Electrophysiology and morphology of vasoactive-intestinal-peptide-immunoreactive neurones of the guinea-pig ileum

Shapes and projections of neurons with immunoreactivity for gamma-aminobutyric acid in the guinea-pig small intestine

The distribution of nerve cell bodies and fibres with immunoreactivity for gamma-aminobutyric acid (GABA) has been studied in the guinea-pig small intestine. Cell bodies were common in myenteric ganglia but were extremely rare in the submucosa. Reactive fibres were numerous in the tertiary component of the myenteric plexus and in the circular muscle but they were rare in both myenteric and submucous ganglia. Reactive nerve fibres were absent from the mucosa. This distribution conforms to previous descriptions. Exposure to exogenous GABA, in vitro, was used to supplement endogenous stores of GABA. The morphology of cell bodies was better defined after this treatment. Nearly all cell bodies had type-I morphology, i.e., the cells had numerous short lamellar dendrites and one axon. Most axons ran anally. Some could be traced to the tertiary component of the myenteric plexus, others to the circular muscle. Removal of the myenteric plexus from a short length of intestine caused a loss of nerve fibres from the circular muscle beneath the site of operation and a decrease in fibre density in the circular muscle that extended anally from the lesion for about 1 mm. The nerve lesions caused no significant changes in the tertiary plexus. It is concluded that GABA is contained in motor neurons supplying the longitudinal and circular muscle, and that the neurons supplying the circular muscle may be inhibitory.

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.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Ultrastructure and synaptic relationships of calbindin-reactive, Dogiel type II neurons, in myenteric ganglia of guinea-pig small intestine

Immunoreactivity for calbindin D 28K was localized ultrastructurally in nerve cell bodies and nerve fibres in myenteric ganglia of the guinea-pig small intestine. Reactive cell bodies had a characteristic ultrastructure: the cytoplasm contained many elongate, electron-dense mitochondria, numerous secondary lysosomes that were peripherally located, peripheral stacks of rough endoplasmic reticulum and dispersed Golgi apparatus. The cells were generally larger than other myenteric neurons and had mainly smooth outlines. The cytoplasmic features of these neurons were shared by a small group of immunonegative cells, but the majority of negative cells had clearly different ultrastructural appearances. Of 310 cells from 16 ganglia that were systematically examined, 38% were immunoreactive for calbindin, 10% were unreactive but similar in ultrastructure to the calbindin-reactive neurons and 51% were unreactive and dissimilar in the appearance of their cytoplasmic organelles. Immunoreactive varicosities with synaptic specializations were found on most unreactive neurons, but were markedly less frequent on the calbindin-immunoreactive cell bodies. Non-reactive presynaptic fibres were also more common on non-reactive neurons than on the calbindin-positive cell bodies. Numerous reactive varicosities, some showing synaptic specializations, were found adjacent to other fibres in the neuropil. Light microscopic studies show calbindin immunoreactive neurons to have Dogiel type-II morphology. Thus the present work links distinguishing ultrastructural features to a specific nerve cell type recognized by light microscopy in the enteric ganglia for the first time.

Reflex changes in circular muscle activity elicited by stroking the mucosa: an electrophysiological analysis in the isolated guinea-pig ileum

A preparation of isolated small intestine from the guinea-pig was studied in which reflex responses of the circular muscle were recorded intracellularly when sensory receptors in the mucosa were stimulated mechanically. This preparation was used to examine the properties of mucosa to muscle reflexes that involve non-cholinergic motor neurons innervating the circular muscle. Reproducible stimulation of the mucosa was achieved by stroking with a motor-driven brush. Gentle brush-strokes applied to the mucosa typically evoked inhibitory junction potentials anal to the stimulus and excitatory junction potentials at recording sites oral to the stimulus. Both events were rapid in onset and up to 25 mV in amplitude. The reflexes were blocked by tetrodotoxin (0.5 microM). Junction potentials declined in amplitude with distance from the stimulus, the amplitude of the excitation 15 mm oral to the stimulus was half that at 5 mm from the stimulus, whereas the amplitude of the inhibitory potential at 40-45 mm was about 60% of that at 5-10 mm anal to the stimulus. Hexamethonium (100-200 microM) blocked the ascending excitation but only slightly reduced the descending inhibition. Ascending excitation was blocked by antagonists for substance P receptors in the muscle, and inhibition was substantially reduced by apamin (0.2 microM), both before and after hexamethonium. Both responses were abolished by removal of the mucosa from the stimulus site and when lesions were made through the myenteric plexus between the stimulation and recording sites, but persisted when similar lesions were made through the submucous plexus. It is concluded that there are neurons with mechanoreceptive nerve endings in the mucosa. Stimulation of such sensory neurons leads to activation of pathways in the myenteric plexus that excite motor neurons to the muscle both oral and anal to the stimulation site. The demonstration that mucosa to muscle reflexes can be consistently evoked in the small intestine in vitro provides an opportunity for close analysis of the reflex pathways. Such analysis is not so readily achieved when reflexes are initiated by distension that, by moving the intestine, can dislodge the recording electrode.

Shapes of nerve cells in the myenteric plexus of the guinea-pig small intestine revealed by the intracellular injection of dye

The shapes of myenteric neurons in the guinea-pig small intestine were determined after injecting living neurons with the dye Lucifer yellow via a microelectrode. The cells were fixed and the distribution of Lucifer yellow rendered permanent by an immunohistochemical method. Each of 204 nerve cells was examined in whole-mount preparations of the myenteric plexus and drawn using a camera lucida at 1250 x magnification. Four cell shapes were distinguished: (1) neurons with several long processes corresponding to type II of Dogiel; (2) neurons with a single long process and lamellar dendrites corresponding to type I of Dogiel; (3) neurons with numerous filamentous dendrites; and (4) small neurons with few processes. About 15% of the neurons could not be placed into these classes or into any single class. The type II neurons (39% of the sample) had generally smooth somata and up to 7 (average 3.3) long processes, most of which ran circumferentially. Dogiel type I neurons (34% of sampled neurons) had characteristic lamellar dendrites, i.e., broad dendrites that were flattened in the plane of the plexus. The filamentous neurons (7% of the sample), had, on average, 14 fine processes up to about 50 microns in length. Small neurons with smooth outlines and a few fine processes made up 5% of the neurons encountered. We conclude that myenteric neurons that have been injected with dye can be separated into morphologically distinct classes and that the different morphological classes probably correspond to different functional groupings of neurons.

Electrophysiology of guinea-pig myenteric neurons correlated with immunoreactivity for calcium binding proteins

Experiments were undertaken to define the electrophysiological characteristics and shapes of neurons in the myenteric plexus of the guinea-pig ileum that are immunoreactive for calcium binding proteins. Recordings were made from the neurons with intracellular microelectrodes containing a mixture of the fluorescent dye Lucifer yellow and KCl solution. The neurons studied were filled with Lucifer yellow so that they could be re-identified after processing the tissue to reveal immunoreactivity for either the calcium binding protein (CaBP), spot 35 protein, or vitamin D-dependent CaBP. Neurons were characterized as being AH-neurons, in which each action potential is followed by a prolonged after-hyperpolarization (greater than 4 s), or S-neurons, in which the prolonged after-hyperpolarizations were not observed and focal stimulation of internodal strands evoked fast excitatory synaptic potentials. S-neurons were never immunoreactive for the CaBPs (108 cells), but most AH-neurons (62 of 74) were immunoreactive. Immunoreactive and non-immunoreactive AH-neurons were indistinguishable on the basis of their electrophysiological properties or their shapes (all the AH-neurons were Dogiel type II in shape, i.e. smooth soma and many long processes). The S-neurons had a variety of shapes, but none could be classified as Dogiel type II. It is concluded that most AH-neurons are immunoreactive for calcium binding proteins, and that these proteins are restricted to AH-neurons.

Differential intracellular labelling of identified neurones with two fluorescent dyes

Electrophysiologically characterised rat dorsal root ganglion (DRG) neurones in vitro were injected with a fluorescent dye, either ethidium bromide or Lucifer yellow. Microelectrodes filled with ethidium bromide had lower resistances and were electrically more stable than comparable electrodes filled with Lucifer yellow. Neither dye affected the electrophysiology of these neurones in this study. We have used these dyes to locate and identify unambiguously the electrophysiologically characterised neurones after histological processing. Neurones filled with ethidium bromide could easily be distinguished from those filled with Lucifer yellow, even when they were closely apposed. Preliminary data, using the antibody RT97, indicates the compatibility of ethidium bromide injection with immunocytochemical studies. Ethidium bromide promises to be an important tool for use alone and alongside Lucifer yellow in the correlation of electrophysiology with histology.

Intracellular recordings from cells in the myenteric plexus of the rat duodenum

Intracellular recordings were made in vitro from neurons in the myenteric plexus of freshly dissected preparations of the duodenum of the rat. Nearly one-quarter of neurons (18 out of 77) had long after-hyperpolarizations following their action potentials. Over 60% of neurons (20 out of 32) which were tested exhaustively by focal stimulation at seven points around the recording site were seen to receive fast excitatory synaptic inputs. These were of very short duration (10-30 ms) and were reversibly blocked by the nicotinic antagonist hexamethonium. Only four out of 18 after-hyperpolarization cells (22%) had visible fast synaptic inputs. Seven out of 32 neurons tested received slow excitatory synaptic inputs lasting up to 60 s that were associated with a decrease in conductance and an increase in excitability. No evidence for muscarinic synaptic potentials was seen; only four cells out of 30 with fast excitatory postsynaptic potentials had slow excitatory synaptic potentials visible after a single-shot stimulus; in none of these were the slow excitatory postsynaptic potentials blocked by atropine (up to 1 x 10(-5) M). No inhibitory postsynaptic potentials were recorded in any of the 77 neurons recorded in this study. The effects of five neurotransmitter candidates (acetylcholine, GABA noradrenaline, 5-hydroxytryptamine and substance P) applied by pressure microejection were studied. It is concluded that most of the neurophysiological features reported in the extensively studied guinea-pig small bowel myenteric plexus are present in the rat duodenum. However, the apparent lack of muscarinic synaptic potentials and inhibitory synaptic potentials suggests that there may be some differences between the two species. Our recordings also differ slightly from recently reported studies of rat myenteric neurons grown in cell culture.

Novel autonomic neurotransmitters and upper gastrointestinal function

The evidence for, and possible roles of, inhibitory and excitatory non-adrenergic, non-cholinergic (NANC) nerves supplying smooth muscle, and the effects of putative transmitter candidates are considered for each of three main regions of the upper gastrointestinal tract: (A) the smooth muscle portion of the oesophagus and the oesophagogastric junction, (B) the stomach (fundus, body and antrum) and gastroduodenal junction and (C) the biliary tract and choledochoduodenal junction. The major points from human tissues are as follows: 1. Inhibitory (NANCI) nerves appear to be present in the muscularis externa of oesophagus, stomach and duodenum, with greater density in the circular than in the longitudinal muscle. 2. NANCI nerves are present in high density at the oesophagogastric and choledochoduodenal junctions. They may also be present at the gastroduodenal junction. The gall-bladder may have a very sparse NANCI innervation. 3. Excitatory (NANCE) nerves appear to be present throughout the upper gastrointestinal tract. 4. Many candidates need at present to be considered for the role of NANCE transmitter(s) in the human upper gastrointestinal tract but substance P still seems a likely contender for this role. 5. Fewer candidates are at present generally available for the role of NANCI transmitter(s), with VIP and ATP being leading contenders. However, in the human upper gastrointestinal tract the evidence for ATP is not good, and VIP still remains the favourite candidate except in the gall-bladder, where its role remains to be elucidated.

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.

Galanin-immunoreactive neurons in the guinea-pig small intestine: their projections and relationships to other enteric neurons

Galanin immunoreactivity was observed in nerve cell bodies and nerve fibres, but not in enteroendocrine cells, in the small intestine of the guinea-pig. Nerve terminals were found in the myenteric plexus, in the circular muscle, in submucous ganglia, around submucous arterioles, and in the mucosa. Lesion studies showed that all terminals were intrinsic to the intestine; those in myenteric ganglia arose from cell bodies in more orally placed ganglia. Myenteric nerve cells were also the source of terminals in the circular muscle. Galanin (GAL) was located in a population of submucous nerve cell bodies that also showed immunoreactivity for vasoactive intestinal peptide (VIP) and in a separate population that was immunoreactive for neuropeptide Y (NPY). Processes of the GAL/VIP neurons supplied submucous arterioles and the mucosal epithelium. Processes of GAL/NPY neurons ran to the mucosa. It is concluded that galanin immunoreactivity occurs in several functionally distinct classes of enteric neurons, amongst which are neurons controlling (i) motility, (ii) intestinal blood flow, and (iii) mucosal water and electrolyte transport.

Neuropeptides and the microcircuitry of the enteric nervous system

The discovery of neuropeptides in enteric neurons has revolutionized the study of the microcircuitry of the enteric nervous system. From immunohistochemistry, it is now clear that some individual enteric neurons contain several different neuropeptides with or without other transmitter-specific markers and that these markers occur in various combinations. There is evidence from experiments in which nerve pathways are interrupted that populations of enteric neurons with different combinations of markers have different projection patterns, sending their processes to distinct targets using different routes. Correlations between the neurochemistry of enteric neurons and the types of synaptic inputs they receive are also beginning to emerge from electrophysiological studies. These findings imply that enteric neurons are chemically coded by the combinations of peptides and other transmitter-related substances they contain and that the coding of each population correlates with its role in the neuronal pathways that control gastrointestinal function.