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

Developmental and age-dependent plasticity of GABAA receptors in the mouse colon: Implications in colonic motility and inflammation

Lifelong functional plasticity of the gastrointestinal (GI) tract is essential for health, yet the underlying molecular mechanisms are poorly understood. The enteric nervous system (ENS) regulates all aspects of the gut function, via a range of neurotransmitter pathways, one of which is the GABA-GABA_A receptor (GABA_AR) system. We have previously shown that GABA_A receptor subunits are differentially expressed within the ENS and are involved in regulating various GI functions. We have also shown that these receptors are involved in mediating stress-induced colonic inflammation. However, the expression and function of intestinal GABA_ARs, at different ages, is largely unexplored and was the focus of this study. Here we show that the impact of GABA_AR activation on colonic contractility changes from early postnatal period through to late adulthood, in an age-dependant manner. We also show that the highest levels of expression for all GABA_AR subunits is evident at postnatal day (P) 10 apart from the α3 subunit which increased with age. This increase in the α3 subunit expression in late adulthood (18 months old) is accompanied by an increase in the expression of inflammatory markers within the mouse colon. Finally, we demonstrate that the deletion of the α3 subunit prevents the increase in the expression of colonic inflammatory markers associated with healthy ageing. Collectively, the data provide the first demonstration of the molecular and functional plasticity of the GI GABA_AR system over the course of a lifetime, and its possible role in mediating the age-induced colonic inflammation associated with healthy ageing.

GABA selectively increases mucin-1 expression in isolated pig jejunum

The inhibitory neurotransmitter GABA (γ-aminobutyric acid) is synthesized by glutamic acid decarboxylase, which is expressed in the central nervous system and in various other tissues including the intestine. Moreover, GABA can be ingested in vegetarian diets or produced by bacterial commensals in the gastrointestinal tract. As previous studies in lung have suggested a link between locally increased GABA availability and mucin 5AC production, the present study sought to test whether the presence or lack of GABA (and its precursor glutamine) has an effect on intestinal mucin expression. Porcine jejunum epithelial preparations were incubated with two different amounts of GABA or glutamine on the mucosal side for 4 h, and changes in the relative gene expression of seven different mucins, enzymes involved in mucin shedding, GABA B receptor, enzymes involved in glutamine/GABA metabolism, glutathione peroxidase 2, and interleukin 10 were examined by quantitative PCR (TaqMan(®) assays). Protein expression of mucin-1 (MUC1) was analyzed by Western blot. On the RNA level, only MUC1 was significantly up-regulated by both GABA concentrations compared with the control. Glutamine-treated groups showed the same trend. On the protein level, all treatment groups showed a significantly higher MUC1 expression than the control group. We conclude that GABA selectively increases the expression of MUC1, a cell surface mucin that prevents the adhesion of microorganisms, because of its size and negative charge, and therefore propose that the well-described positive effects of glutamine on enterocytes and intestinal integrity are partly attributable to effects of its metabolite GABA.

Effect of γ-aminobutyric acid on digestive enzymes, absorption function, and immune function of intestinal mucosa in heat-stressed chicken

To explore the effect of dietary γ-aminobutyric acid (GABA) on digestive enzyme activity, absorption function and immune function of intestinal mucosa in heat-stressed Wenchang chicken were studied. One-day-old male Wenchang chickens were randomly divided into a control group (CK), heat stress group (HS), and GABA+HS group. The chickens from the GABA+HS group were administered with 0.2 mL of GABA solution daily. Chickens from HS and GABA+HS groups were subjected to heat stress treatment at 40 ± 0.5°C for 2 h during 1300 to 1500 h every day. Blood was drawn and 0.5 cm-long duodenum, jejunum, and ileum were collected from the chickens on d 3, 5, 7, 9, 12, and 15. Results showed that the activity of Ca²⁺-Mg²⁺-adenosine triphosphatase (ATPase), Na⁺-K⁺-ATPase, maltase, sucrase, and alkaline phosphatase, the contents of secretory IgA, glutathione, and d-xylose, and the number of lymphocytes in HS group were significantly lower than those in the CK group. Among them, some were rescued after the treatment of GABA as the time extension. For maltase, d-xylose, alkaline phosphatase, and Na⁺-K⁺-ATPase, it required 5 to 7 d for achieving the significant effect. For sucrase, 12 d for the alleviation effect was required. In the case of other parameters, no alleviation was observed during the whole period of the study. We have concluded that HS can inhibit the activity of digestive enzymes and reduce absorption and immune functions of intestinal mucosa. γ-Aminobutyric acid can effectively alleviate these inhibitory effects.

Proliferative effects of gamma-aminobutyric acid on the gastric cancer cell line are associated with extracellular signal-regulated kinase 1/2 activation

BACKGROUND AND AIM

Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the adult mammalian brain. However, GABA is found not only in peripheral neuronal tissue, but also in many peripheral non-neuronal tissues, and is thought to have important physiological functions in addition to neurotransmission. We previously reported that GABA participates in chondrocyte proliferation. In the present study, we investigated the effects of GABA on the proliferation of a gastric cancer cell line, KATO III.

METHODS

Reverse transcription polymerase chain reaction and immunohistochemical analyses were performed to examine the expression of the GABA synthesis enzyme, glutamate decarboxylase (GAD), and that of the GABA(A) and GABA(B) receptor subunits. The production of GABA was confirmed by immunohistochemistry. The proliferative effect of GABA on KATO III cells was analyzed by bromodeoxyuridine incorporation assay, and the activation status of mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase [ERK]-1/2, Jun-N-terminal kinase, and p38) and the expression of cyclin D1 were analyzed by western blotting.

RESULTS

KATO III cells expressed GAD and GABA. More than five GABA(A) receptor subunits, including the pi subunit, were expressed in KATO III cells; however, GABA(B) receptor subunits were not seen. The addition of GABA to the medium promoted KATO III proliferation, and maximum proliferative effects were observed in the presence of 10 or 1 microM GABA. The addition of 1 microM GABA predominantly activated ERK-1/2 among the three MAP kinases in addition to increasing cyclin D1 expression.

CONCLUSION

GABA is able to promote KATO III cell proliferation in an autocrine or a paracrine fashion through GABA(A) receptors followed by MAP kinase activation.

Expression of gamma-aminobutyric acid and glutamic acid decarboxylases in rat descending colon and their relation to epithelial differentiation

OBJECTIVE

To detect the expression of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylases (GADs; including two isoforms GAD65 and GAD67) in the epithelial growth zones of the descending colon in rats, and to investigate their relation to epithelial differentiation and proliferation.

METHODS

The expression of GABA and GADs in rat descending colon was investigated by immunofluorescent staining and confocal laser scanning techniques, and goblet cells were further investigated by wheat-germ agglutinin histochemistry. In addition, GAD65 and GAD67 mRNAs were also detected by reverse transcription-polymerase chain reaction. Furthermore, evaluation of cell kinetics in colonic epithelia was conducted by ABC immunostaining using a monoclonal antibody against proliferating cell nuclear antigen (PCNA).

RESULTS

Immunoreactive GABA and GADs were distributed in the upper third of the crypts and at the luminal surface in the rat descending colon. Strong staining for GABA and GADs was localized mainly in the cytoplasm of epithelial cells near the neck of the crypts and along the luminal surface. In addition, GABA and GAD65 were also detected at the lamina propria in colonic mucosa. No staining for GABA or GADs was found in goblet cells. GAD65 and GAD67 mRNAs were identified in homogenates of rat descending colon. PCNA labeled nuclei were found in the lower two-thirds of the crypts.

CONCLUSIONS

The expression of GABA and GADs in the maturation and function zones of rat descending colon suggests that GABA may be involved in the differentiation of colonic epithelial cells.

Expression of GABAA and GABAB receptors in rat growth plate chondrocytes: Activation of the GABA receptors promotes proliferation of mouse chondrogenic ATDC5 cells

Our previous study showed the local production of gamma-aminobutyrate (GABA) in hypertrophic-zone chondrocytes of the rat tibial growth plate, an important long bone growth site. The aim of this study was to identify the presence of GABA receptors in growth plate chondrocytes by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Chondrocytes expressed both GABA(A) and GABA(B) receptor subunit mRNAs as well as the corresponding proteins necessary for the assembly of functional receptors. The GABA(A) receptor subunits detected included alpha1-alpha4, alpha6, beta1-beta3, and delta, and both R1 and R2 subunits of GABA(B) receptors were detected. All receptor subunits were expressed in chondrocytes of the proliferative and hypertrophic zones. These results suggest that GABA is an autocrine/paracrine factor that regulates the physiological state of the growth plate. Subsequent studies with the mouse chondrogenic cell line ATDC5 showed the presence of mRNAs and the corresponding proteins for GABA(A) receptor alpha1, beta2, and beta3 subunits and GABA(B) receptor R1 and R2 subunits. GABA, muscimol (a GABA(A) receptor agonist), and baclofen (a GABA(B) receptor agonist) increased 5-bromodeoxyuridine (BrdU) incorporation into ATDC5 cells. The effect of muscimol was blocked by bicuculline (a GABA(A) receptor antagonist), and the effect of baclofen was blocked by CGP 35348 (a GABA(B) receptor antagonist). These results suggest that GABA contributes to the ATDC5 cell proliferation via GABA(A) and GABA(B) receptors and these mechanisms may be involved in cartilaginous cell growth.

Characteristic expression of gamma-aminobutyric acid and glutamate decarboxylase in rat jejunum and its relation to differentiation of epithelial cells

AIM

To investigate the expression between gamma-aminobutyric acid (GABA) and glutamate decarboxylase and its relation with differentiation and maturation of jejunal epithelial cells in rat jejunum.

METHODS

Immunohistochemical expression of GABA and glutamate decarboxylase (GAD, including two isoforms, GAD65 and GAD67) was investigated in rat jejunum. Meanwhile, double staining was performed with GAD65 immunohistochemistry, followed by lectin histochemistry of fluorescent wheat germ agglutinin. Furthermore, evaluation of cell kinetics in jejunum was conducted by (3)H-thymidine autoradiography and immunohistochemistry using a monoclonal antibody to proliferating cell nuclear antigen (PCNA).

RESULTS

The cells showing positive immunoreactivity GABA and GAD65 were mainly distributed in the villi in rat jejunum, while jejunal epithelial cells were negative for GAD67. Positive GABA or GAD65 staining was mainly located in the cytoplasm and along the brush border of epithelial cells in the middle and upper portions. In addition, a few GABA and GAD65 strongly positive cells were scattered in the upper two thirds of jejunal villi. Double staining showed that GAD65 immunoreactivity was not found in goblet cells. (3)H-thymidine-labeled nuclei were found in the lower and middle portions of jejunal crypts, which was consistent with PCNA staining. Therefore, GABA and GAD65 were expressed in a maturation or functional zone.

CONCLUSION

The characteristic expression of GABA and GAD suggests that GABA might be involved in regulation of differentiation and maturation of epithelial cells in rat jejunum.

Gamma-amino-butyric acid immunoreactivity in intramucosal colonic tumors

BACKGROUND AND AIM

The level of gamma-amino-butyric acid (GABA) is reported to be increased in colon cancer. Moreover, data suggests that GABA plays a role in the proliferation or maturation of some types of cells. We examined the expression of GABA in intramucosal colonic tumors to clarify the relation between GABA and the degree of atypia.

METHODS

Paraffin sections were prepared from 56 protruded-type colonic neoplasms, which were classified as intramucosal adenocarcinoma (AC), adenoma with severe atypia (ASA), or adenoma with mild to moderate atypia (AMA). Expression of GABA was investigated immunohistochemically, and GABA immunoreactivity was compared to the staining patterns of carcinoembryonic antigen (CEA) and cancer-associated antigen (CA19-9) which were classified into three categories.

RESULTS

Intense GABA immunoreactivity was observed in 73.7%, 54.6%, 13.3%, and 5.4% of AC, ASA, AMA, and normal mucosa specimens, respectively. Kendall's correlation coefficient between GABA immunoreactivity and the degree of atypia was 0.447. Strong, positive CEA staining (pattern 3) was observed in 57.9%, 36.3%, and 13.3% of AC, ASA, and AMA specimens, respectively. Strong, positive CA19-9 staining was observed: 26.3%, 9.1% and 0%, respectively. In AC and ASA, the proportion of glands with strong GABA immunoreactivity was greater than the proportion of glands that were strongly positive for CA19-9.

CONCLUSION

GABA may be useful as a tumor marker in combination with other tumor markers such as CEA and CA19-9.

Transport of 14C-gamma-aminobutyric acid into brain, cerebrospinal fluid and choroid plexus in neonatal and adult rats

In general blood to brain entry of amino acids is greater in the neonatal rats compared to the adults. gamma-Aminobutyric acid (GABA), a neurotransmitter amino acid, shows limited transport across the blood-brain barrier (BBB) in the adult rat. Characteristics of GABA entry into the immature rat brain is yet to be addressed. This investigation was set to study the entry of GABA into brain of the neonatal rat compared to the adult. Using the bilateral in situ brain perfusion technique, the entry of 14C-GABA into brain, cerebrospinal fluid (CSF) and lateral ventricles choroid plexuses was studied in the adult and neonatal rats. 14C-GABA uptake into neonatal rat brain after 20 min perfusion was 0.116+/-0.014 ml g(-1), approximately twice that of the adults (P<0.01). Half saturation constant, K(m), did not change with age (P>0.05), whereas maximal transport into the brain, V(max), was reduced from 0.152 to 0.068 nmol min(-1) g(-1) showing a significant reduction with age (P<0.05). In the neonate the entry of GABA into the CSF was dominant when compared to that into the brain, this could be due to a greater diffusional component, K(d), which was detected to be high in the neonate. In conclusion, the uptake of 14C-GABA into brain of the immature rats exceeded that in the adults which is thought to be due to both greater maximal transport and greater diffusion in the neonate compared to the adult.

Identification of gamma-aminobutyric acid receptor subunit types in human and rat liver

GABA is a potent inhibitory neurotransmitter that binds to heterooligomeric receptors in the mammalian brain. In a previous study, we documented specific GABA binding to isolated rat hepatocytes that resulted in inhibition of hepatocyte proliferation. The purpose of the present study was to define the nature of hepatic GABA(A) receptors and to document their expression during rapid liver growth (after partial hepatectomy). PCRs with gene-specific primers derived from published sequences were performed with Marathon-ready human and rat liver cDNA. Two GABA(A) receptor subunit types (beta3 and epsilon) were expressed in human liver and one subunit type (beta3) in rat liver. PCR amplification of the human GABA(A) receptorbeta3-subunit produced a single product (molecular mass 53-59 kDa). In the case of the epsilon-subunit, two PCR products were identified. After partial hepatectomy, GABA(A) receptorbeta3-subunit expression inversely correlated with regenerative activity (r = -0.527, P = 0.006). In conclusion, these results indicate that in the human liver GABA(A) receptors consist of the beta3- and epsilon-subunit types, whereas in the rat liver only the beta3-subunit type is expressed. The results also support the hypothesis that GABAergic activity serves to maintain hepatocytes in a quiescent state.

Increased GABAergic activity inhibits alpha-fetoprotein mRNA expression and the proliferative activity of the HepG2 human hepatocellular carcinoma cell line

BACKGROUND/AIMS

Gamma aminobutyric acid (GABA) is a potent inhibitory neurotransmitter with growth regulatory properties. Recent data indicate that increased GABAergic activity inhibits hepatocyte proliferation in regenerating livers. In the present study, we aimed to investigate whether GABA inhibits the growth of malignant hepatocytes.

METHODS

Increasing concentrations of muscimol (0.05-50 microM), a specific GABA(A) receptor agonist, were added to HepG2 human hepatocellular carcinoma cells and alpha-fetoprotein (AFP) and albumin mRNA expression were determined for varying periods of time (maximum 24 h) thereafter. Cell proliferation was also documented after 48 h of exposure to muscimol.

RESULTS

Muscimol significantly (p<0.0001) decreased AFP mRNA expression (maximum decrease: 65% below baseline values) without affecting albumin mRNA expression. However, the effect on AFP mRNA was transient (maximum duration: 3-6 h) and not associated with changes in cell proliferation. Because preliminary data indicate that GABA(A) receptor activity is markedly downregulated in malignant hepatocytes, transfection studies were performed wherein HepG2 cells were cotransfected with GABA(A) receptor beta2 and beta2 subunit genes in a pCDM8 expression vector or vector alone followed by re-exposure to either muscimol (5 betaM) or saline. In this series of experiments, in addition to AFP mRNA inhibition being as extensive and more prolonged (maximum duration: 6-12 h) in muscimol-treated, GABA(A) receptor-transfected cells, proliferative activity was also significantly inhibited when compared to saline-treated GABA(A) receptor-transfected controls (p<0.01) and muscimol-treated cells transfected with vector alone (p<0.005).

CONCLUSION

The results of this study indicate that increased GABAergic activity inhibits AFP mRNA expression and cell proliferation in this malignant hepatocyte cell line.

Sodium and chloride-dependent high and low-affinity uptakes of GABA by brain capillary endothelial cells

The mechanisms of carrier-mediated transport of gamma-aminobutyric acid (GABA) at the blood-brain barrier (BBB) were examined by investigating [3H]GABA uptake by isolated bovine brain capillaries, monolayers of primary cultured brain capillary endothelial cells (BCECs) attached to plates or suspended BCECs. The uptake of [3H]GABA was concentration-dependent and saturable. Nonlinear regression analysis of the original data indicated the existence of two distinct high and low-affinity GABA transporters on isolated brain capillaries or suspended BCECs, with Km1, Km2, Vm1 and Vm2 equal to 25.3 microM, 485.2 microM, 3.6 and 8.4 nmol/5 min/mg protein, respectively, for the capillaries, and 21.3 microM, 322.0 microM, 6.1 and 15.7 nmol/5 min/mg protein, respectively, for the suspended BCECs. In contrast, a single low-affinity transporter was found for monolayers of BCECs attached to plates with Km and Vm equal to 338.7 microM and 18.8 nmol/5 min/mg protein, respectively. Subcellular location of the two distinct transporters on BCECs is discussed, suggesting that the low-affinity GABA transporter is probably localized to the luminal membrane of BCECs, and the high-affinity GABA transporter is probably localized to the antiluminal membrane. Low temperature (4 degreesC) and metabolic inhibitors markedly diminished both high and low-affinity uptakes of [3H]GABA by isolated brain capillaries. The substitution of Na+ with choline+, K+ or Li+ with the counter anion Cl- almost completely abolished both uptakes. Substitution of Cl- with Br-, I-, F- or NO3- in the presence of Na+ significantly reduced both uptakes to different extents. Alanine, leucine, phenylalanine, arginine, glutamate and pyruvate had no obvious effect on either uptake. Probenecid, amino-oxyacetic acid, beta-alanine, taurine, betaine, and nipecotic acid significantly reduced both uptakes. These data suggested that both the GABA transporters at the BBB were temperature, metabolic energy, Na+ and Cl--dependent, and may be specific and different from the known monocarboxylic acid, GABA and other amino acid transporters, which may play a role in the disposition of GABA in the brain.

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.

Na+-dependent gamma-aminobutyric acid (GABA) transport in the choroid plexus of rabbit

The goal of this study was to examine the mechanisms of transport of gamma-aminobutyric acid (GABA) in the choroid plexus. Choroid plexus slices from the rabbit were depleted of ATP with 2,4-dinitrophenol. GABA accumulated in the choroid plexus slices in a concentrative manner in the presence of an inwardly-directed Na+ gradient. Uptake occurred in the presence of Cl-; replacement of Cl- with gluconate abolished uptake. SCN-, NO3- or Br- were able to support uptake in the absence of Cl- to a significant extent (80, 68 and 61% of control, respectively). GABA uptake was saturable (Km of 37 +/- 8.5 microM, Vmax of 409 +/- 43 nmol/g/min). Na+-driven GABA uptake was inhibited by beta-alanine (IC50 = 22.9 microM) and hypotaurine (IC50 = 21.9 microM) but less potently by nipecotic acid (IC50 = 244 microM) and hydroxy-nipecotic acid (IC50 = 284 microM). Betaine, L-(2,4)-diaminobutyric acid, guvacine and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol were weak inhibitors (IC50 > 500 microM). GABA inhibited Na+-driven uptake of taurine (IC50 = 230 microM); taurine, however, did not inhibit GABA uptake (IC50 > 1 mM). RT-PCR, using degenerate primers for cloned GABA transporters, did not result in the amplification of a band from rat choroid plexus RNA. The location of the choroid plexus in the ventricles of the brain, and its role in the secretion of the cerebrospinal fluid, suggest a role for the choroid plexus Na+-GABA transporter in the disposition of GABA in the brain.

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.

[3H]GABA uptake and GABA localization in mucosal endocrine cells of the rat stomach and colon

The aim of this study was to characterize the distribution of GABAergic cells in the rat gut mucosa. Thin and/or thick serial sections of segments of rat antrum and distal colon were treated for autoradiographic localization of sites of [3H]GABA (50 nM) high-affinity uptake, or GABA immunoreactivity. Dense accumulations of silver grains were localized to a discrete population of granulated mucosal cells. These appeared to be D-type endocrine cells. These gut regions also displayed strongly GABA-immunoreactive mucosal cells. These results confirm the presence of GABAergic cells in the rat antral mucosa, and reveal that [3H]GABA is accumulated by mucosal endocrine cells in the rat colon.

The effect of gamma-aminobutyric acid on hepatic regenerative activity following partial hepatectomy in rats

BACKGROUND

gamma-Aminobutyric acid (GABA) is a potent inhibitory neurotransmitter with growth-regulatory properties. In fulminant hepatic failure, a condition in which hepatic regeneration may be impaired, systemic serum GABA concentrations are markedly elevated. The present study was designed to determine whether increased amounts of circulating GABA interfere with hepatic regenerative activity.

METHODS

Exogenous GABA or isotonic saline was administered to adult male rats (n = 6-12/group) 16 hours before partial hepatectomy and twice daily for 1-3 days thereafter. The hypertrophic and hyperplastic components of hepatic regeneration were determined by calculation of the restitution of liver mass, [14C]leucine incorporation into protein (protein synthesis), and [3H]thymidine incorporation into hepatic DNA (DNA synthesis).

RESULTS

Exogenous GABA impaired restitution of liver mass (GABA vs. controls, day 3: 76% +/- 7% vs. 90% +/- 9%, mean +/- SD) (P < 0.005) and the rate of protein synthesis (GABA vs. controls, day 1: 379 +/- 39 dpm/mg protein vs. 564 +/- 67 dpm/mg protein) (P < 0.01) without interfering with DNA synthesis. Supplemental administration of corticosterone and putrescine restored protein synthesis rates to normal in GABA-treated rats.

CONCLUSIONS

These results indicate that elevated serum GABA concentrations interfere with the hypertrophic component of hepatic regeneration following partial hepatectomy in rats.

Ethanol-associated alterations in the kinetics of putrescine uptake and metabolism by the regenerating liver

Biosynthesis of the polyamines, putrescine, spermidine, and spermine is required for DNA synthesis and liver regeneration after partial hepatectomy. We have previously reported that chronic ethanol consumption impairs polyamine synthesis and significantly retards liver regeneration after partial hepatectomy. In those studies, supplementation with putrescine restored hepatic DNA synthesis in ethanol-fed rats but exerted no effect in pair-fed controls. These differences in the response to putrescine treatment may have resulted from ethanol-associated differences in hepatic uptake, release, or metabolism of putrescine. To resolve these issues and define more completely how putrescine treatment affects DNA synthesis, we now assess the kinetics of putrescine uptake and metabolism after intraperitoneal or intravenous injection of radiolabeled putrescine (1.2 mmol/kg, specific activity 1 microCi/mmol) into rats fed 36% ethanol diets or isocaloric, nonethanol diets for 6 weeks prior to partial hepatectomy. After putrescine treatment, hepatic putrescine concentrations were greater in ethanol-fed rats than controls. Differences in post-treatment hepatic putrescine levels between ethanol and pair-fed groups could not be explained by differences in the rates of hepatic putrescine uptake or excretion into bile; residual de novo synthesis of putrescine from ornithine or metabolism of hepatic putrescine to its polyamine products, spermidine and spermine. Indeed, supplemental putrescine was not appreciably converted to spermidine or spermine in either ethanol or control rats. Hence, these latter polyamines are unlikely to be responsible for the treatment-associated improvement in DNA synthesis that has been noted in ethanol-fed rats. This suggests that putrescine itself acts to restore hepatic DNA synthesis in ethanol-fed rats.

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.

Localization of GAD-like immunoreactivity in the pancreas and stomach of the rat and mouse

The aim of this study was to localize cells immunoreactive for glutamate decarboxylase (GAD), the enzyme of GABA synthesis, in pyloric and oxyntic regions of the rat stomach as well as in the rat and mouse pancreas. GAD immunocytochemistry was carried out on polyethylene glycol or cryostat sections of alkaline paraformaldehyde fixed tissue, with simultaneous immunolabelling of various gastro-pancreatic hormones for topographical comparison. In the rat stomach, nerve fibers displaying intense GAD-like immunoreactivity were seen in the myenteric plexus, the circular muscular layer, the submucosa and the lamina propria of the mucosa. But, they were absent from the submucous plexus. Colchicine treatment of the rats allowed to detect some labelled perikarya in the myenteric plexus suggesting that the GABAergic innervation is at least partly intrinsic to the stomach. In the oxyntic and pyloric mucosa, endocrine cells appeared immunostained for GAD. However, the nature of their hormones remained unknown since double immunodetections revealed that they were immunoreactive neither for gastrin nor for somatostatin. In the rat and mouse pancreas, GAD-like immunoreactivity was found in islet cells which corresponded only to insulin-secreting cells. Somatostatin-, glucagon- and pancreatic polypeptide-immunopositive cells were devoid of GAD immunolabelling. No GAD-like immunoreactivity was detected in the exocrine tissue and innervation. These results strenghten the hypothesis that GABA is not only a neurotransmitter in the stomach but that it could also be an endocrine or paracrine factor in the stomach and pancreas.

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).

Immunocytochemical and autoradiographic studies of the endocrine cells interacting with GABA in the rat stomach

There are now increasing evidences suggesting that GABA is able of direct interaction with certain endocrine cells. In the present study, highly specific anti-GABA-glutaraldehyde antibodies and 3H-GABA uptake were used at the light and electron microscope levels to investigate the occurrence of cells containing endogenous GABA or taking up exogenous GABA in the mucosal antrum and corpus of the rat stomach. Only certain endocrine cell types of both regions were immunostained or grain-labelled. However, the morphology of their secretory granules did not allow to identify the nature of their hormone with certainty but suggested that somatostatin-like cells could interact with GABA. The combination of gastrin and somatostatin immunodetection with 3H-GABA uptake autoradiography at the light microscope level, revealed that a subpopulation of somatostatin-like cells and other still unidentified endocrine cells are able to take up GABA, while the gastrin-like cells are not. These results reinforce the hypothesis that certain endocrine cell types of the diffuse endocrine system of the digestive tract are able to directly interact with GABA.

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.