Prostaglandin E(2) inhibits calcium current in two sub-populations of acutely isolated mouse trigeminal sensory neurons

Effects of PDE-3 inhibition in persistent post-traumatic headache: evidence of cAMP-dependent signaling

BACKGROUND

Phosphodiesterase 3 (PDE-3) inhibition have been implicated in the neurobiologic underpinnings of migraine. Considering the clinical similarities between migraine and persistent post-traumatic headache (PPTH), we aimed to ascertain whether PDE-3 inhibition can elicit migraine-like headache in persons with PPTH.

METHODS

We tested cilostazol, which inhibits PDE-3, in a randomized, double-blind, placebo-controlled, two-way crossover study involving persons with PPTH attributed to mild traumatic brain injury. The randomized participants were allocated to receive oral administration of either 200-mg cilostazol or placebo (calcium tablet) on two separate experiment days. The primary end point was the incidence of migraine-like headache during a 12-hour observation window post-ingestion. The secondary endpoint was the area under the curve (AUC) for reported headache intensity scores during the same observation window.

RESULTS

Twenty-one persons underwent randomization and completed both experiment days. The mean participants' age was 41.4 years, and most (n = 17) were females. During the 12-hour observation window, 14 (67%) of 21 participants developed migraine-like headache post-cilostazol, in contrast to three (14%) participants after placebo (P =.003). The headache intensity scores were higher post-cilostazol than after placebo (P <.001).

CONCLUSIONS

Our results provide novel evidence showing that PDE-3 inhibition can elicit migraine-like headache in persons with PPTH. Given that PDE-3 inhibition increases intracellular cAMP levels, our findings allude to the potential therapeutic value of targeting cAMP-dependent signaling pathways in the management of PPTH. Further investigations are imperative to substantiate these insights and delineate the importance of cAMP-dependent signaling pathways in the neurobiologic mechanisms underlying PPTH.

CLINICALTRIALS

GOV IDENTIFIER

NCT05595993.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

Effects of prostaglandin E2 on synaptic transmission in the rat spinal trigeminal subnucleus caudalis

The spinal trigeminal subnucleus caudalis (Vc) receives preferentially nociceptive afferent signals from the orofacial area. Nociceptive stimuli to the orofacial area induce cyclooxygenase both peripherally and centrally, which can synthesize a major prostanoid prostaglandin E2 (PGE2) that implicates in diverse physiological functions. To clarify the roles of centrally-synthesized PGE2 in nociception, effects of exogenous PGE2 on synaptic transmission in the Vc neurons were investigated in the rat brainstem slice. Spontaneously occurring excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded, respectively, under pharmacological blockade of inhibitory and excitatory transmission by whole-cell patch-clamp mode. Perfusion of PGE2 (1-5 μM) increased the frequency of sIPSCs in a concentration-dependent manner but had no significant effect on the amplitude. Similarly to the effects on sIPSCs, PGE2 increased the sEPSC frequency without any effect on the amplitude. These facilitatory effects of PGE2 on spontaneous synaptic transmissions were blocked by an EP1 antagonist SC19220 but not by an EP4 antagonist AH23848. Electrical stimulation of the trigeminal tract evoked short latency EPSCs (eEPSCs) in the Vc neurons. PGE2 (5 μM) was ineffective on the eEPSCs. The present study demonstrated that PGE2 facilitated spontaneous synaptic transmissions in the Vc neurons through activating the presynaptic EP1 receptors but had no effect on the trigeminal tract-mediated excitatory transmission. These results suggest that centrally-synthesized PGE2 modifies the synaptic transmission in the Vc region, thereby contributing to the processing of nociceptive signals originated from the orofacial area.

Animal models of chronic migraine

Many animal models of migraine have been described. Some of them have been useful in the development of new therapies. All of them have their shortcomings. Animal models of chronic migraine have been relatively less frequently described. Whether a rigid distinction between episodic and chronic migraine is useful when their underlying pathophysiology is likely to be the same and that migraine frequency probably depends on complex polygenic influences remains to be determined. Any model of chronic migraine must reflect the chronicity of the disorder and be reliable and validated with pharmacological interventions. Future animal models of chronic migraine are likely to involve recurrent activation of the trigeminal nociceptive system. Valid models would provide a means for investigating pathophysiological mechanism of the transformation from episodic to chronic migraine and may also be used to test the efficacy of potential preventive medications.

Effects of eugenol on T-type Ca2+ channel isoforms

Eugenol has been used as an analgesic in dentistry. Previous studies have demonstrated that voltage-gated Na(+) channels and high-voltage-activated Ca(2+) channels expressed in trigeminal ganglion (TG) neurons sensing dental pain are molecular targets of eugenol for its analgesic effects. However, it has not been investigated whether eugenol can affect T-type Ca(2+) channels, which are known to be detected in the afferent neurons. In this report, we investigate how eugenol can influence cloned T-type channel isoforms expressed in HEK293 cells, using whole-cell patch clamp. Application of eugenol inhibited Cav3.1, Cav3.2, and Cav3.3 currents in a concentration-dependent manner with IC50 values of 463, 486, and 708 μM, respectively. Eugenol was found to negatively shift the steady-state inactivation curves of the T-type channel isoforms, but it did not shift their activation curves. In addition, eugenol had little effect on the current kinetics of Cav3.1 and Cav3.2, but it accelerated the inactivation kinetics of Cav3.3 currents. Reduction of channel availability enhanced eugenol inhibition sensitivity for Cav3.1 and Cav3.2, but not for Cav3.3. Moreover, eugenol inhibition of T-type channel isoforms was found to be use dependent. Finally, we show that the T-type currents recorded from rat TG neurons were inhibited by eugenol with a similar potency to Cav3.1 and Cav3.2 isoforms. Taken together, our findings suggest that T-type Ca(2+) channels are additional molecular targets for the pain-relieving effects of eugenol.

T-type Ca2+ channels in normal and abnormal brain functions

Low-voltage-activated T-type Ca(2+) channels are widely expressed in various types of neurons. Once deinactivated by hyperpolarization, T-type channels are ready to be activated by a small depolarization near the resting membrane potential and, therefore, are optimal for regulating the excitability and electroresponsiveness of neurons under physiological conditions near resting states. Ca(2+) influx through T-type channels engenders low-threshold Ca(2+) spikes, which in turn trigger a burst of action potentials. Low-threshold burst firing has been implicated in the synchronization of the thalamocortical circuit during sleep and in absence seizures. It also has been suggested that T-type channels play an important role in pain signal transmission, based on their abundant expression in pain-processing pathways in peripheral and central neurons. In this review, we will describe studies on the role of T-type Ca(2+) channels in the physiological as well as pathological generation of brain rhythms in sleep, absence epilepsy, and pain signal transmission. Recent advances in studies of T-type channels in the control of cognition will also be briefly discussed.

COX-1-derived PGE2 and PGE2 type 1 receptors are vital for angiotensin II-induced formation of reactive oxygen species and Ca(2+) influx in the subfornical organ

Regulation of blood pressure by angiotensin II (ANG II) is a process that involves the reactive oxygen species (ROS) and calcium. We have shown that ANG-II type 1 receptor (AT1R) and prostaglandin E2 (PGE2) type 1 receptors (EP1R) are required in the subfornical organ (SFO) for ROS-mediated hypertension induced by slow-pressor ANG-II infusion. However, the signaling pathway associated with this process remains unclear. We sought to determine mechanisms underlying the ANG II-induced ROS and calcium influx in mouse SFO cells. Ultrastructural studies showed that cyclooxygenase 1 (COX-1) codistributes with AT1R in the SFO, indicating spatial proximity. Functional studies using SFO cells revealed that ANG II potentiated PGE2 release, an effect dependent on AT1R, phospholipase A2 (PLA2) and COX-1. Furthermore, both ANG II and PGE2 increased ROS formation. While the increase in ROS initiated by ANG II, but not PGE2, required the activation of the AT1R/PLA2/COX-1 pathway, both ANG II and PGE2 were dependent on EP1R and Nox2 as downstream effectors. Finally, ANG II potentiated voltage-gated L-type Ca(2+) currents in SFO neurons via the same signaling pathway required for PGE2 production. Blockade of EP1R and Nox2-derived ROS inhibited ANG II and PGE2-mediated Ca(2+) currents. We propose a mechanism whereby ANG II increases COX-1-derived PGE2 through the AT1R/PLA2 pathway, which promotes ROS production by EP1R/Nox2 signaling in the SFO. ANG II-induced ROS are coupled with Ca(2+) influx in SFO neurons, which may influence SFO-mediated sympathoexcitation. Our findings provide the first evidence of a spatial and functional framework that underlies ANG-II signaling in the SFO and reveal novel targets for antihypertensive therapies.

Sex differences in the inflammatory mediator-induced sensitization of dural afferents

Approximately 20% of the adult population suffers from migraine. This debilitating pain disorder is three times more prevalent in women than in men. To begin to evaluate the underlying mechanisms that may contribute to this sex difference, we tested the hypothesis that there is a sex difference in the inflammatory mediator (IM)-induced sensitization of dural afferents. Acutely dissociated retrogradely labeled dural afferents from adult Sprague-Dawley rats were examined with whole cell patch-clamp recordings. Baseline passive and active electrophysiological properties of dural afferents from both sexes were comparable. However, while IM-induced increases in the excitability of dural afferents from male and female rats were also comparable, the proportion of dural afferents from female rats sensitized by IM (~100%) was significantly greater than that of dural afferents from male rats (~50%). This appeared to be due to differences downstream of IM receptors, as tetrodotoxin-resistant sodium current was increased by IM in a majority of male dural afferents (13/14). These data indicate that there are both quantitative and qualitative differences in the IM-induced sensitization of dural afferents that may contribute to the sex difference in the manifestation of migraine.

Airway inflammation and central respiratory control: results from in vivo and in vitro neonatal rat

In infants, respiratory infection elicits tachypnea. To begin to evaluate the role of brainstem cytokine expression in modulation of breathing pattern changes, we compared the pattern generated after endotracheal instillation of lipopolysaccharide (LPS) in in vivo rat pups to local pro-inflammatory cytokine injection in the nucleus tractus solitarius (nTS) in an in vitro en bloc brainstem spinal cord preparation. We hypothesized that both challenges would elicit similar changes in patterning of respiration. In anesthetized, spontaneously breathing rat pups, lipopolysaccharide (LPS) or saline was instilled in the airway of urethane-anesthetized rats (postnatal days 10-11). We recorded diaphragm EMG over the subsequent 2h and saw a 20-30% decrease in interburst interval (Te) at 20-80min post-injection in LPS-instilled animals with no significant change in Ti. In contrast, IL-1β injections into the nTS of en bloc in vitro brainstem-spinal cord preparations from 0 to 5 day-old pups maintained Ti and caused an increase in Te as early as 20min later, decreasing frequency for 80-120min after injection. Our results suggest that the neonatal respiratory response to the cytokine IL-1β mediated inflammatory response depends on the site of the inflammatory stimulus and that the direct effect of IL-1β in the nTS is to slow rather than increase rate.

Ionic mechanisms underlying inflammatory mediator-induced sensitization of dural afferents

Migraineurs experience debilitating headaches that result from neurogenic inflammation of the dura and subsequent sensitization of dural afferents. Given the importance of inflammatory mediator (IM)-induced dural afferent sensitization to this pain syndrome, the present study was designed to identify ionic mechanisms underlying this process. Trigeminal ganglion neurons from adult female Sprague Dawley rats were acutely dissociated 10-14 d after application of retrograde tracer DiI onto the dura. Modulation of ion channels and changes in excitability were measured in the absence and presence of IMs (in mum: 1 prostaglandin, 10 bradykinin, and 1 histamine) using whole-cell and perforated-patch recordings. Fura-2 was used to assess changes in intracellular Ca(2+). IMs modulated a number of currents in dural afferents, including those both expected and/or previously described [i.e., an increase in tetrodotoxin-resistant voltage-gated Na(+) current (TTX-R I(Na)) and a decrease in voltage-gated Ca(2+) current] as well currents never before described in sensory neurons (i.e., a decrease in a Ca(2+)-dependent K(+) current and an increase in a Cl(-) current), and produced a sustained elevation in intracellular Ca(2+). Although several of these currents, in particular TTX-R I(Na), appear to contribute to the sensitization of dural afferents, the Cl(-) current is the primary mechanism underlying this process. Activation of this current plays a dominant role in the sensitization of dural afferents because of the combination of the density and biophysical properties of TTX-R I(Na), and the high level of intracellular Cl(-) in these neurons. These results suggest novel targets for the development of antimigraine agents.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

BKCa currents are enriched in a subpopulation of adult rat cutaneous nociceptive dorsal root ganglion neurons

The biophysical properties and distribution of voltage-dependent, Ca(2+) -modulated K(+) (BK(Ca)) currents among subpopulations of acutely dissociated DiI-labeled cutaneous sensory neurons from the adult rat were characterized with whole-cell patch-clamp techniques. BK(Ca) currents were isolated from total K(+) current with iberiotoxin, charybdotoxin or paxilline. There was considerable variability in biophysical properties of BK(Ca) currents. There was also variability in the distribution of BK(Ca) current among subpopulations of cutaneous dorsal root ganglia (DRG) neurons. While present in each of the subpopulations defined by cell body size, IB4 binding or capsaicin sensitivity, BK(Ca) current was present in the vast majority (> 90%) of small-diameter IB4+ neurons, but was present in only a minority of neurons in subpopulations defined by other criteria (i.e. small-diameter IB4-). Current-clamp analysis indicated that in IB4+ neurons, BK(Ca) currents contribute to the repolarization of the action potential and adaptation in response to sustained membrane depolarization, while playing little role in the determination of action potential threshold. Reverse transcriptase-polymerase chain reaction analysis of mRNA collected from whole DRG revealed the presence of multiple splice variants of the BK(Ca) channel alpha-subunit, rslo and all four of the accessory beta-subunits, suggesting that heterogeneity in the biophysical and pharmacological properties of BK(Ca) current in cutaneous neurons reflects, at least in part, the differential distribution of splice variants and/or beta-subunits. Because even a small decrease in BK(Ca) current appears to have a dramatic influence on excitability, modulation of this current may contribute to sensitization of nociceptive afferents observed following tissue injury.

Tumour necrosis factor alpha activates nuclear factor kappaB signalling to reduce N-type voltage-gated Ca2+ current in postganglionic sympathetic neurons

Inflammation has profound effects on the innervation of affected tissues, including altered neuronal excitability and neurotransmitter release. As Ca(2+) influx through voltage-gated Ca(2+) channels (VGCCs) is a critical determinant of excitation-secretion coupling in nerve terminals, the aim of this study was to characterize the effect of overnight incubation in the inflammatory mediator tumour necrosis factor alpha (TNFalpha; 1 nM) on VGCCs in dissociated neurons from mouse superior mesenteric ganglia (SMG). Voltage-gated Ca(2+) currents (I(Ca)) were measured using the perforated patch clamp technique and the VGCC subtypes present in SMG neurons were estimated based on inhibition by selective VGCC blockers: omega-conotoxin GVIA (300 nM; N-type), nifedipine (10 microM; L-type), and omega-conotoxin MVIIC (300 nM; N-, P/Q-type). We used intracellular Ca(2+) imaging with Fura-2 AM to compare Ca(2+) influx during depolarizations in control and TNFalpha-treated neurons. TNF receptor and VGCC mRNA expression were measured using PCR, and channel alpha subunit (CaV2.2) was localized with immunohistochemistry. Incubation in TNFalpha significantly decreased I(Ca) amplitude and depolarization-induced Ca(2+) influx. The reduction in I(Ca) was limited to omega-conotoxin GVIA-sensitive N-type Ca(2+) channels. Depletion of glial cells by incubation in cytosine arabinoside (5 microM) did not affect I(Ca) inhibition by TNFalpha. Preincubation of neurons with SC-514 (20 microM) or BAY 11-7082 (1 microM), which both inhibit nuclear factor kappaB signalling, prevented the reduction in I(Ca) by TNFalpha. Inhibition of N-type VGCCs following TNFalpha incubation was associated with a decrease in CaV2.2 mRNA and reduced membrane localization of CaV2.2 immunoreactivity. These data suggest that TNFalpha inhibits I(Ca) in SMG neurons and identify a novel role for NF-kappaB in the regulation of neurotransmitter release during inflammatory conditions with elevated circulating TNFalpha, such as Crohn's disease and Guillain-Barré syndrome.

Dual modulation of synaptic transmission in the nucleus tractus solitarius by prostaglandin E2 synthesized downstream of IL-1beta

The activation of the innate immune system induces the production of blood-borne proinflammatory cytokines like interleukin-1beta (IL-1beta), which in turn triggers brain-mediated adaptative responses referred to as sickness behaviour. These responses involve the modulation of neural networks in key regions of the brain. The nucleus tractus solitarius (NTS) of the brainstem is a key nucleus for immune-to-brain signalling. It is the main site of termination of vagal afferents and is adjacent to the area postrema, a circumventricular organ allowing blood-borne action of circulating IL-1beta. Although it is well described that IL-1beta activates cerebral endothelial and glial cells, it is still unknown if and how IL-1beta or downstream-synthesized molecules impact NTS synaptic function. In this study we report that IL-1beta did not modulate NTS synaptic transmission per se, whereas prostaglandin E(2) (PGE(2)), which is produced downstream of IL-1beta, produced opposite effects on spontaneous and evoked release. On the one hand, PGE(2) facilitated glutamatergic transmission between local NTS neurons by enhancing the frequency of spontaneous excitatory postsynaptic currents through a presynaptic receptor different from the classical EP1-4 subtypes. On the other hand, PGE(2) also depressed evoked excitatory input from vagal afferent terminals through presynaptic EP3 receptors coupled to G-proteins linked to adenylyl cyclase and protein kinase A activity. Our data show that IL-1beta-induced PGE(2) can modulate evoked and spontaneous release in the NTS differentially through different mechanisms. These data unravel some molecular mechanisms by which innate immune stimuli could signal to, and be integrated within, the brainstem to produce central adaptative responses.

Prostaglandin E2 potentiates the excitability of small diameter trigeminal root ganglion neurons projecting onto the superficial layer of the cervical dorsal horn in rats

The aim of the present study was to investigate how prostaglandin E2 (PGE2) affects the excitability of trigeminal root ganglion (TRG) neurons, projecting onto the superficial layer of the cervical dorsal horn, using fluorescence retrograde tracing and perforated patch-clamp techniques. TRG neurons were retrogradely labeled with fluorogold (FG). The cell diameter of FG-labeled neurons was small (< 30 microm). Under the voltage-clamp mode, application of PGE2 (0.01-10 microM) concentration-dependently increased the magnitude of the peak tetrodotoxin-resistant sodium current (TTX-R I(Na)) and this current was maximal at a concentration of 1 microM. One micromolar PGE2 application caused a hyperpolarizing shift of 8.3 mV in the activation curve for TTX-R I(Na). In the current-clamp mode, the PGE2 (1 microM) application significantly increased the number of action potentials during the depolarizing step pulses as well as the level of overshoot but had no significant effect on the resting membrane potential. These results suggest that the excitability of small diameter TRG neurons seen after 1 microM PGE2 application is involved in an increase in the

Methanandamide activation of a novel current in mouse trigeminal ganglion sensory neurons in vitro

Anandamide is an endogenous agonist for cannabinoid receptors and produces analgesia by acting at these receptors in several sites in the brain and peripheral nervous system. Anandamide is also an agonist at the TRPV1 receptor, a protein that serves as an important integrator of noxious stimuli in sensory neurons. Although anandamide actions at CB1 and TRPV1 receptors can explain many of its effects on sensory neurons, some apparently CB1- and TRPV1-independent effects of anandamide have been reported. To explore possible mechanisms underlying these effects we examined the actions of the stable anandamide analog methanandamide on the membrane properties of trigeminal ganglion neurons from mice with TRPV1 deleted. We found that methanandamide and anandamide activate a novel current in a subpopulation of small trigeminal ganglion neurons. Methanandamide activated the current (EC(50) 2 microM) more potently than it activates TRPV1 under the same conditions. The methanandamide-activated current reverses at 0 mV and does not inactivate at positive potentials but declines rapidly at negative membrane potentials. Activation of the current is not mediated via cannabinoid receptors and does not appear to involve G proteins. The phytocannabinoid Delta(9)-tetrahydrocannabinol, the endocannabinoid-related molecules N-arachidonoyl dopamine and N-arachidonoyl glycine and the non-specific TRPV channel activator 2-aminoethoxydiphenyl borate do not mimic the effects of methanandamide. The molecular identity of the current remains to be established, but we have identified a potential new effector for endocannabinoids in sensory neurons, and activation of this current may underlie some of the previously reported CB1 and TRPV1-independent effects of these compounds.

Prostaglandin E2 depresses solitary tract-mediated synaptic transmission in the nucleus tractus solitarius

Prostaglandin E(2) (PGE(2)) is a prototypical inflammatory mediator that excites and sensitizes cell bodies [Kwong K, Lee LY (2002) PGE(2) sensitizes cultured pulmonary vagal sensory neurons to chemical and electrical stimuli. J Appl Physiol 93:1419-1428; Kwong K, Lee LY (2005) Prostaglandin E(2) potentiates a tetrodotoxin (TTX)-resistant sodium current in rat capsaicin-sensitive vagal pulmonary sensory neurons. J Physiol 56:437-450] and peripheral nerve terminals [Ho CY, Gu Q, Hong JL, Lee LY (2000) Prostaglandin E (2) enhances chemical and mechanical sensitivities of pulmonary C fibers in the rat. Am J Respir Crit Care Med 162:528-533] of primary vagal sensory neurons. Nearly all central nerve terminals of vagal afferents are in the nucleus tractus solitarius (NTS), where they operate with a high probability of release [Doyle MW, Andresen MC (2001) Reliability of monosynaptic sensory transmission in brain stem neurons in vitro. J Neurophysiol 85:2213-2223]. We studied the effect of PGE(2) on synaptic transmission between tractus solitarius afferent nerve terminals and the second-order NTS neurons in brain stem slices of Sprague-Dawley rats. Whole-cell patch recording in voltage clamp mode was used to study evoked excitatory postsynaptic glutamatergic currents (evEPSCs) from NTS neurons elicited by electrical stimulation of the solitary tract (ST). In 34 neurons, bath-applied PGE(2) (200 nM) decreased the evEPSC amplitude by 49+/-5%. In 22 neurons, however, PGE(2) had no effect. We also tested 15 NTS neurons for capsaicin sensitivity. Seven neurons generated evEPSCs that were equally unaffected by PGE(2) and capsaicin. Conversely, evEPSCs of the other eight neurons, which were PGE(2)-responsive, were abolished by 200 nM capsaicin. Furthermore, the PGE(2-)induced depression of evEPSCs was associated with an increase in the paired pulse ratio and a decrease in both the frequency and amplitude of the spontaneous excitatory postsynaptic currents (sEPSCs) and TTX-independent spontaneous miniature excitatory postsynaptic currents (mEPSCs). These results suggest that PGE(2) acts both presynaptically on nerve terminals and postsynaptically on NTS neurons to reduce glutamatergic responses.

Histamine H3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA2-COX-PGE2-EP3R pathway

We hypothesized that the histamine H(3)-receptor (H(3)R)-mediated attenuation of norepinephrine (NE) exocytosis from cardiac sympathetic nerves results not only from a Galpha(i)-mediated inhibition of the adenylyl cyclase-cAMP-PKA pathway, but also from a Gbetagamma(i)-mediated activation of the MAPK-PLA(2) cascade, culminating in the formation of an arachidonate metabolite with anti-exocytotic characteristics (e.g., PGE(2)). We report that in Langendorff-perfused guinea-pig hearts and isolated sympathetic nerve endings (cardiac synaptosomes), H(3)R-mediated attenuation of K(+)-induced NE exocytosis was prevented by MAPK and PLA(2) inhibitors, and by cyclooxygenase and EP(3)-receptor (EP(3)R) antagonists. Moreover, H(3)R activation resulted in MAPK phosphorylation in H(3)R-transfected SH-SY5Y neuroblastoma cells, and in PLA(2) activation and PGE(2) production in cardiac synaptosomes; H(3)R-induced MAPK phosphorylation was prevented by an anti-betagamma peptide. Synergism between H(3)R and EP(3)R agonists (i.e., imetit and sulprostone, respectively) suggested that PGE(2) may be a downstream effector of the anti-exocytotic effect of H(3)R activation. Furthermore, the anti-exocytotic effect of imetit and sulprostone was potentiated by the N-type Ca(2+)-channel antagonist omega-conotoxin GVIA, and prevented by an anti-Gbetagamma peptide. Our findings imply that an EP(3)R Gbetagamma(i)-induced decrease in Ca(2+) influx through N-type Ca(2+)-channels is involved in the PGE(2)/EP(3)R-mediated attenuation of NE exocytosis elicited by H(3)R activation. Conceivably, activation of the Gbetagamma(i) subunit of H(3)R and EP(3)R may also inhibit Ca(2+) entry directly, independent of MAPK intervention. As heart failure, myocardial ischemia and arrhythmic dysfunction are associated with excessive local NE release, attenuation of NE release by H(3)R activation is cardioprotective. Accordingly, this novel H(3)R signaling pathway may ultimately bear therapeutic significance in hyper-adrenergic states.

Decreased mu-opioid receptor signalling and a reduction in calcium current density in sensory neurons from chronically morphine-treated mice

Sensory neurons are a major site of opioid analgesic action, but the effect of chronic morphine treatment (CMT) on mu-opioid receptor function in these cells is unknown. We examined mu-opioid receptor modulation of calcium channel currents (I(Ca)) in small trigeminal ganglion (TG) neurons from mice chronically treated with morphine and measured changes in mu-opioid receptor mRNA levels in whole TG. Mice were injected subcutaneously with 300 mg kg(-1) of morphine base in a slow release emulsion three times over 5 days, or with emulsion alone (vehicles). CMT mice had a significantly reduced response to the acute antinociceptive effects of 30 mg kg(-1) morphine compared with controls (P=0.035).Morphine inhibited I(Ca) in neurons from CMT (EC(50) 300 nM) and vehicle (EC(50) 320 nM) mice with similar potency, but morphine's maximum effect was reduced from 36% inhibition in vehicle to 17% in CMT (P<0.05). Similar results were observed for the selective mu-opioid agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO). Inhibition of I(Ca) by the GABA(B) agonist baclofen was unaffected by CMT. In neurons from CMT mice, there were significant reductions in P/Q-type (P=0.007) and L-type (P=0.002) I(Ca) density.mu-Opioid receptor mRNA levels were not altered by CMT. These data demonstrate that CMT produces a significant reduction of the effectiveness of mu-opioid agonists to inhibit I(Ca) in TG sensory neurons, accompanied by a reduction in I(Ca) density. Thus, adaptations in sensory neurons may mediate some of the tolerance to the antinociceptive effects of morphine that develop during systemic administration.

Inflammation-mediated hyperexcitability of sensory neurons

One of the most prominent signs of tissue injury and inflammation is pain and pain continues to be the primary reason people seek medical attention. Inflammatory pain reflects, at least in part, an increase in the excitability, or sensitization, of subpopulations of primary afferent neurons. While the sensitization of high threshold afferents was observed almost 40 years ago, the basis for this phenomenon continues to be an active and fertile area of research today. This review will summarize recent advances in our mechanistic understanding of sensitization, focusing on four general areas where re search has been most active or productive. These include: (1) the characterization of second messenger pathways underlying inflammation-induced changes in afferent excitability; (2) the impact of previous injury on the afferent response to subsequent inflammation; (3) the impact of target of innervation on the specific afferent response to inflammation, and (4) the impact of sex hormones on the sensitization of high threshold afferents. Work in these areas highlights how much has been learned about this process as well as how much there is yet to learn.

The effect of prostaglandin E1 on ion currents of NG108-15 cells

The aim of this study was to elucidate the mechanism by which prostaglandin E(1) (PGE(1)) acts on ion currents of whole-cell voltage-clamped NG108-15 neuroblastomaxglioma hybrid cells. Ruptured and perforated patch were used. The holding current at -70 mV, the current-voltage curve produced by ramp pulses from -70 to 0 mV and the T-type and hva (high-voltage-activated) Ca(2+) currents associated with rectangular pulses were recorded. Bath application of PGE(1) (0.2 or 3 microM) reversibly increased the holding current, an effect mimicked by the prostanoid agonist iloprost (5-50 nM). The PGE(1) effect was totally blocked by the cAMP-antagonist Rp-cAMPS whereas H-89, an inhibitor of protein kinase A (PKA), failed to inhibit it, even when applied in the fairly high bath concentration of 30 microM. PGE(1) and iloprost also inhibited the T-type and hva Ca(2+) currents and this effect of PGE(1) was likewise not prevented by H-89. In some of the cells, the PGE(1) effect on holding current could be mimicked by 8-pCPT-2Me-cAMP (100-300 microM), a selective agonist of Epac (exchange protein activated by cAMP), but unlike the PGE(1) effect its action was not abolished by Rp-cAMPS. The effect of PGE(1) on the the holding current and on the T-type Ca(2+) current was diminished when EGTA in the pipette solution was replaced by BAPTA, suggesting that Ca(2+) ions are involved in the PGE(1) effect. It is concluded that the PGE(1) effect is mediated by cAMP and Ca(2+) ions but not by PKA or Epac.

Prostaglandin E2-induced modification of tetrodotoxin-resistant Na+ currents involves activation of both EP2 and EP4 receptors in neonatal rat nodose ganglion neurones

1 The aim of the present study was to investigate which EP receptor subtypes (EP1-EP4) act predominantly on the modification of the tetrodotoxin-resistant Na+ current (I(NaR)) in acutely isolated neonatal rat nodose ganglion (NG) neurones. 2 Of the four EP receptor agonists ranging from 0.01 to 10 muM, the EP2 receptor agonist (ONO-AE1-259, 0.1-10 microM) and the EP4 receptor agonist (ONO-AE1-329, 1 microM) significantly increased peak I(NaR). The responses were associated with a hyperpolarizing shift in the activation curve. 3 Neither the EP1 receptor agonist ONO-DI-004 nor the EP3 receptor agonist ONO-AE-248 significantly modified the properties of I(NaR). 4 In PGE2 applications ranging from 0.01 to 10 microM, 1 microM PGE2 produced a maximal increase in the peak I(NaR) amplitude. The PGE2 (1 microM)-induced increase in the GV(1/2) baseline (% change in G at baseline V(1/2)) was significantly attenuated by either intracellular application of the PKA inhibitor PKI or extracellular application of the protein kinase C inhibitor staurosporine (1 microM). However, the slope factor k was not significantly altered by PGE2 applications at 0.01-10 microM. In addition, the hyperpolarizing shift of V(1/2) by PGE2 was not significantly altered by either PKI or staurosporine. 5 In other series of experiments, reverse transcription-polymerase chain reaction (RT-PCR) of mRNA from nodose ganglia indicated that all four EP receptors were present. 6 The NG contained many neuronal cell bodies (diameter <30 microm) with intense or moderate EP2, EP3, and EP4 receptor-immunoreactivities. 7 These results suggest that the PGE2-induced modification of I(NaR) is mainly mediated by activation of both EP2 and EP4 receptors.

Endogenous PGE2Regulates Membrane Excitability and Synaptic Transmission in Hippocampal CA1 Pyramidal Neurons

The significance of cyclooxygenases (COXs), the rate-limiting enzymes that convert arachidonic acid (AA) to prostaglandins (PGs) in the brain, is unclear, although they have been implicated in inflammatory responses and in some neurological disorders such as epilepsy and Alzheimer's disease. Recent evidence that COX-2, which is expressed in postsynaptic dendritic spines, regulates PGE2signaling in activity-dependent long-term synaptic plasticity at hippocampal perforant path-dentate granule cell synapses, suggests an important role of the COX-2–generated PGE2in synaptic signaling. However, little is known of how endogenous PGE2regulates neuronal signaling. Here we showed that endogenous PGE2selectively regulates fundamental membrane and synaptic properties in the hippocampus. Somatic and dendritic membrane excitability was significantly reduced when endogenous PGE2was eliminated with a selective COX-2 inhibitor in hippocampal CA1 pyramidal neurons in slices. Exogenous application of PGE2produced significant increases in frequency of firing, excitatory postsynaptic potentials (EPSP) amplitude, and temporal summation in slices treated with the COX-2 inhibitor. The PGE2-induced increase in membrane excitability seemed to result from its inhibition of the potassium currents, which in turn, boosted dendritic Ca2+influx during dendritic-depolarizing current injections. In addition, the PGE2-induced enhancement of EPSPs was blocked by eliminating both PKA and PKC activities. These findings indicate that endogenous PGE2dynamically regulates membrane excitability, synaptic transmission, and plasticity and that the PGE2-induced synaptic modulation is mediated via cAMP-PKA and PKC pathways in rat hippocampal CA1 pyramidal neurons.

PGE2 increases the tetrodotoxin-resistant Nav1.9 sodium current in mouse DRG neurons via G-proteins

Inflammation caused by tissue damage results in pain, reflecting an increase in excitability of the primary afferent neurons innervating the area. There is some evidence to suggest that altered function of voltage-gated sodium channels is responsible for the hyperexcitability produced by inflammatory agents, possibly acting through G-proteins, but the role of different channel subtypes has not been fully explored. The tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.9 is expressed selectively in C- and A-fibre nociceptive-type units and is upregulated by G-protein activation. In this study, we examined the effects of the inflammatory agent prostaglandin-E(2) (PGE(2)) on Na(v)1.9 current in both Na(v)1.8-null and wild-type (WT) mice and explored the role of specific G-proteins in modulation. PGE(2) caused a twofold increase in Na(v)1.9 current (p<0.05) in both systems. Steady-state activation was shifted in a hyperpolarizing direction by 6-8 mV and availability of channels by 12 mV. No differences in the activation and inactivation kinetics could be detected. The increase in current was blocked by pertussis toxin (PTX) but not cholera toxin (CTX), showing involvement of G(i/o) but not G(s) subunits. Our data indicate that Na(v)1.9 current can be increased during inflammation via a G-protein dependent mechanism and suggest that this could contribute to the regulation of electrogenesis in dorsal root ganglia (DRG) neurons.

Effects of sumatriptan on rat medullary dorsal horn neurons

This study examined the cellular actions of the anti-migraine drug sumatriptan, on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. Sumatriptan inhibited the miniature EPSC (mEPSC) rate in a dose dependent fashion, with an EC(50) of 250 nM. Sumatriptan (3 microM) inhibited the mEPSC rate by 36%, without altering the mEPSC amplitude. This effect was partially reversed by the 5HT(1D) specific antagonist BRL15572 (10 microM). In contrast, the 5HT(1B) agonist CP93129 (10 microm) did not alter the mEPSC rate. Furthermore, sumatriptan (3 microM) decreased the amplitude of electrically evoked EPSCs (eEPSC) by 40%. After incubating the slices in ketanserin (an antagonist which shows selectivity for 5HT(1D) over 5HT(1B) receptors) sumatriptan had little effect on eEPSC amplitude. In control conditions paired stimuli resulted in paired pulse depression (PPD; the ratio eEPSC(2)/eEPSC(1)=0.7+/-0.01), whilst in the presence of sumatriptan the PPD was blocked (ratio eEPSC(2)/eEPSC(1)=0.9+/-0.1). Sumatriptan produced no post-synaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (-60 to -130 mV). RT-PCR experiments revealed the presence of mRNA for both 5HT(1D) and 5HT(1B) receptor subtypes in the trigeminal ganglia and subnucleus caudalis. These data suggest that sumatriptan acts pre-synaptically on trigeminal primary afferent central terminals to reduce the probability of release of glutamate, and that this action is mediated through 5HT(1D) receptors.

Neuroinflammation, COX-2, and ALS—a dual role?

Although the root cause of many neurodegenerative diseases is unknown, neuroinflammation may play a key role in these types of disease, including amyotrophic lateral sclerosis (ALS). In the context of neurodegeneration, it is unclear if the disease is propagated through inflammation, or whether in contrast, evidence of inflammation reflects an attempt to protect against further cellular injury. Inflammatory pathways involving the cyclooxygenase (COX) enzymes and subsequent generation of prostaglandins are potential target sites for treatments to halt the progression of ALS. In the CNS, COX enzymes are localized to neurons, astrocytes, and microglia and can be induced under various conditions. In addition, there appears to be a dual role for the prostaglandin products of COX enzymes in the nervous system. Some prostaglandins promote the survival of neurons, while others promote apoptosis. In this review, the pathways of COX activity and prostaglandin production form the center of the debate regarding the dual nature of neuroinflammation. We will also discuss how this duality may affect future treatments for neurodegenerative diseases such as ALS.

Effects of hypoxia and putative transmitters on [Ca2+]i of rat glomus cells

Dissociated rat glomus cells were loaded with Fura-2 AM to study the effects of hypoxia, and carotid body transmitters on intracellular calcium, [Ca2+]i. The mean control [Ca2+]i was 55 nM in isolated cells and 67 nM in clusters. The following procedures changed [Ca2+]i:0[Ca2+]o+EGTA reduced [Ca2+]i by about 50%, suggesting that the remaining calcium originated from intracellular organelles. [Ca2+]i increased when [Ca2+]o was doubled. Hypoxia by sodium dithionite (Na2S2O4) induced large [Ca2+]i increases in clustered and isolated cells. Smaller rises occurred with 100% N2 hypoxia. The augmented [Ca2+]i, induced by Na2S2O4, was reduced (not eliminated) in 0[Ca2+]o+EGTA, suggesting that some calcium was intracellularly released. Nifedipine depressed (did not block) the Na2S2O4-induced calcium increase, implying some inflow via other (N, T or P/Q) voltage-dependent or voltage-independent calcium channels.Cholinergic agents (ACh, nicotine, muscarine, bethanechol and pilocarpine) increased [Ca2+]i. The ACh effect was produced exclusively by calcium inflow since it was eliminated in 0[Ca2+]o+EGTA. Cholinergic effects were depressed (not obliterated) by D-tubocurarine (D-TC), hexamethonium (C6) and atropine.ACh, nicotine and pilocarpine potentiated the excitatory effect of Na2S2O4 on [Ca2+]i. Bethanechol depressed this excitation whereas muscarine had inconsistent effects. Atropine and C6 depressed [Ca2+]i increases elicited by Na2S2O4 but the effects of D-TC were variable. Dopamine (DA) had variable effects. It increased [Ca2+]i in 75% of cases, and reduced the Na2S2O4 -induced calcium increase.Thus, calcium increases during Na2S2O4 occur by direct effects on the glomus cells and feedback action through released ACh and DA.

Glutamate-mediated cytosolic calcium oscillations regulate a pulsatile prostaglandin release from cultured rat astrocytes

The synaptic release of glutamate evokes in astrocytes periodic increases in [Ca2+]i, due to the activation of metabotropic glutamate receptors (mGluRs). The frequency of these [Ca2+]i oscillations is controlled by the level of neuronal activity, indicating that they represent a specific, frequency-coded signalling system of neuron-to-astrocyte communication. We recently found that neuronal activity-dependent [Ca2+]i oscillations in astrocytes are the main signal that regulates the coupling between neuronal activity and blood flow, the so-called functional hyperaemia. Prostaglandins play a major role in this fundamental phenomenon in brain function, but little is known about a possible link between [Ca2+]i oscillations and prostaglandin release from astrocytes. To investigate whether [Ca2+]i oscillations regulate the release of vasoactive prostaglandins, such as the potent vasodilator prostaglandin E2 (PGE2), from astrocytes, we plated wild-type human embryonic kidney (HEK)293 cells, which respond constitutively to PGE2 with [Ca2+]i elevations, onto cultured astrocytes, and used them as biosensors of prostaglandin release. After loading the astrocyte-HEK cell co-cultures with the calcium indicator Indo-1, confocal microscopy revealed that mGluR-mediated [Ca2+]i oscillations triggered spatially and temporally coordinated [Ca2+]i increases in the sensor cells. This response was absent in a clone of HEK cells that are unresponsive to PGE2, and recovered after transfection with the InsP3-linked prostanoid receptor EP1. We conclude that [Ca2+]i oscillations in astrocytes regulate prostaglandin releases that retain the oscillatory behaviour of the [Ca2+]i changes. This finely tuned release of PGE2 from astrocytes provides a coherent mechanistic background for the role of these glial cells in functional hyperaemia.

Prostanoid EP3 and TP receptors-mediated inhibition of noradrenaline release from the isolated rat stomach

The postganglionic sympathetic nerves of the isolated rat stomach were electrically stimulated twice at 1 Hz for 1 min. Prostaglandin E(2) and ONO-AE-248 (16S-9-deoxy-9beta-chloro-15-deoxy-16-hyfroxy-17,17-trimethylene-19,20-didehydro prostaglandin F(2)) (an EP(3) receptor agonist) reduced the evoked noradrenaline release, while ONO-DI-004 (17S-2,5-ethano-6-oxo-17,20-dimethyl prostaglandin E(1)) (an EP(1) receptor agonist), ONO-AE1-259-01 (11,15-O-dimethyl prostaglandin E(2)) (an EP(2) receptor agonist) and ONO-AE1-329 [16-(3-methoxymethyl)phenyl-omega-tetranor-3,7-dithia prostaglandin E(1)] (an EP(4) receptor agonist) had no effect. U-46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha)) and I-BOP (7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabicyclo[2,2,1] hept-2-yl]-,[1S[1alpha,2alpha(Z),3beta(1E,3S)4alpha]]-5-heptenoic acid) (TP receptor agonists) also reduced the noradrenaline release and these inhibitory effects were abolished by SQ-29548 (7-[3-[[2-[(phenylamino) carbonyl] hydrazino]methyl]-7-oxabicyclo[2,2,1]hept-2-yl][1S(1alpha,2alpha(Z), 3alpha,4alpha]-5-heptenoic acid) (a TP receptor antagonist). The inhibitory effect of U-46619, but not ONO-AE-248, was abolished by pertussis toxin. These results suggest that the prostanoid EP(3) and TP receptors mediate the inhibition of gastric noradrenaline release; TP, but not EP(3), receptor-mediated inhibition is mediated by a pertussis toxin-sensitive mechanism in rats.

Anandamide is a partial agonist at native vanilloid receptors in acutely isolated mouse trigeminal sensory neurons

1. The endogenous fatty acid anandamide (AEA) is a partial agonist at cannabinoid CB1 receptors and has been reported to be a full agonist at the recombinant vanilloid receptor, VR1. 2. Whole cell voltage clamp techniques were used to examine the efficacy of AEA and related analogues methanandamide and N-(4-hydroxyphenyl)-arachidonylamide (AM404) at native VR1 receptors in acutely isolated mouse trigeminal neurons. 3. Superfusion of the VR1 agonist capsaicin onto small trigeminal neurons voltage clamped at +40 mV produced outward currents in most cells, with a pEC(50) of 6.3+/-0.1 (maximum currents at 10-30 micro M). 4. AEA produced outward currents with a pEC(50) of 5.6+/-0.1. Maximal AEA currents (30-100 micro M) were 38+/-2% of the capsaicin maximum. AEA currents were blocked by the VR1 antagonist capsazepine (30 micro M), but unaffected by the CB1 antagonist SR141716A (1 micro M). 5. Methanandamide and AM404 were less potent than AEA at activating VR1. Methanandamide (100 micro M) produced currents 37+/-6% of the capsaicin maximum, the highest concentration of AM404 tested (100 micro M) produced currents that were 55+/-9% of the capsaicin maximum. 6. Capsazepine abolished the currents produced by AM404 (100 micro M) and strongly attenuated (>70%) those produced by methanandamide (100 micro M). 7. Co-superfusion of AEA (30 micro M, methanandamide (100 micro M) or AM404 (100 micro M) with capsaicin (3 micro M) resulted in a significant reduction of the capsaicin current. 8. These data indicate that AEA, methanandamide and AM404 activate native VR1 receptors, but that all three compounds are partial agonists when compared with capsaicin.