CCK(B) receptors in the periaqueductal grey are involved in electroacupuncture antinociception in the rat cold water tail-flick test

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Exploring the Mechanisms of Electroacupuncture-Induced Analgesia through RNA Sequencing of the Periaqueductal Gray

Electroacupuncture (EA) can relieve various pains. However, its mechanism in terms of the transcriptome is still not well-known. To explore the full profile of EA-induced molecular modification in the central nerve system, three twins of goats were selected for a match-paired experiment: EA stimulation (60 Hz, 30 min) and none-EA (control). Goats in the EA group showed an increased (p < 0.05) nociceptive threshold compared with the control goats. Experimental goats were sacrificed at 4 h of the experiment, and the periaqueductal grays were harvested for RNA sequencing. As a result, 2651 differentially expressed genes (1803 up-regulated and 848 down-regulated genes) were found and enriched in 30 Kyoto Encyclopedia of Genes and Genomes pathways and 149 gene ontology terms. EA-regulated five neuropeptide genes (proenkephalin, proopiomelanocortin, preprodynorphin, diazepam-binding inhibitor and proprotein convertase 1 inhibitor) were validated with quantitative PCR. Furthermore, up-regulated glutamate receptors, glutamate transporters, γ-aminobutyric acid (GABA) receptors, GABA transporters, synaptotagmins or mitogen-activated protein kinase (MAPK) genes might contribute to EA-induced analgesia through regulating the glutamatergic synapse, GABAergic synapse, MAPKs, ribosome or ubiquitin-proteasome pathways. Our findings reveal a full profile of molecular modification in response to EA and provide a solid experimental framework for exploring the mechanisms underlying EA-induced analgesia.

No more pain upon Gq-protein-coupled receptor activation: role of endocannabinoids

Marijuana has been used to relieve pain for centuries. The analgesic mechanism of its constituents, the cannabinoids, was only revealed after the discovery of cannabinoid receptors (CB1 and CB2) two decades ago. The subsequent identification of the endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), and their biosynthetic and degradation enzymes discloses the therapeutic potential of compounds targeting the endocannabinoid system for pain control. Inhibitors of the anandamide and 2-AG degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase, respectively, may be superior to direct cannabinoid receptor ligands as endocannabinoids are synthesized on demand and rapidly degraded, focusing action at generating sites. Recently, a promising strategy for pain relief was revealed in the periaqueductal gray (PAG). It is initiated by Gq-protein-coupled receptor (Gq PCR) activation of the phospholipase C-diacylglycerol lipase enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. Here, we introduce the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, particularly in the PAG. We also review recent studies disclosing the Gq PCR-phospholipase C-diacylglycerol lipase-2-AG retrograde disinhibition mechanism in the PAG, induced by activating several Gq PCRs, including metabotropic glutamatergic (type 5 metabotropic glutamate receptor), muscarinic acetylcholine (M1/M3), and orexin 1 receptors. Disinhibition mediated by type 5 metabotropic glutamate receptor can be initiated by glutamate transporter inhibitors or indirectly by substance P, neurotensin, cholecystokinin and capsaicin. Finally, the putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is discussed.

Mechanisms of acupuncture-electroacupuncture on persistent pain

In the last decade, preclinical investigations of electroacupuncture mechanisms on persistent tissue injury (inflammatory), nerve injury (neuropathic), cancer, and visceral pain have increased. These studies show that electroacupuncture activates the nervous system differently in health than in pain conditions, alleviates both sensory and affective inflammatory pain, and inhibits inflammatory and neuropathic pain more effectively at 2 to 10 Hz than at 100 Hz. Electroacupuncture blocks pain by activating a variety of bioactive chemicals through peripheral, spinal, and supraspinal mechanisms. These include opioids, which desensitize peripheral nociceptors and reduce proinflammatory cytokines peripherally and in the spinal cord, and serotonin and norepinephrine, which decrease spinal N-methyl-D-aspartate receptor subunit GluN1 phosphorylation. Additional studies suggest that electroacupuncture, when combined with low dosages of conventional analgesics, provides effective pain management which can forestall the side effects of often-debilitating pharmaceuticals.

Targeting the cannabinoid system for pain relief?

Marijuana has been used to relieve pain for centuries, but its analgesic mechanism has only been understood during the past two decades. It is mainly mediated by its constituents, cannabinoids, through activating central cannabinoid 1 (CB1) receptors, as well as peripheral CB1 and CB2 receptors. CB2-selective agonists have the benefit of lacking CB1 receptor-mediated CNS side effects. Anandamide and 2-arachidonoylglycerol (2-AG) are two intensively studied endogenous lipid ligands of cannabinoid receptors, termed endocannabinoids, which are synthesized on demand and rapidly degraded. Thus, inhibitors of their degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase (MAGL), respectively, may be superior to direct cannabinoid receptor ligands as a promising strategy for pain relief. In addition to the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, we also review recent studies that revealed a novel analgesic mechanism, involving 2-AG in the periaqueductal gray (PAG), a midbrain region for initiating descending pain inhibition. It is initiated by Gq-protein-coupled receptor (GqPCR) activation of the phospholipase C (PLC)-diacylglycerol lipase (DAGL) enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. This GqPCR-PLC-DAGL-2-AG retrograde disinhibition mechanism in the PAG can be initiated by activating type 5 metabotropic glutamate receptor (mGluR5), muscarinic acetylcholine (M1/M3), and orexin (OX1) receptors. mGluR5-mediated disinhibition can be initiated by glutamate transporter inhibitors, or indirectly by substance P, neurotensin, cholecystokinin, capsaicin, and AM404, the bioactive metabolite of acetaminophen in the brain. The putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is also discussed.

Cholecystokinin exerts an effect via the endocannabinoid system to inhibit GABAergic transmission in midbrain periaqueductal gray

Cholecystokinin modulates pain and anxiety via its functions within brain regions such as the midbrain periaqueductal gray (PAG). The aim of this study was to examine the cellular actions of cholecystokinin on PAG neurons. Whole-cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of cholecystokinin and its effects on synaptic transmission. Sulfated cholecystokinin-(26-33) (CCK-S, 100-300 nM), but not non-sulfated cholecystokinin-(26-33) (CCK-NS, 100-300 nM) produced an inward current in a sub-population of opioid sensitive and insensitive PAG neurons, which did not reverse over a range of membrane potentials. The CCK-S-induced current was abolished by the CCK1 selective antagonist devazepide (100 nM), but not by the CCK2 selective antagonists CI988 (100 nM, 1 μM) and LY225910 (1 μM). CCK-S, but not CCK-NS produced a reduction in the amplitude of evoked GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) and an increase in the evoked IPSC paired-pulse ratio. By contrast, CCK-S had little effect on the rate and amplitude of TTX-resistant miniature IPSCs under basal conditions and when external K(+) was elevated. The CCK-S-induced inhibition of evoked IPSCs was abolished by the cannabinoid CB1 receptor antagonist AM251 (3 μM), the mGluR5 antagonist MPEP (10 μM) and the 1, 2-diacylglycerol lipase (DAGLα) inhibitor tetrahydrolipstatin (10 μM). In addition, CCK-S produced an increase in the rate of spontaneous non-NMDA-mediated, TTX-dependent excitatory postsynaptic currents (EPSCs). These results suggest that cholecystokinin produces direct neuronal depolarisation via CCK1 receptors and inhibits GABAergic synaptic transmission via action potential-dependent release of glutamate and mGluR5-induced endocannabinoid signaling. Thus, cholecystokinin has cellular actions within the PAG that can both oppose and reinforce opioid and cannabinoid modulation of pain and anxiety within this brain structure.

Neural mechanism underlying acupuncture analgesia

Acupuncture has been accepted to effectively treat chronic pain by inserting needles into the specific "acupuncture points" (acupoints) on the patient's body. During the last decades, our understanding of how the brain processes acupuncture analgesia has undergone considerable development. Acupuncture analgesia is manifested only when the intricate feeling (soreness, numbness, heaviness and distension) of acupuncture in patients occurs following acupuncture manipulation. Manual acupuncture (MA) is the insertion of an acupuncture needle into acupoint followed by the twisting of the needle up and down by hand. In MA, all types of afferent fibers (Abeta, Adelta and C) are activated. In electrical acupuncture (EA), a stimulating current via the inserted needle is delivered to acupoints. Electrical current intense enough to excite Abeta- and part of Adelta-fibers can induce an analgesic effect. Acupuncture signals ascend mainly through the spinal ventrolateral funiculus to the brain. Many brain nuclei composing a complicated network are involved in processing acupuncture analgesia, including the nucleus raphe magnus (NRM), periaqueductal grey (PAG), locus coeruleus, arcuate nucleus (Arc), preoptic area, nucleus submedius, habenular nucleus, accumbens nucleus, caudate nucleus, septal area, amygdale, etc. Acupuncture analgesia is essentially a manifestation of integrative processes at different levels in the CNS between afferent impulses from pain regions and impulses from acupoints. In the last decade, profound studies on neural mechanisms underlying acupuncture analgesia predominately focus on cellular and molecular substrate and functional brain imaging and have developed rapidly. Diverse signal molecules contribute to mediating acupuncture analgesia, such as opioid peptides (mu-, delta- and kappa-receptors), glutamate (NMDA and AMPA/KA receptors), 5-hydroxytryptamine, and cholecystokinin octapeptide. Among these, the opioid peptides and their receptors in Arc-PAG-NRM-spinal dorsal horn pathway play a pivotal role in mediating acupuncture analgesia. The release of opioid peptides evoked by electroacupuncture is frequency-dependent. EA at 2 and 100Hz produces release of enkephalin and dynorphin in the spinal cord, respectively. CCK-8 antagonizes acupuncture analgesia. The individual differences of acupuncture analgesia are associated with inherited genetic factors and the density of CCK receptors. The brain regions associated with acupuncture analgesia identified in animal experiments were confirmed and further explored in the human brain by means of functional imaging. EA analgesia is likely associated with its counter-regulation to spinal glial activation. PTX-sesntive Gi/o protein- and MAP kinase-mediated signal pathways as well as the downstream events NF-kappaB, c-fos and c-jun play important roles in EA analgesia.

The maintenance of individual differences in the sensitivity of acute and neuropathic pain behaviors to electroacupuncture in rats

"Responder" Sprague-Dawley (SD) rats that were sensitive to electroacupuncture (EA) in an acute thermal pain test (i.e. tail flick latency [TFL] test) maintained sensitivity to EA in the warm allodynia test after peripheral nerve injury. Similarly, the "non-responder" SD rats that were insensitive to EA in the TFL test were also insensitive to EA in the allodynia test. The EA-induced analgesic effects in the TFL test were significantly higher in CCK-A receptor deficient, Otsuka Long-Evans Tokushima Fatty (OLETF) rats than in their littermates, Long-Evans Tokushima Otsuka (LETO) rats. Similarly, the anti-allodynic effects of EA were significantly greater in OLETF rats than in LETO rats. These results suggest that the individual differences in the sensitivity of acute pain behavior to EA were maintained in neuropathic pain behavior following peripheral nerve injury, and that CCK-A receptor expression plays an important role in mediating this phenomenon.

Periaqueductal gray cholecystokinin infusions block morphine-induced disruption of maternal behavior

Cholecystokinin (CCK) and opiates interaction is critical for maintaining maternal behavior during lactation. Morphine inhibits while CCK restores maternal behavior. Recently we have shown that periaqueductal gray (PAG) is a region critically involved in the opioidergic blockade of maternal behavior. A critical level of morphine-induced activation of the rostral lateral PAG is required to inhibit maternal behavior in lactating rats. Since central CCK injections reverted morphine-induced inhibition of maternal behavior, we tested whether this peptide would act similarly in the PAG. This hypothesis was confirmed in experiments showing that morphine's inhibitory effect on maternal responsiveness was blocked by 1.0 and 0.2 nmol CCK injections into the rostral PAG, but not in nearby regions of the mesencephalic reticular nucleus. To test for possible compensatory changes the CCK2 receptor due to morphine treatments the expression of CCK2 receptor mRNA was evaluated in the PAG. PAG CCK2 receptor cDNA amplification revealed no difference in morphine treated animals. These results broaden understanding of the role played by CCK in the PAG. This CCK action might not depend on changes in its receptor.

CCK(B) receptor antagonist L365,260 potentiates the efficacy to and reverses chronic tolerance to electroacupuncture-induced analgesia in mice

Cholecystokinin octapeptide (CCK-8) is a physiological antagonist of endogenous opioids in the central nervous system (CNS). Our previous work has shown that CCK-8 plays an important role in the development of tolerance to morphine analgesia and electroacupuncture (EA) analgesia in the rat. The present studies were designed to examine whether the CCK(B) receptor is involved in the modulation of EA analgesia and the development of EA tolerance in mice. The latency to flick the tail in the radiant heat was used as index to assess the efficacy of EA analgesia. Subcutaneous (s.c.) injection of the CCK(B) receptor antagonist L365,260 produced a dose-dependent (0.125-2.0 mg/kg) potentiation of the analgesia induced by 100 Hz EA, with a maximal effect occurred at 0.5 mg/kg. In addition, L365,260 (0.5 mg/kg) significantly reversed chronic tolerance to 100 Hz EA in mice. These results suggest that the CCK(B) receptor might play a role in the tonic inhibition of 100 Hz EA-induced analgesia and in the mediation of chronic tolerance to 100 Hz EA in mice. The results opened a way for further investigation of the function of CCK-8 in pain modulation using inbred strains of mice.

Cellular action of cholecystokinin-8S-mediated excitatory effects in the rat periaqueductal gray

The peptide cholecystokinin (CCK) is one of the major neurotransmitters modulating satiety, nociception, and anxiety behavior. Although many behavioral studies showing anti-analgesic and anxiogenic actions of CCK have been reported, less is known about its cellular action in the central nervous system (CNS). Therefore, we examined the action of CCK in rat dorsolateral periaqueductal gray (PAG) neurons using slice preparations and whole-cell patch-clamp recordings. Application of CCK-8S produced an inward current accompanied by increased spontaneous synaptic activities. The CCK-8S-induced inward current (I(CCK)) was recovered after washout and reproduced by multiple exposures. Current-voltage plots revealed that I(CCK) reversed near the equilibrium potential for K(+) ions with a decreased membrane conductance. When several K(+) channel blockers were used, application of CdCl(2), TEA, or apamin significantly reduced I(CCK). I(CCK) was also significantly reduced by the CCK(2) receptor antagonist, L-365,260, while it was not affected by the CCK(1) receptor antagonist, L-364,718. Furthermore, we examined the effects of CCK-8S on miniature excitatory postsynaptic currents (mEPSCs) in order to determine the mechanism of CCK-mediated increase on synaptic activities. We found that CCK-8S increased the frequency of mEPSCs, but had no effect on mEPSC amplitude. This presynaptic effect persisted in the presence of CdCl(2) or Ca(2+)-free bath solution, but was completely abolished by pre-treatment with BAPTA-AM, thapsigargin or L-365,260. Taken together, our results indicate that CCK can excite PAG neurons at both pre- and postsynaptic loci via the activation of CCK(2) receptors. These effects may be important for the effects of CCK on behavior and autonomic function that are mediated via PAG neurons.

The difference in mRNA expressions of hypothalamic CCK and CCK-A and -B receptors between responder and non-responder rats to high frequency electroacupuncture analgesia

The present study was performed to determine whether the expression levels of the hypothalamic cholecystokinin (CCK) and its receptors are associated with the responsiveness to high frequency electroacupuncture (EA) analgesia in rats. EA stimulation (100 Hz, 0.5 ms pulse width, 0.2-0.3 mA) was delivered to the Zusanli (ST36) acupoint of male Sprague-Dawley rats for 20 min without anesthetics or holder restraint. The analgesic effect of EA was quantified using a tail flick latency test, and subsequently animals were allocated to responder or non-responder groups. The hypothalamus of rats in each group was dissected and RNA was purified. The mRNA expressions of CCK, and CCK-A and -B receptor were determined by real-time RT-PCR. CCK mRNA levels were not significantly different in the two groups, whereas both CCK-A and -B receptors were significantly more expressed in non-responders. These results suggest that the level of CCK receptor mRNA expression in the hypothalamus, rather than CCK mRNA, has an important relationship with the individual variations to high frequency EA analgesia in rats.

Neural Basis for the Hyperalgesic Action of Cholecystokinin in the Rostral Ventromedial Medulla

The analgesic actions of opioids can be modified by endogenous “anti-opioid” peptides, among them cholecystokinin (CCK). CCK is now thought to have a broader, pronociceptive role, and contributes to hyperalgesia in inflammatory and neuropathic pain states. The aim of this study was to determine whether anti-opioid and pronociceptive actions of CCK have a common underlying mechanism. We showed previously that a low dose of CCK microinjected into the rostral ventromedial medulla (RVM) blocked the analgesic effect of systemically administered morphine by preventing activation of off-cells, which are the antinociceptive output of this well characterized pain-modulating region. At this anti-opioid dose, CCK had no effect on the spontaneous activity of these neurons or on the activity of on-cells (hypothesized to facilitate nociception) or “neutral cells” (which have no known role in pain modulation). In this study, we used microinjection of a higher dose of CCK into the RVM to test whether activation of on-cells could explain the pronociceptive action of this peptide. Paw withdrawal latencies to noxious heat and the activity of a characterized RVM neuron were recorded in rats lightly anesthetized with methohexital. CCK (30 ng/200 nl) activated on-cells selectively and produced behavioral hyperalgesia. Firing of off-cells and neutral cells was unaffected. These data show that direct, selective activation of RVM on-cells by CCK is sufficient to produce thermal hyperalgesia and indicate that the anti-opioid and pronociceptive effects of this peptide are mediated by actions on different RVM cell classes.

Circuitry underlying antiopioid actions of cholecystokinin within the rostral ventromedial medulla

It is now well established that the analgesic actions of opioids can be modified by "anti-analgesic" or "antiopioid" peptides, among them cholecystokinin (CCK). Although the focus of much recent work concerned with CCK-opioid interactions has been at the level of the spinal cord, CCK also acts within the brain to modify opioid analgesia. The aim of the present study was to characterize the actions of CCK in a brain region in which the circuitry mediating the analgesic actions of opioids is relatively well understood, the rostral ventromedial medulla (RVM). Single-cell recording was combined with local infusion of CCK in the RVM and systemic administration of morphine in lightly anesthetized rats. The tail-flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of RVM neurons with distinct responses to opioids have been identified. OFF cells are activated, indirectly, by morphine and mu-opioid agonists, and there is strong evidence that this activation is crucial to opioid antinociception. ON cells, thought to facilitate nociception, are directly inhibited by opioids. Cells of a third class, NEUTRAL cells, do not respond to opioids, and whether they have any role in nociceptive modulation is unknown. CCK microinjected into the RVM by itself had no effect on tail flick latency or the firing of any cell class but significantly attenuated opioid activation of OFF cells and inhibition of the tail flick. Opioid suppression of ON-cell firing was not significantly altered by CCK. Thus CCK acting within the RVM attenuates the analgesic effect of systemically administered morphine by preventing activation of the putative pain inhibiting output neurons of the RVM, the OFF cells. CCK thus differs from another antiopioid peptide, orphanin FQ/nociceptin, which interferes with opioid analgesia by potently suppressing all OFF-cell firing.