Antagonism of swim-stress-induced antinociception by the delta-opioid receptor antagonist naltrindole in adult and young rats.

1. The availability of the non-peptide delta-opioid receptor antagonist naltrindole has provided the possibility for in vivo studies on the function of delta-opioid receptors. We have studied the effects of naltrindole on swim-stress-induced antinociception in adult and neonatal rats. 2. Adult, 25 and 20 day old rats were stressed by warm water (20 degrees C) swimming for 3 min periods and antinociception was assessed by the tail immersion test (50 degrees C). 3. Naltrindole (0.5 and 1 mg kg-1) antagonized swim-stress-induced antinociception in adult and 25 day old rats but in 20 day old rats naltrindole (1 mg kg-1) was without effect. 4. Antinociception induced by the highly mu-opioid receptor selective agonist alfentanil was completely antagonized by naloxone (1 mg kg-1) but virtually unaffected by naltrindole (1 mg kg-1). 5. Neither naloxone nor naltrindole (1 mg kg-1) antagonized swim-stress-induced rises in plasma corticosterone in adult rats at the time of peak antinociception. 6. In conclusion, naltrindole shows in vivo antagonism of opioid-mediated responses. Swim-stress-induced antinociception is mediated through the delta-opioid receptor in 25 day old and adult rats and through the mu-opioid site in 20 day old animals.     

Effects of the delta-opioid receptor antagonist naltrindole on antinociceptive responses to selective delta-agonists in post-weanling rats.

1. Antagonism, by the selective delta-opioid receptor antagonist naltrindole, of the antinociceptive effects of [D-Pen2, D-Pen5] enkephalin (DPDPE), [D-Ser2, Leu5, Thr6] enkephalin (DSLET) and D-Ala2 deltorphin I (DELT I) has been studied in 25 day old rats. 2. Antinociception was measured by the 50 degrees C tail immersion test following i.p. administration of agonists and/or antagonists. 3. Dose-related antinociception was observed with DPDPE, DSLET and DELT I and ED75 doses were computed (0.66 mg kg-1, 0.65 mg kg-1, 0.032 mg kg-1 respectively) and used for antagonism studies. 4. Naltrindole (0.01 mg kg-1) significantly attenuated the antinociceptive effects of DPDPE and DSLET with 0.1 mg kg-1 producing complete reversal of the effects of the ED75 dose. In contrast, naltrindole at 0.01 and 0.1 mg kg-1 did not alter antinociceptive responses to DELT I. Naltrindole at 1 mg kg-1 significantly attenuated DELT I antinociception. 5. Naloxone (1 mg kg-1) produced equivalent degrees of antagonism of the antinociceptive effects of DPDPE, DSLET and DELT I. ICI 174,864 (1 mg kg-1) also antagonized antinociception with a differential degree of attenuation (DSLET > DPDPE > DELT I). 6. Naltrindole (1 mg kg-1) had no effect on the antinociception induced by the selective mu-agonist alfentanil (60 micrograms kg-1). Naltrindole, naloxone or ICI 174,864 had no effect on nociceptive latencies. 7. The differential antagonism by naltrindole of the effects of three selective delta-agonists suggests delta-receptor heterogeneity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alpha-adrenoceptor involvement in swim stress-induced antinociception in the mouse

Three different intensities of swim stress produced stress-induced antinociception (SIA) in mice which was assessed either by the reduction in the number of abdominal constrictions produced by acetic acid or by an increase in reaction time on a hot-plate. The involvement of alpha-adrenoceptors in the three models of SIA was investigated using selective antagonists. SIA produced by the mild stress of a 30 s warm water swim was attenuated by idazoxan (0.5-1 mg kg-1), and by yohimbine at a dose (1 mg kg-1) which reduced antinociception produced by clonidine (12.5-50 micrograms kg-1). Indoramin (1-2 mg kg-1) did not affect this model of SIA, but reversed phenylephrine induced inhibition of the constrictions. A 3 min room temperature swim increased reaction times on the hot-plate and this naloxone-sensitive SIA was reduced significantly by prazosin (1-2 mg kg-1), idazoxan (0.5-1 mg kg-1) and yohimbine (0.5-1 mg kg-1) but enhanced by clonidine (0.5 mg kg-1) and noradrenaline (NA) (10 micrograms i.c.v.). Mice treated with 6-hydroxydopamine (60 + 60 micrograms i.c.v.) were hypersensitive to the hot-plate and did not develop SIA. Levels of noradrenaline in the brain (minus the cerebellum) were decreased after the room temperature swim SIA. The most severe stress of a cold water swim produced SIA on the hot-plate which was initially naloxone-insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Preweanling naltrindole administration differentially affects clonidine induced antinociception and plasma adrenaline levels in male and female neonatal rats

1. The influence of a chronic treatment with the delta-selective opioid antagonist naltrindole (1 mg kg-1) during the preweanling period (daily injections from birth to postnatal day 19), on the antinociceptive and sympatholytic effects of the alpha2-adrenergic agonist clonidine in male and female rats of 20 and 25 days of age was investigated. 2. Nociception was assessed using the tail immersion test (water at 50 degrees C) and plasma levels of adrenaline were measured by high-performance liquid chromatography. 3. The dose of clonidine (1.5 mg kg-1) and the time point at which nociceptive responses were recorded (30 min after the administration of the drug) were chosen on the basis of dose-response (0.5, 1, 1.5 and 2 mg kg-1) and time-response (5, 10, 15, 30 and 60 min) curves which were previously carried out in naive control neonatal rats. 4. In females, the functional blockade of the delta-receptor by neonatal naltrindole treatment did not modify the sympatholytic effect of clonidine but prevented clonidine induced antinociception. Conversely, in males naltrindole treatment allowed the appearance of clonidine antinociception and the sympatholytic effect of clonidine. 5. The results indicate that the delta-receptor is involved in the modulation of antinociceptive and sympatholytic responses to clonidine in neonatal rats and suggest the existence of sex differences in the interactions between delta-opioid and alpha2-adrenergic receptors.     

Swim-stress-induced antinociception in young rats.

1. Opioid and non-opioid mechanisms have been implicated in the phenomenon of stress-induced antinociception in adult rodents. We have studied stress-induced antinociception in developing rats and characterized differences in the neurochemical basis of this effect in pre- and post-weanling animals. 2. Twenty and 25 day old rats were stressed using warm water (20 degrees C) swimming for 3 or 10 min periods and antinociception was assessed by the tail immersion test (50 degrees C). 3. A 3 min swim in 20 and 25 day old rats produced marked antinociception which was blocked by naloxone, Mr 1452, 16-methyl cyprenorphine and levallorphan but not Mr 1453 or N-methyl levallorphan. The delta-opioid receptor antagonist ICI 174,864 attenuated stress-induced antinociception in 25 day old rats but was without effect in 20 day old animals. 4. A 10 min swim in 25 day old rats produced antinociception which was non-opioid in nature. In contrast, antinociception was not observed in 20 day old rats after a 10 min swim-stress. 5. Pretreatment of animals with dexamethasone blocked 3 min swim-stress antinociception in 20 and 25 day old animals but had no effect on antinociception induced by a 10 min swim. 6. Swim-stress-induced antinociception can be observed in young rats and dissociated into opioid and non-opioid types dependent on the duration of swimming stress. The non-opioid type appears to develop more slowly and cannot be observed in preweanling rats. The opioid type is a predominantly mu-receptor phenomenon in preweanling animals but delta-receptor components are observable in postweanling rats.     

Investigation of the different types of opioid receptor involved in electroconvulsive shock-induced antinociception and catalepsy in the rat.

The effects of novel opioid antagonists on the behavioural syndrome induced by electroconvulsive shock (ECS) in rats have been examined and compared with those of the established agent naloxone. A single ECS produced catalepsy and significantly increased tail immersion response times during the 15 min following the seizure. These responses were inhibited by a low dose of naloxone (1 mg kg-1, i.p.) and also by RX8008M (16-methylcyprenorphine; 1 mg kg-1, i.p.) which blocks mu- and delta- but not kappa-opioid receptor function. In comparison, the antinociception and catalepsy induced by ECS was not attenuated by the selective delta-receptor antagonist naltrindole (1 mg kg-1, i.p.). These results suggest that ECS-induced antinociception and catalepsy may be mediated by endogenous opioids acting at mu-opioid receptors and are consistent with biochemical studies showing the release of beta-endorphin in both animals and man following this procedure.     

Neonatal naltrindole and handling differently affect morphine antinociception in male and female rats

The effects of a daily injection of the delta selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive responses to morphine (2 mg/kg) in 20-day-old rats of both sexes were investigated. The effects of postnatal handling were studied by including two control groups--one group receiving daily injections of saline, and a naive unhandled group. Antinociception was assessed using the tail-immersion test and time-response curves (5, 10, 15, and 30 min) were carried out for all experimental groups. In all treatment groups females showed greater sensitivity to the noxious stimuli compared to males. No significant effect of naltrindole treatment on baseline latencies was found. Postnatal handling increased sensitivity to thermal pain in both sexes, and reduced the effect of morphine in males. No significant effect of chronic naltrindole administration on morphine antinociception was found in this sex. Naltrindole-treated females showed an increased antinociception when compared to unhandled animals of the same gender. The results indicate that preweanling handling stress and chronic naltrindole treatment differentially affected morphine antinociception in male and female neonatal rats.     

The role of minoxidil on endogenous opioid peptides in the spinal cord: a putative co-agonist relationship between K-ATP openers and opioids

ATP-gated K(+) channel openers produce antinociception that is attenuated by opioid receptor antagonists, indicating K-ATP openers produce antinociception, in part, via the release of endogenous opioid peptides. Utilizing the spinal perfusion method, male Sprague-Dawley rats were administered minoxidil intrathecally (i.t.) at doses ranging from 12.5 to 200 microg/rat for 3 min, tested for antinociception using the tail-flick test, and perfused with artificial cerebrospinal fluid (aCSF) to collect endogenous opioid peptides. Endogenous opioid peptide levels were measured by radioimmunoassay. Naltrindole, a delta-opioid receptor antagonist, at 4 mg/kg, subcutaneously (s.c.), blocked minoxidil-induced antinociception. beta-Funaltrexamine, a mu-opioid receptor antagonist, at 100 microg/rat, partially blocked minoxidil, whereas the kappa-opioid receptor antagonist nor-binaltorphimine, at a dose of 100 microg/rat, did not attenuate minoxidil. Although antagonists of the mu- and delta-opioid receptor attenuated minoxidil-induced antinociception, there was no increase in beta-endorphin, an endogenous ligand with affinity for both micro- and delta-opioid receptors or [Leu(5)]enkephalin, an endogenous ligand with affinity for delta-opioid receptors.     

Melatonin reduces formalin-induced nociception and tactile allodynia in diabetic rats

The purpose of this study was to assess the antinociceptive and antiallodynic effect of melatonin as well as its possible mechanism of action in diabetic rats. Streptozotocin (50 mg/kg) injection caused hyperglycemia within 1 week. Formalin-evoked flinching was increased in diabetic rats as compared to non-diabetic rats. Oral administration of melatonin (10-300 mg/kg) dose-dependently reduced flinching behavior in diabetic rats. In addition, K-185 (a melatonin MT(2) receptor antagonist, 0.2-2 mg/kg, s.c.) completely blocked the melatonin-induced antinociception in diabetic rats, whereas that naltrexone (a non-selective opioid receptor antagonist, 1 mg/kg, s.c.) and naltrindole (a selective delta opioid receptor antagonist, 0.5 mg/kg, s.c.), but not 5'-guanidinonaltrindole (a selective kappa opioid receptor antagonist, 1 mg/kg, s.c.), partially reduced the antinociceptive effect of melatonin. Given alone K-185, naltrexone, naltrindole or 5'-guanidinonaltrindole did not modify formalin-induced nociception in diabetic rats. Four to 8 weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. On this condition, oral administration of melatonin (75-300 mg/kg) dose-dependently reduced tactile allodynia in diabetic rats. Both antinociceptive and antiallodynic effects were not related to motor changes as melatonin did not modify number of falls in the rotarod test. Results indicate that melatonin is able to reduce formalin-induced nociception and tactile allodynia in streptozotocin-injected rats. In addition, data suggest that melatonin MT(2) and delta opioid receptors may play an important role in these effects.     

Imipramine-induced antinociception in the formalin test. Receptor mechanisms involved and effect of swim stress

This study concerned the effect of swim stress on imipramine-induced antinociception in mice. The data showed that intraperitoneal (i.p.) administration of different doses of imipramine (10-40 mg/kg) and 0.5-3 min of swim stress (17 degrees C) induced antinociception in the first and second phases of the formalin test. Low period of swim stress (10 s) with low doses of imipramine (2.5, 5 and 10 mg/kg i.p.), which did not have any effect by themselves, in combination showed antinociception in the second phase of the test. Either yohimbine (0.5 mg/kg i.p.) or naloxone (1 mg/kg i.p.) reversed the response induced by the combination of low doses of imipramine plus swim stress. Yohimbine (1 mg/kg i.p.) decreased the response of imipramine (20 mg/kg i.p.) but not that of 30 s swim stress in the second phase. However, naloxone (1 mg/kg i.p.) reduced the antinociception induced by imipramine (20 mg/kg i.p.) or 30 s swim stress in the second phase of the test, the combination of imipramine with swim stress was not altered by yohimbine or naloxone. Prazosin induced antinociception by itself in the first phase of the test and increased swim-stress-induced antinociception with no interaction. It is concluded that antinociception induced by imipramine in the second phase of formalin test may be mediated through alpha(2)-adrenoceptor antagonists. The results indicate that the responses of swim stress and imipramine may be mediated by an opioid mechanism, but the combination of both drugs induced higher antinociceptive effects.     

Delta opioid receptor ligands modulate anxiety-like behaviors in the rat

The role of the delta opioid receptor in regulating anxiety-like behavior in male Sprague-Dawley rats was examined.Using an elevated plus maze, the effects of the selective delta opioid receptor antagonist naltrindole (1 or 5 mg kg(-1)) and agonist SNC80 (1, 5 or 20 mg kg(-1)) on anxiety-like behavior were measured. Anxiety was also measured following administration of diazepam (3 mg kg(-1)) and amphetamine (1 mg kg(-1)) and compared to the effects of SNC80. Locomotor activity following administration of naltrindole, SNC80, diazepam, and amphetamine was measured. Finally, the defensive burying paradigm was used to confirm the findings from the elevated plus maze.Results demonstrated that SNC80 produced dose-dependent anxiolytic effects similar to that of the classical antianxiety agent, diazepam. Administration of naltrindole caused anxiogenic behavior in rats further supporting the involvement of the delta opioid receptor system in regulating anxiety. Naltrindole also blocked the anxiolytic effects of SNC80. Amphetamine had no effect on anxiety-like behavior. SNC80 induced hyperactivity similar to amphetamine at the doses tested, while naltrindole and diazepam did not significantly affect locomotor activity.Although SNC80 can increase locomotor activity, control experiments reported herein indicate that hyperlocomotion is not sufficient to produce an anxiolytic response on the elevated plus maze. Together with the results from the defensive burying paradigm, this suggests that the effects of SNC80 on reducing anxiety are independent of its effects on locomotion. Collectively these data show that the delta opioid receptor system can regulate anxiety-like behavior in an anxiolytic (agonist) and anxiogenic (antagonist) manner.     

Vasopressin and stress-induced antinociception in the mouse.

1. Arginine vasopressin produced antinociception in the hot-plate test after intracerebroventricular injection (0.5 micrograms) and in the acetic acid abdominal constriction test after intraperitoneal injection (0.1 mg kg-1). 2. The antinociception produced by arginine vasopressin was sensitive to deamino(CH2)5Tyr(Me) arginine vasopressin (0.5 micrograms i.c.v.; 0.1 mg kg-1 i.p.) but not to naloxone (5 micrograms i.c.v.; 2 mg kg-1 i.p.) 3. Arginine vasopressin when administered by the intracerebroventricular route, but not by the intraperitoneal route, produced characteristic behaviour which was sensitive to deamino(CH2)5Tyr(Me) arginine vasopressin (0.5 micrograms, i.c.v.). 4. A 3 min swim at 20 degrees C produced antinociception on the hot-plate which was sensitive to naloxone (0.4 mg kg-1, i.p.) but not to deamino(CH2)5Tyr(Me) arginine vasopressin (0.5 micrograms, i.c.v.). 5. The reduction in the number of acetic acid-induced abdominal constrictions produced by a 30 s swim at 30 degrees C was not sensitive to either naloxone (2 mg kg-1, i.p.) or deamino(CH2)5Tyr(Me) arginine vasopressin (0.1 mg kg-1, i.p.). 6. Arginine vasopressin, at high doses, is antinociceptive in the mouse but does not appear to mediate stress-induced antinociception in this species.     

Developmental responses to opioids reveals a lack of effect on stress-induced corticosterone levels in neonatal rats.

The neonate has an unusual capacity for survival and the possibility exists that mechanisms for controlling stress responses may differ in the developing animal. In adults both endogenous and exogenous opioids can modulate the corticosterone responses to stress. We have studied this effect in neonatal rats and found that opioid modulation is absent in early postnatal development. Neonatal rats of either sex were injected with morphine (5-50 mg kg-1), fentanyl (10-100 micrograms kg-1), buprenorphine (0.1-30 mg kg-1) or naloxone (0.1-10 mg kg-1) and plasma corticosterone measured fluorimetrically 15 or 20 min later. In addition naloxone reversibility studies (1 mg kg-1, co-administered) were carried out for the opioid agonists. In adult rats, elevations in plasma corticosterone caused by injection stress were potentiated by morphine, fentanyl and buprenorphine. In neonates, though injection stress-induced rises in plasma corticosterone were absent at 10 days, elevations were observed at 21 days and later. However, significant potentiation of this corticosterone response by fentanyl was absent at 21 days and at later ages (30 and 40 days) for morphine and buprenorphine. The potentiating effect of all three agonists did not become fully effective until day 45. In addition, in animals acclimatized to injection stress by 7 day injection pretreatment, fentanyl did not significantly alter corticosterone levels in 30 day old neonates. High doses of naloxone (10 mg kg-1) significantly increased the corticosterone response to injection stress in adult rats but this effect was absent in 30 day old animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible antinociceptive mechanisms of opioid receptor antagonists in the mouse formalin test

It has been reported that opioid receptor antagonist can induce antinociception in several nociceptive tests. In the intraplantar formalin pain model, however, opioid antagonist-induced antinociception, as well as its underlying mechanism, has not been well characterized. Therefore, in the mouse formalin test, we attempted to characterize the site of action and the possible opioid receptor subtypes. We found that naltrexone (a nonselective opioid antagonist) injected intraperitoneally (i.p., 1-20 mg/kg), intrathecally (i.t., 0.1-10 microg) and intracerebroventricularly (i.c.v., 0.1-10 microg) phase. Administration of beta-funaltrexamine (beta-FNA, 10-40 mg/kg i.p., 1.25-5 microg it or i.c.v.), naltrindole (1-10 mg/kg i.p., 1.25-5 microg it or i.c.v.) and nor-binaltorphimine (nor-BNI, 1-10 mg/kg i.p., 10-40 microg it or i.c.v.), which are selective mu-, delta- and kappa-opioid antagonists, respectively, also produced antinociception during the second phase. Additionally, we examined the involvement of the descending monoaminergic systems in the naltrexone-induced antinociception in the formalin test. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, a serotonergic neurotoxin, 20 microg i.t.), but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, a noradrenergic neurotoxin, 20 microg i.t.), reversed the naltrexone-induced antinociception during the second phase. Our results suggest that blockade of supraspinally or spinally located opioid receptors may play roles in the regulation of antinociception during the tonic painful stage. In addition, opioid receptors localized at the neuroterminal of the descending serotonergic, but not noradrenergic, inhibitory system in the spinal cord appear to be involved in opioid antagonist-induced antinociception during the second tonic phase of the formalin test.     

Perinatal lead exposure impairs opioid but not non-opioid stress-induced antinociception in developing rats.

1. The development of opioid systems has been shown to be sensitive to perinatal exposure to lead. We have studied the effects of such exposure on opioid and non-opioid mediated stress-induced antinociception in developing rats. 2. Lead was administered in the maternal drinking water from conception to postnatal day 14 at 300 and 1000 p.p.m. Twenty and 25 day old rats were subjected to swimming stress and antinociception measured using the tail immersion test. 3. A 3 min swim-stress induced an opioid-mediated antinociceptive response in 20 day old rats which was attenuated by 300 p.p.m. lead and by 1000 p.p.m. lead treatment in a dose-related manner. A 10 min swim-stress induced a non-opioid mediated antinociceptive response in 25 day old rats which was not antagonised by 300 or 1000 p.p.m. lead. 4. Naloxone antagonised the residual antinociception observed in 20 day old animals treated with 300 p.p.m. lead and had no effect on antinociception in control or lead-treated 25 day old rats. 5. Using a lead exposure model considered to represent subclinical lead toxicity in man, it was shown that perinatal lead exposure disrupts opioid but not non-opioid mediated stress antinociception.     

Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding.

1. In rats, the interaction between the mu-opioid agonist dermorphin and the delta-opioid agonist [D-Ala2, Glu4]deltorphin was studied in binding experiments to delta-opioid receptors and in the antinociceptive test to radiant heat. 2. When injected i.c.v., doses of [D-Ala2, Glu4]deltorphin higher than 20 nmol produced antinociception in the rat tail-flick test to radiant heat. Lower doses were inactive. None of the doses tested elicited the maximum achievable response. This partial antinociception was accomplished with an in vivo occupancy of more than 97% of brain delta-opioid receptors and of 17% of mu-opioid receptors. Naloxone (0.1 mg kg-1, s.c.), and naloxonazine (10 mg kg-1, i.v., 24 h before), but not the selective delta-opioid antagonist naltrindole, antagonized the antinociception. 3. In vitro competitive inhibition studies in rat brain membranes showed that [D-Ala2, Glu4]deltorphin displaced [3H]-naltrindole from two delta-binding sites of high and low affinity. The addition of 100 microM Gpp[NH]p produced a three fold increase in the [D-Ala2, Glu4]deltorphin Ki value for both binding sites. The addition of 10 nM dermorphin increased the Ki value of the delta-agonist for the high affinity site five times. When Gpp[NH]p was added to the incubation medium together with 10 nM dermorphin, the high affinity Ki of the delta-agonist increased 15 times. 4. Co-administration into the rat brain ventricles of subanalgesic doses of dermorphin and [D-Ala2, Glu4]deltorphin resulted in synergistic antinociceptive responses. 5. Pretreatment with naloxone or with the non-equilibrium mu-antagonists naloxonazine and beta-funaltrexamine completely abolished the antinociceptive response of the mu-delta agonist combinations. 6. Pretreatment with the delta-opioid antagonists naltrindole and DALCE reduced the antinociceptive response of the dermorphin-[D-Ala2, Glu4]deltorphin combinations to a value near that observed after the mu-agonist alone. At the dosage used, naltrindole occupied more than 98% of brain delta-opioid receptors without affecting mu-opioid-receptors. 7. These data suggest that in the rat tail-flick test to radiant heat, mu- and delta-opioid agonists co-operate positively in evoking an antinociceptive response. Although interactions between different opioid pathways cannot be excluded, in vitro binding results indicate that this co-operative antinociception is probably mediated by co-activation of the delta-opioid receptors at the cellular level by the mu- and delta-agonist.     

Reversal by naloxone of the spinal antinociceptive actions of a systemically-administered NSAID.

1. Possible interactions between non-steroidal anti-inflammatory drugs (NSAIDs) and endogenous opioids were examined in electrophysiological experiments in alpha-chloralose anaesthetized spinalized rats without or with carrageenan-induced acute inflammation of one hindpaw. Spinal reflex responses, monitored as single motor unit discharges, were elicited by noxious pinch and electrical stimuli. 2. The mu-opioid agonist, fentanyl, was an effective depressant of reflexes under all conditions (ED50 6-14 micrograms kg-1, i.v.). In rats without peripheral inflammation the NSAID, flunixin, a niflumic acid derivative, had only a small effect that was not dose-dependent. However, in animals with unilateral inflammation, flunixin reduced spinal reflexes evoked both by noxious pinch stimuli (that activate peripheral nociceptors; ID50 4 mg kg-1, i.v.) and by electrical stimuli (that bypass nociceptor endings; ID50 6.5- 11 mg kg-1, i.v.), indicating that it has a central site of action at doses comparable to those used clinically. 3. The opioid antagonist, naloxone (1 mg kg-1, i.v.), reversed all actions of fentanyl. It did not reverse the small effects that flunixin had in rats without inflammation, showing that the NSAID is not a direct opioid agonist. In rats with carrageenan-induced inflammation of the hindpaw, however, naloxone fully reversed or prevented the antinociception by flunixin, but not that by the alpha 2-adrenoceptor agonist, medetomidine. 4. We conclude that under conditions of peripheral inflammation and the resultant central changes, the NSAID, flunixin, has antinociceptive actions that are mediated by endogenous opioids acting within the spinal cord.     

Involvement of the opioid system in the anxiolytic-like effects induced by Delta(9)-tetrahydrocannabinol

RATIONALE: Recent studies have shown that several pharmacological actions induced by cannabinoids, including antinociception and reward, involve the participation of the endogenous opioid system. OBJECTIVES: The present study was designed to examine the possible involvement of the different opioid receptors in the anxiolytic-like responses induced by Delta(9)-tetrahydrocannabinol (THC). METHODS: The administration of a low dose of THC (0.3 mg/kg) produced clear anxiolytic-like responses in the light-dark box, as previously reported. The effects of the pretreatment with the CB(1) cannabinoid receptor antagonist, SR 141716A (0.5 mg/kg), or the micro -opioid receptor antagonist, beta-funaltrexamine (5 mg/kg), the delta-opioid receptor antagonist, naltrindole (2.5 mg/kg) and the kappa-opioid receptor antagonist, nor-binaltorphimine (2.5 mg/kg) were evaluated on anxiolytic-like responses induced by THC. RESULTS: SR 141716A completely blocked the anxiolytic-like response induced by THC, suggesting that this effect is mediated by CB(1) cannabinoid receptors. The micro -opioid receptor antagonist beta-funaltrexamine and the delta-opioid receptor antagonist naltrindole, but not the kappa-opioid receptor antagonist nor-binaltorphimine, abolished THC anxiolytic-like effects, suggesting an involvement of micro - and delta-opioid receptors in this behavioural response. CONCLUSIONS: These results demonstrate that the endogenous opioid system is involved in the regulation of anxiety-like behaviour by cannabinoids and provide new findings to clarify further the interaction between these two neuronal systems.     

L-arginine exerts a dual role in nociceptive processing in the brain: involvement of the kyotorphin-Met-enkephalin pathway and NO-cyclic GMP pathway.

1. Intracerebroventricular (i.c.v.) administration of L-arginine (L-Arg), at 10-100 micrograms per mouse, produced antinociception in mice, as assessed by the tail flick test; this antinociception was reversed by pretreatment (s.c.) with naltrindole (NTI), a delta-selective opioid antagonist, and by co-administered L-leucyl-L-arginine (Leu-Arg), a kyotorphin (endogenous Met-enkephalin releaser) receptor antagonist. 2. L-NG-nitroarginine methyl ester (L-NAME), a NO synthase inhibitor, but not D-NG-nitroarginine methyl ester, given i.c.v. at 3-10 micrograms per mouse, exhibited antinociceptive activity that was resistant to naloxone (s.c.), NTI (s.c.) and Leu-Arg (i.c.v.). 3. The L-NAME (i.c.v.)-induced antinociception was not reversed by L-Arg (i.c.v.), which was antinociceptive by itself, but was abolished by combined injection of L-Arg plus Leu-Arg (i.c.v.) or by L-Arg (i.c.v.) after NTI (s.c.). 4. Methylene blue (MB), a soluble guanylate cyclase inhibitor, at 0.1-1 microgram per mouse, produced antinociception by i.c.v. administration. The antinociception induced by MB (i.c.v.) or L-NAME (i.c.v.) was reversed by co-administered dibutyryl cyclic GMP. 5. These findings suggest that L-Arg plays a dual role in nociceptive processing in the brain, being antinociceptive via the kyotorphin-Met-enkephalin pathway and nociceptive via the NO-cyclic GMP pathway.     

A LACK OF POTENCY FOR THE δ-OPIOID ANTAGONIST NALTRINDOLE AFTER MICROINJECTION INTO THE ROSTRAL VENTROLATERAL MEDULLA OF RABBITS

1. Naltrindole is a novel delta-opioid antagonist which is highly potent in vitro. We examined the effects on arterial pressure of naltrindole (0.3-300 pmol) after microinjection into the pressor area of the rostral ventrolateral medulla of chloralose-anaesthetized rabbits. 2. Naltrindole itself had no significant effects on arterial pressure, and only slightly attenuated the hypotensive effect of an exogenous agonist ( [D-Ala2, D-Leu5]-enkephalin). This is in contrast to previous demonstration of a marked pressor effect following another delta-antagonist, ICI 174,864. 3. Thus, naltrindole, tested in vivo against endogenous opioid effects, does not appear to be as potent as it is in vitro, and in this case was virtually ineffective.