Endothelin modulates calcium channel current in neurones of rabbit pelvic parasympathetic ganglia

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Endothelin-1 induced desensitization in primary afferent neurons

Endothelin-1 (ET-1) is a known algogen that causes acute pain and sensitization in humans and spontaneous nociceptive behaviors when injected into the periphery in rats, and is elevated during vaso-occlusive episodes (VOEs) in sickle cell disease (SCD) patients. Previously, our lab has shown that a priming dose of ET-1 produces sensitization to capsaicin-induce secondary hyperalgesia. The goal of this study was to determine if the sensitization induced by ET-1 priming is occurring at the level of the primary afferent neuron. Calcium imaging in cultured dorsal root ganglion (DRG) neurons was utilized to examine the effects of ET-1 on primary afferent neurons. ET-1 induces [Ca(2+)]i transients in unprimed cells. ET-1 induced [Ca(2+)]i transients are attenuated by priming with ET-1. This priming effect occurs whether the priming dose is given 0-4 days prior to the challenge dose. Similarly, ET-1 priming decreases capsaicin-induced [Ca(2+)]i transients. At the level of the primary afferent neuron, ET-1 priming has a desensitizing effect on challenge exposures to ET-1 and capsaicin.

Endothelin receptors and pain

The endogenous endothelin (ET) peptides participate in a remarkable variety of pain-relatedprocesses. Pain that is elevated by inflammation, by skin incision, by cancer, during a Sickle Cell Disease crisis and by treatments that mimic neuropathic and inflammatory pain and are all reduced by local administration of antagonists of endothelin receptors. Many effects of endogenously released endothelin are simulated by acute, local subcutaneous administration of endothelin, which at very high concentrations causes pain and at lower concentrations sensitizes the nocifensive reactions to mechanical, thermal and chemical stimuli.

PERSPECTIVE

In this paper we review the biochemistry, second messenger pathways and hetero-receptor coupling that are activated by ET receptors, the cellular physiological responses to ET receptor activation, and the contribution to pain of such mechanisms occurring in the periphery and the CNS. Our goal is to frame the subject of endothelin and pain for a broad readership, and to present the generally accepted as well as the disputed concepts, including important unanswered questions.

Endothelin-1-induced depolarization and hyperpolarization in submandibular ganglion neurons

The effects of endothelin-1 were studied in vitro on neurons in the hamster submandibular ganglion, using the intracellular microelectrode technique. Endothelin (1 microM) caused a depolarization (5.5 +/- 1.2 mV) followed by a hyperpolarization (8.5 +/- 2.8 mV) of the membrane potential. Membrane conductance was increased during the endothelin-induced depolarization and was decreased during the endothelin-induced hyperpolarization. The endothelin-induced depolarization was depressed (mean 43.6%) in a Krebs solution containing zero calcium and high magnesium. The results suggested that the predominant component of the depolarization was mediated by calcium ions. The calcium-insensitive component of depolarization was carried by chloride ions. Endothelin-induced slow rhythmic hyperpolarizations were probably induced by a decrease in chloride ion conductance.

Tip60 and HDAC7 interact with the endothelin receptor a and may be involved in downstream signaling

Endothelins exert their biological effects through G protein-coupled receptors. However, the precise mechanism of downstream signaling and trafficking of the receptors is largely unknown. Here we report that the histone acetyltransferase Tip60 and the histone deacetylase HDAC7 interact with one of the ET receptors, ETA, as determined by yeast two-hybrid analysis, glutathione S-transferase pull-down assays, and co-immunoprecipitation from transfected COS-7 cells. In the absence of ET-1, Tip60 and HDAC7 were localized mainly in the cell nucleus while ETA was predominantly confined to the plasma membrane. Stimulation with ET-1 resulted in the internalization of ETA to the perinuclear compartment and simultaneously in the efflux of Tip60 and HDAC7 from the nucleus to the same perinuclear compartment where each protein co-localized with the receptor. Upon co-transfection with ETA into COS-7 cells, Tip60 strongly increased ET-1-induced ERK1/2 phosphorylation, whereas HDAC7 had no significant effect. We thus suggest that protein acetylase and deacetylase interact with ETA in a ligand-dependent fashion and may participate in ET signal transduction.

Parasympathetic neurotransmission in rabbit isolated bronchus is modulated at prejunctional sites via endothelinB receptor stimulation

OBJECTIVE

The aim of this study was to investigate the mechanism involved in endothelin-induced potentiation of the response to parasympathetic nerve stimulation.

METHODOLOGY

We used autoradiographic and functional studies in rabbit isolated bronchi.

RESULTS

Autoradiography revealed dense binding sites for radiolabelled endothelin-3 over bronchial parasympathetic ganglia. The contractile response of the bronchus to electrical field stimulation was significantly potentiated by endothelin-3, endothelin-1, sarafotoxin S6c and BQ-3020 to 326+/-53%, 293+/-63%, 514+/-119% and 655+/-178%, respectively, of control values. The endothelin-3-induced potentiation of neurally evoked responses was not affected by the presence of propranolol, phentolamine or hexamethonium. The potentiation was also unaltered by pretreatment with the endothelinA receptor antagonist BQ-123 (3 micromol/L), but was significantly reduced in the presence of the combined endothelinA/endothelinB receptor antagonist PD 145065, indicating that the potentiation was mediated via endothelinB receptors. Confirmation of endothelinB receptor involvement in the neuropotentiation was obtained by demonstration of a significant amelioration of the potentiation in the presence of the endothelinB receptor selective antagonist BQ-788, and after endothelinB receptor desensitization by the endothelin, receptor selective agonist sarafotoxin S6b.

CONCLUSIONS

These results suggest that the endothelin-induced potentiation of parasympathetic neural responses in the rabbit bronchus is mediated via endothelinB receptor activation.

A linear endothelin-1 analogue: solution structure of ET-1[Aib1,3,11,15, Nle7] by nuclear magnetic resonance spectroscopy and molecular modelling

Two-dimensional nuclear magnetic resonance techniques and a combination of distance geometry and molecular dynamics calculations were utilised to determine the three dimensional solution structure of an ET-1 analogue, ET-1[Aib1,3,11,15, Nle7], in a methanol-d3/water co-solvent. The modelled structure shows that the peptide folds into a consistent alpha-helical conformation between residues Ser4-His16 while the C-terminus prefers no fixed conformation. Our studies confirm that the disulphide links which are normally associated with the endothelin family of neuropeptides are not important for the formation of a helical conformation in solution. This full length, modified, synthetic linear ET-1 analogue plays a vital role towards designing endothelin receptor agonists. Structure activity relationships are discussed in terms of the conformational features of the calculated structure.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

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.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

The receptors for endothelins and their analogues in SK-N-MC neuroblastoma cells

The potency order of peptides to inhibit [125I]endothelin-1 binding and to stimulate phosphatidylinositol phosphate (PtdInsP) turnover in SK-N-MC cells was consistent with the presence of ETA-endothelin receptors. Divalent cations enhanced [125I]endothelin-1 binding by, in the case of Mn2+, increasing radioligand affinity. Mn2+ did not induce conformational changes in endothelin-1, and its effect was maintained in solubilized receptors. Hence, metal ions may directly interact with endothelin receptors. The effects of BQ-123 and [Ala1,3,11,15]endothelin-1 on PtdInsP turnover were investigated. Concentration-response curves of endothelins were modeled by a second-order equation that assumes pseudoirreversible ligand binding.

Increased production of endothelins in the hippocampus of stroke-prone spontaneously hypertensive rats following transient forebrain ischemia: histochemical evidence

1. The effect of transient forebrain ischemia on endothelin-1 (ET-1) and endothelin-3 (ET-3) production in the hippocampus of stroke-prone spontaneously hypertensive rats (SHRSPs) was investigated using immunohistochemical techniques. 2. In SHRSPs subjected to 10-min bilateral carotid occlusion, neuronal degeneration in the CA1 pyramidal cell layer of the hippocampus was detectable at 4 days and remarkable at 7 days after reperfusion. 3. Coinciding with neuronal degeneration, ET-1- and ET-3-like immunoreactivities were intense in the CA1 pyramidal-cell layer, the stratum lacunosum moleculare, and the CA4 subfield of the hippocampus. Almost all of the immunostained cells had morphological characteristics of astrocytes. 4. The possibility that ET has a role in the development of neuronal cell death following transient forebrain ischemia warrants further attention.

The structure and specificity of endothelin receptors: their importance in physiology and medicine

In addition to involvement in vascular endothelium-smooth muscle communication, the secretion of and receptors for, endothelins are widely distributed. Two cloned receptor subtypes are G-protein-coupled to several intracellular messengers, predominantly inositol phosphates. From a knowledge of structure-activity relationships and peptide conformations, details of receptor architecture and selective agents, including nonpeptides and antagonists, have been discovered. From the nature of the actions of endothelins, receptor distributions (including CNS) and plasma levels, it is concluded that they are paracrine factors normally involved in long-term cellular regulation, but which may be important in several pathologies, many of which are stress-related.

Guanosine 3',5'-cyclic monophosphate regulates calcium channels in neurones of rabbit vesical pelvic ganglia

1. The effects of dibutyryl guanosine 3',5'-cyclic monophosphate (db-cyclic GMP) were studied in vitro on calcium channels of neurones in rabbit vesical parasympathetic ganglia, using intracellular and single-electrode voltage-clamp recordings. 2. Db-cyclic GMP (100 microM) caused membrane depolarization associated with a decrease in membrane input resistance and an after-hyperpolarization associated with an increase in membrane input resistance. 3. Db-cyclic GMP (0.01-1 mM) caused a concentration-dependent, transient inward current followed by a long-lasting outward current. Membrane conductance was increased and decreased during the inward and outward currents, respectively. 4. The db-cyclic GMP-induced inward current was depressed in nominally calcium-free solutions, by cobalt (1 mM) and nicardipine (10 microM). The mean reversal potentials of the inward current were +42 and -20 mV in the presence and absence of calcium in the external solution, respectively. 5. The db-cyclic GMP-induced inward current was not altered by lowering the external sodium concentration, raising external potassium concentration or by intracellular injection of caesium. 6. A calcium-insensitive component of the db-cyclic GMP-induced current was increased by lowering the external chloride concentration and blocked by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid, a chloride channel blocker. 7. Voltage-dependent, high-threshold calcium currents were depressed during the db-cyclic GMP-induced inward current and facilitated during the outward current. 8. Cyclic GMP was less potent than db-cyclic GMP in causing both inward and outward currents or modulation of calcium currents. GTP, GDP, GMP, guanosine, 8-bromoadenosine 3',5'-cyclic monophosphate and forskolin did not alter the holding current or voltage-dependent calcium currents. 9. It is concluded that intracellular cyclic GMP causes not only activation of resting calcium and chloride channels but also a transient depression followed by long-lasting facilitation of voltage-dependent calcium currents in neurones of vesical parasympathetic ganglia.

Potentiation by Ca2+ ionophores and inhibition by extracellular KCl of endothelin-induced phosphoinositide turnover in C6 glioma cells

Interactions between endothelin-1 (ET)-induced phosphoinositide (PI) hydrolysis and agents that increase Ca2+ influx (i.e. A23187 and ionomycin) or induce depolarization (i.e. KCl) were investigated using C6 glioma. A23187 dose-dependently potentiated ET (30 nM)- and ATP (100 microM)-induced [3H]inositol phosphate (IP) accumulation. This potentiation was associated with an increase in the maximal stimulation elicited by both ET and ATP but their EC50 values were unchanged. This effect of A23187 occurred at concentrations that did not affect basal PI turnover; i.e. 10 nM-3 microM. Ionomycin within the range of 1 nM-1 microM also significantly enhanced ET-induced PI breakdown and this effect was associated with an increase of [Ca2+]i. KCl in a concentration-dependent manner (14.7-54.7 mM) markedly inhibited PI breakdown elicited by ET and ATP, but had much less inhibition on basal activity and no effect on A23187- and ionomycin-induced responses. In parallel, KCl added before or after ET, sharply attenuated the increase of ET-induced [Ca2+]i but did not affect basal level or ionomycin-induced [Ca2+]i response. Neither the potentiation by A23187 nor the inhibition by KCl of ET-induced PI turnover was observed in cultured cerebellar astrocytes. Our results suggest that the cell type-specific regulation by Ca2+ ionophores and KCl on ET-induced PI metabolism is closely related to perturbation of [Ca2+]i.

Effect of endothelin 1 on plasma glucose and lipid levels in pygmy goats

The effect of endothelin 1, a recently discovered vasoconstrictor hormone, on levels of plasma glucose and lipids was investigated in pygmy goats. Endothelin 1 was injected intraperitoneally (1.72 micrograms/kg BW) and blood samples were taken by puncturing the jugular vein immediately before and 60, 150 and 240 min post injection. Endothelin 1 in comparison to vehicle significantly increased plasma glucose and FFA levels, whereas plasma triglycerides were significantly reduced. Thus, beside its vasoconstrictive action, endothelin 1 also has metabolic effects. Whether these effects are directly caused by endothelin 1 or whether they are due to a release of a metabolic hormone, e.g. epinephrine, remains to be investigated.

Cerebellar neurons and glia respond differentially to endothelins and sarafotoxin S6b

Endothelins (ETs) and sarafotoxin-S6b belong to a family of extremely potent vasoconstrictors which may also have a role as neuropeptides or neuromodulators in the central nervous system (CNS). We show, using single cell dynamic video imaging of intracellular free calcium ions ([Ca2+]i), that binding of ET to its receptors modulates [Ca2+]i of neurons as well as glial cells in primary cultures of rat cerebellum. At least two receptor subtypes, differing in both their ligand specificity and distribution, appear to be involved in the action of ETs and sarafotoxin S6b on these cells. One of these receptors may be a previously undescribed neuronal form of ET receptor. This is the first demonstration of a direct effect of ETs on neurons as well as glia in the CNS. These data support a possible role for ET as a neurotransmitter or neuromodulator in the CNS.

Specific binding sites for 125I-endothelin-1 in the porcine and human spinal cord

Specific binding sites for 125I-endothelin-1 (125I-ET-1) in the spinal cord were investigated using quantitative receptor autoradiographic and chemical cross-linking methods. The binding sites were highly concentrated in porcine and human spinal cord areas corresponding anatomically to the dorsal horn (Rexed's laminae I-III), an area around the central canal (lamina X) and the principal part of the intermediolateral nucleus (IMLp). The localization of the binding sites differed from those of 125I-omega-conotoxin GVIA (125I-CgTx) and 125I-Bolton-Hunter substance P (125I-BH-SP), with the exception that the IMLp shared 125I-ET-1 with 125I-CgTx and 125I-BH-SP binding sites. Specific 125I-ET-1 binding sites in the areas examined were characteristically single and of high affinity. There were no differences between the potencies of unlabeled ET family peptides, ET-1, ET-2, ET-3 and sarafotoxin S6b at inhibiting 125I-ET-1 binding to the areas. Chemical cross-linking studies showed that 125I-ET-1 and 125I-ET-3 mainly bound to a protein with molecular mass of 43 kDa in the porcine and human thoracic spinal cord membranes. The present finding shows the neuronal significance of this newly discovered peptide in the spinal cord.