The paired homeodomain protein DRG11 is required for the projection of cutaneous sensory afferent fibers to the dorsal spinal cord

Cutaneous sensory neurons that detect noxious stimuli project to the dorsal horn of the spinal cord, while those innervating muscle stretch receptors project to the ventral horn. DRG11, a paired homeodomain transcription factor, is expressed in both the developing dorsal horn and in sensory neurons, but not in the ventral spinal cord. Mouse embryos deficient in DRG11 display abnormalities in the spatio-temporal patterning of cutaneous sensory afferent fiber projections to the dorsal, but not the ventral spinal cord, as well as defects in dorsal horn morphogenesis. These early developmental abnormalities lead, in adults, to significantly attenuated sensitivity to noxious stimuli. In contrast, locomotion and sensori-motor functions appear normal. Drg11 is thus required for the formation of spatio-temporally appropriate projections from nociceptive sensory neurons to their central targets in the dorsal horn of the spinal cord.

PKCgamma contributes to a subset of the NMDA-dependent spinal circuits that underlie injury-induced persistent pain

In previous studies we provided evidence that the gamma isoform of protein kinase C (PKCgamma) is an important contributor to the increased pain sensitivity that occurs after injury. Here we combined electrophysiological and behavioral approaches in wild-type and PKCgamma-null mice to compare the hyperexcitability of wide dynamic range neurons in lamina V of the spinal cord dorsal horn with the behavioral hyperexcitability produced by the same injury [application of a C-fiber irritant, mustard oil (MO), to the hindpaw]. Wild-type and null mice did not differ in their response to mechanical or thermal stimuli before tissue injury, and the magnitude of the response to the MO stimuli was comparable. In wild-type mice, MO produced a dramatic and progressive enhancement of the response of lamina V neurons to innocuous mechanical and thermal stimuli. The time course of the neuronal hyperexcitability paralleled the time course of the MO-induced behavioral allodynia (nocifensive behavior in response to a previously innocuous mechanical stimulus). Neuronal hyperexcitability was also manifest in the PKCgamma-null mice, but it lasted <30 min. By contrast, the behavioral allodynia produced by MO in the PKCgamma-null mice, although reduced to approximately half that of the wild-type mice, persisted long after the lamina V hyperexcitability had subsided. Because the MO-induced behavioral allodynia was completely blocked by an NMDA receptor antagonist, we conclude that PKCgamma mediates the transition from short- to long-term hyperexcitability of lamina V nociresponsive neurons but that the persistence of injury-induced pain must involve activity within multiple NMDA-dependent spinal cord circuits.

Contribution of alpha(2) receptor subtypes to nerve injury-induced pain and its regulation by dexmedetomidine

There is evidence that noradrenaline contributes to the development and maintenance of neuropathic pain produced by trauma to a peripheral nerve. It is, however, unclear which subtype(s) of alpha adrenergic receptors (AR) may be involved. In addition to pro-nociceptive actions of AR stimulation, alpha(2) AR agonists produce antinociceptive effects. Here we studied the contribution of the alpha(2) AR subtypes, alpha(2A), alpha(2B) and alpha(2C) to the development of neuropathic pain. We also examined the antinociceptive effect produced by the alpha(2) AR agonist dexmedetomidine in nerve-injured mice. The studies were performed in mice that carry either a point (alpha(2A)) or a null (alpha(2B) and alpha(2C)) mutation in the gene encoding the alpha(2) AR. To induce a neuropathic pain condition, we partially ligated the sciatic nerve and measured changes in thermal and mechanical sensitivity. Baseline mechanical and thermal withdrawal thresholds were similar in all mutant and wild-type mice; and, after peripheral nerve injury, all mice developed comparable hypersensitivity (allodynia) to thermal and mechanical stimulation. Dexmedetomidine reversed the allodynia at a low dose (3 microg kg(-1), s.c.) and produced antinociceptive effects at higher doses (10 - 30 microg kg(-1)) in all groups except in alpha(2A) AR mutant mice. The effect of dexmedetomidine was reversed by intrathecal, but not systemic, injection of the alpha(2) AR antagonist RS 42206. These results suggest that neither alpha(2A), alpha(2B) nor alpha(2C) AR is required for the development of neuropathic pain after peripheral nerve injury, however, the spinal alpha(2A) AR is essential for the antinociceptive effects of dexmedetomidine.

Urinary bladder hyporeflexia and reduced pain-related behaviour in P2X3-deficient mice

Extracellular ATP is implicated in numerous sensory processes ranging from the response to pain to the regulation of motility in visceral organs. The ATP receptor P2X3 is selectively expressed on small diameter sensory neurons, supporting this hypothesis. Here we show that mice deficient in P2X3 lose the rapidly desensitizing ATP-induced currents in dorsal root ganglion neurons. P2X3 deficiency also causes a reduction in the sustained ATP-induced currents in nodose ganglion neurons. P2X3-null mice have reduced pain-related behaviour in response to injection of ATP and formalin. Significantly, P2X3-null mice exhibit a marked urinary bladder hyporeflexia, characterized by decreased voiding frequency and increased bladder capacity, but normal bladder pressures. Immunohistochemical studies localize P2X3 to nerve fibres innervating the urinary bladder of wild-type mice, and show that loss of P2X3 does not alter sensory neuron innervation density. Thus, P2X3 is critical for peripheral pain responses and afferent pathways controlling urinary bladder volume reflexes. Antagonists to P2X3 may therefore have therapeutic potential in the treatment of disorders of urine storage and voiding such as overactive bladder.

Impaired nociception and pain sensation in mice lacking the capsaicin receptor

The capsaicin (vanilloid) receptor VR1 is a cation channel expressed by primary sensory neurons of the "pain" pathway. Heterologously expressed VR1 can be activated by vanilloid compounds, protons, or heat (>43 degrees C), but whether this channel contributes to chemical or thermal sensitivity in vivo is not known. Here, we demonstrate that sensory neurons from mice lacking VR1 are severely deficient in their responses to each of these noxious stimuli. VR1-/- mice showed normal responses to noxious mechanical stimuli but exhibited no vanilloid-evoked pain behavior, were impaired in the detection of painful heat, and showed little thermal hypersensitivity in the setting of inflammation. Thus, VR1 is essential for selective modalities of pain sensation and for tissue injury-induced thermal hyperalgesia.

Elevated substance-P-like immunoreactivity levels in spinal dialysates during the formalin test in normal and diabetic rats

Pharmacologic studies implicate the involvement of substance P in spinal nociceptive processing during the formalin test. However, no direct measurement of the temporal changes in substance P levels within the spinal cord of conscious animals has been reported. Further, dissociation between substance P levels and formalin-evoked nocifensive behavior may exist in diabetic rats, as exaggerated hyperalgesic behavior coexists with reduced peripheral nerve substance P levels. The present study was performed to directly measure the appearance of substance-P-like immunoreactivity (SP-LI) in spinal CSF of conscious, unrestrained rats using microdialysis techniques following injection of formalin into the hindpaw. The effect of diabetes upon formalin-evoked SP-LI levels in spinal CSF dialysates was also determined. In control rats, SP-LI increased in spinal dialysates following formalin injection and levels were maximal 20-30 min after injection, rising to 325% of basal values (p<0.02). Diabetic rats exhibited reduced (p<0.05) SP-LI in their spinal roots, while basal levels in spinal CSF were not different from controls. Formalin-evoked nocifensive behavior was increased in diabetic rats but SP-LI levels in spinal CSF dialysates after paw formalin injection were significantly (p<0.05) attenuated, reaching a maximum of only 161% of basal levels. This was accompanied by attenuated swelling at the formalin injection site and increased thermal response latencies. While increased SP-LI in spinal CSF coincides with phase 2 behavior in the formalin test and may contribute to spinal nociceptive processing during this period, exaggerated spinal substance P release is unlikely to underlie the increased nocifensive behavior seen in diabetic rats.

Inflammation-induced up-regulation of protein kinase Cgamma immunoreactivity in rat spinal cord correlates with enhanced nociceptive processing

Activation of various second messengers contributes to long-term changes in the excitability of dorsal horn neurons and to persistent pain conditions produced by injury. Here, we compared the time-course of decreased mechanical nociceptive thresholds and the density of protein kinase Cgamma immunoreactivity in the dorsal horn after injections of complete Freund's adjuvant in the plantar surface of the rat hindpaw. Complete Freund's adjuvant significantly increased paw diameter and mechanical sensitivity ipsilateral to the inflammation. The changes peaked one day post-injury, but endured for at least two weeks. In these rats, we recorded a 75-100% increase in protein kinase Cgamma immunoreactivity in the ipsilateral superficial dorsal horn of the L4 and L5 segments at all time-points. Electron microscopy revealed that the up-regulation was associated with a significant translocation of protein kinase Cgamma immunoreactivity to the plasma membrane. In double-label cytochemical studies, we found that about 20% of the protein kinase Cgamma-immunoreactive neurons, which are concentrated in inner lamina II, contain glutamate decarboxylase-67 messenger RNA, but none stain for parvalbumin or nitric oxide synthase. These results indicate that persistent changes in protein kinase Cgamma immunoreactivity parallel the time-course of mechanical allodynia and suggest that protein kinase Cgamma contributes to the maintenance of the allodynia produced by peripheral inflammation. The minimal expression of protein kinase Cgamma in presumed inhibitory neurons suggests that protein kinase Cgamma-mediated regulation of excitatory interneurons underlies the changes in spinal cord activity during persistent nociception.

The cloned capsaicin receptor integrates multiple pain-producing stimuli

Capsaicin, the main pungent ingredient in "hot" chili peppers, elicits buming pain by activating specific (vanilloid) receptors on sensory nerve endings. The cloned vanilloid receptor (VR1) is a cation channel that is also activated by noxious heat. Here, analysis of heat-evoked single channel currents in excised membrane patches suggests that heat gates VR1 directly. We also show that protons decrease the temperature threshold for VR1 activation such that even moderately acidic conditions (pH < or = 5.9) activate VR1 at room temperature. VR1 can therefore be viewed as a molecular integrator of chemical and physical stimuli that elicit pain. Immunocytochemical analysis indicates that the receptor is located in a neurochemically heterogeneous population of small diameter primary afferent fibers. A role for VR1 in injury-induced hypersensitivity at the level of the sensory neuron is presented.

Pain: nocistatin spells relief

Tissue or nerve injury can dramatically alter the transmission of sensory stimuli by spinal cord neurons, so that a light touch produces pain. The discovery that peptide products of prepronociceptin processing either facilitate or inhibit these mechanisms suggests novel approaches to treating these conditions.

Partial sciatic nerve injury in the mouse as a model of neuropathic pain: behavioral and neuroanatomical correlates

The generation of knock-out and transgenic mice offers a promising approach to the identification of novel biochemical factors that contribute to persistent pain conditions. To take advantage of these mice, however, it is important to demonstrate that the traditional models of persistent pain, which were largely developed for studies in the rat, can be used in the mouse. Here, we combined behavioral and anatomical methods to characterize the pathophysiology of a partial nerve injury-evoked pain condition in the 'normal' mouse. In male C57BL6 mice we tied a tight ligature around 1/3 to 1/2 of the diameter of the sciatic nerve and evaluated the time-course and magnitude of the ensuing mechanical and thermal allodynia. We also used immunocytochemistry to analyze nerve injury-induced changes in substance P (SP) and NK-1 (SP) receptor expression in the spinal cord. As in the rat, partial nerve injury markedly decreased paw withdrawal thresholds to both mechanical and thermal stimuli on the injured side. We detected threshold changes one day after the injury. The thermal allodynia resolved by 49 days, but the mechanical allodynia persisted for the duration of the study (70 days). We found no changes contralateral to the nerve injury. Sympatholytic treatment with guanethidine significantly reduced both the thermal and mechanical allodynia. We observed a reduction of SP immunoreactivity in the superficial dorsal horn on the injured side at 7 and 14, but not at 3 or 70 days after the nerve injury, and we observed an increase of NK-1 receptor expression at 3, 7, 14 and 42, but not at 70 days after the injury. We conclude that partial injury to the sciatic nerve produces a comparable allodynia and neurochemical plasticity in the rat and mouse. These results establish a valuable model for future studies of the biochemical basis of neuropathic pain in mice with specific gene modifications.

Preserved acute pain and reduced neuropathic pain in mice lacking PKCgamma

In normal animals, peripheral nerve injury produces a persistent, neuropathic pain state in which pain is exaggerated and can be produced by nonpainful stimuli. Here, mice that lack protein kinase C gamma (PKCgamma) displayed normal responses to acute pain stimuli, but they almost completely failed to develop a neuropathic pain syndrome after partial sciatic nerve section, and the neurochemical changes that occurred in the spinal cord after nerve injury were blunted. Also, PKCgamma was shown to be restricted to a small subset of dorsal horn neurons, thus identifying a potential biochemical target for the prevention and therapy of persistent pain.

Diminished inflammation and nociceptive pain with preservation of neuropathic pain in mice with a targeted mutation of the type I regulatory subunit of cAMP-dependent protein kinase

To assess the contribution of PKA to injury-induced inflammation and pain, we evaluated nociceptive responses in mice that carry a null mutation in the gene that encodes the neuronal-specific isoform of the type I regulatory subunit (RIbeta) of PKA. Acute pain indices did not differ in the RIbeta PKA mutant mice compared with wild-type controls. However, tissue injury-evoked persistent pain behavior, inflammation of the hindpaw, and ipsilateral dorsal horn Fos immunoreactivity was significantly reduced in the mutant mice, as was plasma extravasation induced by intradermal injection of capsaicin into the paw. The enhanced thermal sensitivity observed in wild-type mice after intraplantar or intrathecal (spinal) administration of prostaglandin E2 was also reduced in mutant mice. In contrast, indices of pain behavior produced by nerve injury were not altered in the mutant mice. Thus, RIbeta PKA is necessary for the full expression of tissue injury-evoked (nociceptive) pain but is not required for nerve injury-evoked (neuropathic) pain. Because the RIbeta subunit is only present in the nervous system, including small diameter trkA receptor-positive dorsal root ganglion cells, we suggest that in inflammatory conditions, RIbeta PKA is specifically required for nociceptive processing in the terminals of small-diameter primary afferent fibers.

Neonatal capsaicin treatment abolishes formalin-induced spinal PGE2 release

We investigated the effect of neonatal capsaicin treatment on formalin-evoked pain behavior and spinal levels of nociceptive neuromodulators using in vivo intrathecal microdialysis in conscious adult rats and age-matched controls. Capsaicin-treated rats displayed thermal hypoalgesia and a significant decrease in tissue content of calcitonin gene-related peptide. Paw swelling, flinching and release of spinal prostaglandin E2 induced by injection of formalin into the hindpaw were also reduced in capsaicin-treated rats compared with controls, whereas glutamate, aspartate and taurine release was unaffected. These data suggest that formalin-induced inflammation, pain behavior and spinal prostaglandin E2 release are mediated by mechanisms sensitive to neonatal capsaicin while the formalin-evoked release of amino acids in the spinal cord is not.

Capsaicin-evoked release of pituitary adenylate cyclase activating peptide (PACAP) and calcitonin gene-related peptide (CGRP) from rat spinal cord in vivo

Capsaicin-evoked release of pituitary adenylate cyclase activating peptide (PACAP)-like immunoreactivity (LI) from rat spinal cord was examined in vivo. In anaesthetized rats, a catheter was inserted through the atlanto-occipital membrane into the subarachnoid space at the level of the sacral spinal cord for infusion of artificial cerebrospinal fluid. Another catheter was placed in the cisternal opening for outflow. Blood pressure was monitored and kept stable during the experiment. Perfusion samples were analyzed for PACAP and calcitonin gene-related peptide (CGRP) by radioimmunoassay. The addition of capsaicin (10 microM) to the perfusate elevated the concentrations of PACAP-27-LI in the artificial cerebrospinal fluid by 177%, PACAP-38-LI by 93% and CGRP-LI by 692%. In view of the presence of PACAP-immunoreactive nerve fibres in the superficial layers of the dorsal horn and the expression of PACAP in the small sized neurons in the dorsal root ganglia, the findings suggest that PACAP is released into the artificial cerebrospinal fluid from C-fibres in the spinal cord. PACAP conceivably plays a modulating role in nociception.

Efficacy of spinal NMDA receptor antagonism in formalin hyperalgesia and nerve injury evoked allodynia in the rat

Neuropathic pain remains a significant clinical problem. Current understanding implicates the spinal cord dorsal horn N-methyl-d-aspartate (NMDA) receptor apparatus in its pathogenesis. Previous reports have described NMDA antagonist reduction of nerve injury-induced thermal hyperalgesia and formalin injection-related electrical activity. We examined a panel of spinally administered NMDA antagonists in two models: allodynia evoked by tight ligation of the fifth and sixth lumbar spinal nerves (a model of chronic nerve injury pain), and the formalin paw test (a model wherein pretreatment with drug may preempt the development of a pain state). A wide range of efficacies was observed. In the nerve injury model, order of efficacy (expressed as percent of maximum possible effect +/- S.E.), at the maximum dose not yielding motor impairment, was memantine (96 +/- 5%) = AP5 (91 +/- 7%) > dextrorphan (64 +/- 11%) = dextromethorphan (65 +/- 22%) > MK801 (34 +/- 8%) > ketamine (18 +/- 6%). For the formalin test, the order of efficacy was AP5 (86 +/- 9%) > memantine (74 +/- 5%) > or = MK801 (67 +/- 16%) > dextrorphan (47 +/- 16%) > dextromethorphan (31 +/- 12%) > ketamine (17 +/- 15%). In the nerve injury model, no supraspinal action was seen after intracerebroventricular injections of dextromethorphan and ketamine. NMDA antagonists by the spinal route appear to be useful therapeutic agents for chemically induced facilitated pain as well as nerve injury induced tactile allodynia. It is not known what accounts for the wide range of efficacies.

The effect of alpha-trinositol (D-myo-inositol 1,2,6-trisphosphate) on formalin-evoked spinal amino acid and prostaglandin E2 levels

The effect of the inositol trisphosphate analog alpha-trinositol on noxious-evoked behavior, amino acid and prostaglandin E2 (PGE2) release was examined in unanesthetized rats using intrathecal microdialysis probes. Subcutaneous injection of 50 microliters 5% formalin solution produced two phases of pain-like behavior and significant elevation of glutamate, aspartate, glycine, taurine and serine during phase 1. PGE2 concentrations were increased during both phases 1 and 2. Intraperitoneal delivery of 300 mg/kg alpha-trinositol significantly suppressed both phases 1 and 2 of formalin-induced behavior and the associated elevation of amino acids and PGE2. These data demonstrate that the antinociceptive effect of alpha-trinositol corresponds to suppression of noxious-evoked release of amino acids and PGE2 from the spinal cord.

The effect of pituitary adenylate cyclase activating peptide (PACAP) on the nociceptive formalin test

Recent studies have suggested that pituitary adenylate cyclase activating peptide (PACAP) may be involved in nociceptive transmission. The present study examined the effect of low-dose PACAP-27 on nociceptive behavior using the formalin test. PACAP-27 was administered intrathecally. Twenty minutes later, formalin (50 microliters, 5%) was injected subcutaneously into the dorsal surface of the right hind paw. Intrathecal PACAP-27 at 0.6 pmol suppressed the second phase response to formalin, while 5 pmol depressed both phases. PACAP-27 at 5 pmol did not impair motor function. Hence, the data suggest that the effect of PACAP-27 on formalin-induced pain-related behavior is specific. The findings may lead to a better understanding of the role of PACAP in nociceptive transmission.

Antinociceptive effect of alpha-trinositol, a novel D-myo-inositol phosphate derivative, in the formalin test in rats

The antinociceptive effect of alpha-trinositol was examined in rats using the formalin test following systemic, spinal and local subcutaneous administration. Injection of formalin into the paw evoked two phases (phase 1: 0-9 min; phase 2: 10-60 min) of flinching behavior of the injected paw. Intrathecal administration of alpha-trinositol resulted in a dose-dependent suppression of the first (ED50: 8 microg) and second (ED50: 9 microg) phase of formalin-evoked behavioral response. Similarly, intraperitoneal delivery showed a dose-dependent reduction of the first (ED50: 83 mg/kg) and second (ED50: 56 mg/kg) phase of the formalin test. Subcutaneous injection of 100 microg, but not 10 mu g, alpha-trinositol into the rat paw together with the formalin solution, had no effect on the first phase, but reduced by 20% the second phase of behavior. These data show that alpha-trinositol produces a suppression of acute and prolonged nociceptive behaviors with a central mechanism of action, although some peripheral component may contribute to the reduction of the late phase following systemic administration.

Effects of prostaglandin E2 and capsaicin on behavior and cerebrospinal fluid amino acid concentrations of unanesthetized rats: a microdialysis study

Prostaglandin E2 (PGE2) delivered to the spinal cord produces an increased sensitivity to noxious (hyperalgesia) and innocuous (allodynia) stimuli. The mechanisms that underlie this effect remain unknown, but a PGE2-evoked enhancement of spinal neurotransmitter release may be involved. To address this hypothesis, we examined the effect of PGE2 on CSF concentrations of amino acids and also the modulatory effect of PGE2 on capsaicin-evoked changes of spinal amino acid concentrations using a microdialysis probe placed in the lumbar subarachnoid space. Amino acids were quantified using HPLC with fluorescence detection. Addition of 1 mM, but not 10 or 100 microM, PGE2 to the perfusate for a 10-min period (flow rate, 5 microliters/min) evoked an immediate increase (80-100%) in glutamate (Glu), aspartate (Asp), taurine (Tau), glycine (Gly), and gamma-aminobutyric acid (GABA) concentrations. Similarly, capsaicin infusion (0.1-10 microM) induced a dose-dependent increase in Glu, Asp, Tau, Gly, GABA, and ethanolamine levels. Significant increases in amino acid levels evoked by PGE2 or capsaicin were associated with a touch-evoked allodynia. The combination of PGE2 (10 microM) and capsaicin (0.1 or 1.0 microM) at concentrations that individually had no effect together evoked a significant increase (60-100%) in Glu, Asp, Tau, Gly, and GABA concentrations and produced tactile allodynia. These data demonstrate that spinally delivered PGE2 or capsaicin substantially elevates CSF concentrations of both excitatory and inhibitory amino acids. The capacity of PGE2 to enhance and prolong capsaicin-evoked amino acid concentrations may be one of the mechanisms by which spinal PGE2 produces hyperalgesia and allodynia.

The spinal loop dialysis catheter: characterization of use in the unanesthetized rat

To permit long-term measurement of time-dependent changes in levels of dialyzable drugs and transmitters in the spinal intrathecal (i.t.) space of the unanesthetized rat, we developed a dialysis catheter for chronic placement. This was accomplished by constructing a loop probe 9 cm in length from 0.3-mm-diameter dialysis tubing that was made impermeable except for the distal loop. This loop catheter was readily inserted though an incision in the cisternal membrane and passed to the lumbar enlargement. The ends of the catheter were then externalized on the top of the head. To permit i.t. injections, an additional i.t. catheter could also be inserted simultaneously by the same route. For dialysis, an external end of the loop catheter was connected to a syringe pump and perfused with artificial CSF (10 microliters/min) and the out flow collected. A series of studies were performed to demonstrate the characteristics and utility of this technique. (1) Stability of resting release: glutamate and glucose concentrations in spinal dialysate showed no significant changes from 3 to 10 days after implantation. (2) Spinal cord ischemia: ischemia induced by aortic occlusion or cardiac arrest evoked a time dependent increase in retrieved glutamate. (3) Spinal cord compression caused a time-dependent glutamate, aspartate and PGE2 increase. (4) Noxious afferent stimulation induced by the injection of formalin into the hindpaw resulted in a rapid and transient increase in dialysate glutamate concentration. (5) Direct activation of spinal excitatory amino acids receptors by i.t. injection of kainic acid (1 microgram) evoked a significant increase in aspartate and taurine. (6) Continuous delivery of spinal opiate (alfentanil) via dialysis resulted in a maintained, concentration dependent elevation in the thermal escape latencies in the unanesthetized rat. The loop dialysis catheter provides a robust experimental tool for studying time dependent changes in the concentration of diffusible substances in spinal CSF over an extended post-implantation interval and allows comparison of these changes with concurrently assessed behavioral indices.

Different effects of two aldose reductase inhibitors on nociception and prostaglandin E

This study examined the effect of two structurally dissimilar aldose reductase inhibitors, N-[[5-(trifluoromethyl)-6-methoxy-1- napthalenyl]thioxomethyl]-N-methlyglycine (tolrestat) and 4-amino-2,6-dimethylphenyl-sulphonyl nitromethane (ICI 222155), on formalin-evoked behavioural responses in control and diabetic rats and on capsaicin-evoked release of prostaglandin E from spinal cord slices in vitro. Both compounds, given orally for 4 weeks, prevented hyperalgesia in diabetic rats 5-20 min after hindpaw formalin injection. ICI 222155 also prevented hyperalgesia in diabetic rats 21-60 min after formalin, whereas tolrestat suppressed activity in diabetic rats below controls and also suppressed activity in controls when given orally or intrathecally. Capsaicin-evoked release of prostaglandin E from spinal cord slices of control rats was significantly reduced by tolrestat, but not ICI 222155. These data suggest that hyperalgesia in diabetic rats is related to glucose metabolism by aldose reductase, whereas tolrestat has specific effects on formalin-evoked nociception associated with an ability to reduce spinal prostaglandin release.

Characterization of the spinal antinociceptive activity of constrained peptidomimetic opioids

We examined the in vitro and in vivo bioactivities of several families of peptidomimetic opioids including: constrained linear enkephalin (n = 12 analogs), dermorphin (n = 9 analogs) and morphiceptin (n = 17 analogs). The biological activities were assessed in vitro by examining the inhibitory effects of these agents on the electrically evoked contractions of the guinea pig ileum (GPI) and the mouse vas deferens (MVD) preparations. The in vivo bioactivities were determined from the antinociceptive activity of these agents on the 52.5 degrees C hot-plate test after spinal administration of rats with chronically placed spinal catheters. Examination of the effect of cyclization, incorporation of retro-inverso bonds and substitutions of D- or constrained amino acids reveals systematic changes in the activity of these agents. There was a significant correlation between the potency of these agents in the hot-plate bioassay and their activity in the GPI and, to a lesser extent, in the MVD tests. Examination of the ability of naltrindole (a delta selective antagonist) to reverse the drug action and the respective potency on the GPI and MVD, showed that a correlation exists with actions on the MVD, but not on the GPI, consistent with the likelihood that agents with high MVD/GPI ratios in vitro act at the mu sites, whereas those with low MVD/GPI ratios act at the delta receptor in the spinal cord. The close correlations between activity in the GPI and spinal cord suggest that the structural requirements for potency in the smooth muscle and in the spinal cord are essentially the same as those mu receptors that mediate nociceptive transmission.

Highly potent side chain-main chain cyclized dermorphin-deltorphin analogues: an integrated approach including synthesis, bioassays, NMR spectroscopy and molecular modelling

Our continuing efforts to study structure-activity relationships of peptide opioids have resulted in the synthesis of a series of cyclic opioids related to dermorphins and deltorphins. The biological activities of the compounds have been determined and the conformational analyses carried out using 1H-NMR spectroscopy and molecular modelling. The three compounds in the series Tyr-c[D-Orn-Phe-Ala], Tyr-c[D-Lys-Phe-Ala], and Tyr-c[A2bu-Phe-Ala-Leu] are cyclized via a lactam bridge from the side-chain of the residue at the second position with the carboxyl terminus of each compound. The molecules incorporate 12-, 13- and 14-membered rings, respectively. They include a phenylalanine at the third position which is a distinguishing characteristic of dermorphins and deltorphins. The guinea pig ileum and mouse vas deferens assays show that the compounds are highly active at both mu- and delta-opioid receptors. The compounds are all highly effective antinociceptive agents as measured by the intrathecal rat hot plate test. Conformational analyses of the molecules indicate that they can adopt topochemical arrays required for bioactivity at both mu- and delta-receptors which explains their high activity in both guinea pig ileum and mouse vas deferens in vitro assays. The results support our models for mu- and delta-receptor activity for constrained peptide opioids.

Cyclooxygenase inhibition and the spinal release of prostaglandin E2 and amino acids evoked by paw formalin injection: a microdialysis study in unanesthetized rats

Injection of formalin into the hind paw evokes a biphasic flinching of the injured paw. Pharmacological characterization of this behavior has implicated the spinal release of excitatory amino acids (EAAs) and cyclooxygenase (COX) products. To address this hypothesis, we examined the effect of paw formalin injection on release of EAAs and prostaglandin E2-like immunoreactivity (PGE2-LI) from the spinal cord in unanesthetized rats using a dialysis probe placed in the lumbar subarachnoid space. To assess the contribution of spinal COX products, the effects of S(+)- and R(-)-ibuprofen (active and inactive COX inhibitors) were examined. Paw formalin injection evoked a biphasic spinal release of PGE2-LI with an increase above resting concentrations of 110% in the 0-10 min sample, and of 83% in the 20-30 min sample. Significantly increased release of glutamate (Glu; 110%) and aspartate (Asp; 112%) was only observed in the 0-10 min sample. Saline injection into the paw had no effect on behavior, PGE2-LI, or EAA release. Intraperitoneal administration of 10 mg/kg, but not 1 mg/kg, S(+)-ibuprofen reduced paw flinching, blocked the elevated levels of PGE2-LI, and suppressed Glu and Asp release to 50% of control. Intrathecal delivery of 10 micrograms, but not 1 microgram, S(+)-ibuprofen also suppressed formalin-induced behavior, PGE2-LI, Glu, and Asp release. R(-)-ibuprofen showed no effect on formalin-induced behaviors or spinal release. These data demonstrate that paw formalin injection produces spinal release of PGE2-LI corresponding to the biphasic behavioral response and that the evoked release is blocked by antinociceptive doses of COX inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of morphine on formalin-evoked behaviour and spinal release of excitatory amino acids and prostaglandin E2 using microdialysis in conscious rats

1. In the present study, the object was to examine the effects of morphine on spinal release in vivo of excitatory amino acids (EAA), prostaglandin E2 (PGE2), and a marker for nitric oxide (NO) synthesis, citrulline (Cit), evoked by a protracted noxious stimulus produced by the injection of formalin into the paw. Spinal release was monitored in conscious rats using a microdialysis probe implanted into the subarachnoid space with the active site placed at the level of the lumbar enlargement. In split dialysate samples. EAAs were measured by high performance liquid chromatography (h.p.l.c.) and PGE2 was determined by radioimmunoassay. 2. Resting concentrations in nmol ml-1 for the amino acids (mean +/- s.e.mean, n = 21) were: 4.8 +/- 0.4 for glutamate (Glu), 0.8 +/- 0.1 for aspartate (Asp), 8.8 +/- 0.8 for taurine (Tau), 24 +/- 3 for glycine (Gly), 19 +/- 3 for serine (Ser), 5.2 +/- 0.8 for asparagine (Asn), 64 +/- 4 for glutamine (Gln) and 5.2 +/- 0.4 for Cit. Mean basal release for PGE2 was 12 +/- 1 pmol ml-1. 3. Subcutaneous (s.c.) injection of 5% formalin evoked a biphasic flinching behaviour (phase 1: 0-9 min and phase 2: 10-60 min) of the injected paw. Corresponding to phase 1 behaviour, there was a significant increase (50-100%) in spinal levels of Glu, Asp, Tau, Gly, Cit and PGE2, but not Ser, Asn and Gln. A significant (P < 0.01) second phase increase in release was observed only for Cit and PGE2. However, Glu and Asp levels were increased by approximately 45%. 4. Injection of morphine sulphate (3 mg kg-1, s.c.) had no effect on resting release, but produced a significant suppression of the formalin-evoked behaviour and release of Glu, Asp, Tau, Gly, Cit and PGE2. The effect of morphine was reversed by pretreatment with 1 mg kg-1 naloxone. Naloxone by itself did not change the release or behaviour of the formalin test.5. This study demonstrates that both spinal EAA and PGE2 release patterns correlate with behavioural nociceptive responses in the formalin test and that morphine suppresses the formalin-evoked behaviours and spinal release. The reversal by naloxone of the morphine effect indicates mediation via an opioid receptor.

Capsaicin-evoked prostaglandin E2 release in spinal cord slices: relative effect of cyclooxygenase inhibitors

The release of prostaglandin E2 was examined from superfused spinal cord slices. The addition of capsaicin to the perfusate resulted in a dose-dependent increase of prostaglandin E2-like immunoreactivity. Capsaicin (10 microM) evoked prostaglandin E2 release from basal levels of 5.3 +/- 0.8 to 30 +/- 3 fmol/10 min fraction. The capsaicin-evoked release was blocked by the capsaicin receptor antagonist capsazepine (10 microM), but not by removal of extracellular Ca2+ ions. Addition of non-steroidal anti-inflammatory drugs (NSAIDs) to the perfusate had no effect on resting levels of prostaglandin E2, but resulted in a concentration-dependent suppression of capsaicin-evoked release of prostaglandin E2. The IC50 values (in microM) were: indomethacin: 0.7, S(+)-flurbiprofen: 2.0, acetaminophen: 4.4,ketorolac: 5.0, R(-)-flurbiprofen: 8.7, S(+)-ibuprofen: 9.5, and for R(-)-ibuprofen: > 10. The relative potency for the NSAIDs to reduce capsaicin-evoked prostaglandin E2 release, with the exception of acetaminophen, corresponds to their antinociceptive activity after spinal delivery, a finding which further supports the role of prostanoids in spinal nociceptive processing.

Voltage-sensitive calcium channels in spinal nociceptive processing: blockade of N- and P-type channels inhibits formalin-induced nociception

The role of spinal voltage-sensitive calcium channels (VSCC) in a behavioral model of prolonged nociception was examined in rats. Blockade of VSCC by the trivalent cations neodymium (NdCl3) and lanthanum (LaCl3) resulted in a dose-dependent suppression of both phases of the response to formalin. omega-Conopeptides, which selectively block N-type VSCC, also produced a dose-dependent inhibition of both the initial behavior [phase 1; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.010) > SNX-239 (0.16) >> SNX-159 (> 0.26); SNX-199 (> 0.30)] and the facilitated response [phase 2; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.009) > SNX-239 (0.020) > SNX-159 (0.120) = SNX-199 (0.230)]. In contrast, SNX-231 (0.24 nmol), which is selective for a non-L/non-N site and also the L-type VSCC blockers nifedipine (24 nmol), nimodipine (29 nmol), verapamil (200 nmol), and diltiazem (220 nmol), had minimal effects on either phase of the formalin test at the highest dose examined. The P-type channel blocker omega-agatoxin IVA produced a 40% inhibition of phase 1 at the highest dose and phase 2 was suppressed in a dose-dependent fashion (ED50, 0.001 nmol). The response latency to a high-threshold thermal stimulus (the 52.5 degrees C hot plate) was moderately (20%) increased by NdCl3 (0.30 nmol) and SNX-111 (0.008 nmol), but not verapamil (200 nmol) and omega-agatoxin IVA (0.006 nmol). High doses of the N-type VSCC produced characteristic shaking behavior, serpentine-like tail movements, and impaired coordination. However, at antinociceptive doses there was no significant motor effect, though three of the N-type antagonists produced some tail movements. These studies demonstrate that VSCC of the N- and P-type, but not L-type, are involved in facilitated nociceptive processing at the spinal level.

Antinociceptive effect of spinally delivered prostaglandin E receptor antagonists in the formalin test on the rat

The antinociceptive effect of spinally administered prostaglandin E2 receptor antagonists, SC-51089 and SC-51234A, which are selective for EP1 receptors, was examined in rats using the formalin test. Intrathecal injection of SC-51089 (30-300 micrograms) or SC-51234A (30-300 micrograms) resulted in a significant, dose-dependent, suppression of the second phase (10-60 min), but not the first phase (0-9 min), flinching behavior evoked by formalin injection into the paw. ED25 values and 95% confidence intervals for the second phase were 120 (70-200) micrograms for SC-51089 and 80 (50-140) micrograms for SC-51234A. These data demonstrate that specific nociceptive behaviors are attenuated by spinal prostaglandin E2 receptor antagonists and suggest that prostaglandin E2 is involved in facilitated processing at the spinal level.

Antinociception produced by spinal delivery of the S and R enantiomers of flurbiprofen in the formalin test

The antinociceptive effect of spinally delivered S- and R-flurbiprofen, enantiomers of 2-arylpropionic acid, was studied in rats using the formalin test. Intrathecal injections of S- or R-flurbiprofen produced a significant reduction of the second phase of the formalin test, with no effect on the first phase. The maximal suppression of the second phase was similar for both agents (about 50% at the highest doses). While both agents were active, S-flurbiprofen was significantly more potent than R-flurbiprofen. The potency ratio between the dose-response curves was 10 (6-20; 95% confidence intervals). These results demonstrate that both S- and R-flurbiprofen produce antinociception after spinal administration. The potency difference is similar to that for inhibition of cyclooxygenase in brain tissue and supports the hypothesis that cyclooxygenase products are involved in prolonged spinal nociceptive transmission.

Anti-nociceptive effects of the GM1 ganglioside derivative AGF 44 on the formalin test in normal and streptozotocin-diabetic rats

The effect of AGF 44, an ester derivative of ganglioside GM1, on formalin-induced nociceptive behavior was examined in normal and streptozotocin-diabetic rats. AGF 44 (30 mg/kg/day i.p. for 7 days) produced a significant reduction of the second phase (20-40 min) and a lesser degree of suppression of the first phase (1-2 min) of the formalin test in both control and diabetic rats. Diabetic rats showed an increased response during the quiescent period (5-16 min) which was ameliorated by treatment with AGF 44 (30 mg/kg i.p.) for the last 7 days of a 5 week period of diabetes. These results indicate that AGF 44 can diminish responses to a prolonged nociceptive stimulus in both normal and diabetic rats and reduces the exaggerated nociceptive behavior of diabetic rats.

Pharmacology of the spinal action of ketorolac, morphine, ST-91, U50488H, and L-PIA on the formalin test and an isobolographic analysis of the NSAID interaction

BACKGROUND

Noxious cutaneous stimuli enhance spinal excitability. The behavioral correlate to this response is found in the rat formalin test, in which formalin injection into the hindpaw evokes signs of nociception (flinching and licking of the injected paw) with acute (phase 1) and delayed-hyperalgesic (phase 2) components.

METHODS

The effect of intrathecal morphine (a mu agonist), U50488H (a kappa agonist), ST-91 (an alpha 2 agonist), L-PIA (an adenosine A1 agonist), and ketorolac (a nonsteroidal antiinflammatory drug, or NSAID), were examined in rats undergoing the formalin test. Spinal interactions between ketorolac and the mu, kappa, alpha 2, and adenosine A1 agonists were assessed using isobolographic analysis.

RESULTS

Morphine and ST-91 caused a dose-dependent suppression of phase 1 and phase 2 of the formalin test, while U50488H and L-PIA had little effect on phase 1, but caused dose-dependent depression of phase 2. Intrathecal ketorolac inhibited the phase 2 response, but had limited effect on phase 1. The isobolographic analysis revealed a significant synergy (with fractional dose ratios of less than 1) between ketorolac and morphine or ST-91 for phase 1 and phase 2, but only an additive interaction was found between ketorolac and L-PIA or U50488H.

CONCLUSIONS

These observations offer systematic support for the powerful interaction between NSAIDs and opioids and certain other analgesics in clinical pain states. These studies also demonstrate that spinal synergy is not a common property of all interactions. Thus, the NSAID synergy appears to occur with agents that exert a concurrent action both pre- and postsynaptic to the primary afferents.

Antinociceptive actions of spinal nonsteroidal anti-inflammatory agents on the formalin test in the rat

Subcutaneous injection of formalin into the dorsal surface of the hindpaw evoked a two-phased flinching (phase 1:0-9 min; phase 2: 10-60 min) of the injected paw. Intrathecal administration of the nonsteroidal anti-inflammatory drugs (NSAID) produced minimal effects upon phase 1, but showed a significant, though submaximal, dose-dependent suppression of the phase 2 response. Ordering of i.t. potency was (ID50 in nmol): indomethacin (1.9) > or = flurbiprofen (2.1) > ketorolac (5.2) > or = zomepirac (5.9) > S(+)ibuprofen (16) > or = ibuprofen(racemic) (19) > acetylsalicylic acid (27) > acetaminophen (250) > R(-)ibuprofen (> 270) = 0. Intraperitoneal administration also produced a dose-dependent inhibition of phase 2, but only at doses which were 100 to 1000 times higher than those required to produce similar effects after i.t. injection. Intrathecal and i.p. dose-response curves showed similar distinct plateaus of maximum achievable inhibition (intrinsic activity) of the phase 2 behavior, ranging from 20 to 50% of the control response. Varying the time of drug injection reveals that injection 9 min after formalin yielded effects the same as those observed when the agent was given 2 min before formalin. Pretreatment at longer intervals indicated that the duration of the antinociceptive effect was between 3 to 6 hr after the i.t. injection. The i.t. injection of the highest doses of the several NSAID were without significant effect upon the 52.5 degrees C hot plate test. These studies indicate that NSAID have a powerful effect upon spinal nociceptive processing evoked by the s.c. injection of formalin.

Isobolographic and dose-response analyses of the interaction between intrathecal mu and delta agonists: effects of naltrindole and its benzofuran analog (NTB)

Intrathecal administration of DPDPE, PL017, DAMGO, morphine and DADLE produced a dose-dependent increase in hot plate response latency, with the order of potency (ED50 nmol) being: DAMGO (0.17) > DADLE (0.70) > or = PL017 (1.2) > morphine (15) > DPDPE (130). Characteristics of the spinal mu and delta interaction were determined independently by two methods. 1) In the presence of a fixed dose of DPDPE (150 nmol), there was a left shift in the dose-response curve of the mu agonist, with the magnitude of the shifts being greater than those anticipated from a simple additive interaction: PL017 (31-fold) > or = DAMGO (20-fold) > morphine (6.5-fold) > 4-fold (theoretical additive shift). 2) With an isobolographic analysis, a statistically significant nonlinearity was observed, suggesting a multiplicative interaction upon coadministration of the delta-selective ligand DPDPE together with all the tested mu-selective agonists. Examining antagonist activity, mu agonists were antagonized in a dose-dependent fashion by naloxone and naltrindole-benzofuran analog, whereas DPDPE was reversed by all three antagonists used, naltrindole, naltrindole-benzofuran analog and naloxone. The synergic effect produced by the coadministration of PL017 and DPDPE, was reversed in a dose-dependent fashion by all three antagonists, suggesting that the interaction requires the concurrent agonist occupancy of mu and delta receptors.

Liposome encapsulation prolongs alfentanil spinal analgesia and alters systemic redistribution in the rat

The effect of liposome encapsulation on the analgesia produced by intrathecally administered alfentanil was examined in the rat. In rats prepared with chronic intrathecal catheters, alfentanil in doses of 1-50 micrograms was administered intrathecally in either saline or in multilamellar liposomes (dipalmitoylphosphatidylcholine and cholesterol). Animals were then tested for analgesia by hot-plate and paw-pressure tests. A second group of animals received intrathecal injections of 30 micrograms alfentanil in saline or liposomes, and blood samples were obtained at 5, 15, 45, and 135 min thereafter for measurement of alfentanil plasma concentrations. The liposome preparation markedly prolonged spinal analgesia in the paw-pressure test and to a lesser extent in the hot-plate test. Neither the time to peak analgesia nor the intensity of analgesia differed between the saline and liposome groups. Liposome encapsulation significantly reduced the peak alfentanil plasma concentration at 5 min and prolonged the period in which low but measurable levels of alfentanil could be measured in plasma. These pharmacokinetic data demonstrate that liposome encapsulation resulted in a slow but prolonged appearance of free alfentanil into a diffusible pool available for uptake into the spinal cord. Consistent with the lower peak plasma concentration of alfentanil, the liposome group demonstrated a significantly lower incidence of catalepsy, indicating less systemic redistribution of alfentanil to supraspinal sites. Liposome encapsulation thus appears to produce a significant reduction in peak plasma concentration with a concomitant reduction in systemic side effects and an increase in the duration of action for a given intrathecal dose of the otherwise rapidly cleared alfentanil.

Hyperalgesia mediated by spinal glutamate or substance P receptor blocked by spinal cyclooxygenase inhibition

Inhibition of cyclooxygenase by nonsteroidal anti-inflammatory drugs (NSAIDs) in the periphery is commonly accepted as the primary mechanism by which these agents produce a selective attenuation of pain (analgesia). NSAIDs are now shown to exert a direct spinal action by blocking the excessive sensitivity to pain (hyperalgesia) induced by the activation of spinal glutamate and substance P receptors. These findings demonstrate that the analgesic effects of NSAIDs can be dissociated from their anti-inflammatory actions. Spinal prostanoids are thus critical for the augmented processing of pain information at the spinal level.