Slow synaptic potentials in neurones of the myenteric plexus

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.

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.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

Angiotensin receptors and actions in guinea pig enteric nervous system

Actions of ANG II on electrical and synaptic behavior of enteric neurons in the guinea pig small intestine were studied. Exposure to ANG II depolarized the membrane potential and elevated neuronal excitability. The number of responding neurons was small, with responses to ANG II in 32% of submucosal neurons and 25% of myenteric neurons. Hyperpolarizing responses were evoked by ANG II in 45% of the neurons. The hyperpolarizing responses were suppressed by alpha2-noradrenergic receptor antagonists, which suggested that the hyperpolarizing responses reflected stimulation of norepinephrine release from sympathetic neurons. Exposure to ANG II enhanced the amplitude and prolonged the duration of noradrenergic inhibitory postsynaptic potentials and suppressed the amplitude of both fast and slow excitatory postsynaptic potentials. The selective ANG II(1) receptor (AT1R) antagonists, ZD-7115 and losartan, but not a selective AT2R antagonist (PD-123319), suppressed the actions of ANG II. Western blot analysis and RT-PCR confirmed expression of AT1R protein and the mRNA transcript for the AT1R in the enteric nervous system. No expression of AT2R protein or mRNA was found. Immunoreactivity for AT1R was expressed by the majority of neurons in the gastric antrum and small and large intestine. AT1R immunoreactivity was coexpressed with calbindin, choline acetyltransferase, calretinin, neuropeptide Y, and nitric oxide synthase in subpopulations of neurons. The results suggest that formation of ANG II might have paracrine-like actions in the enteric nervous system, which include alterations in neuronal excitability and facilitated release of norepinephrine from sympathetic postganglionic axons. The enhanced presence of norepinephrine is expected to suppress fast and slow excitatory neurotransmission in the enteric microcircuits and to suppress neurogenic mucosal secretion.

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

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

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.

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

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

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Enhanced excitability of myenteric AH neurones in the inflamed guinea-pig distal colon

The electrical and synaptic properties of myenteric neurones in normal and inflamed guinea-pig distal colons were evaluated by intracellular microelectrode recording. Chronic inflammation was established 6 days following administration of trinitrobenzene sulfonic acid (TNBS). In S neurones, inflammation only altered synaptic inputs as the amplitude of fast excitatory postsynaptic potentials were significantly larger (31 +/- 2 mV compared to 20 +/- 1 mV) and they were more likely to receive slow excitatory synaptic input (85% compared to 55%). AH neurones displayed altered electrical properties in colitis compared to control tissues: they generated more action potentials during a maximal depolarising current pulse (7 +/- 1 compared to 1.6 +/- 0.2); they had a smaller after hyperpolarisation (9 +/- 2 mV s compared to 20 +/- 2 mV s); and they were more likely to receive fast excitatory synaptic input (74% compared to 17%), possess spontaneous activity (46% compared to 3%), and generate anodal break action potentials (58% compared to 19%). Although the resting membrane potential, input resistance and action potential characteristics were unaltered in AH neurones from inflamed tissues, they exhibited an enhanced Cs+-sensitive rectification of the current-voltage relationship. This suggests that the increase in excitability of AH neurones may involve a colitis-induced augmentation of the hyperpolarisation-activated cation current (Ih) in these cells. An increased excitability, selectively in AH neurones, suggests that the afferent limb of intrinsic motor reflexes is disrupted in the inflamed colon and this may contribute to dysmotility associated with inflammatory diseases.

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.

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.

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.

Excitatory synaptic inputs on myenteric Dogiel type II neurones of the pig ileum

The synaptic input on myenteric Dogiel type II neurones (n = 63) obtained from the ileum of 17 pigs was studied by intracellular recording. In 77% of the neurones, electrical stimulation of a fibre tract evoked fast excitatory postsynaptic potentials (fEPSPs) with an amplitude of 6 +/- 5 mV (mean +/- S.D.) and lasting 49 +/- 29 ms. The nicotinic nature of the fEPSPs was demonstrated by superfusing hexamethonium (20 microM). High-frequency stimulation (up to 20 Hz, 3 seconds) did not result in a rundown of the fEPSPs, and did not evoke slow excitatory or inhibitory postsynaptic potentials. The effects of neurotransmitters, possibly involved in these excitatory responses, were investigated. Pressure microejection of acetylcholine (10 mM in pipette) resulted in a fast nicotinic depolarisation in 67%(18/27) of the neurones (13 +/- 9 mV, duration 7.0 +/- 7.2 seconds) as did 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) application (10 mM; 14 +/- 10 mV, duration 4.1 +/- 2.8 seconds) in 76% of the cells. The fast nicotinic response to acetylcholine was sometimes (6/27) followed by a slow muscarinic depolarisation (8 +/- 4 mV; duration 38.7 +/- 10.8 seconds). Immunostaining revealed 5-hydroxytryptamine hydrochloride (5-HT)- and calcitonin gene-related peptide (CGRP)-positive neuronal baskets distributed around and in close vicinity to Dogiel type II neuronal cell bodies. Microejection of 5-HT (10 mM) resulted in a fast nicotinic-like depolarisation (12 +/- 6 mV, duration 3.0 +/- 1.3 seconds) in 4 of 8 neurones tested, whereas microejection of CGRP (20 mM) gave rise to a slow muscarinic-like depolarisation (6 +/- 2 mV, duration 56.0 +/- 27.5 seconds) in 8 of 12 neurones tested. In conclusion, myenteric Dogiel type II neurones in the porcine ileum receive diverse synaptic input. Mainly with regard to the prominent presence of nicotinic responses, these neurones behave contrary to their guinea pig counterparts.

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

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

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

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

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

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

Orally projecting interneurones in the guinea-pig small intestine

1. Orally projecting, cholinergic interneurones are important in mediating ascending excitatory reflexes in the small intestine. We have shown that there is just one major class of orally projecting interneurone, which we have characterized using retrograde labelling in organ culture, combined with immunohistochemistry, intracellular recording and dye filling. 2. Orally projecting interneurones, previously shown to be immunoreactive for choline acetyltransferase, tachykinins, enkephalin, calretinin and neurofilament protein triplet, have axons up to 14 mm long and are the only class of cells with orally directed axons more than 8.5 mm long. 3. They are all small Dogiel type I neurones with short dendrites, usually lamellar in form, and a single axon which sometimes bifurcates. Their axons give rise to short varicose collaterals in myenteric ganglia more than 3 mm oral to their cell bodies. 4. Orally projecting interneurones receive prominent fast excitatory post synaptic potentials (fast EPSPs). A major source of fast EPSPs is other ascending interneurones located further aborally. They also receive fast EPSPs from circumferential pathways. 5. In the stretched preparations used in this study, orally projecting interneurones were highly excitable, firing repeatedly to depolarizing current pulses and had negligible long after-hyperpolarizations following their action potentials. They did not receive measurable non-cholinergic slow excitatory synaptic inputs. 6. Ascending interneurones had a characteristic inflection in their membrane responses to depolarizing current pulses and their first action potential was typically delayed by approximately 30 ms. Under single electrode voltage clamp, ascending interneurones had a transient outward current when depolarized above -70 mV from more hyperpolarized holding potentials. Ascending interneurones also consistently showed marked inward rectification under both current clamp and voltage clamp conditions. 7. This class of cells has consistent morphological, neurochemical and electrophysiological characteristics and are important in mediating orally directed enteric reflexes.

Suppression of cAMP formation by adenosine in myenteric ganglia from guinea-pig small intestine

Effects of the adenosine receptor agonist 2-chloro-N6-cyclopentyl-adenosine (CCPA) on stimulation of cAMP formation by histamine, 5-hydroxytryptamine, substance P and forskolin were determined for enzymatically dissociated ganglia from the myenteric plexus of guinea-pig small intestine. Each of the 4 substances stimulated cAMP production. CCPA blocked the stimulation of cAMP by histamine, but not by 5-hydroxytryptamine or substance P. CCPA marginally suppressed stimulation by forskolin. CCPA alone suppressed basal levels of cAMP. The adenosine receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) reversed the inhibitory action of CCPA on stimulation of cAMP formation by histamine. Exposure to adenosine deaminase or CPT increased cAMP in the ganglia. The results are consistent with a hypothesis that stimulation of adenylate cyclase and elevation of intraneuronal cAMP in enteric neurons are steps in the signal transduction cascade for the excitatory actions of 5-hydroxytryptamine, substance P and histamine. They are consistent also with an original hypothesis from electrophysiologic studies which states that stimulation of adenosine A1 receptors suppresses cAMP formation and thereby slow synaptic excitation in response to histamine, but not to 5-hydroxytryptamine or substance P. The results support evidence from intracellular microelectrode studies which suggested that endogenous adenosine accumulates to levels sufficient for tonic suppression of cAMP formation in myenteric ganglia in vitro.

Influence of the mucosa on the excitability of myenteric neurons

Intracellular microelectrodes were used to examine the active and passive membrane properties of neurons in the myenteric plexus of the guinea-pig small intestine. Neurons of two types were examined: S neurons, which have prominent fast excitatory postsynaptic potentials and in which action potentials are not followed by long-lasting afterhyperpolarizations, and AH neurons, which have long-lasting afterhyperpolarizations following soma action potentials. In preparations in which the myenteric ganglia and longitudinal muscle, but no mucosa, were present, most S neurons (59/64) responded to intracellular depolarizing current with brief bursts of action potentials. Regardless of the strength of a depolarizing current of 500-ms duration, these neurons never fired action potentials beyond the first 250 ms. S neurons in this state were called rapidly accommodating. In contrast, within 600 microm circumferential to the intact mucosa, 26/58 S neurons fired action potentials for most or all of the period of a 500-ms insightful depolarizing pulse. S neurons in this state were called slowly accommodating. Depolarization of S neurons in the rapidly accommodating state caused a rapidly developing reduction in membrane resistance (outward rectification; onset about 7 ms). This rectification was absent from S neurons in the slowly accommodating state. Tetraethylammonium blocked the early rectification and the changed neuronal state from rapidly accommodating to slowly accommodating. Application of tetrodotoxin to neurons in the slowly accommodating state revealed the early rectification, indicating that its absence from these neurons before tetrodotoxin was applied had been due to ongoing activity in axons providing synaptic input to the neurons. After the mucosa was disconnected from the other layers and laid back in its original position, all S neurons close to the mucosa were in the rapidly accommodating state (17/17). Slow excitatory postsynaptic potentials, evoked by electrical stimulation of nerve tracts, converted 17 of 43 S neurons from rapidly accommodating to slowly accommodating and eliminated the early outward rectification in these neurons. These results indicate that the action potential firing properties of S neurons can be changed by external influences, including the activity of synaptic inputs that release a slowly acting transmitter. Spontaneous antidromic action potentials were recorded in 8/62 AH neurons within 600 microm circumferential to the intact mucosa. It is concluded that, when the mucosa is intact, a background firing of sensory neurons occurs which leads to a state change in many S neurons innervated by the active sensory neurons. We conclude that this state change is caused by the block of a voltage-sensitive outward rectification.

Myenteric neurons of the rat descending colon: electrophysiological and correlated morphological properties

Conventional intracellular electrophysiological recordings were made from 502 myenteric neurons of the rat descending colon. Myenteric neurons could be classified into three groups on the basis of distinct electrophysiological properties. The first group of neurons (51% of all neurons) fired tetrodotoxin-sensitive action potentials in response to direct somal depolarization and the majority (98%) of this group generated fast cholinergic excitatory synaptic potentials in response to focal stimulation and were therefore designated S/Type 1 neurons. The second group (40%) of neurons fired tetrodotoxin-insensitive action potentials which were followed by long-lasting membrane afterhyperpolarizations, hence were termed AH neurons. These neurons did not receive fast cholinergic synaptic inputs but ionophoretic application of acetylcholine induced rapid nicotinic cholinoceptor-mediated depolarizations. The final group of neurons (9%), named Type 3 neurons, received fast cholinergic synaptic inputs but could never be made to fire action potentials. Rundown in amplitude of successive fast excitatory synaptic potentials evoked by a short train of presynaptic nerve stimuli was observed in only a small proportion of neurons (8/37; 22%) with the majority of neurons (29/37; 78%) showing no such decrease in amplitude, even at frequencies of stimulation as high as 10 Hz. Superfusion of 5-hydroxytryptamine could induce both an inhibition and a facilitation of cholinergic fast synaptic transmission. Evidence was adduced that these presynaptic inhibitory and facilitatory actions appeared to be mediated via 5-hydroxytryptamine 1A and 5-hydroxytryptamine 4 receptors, respectively. Muscarinic slow excitatory synaptic potentials were not detected (9/9 neurons tested) and non-cholinergic slow excitatory synaptic potentials following repetitive focal presynaptic nerve stimulation were observed in only 39/502 (8%) of all neurons. In those neurons in which a demonstrable change in membrane input resistance was detectable, slow excitatory potentials were accompanied by an increased input resistance. In addition, in a small subset (4%) of S/Type 1 neurons, slow membrane hyperpolarizations accompanied by an increased membrane input resistance were observed following tetanic presynaptic nerve stimulation. Superfusion of 5-hydroxytryptamine induced both membrane depolarizations and hyperpolarizations. Membrane depolarizations were observed in 40% of all neuronal types (34% of S/Type 1 neurons, 58% of AH neurons and 11% of Type 3 neurons) and were accompanied by an increased membrane input resistance and occasionally, in S/Type 1 and AH neurons, by anodal break excitation or spontaneous action potential firing. Membrane hyperpolarizations were observed in S/Type 1 neurons (5%) only and were accompanied, unexpectedly, by an increased membrane input resistance. In those neurons that responded both to application of 5-hydroxytryptamine and tetanic presynaptic nerve stimulation, 5-hydroxytryptamine always mimicked the slow synaptic response indicating that 5-hydroxytryptamine may function as a slow synaptic mediator in some myenteric neurons. Myenteric neurons identified by intracellular injection of the neuronal marker Neurobiotin TM were found to conform to the morphological classification schemes proposed for myenteric neurons of the guinea-pig and porcine intestine, that is, Dogiel Types I and II and Stach Type IV neurons were present. Simultaneous electrophysiological recording and intracellular staining techniques revealed that a correlation existed between the electrophysiological and morphological properties of myenteric neurons of the rat colon, with electrophysiological classified S/Type 1 neurons having Dogiel Type I morphologies (95/108 neurons; 88%) and electrophysiological classified AH neurons having Dogiel Type II morphologies (87/94 neurons; 93%)...

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.

Identification of sensory nerve cells in a peripheral organ (the intestine) of a mammal

It is commonly believed that the cell bodies of mammalian sensory neurons are contained within spinal and cranial sensory ganglia associated with the central nervous system or within the central nervous system itself. However, strong circumstantial evidence implies that some sensory neurons are contained entirely within the gastrointestinal tract. We have investigated this possibility by using intracellular methods to record the responses of myenteric neurons in the guinea-pig small intestine to physiological stimuli applied to the neighbouring mucosa. The results show that the myenteric plexus contains a population of chemosensitive sensory neurons and that these neurons correspond to neurons with AH electrophysiological properties and Dogiel type II morphology. This is the first direct evidence that some sensory neurons are contained entirely within the peripheral nervous system.

Contribution of chloride conductance increase to slow EPSC and tachykinin current in guinea-pig myenteric neurones

1. Single electrode voltage clamp recordings were obtained from myenteric neurones of guinea-pig ileum in vitro. Slow excitatory postsynaptic currents (sEPSCs) were elicited by focal stimulation of interganglionic nerve strands in twenty-four of thirty neurones more than 30 min after impalement. In seventeen of twenty-four neurones, sEPSCs were associated with a conductance decrease and reversed polarity at -96 +/- 3 mV (near the reversal potential for potassium, EK); this response was due to inhibition of resting potassium conductance, gK. In seven of twenty-four neurones, there was either no net conductance change or a biphasic conductance change during the sEPSC; a reversal potential for peak currents could not be determined. 2. Application of senktide (3 microM), a neurokinin-3 receptor agonist, caused an inward current in forty-one of fifty-three neurones more than 30 min after impalement. In twenty of forty-one neurones, senktide-induced currents were due to inhibition of resting gK. In eleven of forty-one neurones there was either no net conductance change or a biphasic conductance change; a reversal potential for peak currents could not be determined. In ten out of forty-one neurones, senktide-induced currents were associated with a conductance increase (ginc); the estimated reversal potential was -17 +/- 3 mV. 3. Application of forskolin (1 microM) caused an inward current that occluded the decrease in gK caused by senktide and the sEPSC. In neurones in which sESPCs and senktide responses were associated with an unclear or biphasic conductance change, forskolin did not reduce the peak current and residual currents were usually associated with a ginc. 4. In neurones in which senktide-induced currents were associated with a ginc, reducing extracellular Cl- to 13 mM reduced senktide-induced currents by 79%. Reducing extracellular Na+, or adding tetraethylammonium (TEA, 50 mM), cobalt (2 mM) or picrotoxin (30 microM) did not change senktide-induced currents. The chloride transport/channel blockers niflumic acid and mefenamic acid (both at 100 microM) blocked senktide-induced currents. It was concluded that senktide increases chloride conductance (gCl). 5. Chord conductance measurements made between -70 and -90 mV during sEPSCs were used to determine the contribution of an increase in gCl to sEPSCs. These measurements indicated that the peak sEPSC is composed of a 90% decrease in gK and a 10% increase in gCl. Similar data were obtained from measurements made during senktide responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Local neural control of intestinal motility: nerve circuits deduced for the guinea-pig small intestine

1. Propulsion of digesta along the intestine appears to occur by the action of a series of local reflexes which cause contraction oral to the digesta and relaxation of circular muscle on the anal side. 2. There is now substantial evidence available about the identities of the enteric neurons that mediate these reflexes. 3. The motor neurons and interneurons of the reflex pathways lie within the myenteric plexus. These neurons can be classified electrophysiologically as S-neurons and have distinctive projections and neurochemistries. 4. The sensory neurons may lie in the myenteric plexus, but there is some evidence for sensory neurons in the submucous plexus. A contribution from extrinsic sensory neurons to local motility reflexes cannot be ruled out. Intrinsic sensory neurons are probably AH-neurons and are large multi-axonal cells.

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.

Characterization of 5-HT receptors mediating contraction and relaxation of the longitudinal muscle of guinea-pig distal colon in vitro

A range of agonists and antagonists were used to characterize the receptors through which 5-hydroxytryptamine (5-HT) contracts and relaxes the longitudinal muscle of segments of guinea-pig distal colon, in vitro. 5-HT contracted the longitudinal muscle over the concentration range 10(-9) to 10(-4) mol/l. The 5-HT3 receptor agonist, 2-methyl-5-HT, produced concentration dependent contractions over the range 10(-6) to 10(-4) mol/l. 5-methoxytryptamine, an agonist at 5-HT4 receptors, caused contractions over a concentration range of 10(-8) to 10(-4) mol/l. The 5-HT4 antagonist, SDZ 205-557 (5 x 10(-7) mol/l) substantially suppressed the responses to low concentrations of 5-HT and to 5-methoxytryptamine, but had no effect on the responses to higher concentrations of 5-HT. In contrast, the 5-HT3 antagonist, granisetron (10(-6) mol/l), blocked the effect of 2-methyl-5-HT and substantially depressed responses to high concentrations of 5-HT, but had no effect on lower concentrations of 5-HT. Granisetron produced a small reduction in the response to 5-methoxytryptamine. Tetrodotoxin (TTX) (3 x 10(-7) mol/l) almost abolished the response to 5-methoxytryptamine and markedly suppressed the response to 2-methyl-5-HT, but the responses to 5-HT were only partially reduced. The 5-HT1 antagonist, methiothepin (10(-6) mol/l) depressed the response to 5-HT (10(-7) to 10(-4) mol/l) and blocked its TTX insensitive component. The 5-HT2 antagonist, ketanserin, in concentrations up to 10(-5) mol/l, had no effect on the contractions evoked by 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous intracellular recordings from enteric neurons reveal that myenteric AH neurons transmit via slow excitatory postsynaptic potentials

Simultaneous intracellular electrical recordings were made from pairs of neurons separated circumferentially by 100-200 microns of the myenteric plexus of the guinea-pig ileum in vitro. The recording electrodes were filled with the dye neurobiotin which was injected into impaled nerve cells, and later revealed histochemically. Intracellular current pulses were used to evoke action potentials via the recording electrode in one type of myenteric neuron, in most cases an AH neuron, while a second electrode was used to record from a simultaneously impaled S neuron or AH neuron. AH neurons are thought to be primary sensory neurons, whereas S neurons are interneurons and motor neurons. Ninety pairs of neurons were adequately tested for interaction. From these, 17 S neurons and three AH neurons that responded to AH neuron stimulation were detected. In each case, the response was a slow depolarization that was seen only in response to a train of stimuli at 10 Hz. The slow depolarizations were enhanced by passing depolarizing current and diminished by hyperpolarization. Responses were also diminished by lowering external Ca.2+ and elevating Mg2+. In all cases in which intracellular recording indicated communication between neurons, morphological evidence of connection was seen. In no case was there communication without connection, but in four instances, morphological connections appeared to exist, although no physiological evidence of communication was obtained.

Corticotropin-releasing hormone excites myenteric neurons in the guinea-pig small intestine

The electrophysiological actions of corticotropin-releasing hormone (CRH) on myenteric neurons from the guinea-pig ileum were studied by intracellular microelectrode recording. CRH, when applied by micropressure ejection or in the medium (0.2-20 nM) evoked prolonged depolarization in 21 of 42 S/type 1 neurons and in 28 of 40 AH/type 2 neurons. These responses were associated with increased input resistance and augmented excitability. The post-spike hyperpolarization in AH/type 2 cells was suppressed during the CRH-evoked responses. The reversal potential of the response to CRH was about -90 mV, consistent with the closure of potassium channels by the peptide. The CRH-induced depolarization was prevented by incubation in 10 microM 5'-N-ethylcarboxamidoadenosine (NECA, an adenosine analog) suggesting that the response was mediated by stimulation of adenylate cyclase and elevation of cAMP. CRH reduced the amplitude of fast nicotinic excitatory postsynaptic potentials. This appeared to be a postsynaptic action because the peptide also reduced the responses to exogenously applied acetylcholine. These results suggest that CRH can directly influence intestinal function by acting on myenteric neurons.

Synaptic behaviour in the myenteric plexus of the guinea-pig gastric antrum

1. Intracellular recording methods were used to study the synaptic behaviour of neurones in the myenteric plexus of the guinea-pig gastric antrum. Synaptic potentials occurred spontaneously or were evoked by focal electrical stimulation of interganglionic fibre tracts. Synaptic events consisted of fast and slow excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 2. Fast EPSPs with durations less than 20 ms were evoked in every antral neurone in a population sample of 370 cells. Most of the ganglion cells received multiple inputs from axons entering the individual ganglia in several different interganglionic fibre tracts. Many of the neurones also received input from multiple axons projecting in individual fibre tracts. The fast EPSPs behaved like nicotinic cholinergic EPSPs. They were evoked at stimulus frequencies up to 60 Hz without evidence of the run-down characteristic of fast EPSPs in the intestine. 3. Slow EPSPs were evoked by repetitive stimulation of the interganglionic connectives. They consisted of a slowly activating depolarization which persisted for several seconds after termination of the stimulus. The depolarizing responses were associated with an increase in the input resistance, with enhanced excitability and with suppression of hyperpolarizing after-potentials in AH/type 2 neurones. They were observed in 14.4% of the neurones of which 89% were AH/type 2 neurones. AH/type 2 neurones, unlike other myenteric neurones, were identified by action potentials with long lasting after-hyperpolarization. 4. IPSPs were hyperpolarizing potentials evoked by repetitive stimulation of interganglionic fibre tracts. The hyperpolarizing responses were associated with decreased input resistance. They occurred in 1.4% of the antral neurones. 5. Application of acetylcholine (ACh) by micro-ejection mimicked the fast EPSPs in all neurones. This fast nicotinic response to ACh was followed by a slowly activating, long-lasting muscarinic depolarization in 32% of the neurones. The slow muscarinic response was associated with increased input resistance, suppression of hyperpolarizing after-potentials and enhanced excitability. 6. Fast EPSPs were not suppressed by accumulation of ACh at presynaptic transmitter release sites. Unlike the intestine, presynaptic muscarinic autoreceptors appeared to be absent from the microcircuits in the antrum. 7. Synaptic behaviour in the local circuits of the gastric antrum differed from the gastric corpus. This may be a reflection of specialization of the circuits for organization of the distinctive patterns of digestive behaviour found in this region of the stomach.

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.

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)

Neuropharmacology of the muscarinic antagonist telenzepine in myenteric ganglia of the guinea-pig small intestine

Intracellular recording methods were used to investigate the actions of the putative M1 muscarinic receptor antagonist telenzepine on the electrical and synaptic behavior of myenteric neurons. Telenzepine had no effect on resting membrane potential, input resistance, excitability and antidromic potentials in both AH/type 2 and S/type 1 neurons, when applied in concentrations of 0.1-2000 nM, although higher concentrations (10-100 microM) did have a significant non-specific effect on the postsynaptic membrane. Micromolar concentrations of telenpzepine (1-2 microM) had no effect on excitatory responses to substance P, vasoactive intestinal peptide, the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium or the nicotinic action of acetylcholine. Nicotinic fast excitatory postsynaptic potentials were also unaffected by 2 microM telenzepine. In contrast, at submicromolar concentrations (100 nM), telenzepine abolished responses to either muscarine or the muscarinic component of the acetylcholine response. The excitatory effect of muscarine at postsynaptic M1 receptors was dose dependently inhibited by telenzepine (0.1-1000 nM) at concentrations which had no effect on the electrical properties of the cells. This effect was slowly reversible, usually requiring more than 60 min for significant recovery. The threshold dose of telenzepine as an antagonist of the muscarinic depolarization in AH/type 2 neurons was in the range of 0.1-1 nM. The IC50 concentration of telenzepine needed to abolish the response was 8.5 nM. A small proportion of stimulus-evoked slow excitatory postsynaptic potentials in both AH/type 2 and S/type 1 cells were abolished by 1 microM telenzepine, while the majority of them remained unaffected, indicating that some slow excitatory postsynaptic potentials are mediated by the muscarinic action of released acetylcholine.(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.

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.

Histamine H2 receptor mediates postsynaptic excitation and presynaptic inhibition in submucous plexus neurons of the guinea-pig

Intracellular recordings were made from submucous plexus neurons of the guinea-pig cecum maintained in vitro. Histamine (0.3-10 microM) produced a dose-dependent membrane depolarization (congruent to 13 mV with 3 microM) in about 28% of the cells tested; most of these cells showed a prominent calcium-activated potassium conductance (AH cells). The depolarization was due primarily to an inactivation of potassium conductance which is available at the resting membrane potential of -60 mV. Peak amplitude of the fast excitatory postsynaptic potential was depressed by histamine (0.1-10 microM) in a dose-dependent manner (congruent to 62% depression with 1 microM). This was observed even in those cells in which histamine did not produce any membrane depolarizations (mostly S cells). The depression of the fast excitatory postsynaptic potential resulted from the presynaptic inhibition of acetylcholine release. Histamine also reduced the amplitude of the non-cholinergic, presumably peptidergic, slow excitatory postsynaptic potential by suppressing peptide release from presynaptic nerve terminals. Peak amplitude of the adrenergic inhibitory synaptic potential was not depressed by histamine suggesting that histamine receptors are not present on presynaptic terminals of sympathetic nerve fibres. Both postsynaptic and presynaptic actions of histamine were blocked by cimetidine or ranitidine but not by pyrilamine implying that H2 receptors are involved.

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)

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

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