Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters.

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

Investigation of the prostacyclin (IP) receptor antagonist RO1138452 on isolated blood vessel and platelet preparations.

BACKGROUND AND PURPOSE: The current study examined the utility of the recently described prostacyclin (prostanoid IP) receptor antagonist RO1138452 (2-(4-(4-isopropoxybenzyl)-phenylamino) imidazoline) as a tool for classifying prostanoid receptors. EXPERIMENTAL APPROACH: pA(2) values were determined on isolated smooth muscle and platelet preparations. KEY RESULTS: RO1138452 antagonized relaxation of human pulmonary artery, guinea-pig aorta and rabbit mesenteric artery induced by the selective IP agonist cicaprost. Schild plots had slopes close to unity, generating pA(2) values of 8.20, 8.39 and 8.12 respectively. Non-surmountable antagonism was sometimes found with the higher concentrations of RO1138452, attributable to the EP(3) contractile action of cicaprost. RO1138452 did not block relaxation of guinea-pig trachea induced by the EP(2)-selective agonist butaprost. In contrast, there was a modest inhibition of butaprost-induced relaxation of human pulmonary artery by RO1138452, implying activation of both EP(2) and IP receptors by butaprost. RO1138452 did not affect relaxation induced by PGE(2) (EP(4) agonist) and substance P (NK(1)/endothelium-dependent agonist) in rabbit mesenteric artery. In human and rat platelet-rich plasmas, RO1138452 antagonized cicaprost-induced inhibition of platelet aggregation in a surmountable manner; pA(2) values may have been affected by binding of RO1138452 to plasma protein. RO1138452 did not affect the inhibitory actions of PGD(2) (DP(1) agonist) and NECA (adenosine A(2A) agonist) in human platelets. CONCLUSIONS AND IMPLICATIONS: The data indicate that RO1138452 is a potent and selective IP receptor antagonist. RO1138452 represents an important addition to our armoury of prostanoid receptor antagonists and a potential clinical agent in situations where prostacyclin has a pathophysiological function.

The role of IP prostanoid receptors in inflammatory pain.

Prostanoid receptor-mediated sensitization of sensory nerve fibres is a key contributor to the generation of hyperalgesia. It is generally thought that prostaglandin (PG) E2 is the principal pro-inflammatory prostanoid. Consequently, prostanoid EP receptors on sensory neurones have been identified as potential therapeutic targets. However, IP prostanoid receptors are also present on sensory neurones, and recent data from transgenic mice lacking the IP receptor demonstrate its importance in the induction of oedema and pain behaviour. PGI2, the primary endogenous agonist for the IP receptor, is rapidly produced following tissue injury or inflammation; thus, it may be of equal, or greater, importance than PGE2 during episodes of inflammatory pain. In this review, Keith Bley, John Hunter, Richard Eglen and Jacqueline Smith compare the roles of EP and IP receptors in nociception and suggest that the IP receptor constitutes a novel target for anti-nociceptive agents.

Tetrodotoxin inhibits neuropathic ectopic activity in neuromas, dorsal root ganglia and dorsal horn neurons.

Neuropathic pain or persistent dysesthesias may be initiated by mechanical, chemical, or ischemic damage to peripheral sensory nerves. In animal models of neuropathic pain, transection or constrictive injury to peripheral nerves produces ectopic discharges originating at both injury sites and related dorsal root ganglia (DRG), and, consequently, hyperexcitability in associated dorsal horn (DH) neurons of the spinal cord. Since ectopic discharges are inhibited by agents that block voltage-sensitive Na+ channels, it has been postulated that accumulation of Na+ channels in the membrane at nerve injury sites may contribute to, or be responsible for, the development of ectopic neuronal activity (ENA). The present study therefore, tested the sensitivity of ENA to intravenously administered tetrodotoxin (TTX), an extremely potent and selective Na+ channel blocker. Comparative effects of TTX on cardiac parameters such as heart rate (HR) and diastolic blood pressure (DBP) were also studied. Experiments were performed on adult male Sprague-Dawley rats in which the common sciatic nerve had been transected 4-10 days earlier. Neuromal activity was measured in fine bundles of microfilaments teased from sciatic nerves, and extracellular microelectrode recordings were made from DRG and DH neurons. Cardiovascular parameters were recorded simultaneously. Intravenously administered TTX induced dose-dependent inhibition of ENA, with that originating from neuromas being the most sensitive; ED50 values (expressed as microg/kg, with 95% confidence limits) for neuromal, DRG and DH neuron activity were: 0.8 (0.6-1.2), 4.3 (2.2-8.4) and 36.2 (16.1-81.3), respectively. Inhibition of ENA in neuromas and DRG did not recover within 10 min after 100 or 300 microg/kg TTX. By comparison, the ED50 value for the initial decrease of HR was 17.9 (15.0-21.5) microg/kg, and partial recovery occurred within approximately 3 min. These data support the hypothesis that Na+ channel accumulation contributes to the generation of ectopic discharges in neuromas and DRG, and suggest that TTX-sensitive Na+ channels located at the nerve injury site and DRG play an important role in the genesis of neuropathic pain.

Characterization of prostanoid receptor-evoked responses in rat sensory neurones.

1. Prostanoid receptor-mediated sensitization, or excitation, of sensory nerve fibres contributes to the generation of hyperalgesia. To characterize the prostanoid receptors present on sensory neurones, biochemical assays were performed on primary cultures of adult rat dorsal root ganglia (DRG) and the F-11 (embryonic rat DRG x neuroblastoma hybrid) cell line. 2. In DRG cultures, the IP receptor agonists, cicaprost and carbaprostacyclin (cPGI2) stimulated cyclic AMP accumulation. Prostaglandin E2 (PGE2) also increased cyclic AMP levels, but to a lesser extent, while carbocyclic thromboxane A2 (cTxA2), PGD2 and PGF2alpha had negligible effects. The rank order of agonist potency was cicaprost>PGE2=BMY45778=cPGI2=PGI2. In the F-11 cells, the rank order of agonist potency for the stimulation of cyclic AMP accumulation was: cicaprost>iloprost=cPGI2=PGI2=BMY45778>PGE2=cTXA2++ +. In DRG cultures, cicaprost induced significantly more accumulation of inositol phosphates than PGE2. 3. To examine the effects of prostanoids on C-fibre activity, extracellular recordings of d.c. potentials from the rat isolated vagus nerve were made with the 'grease-gap' technique. PGI2 (0.1 nM-10 microM) produced the largest depolarizations of the nerve. The rank order of agonist potency was: PGI2=cPGI2=PGE1>cTXA2>PGE2=PGD2=TXB2>PGF2alpha. 4. Prior depolarization of nerves with either forskolin (10 microM) or phorbol dibutyrate (1 microM) alone significantly reduced the response to PGI2 (10 microM), while simultaneous application of both forskolin and phorbol dibutyrate attenuated PGI2 responses almost completely. 5. Putative EP1 and/or TP receptor-selective antagonists had no effect on the responses to PGI2, cPGI2 or PGE2 in the three preparations studied. 6. Collectively, these data are consistent with a positive coupling of IP receptors to both adenylyl cyclase and phospholipase C in sensory neurones. These findings suggest that IP receptors play a major role in the sensitization of rat sensory neurones.

A stable prostacyclin analog enhances ectopic activity in rat sensory neurons following neuropathic injury.

Prostanoids sensitize sensory afferents during inflammation. However, their role in neuropathic pain is still unclear. We analyzed the actions of prostanoids, non-selective (indomethacin) or selective (celecoxib and NS-398) cyclooxygenase-2 (COX or COX-2) inhibitors, on the ectopic activity of dorsal root ganglia (DRG) and dorsal horn (DH) neurons in a model of neuropathic injury. Extracellular recordings of DRG and DH neurons and cardiovascular measurements were performed on anesthetized, paralyzed and artificially ventilated adult male Sprague-Dawley rats whose sciatic nerve had been transected. PGD(2), PGE(2), PGF(2alpha), carbaprostacyclin (cPGI(2); a stable prostacyclin analog), and carbocyclic thromboxane (cTXA(2)) were administered at cumulative doses (0.0001-5 mg/kg, i.p.) at 5 or 10 min intervals. Only cPGI(2) significantly increased the DRG and DH activity in a dose-dependent manner, with ED(50) values of 0.05 (0.01-0.96) and 0.69 (0.11-1.04) mg/kg, respectively. The other prostanoids did not significantly increase activity, although they reduced heart rate for up to 5 min following administration. Time course experiments with single doses of cPGI(2) (1 mg/kg, i.v.) increased DH discharge rate 3-17 min after injection. Indomethacin (3 mg/kg, s.c.), but not celecoxib or NS-398 (both at 6 mg/kg, s.c.), reduced both DRG and DH activity. Our results indicate that cPGI(2) excites DRG and DH neurons of neuropathic rats, and may suggest a role for IP prostanoid receptors in pain episodes associated with nerve injury. The inhibitory effect of indomethacin, but not celecoxib or NS-398, on ectopic activity may suggest that a tonic generation of PGI(2) by COX-1 could contribute to neuropathic pain.

QX-314 inhibits ectopic nerve activity associated with neuropathic pain.

In animal models of neuropathic pain, transection or constrictive injury to peripheral nerves produces ectopic discharges originating at both injury sites and related dorsal root ganglia (DRG). In addition, hyperexcitability is observed in associated dorsal horn (DH) neurons of the spinal cord. As ectopic discharges are inhibited by agents that block voltage-sensitive Na+ channels, it has been postulated that accumulation of Na+ channels in the membrane at nerve injury sites may contribute to the development of ectopic nerve activity (ENA). The goal of the present study was to compare the sensitivity of ENA to lidocaine and QX-314, a positively charged lidocaine derivative, which is frequently assumed to be membrane impermeant. Experiments were performed on adult male Sprague-Dawley rats in which the common sciatic nerve had been transected 4-10 days earlier. Extracellular microelectrode recordings were made from DRG and DH neurons, and neuronal activity was measured in fine bundles of microfilaments teased from sciatic nerves in anesthetized and paralyzed rats. Comparative effects on heart rate (HR) and mean blood pressure (MBP) were also studied. To confirm that externally applied QX-314 is able to inhibit high frequency activity in sensory nerves, QX-314 was superfused over isolated rat vagus nerves during stimulation of compound action potentials in C-fibers (C-spikes). As expected, intravenously administered lidocaine inhibited ENA at all three sites. Lidocaine ED50 values (expressed as mg/kg, with 95% confidence limits) were: 10.2 (7.8-13.3), 1.4 (0.8-2.4) and 0.9 (0.4-2.0) for neuromas, DRG and DH neurons, respectively. QX-314 also induced dose-dependent inhibition of ENA at neuromas and DRG, but produced only a small inhibition of DH neuron ENA. QX-314 had the following ED50 values (mg/kg) for neuromas, DRG and DH neurons, respectively: 2.3 (2.0-2.8), 6.9 (4.7-26.5) and 85.7. QX-314-mediated inhibition of DRG ENA had a slow onset and was long-lasting, relative to lidocaine. Lidocaine or QX-314 also significantly reduced HR and MBP in the same dose range as that which reduced ENA in DRG or neuromas. In isolated rat vagus nerve recordings, QX-314 induced marked use-dependent inhibition of C-spike amplitude, with IC50 values (microM) of 9000 (4600-18,000) and 350 (290-420) for low- (0.03 Hz) and high-frequency (30 Hz) C-spikes, respectively. These data support the hypothesis that Na+ channel accumulation contributes to the generation of ectopic discharges in neuromas and DRG, and suggests that intravenous QX-314 can acutely block Na+ channels at these sites.

Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve.

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT4 receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the 'grease-gap' technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations > or = 1 microM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT3 receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarization were not reduced by 5-HT1 or 5-HT2 receptor antagonists, but were potently inhibited by the 5-HT4 receptor antagonist GR 113808 (pA2 = 9.3), and mimicked by 5-methoxytryptamine (pEC50 = 5.3). 5-HT4-mediated responses were larger at 37 degrees C than at 31 degrees C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 microM. Conversely, 5-HT3 receptor responses were potentiated at lower temperatures (< or = 31 degrees C). Consistent with the reported positive coupling of 5-HT4 receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT4 receptor activation. Pre-depolarization of nerves with 10 microM forskolin or 300 microM 8-Br-cAMP diminished the effect of 5-HT4 receptors. This study has confirmed the presence of 5-HT4 receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the electrophysiological, biochemical and behavioral actions of epibatidine.

Epibatidine has been reported to be a potent, nonopioid analgesic. In this study we further characterized its receptor interactions and its analgesic properties. Radioligand binding assays demonstrated that epibatidine has high affinity for nicotinic receptors (Ki = 0.12 nM) but low affinity for opioid and other receptors (Ki > 3.0 microM). In vitro functional assays demonstrated that the compound is a potent agonist at both neuronal and neuromuscular nicotinic receptors. Epibatidine depolarized rat isolated vagus nerve with an EC50 of 33.1 nM and contracted guinea pig ileum with an EC50 of 6.1 nM. Epibatidine contracted frog rectus abdominis muscle with an EC50 of 18.2 nM. In vivo, epibatidine demonstrated short-lived analgesic actions. Epibatidine (10 and 30 micrograms/kg), at 5 but not 20 min after dosing, increased the threshold for vocalization evoked by foot shock. Epibatidine, at 5 and 20 but not 60 min after dosing, also increased the latency to a nociceptive response in a hot-plate assay. Both (+)- and (-)-enantiomers of epibatidine were active in these assays. The action of epibatidine in the hot-plate test was reversed by the nicotinic receptor antagonist mecamylamine but not by the opioid receptor antagonist naloxone. In contrast to morphine, epibatidine failed to increase locomotor activity. These findings demonstrate that epibatidine is a potent agonist at both neuronal and neuromuscular nicotinic receptors. These findings also demonstrate a short-lived, naloxone-insensitive, analgesic action for both the (+)- and (-)-enantiomers of epibatidine.(ABSTRACT TRUNCATED AT 250 WORDS)