Excitatory synaptic potentials due to activation of neurons with short projections in the myenteric plexus

The extraordinary partnership of Geoff Burnstock and Mollie Holman

Here, we recognise some of the extraordinary accomplishments of the partnership between Geoff Burnstock and Mollie Holman, and the everlasting impact they both made in autonomic neuroscience in Australia. Much of strength today in autonomic neuroscience can be traced back to a time when Geoff and Mollie commenced their seminal studies on autonomic neuroscience, initially at Oxford, then at The University of Melbourne in the mid 1960's. Mollie and Geoff published their first paper together, at Oxford, with their then mentor, and doyenne of smooth muscle, Professor Edith Bülbring. They did not always agree on the interpretation of their own scientific findings. Geoff was convinced early on that Adenosine triphosphate (ATP), or a related purine, was an excitatory neurotransmitter at peripheral sympathetic neuroeffector junctions. Mollie was reticent for decades. However, she began to take the notion seriously that ATP maybe a neurotransmitter, when receptors for purines were identified in the 1990's. What the partnership between Mollie and Geoff taught us in Australia was to not fear respectful criticism, but rather to be receptive to and embrace objective, collegial and constructive scientific peer-review. One of the many great legacies of Geoff and Mollie was the large number of researchers, who were fortunate disciples of their supervision, and who have now themselves gone on to make significant discoveries in autonomic and visceral neuroscience. This review summarizes some of their major legacies and represents a very personal historical perspective of the two authors, pupils respectively of Mollie and Geoff.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

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.

Distribution of neuronal nicotinic acetylcholine receptors containing different alpha-subunits in the submucosal plexus of the guinea-pig

The subunit composition and localisation of nicotinic acetylcholine receptors (nAChRs) in the submucosal plexus of the guinea-pig ileum were studied using both affinity-purified monoclonal and polyclonal antibodies against alpha3, alpha4, alpha5 and alpha7 nAChR subunits and specific alpha7-containing nAChRs blocker methyllycaconitine (MLA). By means of immunohistochemistry performed in non-dissociated preparations, it was found that only 4% of submucosal ganglia expressed nAChRs. Specific staining, associated with cell membranes, was found with alpha3-, alpha5- and alpha7-, but not alpha4-specific antibodies. Double staining using alpha5- and alpha7-specific antibodies demonstrated that about one-half of the nAChR-positive ganglia contained neurons immunoreactive to both antibodies, while the others possessed either alpha5- or alpha7-immunoreactivity. Nanomolar concentrations of MLA prevented alpha7-specific antibody binding and did not influence the alpha5-specific antibody binding even when applied in micromolar concentrations. In electrophysiological experiments performed using a patch-clamp 'whole-cell' recording method, the neurons were identified by their sensitivity to MLA. In conclusion, submucosal neurons of the guinea-pig ileum express nAChRs containing alpha3-, alpha5- and alpha7-subunits. The co localisation of alpha5- and alpha7-subunits found in immunohistochemical experiments as well as kinetic characteristics of MLA-blocked receptors found by electrophysiological experiments allow us to suggest the presence of heteromeric alpha7-containing nAChRs in the submucosal plexus of the guinea-pig ileum.

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.

Slow excitatory metabotropic signal transmission in the enteric nervous system

Metabotropic mechanisms of excitatory signalling in enteric neurones underlie both slow synaptic transmission and paracrine transmission from enteric non-neuronal cells. The type of neurone in which signalling occurs determines the characteristics of synaptic- and paracrine-mediated slow excitatory responses. Slow excitatory responses in neurones with AH-type electrophysiological behaviour and multipolar Dogiel type II morphology are characterized by membrane depolarization associated with closure of Ca2+ -gated K+ channels that is reflected by increased neuronal input resistance. Slow excitatory responses in neurones with S-type electrophysiological behaviour and uniaxonal morphology are characterized by membrane depolarization associated with opening of cationic channels and decreased neuronal input resistance. Postreceptor signalling that involves activation of adenylate cyclase, stimulation of cAMP formation and activation protein kinase A generates excitatory responses characterized by increased neuronal input resistance in AH neurones. Postreceptor signalling that involves activation of phospholipase C, release of IP3 and diacylglycerol and activation of protein kinase C and calmodulin kinases generates excitatory responses characterized by decreased neuronal input resistance in S neurones. Slow excitatory responses that are characterized by increased neuronal input resistance are a property of AH-type neurones that function as interneurones in the neural networks of the ENS. Slow excitatory responses that are characterized by decreased neuronal input resistance are a property of S-type neurones that function either as interneurones or as musculomotor and secretomotor neurones in the neural networks of the ENS.

Slow excitatory synaptic potentials evoked by distension in myenteric descending interneurones of guinea-pig ileum

The functional significance of the slow excitatory synaptic potentials (EPSPs) in myenteric neurones is unknown. We investigated this using intracellular recording from myenteric neurones in guinea-pig ileum, in vitro. In all, 121 neurones responded with fast EPSPs to distension of the intestine oral to the recording site. In 28 of these neurones, distension also evoked depolarizations similar to the slow EPSPs evoked by electrical stimulation in the same neurones. Intracellular injection of biocytin and immunohistochemistry revealed that neurones responding to distension with slow EPSPs were descending interneurones, which were immunoreactive for nitric oxide synthase (NOS). Other neurones, including inhibitory motor neurones and interneurones lacking NOS, did not respond to distension with slow EPSPs, but many had slow EPSPs evoked electrically. Slow EPSPs evoked electrically or by distension in NOS-immunoreactive descending interneurones were resistant to blockade of NK(1) or NK(3) tachykinin receptors (SR 140333, 100 nM; SR 142801, 100 nM, respectively) and group I metabotropic glutamate receptors (PHCCC, 10-30 microM), when the antagonists were applied in the recording chamber of a two-chambered organ bath. However, slow EPSPs evoked electrically in inhibitory motor neurones were substantially depressed by SR 140333 (100 nM). Blockade of synaptic transmission in the stimulation chamber of the organ bath abolished slow EPSPs evoked by distension, indicating that they arose from activity in interneurones, and not from anally directed, intrinsic sensory neurones. Thus, distension evokes slow EPSPs in a subset of myenteric neurones, which may be important for intestinal motility.

Comparison of the effects of neurokinin-3 receptor blockade on two forms of slow synaptic transmission in myenteric AH neurons

AH neurons are intrinsic sensory neurons of the intestine that exhibit two types of slow synaptic event: slow excitatory postsynaptic potentials which increase their excitability for about 2-4 min, and sustained slow postsynaptic excitation which can persist for several hours, and may be involved in long-term changes in the sensitivity of the intestine to sensory stimuli. The effects of the neurokinin-3 tachykinin receptor antagonist, SR142801, on these two types of synaptic event in AH neurons of the myenteric ganglia of guinea-pig small intestine were compared. Slow excitatory postsynaptic potentials were evoked by stimulation of synaptic inputs at 10-20 Hz for 1s, and sustained slow postsynaptic excitation was evoked by stimulation of inputs at 1Hz for 4 min. SR142801 (1microM) reduced the amplitude of the slow excitatory postsynaptic potential to 26% of control, and also reduced the increase in input resistance and the extent of anode break excitation associated with the slow excitatory postsynaptic potential. In contrast, SR142801 did not reduce the increase in excitability, the increase in input resistance or the depolarisation that occur during the sustained slow postsynaptic excitation. SR142801 did not change the resting membrane potential or the resting input resistance. We conclude that tachykinins, acting through neurokinin-3 receptors, are involved in the generation of the slow excitatory postsynaptic potential, but not in the sustained slow postsynaptic excitation, and that the release of transmitters from synaptic inputs to AH neurons is frequency coded.

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.

Purinergic and cholinergic neuro-neuronal transmission underlying reflexes activated by mucosal stimulation in the isolated guinea-pig ileum

1. We present evidence that adenosine triphosphate (ATP) plays a major role in excitatory neuro-neuronal transmission in ascending and descending reflex pathways to the longitudinal (LM) and circular muscle (CM). 2. A partitioned bath was used for the pharmacological isolation of a segment of guinea-pig ileum ( approximately 6 cm in length), allowing drugs to be selectively applied to an intermediate region between the region where mucosal stimulation was applied and that where mechanical recordings were made. 3. Brush stroking the mucosa (3 strokes) elicited a synchronous contraction of the LM and CM both above (ascending excitation) and below (descending excitation) the site of stimulation. All reflexes were abolished when tetrodotoxin (1 microM) was applied to the intermediate chamber. 4. Hexamethonium (300 microM) added to the intermediate chamber abolished the ascending contraction in 15 % of oral preparations (from 26 preparations, 18 animals) and the descending contraction in 13% of anal preparations studied (from 53 preparations, 48 animals). In the remaining 85% of oral preparations, hexamethonium usually attenuated the oral contraction of the LM and CM. However, in the remaining 87% of anal preparations, hexamethonium had no effect on the anal contraction of the LM and CM. 5. Oral and anal reflexes that were hexamethonium resistant were either abolished or attenuated by the further addition of the P2 purinergic receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) or alpha,beta-methylene ATP (50-100 microM) to the intermediate chamber. 6. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP, 20 microM) or alpha,beta-methylene ATP (50-100 microM) stimulated both ascending and descending excitatory pathways, when applied to the intermediate chamber. 7. In conclusion, ascending and descending neuro-neuronal transmission in excitatory nervous pathways to the LM and CM is complex and clearly involves neurotransmitter(s) other than acetylcholine (ACh). We suggest mucosal stimulation releases ACh and ATP in both ascending and descending excitatory reflex pathways that synapse with excitatory motoneurons to the LM and CM.

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.

Review article: roles played by 5-hydroxytryptamine in the physiology of the bowel

The application of 5-HT to the gut elicits a wide variety of effects because of the expression and wide distribution in the bowel of many subtypes of 5-HT. There is, however, no reason to believe that all of these receptors are stimulated by endogenous 5-HT. 5-HT has been found to be the neurotransmitter of a subset of myenteric interneurons, which evoke a slow excitatory postsynaptic response mediated by 5-HT1P receptors. The major enteric depot of 5-HT is found in mucosal enterochromaffin cells, which are sensory transducers that utilize 5-HT to activate both intrinsic (via 5-HT1P and 5-HT4 receptors) and extrinsic (via 5-HT3 receptors) primary afferent nerves. Mucosal 5-HT is inactivated by uptake into epithelial cells mediated by the same 5-HT transporter utilized by serotonergic neurons. Antagonism of 5-HT3 receptors by compounds such as alosetron should be useful in treating functional bowel disease because they can inhibit excitation of extrinsic sensory nerves by 5-HT without interfering with intrinsic enteric reflexes.

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.

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

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

Nitric oxide modulates cholinergic reflex pathways to the longitudinal and circular muscle in the isolated guinea-pig distal colon

1. The involvement of nitric oxide (NO) in enteric neural pathways underlying reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers activated by mucosal stimulation was examined in the isolated guinea-pig distal colon. 2. A segment of colon spanned two partitions (10 mm apart), which divided the organ bath into three chambers: a recording chamber where LM and CM tension was measured; a stimulation chamber where mucosal stimulation was applied; and a middle chamber separating them. 3. Brushing the mucosa anal and oral to the recording site evoked simultaneous oral contraction and anal relaxation of both the LM and CM. 4. N omega-nitro-L-argininel-NA; 100 microM) or N omega-nitro-L-arginine methyl ester (L-NAME; 100 microM) applied to the middle chamber or stimulation chamber decreased the oral contractile response of the LM and CM (by about 30-40 %), but increased the anal relaxation (> 600 %) and exposed an anal contraction (> 1000 % increase) of both muscles. The addition of L-NA to the recording chamber reduced the anal relaxation of the LM and CM and the anal contraction of the LM, but slightly increased the anal contraction of the CM. 5. S-Nitroso-N-acetylpenicillamine (SNAP; 10 microM), an NO donor, reversed the effects of L-NA in the middle or stimulation chambers. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), a soluble guanylate cyclase inhibitor, mimicked the effects of L-NAin the middle chamber or stimulation chamber, but these effects were not reversed by SNAP. 7. The oral contractile responses, and the anal relaxation and contractile responses of the LM and CM produced by L-NA in the stimulation or middle chambers, were blocked by hexamethonium (300 microM) in any chamber. Atropine (1 microM) in the recording chamber reduced the contractile responses of the LM and CM. 8. In conclusion, endogenous NO facilitates and depresses release of acetylcholine from interneurons in ascending and descending nervous pathways, respectively. These NO effects are mediated through soluble guanylate cyclase in cholinergic interneurons

Cellular pathways mediating tachykinin-evoked secretomotor responses in guinea pig ileum

This study characterized tachykinin-evoked secretomotor responses in in vitro submucosal and mucosal-submucosal preparations of the guinea pig ileum using combined intracellular and Ussing chamber recording techniques. Superfusion of endogenous tachykinins substance P (SP), neurokinin A (NKA), and neurokinin B depolarized single submucosal neurons and evoked increased short-circuit current (Isc) responses in Ussing chamber preparations. The NK1-receptor agonist [Sar9,Met(O2)11]SP [50% effective concentration (EC50) = 2 nM] depolarized all submucosal neurons examined. The NK3-receptor agonist senktide (EC50 = 20 nM) depolarized approximately 50% of neurons examined, whereas the NK2-receptor agonist [Ala5,beta-Ala8]NKA-(4-10) had no effect on membrane potential. [Sar9,Met(O2)11]SP and senktide evoked similar increases in Isc that were tetrodotoxin sensitive (91 and 100%, respectively) and were selectively blocked by the NK1 antagonist CP-99,994 and the NK3 antagonist SR-142,801, respectively. Capsaicin-evoked increases in Isc were significantly inhibited (54%, P < 0.05) by CP-99,994 but not by SR-142,801. Neither antagonist inhibited slow excitatory postsynaptic potentials. These findings suggest that tachykinin-evoked secretion in guinea pig ileum is mediated by NK1 and NK3 receptors on submucosal secretomotor neurons and that capsaicin-sensitive nerves release tachykinin(s) that activate the NK1 receptors.

Neuron number in the myenteric plexus of the ascending colon of rats. A comparative study using two staining techniques

We carried out this study with the purpose of comparing the neuronal density in antimesocolic and intermediate regions of the colon of rats. We used the ascending colon of ten seven-months old Wistar rats. With the Giemsa method we found 29,046 neurons/cm2 on the antimesocolic region and 30,968 neurons/cm2 on the intermediate regions. With the NADH-diaphorase technique 12,308 neurons/cm2 on the antimesocolic regions and 8798 neurons/cm2 on the intermediate regions were evidenced. The number of NADH-diaphorase positive neurons is significantly less than the number of Giemsa-stained neurons and that this difference is enhanced on the intermediate regions of the intestinal circumference. Therefore, to compare the number of neurons of an intestinal segment of a same species at the same age, it is necessary to take into consideration the technique employed and the region of the intestinal circumference from where the sample was obtained.

Mediation by protein kinases C and A of Go-linked slow responses of enteric neurons to 5-HT

5-HT activates the peristaltic reflex and is the neurotransmitter of a subset of myenteric interneurons. Hyperpolarizing afterpotential (AH)/type 2 neurons respond to 5-HT with a long-lived depolarization that is caused by the inhibition of a Ca(2+)-activated K+ conductance (gKCa). This effect is mediated by a G-protein-coupled receptor, 5-HT1P. 5-HT1P agonists specifically activate G alpha o, the immunoreactivity of which was found to be highly abundant and membrane-associated in almost all enteric neurons. Responses of hyperpolarizing AH/type 2 neurons to 5-HT were inhibited by intracellular injection of GDP beta S or anti-G alpha o Fab fragments but were potentiated and prolonged by intracellular GTP gamma S. Responses to 5-HT were antagonized by pertussis toxin, downregulation of protein kinase C (PKC) and inhibitors of phosphatidylcholine phospholipase C (PC-PLC), PKC (including pseudosubstrate peptides, chelerythrine, and the alpha/beta isoform-specific inhibitor Go 6976), protein kinase A (PKA), and adenylate cyclase. Responses to 5-HT were mimicked by activators of PKC, and 5-HT induced a concentration-dependent increase in the membrane-associated PKC activity in isolated myenteric ganglia. Immunocytochemical studies suggested that the most abundant isoforms of PKC in enteric neurons are alpha and delta. These data suggest that signal transduction of the 5-HT1P-mediated slow response to 5-HT involves activation of PC-PLC by G alpha o to liberate diacylglycerol, which stimulates PKC (most likely alpha). PKC probably activates adenylate cyclase, which through cAMP, activates PKA. Activation of both PKA and PKC lead to closure of gKCa.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Electrophysiological mapping of fast excitatory synaptic inputs to morphologically and chemically characterized myenteric neurons of guinea-pig small intestine

Neurons within the myenteric plexus of the guinea-pig ileum were impaled using conventional intracellular electrodes. Points of stimulation within the surrounding ganglia and connectives which gave rise to fast excitatory synaptic potentials were mapped using a movable monopolar stimulating electrode. Cells were then injected with the intracellular marker, biocytin, and processed for multiple label immunohistochemistry to reveal their morphologies, chemical contents and, hence, their functional classes. Of 65 neurons belonging to the S electrophysiological class, 53 received fast excitatory synaptic inputs from stimulation at sites at least 2 mm away in a directly circumferential direction. These inputs almost certainly arise from stimulation of the circumferentially-directed axons of the Dogiel type II/AH-neurons, which are thought to be intrinsic sensory neurons. The majority of cells which projected anally and were immunoreactive for nitric oxide synthase (19/25), all neurons which ramified in the tertiary plexus and were identified as longitudinal muscle motor neurons (6/6) and all neurons identified as excitatory motor neurons innervating the circular muscle (12/12) received inputs from these circumferentially-directed pathways. However only one of six descending filamentous interneurons impaled received such inputs, suggesting they may be differentially innervated. The conduction velocities of circumferentially-directed axons giving rise to fast excitatory post synaptic potentials were estimated to be 0.41 +/- 0.10 m/s (mean +/- standard deviation, n = 21). The conduction velocities estimated for longitudinally-directed pathways were 0.55 +/- 0.25 m/s (n = 29). Thus, the majority of myenteric neurons receive fast excitatory synaptic input from putative intrinsic sensory neurons which project circumferentially around the intestine.

Cellular sites of expression of the neurokinin-1 receptor in the rat gastrointestinal tract

In the digestive system, substance P is an excitatory transmitter to muscle, a putative excitatory neuro-neuronal transmitter, a vasodilator, and a mediator in inflammatory processes. Many of the biological effects of substance P are mediated by a high-affinity interaction with the tachykinin receptor neurokinin-1. The aim of the present study was to identify the sites of expression of this receptor in the rat stomach and intestine by immunohistochemistry with a polyclonal antiserum raised to the intracellular C-terminal portion of the rat neurokinin-1 receptor. Neurokinin-1 receptor immunoreactivity is present in a large population of enteric neurons. The relative density of these neurons along the gut is colon > ileum >> stomach. In the intestine, stained neurons have a smooth cell body with processes that can be followed within and between plexuses, and make close approaches to other neuronal cells, but do not appear to project outside the plexuses, suggesting that they are interneurons. In the stomach, neurokinin-1 receptor-immunoreactive neurons are infrequent and have a poorly defined and irregular shape. Neurokinin-1 receptor immunoreactivity is also localized to numerous non-neuronal cells in the inner portion of the circular muscle layer of the small intestine, which have the appearance of small dark smooth muscle cells or interstitial cells of Cajal. These cells are postulated to form a "stretch-sensitive" system with the deep muscular plexus and thus constitute an important site of regulation of muscle activity. Double labeling immunofluorescence was used to simultaneously localize neurokinin-1 receptor and substance P/tachykinin immunoreactivities. These experiments demonstrate that in the enteric plexuses, substance P/tachykinin-immunoreactive varicose fibers encircle the cell bodies of most neurokinin-1 receptor-containing neurons, and in the inner portion of the circular muscle layer of the small intestine they lie close to neurokinin-1 receptor-immunoreactive non-neuronal cells. In addition, some enteric neurons express both neurokinin-1 receptor and substance P/tachykinin immunoreactivities. The present study provides strong evidence that the neurokinin-1 receptor is the tachykinin receptor mediating the actions of substance P on enteric neurons and smooth muscle.

Ultrastructural studies of the myenteric plexus and smooth muscle in organotypic cultures of the guinea-pig small intestine

External muscle and myenteric plexus from the small intestine of adult guinea-pigs were maintained in vitro for 3 or 6 days. Myenteric neurons and smooth muscle cells from such organotypic cultures were examined at the electron-microscopic level. An intact basal lamina was found around the myenteric ganglia and internodal strands. Neuronal membranes, nuclei and subcellular organelles appeared to be well preserved in cultured tissues and ribosomes were abundant. Dogiel type-II neurons were distinguishable by their elongated electron-dense mitochondria, numerous lysosomes and high densities of ribosomes. Vesiculated nerve profiles contained combinations of differently shaped vesicles. Synaptic membrane specializations were found between vesiculated nerve profiles and nerve processes and cell bodies. The majority of nerve fibres were well preserved in the myenteric ganglia, in internodal strands and in bundles running between circular muscle cells. No detectable changes were found in the ultrastructure of the somata and processes of glial cells. Longitudinal and circular muscle cells from cultured tissue had clearly defined membranes with some close associations with neighbouring muscle cells. Caveolae occurred in rows that ran parallel to the long axis of the muscle cells. These results indicate that the ultrastructural features of enteric neurons and smooth muscle of the guinea-pig small intestine are well preserved in organotypic culture.

Ultrastructural examination of the targets of serotonin-immunoreactive descending interneurons in the guinea pig small intestine

Serotonin neurons are descending interneurons in the myenteric plexus of the guinea pig small intestine. Preembedding single- and double-label immunocytochemistries at the ultrastructural level were used to identify the targets of these serotonin interneurons. Serial ultrathin sections were taken through a myenteric ganglion that had been processed for serotonin immunocytochemistry. The ganglion contained the cell bodies of 69 neurons, including 2 serotonin neurons and 6 neurons with the ultrastructural features of Dogiel type II cells. For each cell body in the ganglion, the number of serotonin inputs (synapses and close contacts) was determined. About 59% of the cell bodies did not receive any serotonin inputs. The most abundant serotonin terminals were related to two targets: other serotonin descending interneurons and a population of neurons with Dogiel type I morphology, but whose neurochemistry and function is unknown. The serotonin inputs to the serotonin cell bodies were located predominantly on the lamellar dendrites. Each of the Dogiel type II neurons received 3 or fewer serotonin inputs, and none of the serotonin inputs to Dogiel type II neurons formed a synapse. Overall, about 40% of the serotonin inputs formed synapses. The serotonin inputs to neurons that received many serotonin inputs were more likely to show synaptic specializations than serotonin inputs to neurons that received few serotonin inputs. Inhibitory motor neurons contain nitric oxide synthase (NOS). At the light microscope level, serotonin nerve fibers do not form dense pericellular baskets around NOS cell bodies. To determine whether there are serotonin inputs to NOS neurons, serial ultrathin sections were taken through a myenteric ganglion that had been processed for preembedding double-label immunocytochemistry, in which the NOS neurons were labeled with peroxidase-diaminobenzidine and the serotonin neurons with silver-intensified 1 nm gold. Only 1 out of 9 NOS cells examined in serial section received more than 5 serotonin inputs. The results suggest that, in the guinea pig small intestine, the serotonin descending interneurons are not an essential element of the descending inhibitory reflex.

Effects of 5-HT1A and 5-HT4 receptor agonists on slow synaptic potentials in enteric neurons

Intracellular electrophysiological methods were used to examine the effects of 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT), 4-amino-5-chloro-2-methoxy-N-(4-[1-azabicyclo[3,3,1]nonyl]) benzamide hydrochloride (renzapride), cis-4-amino-5-chloro-N[1-[3- (4-fluorophenoxy)propyl]-3-methoxy-4-piperidinyl[-2-methoxybenzamide monohydrate (cisapride) and endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-3- (1-methyl)ethyl-2-oxo-1 H-benzimidazole-1-carboxamidehydrochloride (BIMU 8) on noncholineric slow excitatory postsynaptic potentials (slow EPSPs) in myenteric afterhyperpolarization (AH) neurons of guinea pig ileum. 5-HT (0.01-1 microM) and 5-CT (0.001-0.1 microM) produced a concentration-dependent inhibition of slow EPSPs. The 5-HT1A receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimidobutyl]piperazine (NAN-190) produced rightward shifts in 5-HT and 5-CT concentration-response curves; facilitation of slow EPSPs was never observed. 5-MeOT caused a depolarization and inhibited spike afterhyperpolarizations in a concentration-dependent manner but this effect was not blocked by the 5-HT3/5-HT4 receptor antagonist, tropisetron (1 microM). Renzapride (0.01-0.3 microM), cisapride (0.01-1.0 microM) and BIMU 8 (0.01-1.0 microM) did not change the membrane potential of any neuron tested. Renzapride and BIMU 8 did not change the amplitude of slow EPSPs. In 13 of 19 neurons cisapride did not change the amplitude of slow EPSPs; in 6 neurons cisapride (1 microM) reversibly inhibited the slow EPSP. Responses to substance P which mimicked the slow EPSP were not affected by cisapride.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Neurokinin A mimics the slow excitatory postsynaptic current in submucous plexus neurons of the guinea-pig caecum

Single microelectrode voltage-clamp recordings were made from submucous neurons of the guinea-pig caecum. The slow excitatory postsynaptic current was compared with the currents induced by neurokinin A and substance P. The current induced by neurokinin A (100-300 nM) was associated with a decreased membrane conductance and reversed in polarity between -90 and -100 mV. The neurokinin A current was reduced by Co2+ (1-2 mM), but was not affected by Cs+ (1-2 mM), Ba2+ (10-100 microM) or low Cl- (20-40 mM) solutions. In about 80% of the neurons, the current induced by substance P (100-300 nM) was associated with a decreased membrane conductance and did not reverse with hyperpolarization of the membrane potential up to -130 mV. The current was reduced by Co2+ (1-2 mM) and augmented by low Cl- (20-40 mM) solutions, but was not affected by Cs+ (1-2 mM) or Ba2+ (10-100 microM)-containing solutions. In about 20% of the neurons, the substance P current reversed in polarity between -100 and -120 mV. The slow excitatory postsynaptic current elicited by repetitive nerve stimulation (10-40 Hz, three to five pulses) was accompanied by a decreased membrane conductance, and reversed in polarity between -90 and -100 mV. The slow excitatory postsynaptic current was abolished by Co2+ (1-2 mM) or low Na+ (12 mM) solutions, but was not affected by Cs+ (1-2 mM), Ba2+ (10-100 microM) or low Cl- (20-40 mM) solutions. In such neurons, the neurokinin A current was reversed at approximately the same potential at which the slow excitatory postsynaptic current was reversed, while the substance P current was not reversed even by much stronger hyperpolarizations. It was concluded that the neurokinin A current was mainly due to depression of potassium conductances, while the substance P current resulted from both increased anion conductance and decreased potassium conductances. The conductance change underlying the slow excitatory postsynaptic current is similar to that caused by neurokinin A.

Origins of synaptic inputs to calretinin immunoreactive neurons in the guinea-pig small intestine

Calretinin immunoreactivity is almost completely confined to two classes of neuron in the myenteric plexus of the guinea-pig small intestine, longitudinal muscle motor neurons and ascending interneurons. Nerve cell bodies of the two classes can be readily identified by their sizes and positions in ganglia. The motor neurons, which are small Dogiel type I neurons, are about 20% and the interneurons, which are medium-sized Dogiel type I neurons, are about 5% of myenteric neurons. In the present work, we have also discovered a minor population (0.1%) of small filamentous neurons. In unoperated regions of intestine, at the light microscopic level, numerous calretinin immunoreactive nerve fibres were found in the tertiary plexus that innervates the longitudinal muscle and a medium density of varicose fibres formed pericellular endings in the myenteric ganglia. After double myotomy operations, in areas of plexus 0.5 to 1.5 mm wide which were isolated from ascending and descending inputs, calretinin-immunoreactive fibres of the tertiary plexus were unchanged, but the pericellular endings in the ganglia disappeared. Both the ascending interneurons and the longitudinal muscle motor neurons received ultrastructurally identified synapses and close axonal contacts that were calretinin-immunoreactive. These were counted in semi-serial sections from normal intestine and from regions between myotomy operations. In unoperated intestine, the proportions of calretinin-immunoreactive synapses on small, calretinin-immunoreactive, Dogiel type I nerve cells and small filamentous nerve cells were 30% and 0.1% respectively and on medium-sized Dogiel type I cells the proportion was 28%. Electron microscopy revealed an almost complete loss of immunoreactive inputs to the small Dogiel type I cells between double myotomies, but the number of unreactive inputs was the same as in normal intestine. This work demonstrates that the ascending calretinin-immunoreactive interneurons connect with one another to form ascending chains in the myenteric plexus and that they also provide about 1/3 of the inputs received by calretinin-immunoreactive longitudinal muscle motor neurons. Many of the remaining inputs to these motor neurons are local; we have deduced that these are mainly from primary sensory neurons.

Presynaptic inhibition by adenosine A1 receptors on guinea pig small intestinal myenteric neurons

BACKGROUND

Adenosine acts at A1 receptors to inhibit the release of most neurotransmitters. This study tested the hypothesis that both exogenous adenosine (ADO) and tonic release of endogenous ADO act at presynaptic A1 receptors to suppress excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in myenteric neurons.

METHODS

Intracellular microelectrodes were used to study actions of ADO, the agonists 2-chloro-N6-cyclopentyl ADO, its 1-deaza derivative, 5'-N-ethylcarboxamido ADO, and CGS 21680 or the antagonists 8-cyclopentyl-1,3-dimethylxanthine, its 1,3-dipropyl analog, and 1,3-dipropyl-8-p-sulfophenylxanthine on synaptic behavior in myenteric neurons.

RESULTS

Each of the agonists suppressed slow EPSPs in all 35 AH/type 2, 8 of 10 S/type 1, and 7 of 7 nonspiking neurons. ADO also decreased neuronal excitability (n = 63) in AH/type 2 neurons. Agonists suppressed fast nicotinic EPSPs in all 20 S/type 1, 10 nonspiking, and 3 AH/type 2 neurons without having any effect on postsynaptic responses to nicotinic agonists. CCPA was more potent than CGS 21680 in suppressing EPSPs. In 30% of neurons, the only action of antagonists was to block the effect of A1 or A2 agonists on EPSPs. Agonists did not inhibit IPSPs, but unmasked robust slow IPSPs by preventing slow EPSPs. Antagonists acted alone to enhance EPSPs in 70% of neurons.

CONCLUSIONS

(1) ADO acts at presynaptic A1 sites to suppress EPSPs in all neurons, (2) IPSPs are revealed by ADO, and (3) ongoing release of endogenous ADO inhibits synaptic transmission.

Ramifications of the axons of AH-neurons injected with the intracellular marker biocytin in the myenteric plexus of the guinea pig small intestine

The projections and terminal ramifications of electrophysiologically characterized myenteric neurons of the guinea pig small intestine were studied after intracellular injection of the marker substance biocytin. Myenteric neurons were impaled with microelectrodes containing 4% biocytin in 2 M KCl (pH 7.4) and characterized electrophysiologically as either AH-neurons or S-neurons. AH-neurons were neurons in which action potentials were followed by prolonged after-hyperpolarizations (lasting greater than 4 seconds). S-neurons were neurons in which such hyperpolarizations were not seen. Electrical stimulation of internodal strands evoked prominent fast excitatory synaptic potentials in S-neurons, but not in AH-neurons. Biocytin was injected electrophoretically into the impaled AH-neurons by passage of hyperpolarizing current (0.6-0.8 nA for 5-15 minutes) through the recording electrode. The preparation was then fixed in Zamboni's fixative, dehydrated, and exposed to avidin coupled to horseradish peroxidase which allowed the injected biocytin to be visualised via a diaminobenzidine reaction. In many cases, the injected biocytin appeared to fill all the processes of injected AH-neurons that ramified within the myenteric plexus. The filled processes included axons running up to 4 mm within the plexus and profuse varicose terminals ramifying within both the ganglion containing the injected cell body and nearby ganglia. Most (90%) cell bodies of the injected AH-neurons had the morphology of Dogiel type II neurons; large, mostly smooth cell bodies with few short processes and several long processes. The other 10% of the AH-neurons had similar cell bodies and long processes but also had prominent short filamentous processes. This population was termed dendritic AH-neurons. The projections and terminals of 28 AH/Dogiel type II neurons and 7 dendritic AH-neurons were analysed in detail. Both types of neurons project circumferentially to provide terminals to nearby ganglia, but the AH/Dogiel type II neurons also provide terminals to their own ganglia while the dendritic AH-neurons typically do not. Although many of the injected AH-neurons had projections orally or anally along the intestine no evidence for a preferential direction of projection was obtained. Analysis of the areas and distributions of the terminal fields of the AH/Dogiel type II neurons suggests that each may contact several other myenteric neurons and that each myenteric neuron may receive input from about ten AH/Dogiel type II neurons.

The mechanism of action of peppermint oil on gastrointestinal smooth muscle. An analysis using patch clamp electrophysiology and isolated tissue pharmacology in rabbit and guinea pig

An investigation of the mechanism of peppermint oil action was performed using isolated pharmacological preparations from guinea pig large intestine and patch clamp electrophysiology techniques on rabbit jejunum. Peppermint oil relaxed carbachol-contracted guinea pig taenia coli (IC50, 22.1 micrograms/mL) and inhibited spontaneous activity in the guinea pig colon (IC50, 25.9 micrograms/mL) and rabbit jejunum (IC50, 15.2 micrograms/mL). Peppermint oil markedly attenuated contractile responses in the guinea pig taenia coli to acetylcholine, histamine, 5-hydroxytryptamine, and substance P. Peppermint oil reduced contractions evoked by potassium depolarization and calcium contractions evoked in depolarizing Krebs solutions in taenia coli. Potential-dependent calcium currents recorded using the whole cell clamp configuration in rabbit jejunum smooth muscle cells were inhibited by peppermint oil in a concentration-dependent manner. Peppermint oil both reduced peak current amplitude and increased the rate of current decay. The effect of peppermint oil resembled that of the dihydropyridine calcium antagonists. It is concluded that peppermint oil relaxes gastrointestinal smooth muscle by reducing calcium influx.

Interactions between reflexes evoked by distension and mucosal stimulation: electrophysiological studies of guinea-pig ileum

Intracellular recording methods were used to examine stereotyped reflexes evoked in the circular muscle of guinea-pig small intestine by distension or repetitive deformation of the mucosal villi, in vitro. Both stimuli evoked compound excitatory junction potentials (EJPs) on the oral side of the site of stimulation and compound inhibitory junction potentials (IJPs) on the anal side. Stimulation of the mucosa by application of 10 microliters of 0.5 M HCl evoked similar reflex responses in the circular muscle. The compound EJPs evoked by mucosal stimulation were depressed, but not abolished, by 1 microM hyoscine, indicating that these responses were partially mediated by release of acetylcholine, as are the equivalent responses evoked by distension. The compound EJPs and the compound IJPs evoked by maintained distension or by repeated mechanical stimulation of the mucosa were transient, lasting in most cases for 3-5 s before the membrane potential returned to resting level. This decline (rundown) occurred in part of the circuit close to the site of stimulation as stimuli applied elsewhere during the period of rundown evoked normal EJPs and IJPs. Mechanical stimuli (brush strokes that deformed the mucosal villi) applied to the mucosa at the site of a maintained distension evoked responses of normal amplitude during the period when the response to the distension had declined to zero. In contrast, during the period when the responses to repetitive mechanical stimulation of the mucosa had disappeared, the reflex responses evoked by distension at the same site were substantially augmented. Chemical stimulation of the mucosa with acid also enhanced the responses to distension.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin: its role and receptors in enteric neurotransmission

Enteric neural 5-HT receptors were analyzed and related to possible physiological actions of 5-HT. Receptors were identified electrophysiologically with intracellular microelectrodes and by studies of the binding of radioligands. Radioligand binding was assessed by rapid filtration and by radioautography. Three subtypes of 5-HT receptor, 5-HT1P, 5-HT3, and 5-HT1A, were identified. 5-HT1P receptors were found to mediate slow depolarizations of myenteric neurons that were associated with a decrease in membrane conductance. These responses were inhibited by 5-HTP-DP and by BRL 24924 and mimicked by 5- and 6-hydroxyindalpine. 5-HT1P receptors were labeled with high affinity by 3H-5-HT and were located on both submucosal and myenteric neurons and on processes of intrinsic neurons in the lamina propria. Serotonergic EPSPs were found to be mediated by 5-HT1P receptors; it is postulated that 5-HT1P receptors may be involved in initiation of the peristaltic reflex and in the regulation of gastic emptying. 5-HT3 receptors have been shown to be responsible for fast depolarizations of myenteric and submucosal neurons associated with a rise in membrane conductance. These responses are antagonized by ICS 205-930 and mimicked by 2-methyl-5-HT. 5-HT1A receptors have been reported by others to mediate hyperpolarizing responses of myenteric neurons associated with a rise in membrane conductance. Hyperpolarizing responses are also elicited by the 5-HT1A agonist, 8-OH-DPAT. No physiological role has yet been identified for 5-HT3 or 5-HT1A receptors in the ENS.

Substance P mediates synaptic transmission between rat myenteric neurones in cell culture

1. Whole-cell patch-clamp recordings were made from pairs of neurones in cell cultures of rat myenteric neurones. In some pairs, action potentials evoked in the first neurone evoked a slow excitatory postsynaptic potential (EPSP) in the second neurone. 2. Action potentials at a frequency of at least 5 Hz were required to evoked slow EPSPs. In one group of cells, the slow EPSP followed a series of nicotinic fast EPSPs; in another group, fast EPSPs did not precede the slow EPSP. 3. The slow EPSPs were 2-16 mV in amplitude and were accompanied by decreased resting potassium conductance. 4. Most (17/28) neurones in which action potentials evoked only slow EPSPs in a follower cell contained substance P (SP)-like immunoreactivity; they were not immunoreactive for 5-hydroxytryptamine (0/15) or vasoactive intestinal peptide (0/22). 5. Postsynaptic responses to SP, neurokinin A and a synthetic tachykinin [( pGlu6, Pro9]SP6-11) mimicked the slow EPSPs. The non-tachykinin peptide vasoactive intestinal polypeptide (VIP), which was not found in neurones that evoked only slow EPSPs, also mimicked the slow EPSPs. Responsiveness to SP decreased significantly during slow EPSPs. 6. Desensitization to either SP or VIP reduced or prevented the slow EPSPs and also responses to each other. Two proposed antagonists of SP receptors, [D-Arg1, D-Pro2,D-Trp7,9,Leu11]substance P and [D-Arg1,D-Trp7,9,Leu11]substance P, did not affect the slow EPSPs significantly. 7. Antisera against SP reversibly blocked or reduced slow EPSPs evoked by eight of thirteen presynaptic neurones that evoked slow EPSPs without evoking fast EPSPs. All eight of the presynaptic neurones that evoked anti-SP-sensitive slow EPSPs contained SP-like immunoreactivity. None of the presynaptic neurones that evoked anti-SP-insensitive slow EPSPs contained detectable SP-like immunoreactivity. Normal sera and anti-VIP antisera did not alter the slow EPSPs detectably. 8. It is concluded that subsets of myenteric neurones release an SP-like transmitter to evoke slow EPSPs. These neurones appear to lack a 'classical' neurotransmitter that evokes fast EPSPs.

Immunocytochemical analysis of potential neurotransmitters present in the myenteric plexus and muscular layers of the corpus of the guinea pig stomach

Recent electrophysiological studies of neurons of the myenteric plexus of the corpus of the guinea pig stomach have revealed that slow synaptic events are extremely rare. In contrast, they are commonly encountered in similar investigations of myenteric ganglia of the guinea pig small intestine. The current immunocytochemical analysis of the myenteric plexus and innervation of the muscularis externa of the corpus of the guinea pig stomach was undertaken in order to determine whether putative neurotransmitters capable of mediating slow synaptic events are present in gastric ganglia. A major difference between the small intestine and the stomach was found in the innervation of the musculature. Whereas the longitudinal muscle layer of the small intestine contains very few nerve fibers and is innervated mainly at its interface with the myenteric plexus, the longitudinal muscle of the corpus of the stomach contained as many varicose substance P (SP)-, vasocative intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive axons as the circular muscle layer. These putative neurotransmitters were also present in the ganglia of the myenteric plexus, where varicose SP-, VIP-, and NPY-immunoreactive fibers encircled nonimmunoreactive neurons. Varicose 5-hydroxytryptamine (5-HT)-immunoreactive terminal axons were essentially limited to the myenteric plexus and were found both in ganglia and in interganglionic connectives, where they were particularly numerous; 5-HT-immunoreactive neurons appeared to be more abundant in the stomach than in the small intestine. Tyrosine hydroxylase (TH)- and calcitonin-gene-related-peptide (CGRP)-immunoreactive axons were also more common in the myenteric plexus than in the musculature, but of these, only the TH-immunoreactive neurites tended, like those of the other putative transmitters, to encircle neurons in myenteric ganglia. Evidence was obtained that, as in the small intestine, at least some of the SP-, VIP-, NPY-, and 5-HT-immunoreactive fibers in the stomach are derived from intrinsic gastric myenteric neurons. In contrast, unlike the small intestine, gastric myenteric ganglia appeared to lack intrinsic CGRP-immunoreactive neurons; therefore, the CGRP-immunoreactive gastric axons are probably of extrinsic origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine 5'-triphosphate modulates membrane potassium conductance in guinea-pig myenteric neurones

1. Intracellular recordings were made from myenteric neurones isolated from the guinea-pig small intestine to study actions of adenosine 5'-triphosphate (ATP). ATP was applied by superfusion (10 nM-100 microM) or pressure ejection from ATP-containing glass pipettes. 2. Myenteric neurones have been classified into two groups: type I/S neurones and type II/AH neurones. ATP produced a membrane hyperpolarization in 80% of AH neurones and a membrane depolarization in 90% of S neurones in a dose-dependent manner. Adenosine caused responses similar to those induced by ATP in both AH and S neurones, but was less effective than ATP. 3. The ATP-induced hyperpolarization was associated with a fall in input resistance, but the ATP-induced depolarization was accompanied by an increase in input resistance. Both responses reversed in polarity near the potassium equilibrium potential (-84 to -87 mV) and the reversal potential varied with extracellular potassium concentration, as predicted by the Nernst equation. These results indicate that the hyperpolarization is due to an increase, while the depolarization is due to a decrease in potassium conductance. 4. Both the hyperpolarization and the depolarization induced by ATP persisted in calcium-free solution containing 1.2 mM-magnesium, but were markedly reduced or abolished in calcium-free solutions containing 3.7-10 mM-magnesium and by 1 mM-nickel or cobalt. Both responses to ATP persisted in tetraethylammonium (1-10 mM) or tetrodotoxin (1-3 microM)-containing solutions. 5. Quinine and quinidine (1-100 microM) reversibly depressed both the ATP-induced responses. Caffeine (100 microM), theophylline (100 microM) and 3-isobutyl-1-methylxanthine (1-10 microM) did not significantly affect the ATP-induced depolarization but did reversibly depress the ATP-induced hyperpolarization. 6. These results suggest that the ATP-induced hyperpolarization may be due to activation, and the ATP-induced depolarization to inactivation, of a calcium-sensitive potassium conductance.

Specificities of afferents reinnervating cat muscle spindles after nerve section

1. We have made quantitative assessments of the sensory reinnervation and recovery of peroneus brevis muscle spindles following section and epineurial repair of the common peroneal nerve. After 6-50 weeks recovery, single-unit, dorsal-root recordings were made of the responses to ramp-and-hold or sinusoidal stretch of the reinnervated spindles, which were subsequently examined in teased, silver preparations. 2. Assessments of recovery used data obtained from cross-union experiments in which foreign afferents (including Ib) were given the opportunity of reinnervating spindles in the absence of their native (Ia, spindle II) afferents; and from an examination of tenuissimus spindles reinnervated by Ia and spindle II afferents in the absence of Ib afferents. These studies revealed: (i) that regenerating Ib afferents can terminate in sites originally occupied by the endings of Ia or spindle II afferents, and respond to stretch like normal Ia and spindle II afferents; (ii) that Ib and spindle II afferents reinnervating spindles are histologically identical apart from diameter range; and (iii) that some cutaneous afferents can reinnervate spindles and give highly abnormal, phasic stretch responses. 3. Recovery of afferents reinnervating spindles was marked by increases in conduction velocity and proportions firing tonically, but their firing rates at the three phases of ramp-and-hold stretch were considerably lower than normal and showed no tendency to increase. 4. Some relatively fast afferents that gave spindle II-type responses were identified as Ib afferents reinnervating secondary-ending sites; conversely, some relatively slow afferents that gave Ia-type responses were identified as spindle II afferents reinnervating primary-ending sites. 5. The estimated loss of spindle afferents from tenuissimus after nerve section (52% Ia, 49% spindle II) was considerably less than the estimated loss of these afferents from peroneus brevis after section of the common peroneal nerve (79% Ia, 86% spindle II). The proportion of spindles in tenuissimus reinnervated by free-ending afferents was also much lower (22%) than in peroneus brevis (73%). These differences are partly attributed to the greater size and degree of afferent complexity of the common peroneal nerve. 6. Similar proportions of spindles in peroneus brevis were reinnervated by Ia and Ib afferents after both partial (27% Ia, 20% Ib) and complete (21% Ia, 20% Ib) section of the common peroneal nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Actions of serotonin and substance P on myenteric neurons of guinea-pig distal colon

Intracellular methods were used to study the effects of serotonin and substance P (SP) on the electrical behavior of myenteric neurons in guinea-pig distal colon in vitro. Serotonin evoked either a short-duration transient depolarization, a long-lasting depolarization or a multiphasic response consisting of a rapid depolarization followed by a short duration hyperpolarizing potential and then a long-lasting depolarization. Application of SP evoked a long-lasting depolarization. Depolarizing potentials to both substances were accompanied by enhanced excitability that was reflected by repetitive spike discharge. Long-lasting depolarizations were associated with increased input resistance. The responses to serotonin or SP were unaltered by the presence of tetrodotoxin, hexamethonium or elevated extracellular Mg2+ and reduced Ca2+. Some neurons responded to both serotonin and SP indicating that both receptors coexisted on the same neuron. The putative SP antagonist, [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP did not affect the responses to SP. It did suppress the slow-depolarizing response to serotonin, while the fast response was unaffected. The responses to serotonin and SP in myenteric neurons of guinea-pig colon resembled the responses reported by others for small intestinal myenteric neurons.

An electrophysiological study of the projections of motor neurones that mediate non-cholinergic excitation in the circular muscle of the guinea-pig small intestine

The projections of neurones that produce the fast non-cholinergic excitatory junction potentials (e.j.p.s) in the circular muscle were analysed in the isolated ileum of the guinea-pig. Standard intracellular microelectrode techniques were used to record the amplitudes of such e.j.p.s in response to short trains of stimuli from transmural electrodes. Projections of the neurones around the circumference of the intestine were determined by plotting the change in e.j.p. amplitude with distance from longitudinally placed electrodes. Projections in the oral and anal directions were examined by recording at varying distances from transversely placed electrodes, and also by recording responses elicited close to longitudinal electrodes at various distances from lesions made 3-5 days earlier to interrupt orally and anally directed pathways. Experiments were performed in the presence of hexamethonium to determine the projections of the final motor neurones and in the absence of the drug to examine the projections of excitatory inputs to these neurones. With hexamethonium present, there was a decline in e.j.p. amplitude to 7.5% of maximum at 12 mm (a half circumference) from longitudinal stimulating electrodes. The decline was much less if hexamethonium was not present, and slightly greater if the myenteric plexus was removed. Thus, excitatory motor neurones and cholinergic neurones that impinge upon them both project circumferentially. When the longitudinal muscle and myenteric plexus were removed, and 3-5 days allowed for terminals to degenerate, no e.j.p.s could be recorded in the circular muscle, indicating that the fibres reach the circular muscle from the myenteric plexus. Following transverse lesions, substantial deficits in excitatory transmission only occurred within 1 mm oral or anal to the lesions indicating that the majority of neurones have only short projections along the intestine. A slight deficit in e.j.p. amplitude, up to 20%, was observed extending to about 5 mm oral; but normal transmission was restored by about 10 mm for the lesions. Thus there is a minority of excitatory motor neurones with oral projections up to about 10 mm in length. Results with transmural stimulation showed that these nerve fibres can cause excitation both when orthodromically and when antidromically stimulated, indicating that they provide collaterals along their lengths. With no hexamethonium present, e.j.p.s exhibited little decrement in amplitude over distances of over 30 mm oral or anal indicating that there are both ascending and descending cholinergic pathways that impinge on the final motor neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenergic innervation of serotoninergic neurons in the myenteric plexus

The monoaminergic innervation of the guinea pig small intestine was investigated to determine if there is an anatomical basis for the hypothesis that serotoninergic and noradrenergic neurons physiologically interact in the enteric nervous system. Initial rates of uptake of tritiated 5-hydroxytryptamine (3H-5-HT) or norepinephrine (3H-NE) by segments of guinea pig small intestine were measured in order to estimate the regional density of the serotoninergic and noradrenergic innervation. No change was found in the uptake of 3H-5-HT as a function of distance between duodenum and ileum, whereas the relative uptake of 3H-NE declined. The pattern of serotoninergic elements demonstrated radioautographically was compared with that obtained by visualizing 5-HT immunoreactivity. Both methods revealed that a small number of serotoninergic neurons, located in 35.3% +/- 1.5% of myenteric ganglia, give rise to many fibers that form thick bundles in interganglionic connectives. Moreover, there was a pronounced heterogeneity in the serotoninergic innervation of individual myenteric neurons and ganglia. In material fixed with aldehydes and postfixed with NaMnO4, noradrenergic axon terminals were identified by their characteristic small dense-cored vesicles. Following incubation with 3H-NE only terminals with small dense-cored vesicles were radioautographically labeled, confirming that these terminals are noradrenergic. When 3H-5-HT was substituted for 3H-NE, noradrenergic terminals were not labeled, showing that nonspecific uptake of 3H-5-HT into noradrenergic axons did not occur in the presence of 5-hydroxydopamine. The combination of aldehyde-NaMnO4 fixation with the radioautographic localization of 3H-5-HT thus permitted the simultaneous identification of serotoninergic and noradrenergic neural elements. Serotoninergic varicosities were found to differ from noradrenergic varicosities in the size, appearance, and packing density of their synaptic vesicles. In addition, recognizable but rudimentary pre- and postsynaptic membrane specializations were associated with serotoninergic but not noradrenergic varicosities. Most serotoninergic neuronal cell bodies were contacted both by serotoninergic synapses and noradrenergic varicosities. Similar appositions of noradrenergic varicosities with nonserotoninergic neurons appeared to be rare. In view of earlier observations that sympathetic nerves affect the release of 5-HT from stimulated enteric serotoninergic neurons, it seems likely that the noradrenergic appositions with serotoninergic neurons are the anatomical substrate for this effect.(ABSTRACT TRUNCATED AT 400 WORDS)