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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
Cell Communication and Signalingvolume 17, Article number: 68 (2019)
Cholecystokinin (CCK) is implicated in the regulation of nociceptive sensitivity of primary afferent neurons. Nevertheless, the underlying cellular and molecular mechanisms remain unknown.
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.
CCK-8 reversibly and concentration-dependently decreased A-type K+ channel (IA) 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 Gαo. Blocking G-protein signaling with pertussis toxin or by the intracellular application of anti-Gβ 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 IA response. CCK-8 application significantly activated JNK mitogen-activated protein kinase. Antagonism of either JNK or c-Src prevented the CCK-BR-mediated IA 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 IA blockade.
Our findings indicate that CCK-8 attenuated IA through CCK-BR that is coupled to the Gβγ-dependent PI3K and c-Src-mediated JNK pathways, thereby enhancing the sensory neuronal excitability in DRG neurons and peripheral pain sensitivity in mice.
Cholecystokinin (CCK), a gastrointestinal polypeptide hormone existing in a variety of amino acid chain lengths, has been isolated from the central nervous system and peripheral tissues . Two types of functional membrane receptors, cholecystokinin A receptor (CCK-AR), located mainly on pancreatic acinar cells, and CCK-BR, mostly in the stomach and nervous system tissues, have been identified as the endogenous receptors of CCK . CCK, acting through its receptors, is found to be involved in the regulation of a variety of physiological functions in the nervous system, including metabolic, neurotrophic and modulatory actions . Additionally, in vitro experiments have suggested that CCK might regulate the sensitivity of nociceptive sensory neurons , where CCK-BRs were abundantly expressed [5, 6]. It has been established that the elevated level of CCK mRNA in the dorsal root ganglia (DRG) induced by peripheral nerve injury sensitizes and excites primary afferent sensory neurons, leading to pain hypersensitivity , with the application of CCK inducing pronociceptive effects . Evidence also suggests that CCK is a potential trigger for increased visceral sensitivity in healthy subjects  as well as in irritable bowel syndrome patients [10, 11]. Moreover, antagonism of CCK receptors can effectively reverse burn-induced mechanical allodynia , and the deletion of the CCK-BR gene attenuates the symptoms of mechanical allodynia in neuropathic pain . So far, however, the mechanisms underlying the CCK-mediated hyperalgesia still remain unclear.
Changes in neuronal excitability of peripheral sensory neurons might directly regulate symptoms of pain, such as allodynia, hyperalgesia and spontaneous pain . Voltage-gated K+ channels (Kv) are one of the major classes of ion channels responsible for driving neuronal excitability in both the central and peripheral nervous system  and in whole-cell patch clamp recordings are separated into two major categories: a large transient component characteristic for fast-inactivating A-type channels and a sustained delayed-rectifying channels, which respectively mediate IA and IDR currents [15, 16]. A-type channels are sensitive to millimolar concentrations of 4-aminopyridine (4-AP) , and they play pivotal roles in the control of the electrical properties and excitability of nociceptive neurons [14, 15]. Recent evidence has suggested that A-type channels have been implicated in pain plasticity and neuropathic conditions , which begin with the aberrant firing of action potential bursts in damaged neuronal tissue. For example, peripheral nerve injury results in the reduction of channel expression, thereby decreasing IA, and enhancing the sensory neuronal excitability and pain sensitivity [18, 19]. Manipulation of IA is, therefore, predicted to affect neuronal excitability and useful for pain treatment.
In the current study, we examined the regulation of CCK-8 on IA in small-sized (< 30 μm in soma diameter) DRG neurons to determine whether A-type K+ channels mediate the nociceptive actions of CCK-BR.
Materials and methods
Adult ICR mice (male, 6–8 weeks of age) were purchased from the Experimental Animal Center of Soochow University. Mice were maintained in specific-pathogen-free facilities on a 12-h light-dark cycle at a room temperature of 22 ± 1 °C, and housed in cages with access to food and water ad libitum. All animal studies were conducted in accordance with the National Institutes of Health’s Guidelines for Animal Care and Use and approved by the Animal Care and Use Committee of Soochow University.
Isolation of DRG neurons
DRG neurons were obtained from lumber L4–6 segments in adult ICR mice (male, 6–8 weeks of age) using enzymatic dissociation procedure as described previously [20, 21]. Dissociated DRG neurons were plated onto Matrigel-coated glass coverslips and maintained in incubators until recording. We sorted DRG neurons into groups based on the soma diameter distribution of small (< 30 μm in soma diameter), medium (30–40 μm in soma diameter), and large-sized (> 40 μm in soma diameter) DRG neurons , and in patch-clamp experiments we only recorded the small-sized neurons. Cells were subjected to whole-cell recordings within 24–48 h of plating.
Whole-cell patch-clamp recordings were performed at room temperature (23 ± 1 °C). Recording electrodes (World Precision Instruments, USA) pulled from borosilicate glass microcapillary tubes (Sutter Instruments) had resistances from 3 to 5 MΩ when filled with internal solution. Recordings were made using a MultiClamp 700B patch-clamp amplifier (Molecular Devices, USA). Whole-cell currents were low-pass filtered at 2–5 kHz. The values of cell capacitance and series resistance were taken directly from readings of the amplifier after electronic subtraction of the capacitive transients. In voltage-clamp mode, 80% of the series resistance was electronically compensated. A P/6 protocol was used for on-line leak subtractions. The external solution used for Kv current recordings contained (mM): KCl (5), choline-Cl (150), MgCl2 (1), CaCl2 (0.03), glucose (10) and HEPES (10) adjusted to pH 7.4 with KOH, 310 mOsm. The internal solution used for Kv current recordings contained (mM): KCl (140), Na-GTP (0.3), Mg-ATP (3), 0.5 CaCl2, MgCl2 (1), EGTA (5) and HEPES (10) adjusted to pH 7.4 with KOH, 295 mOsm. The external solution used for Cav current recordings contained the following (mM): BaCl2 (5), tetraethylammonium chloride (TEA-Cl) (140), CsCl (5), HEPES (10), MgCl2 (0.5) and glucose (5.5) adjusted to pH 7.35 with TEA-OH, 310 mOsm. The internal solution used for Cav current recordings contained the following (mM): CsCl (110), Mg-ATP (4), EGTA (10), Na-GTP (0.3) and HEPES (25) adjusted to pH 7.4 with CsOH, 295 mOsm. To separate T-type channel currents, we applied the L-type channel blocker nifedipine (5 μM) and the N- and P/Q-type channel blocker ω-conotoxin MVIIC (0.2 μM) in the external solution. The external solution used for both Nav current and current-clamp recordings contained the following (mM): KCl (2), MgCl2 (2), CaCl2 (2), NaCl (128), HEPES (25) and glucose (30) adjusted to pH 7.4 with NaOH, 305 mOsm. The internal solution contained the following (in mM): NaCl (10), KCl (110), Na-GTP (0.3), Mg-ATP (4), EGTA (2) and HEPES (25) adjusted to pH 7.3 with KOH, 295 mOsm. Path pipettes had resistance from 2 to 3 MΩ for internal dialysis with compounds. Monoclonal antibodies raised against Gαo (anti-Go-Ab, Santa Cruz Biotechnology) or Gαi (anti-Gi-Ab, Santa Cruz Biotechnology) were diluted to 1: 100 times from a stock solution (200 μg/ml). This antibody used in the present study has been previously shown to specifically recognize the G-protein Gαo but not Gαi subunit [20, 23]. For the whole cell experiments, the antibody was diluted into the intracellular solution and loaded into the pipette. In patch-clamp recording in which cells were dialyzed with inhibitors or activators, protocols were initiated at least 5 min after breaking the membrane patch.
Immunoblot analysis was conducted as described in our previous studies . Briefly, equal amounts of proteins (25 μg) were separated by 10% SDS–PAGE and electroblotted onto PVDF membranes (Merck Millipore, Germany). Blotted proteins were probed with the following primary antibodies: goat anti-CCK-AR (1: 500, Abcam), goat anti-CCK-BR (1: 500, Abcam), rabbit anti-phospho-Akt (1:2000, Cell Signaling Technology), rabbit anti-Akt (1:1000, Cell Signaling Technology), rabbit anti-phospho-p38 MAPK (1:1000, Cell Signaling Technology), rabbit anti-p38 MAPK (1:1000, Cell Signaling Technology), rabbit anti-phospho-ERK1/2 (1:2000, Cell Signaling Technology), rabbit anti-ERK1/2 (1:1000, Cell Signaling Technology), rabbit anti-phospho-SAPK/JNK (1:1000, Cell Signaling Technology), rabbit anti-SAPK/JNK (1:1000, Cell Signaling Technology), rabbit anti-phospho-Src (pTyr418, 1:2000, Abcam), rabbit anti-Src (1: 1000, Abcam) and rabbit anti-GAPDH (1:5000, Abcam). After extensive washing in TBST, membranes were incubated with horseradish peroxidase-conjugated anti-rabbit or anti-goat IgG (1: 8000, Multi Sciences). Chemiluminescent signals were generated using a SuperSignal West Pico trial kit (Pierce) and detected using ChemiDoc XRS System (Bio-Rad Laboratories). The software Quantity One (Bio-Rad Laboratories) was used for background subtraction and for quantification of immunoblotting data.
Total cellular proteins were extracted using homogenization buffer (Tris 20 mM, pH 7.4, NaCl 150 mM, EDTA 1 mM, 0.5% Triton, and DTT 1 mM) supplemented with a cocktail of protease inhibitors. After centrifugation at 21,000 g for 30 min at 4 °C, the supernatant was saved and protein concentration was measured by BCA protein assay (Beyotime, Shanghai, China). Extract containing 500 μg of protein was incubated at 4 °C for 3 h with 3 μg of goat polyclonal antibody against CCK-BR (1: 500, Abcam). Protein A-Sepharose beads (Amersham Biosciences) were added to the samples and gently shaken for 4 h at 4 °C. Beads were then rinsed and removed in lysis buffer. The pellet was boiled with 4 × Laemmli sample buffer and separated by SDS-PAGE. Immunoreactive proteins on membranes were developed as described above.
Immunohistochemistry was performed as previously described [20, 23]. Tissue samples were sectioned into 15 μm thin slices using a cryostat (CM 1950; Leica, San Jose, USA). The sections were blocked with 5% normal goat serum in PBS, plus 0.2% Triton X-100 for 1 h at room temperature then incubated overnight at 4 °C with primary antibody against CCK-BR (goat, 1: 500, Abcam), antibody against NF-200 (mouse, 1:1000, Abcam), or antibody against CGRP (mouse, 1:500, Abcam). Sections were washed three times with PBS at room temperature, followed by Cy3-conjugated donkey anti-goat IgG (1:500, BBI Life Science), FITC-conjugated donkey anti-mouse IgG (1:200, BBI Life Science) or FITC-IB4 (1:200, Sigma) in PBS at room temperature for 2 h. After sections were washed three times with PBS at room temperature, images were captured with a fluorescence microscope (Nikon 104c, Japan). Negative controls, omitting each primary antibody, were used in each case, and no significant staining was observed in these samples (data not shown).
PKA activity assay
PKA activity in homogenates was determined by enzyme-linked immuno sorbent assay (ELISA, Promega), according to the manufacture’s instructions. Briefly, the cells were pretreated with either vehicle or KT-5720 for 30 min, followed by treatment with either vehicle (0.1% DMSO), or forskolin for 15 min. The cells were washed with ice-cold phosphate-buffered saline (PBS), placed on ice, and incubated with 200 μl lysis buffer. After a 10-min incubation on ice, the cells were transferred to microcentrifuge tubes. Cell lysates were centrifuged for 15 min, and aliquots of the supernatants containing 0.2 μg of protein were assayed for PKA activity. The activity is expressed as RLU− 1 (relative light units)/amount of protein.
PI3K activity assay
Cells were stimulated with or without CCK-8 (100 nM) for 15 min. After stimulation, PI3K activity in homogenates was determined with a PI3-Kinase HTRF™ Assay kit (Millipore Corporation, Bedford, MA), using 20 μg of protein for each sample, as stated in the manufacturer’s protocol. HTRF was then measured with an excitation wavelength of 335 nm and emission wave length of 620 and 665 nm with a spectrofluorometer (Tecan, Infinite M1000, Salzburg, Austria).
Behavioral testing was conducted in an appropriately lighted, quiet room, always during the light cycle between 9:00 AM and 4:00 PM in a series and by the same experimenter. The operator who performed the behavioral tests was blinded to all treatments. Animals were allowed to acclimate to a testing room for at least 30 min before performing the assessments. Mechanical sensitivity was determined on paw withdrawal to manual application of graded von Frey hairs (0.02–2.56 g; Stoelting) to the plantar surface as described previously. Thermal sensitivity was tested using a commercially available paw thermal stimulation system (IITC Life Sciences), and are expressed as paw-withdrawal latency (PWL) and tail-flick latency. Animals were gently dropped into an acrylic box with a metal floor that was preheated to a certain temperature. The values of PWL were calculated using a timer that was started when the animal is released onto the preheated plate and stopped at the moment of withdrawal, shaking, or licking of either hind paw. The cutoff latency was set to prevent tissue damage. All animals were tested once for each temperature per session in a random sequence. All drugs or vehicle were injected subcutaneously into the plantar surface of one hind paw in a volume of 10 μl. The pH of the solutions was adjusted at 7.4 to prevent skin irritation.
All drugs were purchased from Sigma (MO, USA) unless otherwise indicated. Stock solutions of 4-aminopyridine (4-AP), pertussis toxin (PTX), cholera toxin (CTX), PMA (Tocris Bioscience, Ellisville, MO) and ω-conotoxin MVIIC (Tocris Bioscience, Ellisville, MO) were prepared in distilled deionized water. Z941 was a kind gift from Dr. Terrance P. Snutch (University of British Columbia, Vancouver, British Columbia, Canada). Stock solutions of cholecystokinin-8 (Tocris Bioscience, Ellisville, MO), LY294002, CCK-4, nifedipine, forskolin, gallein, wortmannin, KT5720 (RD system), devazepide, LY225910, GW5823, BC264, SP600125, SB203580, anisomycin, PP2, PP3, Akt inhibitor III, U0126, and GF109203X were prepared in dimethylsulfoxide (DMSO). The final concentration of DMSO in the bath solution was estimated to be less than 0.01%, and this compound had no functional effects on IA (not shown).
In electrophysiological experiments, data acquisition and analysis were performed with Clampfit 10.2 (Axon Instruments) and/or GraphPad Prism 5.0 software (Prism Software). The amplitude of IA was measured at the peak. The data plots were fit by the Boltzmann equation for the activation and inactivation curves as described previously . All data are presented as means ± SEM. Statistical significance between two groups was determined using a paired or unpaired Student’s t two-tailed test. Comparisons of multiple groups against a pooled control were tested using one-way analysis of variance (ANOVA) followed by a Bonferroni’s post-test. Differences in values over time among groups were done using two-way ANOVA. The criterion for significance in all analyses was considered as p < 0.05.
CCK-8 selectively decreased I A in DRG neurons
The studies in vitro of nociceptive processing usually examined different subtypes of peripheral sensory neurons [25, 26]. In the present study, we limited patch-clamp recordings to small-sized DRG neurons (< 30 μm in soma diameter) as these neurons are primarily involved in nociceptive signaling [19, 26]. Two main types of outward voltage-gated K+ channel (Kv) currents have been characterized in these nociceptive neurons — the transient A-type K+ channel currents (IA) and the sustained and delayed-rectifier K+ channel currents (IDR) [15, 16]. We first isolated these two kinetically different whole-cell currents. A total outward current exhibiting a rapidly inactivating and a more sustained component was elicited by a depolarizing pulse from the holding potential of − 80 mV to + 40 mV (Fig. 1a). Biophysical separation of a delayed-rectifier current (IDR) was obtained by a depolarizing prepulse to − 10 mV, which inactivated the transient channels. IA was then isolated by subtracting IDR from the total current (Fig. 1a). Addition of 5 mM 4-aminopyridine (4-AP) inhibited the remaining outward current by 87.1 ± 5.3% (n = 6, Fig. 1b), further confirming the effective isolation of IA.
Application of 100 nM CCK-8 to small-sized DRG neurons significantly decreased IA by 30.9 ± 3.7% (n = 8, Figs. 1c and e), while IDR was not effectively affected (decreased by 1.2 ± 0.9%, n = 10, Figs. 1d and e). The amplitude of IA partially recovered after CCK-8 washout (Fig. 1c). The CCK-8 effect on IA was concentration-dependent (Fig. 1f). The half-maximal inhibitory concentration (IC50) calculated from a sigmoidal Hill equation [23, 24] observed at 47.3 nM (Fig. 1f). Further, we examined whether CCK-8 would alter the biophysical properties of IA. While no significant changes were observed in the activation properties of IA (Vhalf from 5.8 ± 1.6 mV to 6.5 ± 2.5 mV, n = 9, Figs. 1h and i), CCK-8 shifted the steady-state inactivation curve to the hyperpolarized level by 7.8 mV (Vhalf from − 55.9 ± 3.9 mV to − 63.7 ± 2.8 mV, n = 12, Figs. 1g and i). These findings reveal that the CCK-8-induced reduction in IA is mainly contributed by an increased proportion of channels retained in the inactivated state.
The CCK-BR mediated the CCK-8–induced I A decrease
The CCK-AR and CCK-BR have been identified as the endogenous receptors for CCK-8 . To determine which one is involved in the CCK-8–induced IA reduction, we first examined the protein profile and subcellular expression of these receptors in mouse DRGs. Immunoblot analysis revealed that only CCK-BR (predicted size of 80 kDa), but not CCK-AR (predicted size of 95 kDa), were endogenously expressed (Fig. 2a). Protein lysates prepared from the gallbladder of the same mice were used as a positive control. Small unmyelinated sensory neurons have been classified into isolectin B4 (IB4)-positive (non-peptidergic) subset and peptidergic (IB4 negative) subset expressing calcitonin gene-related peptide (CGRP), while large neurons in myelinated A-fibers express neurofilament 200 (NF200). We analyzed the CCK-BR expression in DRGs subsets by coimmunostaining of CCK-BR with the mentioned markers. CCK-BRs were found to be heavily colocalized with IB4 and CGRP, and less with neurofilament-200 (NF-200), a marker for myelinated A-fibers (Fig. 2b). Next, we determined the participation of CCK-BR in the effect of CCK-8 on IA. While the CCK-8 mediated reduction of currents was not affected by the presence of 1 μM of the CCK-AR antagonist devazepide (decreased by 29.3 ± 3.7%, n = 10, Figs. 2c and e), such effect was completely abolished in the presence of 1 μM of the CCK-BR antagonist LY225910 (decreased by 4.7 ± 0.9%, n = 10, Figs. 2d and e). This evidence undoubtably indicates that the CCK-BR, but not the CCK-AR, is involved in the CCK-8-induced IA reductions. Furthermore, application of 0.1 μM BC-264, a selective CCK-BR agonist, significantly decreased IA by 30.9 ± 2.7% (n = 7, Fig. 2f), while the selective CCK-AR agonist GW5823 (5 μM) elicited no such effect (decreased by 3.1 ± 1.2%, n = 9, Fig. 2f). Since it is known that both CCK-4 and CCK-8 are active forms of CCKs in the nervous system [28, 29], we also test whether application of another selective CCK-BR agonist CCK-4 affects IA in small-sized DRG neurons. Indeed, CCK-4 at 300 nM significantly decreased IA by 32.2 ± 4.9% (n = 7). These findings further support the conclusion that the CCK agonist driven IA decrease was mediated specifically by CCK-BR in small-sized DRG neurons.
The CCK-BR-mediated IA decrease requires the βγ subunits (Gβγ) of go-protein
CCK-BR coupled to heterotrimeric G-proteins, which are key transducers to control numerous cellular processes . We next examined actions of different subtypes of G-proteins in the CCK-BR-mediated IA modulation. Inactivation of Gs by pre-treating DRG neurons with cholera toxin (CTX, 500 ng/ml) had no significant effects on the CCK-8-induced IA decrease (decreased by 32.3 ± 3.6%, n = 11, Fig. 3a and c). Contrastingly, pre-treating cells with pertussis toxin (PTX, 200 ng/ml) to inactivate Gi/o abrogated the CCK-8-induced response (decreased by 3.9 ± 2.2%, n = 7, Fig. 3b and c). This CCK-BR-induced PTX-sensitive, but CTX-insensitive decrease in IA, indicated the involvement of Gi/o, but not Gs in the signaling cascade. Further, dialysis of cells with an antibody specifically against Gαo (2 μg/ml) blocked the effect of CCK-8 on IA reduction (decreased by 1.5 ± 2.9%, n = 8, Fig. 3d), whereas a Gαi-specific antibody (2 μg/ml) had no such effect (decreased by 27.6 ± 4.7%, n = 9, Fig. 3d). Together, these findings suggest that Gαo-protein mediates the response to CCK-8. Moreover, we found that endogenous Gαo (Fig. 3e), but not Gαi (Fig. 3f), was co-immunoprecipitated with an antibody against CCK-BR from DRG tissues, indicating that the CCK-BR and the Gαo subunit form a complex in situ. Further, intracellular application of a Gβ-specific antibody abrogated the CCK-8-induced IA reduction (decreased by 3.3 ± 3.5%, n = 9; Figs. 3g and h), while its denatured form did not elicit such effects (decreased by 32.5 ± 1.3%, n = 8; Fig. 3h). Similar results were obtained with a specific Gβγ inhibitor, gallein. Pretreatment of cells with gallein (10 μM) completely abolished the CCK-8-induced IA reduction (decreased by 2.7 ± 0.9%, n = 9; Fig. 3h). Thus, the Gβγ subunit of Go-protein is also required for the CCK-BR-mediated IA reductions.
The CCK-BR–mediated I A decrease requires PI3K, but independently of Akt
In view of the fact that protein kinase C (PKC) has been shown to act downstream of Gβγ  and regulate IA superficial dorsal horn neurons , we pre-incubated cells with PKC inhibitors and found that pretreating with GF109203X (1 μM) did not affect the CCK-8-induced IA response (decreased by 31.8 ± 4.3%, n = 9, Fig. 4a) while pre-incubation of cells with GF109203X substantially blocked the PKC activator PMA (phorbol 12-myristate 13-acetate)-induced IA reduction (5 μM, n = 9, Fig. 4b). Previous studies have highlighted the critical role of PI3K/Akt cascades in Gβγ–mediated responses . Thus, we investigated whether the inhibitory effect of CCK-8 on IA was PI3K/Akt-dependent. We found that CCK-8 application significantly induced PI3K activation and that pretreating cells with the PI3K inhibitor LY294002 at 20 μM abolished this effect (Fig. 4c). Consistently, pre-treatment with the PI3K inhibitor LY294002 (20 μM) (decreased by 1.9 ± 2.7%, n = 8, Fig. 4d and f) or wortmannin (1 μM) (decreased by 2.7 ± 3.5%, n = 10, Figs. 4e and f) also prevented the CCK-8 effects on IA, indicating the involvement of PI3K in the CCK-BR–mediated IA decrease. Further, we examined whether the CCK-8 action is also mediated by Akt, a major downstream target of the PI3Ks . We measured the Akt activity in DRG cells and found that 100 nM CCK-8 significantly increased the phosphorylated Akt (p-Akt) level, while the total Akt (t-Akt) remained unchanged (Fig. 4g). This effect was abrogated by the Akt inhibitor III (10 μM, Fig. 4g). To further determine the involvement of Akt in the modulation of IA by CCK-8, cells were pretreated with Akt inhibitor III prior to CCK-8 application. Interestingly, in the presence of 10 μM Akt inhibitor III, CCK-8 at 100 nM still induced a significant decrease in IA (decreased by 28.5 ± 3.8%, n = 9, Fig. 4h), revealing that the CCK-8–induced IA decrease was mediated by PI3K, but independently of Akt.
CCK-8 attenuates I A through c-Src-dependent JNK pathway
Mitogen-activated protein kinases (MAPKs), composing a family of protein kinases that play pivotal roles in mediating pain sensitivity , were shown to regulate neuronal IA . Thus, it was of interest to the current study to examine whether the MAPK cascades would be involved in the CCK-BR-induced response. Immunoblot analysis indicated that the exposure of DRG cells to CCK-8 (100 nM) significantly increased the expression of phosphorylated c-Jun N-terminal kinase (p-JNK), while the protein levels of total JNK (t-JNK), p-p38 as well as p-ERK activity remained unchanged (Fig. 5a). Blockade of CCK-BR with LY225910 (1 μM), as well as pretreating cells with the PI3K antagonist LY294002 (20 μM), eliminated the CCK-8–induced JNK activation (Fig. 5b). These findings reveal that the PI3K-mediated JNK signaling was involved in the CCK-8–induced effects. Next, we pretreated cells with 10 μM SP600125, a specific JNK inhibitor, and found that SP600125 abrogated the IA decrease induced by CCK-8 (decreased by 0.7 ± 1.9%, n = 7, Figs. 5c and d). Contrastingly, U0126 (1 μM), a MAPK/ERK (MEK) inhibitor, as well as the p-38 inhibitor SB203580 (10 μM), elicited no such effects (U0126: decreased by 28.3 ± 5.5%, n = 9; SB203580: decreased by 29.3 ± 3.8%, n = 11; Fig. 5d). As complementary support, the application of the JNK agonist anisomycin (25 ng/ml) to DRG neurons induced a significant decrease in IA amplitude (decreased by 32.7 ± 3.9%, n = 11, Fig. 5e).
Protein kinase A (PKA) was involved in the regulation of Kv channel currents [24, 31] and has been suggested to mediate the crosstalk between the PI3K and MAPK pathways . Thus, we asked whether the JNK-dependent regulation of IA by CCK-8 required PKA. Pretreatment of cells with the PKA inhibitor, KT-5720 (1 μM), had no significant effect on the CCK-8–induced IA decrease (decreased by 29.1 ± 3.2%, n = 10, Fig. 5f), while the administration of KT-5720 (1 μM) blocked 20 μM forskolin-induced PKA activation (Fig. 5g), indicating a PKA-independent mechanism involved in CCK-BR-mediated response. Src kinase (Src) has been demonstrated to activate JNK pathways [36, 37]. We assayed the cellular Src activity in DRG cells treated with CCK-8. Figure 5h illustrates that phosphorylated Src (pTyr418, p-Src) increased following treatment with CCK-8 (100 nM), whereas the protein levels of total Src (t-Src) was unchanged (Fig. 5h). This effect was abolished by the PI3K inhibitor LY294002 (20 μM, Fig. 5h). Moreover, cells were treated with the Src-specific inhibitor PP2 (10 μM) prior to CCK-8 exposure, and the JNK activation was monitored. Pretreatment of the cells with the Src inhibitor PP2 (10 μM) completely abolished CCK-8-induced JNK activation, while the inactively structural analog PP3 (10 μM) elicited no such effect (Fig. 5i). Consistent with this, pretreatment of cells with PP2 (10 μM) (decreased by 0.6 ± 2.7%, n = 12, Fig. 5j and k), but not PP3 (decreased by 26.9 ± 3.1%, n = 8, Fig. 5k), completely abolished the CCK-8-induced IA decrease. Collectively, these results suggest that Src, but not PKA, mediated the signaling between PI3K and JNK in the CCK-BR-mediated IA response.
Activation of CCK-BRs induces DRG neuronal hyperexcitability
Kv exerts pivotal effects in modulating neuronal excitability in peripheral sensory neurons . To determine the functional roles of the CCK-BR-mediated IA response, we determined whether the membrane excitability of DRG neurons would be affected by CCK-8. Bath application of CCK-8 (100 nM) had no significant effects on the whole-cell currents of Nav (n = 11, Fig. 6a) and the high voltage-activated (HVA) calcium channel currents (n = 8, Fig. 6b) in small DRG neurons, whereas CCK-8 increased LVA (T-type) channel currents by 8.3% (n = 9, Fig. 6c). Using an external solution including Z941 (10 μM) to block T-type channels, we found that 100 nM CCK-8 significantly increased action potential (AP) firing in response to 1-s current injection (by 56.6 ± 2.9% compared to control, n = 17, Fig. 6d and e). After washout, the firing rate was partially restored (Fig. 6e). Additionally, CCK-8 (100 nM) significantly shortened the first spike latency (Fig. 6f) and decreased AP threshold (n = 17, Fig. 6g). The other membrane properties of neuronal excitability, including resting membrane potential, were not significantly changed by 100 nM CCK-8 (not shown). Pretreating neurons with LY225910 (1 μM) abrogated the CCK-8-induced increase in AP firing rate, indicating the CCK-BR involvement (n = 12, Fig. 6h). To further verify the CCK-BR-induced neuronal hyperexcitability through IA decrease, 4-AP was applied prior to CCK-8. Pre-treatment of DRG neurons with 5 mM 4-AP abrogated the neuronal hyperexcitability induced by 100 nM CCK-8 (n = 12, Fig. 6j and k), indicating that the CCK-BR-mediated IA decrease subsequently induced neuronal hyperexcitability in small DRG neurons.
Involvement of A-type channels encoding IA in CCK-induced pain hypersensitivity
Further, we determined whether CCK-8 would affect in vivo pain sensitivity in animals. Intraplantar injection of CCK-8 (50 ng) markedly increased pain sensitivity to both mechanical and heating stimuli (Figs. 7a and b). The CCK-8-induced pain hypersensitivity to mechanical or heating stimulation was completely abrogated by intraplantar pretreatment of the CCK-BR antagonist LY225910 (0.5 μg, Figs. 7c and d), but not by the CCK-AR antagonist devazepide (1 μg, Figs. 7c and d). Moreover, intraplantar pretreatment with 4-AP (25 nmol) induced a significant increase in mechanical and heat sensitivity as compared with animals received a saline injection (Figs. 7e and f). Sensitivity assessed after intraplantar injection of CCK-8 showed that CCK-8 did not induce any additive effects to that of 4-AP on mechanical (Fig. 7e) and thermal (Fig. 7f) pain sensitivity, strongly suggesting that CCK-8 and 4-AP likely target molecules in the same cellular signaling pathway in vivo. Collectively, these findings reveal that A-type channels encoding IA contribute to the CCK-BR–mediated acute pain hypersensitivity.
The present study provides mechanistic data describing a novel functional role of CCK-8 in modulating transient IA in small-sized DRG neurons, without any concurrent effect on IDR. Based on our findings, we propose a signaling cascade model in which CCK-8–stimulated PI3K recruits the Src-dependent JNK to suppress IA. This attenuation of IA induced by CCK-8 application is mediated by the stimulation of CCK-BR and leads to sensory neuronal hyperexcitability and pain hypersensitivity in mice (see Fig. 8).
The PKC family of isozymes mediates IA responses in a cell-type and tissue-specific manner. For instance, activation of group I metabotropic glutamate receptors led to an inhibition of IA through a PKC-dependent mechanism in striatal cholinergic interneurons, while in large aspiny neurons activation of PKCα increases IA. Interestingly, in murine proximal colonic myocytes, the PKC-independent regulation of IA has also been reported . In this study, the CCK-8-induced IA decrease was independent of PKC and was mediated by PI3K, through the JNK-dependent signaling. These results are supported by previous studies that Kv currents including IA recorded from trigeminal ganglion neurons and pancreatic β cells decreased in response to PI3K pathway activation [21, 40]. Interestingly, the activation of PI3K has also been reported to increase IA in cultured rat cerebellar granule cells . In addition, PI3K-induced activation of Kv4.3 channels through glucocorticoid-inducible kinase-1 (SGK1) was also reported . Although these discrepancies require further clarification, the regulatory effects of PI3K would be variable in tissues/cell types expressing different A-type channel subunits. Another appropriate alternative hypothesis is that the stimulatory PI3K can also phosphorylate an intermediate protein that in turn down-regulates IA in small-sized DRG neurons. Furthermore, different splice variants of KChIP auxiliary subunits of IA channels can engender different, even opposing, modulation of Kv4 channel currents .
A known target of Gβγ is PI3K . In contrast to many other common Gβγ-dependent PI3K signal transduction events, the CCK-8–induced PI3K dependent IA attenuation isnot mediated by Akt, as demonstrated by the specific inhibition of Akt with pharmacological agents. Interestingly, previous studies have shown that Akt both negatively and positively regulates Kv4 [44, 45], which forms one of the major components mediating IA. For example, Akt down-regulates the activity of Kv4 channels in cultured cerebellar granule cells of rats ; in the same neurons, a different study demonstrates that enhanced Akt activity is required for IA amplification and Kv4.2 induction . This Akt-dependent stimulation of Kv4 also occurs in the arcuate nucleus . Thus, it appears that Akt differentially regulates the activity of Kv4 channels in a tissue-specific manner. In our study, the CCK-BR-mediated IA response was found to be independent of Akt; therefore, we went on to investigate what mediating PI3K signals to suppress IA in DRG neurons. Considerable in vivo and in vitro studies indicate that ERK plays pivotal roles in neuropathic pain [34, 47]. Phosphorylated ERK is elevated in DRG cells following peripheral nerve injury . Intrathecal application of ERK inhibitors reduces the pain behavior associated with nerve injury . Moreover, one of the most convincing evidence comes from the direct phosphorylation of the pore-forming channel subunit of Kv4.2 by ERK  that determine a downregulation of IA in superficial dorsal horn neurons . Contrastingly, antagonism of ERK completely abrogated IA increase induced by dopamine in lateral pyloric neurons . However, we found that the CCK-8-induced decrease of IA was unlikely induced by ERK phosphorylation, because the CCK-8 application did not change the ERK activity in DRG cells, whereas the levels of p-JNK were significantly increased. Moreover, the MAPK/ERK inhibitor did not affect the CCK-8–induced IA response. Our findings suggested that PI3K stimulated JNK in DRG neurons and that this signaling is essential for the CCK-BR–mediated IA response. Our results showed that 1) application of the JNK inhibitor SP600125, but not the p38 MAPK inhibitor SB203580 or the MAPK/ERK (MEK) inhibitor U016, blocked the CCK-8–induced IA decrease and 2) antagonism of PI3K blocked CCK-BR–mediated JNK activation. Consistent with these findings, the increased activity of JNK in ventricular myocytes markedly decreased the amplitude of transient outward K+ current density . These observations are in line with an earlier study showing a C-reactive protein (CRP)-induced modulation of intracellular JNK and interactions with voltage-activated K+ channels .
Up till now, it is still relatively unclear how PI3K activates JNK. It has been established that that PI3K may stimulate PKA, subsequently activating the downstream MAPK pathway . In the present study, activation of CCK-BR did not influence the PKA activity in DRG cells, indicating some other mechanisms, but not of PKA, mediate the crosstalk between PI3K and JNK signaling. Src kinases are downstream of PI3K and can facilitate JNK activity , suggesting possible crosstalk between PI3K and JNK signaling. In support of this observation, the current study demonstrated that the Src kinase inhibitor PP2 blocked the CCK-8–induced JNK activation. The blockade of PI3K also abolished the CCK-BR-mediated increase in Src activity, indicating that PI3K may modulate the JNK pathway through c-Src. Therefore, it is likely that CCK-8–activated PI3K recruits Src to up-regulate JNK activity, and thereby regulating CCK-BR–mediated IA response in DRG neurons.
IA, encoded by A-type K+ channels, is important determinants of both the delay of spike onset (first spike latency) and the decrease in the firing frequency . Acute decreases in IA in sensory neurons cause robust increases in neuronal excitability,  and may increase the responsiveness to nociceptive stimulation and contribute to mechanical hypersensitivity and thermal hyperalgesia . Genetic studies have firmly established a prominent role for A-type channels in amplifying nociceptive signals in the periphery and in contributing to central sensitization in the spinal dorsal horn [17, 19, 55]. Further, recent evidence has suggested that modulation of peripheral A-type channels influences somatic and visceral nociceptive inputs and thus an increase of A-type channel currents results in significant anti-nociception in a variety of animal neuropathic pain models . In the current study, consistently with the CCK-8-induced IA decrease, activation of CCK-BR led to increased excitability in DRG neurons with increased spike frequency and shortened first-spike latency, both of which are major parameters determining the timing of neurotransmitter release, and hence pain transmission . In addition, acute mechanical hypersensitivity and thermal hyperalgesia mediated by CCK-BR can be occluded by the A-type K+ channel blockade. As such, our findings are supportive of the reasonable assumption that nociceptive actions of CCK-BR are mediated, at least in part, through the JNK-dependent reduction of IA. Our present results are, indeed, in accordance with previous studies that CCK-8 might induce pro-nociceptive actions [8, 9]. Intrathecal inhibition of JNK, a key modulator IA in the present study, has been found to attenuate the CCI-induced mechanical allodynia and thermal hyperalgesia in rats . Following spinal nerve ligation (SNL), phosphorylated JNK in small-sized DRG neurons have been found to be greatly increased  and the intrathecal infusion of JNK inhibitor can reverse mechanical but not thermal hypersensitivity .
Collectively, this study found that CCK-8 decreases IA through the Gβγ-mediated PI3K/Src/JNK pathway. This mechanism occurred via CCK-BR and mediated the neuronal hyperexcitability in peripheral sensory neurons and pain hypersensitivity in mice. Modulation of IA by CCK-8 in peripheral sensory neurons is of particular interest. The identification of CCK-BR-mediated molecular mechanisms contributing to pain hypersensitization may offer insights into opportunities for analgesic pharmacotherapy.
Availability of data and materials
All data and materials generated in this study are available upon request.
CCK type B receptor
Extracellular regulated protein kinases
G-protein coupled receptor
- Gβγ :
G-protein βγ subunit
- I A :
A-type K+ current
- I DR :
Sustained delayed rectifier K+ current
c-Jun N-terminal kinase
Mitogen-activated protein kinase
Protein kinase A;
Protein kinase C
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We would like to give our thanks to Mrs. Shan Gong and Dr. Yue Wang from the Department of Anatomy for their technical assistance. We also appreciate Dr. Xianmin Yu for his invaluable comments.
This study was supported by the National Natural Science Foundation of China (No. 81873731, No. 81771187, No. 81622014, No. 81671080, No. 81571063, No. 31800879), Qing-Lan Project of Jiangsu Province (J. Tao), the Six Talent Peak Project of Jiangsu Province (No. JY-065), Jiangsu Key Laboratory of Neuropsychiatric Diseases (BM2013003), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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