largely tetrodotoxin-resistant. A GABAergic mechanism could be involved but also other inhibitory transmitter(s) should be taken into account. EDA appears a useful tool to study the inhibitory non-adrenergic non-cholinergic innervation of the rat proximal duodenum.

Influence of GABA and ethylenediamine in the guinea-pig duodenum

1. GABA induced concentration-dependent transient contractions of the guinea-pig duodenum, but only occasionally evoked small relaxatory responses. The GABA-induced contractions were blocked by atropine and tetrodotoxin but were not influenced by hexamethonium; during electrically evoked twitch contractions, GABA had a concentration-dependent inhibitory effect. 2. The concentration-response curve for the contractile effect of GABA was shifted to the right in a dose-dependent manner by bicuculline and picrotoxin, with a clear reduction of the maximal effect in the presence of picrotoxin. 3. Homotaurine and delta-aminovaleric acid but not baclofen mimicked the GABA-induced contractions; the responses induced by these GABAA receptor agonists were antagonized by atropine, tetrodotoxin and bicuculline. Baclofen concentration-dependently inhibited electrically evoked twitch contractions. 4. Ethylenediamine also had a GABA-like effect, and cross-desensitization developed between GABA and ethylenediamine. 5. The ethylenediamine-induced contractions were not antagonized by thiosemicarbazide; they were reduced by 3-mercaptopropionic acid but the GABA-induced contractions were reduced to the same extent. 6. It is concluded that GABA induces contraction of the guinea-pig duodenum by excitation of GABAA receptors on postganglionic cholinergic neurones; a GABAB receptor-mediated inhibitory effect can be observed during electrically evoked twitch contractions. Ethylenediamine mimicks the GABAA receptor-mediated effect probably by a direct effect on the GABAA receptors.

Augmentation of cysteamine-induced ulceration of rat duodenum by systemically administered gamma-aminobutyric acid (GABA)

Subcutaneous administration of a single dose of gamma-aminobutyric acid (GABA, 10 mg/100 g) in conjunction with a pretreatment dose of aminooxyacetic acid (AOAA 2.5 mg/100 g subcutaneously) to prevent the degradation of GABA, significantly augmented the incidence and intensity of cysteamine HCl-induced duodenal ulceration in rats. This effect of GABA could be reduced by the GABA receptor antagonist, bicuculline (30 micrograms/100 g subcutaneously). These results suggest peripheral GABA receptors can modulate cysteamine HCl-induced duodenal ulcer.

GABA receptors are involved in the modulation of the release of 5-hydroxytryptamine from the vascularly perfused small intestine of the guinea-pig

Isolated small intestinal segments of the guinea-pig were perfused arterially and the release of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) into the portal venous effluent was determined by HPLC with electrochemical detection. Test substances were applied intraarterially. Muscimol (1 microM) time dependently first increased then decreased the release of 5-HT and 5-HIAA. The stimulatory effect was prevented by tetrodotoxin (TTx) or scopolamine, indicating that it was mediated by the release of acetylcholine. Bicuculline concentration dependently decreased (1 microM) or increased (10, 50 microM) the release of 5-HT and 5-HIAA, indicating that endogenous GABA also activates stimulatory and inhibitory GABAA receptors. Bicuculline antagonized the stimulatory and inhibitory effect of muscimol. (-)-Baclofen, but not its (+) enantiomer, inhibited the release of 5-HT in the absence and presence of TTx. It was concluded that the release of 5-HT from enterochromaffin cells is directly inhibited by GABAA and GABAB receptors. In addition, acetylcholine released after activation of GABAA receptors stimulates 5-HT release.

The role of GABAA receptor function in peristaltic activity of the guinea-pig ileum: a comparative study with bicuculline, SR 95531 and picrotoxinin

1. The peristaltic activity of the guinea-pig ileum was studied in the absence and in the presence of the blockade of GABAA receptors. 2. Bicuculline (1-30 microM), improved at the highest concentrations the efficiency of peristalsis by enhancing the frequency of propulsive contractions and the amount of fluid ejected per unit of time. 3. Neither SR 95531 (0.3-10 microM), a novel GABAA receptor antagonist, which competitively antagonized 3-aminopropane sulphonic acid induced contractions in myenteric plexus-longitudinal muscle preparations (pA2 value: 6.47), nor picrotoxinin (1-30 microM) modified peristaltic parameters or influenced the potentiating effect of bicuculline on peristaltic activity. 4. In myenteric plexus-longitudinal muscle preparations, bicuculline (1-30 microM) enhanced the amplitude of electrically-induced cholinergic contractions without modifying submaximal contractions to applied acetylcholine. SR 95531 and picrotoxinin had no effect on twitch amplitude. In the presence of each of these compounds, bicuculline retained its potentiating effect. 5. The results obtained with SR 95531 and picrotoxinin question the view that GABAA receptors may exert a critical role in intestinal propulsion by modulating the activity of nerve pathways subserving peristalsis. Bicuculline potentiates the peristaltic activity of the ileum probably via a facilitatory effect on enteric cholinergic transmission that is independent of GABAA receptor blockade.

Immunocytochemical localization of L-glutamate decarboxylase and catecholamine-synthesizing enzymes in the retroperitoneal sympathetic tissue of the newborn rat

The localization of L-glutamate decarboxylase (GAD), the enzyme synthesizing gamma-aminobutyric acid, was studied in newborn rat retroperitoneal sympathetic tissue, i.e. the main retroperitoneal paraganglion, adrenal medullae and abdominal sympathetic ganglia using the indirect immunofluorescence method. The coexistence of GAD with the catecholamine-synthesizing enzymes tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) was analyzed in consecutive sections or by staining one section consecutively with different antisera. GAD immunoreactivity was observed only in some cell types of each organ studied. In the main retroperitoneal paraganglion, the small, intensely TH-immunoreactive, paraganglion-type cells were GAD-immunoreactive, while the larger moderately TH-immunoreactive, neuron-like cells were non-reactive for GAD. In the adrenal medulla, GAD immunoreactivity was localized only in the adrenaline-synthesizing, PNMT-immunoreactive chromaffin cells. The noradrenaline-synthesizing, i.e. the TH-immunoreactive cells with no PNMT immunoreactivity, were non-reactive for GAD. In the abdominal sympathetic ganglia, some small intensely TH-immunoreactive cells were GAD-immunoreactive, while the principal neurons were non-reactive for GAD. These results provide immunohistochemical evidence that GAD is present and is colocalized with catecholamine-synthesizing enzymes in various sympathetic tissues of the newborn rat. The present results indicate that GAD is localized in adrenaline-synthesizing cells of all the sympathetic tissues studied. A fraction of noradrenaline-synthesizing cells of retroperitoneal sympathetic tissues, excluding the adrenal medulla, also contains GAD.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Establishment and properties of separate cultures of enteric neurons and enteric glia

In this paper methods are described for the preparation of two types of culture derived from myenteric explants: (a) highly enriched neuronal cell cultures, and (b) purified glial cells (greater than 98%). Both procedures combine the technique of antibody complement-mediated cytolysis with the use of an antimitotic agent. Immunohistochemical methods were used to compare the purified cells to their counterparts in mixed cultures (see accompanying paper). Antibodies to the glycoprotein Thy-1 and the monoclonal antibody A2B5 which recognizes gangliosides, labelled the cell surface of all enteric neurons in enriched cultures while subpopulations of the neurons expressed the Leu 7 carbohydrate epitope, the neurotransmitter 5-hydroxytryptamine and the neuropeptides substance P, methionine-enkephalin and vasoactive intestinal polypeptide. Autoradiographic experiments show that a subpopulation of enriched neurons exhibit high-affinity uptake sites for gamma-[3H]aminobutyric acid (GABA). All purified enteric glia continue to express the calcium binding protein S100, the basement membrane glycoprotein laminin and the antigens recognized by the A2B5 antibody, and subpopulations of glia are labelled by the monoclonal antibodies LB1 which binds to GD3 gangliosides, and Leu 7. Thus enteric neurons and glia can survive independently of each other and express molecular properties which are present in cultures normally containing both cell types.

Antigenic markers and laminin expression in cultured enteric neural cells

In this study, polyclonal and monoclonal antibodies have been used in conjunction with standard immunohistochemical methods to define markers which can be used to identify and study the main cell types present in the outgrowth area of explant cultures of myenteric plexus from newborn guinea pig. We show that all of the neurons binds antibodies to the glycoprotein Thy-1 and the antibody A2B5 which recognizes gangliosides. All enteric glial cells bind antibodies to the calcium-binding protein S100, and the A2B5 monoclonal antibody and ca. 95% of glia are labeled by the antibody LB1, which recognizes the GD3 ganglioside. Most fibroblasts are labelled by antibodies to Thy-1 and the matrix glycoprotein fibronectin. Thus enteric neurons can be defined serologically as Thy-1+/A2B5+/S100- cells; enteric glia as S100+ cells and fibroblasts as A2B5-/S100-cells. The markers have been used to demonstrate that laminin is made by both enteric glia and fibroblasts. They have also been used to show that ca. 5% of neurons and less than 5% of enteric glia bind the antibody Leu 7 (HNK-1, L2), thus revealing the subpopulations of neurons and glia show differential cell surface expression of the carbohydrate epitope recognized by the antibody. In the accompanying paper, we demonstrate that two of the antibodies (LB1 and Thy-1) can be used to generate purified populations of neurons and glia.

Lack of release of vasoactive intestinal polypeptide and calcitonin gene-related peptide during electrical stimulation of enteric nerves in streptozotocin-diabetic rats

The simultaneous release of endogenous acetylcholine, serotonin, vasoactive intestinal polypeptide, substance P, and calcitonin gene-related peptide was measured during electrical field stimulation of isolated preparations of rat ileum from control and 8-wk streptozotocin-treated diabetic rats. Electrical field stimulation of the control rat ileum caused a significant increase in the release of all the above substances from the enteric nerves. The electrically evoked, but not the basal, release of these substances was inhibited by tetrodotoxin. In the diabetic rat ileum, however, there was no increase in the release of vasoactive intestinal polypeptide and calcitonin gene-related peptide during electrical stimulation, whereas endogenous release of acetylcholine, serotonin, and substance P was unaffected by the diabetic state. This was surprising in view of the increased fluorescence intensity and tissue content of vasoactive intestinal polypeptide-like immunoreactivity in the same tissue reported previously. The lack of increase in evoked release of vasoactive intestinal polypeptide in the diabetic preparations might be due to an impaired mechanism of release at the terminal site or to defective axonal transport of the peptide, whereas in the case of calcitonin gene-related peptide, it might be the result of the low level of the peptide present in the enteric nerve fibers of the diabetic rat ileum. The differential effect of diabetes on enteric nerves is discussed.

GABA-related actions in isolated in vitro preparations of the rat small intestine

Longitudinal organ bath preparations of the rat duodenum, jejunum and ileum were tested for their responsiveness to GABA-receptor agonists. The GABAA-receptor agonists, GABA and 3APS, induced non-adrenergic, non-cholinergic relaxations and/or contractions, although the magnitude and type of response varied depending upon the region tested. All regions relaxed to applied GABA or 3APS, however the jejunum and ileum also responded with cholinergic contractions. These relaxant and contractile actions were neurogenic and sensitive to blockade by the GABA antagonists bicuculline or picrotoxinin, and desensitization to either agonist. The GABAB-receptor agonist baclofen, caused a reduction in electrically evoked cholinergic contractions. These inhibitory actions of baclofen were insensitive to bicuculline or picrotoxinin. Taken together, these results show that GABA-ergic actions in the rat small intestine are mediated by two pharmacologically distinct neural receptor populations, the GABAA and GABAB sites, the distribution and sensitivity of which differ along the length of the small intestine.

Enteric gamma-aminobutyric acid-containing neurons and the relevance to motility of the cat colon

gamma-Aminobutyric acid (GABA)-containing neurons were identified and the functional relevance in the motility of the colon was studied. Autoradiography of the cat colon treated with [3H]GABA demonstrated scattered neurons in the myenteric plexus selectively labeled with [3H]GABA. Electrical transmural stimulation of the isolated cat colon led to an increase in the Ca2+-dependent, tetrodotoxin-sensitive release of endogenous GABA. gamma-Aminobutyric acid increased the amplitude of rhythmic contractions of the circular muscle of the colon and also the release of acetylcholine, which was Ca2+-dependent and tetrodotoxin-sensitive. Scopolamine inhibited the GABA-evoked rhythmic contractions, without effect on the evoked release of acetylcholine. Bicuculline and furosemide reduced the amplitude of spontaneous rhythmic contractions and the tone, which was reversed by GABA. These results suggest that GABA-containing neurons are involved in the control of motility of the cat colon, due to the stimulation of cholinergic neurons.

Localization of high-affinity GABA uptake and GABA content in the rat duodenum during development

The localization of high-affinity uptake sites for 3H gamma-aminobutyric acid (3H-GABA) was investigated in the rat duodenum during ontogenesis and also at the adult stage (from 15.5 days of fetal life up to 105 days post natum) by means of low- and high-resolution autoradiography. At all stages studied, specific endocrine cell types of the epithelium were labelled and an intense uptake was detected in the nervous tissue, especially in glial cells but also in scarce neurones. When the incubation medium was supplemented with beta-alanine (1 mM), a blocker of the glial uptake for GABA, the labelling persisted only in endocrine cells and in few neurones. The intensity and the frequency of the labelling decreased at later periods compared to the earlier developmental stages. The GABA content of the duodenum as measured by a new ion-exchange column chromatography-HPLC-coupled method was higher in the early postnatal period compared to later stages. These observations suggest that GABA, in addition to being a neurotransmitter, may play an important role during development of the duodenum.

An immunohistochemical study of the distribution of enteric GABA-containing neurons in the rat and guinea-pig intestine

gamma-Aminobutyric acid (GABA) antiserum was applied to sections of rat and guinea-pig intestine which were subsequently processed to reveal any immunoreactivity using either fluorescence or peroxidase techniques. Immunopositive fibres were demonstrated in stomach, duodenum, ileum and colon of rat and guinea-pig intestine. Myenteric ganglia and nerve bundles in the circular muscle contained immunopositive nerve fibres, while the longitudinal muscle, submucosa and mucosa were only rarely innervated. In favourable sections, immunopositive fibres could be seen running from the myenteric plexus into the circular muscle, thus suggesting that the GABA-immunopositive nerves in the circular muscle originate from neurons in the myenteric plexus. In both rat and guinea-pig, immunoreactive nerve cell bodies were most numerous in the myenteric plexus of the colon. In the rat, immunopositive fibres in the circular muscle were most abundant in the ileum, whereas in the guinea-pig it was the colon circular muscle that was most richly innervated. The results demonstrate that neurons which show GABA immunoreactivity are present along the length of the gastrointestinal tract. Their distribution in both myenteric ganglia and circular muscle is heterogeneous both within and between the two species studied. It is probable that this heterogeneity reflects the diversity and specificity of function of this class of enteric neurons.

Is plasma GABA of peripheral origin?

Plasma levels of gamma-aminobutyric acid (GABA) appear to be altered in affective disorders and alcoholism. Plasma levels of GABA were not affected by menstrual cycle, exercise, gender, gut flora, nor by cholinergic stimulation by bethanechol. An obvious peripheral source for plasma GABA could not be demonstrated.

Uptake inhibitors potentiate gamma-aminobutyric acid-induced contractile responses in the isolated ileum of the guinea-pig

The gamma-aminobutyric acid (GABA)-induced contractile responses in the guinea-pig isolated ileum, maintained in Krebs-bicarbonate solution (pH 7.4, 37 degrees C), were significantly potentiated by inhibitors of GABA uptake, with a greater potentiation of the responses in the presence of (+/-)-cis-3-aminocyclohexane-carboxylic acid (ACHC) greater than L-2,4-diaminobutyric acid (DABA) greater than (+/-)-nipecotic acid greater than beta-alanine, whilst simultaneous addition of DABA with beta-alanine caused a greater potentiation of the GABA-induced responses than did nipecotic acid with beta-alanine, or any of the uptake blockers applied alone. The concentration-response curves for the GABA-induced ileal contraction were shifted to the left in the presence of the uptake inhibitors, this shift being more prominent over the lower concentration range of GABA (1-20 microM). By contrast, contractile responses to muscimol or 3-amino-1-propanesulphonic acid (3APS) were not potentiated by the uptake blockers, neither were their concentration-response curves altered. Bicuculline methochloride shifted the GABA concentration-response curve to the right, whilst picrotoxinin both shifted the concentration-response curve for GABA to the right and depressed the maximum response. In the presence of the uptake inhibitors, the rightward shift of the concentration-response curves for GABA induced by bicuculline was less than that induced by bicuculline alone. The rightward shift with picrotoxinin was similarly reduced in the presence of the uptake inhibitors, without altering the depression of the maximum by picrotoxinin. Bicuculline caused a rightward shift of the concentration-response curves for 3APS and muscimol, with the curve for 3APS most affected. Picrotoxinin similarly shifted the concentration-response curves for 3APS and muscimol but depressed the maximum, with the curve for 3APS again being most affected. None of the inhibitors of GABA uptake influenced the concentration-response curves for 3APS or muscimol in the presence of bicuculline or picrotoxinin. 5. In conclusion, a saturable GABA uptake system is present in the enteric nervous system of the guinea-pig intestine, where neuronal GABA uptake appears to predominate over glial uptake.