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

GABA in the Mammalian Enteric Nervous System

gamma-Aminobutyric acid (GABA) is a transmitter of enteric interneurons, targeting excitatory GABA(A) or inhibitory GABA(B) receptors that modulate motility and mucosal function. Enteric GABA may also subserve hormonal and paracrine signaling. Disruption in gastrointestinal function following perturbation of enteric GABA receptors presents potential new target sites for drug development.

Intraventricular administration of substance p increases the dendritic arborisation and the synaptic surfaces of Purkinje cells in rat's cerebellum

Substance P was infused in the lateral ventricles of twenty Lewis rats for twenty days. On the twentieth day the animals were sacrificed and the cerebellar cortex was processed for electron microscopy. The ultrastructural morphometric analysis revealed that the Purkinje cell dendritic arborisation and the number of the synapses between the parallel fibres and the Purkinje cell dendritic spines were much higher than in control animals. Numerous unattached spines of the secondary and tertiary dendritic branches of the Purkinje cells were also seen in the molecular layer either free or surrounded by astrocytic sheath. The increased number of synapses between the Purkinje cell dendrites and the parallel fibres in the animals, which received substance P intraventricularly, in correlation to control animals, supports a neurotrophine-like activity of the substance P in the mammalian cerebellum, enforcing the pre-programmed capability of the Purkinje cells to develop new synaptic surfaces.

Autoradiographic distribution of radioactivity from (14)C-GABA in the mouse

We investigated the distribution of radioactivity from (14)C-labeled gamma-aminobutyric acid (GABA) in the mouse by in vivo autoradiography to clarify the tissues that show GABA uptake and/or GABA binding. Male mice were injected intravenously with (14)C-GABA in both the absence and presence of an excess of unlabeled GABA, baclofen and isoguvacine. Whole-body autoradiography of (3)H-baclofen, a GABA(B) receptor agonist was also performed. At short intervals after (14)C-GABA injection ( 3 and 6 minutes), very high radioactivity was detected in the kidney cortex, liver, pineal gland, hypophysis, median eminence of the hypothalamus, and cervical ganglion. The hyaline cartilage and glandular part of the stomach showed moderate radioactivity. In the presence of an excess amount of unlabeled GABA, radioactivity in most of tissues decreased significantly, but no significant difference in radioactivity was observed in the presence of baclofen and isoguvacine, agonists of GABA(A) and GABA(B) receptors, respectively. Autoradiography of (3)H-baclofen showed that the kidney had high level of radioactivity, whereas the activity in other tissues and organs was similar or lower than in the blood except for the content of the urinary bladder and the pancreas at 15 minutes after injection. These data indicate that radioactivity from incorporated (14)C-GABA into a variety of cells is much higher than that from bound (14)C-GABA to the receptor sites. Our results suggest that GABA can be quickly localized in many organs of the mouse body after 3 minutes following injection, and GABA may serve multiple functions in those organs.

Neurochemical characterization and distribution of enteric GABAergic neurons and nerve fibres in the human colon

GABA, somatostatin and enkephalin are neurotransmitters of enteric interneurons and comprise part of the intrinsic neural circuits regulating peristalsis. Within the relaxation phase of reflex peristalsis, nitric oxide (NO) is released by inhibitory motor neurons and perhaps enteric interneurons as well. Previously, we identified by GABA transaminase (GABA-T) immunohistochemistry, a subpopulation of GABAergic interneurons in the human colon which also contain NO synthase activity and hence produce NO. In this study, we have examined further the capacity for cotransmission within the GABAergic innervation in human colon. The expression of two important neuropeptides within GABAergic neurons was determined by combined double-labelled immunocytochemistry using antibodies for GABA-T, enkephalin and somatostatin, together with the demonstration of NO synthase-related NADPH diaphorase staining in cryosectioned colon. Both neuropeptides were found in GABAergic neurons of the colon. The evidence presented herein confirms the colocalization of NO synthase activity and GABA-T immunoreactivity in subpopulations of enteric neurons and further allows the neurochemical classification of GABAergic neurons of the human colon into three subsets: (i) neurons colocalizing somatostatin-like immunoreactivity representing about 40% of the GABAergic neurons, (ii) neurons colocalizing enkephalin-like immunoreactivity, about 9% of the GABAergic neurons and (iii) neurons colocalizing NO synthase activity, about 23% of the GABAergic neurons. This division of GABAergic interneurons into distinct subpopulations of neuropeptide or NO synthase containing cells is consistent with and provides an anatomical correlate for the pharmacology of these transmitters and the pattern of transmitter release during reflex peristalsis.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

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.

The Leu13-motilin (KW-5139)-evoked release of acetylcholine from enteric neurones in the rabbit duodenum

1. Involvement of cholinergic mechanisms in the contractile response to Leu13-motilin (LMT, KW-5139) was investigated in rabbit duodenal segments, and longitudinal muscle-myenteric plexus (LM-MP) preparations preincubated wtih [3H]-choline. 2. Contractile response to LMT (0.1 nM-1 microM) consisted of an initial rapid (phasic) contraction and a tonic contraction slowly fading to a sustained plateau. LMT caused a concentration-dependent phasic contraction of rabbit isolated duodenal segments. The EC50 value was 2.5 nM and the maximum amplitude of the contraction was 103% of the response induced by acetylcholine (ACh, 100 microM). Neither tetrodotoxin nor atropine changed the EC50 value or the maximum amplitude of the response to LMT. 3. Both atropine and tetrodotoxin decreased the amplitude and accelerated fading of the tonic contraction produced by LMT. 4. LMT (30 nM-3 microM) induced an increase of 3H-outflow, in a concentration-dependent manner. The LMT-induced increase of 3H-outflow was prevented by removal of external Ca2+ or by the presence of tetrodotoxin. 5. Porcine motilin (10 nM-1 microM) also stimulated the release of 3H at a similar concentration-range to that seen with LMT. 6. Pretreatment with LMT (3 microM for 20 min) decreased LMT- and the porcine motilin-evoked release of 3H but did not alter the high K(+)-evoked release. 7. Our results suggest that LMT and porcine motilin stimulate the release of ACh from enteric neurones through the same receptor, and that the release of ACh plays a role in tonic components of contraction in the rabbit duodenum.

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

The actions of gamma-aminobutyric acid (GABA) and the receptor selective agonists, muscimol (GABAA) and baclofen (GABAB), on motor activity of the longitudinal muscle-myenteric plexus of guinea-pig distal colon were studied in vitro. Preparations exhibited spontaneous contractions that were blocked by scopolamine (1 microM) or tetrodotoxin (1 microM). GABA (3-100 microM) inhibited these contractions; the EC50 was 8 microM. GABA-induced relaxations were not blocked by picrotoxin (30 microM). The GABAA receptor antagonist, bicuculline (3-30 microM), increased the amplitude of spontaneous contractions; this response was not blocked by tetrodotoxin. Baclofen (3-100 microM; EC50 = 14 microM) mimicked the GABA-induced relaxation. Baclofen-induced relaxations were not blocked by the GABAB antagonist, phaclofen (30-100 microM). Muscimol (10-100 microM) induced a contraction followed by a relaxation; both responses faded in the presence of muscimol. The muscimol EC50's for contraction and relaxation were 12.5 and 11 microM, respectively. The muscimol contraction was blocked by tetrodotoxin, scopolamine and picrotoxin and was reduced by hexamethonium (30 microM). Muscimol relaxations were blocked by tetrodotoxin, picrotoxin and apamin (0.1 microM). Muscimol responses were not altered after preincubation of the tissues with cortisol (10 pM-1 microM). These data indicate that GABA can act at presynaptic GABAB receptors to inhibit acetylcholine release from enteric neurons and reduce spontaneous contractions. There are also GABAA receptors on excitatory and inhibitory neurons and agonist action at these receptors results in contraction and relaxation.

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.

Substance P-evoked release of GABA from isolated spinal cord of the newborn rat

Isolated spinal cords of newborn rats were perfused with artificial cerebrospinal fluid and the effects of substance P and its analogs on the release of endogenous GABA were examined. Application of substance P evoked a dose-dependent release of GABA from spinal cords. The threshold concentration of substance P for induction of a significant increase in the GABA release was 3 microM. The substance P-evoked GABA release was neither blocked by removal of Ca2+ from perfusion medium nor by tetrodotoxin. In contrast, the GABA release evoked by high K+ (90 mM) was abolished in Ca(2+)-free medium, and the GABA release evoked by veratridine (5 microM) was suppressed by tetrodotoxin (1 microM). A GABA uptake inhibitor, cis-4-hydroxynipecotic acid, markedly augmented the GABA release induced by high K+, but not that induced by substance P or veratridine. These results suggest the possibility that a carrier-mediated mechanism might be involved in the GABA release induced by substance P, as well as by veratridine, in the newborn rat spinal cord. Two N-terminal fragments of substance P, substance P free acid and substance P1-10 amide, as well as [D-Arg1,D-Trp7,9,Leu11]substance P (spantide), evoked an increase in the GABA release, whereas substance P1-6, and a C-terminal fragment, substance P5-11 were inactive. Somatostatin and compound 48/80 also evoked a GABA release, which was independent of external Ca2+ and resistant to tetrodotoxin. [D-Pro4,D-Trp7,9,10]substance P4-11 (10-15 microM) inhibited the GABA release evoked by substance P, somatostatin and compound 48/80.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of substance p-like immunoreactivity in nerves of the gastrointestinal tract of the frog Rana esculenta L

Substance P-like immunoreactivity in the alimentary canal of the frog Rana esculenta L. was studied by means of the indirect immunoperoxidase method. In all segments of the gastrointestinal tract, immunoreactivity was revealed in both the myenteric and the submucosa plexus. Stained nerve cells were observed in the myenteric plexus but not in the submucous plexus. The proximal part of the oesophagus and hindgut were free of immunoreactive perkarya. The stained nerve cells were of the Dogiel type I category in the foregut, and type II in the midgut. Both the musculature and gastrointestinal glands were supplied with immune-positive fibres. These results indicate that substance P may play similar roles in the frog gut, as described previously in mammals and fish.

Distribution of GABA-like immunoreactivity in myenteric plexus of carp, frog and chicken

The distribution of GABA-like immunoreactivity was studied by means of indirect immunocytochemical methods in some lower vertebrate species (carp, frog, chicken). An immunoreactive network was revealed in the myenteric plexus of the alimentary canal of carp. GABA-positive nerve cells were attached closely to the fibres in the stomach. In other gut regions immunostained neurons were less frequent. Immunoreactive fibres often formed baskets on the surfaces of immunonegative neurons along the whole length of the alimentary canal. The number of immunopositive nerve fibres and pericellular baskets seemed to be lower in the mid- and hingut than in the foregut region. A similar distribution of GABA-immunoreactivity was revealed in the frog myenteric plexus. The ganglionated foregut region possessed a relatively dense GABAergic innervation. This part of the gut contained immunostained nerve cells and fibres, while the mid- and hindgut possessed only a scanty fibre system. Chicken exhibited an extensive immunoreactive plexus for GABA, although the GABA-stained perikarya were restricted mainly to the duodenum. Further regions of the small intestine were poor in immunoreactive cell bodies, which suggests a segmental origin and arrangement of GABAergic innervation within the plexus. In all three species studied, GABA-positive fibres run into the circular muscle layer. The varicosity suggests their influence on the movement of the smooth muscles through modifying the transmitter release of neighbouring terminals.

Regulation of the substance P-induced contraction via the release of acetylcholine and gamma-aminobutyric acid in the guinea-pig urinary bladder

1. The action of substance P (SP) on the release of gamma-aminobutyric acid (GABA) and acetylcholine (ACh) and on contraction were studied in strips of the guinea-pig urinary bladder. Substance P induced a dose-dependent contraction of strips of guinea-pig urinary bladder (EC50 = 1.2 x 10(-9) M). This contraction was not altered by tetrodotoxin, but with a dose of 10(-9) M and less, there was a complete inhibition by 10(-6) M) atropine. Contractions initiated by 3 x 10(-9) M) SP or more were partly inhibited by atropine. The EC50 value of substance P in the presence of atropine was 7.0 x 10(-9) M. 2. Substance P induced a Ca2+-dependent and tetrodotoxin-resistant release of [3H]-acetylcholine (ACh) from strips of urinary bladder preloaded with [3H]-choline (EC50 = 4.9 x 10(-10) M), and this release was antagonized by [D-Pro2,D-Trp7,9] substance P. 3. Bicuculline increased the substance P-induced contraction and the release of [3H]-ACh from the strips. 4. Substance P induced a Ca2+-dependent and tetrodotoxin-sensitive release of [3H]-gamma-aminobutyric acid (GABA) from strips preloaded with [3H]-GABA (EC50 = 2.6 x 10(-9) M), and this release was antagonized by [D-Pro2,D-Trp7,9] substance P. 5. Therefore, substance P appears to exert excitatory effects on the contractility of urinary bladder predominantly by stimulating its own receptor located on the cholinergic nerve terminals. GABA released by substance P inhibits stimulation of the cholinergic neurone. However, the direct action of substance P on the cholinergic neurone is more potent that the indirect action via GABA release.

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.

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.

Central and peripheral action of GABAA and GABAB agonists on small intestine motility in rats

gamma-Aminobutyric acid (GABA) is known as a neurotransmitter in the central nervous system and in the enteric nervous system. The effects and the sites of action of GABA and of its GABAA (muscimol) and GABAB (baclofen) agonists were determined on intestinal motility of unanesthetized rats chronically fitted with intraparietal electrodes in the duodeno-jejunum and fasted for 8 h. GABA (6 mg/kg i.p.) induced a biphasic response i.e. a primary inhibition followed by a period of irregular spiking activity. Muscimol (4 mg/kg i.p.) inhibited the cyclic motor profile while baclofen (4 mg/kg i.p.) had a stimulatory effect chiefly at the duodenal level. Only baclofen intracerebroventricularly administered (1 microgram i.c.v.) was able to reproduce the intestinal motor effects observed after systemic injections. Bicuculline (a specific GABAA antagonist) blocked the inhibition induced by GABA and muscimol; atropine (i.p. and i.c.v.) antagonized the irregular spiking activity induced by GABA and baclofen. It is concluded that the dual effect of GABA can be explained by an action at 2 subtypes of receptors: GABAA and GABAB. Stimulation of GABAA receptors induced peripherally mediated inhibition of the duodeno-jejunal motility. On the contrary stimulation of the GABAB receptors increased and disrupted duodenal cyclic motility by a central action involving central and peripheral muscarinic receptors.

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.

Involvement of substance P in the excitatory action of GABAA agonists on cholinergic neurons in the guinea-pig ileum

The possible involvement of substance P (SP) in cholinergic contractions induced by GABAA agonists in the guinea-pig ileum was further investigated. Responses evoked by 3-aminopropane sulphonic acid (3-APS) or muscimol consisted of a rapid phasic contraction followed in 70% of preparations by a tonic contraction, usually smaller in amplitude but considerably longer in duration. Phasic and tonic components were sensitive to bicuculline, neurogenic (cholinergic) in nature and susceptible to desensitization. Capsaicin (0.2 microM) pretreatment and SP receptor desensitization caused by 3 different priming SP concentrations (10 nM, 30 nM, 100 nM), depressed both components of the 3-APS-induced response, the magnitude of antagonism being greater for tonic contractions. Similar findings were obtained by using 10 microM (D-Pro4,D-Trp7.9)SP-(4-11), even though the degree of antagonism caused by this SP antagonist was consistently lower. These results indicate that depression of SP receptor function achieved by three different procedures decreases cholinergic contractile responses to GABAA agonists in the guinea-pig ileum. This provides further support for the hypothesis that GABAA receptor activation evokes both direct and indirect stimulation of enteric cholinergic neurons and that SP and/or a related peptide play an important role in mediating the indirect component of the cholinergic response.

Release of gamma-aminobutyric acid and acetylcholine by neurotensin in guinea-pig ileum

The release of gamma-aminobutyric acid (GABA) and acetylcholine (ACh) from the strips of guinea-pig ileum was investigated in the presence of neurotensin. Neurotensin evoked the release of [3H]-GABA from the strips preloaded with [3H]-GABA, and the evoked release was Ca2+-dependent and tetrodotoxin-sensitive. Hexamethonium, scopolamine, [D-Pro2,D-Trp7,9] substance P and pretreatment with substance P did not alter the neurotensin-evoked release of [3H]-GABA. Pretreatment with neurotensin inhibited the release of [3H]-GABA evoked by neurotensin but not by high K+, thereby indicating that neurotensin induced a specific desensitization of its own receptor. These observations indicate that neurotensin may stimulate the GABAergic neurone through its own receptor. Neurotensin evoked the release of [3H]-ACh from strips preloaded with [3H]-choline and this release was Ca2+-dependent and tetrodotoxin-sensitive. The evoked release of [3H]-ACh was not affected by hexamethonium, scopolamine and [D-Pro2,D-Trp7,9] substance P. Bicuculline partly inhibited the neurotensin-evoked release of [3H]-ACh; thus neurotensin seems to induce a release of ACh partly through the release of endogenous GABA. All this evidence indicates that neurotensin induces release of GABA as well as ACh from the myenteric neurones of the guinea-pig ileum.

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.

Inhibitory action of capsaicin on cholinergic responses induced by GABAA agonists in the guinea-pig ileum

Pretreatment of the guinea-pig ileum with capsaicin resulted consistently in depression of the neurogenic cholinergic contractions induced by the GABAA receptor agonists 3-aminopropane sulphonic acid (3-APS) and muscimol. Since capsaicin acts mainly by releasing and depleting substance P from its stores in intestinal nerves, it is likely that substance P plays a role in the response caused by GABAA-mimetic compounds, On the whole, our results suggest that excitatory responses to 3-APS and muscimol result from both direct and indirect activation of intrinsic intestinal cholinergic neurons innervating smooth muscle cells.

Immunohistochemical demonstration of GABA containing neurons in the guinea pig ileum using purified GABA antiserum

Antisera against GABA were prepared by immunizing rabbits with GABA conjugated to bovine serum albumin by glutaraldehyde and the antisera were then purified using a GABA immobilized epoxy-activated affinity column. Affinity chromatography and GABA-immobilized epoxy-activated agarose gels were made use of for the reduction of the cross-reactivities of GABA antiserum against endogenous amino acids. The purified GABA antiserum showed remarkably less cross-reactivity. Using this purified GABA antiserum, we noted numerous GABA-like immunoreactive (GABA-positive) nerve fibers in the myenteric meshworks and a few GABA-positive fibers exiting from the myenteric plexus of guinea pig ileum. In the myenteric ganglia, there were GABA-positive nerve cells and GABA-positive varicose fibers surrounding or running along the non-immunoreactive nerve cells. The direct visualization of enteric GABA neurons provides further support for the proposal that GABA is a neurotransmitter in the guinea pig small intestine.

GABA in the PNS: demonstration in enteric neurons

The hypothesis that gamma-aminobutyric acid (GABA) is a peripheral neurotransmitter was tested by determining whether GABA is present in enteric neurons. GABA was detected in the quail gut using high pressure liquid chromatography. GABA-like immunoreactivity was found in both enteric neural plexuses. Only a few neuronal perikarya, but many fibers were stained in each. GABA-immunoreactive neurons survived for over a week in vitro; therefore, they must be intrinsic to the bowel. These and earlier observations indicate that GABA is a peripheral (enteric) as well as a central neurotransmitter.

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.