Nicotinic Fast Excitatory Postsynaptic Potentials Research Articles

HCN hyperpolarization-activated cation channels strengthen virtual nicotinic EPSPs and thereby elevate synaptic amplification in rat sympathetic neurons.

The influence of hyperpolarization-activated cation current (h-current; Ih) upon synaptic integration in paravertebral sympathetic neurons was studied together with expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) subunit isoforms. All four HCN subunits were detected in homogenates of the rat superior cervical ganglion (SCG) using the PCR to amplify reverse-transcribed messenger RNAs (RT-PCR) and using quantitative PCR. Voltage clamp recordings from dissociated SCG neurons at 35°C detected Ih in all cells, with a maximum hyperpolarization-activated cation conductance of 1.2 ± 0.1 nS, half-maximal activation at -87.6 mV, and reversal potential of -31.6 mV. Interaction between Ih and synaptic potentials was tested with virtual fast nicotinic excitatory postsynaptic potentials (EPSPs) created with dynamic clamp. The blocking of Ih with 15 μM ZD7288 hyperpolarized cells by 4.7 mV and increased the virtual synaptic conductance required to stimulate an action potential from 7.0 ± 0.9 nS to 12.1 ± 0.9 nS. In response to stimulation with 40 s long trains of virtual EPSPs, ZD7288 reduced postsynaptic firing from 2.2 to 1.7 Hz and the associated synaptic amplification from 2.2 ± 0.1 to 1.7 ± 0.2. Cyclic nucleotide binding to HCN channels was simulated by blocking native Ih with ZD7288, followed by reconstitution with virtual Ih using a dynamic clamp model of the voltage clamp data. Over a 30-mV range, shifting the half-activation voltage for Ih in 10 mV depolarizing increments always increased synaptic gain. These results indicate that Ih, in sympathetic neurons, can strengthen nicotinic EPSPs and increase synaptic amplification, while also working as a substrate for cyclic nucleotide-dependent modulation.

Synaptic and membrane properties of parasympathetic ganglionic neurons innervating mouse trachea and bronchi

The pathophysiology of airway diseases, such as asthma, is increasingly studied using transgenic mice and other mouse models of airway inflammation where allergen-induced changes in airway smooth muscle tone and mucous secretion is due, in part, to activation of preganglionic airway parasympathetic nerves. Ganglionic parasympathetic neurons located in the airways in several species, including humans, have anatomical and electrophysiological properties that limit transmission of preganglionic synaptic input. In this study, intracellular recordings were made from neurons in parasympathetic ganglia located on the trachea and bronchi of adult mice to determine electrophysiological properties associated with regulation of transmission of preganglionic input. Ganglionic neurons were characterized as having either tonic or phasic action potential accommodation patterns. Tonic neurons responded with repetitive action potentials sustained throughout a depolarizing current step, whereas phasic neurons generated one or a burst of action potential(s) and accommodated. A small subset displayed both patterns. Phasic neurons could be further differentiated as usually having either short- or long-duration afterhyperpolarizing potential following single and multiple action potentials. In most cells, stimulation of preganglionic nerves elicited one population of nicotinic fast excitatory postsynaptic potentials that were graded in amplitude, usually suprathreshold for action potential generation, and did not decrease in amplitude during higher frequency stimulation. Dye injection into the neurons revealed that dendrites were either absent or very short. These results provide evidence that in contrast to the characteristics of airway parasympathetic neurons reported in other species, including human, the electrophysiological and synaptic properties, and anatomical characteristics of mouse lower airway ganglionic neurons, are less associated with integration of presynaptic input.

Acute effects of nerve growth factor (NGF) on membrane properties of mouse lower airway parasympathetic ganglionic neurons

The neurotrophin, NGF, is located in lower airway smooth muscle and glands, tissue innervated by local parasympathetic ganglionic neurons, and NGF is increased by inflammation in this tissue. However, the acute effect of NGF release on parasympathetic nerves is unknown. Using single cell current clamp recording of tracheal and bronchial ganglionic neurons, NGF (30 ng/ml) decreased the action potential afterhyperpolarization amplitude and duration from 17±2mV and 52±8ms, respectively, in control neurons to 11±3 mV and 34±14ms after treatment (n=7); NGF (1–30 ng/ml) had no effect on accommodation in phasic (n=4) or tonic (n=3) neurons. NGF (1–30 ng/ml) had no effect on the resting membrane potential or input resistance (n=7). Using intracellular QX314, vagus nerve stimulation evoked 16±2mV (range of 13–19 mV; n=6) nicotinic fast excitatory postsynaptic potentials (fEPSPs) in control neurons which were increased to 21±3mV in the presence of 30ng/ml NGF. Using whole cell patch clamp recording, DMPP (1μM, 30s) evoked a 213pA inward current in a control neuron and 258pA in the presence of NGF (10ng/ml). In this study, we determined that NGF affects action potential and synaptic membrane properties of mouse ganglionic neurons; the effect on fEPSPs is possibly postsynaptic as NGF increased the nicotinic current. This study provides additional support that NGF has short‐term, as well as long‐term, effects on adult mammalian neurons. This work was supported by NIH‐NHLBI (ACM).

Nerve growth factor acutely potentiates synaptic transmission in vitro and induces dendritic growth in vivo on adult neurons in airway parasympathetic ganglia

Elevated levels of nerve growth factor (NGF) and NGF-mediated neural plasticity may have a role in airway diseases such as asthma and chronic obstructive pulmonary disease (COPD). Although NGF is known to affect sensory and sympathetic nerves, especially during development, little is known regarding its effect on parasympathetic nerves, especially on adult neurons. The purpose of this study was to analyze the acute and chronic effects of NGF on the electrophysiological and anatomical properties of neurons in airway parasympathetic ganglia from adult guinea pigs. Using single cell recording, direct application of NGF caused a lasting decrease in the cumulative action potential afterhyperpolarization (AHP) and increased the amplitude of vagus nerve-stimulated nicotinic fast excitatory postsynaptic potentials. Neuronal responsiveness to nicotinic receptor stimulation was increased by NGF, which was blocked by the tyrosine kinase inhibitor, K-252a, implicating neurotrophin-specific (Trk) receptors. Neurotrophin-3 and brain-derived neurotrophic factor had no effect on the synaptic potentials, AHP, or nicotinic response; inhibition of cyclooxygenase with indomethacin inhibited the effect of NGF on the cumulative AHP. Forty-eight hours after in vivo application of NGF to the trachealis muscle caused an increase in dendritic length on innervating neurons. These results are the first to demonstrate that NGF increases the excitability of lower airway parasympathetic neurons, primarily through enhanced synaptic efficacy and changes to intrinsic neuron properties. NGF also had dramatic effects on the growth of dendrites in vivo. Such effects may indicate a new role for NGF in the regulation of parasympathetic tone in the diseased or inflamed lower airways.

Short-term synaptic plasticity in rabbit pancreatic ganglia

The extrinsic innervation of the pancreas converges on a plexus of intrinsic pancreatic ganglia whose cholinergic neurons innervate acini, ducts, islets and blood vessels. Therefore, understanding ganglionic transmission is essential for understanding neural control of pancreatic secretion. Intracellular recordings of nicotinic fast excitatory postsynaptic potentials (fEPSPs) and action potentials (APs) were used to characterize and compare transmission in ganglia from the head/neck and body regions of the rabbit pancreas. Paired-pulse facilitation (PPF) or depression (PPD) of fEPSPs was observed in ganglia from both regions with PPF peaking and disappearing at shorter inter-stimulus intervals than PPD. PPF was most frequent in the head/neck (60%) and PPD (50%) in the body. Repetitive stimulation (10 Hz/5 s) evoked multiple forms of mid- and post-train plasticity. Facilitation during the first 1–2 s of train stimulation was reduced or reversed with continued stimulation due to development of synaptic depression and mid-train depression was of greater magnitude in the head/neck region. A brief (∼ 10 s) post-train augmentation was followed by a 1–2 min post-train depression that appeared to result from inhibition of ACh release. Regional differences in the frequency, magnitude, or duration of all forms of synaptic plasticity suggested regional differences in the extrinsic innervation patterns and possibly the function of pancreatic ganglia. In conclusion, rabbit pancreatic ganglia exhibit multiple forms of short-term synaptic plasticity that markedly alter the probability of postsynaptic firing, consistent with these ganglia being critical sites of synaptic integration and autonomic regulation of pancreatic secretion.

Actions of galanin on neurotransmission in the submucous plexus of guinea pig small intestine

Electrophysiologic recording methods were used to study the actions of galanin on synaptic transmission in the submucous plexus of guinea pig ileum. Exposure to galanin resulted in concentration-dependent suppression of slow noradrenergic inhibitory postsynaptic potentials and fast nicotinic excitatory postsynaptic potentials in the majority of neurons. Failure of galanin to suppress nicotinic depolarizing responses to micropressure pulses of acetylcholine and failure to suppress hyperpolarizing responses to micropressure pulses of norepinephrine suggested that galanin acted at presynaptic inhibitory receptors to suppress release of acetylcholine and norepinephrine. Galanin suppressed slow excitatory postsynaptic potentials in eight of eight neurons with AH (after-hyperpolarization) type electrical behavior and in none of 26 neurons with S (synaptic) type electrical behavior. Suppression of excitatory neurotransmission in AH neurons was always associated with membrane hyperpolarization. Excitatory responses caused by experimentally applied substance P were also inhibited by galanin. Galanin-(1–16) and galanin-like peptide mimicked the inhibitory actions of galanin on neurotransmission. The selective galanin GAL2 receptor agonist [ d-Trp 2]galanin was inactive. The chimeric peptides, galanin-(1–13)-spantide I, galantide, galanin-(1–13)-neuropeptide Y(25–36) amide, galanin-(1–13)-bradykinin-(2–9)amide and galanin-(1–13)-Pro-Pro-Ala-Leu-Ala-Leu-Ala amide all produced varying degrees of suppression of the synaptic potentials. The evidence suggests that the galanin GAL1 receptor, but not the galanin GAL2 receptor, mediated the presynaptic and postsynaptic inhibitory actions of galanin.

The integrative membrane properties of human bronchial parasympathetic Ganglia neurons.

Parasympathetic ganglia neurons in the lower airway of laboratory animals have membrane properties associated with integration of signals from the central nervous system. In this study, intracellular recordings were made from parasympathetic ganglia located on bronchi from human lungs in order to determine the level of integration provided by human neurons. Ganglion neurons were characterized as either tonic or phasic: tonic neurons responded with repetitive action potentials sustained throughout a depolarizing current step whereas phasic neurons generated one action potential and accommodated. Phasic neurons could be further differentiated as having either short or long duration after hyperpolarizing potentials following single action potentials. In phasic neurons, stimulation of preganglionic nerves elicited one or two populations of nicotinic fast excitatory postsynaptic potentials (fEPSPs) that were graded in amplitude, subthreshold for action potential generation, and decreased in amplitude during higher frequency stimulation. In tonic neurons, single preganglionic stimuli evoked two to five populations of fEPSPs, one to three of which were at threshold for action potential generation. Dye injection into the neurons revealed multiple, branching dendrites. These results provide evidence that human bronchial ganglion neurons have unique membrane properties and anatomical characteristics associated with integrating presynaptic stimuli. Changes in these properties may thus affect output from these ganglia and, consequently, autonomic tone in the lower airways.

Duodenal neurons provide nicotinic fast synaptic input to sphincter of Oddi neurons in guinea pig.

We have investigated the existence of neural connections between the duodenum and the sphincter of Oddi (SO). Stimulation of duodenal myenteric fiber bundles elicited synaptic responses in SO neurons, which included nicotinic fast excitatory postsynaptic potentials (EPSPs), slow EPSPs, and alpha(2)-adrenoreceptor-mediated inhibitory postsynaptic potentials. After 48 h in organ culture, when extrinsic fibers had diminished, only the fast EPSPs persisted. Duodenal mucosal stimulation also elicited nicotinic fast EPSPs in SO neurons. There was no association between the SO neurons that received duodenal input and their chemical coding. A reciprocal projection also exists from the SO to the duodenum. In acute and cultured preparations, duodenal myenteric stimulation caused antidromic responses in 20% of SO neurons. Furthermore, 45.6 +/- 10.5 neurons in SO ganglia were retrogradely labeled from dye application sites in the duodenum. It is proposed that bidirectional neural communication occurs between the duodenum and the SO and that duodenal neurons provide excitatory fast synaptic input to SO neurons through a reflex that can be activated at the duodenal mucosa.

IL-1beta and IL-6 excite neurons and suppress nicotinic and noradrenergic neurotransmission in guinea pig enteric nervous system.

Conventional intracellular microelectrodes and injection of biocytin were used to study the actions of IL-1beta and IL-6 on electrical and synaptic behavior in morphologically identified guinea pig small intestinal submucous neurons. Exposure to nanomolar concentrations of either IL-1beta or IL-6 stimulated neuronal excitability. The excitatory action consisted of depolarization of the membrane potential, decreased membrane conductance, and increased discharge of action potentials. Excitatory action of IL-1beta was suppressed by the natural IL-1beta human receptor antagonist. Electrical stimulation of sympathetic postganglionic axons evoked inhibitory postsynaptic potentials (IPSPs), and stimulation of cholinergic axons evoked nicotinic fast excitatory postsynaptic potentials (EPSPs). Both kinds of synaptic potentials occurred in neurons with uniaxonal morphology believed to be secretomotor neurons. Either IL-1beta or IL-6 suppressed the noradrenergic IPSPs and the fast EPSPs, and the two acted synergistically when applied in combination. Suppression of the IPSP resulted from presynaptic inhibition of the release of norepinephrine from sympathetic nerves. The results suggest that the presence of either or both inflammatory cytokines will release the sympathetic brake from secretomotor neurons to the intestinal crypts and from nicotinic synapses in the integrative microcircuits, where norepinephrine is known to have a presynaptic inhibitory action. This, in concert with excitation of secretomotor neurons, may lead to neurogenic secretory diarrhea.

Organization of intrinsic cholinergic neurons projecting within submucosal plexus of guinea pig ileum.

Electrophysiological techniques were employed to examine the organization of the projections of submucosal neurons in the submucosal plexus of guinea pig ileum. These neurons were activated by focal pressure-pulse application of 5-hydroxytryptamine (5-HT) to single ganglia in submucosal preparations in vitro, and resulting fast excitatory postsynaptic potentials (EPSPs) were recorded intracellularly in S-type neurons. 5-HT-evoked fast EPSPs were blocked by TTX, hexamethonium, and ICS-205-930 (tropisetron). 5-HT was applied either directly to the ganglion containing the neuron recorded intracellularly or to adjacent ganglia positioned at increasing distances on either side of the impaled cell in circumferential or longitudinal orientations. All S-type neurons recorded in this study (n = 103) received nicotinic fast EPSPs from cholinergic neurons when 5-HT was applied directly to the ganglion containing the impaled neuron. Stimulation of adjacent ganglia also evoked nicotinic fast EPSPs, but the number of neurons that received this input decreased as the distance between the stimulus and the impaled cell increased. Maximal projections were 3 mm in the circumferential and orad-to-aborad orientations. There were no significant projections in the aborad-to-orad direction. These findings suggest that S-type neurons in the submucosal plexus are innervated by intrinsic cholinergic neurons that project over relatively short distances and have a distinct orad-to-aborad polarity.

An Electrophysiological Study of Pterygopalatine Ganglion Neurons in the Rabbit

A preparation was developed to investigate the synaptic connections of the pterygopalatine ganglion (P.P.G.) neurons of the rabbit. Many neurons received synaptic inputs from more than one preganglionic fiber in the vidian nerve (preganglionic nerve) with a wide range of conduction velocities. It is assumed that P.P.G. neurons integrate synaptic inputs from the higher centers. In some neurons, nicotinic fast excitatory postsynaptic potentials (e.p.s.p.s) were evoked in response to stimulation of one posterior nasal nerve, and also an antidromic action potential occurred in response to stimulation of the other posterior nasal nerve. Fast e.p.s.p.s were recorded from a ganglion neuron in response to the cooling stimulation of the nasal mucosa. These results revealed that the ganglionic reflex arc mediated through the nasal afferent fibers exists in the P.P.G. Moreover, the appearance of slow inhibitory postsynaptic potentials (slow i.p.s.p.s) and slow excitatory postsynaptic potentials (slow e.p.s.p.s) in response to repetitive stimuli of the vidian nerve may influence the synaptic transmissions of P.P.G. neurons. The P.P.G. plays a significant role as a complicated key point of signal transmissions from both the periphery and higher centers.

Synaptic inputs to morphologically identified myenteric neurons in guinea pig rectum from pelvic nerves.

Neurobiotin-filled microelectrodes were used to investigate electrical and synaptic behavior and morphological characteristics of rectal myenteric neurons that received synaptic inputs from the pelvic nerves. Stimulation of the pelvic nerve at low frequencies (< 3.3 Hz) evoked nicotinic fast excitatory postsynaptic potentials (fast EPSPs) in 45.3% of rectal neurons. Pelvic fast EPSPs were found in S/type 1, AH/type 2, type 3, or single-spike neurons that had a single long process preferentially projecting in the orad direction. Stimulation of the pelvic nerve at higher frequencies (5-20 Hz) elicited slow membrane excitation in 13.9% of the neurons. They were either AH/type 2 neurons with Dogiel II morphology or S/type 1 neurons with a single long process. Hexamethonium (100 microM) blocked pelvic fast EPSPs more quickly than those evoked by fiber tract stimulation but did not affect slow excitatory response. The results suggested the presence of more than one nicotinic-cholinergic synapse in the pelvic nerve pathway and the possible release of a noncholinergic excitatory substance from the afferent nerve terminals. It is possible that a subpopulation of rectal neurons, which receive a fast EPSP and have a single long process that projects in the orad direction, might be interneurons that mediate the defecation reflex.

Effects of an indene-derivative, TN-871, on synaptic transmission in a sympathetic ganglion : presynaptic actions on neurotransmitter release

Intracellular recordings were made from bullfrog sympathetic ganglion cells to elucidate effects of 2-n-butyl-1-(4-methylpiperazinyl)-5,6-methylenedioxyindene.2 HCl (TN-871) on synaptic transmission. TN-871 at 30 nM augmented cholinergic nicotinic fast excitatory postsynaptic potentials (fast EPSPs), whereas the drug at 3 microM reversibly depressed them, without affecting acetylcholine-induced depolarizations. TN-871 did not affect active and passive electrical properties of the ganglion cells. The quantal analysis method was applied to the fast EPSPs in a 0.54 mM Ca2+/7.56 mM Mg2+ Ringer's solution. The mean quantal content was significantly increased by TN-871 at 30 nM but significantly decreased at 3 microM. TN-871 at 300 nM either increased or decreased the mean quantal content. The mean quantal size of the fast EPSPs was not changed by TN-871 at the concentrations examined. Fast EPSPs in a 0.99 mM Ca2+/4.86 mM Mg2+ Ringer's solution were not affected by nicardipine, but were inhibited in amplitude by omega-conotoxin in a concentration-dependent manner. It is likely that TN-871, in high concentrations, might block omega-conotoxin-sensitive N-type calcium channels in the presynaptic terminals. These results indicate that TN-871 modulates transmitter release from preganglionic nerve terminals without changing the postsynaptic sensitivity of the ganglion cells to ACh.

Sympathetic input to ganglia of the guinea pig sphincter of Oddi

Intracellular recording and immunohistochemical staining techniques were used to establish whether sphincter of Oddi (SO) ganglia are a target of sympathetic input to this region. Norepinephrine (0.01-10.0 microM) decreased the amplitude of the nicotinic fast excitatory postsynaptic potential (EPSP) evoked by stimulation of interganglionic fiber tracts, with a half-maximal inhibitory concentration (EC50) of 300 nM. Norepinephrine did not alter the responsiveness of the neurons to acetylcholine. The alpha 2-adrenoreceptor agonist UK-14304 mimicked the norepinephrine-induced effect with a EC50 of 2.5 nM, whereas alpha 1- and beta-adrenoreceptor agonists had no effect on the EPSP. The alpha 2-adrenoreceptor antagonist idazoxan (1.0 microM) inhibited the UK-14304 response, with a dissociation constant of 1.0 nM. Release of endogenous catecholamines, by the addition of tyramine (100 microM) to the bath, caused an idazoxan-sensitive decrease in the amplitude of the fast EPSP. In the minority of SO neurons that exhibited inhibitory postsynaptic potentials (IPSPs), norepinephrine caused a hyperpolarization of the membrane potential. The IPSP and the norepinephrine-induced hyperpolarization were inhibited by alpha 2-adrenoreceptor antagonists. Desipramine (1.0 microM), an uptake inhibitor, reversibly increased the amplitude of the IPSP. Immunoreactivities for tyrosine hydroxylase and dopamine beta-hydroxylase were coexistent in nerve fibers and nonexistent in cell bodies in the ganglionated plexus of the SO. The results of this study indicate that norepinephrine acts pre- and postsynaptically as an inhibitory neurotransmitter in SO ganglia.

A physiologically-evoked M 1-muscarinic depolarization in guinea-pig inferior mesenteric ganglion neurons

The possibility of physiologically-evoked muscarinic excitatory synaptic potentials was examined in the inferior mesenteric ganglion (IMG) with intracellular microelectrodes in vitro. Three types of depolarizing responses were evoked concurrently by colonic distension: (1) fast nicotinic excitatory postsynaptic potentials (EPSPs); (2) an ‘intermediate’ time course depolarization and (3) a long time course potential that persisted throughout the period of distension. After hexamethonium was superfused over the ganglia the long time course potential was observed in 91% of IMG neurons. Intermediate time course depolarizations were observed in 100% of IMG neurons and correlated with each propulsive contraction of the distal colon. The intermediate depolarizations had an average amplitude of 1.8 ± 0.1 mV ( n = 175 individual events; 27 preparations) with an average duration of 11.9 ± 0.8 sec ( n = 28 individual events). The intermediate time course synaptic potentials were accompanied by an increase in input resistance of 15% ( n = 6). Superfusion of atropine (1 μM; n = 6) on the IMG or the M 1-selective antagonist pirenzepine (1 μM; n = 5) abolished the intermediate time course synaptic potentials during distension. Superfusion of the M 2-selective antagonist AF-DX 116 (1 μM; n = 4) had no effect. In all preparations examined, distension-induced intermediate time course depolarizations were blocked by tetrodotoxin (TTX) (3 μM). Pressure ejection of carbachol (1 mM, 60–100 ms pulses) evoked depolarizations of similar amplitude (6.5 ± 0.7 mV; n = 18) and duration to the intermediate depolarizations observed during propulsive contractions. In all preparations examined, carbachol-induced depolarizations were antagonized by atropine (1 μM) and pirenzepine (1 μM) but not by AF-DX 116 (1 μM). These experiments demonstrate the existence of a physiologically-evoked muscarinic excitatory synaptic potential in IMG neurons of the guinea pig which is mediated through the M 1-receptor subtype.

The electrophysiological effects of endogenous GABA in the guinea-pig inferior mesenteric ganglion.

1. GABA receptor-modulating drugs and intracellular recording techniques were used to determine the functional significance of peripheral afferent GABA-containing nerves projecting from the distal colon to sympathetic neurones in the inferior mesenteric ganglion of the guinea-pig. 2. GABAA receptor-modulating drugs added selectively to the inferior mesenteric ganglion side of a two-compartment bath had pronounced effects on on-going colonic afferent cholinergic synaptic input. Bicuculline (20 microM) decreased the amplitude and frequency of fast excitatory postsynaptic potentials (EPSPs) by 40% whereas diazepam (5 microM) increased cholinergic input by 43%. Neither drug had any effect on the resting membrane potential or membrane input resistance of ganglion cells. 3. Bicuculline (20 microM) significantly reduced, whereas diazepam (5 microM) significantly enhanced, distension-induced increases in nicotinic fast EPSPs and action potentials. 4. Slow EPSPs evoked by colonic distension were not affected by bicuculline or diazepam. 5. Manual voltage clamp of the postsynaptic depolarizing response to exogenous GABA revealed GABA-induced presynaptic facilitation of colonic afferent but not central preganglionic efferent cholinergic synaptic input. 6. The data suggest that endogenously released GABA participates in on-going colo-colonic reflex activity by acting on presynaptic GABAA receptors to facilitate release of acetylcholine from colonic mechanosensory nerves.

Physiological and morphological properties of neurons in sphincter of Oddi region of the guinea pig.

Intracellular recordings and dye injections were used to investigate neurons located in ganglia of the sphincter of Oddi (SO) region in guinea pigs. Four types of neurons were encountered based on physiological properties. The two most abundant types, tonic and phasic, had similar membrane characteristics and morphologies but yet could be differentiated by their spiking characteristics. Tonic cells spiked throughout a depolarizing current pulse and were sometimes spontaneously active. Phasic cells fired only a single action potential at the onset of a current pulse regardless of stimulus amplitude or duration. Both tonic and phasic cells had Dogiel type I morphologies. They typically had a single long process and several very short processes emanating from the soma. NADPH diaphorase activity was demonstrated in cells with similar morphologies, indicating that nitric oxide may be an intrinsic transmitter in some of these cells. Cells with a prolonged afterhyperpolarization (AH cells), similar to the type 1/AH cells of the gut, were rarely encountered. This finding was consistent with the observation that very few calbindin D-immunoreactive neurons exist in this region. Action potentials could not be generated in the fourth type of neuron, called nonspiking neurons, even though they did receive synaptic input. Most tonic and phasic cells received at least one nicotinic fast excitatory postsynaptic potential (EPSP). In addition, both slow EPSPs and inhibitory postsynaptic potentials were observed. Most AH cells received only slow excitatory synaptic input.

Presynaptic inhibitory effects of the peptides NPY, PYY and PP on nicotinic EPSPs in guinea-pig gastric myenteric neurones.

1. Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) affect gastrointestinal effector systems. Although their precise mode of action is unknown it is suggested that their effects are partly mediated by enteric neurones. Therefore, the aim of this study was to investigate the effects of NPY, PYY, avian PP (aPP) and bovine PP (bPP) on the electrophysiological behaviour of gastric myenteric neurones using intracellular recording methods. 2. In all thirty-one neurones tested, electrical stimulation of interganglionic fibre tracts evoked cholinergic, nicotinic fast excitatory postsynaptic potentials (fEPSPs). NPY, PYY and bPP (10-500 nM) inhibited the fEPSPs in a concentration-dependent manner. Additionally, these peptides reversibly abolished spontaneously occurring nicotinic fEPSPs. None of the peptides exhibited any effect on the response to exogenously applied acetylcholine or on the postsynaptic excitability of the neurones. The inhibitory effects on the fEPSPs could not be reversed by perfusion of the alpha-adrenoceptor antagonist, phentolamine (1 microM). 3. In contrast, aPP had no significant effect on fEPSPs even when perfused at the high concentration of 1 microM. 4. The results suggest that NPY, PYY and bPP, but not aPP, act presynaptically to inhibit acetylcholine release from myenteric neurones of the gastric corpus, thereby suppressing fEPSPs. The study indicates a modulatory role for NPY, PYY and bPP in synaptic interactions within the enteric nervous system.

Introduction to slow synaptic potentials and their neuromodulation by dopamine.

The existence of two muscarinically mediated slow postsynaptic potentials (PSPs) and a noncholinergic (peptidergic) late-slow PSP was established in the 1960s. These have synaptic delays and PSP durations 100-10,000 times those for the nicotinic (fast) excitatory post-synaptic potential (EPSP). Evidence is reviewed for an against the proposal that, in rabbit superior cervical ganglia, the slow (s-) inhibitory postsynaptic potential requires a second transmitter, dopamine, released by muscarinic action on interneurones (the small, intensely fluorescent cells). The s-EPSP in frog ganglia appears only in already depolarized cells by a muscarinic closure of the M (voltage-sensitive K+) channels. But the large s-EPSP in mammalian neurones, not depolarized, is generated largely via other mechanisms, especially one involving cyclic GMP. Dopamine also produces a long-term enhancement (LTE) of the muscarinic slow PSPs in rabbit superior cervical ganglia, whether dopamine is applied exogenously or released intraganglionically by preganglionic nerve impulses at 10 s-1. LTE is producible heterosynaptically, and it persists well over 3 h; a noncholinergic (peptide?) transmitter may contribute to the initial 30 min of LTE. LTE is mediated by a D1 receptor coupled to cyclic AMP; it is blocked by cyclic GMP or low Ca2+ or calmidazolium (a calmodulin inhibitor). The modulatory process of LTE has certain similarities to, but also fundamental differences from, the long-term potentiation known in the hippocampus.

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 cell 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 μM 5′-N-ethylcarboxamidoadesosine (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.