Synaptic events in sympathetic ganglia

Optimization of an autaptic culture system for studying cholinergic synapses in sympathetic ganglia

An autaptic synapse (or 'autapse') is a functional connection between a neuron and itself, commonly used in studying the molecular mechanisms underlying synaptic transmission and plasticity in central neurons. Most previous studies on autonomic synaptic functions have relied on spontaneous connections among neurons in mass cultures. However, growing evidence supports the utility of microcultures cultivating autaptic neurons for examining cholinergic transmission within sympathetic ganglia. Despite these advancements, standardized protocols for culturing autaptic sympathetic neurons have yet to be established. Drawing on historical literature, this study delineates optimal experimental conditions to efficiently and reliably produce cholinergic synapses in sympathetic neurons within a short time frame. Our research emphasizes five key factors: (i) the generation of uniformly sized microislands of growth permissive substrates; (ii) the addition of nerve growth factor, ciliary neurotrophic factor (CNTF), and serum to the culture medium; (iii) independence from specific serum and neuronal medium types; (iv) the reciprocal roles of CNTF and glial cells; and (v) the promotion of cholinergic synaptogenesis in SCG neurons through indirect glia co-cultures, rather than direct glial feeder layer cultures. In conclusion, glia-free monocultures of SCG neurons are relatively simple to prepare and yield robust and reliable synaptic currents. This makes them an effective model system for straightforwardly addressing fundamental questions about neurogenic mechanisms involved in cholinergic synaptic transmission in autonomic ganglia. Furthermore, autaptic culture experiments could eventually be implemented to investigate the roles of functional neuron-satellite glia units in regulating cholinergic functions under physiological and pathological conditions.

Muscarinic acetylcholine activity modulates cortical silent period, but not motor evoked potentials, during muscle contractions

This study used transcranial magnetic stimulation (TMS) to determine if muscarinic receptor blockade affects muscle responses during voluntary contractions. Motor evoked potentials (MEPs) were recorded from biceps brachii in 10 subjects (age: 23 ± 2) during 10%, 25%, 50%, 75%, and 100% maximal voluntary contractions (MVCs). Each contraction intensity was examined under non-fatigued and fatigued conditions. All measurements were obtained post-ingestion of 25 mg promethazine or placebo. MEP area and the duration of the TMS-evoked silent period (SP) were calculated for all contractions. No drug-related differences were detected for MEP area during non-fatigued or fatigued contractions. A main effect of drug was detected for the SP (p = 0.019) where promethazine increased SP duration by an average of 0.023 [Formula: see text] 0.015 s. This drug effect was only identified for the unfatigued contractions and not following the sustained fatiguing contractions (p = 0.105). The cholinergic system does not influence corticospinal excitability during voluntary muscle contractions, but instead affects neural circuits associated with the TMS-evoked SP. Given the prevalence of cholinergic properties in prescription and over-the-counter medications, the current study enhances our understanding of mechanisms that may contribute to motor side-effects.

Effects of cevimeline on excitability of parasympathetic preganglionic neurons in the superior salivatory nucleus of rats

The superior salivatory nucleus (SSN) contains parasympathetic preganglionic neurons innervating the submandibular and sublingual salivary glands. Cevimeline, a muscarinic acetylcholine receptor (mAChR) agonist, is a sialogogue that possibly stimulates SSN neurons in addition to the salivary glands themselves because it can cross the blood-brain barrier (BBB). In the present study, we examined immunoreactivities for mAChR subtypes in SSN neurons retrogradely labeled with a fluorescent tracer in neonatal rats. Additionally, we examined the effects of cevimeline in labeled SSN neurons of brainstem slices using a whole-cell patch-clamp technique. Mainly M1 and M3 receptors were detected by immunohistochemical staining, with low-level detection of M4 and M5 receptors and absence of M2 receptors. Most (110 of 129) SSN neurons exhibited excitatory responses to application of cevimeline. In responding neurons, voltage-clamp recordings showed that 84% (101/120) of the neurons exhibited inward currents. In the neurons displaying inward currents, the effects of the mAChR antagonists were examined. A mixture of M1 and M3 receptor antagonists most effectively reduced the peak amplitude of inward currents, suggesting that the excitatory effects of cevimeline on SSN neurons were mainly mediated by M1 and M3 receptors. Current-clamp recordings showed that application of cevimeline induced membrane depolarization (9/9 neurons). These results suggest that most SSN neurons are excited by cevimeline via M1 and M3 muscarinic receptors.

SAD kinases control the maturation of nerve terminals in the mammalian peripheral and central nervous systems

Axons develop in a series of steps, beginning with specification, outgrowth, and arborization, and terminating with formation and maturation of presynaptic specializations. We found previously that the SAD-A and SAD-B kinases are required for axon specification and arborization in subsets of mouse neurons. Here, we show that following these steps, SAD kinases become localized to synaptic sites and are required within presynaptic cells for structural and functional maturation of synapses in both peripheral and central nervous systems. Deleting SADs from sensory neurons can perturb either axonal arborization or nerve terminal maturation, depending on the stage of deletion. Thus, a single pair of kinases plays multiple, sequential roles in axonal differentiation.

Sodium leak channels in neuronal excitability and rhythmic behaviors

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

Muscarinic acetylcholine receptors (mAChRs) in the nervous system: some functions and mechanisms

This article summarizes some of the effects of stimulating muscarinic receptors on nerve cell activity as observed by recording from single nerve cells and cholinergic synapses in the peripheral and central nervous sytems. It addresses the nature of the muscarinic receptor(s) involved and the ion channels and subcellular mechanisms responsible for the effects. The article concentrates on three effects: postsynaptic excitation, postsynaptic inhibition, and presynaptic (auto) inhibition. Postsynaptic excitation results primarily from the inhibition of potassium currents by M(1)/M(3)/M(5) receptors, consequent upon activation of phospholipase C by the G protein Gq. Postsynaptic inhibition results from M2-activation of inward rectifier potassium channels, consequent upon activation of Gi. Presynaptic inhibition results from M(2) or M(4) inhibition of voltage-gated calcium channels, consequent upon activation of Go. The segregation receptors, G proteins and ion channels, and the corelease of acetylcholine and glutamate from cholinergic fibres in the brain are also discussed.

Peptide neurotransmitters activate a cation channel complex of NALCN and UNC-80

Several neurotransmitters act through G-protein-coupled receptors to evoke a 'slow' excitation of neurons. These include peptides, such as substance P and neurotensin, as well as acetylcholine and noradrenaline. Unlike the fast (approximately millisecond) ionotropic actions of small-molecule neurotransmitters, the slow excitation is not well understood at the molecular level, but can be mainly attributed to suppressing K(+) currents and/or activating a non-selective cation channel. The molecular identity of this cation channel has yet to be determined; similarly, how the channel is activated and its relative contribution to neuronal excitability induced by the neuropeptides are unknown. Here we show that, in the mouse hippocampal and ventral tegmental area neurons, substance P and neurotensin activate a channel complex containing NALCN and a large previously unknown protein UNC-80. The activation by substance P through TACR1 (a G-protein-coupled receptor for substance P) occurs by means of a unique mechanism: it does not require G-protein activation but is dependent on Src family kinases. These findings identify NALCN as the cation channel activated by substance P receptor, and suggest that UNC-80 and Src family kinases, rather than a G protein, are involved in the coupling from receptor to channel.

Dynamic Clamp Analysis of Synaptic Integration in Sympathetic Ganglia

Advances in modern neuroscience require the identification of principles that connect different levels of experimental analysis, from molecular mechanisms to explanations of cellular functions, then to circuits, and, ultimately, to systems and behavior. Here, we examine how synaptic organization of the sympathetic ganglia may enable them to function as use-dependent amplifiers of preganglionic activity and how the gain of this amplification may be modulated by metabotropic signaling mechanisms. The approach combines a general computational model of ganglionic integration together with experimental tests of the model using the dynamic clamp method. In these experiments, we recorded intracellularly from dissociated bullfrog sympathetic neurons and then mimicked physiological synapses with virtual computer-generated synapses. It thus became possible to analyze the synaptic gain by recording cellular responses to complex patterns of synaptic activity that normally arise in vivo from convergent nicotinic and muscarinic synapses. The results of these studies are significant because they illustrate how gain generated through ganglionic integration may contribute to the feedback control of important autonomic behaviors, in particular to the control of the blood pressure. We dedicate this paper to the memory of Professor Vladimir Skok, whose rich legacy in synaptic physiology helped establish the modern paradigm for connecting multiple levels of analysis in studies of the nervous system.

Protein kinase C bound with A-kinase anchoring protein is involved in muscarinic receptor-activated modulation of M-type KCNQ potassium channels

The second messenger for closure of M/KCNQ potassium channels in post-ganglionic neurons and central neurons had remained as a 'mystery in the neuroscience field' for over 25 years. However, recently the details of the pathway leading from muscarinic acetylcholine receptor (mAChR)-stimulation to suppression of the M/KCNQ-current were discovered. A key molecule is A-kinase anchoring protein (AKAP; AKAP79 in human, or its rat homolog, AKAP150) which forms a trimeric complex with protein kinase C (PKC) and KCNQ channels. AKAP79 or 150 serves as an adapter that brings the anchored C-kinase to the substrate KCNQ channel to permit the rapid and 'definitive' phosphorylation of serine residues, resulting in avoidance of signal dispersion. Thus, these findings suggest that mAChR-induced short-term modulation (or memory) does occur within the already well-integrated molecular complex, without accompanying Hebbian synapse plasticity. However, before this identity is confirmed, many other modulators which affect M-currents remain to be addressed as intriguing issues.

Long-term potentiation of transmitter exocytosis expressed by Ca2+-induced Ca2+ release from thapsigargin-sensitive Ca2+ stores in preganglionic nerve terminals

We have studied whether Ca(2+)-induced Ca(2+) release (CICR) is involved in the mechanism of long-term potentiation (LTP) at nicotinic synapses of bullfrog sympathetic ganglia. Fast excitatory postsynaptic potentials (fast EPSPs) were recorded in a low-Ca(2+), high-Mg(2+) solution and quantal analysis was applied. The conditioning stimulation of the B-type preganglionic nerve at 20 Hz for 4 min consistently enhanced the amplitude and quantal content of fast EPSP for > 2 h, but only sometimes enhanced the quantal size. The LTP of quantal content produced by the conditioning tetanus was blocked by thapsigargin, a blocker of Ca(2+) pumps at Ca(2+) stores, applied before or after the conditioning tetanus, and by Xestospongin C, a blocker of inositoltrisphosphate (IP(3)) receptors, applied before the tetanus. It was not, however, blocked by ryanodine, a blocker and/or activator of ryanodine receptors, or by propranolol, a blocker of beta-adrenergic receptors. Thus the long-lasting activity of the preganglionic nerve at a high frequency causes the LTP of impulse-evoked transmitter release by the activation of CICR from thapsigargin-sensitive Ca(2+) stores in the nerve terminals. It is likely that a large Ca(2+) entry into the nerve terminals during tetanic activity primes ryanodine-insensitive Ca(2+) release channels for activation.

Estimating use-dependent synaptic gain in autonomic ganglia by computational simulation and dynamic-clamp analysis

Biological gain mechanisms regulate the sensitivity and dynamics of signaling pathways at the systemic, cellular, and molecular levels. In the sympathetic nervous system, gain in sensory-motor feedback loops is essential for homeostatic regulation of blood pressure and body temperature. This study shows how synaptic convergence and plasticity can interact to generate synaptic gain in autonomic ganglia and thereby enhance homeostatic control. Using a conductance-based computational model of an idealized sympathetic neuron, we simulated the postganglionic response to noisy patterns of presynaptic activity and found that a threefold amplification in postsynaptic spike output can arise in ganglia, depending on the number and strength of nicotinic synapses, the presynaptic firing rate, the extent of presynaptic facilitation, and the expression of muscarinic and peptidergic excitation. The simulations also showed that postsynaptic refractory periods serve to limit synaptic gain and alter postsynaptic spike timing. Synaptic gain was measured by stimulating dissociated bullfrog sympathetic neurons with 1-10 virtual synapses using a dynamic clamp. As in simulations, the threshold synaptic conductance for nicotinic excitation of firing was typically 10-15 nS, and synaptic gain increased with higher levels of nicotinic convergence. Unlike the model, gain in neurons sometimes declined during stimulation. This postsynaptic effect was partially blocked by 10 microM Cd2+, which inhibits voltage-dependent calcium currents. These results support a general model in which the circuit variations observed in parasympathetic and sympathetic ganglia, as well as other neural relays, can enable functional subsets of neurons to behave either as 1:1 relays, variable amplifiers, or switches.

5-HT modulates multiple conductances in immature rat rostral ventrolateral medulla neurones in vitro

1. Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0.5 microM), 5-HT (50 microM) elicited an outward current (I5-HT,outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I5-HT,inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. 2. The steady-state I-V relationship of I5-HT,outward showed an inward rectification; the 5-HT-induced current, which reversed at -87.9 +/- 3.0 mV, was suppressed by 0.1 mM Ba2+. 3. Two types of steady-state I-V relationship for I5-HT,inward were noted: type I I5-HT,inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (EK), -94 mV, in 8/17 neurones; type II I5-HT,inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. 4. Both type I and type II I5-HT,inward were significantly reduced in a low [Na+]o solution. In this solution, I5-HT,inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I5-HT,inward shifted to about -80 mV as the [K+]o was increased from 3.1 to 7.0 mM in low [Na+]o solution. The type II I5-HT,inward did not reverse at the estimated EK in the same solution. 5. While not affected by externally applied Cs+ (1 mM), I5-HT,inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11.9 +/- 6.4 mV in 8/15 responsive neurones. 6. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I5-HT, outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I5-HT,inward.

Optical monitoring of synaptic transmission in bullfrog sympathetic ganglia using a voltage-sensitive dye

Multiple-site optical recording of neural activity, using a voltage-sensitive merocyanine-rhodanine dye (NK2761) and a 12 x 12-element photodiode array, was employed to monitor the synaptic transmission in the bullfrog lumbar sympathetic ganglion. When the presynaptic nerve fibers were stimulated, the signals had two or three peaks, and their later phase was reduced in a low calcium bathing solution or in a solution containing D-tubocurarine. We conclude that the first phase of the optical signals evoked by the presynaptic stimulation corresponds to the electrical activity of the presynaptic neurons. The signals evoked by the postsynaptic stimulation showed relatively simple shape reflecting the action potential in the ganglion cells.

Voltage-gated and inwardly rectifying potassium channels

This lecture is dedicated to Max Delbrück and Seymour Benzer. Max Delbrück was our graduate advisor. He introduced us to a variety of biophysical problems, and taught us ways of thinking about these problems by example. Potassium channels was one of the topics included in his journal club in the early seventies; Max also carefully considered the feasibility of purifying potassium channels then. It was in Seymour Benzer's laboratory that we began to look for Drosophila mutants that affect synaptic transmission at the larval neuromuscular junction. Shaker was the first behavioural mutant we tested that gave a robust phenotype, a phenotype that could be mimicked by treating wild-type preparations with a potassium channel blocker. This mutant fly has led us to our subsequent molecular studies of potassium channels. Since we settled in the University of California, San Francisco, and began to study neural development as well as potassium channels, we have settled into the pattern of each attending meetings and presenting our studies on one of these two areas so as to avoid both being away from home and our children at the same time. In following this pattern, I will be presenting the studies of potassium channels as part of our long-term collaboration. In this talk I will first briefly take you through the path that led us to the molecular studies of potassium channels and then discuss the diversity and modulation of these potassium channels at the molecular and physiological level.

The cardiovascular response to medullary cholinergic and corticoid stimulation is calcium channel dependent in rats

Experiments were performed on anaesthetized Wistar or Sprague-Dawley rats of both sexes. Microinjection of an acetylcholinesterase inhibitor physostigmine (0.4 microgram/0.1 microliter/site) or acetylcholine (ACh, 25 ng/0.1 microliter/site) into the rostral ventrolateral medulla (rVLM) caused an increase in blood pressure (BP), heart rate (HR) and the pressor response produced by stimulation of the dorsal periaqueductal grey (dPAG) in the midbrain. Prior microinjection of the calcium channel blocker verapamil (0.25 microgram/0.1 microliter/site) into the same sites blocked the cardiovascular effect in response to the respective microinjection of the drugs mentioned above. Moreover, verapamil pretreatment blocked the pressor and tachycardiac effect induced by respective microinjection of corticosterone (40 ng/0.1 microliter/site) or aldosterone (40 ng/0.1 microliter/site) into the rVLM, as well as the enhancement of the pressor response to stimulation of the dPAG induced by microinjection of corticoids into the rVLM. These results suggest that the enhancement of cardiovascular activities mediated by cholinergic mechanisms may be due to the activation of postsynaptic calcium channels of neurons in the rVLM. The corticosteroid effect seems to be mediated by similar mechanisms.

Corticotropin-releasing hormone excites adrenocorticotropin-secreting human pituitary adenoma cells by activating a nonselective cation current

The mechanisms of corticotropin-releasing hormone (CRH) induced excitation of ACTH-secreting adenoma cells were investigated using the perforated whole-cell clamp technique and intracellular Ca2+ concentration ([Ca2+]i) measurement. CRH depolarized ACTH-secreting adenoma cells by activating a nonselective cation current that showed slight inward rectification. This channel did not seem to be a member of the Ca(2+)-activated cation currents because it was activated even when the [Ca2+]i was chelated below 50 nM. The activation of the current was induced by protein kinase A-mediated pathways. By [Ca2+]i measurement, CRH increased [Ca2+]i of these cells dependently on voltage-gated Ca2+ current. This CRH-induced [Ca2+]i increase was abolished in Na(+)-free extracellular solution, but was not abolished by the addition of 5 microM tetrodotoxin to the extracellular solution. CRH-induced ACTH secretion from the cultured adenoma cells was also abolished in Na(+)-free extracellular solution, but not in tetrodotoxin-containing extracellular solution. These data indicate that a Na+ current (maybe the nonselective cation current) other than voltage-gated Na+ current plays an important role in CRH-induced [Ca2+]i increase and ACTH secretion. CRH also activated a nonselective cation current in nonadenoma human corticotrophs, suggesting that the activation of a nonselective cation current is a physiological mechanism of CRH-induced excitation in human corticotrophs.

Calcium-dependent after-hyperpolarization in dissociated bullfrog sympathetic neurons

Whole-cell recordings were made from dissociated bullfrog sympathetic neurons. Tetraethylammonium (30 mM) and apamin (100 nM) were added to the superfusate to eliminate the known calcium-activated potassium currents termed Ic and IAHP. Under these conditions, the action potential carried by calcium ions was followed by a prolonged (10-60 s) after-hyperpolarization. A current component (IAC) underlying the after-hyperpolarization was eliminated by barium (2 mM) and showed voltage-dependence identical to that of a M-type potassium current. I concluded that the after-hyperpolarization is caused not only by IAHP but also by the calcium-dependent potentiation of M-current.

Calcium-dependent potentiation of M-current in bullfrog sympathetic neurons

Whole-cell voltage-clamp recordings were made from cultured bullfrog sympathetic neurons to measure the steady-state activation curve of M-type potassium current. When measured with a calcium-deficient (10 nM) pipette solution M-conductance was 4.8 nS at -35 mV having the 50%-activation voltage at-20 mV. Respective values were 17.2 nS at -35 mV with the 50%-activation voltage at -42 mV when measured with a calcium-rich (1 microM) solution, indicating the hyperpolarizing displacement of the activation curve with high internal calcium. It is suggested that intracellular calcium ions can modulate kinetics of M-current which thereby regulate the number of M-channels being open at given membrane potentials.

The mammalian sympathetic prevertebral ganglia: integrative properties and role in the nervous control of digestive tract motility

The prevertebral ganglia which are a constitutive part of the sympathetic system have long been considered as a simple relay on this efferent pathway. In fact, these ganglia must be considered as true peripheral nervous centres. They possess various integrative properties, such as projections of central and peripheral inputs onto the ganglionic neurones, gating of these projections and pacemaker activity of the ganglionic neurones. These properties explain the ability of these ganglia to participate in the regulation of various visceral functions, including digestive tract motility.

Correlation between dendrodendritic synapses of adrenergic type and synaptically evoked hyperpolarization in the sympathetic ganglion of adult rats

Intracellular recording and labeling with biocytin followed by electron microscopic observation were used to examine the nature and the morphological basis of a synaptically evoked hyperpolarization following spikes in the rat superior cervical ganglion neurons. A large hyperpolarization (the amplitude > 8 mV; the duration > 1 s following spikes) was elicited by repetitive stimulation of the preganglionic nerves in 8% of cells examined (n = 50). The alpha 2-adrenoceptor antagonist, yohimbine, reversibly attenuated the hyperpolarization, without affecting spikes. A nicotinic antagonist, hexamethonium, blocked both the hyperpolarization and spikes. Atropine had no effect of these responses. Electron microscopic observation of dendrites of these cells revealed that they received synaptic inputs of adrenergic type besides a cholinergic one from the preganglionic axons. Some dendrites served as presynaptic elements. These results strongly suggest that the hyperpolarization is an inhibitory postsynaptic potential and that this disynaptic response to the preganglionic stimulation is mediated mainly by two transmitters, acetylcholine and noradrenaline that are released from axodendritic and dendrodendritic synapses, respectively. We conclude that there appears to be an adrenergic inhibitory local circuit that modulates cholinergic transmission in the sympathetic ganglia.

In vivo activity of B- and C-neurones in the paravertebral sympathetic ganglia of the bullfrog

1. Spontaneous, in vivo synaptic activity was recorded from 146 B-cells and 60 C-cells in the IXth and Xth paravertebral sympathetic ganglia of the urethane-anaesthetized bullfrog. Sympathetic outflow to the blood vessels, which are innervated by C-cells, is different from that received by targets in the skin, which are innervated by B-cells. 2. B-cells were divided into three groups: the first (61 cells) exhibited only action potentials (APs) at 0.01-0.3 s-1; the second (59 cells) exhibited APs and EPSPs and the third (26 cells) were silent. In addition to their usual suprathreshold input from the ipsilateral sympathetic chain, 53% of B-cells received subthreshold input which probably arose from fibres in the contralateral chain. 'Slow' B-cells exhibited less subthreshold activity and a slightly higher AP frequency than 'fast' B-cells. All B-cells are involved in a sympathetic reflex which is activated by tactile stimulation of the skin of the hindlimb. Activation of this reflex increased AP frequency without promoting long-lasting depolarization. 3. Sixty-seven per cent of C-cells exhibited rhythmic bursting activity with or without small intraburst EPSPs. Bursts tended to correlate with electrocardiographic (ECG) activity. The remainder exhibited an irregular pattern of activity which was not correlated with ECG activity and which included one to three APs and EPSPs interspersed between the bursts. Activity of both types of C-cell was inhibited following stimulation of the skin. 4. An average of twenty-three B-cells and twenty-one C-cells discharge simultaneously in vivo. This reflects branching of preganglionic fibres and results in synchrony of discharge in both postganglionic B- and C-fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of calcium-dependent chloride channels causes post-tetanic depolarization in rabbit parasympathetic neurons

Intracellular recordings were made from neurons in rabbit and feline vesical parasympathetic ganglia in vitro. In response to cathodal current injection (0.1-1 nA for 2-20 ms) the majority of rabbit neurons (229 out of 250) exhibited a single action potential that was followed by a fast and slow after-hyperpolarization (sAHP neuron). The remainder of the cells exhibited an action potential followed by only a fast after-hyperpolarization (fAHP neuron). fAHP neurons did not exhibit anomalous rectification and a spontaneous rhythmic hyperpolarization, which were common membrane properties in sAHP neurons. In response to a train of cathodal current pulses (5-20 Hz for 0.1-10 s), fAHP neurons exhibited action potentials followed by a post-tetanic depolarization (PTD). The PTD was associated with a decrease in membrane input resistance. The amplitude and duration of the PTD were a function of the number of action potentials in the train. The amplitude of the PTD was increased by membrane hyperpolarization and its estimated reversal potential was approximately -30 mV. Low-chloride solution and intracellular injection of chloride ions augmented the amplitude and duration of the PTD, whereas low-sodium, high-potassium and low-potassium solutions did not affect them. Tetraethylammonium (5-10 mM) and barium (0.5-1 mM) increased the amplitude and duration of the PTD. Nominal calcium-free solutions and omega-conotoxin (500 nM) abolished the PTD. The data suggest that activation of chloride channels by calcium influx through omega-conotoxin-sensitive calcium channels mediates the PTD. Repetitive stimulation of the pelvic nerve evoked a train of orthodromic action potentials followed by the PTD of fAHP neurons. (+)-Tubocurarine (10 microM) and hexamethonium (200 microM), but not atropine (1 microM), abolished orthodromic action potentials and the PTD, whereas these cholinergic antagonists did not depress the PTD evoked by direct action potentials. In summary, the data suggest that the PTD may function as a slow synaptic potential in fAHP neurons. This appears likely because neither slow excitatory nor inhibitory postsynaptic potentials are present in neurons of rabbit vesical parasympathetic ganglia. In contrast, slow inhibitory and excitatory postsynaptic potentials were recorded from neurons in feline vesical parasympathetic ganglia.

Delayed onset and slow time course of the non-M-type muscarinic current in bullfrog sympathetic neurons

The onset and time course of the muscarinic currents induced by brief applications of acetylcholine (ACh) were examined in voltage-clamped neurons of bullfrog sympathetic ganglia bathed in a solution containing d-tubocurarine. At a potential of -40 mV, the ACh-induced current (IACh) appeared within 1.2 s and rapidly increased to its peak with a half-activation time of 2.2 s. This initial current was termed the fast IACh and was blocked by 4 mM Ba2+. At a potential more negative than -60 mV, the fast IACh disappeared and the remaining IACh activated with a delay of 3.9 s and slowly increased to its peak with a half-activation time of 8.2 s. This delayed current was termed the slow IACh and is thought to be associated with inhibition of a K+ current, or IM, as well as activation of an inward current through non-M-type muscarinic cation channels. The slow IACh was not inhibited by Ba2+, but its amplitude was reduced with depolarization (the extrapolated reversal potential was +3 mV). In Na(+)-free solution, the amplitude of the slow IACh reduced, but its polarity did not reverse in the voltage region examined (-30 to -100 mV). The slow excitatory postsynaptic current was also recorded, and was shown to have a similar delay in onset and slow time course. The results demonstrate that ACh activates the non-M-type muscarinic current three times more slowly than it inhibits IM.

Effects of muscarine on K(+)-channel currents in the C-cells of bullfrog sympathetic ganglion

The effects of muscarine on small, putative C-cells and large, putative B-cells dissociated from bullfrog paravertebral sympathetic ganglia were studied by whole cell and single channel recording techniques. The dominant action of muscarine was to activate an inwardly-rectifying K+ current (IK(G)) in C-cells and to suppress M-current (IM) in B-cells. However, both IM and IK(G) were affected by muscarine in 5 out of 78 putative C-cells and in 8 others only IM was affected. By contrast, IK(G) was only activated in 1 out of 105 B-cells. This predicts that the muscarinic slow IPSP, which can be evoked by preganglionic stimulation, occurs exclusively in C-cells. 6% of these cells could, however, generate a muscarinic slow EPSP in addition to a slow IPSP and 10% could generate a slow EPSP without a slow IPSP. The rectification associated with IK(G) was neither a direct consequence of the direction of movement of K+ ions nor a simple consequence of channel block by intracellular Mg2+ or Na+ ions. The fit of the activation curve by a Boltzmann equation suggests that the conductance underlying IK(G) is controlled by a voltage-dependent gating charge (valency approximately -2). Muscarine activated no new channels in outside-out or cell-attached patches but increased the opening probability of two types of K+ channels (unitary conductances approximately 20 pS and approximately 55 pS). The possible role of these channels in the generation of IK(G) is discussed.

Neurotensin excitation of rat ventral tegmental neurones

1. Whole-cell patch-clamp recordings were made from ventral tegmental area neurones in rat midbrain slices in vitro. In principal cells, which are presumed to contain dopamine, neurotensin (< or = 1 microM) caused an inward current at -60 mV in thirty of forty-seven neurones and had no effect on the remainder. In secondary neurones, neurotensin caused an inward current in twelve of thirty-three cells. 2. The inward current evoked by neurotensin reached a maximum amplitude of about 80 pA, and declined over several minutes when the application was discontinued. The current was most commonly accompanied by a decrease in membrane conductance and reversed polarity at a strongly hyperpolarized potential; this reversal potential was less negative in a higher extracellular potassium concentration. Neurotensin also caused an inward current even in potassium-free internal and external solutions; this current was accompanied by a conductance increase, reversed close to 0 mV and was inhibited by reduction of the extracellular sodium concentration (from 150 to 20 mM). 3. The inward current was associated with a large increase in noise; this persisted in calcium-free solutions but was inhibited by low sodium concentration. The increase in noise was more prominent at hyperpolarized potentials. The amplitude of the unitary current underlying the increase in noise was estimated from the ratio of the variance to the mean as about 1.5 pA at -100 mV. 4. When the recording was made with an electrode containing guanosine 5'-thio-triphosphate, the steady inward current evoked by neurotensin did not reverse when the application was discontinued. When the recording electrode contained pertussis toxin, the action of neurotensin was not different although outward currents evoked by dopamine and baclofen declined with time. 5. It is concluded that neurotensin excites ventral tegmental area neurones by activating a pertussis toxin-insensitive guanosine nucleotide-binding protein. This leads to a reduction in membrane potassium conductance and an increase in membrane sodium conductance, the relative contribution of which varies from cell to cell.

Common ionic mechanisms of excitation by substance P and other transmitters in guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones and single-electrode voltage-clamp methods were used to record membrane currents. The actions of substance P (SP), 5-hydroxytryptamine (5-HT), muscarine, vasoactive intestinal polypeptide (VIP), forskolin and nerve stimulation were studied. 2. Substance P, 5-HT (in the presence of 5-HT3 receptor antagonists), muscarine, VIP, forskolin and slow excitatory synaptic transmission all produced identical responses: an inward current associated with a membrane conductance decrease at the resting potential. The actions of any one occluded the actions of any other and all responses were pertussis-toxin insensitive. 3. These agonists produced a voltage-independent decrease in a 'leak' potassium conductance between -40 and -120 mV in 14% of neurones. 4. These agonists decreased a voltage-dependent, calcium-activated potassium conductance between -40 and -80 mV in all other (86%) neurones. The agonists still evoked an inward current without apparent conductance change at potentials between -90 and -130 mV. 5. In a low calcium solution containing cobalt or cadmium, the agonists produced an inward current associated with a conductance increase from -40 to -120 mV. Ion replacement studies indicated this current was due to an increase in a cation-selective (mainly sodium) conductance. 6. The agonists also reduced the inwardly rectifying potassium current that is activated by somatostatin and alpha 2-adrenoceptor agonists in these neurones. The agonists did not alter the inwardly rectifying potassium current that is present in these neurones in the absence of somatostatin or alpha 2-agonists. 7. Thus, SP, 5-HT, muscarine, VIP and the release of slow excitatory transmitters all appear to act through a common intracellular transduction pathway, an increase in adenylate cyclase. This results in an activation of a sodium-selective cation current and an inhibition of three distinct potassium conductances: the background potassium conductance, the calcium-activated potassium conductance and the inwardly rectifying potassium conductance activated by somatostatin and alpha 2-adrenoceptor agonists.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Substance P and somatostatin inhibit calcium channels in rat sympathetic neurons via different G protein pathways

We studied inhibition of N-type Ca2+ channels in rat superior cervical ganglion neurons by substance P (SP) and somatostatin-14 (Som). In whole-cell clamp, 70 of 82 acutely dissociated neurons showed inhibition (mean 37%) by 500 nM SP, and 54 of 61 showed inhibition by 240 nM Som (mean 57%). Pertussis toxin (PTX) blocked Som but not SP inhibition; intracellular dialysis with 2 mM GDP-beta-S attenuated inhibition with either peptide. Inhibition was voltage dependent with Som but not with SP. Neurokinin A (1 microM) or B was without effect, implicating NK1 tachykinin receptors. In cell-attached patches with bath-applied drugs, to test for a diffusible messenger, inhibition by SP or Som was only 8%. Thus, SP signaling is voltage independent and PTX insensitive; Som inhibition is voltage dependent and PTX sensitive; and both are membrane delimited.

Intracellular calcium dynamics in response to action potentials in bullfrog sympathetic ganglion cells

1. Dynamic changes in the intracellular free Ca2+ concentration ([Ca2+]i) following electrical membrane activity, were recorded from the neurone soma of the excised bullfrog sympathetic ganglion, using Fura-2 fluorescence and compared with the accompanying Ca(2+)-dependent electrical membrane responses. 2. The resting [Ca2+]i was about 100 nM, a value little changed by penetration with an intracellular electrode. 3. A net rise in fluorescence at a wavelength of 340 nm (Ca2+ transient) induced by a single action potential in Ringer solution rose almost in parallel with the initial decay phase of a slow Ca(2+)-dependent after-hyperpolarization; decayed in parallel with the late phase; and increased in amplitude and duration in the presence of tetraethylammonium (20 mM). 4. A Ca2+ transient induced by repetitive action potentials was increased asymptotically in amplitude and progressively in duration by increasing the number of spikes, and was slower in time course than the associated Ca(2+)-dependent K+ current. 5. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording Indo-1 fluorescence with a confocal microscope demonstrated an inward spread of a rise in [Ca2+]i following a tetanus. 6. Both single spike- and tetanus-induced Ca2+ transients were abolished in a Ca(2+)-free solution, while single or repetitive transient rises in [Ca2+]i induced by caffeine (5-10 mM) were generated under the same conditions. 7. Ryanodine (10-50 microM) did not affect tetanus-induced Ca2+ transients, whereas it blocked completely the caffeine-induced oscillation of [Ca2+]i. 8. Ca2+ transients induced by a tetanus in Ringer solution were independent of the interval from the preceding tetanus. The amplitude of Ca2+ transients induced by a tetanus in the presence of caffeine (5 mM) was equal to, or greater than, that generated in Ringer solution in any of the phases of [Ca2+]i oscillation. 9. It is suggested that under the physiological conditions here, the induction of action potentials does not cause the release of Ca2+ in the cells of the freshly excised bullfrog sympathetic ganglion, and that Ca(2+)-buffering systems contribute not only to lowering a transient rise in [Ca2+]i but also to sustaining an increased [Ca2+]i after a large Ca2+ load into the cell.

Muscarine increases cation conductance and decreases potassium conductance in rat locus coeruleus neurones

1. Whole-cell patch-clamp recordings were made from rat locus coeruleus neurones in slices of brain tissue in vitro. Muscarine (30 microM) caused an inward current of about 100 pA in neurones voltage clamped at -60 mV. 2. In about 75% of cells, the current elicited by muscarine was independent of potential in the range -60 to -120 mV and had no associated conductance change. 3. In about 25% of cells, the current became smaller with hyperpolarization, was associated with a decreased conductance, and reversed polarity between -100 and -140 mV. The reversal potential changed with the logarithm of the extracellular potassium concentration. Barium and caesium blocked inward rectification and also prevented reversal of the muscarine current. 4. When potassium ions of the intracellular and extracellular solutions were replaced by caesium, the current evoked by muscarine became smaller with depolarization at reversed polarity at +9 mV. This current was associated with an increase in conductance, and was greatly reduced when the extracellular sodium concentration was reduced to 20 mM. 5. The results could be quantitatively accounted for by a model in which muscarine both increases a voltage-independent cation conductance and decreases the inward rectifier potassium conductance.

Neurotransmission through sympathetic ganglia of spontaneously hypertensive rats

Transmission of neuronal activity was assessed by recording preganglionic and postganglionic compound action potentials in superior cervical ganglia isolated from adult spontaneously hypertensive rats (SHR), Wistar-Kyoto (WKY) rats, and Wistar rats as well as young SHR and WKY rats to determine if previously observed alterations of membrane excitability, synaptic transmission, or both, have an effect on the transmission of preganglionic activity in SHR. Single stimuli induced more postganglionic neurons to fire over a wide range of preganglionic stimulation intensities in superior cervical ganglia from adult SHR as compared with those from adult normotensive controls. Short stimulation trains confirmed that SHR are able to maintain this greater number of active postganglionic neurons during low-frequency stimulation (1-20 Hz). However, by the end of a train of high-frequency stimulation (70-100 Hz) fewer neurons fired in ganglia from SHR compared with those from normotensive controls. These differences in transmission were not observed in the young rats. The results from the present study demonstrate that physiological frequencies of preganglionic activity are more effectively transmitted through sympathetic ganglia from adult SHR compared with those from normotensive controls, and this enhanced transmission through ganglia may contribute to the elevated sympathetic activity and the consequent hypertension seen in this model.

A patch-clamp study on the muscarine-sensitive potassium channel in bullfrog sympathetic ganglion cells

1. A voltage-independent K+ channel was characterized and effects of muscarine were studied in cultured bullfrog sympathetic ganglion cells using the cell-attached patch-clamp configuration. 2. Three types of single-channel current were recorded from 2- to 10-day-old cultured cells in the presence of tetraethylammonium (2-20 mM), tetrodotoxin (1-2 microM), Cd2+ (0.1 mM) and apamin (20 nM). 3. The most frequently observed channel was a voltage-independent K+ channel which was open at the resting membrane potential and had a conductance of 52.6, 78.9 and 114.9 pS at a [K+]o of 2, 40 and 100 mM, respectively. This channel was designated background K+ channel. 4. Two other channel types were observed less frequently. One had a conductance of 26 pS (external K+, 118 mM) and a long open time of several seconds at the resting membrane potential. The second channel had a smaller conductance (20 pS) and displayed a voltage-dependent activation. 5. The open probability of the background K+ channel varied between patches, ranging from 0.0005 to 0.486. The open time distribution was fitted by a single exponential with a time constant of 0.51 ms. Both of these parameters were independent of the membrane potential. The closed time distribution consisted of at least four exponentials having time constants of 0.17, 3.7, 120 ms and several seconds. 6. Muscarine (10-20 microM) applied to the membrane outside the patch pipette reversibly enhanced the activity of the background K+ channel. This effect was associated with an increase in the open probability, which resulted from an increase in the mean open time concomitant with a decrease in the mean closed time. Muscarine did not change the single-channel conductance of this channel. 7. The effects of muscarine were blocked by atropine (1 microM). 8. It is concluded that there exists a muscarine-sensitive, voltage-independent K+ channel in cultured bullfrog ganglion cells. This K+ channel appears to contribute to the generation of the resting membrane potential and underlie the slow inhibitory postsynaptic potential of these neurones in situ.

Vx enhances neuronal excitability and alters membrane properties of Rana catesbeiana sympathetic ganglion neurons

1. The effects of VX (10 microM) were examined on sympathetic ganglion neurons from the bullfrog using intracellular recording techniques. 2. VX significantly increased the amplitude of the residual EPSP from 4.8 +/- 0.86 mV (n = 4) to 13.7 +/- 1.23 mV (n = 4). 3. VX significantly decreased the membrane potential 5.2 +/- 0.75 mV (n = 6). The input resistance and the duration of the spike afterhyperpolarization (AHP) were also reduced 69.8% and 69.6% of control, respectively. 4. VX increased neuronal excitability greater than 200% (n = 5) of control. 5. The VX-induced neuronal excitability may result from a reduction in the duration of the AHP and contribute to the CNS toxicity.

The effects of soman on the electrical properties and excitability of bullfrog sympathetic ganglion neurones

1. The effects of soman (0.1-10 microM), an irreversible inhibitor of acetylcholinesterase (AChE), were examined on the electrical properties of ganglion neurones of the paravertebral sympathetic chain of the bullfrog, Rana catesbeiana. 2. Soman (10 microM) depolarized 29 of 35 (83%) ganglion neurones studied by 6.4 +/- 0.65 mV within 10 min of application and reduced the cell input resistance in 9 of 11 neurones examined (82%) to 55 +/- 5.3% of control. 3. Soman (10 microM) significantly reduced the maximum amplitude and the maximum rate of rise of the action potential and the duration, but not the amplitude, of the after-hyperpolarization (AHP) following the action potential elicited by either direct or antidromic stimulation. The maximum rate of fall and the duration of the action potential were not significantly affected by soman. These actions of soman were independent of the agent-induced depolarization. When examined by a single microelectrode voltage clamp, soman reduced the amplitude and the time constant of the current underlying the slow AHP, IAHs. 4. Soman (1-10 microM) produced an increase in neuronal excitability which was evidenced as either an increase in the number of action potentials or a decrease in the interspike interval in response to constant-current depolarizing pulses. The soman-induced increase in excitability occurred independently of both the agent-induced depolarization and the decrease in input resistance, was reversible with washing, was not caused by an inhibition of the M-current and was also recorded in dissociated sympathetic ganglion neurones.5. The effects of soman on the membrane potential, input resistance and the duration of the AHP but not cell excitability were blocked by pretreatment with atropine (10 microM). Pretreatment with dihydro-/J-erythroidine (DHbetalE) (10 microM) was ineffective in blocking or reversing the effects of soman. These results suggest that the direct actions of soman on the electrical properties of these neurones are mediated by activation of muscarinic receptors.

Time course of receptor-channel coupling in frog sympathetic neurons

The M-type potassium current and the N-type calcium current are inhibited by several different neurotransmitters in frog sympathetic neurons. These effects seem to be mediated via G proteins, but it is not clear whether diffusible second messengers are involved. Using a rapid (approximately 100 ms) flow tube perfusion system to apply agonists, the inhibition of calcium current develops and recovers rapidly but not instantaneously (t1/2 = 1-2 s). M-current inhibition is considerably slower, with t1/2 approximately 30 s for recovery from inhibition. At least for M-current inhibition, there appears to be sufficient time for involvement of an enzymatic cascade in receptor-channel coupling.

The effects of irreversible acetylcholinesterase inhibitors on transmission through sympathetic ganglia of the bullfrog

The effects of soman, sarin and VX were examined on ganglionic transmission through paravertebral chain ganglia of the bullfrog, Rana catesbeiana. Low frequency (0.1 Hz), short (2 sec) and long (10 sec) trains of preganglionic stimulation, after exposure to the agents, induced repetitive activity in the extracellularly recorded compound action potential. An irreversible transient depression was observed after exposure to the agents during the first second of short and long stimulus trains. Long stimulus trains of high frequency were required to produce a rundown in the amplitude of the compound action potential, whether recorded in the presence of each agent (10 microM) or following a wash with agent-free solution. The rundown of the compound action potential was use-dependent and not blocked or reversed by atropine (10 microM). Intracellular recordings, in the presence of either soman or VX, demonstrated (1) an increase in the amplitude of the residual excitatory postsynaptic potential or current evoked by synaptic stimulation, (2) an increase in the amplitude and duration of the acetylcholine-induced potential, (3) no increase in either the amplitude or duration of the carbachol-induced potential, (4) repetitive firing with orthodromic but not antidromic stimulation and (5) a concentration- and frequency-dependent depolarization of individual ganglion neurons with orthodromic stimulation which resulted in a decrease in the generation of action potentials. These results suggest that the agent-induced decrease in the compound action potential occurred as a consequence of activity-dependent depolarization of ganglion neurons, which occurs after inhibition of cholinesterase.

A muscarine-activated voltage-independent K+ channel in cultured bullfrog sympathetic neurones

In cultured bullfrog sympathetic neurones, cell-attached patch clamp revealed a voltage-independent K+ channel having a conductance of 46-113 pS at an external K+ of 2-120 mM. Muscarine (10-20 microM), applied to the cell membrane outside a recording pipette, increased its open probability and mean open time in an atropine-sensitive manner. This muscarine-activated K+ channel could underlie some of the muscarinic inhibitory postsynaptic potentials in both central and peripheral nervous systems.

The localization of the beta-subtype of protein kinase C (PKC-beta) in rat sympathetic neurons

The localization of PKC-beta was studied in rat sympathetic neurons using a polyclonal antibody specific for the beta 1- and beta 2-subspecies. The tissues studied included the superior cervical (SCG) and hypogastric (HGG) ganglia and the target tissues of the SCG and HGG neurons: the submandibular gland, iris, prostate and vas deferens. PKC-beta-LI was found in nerve fibers in both ganglia. A proportion of the fibers in the SCG disappeared after decentralization, suggesting that the fibers were of both pre- and postganglionic origin. The somata of the HGG and SCG neurons expressed varying amounts of PKC-beta-LI, the majority of SCG neurons being labelled only after colchicine treatment. In all target tissues there were PKC-beta-immunoreactive nerve fibers in bundles, but the most peripheral branches of the fibers were negatively labelled. The results show that PKC-beta-LI is widely present in sympathetic postganglionic neurons with mainly quantitative differences. The lack of PKC-beta in the most peripheral branches of nerve fibers might be a general feature of sympathetic postganglionic neurons, suggesting that the participation of PKC-beta in neurotransmitter release and in other functions in nerve terminals in sympathetic adrenergic neurons is unlikely.

Intracellular recordings from pancreatic ganglia of the cat

1. The anatomy, morphology, and electrophysiology of parasympathetic ganglia of cat pancreas were studied in vitro. 2. Pancreatic ganglia existed as an interconnected plexus of small ganglia (ten to fifty cells) lying in the interlobular connective tissue. Occasionally smaller ganglia (four to ten cells) were observed lying on or within nerve trunks. 3. Electron micrographs revealed the presence of neurones and satellite cells as well as unmyelinated axons and nerve terminals. Nerve terminals contained small clear vesicles and/or large, dense-cored vesicles. 4. Intracellular recording of electrical activity revealed the presence of two types of ganglion cells. Type I ganglion cells exhibited resting membrane potentials that ranged from -40 to -63 mV and input resistances that ranged from 8 to 168 M omega. They responded to intracellular depolarizing current with action potentials, and received synaptic inputs which when activated caused fast and slow depolarizing responses. Type I cells were considered to be ganglionic neurones. Type II ganglion cells had higher resting membrane potentials that ranged from -61 to -83 mV, lower input resistances that ranged from 5 to 83 M omega and were electrically unexcitable. Repetitive stimulation of preganglionic nerves evoked a slow depolarization that was frequency dependent. Type II cells were considered to be satellite cells. 5. Stimulation of nerve trunks both central and peripheral to the ganglia evoked multiple, subthreshold, fast EPSPs in all type I cells tested. Fast EPSPs were blocked by the nicotinic antagonist hexamethonium. 6. Antidromic potentials were also observed following stimulation of either central or peripheral nerve trunks but never both. 7. In type I cells repetitive stimulation of both central and peripheral nerve trunks resulted in a slow, synaptically mediated depolarization which persisted during superfusion with nicotinic and muscarinic receptor antagonists. 8. Periods of low-frequency, spontaneous fast EPSPs and action potentials were observed in all type I cells tested. 9. It was concluded that parasympathetic neurones in cat pancreatic ganglia receive convergent fast and slow synaptic inputs from central and possibly peripheral sources and may function in vivo as sites of integration. The occurrence of spontaneous synaptic potentials in pancreatic ganglia suggests the possibility of intrinsic neural control of pancreatic function.

Synaptic structure and axon collaterals of type B neurons in bullfrog sympathetic ganglia: intracellular horseradish peroxidase (HRP)-labeling study

Type B neurons of the bullfrog sympathetic ganglia were examined to confirm the existence of axon collaterals and the distribution of synaptic contacts using the intracellular horseradish peroxidase (HRP) labeling method. The mean diameter of the perikarya was 60.8 (+/- 11.5 standard deviation; n = 36) X 43.8 (+/- 11.3) microns and the mean diameter of the initial segments of axons was 6.0 (+/- 1.8; n = 36) microns. Axon collaterals were found in 6 cells among 36 examined. They branched from axons at 61-167 microns from the perikaryon of origin. Short-axon collaterals containing vesicles (diameter: about 70 nm) were also observed to protrude from the stem axons. Spine-like processes were observed from the cell soma, axon hillock and the initial segment of the axon. They enclosed synaptic axon varicosities, or extended into the extracellular space without any synaptic contact. Serial sections revealed 171 axon varicosities in contact with a single ganglion cell; 32 (18.7%) varicosities were seen on the somata. 66 (38.6%) on the axon hillock and 73 (42.7%) on the initial segment of the axon which extended 100 microns from the perikaryon. Synaptic terminals were also found on the axon as far as 494 microns from the cell body of origin. These findings would provide a morphological basis for interaction between bullfrog sympathetic neurons at pre- or postsynaptic sites.

A G Protein Mediates the Inhibition of the Voltage-Dependent Potassium M Current by Muscarine, LHRH, Substance P and UTP in Bullfrog Sympathetic Neurons

The involvement of G proteins in the transduction mechanism of M current (Im) inhibition by extracellular ligands in bullfrog sympathetic neurons was examined using the hydrolysis resistant nucleotide analogues GTPgammaS and GDPbetaS. Im was recorded in large (40 - 60 microm) isolated neurons using the patch-clamp technique in the whole-cell configuration, as well as in neurons from the intact ganglion impaled with conventional microelectrodes. In whole-cell recordings Im could be recorded without significant loss for 1 h or more provided ATP was present in the patch pipette. Muscarine, D-Ala6-LHRH, substance P and UTP reversibly inhibited Im in isolated control neurons, with full and rapid recovery of the current following agonist washout. Dialysis of isolated neurons with various concentrations of GTPgammaS (1 - 100 microM) affected, in a dose-dependent manner, the recovery of Im after its inhibition by brief agonist application. With 50 microM GTPgammaS, Im inhibition became completely irreversible. Similarly, the reversibility of Im inhibition by muscarine was reduced or abolished by the iontophoretic injection of GTPgammaS through a second microelectrode into neurons of the intact ganglion. GTPgammaS by itself caused a slow, agonist-independent suppression of Im in dialysed neurons, thus mimicking agonist action. Dialysis of isolated neurons with GDPbetaS (100 - 500 microM) attenuated by half or more the magnitude of Im inhibition by agonist as compared to control neurons. In addition, GDPbetaS attenuated the response of a given neuron to muscarine and D-Ala6-LHRH, and caused slow increase of Im, as a function of dialysis time. Incubation (2 - 72 h, 4 - 36 degrees C) of isolated neurons or intact ganglions with activated pertussis toxin had no effect on the response to muscarine. Toxin injections to experimental animals were equally ineffective. In contrast to Im, the additional inward current with increase in conductance induced by muscarine and D-Ala6-LHRH reversed with agonist washout in GTPgammaS-dialysed neurons, although more slowly than in control neurons. The results in this study indicate that a G protein, possibly pertussis toxin-insensitive, provides a common coupling step linking muscarinic, substance P, D-Ala6-LHRH and UTP receptors to the inhibition of M current.

Interaction of telenzepine with muscarinic receptors in mammalian sympathetic ganglia

The interaction of the antimuscarinic drug telenzepine with muscarinic receptors was studied in rabbit and rat isolated superior cervical sympathetic ganglia. Radioligand binding demonstrated two muscarinic receptor sites in rabbit ganglia, with the characteristics of M1- and M2-receptors. Telenzepine bound to the M1 sites with a KI of 0.94 nmol/l and to the M2 sites with a KI of 17.8 nmol/l; the corresponding values for pirenzepine were 18.6 and 588 nmol/l; for AF-DX 116 the values were 891 and 33 nmol/l respectively. [3H]Telenzepine dissociated from the M1-receptors with a half time of 46 min at 37 degrees C. Electrophysiological experiments demonstrated that telenzepine reduced the amplitude of the extracellularly recorded slow excitatory postsynaptic potential and the slow inhibitory postsynaptic potential (ED50: 38 and 253 nmol/l respectively). In rat ganglia, application of muscarine or the M1-receptor agonist McN-A-343 increased the amplitude of submaximal population action potentials. This facilitation of synaptic transmission was potently blocked by telenzepine and pirenzepine but only weakly by AF-DX 116 (ED50: ca. 30, 150 and 20 mumol/l, respectively). It is concluded that telenzepine blocks the generation of the slow excitatory postsynaptic potential and the excitatory action of muscarine and McN-A-343 via an action on muscarinic M1-receptors.