delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

δ-Opioid receptors in primary sensory neurons tonically restrain nociceptive input in chronic pain but do not enhance morphine analgesic tolerance

δ-Opioid receptors (DORs, encoded by the Oprd1 gene) are expressed throughout the peripheral and central nervous system, and DOR stimulation reduces nociception. Previous studies suggest that DORs promote the development of analgesic tolerance of μ-opioid receptor (MOR) agonists. It is uncertain whether DORs expressed in primary sensory neurons are involved in regulating chronic pain and MOR agonist-induced tolerance. In this study, we generated Oprd1 conditional knockout (Oprd1-cKO) mice by crossing Advillin-Cre mice with Oprd1-floxed mice. DOR expression in the dorsal root ganglion was diminished in Oprd1-cKO mice. Systemic or intrathecal injection of the DOR agonist SNC-80 produced analgesia in wild-type (WT), but not Oprd1-cKO, mice. In contrast, intracerebroventricular injection of SNC-80 produced a similar analgesic effect in WT and Oprd1-cKO mice. However, morphine-induced analgesia, hyperalgesia, or analgesic tolerance did not differ between WT and Oprd1-cKO mice. Compared with WT mice, Oprd1-cKO mice showed increased mechanical and heat hypersensitivity after nerve injury or tissue inflammation. Furthermore, blocking DORs with naltrindole increased nociceptive sensitivity induced by nerve injury or tissue inflammation in WT, but not Oprd1-cKO, mice. In addition, naltrindole potentiated glutamatergic input from primary afferents to spinal dorsal horn neurons increased by nerve injury or CFA in WT mice; this effect was absent in Oprd1-cKO mice. Our findings indicate that DORs in primary sensory neurons are critically involved in the analgesic effect of DOR agonists but not morphine-induced analgesic tolerance. Presynaptic DORs at primary afferent central terminals constitutively inhibit inflammatory and neuropathic pain by restraining glutamatergic input to spinal dorsal horn neurons.

Vitamin D and Its Potential Interplay With Pain Signaling Pathways

About 50 million of the U.S. adult population suffer from chronic pain. It is a complex disease in its own right for which currently available analgesics have been deemed woefully inadequate since ~20% of the sufferers derive no benefit. Vitamin D, known for its role in calcium homeostasis and bone metabolism, is thought to be of clinical benefit in treating chronic pain without the side-effects of currently available analgesics. A strong correlation between hypovitaminosis D and incidence of bone pain is known. However, the potential underlying mechanisms by which vitamin D might exert its analgesic effects are poorly understood. In this review, we discuss pathways involved in pain sensing and processing primarily at the level of dorsal root ganglion (DRG) neurons and the potential interplay between vitamin D, its receptor (VDR) and known specific pain signaling pathways including nerve growth factor (NGF), glial-derived neurotrophic factor (GDNF), epidermal growth factor receptor (EGFR), and opioid receptors. We also discuss how vitamin D/VDR might influence immune cells and pain sensitization as well as review the increasingly important topic of vitamin D toxicity. Further in vitro and in vivo experimental studies will be required to study these potential interactions specifically in pain models. Such studies could highlight the potential usefulness of vitamin D either alone or in combination with existing analgesics to better treat chronic pain.

Opioid Receptors in Immune and Glial Cells-Implications for Pain Control

Opioid receptors comprise μ (MOP), δ (DOP), κ (KOP), and nociceptin/orphanin FQ (NOP) receptors. Opioids are agonists of MOP, DOP, and KOP receptors, whereas nociceptin/orphanin FQ (N/OFQ) is an agonist of NOP receptors. Activation of all four opioid receptors in neurons can induce analgesia in animal models, but the most clinically relevant are MOP receptor agonists (e.g., morphine, fentanyl). Opioids can also affect the function of immune cells, and their actions in relation to immunosuppression and infections have been widely discussed. Here, we analyze the expression and the role of opioid receptors in peripheral immune cells and glia in the modulation of pain. All four opioid receptors have been identified at the mRNA and protein levels in immune cells (lymphocytes, granulocytes, monocytes, macrophages) in humans, rhesus monkeys, rats or mice. Activation of leukocyte MOP, DOP, and KOP receptors was recently reported to attenuate pain after nerve injury in mice. This involved intracellular Ca²⁺-regulated release of opioid peptides from immune cells, which subsequently activated MOP, DOP, and KOP receptors on peripheral neurons. There is no evidence of pain modulation by leukocyte NOP receptors. More good quality studies are needed to verify the presence of DOP, KOP, and NOP receptors in native glia. Although still questioned, MOP receptors might be expressed in brain or spinal cord microglia and astrocytes in humans, mice, and rats. Morphine acting at spinal cord microglia is often reported to induce hyperalgesia in rodents. However, most studies used animals without pathological pain and/or unconventional paradigms (e.g., high or ultra-low doses, pain assessment after abrupt discontinuation of chronic morphine treatment). Therefore, the opioid-induced hyperalgesia can be viewed in the context of dependence/withdrawal rather than pain management, in line with clinical reports. There is convincing evidence of analgesic effects mediated by immune cell-derived opioid peptides in animal models and in humans. Together, MOP, DOP, and KOP receptors, and opioid peptides in immune cells can ameliorate pathological pain. The relevance of NOP receptors and N/OFQ in leukocytes, and of all opioid receptors, opioid peptides and N/OFQ in native glia for pain control is yet to be clarified.

Nociceptor Signalling through ion Channel Regulation via GPCRs

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

Suramin-Induced Neurotoxicity: Preclinical Models and Neuroprotective Strategies

Suramin is a trypan blue analogon originally developed to treat protozoan infections, which was found to have diverse antitumor effects. One of the most severe side effects in clinical trials was the development of a peripheral sensory-motor polyneuropathy. In this study, we aimed to investigate suramin-induced neuropathy with a focus on calcium (Ca²⁺) homeostasis as a potential pathomechanism. Adult C57Bl/6 mice treated with a single injection of 250 mg/kg bodyweight suramin developed locomotor and sensory deficits, which were confirmed by electrophysiological measurements showing a predominantly sensory axonal-demyelinating polyneuropathy. In a next step, we used cultured dorsal root ganglia neurons (DRGN) as an in vitro cell model to further investigate underlying pathomechanisms. Cell viability of DRGN was significantly decreased after 24-hour suramin treatment with a calculated IC₅₀ of 283 µM. We detected a suramin-induced Ca²⁺ influx into DRGN from the extracellular space, which could be reduced with the voltage-gated calcium channel (VGCC) inhibitor nimodipine. Co-incubation of suramin and nimodipine partially improved cell viability of DRGN after suramin exposure. In summary, we describe suramin-induced neurotoxic effects on DRGN as well as potentially neuroprotective agents targeting intracellular Ca²⁺ dyshomeostasis.

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.

Targeting G protein-coupled receptor for pain management

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage. Great progress has been made in understanding the important roles of various G protein-coupled receptors in the regulation of pain transmission. However, many important questions remain uncertain about the precise signal transduction mechanisms. This review focuses opioid receptor and CXC receptor 4 on the effects and mechanisms of pain. Taken together, chemokines and their receptors are potential targets for the development of novel pain management and therapy.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

GABAB receptors inhibit low-voltage activated and high-voltage activated Ca(2+) channels in sensory neurons via distinct mechanisms

Growing evidence suggests that mammalian peripheral somatosensory neurons express functional receptors for gamma-aminobutyric acid, GABAA and GABAB. Moreover, local release of GABA by pain-sensing (nociceptive) nerve fibres has also been suggested. Yet, the functional significance of GABA receptor triggering in nociceptive neurons is not fully understood. Here we used patch-clamp recordings from small-diameter cultured DRG neurons to investigate effects of GABAB receptor agonist baclofen on voltage-gated Ca(2+) currents. We found that baclofen inhibited both low-voltage activated (LVA, T-type) and high-voltage activated (HVA) Ca(2+) currents in a proportion of DRG neurons by 22% and 32% respectively; both effects were sensitive to Gi/o inhibitor pertussis toxin. Inhibitory effect of baclofen on both current types was about twice less efficacious as compared to that of the μ-opioid receptor agonist DAMGO. Surprisingly, only HVA but not LVA current modulation by baclofen was partially prevented by G protein inhibitor GDP-β-S. In contrast, only LVA but not HVA current modulation was reversed by the application of a reducing agent dithiothreitol (DTT). Inhibition of T-type Ca(2+) current by baclofen and the recovery of such inhibition by DTT were successfully reconstituted in the expression system. Our data suggest that inhibition of LVA current in DRG neurons by baclofen is partially mediated by an unconventional signaling pathway that involves a redox mechanism. These findings reinforce the idea of targeting peripheral GABA receptors for pain relief.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Effects of intrathecal SNC80, a delta receptor ligand, on nociceptive threshold and dorsal horn substance p release

Delta-opioid receptors (DOR) are present in the superficial dorsal horn and are believed to regulate the release of small afferent transmitters as evidenced by the effects of spinally delivered delta-opioid preferring peptides. Here we examined the effects of intrathecal SNC80 [(+)-4-[α(R)-α-[(2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-(methoxybenzyl)-N,N-diethylbenzamide], a selective nonpeptidic DOR agonist, in three preclinical pain models, acute thermal escape, intraplantar carrageenan-tactile allodynia, and intraplantar formalin flinches, and on the evoked release of substance P (SP) from small primary afferents. Rats with chronic intrathecal catheters received intrathecal vehicle or SNC80 (100 or 200 μg). Intrathecal SNC80 did not change acute thermal latencies or carrageenan-induced thermal hyperalgesia. However, SNC80 attenuated carrageenan-induced tactile allodynia and significantly reduced both phase 1 and phase 2 formalin-induced paw flinches, as assessed by an automatic flinch counting device. These effects were abolished by naltrindole (3 mg/kg i.p.), a selective DOR antagonist, but not CTOP (10 µg i.t.), a selective MOR antagonist. Furthermore, intrathecal SNC80 (200 μg) blocked formalin-induced substance P release otherwise evoked in the ispilateral superficial dorsal horn as measured by NK1 receptor internalization. In conclusion, intrathecal SNC80 alleviated pain hypersensitivity after peripheral inflammation in a fashion paralleling its ability to block peptide transmitter release from small peptidergic afferents, which by its pharmacology appears to represent an effect mediated by a spinal DOR.

Functional role of peripheral opioid receptors in the regulation of cardiac spinal afferent nerve activity during myocardial ischemia

Thinly myelinated Aδ-fiber and unmyelinated C-fiber cardiac sympathetic (spinal) sensory nerve fibers are activated during myocardial ischemia to transmit the sensation of angina pectoris. Although recent observations showed that myocardial ischemia increases the concentrations of opioid peptides and that the stimulation of peripheral opioid receptors inhibits chemically induced visceral and somatic nociception, the role of opioids in cardiac spinal afferent signaling during myocardial ischemia has not been studied. The present study tested the hypothesis that peripheral opioid receptors modulate cardiac spinal afferent nerve activity during myocardial ischemia by suppressing the responses of cardiac afferent nerve to ischemic mediators like bradykinin and extracellular ATP. The nerve activity of single unit cardiac afferents was recorded from the left sympathetic chain (T₂-T₅) in anesthetized cats. Forty-three ischemically sensitive afferent nerves (conduction velocity: 0.32-3.90 m/s) with receptive fields in the left and right ventricles were identified. The responses of these afferent nerves to repeat ischemia or ischemic mediators were further studied in the following protocols. First, epicardial administration of naloxone (8 μmol), a nonselective opioid receptor antagonist, enhanced the responses of eight cardiac afferent nerves to recurrent myocardial ischemia by 62%, whereas epicardial application of vehicle (PBS) did not alter the responses of seven other cardiac afferent nerves to ischemia. Second, naloxone applied to the epicardial surface facilitated the responses of seven cardiac afferent nerves to epicardial ATP by 76%. Third, administration of naloxone enhanced the responses of seven other afferent nerves to bradykinin by 85%. In contrast, in the absence of naloxone, cardiac afferent nerves consistently responded to repeated application of ATP (n = 7) or bradykinin (n = 7). These data suggest that peripheral opioid peptides suppress the responses of cardiac sympathetic afferent nerves to myocardial ischemia and ischemic mediators like ATP and bradykinin.

Protective effects of the delta opioid peptide [D-Ala2, D-Leu5]enkephalin in an ex vivo model of ischemia/reperfusion in brain slices

INTRODUCTION

The delta opioid peptide [D-Ala2, D-Leu5]enkephalin (DADLE) plays a key role in neuronal protection against both hypoxic and ischemic conditions. However, the cellular mechanisms of action of DADLE under these conditions remain unclear.

METHODS

Ischemia was simulated with perfusing the brain slices with glucose-free artificial cerebrospinal fluid. Apoptosis was examined using an in situ cell death detection kit and expressed as the percentage of positively labeled neurons relative to total number of neurons. PCR was performed by adding cDNA, 5 pm dNTP, 1 μL Taqase, and primers. PCR products were separated with electrophoresis, stained with ethidium bromide, and visualized under ultraviolet light.

AIMS

To investigate the potential effects of DADLE in an ex vivo model of cerebral ischemia/reperfusion.

RESULTS

DADLE attenuated lactic dehydrogenase release and neuronal apoptosis in a concentration-dependent manner. The protective effects of DADLE were attenuated by representative selective delta2, but not delta1 opioid antagonists. Treatment with PD98059, a selective inhibitor of ERK kinase (MEK), also blocked the protective effect of DADLE as well as ERK phosphorylation induced by DADLE.

CONCLUSIONS

Endogenous opioid peptides could promote cell survival via delta2 opioid receptors, possibly through the downstream MEK-ERK pathway.

Dissociation of μ- and δ-opioid inhibition of glutamatergic synaptic transmission in superficial dorsal horn

BACKGROUND

There is anatomical and behavioural evidence that μ- and δ-opioid receptors modulate distinct nociceptive modalities within the superficial dorsal horn. The aim of the present study was to examine whether μ- and δ-opioid receptor activation differentially modulates TRP sensitive inputs to neurons within the superficial dorsal horn. To do this, whole cell patch clamp recordings were made from lamina I - II neurons in rat spinal cord slices in vitro to examine the effect of opioids on TRP agonist-enhanced glutamatergic spontaneous miniature excitatory postsynaptic currents (EPSCs).

RESULTS

Under basal conditions the μ-opioid agonist DAMGO (3 μM) reduced the rate of miniature EPSCs in 68% of neurons, while the δ- and κ-opioid agonists deltorphin-II (300 nM) and U69593 (300 nM) did so in 13 - 17% of neurons tested. The TRP agonists menthol (400 μM) and icilin (100 μM) both produced a Ca2+-dependent increase in miniature EPSC rate which was unaffected by the voltage dependent calcium channel (VDCC) blocker Cd2+. The proportion of neurons in which deltorphin-II reduced the miniature EPSC rate was enhanced in the presence of icilin (83%), but not menthol (0%). By contrast, the proportion of DAMGO and U69593 responders was unaltered in the presence of menthol (57%, 0%), or icilin (57%, 17%).

CONCLUSIONS

These findings demonstrate that δ-opioid receptor activation selectively inhibits inputs activated by icilin, whereas μ-opioid receptor activation has a more widespread effect on synaptic inputs to neurons in the superficial dorsal horn. These findings suggest that δ-opioids may provide a novel analgesic approach for specific, TRPA1-like mediated pain modalities.

Coexpression of delta- and mu-opioid receptors in nociceptive sensory neurons

Morphine-induced analgesia and antinociceptive tolerance are known to be modulated by interaction between delta-opioid receptors (DORs) and mu-opioid receptors (MORs) in the pain pathway. However, evidence for expression of DORs in nociceptive small-diameter neurons in dorsal root ganglia (DRG) and for coexistence of DORs with MORs and neuropeptides has recently been challenged. We now report, using in situ hybridization, single-cell PCR, and immunostaining, that DORs are widely expressed not only in large DRG neurons but in small ones and coexist with MORs in peptidergic small DRG neurons, with protachykinin-dependent localization in large dense-core vesicles. Importantly, both DOR and MOR agonists reduce depolarization-induced Ca(2+) currents in single small DRG neurons and inhibit afferent C-fiber synaptic transmission in the dorsal spinal cord. Thus, coexistence of DORs and MORs in small DRG neurons is a basis for direct interaction of opioid receptors in modulation of nociceptive afferent transmission and opioid analgesia.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Neuroprotective role of delta-opioid receptors against mitochondrial respiratory chain injury

It is recognized in recent years that activation of delta-opioid receptor (DOR) elicits neuroprotection against hypoxia and ischemia. However, the underlying mechanisms are not well understood yet. Mitochondrial dysfunction plays a key role in hypoxic neuronal injury, but the effect of DOR activation on neurons with a mitochondrial respiratory chain deficiency is poorly elucidated. In this study we tested the effects of DOR activation and inhibition on cultured cortical neurons after inhibiting mitochondrial respiratory chain with sodium azide (NaN(3)) in days 8 cultures. Neuronal injury was assessed by lactate dehydrogenase release. Changes in DOR proteins were investigated using an antibody against the N-terminus of the DOR, which recognizes the 60, 48, and 32 kDa proteins. Our main findings are that 1) delta- but not mu-opioid receptor activation reduces NaN(3)-induced neuronal damage, and this neuroprotective effect is abolished by DOR antagonist (naltrindole, NTI); 2) prolonged DOR inhibition with NTI further increases NaN(3)-induced neuronal damage; 3) NaN(3) treatment down-regulates DOR protein levels in neurons, and the 60 and 32 kDa proteins are particularly sensitive; 4) DADLE, besides activating DOR directly, also reverses the decrease of neuronal DOR protein levels induced by NaN(3), which may contribute greatly to its neuroprotective effect; 5) NTI reverses NaN(3)-induced down-regulation of DOR proteins as well, the effect of NTI amplifying NaN(3)-induced neuronal damage therefore is probably due to its inhibition on DOR activity only. In conclusion, these data suggest that DOR activation plays an important role in neuroprotection against mitochondrial respiratory chain injury.

Removing TRPV1-expressing primary afferent neurons potentiates the spinal analgesic effect of delta-opioid agonists on mechano-nociception

Most delta-opioid receptors are located on the presynaptic terminals of primary afferent neurons in the spinal cord. However, their presence in different phenotypes of primary afferent neurons and their contribution to the analgesic effect of delta-opioid agonists are not fully known. Resiniferatoxin (RTX) is an ultra-potent transient receptor potential vanilloid type 1 channel (TRPV1) agonist and can selectively remove TRPV1-expressing primary afferent neurons. In this study, we determined the role of delta-opioid receptors expressed on TRPV1 sensory neurons in the antinociceptive effect of the delta-opioid receptor agonists [D-Pen(2),D-Pen(5)]-enkephalin and [D-Ala(2),Glu(4)]-deltorphin. Nociception was measured by testing the mechanical withdrawal threshold in the hindpaw of rats. Changes in the delta-opioid receptors were assessed using immunocytochemistry and the [(3)H]-naltrindole radioligand binding. In RTX-treated rats, the delta-opioid receptor on TRPV1-immunoreactive dorsal root ganglion neurons and afferent terminals in the spinal cord was diminished. RTX treatment also significantly reduced the maximal specific binding sites (31%) of the delta-opioid receptors in the dorsal spinal cord. Interestingly, intrathecal injection of [D-Pen(2),d-Pen(5)]-enkephalin or [D-Ala(2),Glu(4)]-deltorphin produced a large and prolonged increase in the nociceptive threshold in RTX-treated rats. These findings indicate that loss of TRPV1-expressing afferent neurons leads to a substantial reduction in presynaptic delta-opioid receptors in the spinal dorsal horn. However, the effect of delta-opioid agonists on mechano-nociception is paradoxically potentiated in the absence of TRPV1-expressing sensory neurons. This information is important to our understanding of the cellular sites and mechanisms underlying the spinal analgesic effect of delta-opioid agonists.

Bacterial lipopolysaccharide regulates nociceptin expression in sensory neurons

Nociceptin/orphanin FQ (N/OFQ) is an opioid-related peptide that is markedly up-regulated in sensory neurons in vivo following peripheral inflammation and plays a key role in pain physiology. To identify substances that up-regulate N/OFQ expression in sensory neurons, we carried out an in vitro screen using purified adult mouse dorsal root ganglion (DRG) neurons and identified the potent proinflammatory agent bacterial lipopolysaccharide (LPS) as a very effective inducer of N/OFQ. The robust response of these neurons to LPS enabled us to identify the components of a putative neuronal LPS receptor complex. In contrast to the immune system, where the functional LPS receptor complex is composed of CD-14 together with either MD-2 and TLR4 on myeloid cells or the homologous receptors MD-1 and RP105 on mature B cells, DRG neurons express the unusual combination of CD-14, TLR4, and MD-1. Blocking antibodies against TLR4 and MD-1 prevented induction of N/OFQ by LPS, and, in immunoprecipitation experiments, MD-1 coprecipitated with TLR4. Our findings suggest that LPS regulates N/OFN expression in sensory neurons via a novel combination of LPS receptor components and demonstrate for the first time a direct action of a key initiator of innate immune responses on neurons.

Distinct inhibition of voltage-activated Ca2+ channels by delta-opioid agonists in dorsal root ganglion neurons devoid of functional T-type Ca2+ currents

Both mu- and delta-opioid agonists selectively inhibit nociception but have little effect on other sensory modalities. Voltage-activated Ca(2+) channels in the primary sensory neurons are important for the regulation of nociceptive transmission. In this study, we determined the effect of delta-opioid agonists on voltage-activated Ca(2+) channel currents (I(Ca)) in small-diameter rat dorsal root ganglion (DRG) neurons that do and do not bind isolectin B(4) (IB(4)). The delta-opioid agonists [d-Pen(2),d-Pen(5)]-enkephalin (DPDPE) and deltorphin II produced a greater inhibition of high voltage-activated I(Ca) in IB(4)-negative than IB(4)-positive neurons. Furthermore, DPDPE produced a greater inhibition of N-, P/Q-, and L-type I(Ca) in IB(4)-negative than IB(4)-positive neurons. However, DPDPE had no significant effect on the R-type I(Ca) in either type of cells. We were surprised to find that DPDPE failed to inhibit either the T-type or high voltage-activated I(Ca) in all the DRG neurons with T-type I(Ca). Double immunofluorescence labeling showed that the majority of the delta-opioid receptor-immunoreactive DRG neurons had IB(4) labeling, while all DRG neurons immunoreactive to delta-opioid receptors exhibited Cav(3.2) immunoreactivity. Additionally, DPDPE significantly inhibited high voltage-activated I(Ca) in Tyrode's or N-methyl-d-glucamine solution but not in tetraethylammonium solution. This study provides new information that delta-opioid agonists have a distinct effect on voltage-activated Ca(2+) channels in different phenotypes of primary sensory neurons. High voltage-activated Ca(2+) channels are more sensitive to inhibition by delta-opioid agonists in IB(4)-negative than IB(4)-positive neurons, and this opioid effect is restricted to DRG neurons devoid of functional T-type Ca(2+) currents.

Effects of delta-opioid agonist SNC80 on white matter injury following spinal cord ischemia in normothermic and mildly hypothermic rats

PURPOSE

Although the delta-opioid agonist SNC80 has been shown to attenuate hind-limb motor function and gray matter injury in normothermic rats subjected to spinal cord ischemia (SCI), its effects on white matter injury remain undetermined. In the present study, we investigated whether SNC80 could attenuate white matter injury in normothermic and mildly hypothermic rats.

METHODS

Forty rats were randomly allocated to one of following five groups: vehicle or SNC80 with 10 min of SCI at 38 degrees C (V-38-10m or SNC-38-10m, respectively), vehicle or SNC80 with 22 min of SCI at 35 degrees C (V-35-22m or SNC-35-22m, respectively), or sham. SNC80 or vehicle was intrathecally administered 15 min before SCI. Forty-eight hours after reperfusion, the white matter injury was evaluated by the extent of vacuolation.

RESULTS

The percent area of vacuolation in the ventral white matter was significantly lower in the SNC-38-10m and SNC-35-22m groups compared with that in the V-38-10m and V-35-22m groups, respectively (P < 0.05).

CONCLUSION

The results indicate that intrathecal treatment with the delta-opioid agonist SNC80 can attenuate the ventral white matter injury following SCI in rats under normothermic and mildly hypothermic conditions.

Activation of DOR attenuates anoxic K+ derangement via inhibition of Na+ entry in mouse cortex

We have recently found that in the mouse cortex, activation of delta-opioid receptor (DOR) attenuates the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation. This novel observation suggests that DOR may protect neurons from hypoxic/ischemic insults via the regulation of K(+) homeostasis because the disruption of K(+) homeostasis plays a critical role in neuronal injury under hypoxic/ischemic stress. The present study was performed to explore the ionic mechanism underlying the DOR-induced neuroprotection. Because anoxia causes Na(+) influx and thus stimulates K(+) leakage, we investigated whether DOR protects the cortex from anoxic K(+) derangement by targeting the Na(+)-based K(+) leakage. By using K(+)-sensitive microelectrodes in mouse cortical slices, we showed that 1) lowering Na(+) concentration and substituting with impermeable N-methyl-D-glucamine caused a concentration-dependent attenuation of anoxic K(+) derangement; 2) lowering Na(+) concentration by substituting with permeable Li(+) tended to potentiate the anoxic K(+) derangement; and 3) the DOR-induced protection against the anoxic K(+) responses was largely abolished by low-Na(+) perfusion irrespective of the substituted cation. We conclude that external Na(+) concentration greatly influences anoxic K(+) derangement and that DOR activation likely attenuates anoxic K(+) derangement induced by the Na(+)-activated mechanisms in the cortex.

Effects of delta-opioid receptor stimulation and inhibition on hippocampal survival in a rat model of forebrain ischaemia

BACKGROUND

It has been reported that delta-opioid (DOP) receptor agonists may be neuroprotective in the central nervous system. However, the DOP agonist [d-Ala(2), d-Leu(5)]enkephalin (DADLE) does not produce neuroprotection in severe forebrain ischaemia. The aim of this study was to examine the effects of DADLE on hippocampal neurone survival against less severe forebrain ischaemia.

METHODS

Intraperitoneal injection of DADLE (0 or 16 mg kg(-1)) in male Sprague-Dawley rats was performed 30 min before ischaemia. Severe (10 min), moderate (8 min), or mild (6 min) forebrain ischaemia was produced by bilateral carotid occlusion combined with hypotension (35 mm Hg) under isoflurane (1.5%) anaesthesia. Naltrindole (10 mg kg(-1)) (DOP antagonist) was administered 30 min before DADLE in order to confirm DOP receptor activation in the neuroprotective efficacy of DADLE. Naltrindole alone was also administered 30 min before ischaemia to examine endogenous DOP agonism as a self-protecting mechanism against ischaemia. All animals were evaluated neurologically and histologically after a 1 week recovery period.

RESULTS

DADLE improved neurone survival in hippocampal CA3 and dentate gyrus (DG) sectors. CA1 neurones were not protected against moderate and mild ischaemia. Naltrindole abolished DADLE neuroprotection in the CA3 and DG after both moderate and mild ischaemia. Interestingly, regardless of co-administration of DADLE, naltrindole significantly worsened neuronal injury in the CA1 region after mild ischaemia.

CONCLUSIONS

These results suggest that DADLE provides limited neuroprotection to relatively ischaemia-resistant regions but not to selectively vulnerable regions. This was probably mediated by DOP stimulation. Pre-ischaemic treatment with a DOP antagonist, regardless of co-administration of DADLE, worsened neuronal damage at the selectively vulnerable regions only after mild forebrain ischaemia. These data suggest that DOP activation with endogenous DOP ligand may be involved in self-protecting ischaemia-sensitive regions of the brain.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

delta-, but not mu-, opioid receptor stabilizes K(+) homeostasis by reducing Ca(2+) influx in the cortex during acute hypoxia

Past work has shown that delta-opioid receptor (DOR) activation by [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) attenuated the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K(+) homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K(+) homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from micro-opioid receptors (MOR) in stabilizing K(+) homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 microM), H-Dmt-Tic-NH-CH (CH(2)--COOH)-Bid (1-10 microM), a more specific and potent DOR agonist significantly attenuated anoxic K(+) derangement in cortical slice; (2) [D-Ala(2), N-Me-Phe(4), glycinol(5)]-enkephalin (DAGO; 10 microM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K(+) homeostasis; (3) absence of Ca(2+) greatly attenuated anoxic K(+) derangement; (4) inhibition of Ca(2+)-activated K(+) (BK) channels with paxilline (10 microM) reduced anoxic K(+) derangement; (5) DADLE (10 microM) could not further reduce anoxic K(+) derangement in the Ca(2+)-free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 microM), a K(ATP) channel blocker, decreased anoxia-induced K(+) derangement, but DADLE (10 microM) could further attenuate anoxic K(+) derangement in the glybenclamide-perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K(+) derangement in the cortex, at least partially via an inhibition of hypoxia-induced increase in Ca(2+) entry-BK channel activity.

Cortical delta-opioid receptors potentiate K+ homeostasis during anoxia and oxygen-glucose deprivation

Central neurons are extremely vulnerable to hypoxic/ischemic insult, which is a major cause of neurologic morbidity and mortality as a consequence of neuronal dysfunction and death. Our recent work has shown that delta-opioid receptor (DOR) is neuroprotective against hypoxic and excitotoxic stress, although the underlying mechanisms remain unclear. Because hypoxia/ischemia disrupts ionic homeostasis with an increase in extracellular K(+), which plays a role in neuronal death, we asked whether DOR activation preserves K(+) homeostasis during hypoxic/ischemic stress. To test this hypothesis, extracellular recordings with K(+)-sensitive microelectrodes were performed in mouse cortical slices under anoxia or oxygen-glucose deprivation (OGD). The main findings in this study are that (1) DOR activation with [D-Ala(2), D-Leu(5)]-enkephalinamide attenuated the anoxia- and OGD-induced increase in extracellular K(+) and decrease in DC potential in cortical slices; (2) DOR inhibition with naltrindole, a DOR antagonist, completely abolished the DOR-mediated prevention of increase in extracellular K(+) and decrease in DC potential; (3) inhibition of protein kinase A (PKA) with N-(2-[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide dihydrochloride had no effect on the DOR protection; and (4) inhibition of protein kinase C (PKC) with chelerythrine chloride reduced the DOR protection, whereas the PKC activator (phorbol 12-myristate 13-acetate) mimicked the effect of DOR activation on K(+) homeostasis. These data suggest that activation of DOR protects the cortex against anoxia- or ODG-induced derangement of potassium homeostasis, and this protection occurs via a PKC-dependent and PKA-independent pathway. We conclude that an important aspect of DOR-mediated neuroprotection is its early action against derangement of K(+) homeostasis during anoxia or ischemia.

Delta opioid receptors stimulation with [D-Ala2, D-Leu5] enkephalin does not provide neuroprotection in the hippocampus in rats subjected to forebrain ischemia

It has been reported that delta opioid agonists can have neuroprotective efficacy in the central nervous system. This study was conducted to test the hypothesis that a delta opioid receptor (DOR) agonist, [D-Ala2, D-Leu5] enkephalin (DADLE), can improve neuron survival against experimental forebrain ischemia in rats. Using male rats (n=125), intraperitoneal injection of DADLE (0, 0.25, 1, 4, 16 mg kg-1) was performed 30 min before ischemia. Ten minutes interval forebrain ischemia was provided by the bilateral carotid occlusion combined with hypotension (35 mm Hg) under isoflurane (1.5%) anesthesia. All animals were neurologically and histologically evaluated after a recovery period of 1 week. As histological evaluation, percentages of ischemic neurons in the CA1, CA3, dentate gyrus (DG) were measured. During the recovery period, 27 rats died because of apparent upper airway obstruction, seizure, or unidentified causes. There were no differences in the motor activity score among the groups. Ten minutes forebrain ischemia induced approximately 75, 20, and 10% neuronal death in the CA1, CA3, and DG, respectively. Any doses of DADLE did not attenuate neuronal injury in the hippocampus after ischemia. Pre-ischemic treatment of DORs agonism with DADLE did not provide any neuroprotection to the hippocampus in rats subjected to forebrain ischemia.

nPKCepsilon and NMDA receptors participate in neuroprotection induced by morphine pretreatment

Morphine pretreatment induces ischemic tolerance in neurons, but it remains uncertain whether novel protein kinase C epsilon isoform (nPKCepsilon) and N-methyl-D-aspartate (NMDA) receptors are involved in this neuroprotection. The present study examined this issue. Hippocampal slices from adult BALB/C mice were incubated with morphine at 0.1-10.0 muM in the presence or absence of various antagonists for 30 minutes and then kept in morphine- and antagonist-free buffer for 30 minutes before being subjected to oxygen-glucose deprivation for 20 minutes. After recovery in oxygenated artificial fluid for 5 hours, assessment of slice injury was done by determination of the intensity of slice stain after they were incubated with 2% 2,3,5-triphenyltetrazolium chloride for 30 minutes and extracted by organic solvent for 24 hours. At designated periods, slices were preserved for immunoblot analysis to observe effects of morphine pretreatment on membrane translocation and total protein expression of nPKCepsilon and phosphorylation of NR1 subunits of NMDA receptors. The neuroprotection induced by morphine pretreatment was partially blocked by chelerythrine (a nonselective PKC blocker), epsilonv(1-2) (a selective nPKCepsilon antagonist), MK-801 (a noncompetitive NMDA receptor blocker), chelerythrine combined with MK-801, and epsilonv(1-2) with MK-801. Morphine pretreatment significantly inhibited nPKCepsilon membrane translocation and phosphorylation of NR1 subunits of NMDA receptors during reperfusion injury. However, epsilonv(1-2) blocked these effects induced by morphine pretreatment. These findings suggested that nPKCepsilon and NMDA receptors might participate in neuroprotection induced by morphine pretreatment, and NMDA receptors might be downstream targets of nPKCepsilon.

Role of delivery and trafficking of delta-opioid peptide receptors in opioid analgesia and tolerance

Changes in the number of receptors on the cell surface lead to modulations of physiological functions and pharmacological responses of neurons. Recent studies show that delta-opioid peptide (DOP) and mu-opioid peptide (MOP) receptors have distinct subcellular localizations in neurons. In nociceptive small neurons in the dorsal root ganglia, DOP receptors are sorted into neuropeptide-containing secretory vesicles, enabling the stimulus-induced cell surface expression of these receptors. MOP receptors are constitutively expressed on the cell surface. The physical interaction between DOP receptors and MOP receptors seems to be an important mechanism for the modulation of receptor functions. Experiments in animals show that MOP-receptor-mediated spinal analgesia is enhanced and morphine tolerance does not develop when DOP receptor functions are pharmacologically or genetically attenuated. Thus, the delivery and trafficking of DOP receptors are crucial processes that modulate opioid analgesia and tolerance.

Delta (delta) opioid receptors in small and medium-sized trigeminal neurons supporting the dental pulp of rats

The control of pain perception is a challenge in clinical dentistry, most prominent during tooth pulp inflammation. The tooth pulp is a well-defined target, and is densely supplied by a sensory trigeminal innervation. Opioids are signaling molecules that are suggested to participate in pain perception. Here we analysed the presence of delta opioid receptor (DOR) in trigeminal neurons innervating the tooth pulp of rat molars. Immunohistochemical and ultrastructural analysis revealed that DOR was identified in peripheral nerves in the molar dental pulp, both in the root and the coronal pulpal parts, with branching in the highly innervated subodontoblast layer. DOR was localised in about one third of all the trigeminal dental neurons, identified by means of retrograde neuronal transport of fluorogold (FG) from the dental pulp. Of the DOR-labeled neurons, nearly all were small and medium-sized (147.5-1,810.2 microm(2), mean 749.1 +/- 327.3 microm(2)). Confocal microscopy confirmed that DOR-immunoreactivity was distributed as granules in the neuronal cytoplasm. Approximately 70% of the DOR-immunoreactive neurons were also immunopositive for vanilloid receptor 1 (TRPV1). Ultrastructural analysis demonstrated DOR-immunoreactivity in the unmyelinated and in some of the myelinated nerve fibers in the dental pulp. These results indicate that DOR may influence the function in a subset of small and medium-sized trigeminal sensory neurons supporting the tooth, which are mainly known for their ability to mediate nociceptive stimuli. Agonists, acting on DOR, may thus have an influence on a subpopulation of nociceptive neurons supporting the rat tooth.

A neuropeptide courier for delta-opioid receptors?

The delta-opioid receptor and the precursor protein of a neuropeptide, substance P, are colocalized in the large dense-core vesicles of pain-sensing neurons. In this issue of Cell, report that trafficking of the delta-opioid receptor to these vesicles depends on its physical interaction with the substance P domain of its precursor polyprotein (protachykinin). Moreover, in mice lacking this precursor, the contribution of the delta-opioid receptor to pain processing is dramatically altered. These observations suggest a new role for peptide precursors as sorting signals in vesicular transport.

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Induction of δ Opioid Receptor Function by Up-Regulation of Membrane Receptors in Mouse Primary Afferent Neurons

It is not clear whether primary afferent neurons express functional cell-surface opioid receptors. We examined delta receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after receptor up-regulation. [D-Ala2,Phe4,Gly5-ol]-enkephalin (DAMGO), [D-Ala2,D-Leu5]-enkephalin (DADLE), trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzene-acetamide methanesulfonate (U-50,488H; 1 microM), and baclofen (50 microM) inhibited Ca2+ currents, whereas the -selective ligands [D-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 microM) did not. The effect of DADLE (1 microM) was blocked by the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the -antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating mu receptors. Despite a lack of functional delta receptors, flow cytometry revealed cell-surface receptors. We used this approach to identify conditions that up-regulate receptors, including mu receptor gene deletion in dorsal root ganglion neurons of mu-/- mice and 18-h incubation of mu+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface delta receptors was up-regulated to 149 +/- 9 and 139 +/- 5%, respectively; furthermore, DPDPE and deltorphin II (1 microM) inhibited Ca2+ currents in both cases. Viral replacement of mu receptors in mu-/- neurons reduced delta receptor expression to mu+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished receptor coupling. Our observations suggest that receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which mu receptors predominate. However, up-regulation of cell-surface delta receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional delta receptor expression may reveal a novel target for analgesic therapy.

Ca2+ sparks and secretion in dorsal root ganglion neurons

Ca(2+) sparks as the elementary intracellular Ca(2+) release events are instrumental to local control of Ca(2+) signaling in many types of cells. Here, we visualized neural Ca(2+) sparks in dorsal root ganglion (DRG) sensory neurons and investigated possible role of DRG sparks in the regulation of secretion from the somata of the cell. DRG sparks arose mainly from type 3 ryanodine receptor Ca(2+) release channels on subsurface cisternae of the endoplasmic reticulum, rendering a striking subsurface localization. Caffeine- or 3,7-dimethyl-1-(2-propynyl)xanthine-induced store Ca(2+) release, in the form of Ca(2+) sparks, triggered exocytosis, independently of membrane depolarization and external Ca(2+). The spark-secretion coupling probability was estimated to be between 1 vesicle per 6.6 sparks and 1 vesicle per 11.4 sparks. During excitation, subsurface sparks were evoked by physiological Ca(2+) entry via the Ca(2+)-induced Ca(2+) release mechanism, and their synergistic interaction with Ca(2+) influx accounted for approximately 60% of the Ca(2+)-dependent exocytosis. Furthermore, inhibition of Ca(2+)-induced Ca(2+) release abolished endotoxin-induced secretion of pain-related neuropeptides. These findings underscore an important role for Ca(2+) sparks in the amplification of surface Ca(2+) influx and regulation of neural secretion.

N-methyl-D-aspartate receptors enhance mechanical responses and voltage-dependent Ca2+ channels in rat dorsal root ganglia neurons through protein kinase C

N-methyl-D-aspartate (NMDA)receptors (NMDARs) located on peripheral terminals of primary afferents are involved in the transduction of noxious mechanical stimuli. Exploiting the fact that both NMDARs and stretch-activated channels are retained in short-term culture and expressed on the soma of dorsal root ganglia (DRG) neurons, we examined the effect of NMDA on mechanically mediated changes in intracellular calcium concentration ([Ca2+]i). Our aims were to determine whether NMDARs modulate the mechanosensitivity of DRG neurons. Primary cultures of adult rat lumbosacral DRG cells were cultured for 1-3 days. [Ca2+]i responses were determined by Fura-2 ratio fluorescence. Somas were mechanically stimulated with fire-polished glass pipettes that depressed the cell membrane for 0.5 s. Voltage-activated inward Ca2+ currents were measured by the whole cell patch clamp. Stimulation of neurons with 100 microM NMDA in the presence, but not the absence, of co-agonist (10 microM D-serine) caused transient [Ca2+]i responses (101+/-9 nM) and potentiated [Ca2+]i peak responses to subsequent mechanical stimulation more than two-fold (P < 0.001). NMDA-mediated potentiation of mechanically induced [Ca2+]i responses was inhibited by the selective protein kinase C (PKC) inhibitor GF109203X (GFX; 10 microM), which had no independent effects on NMDA- or mechanically induced responses. Short-term treatment with the PKC activator phorbol dibutyrate (1 microM PDBu for 1-2 min) also potentiated mechanically induced [Ca2+]i responses nearly two-fold (P < 0.001), while longer exposure (>10 min) inhibited the [Ca2+]i transients by 44% (P < 0.001). Both effects of PDBu were prevented by prior treatment with GFX. Inhibition of voltage-dependent Ca2+ channels with 25 microM La3+ had no effect on mechanically induced [Ca2+]i transients prior to NMDA, but prevented enhancement of the transients by NMDA and PDBu. NMDA pretreatment transiently enhanced nifedipine-sensitive, voltage-activated Ca2+ currents by a process that was sensitive to GFX. In conclusion, activation of NMDARs on cultured DRG neurons sensitize voltage-dependent L-type Ca2+ channels which contribute to mechanically induced [Ca2+]i transients through a PKC-mediated process.

Modulation of voltage-dependent calcium channels by neurotransmitters and neuropeptides in parasympathetic submandibular ganglion neurons

The control of saliva secretion is mainly under parasympathetic control, although there also could be a sympathetic component. Sympathetic nerves are held to have a limited action in secretion in submandibular glands because, on electrical stimulation, only a very small increase to the normal background, basal secretion occurs. Parasympathetic stimulation, on the other hand, caused a good flow of saliva with moderate secretion of acinar mucin, plus an extensive secretion of granules from the granular tubules. The submandibular ganglion (SMG) is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. Neurotransmitters and neuropeptides acting via G-protein coupled receptors (GPCRs) change the electrical excitability of neurons. In these neurons, many neurotransmitters and neuropeptides modulate voltage-dependent calcium channels (VDCCs). The modulation is mediated by a family of GPCRs acting either directly through the membrane delimited G-proteins or through second messengers. However, the mechanism of modulation and the signal transduction pathway linked to an individual GPCRs depend on the animal species. This review reports how neurotransmitters and neuropeptides modulate VDCCs and how these modulatory actions are integrated in SMG systems. The action of neurotransmitters and neuropeptides on VDCCs may provide a mechanism for regulating SMG excitability and also provide a cellular mechanism of a variety of neuronal Ca(2+)-dependent processes.

High voltage-activated Ca2+ channel currents in isolectin B4-positive and -negative small dorsal root ganglion neurons of rats

Voltage-gated Ca(2+) channels in the primary sensory neurons are important for neurotransmitter release and regulation of nociceptive transmission. Although multiple classes of Ca(2+) channels are expressed in the dorsal root ganglion (DRG) neurons, little is known about the difference in the specific channel subtypes among the different types of DRG neurons. In this study, we determined the possible difference in high voltage-activated Ca(2+) channel currents between isolectin B(4) (IB(4))-positive and IB(4)-negative small-sized (15-30 microm) DRG neurons. Rat DRG neurons were acutely isolated and labeled with IB(4) conjugated to a fluorescent dye. Whole-cell patch clamp recordings of barium currents flowing through calcium channels were performed on neurons with and without IB(4). The peak current density of voltage-gated Ca(2+) currents was not significantly different between IB(4)-positive and IB(4)-negative neurons. Also, both nimodipine and omega-agatoxin IVA produced similar inhibitory effects on Ca(2+) currents in these two types of neurons. However, block of N-type Ca(2+) channels with omega-conotoxin GVIA produced a significantly greater reduction of Ca(2+) currents in IB(4)-positive than IB(4)-negative neurons. Furthermore, the IB(4)-positive neurons had a significantly smaller residual Ca(2+) currents than IB(4)-negative neurons. These data suggest that a higher density of N-type Ca(2+) channels is present in IB(4)-positive than IB(4)-negative small-sized DRG neurons. This differential expression of the subtypes of high voltage-activated Ca(2+) channels may contribute to the different function of these two classes of nociceptive neurons.

The effects of the delta-opioid agonist SNC80 on hind-limb motor function and neuronal injury after spinal cord ischemia in rats

Recent investigation suggested neuroprotective efficacy of a delta-opioid agonist in the brain. We investigated the effects of intrathecal treatment with a delta-opioid agonist (SNC80) on spinal cord ischemia (SCI) in rats. SCI was induced with an intraaortic balloon catheter. The animals were randomly allocated to one of the following five groups: 1) SNC80 before 9 min of SCI (SNC-9; n = 12), 2) vehicle before 9 min of SCI (V-9; n = 12), 3) SNC80 before 11 min of SCI (SNC-11; n = 10), 4) vehicle before 11 min of SCI (V-11; n = 12), or 5) sham (n = 12). SNC80 (400 nmol) or vehicle was administered 15 min before SCI. Forty-eight hours after reperfusion, hind-limb motor function was assessed by using the Basso, Beattie, Bresnahan (BBB) scale (0 = paraplegia; 21 = normal) and histological assessment of the L4 and L5 spinal segments was performed. BBB scores in the SNC-9 group were higher compared with those in the V-9 group (P < 0.05), whereas there were no differences in BBB scores between the SNC-11 and V-11 groups. There were significantly more normal neurons in the SNC-9 and SNC-11 groups than in the V-9 and V-11 groups (P < 0.05). The results indicate that intrathecal treatment with the delta-opioid agonist SNC80 can attenuate hind-limb motor dysfunction and neuronal injury after SCI in rats.

Central terminals of nociceptors are targets for nicotine suppression of inflammation

Spinal intrathecal administration of nicotine inhibits bradykinin-induced plasma extravasation, a component of the inflammatory response, in the knee joint of the rat in a dose-related fashion. Nociceptors contain nicotinic receptors and activation of a nociceptor at its peripheral terminal, by capsaicin, also produces inhibition of inflammation. Therefore the aim of this study was to test the hypothesis that the spinal target for this effect of nicotine is the central terminal of the primary afferent nociceptor. Intrathecal administration of the neurokinin-1 receptor antagonist, (3aR,7aR)-7,7-diphenyl-2-(1-imino-2(2-methoxyphenyl)-ethyl) perhydroisoindol-4-1 hydrochloride or the N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid, both antagonists of the action of primary afferent neurotransmitters, markedly attenuated the inhibition of bradykinin-induced plasma extravasation produced by both intrathecal nicotine and intraplantar capsaicin.Conversely, intrathecal administration of an alpha-adrenoceptor antagonist, phentolamine or an opioid receptor antagonist, naloxone, to block descending antinociceptive controls, which provide inhibitory input to primary afferent nociceptors, enhanced the action of both nicotine and capsaicin. These findings support the hypothesis that the central terminal of the primary afferent nociceptor is a CNS target at which nicotine acts to inhibit inflammation.

Intrathecally administered cholera toxin blocks allodynia and hyperalgesia in persistent pain models

In persistent pain, the spinal cord concentration of the opioid peptide dynorphin increases dramatically, yet the function of dynorphin remains unknown. If prodynorphin expression could be manipulated in vivo, it might be possible to determine what role dynorphin plays in persistent pain. Previous work in our laboratory showed that prodynorphin expression is regulated through the cyclic adenosine monophosphate pathway. Therefore, we attempted to enhance prodynorphin expression in the spinal cord of rats by stimulating adenylate cyclase with cholera toxin; however, contrary to our hypothesis, intrathecally administered cholera toxin did not enhance prodynorphin expression. Rather, cholera toxin suppressed the increase in prodynorphin produced by inflammation. Cholera toxin also inhibited the allodynia and hyperalgesia associated with inflammation and nerve injury. Interestingly, the antiallodynic and antihyperalgesic actions of cholera toxin were reversed with the opioid receptor antagonist, naloxone. These findings suggest that cholera toxin enhances or unmasks an endogenous opioid pathway to produce its antiallodynic and antihyperalgesic effects. Furthermore, these data indicate that the suppression of the inflammation-induced increase in spinal cord prodynorphin is caused by the opioid-mediated decrease in the nociceptive stimulus.

mu-Opioid receptor modulates peptide release from rat neurohypophysial terminals by inhibiting Ca(2+) influx

The activation of opioid receptors in neurones of the central nervous system leads to a variety of effects including the modulation of diuresis and parturition, processes that are directly controlled by the hypothalamic-neurohypophysial system (HNS). The effects of mu-opioid receptor activation on peptide release, voltage-gated Ca2+ currents and intracellular calcium levels ([Ca2+]i) were studied in isolated nerve terminals of the HNS. The mu-receptor agonist, DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin) inhibited high K+-induced peptide release in a dose-dependent manner, with oxytocin release being more sensitive to block than vasopressin release at all concentrations tested. The addition of the mu-receptor antagonist CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide) was able to overcome the inhibitory effects of DAMGO. By contrast to previous results, voltage-gated Ca2+ currents were sensitive to blockage by DAMGO and this inhibition was also prevented by CTOP. Furthermore, [Ca2+]i measurements with Fura-2 corroborated the inhibition by DAMGO of calcium entry and its reversal by the micro -receptor antagonist in these nerve terminals. Thus, the decrease in neuropeptide release, particularly for oxytocin, induced by the activation of mu-opioid receptors in neurohypophysial terminals is mediated, at least in part, by a corresponding decrease in Ca2+ entry due to the inhibition of voltage-gated Ca2+ channels.

The same cellular signaling pathways mediate survival in sensory neurons that switch their trophic requirements during development

A distinct subpopulation of rat dorsal root sensory (DRG) neurons, termed P-neurons, switch their trophic requirements for survival during development from nerve growth factor (NGF) at embryonic stages to basic fibroblast growth factor (bFGF) just after birth. We investigated in cultured P-neurons the intracellular signaling pathways mediating survival before and after this switch. The NGF-induced survival was completely blocked by either wortmannin (100 nM) or PD98059 (25-50 nM), which selectively inhibit the phosphatidylinositol 3-kinase-AKT (PI3 kinase-AKT) and mitogen-activated kinase kinase extracellular regulated kinase (MEK-ERKs) pathways, respectively. NGF activated AKT and ERKs in single embryonic P-neurons, as assayed by immunofluorescence of phosphorylated proteins. In concordance with the survival assays, wortmannin and PD98059 blocked AKT and ERKs activation, respectively. Following the trophic switch, bFGF used the same signaling pathways to promote survival of post-natal P-neurons, as either wortmannin or PD98059 blocked its effect. Also, bFGF activated AKT and ERKs in single P-neurons, and this activation was blocked by the same inhibitors. These results strongly suggest that both pathways concurrently mediate the action of NGF and bFGF during embryonic and post-natal periods, respectively. Thus, we report the novel result that the switch in trophic requirements occurs with conservation of the signaling pathways mediating survival.

Opioid receptor antagonists modulate Ca2+-activated K+ channels in mesenteric arterial smooth muscle cells of rats in hemorrhagic shock

Previous study has indicated a significant enhancement of activity of large-conductance Ca2+-activated K+ channel (BKCa) in mesenteric arterial vascular smooth muscle cells isolated from rats in vascular hyporesponsive stage of hemorrhagic shock. In the present study, the effect of opioid receptor antagonism on BKCa activity in the vascular smooth muscle cells of rats in the hyporesponse stage of hemorrhagic shock was investigated by using inside-out configuration of the patch-clamp technique. The results showed that naloxone (10 microM) down-regulated the activity of BKCa by reducing open probability (Po) and open frequency of the channels. The reduction of Po resulted from a decrease of mean open time and an increase of the slow closed time constant. Naltrindole and nor-binaltorphimine (100 nM) had the similar effects to that of naloxone, but no significant effect of beta-funaltrexamine (100 nM) on the activity of the channels could be found. These results suggest that delta- and kappa-opioid receptors, but not mu-receptors, may be involved in the regulation of BKCa in vascular hyporesponse stage, and that inhibition of BKCa may be one of the mechanisms of the opioid receptor antagonists improving the response of resistance arteries to vasoactive stimulants during the decompensatory stage of hemorrhagic shock.

Opioid receptor subtypes differentially modulate serotonin efflux in the rat central nervous system

Opioid receptor subtypes may have site-specific effects and play different roles in modulating serotonergic neurotransmission in the mammalian central nervous system. To test this hypothesis, we used in vivo microdialysis to measure changes in extracellular serotonin (5-hydroxytryptamine; 5-HT) in response to local infusion of mu-, delta-, and kappa-opioid receptor ligands into the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), and nucleus accumbens (NAcc) of freely behaving rats. The mu-opioids [D-Ala(2)-N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), endomorphin-1, and endomorphin-2 were administered by reverse dialysis infusion into the DRN. In response, extracellular 5-HT was increased in the DRN, an effect that was blocked by the selective mu-receptor antagonist beta-funaltrexamine, but not by the delta-receptor antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174,864). Infusion of delta-receptor agonists, [D-Ala(2),D-Len(5)]enkephalin (DADLE), [D-Pen(2,5)]enkephalin (DPDPE), and deltophin-II into the DRN also increased extracellular 5-HT, an effect that was blocked by selective delta-receptor antagonists. In contrast to the DRN, local infusion of mu- and delta-opioids had no effect on 5-HT in the MRN or NAcc. These data indicate that mu- and delta-opioid ligands have a selective influence on serotonergic neurons in the DRN. Finally, the kappa-receptor agonist U-50,488 [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide] produced similar decreases in 5-HT during local infusion into the DRN, MRN, and NAcc. These results provide evidence of differential regulation of 5-HT release by opioid receptor subtypes in the midbrain raphe and forebrain.

Modulation of Ca2+ channels by opioid receptor antagonists in mesenteric arterial smooth muscle cells of rats in hemorrhagic shock

The effects of hemorrhagic shock on Ba currents ( ) via Ca channels and the regulation of the channels in the vascular hyporesponse stage of hemorrhagic shock by opioid receptor antagonists were examined by using the whole-cell recording of patch-clamp technique in mesenteric arterial smooth muscle cells of rats. The results showed that hemorrhagic shock induced an inhibition of Ca channels in the cells; 10 micro M of naloxone and 100 n of naltrindole, nor-binaltorphimine, and beta-funaltrexamine increased the in the cells of rats in shock. After inhibition of protein kinase C by using 1-(5-isoquindinesulfonyl)-2-methylpiperazine via electrodes, the enhancement of by the antagonists was not observed. These results suggested that the inhibition of Ca channel induced by hemorrhagic shock was mediated by delta-, kappa-, and mu -opioid receptors in the cells and may be partly responsible for vascular hyporesponse. The enhancement of was mediated by activation of protein kinase C and may be responsible for the antagonist-caused improvement in the response of resistance arteries to vasoactive stimulants at the decompensatory stage of hemorrhagic shock.