Changes of opioid binding density in the rat spinal cord following unilateral dorsal rhizotomy

μ-Opioid receptors in primary sensory neurons are involved in supraspinal opioid analgesia

Both inhibiting ascending nociceptive transmission and activating descending inhibition are involved in the opioid analgesic effect. The spinal dorsal horn is a critical site for modulating nociceptive transmission by descending pathways elicited by opioids in the brain. μ-Opioid receptors (MORs, encoded by Oprm1) are highly expressed in primary sensory neurons and their central terminals in the spinal cord. In the present study, we tested the hypothesis that MORs expressed in primary sensory neurons contribute to the descending inhibition and supraspinal analgesic effect induced by centrally administered opioids. We generated Oprm1 conditional knockout (Oprm1-cKO) mice by crossing Advillin^Cre/+ mice with Oprm1^flox/flox mice. Immunocytochemical labeling in Oprm1-cKO mice showed that MORs are completely ablated from primary sensory neurons and are profoundly reduced in the superficial spinal dorsal horn. Intracerebroventricular injection of morphine or fentanyl produced a potent analgesic effect in wild-type mice, but such an effect was significantly attenuated in Oprm1-cKO mice. Furthermore, the analgesic effect produced by morphine or fentanyl microinjected into the periaqueductal gray was significantly greater in wild-type mice than in Oprm1-cKO mice. Blocking MORs at the spinal cord level diminished the analgesic effect of morphine and fentanyl microinjected into the periaqueductal gray in both groups of mice. Our findings indicate that MORs expressed at primary afferent terminals in the spinal cord contribute to the supraspinal opioid analgesic effect. These presynaptic MORs in the spinal cord may serve as an interface between ascending inhibition and descending modulation that are involved in opioid analgesia.

μ-Opioid receptors in primary sensory neurons are essential for opioid analgesic effect on acute and inflammatory pain and opioid-induced hyperalgesia

KEY POINTS

μ-Opioid receptors (MORs) are expressed peripherally and centrally, but the loci of MORs responsible for clinically relevant opioid analgesia are uncertain. Crossing Oprm1^flox/flox and Advillin^Cre/+ mice completely ablates MORs in dorsal root ganglion neurons and reduces the MOR expression level in the spinal cord. Presynaptic MORs expressed at primary afferent central terminals are essential for synaptic inhibition and potentiation of sensory input by opioids. MOR ablation in primary sensory neurons diminishes analgesic effects produced by systemic and intrathecal opioid agonists and abolishes chronic opioid treatment-induced hyperalgesia. These findings demonstrate a critical role of MORs expressed in primary sensory neurons in opioid analgesia and suggest new strategies to increase the efficacy and reduce adverse effects of opioids.

ABSTRACT

The pain and analgesic systems are complex, and the actions of systemically administered opioids may be mediated by simultaneous activation of μ-opioid receptors (MORs, encoded by the Oprm1 gene) at multiple, interacting sites. The loci of MORs and circuits responsible for systemic opioid-induced analgesia and hyperalgesia remain unclear. Previous studies using mice in which MORs are removed from Nav1.8- or TRPV1-expressing neurons provided only an incomplete and erroneous view about the role of peripheral MORs in opioid actions in vivo. In the present study, we determined the specific role of MORs expressed in primary sensory neurons in the analgesic and hyperalgesic effects produced by systemic opioid administration. We generated Oprm1 conditional knockout (Oprm1-cKO) mice in which MOR expression is completely deleted from dorsal root ganglion neurons and substantially reduced in the spinal cord, which was confirmed by immunoblotting and immunocytochemical labelling. Both opioid-induced inhibition and potentiation of primary sensory input were abrogated in Oprm1-cKO mice. Remarkably, systemically administered morphine potently inhibited acute thermal and mechanical nociception and persistent inflammatory pain in control mice but had little effect in Oprm1-cKO mice. The analgesic effect of intrathecally administered morphine was also profoundly reduced in Oprm1-cKO mice. Additionally, chronic morphine treatment-induced hyperalgesia was absent in Oprm1-cKO mice. Our findings directly challenge the notion that clinically relevant opioid analgesia is mediated mostly by centrally expressed MORs. MORs in primary sensory neurons, particularly those expressed presynaptically at the first sensory synapse in the spinal cord, are crucial for both opioid analgesia and opioid-induced hyperalgesia.

Subcellular localization of mu-opioid receptor G(s) signaling

In membranes obtained from mu-opioid receptor (MOR) expressing Chinese hamster ovary (CHO) cells (MOR-CHO), the MOR-selective agonist sufentanil produced a concentration-dependent stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate binding to G(s)alpha that was abolished by blocking MOR with naloxone. This unequivocally demonstrates the long-debated functionality of the previously described association of MOR with G(s)alpha. Several complementary observations indicate the relevance of caveolae to MOR-coupled G(s)alpha signaling. 1) In MOR-CHO membranes, sufentanil stimulated the translocation of G(s)alpha into Triton-insoluble membrane compartments. 2) Sufentanil enhanced the coimmunoprecipitation (co-IP) of G(s)alpha and adenylyl cyclase (AC) with caveolin-1 (a marker for caveolae) from the Triton-insoluble membrane fraction of spinal cord and MOR-CHO. 3) MOR blockade (via naloxone) or G(s) inactivation (via cholera toxin) abolished both the increased trafficking of G(s)alpha into the Triton-insoluble membrane fraction of MOR-CHO and the augmented co-IP from spinal cord membranes of G(s)alpha and AC with caveolin-1. This indicates that these events occurred subsequent to activation of MOR and G(s)alpha. Strikingly, lesser-phosphorylated G(s)alpha, which preferentially couple to MOR (Mol Brain Res 135:217-224, 2005; Mol Pharmacol 72:753-760, 2007; Mol Pharmacol 73:868-879, 2008), are concentrated in caveolae, underscoring their relevance to MOR G(s)alpha signaling. MOR-stimulated trafficking of G(s)alpha and AC into caveolae and the likelihood of increased MOR G(s)alpha coupling within caveolae could suggest that they contain the downstream effectors for MOR G(s)alpha AC signaling.

Enhancement of morphine antinociception with the peptide N-methyl-d-aspartate receptor antagonist [Ser1]-histogranin in the rat formalin test

Opiates may be used to attenuate chronic pain, but long-term use is complicated by the possible increase in pain over time, escalating dose requirements, and untoward side effects. Adjuncts such as ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, may be added to opiates to provide more consistent analgesia. However, the unwanted motor side effects of NMDA receptor antagonists prevent their widespread clinical usage. In the current study, an analogue of the naturally-derived peptide histogranin, [Ser(1)]histogranin (SHG), an NMDA receptor modulator without adverse side effects like those in current clinical use, was evaluated for its potential to enhance the antinociceptive effect of intrathecal morphine in the rat formalin test. Intrathecal injection of a combination of SHG and morphine resulted in significantly reduced hind paw flinching compared with morphine alone in the first and second phases. The effective dose of SHG used in the combination had no efficacy when tested alone. These results were similar to the increased efficacy that was obtained with a combination of ketamine and morphine. Thus, enhancement of opiate efficacy is possible using a novel peptide NMDA receptor modulator with a potentially improved safety profile.

Mu-opioid receptors are involved in the tolerance to nicotine antinociception

Several studies have shown the participation of the endogenous opioid system on the antinociceptive effects and addictive properties of nicotine. The aim of the present study was to explore the involvement of the mu-opioid receptors in the development of tolerance to nicotine antinociception. Chronic treatment of C57BL/6 mice with nicotine (5 mg/kg s.c., three times daily during 12 days) resulted in tolerance to its antinociceptive responses in the tail-immersion test. We investigated the possible existence of adaptive changes in the expression and/or functional activity of mu-opioid receptors in these tolerant mice by using autoradiography of [(3)H]D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H]DAMGO) binding and DAMGO-stimulated guanosine [(35)S]5'-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) binding. The density of mu-opioid receptors in the spinal cord was not modified in nicotine-tolerant mice, whereas a decrease was found in the caudate-putamen, as well as in the core and the shell of the nucleus accumbens. However, the functional activity of these receptors was significantly increased in the spinal cord as a consequence of nicotine treatment. To further investigate the role of mu-opioid receptors in the tolerance to nicotine-induced antinociception, we evaluated this response in C57BL/6 mu-opioid receptor knockout mice. Chronic nicotine treatment produced tolerance in both wild-type and knockout animals, but tolerance developed faster in mice lacking mu-opioid receptors. These results indicate that mu-opioid receptors play an important role in the development of tolerance to nicotine antinociceptive effects.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Contralateral cytokine gene induction after peripheral nerve lesions: dependence on the mode of injury and NMDA receptor signaling

There is increasing evidence that unilateral nerve injury evokes contralateral responses, but the underlying mechanisms are largely unknown. In the present investigation, we analyzed cytokine and chemokine gene induction in contralateral, non-lesioned nerves after sciatic nerve crush and chronic constriction injury (CCI) by quantitative reverse transcriptase polymerase chain reaction in mice. After sciatic nerve crush, contralateral changes in cytokine gene expression were restricted to interleukin (IL)-1beta, which showed a monophasic peak at the first postoperative day. Following CCI, contralateral transcripts for IL-1beta, IL-10 and monocyte chemoattractant protein-1 (MCP-1) were significantly increased already at day 1 and upregulation persisted over the next 4 weeks. In contrast, tumor necrosis factor alpha (TNF-alpha) levels remained unchanged. Contralateral gene induction was restricted to the homonymous opposite sciatic nerve, but spared the femoral nerve. NMDA receptor blockade completely abolished contralateral cytokine expression after CCI on the mRNA level. In contralateral dorsal root ganglia, only IL-10 mRNA levels were modified after nerve injury. Sham operation significantly increased the cytokine and chemokine gene expression at the ipsilateral side, but could not mediate contralateral effects. Our study confirms that nerve injury evokes contralateral responses and identifies NMDA-mediated signaling as one underlying mechanism.

Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord

An in vivo fluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available delta opioid receptors (deltaORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous deltaOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of deltaOR-expressing neurons, as revealed by in situ hybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered deltaOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either deltaOR mRNA or (125)I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface deltaOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that deltaOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that microOR-induced changes in the cell surface availability of deltaOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of deltaOR responsiveness through microOR activation in this region is linked to somatosensory information processing.

Suppression of Acute Herpetic Pain-Related Responses by the κ-Opioid Receptor Agonist (-)-17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[N-methyl-3-trans-3-(3-furyl) Acrylamido] Morphinan Hydrochloride (TRK-820) in Mice

(-)-17-Cyclopropylmethyl-3,14beta-dihydroxy-4,5alpha-epoxy-6beta-[N-methyl-3-trans-3-(3-furyl) acrylamido] morphinan hydrochloride (TRK-820) is a kappa-opioid receptor agonist that has pharmacological characteristics different from typical kappa-opioid receptor agonists. This study was conducted to determine the antiallodynic and antihyperalgesic effects of TRK-820 in a mouse model of acute herpetic pain and to compare them with those of the kappa-opioid receptor agonist enadoline and the mu-opioid receptor agonist morphine. Percutaneous inoculation with herpes simplex virus type-1 induced tactile allodynia and mechanical hyperalgesia in the hind paw on the inoculated side. TRK-820 (0.01-0.1 mg/kg p.o.), enadoline (1-10 mg/kg p.o.) and morphine (5-20 mg/kg p.o.) dose dependently inhibited the allodynia and hyperalgesia, but the antiallodynic and antihyperalgesic dose of enadoline markedly decreased spontaneous locomotor activity. The antinociceptive action of TRK-820 (0.1 mg/kg) was completely antagonized by pretreatment with norbinaltorphimine, a kappa-opioid receptor antagonist, but not by naltrexone, a mu-opioid receptor antagonist. Repeated treatment with morphine (20 mg/kg, four times) resulted in the reduction of antiallodynic and antihyperalgesic effects, whereas the inhibitory potency of TRK-820 (0.1 mg/kg) was almost the same even after the fourth administration. There was no cross-tolerance in antinociceptive activities between TRK-820 and morphine. Intrathecal and intracerebroventricular, but not intraplantar, injections of TRK-820 (10-100 ng/site) suppressed the allodynia and hyperalgesia. These results suggest that TRK-820 inhibits acute herpetic pain through kappa-opioid receptors in the spinal and supraspinal levels. TRK-820 may have clinical efficacy in acute herpetic pain with enough safety margins.

Phylogenetic changes in the expression of delta opioid receptors in spinal cord and dorsal root ganglia

To assess the validity of rodent models for investigating the role of delta opioid receptors (DOR) in analgesia, the distribution of DOR binding and mRNA were compared between rodent and primate spinal cord and dorsal root ganglia (DRG), using receptor autoradiography and in situ hybridization, respectively. In mouse and rat spinal cord, [(125)I]-deltorphin-labeled DOR binding sites were detected throughout the gray matter. In contrast, in primate and particularly in human spinal cord, DOR binding was mainly present in laminae I-II, with little to no binding in deeper layers. Accordingly, in rodent spinal cord, DOR mRNA was expressed by a large number of neurons distributed throughout the ventral and dorsal horns, whereas in the primate, DOR expression was significantly lower, as evidenced by a moderate number of labeled cells throughout the gray matter in monkey and by only few labeled cells in human, mainly in Clarke's column and lamina IX. Major species differences in DOR expression were also observed in primary afferent cells bodies. In rat DRG, intense DOR mRNA hybridization was primarily observed over large ganglion cells immunopositive for neurofilament 200. In contrast, in monkey and human DRG, DOR mRNA was primarily detected over small and medium-sized ganglion cells. These results demonstrate major differences in the expression and distribution of DOR in the spinal cord and DRG between mammalian species. Specifically, they point to a progressive specialization of DOR toward the regulation of primary somatosensory, namely nociceptive, inputs during phylogeny and suggest that the effects of DOR agonists in rodents may not be entirely predictive of their action in humans.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Evidence for a localization of [(3)H]nociceptin binding sites on medullar primary afferent fibers

The ORL1 receptor (opioid receptor-like 1) and its endogenous ligand, nociceptin, are involved in nociperception. We have studied, in a deafferented animal model, the modification of medullar [(3)H]nociceptin binding site density. A rhizotomy was carried out in rats at the cervicothoracic level, and the dorsal afferent fibers from C5 to T1 were lesioned. Seven days after surgery, animals were sacrificed, and the binding of [(3)H]nociceptin (2 nM) was then performed on spinal cord sections. An autoradiographic analysis revealed a significant reduction (-18%) of [(3)H]nociceptin binding site density in the dorsal horn ipsilateral to the deafferentation compared with the contralateral side of the lesion. In the ventral horn, no significant difference (-5%) of binding was observed in the ipsilateral side of the deafferentation compared with the contralateral side. Thus, [(3)H]nociceptin binding sites appear to be located mainly on either interneurons or deutoneurons of the spinal cord, because the bulk of the labeling is spared by the lesion. However, the significant reduction of labeling that occurs on the dorsal part of the ipsilateral side to the lesion indicates that [(3)H]nociceptin binding sites are also present on these dorsal afferent fibers.

Immunohistochemical distribution of delta opioid receptors in the rat central nervous system: evidence for somatodendritic labeling and antigen-specific cellular compartmentalization

Many studies have reported on the distribution of delta opioid receptors (delta OR) in the mammalian central nervous system (CNS) by using a variety of techniques. However, no general consensus has emerged with regards to the localization of this receptor due to inconsistencies in the immunohistochemical literature. In the present study, we analyzed the cellular and subcellular distribution of immunoreactive delta OR in the rat CNS using two different antibodies (directed against a sequence in the C-terminus or N-terminus of the rat delta OR). By using Western blotting, these two antibodies recognized similar forms of the delta OR in COS-7 cells transfected with this receptor, but distinct forms in membranes from the rat spinal cord. By using light microscopic immunohistochemistry, both antibodies recognized identical populations of nerve cell bodies throughout the CNS; the distribution of these cell bodies conformed to that of delta OR mRNA-expressing cells detected by in situ hybridization. However, whereas the C-terminus-directed antibody recognized predominantly perikarya and proximal dendrites, the N-terminus-directed antibody also labeled extensively dendritic and terminal arbors. Furthermore, by using electron microscopy, the two antibodies were found not only to label differentially somatodendritic versus axonal compartments, but also plasma membrane versus cytoplasmic ones, suggesting that distinct immunological forms of the receptor are being targeted preferentially to different cellular and subcellular domains.

Equal proportions of small and large DRG neurons express opioid receptor mRNAs

Previous studies have reported that the mRNAs encoding the cloned mu-opioid receptor (MOR1) and the cloned delta-opioid receptor (DOR1) are expressed in the dorsal root ganglia (DRG) of rats. In the present study, we determined the sizes of DRG neurons expressing DOR1 and MOR1 mRNAs and examined whether or not DRG neurons were likely to be the source of the DOR1 and MOR1 immunoreactivity previously observed in the spinal dorsal horn. DRG neurons were labeled in five male Sprague-Dawley rats by applying Fluoro-Gold (FG) topically to the dorsal root entry zone. Five-micrometer cryostat sections were cut, and in situ hybridization was performed using full-length cRNA probes labeled with 35S-UTP. The distribution of sizes of DRG neuronal profiles (1372 neuronal profiles were evaluated) ranged from 98 to 2081 microm(2) and was similar to those found in previous reports. Of 583 retrogradely labeled neuronal profiles in DRGs, 246 (40 +/- 14%, mean +/- SD, n = 5) expressed MOR1 mRNA. Of 789 DRG cell profiles from sections that were hybridized for DOR1 mRNA, 687 (85 +/- 18%, mean +/- SD, n = 5) were labeled for DOR1. The proportion of DRG cell profiles expressing DOR1 mRNA was significantly higher than that expressing MOR1 mRNA (P < 0.0001, chi-square test). No significant differences were observed between small (less than or = 700 microm(2)) and large (> 700 microm(2)) FG-labeled neurons in the proportions labeled for either MOR1 mRNA (202/497 vs. 44/86, P > 0.2, chi-square test) or DOR1 mRNA (555/651 vs. 132/138, P > 0.3, chi-square test). Most FG-labeled neurons that expressed either MOR1 mRNA or DOR1 mRNA (82.1 and 80.8%, respectively) were smaller than 700 microm(2). In addition to cells expressing a single opioid receptor, individual DRG neurons were observed that expressed both MOR1 and DOR1. In a sample of 25 DRG neurons expressing MOR1-mRNA, 23 also expressed DOR1 mRNA. Within the spinal cord itself, DOR1 and MOR1 mRNAs had different patterns of expression. Both were expressed in the dorsal horn, but of the two, only MOR1 message was expressed in the superficial dorsal horn. We conclude that both small and large DRG neurons express DOR1 and MOR1 mRNAs, but most cells expressing these mRNAs are small. In addition, some DRG neurons express both MOR1 and DOR1 mRNAs. Finally, both DOR1 and MOR1 in the spinal dorsal horn originate, at least in part, from DRG neurons.

Immunohistochemical localization of delta opioid receptors in peripheral tissues

The distribution of delta opioid receptor (DOR) immunoreactivity (ir) was examined in various peripheral tissues of Sprague-Dawley rats and macaque monkeys, including glabrous and hairy skin, corneas, eyelids, and the lip. DOR-ir was observed in all tissues examined. In addition to the presence of DOR-immunoreactive fibers in subcutaneous nerve bundles and the papillary dermis, we report the existence of positively labeled fibers and terminals in close association with peripheral structures not traditionally assigned a primarily nociceptive function, such as hair follicles, glandular apparatus, and blood vessels. In every case, staining was restricted to small-diameter axons that appeared to terminate as free nerve endings. To further classify DOR-immunoreactive fibers, we examined the extent of colocalization between DOR and three commonly used neuronal subtype markers; tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), and RT-97, a monoclonal antibody which preferentially labels neurons with myelinated axons. Additional double-labeling experiments using the nonspecific neuronal marker Protein Gene Product 9.5 were performed in glabrous skin to determine the percentage of total fiber count that displayed DOR-ir. No colocalization was observed between DOR and RT-97, indicating that DOR-ir is localized to unmyelinated axons. In addition, DOR colocalized with CGRP, but did not colocalize with TH. Taken together, these data support the hypothesis that delta opioid receptors in peripheral tissues are associated with sensory fibers, but not with the terminals of postganglionic sympathetic neurons.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Pre- and postsynaptic distribution of cannabinoid and mu opioid receptors in rat spinal cord

In vitro receptor binding and quantitative autoradiography were used to assess the pre- and postsynaptic distribution of cannabinoid receptors in the cervical dorsal horn of the rat spinal cord. An extensive unilateral dorsal rhizotomy was performed across seven or eight successive spinal segments from C3 to T1 or T2. The densities of cannabinoid and mu opioid receptors in the central (C6) spinal segment were assessed 2, 4, 8, and 16 days post rhizotomy and compared with those of untreated rats. Rhizotomy induced approximately a 50% ipsilateral loss in the [3H]CP55,940 binding to spinal cannabinoid receptors that was maximal at 8 days post-rhizotomy. By comparison, the binding of [3H][d-Ala2-MePhe4, Gly-ol5]enkephalin (DAMGO) to mu receptors was depleted approximately 60% in near-adjacent sections. By contrast, changes in [3H]CP55,940 binding contralateral to the deafferentation were largely absent at all post-lesion delays. These data suggest that under conditions in which a spinal segment is completely deafferented, approximately 50% of cannabinoid receptors in the cervical (C6) dorsal horn reside presynaptically on central terminals of primary afferents. The present data provide anatomical evidence for presynaptic as well as postsynaptic localization of cannabinoid receptors in the spinal dorsal horn.

Does the right side know what the left is doing?

Following peripheral-nerve lesions there are well-documented events that affect the contralateral nonlesioned structures. These contralateral effects are qualitatively similar to those occurring at the ipsilateral side, but are usually smaller in magnitude and have a briefer time course. It is unclear whether the findings are an epiphenomenon or serve a biological purpose, but in either case the existence of these effects implies the presence of unrecognized signalling mechanisms that link the two sides of the body. Strong circumstantial evidence argues against a peripheral mechanism (for example, via circulating factors) and in favour of a central mechanism, in particular signalling via the system of commissural interneurons that is present in spinal cord and brainstem. While an altered pattern of activity in this system might underlie the phenomenon, there are several reasons for proposing that the changes depend upon chemical signals, possibly growth factors. Because of its relative easy access for experimental manipulation, the spinal cord could serve as a model system to study these transmedian signalling systems.

Regulation of cannabinoid and mu opioid receptors in rat lumbar spinal cord following neonatal capsaicin treatment

In vitro receptor binding and quantitative autoradiography were used to determine whether cannabinoid receptors in rat lumbar spinal cord are localized to the central terminals of nociceptive primary afferents. Rats were treated as neonates with capsaicin to destroy sensory C-fibers. The densities of cannabinoid and mu opioid receptors in the spinal cord of the adult rats were compared with age-matched vehicle controls. Neonatal capsaicin produced a moderate but reliable suppression (16%) of [3H]CP55,940 binding to cannabinoid receptors. By contrast, the binding of [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin (DAMGO) to mu receptors was depleted by approximately 60% in near adjacent sections. These data suggest that only a subpopulation of cannabinoid receptors is situated on the central terminals of primary afferent C-fibers. The present data provide anatomical evidence for a dissociation between cannabinoid and mu opioid modulation of sensory transmission at the level of the primary afferent inputs to the spinal cord.

Treatment of chronic allodynia in spinally injured rats: effects of intrathecal selective opioid receptor agonists

We examined the effects of intrathecal (i.t.) selective opioid receptor agonists in alleviating mechanical and cold allodynia in spinally injured rats. Both DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a mu-opioid receptor agonist) and DPDPE ([D-Phe2,D-Phe5]-enkephalin, a delta-opioid receptor agonist) dose-dependently relieved the chronic allodynia-like behavior at doses selective for their respective receptors. The anti-allodynic effect of DAMGO and DPDPE was reversed by the selective mu- and delta-opioid receptor antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) and naltrindole, respectively. In contrast, the selective kappa-opioid receptor agonist U50488H did not alleviate the allodynia-like behavior, but rather enhanced it. The anti-nociceptive and anti-allodynic effect of i.t. DAMGO was blocked by U50488H. Thus, activation of spinal mu- and delta-, but not kappa-opioid receptors produced anti-allodynic effect in this model of central pain. Drugs which act selectively on opioid receptor subtypes may be useful in managing chronic central pain of spinal cord origin.

Immunoreactivity for endomorphin-2 occurs in primary afferents in rats and monkey

Antisera were raised against endomorphin-2, a recently isolated endogenous opioid peptide that binds potently and selectively to the mu-opioid receptor. When sections of spinal cord were stained immunocytochemically, a dense plexus of fibres and varicosities was visualized in the superficial dorsal horn of rats and one monkey. Following unilateral multiple dorsal rhizotomy, labeling for endomorphin-2 was markedly reduced ipsilateral to the lesion. In sections stained for both endomorphin-2 and CGRP, double-labeling was observed. Taken together, these data suggest that endomorphin-2 occurs in small diameter primary afferent fibres in rodents and primates. It appears possible that the release of neurotransmitters from nociceptive primary afferents might be regulated by release of endomorphin-2 from primary afferent terminals.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

Pharmacology and mechanisms of opioid analgesic activity

Opiates by an action at specific receptors can induce a highly selective alteration in the response of humans and animals to strong and otherwise aversive chemical, mechanical or thermal stimuli. Specific investigations in a variety of species from rodent to primate using microinjection techniques to examine the pharmacology of local drug action have shown potent antinociceptive actions to be mediated by a receptor specific action at a number of sites within the brain, including the periaqueductal gray (PAG: mu receptor), the rostral ventral medulla (mu/delta receptor) and the substantia nigra (mu receptor) and within the spinal dorsal horn (mu/delta/kappa receptor). Mechanistic studies have shown these actions in the different sites to be mediated by several discrete mechanisms. For example, in the PAG, the local opiate effect is likely mediated by the indirect activation of bulbospinal pathways, rostral projections to forebrain sites and by a local alteration in afferent input into the brainstem core. In the spinal cord, this effect is mediated by an action presynaptic to the primary afferent and by a post-synaptic effect to hyperpolarize projection neurons. In addition, it is now appreciated that mu and kappa receptors in the periphery can modulate the sensitized state of the small afferent terminal innervating inflamed tissue and exert an anti-hyperalgesic action. After systemic delivery of an opiate, it is thus clear that a wide array of central and peripheral systems serve to explain the powerful analgesic effect exerted by this class of agents.

Presence of neuropeptide FF receptors on primary afferent fibres of the rat spinal cord

A radioiodinated analogue of neuropeptide FF, [125I][D. Tyr1,(NMe) Phe3]neuropeptide FF, was used as a selective probe to label neuropeptide FF receptors in the rat spinal cord. Following neonatal capsaicin treatment, dorsal rhizotomy or sciatic nerve section, the distribution and possible alterations of spinal cord specific [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF binding sites were evaluated using in vitro quantitative receptor autoradiography. In normal rats, the highest densities of sites were observed in the superficial layers of the dorsal horn (laminae I-II) whereas moderate to low amounts of labelling were seen in the deeper (III-VI) laminae, around the central canal, and in the ventral horn. Capsaicin-treated rats showed a bilateral decrease (47%) in [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF binding in all spinal areas. Unilateral sciatic nerve section and unilateral dorsal rhizotomy induced significant depletions (15-27%) in [125I][D.Tyr1,(NMe)Phe3]neuropeptide FF labelling in the ipsilateral dorsal horn. These results suggest that a proportion of neuropeptide FF receptors is located on primary afferent terminals of the dorsal horn and could thus play a role in the modulation of nociceptive transmission.