Antinociceptive effect produced by intracerebroventricularly administered dynorphin A is potentiated by p-hydroxymercuribenzoate or phosphoramidon in the mouse formalin test

The antinociceptive effects of intracerebroventricularly (i.c.v.) administered dynorphin A, an endogenous agonist for kappa-opioid receptors, in combination with various protease inhibitors were examined using the mouse formalin test in order to clarify the nature of the proteases involved in the degradation of dynorphin A in the mouse brain. When administered i.c.v. 15 min before the injection of 2% formalin solution into the dorsal surface of a hindpaw, 1-4 nmol dynorphin A produced a dose-dependent reduction of the nociceptive behavioral response consisting of licking and biting of the injected paw during both the first (0-5 min) and second (10-30 min) phases. When co-administered with p-hydroxymercuribenzoate (PHMB), a cysteine protease inhibitor, dynorphin A at the subthreshold dose of 0.5 nmol significantly produced an antinociceptive effect during the second phase. This effect was significantly antagonized by nor-binaltorphimine, a selective kappa-opioid receptor antagonist, but not by naltrindole, a selective delta-opioid receptor antagonist. At the same dose of 0.5 nmol, dynorphin A in combination with phosphoramidon, an endopeptidase 24.11 inhibitor, produced a significant antinociceptive effect during both phases. The antinociceptive effect was significantly antagonized by naltrindole, but not by nor-binaltorphimine. Phenylmethanesulfonyl fluoride (PMSF), a serine protease inhibitor, bestatin, a general aminopeptidase inhibitor, and captopril, an angiotensin-converting enzyme inhibitor, were all inactive. The degradation of dynorphin A by mouse brain extracts in vitro was significantly inhibited only by the cysteine protease inhibitors PHMB and N-ethylmaleimide, but not by PMSF, phosphoramidon, bestatin or captopril. The present results indicate that cysteine proteases as well as endopeptidase 24.11 are involved in two steps in the degradation of dynorphin A in the mouse brain, and that phosphoramidon inhibits the degradation of intermediary delta-opioid receptor active fragments enkephalins which are formed from dynorphin A.     

Cysteine protease inhibitors suppress the development of tolerance to morphine antinociception

The effects of various protease inhibitors on the development of antinociceptive tolerance to morphine were examined in mice. Intrathecal (i.t.) administration of morphine (0.01-1 nmol) produced a dose-dependent and significant antinociceptive effect in the 0.5% formalin test. When the doses of morphine (mg/kg, s.c. per injection) were given as pretreatment twice daily for two days [first day (30) and second day (60)], i.t. administration of morphine (0.1 nmol) was inactive due to antinociceptive tolerance on the third day. Tolerance to i.t. morphine was significantly suppressed by the i.t. injection of N-ethylmaleimide or Boc-Tyr-Gly-NHO-Bz, inhibitors of cysteine proteases involved in dynorphin degradation, as well as by dynorphin A, dynorphin B and (-) U-50,488, a selective kappa-opioid receptor agonist. On the other hand, amastatin, an aminopeptidase inhibitor, phosphoramidon, an endopeptidase 24.11 inhibitor, lisinopril, an angiotensin-converting enzyme inhibitor, and phenylmethanesulfonyl fluoride, a serine protease inhibitor, were inactive. These results suggest that cysteine protease inhibitors suppress the development of morphine tolerance presumably through the inhibition of dynorphin degradation.     

Differential effects of N-peptidyl-O-acyl hydroxylamines on dynorphin-induced antinociception in the mouse capsaicin test

In the capsaicin test, intrathecal (i.t.) dynorphins are antinociceptive. Cysteine protease inhibitors such as p-hydroxymercuribenzoate (PHMB) given i.t. augment and prolong their activity. The effect of two novel cysteine protease inhibitors, N-peptidyl-O-acyl hydroxylamines, on the antinociception induced by i.t. administered dynorphin A or dynorphin B has been investigated. When administered i.t. 5 min before the injection of capsaicin (800 ng) into the plantar surface of the hindpaw, dynorphin A (62.5-1000 pmol) or dynorphin B (0.5-4 nmol) produced a dose-dependent and significant antinociceptive effect. The effect of dynorphin A (1 nmol) and dynorphin B (4 nmol) disappeared completely within 180 and 60 min, respectively. PHMB (2 nmol) and Boc-Tyr-Gly-NHO-Bz (BYG-Bz) (2 nmol) co-administered with dynorphin A or dynorphin B significantly prolonged antinociception induced by both. On the other hand, Z-Phe-Phe-NHO-Bz (ZFF-Bz) (1 and 2 nmol) only prolonged antinociception induced by dynorphin A. The results suggest that Z-Phe-Phe-NHO-Bz is an inhibitor of cysteine proteases preferring cleavage of dynorphin A, with less specificity towards dynorphin B in the mouse spinal cord.     

Dynorphin A (1–13) Neurotoxicity in Vitro: Opioid and Non-Opioid Mechanisms in Mouse Spinal Cord Neurons

Dynorphin A is an endogenous opioid peptide that preferentially activates κ-opioid receptors and is antinociceptive at physiological concentrations. Levels of dynorphin A and a major metabolite, dynorphin A (1–13), increase significantly following spinal cord trauma and reportedly contribute to neurodegeneration associated with secondary injury. Interestingly, both κ-opioid and N-methyl- -aspartate (NMDA) receptor antagonists can modulate dynorphin toxicity, suggesting that dynorphin is acting (directly or indirectly) through κ-opioid and/or NMDA receptor types. Despite these findings, few studies have systematically explored dynorphin toxicity at the cellular level in defined populations of neurons coexpressing κ-opioid and NMDA receptors. To address this question, we isolated populations of neurons enriched in both κ-opioid and NMDA receptors from embryonic mouse spinal cord and examined the effects of dynorphin A (1–13) on intracellular calcium concentration ([Ca²⁺]_i) and neuronal survival in vitro. Time-lapse photography was used to repeatedly follow the same neurons before and during experimental treatments. At micromolar concentrations, dynorphin A (1–13) elevated [Ca²⁺]_i and caused a significant loss of neurons. The excitotoxic effects were prevented by MK-801 (Dizocilpine) (10 μM), 2-amino-5-phosphopentanoic acid (100 μM), or 7-chlorokynurenic acid (100 μM)—suggesting that dynorphin A (1–13) was acting (directly or indirectly) through NMDA receptors. In contrast, cotreatment with (−)-naloxone (3 μM), or the more selective κ-opioid receptor antagonist nor-binaltorphimine (3 μM), exacerbated dynorphin A (1–13)-induced neuronal loss; however, cell losses were not enhanced by the inactive stereoisomer (+)-naloxone (3 μM). Neuronal losses were not seen with exposure to the opioid antagonists alone (10 μM). Thus, opioid receptor blockade significantly increased toxicity, but only in the presence of excitotoxic levels of dynorphin. This provided indirect evidence that dynorphin also stimulates κ-opioid receptors and suggests that κ receptor activation may be moderately neuroprotective in the presence of an excitotoxic insult. Our findings suggest that dynorphin A (1–13) can have paradoxical effects on neuronal viability through both opioid and non-opioid (glutamatergic) receptor-mediated actions. Therefore, dynorphin A potentially modulates secondary neurodegeneration in the spinal cord through complex interactions involving multiple receptors and signaling pathways.     

Effects of the subcutaneous administration of enkephalins on tail-flick response and righting reflex of developing rats.

The s.c. administration of [Met5]-enkephalin to 10-day-old rats pretreated with the mixture of 3 peptidase inhibitors, amastatin, captopril and phosphoramidon, produced the inhibition of tail-flick response and loss of righting reflex. When infant rats were pretreated with the mixture of any combination of two peptidase inhibitors, however, the change in both the response and the reflex were not produced at all by enkephalin injection, indicating that 3 kinds of enzymes, amastatin-sensitive aminopeptidase(s), captopril-sensitive peptidyl dipeptidase A and phosphoramidon-sensitive endopeptidase 24.11, played an important role in the inactivation of enkephalin after its systemic administration. Additionally, the fact that the two enkephalin-induced effects were more effectively antagonized by naloxone, a relatively selective mu-opioid antagonist, than by naltrindole, a specific delta-antagonist, or by nor-binaltorphimine, a specific kappa-antagonist, showed that these two effects were produced by the interaction of enkephalin with mu receptors. Moreover the involvement of mu receptors in the production of these two effects was shown by the fact that the s.c. administration of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a selective mu agonist, also produced these two effects which were more effectively antagonized by naloxone than by naltrindole or nor-binaltorphimine. Since the magnitude of the two effects induced by enkephalins in 15-day-old rats was significantly lower than that in 10-day-old rats, and the two enkephalin-induced effects were not produced at all in 20-day-old rats, a maturation-induced decrease in the permeability of the blood-brain barrier against opioid peptides was indicated.     

Pharmacological characterization of the ameliorating effect on short-term memory impairment and antinociceptive effect of KT-90 in mice

(−)-3-Acetyl-6β-acetylthio-N-cyclopropylmethyl-normorphine (KT-90) is a synthesized compound that binds to μ-, δ- and κ-opioid receptors in vitro. KT-90 induces analgesia in the tail-flick test and this effect is antagonized by nor-BNI, a selective κ-opioid receptor antagonist. However, lower doses of KT-90 antagonize morphine-induced analgesia. We reported that κ-opioid receptor agonists such as U-50,488H and dynorphin A (1-13), improved scopolamine-induced impairment of learning and memory in mice and/or rats. In this study, the effects of KT-90 were investigated in an acetic acid-induced writhing test and scopolamine-induced memory impairment test using spontaneous alternation performance in a Y-maze. Male ddY mice were treated with scopolamine (1.65 μmol/kg, s.c.) 30 min before the behavioral test. KT-90 (0.07–2.35 μmol/kg, s.c.) was injected 30 min before testing. In the writhing test, the antinociceptive effect of KT-90 (0.71 μmol/kg) was completely antagonized by a selective μ-opioid receptor antagonist, β-funaltrexamine (10.2 nmol/mouse, i.c.v.) and partially antagonized by nor-BNI (4.9 nmol/mouse, i.c.v.), but it was not antagonized by a selective δ-opioid receptor antagonist, naltrindole (9.1 pmol/mouse, i.c.v.). KT-90 significantly improved the impairment of spontaneous alternation induced by scopolamine. The ameliorating effect of KT-90 was not antagonized by nor-BNI, but was almost completely antagonized by a selective σ receptor antagonist, NE-100 (2.6 μmol/kg, i.p.). These results suggested that the KT-90-induced antinociceptive effect was mediated by μ- and partially by κ-opioid receptors, and the KT-90-induced improvement in scopolamine-induced impairment of spontaneous alternation was mediated mainly via σ receptors.     

Des-tyrosine dynorphin A-(2–13) improves carbon monoxide-induced impairment of learning and memory in mice

The effects of des-tyrosine¹ dynorphin A-(2–13) (dynorphin A-(2–13)) on carbon monoxide (CO)-induced impairment of learning and memory in mice were investigated using a Y-maze task and a passive avoidance test. The lower percentage alternation and shorter step-down latency of the CO-exposed group indicated that learning and/or memory impairment occurred in mice 5 and 7 days after CO exposure, respectively. Administration of dynorphin A-(2–13) (1.5 and/or 5.0 nmol/mouse, intracerebroventricularly (i.c.v.)) 30 min before behavioral tests improved the CO-induced impairment in alternation performance and the CO-induced shortened step-down latency. We previously reported that dynorphin A-(1–13) improved the impairment of learning and/or memory via kappa opioid receptor mediated mechanisms. To determine whether the effect of dynorphin A-(2–13) was also mediated via kappa opioid receptors, we attempted to block its action using a selective kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI). Nor-BNI (4.9 nmol/mouse, i.c.v.) did not block the effects of dynorphin A-(2–13) on the CO-induced impairment of learning and/or memory. These results indicate that dynorphin A-(2–13) improves impairment of learning and/or memory via a non-opioid mechanism.     

Dynorphin B and spinal analgesia: induction of antinociception by the cannabinoids CP55,940, Δ-THC and anandamide

The endogenous opioid dynorphin B was evaluated for its role in cannabinoid-induced antinociception. Previous work in our laboratory has shown that the synthetic, bicyclic cannabinoid, CP55,940, induces the release of dynorphin B whilst the naturally occurring cannabinoid, Δ⁹-tetrahydrocannabinol (Δ⁹-THC), releases dynorphin A. The dynorphins contribute in part to the antinociceptive effects of both cannabinoids at the level of the spinal cord. The present study compares dynorphin B released from perfused rat spinal cord in response to acute administration of anandamide (AEA), Δ⁹-THC and CP55,940 at two time points, 10 min and 30 min post administration, and attempts to correlate such release with antinociceptive effects of the drugs. Dynorphin B was collected from spinal perfusates of rats pretreated with Δ⁹-THC, CP55,940 or AEA. The supernatant was lyophilized and the concentrations of dynorphin B were measured via radioimmunoassay. At a peak time of antinociception (10 min), CP55,940 and Δ⁹-THC induced significant two-fold increases in the release of dynorphin B. AEA did not significantly release dynorphin B. Upon a 30-min pretreatment with the drugs, no significant dynorphin B release was observed, although antinociceptive effects persisted for CP55,940 and Δ⁹-THC. Previous work indicates that Δ⁹-THC releases dynorphin A while AEA releases no dynorphin A. This study confirms that although all three test drugs produced significant antinociception at 10 min, the endocannabinoid, AEA, does not induce antinociception via dynorphin release. Thus, our data indicate a distinct mechanism which underlies AEA-induced antinociception.     

Stress-induced analgesia in μ-opioid receptor knockout mice reveals normal function of the δ-opioid receptor system

Stress-induced analgesia (SIA) was examined in wildtype and μ-opioid receptor knockout mice. We used thermal paw withdrawal (TPW) latency following a continuous 3-min swim in 20°C water, and found a significant increase in TPW latency in both wild-type and knockout mice. Pre-treatment prior to the swim with naltrindole, a selective δ-opioid receptor antagonist, blocked the increase in TPW latency in knockout mice. These results demonstrate an intact δ-receptor-mediated function of a physiologically-released endogenous agonist in the μ-opioid receptor knockout mouse. The present findings are in contrast with previous reports that analgesia induced by exogenous delta agonists is reduced in the knockout mice.     

Dynorphin A-induced rat hindlimb paralysis and spinal cord injury are not altered by the kappa opioid antagonist nor-binaltorphimine

The selective kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) was used to distinguish a kappa opioid component in the mechanisms underlying the hindlimb paralysis, ischemia, and neuronal injury induced in the rat by the kappa opioid agonist dynorphin A. Spinal intrathecal (i.t.) injection of nor-BNI (20 nmol) either 15 min or immediately before i.t. injections of 5 or 20 nmol of dynorphin A failed to alter the dynorphin A-induced disruption of hindlimb motor function and nociceptive responsiveness. Nor-BNI also did not change the 3-fold increases in cerebrospinal fluid lactate concentrations produced by 20 nmol of dynorphin A. Neuroanatomical evaluations revealed that the cell loss, fiber degeneration, and central gray necrosis in lumbosacral spinal cords of rats treated with 20 nmol of dynorphin A were not altered by nor-BNI (20 nmol, i.t.). Thus, the spinal cord injury and associated neurological deficits resulting from i.t. injection of dynorphin A appear to be primarily, if not totally, attributable to its non-kappa opioid action(s).     

Inhibition of neutral endopeptidase (NEP) facilitates neurogenic inflammation

Neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE) are involved in neuropeptide degradation and may modulate neurogenic inflammation. We therefore explored the effect of specific blockers of NEP and ACE on the intensity of neurogenic inflammation. We investigated eight subjects on three occasions. Two pairs of microdialysis fibers equipped with intraluminal wires were inserted intracutaneously into the volar forearms and electrical stimuli were delivered via the intraluminal electrodes. The microdialysis fibers were perfused either with normal saline, phosphoramidon (NEP inhibitor), or captopril (ACE inhibitor). CGRP release was assessed in the microdialysis eluate via a specific EIA and by evaluating the extent and intensity of the neurogenic flare via a laser Doppler imager. The area of hyperalgesia and allodynia was assessed during electrical stimulation. Inhibition of NEP with phosphoramidon increased flare intensity (P < 0.002) and size (P < 0.01), while blocking ACE had no effect on neurogenic vasodilation. CGRP release could be measured in microdialysis samples after phosphoramidon perfusion only (P < 0.03), not in samples with captopril or saline perfusion. No effect on the areas of hyperalgesia and allodynia could be detected. Our findings suggest that NEP but not ACE is most important for CGRP degradation in human skin. This may be of particular importance for the understanding of pain disorders like migraine or complex regional pain syndrome.     

Repeated massage-like stimulation induces long-term effects on nociception: contribution of oxytocinergic mechanisms

Massage-like stroking induces acute antinociceptive effects that can be reversed by an oxytocin antagonist, indicating activation of oxytocin on endogenous pain controlling systems. We now demonstrate an increase in hindpaw withdrawal latencies (HWLs), in response to thermal and mechanical stimuli, which was present after six treatments of massage-like stroking every other day and which continued to increase through the remaining seven treatments. Repeated massage-like stroking also resulted in increased oxytocin-like immunoreactivity (oxytocin-LI) levels in plasma and periaquaductal grey matter (PAG). Furthermore, increases in HWLs were also present after injections of oxytocin into the PAG (0.1, 0.5 and 1.0 nmol). Intra-PAG oxytocin injection of 1 nmol followed by 1 or 20 nmol of naloxone attenuated the increments in HWL. Also, there was a dose-dependent attenuation of the oxytocin-induced antinociceptive effects following intra-PAG injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) but not the delta-antagonist naltrindole. The long-term antinociceptive effects of massage-like stroking may be attributed, at least partly, to the oxytocinergic system and its interaction with the opioid system, especially the mu- and the kappa-receptors in the PAG.     

Antinociception produced by receptor selective opioids. Modulation of supraspinal antinociceptive effects by spinal opioids

This study evaluated the antinociceptive effects produced when different combinations of supraspinal μ- and δ-opioid agonist were co-administered with spinal μ-, δ-, and κ-opioid agonist. Using the Randall-Selitto paw-withdrawal test, in the rat, changes in nociceptive thresholds were measured following co-administration of sequentially increasing i.c.b. doses of either DAMGO or DPDPE with a low-antinociceptive dose of intrathecal DAMGO, DPDPE, or U50,488H. Antinociceptive synergy (i.e. a more than additive antinociceptive effect) was demonstrated with all of the combinations tested except for supraspinal DPDPE co-administered with spinal DAMGO. The results of this study provide support for the suggestion that supraspinal and spinal antinociceptive mechanisms share, in part, common neural circuits. Marked differences in the overall magnitude of the antinociceptive effects produced by the various combinations of opioid agonists were demonstrated through a secondary analysis of the data. When sequentially increasing i.c.v. doses of DAMGO were administered, significantly larger increases in nociceptive thresholds were observed with co-administration of intrathecal injections of low antinociceptive doses of either DAMGO or U50,488H compared to DPDPE. In contrast, when DPDPE was administered supraspinally, the largest increases in nociceptive thresholds were demonstrated with co-administration of DPDPE at the spinal site. The results of the secondary analysis provide support for the hypothesis that descending antinociceptive control systems activated by supraspinal administration of selective μ- and δ-opioid agonists interact, differently, with spinal μ-, δ, and κ-opioidergic mechanisms.     

Effects of intracerebroventricularly administered chimeric peptide of metenkephalin and FMRFa—[D-Ala]YFa—on antinociception and its modulation in mice

An enzymatically stable analog of YGGFMKKKFMRFamide (YFa), a chimeric peptide of metenkephalin and FMRFa, was synthesised. The antinociceptive effects of intracerebroventricular injections of this analog—[D-Ala²]YAGFMKKKFMRFamide ([D-Ala²]YFa)—was then investigated using the mouse radiant-heat tail-flick test. [D-Ala²]YFa produced modest to good antinociception at 1, 2, and 5 μg/mouse (0.64, 1.28, and 3.22 nmol, respectively). This antinociceptive effect was completely reversed by the opioid receptor antagonist naloxone (1.5 μg/mouse: 4.12 nmol, intracerebroventricular [i.c.v.]), administered 5 min prior. Pretreatment (5 min) with either neuropeptides FF (1 μg/mouse: 0.92 nmol, i.c.v.) or FMRFa (1 μg/mouse: 1.69 nmol, i.c.v.) significantly attenuated the antinociceptive effects induced by [D-Ala²]YFa (1 μg/mouse, i.c.v.). Intracerebroventricular administration of [D-Ala²]YFa at 1 μg/mouse dose with morphine (2 μg/mouse: 5.86 nmol, i.c.v.) produced an additive antinociceptive effect, suggesting that [D-Ala²]YFa may have a modulatory effect on opioid (morphine) analgesia. These results provide further support for a role of such amphiactive sequences in antinociception and its modulation.     

The effects of aging on mu and delta opioid receptors in the spinal cord of Fischer-344 rats

Previous research has demonstrated that the antinociceptive efficacy of opioids decreases with advancing age. This study utilized radioligand binding techniques to determine if this decline is due to a change in the receptor density (B_max) and/or affinity (measured as K_d) of the mu (μ) and/or delta (δ) opioid receptors in the spinal cord with advancing age. Saturation binding analysis with [³H][ -Ala²,N-methyl-Phe⁴,Gly⁵-ol]enkephalin (DAMGO: a μ-opioid selective agonist) and [³H]naltrindole (a δ-opioid selective antagonist) revealed no age-related changes in B_max for either the μ or δ-opioid receptors. The K_d value for naltrindole was likewise unaffected by age. The K_d value for DAMGO however, was significantly higher in the aged group as compared with the young and mature groups, indicating a decreased affinity of spinal μ-opioid receptors for DAMGO.     

Suppression of morphine withdrawal by electroacupuncture in rats: dynorphin and κ-opioid receptor implicated

Our previous work has demonstrated that 100-Hz electroacupuncture (EA) or 100-Hz transcutaneous electrical nerve stimulation (TENS) was very effective in ameliorating the morphine withdrawal syndrome in rats and humans. The mechanism was obscure. (1) Rats were made dependent on morphine by repeated morphine injections (5–140 mg/kg, s.c., twice a day) for eight days. They were then given 100-Hz EA for 30 min 24 h after the last injection of morphine. A marked increase in tail flick latency (TFL) was observed. This effect of 100-Hz EA could be blocked by naloxone (NX) at 20 mg/kg, but not at 1 mg/kg, suggesting that 100-Hz EA-induced analgesia observed in morphine-dependent rats is mediated by κ-opioid receptors. (2) A significant decrease of the concentration of dynorphin A (1–17) immunoreactivity (-ir) was observed in the spinal perfusate in morphine-dependent rats, that could be brought back to normal level by 100-Hz EA. (3) 100-Hz EA was very effective in suppressing NX-precipitated morphine withdrawal syndrome. This effect of EA could be prevented by intrathecal administration of nor-BNI (2.5 μg/20 μl), a κ-opioid receptor antagonist, or dynorphin A (1–13) antibodies (25 μg/20 μl) administered 10 min prior to EA. In conclusion, while the steady-state spinal dynorphin release is low in morphine-dependent rats, it can be activated by 100-Hz EA stimulation, which may be responsible for eliciting an analgesic effect and ameliorating morphine withdrawal syndrome, most probably via interacting with κ-opioid receptor at spinal level.     

Involvement of μ- and δ-opioid receptors in the ethanol-associated place preference in rats exposed to foot shock stress

The purpose of this study was to establish the ethanol-induced place preference in rats exposed to foot shock stress using the conditioned place preference paradigm. We also investigated the role of the endogenous opioid system in the development of the ethanol-induced place preference. The administration of ethanol (300 mg/kg, i.p.) with foot shock stress, but not without such stress, induced a marked and significant place preference. Naloxone (1 and 3 mg/kg, s.c.), a non-selective opioid receptor antagonist, significantly attenuated the ethanol-induced place preference. Moreover, the selective μ-opioid receptor antagonist β-funaltrexamine (3 and 10 mg/kg, i.p.) and selective δ-opioid receptor antagonist naltrindole (1 and 3 mg/kg, s.c.), but not the selective κ-opioid receptor antagonist nor-binaltorphimine (1 and 3 mg/kg, i.p.), significantly attenuated the ethanol-induced place preference. Furthermore, 150 mg/kg ethanol (which tended to produce a place preference, although not significantly) combined with each dose (that did not produce a place preference) of the μ-opioid receptor agonist morphine (0.1 mg/kg, s.c.) or selective δ-opioid receptor agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydroquinolino [2,3,3-g] isoquinoline (TAN-67; 20 mg/kg, s.c.), but not the selective κ-opioid receptor agonist trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)cyclohexyl)benzenacetamide methanesulfonate (U50,488H; 1 mg/kg, s.c.), produced a significant place preference. These data indicate that stress may be important for development of the rewarding effect of ethanol, and that μ- and δ-opioid receptors may be involved in the rewarding mechanism of ethanol under stressful conditions.     

Antinociception induced by opioid or 5-HT agonists microinjected into the anterior pretectal nucleus of the rat

The changes in the latency for tail withdrawal in response to noxious heating of the skin induced by microinjection of opioid or serotonergic agonists into the anterior pretectal nucleus (APtN) was studied in rats. The μ-opioid agonist DAMGO (78 and 156 ρmol), but not the δ-opioid agonist DADLE (70 and 140 ρmol), the κ-opioid agonist bremazocine (0.24 and 0.48 ηmol) or the σ-opioid agonist N-allylnormetazocine (0.54 ηmol), produced a dose-dependent antinociceptive effect. The 5-HT₁ agonist 5-carboxamidotryptamine (19 and 38 ηmol) and the 5-HT_1B agonist, CGS 12066B (1.12 and 2.24 ηmol), but not the non-selective 5-HT agonist m-CPP (41 to 164 ηmol), 5-HT₂ agonist α-methylserotonin (36 and 72 ηmol) and 5-HT₃ agonist 2-methylserotonin (36 and 72 ηmol), produced a dose-dependent antinociceptive effect. These results indicate that the antinociceptive effects of opioid or serotonergic agonists microinjected into the APtN depend on drug interaction with local μ or 5-HT_1B receptors, respectively.     

The NSAID dexketoprofen trometamol is as potent as μ-opioids in the depression of wind-up and spinal cord nociceptive reflexes in normal rats

The aim of this study was to examine the potency of the antinociceptive effects of the non-steroidal antiinflammatory drug (NSAID), Dexketoprofen Trometamol (the active enantiomer of ketoprofen) on spinal cord nociceptive reflexes. These effects were compared with those of the μ-opioid receptor agonist fentanyl in normal animals. The experiments were performed in male Wistar rats anaesthetised with alpha-chloralose. The nociceptive reflexes were recorded as single motor units in peripheral muscles, activated by mechanical and electrical stimulation. Both dexketoprofen and fentanyl inhibited responses evoked by mechanical and electrical stimulation with doses in the same nanomolar range (dexketoprofen ID50s: 100 and 762 nmol kg⁻¹ and fentanyl: 40 and 51 nmol kg⁻¹, respectively). Dexketoprofen and fentanyl also significantly inhibited wind-up. Since fentanyl has been shown to be some 1000 times more potent than morphine in this type of experiments, we conclude that dexketoprofen has central analgesic actions in normal animals and depresses nociceptive responses with a potency similar to that of μ-opioid agonists.     

Des-tyrosine(1) dynorphin A-(2-13) improves carbon monoxide-induced impairment of learning and memory in mice

The effects of des-tyrosine(1) dynorphin A-(2-13) (dynorphin A-(2-13)) on carbon monoxide (CO)-induced impairment of learning and memory in mice were investigated using a Y-maze task and a passive avoidance test. The lower percentage alternation and shorter step-down latency of the CO-exposed group indicated that learning and/or memory impairment occurred in mice 5 and 7 days after CO exposure, respectively. Administration of dynorphin A-(2-13) (1.5 and/or 5.0 nmol/mouse, intracerebroventricularly (i.c.v.)) 30 min before behavioral tests improved the CO-induced impairment in alternation performance and the CO-induced shortened step-down latency. We previously reported that dynorphin A-(1-13) improved the impairment of learning and/or memory via kappa opioid receptor mediated mechanisms. To determine whether the effect of dynorphin A-(2-13) was also mediated via kappa opioid receptors, we attempted to block its action using a selective kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI). Nor-BNI (4.9 nmol/mouse, i.c.v.) did not block the effects of dynorphin A-(2-13) on the CO-induced impairment of learning and/or memory. These results indicate that dynorphin A-(2-13) improves impairment of learning and/or memory via a non-opioid mechanism.