Nicotinic Fast Excitatory Postsynaptic Potentials Research Articles

Effect of nifedipine on neurons of guinea pig celiac ganglion

The effect of nifedipine on electrophysiological membrane properties and nicotinic neurotransmission of guinea pig celiac ganglion neurons was studied using intracellular recordings in vitro. Nifedipine in concentrations of 0.1–10 μM did not affect membrane potential, membrane input resistance or the amplitude and duration of action potentials induced by intracellular current injection. Higher doses of nifedipine (0.1–1 mM) significantly reduced the amplitude and extended the duration of action potentials induced by intracellular current injection. Superfusion of the ganglia with nifedipine in concentrations of 0.1–10 μM significantly inhibited nicotinic fast excitatory postsynaptic potentials (f-EPSPs) and orthodromic action potentials evoked by nerve stimulation. This depressant effect of nifedipine on synaptic transmission was eliminated with high Ca 2+ (12.5 mM). Nifedipine (10 μM) did not affect the postsynaptic effect of exogenous acetylcholine (ACh), but significantly reduced the quantal content but not the quantal size of evoked f-EPSPs in a low Ca 2+ (0.5 mM), high Mg 2+ (5.5 mM) Krebs solution. Nifedipine in concentration of 10 μM did not affect afterspike hyperpolarization (AH) and post-tetanic hyperpolarization (PTH), which have been recognized to be generated mainly by an increase of calcium-dependent potassium conductance. Higher doses of nifedipine (0.1–1 mM) significantly depressed AH and PTH. These experimental results suggest that nifedipine in concentrations of 0.1–10 μM exerts an inhibitory effect on nicotinic neurotransmission without affecting the membrane properties of the guinea pig celiac ganglion neurons. This inhibitory effect of nifedipine on synaptic transmission may result from blocking L-type calcium channels and reducing the quantal release of ACh from the presynaptic nerve terminals.

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 M i 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 l neurons, when applied in concentrations of 0.1–2000 nM, although higher concentrations (10–100 μM) did have a significant non-specific effect on the postsynaptic membrane. Micromolar concentrations of telenzepine (1–2 M) 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 μiM telenzepine. In contrast, at submicromolar concentrations (100 nM). telenzepine abolished responses to either muscarine or the muscarinic component of the acetylcholin0e response. The excitatory effect of muscarine at postsynaptic M 1 receptors was dese 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 IC 50 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 μM telenzepine, while the majority of them remained unaffected, indicating that some slow excitatory postsynaptic potentials are mediated by the muscarinic action of released acetylcholine. The antagonist profile displayed by telenzepine at postsynaptic M 1 receptors confirms that this compound has specificity and high affinity for the postsynaptic m 1 receptors located on myenteric neurons of the small intestine.

Intracellular recordings from gallbladder ganglia of opossums

Postganglionic parasympathetic neurons of the opossum gallbladder were studied using morphological and intracellular electrophysiological recording techniques. On average there were 17 ganglia/cm2 with 8 neurons/ganglion arranged as loosely or densely packed clusters. Intracellular injection of horseradish peroxidase identified two types of neurons. Most of the neurons (approximately 80%) had a single long process that extended beyond the ganglion border with few or no short additional neurites. The other type was multipolar, having several processes that did not appear to extend beyond the ganglion border. Gallbladder neurons had passive and active electrical properties similar to other parasympathetic ganglion cells. Most of neurons studied (71%) responded to intracellular injection of suprathreshold depolarizing current with a short burst of action potentials. The remaining neurons (29%) responded with only an initial action potential. Seventy percent of the neurons tested received nicotinic fast excitatory postsynaptic potentials (EPSPs). Nineteen percent of these neurons also exhibited a slow depolarizing response following repetitive orthodromic nerve stimulation. Cholecystokinin octapeptide sulfate (CCK-8, 0.1-500 nM) had no effect on resting membrane potential or membrane input resistance. However, CCK-8 increased the amplitude of fast EPSPs evoked by orthodromic nerve stimulation by facilitating release of acetylcholine because CCK-8 had no effect on the postsynaptic response to exogenously applied acetylcholine. These data suggest that the ganglionated plexus of the gallbladder may mediate some of the observed effects of CCK-8 on gallbladder motility.

Use of stereoisomers of zacopride to analyze actions of 5-hydroxytryptamine on enteric neurons

Two subtypes of excitatory 5-hydroxytryptamine (5-HT) receptor, 5-HT1P and 5-HT3, are found on type 2-AH neurons of the guinea pig myenteric plexus. The 5-HT1P receptor mediates a slow and the 5-HT3 receptor a fast depolarization of these cells, however, the role of these receptors in the physiology of the gut is unknown. Renzapride (BRL 24924), a substituted benzamide, has previously been found to antagonize responses of myenteric neurons mediated by both 5-HT1P and 5-HT3 receptors. The effects on myenteric type 2-AH neurons of a structurally similar benzamide, zacopride, which unlike renzapride has S and R stereoisomers, were investigated to gain further insight into 5-HT receptor function. In contrast to renzapride, S-, but not R-zacopride, was found to mimic the 5-HT1P receptor-mediated slow response to 5-HT. Desensitization of 5-HT1P receptors with 5-HT inhibited slow depolarizing responses to S-zacopride, and desensitization with S-zacopride antagonized slow responses to 5-HT. Responses to S-zacopride were also inhibited by renzapride and the 5-HT1P receptor antagonist N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP). S-zacopride, like renzapride and 5-HT, presynaptically inhibited nicotinic fast excitatory postsynaptic potentials, an effect that can be mediated by 5-HT1P or 5-HT1A receptors. Both S and R stereoisomers of zacopride antagonized 5-HT3 receptor-mediated fast responses to 5-HT. Unlike 5-HTP-DP, neither zacopride or its stereoisomers nor renzapride inhibited the binding of 5-[3H]HT to 5-HT1P receptors. [3H]zacopride (5-10 nM) was found to bind to a site in the gut from which it could be displaced by a 1,000-fold excess of renzapride and S-zacopride (but not R-zacopride) greater than 5-HTP-DP much greater than the 5-HT3 receptor antagonist ICS 205-930. These observations suggest that, in addition to 5-HT3 receptors, there is a benzamide binding site on myenteric neurons that interacts with, but is distinct from, the 5-HT recognition site of 5-HT1P receptors. Benzamides may affect coupling of the 5-HT1P receptor to its effector.

Effects of Cisapride on Cholinergic Neurotransmission and Propulsive Motility in the Guinea Pig Ileum

The actions of cisapride on electrical behavior of single myenteric neurons and on the propulsive activity of segments of ileum were studied in vitro. Cisapride (10-300 nM) did not affect the membrane potential, resting properties, or active properties of S neurons. The amplitude of fast nicotinic excitatory postsynaptic potentials recorded from S cells was increased by cisapride in the concentration range of 10 nM to 1 microM. Higher cisapride concentrations (3-10 microM) reduced the amplitude of fast excitatory postsynaptic potentials. Potentiation of fast excitatory postsynaptic potentials by cisapride was antagonized by ICS 205-930 (1 microM) but was unaffected by GR 38032F (1 microM), both compounds being 5-HT3-receptor antagonists. Cisapride did not modify the electrical behavior of AH neurons except at the highest concentrations (3-10 microM), which caused hyperpolarization of some neurons. The propulsive efficiency (i.e., number of peristalses and total amount of fluid ejected per unit of time) of isolated segments of ileum was enhanced by cisapride (100 nM to 3 microM). Higher cisapride concentrations (6 or 10 microM) had a depressant action on propulsive activity. The stimulatory effect of cisapride on propulsion was not antagonized by ICS 205-930 (300 nM or 1 microM). These data indicate that cisapride facilitates cholinergic transmission in the myenteric plexus of guinea pig ileum and that this effect may be at least partially responsible for the increased propulsive efficiency observed in ileal segments.

Modulation of synaptic transmission in the rabbit coeliac ganglia by gastric and duodenal mechanoreceptors

The involvement of duodenal and gastric mechanoreceptors in the modulation of synaptic transmission was investigated in a rabbit sympathetic prevertebral ganglion. The present study was performed in vitro onp the coeliac plexus connected to the stomach and the duodenum. The electrical activity of ganglionic neurons was recorded using intracellular recording techniques. The patterns of synaptic activation of these ganglionic neurons in response to the activation of mechanoreceptors by gastric or duodenal distension were investigated. Although gastric or duodenal distension was unable to elicit any fast synaptic activity in ganglionic neurons, it produced either an inhibition or a facilitation of the fast nicotinic excitatory postsynaptic potentials elicited by stimulation of the thoracic splanchnic nerves. In addition, this distension triggered long-lasting (3–11 min) modifications in the electrical properties of the ganglionic neurons, i.e. slow depolarizations (6–18 mV) or slow hyperpolarizations (3–6 mV), which were sometimes associated with a decrease in the input membrane resistance. After cooling of the nerves connecting the coeliac ganglia to the stomach, the activation of gastric or duodenal mechanoreceptors was no longer able to modify the fast synaptic activation or the electrical properties of the ganglionic neurons. The results demonstrate that gastric and duodenal mechanoreceptors project onto neurons of the coeliac ganglia and change their excitability as well as the central inputs they receive. The long duration of these modifications suggests that gastric and duodenal mechanoreceptors can modulate the activity of the neurons of the coeliac ganglia.

The effect of serotonergic agents on haloperidol-induced catalepsy

The actions of cisapride on electrical behavior of single myenteric neurons and on the propulsive activity of segments of ileum were studied in vitro. Cisapride (10–300 nM) did not affect the membrane potential, resting properties, or active properties of S neurons. The amplitude of fast nicotinic excitatory postsynaptic potentials recorded from S cells was increased by cisapride in the concentration range of 10 nM to 1 μM. Higher cisapride concentrations (3-10 μM) reduced the amplitude of fast excitatory postsynaptic potentials. Potentiation of fast excitatory postsynaptic potentials by cisapride was antagonized by ICS 205-930 (1 μM) but was unaffected by GR 38032F (1 μM), both compounds being 5-HT3-receptor antagonists. Cisapride did not modify the electrical behavior of AH neurons except at the highest concentrations (3-10 μM), which caused hyperpolarization of some neurons. The propulsive efficiency (i.e., number of peristalses and total amount of fluid ejected per unit of time) of isolated segments of ileum was enhanced by cisapride (100 nM to 3 μM). Higher cisapride concentrations (6 or 10 μM) had a depressant action on propulsive activity. The stimulatory effect of cisapride on propulsion was not antagonized by ICS 205-930 (300 nM or 1 μM). These data indicate that cisapride facilitates cholinergic transmission in the myenteric plexus of guinea pig ileum and that this effect may be at least partially responsible for the increased propulsive efficiency observed in ileal segments.

Differential localization of 5-HT1 receptors on myenteric and submucosal neurons.

Intracellular recordings were made from guinea pig enteric neurons, and the effects of 5-hydroxytryptamine (5-HT) and the 5-HT1 selective agonists 5-carboxyamidotryptamine (5-CT) and 8-hydroxy-2-(n-dipropylamino)tetralin (DPAT) were studied on membrane potential and synaptic potentials. Most myenteric AH neurons were hyperpolarized when these agonists were applied by superfusion; this hyperpolarization was due to an increase in potassium conductance. Membrane hyperpolarizations to 5-HT, 5-CT, or DPAT were never observed in submucous neurons. Fast nicotinic excitatory postsynaptic potentials (EPSPs) and slow EPSPs recorded from S neurons in the myenteric plexus were suppressed by 5-HT, 5-CT, and DPAT; slow EPSPs in myenteric AH neurons were also inhibited by these agonists. Fast and slow EPSPs recorded from submucous S neurons were not affected by 5-CT or DPAT. However, slow EPSPs recorded from submucous AH neurons were readily blocked by 5-CT and DPAT. The results indicate that 5-HT1 receptors are located on the cell bodies of myenteric but not submucosal neurons. The nerve terminals that release the mediator or mediators of fast and slow synaptic potentials in myenteric neurons also have 5-HT1 receptors and presumably arise from other myenteric neurons; the nerve terminals responsible for the slow EPSP to AH neurons seem to be the only elements of the submucous plexus that express 5-HT1 receptors.

Long-term potentiation at nicotinic synapses in the rat superior cervical ganglion.

1. Nicotinic fast excitatory postsynaptic potentials (fast EPSPs) were recorded intracellularly from postganglionic neurones in the isolated rat superior cervical ganglion. 2. An hours-long potentiation of the fast EPSP could be induced by brief tetanic stimulation of the preganglionic nerve (5 Hz for 5 s to 20 Hz for 20 s). While long-term potentiation (LTP) can be detected in every ganglion by extracellular techniques, LTP was induced in only two-thirds of the nicotinic synaptic responses. 3. Muscarinic blockade with atropine did not prevent LTP of the fast EPSP. 4. LTP of the fast EPSP did not correlate with changes in input resistance nor cell potential, as recorded in the soma. 5. The formation of nicotinic LTP appeared to depend upon stimulation of the nerve terminals. Non-synaptic tetanic depolarization of the postganglionic neurone, effected by injecting depolarizing current pulses through the intracellular microelectrode, was not sufficient. LTP could be induced by synaptic tetani in two-thirds of the same neurones. 6. The response to exogenous 1,1-dimethyl-4-phenylpiperazinium (DMPP), a selective nicotinic agonist, was not increased during nicotinic synaptic LTP. This was true whether DMPP was applied by pressure-ejection from an extracellular micropipette during intracellular recording, or by brief superfusion during sucrose-gap recording of postganglionic responses. 7. Responses to exogenous acetylcholine and carbachol were increased during nicotinic LTP when these non-selective cholinergic agonists were applied by pressure-ejection during intracellular recording. However, the potentiation of the fast EPSP was always at least twofold greater than the potentiation of the response to these exogenous agonists. 8. Potentiation of the responses to acetylcholine and carbachol may have been due to long-term enhancement of muscarinic responses. Thus, no postsynaptic basis for nicotinic LTP was uncovered in these studies.

Neuropeptides mark functionally distinguishable cholinergic enteric neurons

Subpopulations of physiologically identified cholinergic enteric neurons in cell culture contain somatostatin (SOM)- or vasoactive intestinal peptide (VIP)-like immunoreactivity (LIR). These subpopulations differ in their synaptic effects on other neurons: cholinergic neurons that contains SOM-LIR cause fast nicotinic excitatory postsynaptic potentials (EPSPs) that have significantly larger amplitudes than do EPSPs caused by cholinergic neurons that lack SOM-LIR. Cholinergic neurons containing VIP-LIR cause slow non-cholinergic depolarizations in addition to fast nicotinic EPSPs. These findings are the first correlation between neuropeptide content and functional differences in the synaptic effects of subpopulations of cholinergic enteric neurons.

Pre- and postsynaptic actions of noradrenaline and clonidine on myenteric neurons

Alpha-2 adrenergic agonists inhibit nicotinic excitatory postsynaptic potentials and reduce calcium dependent action potentials in myenteric neurons. To test the hypothesis that adrenergic inhibitory effects on action potential configuration and on inhibition of acetyicholine release from the nerve terminal are analogous processes, the pharmacological characteristics and underlying mechanisms of these two effects were compared in neurons of the myenteric plexus in the guinea pig. Both clonidine and noradrenaline reduced the nicotinic fast excitatory postsynaptic potential in a concentration dependent manner, although the maximum effect produced by noradrenaline was greater. The specific alpha-2 antagonist RX781094 blocked the action of noradrenaline, with an apparent K d value of3.8 ± 1 nM. Clonidine was similarly antagonized by low concentrations of this compound. The potassium channel blocker barium prevented inhibition of the fast excitatory postsynaptic potential by clonidine but not by noradrenaline. Action potentials recorded from after-hyperpolarization neurons with cesium chloride filled electrodes were prolonged in duration due to the blockade by cesium of outward potassium movement. Under these conditions, noradrenaline reduced action potential duration and slowed the rate of rise of the calcium dependent component in the presence of tetrodotoxin. RX781094 antagonized the latter effect with an estimated apparent K d of5.8 ± nM. The rate of rise of the calcium dependent action potential was not affected by clonidine (30 nM to 1 /gmM). In the absence of potassium channel blockers noradrenaline caused hyperpolarizations which were blocked by RX781094. It has been previously shown that clonidine hyperpolarizes myenteric neurons via an adrenergically mediated increase in potassium conductance. 17 It was concluded that the characteristics of presynaptic inhibition of release by noradrenaline and clonidine parallel the respective actions of these agonists on action potential configuration. While all clonidine effects could be explained on the basis of an increase in potassium conductance, noradrenaline exerted an additional inhibitory action which persisted in the presence of potassium channel blockade by barium or cesium.

An intracellular study of synaptic transmission and dendritic morphology in sympathetic neurons of the chick embryo

The characteristics of synaptic transmission in whole embryonic avian sympathetic ganglia have been examined by intracellular recording. Neurons in lumbar paravertebral ganglia of chick embryos exhibit both fast nicotinic excitatory postsynaptic potentials (EPSPs) and non-cholinergic slow EPSPs. Fast nicotinic transmission is mediated by at least 3-5 convergent preganglionic inputs and can be detected at the earliest embryonic stage examined (Stage 38; 12 days of incubation). Two types of non-cholinergic slow EPSPs have been observed and distinguished by their time course and the resulting changes in input resistance. One of these slow synaptic potentials is mimicked by exogenously applied substance P, but not by exogenous luteinizing hormone-releasing hormone (LH-RH). Muscarinic agonists also evoke slow depolarizations in the ganglia, mediated at least in part by inhibition of the M-current. Intracellular labeling with horseradish peroxidase reveals cells with 5-10 primary dendrites at Stage 42 (16 days of incubation), the earliest stage to exhibit slow EPSPs. The active and passive membrane properties of avian sympathetic neurons, including the presence of the M-current, generally resemble those of adult mammalian and amphibian sympathetic neurons. Functional activity in chick sympathetic neurons is present at a developmental stage where both biochemical and morphological indices of synapse maturation are at low levels. Since this progression has also been observed in the avian ciliary ganglion, it may be of general relevance to neuronal development.

Differential and selective antagonism of the slow-inhibitory postsynaptic potential and slow-excitatory postsynaptic potential by gallamine and pirenzepine in the superior cervical ganglion of the rabbit

Two cholinergic antagonists, gallamine and pirenzepine, agents that have been shown to bind selectively to different subpopulations of the muscarinic receptor, were found to antagonize selectively and differentially the amplitudes of the slow-inhibitory and slow-excitatory postsynaptic potentials in the superior cervical ganglion of the rabbit. Incubation of ganglia with gallamine resulted in a concentration-dependent suppression of the slow-inhibitory postsynaptic potential. The pharmacological action of gallamine was highly specific. At concentrations which reduced the amplitude of the slow-inhibitory postsynaptic potential by as much as 70–90%, there was no reduction of the amplitudes of the muscarinic slow-excitatory postsynaptic potential, the nicotinic fast-excitatory postsynaptic potential, noncholinergic slow-slow-excitatory postsynaptic potential, or post-stimulus hyperpolarizing afterpotentials. The amplitude of the slow-excitatory postsynaptic potential was actually facilitated in the presence of gallamine, presumably as a result of suppression of the overlapping slow-inhibitory postsynaptic potential. In contrast to the action of gallamine, pirenzepine produced a selective suppression of the amplitude of the slow-excitatory postsynaptic potential. Pirenzepine had very little influence on the amplitude of the slow-inhibitory postsynaptic potential at concentrations sufficient to reduce the amplitude of the slow-excitatory postsynaptic potential by as much as 50%, and had no effect on the amplitudes of the nicotinic fast-excitatory postsynaptic potential or noncholinergic slow-slow-excitatory postsynaptic potential. The evidence presented suggests that multiple muscarinic recognition sites, previously identified by-studies of the affinities of pharmacological agents for the muscarinic receptor, may actually be involved in synaptic transmission and functionally coupled to cellular effector mechanisms.

Serotonin mediates a slow excitatory potential in mammalian celiac ganglia

Neuropharmacological and histochemical evidence presented here indicates that serotonin (5-HT) is the transmitter mediating one of the postsynaptic potentials in the guinea pig celiac ganglion. Repetitive stimulation elicited in celiac neurons, in addition to the nicotinic fast excitatory postsynaptic potential (EPSP), a slow EPSP that was resistant to cholinergic antagonist. Application of 5-HT caused a depolarization with membrane characteristics similar to those of the slow EPSP, furthermore, the latter was reversibly suppressed by 5-HT. The slow depolarization evoked by either nerve stimulation or 5-HT was augmented by fluoxetine, a 5-HT reuptake blocker, and depressed by cyproheptadine, a 5-HT receptor blocker; in addition, tryptophan, a precursor of 5-HT, enhanced differentially the slow EPSP. Lastly, histochemical study revealed dense networks of 5-HT immunoreactivity nerve fibers encircling many ganglionic neurons.

Clonidine activates membrane potassium conductance in myenteric neurones.

1 Intracellular recordings were made from neurones in the myenteric plexus of the ileum removed from guinea-pigs. The effects of clonidine and adrenaline on membrane potential and resistance were observed.2 Clonidine (100 pM-30 nM) caused a concentration-dependent membrane hyperpolarization associated with a fall in neurone input resistance.3 The amplitude of the clonidine hyperpolarization, but not the conductance increase, was greater in cells with lower resting potentials and smaller in more polarized neurones. In a given cell, membrane hyperpolarization decreased and membrane depolarization increased the clonidine effect.4 Low potassium solutions enhanced and high potassium solutions reduced the hyperpolarizing action of clonidine but did not significantly change the conductance increase caused by clonidine.5 The concentration-effect curve for clonidine was displaced to the left when the extracellular calcium concentration was reduced. Conversely, clonidine was almost ineffective in elevated calcium concentrations. This was true for both the hyperpolarization and the conductance increase.6 It is suggested that clonidine activates a potassium conductance by causing an elevation in the free intracellular calcium concentration.7 Clonidine reversibly depressed the amplitude of the nicotinic fast excitatory postsynaptic potential and the noncholinergic slow excitatory postsynaptic potential.8 All the effects of clonidine were shared by adrenaline and the actions of both were reversed or prevented by phentolamine (100 nM-1 muM).