Some effects of 5-hydroxytryptamine, dopamine and noradrenaline on neurones in the submucous plexus of guinea-pig small intestine

An In Vitro Approach to Studying the Microbial Community and Impact of Pre and Probiotics under Anorexia Nervosa Related Dietary Restrictions

Individuals with anorexia nervosa (AN) often suffer psychological and gastrointestinal problems consistent with a dysregulated gut microbial community. Psychobiotics have been postulated to modify microbiota and improve mental well-being and gut symptoms, but there is currently a lack of evidence for such approaches in AN. The aim of this study was to use an in vitro colonic model to evaluate the impact of dietary restrictions associated with AN on the intestinal ecosystem and to assess the impact of pre and probiotic intervention. Bacteriology was quantified using flow cytometry combined with fluorescence in situ hybridisation and metabolic end products (including neurotransmitters) by gas chromatography and liquid chromatography mass spectrometry Consistent with previous research, the nutritional changes significantly reduced total microbiota and metabolites compared with healthy conditions. Pre and probiotic supplementation on restricted conditions enhanced the microbial community and modulated metabolic activity to resemble that of a healthy diet. The model system indicates that nutritional changes associated with AN can impact the microbial community, and that these changes can, at least in part, be restored through the use of pre and probiotic interventions.

Neurotransmitters: The Critical Modulators Regulating Gut-Brain Axis

Neurotransmitters, including catecholamines and serotonin, play a crucial role in maintaining homeostasis in the human body. Studies on these neurotransmitters mainly revolved around their role in the "fight or flight" response, transmitting signals across a chemical synapse and modulating blood flow throughout the body. However, recent research has demonstrated that neurotransmitters can play a significant role in the gastrointestinal (GI) physiology. Norepinephrine (NE), epinephrine (E), dopamine (DA), and serotonin have recently been a topic of interest because of their roles in the gut physiology and their potential roles in GI and central nervous system pathophysiology. These neurotransmitters are able to regulate and control not only blood flow, but also affect gut motility, nutrient absorption, GI innate immune system, and the microbiome. Furthermore, in pathological states, such as inflammatory bowel disease (IBD) and Parkinson's disease, the levels of these neurotransmitters are dysregulated, therefore causing a variety of GI symptoms. Research in this field has shown that exogenous manipulation of catecholamine serum concentrations can help in decreasing symptomology and/or disease progression. In this review article, we discuss the current state-of-the-art research and literature regarding the role of neurotransmitters in regulation of normal GI physiology, their impact on several disease processes, and novel work focused on the use of exogenous hormones and/or psychotropic medications to improve disease symptomology. J. Cell. Physiol. 232: 2359-2372, 2017. © 2016 Wiley Periodicals, Inc.

Gastrointestinal Parasites and the Neural Control of Gut Functions

Gastrointestinal motility and transport of water and electrolytes play key roles in the pathophysiology of diarrhea upon exposure to enteric parasites. These processes are actively modulated by the enteric nervous system (ENS), which includes efferent, and afferent neurons, as well as interneurons. ENS integrity is essential to the maintenance of homeostatic gut responses. A number of gastrointestinal parasites are known to cause disease by altering the ENS. The mechanisms remain incompletely understood. Cryptosporidium parvum, Giardia duodenalis (syn. Giardia intestinalis, Giardia lamblia), Trypanosoma cruzi, Schistosoma species and others alter gastrointestinal motility, absorption, or secretion at least in part via effects on the ENS. Recent findings also implicate enteric parasites such as C. parvum and G. duodenalis in the development of post-infectious complications such as irritable bowel syndrome, which further underscores their effects on the gut-brain axis. This article critically reviews recent advances and the current state of knowledge on the impact of enteric parasitism on the neural control of gut functions, and provides insights into mechanisms underlying these abnormalities.

5-HT(1A), SST(1), and SST(2) receptors mediate inhibitory postsynaptic potentials in the submucous plexus of the guinea pig ileum

Vasoactive intestinal peptide (VIP) immunoreactive neurons are important secretomotor neurons in the submucous plexus. They are the only submucosal neurons to receive inhibitory inputs and exhibit both noradrenergic and nonadrenergic inhibitory synaptic potentials (IPSPs). The former are mediated by alpha(2)-adrenoceptors, but the receptors mediating the latter have not been identified. We used standard intracellular recording, RT-PCR, and confocal microscopy to test whether 5-HT(1A), SST(1), and/or SST(2) receptors mediate nonadrenergic IPSPs in VIP submucosal neurons in guinea pig ileum in vitro. The specific 5-HT(1A) receptor antagonist WAY 100135 (1 microM) reduced the amplitude of IPSPs, an effect that persisted in the presence of the alpha(2)-adrenoceptor antagonist idazoxan (2 microM), suggesting that 5-HT might mediate a component of the IPSPs. Confocal microscopy revealed that there were many 5-HT-immunoreactive varicosities in close contact with VIP neurons. The specific SSTR(2) antagonist CYN 154806 (100 nM) and a specific SSTR(1) antagonist SRA 880 (3 microM) each reduced the amplitude of nonadrenergic IPSPs and hyperpolarizations evoked by somatostatin. In contrast with the other antagonists, CYN 154806 also reduced the durations of nonadrenergic IPSPs. Effects of WAY 100135 and CYN 154806 were additive. RT-PCR revealed gene transcripts for 5-HT(1A), SST(1), and SST(2) receptors in stripped submucous plexus preparations consistent with the pharmacological data. Although the involvement of other neurotransmitters or receptors cannot be excluded, we conclude that 5-HT(1A), SST(1), and SST(2) receptors mediate nonadrenergic IPSPs in the noncholinergic (VIP) secretomotor neurons. This study thus provides the tools to identify functions of enteric neural pathways that inhibit secretomotor reflexes.

5-HT antagonists NAN-190 and SB 269970 block alpha2-adrenoceptors in the guinea pig

Serotonin (5-HT) plays a significant role in the regulation of intestinal secretion of water and electrolytes. The initial aim of this study was to use intracellular recording and specific antagonists to identify roles of 5-HT1A and 5-HT7 receptors of submucosal noncholinergic secretomotor neurons of guinea pig ileum, in vitro. However, it was found that the widely used 5-HT receptor antagonists NAN-190 (5-HT1A) and SB 269970 (5-HT7) both blocked alpha2-adrenoceptors, and hence depressed inhibitory synaptic potentials and hyperpolarizations evoked by noradrenaline, in these neurons. Both compounds enhanced neurally evoked contractions of the guinea pig vas deferens, an effect characteristic of blockade of alpha2-adrenoceptors. These results raise significant concerns about studies using NAN-190 and SB 269970 as specific antagonists of serotonin receptors.

Stimulation of adenosine A1 and A2A receptors by AMP in the submucosal plexus of guinea pig small intestine

Actions of adenosine 5'-monophosphate (AMP) on electrical and synaptic behavior of submucosal neurons in guinea pig small intestine were studied with "sharp" intracellular microelectrodes. Application of AMP (0.3-100 microM) evoked slowly activating depolarizing responses associated with increased excitability in 80.5% of the neurons. The responses were concentration dependent with an EC(50) of 3.5 +/- 0.5 microM. They were abolished by the adenosine A(2A) receptor antagonist ZM-241385 but not by pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid, trinitrophenyl-ATP, 8-cyclopentyl-1,3-dimethylxanthine, suramin, or MRS-12201220. The AMP-evoked responses were insensitive to AACOCF3 or ryanodine. They were reduced significantly by 1) U-73122, which is a phospholipase C inhibitor; 2) cyclopiazonic acid, which blocks the Ca(2+) pump in intraneuronal membranes; and 3) 2-aminoethoxy-diphenylborane, which is an inositol (1,4,5)-trisphosphate receptor antagonist. Inhibitors of PKC or calmodulin-dependent protein kinase also suppressed the AMP-evoked excitatory responses. Exposure to AMP suppressed fast nicotinic ionotropic postsynaptic potentials, slow metabotropic excitatory postsynaptic potentials, and slow noradrenergic inhibitory postsynaptic potentials in the submucosal plexus. Inhibition of each form of synaptic transmission reflected action at presynaptic inhibitory adenosine A(1) receptors. Slow excitatory postsynaptic potentials, which were mediated by the release of ATP and stimulation of P2Y(1) purinergic receptors in the submucosal plexus, were not suppressed by AMP. The results suggest an excitatory action of AMP at adenosine A(2A) receptors on neuronal cell bodies and presynaptic inhibitory actions mediated by adenosine A(1) receptors for most forms of neurotransmission in the submucosal plexus, with the exception of slow excitatory purinergic transmission mediated by the P2Y(1) receptor subtype.

Physiological modulation of intestinal motility by enteric dopaminergic neurons and the D2 receptor: analysis of dopamine receptor expression, location, development, and function in wild-type and knock-out mice

Dopaminergic neurons are present in both plexuses of the murine bowel and are upregulated after extrinsic denervation but play unknown roles in enteric nervous system (ENS) physiology. Transcripts encoding dopamine (DA) receptors D1-D5 were analyzed by reverse transcription-PCR in stomach approximately duodenum approximately ileum approximately proximal > > distal colon. Dissected muscle and myenteric plexus contained transcripts encoding D1-D3 and D5, whereas mucosa contained D1 and D3-D5. D1-D5 expression began in fetal gut [embryonic day 10 (E10)], before the appearance of neurons (E12), and was sustained without developmental regulation through postnatal day 1. In situ hybridization revealed that subsets of submucosal and myenteric neurons contained mRNA encoding D2 or D3. Immunoblots confirmed that D1, D2, and D5 receptor proteins were present from stomach through distal colon. Subsets of submucosal and myenteric neurons were also D1, D2, or D3 immunoreactive. When double labeled by in situ hybridization, these neurons contained mRNA encoding the respective receptors. Total gastrointestinal transit time (TGTT) and colonic transit time (CTT) were measured in mice lacking D2, D3, or D2 plus D3. Both TGTT and CTT were decreased significantly (motility increased) in D2 and D2 plus D3, but not D3, knock-out animals. Mice lacking D2 and D2 plus D3 but not D3 were smaller than wild-type littermates, yet ate significantly more and had greater stool frequency, water content, and mass. Because motility is abnormal when D2 is absent, the net inhibitory DA effect on motility is physiologically significant. The early expression of DA receptors is also consistent with the possibility that DA affects ENS development.

Angiotensin receptors and actions in guinea pig enteric nervous system

Actions of ANG II on electrical and synaptic behavior of enteric neurons in the guinea pig small intestine were studied. Exposure to ANG II depolarized the membrane potential and elevated neuronal excitability. The number of responding neurons was small, with responses to ANG II in 32% of submucosal neurons and 25% of myenteric neurons. Hyperpolarizing responses were evoked by ANG II in 45% of the neurons. The hyperpolarizing responses were suppressed by alpha2-noradrenergic receptor antagonists, which suggested that the hyperpolarizing responses reflected stimulation of norepinephrine release from sympathetic neurons. Exposure to ANG II enhanced the amplitude and prolonged the duration of noradrenergic inhibitory postsynaptic potentials and suppressed the amplitude of both fast and slow excitatory postsynaptic potentials. The selective ANG II(1) receptor (AT1R) antagonists, ZD-7115 and losartan, but not a selective AT2R antagonist (PD-123319), suppressed the actions of ANG II. Western blot analysis and RT-PCR confirmed expression of AT1R protein and the mRNA transcript for the AT1R in the enteric nervous system. No expression of AT2R protein or mRNA was found. Immunoreactivity for AT1R was expressed by the majority of neurons in the gastric antrum and small and large intestine. AT1R immunoreactivity was coexpressed with calbindin, choline acetyltransferase, calretinin, neuropeptide Y, and nitric oxide synthase in subpopulations of neurons. The results suggest that formation of ANG II might have paracrine-like actions in the enteric nervous system, which include alterations in neuronal excitability and facilitated release of norepinephrine from sympathetic postganglionic axons. The enhanced presence of norepinephrine is expected to suppress fast and slow excitatory neurotransmission in the enteric microcircuits and to suppress neurogenic mucosal secretion.

Enteric dopaminergic neurons: definition, developmental lineage, and effects of extrinsic denervation

The existence of enteric dopaminergic neurons has been suspected; however, the innervation of the gut by sympathetic nerves, in which dopamine (DA) is the norepinephrine precursor, complicates analyses of enteric DA. We now report that transcripts encoding tyrosine hydroxylase (TH) and the DA transporter (DAT) are present in the murine bowel (small intestine > stomach or colon; proximal colon > distal colon). Because sympathetic neurons are extrinsic, transcripts encoding TH and DAT in the bowel are probably derived from intrinsic neurons. TH protein was demonstrated immunocytochemically in neuronal perikarya (submucosal >> myenteric plexus; small intestine > stomach or colon). TH, DA, and DAT immunoreactivities were coincident in subsets of neurons (submucosal > myenteric) in guinea pig and mouse intestines in situ and in cultured guinea pig enteric ganglia. Surgical ablation of sympathetic nerves by extrinsic denervation of loops of the bowel did not affect DAT immunoreactivity but actually increased numbers of TH-immunoreactive neurons, expression of mRNA encoding TH and DAT, and enteric DOPAC (the specific dopamine metabolite). The fetal gut contains transiently catecholaminergic (TC) cells. TC cells are the proliferating crest-derived precursors of mature neurons that are not catecholaminergic and, thus, disappear after embryonic day (E) 14 (mouse) or E15 (rat). TC cells appear early in ontogeny, and their development/survival is dependent on mash-1 gene expression. In contrast, the intrinsic TH-expressing neurons of the murine bowel appear late (perinatally) and are mash-1 independent. We conclude that the enteric nervous system contains intrinsic dopaminergic neurons that arise from a mash-1-independent lineage of noncatecholaminergic precursors.

Ongoing nicotinic and non-nicotinic inputs to inhibitory neurons in the mouse colon

1. Intracellular microelectrodes were used to record spontaneous and evoked inhibitory junction potentials (IJP) from the circular muscle layer of the mid-distal region of the mouse isolated colon in the presence of nifedipine (1 micromol/L) and hyoscine (1 micromol/L). 2. The length of the tissue preparation (> 1 cm) or the presence of the mucosa had no effect on the frequency of spontaneous IJP. 3. Hexamethonium (500 micromol/L) reduced the frequency of spontaneous IJP to approximately 70% of the control frequency, whereas D-tubocurarine (280 micromol/L) reduced the frequency to approximately 17% of control. Apamin (250 nmol/L) abolished all spontaneous IJP activity. 4. The greater inhibition of spontaneous IJP in the presence of D-tubocurarine compared with hexamethonium is discussed as a possible 'apamin-like' effect. 5. Although electrically evoked IJP (single pulse at 15 V, 0.6 msec) were not significantly affected by hexamethonium, D-tubocurarine and apamin reduced the amplitude of evoked IJP to approximately 65 and 50% of control, respectively. 6. These results suggest that the properties of spontaneous IJP cannot be inferred by a study of evoked IJP alone.

Histamine H3 receptor-mediated suppression of inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine

Conventional intracellular microelectrodes and marker injection techniques were used to study the actions of histamine on inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine. Bath application of histamine (1-300 microM) reversibly suppressed both noradrenergic and non-adrenergic slow inhibitory postsynaptic potentials in a concentration-dependent manner. These effects of histamine were mimicked by the selective histamine H(3) receptor agonist R(-)-alpha-methylhistamine but not the selective histamine H(1) receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl) heptanecarboxamide (HTMT dimaleate), or the selective histamine H(2) receptor agonist, dimaprit. The histamine H(3) receptor antagonist, thioperamide, blocked the effects of histamine. Histamine H(1) and H(2) receptor antagonists did not change the action of histamine. Hyperpolarizing responses to focal application of norepinephrine or somatostatin by pressure ejection from micropipettes were unaffected by histamine and R(-)-alpha-methylhistamine. The results suggest that histamine acts at presynaptic histamine H(3) receptors on the terminals of sympathetic postganglionic fibers and intrinsic somatostatinergic nerves in the small intestine to suppress the release of the inhibitory neurotransmitters, norepinephrine and somatostatin.

Immunohistochemical and electrophysiological characterization of submucous neurons from the guinea-pig small intestine in organ culture

Immunohistochemical and electrophysiological properties of submucous neurons were investigated in organ cultures of the guinea-pig small intestine. Preparations of submucosa, with or without the myenteric plexus attached, were maintained in vitro for 3 to 5 days. Immunohistochemical labelling for peptides revealed that the cultured submucous plexus remained substantially intact and the immunoreactivity of cell bodies was well preserved. Substantial sprouting of nerve fibers immunoreactive for vasoactive intestinal peptide (VIP) or neuropeptide Y (NPY) was evident in submucous ganglia after 5 days in organ culture. Nerve fibers immunoreactive for substance P. somatostatin, 5-hydroxytryptamine or tyrosine hydroxylase were substantially depleted in submucous ganglia or perivascular nerves at 3 days and had virtually disappeared after 5 days in cultures of isolated submucosa. During intracellular recording from submucous neurons, action potentials were initiated by depolarizing current pulses in all neurons cultured with or without the myenteric plexus and muscle layers. Electrical stimulation of internodal strands evoked fast excitatory synaptic potentials (fast EPSPs) in nearly all neurons whether or not the myenteric plexus was present during the culture period up to 5 days. The removal of myenteric plexus and extrinsic nerves did not abolish fast EPSPs from submucous neurons, suggesting that some fast EPSPs may originate from neurons in the submucous plexus, although the possibility that new synapses formed by sprouting, or surviving axons severed from myenteric or sympathetic ganglia may have been functional, cannot be entirely excluded. This work demonstrates that the immunohistochemical and electrophysiological characteristics of submucous neurons are largely maintained in organ cultures of the submucosa.

Electrophysiological studies of 5-hydroxytryptamine receptors on enteric neurons

Enteric nerves express multiple receptors for 5-hydroxytryptamine (5-HT). Three excitatory and 1 inhibitory receptor for 5-HT can be identified using electrophysiological methods. The excitatory receptors are the 5-HT1P, 5-HT3 and 5-HT4 subtypes. The 5-HT1P mediates slow depolarizations (> 10 s duration) of many enteric nerves and 5-HT1P receptors mediate some slow excitatory synaptic potentials. The 5-HT3 receptor is a ligand-gated cation channel that mediates fast depolarizations (< 2 s). The 5-HT4 receptor mediates presynaptic facilitation of fast excitatory neurotransmission. The inhibitory receptor is the 5-HT1A receptor. 5-HT1A receptors mediate hyperpolarizations in AH neurons and presynaptic inhibition of fast and slow excitatory neurotransmission.

Substance P effects on blood flow, fluid transport and vasoactive intestinal polypeptide release in the feline small intestine

1. Substance P (SP) infusions were given close I.A. to the feline small intestine in vivo in a dose that produced plasma concentrations of 1-5 microM. This infusion regularly evoked a net fluid secretion measured with a gravimetric technique. Concomitantly, the release into blood of vasoactive intestinal polypeptide (VIP), a putative neurotransmitter of the enteric nervous system, increased. 2. The SP-induced fluid secretion was blocked by tetrodotoxin (7 micrograms close I.A.), a blocker of fast sodium channels in excitable tissues, and hexamethonium (10 mg (kg body wt)-1, I.V.), a nicotinic receptor antagonist, suggesting that the SP effect was mediated by the enteric nervous system. In line with this it was shown that the SP-evoked release of VIP was also significantly diminished by hexamethonium. 3. Close I.A. infusions of methionine enkephalin (Met-enkephalin; 7-23 nmol min-1) or electrical stimulation of the sympathetic nerve fibres (6 Hz) to the intestine markedly diminished net fluid secretion and the release of VIP caused by SP given close I.A. 4. The cyclo-oxygenase inhibitor diclofenac (5 mg (kg body wt)-1, I.V.) or the histamine-1 receptor antagonist pyrilamine (10 mg (kg body wt)-1, I.V.) did not influence the fluid secretion caused by SP, indicating that the effects of SP were not due to the actions of prostaglandins or histamine. 5. It is proposed that SP activates a nervous reflex arch that we have shown to be activated by various luminal stimuli, including cholera toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of 5-hydroxytryptamine receptors mediating mucosal secretion in guinea-pig ileum

1. The receptor subtypes through which 5-hydroxytryptamine (5-HT) increases electrolyte secretion across the mucosa of guinea-pig ileum were studied. 2. Flat sheep preparations of guinea-pig mucosa plus submucosa were placed in Ussing chambers and the short circuit current (ISC), an index of net electrogenic electrolyte transport across the mucosa, was measured under voltage clamp conditions. 3. Low concentrations of 5-HT (10-300 nM) evoked monophasic increases in ISC which were significantly reduced by hyoscine (100 nM), tetrodotoxin (TTX, 300 nM) and the 5-HT2 receptor antagonist, ketanserin (3-300 nM). 4. Higher concentrations of 5-HT (1-10 microM) produced biphasic responses which were reduced by hyoscine (100 nM), TTX (300 nM), ketanserin (3-300 nM) and also by the 5-HT3 receptor antagonists, granisetron (1 microM) and ICS 205-930 (100 nM). 5. 2-Methyl-5-HT (1-100 microM) and alpha-methyl-5-HT (30 nM-30 microM), agonists at 5-HT3 and 5-HT2 receptors respectively, also evoked ISC increases. These responses were reduced by hyoscine (100 nM) and abolished by TTX (300 nM) and the respective receptor antagonists, granisetron (1 microM) and ketanserin (30 nM). 6. The 5-HT4 receptor antagonist, SDZ 205-557 (300 nM) had no effect on the response to 5-HT. 7. The TTX-resistant response to 5-HT was not affected by 5-HT2,3 or 4 receptor antagonists. 8. These results indicate that 5-HT mediates secretion partly by an action on 5-HT3 receptors located on cholinergic and noncholinergic secretomotor neurones, partly by an action on higher affinity'5-HT2-like' receptors predominantly on noncholinergic neurones, and partly by a direct action on the epithelium.

Noradrenaline as a presynaptic inhibitory neurotransmitter in ganglia of the guinea-pig gall-bladder

1. The effects of noradrenaline on guinea-pig gall-bladder ganglia were investigated with intracellular recording techniques. 2. Noradrenaline (0.01-100 microM) decreased the amplitude of the fast excitatory postsynaptic potential (EPSP) that was evoked by stimulation of interganglionic fibre tracts. High concentrations of noradrenaline (10-100 microM) caused an inhibition ranging from 93-100%. The noradrenaline concentration that resulted in half-maximal inhibition (EC50) of the EPSP was 280 nM. 3. Experiments with selective agonists and antagonists indicated that the alpha 2-adrenoreceptor was involved in the inhibition of the EPSP. Clonidine (0.001-100 microM) reduced the EPSP in a concentration-dependent manner with an EC50 of 30 nM. Yohimbine (100-300 nM) caused a rightward shift of the noradrenaline concentration-effect relationship, with a dissociation equilibrium constant of 1.4 nM. 4. Release of endogenous catecholamines by tyramine (100 microM) in the presence of desipramine (1.0 microM), caused a yohimbine-sensitive decrease in the amplitude of the EPSP. Treatment with tyramine did not affect the amplitude of the EPSP in tissue that had undergone prior chemical sympathectomy with 6-hydroxydopamine. 5. Electrical stimulation of the vascular plexus (1-3 s; 10-20 Hz; 10 mA) decreased the amplitude of the EPSP. In some cases suprathreshold responses were reduced to subthreshold EPSPs following stimulation of the vascular plexus. Yohimbine (300 nM) reversibly inhibited the effects of vascular plexus stimulation. 6. Noradrenaline did not modify the responses of gall-bladder neurones to exogenously applied acetylcholine. Also, application of noradrenaline, by superfusion (0.001-100 microM) or by pressure microejection (1.0 mM), had no effect on the resting membrane potential, membrane conductance, or action potential characteristics of gall-bladder neurones. 7. Immunoreactivity for type A monoamine oxidase (MAO-A) was found in the vascular plexus and the ganglionated plexus of the gall-bladder. 8. These results show that noradrenaline has an alpha 2-adrenoreceptor-mediated presynaptic inhibitory effect on fast synaptic transmission in the ganglia of the guinea-pig gall-bladder. It is proposed that vagal terminals may be an important target of this adrenergic inhibitory input to the gall-bladder.

Effects of noradrenaline on rat paratracheal neurones and localization of an endogenous source of noradrenaline

1. Intracellular recording techniques were used to study the actions of exogenous noradrenaline (NA) on rat paratracheal neurones in situ. The receptor subtypes underlying these actions were investigated by application of selective adrenoceptor antagonists. 2. Application of NA (0.1-10 microM) by superfusion evoked a membrane depolarization in 85% (52 out of 61) of all paratracheal neurones studied. The response consisted of a slow depolarization which was sometimes accompanied by action potential discharge. In 26 out of 31 cells the response was associated with a change in input resistance of the cell membrane. In 22 out of 26 cells there was a 30% increase, whilst in a further 4 cells there was a 15% decrease in input resistance. The amplitude of the NA depolarization was concentration-dependent. 3. The depolarization evoked by NA was reversibly antagonized by prazosin (1 microM) but unaffected by yohimbine (1 microM) or propranolol (1-10 microM). 4. High performance liquid chromatography with electrochemical detection (h.p.l.c.-e.c.d.) was used to assay for NA and dopamine in samples containing mainly paratracheal ganglia and in samples of tracheal smooth muscle with mucosa. NA was present in all samples assayed at a level of 1.6 micrograms NA g-1 and 0.5 microgram NA g-1 wet weight of the two sample types respectively. Dopamine was not detected in any samples of either ganglia or smooth muscle with mucosa. 5. It is concluded that NA-evoked depolarizations of rat paratracheal neurones result from stimulation of alpha 1-adrenoceptors, and that local levels of NA may be sufficiently high to activate these receptors directly.

[3H] GR67330, a very high affinity ligand for 5-HT3 receptors

GR67330 potently inhibited 5-hydroxytryptamine (5-HT)-induced depolarizations of the rat isolated vagus nerve. At the higher concentrations used (0.3 nmol/l-1 nmol/l) this was accompanied by a marked reduction in the maximum response to 5-HT. The calculated pKB value was 10.2. The binding of the tritiated derivative of GR67330 to homogenates of rat entorhinal cortex was examined. Kinetic analysis revealed that specific [3H] GR67330 (0.1 nmol/l) binding was rapid and reversible. Association and dissociation rate constants were 1.48 +/- 0.36 x 10(8) mol/l-1 s-1 and 7.85 +/- 0.41 x 10(-3) s-1 respectively. Equilibrium saturation analysis revealed specific binding was to a single site (Bmax 22.6 +/- 0.21 fmol/mg protein) of high affinity (Kd 0.038 +/- 0.003 nmol/l). At low ligand concentrations, specific binding was up to 90% of total binding. If unlabelled GR67330 was used to define non-specific binding two sites were evident (Kd1 0.066 +/- 0.007 nmol/l, Kd2 20.1 +/- 9.7 nmol/l; Bmax1 31.5 +/- 3.2 fmol/mg protein, Bmax2 1110 +/- 420 fmol/mg protein). [3H] GR67330 binding was inhibited potently by 5-HT3 antagonists and agonists. Ligands for other 5-HT receptors and other neurotransmitter receptors were either only weakly active or inactive at inhibiting binding. Hill numbers for antagonist inhibition of binding were close to unity, except for quipazine which was significantly greater than one. In common with other 5-HT3 binding studies, all 5-H-agonist tested had Hill numbers greater than one (1.51-1.71). GR38032 and GR65630 inhibited a greater proportion of binding than other 5-HT3 antagonists, this additional binding was interpreted as inhibition from a second saturable site unrelated to the 5-HT3 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

An in vitro study of the projections of enteric vasoactive intestinal polypeptide-immunoreactive neurons in the human colon

The anatomical basis of the peptidergic neural control of the human colon is largely unknown. In this study, in vitro retrograde tracing methods have been used on fresh human colon to determine the projection pathways of the enteric nerves and, in particular, those containing vasoactive intestinal polypeptide, one of the most abundant and potent of the gut neuropeptides. Two components of the submucous plexus were identified, the inner one projecting to the lamina propria, and the outer to the circular muscle. The lengths of projections within the submucous plexus were up to 5-14 mm in all directions. Myenteric ganglion cells projected to both longitudinal and circular muscles, for distances of up to only 5 mm. The subpopulation of nerves containing vasoactive intestinal polypeptide arose mainly from the submucous plexus and projected up to 6.5 mm anally, 5 mm orally, and 14 mm within the submucous layer to the mucosa or circular muscle. These findings provide entirely new data on the neuroanatomy of the human colon and may help in the understanding of the neural control of colonic secretion and motility.

Effects of 5-HT3 receptor antagonists on 5-HT and nicotinic depolarizations in guinea-pig submucosal neurones

1. Intracellular recordings were made from neurones of the guinea-pig submucosal plexus. The effects of several 5-hydroxytryptamine3 (5-HT3) receptor antagonists on depolarizations produced by ionophoretic application of 5-HT and acetylcholine, as well as on fast excitatory postsynaptic potentials (fast e.p.s.ps) produced by nerve stimulation were examined. 2. ICS 205-930, GR 38032F, MDL 72222, cocaine and curare all inhibited the fast e.p.s.p. as well as the depolarizations in response to 5-HT and acetylcholine (ACh) ionophoresis in a dose-dependent fashion. 3. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the 5-HT mediated depolarizations were 12 nM, 100 nM, 3 microM, 3 microM and 20 microM, respectively. 4. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the nicotinic depolarizations were 4 microM, 12 microM, 11 microM, 6 microM and 17 microM, respectively. Similar IC50 values were obtained for inhibition of the fast e.p.s.ps by these antagonists. 5. The nicotinic receptor blocker, hexamethonium, inhibited the nicotinic depolarization and the fast e.p.s.p. with IC50 values of 10 microM. Hexamethonium (10 microM-5 mM) did not alter the depolarization induced by 5-HT. 6. These results demonstrate that the pharmacological profile of 5-HT3 receptors present on submucosal neurones is identical to that of 5-HT3 receptors on myenteric neurones and, thus, provide evidence that the enteric neuronal 5-HT3 receptor forms a receptor subtype distinct from that characterized in other parts of the autonomic nervous system.

Evidence for two types of 5-hydroxytryptamine receptor on secretomotor neurons of the guinea-pig ileum

Flat sheet preparations of the mucosa plus submucosa from the guinea-pig ileum were placed in Ussing chambers so that short circuit current (Isc), an index of electrolyte movement across the mucosa, could be measured. In these preparations, 5-hydroxytryptamine (5-HT) increases Isc indirectly by stimulating both cholinergic and non-cholinergic secretomotor neurons. The 5-HT3 receptor antagonist, ICS 205-930 (10(-13)-10(-5 M), substantially depressed the secretory response due to 5-HT (10(-6) M), but not that produced by direct activation of muscarinic receptors on the mucosal epithelium with carbachol (10(-6) M), or by stimulation of secretomotor neurons with substance P (10(-8) M) or 1,1-dimethyl-4-phenylpiperazinium (10(-5)M). The residual response to 5-HT, after the addition of a maximally effective concentration of ICS 205-930 (10(-6) M), was further reduced by hyoscine (10(-7) M). When that part of the 5-HT response attributable to the release of acetylcholine was blocked by hyoscine (10(-7) M), ICS 205-930 did not further modify the response to 5-HT. The hyoscine-resistant component was, however, substantially depressed by tetrodotoxin (3.5 x 10(-7) M). The response remaining after ICS 205-930 and hyoscine was not affected by methysergide (2 x 10(-5) M) or cyproheptadine (10(-7) M). We conclude that there are ICS 205-930 sensitive 5-HT receptors on cholinergic secretomotor neurons, and ICS 205-930, methysergide, and cyproheptadine insensitive 5-HT receptors on non-cholinergic secretomotor neurons.

Histamine H2 receptor mediates postsynaptic excitation and presynaptic inhibition in submucous plexus neurons of the guinea-pig

Intracellular recordings were made from submucous plexus neurons of the guinea-pig cecum maintained in vitro. Histamine (0.3-10 microM) produced a dose-dependent membrane depolarization (congruent to 13 mV with 3 microM) in about 28% of the cells tested; most of these cells showed a prominent calcium-activated potassium conductance (AH cells). The depolarization was due primarily to an inactivation of potassium conductance which is available at the resting membrane potential of -60 mV. Peak amplitude of the fast excitatory postsynaptic potential was depressed by histamine (0.1-10 microM) in a dose-dependent manner (congruent to 62% depression with 1 microM). This was observed even in those cells in which histamine did not produce any membrane depolarizations (mostly S cells). The depression of the fast excitatory postsynaptic potential resulted from the presynaptic inhibition of acetylcholine release. Histamine also reduced the amplitude of the non-cholinergic, presumably peptidergic, slow excitatory postsynaptic potential by suppressing peptide release from presynaptic nerve terminals. Peak amplitude of the adrenergic inhibitory synaptic potential was not depressed by histamine suggesting that histamine receptors are not present on presynaptic terminals of sympathetic nerve fibres. Both postsynaptic and presynaptic actions of histamine were blocked by cimetidine or ranitidine but not by pyrilamine implying that H2 receptors are involved.

Correlated electrophysiological and histochemical studies of submucous neurons and their contribution to understanding enteric neural circuits

Neither submucous ganglia, nor intestinal secretomotor reflexes are mentioned in the majority of the textbooks of physiology; because it has been realized only very recently that the submucous neurons may have important influences on whole body water and electrolyte balance. In the present review, we trace the rapid progress that has been made in determining the physiological properties of submucous neurons with known chemistry and projections in the guinea-pig small intestine, and we analyze how the work relates to studies in vivo of the neuronal control of intestinal trans-epithelial fluid transport. Four types of submucous neurons, which appear to be the full complement in the guinea-pig small intestine, have been identified through electrophysiological and histochemical analysis. (1) Cholinergic secretomotor neurons contain immunoreactivity for choline-acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neuropeptide Y (NPY), somatostatin (SOM), and in the majority of cases galanin (GAL); these neurons project to the mucosal epithelium. (2) Non-cholinergic secretomotor neurons contain dynorphin (DYN), GAL and vasoactive intestinal peptide (VIP); these neurons project to the mucosa and provide collaterals to submucous arterioles. (3) Cholinergic interneurons contain ChAT alone; these neurons connect with the secretomotor neurons. (4) Presumed sensory neurons contain ChAT and substance P (SP) and have nerve endings in the mucosa. The two groups of secretomotor neurons receive cholinergic synaptic inputs from both myenteric and submucous ganglia. In addition, the DYN/GAL/VIP neurons receive sympathetic inhibitory inputs as well as inhibitory and non-cholinergic excitatory inputs from myenteric ganglia. The ChAT/SP nerve cells in submucous ganglia receive no or very ineffective inputs. From these data, from experiments on transmission from the neurons to the intestinal epithelium, and from studies of secretomotor reflexes in vivo, a correlated functional and structural circuitry of the submucous ganglia and their connections has been deduced. It is concluded that secretomotor reflexes are stimulated by the contents of the lumen during the digestion and absorption of food and that these reflexes cause a proportion of water and electrolytes that are absorbed with nutrients such as glucose to be returned to the lumen. The balance of absorption and secretion of water and electrolytes is controlled by sympathetic inhibitory inputs to secretomotor neurons, the activity in sympathetic pathways being varied to contribute to whole body water and electrolyte balance.

Intrinsic and extrinsic inhibitory synaptic inputs to submucous neurones of the guinea-pig small intestine

1. The sources of inhibitory synaptic inputs to neurones in submucous ganglia of the guinea-pig small intestine were examined by making lesions to cause selective degeneration of nerve terminals of sympathetic or intrinsic origin. Intracellular recordings were used to evaluate the effects of lesions on the inhibitory inputs. Immunohistochemical techniques were used to identify the neurochemical classes of the impaled neurones and to confirm the efficacy of the lesions. 2. The neurones from which recordings were taken were filled with the fluorescent dye Lucifer Yellow. The preparations were then fixed and processed for immunohistochemistry. 3. Thirty-one neurones reactive for vasoactive intestinal polypeptide (VIP) were examined in control submucous ganglia and all exhibited inhibitory synaptic potentials. In preparations extrinsically denervated by severing the mesenteric nerves, twenty-seven of twenty-eight VIP-reactive neurones had inhibitory synaptic potentials. This indicates that these neurones receive inhibitory synaptic inputs from intrinsic neurones. However, significantly more stimuli were required to evoke a detectable inhibitory synaptic potential in extrinsically denervated preparations than in normal intestine. 4. Extrinsic denervations were combined with removal of the myenteric plexus so that nerve terminals arising from both cell bodies in extrinsic ganglia and in the myenteric plexus degenerated. Under these conditions no inhibitory synaptic potentials could be recorded in any of the nine VIP-reactive neurones studied. 5. The conductance change underlying the intrinsic inhibitory synaptic potentials appeared to be similar to that underlying the responses in normal intestine. 6. The time courses of the intrinsic inhibitory synaptic potentials differed from those of the control responses. The responses to short trains of stimuli were significantly briefer and the responses to long trains significantly more prolonged in the extrinsically denervated preparations than in normal preparations. 7. The intrinsic inhibitory synaptic potentials were not significantly affected by phentolamine (0.2 microM), guanethidine (1 microM) or naloxone (1 microM), although the first two drugs markedly depressed control inhibitory synaptic potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug receptors on single enteric neurons

There are many substances contained within enteric nerves which excite or inhibit other nerves when these substances are applied to single neurons. The actions of these substances and of drugs which mimic these actions is to open or close membrane ion channels. The effects on membrane potential are dependent on the nature of the ions which pass through the channel and whether the channel is opened or closed. In the enteric nervous system, drugs can act at one of three broad classes of receptors: [1] those which are part of an ion channel complex and which open either cation channels or chloride channels, both of which result in membrane depolarization [2] those which open potassium channels resulting in hyperpolarization or [3] those which close potassium channels resulting in depolarization. Receptors which open potassium channels are coupled to the channel via a G-protein while receptors which close potassium channels are coupled to the channel, in some cases, via a cyclic AMP-dependent system while in other cases another second messenger system is involved.

Electrophysiological characterization of functionally distinct 5-hydroxytryptamine receptors on guinea-pig submucous plexus

Intracellular recordings were made from neurons of the guinea-pig submucous plexus and the actions of 5-hydroxytryptamine on the postsynaptic membrane and on evoked synaptic potentials were examined. 5-Hydroxytryptamine produced two types of direct postsynaptic responses: (1) A depolarization associated with a fall in input resistance was observed in all cells. Voltage-clamp and ion substitutions showed that this depolarization resulted primarily from an inward sodium current. This response could be as brief as 30 ms; it showed desensitization and was selectively abolished by 0.2-2 microM ICS 205-930. (2) A depolarization (or inward current) associated with a decreased conductance was observed in about 50% of neurons, usually after the first response was blocked by ICS 205-930. This response was due to a decreased potassium conductance; the minimum time course of this response was 8-10 s. It did not show desensitization and was not sensitive to blockade by currently available antagonists of 5-hydroxytryptamine, nicotinic and/or muscarinic receptors. Higher concentrations of 5-hydroxytryptamine were required to produce the sodium conductance increase than the potassium conductance decrease; 2-methyl-5-hydroxytryptamine was equally effective in producing these responses. 5-Hydroxytryptamine also caused a barrage of "spontaneous" nicotinic excitatory post-synaptic potentials which were sensitive to tetrodotoxin. This response desensitized, was blocked by ICS 205-930 and is presumed to reflect excitation of other cholinergic cell bodies in the plexus by the sodium conductance increase mechanism described. The evoked nicotinic excitatory postsynaptic potential and the adrenergic inhibitory postsynaptic potential were decreased by 5-hydroxytryptamine; a portion of this inhibition showed desensitization and was blocked by ICS 205-930 as well as by the muscarinic receptor antagonists, atropine and pirenzepine. The ICS 205-930-insensitive portion of this inhibition could not be attributed to activation of 5-hydroxytryptamine-1 or 5-hydroxytryptamine-2 receptors. Thus, the following conclusions are drawn: 5-hydroxytryptamine excites submucous plexus neurons by activating two distinct 5-hydroxytryptamine receptors. Activation of the 5-hydroxytryptamine-3 receptor (sensitive to ICS 205-930) produces a depolarization mediated by an increased sodium conductance. The same effect occurring in other cholinergic cell bodies initiates action potentials which are responsible for the 5-hydroxytryptamine-induced release of acetylcholine.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic control of the gut

In most mammals (except ruminants) activity in the gastrointestinal (GI) tract depends upon the condition or state of the animal, namely, fasted or fed. The fasted state is characterized by a caudally migrating, cycling motor complex, showing periods of intense contractile and secretory activity alternating with periods of quiescence. Although the mechanisms involved in the transition from the fasted to the fed state are not fully understood it seems likely that both states utilize intrinsically located neural control mechanisms and common neuronal pathways to the effector tissues. We have commented on the reported properties of the myenteric neurones and their projections to the muscle layers. The data suggests that there are both cholinergic and non-cholinergic excitatory motor neurones supplying the muscle layers. In the guinea-pig, at least, the projections of the neurones to the circular muscle layer run for relatively short distances in oral-aboral axis of the gut. The non-cholinergic excitatory transmitter substance may be Substance P or a similar tachykinin. Other excitatory nerves may well be present. There are at least two mechanisms used by non-cholinergic non-adrenergic inhibitory nerves supplying the muscle layers. In the guinea-pig ileum, there are at least two distinct projections of inhibitory motor neurones; both have aborally directed projections. The first of these is relatively short and the other long (greater than 10 mm). Individual myenteric neurones appear to contain unique and perhaps identifying groups of peptides. The functional role of many of these peptides, either within the myenteric plexus or their projections to the muscle layers, remains to be elucidated. The projections of the neurones of the submucous plexus run primarily to the mucosa. Both cholinergic and non-cholinergic secretomotor neurones appear to be present. The activation of local neural reflexes, which results in secretomotor activity, may involve submucous sensory neurones containing acetylcholine and Substance P together with cholinergic interneurones. Projections from the myenteric to the submucous plexus are likely to be involved in the coordination of intestinal movement and secretomotor activity. A simplified schematic diagram of some of the neuronal circuitry of the submucous plexus has been developed and includes the findings from immunocytochemical and electrophysiological studies.

Fast and slow depolarizing responses of guinea-pig coeliac ganglion cells to 5-hydroxytryptamine

Responses to 5-hydroxytryptamine (5-HT) applied by pressure ejection to guinea-pig coeliac ganglion cells were recorded intracellularly in vitro. Three types of responses were observed: a fast depolarization (43% of cells), a slow depolarization (25%) and a biphasic response comprising an initial fast followed by a slow depolarization (30%). Fast depolarizations had a mean duration of 6.4 s and were associated with a decrease in membrane input resistance. On hyperpolarizing the membrane, responses increased in amplitude; the mean reversal potential was -23 mV. Fast responses were nearly eliminated in a Na-free medium but were unaffected by low Ca/high Mg solution or by low Cl medium. Fast responses were depressed by MDL 72222, quipazine, metoclopramide, and by relatively high concentrations of D-tubocurarine, but unaffected by methysergide. Slow responses had a mean duration of 62 s and were associated either with no change or a slight fall in membrane resistance. hyperpolarizing the membrane, slow responses were increased in most but not all cells tested. Slow depolarizations were depressed by methysergide but unaffected by agents that suppressed the fast responses. The biphasic response displayed electrophysiological and pharmacological characteristics of both the fast and slow depolarizations. It is concluded that guinea-pig coeliac ganglion cells display either a rapid depolarization, a slow depolarization or a biphasic response to 5-HT, that the fast depolarization appears to be mediated by a conductance increase to Na and K, while a different mechanism appears to underlie the slow depolarization, and that the two responses are mediated by different kinds of 5-HT receptor.

Mianserin blocks alpha 2 adrenoceptors in submucous neurones of the guinea-pig caecum

Intracellular recordings were made from submucous plexus neurones of the guinea-pig caecum in vitro. The peak amplitude of the adrenergic inhibitory postsynaptic potential (IPSP) was depressed by mianserin in a dose-dependent manner (300 nM-100 microM). This was due to a direct blockade of postsynaptic alpha 2 adrenoceptors. The nicotinic excitatory postsynaptic potential (EPSP) and the non-cholinergic EPSP were not affected by mianserin (100 microM). The presynaptic inhibition of the release of acetylcholine, mediated by presynaptic alpha 2 receptors, was also blocked by mianserin (30 microM). The results suggest that mianserin antagonizes both pre- and post-synaptic alpha 2 adrenoceptors in enteric plexus neurones.

Evidence that 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT) is a selective alpha 2-adrenoceptor antagonist on guinea-pig submucous neurones

1 Intracellular recordings were made from neurones of the submucous plexus and from submucosal arteriolar smooth muscle of guinea-pig ileum for the purpose of examining the the actions of 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT). 2 8-OH-DPAT (10 nM-20 microM) had no direct presynaptic or postsynaptic actions on submucous plexus neurones. 3 Membrane hyperpolarizations induced in neurones by noradrenaline or UK 14304 were competitively antagonized by 8-OH-DPAT. For dose-ratios up to 40, Schild plots were linear with slopes not significantly different from unity; pA2 values for the 8-OH-DPAT antagonism of postsynaptic alpha 2-adrenoceptors were 6.9-7.2. 4 The inhibitory synaptic potential, which is due to activation of alpha 2-adrenoceptors located on submucous plexus neurones, was selectively inhibited by 8-OH-DPAT; the IC50 value for inhibition of the inhibitory synaptic potential was 250 nM. 5 Neuronal hyperpolarizations mediated through activation of delta-opioid receptors or somatostatin receptors were unaffected by 8-OH-DPAT (0.1-1 microM). 6 The ability of noradrenaline and UK 14304 to inhibit the release of acetylcholine at synapses in the submucous plexus, and to inhibit the release of the transmitter which mediates the excitatory junction potential in the submucosal arteriolar smooth muscle, was also blocked by 8-OH-DPAT. 7 These results suggest that some of the actions of 8-OH-DPAT previously ascribed to agonism at 5-hydroxytryptamine (5-HT) receptors may actually result from blockade of the actions of endogenously released noradrenaline acting on alpha 2-adrenoceptors.

Neural control of electrogenic transport in the rat jejunum; interaction between intramural and adrenergic mechanisms

The aim of the study was to test the hypothesis that extrinsic adrenergic neurons modulate electrogenic ion transport in the jejunum by inhibition of intramural secretory nerve activity. Isolated pieces of rat jejunum were mounted in the Ussing chamber. Noradrenaline (NA) was released from sympathetic nerve endings in the tissue by electrical stimulation of the mesenteric nerves or by the indirect sympathomimetic agent tyramine. Intramural neurons were activated by electrical field stimulation (EFS). Mesenteric nerve stimulation induced a frequency-related decrease in the transepithelial potential difference (PD) and short circuit current (SCC), effects which were abolished by the alpha-adrenergic antagonist phentolamine. A similar response was induced by the serosal application of tyramine and by exogenous NA. Tetrodotoxin (TTX), a nerve conduction inhibitor, had no significant effect on the response to NA, but attenuated the response to tyramine. Both NA and tyramine markedly attenuated the secretory response to EFS, but there was no significant effect of catecholamines on the secretory response to serosal theophylline. The results suggest at least two mechanisms behind the adrenergic modulation of electrogenic chloride secretion: (I) a ganglionic or presynaptic modulation of secretory nerve activity via innervated alpha-receptors, and (2) a modulation at the level of the enterocytes via non-innervated alpha-receptors. The latter effect does not seem to be mediated by the cAMP system.

A non-adrenergic, non-cholinergic slow inhibitory post-synaptic potential in neurones of the guinea-pig submucous plexus

1. Intracellular recordings were made from neurones in the submucous plexus of guinea-pig ileum and caecum. The responses to electrical stimulation of fibre strands entering the nodes of the plexus were studied. 2. Stimuli comprising trains of pulses (20 Hz, 1-5 s) produced nicotinic excitatory post-synaptic potentials (fast e.p.s.p.s), an adrenergic inhibitory post-synaptic potential (i.p.s.p.), a slow excitatory post-synaptic potential (slow e.p.s.p.) and a fourth, hitherto unnoticed, slow hyperpolarization which followed the slow e.p.s.p. All these responses were abolished by tetrodotoxin or solutions containing a low calcium concentration. 3. The slow hyperpolarization (slow i.p.s.p.) was examined in the presence of blockers of the nicotinic and adrenergic responses, and in conditions in which the slow e.p.s.p. was prevented by desensitizing concentrations of substance P or vasoactive intestinal polypeptide. The slow i.p.s.p. was unaffected by prazosin (0.1-1 microM), propranolol (0.1-1 microM), atropine (1 microM) or naloxone (1 microM). 4. The amplitude and duration of the slow i.p.s.p. increased with increasing numbers of stimulus pulses; it had an amplitude of 17 mV and a duration of 70 s when evoked by a stimulus of 20 Hz for 3 s. 5. The slow i.p.s.p. was associated with a decrease in the input resistance of the cell. It reversed polarity at -90 mV in 4.7 mM-potassium and the extrapolated reversal potential in 0.47 mM-potassium was -145 mV; these findings indicate that the slow i.p.s.p. results from an increase in membrane potassium conductance. 6. The slow i.p.s.p. could still be recorded from submucous plexus neurones in segments of ileum which had been extrinsically denervated 6-11 days previously.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of 5-HT3 receptor subtypes causes rapid excitation of rabbit parasympathetic neurones

Intracellular recordings were made from parasympathetic neurones of the rabbit vesical pelvic ganglia maintained in vitro. 5-Hydroxytryptamine (5-HT) caused a membrane depolarization which was antagonized by ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester) but not by methysergide. ICS 205-930 caused a parallel shift to the right of the dose-response curve for 5-HT. These results suggest that the 5-HT3 receptor is involved in the membrane depolarization.

Development of neural receptors for serotonin in the murine bowel

During the ontogeny of the enteric nervous system, the varicosities of mature neurons contact dividing neural precursors, which persist in the developing murine gut for several weeks postnatally. This phenomenon has led to the hypothesis that the release of transmitter from mature neurons may influence the subsequent development of uncommitted neuroblasts. In order to test this hypothesis, it is necessary to know the timing of the expression of postsynaptic receptors for enteric neurotransmitters. Since serotoninergic neurons are among the earliest of enteric neurons to develop (appearing on day E12 of development in the mouse), the ontogeny of enteric neural serotonin receptors (5-HTR) was studied. These receptors have previously been characterized in adult guinea pigs and rabbits. In the current experiments, 5-HTR were identified in the adult murine bowel; their properties were compared with the 5-HTR of guinea pig and rabbits; their ontogeny was followed throughout the length of the developing mouse gut; and the properties of 5-HTR in the developing murine bowel were compared with those of the mature gut. The 5-HTR were assayed by measuring the binding of 3H-serotonin (3H-5-HT) to isolated enteric membranes by rapid filtration, and to frozen sections of bowel by radioautography. A single saturable, high affinity 3H-5-HT binding site was found in membranes from the adult mouse gut (KD = 3.9 +/- 0.5 nM; Bmax = 1.6 +/- 0.3 pmoles/mg protein). Binding of 3H-5-HT at this site was not antagonized by compounds known to be antagonists at receptors for other neurotransmitters or at the 5-HT1 or 5-HT2 class of CNS 5-HTR. Hydroxylation of the indole ring of analogues of serotonin was required for affinity at the enteric 3H-5-HT binding site. Binding of 3H-5-HT was inhibited by N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan dipeptide, a compound that has been demonstrated to antagonize those responses of myenteric neurons to serotonin that resemble slow excitatory postsynaptic potentials, but not by ICS 205-930 (Sandoz), a serotonin antagonist that does not block these responses. All of these properties of adult murine 3H-5-HT binding sites are virtually identical of those of guinea pigs and rabbits, which have previously been shown to be 5-HTR; therefore, murine enteric 3H-5-HT binding sites are probably 5-HTR as well.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory synaptic potentials recorded from mammalian neurones prolonged by blockade of noradrenaline uptake

1. Intracellular recordings of membrane potential and membrane current were made from neurones of the rat nucleus locus coeruleus and the guinea-pig submucous plexus. These neurones exhibit inhibitory post-synaptic potentials (i.p.s.p.s) which result from noradrenaline acting on alpha 2-adrenoceptors to cause an increase in potassium conductance. 2. Cocaine (0.2-30 microM) reversibly increased the duration of the i.p.s.p. or inhibitory post-synaptic current (i.p.s.c.) in locus coeruleus neurones and submucous plexus neurones by approximately 750% and 350% respectively. The concentrations of cocaine causing half-maximal prolongation of the synaptic current were 3 microM in locus coeruleus and 0.5 microM in submucous plexus. The prolongation was due entirely to a slower rate of decay of the synaptic response. 3. Cocaine (10 microM) produced a maintained hyperpolarization (2-10 mV) or outward current (20-120 pA) in locus coeruleus neurones; in submucous plexus neurones cocaine increased the amplitude and duration of spontaneous i.p.s.p.s. 4. Outward currents produced by superfusion with noradrenaline were increased by cocaine with maximum effects being observed at 10-30 microM-cocaine. The maximum leftward shift in the relation between outward current or membrane hyperpolarization and noradrenaline concentration was 18- to 100-fold in locus coeruleus neurones and 4-fold in submucous plexus neurones. The concentrations of cocaine which caused a half-maximal increase in sensitivity to superfused noradrenaline were similar in both tissues, being 4 microM in locus coeruleus and 2 microM in submucous plexus. 5. These results show that neuronal uptake of noradrenaline released from adrenergic nerves plays a significant role in determining the time course of synaptic potentials mediated by noradrenaline.

Synaptic inputs to immunohistochemically identified neurones in the submucous plexus of the guinea-pig small intestine

1. Electrophysiological recordings were made from neurones in the submucous plexus of the guinea-pig small intestine, and these neurones were classified according to their synaptic inputs. 2. The neurones from which recording were made were filled during the recording period with the fluorescent dye, Lucifer Yellow, so they could be re-identified after processing for immunohistochemical localization of vasoactive intestinal peptide (VIP). 3. The presence or absence of VIP-like immunoreactivity was determined for a total of 130 neurones whose synaptic inputs had been fully characterized and eighty-two were found to be VIP reactive. After the VIP reactivity had been assessed, the preparations were reprocessed to reveal immunoreactivity for neuropeptide Y (NPY) and a further twenty-three neurones (none of which were reactive for VIP) were found to be reactive for this peptide. Of the remaining twenty-five neurones, nineteen were not reactive for either VIP or NPY and six could not be re-identified after reprocessing. 4. Electrical stimulation of internodal strands evoked excitatory synaptic potentials lasting 20-30 ms (fast responses) in all but one of the 130 neurones studied. 5. Almost all the VIP-reactive neurones (seventy-eight of eighty-two cells) exhibited inhibitory synaptic potentials, ranging in amplitude from 2 to 30 mV and lasting 150-1500 ms, but few of the VIP-negative neurones had such responses (six of forty-eight cells). No inhibitory synaptic potentials could be evoked in any of the NPY-reactive neurones. 6. Most VIP-reactive neurones (sixty-nine) had a slow excitatory synaptic potential which could be evoked by a single stimulus, lasted 5-20 s and was associated with an increase in input resistance. Only one NPY-reactive neurone had a slow excitatory potential, but such potentials were seen in nine of the nineteen VIP-negative, NPY-negative neurones. 7. In nine of the twenty-three NPY-reactive neurones a single stimulus evoked an excitatory synaptic potential (intermediate excitatory synaptic potential) lasting 500-1500 ms and associated with a fall in the input resistance. None of the VIP-negative, NPY-negative neurones exhibited the intermediate excitatory potentials but it was not possible to determine whether such potentials could be evoked in VIP-reactive neurones because the inhibitory synaptic potentials would obscure such events. 8. It is concluded that neurochemically distinct populations of submucous neurones can be distinguished physiologically on the basis of the differing combinations of types of synaptic input they receive.

Modification of electrical field stimulation-induced contractions in the guinea-pig ileum by metoclopramide and ICS 205-930 depends on the integrity of the mucosa

Contractions induced by electrical field stimulation of guinea-pig ileum longitudinal muscle strips were enhanced by metoclopramide and ICS 205-930 at concentrations similar to those required to antagonize at 5-hydroxytryptamine 'M' receptors. The enhancement of contraction was observed in intact ileum strips but was not recorded in the longitudinal muscle myenteric plexus preparation or from the ileum with the mucosal layer removed. It is concluded that an intact mucosal layer is required for metoclopramide and ICS 205-930 to enhance electrical field stimulation-induced contractions of the guinea-pig ileum.

5-Hydroxytryptamine decreases the sensitivity of nicotinic acetylcholine receptor in bull-frog sympathetic ganglion cells

The post-synaptic effects of 5-hydroxytryptamine (5-HT) were examined in neurones of bull-frog sympathetic ganglia with intracellular micro-electrode and voltage-clamp recording techniques. Atropine (1 microM) was used to block the muscarinic cholinoceptors. 5-HT reduced the amplitude of the fast excitatory post-synaptic potential (fast e.p.s.p.). 5-HT also reduced the mean amplitude of the miniature excitatory post-synaptic potentials (m.e.p.s.p.s) without affecting their frequency. Voltage-clamp studies showed that 5-HT decreased in a dose-dependent manner the amplitude of the acetylcholine (ACh) current produced by ionophoretic application of ACh to sympathetic neurones. The relationship between the log of the ACh dose, applied ionophoretically, and the peak ACh current (the dose-response curve) was examined in voltage-clamped neurones. 5-HT caused a parallel shift to the right of the dose-response curve for ACh. Analysis using a double reciprocal plot (Lineweaver-Burk plot) revealed that 5-HT increased the apparent dissociation constant (Km) of ACh for the receptor without changing the maximum ACh current (Vmax), suggesting a competitive antagonism. The relationship between the 5-HT dose and the magnitude of inhibition of the ACh current was obtained using two different amplitudes for the ACh response. The dose-response curve of 5-HT-induced inhibition using a relatively high amplitude ACh current, S1, was parallel with that for a relatively low amplitude ACh current, S2. The Dixon plot of these two curves yielded an apparent inhibition constant (Ki) of 42 microM. Both fast excitatory post-synaptic currents (fast e.p.s.c.s) and miniature excitatory post-synaptic currents (m.e.p.s.c.s) had single-exponential decay time courses. The time constants of fast e.p.s.c. decay (tau e) and m.e.p.s.c. decay (tau m) were not altered by 5-HT, suggesting that 5-HT does not change the kinetics of opening and closing of the ionic channel associated with the nicotinic receptor. 5-HT did not alter the reversal potential of the fast e.p.s.c. These results suggest that 5-HT decreases the sensitivity of the nicotinic receptor of sympathetic neurones, by interfering with ACh binding at the active site on the receptor-ionic-channel complex. 5-HT may physiologically inhibit cholinergic transmission as it is an endogenous substance which antagonizes the nicotinic receptor in post-ganglionic neurones of bull-frog sympathetic ganglia.

Pathogenesis and pharmacology of diarrhea

The etiological factors involved in diarrhea are multiple. Also the mechanisms and mediators involved are multiple: intracellular mediators (Ca, cAMP, cGMP, calmodulin, phospholipids), extracellular mediators (hormones, neurotransmitters, prostaglandins, enterotoxins...), intramural blood flow and oxygen, intestinal motility (local- and peristaltic motility). Till now, antidiarrheals are not so versatile that they provide a solution to all types of diarrhea. The mechanisms of action of fluid replacement therapy, loperamide, alpha 2 agonist and some nonsteroidal anti-inflammatory substances are reviewed.