Influence of the mucosa on the excitability of myenteric neurons

Activation of ENS Circuits in Mouse Colon: Coordination in the Mouse Colonic Motor Complex as a Robust, Distributed Control System

The characteristic motor patterns of the colon are coordinated by the enteric nervous system (ENS) and involve enterochromaffin (EC) cells, enteric glia, smooth muscle fibers, and interstitial cells. While the fundamental control mechanisms of colonic motor patterns are understood, greater complexity in the circuitry underlying motor patterns has been revealed by recent advances in the field. We review these recent advances and new findings from our laboratories that provide insights into how the ENS coordinates motor patterns in the isolated mouse colon. We contextualize these observations by describing the neuromuscular system underling the colonic motor complex (CMC) as a robust, distributed control system. Framing the colonic motor complex as a control system reveals a new perspective on the coordinated motor patterns in the colon. We test the control system by applying electrical stimulation in the isolated mouse colon to disrupt the coordination and propagation of the colonic motor complex.

Optogenetic activation of the distal colon epithelium engages enteric nervous system circuits to initiate motility patterns

Digestive functions of the colon depend on sensory-motor reflexes in the enteric nervous system (ENS), initiated by intrinsic primary afferent neurons (IPANs). IPAN terminals project to the mucosal layer of the colon, allowing communication with epithelial cells comprising the colon lining. The chemical nature and functional significance of this epithelial-neural communication in regard to secretion and colon motility are of high interest. Colon epithelial cells can produce and release neuroactive substances such as ATP and 5-hydroxytryptamine (5-HT), which can activate receptors on adjacent nerve fibers, including IPAN subtypes. In this study, we examined if stimulation of epithelial cells alone is sufficient to activate neural circuits that control colon motility. Optogenetics and calcium imaging were used in ex vivo preparations of the mouse colon to selectively stimulate the colon epithelium, measure changes in motility, and record activity of neurons within the myenteric plexus. Light-mediated activation of epithelial cells lining the distal, but not proximal, colon caused local contractions and increased the rate of colonic migrating motor complexes. Epithelial-evoked local contractions in the distal colon were reduced by both ATP and 5-HT receptor antagonists. Our findings indicate that colon epithelial cells likely use purinergic and serotonergic signaling to initiate activity in myenteric neurons, produce local contractions, and facilitate large-scale coordination of ENS activity responsible for whole colon motility patterns.NEW & NOTEWORTHY Using an all-optical approach to measure real-time cell-to-cell communication responsible for colon functions, we show that selective optogenetic stimulation of distal colon epithelium produced activity in myenteric neurons, as measured with red genetically encoded calcium indicators. The epithelial-induced neural response led to local contractions, mediated by both purinergic and serotonergic signaling, and facilitated colonic motor complexes that propagate from proximal to distal colon.

The α isoform of cGMP-dependent protein kinase 1 (PKG1α) is expressed and functionally important in intrinsic primary afferent neurons of the guinea pig enteric nervous system

BACKGROUND

Intrinsic primary afferent neurons (IPANs) enable the gut to manifest reflexes in the absence of CNS input. PKG1α is selectively expressed in a subset of neurons in dorsal root ganglia (DRG) and has been linked to nociception and long-term hyperexcitability.

METHODS

We used immunoblotting, immunocytochemistry, and in vitro assays of IPAN-dependent enteric functions to test hypotheses that subsets of primary neurons of the ENS and DRG share a reliance on PKG1α expression.

KEY RESULTS

PKG1α immunoreactivity was demonstrated in immunoblots from isolated myenteric ganglia. PKG1α, but not PKG1β, immunoreactivity, was coincident with that of neuronal markers (HuC/D; β3-tubulin) in both enteric plexuses. PKG1α immunoreactivity also co-localized with the immunoreactivities of the IPAN markers, calbindin (100%; myenteric plexus) and cytoplasmic NeuN (98 ± 1% submucosal plexus). CGRP-immunoreactive DRG neurons, identified as visceral afferents by retrograde transport, were PKG1α-immunoreactive. We used intraluminal cholera toxin to determine whether PKG1α was necessary to enable stimulation of the mucosa to activate Fos in enteric neurons. Tetrodotoxin (1.0 µM), low Ca2+ /high Mg2+ media, and the PKG inhibitor, N46 (100 µM), all inhibited Fos activation in myenteric neurons. N46 also concentration dependently inhibited peristaltic reflexes in isolated preparations of distal colon (IC50  = 83.3 ± 1.3 µM).

CONCLUSIONS & INFERENCES

These data suggest that PKG1α is present and functionally important in IPANs and visceral afferent nociceptive neurons.

A model of the enteric neural circuitry underlying the generation of rhythmic motor patterns in the colon: the role of serotonin

We discuss the role of multiple cell types involved in rhythmic motor patterns in the large intestine that include tonic inhibition of the muscle layers interrupted by rhythmic colonic migrating motor complexes (CMMCs) and secretomotor activity. We propose a model that assumes these motor patterns are dependent on myenteric descending 5-hydroxytryptamine (5-HT, serotonin) interneurons. Asynchronous firing in 5-HT neurons excite inhibitory motor neurons (IMNs) to generate tonic inhibition occurring between CMMCs. IMNs release mainly nitric oxide (NO) to inhibit the muscle, intrinsic primary afferent neurons (IPANs), glial cells, and pacemaker myenteric pacemaker interstitial cells of Cajal (ICC-MY). Mucosal release of 5-HT from enterochromaffin (EC) cells excites the mucosal endings of IPANs that synapse with 5-HT descending interneurons and perhaps ascending interneurons, thereby coupling EC cell 5-HT to myenteric 5-HT neurons, synchronizing their activity. Synchronized 5-HT neurons generate a slow excitatory postsynaptic potential in IPANs via 5-HT7 receptors and excite glial cells and ascending excitatory nerve pathways that are normally inhibited by NO. Excited glial cells release prostaglandins to inhibit IMNs (disinhibition) to allow full excitation of ICC-MY and muscle by excitatory motor neurons (EMNs). EMNs release ACh and tachykinins to excite pacemaker ICC-MY and muscle, leading to the simultaneous contraction of both the longitudinal and circular muscle layers. Myenteric 5-HT neurons also project to the submucous plexus to couple motility with secretion, especially during a CMMC. Glial cells are necessary for switching between different colonic motor behaviors. This model emphasizes the importance of myenteric 5-HT neurons and the likely consequence of their coupling and uncoupling to mucosal 5-HT by IPANs during colonic motor behaviors.

Colonic migrating motor complexes, high amplitude propagating contractions, neural reflexes and the importance of neuronal and mucosal serotonin

The colonic migrating motor complex (CMMC) is a critical neurally mediated rhythmic propulsive contraction observed in the large intestine of many mammals. It seems to be equivalent to the high amplitude propagating contractions (HAPCs) in humans. This review focuses on the probable neural mechanisms involved in producing the CMMC or HAPC, their likely dependence on mucosal and neuronal serotonin and pacemaker insterstitial cells of Cajal networks and how intrinsic neural reflexes affect them. Discussed is the possibility that myenteric 5-hydroxytryptamine (5-HT) neurons are not only involved in tonic inhibition of the colon, but are also involved in generating the CMMC and modulation of the entire enteric nervous system, including coupling motility to secretion and blood flow. Mucosal 5-HT appears to be important for the initiation and effective propagation of CMMCs, although this mechanism is a longstanding controversy since the 1950s, which we will address. We argue that the slow apparent propagation of the CMMC/HAPC down the colon is unlikely to result from a slowly conducting wave front of neural activity, but more likely because of an interaction between ascending excitatory and descending (serotonergic) inhibitory neural pathways interacting both within the myenteric plexus and at the level of the muscle. That is, CMMC/HAPC propagation appears to be similar to esophageal peristalsis. The suppression of inhibitory (neuronal nitric oxide synthase) motor neurons and mucosal 5-HT release by an upregulation of prostaglandins has important implications in a number of gastrointestinal disorders, especially slow transit constipation.

The microbiome is essential for normal gut intrinsic primary afferent neuron excitability in the mouse

BACKGROUND

The role of intestinal microbiota in the development and function of host physiology is of high interest, especially with respect to the nervous system. While strong evidence has accrued that intestinal bacteria alter host nervous system function, mechanisms by which this occurs have remained elusive. For this reason, we have carried out experiments examining the electrophysiological properties of neurons in the myenteric plexus of the enteric nervous system (ENS) in germ-free (GF) mice compared with specific pathogen-free (SPF) control mice and adult germ-free mice that have been conventionalized (CONV-GF) with intestinal bacteria.

METHODS

Segments of jejunum from 8 to 12 week old GF, SPF, and CONV-GF mice were dissected to expose the myenteric plexus. Intracellular recordings in current-clamp mode were made by impaling cells with sharp microelectrodes. Action potential (AP) shapes, firing thresholds, the number of APs fired at 2× threshold, and passive membrane characteristics were measured.

KEY RESULTS

In GF mice, excitability was decreased in myenteric afterhyperpolarization (AH) neurons as measured by a lower resting membrane potential and by the number of APs generated at 2× threshold. The post AP slow afterhyperpolarization (sAHP) was prolonged for GF compared with SPF and CONV-GF animals. Passive membrane characteristics were also altered in GF mice by a decrease in input resistance.

CONCLUSIONS & INFERENCES

Here, we report the novel finding that commensal intestinal microbiota are necessary for normal excitability of gut sensory neurons and thus provide a potential mechanism for the transfer of information between the microbiota and nervous system.

Role of interstitial cells of Cajal in the generation and modulation of motor activity induced by cholinergic neurotransmission in the stomach

BACKGROUND

Interstitial cells of Cajal (ICC) are intimately linked to the enteric nervous system and a better understanding of the interactions between the two systems is going to advance our understanding of gut motor control. The objective of the present study was to investigate the role of ICC in the generation of gastric motor activity induced by cholinergic neurotransmission.

METHODS

Gastric motor activity was evoked through activation of intrinsic cholinergic neural activity, in in vitro muscle strips by electrical field stimulation, in the in vitro whole stomach by distension and in vivo by fluoroscopy after gavaging the stomach with barium sulfate. The cholinergic activity was assessed as that component of the effect of the stimulus that was sensitive to atropine. These experiments were carried out in wild-type and Ws/Ws rats that have few intramuscular ICC (ICC-IM) in the stomach.

KEY RESULTS

Under all three experimental conditions, cholinergic activity was prominent in both wild-type and W mutant rats providing evidence against the hypothesis that cholinergic neurotransmission to smooth muscle is primarily mediated by ICC-IM. Strong cholinergic activity in Ws/Ws rats was not due to upregulation of muscarinic receptors in ICC but possibly in smooth muscle of the antrum.

CONCLUSIONS & INFERENCES

Pacemaker ICC play a prominent role in the expression of motor activity induced by cholinergic activity and our data suggest that cholinergic neurotransmission to ICC affects the pacemaker frequency.

The neurochemistry and innervation patterns of extrinsic sensory and sympathetic nerves in the myenteric plexus of the C57Bl6 mouse jejunum

In vitro anterograde tracing of axons in mesenteric nerve trunks using biotinamide in combination with immunohistochemical labelling was used to characterize the extrinsic nerve projections in the myenteric plexus of the mouse jejunum. Anterogradely-labelled spinal sensory fibres innervating the enteric nervous system were identified by their immunoreactivity for calcitonin gene-related peptide (CGRP), while sympathetic noradrenergic fibres were detected with tyrosine hydroxylase (TH), using confocal microscopy. The presence of these markers has been previously described in the spinal sensory and sympathetic fibres. Labelled extrinsic nerve fibres in the myenteric plexus were identified apposing enteric neurons that were immunoreactive for either calretinin (CalR), calbindin (CalB) or nitric oxide synthase (NOS). Of the total anterogradely labelled axons in the myenteric plexus, 20% were CGRP-immunoreactive. Labelled CGRP-immunoreactive varicosities were closely apposed to CalR-immunoreactive myenteric cells, many of which were Dogiel type I (40%; interneurons) or type II (20%; intrinsic sensory) neurons. Labelled CGRP-immunoreactive varicosities were also observed in close appositions to CalB-immunoreactive myenteric cell bodies, of which a small subset had type II morphology (18%; intrinsic sensory neurons). A further 43% of all biotinamide-filled fibres were immunoreactive for TH and these fibres were apposed to CalR-immunoreactive cell bodies (small-sized; excitatory motor neurons) and NOS-immunoreactive cell bodies (either type I or small neurons; inhibitory motor neurons and interneurons) in the myenteric plexus. The results provide a neurochemical and neuroanatomical basis for connections between dorsal root afferent neurons and myenteric neurons and suggest an anatomical substrate for the well-known modulation of enteric circuits from sympathetic nerves. No anterogradely-labelled fibres were stained for NOS-immunoreactivity, despite more than 60% of dorsal root ganglion (DRG) neurons retrogradely labelled from the jejunum showing NOS-immunoreactivity. This was due to a substantial, time-dependent, and apparently selective, loss of NOS from extrinsic axons under in vitro conditions. Lastly, a small population of non-immunoreactive biotinamide-filled fibres (<1%) gave rise to dense terminal structures around individual myenteric cell bodies lacking CalR, CalB or NOS. These specialized endings may represent vagal fibres or a subset of spinal sensory neurons that do not contain CGRP.

Plasticity and ambiguity of the electrophysiological phenotypes of enteric neurons

Advances in knowledge of enteric neurons electrophysiological characteristics have led to the realisation that the properties of the neurons are dependent on the state of the intestine, the region, the method of recording and the species. Thus, under different experimental conditions, electrophysiological studies cannot provide a reliable signature that identifies the functional type of neuron. In the normal guinea-pig small intestine, taken as a model tissue, neurons can be separated into two electrophysiological groups, S and AH neurons. Combined morphological and physiological studies place several classes of motor and interneurons in the S group, and intrinsic primary afferent neurons in the AH group. There is some evidence for subgroups of S neurons, in which electrophysiological differences are correlated with functional subtypes, but these subgroups have been incompletely investigated. Morphologically characterized Dogiel type II (DII) neurons are recognisable in many species, from mouse to human, but their electrophysiological characteristics are only partly conserved across species or cannot be satisfactorily defined due to technical difficulties. There is a strong need for a comprehensive analysis of channels and currents of S/Dogiel type I neuron subtypes, similar to the comprehensive analysis of AH/DII neurons in the guinea-pig, and similar studies need to be conducted in human and other species. The purpose of this review is to highlight that criteria used for electrophysiological definition of enteric neurons might not be sufficient to distinguish between functional classes of neurons, due to intrinsic properties of neuronal subpopulations, plasticity in pathological conditions and differences in recording techniques.

Between celiac disease and irritable bowel syndrome: the "no man's land" of gluten sensitivity

The repertoire of gastrointestinal (GI) symptoms is finite; however, the etiologies and mechanisms underlying symptom generation and perception are diverse and, in many cases, unknown. This review examines the clinical and experimental evidence exploring the putative relationship between gluten sensitivity (GS) and the generation of GI symptoms. It explores the hypothesis that, in a proportion of patients, GS causes functional bowel disorder (FBD)-like symptoms. We propose a model for investigating and understanding the induction of GI symptoms and dysfunction by gluten in FBD and organic disease. We hypothesize that, even in the absence of fully developed celiac disease, gluten can induce symptoms similar to FBD. We discuss the hypothesis that GS and post-infectious irritable bowel syndrome (IBS) provide two triggers that can explain at least part of the spectrum that constitutes IBS, further advancing an understanding of the role of mucosal responses to luminal factors in FBDs. We propose that the animal model of GS in human leukocyte antigen (HLA)- DQ8 mice allows investigation of mucosal pathophysiological changes that occur before the onset of full-blown inflammation in a GS host. A better understanding of how gluten can cause symptoms in sensitive individuals will illuminate the interaction between host genotype, diet, and intestinal microbiota in generating one of the most common GI conditions.

Plasticity of enteric nerve functions in the inflamed and postinflamed gut

Inflammation of the gut alters the properties of the intrinsic and extrinsic neurons that innervate it. While the mechanisms of neuroplasticity differ amongst the inflammatory models that have been used, amongst various regions of the gut, and between intrinsic vs extrinsic neurons, a number of consistent features have been observed. For example, intrinsic and extrinsic primary afferent neurons become hyperexcitable in response to inflammation, and interneuronal synaptic transmission is facilitated in the enteric circuitry. These changes contribute to alterations in gut function and sensation in the inflamed bowel as well as functional disorders, and these changes persist for weeks beyond the point at which detectable inflammation has subsided. Thus, gaining a more thorough understanding of the mechanisms responsible for inflammation-induced neuroplasticity, and strategies to reverse these changes are clinically relevant goals. The purpose of this review is to summarize our current knowledge regarding neurophysiological changes that occur during and following intestinal inflammation, and to identify and address gaps in our knowledge regarding the role of enteric neuroplasticity in inflammatory and functional gastrointestinal disorders.

Purinergic receptors and synaptic transmission in enteric neurons

Purines such as ATP and adenosine participate in synaptic transmission in the enteric nervous system as neurotransmitters or neuromodulators. Purinergic receptors are localized on the cell bodies or nerve terminals of different functional classes of enteric neurons and, with other receptors, form unique receptor complements. Activation of purinergic receptors can regulate neuronal activity by depolarization, by regulating intracellular calcium, or by modulating second messenger pathways. Purinergic signaling between enteric neurons plays an important role in regulating specific enteric reflexes and overall gastrointestinal function. In the present article, we review evidence for purine receptors in the enteric nervous system, including P1 (adenosine) receptors and P2 (ATP) receptors. We will explore the role they play in mediating fast and slow synaptic transmission and in presynaptic inhibition of transmission. Finally, we will examine the molecular properties of the native receptors, their signaling mechanisms, and their role in gastrointestinal pathology.

Sensory stimuli directly acting at the central nervous system regulate gastric ghrelin secretion. an ex vivo organ culture study

Ghrelin, a novel gastrointestinal hormone involved in GH regulation, has been postulated as a relevant orexigenic peptide released by splanchnic tissues. Descriptive studies have shown that plasma ghrelin levels increase in states of negative energy balance or fasting, while decreasing in obesity and after feeding. In the present study, a novel organ-culture model of gastric tissue explants obtained from rat donors has been validated for ex vivo experiments. Fasting induced gastric ghrelin release as well as ghrelin mRNA expression that were reflected in plasma. Interestingly, those changes were fully reverted by 15 min of refeeding before stomach extraction. Unexpectedly, when animals were allowed 15 min before explant extraction to see or smell, but not eat, the food (tease feeding), ghrelin secretion was suppressed just like in gastric explants from refed animals. This effect was blocked when the animals were subjected to surgical vagotomy or treated with atropine sulphate. In conclusion, gastric explants were a suitable model for testing ghrelin mechanism of secretion in vitro, and they were found to maintain memory of the previously received signals. Similar to feeding, tease feeding resulted in suppression of ghrelin discharge by explants.

Phenotypic changes of morphologically identified guinea-pig myenteric neurons following intestinal inflammation

We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca(2+)-dependent K(+) channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I(h)) antagonist Cs(+). It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.

Mucosal stimulation activates secretomotor neurons via long myenteric pathways in guinea pig ileum

This study examined whether mucosal stimulation activates long secretomotor neural reflexes and, if so, how they are organized. The submucosa of in vitro full thickness guinea pig ileal preparations was exposed in the distal portion and intracellular recordings were obtained from electrophysiologically identified secretomotor neurons. Axons in the intact mucosa of the oral segment were stimulated by a large bipolar stimulating electrode. In control preparations, a single stimulus pulse evoked a fast excitatory postsynaptic potential (EPSP) in 86% of neurons located 0.7-1.0 cm anal to the stimulus site. A stimulus train evoked multiple fast EPSPs, but slow EPSPs were not observed. To examine whether mucosal stimulation specifically activated mucosal sensory nerve terminals, the mucosa/submucosa was severed from the underlying layers and repositioned. In these preparations, fast EPSPs could not be elicited in 89% of cells. Superfusion with phorbol dibutyrate enhanced excitability of sensory neurons and pressure-pulse application of serotonin to the mucosa increased the fast EPSPs evoked by mucosal stimulation, providing further evidence that sensory neurons were involved. To determine whether these reflexes projected through the myenteric plexus, this plexus was surgically lesioned between the stimulus site and the impaled neuron. No fast EPSPs were recorded in these preparations following mucosal stimulation whereas lesioning the submucosal plexus had no effect. These results demonstrate that mucosal stimulation triggers a long myenteric pathway that activates submucosal secretomotor neurons. This pathway projects in parallel with motor and vasodilator reflexes, and this common pathway may enable coordination of intestinal secretion, blood flow, and motility.

Smoothelin-a is essential for functional intestinal smooth muscle contractility in mice

BACKGROUND & AIMS

In patients with chronic intestinal pseudo-obstruction, intestinal motility is disturbed by either nervous or myogenic aberrations. The cause of the myogenic form is unknown, but it is likely to originate in the contractile apparatus of the smooth muscle cells. Smoothelins are actin-binding proteins that are expressed abundantly in visceral (smoothelin-A) and vascular (smoothelin-B) smooth muscle. Experimental data indicate a role for smoothelins in smooth muscle contraction. A smoothelin-deficient mouse model may help to establish the role of smoothelin-A in intestinal contraction and provide a model for myogenic chronic intestinal pseudo-obstruction.

METHODS

We used gene targeting to investigate the function of smoothelin-A in intestinal tissues. By deletion of exons 18, 19, and 20 from the smoothelin gene, the expression of both smoothelin isoforms was disrupted. The effects of the deficiency were evaluated by pathologic and physiologic analyses.

RESULTS

In smoothelin-A/B knockout mice, the intestine was fragile and less flexible compared with wild-type littermates. The circular and longitudinal muscle layers of the intestine were hypertrophic. Deficiency of smoothelin-A led to irregular slow wave patterns and impaired contraction of intestinal smooth muscle, leading to hampered transport in vivo. This caused obstructions that provoked intestinal diverticulosis and occasionally intestinal rupture.

CONCLUSIONS

Smoothelin-A is essential for functional contractility of intestinal smooth muscle. Hampered intestinal transit in smoothelin-A/B knockout mice causes obstruction, starvation, and, ultimately, premature death. The pathology of mice lacking smoothelin-A is reminiscent of that seen in patients with chronic intestinal pseudo-obstruction.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

Bursts of recurrent excitation in the activation of intrinsic sensory neurons of the intestine

Sensory neurons intrinsic to the wall of the intestine receive input from stimuli in the lumen. These stimuli interact with the mucosal epithelium causing release of sensory mediators that depolarize the sensory nerve terminals. The depolarization and the subsequent pattern of action potential (AP) discharge controls the type and magnitude of the reflex evoked. The characteristics of this AP discharge were investigated in 60 intrinsic sensory neurons from the myenteric plexus of the guinea-pig ileum. Intracellular electrophysiological recordings were made from neurons near intact mucosa during electrical stimulation of the mucosa and/or neuronal soma. Most neurons (87%) responded to mucosal stimulation with a burst of 3.8+/-0.3 APs (average instantaneous frequency, fINT 39+/-4 Hz). In 38%, a somatically evoked AP triggered a similar burst of 2.9+/-0.3 APs (fINT 52+/-6 Hz) while in 50% of neurons, there was ongoing spontaneous bursting (3.8+/-0.2 APs, fINT 48+/-6 Hz). APs in all of these bursts had an inflection on the rising phase and they persisted during somatic hyperpolarization indicating they were generated in a distal process rather than the soma. Collision experiments confirmed this and suggested that bursts originated near the mucosal sensory nerve terminals. A reduction in membrane excitability reduced the number of APs in a burst suggesting a brief depolarizing event, such as a voltage- or ligand-gated ion channel, was responsible. Bursting behavior in the intrinsic sensory neurons is common for mucosal stimuli and may involve a novel transmitter acting at the sensory nerve terminal. Further, some bursting involves positive feedback between the nerve terminals and other elements in the epithelium. This is a novel and potentially important component of intestinal sensory transduction.

MULTIPLE LEVELS OF SENSORY INTEGRATION IN THE INTRINSIC SENSORY NEURONS OF THE ENTERIC NERVOUS SYSTEM

1. The enteric nervous system (ENS) is present in the wall of the gastrointestinal tract and contains all the functional classes of neuron required for complete reflex arcs. One of the most important and intriguing classes of neuron is that responsive to sensory stimuli: sensory neurons with cell bodies intrinsic to the ENS. 2. These neurons have three outstanding and interrelated features: (i) reciprocal connections with each other; (ii) a slow excitatory post-synaptic potential (EPSP) resulting from high-speed firing in other sensory neurons; and (iii) a large after-hyperpolarizing potential (AHP) at the soma. Slow EPSP depolarize the cell body, generate action potentials (APs) and reduce the AHP. Conversely, the AHP limits the firing rate and, hence, reduces transmission of slow EPSP. 3. Processing of sensory information starts at the input terminals as different patterns of APs depending on the sensory modality and recent sensory history. At the soma, the ability to fire APs and, hence, drive outputs is also strongly determined by the recent firing history of the neuron (through the AHP) and network activity (through the slow EPSP). Positive feedback within the population of intrinsic sensory neurons means that the network is able to drive outputs well beyond the duration of the stimuli that triggered them. 4. Thus, sensory input and subsequent reflex generation are integrated over several hierarchical levels within the network on intrinsic sensory neurons.

Intrinsic primary afferent neurons and nerve circuits within the intestine

Intrinsic primary afferent neurons (IPANs) of the enteric nervous system are quite different from all other peripheral neurons. The IPANs are transducers of physiological stimuli, including movement of the villi or distortion of the mucosa, contraction of intestinal muscle and changes in the chemistry of the contents of the gut lumen. They are the first neurons in intrinsic reflexes that influence the patterns of motility, secretion of fluid across the mucosal epithelium and local blood flow in the small and large intestines. In the guinea pig small intestine, where they have been characterized in detail, IPANs have Dogiel type II morphology, that is they are large round or oval neurons with multiple processes, some of which end close to the luminal surface of the intestine, and some of which form synapses with enteric interneurons, motor neurons and with other IPANs. The IPANs have well-defined ionic currents through which their excitability, and their functions in enteric nerve circuits, is determined. These include voltage-gated Na(+) and Ca(2+) currents, a long lasting calcium-activated K(+) current, and a hyperpolarization-activated cationic current. The IPANs exhibit long-term changes in their states of excitation that can be induced by extended periods of low frequency activity in synaptic inputs and by inflammatory mediators, either applied directly or released during an inflammatory challenge. The IPANs may be involved in pathological changes in enteric function following inflammation.

Intrinsic primary afferent neurones of the digestive tract

Intrinsic primary afferent neurones of the intestine are specialized neurones that encode information about the state of the intestine by transducing mechanical and chemical stimuli that reflect tension in the gut wall and the chemical nature of its contents. They connect with interneurones and motor neurones in the gut to form the circuits of intrinsic muscle motor, secretomotor and vasomotor reflexes. A large range of ionic currents occur in these neurones. The neurones have voltage-activated inward sodium currents (both tetrodotoxin-sensitive and tetrodotoxin-insensitive) and inward calcium currents. Calcium entering during the action potential activates a slow after hyperpolarizing potassium current that has a profound influence on subsequent action potential firing. They also exhibit a prominent hyperpolarization-activated nonspecific cation current. The excitability of these neurones and sensory transduction are altered when the gut is inflamed. Changed excitability can persist after the inflammatory state has subsided. Intrinsic primary afferent neurones are thus important, both to the normal physiology and to pathophysiology of the small and large intestines.

Computational model of the migrating motor complex of the small intestine

The migrating motor complex (MMC) is a cyclic motor pattern with several phases enacted over the entire length of the small intestine. This motor pattern is initiated and coordinated by the enteric nervous system and modulated by extrinsic factors. Because in vitro preparations of the MMC do not exist, it has not been possible to determine the intrinsic nerve circuits that manage this motor pattern. We have used computer simulation to explore the possibility that the controlling circuit is the network of AH/Dogiel type II (AH) neurons. The basis of the model is that recurrent connections between AH neurons cause local circuits to enter a high-firing-rate state that provides the maximal motor drive observed in phase III of the MMC. This also drives adjacent segments of the network causing slow migration. Delayed negative feedback within the circuit, provided by activity-dependent synaptic depression, forces the network to return to rest after passage of phase III. The anal direction of propagation is a result of slight anal bias observed in projections of AH neurons. The model relates properties of neurons to properties of the MMC cycle: phase III migration speed is governed by neuron excitability, MMC cycle length is governed by the rate of recovery of synaptic efficacy, and phase III duration is governed by duration of slow excitatory postsynaptic potentials in AH neurons. In addition, the model makes experimental predictions that can be tested using standard techniques.

Inhibitory cotransmission or after-hyperpolarizing potentials can regulate firing in recurrent networks with excitatory metabotropic transmission

Recurrent networks of neurons communicating via excitatory connections are common in the nervous system. In the absence of mechanisms to control firing (collectively termed negative feedback), these networks are likely to be bistable and unable to meaningfully encode input signals. In most recurrent circuits, negative feedback is provided by a specialized subpopulation of interneurons, but such neurons are absent from some systems, which therefore require other forms of negative feedback. One such circuit is found within the enteric nervous system of the intestine, where AH/Dogiel type II neurons are interconnected via excitatory synapses acting through metabotropic receptors to produce slow excitatory postsynaptic potentials (slow EPSPs). Negative feedback in this recurrent network may come from either inhibitory postsynaptic potentials arising from the terminals that produce slow EPSPs or from the after hyperpolarizing potentials (AHPs) characteristic of these neurons. We have examined these possibilities using mathematical analysis, based on the Wilson-Cowan model, and computer simulations. Analysis of steady states showed that, under appropriate conditions, both types of negative feedback can provide robust regulation of firing allowing the networks to encode input signals. Numerical simulations were performed using large, anatomically realistic networks with realistic models for metabotropic transmission and suppression of the AHP. In the presence of constant exogenous input, parameters controlling aspects of synaptic events were varied, confirming the analytical results for static stimuli. The simulated networks also responded to time varying inputs in a manner consistent with known physiology. In addition, simulation revealed that neurons in networks with inhibitory contransmission fired in erratic bursts, a phenomenon observed in neurons in unparalysed tissue. Thus, either inhibitory contransmission or AHPs, or both, can allow recurrent networks of AH/Dogiel type II neurons to encode ongoing inputs in a biologically useful way. These neurons appear to be intrinsic primary afferent neurons (IPANs), which implies that the IPANs in a region act in a coordinated fashion.

Long vasodilator reflexes projecting through the myenteric plexus in guinea-pig ileum

This study examined enteric neural reflexes activating submucosal cholinergic vasodilator motoneurons, which innervate the final resistance vessels regulating mucosal blood flow. Videomicroscopy was employed to monitor dilatation of submucosal arterioles in in vitro preparations from guinea-pig ileum. Balloon distension of intact lumen evoked reflex vasodilatation and flat sheet preparations were employed to separate mucosal mechanical stimulation from intestinal distension. Mucosal stroking and balloon distension of the orad segment evoked vasodilatations > 1.5 cm from the stimulating site. Mucosal stimulation was blocked by combined 5-HT3/5HT4 antagonists but distension-evoked responses were unaffected. Distension-evoked responses were also unaffected by nifedipine (5 microM) or nifedipine (1 microM) and wortmannin (300 nM), suggesting stretch activation rather than stretch-activated contraction was involved. Mucosal and distension-evoked responses were completely blocked when the myenteric plexus was surgically lesioned and were significantly inhibited by hexamethonium. The muscarinic antagonist 4-DAMP, which inhibits vasodilatations evoked by submucosal cholinergic vasodilator neurons, blocked dilatations elicited by mucosal stimulation and balloon distension. Maximal dilatations evoked with either sensory modality could be further enhanced when stimulated with the second modality. Dilatations evoked by stimulation of the aborad segment were similar to those elicited in the orad segment. In conclusion, sensory mechanisms in the mucosa and muscularis propria activate vasodilator pathways in the myenteric plexus which project for significant distances in both ascending and descending directions before innervating submucosal arterioles. These reflexes could co-ordinate mucosal blood flow during multiple motor events such as peristalsis and intestinal mixing between propulsive events.

Localisation of cannabinoid CB(1) receptor immunoreactivity in the guinea pig and rat myenteric plexus

Activation of cannabinoid CB(1) receptors inhibits gastrointestinal motility, propulsion, and transit, whereas selective antagonism of these receptors has the opposite effects, suggesting the presence of endocannabinoid tone. Supporting evidence for presynaptic CB(1) receptors on myenteric neurons has been found in vitro. In this study, selective CB(1) receptor antibodies and neuronal markers were used to identify and characterise myenteric neurons expressing cannabinoid receptors. Whole mounts of rat and guinea pig myenteric preparations were dually labelled with antibodies against the CB(1) receptor and choline acetyltransferase, neurofilament proteins, calbindin, calretinin, synapsin I, microtubule-associated protein-2, calcitonin gene-related peptide, or substance P. The pattern of CB(1) receptor labelling and the neurochemical classification of CB(1) receptor-positive cells were markedly influenced by the species and fixation procedure. Virtually all choline acetyltransferase-immunoreactive myenteric neurons expressed CB(1) receptors in ganglia from both species. Subpopulations of neurons identified with calbindin, calretinin, and microtubule-associated protein-2 did not express CB(1) receptors. A few calcitonin gene-related peptide- and substance P-positive somata coexpressed CB(1) receptor immunoreactivity but showed little colocalisation on individual fibres. There was a close association between CB(1) receptor immunoreactivity and fibres labelled for synaptic protein, suggesting a role in the modulation of transmitter release. Functional responses to cannabinoids in the presence of hexamethonium suggest further that CB(1) receptors occur on excitatory motoneurons. In conclusion, CB(1) receptors are expressed on a variety of cholinergic sensory, interneuronal, and motor neurons in myenteric ganglia.

Ca2+ involvement in the action potential generation of myenteric neurones in the rat oesophagus

Intracellular recordings were used to study the physiological behaviour of rat oesophageal myenteric neurones, which are embedded in striated muscle. Injection of depolarizing pulses evoked action potentials with a clear 'shoulder' in all neurones. This shoulder disappeared under low Ca2+/high Mg2+ conditions. Tetrodotoxin (TTX; 1 micromol L-1) did not impede spike firing, whereas under combined TTX and low Ca2+/high Mg2+ conditions the action potentials were completely abolished, indicating that TTX- resistant action potentials are mediated by a Ca2+ current. Further experiments with omega-conotoxin GVIA (100 nmol L-1) revealed that these Ca2+ currents enter the cell via N-type voltage-activated Ca2+ channels (see also accompanying paper). Tetraethylammonium (10 mmol L-1) caused broadening of the action potentials, which probably resulted from prolonged Ca2+ influx due to blockade of the delayed rectifier K+ channel. Although Ca2+ appears to be involved in the spike generation of all rat oesophageal myenteric neurones, only a minority (14%) shows a slow afterhyperpolarization. Thus, no strict correlation exists between the presence of a shoulder and a slow afterhyperpolarization. Furthermore, morphological identification of 25 of the impaled neurones revealed that there was no strict correlation between morphology and electrophysiological behaviour. Consequently, rat oesophageal myenteric neurones appear to differ in several aspects from myenteric neurones in smooth muscle regions of the gastrointestinal tract.

Electrophysiological characteristics distinguish three classes of neuron in submucosal ganglia of the guinea-pig distal colon

Intracellular recordings were made from neurons in the submucosal ganglia of the guinea-pig distal colon. The recording electrode contained the intracellular marker biocytin, which was injected into neurons so that their electrophysiological characteristics could be correlated with their shape. Correlations of electrophysiology and shape have not been reported previously for neurons in this region. Three types of neuron were identified on electrophysiological grounds. Neurons of the first type (S neurons) had tetrodotoxin-sensitive soma action potentials, and received fast and slow excitatory synaptic inputs. They had uniaxonal morphologies and may function as secretomotor or possibly vasomotor neurons. The second type (AH neurons) received only slow synaptic input, while the soma action potential had tetrodotoxin-sensitive and -insensitive components with a shoulder on the falling phase and a long-lasting afterhyperpolarisation of the membrane potential following a single action potential. Neurons of this type had multipolar morphologies and provided dense innervation of adjacent submucosal ganglia. These neurons are similar to the submucosal intrinsic primary afferent neurons of the guinea-pig small intestine. The final type of neuron [the low-threshold (LT) neuron] had electrophysiological characteristics that set it apart from those described previously within enteric plexuses. They expressed tetrodotoxin-insensitive voltage-gated soma currents, did not have long-lasting afterhyperpolarisations and received only slow synaptic input. In addition, these neurons were very excitable: they had large input resistances and low thresholds for action potential discharge, and often fired action potentials in the absence of stimulation. Neurons with these characteristics were uniaxonal and thus are likely to be secretomotor or possibly vasomotor neurons. This study has shown that submucosal neurons of the distal colon fall into three distinct types, which can be distinguished by a combination of electrophysiological and morphological criteria.

The terminals of myenteric intrinsic primary afferent neurons of the guinea-pig ileum are excited by 5-hydroxytryptamine acting at 5-hydroxytryptamine-3 receptors

The aim of this study was to identify the receptor type(s) by which 5-hydroxytryptamine applied to the intestinal mucosa excites the terminals of myenteric AH neurons. The AH neurons have been identified as the intrinsic primary afferent (sensory) neurons in guinea-pig small intestine and 5-hydroxytryptamine has been identified as a possible intermediate in the sensory transduction process. Intracellular recordings were taken from AH neurons located within 1mm of intact mucosa to which 5-hydroxytryptamine was applied. Trains of action potentials and/or slow depolarizing responses were recorded in AH neurons in response to mucosal application of 5-hydroxytryptamine (10 or 20microM) or the 5-hydroxytryptamine-3 receptor agonist, 2-methyl-5-hydroxytryptamine (1 or 3mM), and to electrical stimulation of the mucosa. The 5-hydroxytryptamine-2 receptor agonist, alpha-methyl-5-hydroxytryptamine (100microM), and the 5-hydroxytryptamine-1,2,4 receptor agonist, 5-methoxytryptamine (10microM), did not elicit such responses. The 5-hydroxytryptamine-3 receptor-selective antagonist, granisetron (typically 1microM), and the 5-hydroxytryptamine-3,4 receptor antagonist, tropisetron (typically 1microM), each reduced or abolished the responses to 5-hydroxytryptamine, while the selective 5-hydroxytryptamine-4 receptor antagonist, SB 204070 (1microM), did not. It is concluded that application of 5-hydroxytryptamine to the mucosa activates a 5-hydroxytryptamine-3 receptor that triggers action potential generation in the mucosal nerve terminals of myenteric AH neurons.

Classes of enteric nerve cells in the guinea-pig small intestine

The guinea-pig small intestine has been very widely used to study the physiology, pharmacology and morphology of the enteric nervous system. It also provides an ideal, simple mammalian preparation for studying how nerve cells are organised into functional circuits underlying simple behaviours. Many different types of nerve cells are present in the enteric nervous system and they show characteristic combinations of morphological features, projections, biophysical properties, neurochemicals, and receptors. To identify the different functional classes is an important prerequisite for systematic analysis of how the enteric nervous system controls normal gut behaviour. Based on combinations of multiple-labelling immunohistochemistry and retrograde tracing, it has been possible to account quantitatively for all of the neurones in the guinea-pig small intestine. This article summarises that account and updates it in the light of recent data. A total of 18 classes of neurones are currently distinguishable, including primary afferent neurones, motor neurones, interneurones, secretomotor and vasomotor neurones. It is now possible to take an individual nerve cell and use a few carefully chosen criteria to assign it to a functional class. This provides a firm anatomical foundation for the systematic analysis of how the enteric nervous system normally functions and how it goes wrong in various clinically important disorders.

Outer submucous plexus: an intrinsic nerve network involved in both secretory and motility processes in the intestine of large mammals and humans

The architecture of the enteric nerve networks in the gastrointestinal tract appears to be more complex in large mammals, including humans, than in small laboratory animals. At least two distinct ganglionic nerve plexuses could be identified in the submucous layer in the digestive tract of large mammals. While functionally and morphologically similar neuron populations are found in the intestinal wall of both small and large mammals, significant differences in their topographical organization and neurochemical features may be present. This short review clearly illustrates that the close and exclusive association, which has been assumed so far between the efferent pathways of the submucous plexus and regulation of intestinal secretion/absorption on the one hand and between the myenteric plexus and regulation of intestinal motility on the other hand, cannot be interpreted that strictly. An attempt has been made to give a briefoverview of the current status of the identification of distinct functional enteric neuronal classes in the gastrointestinal tract of large mammals using the pig and human intestine as references, and to compare these data with the more extensive information gathered from the guinea-pig intestine.

Types of neurons in the enteric nervous system

This paper, written for the symposium in honour of more than 40 years' contribution to autonomic research by Professor Geoffrey Burnstock, highlights the progress made in understanding the organisation of the enteric nervous system over this time. Forty years ago, the prevailing view was that the neurons within the gut wall were post-ganglionic neurons of parasympathetic pathways. This view was replaced as evidence accrued that the neurons are part of the enteric nervous system and are involved in reflex and integrative activities that can occur even in the absence of neuronal influence from extrinsic sources. Work in Burnstock's laboratory led to the discovery of intrinsic inhibitory neurons with then novel pharmacology of transmission, and precipitated investigation of neuron types in the enteric nervous system. All the types of neurons in the enteric nervous system of the small intestine of the guinea-pig have now been identified in terms of their morphologies, projections, primary neurotransmitters and physiological identification. In this region there are 14 functionally defined neuron types, each with a characteristic combination of morphological, neurochemical and biophysical properties. The nerve circuits underlying effects on motility, blood flow and secretion that are mediated through the enteric nervous system are constructed from these neurons. The circuits for simple motility reflexes are now known, and progress has been made in analysing those involved in local control of blood flow and transmucosal fluid movement in the small intestine.

Electrophysiological features of morphological Dogiel type II neurons in the myenteric plexus of pig small intestine

By intracellular recording, 99 myenteric neurons with Dogiel type II morphology were electrophysiologically characterized in the porcine ileum and further subdivided into three groups based on their different types of afterhyperpolarization (AHP). In response to a depolarizing current injection, a fast AHP (fAHP; duration 34 +/- 11 ms; amplitude -11 +/- 6 mV; mean +/- SD) immediately followed every action potential in all neurons. In 32% of the neurons, this fAHP was the sole type of hyperpolarization recorded. Statistical analysis revealed the presence of two neuronal subpopulations that displayed either a long-lasting medium AHP (mAHP; duration after a single spike 773 +/- 753 ms; 51% of neurons) or a slow AHP (sAHP; 4, 205 +/- 1,483 ms; 17%). Slow AHP neurons also differed from mAHP neurons in the delayed onset of the AHP (mAHP 0 ms; sAHP 100-200 ms), as well as in maximum amplitude values and in the time to reach this amplitude (t(max); 148 +/- 11 ms vs. 628 +/- 108 ms). Medium AHP neurons further differed from the sAHP neurons in the occurrence of the AHP following subthreshold current injection and in their resting membrane potential (mAHP, -53 +/- 8 mV; sAHP, -62 +/- 10 mV). Medium AHP and sAHP behaved similarly in that a higher number of spikes increased their amplitude and duration, but not t(max). The majority of neurons fired multiple spikes (up to 25) in response to a 500-ms current injection (81/99) and showed a clear TTX-resistant shoulder on the repolarizing phase of the action potential (77/99), irrespective of the presence of sAHP or mAHP. These results demonstrate that the porcine Dogiel type II neurons differ in various essential electrophysiological properties from their morphological counterparts in the guinea pig ileal myenteric plexus. The most striking interspecies differences were the low occurrence of sAHP (17% vs. 80-90% in guinea pig) with relatively small amplitude (-5 vs. -20 mV), the high occurrence of mAHPs (unusual in guinea pig) and the ability to fire long spike trains (up to 25 spikes vs. 1-3 in guinea pig). In fact, Dogiel type II neurons in porcine ileum combine distinct electrophysiological features considered typical of either S-type or sAHP-type neurons in guinea pig. It can therefore be concluded that in spite of a similar morphology, Dogiel type II neurons do not behave electrophysiologically in a universal way in large and small mammals.

A computer simulation of recurrent, excitatory networks of sensory neurons of the gut in guinea-pig

Intrinsic sensory neurons of the intestine are connected together to form a recurrent network. They interact by slow excitatory post synaptic potentials (EPSPs), which have a complex dependence on the pattern of input. These networks are unstable and unable to give graded responses to sensory input without some form of inhibition, but inhibitory synaptic potentials are rare in this system. Intrinsic sensory neurons have a characteristic after-hyperpolarization (AHP), but this is depressed during slow EPSPs. To test whether AHPs can provide the inhibition necessary for stability, AHPs, slow EPSPs and their interactions were included in a computer simulation of realistic sensory neuron networks. Residual AHPs as small as 1% of control were found to lead to stable networks capable of giving graded responses.

A parallel algorithm for simulation of large neural networks

The simulation of biologically realistic neural networks requires the numerical solution of very large systems of differential equations. Variables within the system can be changing at rates that vary by orders of magnitude, not only at different times of the solution, but at the same time in different parts of the network. Therefore, an efficient implementation must be able to vary the solution step size, and do so independently in different subsystems. A single processor algorithm is presented in which each neuron can be solved with its own step size by using a priority queue to integrate them in the correct order. But this leaves the problem of how communication and synchronisation between neurons should be managed when executing in parallel. The proposed solution uses an algorithm based on waveform relaxation, which allows groups of neurons on different processors to be solved independently and hence in parallel, for substantial parts of the computation. Realistic test problems were run on a distributed memory parallel computer and results show that speedups of 10 using 16 processors are achievable, and indicate that further speedups may be possible.

Recording ionic events from cultured, DiI-labelled myenteric neurons in the guinea-pig proximal colon

To date investigations of enteric neurons by patch clamping/calcium imaging have been limited by studying unidentified heterogeneous populations of neurons. In DiI-labelled colonic myenteric neurons, the feasibility of recording ionic events was determined by applying DiI either to the mucosa or the circular muscle, dispersing neurons after 48 h organotypic culture, and patch-clamping/calcium imaging labeled neurons after 3-7 days in culture. Myenteric neurons with diffuse DiI fluorescence were typically smooth and agranular. Neurons labeled after DiI was applied to circular muscle, fired in either a phasic or a tonic manner, and exhibited fast afterhyperpolarizations (100-300 ms duration) at the end of a depolarizing pulse. They expressed a fast inward current and at least three different outward currents. Action potentials elicited in DiI-labeled sensory neurons were followed by a prolonged afterhyperpolarization (AH, 4-6 s). The offset of a suprathreshold depolarizing step elicited a prolonged outward tail current that approximated the timecourse of the prolonged AH. In addition, in response to membrane depolarization in DiI-labeled neurons loaded with fura-2, robust Ca(2+) transients were recorded using the perforated patch technique. These results demonstrate that DiI labeling of cultured myenteric neurons is feasible, and patch clamp/Ca(2+) fluorescence recordings can be made from specific populations of cultured DiI-labeled colonic myenteric neurons.

A simple mathematical model of second-messenger mediated slow excitatory postsynaptic potentials

We have developed a novel and simple mathematical model of a slow excitatory postsynaptic potential (EPSP) based on an abstraction of the processes of activation, inactivation, and summation of a cAMP, protein kinase A (PKA)-dependent second-messenger cascade. The model describes the activation of receptors, G-proteins, and production of cAMP as the first stage and uses first-order, non-rate-limited kinetics. The second stage corresponds to the release of active, PKA catalytic subunit and can use first- or higher-order kinetics. The third stage represents simple phosphorylation of ion channels and is limited by the number of channels available. The decay of each stage is based on first-order, mass-action kinetics. These equations and some variations were solved numerically and values of the parameters were determined by fitting to a variety of experimental data from myenteric neurons of the guinea-pig ileum. The model produced a slow EPSP with a nonlinear stimulus-response relationship that resulted from the underlying kinetics of the signaling cascade. This system of equations is suitable for incorporation into a large-scale computer simulation, and the methodology should be generalizable to other pathways.

Diverse ionic currents and electrical activity of cultured myenteric neurons from the guinea pig proximal colon

The aim of this study was to perform a patch-clamp analysis of myenteric neurons from the guinea pig proximal colon. Neurons were enzymatically dispersed, cultured for 2-7 days, and recorded from using whole cell patch clamp. The majority of cells fired phasically, whereas about one-quarter of the neurons fired in a tonic manner. Neurons were divided into three types based on the currents activated. The majority of tonically firing neurons lacked an A-type current, but generated a large fast transient outward current that was associated with the rapid repolarizing phase of an action potential. The fast transient outward current was dependent on calcium entry and was blocked by tetraethylammonium. Cells that expressed both an A-type current and a fast transient outward current were mostly phasic. Depolarization of these cells to suprathreshold potentials from less than -60 mV failed to trigger action potentials, or action potentials were only triggered after a delay of >50 ms. However, depolarizations from more positive potentials triggered action potentials with minimal latency. Neurons that expressed neither the A-type current or the fast transient outward current were all phasic. Sixteen percent of neurons were similar to AH/type II neurons in that they generated a prolonged afterhyperpolarization following an action potential. The current underlying the prolonged afterhyperpolarization showed weak inward rectification and had a reversal potential near the potassium equilibrium potential. Thus cultured isolated myenteric neurons of the guinea pig proximal colon retain many of the diverse properties of intact neurons. This preparation is suitable for further biophysical and molecular characterization of channels expressed in colonic myenteric neurons.

Contractile activity in intestinal muscle evokes action potential discharge in guinea-pig myenteric neurons

1. The process by which stretch of the external muscle of the intestine leads to excitation of myenteric neurons was investigated by intracellular recording from neurons in isolated longitudinal muscle-myenteric plexus preparations from the guinea-pig. 2. Intestinal muscle that was stretched by 40 % beyond its resting size in either the longitudinal or circular direction contracted irregularly. Both multipolar, Dogiel type II, neurons and uniaxonal neurons generated action potentials in stretched tissue. Action potentials persisted when the membrane potential was hyperpolarized by passing current through the recording electrode for 10 of 14 Dogiel type II neurons and 1 of 18 uniaxonal neurons, indicating that the action potentials originated in the processes of these neurons. For the remaining four Dogiel type II and 17 uniaxonal neurons, the action potentials were abolished, suggesting that they were the result of synaptic activation of the cell bodies. 3. Neurons did not fire action potentials when the muscle was paralysed by nicardipine (3 microM), even when the preparations were simultaneously stretched by 50 % beyond resting length in longitudinal and circular directions. Spontaneous action potentials were not recorded in unstretched (slack) tissue, but when the L-type calcium channel agonist (-)-Bay K 8644 (1 microM) was added, the muscle contracted and action potentials were observed in Dogiel type II neurons and uniaxonal neurons. 4. The proteolytic enzyme dispase (1 mg ml-1) added to preparations that were stretched 40 % beyond slack width caused the myenteric plexus to lift away from the muscle, but did not prevent muscle contraction. In the presence of dispase, the neurons ceased firing action potentials spontaneously, although action potentials could still be evoked by intracellular current pulses. After the action of dispase, (-)-Bay K 8644 (1 microM) contracted the muscle but did not cause neurons to fire action potentials. 5. Gadolinium ions (1 microM), which block some stretch activated ion channels, stopped muscle contraction and prevented action potential firing in tissue stretched by 40 %. However, when (-)-Bay K 8644 (1 microM) was added in the presence of gadolinium, the muscle again contracted and action potentials were recorded from myenteric neurons. 6. Stretching the tissue 40 % beyond its slack width caused action potential firing in preparations that had been extrinsically denervated and in which time had been allowed for the cut axons to degenerate. 7. The present results lead to the following hypotheses. The neural response to stretching depends on the opening of stretch activated channels in the muscle, muscle contraction in response to this opening, and mechanical communication from the contracting muscle to myenteric neurons. Distortion of sensitive sites in the processes of the neurons opens channels to initiate action potentials that are propagated to the soma, where they are recorded. Neurons are also excited indirectly by slow synaptic transmission from neurons that respond directly to distortion.

Action potential-dependent calcium transients in myenteric S neurons of the guinea-pig ileum

Simultaneous intracellular microelectrode recording and Fura-2 imaging was used to investigate the relationship between intracellular calcium ion concentration ([Ca2+]i) and excitability of tonic S neurons in intact myenteric plexus of the guinea-pig ileum. S neurons were impaled in myenteric ganglia, at locations near connections with internodal strands. The calcium indicator Fura-2 was loaded via the recording microelectrode. The estimated [Ca2+]i of these neurons was approximately 95 nM (n = 25). Intracellular current injection (200 ms pulses, 0.2 nA, delivered at 0.05 Hz) resulted in action potential firing throughout the stimulus pulse, accompanied by transient increases in [Ca2+]i (to approximately 240 nM, n = 12). Increasing the number of evoked action potentials by increasing stimulus duration (100-500 ms) or intensity (0.05-0.3 nA) produced correspondingly larger [Ca2+]i transients. Single action potentials rarely produced resolvable [Ca2+]i events, while short bursts of action potentials (three to five events) invariably produced resolvable [Ca2+]i increases. Some neurons demonstrated spontaneous action potential firing, which was accompanied by sustained [Ca2+]i increases. Action potential firing and [Ca2+]i increases were also observed by activation of slow synaptic input to these neurons, in cases where the slow depolarization initiated action potential firing. Action potentials (evoked or spontaneous) and associated [Ca2+]i transients were abolished by tetrodotoxin (1 microM). Omega-conotoxin GVIA (100 nM) reduced [Ca2+]i transients by approximately 67%, suggesting that calcium influx through N-type calcium channels contributes to evoked [Ca2+]i increases. The S neurons in this study showed prominent afterhyperpolarizations following bursts of action potential firing. The time-course of afterhyperpolarizations was correlated with the time-course of evoked [Ca2+]i transients. Afterhyperpolarizations were blocked by tetrodotoxin and reduced by omega-conotoxin GVIA, suggesting that calcium influx through N-type channels contributes to these events. The electrical properties of Fura-2-loaded neurons were not significantly different from properties of neurons recorded without Fura-2 injection, suggesting that Fura-2 injection alone does not significantly influence the electrical properties of these cells. These data indicate that myenteric S neurons in situ show prominent, activity-dependent increases in [Ca2+]i. These events can be generated spontaneously, or be evoked by intracellular current injection or synaptic activation. [Ca2+]i transients in these neurons appear to involve action potential-dependent opening of N-type calcium channels, and the elevation in [Ca2+]i increase may underlie afterhyperpolarizations and regulate excitability of these enteric neurons.

Topographical and electrophysiological characteristics of highly excitable S neurones in the myenteric plexus of the guinea-pig ileum

1. Most intracellular electrical recordings from myenteric neurones have been made from the centre of large ganglia. In this study, we examined the electrophysiological properties of neurones at the corners of large ganglia close to internodal strands and in microganglia. 2. Of 150 neurones in these locations: 111 were tonic S neurones; 9 were phasic S neurones and 30 were AH neurones. 3. Tonic S neurones were characterized by: (i) low resting membrane potentials (-50 +/- 1 mV, mean +/- s.e.m.); (ii) high input impedance (522 +/- 23 MOmega); (iii) low threshold for action potential (AP) generation (0.012 +/- 0.004 nA); (iv) firing of APs throughout a depolarizing pulse (duration <= 1 s) and one to four APs following a hyperpolarizing pulse and (v) spontaneous fast excitatory postsynaptic potentials (FEPSPs). A substantial proportion of tonic S neurones (43 %) also fired APs spontaneously (7.6 +/- 0.6 Hz; range, 0.3-19 Hz). All APs were blocked by tetrodotoxin (1 microM). 4. Tonic S neurones were subclassified, according to their post-stimulus responses, as SAH or SAD neurones. Following a burst of APs, SAH neurones exhibited a prominent after-hyperpolarization (duration, 711 +/- 10 ms) and SAD neurones an after-depolarization (duration, 170 +/- 10 ms). The after-hyperpolarization was reduced in four of ten neurones by apamin (0.3 microM). 5. FEPSPs were evoked in 20 of 38 S neurones by electrical stimulation applied both oral and anal to the recording site. Repetitive stimuli evoked slow excitatory postsynaptic potentials (SEPSPs) in some tonic S neurones. 6. Three functional classes of S neurones were identified after injection of neurobiotin through the recording microelectrode: (i) longitudinal muscle motor neurones, (ii) short circular muscle motor neurones, and (iii) ascending interneurones. 7. In conclusion, there appears to be topographical organization of highly excitable, tonic S neurones within the myenteric plexus, since, in contrast to other S neurones, they can be readily impaled in myenteric ganglia close to internodal strands and in microganglia.

Internalization of the neurokinin 1 receptor in rat myenteric neurons

Immunoreactivity for the neurokinin 1 receptor is contained in nerve cell bodies that have been deduced to be intrinsic primary afferent neurons in the myenteric plexus of the rat ileum. This study shows that neurokinin 1 receptor immunoreactivity on these neurons represents receptors that can bind agonist and undergo endocytosis, and explores the properties of that endocytosis. Segments of rat ileum were incubated in Hanks' balanced salt solution for 1 h at 4 degrees C, followed by 1 h at 37 degrees C in physiological saline solution with nicardipine and tetrodotoxin, in the presence or absence of substance P. Tissue was then fixed and whole-mount preparations were processed for fluorescence immunohistochemistry, using antibodies raised against the C-terminus of the neurokinin 1 receptor. The intracellular and surface distributions of receptor immunoreactivity were analysed using confocal microscopy and quantified by computer analysis. In tissue not exposed to substance P, most neurokinin 1 receptor immunoreactivity was confined to the surfaces of nerve cells, and 29% was intracellular. Exogenous substance P (10(-6) M) caused an increase in the amount of intracellular receptor to 72%. This internalization was concentration dependent, and maximum receptor internalization was achieved between 10(-6) M and 10(-5) M substance P (EC50 = 4.9 +/-1.6 x 10(-7) M). The specific neurokinin 1 receptor antagonist, SR104333 (10(-6) M), inhibited substance P-induced endocytosis. In tissue that was incubated in 5 x 10(-5) M monensin (to trap endocytosed receptor in the cell), without the addition of substance P, a high level of intracellular neurokinin 1 receptor immunoreactivity (81%) was also present. We deduce that endocytosis in the presence of monensin was stimulated by the release of tachykinins from intrinsic nerve endings, based on the following evidence: when endogenous release of tachykinin was blocked using a high magnesium/low calcium solution, or binding of tachykinins to the receptor was prevented using 10(-6) M SR140333, the intracellular receptor immunoreactivity remained at approximately 40%. Incubation with hypertonic sucrose also trapped receptors on the cell surface. Use of these protocols that modify receptor trafficking showed that agonist induced the neurokinin 1 receptors to aggregate, accumulate in endocytotic vesicles, move to perinuclear organelles and recycle to the surface in less than 1 h. This study indicates that there is sufficient release of endogenous tachykinins in the rat ileum to cause receptor internalization and implies that these intrinsic primary afferent neurons are likely to be under continuous influence from tachykinins in the normal intestine.

Correlation of morphology, electrophysiology and chemistry of neurons in the myenteric plexus of the guinea-pig distal colon

Intracellular recordings were made from myenteric neurons of the guinea-pig distal colon to determine their electrical behaviour in response to intracellular current injection and stimulation of synaptic inputs. The recording microelectrode contained the intracellular marker biocytin, which was injected into impaled neurons so that electrophysiology, shape and immunohistochemistry could be correlated. Myenteric neurons in the distal colon were divided into four morphological groups based on their shapes and projections. One group (29 of the 78 that were characterized electrophysiologically, morphologically and immunohistochemically) was the multiaxonal Dogiel type II neurons, the majority (25/29) of which were calbindin immunoreactive. Each of these neurons had an inflection on the falling phase of the action potential that, in 24/29 neurons, was followed by a late afterhyperpolarizing potential (AHP). Slow excitatory postsynaptic potentials were recorded in 20 of 29 Dogiel type II neurons in response to high frequency internodal strand stimulation and two neurons responded with slow inhibitory postsynaptic potentials. Low amplitude fast excitatory postsynaptic potentials occurred in 3 of 29 Dogiel type II neurons. Neurons of the other three groups were all uniaxonal: neurons with Dogiel type I morphology, filamentous ascending interneurons and small filamentous neurons with local projections to the longitudinal or circular muscle or to the tertiary plexus. Dogiel type I neurons were often immunoreactive for nitric oxide synthase or calretinin, as were some small filamentous neurons, while all filamentous ascending interneurons tested were calretinin immunoreactive. All uniaxonal neurons exhibited prominent fast excitatory postsynaptic potentials and did not have a late AHP following a single action potential, that is, all uniaxonal neurons displayed S type electrophysiological characteristics. However, in 6/19 Dogiel type I neurons and 2/8 filamentous ascending interneurons, a prolonged hyperpolarizing potential ensued when more than one action potential was evoked. Slow depolarizing postsynaptic potentials were observed in 20/29 Dogiel type I neurons, 6/8 filamentous ascending interneurons and 8/12 small filamentous neurons. Six of 29 Dogiel type I neurons displayed slow inhibitory postsynaptic potentials, as did 2/8 filamentous ascending interneurons and 4/12 small filamentous neurons. These results indicate that myenteric neurons in the distal colon of the guinea-pig are electrophysiologically similar to myenteric neurons in the ileum, duodenum and proximal colon. Also, the correlation of AH electrophysiological characteristics with Dogiel type II morphology and S electrophysiological characteristics with uniaxonal morphology is preserved in this region. However, filamentous ascending interneurons have not been encountered in other regions of the gastrointestinal tract and there are differences between the synaptic properties of neurons in this region compared to other regions studied, including the presence of slow depolarizing postsynaptic potentials that appear to involve conductance increases and frequent slow inhibitory postsynaptic potentials.

Long-term effects of synaptic activation at low frequency on excitability of myenteric AH neurons

Intracellular microelectrodes were used to record the effects of extended periods (1-30 min) of synaptic activation on AH neurons in the myenteric ganglia of the guinea-pig ileum. Low-frequency (1 Hz) stimulation gave rise to a slowly developing, sustained increase in excitability of the neurons associated with depolarization and increased input resistance. The increased excitability lasted for up to 3.5 h following the stimulus period. Successive stimulus trains (1-4 min) elicited successively greater increases in excitability. The neurons went through stages of excitation. Before stimulation, 500-ms depolarizing pulses evoked up to three action potentials (phasic response) and anode break action potentials were not observed. As excitability increased, more action potentials were evoked by depolarization (the responses became tonic), anode break action potentials were observed, prolonged after hyperpolarizing potentials that follow multiple action potentials were diminished and, with substantial depolarization of the neurons, invasion by antidromic action potentials was suppressed. It is concluded that a state of elevated excitability is induced in myenteric AH neurons by synaptic activation at low frequency and that changes in excitability can outlast stimulation by several hours.

The enteric nervous system and regulation of intestinal motility

The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intestine. The IPANs that have been physiologically characterized are in the intrinsic myenteric ganglia. They are numerous, about 650/mm length of small intestine in the guinea pig, and communicate with each other through slow excitatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the muscle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and with ascending and descending interneurons. The circular muscle contracts as an annulus, about 2-3 mm in minimum oral-to-anal extent in the guinea pig small intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how they are programmed to generate the motility patterns that are observed.

Roles of neuronal NK1 and NK3 receptors in synaptic transmission during motility reflexes in the guinea-pig ileum

1. The role of NK1 and NK3 receptors in synaptic transmission between myenteric neurons during motility reflexes in the guinea-pig ileum was investigated by recording intracellularly the reflex responses of the circular muscle to distension or compression of the mucosal villi. Experiments were performed in a three-chambered organ bath that enabled drugs to be selectively applied to different sites along the reflex pathways. 2. When applied in the recording chamber, an NK1 receptor antagonist, SR140333 (100 nM), reduced by 40-50% the amplitudes of inhibitory junction potentials (i.j.ps) evoked in the circular muscle by activation of descending reflex pathways. This effect was abolished when synaptic transmission in the stimulus region was blocked with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+, leaving only the component of the descending reflex pathway conducted via long anally directed collaterals of intrinsic sensory neurons. 3. SR140333 (100 nM) had no effect on descending reflex i.j.ps when applied to the stimulus region. Ascending reflexes were also unaffected by SR140333 in the stimulus region or between the stimulus and recording sites. 4. Septide (10 nM), an NK1 receptor agonist, enhanced descending reflexes by 30-60% when in the recording chamber. [Sar9,Met(O2)11]substance P had no effect at 10 nM, but potentiated distension-evoked reflexes at 100 nM. 5. A selective NK3 receptor antagonist, SR142801 (100 nM), when applied to the stimulus region, reduced the amplitude of descending reflex responses to compression by 40%, but had no effect on responses to distension. SR142801 (100 nM) had no effect when applied to other regions of the descending reflex pathways. 6. SR142801 (100 nM) only inhibited ascending reflexes when applied at the recording site. However, after nicotinic transmission in the stimulus region was blocked, SR142801 (100 nM) at this site reduced responses to compression. 7. Contractions of the circular muscle of isolated rings of ileum evoked by low concentrations of septide, but not [Sar9,Met(O2)11]substance P, were potentiated by tetrodotoxin (300 nM). 8. Contractile responses evoked by an NK3 receptor agonist, senktide, were non-competitively inhibited by SR142801. After excitatory neuromuscular transmission was blocked, senktide produced inhibitory responses that were also antagonised by SR142801, but to a lesser extent and in an apparently competitive manner. 9. These results indicate that tachykinins acting via NK1 receptors partly mediate transmission to inhibitory motor neurons. NK3 receptors play a role in transmission from intrinsic sensory neurons and from ascending interneurons to excitatory motor neurons during motility reflexes.

Correlation of electrophysiology, neurochemistry and axonal projections of guinea-pig sphincter of Oddi neurones

Sphincter of Oddi (SO) ganglia are comprised of two main types of neurones based either on their electrical or neurochemical properties. This study investigated whether any correlation exists between the electrical and neurochemical properties of these cells. SO neurones were characterized electrically as either Tonic or Phasic cells, labelled with neurobiotin, fixed, and processed for beta-nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-DA) staining and choline acetyltransferase immuno-reactivity to identify whether electrically characterized neurones were nitrergic or cholinergic. A total of 119 cells were analysed in this manner; 45% of cells were Tonic and 37% were Phasic. An equivalent number of Tonic (58.1%, 18/31) and Phasic cells (60%, 21/35) were choline acetyltransferase (ChAT) positive. Three of 34 Phasic cells were NADPH-DA positive, whereas 11/33 Tonic cells were NADPH-DA positive. In none of the preparations was ChAT immunoreactivity and NADPH-DA reactivity ever observed in the same neurone. Calretinin immunoreactivity was present in a subpopulation of both Tonic and Phasic neurones. No correlation was observed between the direction of axon projections and the electrophysiological or neurochemical properties of the cell. These results suggest that there is a lack of correlation between the electrical properties and the neurochemical content of SO neurones. Various explanations for these findings are discussed.

Correlation of electrophysiological and morphological characteristics of myenteric neurons of the duodenum in the guinea-pig

Intracellular recording, dye filling and immunohistochemistry were used to investigate neurons of the proximal duodenum of the guinea-pig. Recordings were made from neurons of the myenteric plexus in the presence of nicardipine to quell muscle contractions, using microelectrodes that contained the marker substance Neurobiotin. Preparations were subsequently processed histochemically to reveal nerve cell shapes and immunoreactivity for calbindin, calretinin or nitric oxide synthase. Neurons were distinguished by their shapes and axonal projections as Dogiel type II, Dogiel type I, filamentous descending interneurons and small filamentous neurons. Dogiel type II cells had large cell bodies and multiple axon processes. They each had a broad action potential (mean half-width, 2.9 ms) and a prominent inflection (hump) on the falling phase of the action potential. The majority (70%) of Dogiel type II cells were AH neurons, defined by their having a prolonged hyperpolarizing potential that followed a soma action potential and lasted more than 2 s. Fast excitatory postsynaptic potentials were not recorded from Dogiel type II neurons. Two thirds of Dogiel type II neurons fired phasically in response to intracellularly injected 500 ms depolarizing current pulses and one-third fired tonically. Calbindin immunoreactivity occurred in 70% of Dogiel type II neurons. Dogiel type I neurons had lamellar dendrites and a single axon. They had brief action potentials (mean half-width, 1.7 ms) with no, or a slight hump. They responded to fibre tract stimulation with fast excitatory postsynaptic potentials. Only 2/21 exhibited a prolonged hyperpolarization following action potentials. The majority of Dogiel type I neurons thus belong to the S neuron category. Nine Dogiel type I neurons fired phasically in response to 500 ms depolarizing current pulses, while 12 fired tonically. Filamentous descending interneurons had long, branching filamentous dendrites and a single anally-projecting axon which gave rise to varicose branches in myenteric ganglia. Action potential characteristics of filamentous interneurons ranged between those of Dogiel type II and type I neurons. Small neurons. Small neurons with short filamentous, or few simple dendrites were also characterized. They had single axons, which could be traced either locally to the circular muscle, or to the longitudinal muscle. None of 12 filamentous interneurons or of 10 small filamentous neurons exhibited a prolonged post-spike hyperpolarization, whereas fast excitatory postsynaptic potentials were recorded from a majority. It is concluded that the morphological types of neuron that are encountered in the ileum also occur in the duodenum, but the electrophysiological characteristics of the neurons are more variable for each morphological class. Thus, it is not always possible to predict the morphology of myenteric neurons in the duodenum from their electrophysiological properties. Part of the electrophysiological variability appears to be due to duodenal neurons being more excitable than ileal neurons.

Intracellular recording from myenteric neurons of the guinea-pig ileum that respond to stretch

1. Isolated longitudinal muscle-myenteric plexus preparations from guinea-pig ileum were used to investigate the activity of myenteric neurons when the tissue was stretched in the circumferential direction. Membrane potentials were recorded via flexibly mounted intracellular recording electrodes containing Neurobiotin in 1 M KCl. The preparations were stretched to constant widths (+20% and +40% beyond slack width). 2. Multipolar neurons (Dogiel type II morphology) discharged spontaneous action potentials and proximal process potentials during maintained stretching, three of twenty-one at +20% stretch and seven of nine at +40% stretch. At the maximum extent of stretch tried, +40% beyond slack tissue width, action potentials in Dogiel type II neurons occurred at 10-33 Hz. Neurons with other morphologies were all uniaxonal. Some displayed spontaneous fast EPSPs or action potentials, three of forty one at +20% stretch and seven of nineteen at +40% stretch. 3. In seven of eight Dogiel type II neurons, action potentials or proximal process potentials persisted when membrane hyperpolarization was imposed via the recording electrode. Action potential discharge was abolished by hyperpolarization in seven of nine uniaxonal neurons; the exceptions were two orally projecting neurons. 4. Dogiel type II and uniaxonal neurons were classified as rapidly accommodating if they discharged action potentials only at the beginning of a 500 ms intracellular depolarizing pulse and slowly accommodating if they discharged for more than 250 ms. For Dogiel type II neurons, three of thirteen were slowly accommodating at +20% stretch and two of four at 40% stretch. For uniaxonal neurons the corresponding data were twelve of twenty-six and fifteen of nineteen neurons. The slowly accommodating state was associated with increased cell input resistance in uniaxonal neurons. 5. The spontaneous action potential discharge in Dogiel type II and uniaxonal neurons ceased when the muscle was relaxed pharmacologically by nicardipine (3 microM) or isoprenaline (1 microM), although the applied stretch was maintained. At the same time, evoked spike discharge became rapidly accommodating. 6. We conclude that many Dogiel type II neurons, and possibly some orally projecting uniaxonal neurons, are intrinsic, stretch-sensitive, primary afferent neurons that respond to muscle tension with sustained action potential discharge.