Mediators of slow synaptic potentials in the myenteric plexus of the guinea-pig ileum

Computational simulations and Ca2+ imaging reveal that slow synaptic depolarizations (slow EPSPs) inhibit fast EPSP evoked action potentials for most of their time course in enteric neurons

Transmission between neurons in the extensive enteric neural networks of the gut involves synaptic potentials with vastly different time courses and underlying conductances. Most enteric neurons exhibit fast excitatory post-synaptic potentials (EPSPs) lasting 20–50 ms, but many also exhibit slow EPSPs that last up to 100 s. When large enough, slow EPSPs excite action potentials at the start of the slow depolarization, but how they affect action potentials evoked by fast EPSPs is unknown. Furthermore, two other sources of synaptic depolarization probably occur in enteric circuits, activated via GABA_A or GABA_C receptors; how these interact with other synaptic depolarizations is also unclear. We built a compartmental model of enteric neurons incorporating realistic voltage-dependent ion channels, then simulated fast EPSPs, slow EPSPs and GABA_A or GABA_C ligand-gated Cl^- channels to explore these interactions. Model predictions were tested by imaging Ca²⁺ transients in myenteric neurons ex vivo as an indicator of their activity during synaptic interactions. The model could mimic firing of myenteric neurons in mouse colon evoked by depolarizing current during intracellular recording and the fast and slow EPSPs in these neurons. Subthreshold fast EPSPs evoked spikes during the rising phase of a slow EPSP, but suprathreshold fast EPSPs could not evoke spikes later in a slow EPSP. This predicted inhibition was confirmed by Ca²⁺ imaging in which stimuli that evoke slow EPSPs suppressed activity evoked by fast EPSPs in many myenteric neurons. The model also predicted that synchronous activation of GABA_A receptors and fast EPSPs potentiated firing evoked by the latter, while synchronous activation of GABA_C receptors with fast EPSPs, potentiated firing and then suppressed it. The results reveal that so-called slow EPSPs have a biphasic effect being likely to suppress fast EPSP evoked firing over very long periods, perhaps accounting for prolonged quiescent periods seen in enteric motor patterns.

The gastrointestinal tract is the only organ with an extensive semi-autonomous nervous system that generates complex contraction patterns independently. Communication between neurons in this “enteric” nervous system is via depolarizing synaptic events with dramatically different time courses including fast synaptic potentials lasting around 20–50 ms and slow depolarizing synaptic potentials lasting for 10–120 s. Most neurons have both. We explored how slow synaptic depolarizations affect generation of action potentials by fast synaptic potentials using computational simulation of small networks of neurons implemented as compartmental models with realistic membrane ion channels. We found that slow synaptic depolarizations have biphasic effects; they initially make fast synaptic potentials more likely to trigger action potentials, but then actually prevent action potential generation by fast synaptic potentials with the inhibition lasting several 10s of seconds. We confirmed the inhibitory effects of the slow synaptic depolarizations using live Ca²⁺ imaging of enteric neurons from mouse colon in isolated tissue. Our results identify a novel form of synaptic inhibition in the enteric nervous system of the gut, which may account for the vastly differing time courses between signalling in individual gut neurons and rhythmic contractile patterns that often repeat at more than 60 s intervals.

Circuit-specific enteric glia regulate intestinal motor neurocircuits

Glia in the central nervous system exert precise spatial and temporal regulation over neural circuitry on a synapse-specific basis, but it is unclear if peripheral glia share this exquisite capacity to sense and modulate circuit activity. In the enteric nervous system (ENS), glia control gastrointestinal motility through bidirectional communication with surrounding neurons. We combined glial chemogenetics with genetically encoded calcium indicators expressed in enteric neurons and glia to study network-level activity in the intact myenteric plexus of the proximal colon. Stimulation of neural fiber tracts projecting in aboral, oral, and circumferential directions activated distinct populations of enteric glia. The majority of glia responded to both oral and aboral stimulation and circumferential pathways, while smaller subpopulations were activated only by ascending and descending pathways. Cholinergic signaling functionally specifies glia to the descending circuitry, and this network plays an important role in repressing the activity of descending neural pathways, with some degree of cross-inhibition imposed upon the ascending pathway. Glial recruitment by purinergic signaling functions to enhance activity within ascending circuit pathways and constrain activity within descending networks. Pharmacological manipulation of glial purinergic and cholinergic signaling differentially altered neuronal responses in these circuits in a sex-dependent manner. Collectively, our findings establish that the balance between purinergic and cholinergic signaling may differentially control specific circuit activity through selective signaling between networks of enteric neurons and glia. Thus, enteric glia regulate the ENS circuitry in a network-specific manner, providing profound insights into the functional breadth and versatility of peripheral glia.

Deletion of choline acetyltransferase in enteric neurons results in postnatal intestinal dysmotility and dysbiosis

Acetylcholine (ACh)-synthesizing neurons are major components of the enteric nervous system (ENS). They release ACh and peptidergic neurotransmitters onto enteric neurons and muscle. However, pharmacological interrogation has proven inadequate to demonstrate an essential role for ACh. Our objective was to determine whether elimination of ACh synthesis during embryogenesis alters prenatal viability, intestinal function, the neurotransmitter complement, and the microbiome. Conditional deletion of choline acetyltransferase ( ChAT), the ACh synthetic enzyme, in neural crest-derived neurons ( ChAT-Null) was performed. Survival, ChAT activity, gut motility, and the microbiome were studied. ChAT was conditionally deleted in ENS neural crest-derived cells. Despite ChAT absence, mice were born live and survived the first 2 wk. They failed to gain significant weight in the third postnatal week, dying between postnatal d 18 and 30. Small intestinal transit of carmine red was 50% slower in ChAT-Nulls vs. WT and ChAT- Het. The colons of many neonatal ChAT-Null mice contained compacted feces, suggesting dysmotility. Microbiome analysis revealed dysbiosis in ChAT-Null mice. Developmental deletion of ChAT activity in enteric neurons results in proximal gastrointestinal tract dysmotility, critically diminished colonic transit, failure to thrive, intestinal dysbiosis, and death. ACh is necessary for sustained gut motility and survival of neonatal mice after weaning.-Johnson, C. D., Barlow-Anacker, A. J., Pierre, J. F., Touw, K., Erickson, C. S., Furness, J. B., Epstein, M. L., Gosain, A. Deletion of choline acetyltransferase in enteric neurons results in postnatal intestinal dysmotility and dysbiosis.

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.

Electrical stimulation of the mucosa evokes slow EPSPs mediated by NK1 tachykinin receptors and by P2Y1 purinoceptors in different myenteric neurons

Slow excitatory postsynaptic potentials (EPSPs) in enteric neurons arise from diverse sources, but which neurotransmitters mediate specific types of slow EPSPs is unclear. We investigated transmitters and receptors mediating slow EPSPs in myenteric neurons evoked by electrical stimulation of the mucosa in guinea pig small intestine. Segments of ileum or jejunum were dissected to allow access to the myenteric plexus adjacent to intact mucosa, in vitro. AH and S neurons were impaled with conventional intracellular electrodes. Trains of stimuli delivered to the mucosa evoked slow EPSPs in AH neurons that were blocked or depressed by the neurokinin-1 (NK1) tachykinin antagonist SR140333 (100 nM) in 10 of 11 neurons; the NK3 tachykinin receptor antagonist SR142801 (100 nM) had no effect on slow EPSPs in seven of nine AH neurons. Single pulses to the mucosa evoked fast EPSPs and slow depolarizations in S neurons. The depolarizations were divided into intermediate (durations 300-900 ms) or slow (durations 1.3-9 s) EPSPs. The slow EPSPs were blocked by pyridoxal phosphate-6-axophenyl-2-4-disulfonic acid (30 microM, N = 3) or the specific P2Y(1) antagonist MRS 2179 (10 microM, N = 6) and were predominantly in anally projecting S neurons that were immunoreactive for nitric oxide synthase (NOS). In contrast, intermediate EPSPs were predominantly evoked in NOS-negative neurons; these were abolished by MRS 2179 (N = 8). Thus activation of pathways running from the mucosa excites three different types of slow EPSP in myenteric neurons, which are mediated by either a tachykinin (NK1, AH neurons) or a purine nucleotide (P2Y(1), S neurons).

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.

Analysis of whole-cell currents by patch clamp of guinea-pig myenteric neurones in intact ganglia

Whole-cell patch-clamp recordings taken from guinea-pig duodenal myenteric neurones within intact ganglia were used to determine the properties of S and AH neurones. Major currents that determine the states of AH neurones were identified and quantified. S neurones had resting potentials of -47 +/- 6 mV and input resistances (R(in)) of 713 +/- 49 MOmega at voltages ranging from -90 to -40 mV. At more negative levels, activation of a time-independent, caesium-sensitive, inward-rectifier current (I(Kir)) decreased R(in) to 103 +/- 10 MOmega. AH neurones had resting potentials of -57 +/- 4 mV and R(in) was 502 +/- 27 MOmega. R(in) fell to 194 +/- 16 MOmega upon hyperpolarization. This decrease was attributable mainly to the activation of a cationic h current, I(h), and to I(Kir). Resting potential and R(in) exhibited a low sensitivity to changes in [K(+)](o) in both AH and S neurones. This indicates that both cells have a low background K(+) permeability. The cationic current, I(h), contributed about 20 % to the resting conductance of AH neurones. It had a half-activation voltage of -72 +/- 2 mV, and a voltage sensitivity of 8.2 +/- 0.7 mV per e-fold change. I(h) has relatively fast, voltage-dependent kinetics, with on and off time constants in the range of 50-350 ms. AH neurones had a previously undescribed, low threshold, slowly inactivating, sodium-dependent current that was poorly sensitive to TTX. In AH neurones, the post-action-potential slow hyperpolarizing current, I(AHP), displayed large variation from cell to cell. I(AHP) appeared to be highly Ca(2+) sensitive, since its activation with either membrane depolarization or caffeine (1 mM) was not prevented by perfusing the cell with 10 mM BAPTA. We determined the identity of the Ca(2+) channels linked to I(AHP). Action potentials of AH neurones that were elongated by TEA (10 mM) were similarly shortened and I(AHP) was suppressed with each of the three omega-conotoxins GVIA, MVIIA and MVIIC (0.3-0.5 microM), but not with omega-agatoxin IVA (0.2 microM). There was no additivity between the effects of the three conotoxins, which indicates the presence of N- but not of P/Q-type Ca(2+) channels. A residual Ca(2+) current, resistant to all toxins, but blocked by 0.5 mM Cd(2+), could not generate I(AHP). This patch-clamp study, performed on intact ganglia, demonstrates that the AH neurones of the guinea-pig duodenum are under the control of four major currents, I(AHP), I(h), an N-type Ca(2+) current and a slowly inactivating Na(+) current.

Receptor-induced Ca(2+) signaling in cultured myenteric neurons

We studied the effect of excitatory neurotransmitters (10(-5) M) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of cultured myenteric neurons. ACh evoked a response in 48.6% of the neurons. This response consisted of a fast and a slow component, respectively mediated by nicotinic and muscarinic receptors, as revealed by specific agonists and antagonists. Substance P evoked a [Ca(2+)](i) rise in 68.2% of the neurons, which was highly dependent on Ca(2+) release from intracellular stores, since after thapsigargin (5 microM) pretreatment only 8% responded. The responses to serotonin, present in 90.7%, were completely blocked by ondansetron (10(-5) M), a 5-HT(3) receptor antagonist. Specific agonists of other serotonin receptors were not able to induce a [Ca(2+)](i) rise. Removing extracellular Ca(2+) abolished all serotonin and fast ACh responses, whereas substance P and slow ACh responses were more persistent. We conclude that ACh-induced signaling involves both nicotinic and muscarinic receptors responsible for a fast and a more delayed component, respectively. Substance P-induced signaling requires functional intracellular Ca(2+) stores, and the 5-HT(3) receptor mediates the serotonin-induced Ca(2+) signaling in cultured myenteric neurons.

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.

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.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Anatomical and electrophysiological comparison of the sensory innervation of bronchial and tracheal parasympathetic ganglion neurons

Anatomical and electrophysiological techniques were used to examine differences in the level of sensory nerve terminals localized in parasympathetic ganglia in guinea pig trachea and bronchus. We quantified substance-P-immunoreactive nerve terminal profiles within the ganglia and the amplitude of capsaicin-sensitive depolarization (slow excitatory postsynaptic potential or sEPSP) evoked by vagus (bronchial) or recurrent (tracheal) nerve-stimulation. Substance-P-immunoreactive axon profiles per intrinsic tracheal neuron were fewer in number than per bronchial ganglion neuron. The average amplitude and duration of the capsaicin-sensitive sEPSPs were similar in tracheal and bronchial ganglion neurons. Furthermore, capsaicin evoked a nearly equal depolarization of bronchial and tracheal ganglion neurons. The sEPSPs were reduced in both areas by a selective neurokinin-3 receptor antagonist. SR142801. These results demonstrate that although the number of sensory nerve fibers in tracheal ganglia are significantly less than those in the bronchus, this did not translate to an obvious physiological difference in sEPSP amplitude.

Analysis of contributions of acetylcholine and tachykinins to neuro-neuronal transmission in motility reflexes in the guinea-pig ileum

1. The roles of acetylcholine (ACh) and tachykinins in neuro-neuronal transmission during ascending excitatory and descending inhibitory reflexes were studied by recording intracellular reflex responses of the circular muscle to physiological stimuli. Experiments were carried out in opened segments of guinea pig ileum in an organ bath that was partitioned so that three regions could be independently exposed to drugs. 2. Ascending excitatory reflexes evoked by either distension from the serosal side or compression of the mucosa were depressed by 55% and 85%, respectively, in the presence of hexamethonium (200 microM) and by 30% and 45%, respectively, by a desensitizing concentration of the selective NK3 receptor agonist, senktide (1 microM), in the chamber in which reflexes were initiated. Together, hexamethonium and senktide abolished responses to compression. A residual response to distension persisted. This was abolished by hyoscine (1 microM). 3. Hexamethonium (200 microM) abolished ascending reflexes when applied to the region between the stimulus and the recording sites, or to the recording chamber. 4. Descending reflex responses were reduced by 35% by synaptic blockade in the stimulus chamber with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+. Senktide (1 microM) in the stimulus chamber reduced distension reflexes to the same extent as synaptic blockade, whereas hexamethonium (200 microM) and hyoscine (1 microM) depressed responses by less than 20%. Responses to compression were reduced by 40% by senktide alone, while senktide and hexamethonium together reduced responses by 60%, an effect similar to synaptic blockade. Under these conditions, hyoscine in the stimulus chamber restored reflexes evoked by distension, but did not alter those evoked by mucosal compression. 5. Total synaptic blockade in the intermediate chamber, between stimulus and recording sites, reduced descending reflex responses by more than 90%. In contrast, hexamethonium (200 microM) had no effect and hyoscine (1 microM) reduced only the responses to distension (by 30%). Senktide (1 microM) depressed responses to both stimuli by approximately 80%. 6. Application of hexamethonium (200 microM) to the recording chamber depressed descending reflex responses to distension applied in the near stimulation chamber by 15%, but had no effect on responses to compression in the near chamber or to either stimulus applied in the far chamber. 7. Descending reflexes evoked by near chamber stimuli were unaffected by hyoscine (1 microM) or senktide (1 microM) applied to the recording chamber; hyoscine enhanced reflexes evoked by compression in the far chamber by 50%. 8. For the ascending excitatory reflex pathway, it is concluded that transmission from sensory neurones is mediated by ACh acting via both nicotinic and muscarinic receptors, and by tachykinins acting at NK3 receptors. Transmission from ascending interneurones appears to be predominantly via nicotinic receptors. The descending inhibitory pathways are more complex, and while transmission from sensory neurones involves nicotinic, muscarinic and NK3 receptor-dependent components, transmission from descending interneurones to inhibitory motor neurones is neither cholinergic nor due to tachykinins acting via NK3 receptors.

Tachykininergic slow depolarization of motoneurones evoked by descending fibres in the neonatal rat spinal cord

1. In the isolated spinal cord of the neonatal rat, repetitive electrical stimulation of the upper cervical region elicited a prolonged depolarization of lumbar motoneurones (L3-5) lasting 1-2 min, which was recorded extracellularly from ventral roots, or intracellularly. 2. This depolarizing response was markedly depressed by the excitatory amino acid receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (D-APV, 30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). The remaining response was further depressed by a 5-hydroxytryptamine (5-HT) receptor antagonist, ketanserin (3 microM). 3. In the presence of these antagonists, a small part of the depolarizing response of slow time course remained, and this response was partially blocked by the tachykinin NK1 receptor antagonists GR71251 (0.3-5 microM) and RP67580 (0.3-1 microM). In contrast, RP68651 (0.3-1 microM), the inactive enantiomer of RP67580, had no effect on the depolarizing response. 4. The slow depolarizing response in the presence of D-APV, CNQX and ketanserin was markedly potentiated by a peptidase inhibitor, thiorphan (1 microM). 5. This descending fibre-evoked slow depolarization became smaller after prolonged treatment (5-7 h) with 5,7-dihydroxytryptamine (10 microM), a neurotoxin for 5-HT neurones. Under such conditions, the effects of thiorphan and GR71251 on the slow depolarization were virtually absent. 6. Under the action of D-APV, CNQX and ketanserin, applications of tachykinins, substance P and neurokinin A produced depolarizing responses of lumbar motoneurones, and the responses were depressed by GR71251 and potentiated by thiorphan. 7. These results suggest that tachykinins contained in serotonergic fibres serve as neurotransmitters mediating the descending fibre-evoked slow excitatory postsynaptic potentials in motoneurones.

Tachykinins as mediators of slow EPSPs in guinea-pig gall-bladder ganglia: involvement of neurokinin-3 receptors

1. The effects of endogenous tachykinins and related peptides on intact guinea-pig gall-bladder neurones were investigated with single-electrode voltage- and current-clamp recording techniques. 2. Pressure ejection of substance P (100 microM) caused a long lasting membrane depolarization that was associated with a decrease in input resistance. In cells that were voltage-clamped to their resting membrane potential, substance P activated an inward current. 3. The reversal potentials of the substance P-induced depolarization and inward current were congruent to 0 mV. In a low-Na+ solution, the substance P-induced depolarization and inward current were reduced in amplitude. 4. Substance P increased the excitability of neurones, as evidenced by a greater anodal break activity and an increase in the number of action potentials generated during a depolarizing current pulse. 5. Substance P, neurokinin A (NKA) and neurokinin B (NKB) were applied by superfusion to determine the relative potencies of these tachykinins. NKB was the most potent, with an EC50 of 24 nM. The EC50 values for NKA and substance P were 47.8 and 281 nM, respectively. 6. The neurokinin-3 (NK-3) receptor agonist senktide depolarized neurones with an EC50 of 6.3 nM. Neither the NK-1 receptor agonist [Sar9,Met(O2)11]-substance P nor the NK-2 receptor agonist [beta-Ala8]-NKA(4-10) caused a measurable depolarization. 7. The NK-3 antagonist [Trp7,beta-Ala8]-NKA (4-10) inhibited the responsiveness of gall-bladder neurones to substance P with a KB (dissociation constant of receptor antagonist) of 49 nM, and depressed both capsaicin-induced depolarizations and stimulus-evoked slow EPSPs. 8. These data indicate that tachykinins mediate slow EPSPs in guinea-pig gall-bladder ganglia by activating NK-3 receptors on gall-bladder neurones. It is proposed that in response to inflammation or high intraluminal pressure, tachykinins may be released within ganglia by sensory fibres and act directly on intrinsic neurones to facilitate ganglionic transmission.

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)

Modelling of the enteric nervous network: 3. Adrenergic neuron

A mathematical model is developed to investigate the coupled electrochemical processes of nerve-pulse transmission via adrenergic synapse. Based on pharmacological and morphophysiological data, the model describes the dynamics of the propagation of the electric signal along the unmyelinated geometrically non-uniform axon of the neuron and the chemical mechanisms of the transformation of the electrical signal in the synaptic zone into the post-synaptic output. The combined nonlinear system of partial and ordinary differential equations has been obtained and solved numerically. The results of computer simulation of the function of the idealized adrenergic neuron quantitatively and qualitatively describe the dynamics of Ca2+ ion influx into the terminal, noradrenaline release from the free 'releasable' store, its diffusion into the synaptic cleft, binding with the adrenoceptors on the pre- and post-synaptic structures with the generation of the inhibitory post-synaptic potential, and utilization of noradrenaline by neuronal and non-neuronal capture mechanisms.

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.

Electrophysiological and morphological classification of myenteric neurons in the proximal colon of the guinea-pig

Intracellular recordings were made from myenteric neurons in the proximal colon of the guinea-pig. The electrical behaviour of the neurons in response to intracellular depolarizing current pulses, and to internodal strand stimulation, was recorded. The intracellular electrode contained the intracellular marker biocytin which was injected into impaled neurons for subsequent histochemistry. Proximal colon myenteric neurons displayed electrophysiological properties similar to myenteric neurons in the small intestine, and were classified as either AH- or S-neurons. AH-neurons were characterized by the presence of a slow afterhyperpolarization following an action potential. Internodal strand stimulation evoked slow excitatory synaptic potentials in five out of six AH-neurons tested, but did not evoke fast excitatory synaptic potentials in 26 AH-neurons tested. S-neurons lacked a slow afterhyperpolarization, but internodal strand stimulation evoked fast excitatory synaptic potentials in all 113 neurons and slow excitatory synaptic potentials in seven out of 17 tested. A subpopulation of AH-neurons displayed a rhythmic oscillation in membrane potential which could be triggered by an action potential. S-neurons could be subdivided into those that fired tonically and those that fired phasically in response to long depolarizing current pulses. About 80% of the AH-neurons were immunoreactive for calbindin, as were 10% of S-neurons. A further 17% of S-neurons, but no AH neurons, were calretinin immunoreactive. Morphological analysis of filled neurons revealed eight distinct classes. Neurons electrophysiologically classified as AH typically had a large, oval soma and several long tapering processes. Processes of AH-neurons branched into many adjacent ganglia. Almost all S-neurons were uniaxonal and many axons ended in an expansion bulb in the myenteric plexus. S-neurons typically had broad, lamellar processes, or short, spiny processes. Roughly equal proportions of S-neurons had oral or anal projection. However, almost all S-neurons that were immunoreactive for calbindin or calretinin projected orally. The results indicate that myenteric neurons in the proximal colon of the guinea-pig are electrophysiologically similar to myenteric neurons in the small intestine, but there are a greater number of morphological and chemical categories.

ATP regulates synaptic transmission by pre- and postsynaptic mechanisms in guinea-pig myenteric neurons

Intracellular recordings were made from myenteric neurons of the guinea-pig ileum in vitro; they were classified into S and AH neurons according to electrophysiological criteria. ATP (10 nM-100 microM) inhibited excitatory synaptic potentials in the myenteric plexus; fast excitatory postsynaptic potentials and slow excitatory postsynaptic potentials of S neurons and slow excitatory postsynaptic potentials in AH neurons. This inhibitory action was reversible and dose-dependent, and was usually followed by a transient augmentation of the synaptic potentials after washing of ATP. The actions of ATP on the synaptic potentials were prevented by pretreatment with theophylline, caffeine, quinidine and 8-phenyl theophylline. The ATP analogues, ATP-gamma-s (100 nM-100 microM) and alpha-beta-methylene ATP (100 nM-100 microM) also depressed the synaptic potentials recorded from both types of neurons. The inhibitory effect of adenosine on the synaptic potentials was 10 times weaker than that of ATP. Thus, it seems clear that the presynaptic inhibition is not occurring through adenosine A1 or A2 receptors. Furthermore, ATP at high concentrations ( > or = 1 microM) augmented nicotinic fast depolarizations of S neurons produced by extracellular acetylcholine. However, ATP at the same concentrations inhibited the slow depolarizations of S and AH neurons caused by exogenous acetylcholine (muscarinic) and substance P. It is concluded that ATP regulates synaptic transmission in the myenteric plexus of the guinea-pig ileum and the sites of ATP actions are pre- and postsynaptic.

Substance P inhibits activation of calcium-dependent potassium conductances in guinea-pig myenteric neurones

1. Intracellular recordings were made from myenteric AH neurones of the guinea-pig ileum in vitro. Some experiments were done with a single-electrode voltage clamp to measure membrane currents. 2. Substance P (SP) applied by superfusion (10 nM-300 nM), pressure ejection (100 nM-10 microM, 760 mmHg, for 10-20 ms) or ionophoresis (1 mM, 100 nA, for 0.2 s) caused a membrane depolarization and an inward current, associated with a decrease in potassium conductance. 3. The SP-induced depolarization was abolished within 15 min by superfusion with calcium-free/high-magnesium (10 mM) solution or solutions containing cobalt, manganese or nickel at 1-3 mM. The response persisted even after 40-60 min of superfusion with calcium-free/normal-magnesium (1.2 mM) solution. In all these solutions, synaptic potentials were abolished within 5 min. 4. SP inhibited a slowly developing outward current and an outward tail current during and after a long depolarizing command pulse (2-10 s), and an outward after-current following single or multiple brief depolarizing command pulses (10-50 ms). These outward currents were suppressed in calcium-free/high-magnesium solution. 5. SP depressed both a calcium-dependent slow after-hyperpolarization following the action potential and an outward after-current preceded by a brief depolarizing command. Both the SP-induced depolarization and the SP-induced inward current were augmented when the peptide was pressure-ejected during the recovery phase of the slow after-hyperpolarization and during that of the slow outward after-current, but both of them were inhibited or almost abolished when SP was applied immediately after spike initiation or a brief depolarizing command. 6. The SP-induced response was depressed by barium (1-2 mM). The SP response was not inhibited by tetraethylammonium at low concentrations (5-10 mM), but was depressed at high concentration (20 mM). 7. Superfusion (1-10 nM) or pressure application of a calcium ionophore, A23187, inhibited or even reversed the SP depolarization and the SP-induced inward current. 8. These results indicate that SP inhibits activation of a calcium-dependent potassium conductance which contributes to both the slow after-hyperpolarization and the resting membrane potential. SP may affect the process by which calcium activates this potassium conductance.

[Role of substance P in the nervous system control of digestive motility]

Substance P is a 11 amino-acids peptide which belongs to the tachykinins, a family of peptide which induces a rapid contraction of the smooth muscle of the digestive tract. The occurrence of substance P has been demonstrated by immunohistochemical and radioimmunological techniques in most parts of the central and peripheral nervous system. Substance P exerts on the smooth muscle of all the areas of the digestive tract a strong excitatory effect which is either direct or relayed by the cholinergic intramural neurones. Numerous electrophysiological, pharmacological and immunohistochemical data lead to the conclusion that substance P is released by intrinsic neurones of the digestive tract or by extrinsic nerves (vagus and splanchnic nerves, etc...). This release is enhanced by acetylcholine, cholecystokinin, serotonin and neurotensin, it is reduced by opioid peptides and noradrenaline. Substance P participates in the intestinal peristaltic reflex by the activation of the smooth muscle cells of the intestine, either directly or through the activation of the cholinergic intrinsic neurones. Substance P is also involved in the genesis of a non-cholinergic ascending excitatory activity likely occurring during vomiting. Lastly, substance P participates in the reflex contraction of the lower oesophageal sphincter following acidification of the distal part of the oesophagus.

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.

The effect of substance P on nerve action potential propagation and cholinergic transmission in the myenteric plexus of the guinea-pig ileum

The effect of substance P on nerve terminals in myenteric plexus of the guinea-pig ileum was investigated. Neurogenic twitches of the myenteric plexus longitudinal muscle strip were recorded. Twitches of the strip portion where excitation involved the most distal parts of cholinergic nerve terminals were more increased by local application of substance P (0.1 and 0.4 nmol/l) than twitches of the portion where excitation involved both distal and proximal parts of nerve terminals. Substance P addition to a portion of the strip conducting nerve action potentials to invade the neighbouring strip portion also augmented twitches of the latter portion so that the interference with the propagation process was considered. The effect of substance P was poorly antagonized by the addition of a substance P antagonist, (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-substance P. Compound nerve action potentials were evoked in strands of fibres of the myenteric plexus by low-frequency train stimulation (1 Hz). The addition of substance P prevented a decrease of the amplitude of responses observed under control conditions. Using high-frequency train stimulation (30 Hz) the amplitude of responses to impulses 2-7 was augmented over that to the first impulse; substance P further increased such facilitation regularly. It seems that substance P might promote nerve action potential invasion of the distal parts of nerve terminals.

Contractions of the guinea-pig ileum evoked by stimulation of the submucous plexus

Neural pathways from the submucous plexus to the longitudinal muscle of an adjacent segment of isolated guinea-pig ileum were studied. It was found that electrical field stimulation of a strip of submucosa-submucous plexus produced frequency-dependent longitudinal contractions of an intact segment of intestine lying oral to the point of stimulation. The responses were reduced to less than 10% of control by tetrodotoxin, atropine, morphine and chymotrypsin and by desensitization to substance P (SP). The responses were only inhibited by one-third by hexamethonium and were not affected by desensitization to 5-hydroxytryptamine. The effect of desensitization to SP was reversible, but the effect of chymotrypsin was irreversible. SP-induced desensitization and chymotrypsin did not inhibit the twitch response produced by field stimulation of the whole ileal segment. The same results were observed with preparations made from ileal segments that had been extrinsically denervated. The results suggest that intrinsic neurons with processes in the submucous plexus can excite cholinergic and SP-containing neurons in the myenteric plexus, thereby causing the longitudinal muscle to contract.

Novel autonomic neurotransmitters and intestinal function

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

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)

Opioid dependence in myenteric neurons innervating the circular muscle of guinea-pig ileum

Guinea-pigs were treated with morphine for 6-8 days by subcutaneous implantation of pellets, each containing a mixture of morphine base (120 mg) and morphine hydrochloride (35 mg). Each guinea-pig received a single pellet. Mechanical activity of the circular muscle was recorded in vitro in preparations comprising the circular muscle and myenteric plexus. Exposure to morphine was maintained by addition of 1 microM morphine to the organ baths. After 90 min, morphine was withdrawn, either by repeatedly washing tissues in morphine-free Krebs' solution, or by addition of naloxone to reduce the occupancy of the opioid receptors by morphine. Withdrawal of morphine resulted in markedly enhanced contractile activity compared with that in circular muscle-myenteric plexus preparations from untreated control guinea-pigs. The withdrawal contractions were abolished by tetrodotoxin (600 nM) and greatly reduced by hyoscine (1 microM), indicating that they resulted from action potential discharge in myenteric neurons that release acetylcholine onto the circular muscle. Activation of the cholinergic excitatory motor neurons was not secondary to synaptic activation by cholinergic interneurons, because hexamethonium (100 microM) did not affect withdrawal contractions. The withdrawal response may therefore arise in the cholinergic excitatory motor neurons themselves, or in neurons that activate them via noncholinergic mechanisms.

Distribution of calcitonin gene-related peptide immunoreactive nerve fibers in the mudpuppy cardiac septum

An immunohistochemical study was undertaken to determine the distribution of calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers in the cardiac septum of the mudpuppy, Necturus maculosus. Numerous long, CGRP-immunoreactive nerve fibers course across the septum, run in the nerve trunks connecting clusters of postganglionic parasympathetic cells, form complexes over groups of ganglion cells and make pericellular networks around individual ganglion cells. The postganglionic parasympathetic neurons and small intensely fluorescent (SIF)-like cells did not exhibit CGRP immunoreactivity. Most of the CGRP-immunoreactive nerve fibers also are labeled for substance P. In freshly dissected preparations, the staining pattern for CGRP was not similar to that obtained using an antiserum against synaptic vesicle membrane, which appears to preferentially label cholinergic preganglionic terminals on all postganglionic parasympathetic cells in the mudpuppy preparation. Further, in explanted ganglia (maintained 10 days in culture) almost no reactivity was obtained with the antivesicle antiserum whereas numerous nerve fibers still exhibited CGRP-immunoreactivity. These observations demonstrate that the CGRP-immunoreactive nerve fibers are not parasympathetic preganglionic axons. Rather we suggest that the CGRP-immunoreactive nerve fibers are processes of primary sensory fibers.

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)

Naloxone-induced depolarization and synaptic activation of myenteric neurons in morphine-dependent guinea pig ileum

To investigate the cellular basis of opiate dependence, intracellular microelectrodes were used to record from both electrophysiologically defined classes of neurons (S and AH) in myenteric plexus longitudinal muscle preparations from morphine pretreated guinea pigs. These preparations responded to naloxone with the characteristic contraction of the longitudinal smooth muscle, indicative of morphine dependence. Depolarization in response to naloxone was observed in 42% of S neurons, but there were no consistent changes in input resistance. In some cells the depolarization was reduced or abolished after blockade of synaptic transmission, suggesting that it was due in part to the release of an excitatory transmitter producing a slow depolarization in the impaled neuron. Synaptic activation of S neurons during withdrawal was further indicated by the observation that fast postsynaptic potentials appeared after abrupt displacement of morphine from its receptors by naloxone. Morphine withdrawal, therefore, involves both the final motor neurons and interneurons. During naloxone-induced withdrawal, 25% of S neurons discharged action potentials. In contrast, no action potentials were discharged in AH neurons. Furthermore, naloxone did not alter the resting membrane potential, input resistance, soma action potential configuration, or slow hyperpolarization following a soma spike in AH neurons. The specificity of the withdrawal response for S neurons and the relatively small proportion of neurons involved suggests that morphine withdrawal occurs in quite specific neuronal circuits in the myenteric plexus.

Observations on the actions of substance P and [D-Arg1,D-Pro2,D-Trp7,9,Leu11)substance P on single neurons of the guinea pig submucous plexus

Intracellular recordings were made from neurons of the guinea pig submucosal plexus and the effects of substance P and the substance P analogue [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P were examined. Substance P (20-200 nM) depolarized all submucosal neurons; these depolarizations were shown to be due to a decrease in the resting (or "leak") potassium conductance of the membrane. In approximately 50% of the 46 neurons tested, superfusion with [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P (0.2-20 microM) produced a dose-dependent membrane hyperpolarization. This hyperpolarization was prevented by the alpha 2-adrenoceptor antagonist idazoxan (300 nM) or by concentrations of cobalt which abolished all spontaneous and evoked synaptic potentials, indicating that it resulted from release of noradrenaline from sympathetic nerve terminals. [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P depressed the amplitude of the three synaptic potentials recorded from submucosal neurons; the concentrations that caused 50% of the maximal inhibition of the fast excitatory postsynaptic potential, the inhibitory postsynaptic potential, and slow excitatory postsynaptic potential were 40 microM, 600 nM and 20 microM, respectively. When idazoxan was present, the substance P analogue was less effective in depressing the amplitudes of the fast and slow excitatory synaptic potentials suggesting that much of its presynaptic inhibition also resulted from release of noradrenaline. These results provide evidence that [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P releases noradrenaline from sympathetic nerves in the submucosal plexus. One effect of this is a membrane hyperpolarization; another is a presynaptic inhibition of transmitter release. These actions much limit the usefulness of this "substance P antagonist" in efforts to show that synaptic potentials, such as the slow excitatory synaptic potential, are mediated by substance P.

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

Simultaneous intracellular staining and electrophysiological recording techniques have been applied to neurones of guinea-pig myenteric plexus-longitudinal muscle preparations. With micro-electrodes filled with a solution of the fluorescent dye Lucifer Yellow, neurones were first characterized morphologically and electrophysiologically, and subsequently subjected to an indirect immunohistochemical method for the detection of vasoactive intestinal peptide (VIP)-like immunoreactivity. Cross-correlations of morphology, electrophysiology and VIP immunoreactivity were successfully achieved in a total of 164 neurones. Sixty-three had the slow after-hyperpolarization characteristic of AH neurones; 101 cells displayed fast excitatory post-synaptic potentials (e.p.s.p.s) in response to transmural or focal stimulation and were therefore, by definition, S neurones. Unequivocal VIP immunoreactivity was observed in 25 (25%) S neurones, which, with only one exception, had Dogiel Type I morphology (i.e. many short soma processes and a single long process). In contrast, AH neurones had Dogiel Type II morphology (i.e. smooth soma with several long processes) and none showed VIP immunoreactivity. In addition to cholinergic fast e.p.s.p.s., non-cholinergic slow synaptic inputs were evoked in seventeen of the twenty-two VIP-immunoreactive S neurones tested. Both the fast and slow e.p.s.p.s could be elicited by stimulation of the preparation, oral or aboral to the site of recording. These observations demonstrate that, in the guinea-pig ileum, myenteric plexus neurones showing VIP immunoreactivity are of a single electrophysiological type (S neurones) and belong to essentially one morphological class (Dogiel Type I).

The actions of substance P and serotonin on myenteric neurons in tissue culture

Intracellular recordings were made from guinea-pig myenteric neurons in tissue culture. The responses to substance P (SP) and serotonin were compared. Putative transmitters were ejected by pressure from micropipettes. The response to SP was, in the majority of cases, a prolonged depolarization, with a latency of 1-5 s and a duration of 1-2 min. During the depolarization there was an increase in the input resistance of the cells, and the excitability was augmented. In two neurons the response to SP was hyperpolarization followed by depolarization. In about 30% of the cells the response to SP was relatively fast, with a duration of less than 10 s and a latency of less than 100 ms. These responses did not show desensitization. Serotonin induced relatively fast responses only, with durations of 0.3-3 s and latencies of 20-50 ms. These responses were usually depolarizing, but in 3 cells were hyperpolarizing. Two types of behaviour were observed during repetitive ejection of serotonin. Some cells showed strong desensitization while in others there was no desensitization and there was even summation. The actions of SP and serotonin were compared by their ejection in the same experiments. The same general properties described above were observed. It is concluded that the two putative transmitters act via distinct response mechanisms. The SP-induced depolarizations in tissue culture resemble in several respects the slow synaptic potentials recorded previously in myenteric neurons in freshly isolated preparations.

Slow postsynaptic potentials in neurones of submucous plexus of guinea-pig caecum and their mimicry by noradrenaline and various peptides

Intracellular recordings of membrane potential and membrane currents were made from neurones in the submucous plexus of the guinea-pig caecum in vitro. Fast and slow excitatory postsynaptic potentials and slow inhibitory postsynaptic potentials were recorded from the majority of neurones following focal stimulation of presynaptic fibres in the plexus. The slow inhibitory postsynaptic potential was associated with an increase in membrane conductance and reversed its polarity at -90 mV; it was reversibly blocked by yohimbine. The slow excitatory postsynaptic potential and its underlying current was associated with a decrease in membrane conductance. Two kinds of voltage-dependence both of the slow excitatory postsynaptic potential and current were observed; in 80% of cells, the excitatory postsynaptic potential and current became smaller with membrane hyperpolarization and reversed polarity at -90 mV (reversing type) but in 20% of cells both the excitatory postsynaptic potential and current simply disappeared when the membrane potential reached -70 mV (non-reversing type). The effects of acetylcholine, adenosine 5'-triphosphate, bombesin, 5-hydroxytryptamine, neurotensin, noradrenaline, substance P and vasoactive intestinal polypeptide were examined. The only substance which mimicked the slow inhibitory postsynaptic potential was noradrenaline; brief applications of noradrenaline caused hyperpolarizations which had the same time-course, reversal potential and sensitivity to yohimbine as the slow inhibitory postsynaptic potential. The non-reversing type of slow excitatory postsynaptic potential was mimicked only by adenosine 5'-triphosphate. The reversing type of slow excitatory postsynaptic potential was mimicked by bombesin, neurotensin, substance P and vasoactive intestinal polypeptide. 5-Hydroxytryptamine and vasoactive intestinal polypeptide (in some neurones) caused a depolarization with an increase in membrane conductance. All three synaptic potentials were reversibly depressed by superfusion of noradrenaline but noradrenaline did not affect the potential changes evoked by brief application of exogenous acetylcholine or substance P. It is concluded that, in guinea-pig submucous plexus neurones, the slow inhibitory postsynaptic potential is mediated by noradrenaline and results from a potassium conductance increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic modulation by noradrenaline and an opioid of the substance P-induced release of [3H]acetylcholine from the myenteric plexus

Substance P (7.5-750 nM) applied in superfusion dose-dependently released 3H from isolated strips of myenteric plexus-longitudinal muscle of the guinea-pig ileum loaded with [3H]choline. Separation of the [3H]acetylcholine and [3H]choline components of the released radioactivity revealed that in response to substance P (SP) administration only the release of [3H]acetylcholine increased above resting level. A slowly developing tachyphylaxis to the effect of SP was observed. Evidence has been obtained that the slow tachyphylaxis developed to the acetylcholine-releasing effect of SP was not due to the exhaustion of releasable acetylcholine pool. Release of acetylcholine by 150 nM SP was completely prevented by tetrodotoxin or in a Ca2+-free medium and greatly reduced in the presence of noradrenaline or the opioid receptor agonist (D-Met2,Pro5)-enkephalinamide. The effect of noradrenaline and the opioid peptide was apparently prevented by yohimbine and naloxone, respectively.

Muscarinic M1 and M2 receptors mediate depolarization and presynaptic inhibition in guinea-pig enteric nervous system

Intracellular recordings were made from guinea-pig myenteric and submucous plexus neurones. Nicotinic excitatory post-synaptic potentials (fast e.p.s.p.s) and slow e.p.s.p.s were recorded in both plexuses; adrenergic inhibitory post-synaptic potentials (i.p.s.p.s) were recorded from submucous plexus neurones. The effects of muscarinic agonists and antagonists were examined on the synaptic potentials in those neurones in which these substances did not change the membrane potential. Muscarine, oxotremorine, methylfurmethide and McNeil A343 reversibly depressed the amplitude of the fast e.p.s.p. in a concentration-dependent way. Hyoscine, pirenzepine and 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP) caused a parallel shift to the right of the agonist dose-response curves. These muscarinic antagonists themselves did not alter the amplitudes of fast e.p.s.p.s evoked by low frequency (0.05-0.1 Hz) stimulation. Antagonist pA2 values (the negative logarithm of the dissociation equilibrium constant) were determined while recording from individual neurones. pA2 values were: pirenzepine 7.0, hyoscine 8.9, and 4-DAMP 8.7. I.p.s.p.s in the submucous plexus were also depressed by muscarinic agonists, and this was competitively reversed by pirenzepine and 4-DAMP, with apparent pA2 values of 6.9 and 8.7 respectively. Muscarinic antagonists alone increased the amplitude of the i.p.s.p. evoked either by single or repeated stimuli. This enhancement was observed with low concentrations of antagonists and did not become greater when the concentrations were increased. Muscarinic agonists depolarized about one-quarter of myenteric and submucous plexus neurones. Low concentrations of pirenzepine antagonized these depolarizations; the pA2 value was 8.4. Cholinergic slow e.p.s.p.s recorded in some myenteric neurones were depressed or abolished by pirenzepine; concentrations that caused 50% inhibition (IC50) for this action ranged from 10 to 60 nM. It is concluded that presynaptic muscarinic receptors, activation of which inhibits the release of acetylcholine and noradrenaline, are the m2 type. Post-synaptic muscarinic receptors, activation of which depolarizes the membrane, are of the m1 type. The results also suggest that acetylcholine may exert a tonic inhibition of noradrenaline release in the submucous plexus through m2 receptors, and mediates the slow e.p.s.p. in the myenteric plexus through m1 receptors.

Thyrotropin-releasing hormone mimics descending slow synaptic potentials in rat spinal motoneurons

Thyrotropin-releasing hormone (TRH) produced a depolarization in lumbar motoneurons of neonatal rats. The depolarization by TRH persisted after extracellular Ca2+ was replaced by Mg2+ or Mn2+, indicating its direct action upon motoneurons. Stimulation of the ventral descending tract at the lower thoracic segment evoked slow excitatory postsynaptic potentials (e.p.s.ps) lasting 20-30 s in every motoneuron. Both the TRH-induced depolarization and descending slow e.p.s.p. were accompanied by a decrease in input conductance of motoneurons. When the membrane potential of the motoneuron was shifted, both the TRH-induced depolarization and slow e.p.s.p. became larger in amplitude during depolarization and smaller during hyperpolarization. However, they could not be reversed in polarity by hyperpolarization. During the depolarization of motoneuron produced by TRH application, the slow e.p.s.p. was markedly reduced in amplitude, suggesting the involvement of identical ionic mechanisms in the two responses. After incubation of the isolated spinal cord with antisera to TRH, the depolarizing response produced by TRH as well as the descending slow e.p.s.p. was greatly diminished. In contrast, monosynaptic reflexes evoked by dorsal root stimulation remained unchanged under this condition. These results suggest that TRH serves as a neurotransmitter mediating the descending slow e.p.s.p. in motoneurons.

Neurons dissociated from rat myenteric plexus retain differentiated properties when grown in cell culture. III. Synaptic interactions and modulatory effects of neurotransmitter candidates

We have used intracellular recordings to study synaptic interactions between myenteric neurons grown in dissociated cell culture. Intracellular stimulation of individual myenteric neurons caused several types of synaptic effects in nearby neurons: fast excitatory synaptic potentials mediated by nicotinic acetylcholine receptors; slow, non-cholinergic synaptic potentials; dual transmission having both fast cholinergic and slow non-cholinergic components and inhibition of spontaneously occurring fast nicotinic synaptic potentials. Fast nicotinic synaptic potentials were elicited by about 40% of neurons tested and often occurred spontaneously. The fast synaptic potentials were similar to those that have been studied in other autonomic neurons with respect to their estimated reversal potential and their sensitivity to cholinergic antagonists. The amplitudes of the fast synaptic potentials declined if evoked at frequencies greater than 0.5 Hz. Potentiation of the fast synaptic potentials was observed following high-frequency stimulation of presynaptic neurons. Several transmitter candidates modulated fast cholinergic transmission. Substance P and vasoactive intestinal peptide promoted nicotinic transmission by causing increased amplitudes of evoked and spontaneous fast synaptic potentials and an increased frequency of spontaneous synaptic potentials. gamma-Aminobutyrate and [Met]enkephalin both caused decreased amplitudes and frequency of nicotinic synaptic potentials. Serotonin depressed synaptic potentials in some neurons while enhancing them or having no effect in others. Slow, non-cholinergic, synaptic potentials were elicited by about 10% of neurons tested. These synaptic effects lasted 15-300s, caused depolarizations of 3-15 mv and were accompanied by increased neuronal input resistance. The transmitter(s) causing these slow synaptic potentials has not yet been identified.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological responses of guinea-pig myenteric neurons secondary to the release of endogenous serotonin by tryptamine

Intracellular recordings showed that administration of pulses of tryptamine mimicked one of the actions of serotonin (a slow depolarization associated with an increased input resistance) on type II/AH neurons of the myenteric plexus. After superfusion at high concentration tryptamine initially acted like serotonin, but then blocked the action of serotonin on these cells. Measurements of the release of preloaded [3H]serotonin or [3H]norepinephrine revealed that tryptamine is a potent releaser of these labeled amines; this release is Ca2+ independent but temperature dependent. Moreover, incubation with tryptamine depleted the myenteric plexus of endogenous serotonin. Since tryptamine has previously been demonstrated not to inhibit the binding of [3H]serotonin to its enteric neural receptor we framed the hypothesis that the serotonin-releasing action of tryptamine is responsible for its ability to mimic serotonin when given in pulses or to desensitize serotonin receptors through the prolonged release of serotonin when it is superfused. This hypothesis was tested by examining the action of tryptamine on the serotonin-mediated slow excitatory postsynaptic potentials evoked in type II/AH neurons by fiber tract stimulation. Tryptamine superfusion antagonized these slow potentials as predicted. Moreover, after a long time when endogenous serotonin was depleted, the response of type II/AH neurons to exogenous serotonin recovered but the slow synaptic potential did not. The action of tryptamine on this neuron was relatively specific. When the slow synaptic potential and serotonin responses were blocked by tryptamine the type II/AH neurons still responded to acetylcholine. Fast excitatory postsynaptic potentials were not affected by tryptamine. Furthermore, other types of neurons (I/S) and other neuronal responses to serotonin (such as a fast depolarization with decreased input resistance or presynaptic inhibition of acetylcholine release) were not blocked by tryptamine. Finally, radioautographic studies revealed a neural uptake of tryptamine in the chemically sympathectomized myenteric plexus; however, the distribution of tryptamine in the plexus was different from that of serotonin and was not blocked by excess non-radioactive serotonin. Therefore tryptamine does not enter myenteric neurons via the specific serotonin uptake mechanism; however, zimelidine, found to be a selective inhibitor of the enteric uptake of serotonin, antagonized the release of serotonin by tryptamine and attenuated the effect of tryptamine on responses to serotonin.(ABSTRACT TRUNCATED AT 400 WORDS)