Contribution of GIRK2-mediated postsynaptic signaling to opiate and alpha 2-adrenergic analgesia and analgesic sex differences

Ferroptosis inhibitor ferrostatin-1 attenuates morphine tolerance development in male rats by inhibiting dorsal root ganglion neuronal ferroptosis

Background

Ferrostatin-1 and liproxstatin-1, both ferroptosis inhibitors, protect cells. Liproxstatin-1 decreases morphine tolerance. Yet, ferrostatin-1's effect on morphine tolerance remains unexplored. This study aimed to evaluate the influence of ferrostatin-1 on the advancement of morphine tolerance and understand the underlying mechanisms in male rats.

Methods

This experiment involved 36 adult male Wistar albino rats with an average weight ranging from 220 to 260 g. These rats were categorized into six groups: Control, single dose ferrostatin-1, single dose morphine, single dose ferrostatin-1 + morphine, morphine tolerance (twice daily for five days), and ferrostatin-1 + morphine tolerance (twice daily for five days). The antinociceptive action was evaluated using both the hot plate and tail-flick tests. After completing the analgesic tests, tissue samples were gathered from the dorsal root ganglia (DRG) for subsequent analysis. The levels of glutathione, glutathione peroxidase 4 (GPX4), and nuclear factor erythroid 2-related factor 2 (Nrf2), along with the measurements of total oxidant status (TOS) and total antioxidant status (TAS), were assessed in the tissues of the DRG.

Results

After tolerance development, the administration of ferrostatin-1 resulted in a significant decrease in morphine tolerance (P < 0.001). Additionally, ferrostatin-1 treatment led to elevated levels of glutathione, GPX4, Nrf2, and TOS (P < 0.001), while simultaneously causing a decrease in TAS levels (P < 0.001).

Conclusions

The study found that ferrostatin-1 can reduce morphine tolerance by suppressing ferroptosis and reducing oxidative stress in DRG neurons, suggesting it as a potential therapy for preventing morphine tolerance.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

The therapeutic potential of ultra-short-acting β-receptor antagonists in perioperative analgesic: Evidence from preclinical and clinical studies

Perioperative multimodal analgesia can reduce the side effects of a high concentration of opioids, improving the comfort of the patient. However, insufficient analgesia of this model has prompted researchers to explore new adjuvant analgesics. Recently, an increasing number of studies have found a low-grade analgesic effect in the clinical application of ultra-short-acting β-adrenergic receptor antagonists, which are conventionally used as pharmacologic agents in the cardiovascular system. The mechanism by which ultra-short-acting β-antagonists exert antinociceptive effects has not been clarified yet. In this review, we intend to address its potential reasons from the side of neurotransmitters, inflammatory cytokines, and signaling pathways, providing theoretical proof for the application of β-adrenergic receptor antagonists in analgesia.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

GIRK3 deletion facilitates kappa opioid signaling in chondrocytes, delays vascularization and promotes bone lengthening in mice

Long bones are formed and repaired through the process of endochondral ossification. Activation of G protein-coupled receptor (GPCR) signaling pathways is crucial for skeletal development and long bone growth. G protein-gated inwardly-rectifying K⁺ (GIRK) channel genes are key functional components and effectors of GPCR signaling pathways in excitable cells of the heart and brain, but their roles in non-excitable cells that directly contribute to endochondral bone formation have not been studied. In this study, we analyzed skeletal phenotypes of Girk2^-/-, Girk3^-/- and Girk2/3^-/- mice. Bones from 12-week-old Girk2^-/- mice were normal in length, but femurs and tibiae from Girk3^-/- and Girk2/3^-/- mice were longer than age-matched controls at 12-weeks-old. Epiphyseal chondrocytes from 5-day-old Girk3^-/- mice expressed higher levels of genes involved in collagen chain trimerization and collagen fibril assembly, lower levels of genes encoding VEGF receptors, and produced larger micromasses than wildtype chondrocytes in vitro. Girk3^-/- chondrocytes were also more responsive to the kappa opioid receptor (KOR) ligand dynorphin, as evidenced by greater pCREB expression, greater cAMP and GAG production, and upregulation of Col2a1 and Sox9 transcripts. Imaging studies showed that Kdr (Vegfr2) and endomucin expression was dramatically reduced in bones from young Girk3^-/- mice, supporting a role for delayed vasculogenesis and extended postnatal endochondral bone growth. Together these data indicate that GIRK3 controls several processes involved in bone lengthening.

Acute Cannabigerol Administration Lowers Blood Pressure in Mice

Cannabigerol is a cannabinoid compound synthesized by Cannabis sativa, which in its acid form acts as the substrate for both Δ⁹-tetraydrocannabinol and cannabidiol formation. Given its lack of psychoactive effects, emerging research has focused on cannabigerol as a potential therapeutic for health conditions including algesia, epilepsy, anxiety, and cancer. While cannabigerol can bind to classical cannabinoid receptors, it is also an agonist at α2-adrenoreceptors (α2AR) which, when activated, inhibit presynaptic norepinephrine release. This raises the possibility that cannabigerol could activate α2AR to reduce norepinephrine release to cardiovascular end organs to lower blood pressure. Despite this possibility, there are no reports examining cannabigerol cardiovascular effects. In this study, we tested the hypothesis that acute cannabigerol administration lowers blood pressure. Blood pressure was assessed via radiotelemetry at baseline and following intraperitoneal injection of cannabigerol (3.3 and 10 mg/kg) or vehicle administered in a randomized crossover design in male C57BL/6J mice. Acute cannabigerol significantly lowered mean blood pressure (-28 ± 2 mmHg with 10 mg/kg versus -12 ± 5 mmHg vehicle, respectively; p = 0.018), with no apparent dose responsiveness (-22 ± 2 mmHg with 3.3 mg/kg). The depressor effect of cannabigerol was lower in magnitude than the α2AR agonist guanfacine and was prevented by pretreatment with the α2AR antagonist atipamezole. These findings suggest that acute cannabigerol lowers blood pressure in phenotypically normal mice likely via an α2AR mechanism, which may be an important consideration for therapeutic cannabigerol administration.

Attenuated G protein signaling and minimal receptor phosphorylation as a biochemical signature of low side-effect opioid analgesics

Multi-receptor targeting has been proposed as a promising strategy for the development of opioid analgesics with fewer side effects. Cebranopadol and AT-121 are prototypical bifunctional ligands targeting the nociceptin/orphanin FQ peptide receptor (NOP) and µ-opioid receptor (MOP) that elicit potent analgesia in humans and nonhuman primates, respectively. Cebranopadol was reported to produce typical MOP-related side effects such as respiratory depression and reward, whereas AT-121 appeared to be devoid of these liabilities. However, the molecular basis underlying different side effect profiles in opioid analgesics remains unknown. Here, we examine agonist-induced receptor phosphorylation and G protein signaling profiles of a series of chemically diverse mixed MOP/NOP agonists, including cebranopadol and AT-121. We found that these compounds produce strikingly different MOP phosphorylation profiles. Cebranopadol, AT-034 and AT-324 stimulated extensive MOP phosphorylation, whereas AT-201 induced selective phosphorylation at S375 only. AT-121, on the other hand, did not promote any detectable MOP phosphorylation. Conversely, none of these compounds was able to elicit strong NOP phosphorylation and low NOP receptor phosphorylation correlated with partial agonism in a GIRK-channel assay. Our results suggest a close correlation between MOP receptor phosphorylation and side effect profile. Thus, bifunctional MOP/NOP opioid ligands combining low efficacy G protein signaling at both NOP and MOP with no detectable receptor phosphorylation appear to be devoid of side-effects such as respiratory depression, abuse liability or tolerance development, as with AT-121.

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

Pharmacogenetics of Addiction Therapy

Drug addiction is a serious relapsing disease that has high costs to society and to the individual addicts. Treatment of these addictions is still in its nascency, with only a few examples of successful therapies. Therapeutic response depends upon genetic, biological, social, and environmental components. A role for genetic makeup in the response to treatment has been shown for several addiction pharmacotherapies with response to treatment based on individual genetic makeup. In this chapter, we will discuss the role of genetics in pharmacotherapies, specifically for cocaine, alcohol, and opioid dependences. The continued elucidation of the role of genetics should aid in the development of new treatments and increase the efficacy of existing treatments.

Chemokine CCL2 prevents opioid-induced inhibition of nociceptive synaptic transmission in spinal cord dorsal horn

Background

Opioid analgesics remain widely used for pain treatment despite the related serious side effects. Some of those, such as opioid tolerance and opioid-induced hyperalgesia may be at least partially due to modulation of opioid receptors (OR) function at nociceptive synapses in the spinal cord dorsal horn. It was suggested that increased release of different chemokines under pathological conditions may play a role in this process. The goal of this study was to investigate the crosstalk between the µOR, transient receptor potential vanilloid 1 (TRPV1) receptor and C–C motif ligand 2 (CCL2) chemokine and the involvement of spinal microglia in the modulation of opioid analgesia.

Methods

Patch-clamp recordings of miniature excitatory postsynaptic currents (mEPSCs) and dorsal root evoked currents (eEPSC) in spinal cord slices superficial dorsal horn neurons were used to evaluate the effect of µOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO), CCL2, TRPV1 antagonist SB366791 and minocycline. Paw withdrawal test to thermal stimuli was combined with intrathecal (i.t.) delivery of CCL2 and DAMGO to investigate the modulation in vivo.

Results

Application of DAMGO induced a rapid decrease of mEPSC frequency and eEPSC amplitude, followed by a delayed increase of the eESPC amplitude, which was prevented by SB366791. Chemokine CCL2 treatment significantly diminished all the DAMGO-induced changes. Minocycline treatment prevented the CCL2 effects on the DAMGO-induced eEPSC depression, while mEPSC changes were unaffected. In behavioral experiments, i.t. injection of CCL2 completely blocked DAMGO-induced thermal hypoalgesia and intraperitoneal pre-treatment with minocycline prevented the CCL2 effect.

Conclusions

Our results indicate that opioid-induced inhibition of the excitatory synaptic transmission could be severely attenuated by increased CCL2 levels most likely through a microglia activation-dependent mechanism. Delayed potentiation of neurotransmission after µOR activation is dependent on TRPV1 receptors activation. Targeting CCL2 and its receptors and TRPV1 receptors in combination with opioid therapy could significantly improve the analgesic properties of opioids, especially during pathological states.

Analgesic α-conotoxins modulate native and recombinant GIRK1/2 channels via activation of GABAB receptors and reduce neuroexcitability

BACKGROUND AND PURPOSE

Activation of GIRK channels via G protein-coupled GABA_B receptors has been shown to attenuate nociceptive transmission. The analgesic α-conotoxin Vc1.1 activates GABA_B receptors resulting in inhibition of Ca_v 2.2 and Ca_v 2.3 channels in mammalian primary afferent neurons. Here, we investigated the effects of analgesic α-conotoxins on recombinant and native GIRK-mediated K⁺ currents and on neuronal excitability.

EXPERIMENTAL APPROACH

The effects of analgesic α-conotoxins, Vc1.1, RgIA, and PeIA, were investigated on inwardly-rectifying K⁺ currents in HEK293T cells recombinantly co-expressing either heteromeric human GIRK1/2 or homomeric GIRK2 subunits, with GABA_B receptors. The effects of α-conotoxin Vc1.1 and baclofen were studied on GIRK-mediated K⁺ currents and the passive and active electrical properties of adult mouse dorsal root ganglion neurons.

KEY RESULTS

Analgesic α-conotoxins Vc1.1, RgIA, and PeIA potentiate inwardly-rectifying K⁺ currents in HEK293T cells recombinantly expressing human GIRK1/2 channels and GABA_B receptors. GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen occurs via a pertussis toxin-sensitive G protein and is inhibited by the selective GABA_B receptor antagonist CGP 55845. In adult mouse dorsal root ganglion neurons, GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen hyperpolarizes the cell membrane potential and reduces excitability.

CONCLUSIONS AND IMPLICATIONS

This is the first report of GIRK channel potentiation via allosteric α-conotoxin Vc1.1-GABA_B receptor agonism, leading to decreased neuronal excitability. Such action potentially contributes to the analgesic effects of Vc1.1 and baclofen observed in vivo.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Identification of a G-Protein-Independent Activator of GIRK Channels

G-protein-gated inwardly rectifying K⁺ (GIRK) channels are essential effectors of inhibitory neurotransmission in the brain. GIRK channels have been implicated in diseases with abnormal neuronal excitability, including epilepsy and addiction. GIRK channels are tetramers composed of either the same subunit (e.g., homotetramers) or different subunits (e.g., heterotetramers). Compounds that specifically target subsets of GIRK channels in vivo are lacking. Previous studies have shown that alcohol directly activates GIRK channels through a hydrophobic pocket located in the cytoplasmic domain of the channel. Here, we report the identification and functional characterization of a GIRK1-selective activator, termed GiGA1, that targets the alcohol pocket. GiGA1 activates GIRK1/GIRK2 both in vitro and in vivo and, in turn, mitigates the effects of a convulsant in an acute epilepsy mouse model. These results shed light on the structure-based development of subunit-specific GIRK modulators that could provide potential treatments for brain disorders.

Gender-related differences in migraine

Migraine is considered mostly a woman’s complaint, even if it affects also men. Epidemiological data show a higher incidence of the disease in women, starting from puberty throughout life. The sex-related differences of migraine hold clinical relevance too. The frequency, duration, and disability of attacks tend to be higher in women. Because of this, probably, they also consult specialists more frequently and take more prescription drugs than men. Different mechanisms have been evaluated to explain these differences. Hormonal milieu and its modulation of neuronal and vascular reactivity is probably one of the most important aspects. Estrogens and progesterone regulate a host of biological functions through two mechanisms: nongenomic and genomic. They influence several neuromediators and neurotransmitters, and they may cause functional and structural differences in several brain regions, involved in migraine pathogenesis. In addition to their central action, sex hormones exert rapid modulation of vascular tone. The resulting specific sex phenotype should be considered during clinical management and experimental studies.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Qualitative sex differences in pain processing: emerging evidence of a biased literature

Although most patients with chronic pain are women, the preclinical literature regarding pain processing and the pathophysiology of chronic pain has historically been derived overwhelmingly from the study of male rodents. This Review describes how the recent adoption by a number of funding agencies of policies mandating the incorporation of sex as a biological variable into preclinical research has correlated with an increase in the number of studies investigating sex differences in pain and analgesia. Trends in the field are analysed, with a focus on newly published findings of qualitative sex differences: that is, those findings that are suggestive of differential processing mechanisms in each sex. It is becoming increasingly clear that robust differences exist in the genetic, molecular, cellular and systems-level mechanisms of acute and chronic pain processing in male and female rodents and humans.

The role of catecholamines in modulating responses to stress: Sex-specific patterns, implications, and therapeutic potential for post-traumatic stress disorder and opiate withdrawal

Emotional arousal is one of several factors that determine the strength of a memory and how efficiently it may be retrieved. The systems at play are multifaceted; on one hand, the dopaminergic mesocorticolimbic system evaluates the rewarding or reinforcing potential of a stimulus, while on the other, the noradrenergic stress response system evaluates the risk of threat, commanding attention, and engaging emotional and physical behavioral responses. Sex-specific patterns in the anatomy and function of the arousal system suggest that sexually divergent therapeutic approaches may be advantageous for neurological disorders involving arousal, learning, and memory. From the lens of the triple network model of psychopathology, we argue that post-traumatic stress disorder and opiate substance use disorder arise from maladaptive learning responses that are perpetuated by hyperarousal of the salience network. We present evidence that catecholamine-modulated learning and stress-responsive circuitry exerts substantial influence over the salience network and its dysfunction in stress-related psychiatric disorders, and between the sexes. We discuss the therapeutic potential of targeting the endogenous cannabinoid system; a ubiquitous neuromodulator that influences learning, memory, and responsivity to stress by influencing catecholamine, excitatory, and inhibitory synaptic transmission. Relevant preclinical data in male and female rodents are integrated with clinical data in men and women in an effort to understand how ideal treatment modalities between the sexes may be different.

Replacement of current opioid drugs focusing on MOR-related strategies

The scarcity and limited risk/benefit ratio of painkillers available on the market, in addition to the opioid crisis, warrant reflection on new innovation strategies. The pharmacopoeia of analgesics is based on products that are often old and derived from clinical empiricism, with limited efficacy or spectrum of action, or resulting in an unsatisfactory tolerability profile. Although they are reference analgesics for nociceptive pain, opioids are subject to the same criticism. The use of opium as an analgesic is historical. Morphine was synthesized at the beginning of the 19th century. The efficacy of opioids is limited in certain painful contexts and these drugs can induce potentially serious and fatal adverse effects. The current North American opioid crisis, with an ever-rising number of deaths by opioid overdose, is a tragic illustration of this. It is therefore legitimate to develop research into molecules likely to maintain or increase opioid efficacy while improving their tolerability. Several avenues are being explored including targeting of the mu opioid receptor (MOR) splice variants, developing biased agonists or targeting of other receptors such as heteromers with MOR. Ion channels acting as MOR effectors, are also targeted in order to offer compounds without MOR-dependent adverse effects. Another route is to develop opioid analgesics with peripheral action or limited central nervous system (CNS) access. Finally, endogenous opioids used as drugs or compounds that modify the metabolism of endogenous opioids (Dual ENKephalinase Inhibitors) are being developed. The aim of the present review is to present these various targets/strategies with reference to current indications for opioids, concerns about their widespread use, particularly in chronic non-cancer pains, and ways of limiting the risk of opioid abuse and misuse.

Structural Determinants Mediating Tertiapin Block of Neuronal Kir3.2 Channels

Tertiapin (TPN) is a 21 amino acid venom peptide from Apis mellifera that inhibits certain members of the inward rectifier potassium (Kir) channel family at a nanomolar affinity with limited specificity. Structure-based computational simulations predict that TPN behaves as a pore blocker; however, the molecular determinants mediating block of neuronal Kir3 channels have been inconclusive and unvalidated. Here, using molecular docking and molecular dynamics (MD) simulations with 'potential of mean force' (PMF) calculations, we investigated the energetically most favored interaction of TPN with several Kir3.x channel structures. The resulting binding model for Kir3.2-TPN complexes was then tested by targeted mutagenesis of the predicted contact sites, and their impact on the functional channel block was measured electrophysiologically. Together, our findings indicate that a high-affinity TPN block of Kir3.2 channels involves a pore-inserting lysine side chain requiring (1) hydrophobic interactions at a phenylalanine ring surrounding the channel pore and (2) electrostatic interactions with two adjacent Kir3.2 turret regions. Together, these interactions collectively stabilize high-affinity toxin binding to the Kir3.2 outer vestibule, which orients the ε-amino group of TPN-K21 to occupy the outermost K⁺ binding site of the selectivity filter. The structural determinants for the TPN block described here also revealed a favored subunit arrangement for assembled Kir3.x heteromeric channels, in addition to a multimodal binding capacity of TPN variants consistent with the functional dyad model for polybasic peptide pore blockers. These novel findings will aid efforts in re-engineering the TPN pharmacophore to develop peptide variants having unique and distinct Kir channel blocking properties.

Role of Spinal Cord α2-Adrenoreceptors in Noradrenergic Inhibition of Nociceptive Transmission During Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a problem during cancer treatment and for cancer survivors but the central mechanisms underlying CIPN remain understudied. This study aims to determine if CIPN is associated with alterations of noradrenergic modulation of nociceptive transmission at the spinal cord. CIPN was induced in male Wistar rats by paclitaxel injections. One month after CIPN induction, the behavioral effects of the administration of reboxetine (noradrenaline reuptake inhibitor), clonidine (agonist of α₂-adrenoreceptors; α_2–AR) and atipamezole (antagonist of α_2–AR) were evaluated using the von Frey and cold plate tests. Furthermore, we measured the expression of the noradrenaline biosynthetic enzyme dopamine-β-hydroxylase (DBH) and of α_2–AR in the spinal dorsal horn. Reboxetine and clonidine reversed the behavioral signs of CIPN whereas the opposite occurred with atipamezole. In the 3 pharmacological approaches, a higher effect was detected in mechanical allodynia, the pain modality which is under descending noradrenergic control. DBH expression was increased at the spinal dorsal horn of paclitaxel-injected animals. The enhanced noradrenergic inhibition during CIPN may represent an adaptation of the descending noradrenergic pain control system to the increased arrival of peripheral nociceptive input. A potentiation of the α_2–AR mediated antinociception at the spinal cord may represent a therapeutic opportunity to face CIPN.

Apical periodontitis-induced mechanical allodynia: A mouse model to study infection-induced chronic pain conditions

Infection-induced chronic pain is an under-studied pain condition. One example is apical periodontitis, which evokes considerable mechanical allodynia that persists after treatment in 7% to 12% of patients. Available analgesics often provide incomplete relief. However, a preclinical model to study pain mechanisms associated with apical periodontitis is not available. Here, we report a mouse model of apical periodontitis to facilitate studies determining mechanisms mediating persistent infection-induced pain. Mice were anesthetized and the left first molar was exposed to the oral environment for six weeks. Bone resorption, as an indicator of apical periodontitis, was quantified using microcomputed tomography. Mechanical allodynia was determined using extraoral von-Frey filaments in both male and female mice. The expression of c-fos in the medullary dorsal horn was assessed using immunohistochemistry. Mice with apical periodontitis developed significant mechanical allodynia by day 7 that was maintained for 42 days. Mechanical thresholds were significantly lower in females compared to males. Administration of ibuprofen, morphine, or MK-801 reversed mechanical allodynia. Finally, apical periodontitis triggered an upregulation of c-fos in the medullary dorsal horn. Collectively, this model simulates signs of clinical pain experienced by patients with apical periodontitis, detects sex differences in allodynia, and permits the study of peripheral and central trigeminal pain mechanisms.

The multifunctional peptide DN-9 produced peripherally acting antinociception in inflammatory and neuropathic pain via μ- and κ-opioid receptors

BACKGROUND AND PURPOSE

Considerable effort has recently been directed at developing multifunctional opioid drugs to minimize the unwanted side effects of opioid analgesics. We have developed a novel multifunctional opioid agonist, DN-9. Here, we studied the analgesic profiles and related side effects of peripheral DN-9 in various pain models.

EXPERIMENTAL APPROACH

Antinociceptive effects of DN-9 were assessed in nociceptive, inflammatory, and neuropathic pain. Whole-cell patch-clamp and calcium imaging assays were used to evaluate the inhibitory effects of DN-9 to calcium current and high-K⁺ -induced intracellular calcium ([Ca²⁺ ]_i ) on dorsal root ganglion (DRG) neurons respectively. Side effects of DN-9 were evaluated in antinociceptive tolerance, abuse, gastrointestinal transit, and rotarod tests.

KEY RESULTS

DN-9, given subcutaneously, dose-dependently produced antinociception via peripheral opioid receptors in different pain models without sex difference. In addition, DN-9 exhibited more potent ability than morphine to inhibit calcium current and high-K⁺ -induced [Ca²⁺ ]_i in DRG neurons. Repeated treatment with DN-9 produced equivalent antinociception for 8 days in multiple pain models, and DN-9 also maintained potent analgesia in morphine-tolerant mice. Furthermore, chronic DN-9 administration had no apparent effect on the microglial activation of spinal cord. After subcutaneous injection, DN-9 exhibited less abuse potential than morphine, as was gastroparesis and effects on motor coordination.

CONCLUSIONS AND IMPLICATIONS

DN-9 produces potent analgesia with minimal side effects, which strengthen the candidacy of peripherally acting opioids with multifunctional agonistic properties to enter human studies to alleviate the current highly problematic misuse of classic opioids on a large scale.

Analgesic Effects of the GIRK Activator, VU0466551, Alone and in Combination with Morphine in Acute and Persistent Pain Models

G protein-gated inwardly rectifying potassium (GIRK) channels are potassium-selective ion channels. As their name suggests, GIRK channels are effectors of G_i/o G protein-couple receptors whereby activation of these GPCRs leads to increased GIRK channel activity resulting in decreased cellular excitability. In this way, GIRK channels play diverse roles in physiology as effectors of G_i/o-coupled GPCRs: peacemaking in the heart rate, modulation of hormone secretion in endocrine tissues, as well as numerous CNS functions including learning, memory, and addiction/reward. Notably, GIRK channels are widely expressed along the spinothalamic tract and are positioned to play roles in both ascending and descending pain pathways. More notably, GIRK channel knockout and knock-down studies have found that GIRK channels play a major role in the action of opioid analgesics which act predominantly through G_i/o-coupled, opioid-activated GPCRs (e.g., μ-opioid receptors). Recent advances in GIRK channel pharmacology have led to the development of small molecules that directly and selectively activate GIRK channels. Based on research implicating the involvement of GIRK channels in pain pathways and as effectors of opioid analgesics, we conducted a study to determine whether direct pharmacological activation of GIRK channels could produce analgesic efficacy and/or augment the analgesic efficacy morphine, an opioid receptor agonist capable of activating μ-opioid receptors as well as other opioid receptor subtypes. In the present study, we demonstrate that the small-molecule GIRK activator, VU0466551, has analgesic effects when dosed alone or in combination with submaximally effective doses of morphine.

Candidate gene analyses for acute pain and morphine analgesia after pediatric day surgery: African American versus European Caucasian ancestry and dose prediction limits

Acute pain and opioid analgesia demonstrate inter-individual variability and polygenic influence. In 241 children of African American and 277 of European Caucasian ancestry, we sought to replicate select candidate gene associations with morphine dose and postoperative pain and then to estimate dose prediction limits. Twenty-seven single-nucleotide polymorphisms (SNPs) from nine genes (ABCB1, ARRB2, COMT, DRD2, KCNJ6, MC1R, OPRD1, OPRM1, and UGT2B7) met selection criteria and were analyzed along with TAOK3. Few associations replicated: morphine dose (mcg/kg) in African American children and ABCB1 rs1045642 (A allele, β = -9.30, 95% CI: -17.25 to -1.35, p = 0.02) and OPRM1 rs1799971 (G allele, β = 23.19, 95% CI: 3.27-43.11, p = 0.02); KCNJ6 rs2211843 and high pain in African American subjects (T allele, OR 2.08, 95% CI: 1.17-3.71, p = 0.01) and in congruent European Caucasian pain phenotypes; and COMT rs740603 for high pain in European Caucasian subjects (A allele, OR: 0.69, 95% CI: 0.48-0.99, p = 0.046). With age, body mass index, and physical status as covariates, simple top SNP candidate gene models could explain theoretical maximums of 24.2% (European Caucasian) and 14.6% (African American) of morphine dose variances.

Mu-Opioid Receptor Agonist Induces Kir3 Currents in Mouse Peripheral Sensory Neurons - Effects of Nerve Injury

Neuropathic pain often arises from damage to peripheral nerves and is difficult to treat. Activation of opioid receptors in peripheral sensory neurons is devoid of respiratory depression, sedation, nausea, and addiction mediated in the brain, and ameliorates neuropathic pain in animal models. Mechanisms of peripheral opioid analgesia have therefore gained interest, but the role of G protein-coupled inwardly rectifying potassium (Kir3) channels, important regulators of neuronal excitability, remains unclear. Whereas functional Kir3 channels have been detected in dorsal root ganglion (DRG) neurons in rats, some studies question their contribution to opioid analgesia in inflammatory pain models in mice. However, neuropathic pain can be diminished by activation of peripheral opioid receptors in mouse models. Therefore, here we investigated effects of the selective μ-opioid receptor (MOR) agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) on potassium conductance in DRG neurons upon a chronic constriction injury (CCI) of the sciatic nerve in mice. For verification, we also tested human embryonic kidney (HEK) 293 cells transfected with MOR and Kir3.2. Using patch clamp, we recorded currents at -80 mV and applied voltage ramps in high extracellular potassium concentrations, which are a highly sensitive measures of Kir3 channel activity. We found a significantly higher rate of HEK cells responding with potassium channel blocker barium-sensitive inward current (233 ± 51 pA) to DAMGO application in transfected than in untransfected group, which confirms successful recordings of inward currents through Kir3.2 channels. Interestingly, DAMGO induced similar inward currents (178 ± 36-207 ± 56 pA) in 15-20% of recorded DRG neurons from naïve mice and in 4-27% of DRG neurons from mice exposed to CCI, measured in voltage clamp or voltage ramp modes. DAMGO-induced currents in naïve and CCI groups were reversed by barium and a more selective Kir3 channel blocker tertiapin-Q. These data indicate the coupling of Kir3 channels with MOR in mouse peripheral sensory neuron cell bodies, which was unchanged after CCI. A comparative analysis of opioid-induced potassium conductance at the axonal injury site and peripheral terminals of DRG neurons could clarify the role of Kir3 channel-MOR interactions in peripheral nerve injury and opioid analgesia.

Gender, genetics, and analgesia: understanding the differences in response to pain relief

Genetic variations and gender contribute significantly to the large interpatient variations in opioid-related serious adverse effects and differences in pain relief with other analgesics. Opioids are the most commonly used analgesics to relieve moderate-to-severe postoperative pain. Narrow therapeutic index and unexplained large interpatient variations in opioid-related serious adverse effects and analgesia negatively affect optimal perioperative outcomes. In surgical, experimental, chronic, and neuropathic pain models, females have been reported to have more pain than males. This review focuses on literature evidence of differences in pain relief due to multiple genetic variations and gender of the patient.

Preclinical and Clinical Pharmacology of Hydrocodone for Chronic Pain: A Mini Review

Hydrocodone is one of the most prescribed oral analgesic drugs and it is one of the most abused drugs in general population. It is a mu-opioid agonist predominantly metabolized to the O-demethylated product hydromorphone and to the N-demethylated product norhydrocodone. The purpose of the study is to summarize the preclinical and clinical characteristics of hydrocodone. Pharmacokinetic aspect (terminal half-life, maximum serum concentration, and time to maximum serum concentration) of hydrocodone and the influence of metabolic genetic polymorphism in analgesic response to hydrocodone are also illustrated and commented. Literature on experimental preclinical pharmacology investigating analgesic activity in laboratory animals is furtherly discussed. Moreover, the authors discuss and comment on the updated data regarding safety profile and effectiveness of hydrocodone in the treatment of chronic pain. A bibliographic research was carried out (from February 01, 2018 to August 28, 2018) independently by two researchers (blinded to the authors and initially on results) in the major scientific databases and research engines of peer-reviewed literature on life sciences and biomedical topics, starting from January 1990 to August 2018. Analysis of results of clinical studies suggests that abuse-deterrent extended-release (ER) hydrocodone formulations can be effective and they are well tolerated in the treatment of chronic low back pain. Weaker is the evidence of the analgesic effectiveness of ER hydrocodone on other chronic pain syndromes and non-cancer non-neuropathic chronic pain. In these conditions, hydrocodone showed to have positive effects in non-controlled open studies and needs to be further studied to assess the real strength of results.

Gi protein functions in thalamic neurons to decrease orofacial nociceptive response

Orofacial pain includes neuronal pathways that project from the trigeminal nucleus to and through the thalamus. What role the ventroposterior thalamic complex (VP) has on orofacial pain transmission is not understood. To begin to address this question an inhibitory G protein (Gi) designer receptor exclusively activated by a designer drug (DREADD) was transfected in cells of the VP using adeno-associated virus isotype 8. Virus infected cells were identified by a fluorescent tag and immunostaining. Cells were silenced after injecting the designer drug clozapine-n-oxide, which binds the designer receptor activating Gi. Facial rubbing and local field potentials (LFP) in the VP were then recorded in awake, free moving Sprague Dawley rats after formalin injection of the masseter muscle to induce nociception. Formalin injection significantly increased LFP and the nociceptive behavioral response. Activation of DREADD Gi with clozapine-n-oxide significantly reduced LFP in the VP and reduced the orofacial nociceptive response. Because DREADD silencing can result from Gi-coupled inwardly-rectifying potassium channels (GIRK), the GIRK channel blocker tertiapin-Q was injected. Injection of GIRK blocker resulted in an increase in the nociceptive response and increased LFP activity. Immunostaining of the VP for glutamate vesicular transporter (VGLUT2) and gamma-aminobutyric acid vesicular transporter (VGAT) indicated a majority of the virally transfected cells were excitatory (VGLUT2 positive) and a minority were inhibitory (VGAT positive). We conclude first, that inhibition of the excitatory neurons within the VP reduced electrical activity and the orofacial nociceptive response and that the effect on excitatory neurons overwhelmed any change resulting from inhibitor neurons. Second, inhibition of LFP and nociception was due, in part, to GIRK activation.

Estrogen Regulation of GRK2 Inactivates Kappa Opioid Receptor Signaling Mediating Analgesia, But Not Aversion

Activation of κ opioid receptors (KORs) produces analgesia and aversion via distinct intracellular signaling pathways, but whether G protein-biased KOR agonists can be designed to have clinical utility will depend on a better understanding of the signaling mechanisms involved. We found that KOR activation produced conditioned place aversion and potentiated CPP for cocaine in male and female C57BL/6N mice. Consistent with this, males and females both showed arrestin-mediated increases in phospho-p38 MAPK following KOR activation. Unlike in males, however, KOR activation had inconsistent analgesic effects in females and KOR increased Gβγ-mediated ERK phosphorylation in males, but not females. KOR desensitization was not responsible for the lack of response in females because neither Grk3 nor Pdyn gene knock-out enhanced analgesia. Instead, responsiveness was estrous cycle dependent because KOR analgesia was evident during low estrogen phases of the cycle and in ovariectomized (OVX) females. Estradiol treatment of OVX females suppressed KOR-mediated analgesia, demonstrating that estradiol was sufficient to blunt Gβγ-mediated KOR signals. G protein-coupled receptor kinase 2 (GRK2) is known to regulate ERK activation, and we found that the inhibitory, phosphorylated form of GRK2 was significantly higher in intact females. GRK2/3 inhibition by CMPD101 increased KOR stimulation of phospho-ERK in females, decreased sex differences in KOR-mediated inhibition of dopamine release, and enhanced mu opioid receptor and KOR-mediated analgesia in females. In OVX females, estradiol increased the association between GRK2 and Gβγ. These studies suggest that estradiol, through increased phosphorylation of GRK2 and possible sequestration of Gβγ by GRK2, blunts G protein-mediated signals.SIGNIFICANCE STATEMENT Chronic pain disorders are more prevalent in females than males, but opioid receptor agonists show inconsistent analgesic efficacy in females. κ opioid receptor (KOR) agonists have been tested in clinical trials for treating pain disorders based on their analgesic properties and low addictive potential. However, the molecular mechanisms underlying sex differences in KOR actions were previously unknown. Our studies identify an intracellular mechanism involving estradiol regulation of G protein-coupled receptor kinase 2 that is responsible for sexually dimorphic analgesic responses following opioid receptor activation. Understanding this mechanism will be critical for developing effective nonaddictive opioid analgesics for use in women and characterizing sexually dimorphic effects in other inhibitory G protein-coupled receptor signaling responses.

A pharmacogenetics approach to pain management

Introduction

Opioid analgesics are widely used as effective analgesics for the treatment of moderate‐to‐severe pain. However, the analgesic efficacy of opioids is well known to vary widely among individuals, and effective pain treatment is hampered by vast individual differences. Although these differences in opioid requirements have been attributed to various factors, genetic factors are becoming increasingly relevant to the development of genome science.

Aim

This review covers the association between opioid analgesic requirements and particularly gene polymorphisms.

Future perspectives

Personalized pain treatment has begun using prediction formulas based on associated gene polymorphisms. Improvements in personalized pain treatment are expected as scientific knowledge further expands in the future.

The analgesic efficacy of opioids is well known to vary widely among individuals, and effective pain treatment is hampered by vast individual differences. Although these differences in opioid requirements have been attributed to various factors, genetic factors are becoming increasingly relevant to the development of genome science. This review covers the association between opioid analgesic requirements and particularly gene polymorphisms.

Gi-DREADD Expression in Peripheral Nerves Produces Ligand-Dependent Analgesia, as well as Ligand-Independent Functional Changes in Sensory Neurons

Designer receptors exclusively activated by designer drugs (DREADDs) are an advanced experimental tool that could potentially provide a novel approach to pain management. In particular, expression of an inhibitory (Gi-coupled) DREADD in nociceptors might enable ligand-dependent analgesia. To test this possibility, TRPV1-cre mice were used to restrict expression of Gi-DREADDs to predominantly C-fibers. Whereas baseline heat thresholds in both male and female mice expressing Gi-DREADD were normal, 1 mg/kg clozapine-N-oxide (CNO) produced a significant 3 h increase in heat threshold that returned to baseline by 5 h after injection. Consistent with these behavioral results, CNO decreased action potential firing in isolated sensory neurons from Gi-DREADD mice. Unexpectedly, however, the expression of Gi-DREADD in sensory neurons caused significant changes in voltage-gated Ca²⁺ and Na⁺ currents in the absence of CNO, as well as an increase in Na⁺ channel (NaV1.7) expression. Furthermore, CNO-independent excitatory and inhibitory second-messenger signaling was also altered in these mice, which was associated with a decrease in the analgesic effect of endogenous inhibitory G-protein-coupled receptor activation. These results highlight the potential of this exciting technology, but also its limitations, and that it is essential to identify the underlying mechanisms for any observed behavioral phenotypes.

SIGNIFICANCE STATEMENT

DREADD technology is a powerful tool enabling manipulation of activity and/or transmitter release from targeted cell populations. The purpose of this study was to determine whether inhibitory DREADDs in nociceptive afferents could be used to produce analgesia, and if so, how. DREADD activation produced a ligand-dependent analgesia to heat in vivo and a decrease in neuronal firing at the single-cell level. However, we observed that expression of Gi-DREADD also causes ligand-independent changes in ion channel activity and second-messenger signaling. These findings highlight both the potential and the limitations of this exciting technology as well as the necessity to identify the mechanisms underlying any observed phenotype.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Evidence that increased Kcnj6 gene dose is necessary for deficits in behavior and dentate gyrus synaptic plasticity in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS), trisomy 21, is caused by increased dose of genes present on human chromosome 21 (HSA21). The gene-dose hypothesis argues that a change in the dose of individual genes or regulatory sequences on HSA21 is necessary for creating DS-related phenotypes, including cognitive impairment. We focused on a possible role for Kcnj6, the gene encoding Kir3.2 (Girk2) subunits of a G-protein-coupled inwardly-rectifying potassium channel. This gene resides on a segment of mouse Chromosome 16 that is present in one extra copy in the genome of the Ts65Dn mouse, a well-studied genetic model of DS. Kir3.2 subunit-containing potassium channels serve as effectors for a number of postsynaptic metabotropic receptors including GABAB receptors. Several studies raise the possibility that increased Kcnj6 dose contributes to synaptic and cognitive abnormalities in DS. To assess directly a role for Kcnj6 gene dose in cognitive deficits in DS, we produced Ts65Dn mice that harbor only 2 copies of Kcnj6 (Ts65Dn:Kcnj6++- mice). The reduction in Kcnj6 gene dose restored to normal the hippocampal level of Kir3.2. Long-term memory, examined in the novel object recognition test with the retention period of 24h, was improved to the level observed in the normosomic littermate control mice (2N:Kcnj6++). Significantly, both short-term and long-term potentiation (STP and LTP) was improved to control levels in the dentate gyrus (DG) of the Ts65Dn:Kcnj6++- mouse. In view of the ability of fluoxetine to suppress Kir3.2 channels, we asked if fluoxetine-treated DG slices of Ts65Dn:Kcnj6+++ mice would rescue synaptic plasticity. Fluoxetine increased STP and LTP to control levels. These results are evidence that increased Kcnj6 gene dose is necessary for synaptic and cognitive dysfunction in the Ts65Dn mouse model of DS. Strategies aimed at pharmacologically reducing channel function should be explored for enhancing cognition in DS.

Activation of the trigeminal α2-adrenoceptor produces sex-specific, estrogen dependent thermal antinociception and antihyperalgesia using an operant pain assay in the rat

Higher prevalence of several pain disorders in women and sexual dimorphism in G-protein coupled receptor-induced analgesia has been reported. We have previously shown that α2-adrenoceptor-induced antinociception is sex-specific and attenuated by estrogen in the female rat. However, this evidence was obtained using reflexive withdrawal-based nociceptive assays conducted on restrained animals that may not involve cerebral processing. Hence, we evaluated whether activation of the trigeminal α2-adrenoceptor produces sex-specific antinociceptive and antihyperalgesic effects in the orofacial region of the rat using a reward conflict-based operant paradigm in which animals must tolerate nociceptive thermal stimulation to be rewarded. Male and ovariectomized (OVX) Sprague-Dawley rats were implanted intracisternally with a PE10 cannula for drug injections. A group of OVX rats (OVX+E) was administered subcutaneously with estradiol 48h before the test. Effect of clonidine, an α2-adrenoceptor agonist, was determined on the operant pain assay using a fully automated Orofacial Pain Assessment Device. Number of spout licks, thermode contacts, and amount of reward intake were automatically recorded by the ANY-maze software. Using acute pain modeling, clonidine produced a dose-dependent increase in all three parameters in male and OVX groups, however, it was ineffective in the OVX+E group. Similarly, using inflammatory pain modeling, clonidine significantly increased these parameters in carrageenan-treated male and OVX groups but not in the OVX+E group. Thus, α2-adrenoceptor activation produces sex-specific antinociception and antihyperalgesia and estrogen attenuates these effects in female rats using an operant pain assay. These findings may help the discovery of effective analgesics for each sex.

Isolation of proflavine as a blocker of G protein-gated inward rectifier potassium channels by a cell growth-based screening system

The overexpression of Kir3.2, a subunit of the G protein-gated inwardly rectifying K(+) channel, is implicated in some of the neurological phenotypes of Down syndrome (DS). Chemical compounds that block Kir3.2 are expected to improve the symptoms of DS. The purpose of this study is to develop a cell-based screening system to identify Kir3.2 blockers and then investigate the mode of action of the blocker. Chemical screening was carried out using a K(+) transporter-deficient yeast strain that expressed a constitutively active Kir3.2 mutant. The mode of action of an effective blocker was electrophysiologically analyzed using Kir channels expressed in Xenopus oocytes. Proflavine was identified to inhibit the growth of Kir3.2-transformant cells and Kir3.2 activity in a concentration-dependent manner. The current inhibition was strong when membrane potentials (Vm) was above equilibrium potential of K(+) (EK). When Vm was below EK, the blockage apparently depended on the difference between Vm and [K(+)]. Furthermore, the inhibition became stronger by lowering extracellular [K(+)]. These results indicated that the yeast strain serves as a screening system to isolate Kir3.2 blockers and proflavine is a prototype of a pore blocker of Kir3.2.

[BIOELECTRIC ACTIVITY OF INTERNEURONES OF SPINAL CORD IN THE EXPERIMENTAL MENOPAUSE IN FEMAL RATS]

We analyzed the amplitude-time characteristics of the components of the potential dorsal surface of the spinal cord (PDS SC) in experimental menopause. The excitation threshold increased at 102.14% and the latent period at 94.12% in animals with experimental menopause. In the context of supramaximal stimulation of dorsal root L5 identified N1 amplitude growth at 10.14%, N2 at 11.82%, N3 at 48.28%, and P-wave 31.58% and to increase component N3 26.54% in the experimental group. At condition of paired stimulation pulses on the time interval from 2 to 3 ms there was a significant increase, and from 6 to 30 ms--a depression of N1-component of the second PDS SC in the group of animals with experimental menopause. Thus, our results suggest the presence of changes in nerve structures of the posterior horn of the spinal cord at conditions of estrogen deficiency.

GIRK Channels Modulate Opioid-Induced Motor Activity in a Cell Type- and Subunit-Dependent Manner

G-protein-gated inwardly rectifying K(+) (GIRK/Kir3) channel activation underlies key physiological effects of opioids, including analgesia and dependence. GIRK channel activation has also been implicated in the opioid-induced inhibition of midbrain GABA neurons and consequent disinhibition of dopamine (DA) neurons in the ventral tegmental area (VTA). Drug-induced disinhibition of VTA DA neurons has been linked to reward-related behaviors and underlies opioid-induced motor activation. Here, we demonstrate that mouse VTA GABA neurons express a GIRK channel formed by GIRK1 and GIRK2 subunits. Nevertheless, neither constitutive genetic ablation of Girk1 or Girk2, nor the selective ablation of GIRK channels in GABA neurons, diminished morphine-induced motor activity in mice. Moreover, direct activation of GIRK channels in midbrain GABA neurons did not enhance motor activity. In contrast, genetic manipulations that selectively enhanced or suppressed GIRK channel function in midbrain DA neurons correlated with decreased and increased sensitivity, respectively, to the motor-stimulatory effect of systemic morphine. Collectively, these data support the contention that the unique GIRK channel subtype in VTA DA neurons, the GIRK2/GIRK3 heteromer, regulates the sensitivity of the mouse mesolimbic DA system to drugs with addictive potential.

G protein-gated inwardly rectifying potassium channel subunits 1 and 2 are down-regulated in rat dorsal root ganglion neurons and spinal cord after peripheral axotomy

Background

Increased nociceptive neuronal excitability underlies chronic pain conditions. Various ion channels, including sodium, calcium and potassium channels have pivotal roles in the control of neuronal excitability. The members of the family of G protein-gated inwardly rectifying potassium (GIRK) channels, GIRK1–4, have been implicated in modulating excitability. Here, we investigated the expression and distribution of GIRK1 and GIRK2 in normal and injured dorsal root ganglia (DRGs) and spinal cord of rats.

Results

We found that ~70% of the DRG neurons expressed GIRK1, while only <10% expressed GIRK2. The neurochemical profiles of GIRK1- and GIRK2-immunoreactive neurons were characterized using the neuronal markers calcitonin gene-related peptide, isolectin-B4 and neurofilament-200, and the calcium-binding proteins calbindin D28k, calretinin, parvalbumin and secretagogin. Both GIRK subunits were expressed in DRG neurons with nociceptive characteristics. However, while GIRK1 was widely expressed in several sensory neuronal subtypes, GIRK2 was detected mainly in a group of small C-fiber neurons. In the spinal dorsal horn, GIRK1- and -2-positive cell bodies and processes were mainly observed in lamina II, but also in superficial and deeper layers. Abundant GIRK1-, but not GIRK2-like immunoreactivity, was found in the ventral horn (laminae VI–X). Fourteen days after axotomy, GIRK1 and GIRK2 were down-regulated in DRG neurons at the mRNA and protein levels. Both after axotomy and rhizotomy there was a reduction of GIRK1- and -2-positive processes in the dorsal horn, suggesting a presynaptic localization of these potassium channels. Furthermore, nerve ligation caused accumulation of both subunits on both sides of the lesion, providing evidence for anterograde and retrograde fast axonal transport.

Conclusions

Our data support the hypothesis that reduced GIRK function is associated with increased neuronal excitability and causes sensory disturbances in post-injury conditions, including neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0044-z) contains supplementary material, which is available to authorized users.

Behavioral and Genetic Evidence for GIRK Channels in the CNS: Role in Physiology, Pathophysiology, and Drug Addiction

G protein-coupled inwardly rectifying potassium (GIRK) channels are widely expressed throughout the brain and mediate the inhibitory effects of many neurotransmitters. As a result, these channels are important for normal CNS function and have also been implicated in Down syndrome, Parkinson's disease, psychiatric disorders, epilepsy, and drug addiction. Knockout mouse models have provided extensive insight into the significance of GIRK channels under these conditions. This review examines the behavioral and genetic evidence from animal models and genetic association studies in humans linking GIRK channels with CNS disorders. We further explore the possibility that subunit-selective modulators and other advanced research tools will be instrumental in establishing the role of individual GIRK subunits in drug addiction and other relevant CNS diseases and in potentially advancing treatment options for these disorders.

Deletion of GIRK2 Subunit of GIRK Channels Alters the 5-HT1A Receptor-Mediated Signaling and Results in a Depression-Resistant Behavior

BACKGROUND

Targeting dorsal raphe 5-HT1A receptors, which are coupled to G-protein inwardly rectifying potassium (GIRK) channels, has revealed their contribution not only to behavioral and functional aspects of depression but also to the clinical response to its treatment. Although GIRK channels containing GIRK2 subunits play an important role controlling excitability of several brain areas, their impact on the dorsal raphe activity is still unknown. Thus, the goal of the present study was to investigate the involvement of GIRK2 subunit-containing GIRK channels in depression-related behaviors and physiology of serotonergic neurotransmission.

METHODS

Behavioral, functional, including in vivo extracellular recordings of dorsal raphe neurons, and neurogenesis studies were carried out in wild-type and GIRK2 mutant mice.

RESULTS

Deletion of the GIRK2 subunit promoted a depression-resistant phenotype and determined the behavioral response to the antidepressant citalopram without altering hippocampal neurogenesis. In dorsal raphe neurons of GIRK2 knockout mice, and also using GIRK channel blocker tertiapin-Q, the basal firing rate was higher than that obtained in wild-type animals, although no differences were observed in other firing parameters. 5-HT1A receptors were desensitized in GIRK2 knockout mice, as demonstrated by a lower sensitivity of dorsal raphe neurons to the inhibitory effect of the 5-HT1A receptor agonist, 8-OH-DPAT, and the antidepressant citalopram.

CONCLUSIONS

Our results indicate that GIRK channels formed by GIRK2 subunits determine depression-related behaviors as well as basal and 5-HT1A receptor-mediated dorsal raphe neuronal activity, becoming alternative therapeutic targets for psychiatric diseases underlying dysfunctional serotonin transmission.