Distribution of neurons with high-affinity uptake sites for GABA in the myenteric plexus of the guniea-pig, rat and chicken

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.

Effect of GABA-ergic mechanisms on synaptosomal NO synthesis and the nitrergic component of NANC relaxation in rat ileum

BACKGROUND

γ-Aminobutyric acid (GABA) acts on specific neural receptors [A, B and C(Aρ)] to modulate gastrointestinal function. The precise role of GABA receptor activation in the regulation of presynaptic nitric oxide (NO) synthesis in nerve terminals is unknown.

METHODS

Rat ileal nerve terminals were isolated by differential centrifugation. Nitric oxide synthesis was analysed using a L-[(3) H]arginine assay. In vitro studies were performed under non-adrenergic non-cholinergic (NANC) conditions on isolated ileal segments.

KEY RESULTS

γ-Aminobutyric acid inhibited NO synthesis significantly (n = 6, P < 0.05) [(fmol mg(-1) min(-1)) control: 27.7 ± 1.5, 10(-6) mol L(-1): 19.7 ± 1.3; 10(-5) mol L(-1): 17.5 ± 3.0]. This effect was antagonized by the GABA A receptor antagonist bicuculline and the GABA C receptor antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), but not by the GABA B receptor antagonist SCH 50911. The GABA A receptor agonist muscimol [(fmol mg(-1) min(-1)) control: 27.6 ± 1.0, 10(-6) mol L(-1): 19.1 ± 1.7, n = 5, P < 0.05] and the GABA C receptor agonist cis-4-aminocrotonic acid (CACA) [(fmol mg(-1) min(-1)) control: 29.5 ± 3.2, 10(-3) mol L(-1): 20.3 ± 2.5, n = 6, P < 0.05], mimicked the GABA-effect, whereas the GABA B agonist baclofen was ineffective. Bicuculline reversed the inhibitory effect of muscimol, TPMPA antagonized the effect of CACA. In functional experiments the GABA A and C receptor agonists reduced the NANC relaxation induced by electrical field stimulation in rat ileum by about 40%. After NOS-inhibition by Nε-nitro-L-arginine methyl ester (L-NAME) the GABA A receptor agonist had no effect, whereas the GABA C receptor agonist still showed a residual response.

CONCLUSIONS & INFERENCES

γ-Aminobutyric acid inhibits neural NO synthesis in rat ileum by GABA A and GABA C(Aρ) receptor-mediated mechanisms.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

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.

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.

Gene expression and localization of GABA(C) receptors in neurons of the rat gastrointestinal tract

The effects of GABA in the CNS are mediated by three different GABA receptors: GABA(A), GABA(B) and GABA(C) receptors. GABA(A) and GABA(B) receptors, but not yet GABA(C) receptors, have been demonstrated in the enteric nervous system, where GABA has been proposed to be a transmitter. The purpose of this study was to determine whether GABA(C) receptors are present and thus may play a role in mediating the effects of GABA in the myenteric plexus of the rat gastrointestinal tract. We examined the expression of the three known GABA(C) receptor subunits, rho1, rho2 and rho3, in the rat duodenum, ileum and colon using the reverse transcriptase-polymerase chain reaction. We determined the localization of GABA(C) receptors in the myenteric plexus of these regions using two different antisera directed against GABA(C) receptor subunits. The polymerase chain reaction revealed that all three subunits were expressed in the gastrointestinal tract. When the layers of the intestine were separated and the layer containing myenteric neurons was assayed, the rho3 subunit was found in the ileum and colon, whereas rho1 was expressed in the duodenum and weakly in the colon and rho2 was expressed in the ileum. Immunocytochemistry revealed numerous labeled neurons in the myenteric plexus of each region. Colocalization showed that a large proportion of calbindin plus calretinin immunoreactive neurons (intrinsic primary afferent neurons) were immunoreactive for the GABA(C) receptor, and that 56% of nitric oxide synthase immunoreactive neurons (inhibitory motor neurons) exhibited the receptor. These results indicate that GABA(C) receptors of differing subunit compositions are expressed by neurons in the rat gastrointestinal tract. The effects of GABA on intrinsic sensory and on inhibitory motor neurons are likely to be mediated in part through GABA(C) 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.

Relationship between appearance of GABA, fluorogenic monoamines and cytochrome oxidase activity during prenatal morphogenesis of chick myenteric plexus

The basic histology of the developing embryonic gut wall of the chick was examined on haematein and eosin-stained paraffin sections. In parallel with this, the ontogenic sequence of myenteric plexus formation was followed on whole mounts after NADH diaphorase histochemistry. The presence of nerve elements was verified also by electron microscopy. The appearance of enteric gamma-aminobutyric acid-containing neurons, as an example of an intrinsic inhibitory neuronal system, was studied by using an antiserum against the gamma-aminobutyric acid glutaraldehyde bovine serum albumin conjugate. The development of noradrenergic innervation as an extrinsic inhibitory supply was followed by means of a glyoxylic acid-induced fluorescence method. Cytochrome oxidase activity was detected histochemically. Three consecutive steps of the morphogenesis of the myenteric plexus were revealed; first the appearance of a cellular crest at the mesenteric border on embryonic day 9; second the migration and clustering of nerve cells between embryonic days 10 and 16; then the elongation of neurites on embryonic days 16 and 21. Immunoreactive and also fluorescent fibres were first detected on the 14th day of incubation, while immunopositive cell bodies appeared only after hatching. In the early stages the cytochrome oxidase activity was restricted to the perikarya, while at the end of embryonic development the activity also appeared in the ganglionic neuropile. On the basis of these observations, we concluded that there is a close time relation between the morphogenesis and the biochemical and functional maturation of the myenteric plexus.

A comparative study of the effects of delta-aminolaevulinic acid and the GABAA agonist, muscimol, in rat jejunal preparations

Preparations of rat jejunum were tested for their responsiveness to the GABAA receptor agonist, muscimol, and to the haem precursor, delta-aminolaevulinic acid (ALA). Both muscimol (1.0-30 microM) and ALA (1.0 microM-3.0 mM) elicited a concentration-dependent increase in tone. Pretreatment with the GABAA antagonist, bicuculline (10(-5) M), blocked effects of muscimol at all concentrations tested and attenuated effects of 0.3 mM ALA. However, bicuculline enhanced responsiveness of the preparations to ALA at low concentrations (0.01-0.05 microM), as also did picrotoxin (10(-5) M), eliciting a significant increase of tone. The significance of these findings is discussed. This finding of pharmacological activity by ALA at concentrations comparable with its blood levels during acute attacks of intermittent porphyria provides support for the proposal that is may play a role in the aetiology of the gastrointestinal manifestations of this disease.

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).

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).

Autoradiographic localization of [3H] gamma-aminobutyric acid in neuronal elements of the rat gastric antrum and intestine

High-affinity uptake and localization of radiolabelled gamma-aminobutyric acid (GABA) has been examined using light microscopic autoradiography in laminar preparations and transverse paraffin sections of the rat stomach, and small and large intestine. In the presence of beta-alanine (10(-3) M), a substrate specific inhibitor of high-affinity GABA transport into glia, tritiated GABA was accumulated by a high-affinity uptake system into myenteric ganglia and a subpopulation of mucosal cells. In the small and large intestine high-affinity uptake of [3H]GABA was evident in myenteric ganglion cells, extra-ganglionic sites and in the deep muscular nerve plexus of the circular muscle layer. Such labelling could be prevented in tissue treated with the specific neuronal high-affinity uptake blocker, L-2,4-diaminobutyric acid dihydrochloride (L-DABA; 10(-3) M), and therefore represented the selective distribution of [3H]GABA uptake sites to intrinsic neuronal elements of the rat gastrointestinal tract.

Light and electron microscopic localization of GABA-like immunoreactivity in myenteric plexus of chicken

Whole-mounts of 1-day-old chicken midgut were incubated with an antiserum against GABA-glutaraldehyde-BSA conjugate. The immunoreaction was visualized by using the peroxidase-antiperoxidase method, and processed for consecutive light and electronmicroscopic observation. GABA was selectively localized in some of the varicose and nonvaricose nerve fibres of the myenteric plexus. The varicose fibres formed dense networks within the myenteric ganglia, some of which--mainly in duodenum--also contained immunopositive nerve cell bodies. Some of the varicose fibres projected out from the myenteric plexus into the circular muscle layer. At the electronmicroscopic level, labelled axon terminals formed synaptic contact with unlabelled perikarya and vica versa. At the same time, no labelled terminals were found on immunostained cells. In a few cases, axon terminals with GABA positivity were situated close to the smooth muscle cells in the circular muscle layer, suggesting a prejunctional GABA effect on the neighbouring nerve terminals on the release of their transmitters.

GABAA receptors in the rat stomach may mediate mucoprotective effects

The occurrence and characteristics of binding sites specific for gamma-aminobutyric acid (GABA) and muscimol in the rat stomach were examined by biochemical and autoradiographic techniques, and the effects of GABAergic model compounds on gastric ulceration induced by chemical irritation was studied in intact and unilaterally vagotomized rats. Specific binding sites for [3H]GABA and [3H]muscimol, which showed the characteristics of GABAA receptors, were demonstrated on gastric membranes. Specific muscimol binding sites were found in all regions of the stomach and were present in both the mucosal layer and the remaining tissue of the stomach. Oral pretreatment of the rats with GABA, selective GABAA receptor agonists, or inhibitors of GABA degradation protected the gastric mucosa against the ulcers induced by acidified ethanol (chemical irritant), in both intact and vagotomized rats. These findings are consistent with the view that a subpopulation of GABAA receptors in the rat stomach may mediate the anti-ulcer effect.

Demonstration of GABA-like immunoreactivity in myenteric plexus of frog stomach

The GABAergic innervation of the frog stomach was studied by means of an indirect immunohistochemical method. Whole mount preparations were obtained from frog stomachs after the animals had been perfused with a mixture of picric acid, glutaraldehyde and glacial acetic acid. Samples were incubated with an antiserum specific for GABA coupled to BSA with glutaraldehyde. Anti-rabbit IgG-HRP was processed by the two step method (Eckert and Ude 1983). GABA-positive varicose fibers and also nerve cell bodies were revealed within the myenteric plexus. The density of GABA-immunoreactive neurons was not higher than 4-8 cell/cm2, which is approximately 1% of the total nerve cell number in the myenteric plexus.

Releasable, non-neuronal GABA pool in rat stomach

The uptake and K+-evoked release of [3H] gamma-aminobutyric acid (GABA) was examined in tissue slices from rat gastric antrum. Active [3H]GABA accumulation was observed that could only be partially inhibited by L-diaminobutyric acid (L-DABA), a blocker of neuronal GABA uptake. The L-DABA-insensitive uptake component was saturable, sodium-dependent and of high affinity. Moreover, [3H]GABA could be released from this L-DABA-resistant pool by a high K+ concentration. It is concluded that a releasable, non-neuronal GABA pool, which may have some functional significance, is present in the gastric antrum.

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.

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.

Immunocytochemical localization of GABA in cat myenteric plexus

Transverse sections through various levels of the feline gastrointestinal tract (antrum pyloricum, duodenum, jejunum, ileum and colon) were obtained from animals perfused with a mixture of glutaraldehyde and paraformaldehyde, and incubated with an antiserum specific for glutaraldehyde-fixed GABA. This antiserum has previously been characterized and shown to react selectively with presumed GABAergic neurons in the CNS. At all levels of the gastrointestinal tract, staining was observed in varicose fibers in the myenteric plexus and the circular muscle layer. Staining was abolished by glutaraldehyde complexes of GABA previously added to the antiserum, but not by similar complexes of related amino acids. The findings strongly suggest that GABA is concentrated in a subpopulation of gastrointestinal nerve fibers and add to previous evidence favoring a transmitter role for GABA in the enteric nervous system.

Presynaptic muscarinic and alpha-adrenoceptor-mediated regulation of GABA release from myenteric neurones of the guinea-pig small intestine

The effects of cholinomimetic and sympathomimetic drugs on the release of [3H]-gamma-aminobutyric acid ([3H]-GABA) evoked by high K+ from the isolated small intestine of the guinea-pig were investigated, in the presence of tetrodotoxin. Acetylcholine and oxotremorine, at concentrations ranging from 10(-9) to 10(-6) M inhibited the evoked release of [3H]-GABA in a concentration-dependent manner, while nicotine was without effect. Scopolamine and pirenzepine inhibited the effect of oxotremorine, while hexamethonium had no effect. The IC50 values for scopolamine and pirenzepine of the oxotremorine (3 X 10(-8) M)-induced inhibition were 1.02 X 10(-9) M and 9.78 X 10(-10) M, respectively. Noradrenaline, but not isoprenaline inhibited the evoked release of [3H]-GABA. Clonidine (10(-10)-10(-6) M) reduced the evoked release of [3H]-GABA in a concentration-dependent manner, but phenylephrine had no effect. The inhibitory effect of clonidine was antagonized by yohimbine but not by prazosin. These findings provide evidence for the localization of M1-muscarinic and alpha 2-adrenoceptors on GABAergic nerve terminals and their involvement in the presynaptic control of the release of GABA from the guinea-pig small intestine.

gamma-Aminobutyric acid in peripheral tissues

Significant amounts of gamma-aminobutyric acid (GABA), an endogenous amino acid, are present in mammalian peripheral tissues. This finding led to the suggestion that GABA may act as a neurotransmitter in the peripheral nervous system as it does in the central nervous system. This review deals with recent identification of GABA in the autonomic nervous system and the possible functional role of GABA in neuronal and non-neuronal tissues. The identification of GABA in the autonomic nervous system has paved the way for new approaches in pharmacological investigations.

Neurons dissociated from rat myenteric plexus retain differentiated properties when grown in cell culture. I. Morphological properties and immunocytochemical localization of transmitter candidates

We have developed procedures for dissociating neurons from the myenteric plexus of the small intestine of newborn rats and for growing those neurons in cell cultures for up to 3 months. Neurons in these cultures retain many of the differentiated properties of myenteric neurons in vivo. This is the first of a series of 3 papers describing those properties. In this paper, we describe the morphology of cultured neurons that we have observed with light and electron microscopy; we also describe the patterns of straining observed when immunocytochemical techniques were used to localize neurotransmitter candidates in the cultured neurons. Intracellular injections of a fluorescent dye, Lucifer yellow, revealed that many of the cultured neurons had morphologies similar to those of myenteric neurons in vivo. When thin sections of cultures were viewed in an electron microscope, many neurons were observed to have numerous small (40-60 nm), clear synaptic vesicles and/or large (80-150 nm), opaque-cored (p-type) vesicles. Synaptic profiles were most often observed on neuronal somata. Neurons containing immunoreactive serotonin, substance P, somatostatin, enkephalin, bombesin and gastrin/cholecystokinin were observed in about the same proportions as they occur in the intact myenteric plexus. Neurons containing immunoreactive vasoactive intestinal polypeptide were found in higher numbers than reported in vivo. Neurons containing immunoreactive neurotensin, secretin and glutamate decarboxylase were not observed. An antiserum directed against choline acetyltransferase stained 40-50% of the neurons. We conclude that myenteric neurons continue to express much of their normal differentiated properties even when they are removed from the gut, dissociated into a suspension of single cells and grown in culture. Such cultures will be useful for correlating the morphological, biophysical, pharmacological and synaptic properties of individual myenteric neurons and for testing the ability of altered environmental conditions to change those properties.

Substance P provoked gamma-aminobutyric acid release from the myenteric plexus of the guinea-pig small intestine

The release of [3H]gamma-aminobutyric acid (GABA) from the isolated small intestine of the guinea-pig pre-loaded with [3H]GABA was measured in the presence of substance P and vasoactive intestinal polypeptide (VIP). Substance P (10(-10)-10(-7) M) produced a dose-dependent increase in the fractional rate of [3H]GABA release. VIP, even at 10(-7) M, did not affect the spontaneous [3H]GABA release nor the release of [3H]GABA evoked by electrical transmural stimulation (0.5 ms, 15 V, 10 Hz for 30 s). The release of endogenous GABA from the isolated small intestine was measured in the presence of substance P (10(-9) M). After 60 min superfusion, the spontaneous release of GABA was 4.61 +/- 0.14 pmol min-1 g-1 wet wt. (n = 20). Substance P (10(-9) M) produced an approximate 2-fold spontaneous release of endogeneous GABA (8.74 +/- 0.21 pmol min-1 g-1 wet wt. (n = 10)). Perfusion with Ca-free medium containing 1 mM-EGTA and tetrodotoxin (3 X 10(-7) M) inhibited the release of endogenous GABA evoked by substance P (10(-9) M). (D-Pro2, D-Trp7,9) substance P (10(-6) M) antagonized the release of endogenous GABA evoked by substance P (10(-9) M). These results indicate that substance P induces a neuronal release of GABA through its receptor located in the guinea-pig small intestine. Substance P (10(-11)-10(-7) M) produced a dose-dependent increase in the fractional rate of [3H]acetylcholine (ACh) release from the isolated small intestine pre-loaded with [3H]choline. The release of [3H]ACh evoked by substance P (10(-9) M) was inhibited by perfusion with Ca-free medium containing 1 mM-EGTA, tetrodotoxin (3 X 10(-7) M) and (D-Pro2, D-Trp7,9)substance P (10(-6) M). Bicuculline (10(-6) M) inhibited the release of [3H]ACh evoked by substance P (10(-9) M) by 68.1 +/- 4.6% (n = 5), thereby suggesting that the substance P-evoked ACh release is partly mediated through the endogenous GABA released by substance P. These results provide evidence for the neurotransmitter role of GABA and a possible excitatory role of substance P on the GABAergic neurones in the myenteric plexus of the guinea-pig small intestine.