The origins, pathways and terminations of neurons with VIP-like immunoreactivity in the guinea-pig small intestine

Molecular Mechanisms behind Obesity and Their Potential Exploitation in Current and Future Therapy

Obesity is a chronic disease caused primarily by the imbalance between the amount of calories supplied to the body and energy expenditure. Not only does it deteriorate the quality of life, but most importantly it increases the risk of cardiovascular diseases and the development of type 2 diabetes mellitus, leading to reduced life expectancy. In this review, we would like to present the molecular pathomechanisms underlying obesity, which constitute the target points for the action of anti-obesity medications. These include the central nervous system, brain-gut-microbiome axis, gastrointestinal motility, and energy expenditure. A significant part of this article is dedicated to incretin-based drugs such as GLP-1 receptor agonists (e.g., liraglutide and semaglutide), as well as the brand new dual GLP-1 and GIP receptor agonist tirzepatide, all of which have become "block-buster" drugs due to their effectiveness in reducing body weight and beneficial effects on the patient's metabolic profile. Finally, this review article highlights newly designed molecules with the potential for future obesity management that are the subject of ongoing clinical trials.

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Sensory representation and detection mechanisms of gut osmolality change

Ingested food and water stimulate sensory systems in the oropharyngeal and gastrointestinal areas before absorption1,2. These sensory signals modulate brain appetite circuits in a feed-forward manner3-5. Emerging evidence suggests that osmolality sensing in the gut rapidly inhibits thirst neurons upon water intake. Nevertheless, it remains unclear how peripheral sensory neurons detect visceral osmolality changes, and how they modulate thirst. Here we use optical and electrical recording combined with genetic approaches to visualize osmolality responses from sensory ganglion neurons. Gut hypotonic stimuli activate a dedicated vagal population distinct from mechanical-, hypertonic- or nutrient-sensitive neurons. We demonstrate that hypotonic responses are mediated by vagal afferents innervating the hepatic portal area (HPA), through which most water and nutrients are absorbed. Eliminating sensory inputs from this area selectively abolished hypotonic but not mechanical responses in vagal neurons. Recording from forebrain thirst neurons and behavioural analyses show that HPA-derived osmolality signals are required for feed-forward thirst satiation and drinking termination. Notably, HPA-innervating vagal afferents do not sense osmolality itself. Instead, these responses are mediated partly by vasoactive intestinal peptide secreted after water ingestion. Together, our results reveal visceral hypoosmolality as an important vagal sensory modality, and that intestinal osmolality change is translated into hormonal signals to regulate thirst circuit activity through the HPA pathway.

Enteric Control of the Sympathetic Nervous System

The autonomic nervous system that regulates the gut is divided into sympathetic (SNS), parasympathetic (PNS), and enteric nervous systems (ENS). They inhibit, permit, and coordinate gastrointestinal motility, respectively. A fourth pathway, "extrinsic sensory neurons," connect gut to the central nervous system, mediating sensation. The ENS resides within the gut wall and its activities are critical for life; ENS failure to populate the gut in development is lethal without intervention."Viscerofugal neurons" are a distinctive class of enteric neurons, being the only type that escapes the gut wall. They form a unique circuit: their axons project out of the gut wall and activate sympathetic neurons, which then project back to the gut, and inhibit gut movements.For 80 years viscerofugal/sympathetic circuits were thought to have a restricted role, mediating simple sensory-motor reflexes. New data shows viscerofugal and sympathetic neurons behaving unexpectedly, compelling a re-evaluation of these circuits: both viscerofugal and sympathetic neurons transmit higher order, synchronized firing patterns that originate within the ENS. This identifies them as driving long-range motility control between different gut regions.There is need for gut motor control over distances beyond the range of ENS circuits, yet no mechanism has been identified to date. The entero-sympathetic circuits are ideally suited to meet this need. Here we provide an overview of the structure and functions of these peripheral sympathetic circuits, including new data showing the firing patterns generated by enteric networks can transmit through sympathetic neurons.

Alterations in Small Intestine and Liver Morphology, Immunolocalization of Leptin, Ghrelin and Nesfatin-1 as Well as Immunoexpression of Tight Junction Proteins in Intestinal Mucosa after Gastrectomy in Rat Model

The stomach is responsible for the processing of nutrients as well as for the secretion of various hormones which are involved in many activities throughout the gastrointestinal tract. Experimental adult male Wistar rats (n = 6) underwent a modified gastrectomy, while control rats (n = 6) were sham-operated. After six weeks, changes in small intestine (including histomorphometrical parameters of the enteric nervous plexuses) and liver morphology, immunolocalization of leptin, ghrelin and nesfatin-1 as well as proteins forming adherens and tight junctions (E-cadherin, zonula occludens-1, occludin, marvelD3) in intestinal mucosa were evaluated. A number of effects on small intestine morphology, enteric nervous system ganglia, hormones and proteins expression were found, showing intestinal enteroplasticity and neuroplasticity associated with changes in gastrointestinal tract condition. The functional changes in intestinal mucosa and the enteric nervous system could be responsible for the altered intestinal barrier and hormonal responses following gastrectomy. The results suggest that more complicated regulatory mechanisms than that of compensatory mucosal hypertrophy alone are involved.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Recent advances in vasoactive intestinal peptide physiology and pathophysiology: focus on the gastrointestinal system

Vasoactive intestinal peptide (VIP), a gut peptide hormone originally reported as a vasodilator in 1970, has multiple physiological and pathological effects on development, growth, and the control of neuronal, epithelial, and endocrine cell functions that in turn regulate ion secretion, nutrient absorption, gut motility, glycemic control, carcinogenesis, immune responses, and circadian rhythms. Genetic ablation of this peptide and its receptors in mice also provides new insights into the contribution of VIP towards physiological signaling and the pathogenesis of related diseases. Here, we discuss the impact of VIP on gastrointestinal function and diseases based on recent findings, also providing insight into its possible therapeutic application to diabetes, autoimmune diseases and cancer.

Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl^--free and HCO3- -free solutions. The responses were almost completely blocked by TTX (10^-6 M) but not by atropine (10^-5 M) or hexamethonium (10^-4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT₃ and 5-HT₄), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors (Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl^-/ HCO3- secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl^-/ HCO3- secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl^-/ HCO3- secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

Immunohistochemical analysis of the mouse celiac ganglion: An integrative relay station of the peripheral nervous system

Celiac ganglia are important sites of signal integration and transduction. Their complex neurochemical anatomy has been studied extensively in guinea pigs but not in mice. The goal of this study was to provide detailed neurochemical characterization of mouse celiac ganglia and noradrenergic nerves in two target tissues, spleen and stomach. A vast majority of mouse celiac neurons express a noradrenergic phenotype, which includes tyrosine hydroxylase (TH), vesicular monoamine transporter 2, and the norepinephrine transporter. Over 80% of these neuron also express neuropeptide Y (NPY), and this coexpression is maintained by dissociated neurons in culture. Likewise, TH and NPY were colocalized in noradrenergic nerves throughout the spleen and in stomach blood vessels. Somatostatin was not detected in principal neurons but did occur in small, TH-negative cells presumed to be interneurons and in a few varicose nerve fibers. Cholinergic nerves provided the most abundant input to the ganglia, and small percentages of these also contained nitric oxide synthase or vasoactive intestinal polypeptide. A low-to-moderate density of nerves also stained separately for the latter markers. Additionally, nerve bundles and varicose nerve fibers containing the sensory neuropeptides, calcitonin gene-related polypeptide, and substance P, occurred at variable density throughout the ganglia. Collectively, these findings demonstrate that principal neurons of mouse celiac ganglia have less neurochemical diversity than reported for guinea pig and other species but receive input from nerves expressing an array of neurochemical markers. This profile suggests celiac neurons integrate input from many sources to influence target tissues by releasing primarily norepinephrine and NPY.

Calcimimetic R568 inhibits tetrodotoxin-sensitive colonic electrolyte secretion and reduces c-fos expression in myenteric neurons

AIMS

Calcium-sensing receptor (CaSR) is expressed on neurons of both submucosal and myenteric plexuses of the enteric nervous system (ENS) and the CaSR agonist R568 inhibited Cl^- secretion in intestine. The purpose of this study was to localize the primary site of action of R568 in the ENS and to explore how CaSR regulates secretion through the ENS.

MATERIALS AND METHODS

Two preparations of rat proximal and distal colon were used. The full-thickness preparation contained both the submucosal and myenteric plexuses, whereas for the "stripped" preparation the myenteric plexus with the muscle layers was removed. Both preparations were mounted onto Ussing chambers and Cl^- secretory responses were compared by measuring changes in short circuit current (I_sc). Two tissue-specific CaSR knockouts (i.e., neuron-specific vs. enterocyte-specific) were generated to compare the effect of R568 on expression of c-fos protein in myenteric neurons by immunocytochemistry.

KEY FINDINGS

In full-thickness colons, tetrodotoxin (TTX) inhibited I_sc, both in proximal and distal colons. A nearly identical inhibition was produced by R568. However, in stripped preparations, while the effect of TTX on I_sc largely remained, the effect of R568 was nearly completely eliminated. In keeping with this, R568 reduced c-fos protein expression only in myenteric neurons of wild type mice and mutant mice that contained CaSR in neurons (i.e., ^villinCre/Casr^flox/flox mice), but not in myenteric neurons of ^nestinCre/Casr^flox/flox mice in which neuronal cell CaSR was eliminated.

SIGNIFICANCE

These results indicate that R568 exerts its anti-secretory effects predominantly via CaSR-mediated inhibition of neuronal activity in the myenteric plexus.

The Gastrointestinal Circulation: Physiology and Pathophysiology

The gastrointestinal (GI) circulation receives a large fraction of cardiac output and this increases following ingestion of a meal. While blood flow regulation is not the intense phenomenon noted in other vascular beds, the combined responses of blood flow, and capillary oxygen exchange help ensure a level of tissue oxygenation that is commensurate with organ metabolism and function. This is evidenced in the vascular responses of the stomach to increased acid production and in intestine during periods of enhanced nutrient absorption. Complimenting the metabolic vasoregulation is a strong myogenic response that contributes to basal vascular tone and to the responses elicited by changes in intravascular pressure. The GI circulation also contributes to a mucosal defense mechanism that protects against excessive damage to the epithelial lining following ingestion of toxins and/or noxious agents. Profound reductions in GI blood flow are evidenced in certain physiological (strenuous exercise) and pathological (hemorrhage) conditions, while some disease states (e.g., chronic portal hypertension) are associated with a hyperdynamic circulation. The sacrificial nature of GI blood flow is essential for ensuring adequate perfusion of vital organs during periods of whole body stress. The restoration of blood flow (reperfusion) to GI organs following ischemia elicits an exaggerated tissue injury response that reflects the potential of this organ system to generate reactive oxygen species and to mount an inflammatory response. Human and animal studies of inflammatory bowel disease have also revealed a contribution of the vasculature to the initiation and perpetuation of the tissue inflammation and associated injury response.

The Gut's Little Brain in Control of Intestinal Immunity

The gut immune system shares many mediators and receptors with the autonomic nervous system. Good examples thereof are the parasympathetic (vagal) and sympathetic neurotransmitters, for which many immune cell types in a gut context express receptors or enzymes required for their synthesis. For some of these the relevance for immune regulation has been recently defined. Earlier and more recent studies in neuroscience and immunology have indicated the anatomical and cellular basis for bidirectional interactions between the nervous and immune systems. Sympathetic immune modulation is well described earlier, and in the last decade the parasympathetic vagal nerve has been put forward as an integral part of an immune regulation network via its release of Ach, a system coined "the cholinergic anti-inflammatory reflex." A prototypical example is the inflammatory reflex, comprised of an afferent arm that senses inflammation and an efferent arm: the cholinergic anti-inflammatory pathway, that inhibits innate immune responses. In this paper, the current understanding of how innate mucosal immunity can be influenced by the neuronal system is summarized, and cell types and receptors involved in this interaction will be highlighted. Focus will be given on the direct neuronal regulatory mechanisms, as well as current advances regarding the role of microbes in modulating communication in the gut-brain axis.

Nitric oxide enhances inhibitory synaptic transmission and neuronal excitability in Guinea-pig submucous plexus

Varicosities immunoreactive for nitric oxide synthase (NOS) make synaptic connections with submucosal neurons in the guinea-pig small intestine, but the effects of nitric oxide (NO) on these neurons are unknown. We used intracellular recording to characterize effects of sodium nitroprusside (SNP, NO donor) and nitro-l-arginine (NOLA, NOS inhibitor), on inhibitory synaptic potentials (IPSPs), slow excitatory synaptic potentials (EPSPs) and action potential firing in submucosal neurons of guinea-pig ileum in vitro. Recordings were made from neurons with the characteristic IPSPs of non-cholinergic secretomotor neurons. SNP (100 μM) markedly enhanced IPSPs evoked by single stimuli applied to intermodal strands and IPSPs evoked by trains of 2–10 pulses (30 Hz). Both noradrenergic (idazoxan-sensitive) and non-adrenergic (idazoxan-insensitive) IPSPs were affected. SNP enhanced hyperpolarizations evoked by locally applied noradrenaline or somatostatin. SNP did not affect slow EPSPs evoked by single stimuli, but depressed slow EPSPs evoked by stimulus trains. NOLA (100 μM) depressed IPSPs evoked by one to three stimulus pulses and enhanced slow EPSPs evoked by trains of two to three stimuli (30 Hz). SNP also increased the number of action potentials and the duration of firing evoked by prolonged (500 or 1000 ms) depolarizing current pulses, but NOLA had no consistent effect on action potential firing. We conclude that neurally released NO acts post-synaptically to enhance IPSPs and depress slow EPSPs, but may enhance the intrinsic excitability of these neurons. Thus, NOS neurons may locally regulate several secretomotor pathways ending on common neurons.

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.

Synaptic transmission from the submucosal plexus to the myenteric plexus in Guinea-pig ileum

Stimulation of the myenteric plexus results in activation of submucosal neurons and dilation of arterioles, one way that motility and secretion can be coupled together. The present study aimed to examine the converse, whether myenteric neurons receive synaptic input from the submucosal plexus (SMP). Intracellular recordings were made from guinea-pig ileal myenteric neurons while the SMP was electrically stimulated. Of the 29 neurons studied (13 S and 16 AH neurons), stimulation of the SMP evoked a synaptic potential in only seven cells, or 24% of neurons. When the SMP was situated oral to the myenteric plexus, 4 of 13 (31%) myenteric neurons had synaptic input. When it was situated circumferential, 2 of 8 (25%) had input, and when the SMP was situated anal 1 of 8 (13%) had input. Overall, 5 of the 13 (38%) S neurons responded with fast excitatory post-synaptic potentials (EPSPs), one of which also showed a slow EPSP, while 2 of the 16 (13%) AH neurons responded with a slow EPSP. This study indicates that the synaptic input from the SMP to myenteric neurons is relatively sparse. Whether this input is less important than the myenteric to submucosal input or simply represents a more selective form of control is unknown.

Effect of hydrogen peroxide on VIP-induced relaxation of the cat lower esophageal sphincter

We investigated the effects of hydrogen peroxide (H2O2) on relaxation of the cat lower esophageal sphincter (LES). Vasoactive intestinal peptide (VIP) caused dose-dependent relaxation of LES, and H2O2 reduced VIP-induced relaxation. Relaxation was also attenuated by pertussis toxin (PTX), indicating a Gi/o component. VIP treatment increased [35S]GTPgammaS binding to Gs and Gi3 protein, but not to Go, Gq, Gil or Gi2. This increase in Gs or Gi3 binding was reduced by H2O2. However, the relaxation induced by sodium nitroprusside (SNP), 3-morpholino sydnomine (SIN-1), 8-br cGMP (cGMP analog), forskolin (adenylate cyclase activator), and dibutyryl-cAMP (a stable cAMP analog) was not reduced by H2O2. These data suggest that H202 inhibits VIP-induced relaxation via a Gi-dependent pathway, perhaps by inhibiting the activation of G(i3) or Gs downstream of the VIP receptor and independent of cAMP or NO-cGMP signaling.

Morphologies and projections of defined classes of neurons in the submucosa of the guinea-pig small intestine

Four types of neurons have previously been identified by neurochemical markers in the submucosal ganglia of the guinea-pig small intestine, and functional roles have been ascribed to each type. However, morphological differences among the classes have not been determined, and there is only partial information about their projections within the submucosa. In the present work, we used intracellular microelectrodes to fill neurons of each type with biocytin, which was then converted to a permanent dye, so that the shapes of the neurons could be determined and their projections within the submucosa could be followed. Cell bodies of noncholinergic secretomotor/ vasodilator neurons had Dogiel type I morphology. These neurons, which are vasoactive intestinal peptide immunoreactive, had single axons that ran through many ganglia without providing terminals around other neurons. Cholinergic secretomotor neurons with neuropeptide Y immunoreactivity had Stach type IV morphology, and cholinergic secretomotor/vasodilator neurons had stellate cell bodies. The axons of these two types ran short distances in the plexus and did not innervate other submucosal neurons. Neurons of the fourth type, intrinsic primary afferent neurons, had cell bodies with Dogiel type II morphology and their processes supplied networks of varicose processes around other nerve cells. It is concluded that each functionally defined type of submucosal neuron has a characteristic morphology and that intrinsic primary afferent neurons synapse with secretomotor neurons to form monosynaptic secretomotor reflex circuits.

PACAP-(6-38) inhibits the effects of vasoactive intestinal polypeptide, but not PACAP, on the small intestinal circular muscle

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating peptide-(1-38) (PACAP) have been found to stimulate distension-induced peristaltic motility in the guinea-pig isolated small intestine. In this study, we tested whether the putative VIP/PACAP receptor antagonist PACAP-(6-38) counteracts the properistaltic effect of VIP and PACAP in isolated segments of the guinea-pig small intestine. VIP (100 nM) and PACAP (30 nM) had a stimulatory effect, i.e., lowered the peristaltic pressure threshold at which peristaltic waves were triggered and enhanced the frequency of peristaltic waves. PACAP-(6-38) (3 microM) was per se without effect on peristalsis but prevented or reversed the peristaltic motor stimulation caused by VIP, when it was given before or after the agonist, respectively. PACAP-(6-38), however, failed to antagonize the properistaltic effect of PACAP. In ileal circular strips treated with tetrodotoxin (1 microM) and indomethacin (3 microM), spontaneous myogenic activity was inhibited by VIP (5-30 nM). This effect was significantly reduced by a pretreatment with PACAP-(6-38) (3 microM). A similar inhibition by PACAP-(1-38) (10-500 nM) was not influenced by the antagonist. It is concluded that PACAP-(6-38) is a VIP receptor antagonist, both in the peristaltic motor pathways and at the level of the circular muscle of the guinea-pig small intestine. The lack of a motor effect of PACAP-(6-38) on its own indicates that VIP acting on PACAP-(6-38)-sensitive receptors (located on neurons and/or the smooth muscle) is unlikely to participate in peristaltic motor regulation.

Molecular cloning and expression analysis of the turkey vasoactive intestinal peptide receptor

Vasoactive intestinal peptide (VIP) is a prolactin (PRL)-releasing factor whose activity in avian species is believed to be mediated by a specific VIP receptor (VIP-R). Circulating PRL levels are closely related to hypothalamic VIP immunoreactivity, hypothalamic VIP mRNA content, and hypophysial-portal blood VIP concentrations in turkeys. In the present study, a turkey VIP-R (tVIP-R) cDNA was cloned and its mRNA abundance was quantified in various tissues during different reproductive stages. The 2347-bp tVIP-R cDNA encoded a 457 amino acid protein, with a predicted Mr of 52 kDa. The full-length cDNA shares approximately 55% similarity with the mammalian VIP receptor-1. Northern blot analysis revealed that a major 2.7-kb transcript was expressed in laying hen pituitaries. Furthermore, two minor tVIP-R transcripts of 3.7 and 3.4 kb were observed. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) was performed using RNA from various turkey brain and peripheral tissues throughout the reproductive cycle. The steady-state levels of pituitary tVIP-R mRNA changed during the reproductive cycle, whereas mRNA expression in other tissues was not affected. The steady-state levels of tVIP-R mRNA were only affected in the pituitary, whereas mRNA expression in any of the other tissues examined following the immunization of turkeys against VIP were not affected.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

Properties of synaptic inputs from myenteric neurons innervating submucosal S neurons in guinea pig ileum

This study examined synaptic inputs from myenteric neurons innervating submucosal neurons. Intracellular recordings were obtained from submucosal S neurons in guinea pig ileal preparations in vitro, and synaptic inputs were recorded in response to electrical stimulation of exposed myenteric plexus. Most S neurons received synaptic inputs [>80% fast (f) excitatory postsynaptic potentials (EPSP), >30% slow (s) EPSPs] from the myenteric plexus. Synaptic potentials were recorded significant distances aboral (fEPSPs, 25 mm; sEPSPs, 10 mm) but not oral to the stimulating site. When preparations were studied in a double-chamber bath that chemically isolated the stimulating "myenteric chamber" from the recording side "submucosal chamber," all fEPSPs were blocked by hexamethonium in the submucosal chamber, but not by a combination of nicotinic, purinergic, and 5-hydroxytryptamine-3 receptor antagonists in the myenteric chamber. In 15% of cells, a stimulus train elicited prolonged bursts of fEPSPs (>30 s duration) that were blocked by hexamethonium. These findings suggest that most submucosal S neurons receive synaptic inputs from predominantly anally projecting myenteric neurons. These inputs are poised to coordinate intestinal motility and secretion.

Electrophysiological and morphological diversity of mouse sympathetic neurons

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the morphological and electrical properties of sympathetic neurons in the superior cervical, thoracic, and celiac ganglia of mice. Neurochemical and morphological characteristics of neurons varied between ganglia. Thoracic sympathetic ganglia contained three main populations of neurons based on differential patterns of expression of immunoreactivity to tyrosine hydroxylase, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). In the celiac ganglion, nearly all neurons contained immunoreactivity to both tyrosine hydroxylase and NPY. Both the overall size of the dendritic tree and the number of primary dendrites were greater in neurons from the thoracic and celiac ganglia compared with those from the superior cervical ganglion. The electrophysiological properties of sympathetic neurons depended more on their ganglion of origin rather than their probable targets. All neurons in the superior cervical ganglion had phasic firing properties and large afterhyperpolarizations (AHPs). In addition, 34% of these neurons displayed an afterdepolarization preceding the AHP. Superior cervical ganglion neurons had prominent I(M), I(A), and I(H) currents and a linear current-voltage relationship between -60 and -110 mV. Neurons from the thoracic ganglia had significantly smaller action potentials, AHPs, and apparent cell capacitance compared with superior cervical ganglion neurons, and only 18% showed an afterdepolarization. All neurons in superior cervical ganglia and most neurons in celiac ganglia received at least one strong preganglionic input. Nearly one-half the neurons in the celiac ganglion had tonic firing properties, and another 15% had firing properties intermediate between those of tonic and phasic neurons. Most celiac neurons showed significant inward rectification below -90 mV. They also expressed I(A), but with slower inactivation kinetics than that of superior cervical or thoracic neurons. Both phasic and tonic celiac ganglion neurons received synaptic inputs via the celiac nerves in addition to strong inputs via the splanchnic nerves. Multivariate statistical analysis revealed that the properties of the action potential, the AHP, and the apparent cell capacitance together were sufficient to correctly classify 80% of neurons according to their ganglion of origin. These results indicate that there is considerable heterogeneity in the morphological, neurochemical, and electrical properties of sympathetic neurons in mice. Although the morphological and neurochemical characteristics of the neurons are likely to be related to their peripheral projections, the expression of particular electrophysiological traits seems to be more closely related to the ganglia within which the neurons occur.

Analysis of fast synaptic pathways in myenteric plexus of guinea pig ileum

Most fast excitatory postsynaptic potentials (fEPSPs) recorded from guinea pig ileum myenteric plexus are mediated by acetylcholine acting at nicotinic receptors and ATP acting at P2X receptors. These studies examine length and polarity of projection of neurons releasing mediators of fEPSPs. Under ketamine-xylazine anesthesia, animals were sham treated or myenteric pathways were interrupted. After severed axons degenerated, fEPSPs were recorded at the operated site using conventional, intracellular electrophysiological methods and were classified as nicotinic or mixed on the basis of sensitivity to hexamethonium. Cholinergic and noncholinergic fEPSPs were recorded from small, operated segments, suggesting that some neurons have projections between adjacent ganglia. The mean amplitudes of nicotinic and mixed fEPSPs were reduced after circumferential and descending pathways degenerated. The proportion of nicotinic vs. mixed fEPSPs recorded from tissues lacking descending projections was greater than that recorded from sham-treated tissues, suggesting that fibers releasing noncholinergic mediators project aborally. Descending projections communicate with neurons in ganglia at least three rows aboral to their origin. The data suggest that fast noncholinergic neurotransmission could contribute to hexamethonium-resistant descending inhibition during the peristaltic reflex.

Projections of submucous neurons to the myenteric plexus in the guinea pig small intestine

The distribution of submucous neurons that project to the myenteric plexus of the guinea pig small intestine was established by retrograde transport of the carbocyanine dye 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) from myenteric ganglia in organ culture in combination with immunohistochemistry. Following the application of DiI to the serosal surface of a single myenteric ganglion, from 2 to 15 DiI-labelled nerve cell bodies were labelled in the submucous plexus up to 7.9 mm circumferentially, 4.5 mm orally, and 3.4 mm aborally to the DiI application site. No cells were labelled in preparations in which connections between myenteric and submucous plexuses had been severed prior to DiI application. Cells that were immunoreactive for vasoactive intestinal polypeptide (VIP) or for substance P (SP) accounted for about 75% and 11% of DiI-labelled cells, respectively. Neither neuropeptide Y- nor calretinin-immunoreactive submucous neurons were labelled by DiI, indicating that these classes of neurons do not project to the myenteric plexus. Retrograde tracing from the myenteric plexus with Neurobiotin revealed that labelled VIP-immunoreactive neurons had several short, filamentous processes and a single long axon that could be followed through the circular muscle to myenteric ganglia without branches to the mucosa. The previously described projection of submucous, SP-immunoreactive putative sensory neurons to the myenteric plexus was confirmed. However, this study has identified a considerably larger population of presumed interneurons that are immunoreactive for VIP that likely transmit information from the submucous plexus to the myenteric plexus and presumably coordinate activity between the two ganglionated plexuses.

Innervation of intestinal arteries by axons with immunoreactivity for the vesicular acetylcholine transporter (VAChT)

The presence of a cholinergic innervation of arterioles within the gut wall is suggested by pharmacological studies of nerve mediated vasodilatation, but attempts to identify nerve cells that give rise to cholinergic vasodilator fibres have yielded discrepant results. In the present work, antibodies to the vesicular acetylcholine transporter protein (VAChT) were used to investigate the relationships of immunoreactive nerve fibres to submucosal arterioles. Comparison was made with cerebral arteries, which are known to be cholinergically innervated. Double labelling immunohistochemical techniques revealed separate VAChT and tyrosine hydroxylase (TH) immunoreactive (IR) fibres innervating all sizes of arteries of the submucosa of the stomach, ileum, proximal colon, distal colon and rectum as well as the cerebral arteries. Arterioles of all digestive tract regions had greater densities of TH-IR innervation than VAChT-IR innervation. In the ileum, double labelling for VAChT-IR and VIP-IR or calretinin-IR showed more VAChT-IR than either VIP-IR or calretinin-IR fibres. Calretinin-IR and VAChT-IR were colocalised in a majority of calretinin-IR axons, but VIP-IR and VAChT-IR were not colocalised. All calretinin-IR nerve cells in submucous ganglia were immunoreactive for choline acetyltransferase, but only 1-2% of VIP-IR nerve cells were immunoreactive. Extrinsic denervation of the ileum did not alter the distribution of VAChT-IR fibres, but it eliminated TH-IR fibres. Removal of myenteric ganglia (myectomy) did not alter the distribution of fibres with VAChT or TH-IR. This work thus provides evidence for cholinergic innervation of intrinsic arterioles throughout the digestive tract and indicates that the fibres in the small intestine originate from submucosal nerve cells.

Characterization of myenteric interneurons with somatostatin immunoreactivity in the guinea-pig small intestine

The projections, connections, morphology and electrophysiological features of the myenteric interneurons with somatostatin immunoreactivity in the guinea-pig small intestine have been established using retrograde tracing, immunohistochemistry, confocal microscopy and intracellular recording. After application of the fluorescent dye, 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI), to the myenteric plexus, up to 900 nerve cell bodies were labelled in each preparation. Somatostatin-immunoreactive neurons accounted for 13% of all retrogradely labelled cells and were located up to 70 mm orally. When DiI was applied to the submucous ganglia, many myenteric neurons were labelled and 8% of all retrogradely labelled cells were somatostatin immunoreactive and were located up to 60 mm oral to the DiI application sites. These neurons had ovoid cell bodies, a single axon, several long filamentous dendrites and received close contacts from 40-200 somatostatin-immunoreactive varicosities. Intracellular recordings revealed that these cells had features of both S (i.e. with Synaptic inputs) and AH (i.e. neurons with After Hyperpolarization) cells, receiving fast excitatory synaptic inputs, having characteristic "sag" in their response to hyperpolarizing current pulses and sometimes a long afterhyperpolarization following soma action potentials. It is concluded that somatostatin-immunoreactive neurons have distinct electrophysiological features and form very long anally directed interneuronal chains that connect with both myenteric and submucous neurons.

Projections of chemically identified myenteric neurons of the small and large intestine of the mouse

The projections of different subpopulations of myenteric neurons in the mouse small and large intestine were examined by combining immunohistological techniques with myotomy and myectomy operations. The myotomies were used to examine the polarity of neurons projecting within the myenteric plexus and showed that neurons containing immunoreactivity for nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), calbindin and 5-HT projected anally, while neurons with substance P (SP)-immunoreactivity projected orally, in both the small and large intestine. Neurons containing neuropeptide Y (NPY)- and calretinin-immunoreactivity projected locally. In the large intestine, GABA-immunoreactive neurons projected both orally and anally, with more axons tending to project anally. Myectomy operations revealed that circular muscle motor neurons containing NOS/VIP/ +/-NPY and calretinin neurons projected anally both in the small and large intestine, while SP-immunoreactive circular muscle motor neurons projected orally. In the large intestine, GABA-IR circular muscle motor neurons projected both orally and anally. This study showed that although some neurons, such as the NOS/VP inhibitory motor neurons and interneurons, SP excitatory motor neurons and 5-HT interneurons had similar projections to those in other species, the projections of other chemical classes of neurons in the mouse intestine differed from those reported in other species.

Nitrergic inhibition of migrating myoelectric complex in the rat is mediated by vasoactive intestinal peptide

The role of vasoactive intestinal peptide (VIP) for the action of nitric oxide (NO) as a nonadrenergic noncholinergic inhibitory mediator was investigated regarding effects on migrating myoelectric complex (MMC) in rat. Animals were supplied with implanted bipolar electrodes at 5, 15 and 25 cm distal to pylorus for electromyography of small intestine. First, basal recordings with saline were followed by intravenous infusions of glyceryl trinitrate or VIP at different infusion rates to achieve dose-response relationships. Second, effects of different doses of the nitric oxide synthase inhibitor, N omega-nitro-L-arginine (L-NNA) were studied. Third, the action of L-NNA (1 mg kg-1) on the effect of VIP (500 pmol kg-1 min-1), and of the VIP receptor antagonist (4-Cl-D-Phe6, Leu17), VIP (45 nmol 20 min-1), on the action of glyceryl trinitrate (44 nmol kg-1 min-1) was investigated. Glyceryl trinitrate prolonged the MMC cycle length from 16.3 +/- 1.3 to 44.9 +/- 8.0 min (P < 0.001), while VIP completely disrupted the MMC for the whole infusion period (P < 0.05). Higher doses of either compound induced quiescence. L-NNA shortened MMC cycle length from 14.7 +/- 1.2 to 8.6 +/- 1.4 min (P < 0.05), increased its propagation velocity from 2.0 +/- 0.4 to 18.3 +/- 8.4 cm min-1 (P < 0.01) and increased calculated length from 6.3 +/- 1.0 to 55.4 +/- 18.4 cm (P < 0.01). Pretreatment with (4-Cl-D-Phe6, Leu17) VIP blocked the inhibitory action of glyceryl trinitrate and preserved MMC pattern (P < 0.05). In contrast, L-NNA had no effect on the inhibition of MMC caused by VIP. Our results indicate that inhibition of MMC is related to production of NO, which may mediate its actions through VIP.

Neurochemical classification of myenteric neurons in the guinea-pig ileum

A strategy has been developed to identify and quantify the different neurochemical populations of myenteric neurons in the guinea-pig ileum using double-labelling fluorescence immunohistochemistry of whole-mount preparations. First, six histochemical markers were used to identify exclusive, non-overlapping populations of nerve cell bodies. They included immunoreactivity for the calcium binding proteins calbindin and calretinin, the neuropeptides vasoactive intestinal polypeptide, substance P and somatostatin, and the amine, 5-hydroxytryptamine. The sizes of these populations of neurons were established directly or indirectly in double-labelling experiments using a marker for all nerve cell bodies. Each of these exclusive populations was further subdivided into classes by other markers, including immunoreactivity for enkephalins and neurofilament protein triplet. The size of each class was then established directly or by calculation. These distinct, neurochemically-identified classes were related to other published work on the histochemistry, electrophysiology and retrograde labelling of enteric neurons and to the simple Dogiel morphological classification. A classification scheme, consistent with previous studies, is proposed. It includes 14 distinct classes of myenteric neurons and accounts for nearly all neurons in the myenteric plexus of the guinea-pig ileum.

Non-adrenergic non-cholinergic neuron stimulation in the cat lower esophageal sphincter

Both electrical field stimulation and nicotine produced non-adrenergic non-cholinergic (NANC) relaxation of the circular muscle strips from the cat lower esophageal sphincter in the presence of 5 microM guanethidine and 5 microM scopolamine. Low-frequency stimulation (2 Hz, 0.2 ms duration, supramaximal current intensity, 20-s train) provoked a transient relaxation, while at high-frequency stimulation (20 Hz) a slow restoration to the resting tone was observed. Blockade of nitric oxide (NO) synthesis by 1 mM N omega -nitro-L-arginine decreased by 20% the amplitude of the 20 Hz-induced relaxation and changed the pattern of relaxation, making it similar to the sustained relaxation evoked by exogenously applied vasoactive intestinal peptide (VIP). After chymotrypsin (4 U/ml), the pattern of the high-frequency-induced relaxation resembled that of the low-frequency-induced relaxation. Similarly, chymotrypsin changed the shape of nicotine-provoked relaxation, increasing the speed of restoration to the resting tone. We suggest that the fast relaxation elicited in cat lower esophageal sphincter by electrical field stimulation or nicotine is initiated by NO. The slow restoration to the resting tone in the case of high-frequency- or nicotine-induced relaxation seems to be due to the release of VIP or VIP-like peptides. The possibility of participation of another transmitter(s) involved in NANC relaxation should not be excluded.

Why do some nematode parasites of the alimentary tract secrete acetylcholinesterase?

Many gastrointestinal nematodes secrete large amounts of acetylcholinesterases. Antibodies are produced against these secreted acetylcholinesterases and appear to give some protection against infection with some nematodes. The theory that acetylcholinesterase secreted by gastrointestinal nematodes may act as a biochemical holdfast by reducing contractions of the alimentary system has not been substantiated; a vasoactive intestinal polypeptide-like protein is secreted by some species and may be the biochemical holdfast. Secreted acetylcholinesterases may alter host cell permeability, have an anti-coagulant role, affect glycogenesis, and/or be important in certain aspects of acetate and choline metabolism. Probably the most important role for acetylcholinesterase secreted by nematodes is immune modulation and/or reduction of inflammation in the vicinity of the nematode. The reason why some species of gastrointestinal nematodes resistant to benzimidazoles contain elevated amounts of acetylcholinesterase is unclear.

A vasoactive intestinal polypeptide-like protein excreted/secreted by Nippostrongylus brasiliensis and its effect on contraction of uninfected rat intestine

The 50-30 kDa fraction isolated from the excretory/secretory products (E/S) of the nematode Nippostrongylus brasiliensis significantly decreased the amplitude of contraction of segments of uninfected rat intestine when injected into the lumen of the segments maintained in an organ bath. Dot blot analysis of the fraction suggested that it was similar in immunoreactivity to porcine vasoactive intestinal polypeptide (VIP). When antiserum to porcine VIP was mixed with N. brasiliensis E/S and the mixtures were injected into the lumen of segments of rat intestine, the inhibitory effect of the E/S on amplitude of contraction decreased. When physiological concentrations of porcine VIP (12.9 pmol/ml) were injected into the lumen of segments of uninfected rat intestine the amplitude of contraction decreased significantly. Western blot analysis of the E/S, using antiserum to porcine VIP, recognized a 30 kDa protein in the E/S and also in whole worm homogenate suggesting that synthesis of the peptide occurs inside the nematode. Peptide histidine isoleucine (PHI)-like immunoreactivity was detected in a 68 kDa fraction of the E/S and the homogenate but this fraction did not affect the amplitude of contractions of the intestine.

Expression of mRNA for the N-methyl-D-aspartate (NMDAR1) receptor and vasoactive intestinal polypeptide (VIP) co-exist in enteric neurons of the rat

Anatomical, physiological and pharmacological evidence suggests that vasoactive intestinal polypeptide neurons are important mediators of the descending inhibitory component of intestinal peristalsis. Pharmacological data also indicate that glutamate may modulate intestinal motility. Recently, we demonstrated that mRNA coding for the glutamate N-methyl-D-aspartate (NMDA) receptor is expressed by rat enteric neurons. In order to ascertain whether NMDA receptor message is expressed by vasoactive intestinal polypeptide (VIP) neurons, we employed a double in situ hybridization technique. We hybridized tissue sections from the stomach, ileum and descending colon of adult rats with an NMDAR1 oligoprobe and a VIP oligonucleotide. Enteric neurons expressing mRNA for both NMDA and VIP were found in the myenteric and submucosal ganglia at all of the sampling sites. These data suggest that the mechanism for glutamatergic excitation is present in VIP-containing enteric neurons.

Neuropeptide Y in submucosal ganglia: regional differences in the innervation of guinea-pig large intestine

Since information about possible regional differences in the innervation of the guinea-pig large intestine is incomplete, a comparative study was made of the occurrence of neurones and nerve fibres of the submucosa showing immunoreactivity (IR) to neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). In addition, a quantitative analysis was made of submucosal neurones in regions of guinea-pig large intestine selected for probable differences in their function. There were two principal findings: First, the density of NPY-IR neurone somata was high in the ascending colon (mean +/- SEM 3148 +/- 464 neurones/cm2; n = 5 animals) and progressively declined in an anal direction, the descending colon having 348 +/- 125 neurones/cm2 (in the same 5 animals); immunoreactive cell bodies were rare in the rectum. The reduced density was also reflected in a fall in the number of NPY-IR neurones/ganglion from 3.0 +/- 0.3 in the ascending colon to 0.5 +/- 0.2 in the descending colon. Second, varicose NPY-IR intraganglionic fibres were a conspicuous feature of the duodenum, caecum, transverse colon, descending colon and rectum, but not of the ileum, ascending colon or distal spiral. Moreover, in the descending colon and rectum the fibres were arranged in a loose 'cobweb' structure around non-NPY-IR neurone somata; in the caecum, there was an apparent paucity of NPY-IR somata but the exceptionally dense intraganglionic varicose fibre network may have obscured NPY-IR somata. In all regions, fibre baskets were rare. In the ascending colon, only 25 +/- 5% of ganglia (compared to 92 +/- 2% of ganglia in the descending colon) showed any intraganglionic nerve fibres; furthermore, when they occurred, these were not of the 'cobweb' type but, rather, they gave the ganglia a speckled appearance. In very immature fetuses at a stage of development when no neuropeptide somata could be found in either the myenteric or submucosal plexuses, many NPY-IR nerve fibres were present in the submucosa with a distribution similar to that of adult guinea pigs. With respect to the density of VIP-IR neurones in the large intestine, there was only a 40% reduction in the number of neurones/cm2 from proximal to distal colon, in contrast to the corresponding 90% reduction in the density of NPY-IR neurones. The number of VIP-IR neurones/ganglion (6.4) and the proportion of ganglia with VIP-IR fibres (> 90%) were constant. It is concluded that the striking regional dissimilarities in (i) the occurrence of NPY-IR neurone somata and (ii) in the disposition of intraganglionic NPY-IR nerve fibres indicate potentially important regional differences in the functions of neuropeptide Y as an antisecretory peptide in the local regulation of chloride transport in the mucosa and as a modulator of ganglionic transmission, respectively.

Colocalization of NADPH-diaphorase staining and VIP immunoreactivity in neurons in opossum internal anal sphincter

Nitric oxide and vasoactive intestinal polypeptide (VIP) are important inhibitory neurotransmitters mediating relaxation of the internal anal sphincter. The location and coexistence of these two neurotransmitters in the internal anal sphincter has not been examined. We performed a double-labeling study to examine the coexistence of nitric oxide synthase and VIP in the opossum internal anal sphincter using the NADPH-diaphorase technique which is a histochemical stain for nitric oxide synthase. In perfusion-fixed, frozen-sectioned tissue, VIP-immunoreactive neurons were labeled using immunofluorescence histochemistry. After photographing the VIP-immunoreactive neurons, nitric oxide synthase was labeled using the NADPH-diaphorase technique. Ganglia containing neuronal cell bodies were present in the myenteric plexus for the entire extent of the internal anal sphincter. VIP-immunoreactive and NADPH-diaphorase-positive neurons were present in ganglia in the myenteric as well as the submucosal plexuses. Most of the VIP-immunoreactive neurons were also NADPH-diaphorase positive. VIP and nitric oxide synthase are present and frequently coexist in neurons in the internal anal sphincter of the opossum. These neurons may be an important source of inhibitory innervation mediating the rectoanal reflex-induced relaxation of the sphincter. The demonstration of the coexistence of these two neurotransmitters will be of fundamental importance in unraveling their relationship and interaction in the internal anal sphincter as well as other systems.

Projections of neurochemically specified neurons in the porcine colon

The intramural projections of nerve cells containing serotonin (5-HT), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and nitric oxide synthase or reduced nicotinamide adenine dinucleotide phosphate diaphorase (NOS/NADPHd) were studied in the ascending colon of 5- to 6-week-old pigs by means of immunocytochemistry and histochemistry in combination with myectomy experiments. In control tissue of untreated animals, positive nerve cells and fibres were common in the myenteric and outer submucous plexus and, except for 5-HT-positive perikarya, immunoreactive cell bodies and fibres were also observed in the inner submucous plexus. VIP- and NOS/NADPHd-positive nerve fibres occurred in the ciruclar muscle layer while VIP was also abundant in nerve fibres of the mucosal layer. 5-HT- and CGRP-positive nerve fibres were virtually absent from the aganglionic nerve networks. In the submucosal layer, numerous paravascular CGRP-immunoreactive (IR) nerve fibres were encountered. Myectomy studies revealed that 5-HT-, CGRP-, VIP- and NOS/NADPHd-positive myenteric neurons all displayed anal projections within the myenteric plexus. In addition, some of the serotonergic myenteric neurons projected anally to the outer submucous plexus, whereas a great number of the VIP-ergic and nitrergic myenteric neurons send their axons towards the circular muscle layer. The possible function of these nerve cells in descending nerve pathways in the porcine colon is discussed in relation to the distribution pattern of their perikarya and processes and some of their morphological characteristics.

Projections of 5-hydroxytryptamine-immunoreactive neurons in guinea-pig distal colon

The presence of 5-hydroxytryptamine in enteric neurons of the guinea-pig distal colon was demonstrated by immunohistochemistry and the projections of the neurons were determined. 5-Hydroxytryptamine-containing nerve cells were observed in the myenteric plexus but no reactive nerve cells were found in submucous ganglia. Varicose reactive nerve fibres were numerous in the ganglia of both the myenteric and submucous plexuses, but were infrequent in the longitudinal muscle, circular muscle, muscularis mucosae and mucosa. Reactivity also occurred in enterochromaffin cells. Lesion studies showed that the axons of myenteric neurons projected anally to provide innervation to the circular muscle and submucosa and to other more anally located myenteric ganglia. The results suggest that a major population of 5-hydroxytryptamine neurons in the colon is descending interneurons, most of which extend for 10 to 15 mm in the myenteric plexus and innervate both 5-hydroxytryptamine and non-5-hydroxytryptamine neurons.

Co-localization of nitric oxide synthase and vasoactive intestinal peptide immunoreactivity in neurons of the major pelvic ganglion projecting to the rat rectum and penis

Nitric oxide synthase (NOS)- and vasoactive intestinal peptide (VIP)-immunoreactive neurons projecting to the upper rectum or penis were examined using retrograde tracing combined with immunohistochemistry in the major pelvic ganglion of male rats. Five days after injection of Fluoro-Gold (FG) into the upper rectum or penis, the major pelvic ganglion was treated with colchicine. FG injected into the upper rectum labelled many ganglion neurons in the major pelvic ganglion. Immunohistochemistry showed that 37% of FG-labelled neurons were immunoreactive for NOS and 33% for VIP. After injection of FG into the penis, 41% of FG-labelled neurons were immunoreactive for NOS and 25% for VIP. Serial cryostat sections stained for NOS and VIP, respectively, showed the co-localization of NOS and VIP in the ganglion cells projecting to the rectum and penis. In the major pelvic ganglion of the colchicine-treated animals, about 17% of the ganglion cells were immunoreactive for NOS and 32% were immunoreactive for VIP. These neurons were small in diameter (less than 30 microns). A histogram showing cell sizes in cross-sectional areas of NOS-immunoreactive neurons coincided with that of VIP-immunoreactive neurons. Most of the NOS- and VIP-immunoreactive neurons were less than 600 microns. These results indicate that small neurons containing both NOS and VIP in the major pelvic ganglion project to the rectum and penis. In the penile erectile tissues and enteric ganglia, NO and VIP may be released from the same axons and may act concomitantly on the target tissue.

Progressive reorganization of the myenteric plexus during one year following reanastomosis of the ileum of the guinea pig

The enteric nervous system appears to play a pivotal role in the functional recovery of the gastrointestinal tract after partial resection and reanastomosis, but the structural changes following surgery are not fully understood. The present study was designed to clarify the processes of myenteric plexus regeneration up to one year after transection and reanastomosis of the ileum of the guinea pig. The following techniques were used: nicotinamide adenine dinucleotide (NADH) diaphorase histochemistry, immunostaining of neuron-specific enolase (NSE) in whole-mount preparations, and transmission electron microscopy. Two months after transection and reanastomosis, myenteric ganglion cells with NADH diaphorase reactions were scarce in the center of the lesion, and were less numerous in adjacent areas (3 mm in width) than in the control ileum. In the areas adjacent to the lesion, a few large extraganglionic neurons that did not completely compensate for the loss of ganglion neurons were observed. The remaining ileum showed no changes in NADH diaphorase staining pattern at this stage. Two to 12 months after transection and reanastomosis, ectopic large neurons gradually increased in number not only in the areas adjacent to the lesion but also in part of the remaining ileum, up to 10 cm from the lesion. Concomitantly, large ganglion neurons decreased in number in these areas. In other ileal regions (more than 10 cm distant from the site of transection), no obvious changes in NADH diaphorase staining were noted throughout the observation period. The outgrowth of NSE-containing nerve fibers from the severed stumps was seen two weeks after transection. Six weeks later, numerous bundles of fine nerve fibers with NSE were shown to interconnect the oral and anal cut ends of the myenteric plexus, but they exhibited no subsequent alterations. Transmission electron microscopy revealed that regenerating nerve fiber bundles appeared initially among irregularly arranged smooth muscle cells eight weeks after the operation, as expected from light-microscopic observations. These findings suggest that myenteric ganglion cell bodies, unlike myenteric nerve fibers, require a longer term of reconstruction than previously believed after transection and reanastomosis of the ileum of the guinea pig.