Background
Chronic migraine (CM) is one of the most common headache disorders and often affects patients’ quality of life and imposes substantial individual and societal burdens [
1]. The treatment of chronic migraine remains a substantial challenge due to the complex mechanisms involved.
Accumulating evidence shows that central sensitization of the trigeminal nucleus caudalis (TNC) region is a key element in the pathogenesis of chronic migraine, and persistent excitatory transmission mediated by glutamatergic neurons is generally considered to be the major factor in central sensitization [
2‐
4]. The excitatory postsynaptic actions of glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), are carried out by its receptors, which include the N-methyl-D-aspartate receptor (NMDAR) and the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) [
3,
5,
6].
AMPAR consists of four subunits (GLUA1-GLUA4) [
7]. Modulation of synaptic plasticity by dynamic trafficking of GLUA1-containing AMPARs has been shown to be important for neuropathic pain and noxious invasion [
7,
8]. A recent study indicated that phosphorylation of the GLUA1 subunit, which is related to its recruitment to the cell membrane, may make a significant contribution to chronic migraine [
9]. However, the specific molecular mechanism of GLUA1-containing AMPAR trafficking still needs to be explored.
Dopamine receptors belong to the G protein-coupled receptor (GPCR) superfamily, which is divided into two subfamilies: D1 (primarily DRD1) and D2 (primarily DRD2) [
8]. Among dopamine receptors, DRD2 is one of the most important in the CNS, with roles in a series of physiological and pathological processes, such as pain, addiction, learning, and memory [
10‐
14]. Previous research has proven that DRD2 can modulate the firing of spinal neurons and thus alleviate pain behavior in neuropathic pain models [
15‐
17]. An in-vitro study proved that DRD2 can regulate the dendritic density and dendritic spine morphology in striatal and hippocampal neurons [
18,
19]. In addition, several studies have reported that DRD2 can regulate the phosphorylation of the AMPA receptor [
20] and alleviate AMPA receptor-mediated neurotoxicity [
21]. These studies indicate that DRD2 might be involved in central sensitization and regulate AMPARs in chronic migraine. However, the changes in DRD2 expression and its modulatory effect on GLUA1-containing AMPARs in chronic migraine remain unclear.
The PI3K/AKT signaling pathway, a downstream pathway of GPCRs, including dopamine receptors, is actively in a wide range of physiological processes [
22‐
24]. It has been claimed that the PI3K/AKT pathway is involved in regulating rat hippocampal long-term potentiation (LTP) as well as synaptic plasticity and increases the phosphorylation level of GLUA1-containing AMPARs [
25,
26]. The PI3K pathway has also been shown to be activated in chronic migraine [
26] and to have a role in the development of central sensitization in neuropathic pain [
27,
28].
The PI3K pathway has been reported to be regulated by Src family kinases (SFKs), a group of tyrosine kinases participating in many cellular processes [
29,
30]. Several studies have demonstrated that DRD2 can regulate the activity and level of Src family kinases in the brain [
31,
32]. Moreover, we previously reported that Src family kinases could regulate the function of NMDARs and central sensitization in CM rat models [
33]. In summary, we hypothesized that the dopamine D2 receptor participates in central sensitization and regulates dynamic GLUA1-containing AMPAR trafficking as well as synaptic modifications through the PI3K/AKT pathway in a Src family kinase-dependent manner in chronic migraine.
This study characterized the changes in DRD2 expression in chronic migraine and its effects on GLUA1-containing AMPAR trafficking as well as central sensitization. Specifically, we observed a significant reduction in the expression of DRD2 in the TNC and summarized that DRD2 agonist treatment can alleviate migraine-like pain behaviors and reduce central sensitization in CM rats and reduce membrane insertion of GLUA1-containing AMPARs via the PI3K/AKT pathway in a Src family kinase-dependent manner. Therefore, this study may provide a new potential therapeutic option for chronic migraine.
Materials and methods
Animals
Adult male Sprague‒Dawley (SD) rats (250 g-300 g; specific pathogen-free) and pregnant rats provided by Chongqing Medical University were used for this work. All procedures were carried out following the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Rats were kept at room temperature (24 ± 1 °C) on a 12-h light/12-h dark cycle. No limitations for food and water accession. All rats were acclimated for 30 min before each experiment. The rats were grouped randomly except for those with an abnormal baseline pain threshold.
Materials
All antibodies used are listed in Table
1. L-glutamine, deoxyribonuclease (DNase), poly-L-lysine (PLL), bradykinin, histamine, serotonin, and prostaglandin E2 were provided by Sigma‒Aldrich (Missouri, USA). Fluo-4 AM was purchased from Beyotime (Beijing, China). Fetal bovine serum, neurobasal medium, DMEM-HG medium, and B27 were purchased from Thermo (Waltham, USA). LY294002, sulpiride, NASPM, PP2, and 740YP were obtained from MCE (Shanghai, China), and quinpirole was purchased from Sigma‒Aldrich (Missouri, USA). Quinpirole, LY294002, PP2, sulpiride, NASPM, and 740YP were dissolved in 5% DMSO, and the inflammatory soup (IS) was produced with bradykinin (1 mM), histamine (1 mM), serotonin (1 mM), and prostaglandin E2(0.1 mM) (all from Sigma‒Aldrich, Missouri, USA), which were mixed in phosphate-buffered saline (PBS) [
34,
35].
Table 1
Information on antibodies
Anti-ERK1/2 | CST, USA | Rabbit | 1:1000 |
Anti-p-ERK1/2 | CST, USA | Rabbit | 1:1000 |
Anti-AKT | CST, USA | Rabbit | 1:1000 |
Anti-p-AKT | CST, USA | Rabbit | 1:1000 |
Anti-p-Src | CST, USA | Rabbit | 1:1000 |
Anti-p-GLUA1 | Abcam, UK | Rabbit | 1:1000 |
GAPDH | ZEN-BIOSCIENCE, China | Mouse | 1:8000 |
Anti-GLUA1 | Abcam, UK | Rabbit | 1:1000 |
Anti-Src | CST, USA | Rabbit | 1:1000 |
Anti-DRD2(For WB) | Proteintech, USA | Rabbit | 1:1000 |
Anti-p-P85α | Affinity, China | Rabbit | 1:1000 |
Anti-PI3K P110β | Bioss, China | Rabbit | 1:500 |
Anti-PI3K P85α | Proteintech, USA | Mouse | 1:10,000 |
Anti-PSD95 | CST, USA | Mouse | 1:1000 |
Anti-DRD2(For IF) | Santa Cruz, USA | Mouse | 1:100 |
Anti-rabbit IgG | ZEN-BIOSCIENCE, China | Goat | 1:5000 |
Anti-mouse IgG | ZEN-BIOSCIENCE, China | Goat | 1:5000 |
Alexa Fluor 488 anti-mouse IgG | Beyotime, China | Goat | 1: 400 |
Cy3 anti-rabbit IgG | Beyotime, China | Goat | 1: 400 |
Surgery for establishing the CM model
The surgical procedure was performed as described previously [
36]. First, rats under anesthetized with sevoflurane were put on a stereoscopic rack (Stoelting Co, Chicago, USA). A notch was made above the skull and the periosteum was removed to uncover the bregma. A hole with an approximate diameter of 1-mm was made over the left dura anterior fontanelle using a cranial drill (-1.0 mm posterior and + 1.5 mm lateral to the bregma). Next, a sterile catheter was placed at the eyelet with dental cement. After suturing, the rats were positioned on a warm mat until consciousness was restored. Thereafter, the rats were allowed to recover for at least 1 week to assure that their pain thresholds returned to baseline levels. During this time, the surgical area was disinfected daily with iodophor. CM rats were treated with IS (5 μL) daily for 7 days, while the IS was replaced with PBS in the Sham group.
Animal grouping and drug delivery
As needed for experiments, rats were assigned to the following groups: Sham group, Sham + DMSO group, Sham + quinpirole group, Sham + sulpiride group, Sham + LY294002 group, Sham + PP2 group, Sham + NASPM group, CM group, CM + DMSO group, CM + quinpirole group, CM + sulpiride group, CM + LY294002 group, CM + PP2 group, CM + NASPM group, CM + quinpirole + 740YP group, CM + quinpirole + DMSO group, CM + quinpirole + sulpiride group. For the CM + quinpirole group, CM + sulpiride group, CM + PP2 group, CM + NASPM group, and CM + LY294002 group, all drugs were administered on the 7th day after IS infusion. For the CM + quinpirole + 740YP group, on Day 7 after IS injection, quinpirole was administered first, followed by 740YP 30 min later; the vehicle control was 5% DMSO. The doses of quinpirole, LY294002, PP2, sulpiride, NASPM, and 740YP were determined according to previous studies [
19,
25,
33,
37,
38]. All doses of the drugs were administered by intracerebroventricular injection.
Pain behavior test
All behavioral trials were conducted under light conditions between 09:00 and 18:00. Before the tests, the rats were acclimated to the environment for at least 30 min. Baseline testing of pain thresholds was performed before the infusion of IS or PBS. Subsequently, pain thresholds were tested daily after IS injection or after drug administration on Day 7.
First, rats were staged on a test device, and a von Frey monofilament (ranging from 1 to 26 g) was then applied vertically to the hind paw or the periorbital region (rostral area on the left or right side of the face) of each rat using an up-and-down approach to measure the mechanical threshold as described previously [
34,
36].
The paw twitch latency (PWL) is thought to represent the thermal pain threshold [
39]. Briefly, rats were positioned in the cage, then the radiant heat was applied from the bottom to the plantar surface of the hind paw. The time at which the rat responded to the stimulus was recorded and considered as the PWL. The maximum time was set to 25 s to protect the rats.
All tests were repeated 3 times at 5-min intervals, and the average thresholds were calculated. Throughout the experiment, the experimenter was blinded to the experimental group.
Quantitative real-time polymerase chain reaction (qRT‒PCR)
Briefly, RNA was acquired from fresh TNC tissue using RNAiso reagent (TaKaRa, Tokyo, Japan). A PrimeScript RT Kit (TaKaRa, Tokyo, Japan) was used for reverse transcription. Then the qRT‒PCR was conducted on a thermocycler (Bio-Rad, USA) using SYBR Premix Ex Taq TM II (TaKaRa, Tokyo, Japan) to evaluate DRD1 and DRD2 mRNA expression levels. Specific primers were provided by Sangon Biotech (Shanghai, China) and are listed in Table
2. Gene expression was analyzed by the standard 2 − ΔΔCT method.
Table 2
Information on primers
DRD1 | AAGCAGCCTTCATCCTGATTAGCG | TTGTCATCCTCGGTGTCCTCCAG |
DRD2 | CAGTGAACAGGCGGAGAATGGATG | GTGGTGGGATGGATCAGGGAGAG |
GAPDH | ATGACTCTACCCACGGCAAGCT | GGATGCAGGGATGATGTTCT |
Western blot (WB)
In brief, TNCs were removed after rats were euthanized and were then lysed with RIPA buffer containing PMSF (Beyotime, Beijing, China) to obtain total protein. A Plasma Membrane Protein Isolation and Cell Fractionation Kit (Invent Biotechnologies, USA) was applied to isolate the plasma membrane fraction. SDS-polyacrylamide gels were prepared to separate the proteins. After that, the proteins were transferred onto PVDF membranes, which were then incubated with specific primary antibodies overnight at 4 ℃ after 2 hours of blocking with 5% nonfat milk. After being incubated with secondary antibodies, the membranes were then developed by an imaging system (Fusion, Germany) using ECL reagents (Beyotime, Shanghai, China) to detect signals. Finally, the immunoblots were analyzed with ImageJ software, and the levels of the target proteins were normalized to the corresponding GAPDH level.
Immunofluorescence staining (IF)
Rats under deep anesthesia were subjected to cardiac perfusion with PBS followed by 4% paraformaldehyde. The intact TNC was then immediately isolated and postfixed with 4% paraformaldehyde at 4 °C. After dehydration using graded concentrations of sucrose solution (20% and 30%), the tissues were sectioned using a cryostat (Leica, Japan) into 15-μm slices, which were placed on carrier slides. Antigen repair with sodium citrate (Beyotime, Beijing, China) and blocking with 10% goat serum (Boster, Beijing, China) were then executed. For fluorescence colabeling experiments, the primary antibodies were mixed and diluted with 1% PBS and were subsequently incubated with the sections at 4 °C overnight. After incubation with the fluorescent secondary antibody and counterstaining of the nuclei with 4’,6-diamidino-2-phenylindole (DAPI), a confocal laser scanning fluorescence microscope (Zeiss, Germany) was used to acquire images, which were analyzed with ImageJ.
Golgi-Cox staining
An FD Rapid Golgi Staining Kit TM (NeuroTechnologies, USA) was utilized to observe the dendritic spines in the TNC. After rats were sacrificed, TNCs were harvested and then washed quickly with double-distilled water and immersed in a premixed solution (liquid A and liquid B at a ratio of 1:1) which was refreshed once in 24 h and then kept for 2 weeks (in a dark environment). Then the tissues were transferred to liquid C and incubated for 3 days (in a dark environment). A vibratome (Leica VT 1200S, Japan) was used to generate 150-μm-thick tissue slices, which were then stained in a mixture of liquid D, liquid E, and double-distilled water at a ratio of 1:1:2 for 10 min. Next, after being dehydrated with increasing concentrations of ethanol (50%, 75%, 95%, and 100%) and permeating with xylene, the slices were sealed with neutral resin. Images were acquired using a microscope (Axio Imager A2) and were analyzed with ImageJ. All steps were carried out at room temperature.
Transmission Electron Microscopy (TEM)
The relevant steps can be found in our previous work [
40]. Briefly, rats were anesthetized and then subjected to cardiac perfusion with PBS followed by 2.5% glutaraldehyde. TNCs were separated and cut into 1 m
3 pieces using a blade. Next, the pieces were soaked in 4% glutaraldehyde at 4 °C and then post-treated at Chongqing Medical University. Finally, images were acquired using a JEM-1400 PLUS transmission electron microscope at 50,000 X or 30,000 X magnification and then statistically analyzed with Image-Pro 6.0.
Culture of TNC primary neurons
TNCs were removed from embryos on Day 18 of pregnancy sterilely, and then the meninges and blood were removed as described previously [
41]. Next, the tissues were digested with papain (2 mg/ml) for 25 min. The tissues were gently and repeatedly disrupted by pipetting until the large cell clumps disappeared at the end of the digestion step. After centrifugation, the cells were resuspended in DMEM-HG medium. Finally, the cells were plated (2.5 X 10
5 cells/dish) on confocal dishes that were precoated with poly-L-lysine (PLL) at 37 °C and 5% CO2. About 4–6 h later, the DMEM-HG was replaced with neurobasal containing B27(2%) and L-glutamine (0.5 mM/L), followed by half volume changes every 3 days. Cells were incubated until Day 7 for subsequent experiments.
Determination of intracellular Ca2+ concentrations
The primary neurons were pretreated with quinpirole (1 μM) [
19] for 12 h and LY294002 (20 μM) [
42] for 1 h at 37 °C and 5% CO2, and were then incubated with 4 μM Fluo-4-AM (150 μl/well, diluted in PBS buffer) for 30 min at 37 °C. Then, 900 μl of HBSS was left in each well after washing three times with warm (37 °C) HBSS solution. Then, 100 μl of 10X NMDA was added at a specific time (the final concentration is 50 μM), and the changes in the intracellular calcium concentration were determined using the confocal microscope described above (Zeiss, Germany) at a detection wavelength of 488 nm over a total of 500 s for each group. The fluorescence intensity in the collected images was analyzed by ZEN software [
32].
Statistical analysis
All data were analyzed, and graphs were generated using GraphPad Prism 8 (GraphPad Software Inc., San Diego, CA, USA). Data in this paper are expressed as mean ± SEM. The Kolmogorov‒Smirnov test was used to check for normality. Significant differences for two-group and multiple-group comparisons were analyzed by the independent‒sample t-test and one-way ANOVA followed by Dunnett’s multiple comparison test, respectively. Two-factor analysis followed by the Bonferroni post hoc test was used to analyze the behavioral data. P values < 0.05 were thought to be statistically significant.
Discussion
The anti-injury effects of DRD2 and its probable mechanisms of action in rats with IS-induced chronic migraine were examined in this paper. We demonstrated that (1) the periorbital, hind paw mechanical, and thermal pain thresholds were markedly reduced in rats after 7 consecutive days of IS titration, accompanied by a decrease in DRD2 expression and an increase in the membrane insertion of GLUA1-containing AMPARs in the TNC; (2) both the DRD2 agonist quinpirole and the PI3K inhibitor LY293002 increased the pain thresholds, inhibited the membrane translocation of GLUA1-containing AMPARs and reversed the alterations in the dendritic spine density as well as the synaptic ultrastructure, thereby attenuating central sensitization in CM rats; (3)the DRD2 inhibitor sulpiride exacerbated central sensitization and GLUA1 trafficking in CM rats; (4) Blockade of GLUA1 by NASPM reduced GLUA1 trafficking and attenuates central sensitization in CM rats; (5) agonism of DRD2 and inhibition of the PI3K pathway inhibited NMDA-induced calcium influx in cultured primary neurons in vitro; (6) DRD2 agonist alleviated pain behaviors, inhibited the membrane transport of GLUA1-containing AMPARs and reduced central sensitization in CM rats, probably through the PI3K pathway; and (7) DRD2 regulated PI3K signaling possibly through a mechanism involving Src family kinases. These results imply that DRD2 may play a substantial role in the pathophysiological process of chronic migraine and therefore could be a reasonable pharmaceutical candidate for the treatment of chronic migraine.
The rat CM model was established by infusing IS and stimulating the trigeminal vascular system using an internationally approved and reliable surgical cannula [
35,
36]. Estrogen is strongly associated with migraine attacks, as has been reported previously [
45,
46]. Estrogen has a complex relationship with the function of the dopamine system in the brain; females have been reported to have a more active dopamine system than males, which may have unanticipated implications for this study [
46,
47]. In addition, rodents with migraine show great differences in nociceptive processing [
48]. Therefore, in the present study, male rats were eventually chosen to control for possible bias introduced by the presence of estrogen.
As a part of the descending nociceptive inhibitory system, the role of the dopamine system in chronic migraine has gradually garnered research focus [
35,
49]. The involvement of dopamine receptors in chronic migraine has been widely reported, and bioactive ingredients derived from traditional Chinese medicine have been reported to alleviate migraine by modulating dopamine receptors [
49‐
51]. However, the changes in dopamine receptor expression in CM and how they function are still not fully explored. In the present research, we reported that DRD2 was downregulated in CM rats and that the application of a DRD2 agonist alleviated pain behaviors and central sensitization in CM rats, while the administration of a DRD2 antagonist had the opposite effect. These results are certainly encouraging; the underlying mechanisms, however, have rarely been discussed. In addition, this study focused on the mechanism by which DRD2 in the TNC acts to alleviate chronic migraine. The alterations and functions of the dopamine system in other regions of the brain, particularly those of the descending pain inhibitory system in CM rats will be addressed in future research.
Postsynaptic membrane recruitment of GLUA1-containing AMPARs but not GLUA2/3-containing AMPARs is a crucial step in the transmission of nociceptive messages and central sensitization [
8,
52]. It has been previously reported that GLUA1-containing AMPARs in the TNC contribute greatly to chronic migraine [
9,
43]. In this study, we confirmed that inhibition of GLUA1 trafficking attenuates central sensitization and pain hyperesthesia in CM rats. We also clarified that DRD2 acts as an important upstream factor of GLUA1-containing AMPARs and is an important factor regulating central sensitization in CM rats. Consistent with earlier results, we found that the increased recruitment and phosphorylation level of GLUA1-containing AMPARs in the TNC in CM rats was reversed by the administration of quinpirole (a DRD2-specific agonist) and enhanced by sulpiride (a DRD2 antagonist). However, the potential links between DRD2 and GLUA1-containing AMPARs in CM rats remain uncertain.
Phosphatidylinositol 3-kinase (PI3K) is commonly expressed in various brain areas and is essential in the onset and maintenance of chronic pain [
27,
53,
54]. In addition, the PI3K/AKT signaling pathway has been reported to modulate the synaptic plasticity of neurons and the phosphorylation of GLUA1-containing AMPARs and their distribution at synapses [
25,
26]. Our study revealed that the expression of the PI3K P110β subunit was increased in CM rats. Inhibition of the PI3K/AKT pathway significantly alleviated pain behaviors and inhibited the phosphorylation and membrane surface expression of GLUA1-containing AMPAR in CM rats. Here, we hypothesized that DRD2 attenuates central sensitization in CM in association with the PI3K pathway, and this hypothesis was confirmed by our finding that the aberrant activation of the PI3K/AKT pathway was inhibited by the DRD2 agonist quinpirole but further augmented by the DRD2 antagonist sulpiride. In addition, the PI3K agonist 740YP abolished the anti-injury effect of the DRD2 agonist quinpirole. Moreover, the effects of DRD2 on inhibiting GLUA1-containing AMPAR translocation to the synaptic membrane and attenuating central sensitization were disrupted by 740YP. Overall, our findings indicate that there is a functional link between DRD2 and the PI3K pathway and that the PI3K pathway plays a critical hub role in the inhibitory effects of DRD2 on GLUA1-containing AMPAR postsynaptic recruitment, central sensitization, and nociceptive transmission. Furthermore, our results indicate that Src family kinases are probably the mediators linking DRD2 and the PI3K signaling pathway.
In general, the development of central sensitization is usually accompanied by enhanced neuronal transmission and structural plasticity, including the plasticity of dendritic spines and enhanced synaptic connections [
55‐
57]. Therefore, in this study, to verify the impact of DRD2 and the PI3K pathway on central sensitization, in addition to using western blotting to detect ERK and PSD95 expression, we used Golgi-Cox staining and transmission electron microscopy to visualize dendritic spine complexity and the synaptic ultrastructure, and Fluo-4-AM was used to monitor the calcium concentration. The results provided strong evidence for the influential roles of DRD2 and PI3K signaling in regulating neuronal excitability and central sensitization [
58‐
60], in cultured primary neurons in vitro. On the other hand, excitatory postsynaptic currents mediated by glutamate receptors drive central sensitization and the progression of chronic pain [
5,
56,
57]. However, the effects of DRD2 and the PI3K pathway on electrophysiological activity in the TNC in CM rats have not been explored, which is a limitation of this study and will be our focus in future studies. In addition, we used NMDA instead of AMPA to establish the model of neuronal excitation [
32] taking into account the neurotoxicity mediated by AMPA [
61] and the observation that NMDA can also induce the increased expression of GLUA1-containing AMPARs on the membrane surface of neurons, which is pertinent to the purpose of this study [
18,
62].
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