Neuromedin U type 1 receptor stimulation of A-type K+ current requires the βγ subunits of Go protein, protein kinase A, and extracellular signal-regulated kinase 1/2 (ERK1/2) in sensory neurons

Trace amine-associated receptor 1 regulation of Kv1.4 channels in trigeminal ganglion neurons contributes to nociceptive behaviors

BACKGROUND

Trace amines, such as tyramine, are endogenous amino acid metabolites that have been hypothesized to promote headache. However, the underlying cellular and molecular mechanisms remain unknown.

METHODS

Using patch-clamp recording, immunostaining, molecular biological approaches and behaviour tests, we elucidated a critically functional role of tyramine in regulating membrane excitability and pain sensitivity by manipulating Kv1.4 channels in trigeminal ganglion (TG) neurons.

RESULTS

Application of tyramine to TG neurons decreased the A-type K⁺ current (I_A) in a manner dependent on trace amine-associated receptor 1 (TAAR1). Either siRNA knockdown of Gαo or chemical inhibition of βγ subunit (G_βγ) signaling abrogated the response to tyramine. Antagonism of protein kinase C (PKC) prevented the tyramine-induced I_A response, while inhibition of conventional PKC isoforms or protein kinase A elicited no such effect. Tyramine increased the membrane abundance of PKC_θ in TG neurons, and either pharmacological or genetic inhibition of PKC_θ blocked the TAAR1-mediated I_A decrease. Furthermore, PKC_θ-dependent I_A suppression was mediated by Kv1.4 channels. Knockdown of Kv1.4 abrogated the TAAR1-induced I_A decrease, neuronal hyperexcitability, and pain hypersensitivity. In a mouse model of migraine induced by electrical stimulation of the dura mater surrounding the superior sagittal sinus, blockade of TAAR1 signaling attenuated mechanical allodynia; this effect was occluded by lentiviral overexpression of Kv1.4 in TG neurons.

CONCLUSION

These results suggest that tyramine induces Kv1.4-mediated I_A suppression through stimulation of TAAR1 coupled to the G_βγ-dependent PKC_θ signaling cascade, thereby enhancing TG neuronal excitability and mechanical pain sensitivity. Insight into TAAR1 signaling in sensory neurons provides attractive targets for the treatment of headache disorders such as migraine.

(-)-Naringenin 4',7-dimethyl Ether Isolated from Nardostachys jatamansi Relieves Pain through Inhibition of Multiple Channels

(-)-Naringenin 4',7-dimethyl ether ((-)-NRG-DM) was isolated for the first time by our lab from Nardostachys jatamansi DC, a traditional medicinal plant frequently used to attenuate pain in Asia. As a natural derivative of analgesic, the current study was designed to test the potential analgesic activity of (-)-NRG-DM and its implicated mechanism. The analgesic activity of (-)-NRG-DM was assessed in a formalin-induced mouse inflammatory pain model and mustard oil-induced mouse colorectal pain model, in which the mice were intraperitoneally administrated with vehicle or (-)-NRG-DM (30 or 50 mg/kg) (n = 10 for each group). Our data showed that (-)-NRG-DM can dose dependently (30~50 mg/kg) relieve the pain behaviors. Notably, (-)-NRG-DM did not affect motor coordination in mice evaluated by the rotarod test, in which the animals were intraperitoneally injected with vehicle or (-)-NRG-DM (100, 200, or 400 mg/kg) (n = 10 for each group). In acutely isolated mouse dorsal root ganglion neurons, (-)-NRG-DM (1~30 μM) potently dampened the stimulated firing, reduced the action potential threshold and amplitude. In addition, the neuronal delayed rectifier potassium currents (IK) and voltage-gated sodium currents (INa) were significantly suppressed. Consistently, (-)-NRG-DM dramatically inhibited heterologously expressed Kv2.1 and Nav1.8 channels which represent the major components of the endogenous IK and INa. A pharmacokinetic study revealed the plasma concentration of (-)-NRG-DM is around 7 µM, which was higher than the effective concentrations for the IK and INa. Taken together, our study showed that (-)-NRG-DM is a potential analgesic candidate with inhibition of multiple neuronal channels (mediating IK and INa).

Neuromedin B receptor stimulation of Cav3.2 T-type Ca2+ channels in primary sensory neurons mediates peripheral pain hypersensitivity

Background: Neuromedin B (Nmb) is implicated in the regulation of nociception of sensory neurons. However, the underlying cellular and molecular mechanisms remain unknown.

Methods: Using patch clamp recording, western blot analysis, immunofluorescent labelling, enzyme-linked immunosorbent assays, adenovirus-mediated shRNA knockdown and animal behaviour tests, we studied the effects of Nmb on the sensory neuronal excitability and peripheral pain sensitivity mediated by Cav3.2 T-type channels.

Results: Nmb reversibly and concentration-dependently increased T-type channel currents (I_T) in small-sized trigeminal ganglion (TG) neurons through the activation of neuromedin B receptor (NmbR). This NmbR-mediated I_T response was G_q protein-coupled, but independent of protein kinase C activity. Either intracellular application of the QEHA peptide or shRNA-mediated knockdown of G_β abolished the NmbR-induced I_T response. Inhibition of protein kinase A (PKA) or AMP-activated protein kinase (AMPK) completely abolished the Nmb-induced I_T response. Analysis of phospho-AMPK (p-AMPK) revealed that Nmb significantly activated AMPK, while AMPK inhibition prevented the Nmb-induced increase in PKA activity. In a heterologous expression system, activation of NmbR significantly enhanced the Cav3.2 channel currents, while the Cav3.1 and Cav3.3 channel currents remained unaffected. Nmb induced TG neuronal hyperexcitability and concomitantly induced mechanical and thermal hypersensitivity, both of which were attenuated by T-type channel blockade. Moreover, blockade of NmbR signalling prevented mechanical hypersensitivity in a mouse model of complete Freund's adjuvant-induced inflammatory pain, and this effect was attenuated by siRNA knockdown of Cav3.2.

Conclusions: Our study reveals a novel mechanism by which NmbR stimulates Cav3.2 channels through a G_βγ-dependent AMPK/PKA pathway. In mouse models, this mechanism appears to drive the hyperexcitability of TG neurons and induce pain hypersensitivity.

The neuronal potassium current IA is a potential target for pain during chronic inflammation

Voltage-gated ion channels play a key role in the action potential (AP) initiation and its propagation in sensory neurons. Modulation of their activity during chronic inflammation creates a persistent pain state. In this study, we sought to determine how peripheral inflammation caused by complete Freund's adjuvant (CFA) alters the fast sodium (INa ), L-type calcium (ICaL ), and potassium (IK ) currents in primary afferent fibers to increase nociception. In our model, intraplantar administration of CFA induced mechanical allodynia and thermal hyperalgesia at day 14 post-injection. Using whole-cell patch-clamp recording in dissociated small (C), medium (Aδ), and large-sized (Aβ) rat dorsal root ganglion (DRG) neurons, we found that CFA prolonged the AP duration and increased the amplitude of the tetrodotoxin-resistant (TTX-r) INa in Aβ fibers. In addition, CFA accelerated the recovery of INa from inactivation in C and Aδ nociceptive fibers but enhanced the late sodium current (INaL ) only in Aδ and Aβ neurons. Inflammation similarly reduced the amplitude of ICaL in each neuronal cell type. Fourteen days after injection, CFA reduced both components of IK (IKdr and IA ) in Aδ fibers. We also found that IA was significantly larger in C and Aδ neurons in normal conditions and during chronic inflammation. Our data, therefore, suggest that targeting the transient potassium current IA represents an efficient way to shift the balance toward antinociception during inflammation, since its activation will selectively decrease the AP duration in nociceptive fibers. Altogether, our data indicate that complex interactions between IK , INa , and ICaL reduce pain threshold by concomitantly enhancing the activity of nociceptive neurons and reducing the inhibitory action of Aβ fibers during chronic inflammation.

Satellite Glial Cells Give Rise to Nociceptive Sensory Neurons

Dorsal root ganglia (DRG) sensory neurons can transmit information about noxious stimulus to cerebral cortex via spinal cord, and play an important role in the pain pathway. Alterations of the pain pathway lead to CIPA (congenital insensitivity to pain with anhidrosis) or chronic pain. Accumulating evidence demonstrates that nerve damage leads to the regeneration of neurons in DRG, which may contribute to pain modulation in feedback. Therefore, exploring the regeneration process of DRG neurons would provide a new understanding to the persistent pathological stimulation and contribute to reshape the somatosensory function. It has been reported that a subpopulation of satellite glial cells (SGCs) express Nestin and p75, and could differentiate into glial cells and neurons, suggesting that SGCs may have differentiation plasticity. Our results in the present study show that DRG-derived SGCs (DRG-SGCs) highly express neural crest cell markers Nestin, Sox2, Sox10, and p75, and differentiate into nociceptive sensory neurons in the presence of histone deacetylase inhibitor VPA, Wnt pathway activator CHIR99021, Notch pathway inhibitor RO4929097, and FGF pathway inhibitor SU5402. The nociceptive sensory neurons express multiple functionally-related genes (SCN9A, SCN10A, SP, Trpv1, and TrpA1) and are able to generate action potentials and voltage-gated Na⁺ currents. Moreover, we found that these cells exhibited rapid calcium transients in response to capsaicin through binding to the Trpv1 vanilloid receptor, confirming that the DRG-SGC-derived cells are nociceptive sensory neurons. Further, we show that Wnt signaling promotes the differentiation of DRG-SGCs into nociceptive sensory neurons by regulating the expression of specific transcription factor Runx1, while Notch and FGF signaling pathways are involved in the expression of SCN9A. These results demonstrate that DRG-SGCs have stem cell characteristics and can efficiently differentiate into functional nociceptive sensory neurons, shedding light on the clinical treatment of sensory neuron-related diseases.

Effect of neuromedin U on allergic airway inflammation in an asthma model

Asthma is a major inflammatory airway disease with high incidence and mortality rates. The Global Initiative for Asthma released a report called ‘The Global Burden of Asthma’ in 2004. However, the specific pathogenesis of asthma remains unclear. An increasing number of studies have demonstrated that neuromedin U (NMU) plays a pleiotropic role in the pathogenesis of asthma. NMU is a highly structurally conserved neuropeptide that was first purified from porcine spinal cord and named for its contractile effect on the rat uterus. NMU amplifies type 2 innate lymphoid cell (ILC2)-driven allergic lung inflammation. The NMU receptors (NMURs), designated as NMUR1 and NMUR2, belong to the G protein-coupled receptor family. NMUR1 has also been found in immune cells, including ILC2s, mast cells and eosinophils. In view of the important roles of NMU in the pathogenesis of asthma, the present review evaluates the potential mechanisms underlying the impact of NMU on asthma and its association with asthma therapy.

Cholecystokinin type B receptor-mediated inhibition of A-type K+ channels enhances sensory neuronal excitability through the phosphatidylinositol 3-kinase and c-Src-dependent JNK pathway

Background

Cholecystokinin (CCK) is implicated in the regulation of nociceptive sensitivity of primary afferent neurons. Nevertheless, the underlying cellular and molecular mechanisms remain unknown.

Methods

Using patch clamp recording, western blot analysis, immunofluorescent labelling, enzyme-linked immunosorbent assays, adenovirus-mediated shRNA knockdown and animal behaviour tests, we studied the effects of CCK-8 on the sensory neuronal excitability and peripheral pain sensitivity mediated by A-type K⁺ channels.

Results

CCK-8 reversibly and concentration-dependently decreased A-type K⁺ channel (I_A) in small-sized dorsal root ganglion (DRG) neurons through the activation of CCK type B receptor (CCK-BR), while the sustained delayed rectifier K⁺ current was unaffected. The intracellular subunit of CCK-BR coimmunoprecipitated with Gα_o. Blocking G-protein signaling with pertussis toxin or by the intracellular application of anti-G_β antibody reversed the inhibitory effects of CCK-8. Antagonism of phosphatidylinositol 3-kinase (PI3K) but not of its common downstream target Akts abolished the CCK-BR-mediated I_A response. CCK-8 application significantly activated JNK mitogen-activated protein kinase. Antagonism of either JNK or c-Src prevented the CCK-BR-mediated I_A decrease, whereas c-Src inhibition attenuated the CCK-8-induced p-JNK activation. Application of CCK-8 enhanced the action potential firing rate of DRG neurons and elicited mechanical and thermal pain hypersensitivity in mice. These effects were mediated by CCK-BR and were occluded by I_A blockade.

Conclusion

Our findings indicate that CCK-8 attenuated I_A through CCK-BR that is coupled to the G_βγ-dependent PI3K and c-Src-mediated JNK pathways, thereby enhancing the sensory neuronal excitability in DRG neurons and peripheral pain sensitivity in mice.

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

The short interference RNA (siRNA) targeting NMUR2 relieves nociception in a bone cancer pain model of rat through PKC-ERK and PI3K-AKT pathways

BACKGROUND/AIM

Bone cancer pain (BCP) causes troubles and burdens to patients globally. Increasing evidence proved that neuromedin U receptor 2 (NMUR2) was involved in pains. Our study was performed to investigate the role of NMUR2 on BCP and the underlying mechanism.

METHODS

The rats were raised and BCP rat model was established by injection with Walker 256 cells. The RNA and protein expression levels of NMUR2 in rat neurons-dorsal spinal cord cells, RNdsc cells were detected by qRT-PCR and western blot. The administration with NMUR2 was via intrathecal injection with siRNA to silence NMUR2. The tolerance of rat to pain was measured by mechanical allodynia test and presented by paw withdrawal threshold (PWT) value. The effects on protein kinase C (PKC)/extracellular regulated protein kinases (ERK) and phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT) signal pathways were examined by western blot.

RESULTS

The expression of NMUR2 in both mRNA and protein levels was upregulated in BCP rat model. In addition, siRNA injection significantly decreased the expression of NMUR2 on the 3rd, 7th and 14th day. BCP group revealed lower PWT value compared with control while NMUR2 silence increased the PWT value compared with negative control. The phosphorylation of PKC, ERK, PI3K and AKT was increased in BCP model while was decreased by si-NMUR2. PKC/ERK and PI3K/AKT inhibitor administration increased the PWT value compared with BCP group.

CONCLUSION

si-NMUR2 alleviates BCP via inactivation of PKC/ERK and PI3K/AKT signal pathways.

17β-estradiol potentiates TREK1 channel activity through G protein-coupled estrogen receptor

TWIK-related potassium channel 1 (TREK1), a two-pore domain potassium channel, is modulated by various hormones and neurotransmitters by activation of membrane receptor - coupled second messengers. 17β-estradiol is a neuromodulator capable of regulating several cellular processes including the activity of ion channels, in a rapid and non-genomic manner. The G protein-coupled estrogen receptor (GPER) is known to facilitate rapid actions of 17β-estradiol, though its role in modulation of ion channels is not widely explored. Several studies have shown both TREK1 and 17β-estradiol to be neuromodulatory but the interaction between them is not known. In the present study, using single channel cell-attached patch clamp electrophysiology in HEK293 cells, we show that 17β-estradiol increases the activity of hTREK1 channel by acting through hGPER and increasing the channel opening probability within minutes. The potentiation induced by 17β-estradiol is pertussis toxin - sensitive involving action of Gβγ subunits while the inhibitory effect of cAMP-PKA pathway on TREK1 is reduced. Protein phosphatases were also found to be important for the action of 17β-estradiol, which in concert with reduced activity of PKA, may alter the phosphorylation state of the channel and thus increase channel activity. Mutational studies revealed the serines at positions 315 and 348 in the C-terminal domain of hTREK1 to be the target sites for dephosphorylation induced by 17β-estradiol action through hGPER. Elucidation of the pathway for the potentiating action of 17β-estradiol via hGPER on hTREK1 channel activity will help us understand better one of the many possible neuroprotective mechanisms of 17β-estradiol and hTREK1 channel.

Inhibition of A-Type K+ Channels by Urotensin-II Induces Sensory Neuronal Hyperexcitability Through the PKCα-ERK Pathway

Previous studies have implicated urotensin-II in the nociception of sensory neurons. However, to date the relevant mechanisms remain unknown. In the current study we determined the role of urotensin-II in the regulation of transient outward A-type potassium currents (IA) and neuronal excitability in trigeminal ganglion (TG) neurons. We found that application of urotensin-II to small-diameter TG neurons decreased IA in a dose-dependent manner, whereas the delayed rectifier potassium current was unaffected. The IA decrease induced by urotensin-II depended on the urotensin-II receptor (UT-R) and was associated with a hyperpolarizing shift in the steady-state inactivation curve. Exposure of TG cells to urotensin-II markedly increased protein kinase C (PKC) activity, and PKC inhibition eliminated the UT-R-mediated IA decrease. Antagonism of PKCα, either pharmacologically or genetically, but not of PKCβ prevented the decrease in IA induced by urotensin-II. Analysis of phospho-extracellular signal-regulated kinase (p-ERK) revealed that urotensin-II significantly increased the expression level of p-ERK, whereas p-p38 and p-c-Jun N-terminal kinase remained unchanged. Inhibition of mitogen-activated protein kinase/ERK signaling by the kinase antagonist U0126 and PD98059 completely abolished the UT-R-mediated IA decrease. Moreover, urotensin-II significantly increased the action potential firing rate of small TG neurons; pretreatment with 4-aminopyridine prevented this effect. In summary, our findings suggest that urotensin-II selectively attenuated IA through stimulation of the PKCα-dependent ERK1/2 signaling pathway. This UT-R-dependent mechanism might contribute to neuronal hyperexcitability in TG neurons.

Melatonin-mediated inhibition of Cav3.2 T-type Ca2+ channels induces sensory neuronal hypoexcitability through the novel protein kinase C-eta isoform

Recent studies implicate melatonin in the antinociceptive activity of sensory neurons. However, the underlying mechanisms are still largely unknown. Here, we identify a critical role of melatonin in functionally regulating Cav3.2 T-type Ca²⁺ channels (T-type channel) in trigeminal ganglion (TG) neurons. Melatonin inhibited T-type channels in small TG neurons via the melatonin receptor 2 (MT₂ receptor) and a pertussis toxin-sensitive G-protein pathway. Immunoprecipitation analyses revealed that the intracellular subunit of the MT₂ receptor coprecipitated with Gα_o . Both shRNA-mediated knockdown of Gα_o and intracellular application of QEHA peptide abolished the inhibitory effects of melatonin. Protein kinase C (PKC) antagonists abolished the melatonin-induced T-type channel response, whereas inhibition of conventional PKC isoforms elicited no effect. Furthermore, application of melatonin increased membrane abundance of PKC-eta (PKC_η ) while antagonism of PKC_η or shRNA targeting PKC_η prevented the melatonin-mediated effects. In a heterologous expression system, activation of MT₂ receptor strongly inhibited Cav3.2 T-type channel currents but had no effect on Cav3.1 and Cav3.3 current amplitudes. The selective Cav3.2 response was PKC_η dependent and was accompanied by a negative shift in the steady-state inactivation curve. Furthermore, melatonin decreased the action potential firing rate of small TG neurons and attenuated the mechanical hypersensitivity in a mouse model of complete Freund's adjuvant-induced inflammatory pain. These actions were inhibited by T-type channel blockade. Together, our results demonstrated that melatonin inhibits Cav3.2 T-type channel activity through the MT₂ receptor coupled to novel G_βγ -mediated PKC_η signaling, subsequently decreasing the membrane excitability of TG neurons and pain hypersensitivity in mice.

Oncogenic features of neuromedin U in breast cancer are associated with NMUR2 expression involving crosstalk with members of the WNT signaling pathway

Neuromedin U (NMU) has been shown driving the progression of various tumor entities, including breast cancer. However, the expression pattern of NMU and its receptors in breast cancer tissues as well as systematic insight into mechanisms and downstream targets of the NMU-driven signaling pathways are still elusive. Here, NMU expression was found up-regulated in all breast cancer subtypes when compared to healthy breast tissue. Using an in silico dataset comprising 1,195 samples, high NMU expression was identified as an indicator of poor outcome in breast tumors showing strong NMUR2 expression. Next, the biological impact of NMU on breast cancer cells in relation to NMUR2 expression was analyzed. Ectopic NMU expression reduced colony growth while promoting a motile phenotype in NMUR2-positive SKBR3 but not NMUR2-negative Hs578T cells. To uncover signaling pathways and key molecules affected by NMU in SKBR3 cells, Affymetrix microarray analysis was applied. Forced NMU expression affected molecules involved in WNT receptor signaling among others. As such we demonstrated enhanced activation of the WNT/planar cell polarity (PCP) effector RAC1 and down-regulation of canonical WNT targets such as MYC. In summary, NMU might contribute to progression of NMUR2-positive breast cancer representing a potential druggable target for future personalized strategies.

Testosterone rapidly increases Ca2+-activated K+ currents causing hyperpolarization in human coronary artery endothelial cells

Testosterone has endothelium-dependent vasodilatory effects on the coronary artery, with some reports suggesting endothelial ion channel involvement. This study employed the whole-cell patch clamp technique to investigate the effect of testosterone on ion channels in human coronary artery endothelial cells (HCAECs) and the mechanisms involved. We found that 0.03-3μM testosterone significantly induced a rapid, concentration-dependent increase in total HCAEC current (EC₅₀, 71.96±1.66nM; maximum increase, 59.13±8.37%; mean±SEM). The testosterone-enhanced currents consisted of small- and large-conductance Ca²⁺-activated K⁺ currents (SK_Ca and BK_Ca currents), but not Cl^- and nonselective cation currents. Either a non-permeant testosterone conjugate or the non-aromatizable androgen dihydrotestosterone (DHT) could increase HCAEC currents as well. The androgen receptor antagonist flutamide prevented this testosterone, testosterone conjugate, and DHT effect, while the estrogen receptor antagonist fulvestrant did not. Incubating HCAECs with pertussis toxin or protein kinase A inhibitor H-89 largely inhibited the testosterone effect, while pre-incubation with phospholipase C inhibitor U-73122, prostacyclin inhibitor indomethacin, nitric oxide synthase inhibitor L-NAME or cytochrome P450 inhibitor MS-PPOH, did not. Finally, testosterone application induced HCAEC hyperpolarization within minutes; this effect was prevented by SK_Ca and BK_Ca current inhibitors apamin and iberiotoxin. This is the first electrophysiological demonstration of androgen-induced K_Ca current increase, leading to hyperpolarization, in any endothelial cell, and the first report of SK_Ca as a testosterone target. Our data show that testosterone rapidly increased whole-cell HCAEC SK_Ca and BK_Ca currents via a surface androgen receptor, G_i/o protein, and protein kinase A. This mechanism may explain rapid testosterone-induced coronary vasodilation seen in vivo.

Serotonin type-1D receptor stimulation of A-type K(+) channel decreases membrane excitability through the protein kinase A- and B-Raf-dependent p38 MAPK pathways in mouse trigeminal ganglion neurons

Although recent studies have implicated serotonin 5-HT1B/D receptors in the nociceptive sensitivity of primary afferent neurons, the underlying molecular and cellular mechanisms remain unclear. In this study, we identified a novel functional role of the 5-HT1D receptor subtype in regulating A-type potassium (K(+)) currents (IA) as well as membrane excitability in small trigeminal ganglion (TG) neurons. We found that the selective activation of 5-HT1D, rather than 5-HT1B, receptors reversibly increased IA, while the sustained delayed rectifier K(+) current was unaffected. The 5-HT1D-mediated IA increase was associated with a depolarizing shift in the voltage dependence of inactivation. Blocking G-protein signaling with pertussis toxin or by intracellular application of a selective antibody raised against Gαo or Gβ abolished the 5-HT1D effect on IA. Inhibition of protein kinase A (PKA), but not of phosphatidylinositol 3-kinase or protein kinase C, abolished the 5-HT1D-mediated IA increase. Analysis of phospho-p38 (p-p38) revealed that activation of 5-HT1D, but not 5-HT1B, receptors significantly activated p38, while p-ERK and p-JNK were unaffected. The p38 MAPK inhibitor SB203580, but not its inactive analogue SB202474, and inhibition of B-Raf blocked the 5-HT1D-mediated IA response. Functionally, we observed a significantly decreased action potential firing rate induced by the 5-HT1D receptors; pretreatment with 4-aminopyridine abolished this effect. Taken together, these results suggest that the activation of 5-HT1D receptors selectively enhanced IA via the Gβγ of the Go-protein, PKA, and the sequential B-Raf-dependent p38 MAPK signaling cascade. This 5-HT1D receptor effect may contribute to neuronal hypoexcitability in small TG neurons.

Go is required for the release of IL-8 and TNF-α, but not degranulation in human mast cells

Mast cells activated by IgE-dependent and -independent mechanisms play important roles in innate and acquired immune responses. Activation of pertussis toxin (PTX)-sensitive Gi/o proteins is the key step in mast cell degranulation and release of de novo synthesized inflammatory mediators through IgE-independent mechanism. However, the roles of Gi and Go proteins in mast cells activation have not yet been differentiated. In the current study, the functional roles of Go proteins in the activities of LAD2 cells, a human mast cell line, are identified. Knockdown of Gαo expression significantly inhibited the synthesis of IL-8 and TNF-α from substance P activated LAD2 cells but demonstrated no effect on degranulation. This effect was associated with the activation of Erk and JNK/MAPKs signaling, whereas PI3K-Akt, calcium mobilization and NFAT translocation remained unchanged. These results suggest that Gi and Go proteins differentially regulate human mast cells activities through activating distinct signaling cascades.

Melatonin-mediated inhibition of Purkinje neuron P-type Ca²⁺ channels in vitro induces neuronal hyperexcitability through the phosphatidylinositol 3-kinase-dependent protein kinase C delta pathway

Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P-type Ca(2+) channels and action-potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (I(Ba)) by 32.9%. This effect was melatonin receptor 1 (MT(R1)) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibitory effects of melatonin. Pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitors abolished the melatonin-induced decrease in I(Ba). Surprisingly, melatonin responses were not regulated by Akt, a common downstream target of PI3K. Melatonin treatment significantly increased protein kinase C (PKC) activity 2.1-fold. Antagonists of PKC, but not of protein kinase A, abolished the melatonin-induced decrease in I(Ba). Melatonin application increased the membrane abundance of PKCδ, and PKCδ inhibition (either pharmacologically or genetically) abolished the melatonin-induced IBa response. Functionally, melatonin increased spontaneous action-potential firing by 53.0%; knockdown of MT(R1) and blockade of P-type channels abolished this effect. Thus, our results suggest that melatonin inhibits P-type channels through MT(R1) activation, which is coupled sequentially to the βγ subunits of G(i/o)-protein and to downstream PI3K-dependent PKCδ signaling. This likely contributes to its physiological functions, including spontaneous firing of cerebellar Purkinje neurons.

Urotensin-II receptor stimulation of cardiac L-type Ca2+ channels requires the βγ subunits of Gi/o-protein and phosphatidylinositol 3-kinase-dependent protein kinase C β1 isoform

Recent studies have demonstrated that urotensin-II (U-II) plays important roles in cardiovascular actions including cardiac positive inotropic effects and increasing cardiac output. However, the mechanisms underlying these effects of U-II in cardiomyocytes still remain unknown. We show by electrophysiological studies that U-II dose-dependently potentiates L-type Ca(2+) currents (ICa,L) in adult rat ventricular myocytes. This effect was U-II receptor (U-IIR)-dependent and was associated with a depolarizing shift in the voltage dependence of inactivation. Intracellular application of guanosine-5'-O-(2-thiodiphosphate) and pertussis toxin pretreatment both abolished the stimulatory effects of U-II. Dialysis of cells with the QEHA peptide, but not scrambled peptide SKEE, blocked the U-II-induced response. The phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin as well as the class I PI3K antagonist CH132799 blocked the U-II-induced ICa,L response. Protein kinase C antagonists calphostin C and chelerythrine chloride as well as dialysis of cells with 1,2bis(2aminophenoxy)ethaneN,N,N',N'-tetraacetic acid abolished the U-II-induced responses, whereas PKCα inhibition or PKA blockade had no effect. Exposure of ventricular myocytes to U-II markedly increased membrane PKCβ1 expression, whereas inhibition of PKCβ1 pharmacologically or by shRNA targeting abolished the U-II-induced ICa,L response. Functionally, we observed a significant increase in the amplitude of sarcomere shortening induced by U-II; blockade of U-IIR as well as PKCβ inhibition abolished this effect, whereas Bay K8644 mimicked the U-II response. Taken together, our results indicate that U-II potentiates ICa,L through the βγ subunits of Gi/o-protein and downstream activation of the class I PI3K-dependent PKCβ1 isoform. This occurred via the activation of U-IIR and contributes to the positive inotropic effect on cardiomyocytes.

Ghrelin suppresses Purkinje neuron P-type Ca(2+) channels via growth hormone secretagogue type 1a receptor, the βγ subunits of Go-protein, and protein kinase a pathway

Although ghrelin receptors have been demonstrated to be widely expressed in the central nervous system and peripheral tissues of mammals, it is still unknown whether ghrelin functions in cerebellar Purkinje neurons. In this study, we identified a novel functional role for ghrelin in modulating P-type Ca(2+) channel (P-type channel) currents (IBa) as well as action-potential firing in rat Purkinje neurons. Our results show that ghrelin at 0.1μM reversibly decreased IBa by ~32.3%. This effect was growth hormone secretagogue receptor 1a (GHS-R1a)-dependent and was associated with a hyperpolarizing shift in the voltage-dependence of inactivation. Intracellular application of GDP-β-S and pretreatment with pertussis toxin abolished the inhibitory effects of ghrelin. Dialysis of cells with the peptide QEHA (but not the scrambled peptide SKEE), and a selective antibody raised against the G-protein αo subunit both blocked the ghrelin-induced response. Ghrelin markedly increased protein kinase A (PKA) activity, and intracellular application of PKI 5-24 as well as pretreatment of the cells with the PKA inhibitor KT-5720 abolished ghrelin-induced IBa decrease, while inhibition of PKC had no such effects. At the cellular level, ghrelin induced a significant increase in action-potential firing, and blockade of GHS-R1a by BIM-28163 abolished the ghrelin-induced hyperexcitability. In summary, these results suggest that ghrelin markedly decreases IBa via the activation of GHS-R1a, which is coupled sequentially to the activities of Go-protein βγ subunits and the downstream PKA pathway. This could contribute to its physiological functions, including the spontaneous firing of action potentials in cerebellar Purkinje neurons.

17β-estradiol rapidly attenuates P2X3 receptor-mediated peripheral pain signal transduction via ERα and GPR30

Estrogen has been reported to affect pain perception, although the underlying mechanisms remain unclear. In this investigation, pain behavior testing, patch clamp recording, and immunohistochemistry were used on rats and transgenic mice to determine which estrogen receptors (ERs) and the related signaling pathway are involved in the rapid modulation of estrogen on P2X3 receptor-mediated events. The results showed that 17β-estradiol (E2) rapidly inhibited pain induced by α,β-methylene ATP (α,β-me-ATP), a P2X1 and P2X3 receptor agonist in ovariectomized rats and normal rats in diestrus. The ERα agonist 4,49,499-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT) and G protein-coupled receptor 30 (GPR30) agonist G-1 mimicked the estrogen effect, whereas the ERβ agonist diarylpropionitrile (DPN) had no effect. In cultured rat dorsal root ganglion (DRG) neurons, PPT and G-1 but not DPN significantly attenuated α,β-me-ATP-mediated currents, with the dose-response curve of these currents shifted to the right. The inhibitory effect of E2 on P2X3 currents was blocked by G-15, a selective antagonist to the GPR30 estrogen receptor. E2 lacked this effect in DRG neurons from ERα-knockout mice but partly remained in those from ERβ-knockout mice. The P2X3 and GPR30 receptors were coexpressed in the rat DRG neurons. Furthermore, the ERK1/2 inhibitor U0126 reversed the inhibitory effect of E2 on α,β-me-ATP-induced pain and of PPT or G-1 on P2X3 receptor-mediated currents. The cAMP-protein kinase A (PKA) agonist forskolin, but not the PKC agonist phorbol-12-myristate-13-acetate (PMA), mimicked the estrogen-inhibitory effect on P2X3 receptor currents, which was blocked by another ERK1/2 inhibitor, PD98059. These results suggest that estrogen regulates P2X3-mediated peripheral pain by acting on ERα and GPR30 receptors expressed in primary afferent neurons, which probably involves the intracellular cAMP-PKA-ERK1/2 pathway.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Neuritin activates insulin receptor pathway to up-regulate Kv4.2-mediated transient outward K+ current in rat cerebellar granule neurons

Neuritin is a new neurotrophic factor discovered in a screen to identify genes involved in activity-dependent synaptic plasticity. Neuritin also plays multiple roles in the process of neural development and synaptic plasticity. The receptors for binding neuritin and its downstream signaling effectors, however, remain unclear. Here, we report that neuritin specifically increases the densities of transient outward K(+) currents (I(A)) in rat cerebellar granule neurons (CGNs) in a time- and concentration-dependent manner. Neuritin-induced amplification of I(A) is mediated by increased mRNA and protein expression of Kv4.2, the main α-subunit of I(A). Exposure of CGNs to neuritin markedly induces phosphorylation of ERK (pERK), Akt (pAkt), and mammalian target of rapamycin (pmTOR). Neuritin-induced I(A) and increased expression of Kv4.2 are attenuated by ERK, Akt, or mTOR inhibitors. Unexpectedly, pharmacological blockade of insulin receptor, but not the insulin-like growth factor 1 receptor, abrogates the effect of neuritin on I(A) amplification and Kv4.2 induction. Indeed, neuritin activates downstream signaling effectors of the insulin receptor in CGNs and HeLa. Our data reveal, for the first time, an unanticipated role of the insulin receptor in previously unrecognized neuritin-mediated signaling.