Role of spinal nitric oxide in the inhibitory effect of [D-Pen2, D-Pen5]-enkephalin on ascending dorsal horn neurons in normal and diabetic rats

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.

The induction of the transcription factor Nrf2 enhances the antinociceptive effects of delta-opioid receptors in diabetic mice

The involvement of heme oxygenase 1 (HO-1) in the modulation of the antinociceptive effects of opioids in type 1 diabetes has been demonstrated but the role played by the transcription factor Nrf2 in the regulation of painful neuropathy and in the effects and expression of δ-opioid receptors (DOR) in type 2 diabetes, has not been studied. In male BKS.Cg-m+/+Leprdb/J (db/db) mice, the anti-allodynic effects produced by a Nrf2 transcription factor activator, sulforaphane (SFN) administered alone and combined with two DOR agonists, [d-Pen(2),d-Pen(5)]-Enkephalin (DPDPE) and (+)-4-[(αR)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N diethylbenzamide (SNC-80), were evaluated. The effects of SFN on glucose levels and body weight as well as on the proteins levels of Nrf2, HO-1, NAD(P)H: quinone oxidoreductase 1 (NQO1), MAPKs (JNK) and DOR in sciatic nerve from db/db mice were also assessed. This study showed that the administration of SFN dose dependently reversed mechanical allodynia, reduced hyperglycemia and body weight gain associated to type 2 diabetes and significantly increased the anti-allodynic effects of DPDPE and SNC-80 in db/db mice. This treatment normalized the down regulation of Nrf2 and NQO1 and enhanced the protein levels of HO-1 in db/db mice. Moreover, the administration of SFN also inhibited the JNK phosphorylation and DOR down-regulation in the sciatic nerve of diabetic mice. Our data indicated that SFN treatment is effective in reversing mechanical allodynia and enhancing DOR antinociceptive effects in db/db mice which effects might be mediated by activating Nrf2 signaling, reducing hyperglycemia, inhibiting JNK phosphorylation and avoiding DOR down-regulation in the sciatic nerve of these animals. These results propose SFN, alone and/or combined with DOR agonists, as interesting approaches for the treatment of painful diabetic neuropathy associated to type 2 diabetes in mice.

The antinociceptive effects of a δ-opioid receptor agonist in mice with painful diabetic neuropathy: Involvement of heme oxygenase 1

Diabetic neuropathy is poorly controlled by classical analgesics and the research of new therapeutic alternatives is indispensable. Our aim is to investigate if treatment with a carbon monoxide-releasing molecule (tricarbonyldichlororuthenium(II) dimer; CORM-2) or an inducible heme oxygenase (HO-1) inducer (cobalt protoporphyrin IX; CoPP) could enhance the antinociceptive effects produced by a δ-opioid receptor (DOR) agonist in mice with painful diabetic neuropathy. In diabetic mice induced by streptozotocin (STZ) injection, the antiallodynic and antihyperalgesic effects produced by the subcutaneous administration of a DOR agonist ([d-Pen(2),d-Pen(5)]-Enkephalin; DPDPE) and the reversion of its effects with the administration of an HO-1 inhibitor (tin protoporphyrin IX; SnPP) were evaluated. Moreover, the antinociceptive effects produced by the intraperitoneal administration of 10mg/kg of CORM-2 or CoPP, alone or combined, with a subanalgesic dose of DPDPE were also assessed. Our results demonstrated that the subcutaneous administration of DPDPE inhibited the mechanical and thermal allodynia as well as the thermal hyperalgesia induced by diabetes in a dose-dependent manner. Moreover, while the antinociceptive effects produced by a low dose of DPDPE were enhanced by CORM-2 or CoPP co-treatments, the inhibitory effects produced by a high dose of DPDPE were completely reversed by the administration of an HO-1 inhibitor, SnPP, indicating the involvement of HO-1 in the antinociceptive effects produced by this DOR agonist during diabetic neuropathic pain in mice. In conclusion, this study shows that the administration of CORM-2 or CoPP combined with a DOR agonist could be an interesting strategy for the treatment of painful diabetic neuropathy.

[The evoked activity of the dorsal root afferent fibres of the spinal cord of white rats in experimental diabetes mellitus]

We analyzed the functional status of the dorsal root of spinal cord in experimental diabetes mellitus (DM), for instance the parameters of the action potential (AP): threshold, chronaxy and the dynamics of the dorsal root excitability. It was revealed the 1,5 times increase in threshold of excitation when compared to the control animals (P<0,001), the amplitude of AP decreased by 21,7% (P<0,05). It was also revealed a significant increase in response to the 2nd stimulus under applying the paired stimuli on the sciatic nerve in animals with experimental DM. Under applying the stimuli of increasing intensity there was found significant decrease in the amplitude of the AP in animals of the experimental group. It was concluded that hyperglycemia made changes to the processes of excitability and refractoriness in the afferent fibers of the spinal cord.

Variants of neural nitric oxide synthase in the spinal cord of neuropathic rats and their effects on nuclear factor-kappaB (NF-kappaB) activity in PC12 cells

Neuropathic pain due to nerve injury is associated with overactivity of spinal N-methyl-D-aspartate (NMDA) receptors and nitric oxide synthases (NOS). Spinal NOS and NMDA receptors could act in a concerted manner to excite each other in nociceptive signaling. Among the 3 major NOS isoforms, neuronal NOS (nNOS) has the most functional relationship with NMDA receptors through a PDZ-PDZ (PSD-95, Dlg, ZO-1 homology) postsynapse interaction. However, some nNOS variants lack the PDZ domain, which may result in the changes in the interaction with the NMDA receptor and subsequent localization and enzymatic activity. The aim of this study was to determine which nNOS variants are expressed in the spinal cord in neuropathic rats and deduce their role in neuropathic pain by testing the effects of these kinds of nNOS on nuclear factor-kappaB (NF-kappaB) activity in PC12 cells. Western blot analysis revealed that there were at least 3 bands of nNOS (155, 135, and 125 kDa) in the spinal cord and, moreover, that nNOS at 135 kDa decreased significantly after development of neuropathic pain. 5'-RACE-PCR and Southern blots determined that the nNOS at 155 and 135 kDa corresponded to nNOSalpha and nNOSbeta, respectively, which was confirmed by RT-PCR. PC12 cells transfected with the nNOSalpha gene had no effect on NF-kappaB activity, but nNOSbeta without the PDZ domain significantly decreased that in PC12 cells. Considering the importance of spinal NF-kappaB signaling in neuropathic rat, it could be concluded that changes in spinal nNOS variants and quantity after peripheral nerve injury implicate nNOS in the generation of neuropathic pain.

PERSPECTIVE

This article presents data demonstrating that nNOS variants change in the spinal cord of the rats after neuropathic pain and result in differential effects on NF-kappaB activity in PC12 cells. These changes in nNOS variants and their different characteristics may account for the spinal NO paradox role in neuropathic pain. Furthermore, these data suggest that nNOSbeta may represent a new therapeutic target for the treatment of chronic neuropathic pain.

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.

Recombinant herpes vector-mediated analgesia in a primate model of hyperalgesia

Some chronic pain syndromes are characterized by episodes of intense burning and hyperalgesia in localized areas of skin. These sensations are thought to be mediated, at least in part, by the activity of damaged, unmyelinated C nociceptors. These phenomena were modeled by assaying responses of macaques to thermal and chemical stimuli that produced periodic activation and sensitization of C nociceptors. Upon validation of this method, a recombinant herpes simplex vector encoding human preproenkephalin was topically applied to the dorsal surface of the feet of the monkeys. Immunohistochemistry and radioimmunoassay revealed that enkephalin peptides were being produced in releasable pools in sensory neurons innervating the treated skin area. Behavioral responses evoked by periodic sensitization and activation of C nociceptors innervating the vector-treated skin area revealed a substantial and long-lasting (at least 20 weeks) antihyperalgesic and analgesic effect limited to the areas to which the virus was applied. This approach may be a viable means of treating localized cutaneous burning pain and hyperalgesia.

Systemic Morphine Inhibits Dorsal Horn Projection Neurons through Spinal Cholinergic System Independent of Descending Pathways

Cholinergic circuitry and muscarinic receptors within the spinal cord have been proposed to contribute to the analgesic effects of systemic morphine. In this study, we determined whether the descending pathways are involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons mediated by activation of the spinal cholinergic system. Single-unit activity of dorsal horn projection neurons was recorded in anesthetized rats. The neuronal responses to mechanical stimuli applied to the receptive field were determined before and after intravenous injection of morphine. The inhibitory effect of intravenous morphine on dorsal horn neurons was also tested before and after topical spinal application of the muscarinic antagonist atropine in both intact and spinally transected rats. Intravenous injection of 2.5 mg/kg morphine significantly inhibited the evoked response of dorsal horn neurons in both intact and spinally transected rats. Spinal topical application of the mu opioid antagonist H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) completely blocked the effect of morphine on dorsal horn neurons. In addition, spinal application of 10 microM atropine significantly attenuated the effect of systemic morphine. In rats subjected to cervical spinal transection, atropine produced a similar attenuation of the inhibitory effect of systemic morphine on dorsal horn neurons. Data from this electrophysiological study suggest that systemic morphine inhibits ascending dorsal horn neurons through stimulation of spinal mu opioid receptors. Furthermore, activation of the local spinal cholinergic circuitry and muscarinic receptors is involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons independent of descending pathways.

Activation of muscarinic receptors inhibits spinal dorsal horn projection neurons: role of GABAB receptors

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors produce efficacious analgesia. However, the mechanisms of the antinociceptive actions of muscarinic agonists in the spinal cord are not fully known. Previous in vitro studies have shown that muscarinic agonists increase GABA release and reduce the glutamatergic synaptic input to lamina II interneurons through GABAB receptors in the spinal cord. In the present study, we studied the effect of muscarinic agents on dorsal horn projection neurons and the role of spinal GABAB receptors in their action. Single-unit activity of ascending dorsal horn neurons was recorded in the lumbar spinal cord of anesthetized rats. The responses of dorsal horn neurons to graded mechanical stimuli were determined before and after topical spinal application of muscarine and neostigmine. We found that topical application of 0.1-5 microM muscarine or 0.5-5 microM neostigmine significantly suppressed the evoked response of dorsal horn neurons in a concentration-dependent manner. The inhibitory effect of muscarine or neostigmine on dorsal horn neurons was completely abolished in the presence of 1 microM atropine and by intrathecal pretreatment with 1 microg pertussis toxin to inactivate inhibitory G proteins. Furthermore, the inhibitory effect of both muscarine and neostigmine on the evoked response of dorsal horn neurons was significantly attenuated in the presence of 1 microM CGP55845, a GABAB receptor antagonist. Collectively, these data suggest that muscarinic agents inhibit dorsal horn projection neurons through muscarinic receptors coupled to pertussis toxin-sensitive Gi/o proteins. The inhibitory action of muscarinic agonists on these dorsal horn neurons is mediated in part by spinal GABAB receptors.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).