Magnesium chloride and ruthenium red attenuate the antiallodynic effect of intrathecal gabapentin in a rat model of postoperative pain

BACKGROUND

Gabapentin, a gamma-aminobutyric acid analog anticonvulsant, has been shown to possess antinociceptive effects in animal models and clinical trials. An endogenous binding site of [3H]gabapentin has been revealed to be the alpha(2)delta subunit of voltage-dependent Ca2+ channels. Magnesium chloride, ruthenium red, and spermine have been shown to modulate [3H]gabapentin binding to this binding site in vitro. In this study, the authors examined whether intrathecal magnesium chloride, ruthenium red, or spermine could affect the antiallodynic effect of intrathecal gabapentin in a rat model of postoperative pain.

METHODS

Under isoflurane anesthesia, male Sprague-Dawley rats received an incision over the plantar surface of the right hind paw to produce punctate mechanical allodynia. Withdrawal thresholds to von Frey filament stimulation near the incision site were measured before incision, 2 h after incision, and every 30 min after intrathecal coadministration of gabapentin with normal saline or different doses of magnesium chloride, ruthenium red, or spermine for 2 h.

RESULTS

Intrathecal gabapentin (30, 100, 200 microg) dose-dependently reduced incision-induced allodynia. Hexahydrated magnesium chloride (5, 10, 20 microg) and ruthenium red (0.2, 2, 20 ng) noncompetitively inhibited the antiallodynic effect of gabapentin. Spermine at doses not inducing motor weakness (30, 60 microg) did not affect the antiallodynic effect of gabapentin. The antiallodynic effect of intrathecal morphine (1.5 microg) was not affected by hexahydrated magnesium chloride (20 microg), ruthenium red (20 ng), or spermine (60 microg).

CONCLUSIONS

These results provide behavioral evidence to support that the alpha(2)delta subunit of Ca2+ channels may be involved in the antiallodynic action of intrathecal gabapentin in the postoperative pain model.

Intrathecal Treatment With 6-Hydroxydopamine or 5,7-Dihydroxytryptamine Blocks the Antinociception Induced by Endomorphin-1 and Endomorphin-2 Given Intracerebroventricularly in the Mouse

The involvement of spinopetal noradrenergic and serotonergic systems in antinociception induced by endomorphin-1 (EM-1) and endomorphin-2 (EM-2) given supraspinally or spinally were investigated in male CD-1 mice. Groups of mice were pretreated intrathecally (i.t.) with 6-hydroxydopamine (6-OHDA, 20 microg) or 5,7-dihydroxytryptamine (5,7-DHT, 50 microg) for 3 days before intracerebroventricular (i.c.v.) or i.t. injection of different doses of EM-1 or EM-2, and the tail-flick response was measured for antinociceptive effects. I.t. pretreatment with 6-OHDA for 3 days, which markedly depleted noradrenaline (NA) contents by more than 90%, but not serotonin (5-HT) in the spinal cord, completely abolished the antinociception induced by i.c.v.-administered EM-1 or EM-2. Intrathecal pretreatment with 5,7-DHT for 3 days, which markedly reduced 5-HT contents by more than 92%, but only reduced NA by 14 - 25% in the spinal cord, also markedly attenuated the antinociception induced by i.c.v.-administered EM-1 or EM-2. However, the antinociception induced by i.t.-administered EM-1 or EM-2 was not affected in either 6-OHDA or 5,7-DHT pretreated mice. It is concluded that NA and 5-HT in the spinal cord are involved in the antinociception induced by supraspinally, but not spinally administered EM-1 and EM-2.

Attenuation of nicotine-induced antinociception, rewarding effects, and dependence in mu-opioid receptor knock-out mice

The involvement of mu-opioid receptors in different behavioral responses elicited by nicotine was explored by using mu-opioid receptor knock-out mice. The acute antinociceptive responses induced by nicotine in the tail-immersion and hot-plate tests were reduced in the mutant mice, whereas no difference between genotypes was observed in the locomotor responses. The rewarding effects induced by nicotine were then investigated using the conditioning place-preference paradigm. Nicotine produced rewarding responses in wild-type mice but failed to produce place preference in knock-out mice, indicating the inability of this drug to induce rewarding effects in the absence of mu-opioid receptors. Finally, the somatic expression of the nicotine withdrawal syndrome, precipitated in dependent mice by the injection of mecamylamine, was evaluated. Nicotine withdrawal was significantly attenuated in knock-out mutants when compared with wild-type mice. In summary, the present results show that mu-opioid receptors are involved in the rewarding responses induced by nicotine and participate in its antinociceptive responses and the expression of nicotine physical dependence.

Inhibition of improgan antinociception by the cannabinoid (CB)(1) antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A): lack of obligatory role for endocannabinoids acting at CB(1) receptors

Improgan, a nonopioid antinociceptive agent, activates descending, pain-relieving mechanisms in the brain stem, but the receptor for this compound has not been identified. Because cannabinoids also activate nonopioid analgesia by a brain stem action, experiments were performed to assess the significance of cannabinoid mechanisms in improgan antinociception. The cannabinoid CB(1) antagonist N-(piperidin-1-yl)-5-(4-chloro phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) induced dose-dependent inhibition of improgan antinociception on the tail-flick test after i.c.v. administration in rats. The same treatments yielded comparable inhibition of cannabinoid [R-(+)-(2,3-dihydro-5-methyl-3-[(4-mor pholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl)methanone monomethanesulfonate, WIN 55,212-2] analgesia. Inhibition of improgan and WIN 55,212-2 antinociception by SR141716A was also observed in Swiss-Webster mice. Radioligand binding studies showed no appreciable affinity of improgan on rat brain, mouse brain, and human recombinant CB(1) receptors, ruling out a direct action at these sites. To test the hypothesis that CB(1) receptors indirectly participate in improgan signaling, the effects of improgan were assessed in mice with a null mutation of the CB(1) gene with and without SR141716A pretreatment. Surprisingly, improgan induced complete antinociception in both CB(1) (-/-) and wild-type control [CB(1) (+/+)] mice. Furthermore, SR141716A inhibited improgan antinociception in CB(1) (+/+) mice, but not in CB(1) (-/-) mice. Taken together, the results show that SR141716A reduces improgan antinociception, but neither cannabinoids nor CB(1) receptors seem to play an obligatory role in improgan signaling. Present and previous studies suggest that Delta(9)-tetrahydrocannabinol may act at both CB(1) and other receptors to relieve pain, but no evidence was found indicating that improgan uses either of these mechanisms. SR141716A will facilitate the study of improgan-like analgesics.

(−)6-n-Propylnicotine antagonizes the antinociceptive effects of (−)Nicotine

Several 6-alkyl analogues of nicotine were examined in radioligand binding and in vivo functional assays. Although (-)6-ethylnicotine (3) binds with high affinity at nACh receptors (Ki=5.6 nM) and produces nicotine-like actions, its n-propyl homologue (-)4 (Ki=22 nM) failed to produce such effects. In fact, (-)4 antagonized the antinociceptive effects of (-)nicotine in the tail-flick assay in mice, but not the spontaneous activity or discriminative stimulus effects of (-)nicotine. Compound (-)4 appears to selectively antagonize only one of the three effects examined and is an interesting cholinergic agent for subsequent investigation.

Antagonism of the antinociceptive and discriminative stimulus effects of heroin and morphine by 3-methoxynaltrexone and naltrexone in rhesus monkeys

It has been suggested that heroin and morphine may act on different opioid receptor populations in rodents. In support of this hypothesis, the opioid antagonist 3-methoxynaltrexone was reported to be more potent as an antagonist of the antinociceptive effects of heroin than of morphine in mice and rats. To assess the generality of this finding across species and experimental endpoints, the present study compared the potencies of naltrexone and 3-methoxynaltrexone as antagonists of heroin and morphine in two behavioral assays in rhesus monkeys. In the thermal nociception study, tail-withdrawal latencies were measured from water heated to 50 degrees C. In the heroin discrimination study, monkeys were trained to discriminate 0.1 mg/kg heroin from saline in a two-key, food-reinforced drug discrimination procedure, and percentage of heroin-appropriate responding and response rates were measured. Both heroin and morphine produced dose-dependent antinociception, increases in percentage of heroin-appropriate responding, and decreases in response rates. Heroin was approximately 20-fold more potent than morphine. Naltrexone (0.032-0.1 mg/kg) was equipotent in antagonizing all effects of heroin and morphine (pA(2) values = 7.90-8.22). 3-Methoxynaltrexone (0.1-3.2 mg/kg) was also equipotent in antagonizing the antinociceptive, discriminative stimulus, and rate-suppressant effects of heroin and morphine; however, 3-methoxynaltrexone was approximately 100-fold less potent than naltrexone (pA(2)/pK(B) values = 5.96-6.36). These results suggest that heroin and morphine act on pharmacologically similar populations of opioid receptors in rhesus monkeys, and also indicate that 3-methoxynaltrexone does not differentially antagonize the effects of heroin and morphine in rhesus monkeys.

Differential antagonism of endomorphin-1 and endomorphin-2 supraspinal antinociception by naloxonazine and 3-methylnaltrexone

To determine if different subtypes of mu-opioid receptors were involved in antinociception induced by endomorphin-1 and endomorphin-2, the effect of pretreatment with various mu-opioid receptor antagonists beta-funaltrexamine, naloxonazine and 3-methylnaltrexone on the inhibition of the paw-withdrawal induced by endomorphin-1 and endomorphin-2 given intracerebroventricularly (i.c.v.) were studied in ddY male mice. The inhibition of the paw-withdrawal induced by i.c.v. administration of endomorphin-1, endomorphin-2 or DAMGO was completely blocked by the pretreatment with a selective mu-opioid receptor antagonist beta-funaltrexamine (40 mg/kg), indicating that the antinociception induced by all these peptides are mediated by the stimulation of mu-opioid receptors. However, naloxonazine, a mu1-opioid receptor antagonist pretreated s.c. for 24h was more effective in blocking the antinociception induced by endomorphin-2, than by endomorphin-1 or DAMGO given i.c.v. Pretreatment with a selective morphine-6 beta-glucuronide blocker 3-methylnaltrexone 0.25mg/kg given s.c. for 25 min or co-administration of 3-methylnaltrexone 2.5 ng given i.c.v. effectively attenuated the antinociception induced by endomorphin-2 given i.c.v. and co-administration of 3-methylnaltrexone shifted the dose-response curves for endomorphin-2 induced antinociception to the right by 4-fold. The administration of 3-methylnaltrexone did not affect the antinociception induced by endomorphin-1 or DAMGO given i.c.v. Our results indicate that the antinociception induced by endomorphin-2 is mediated by the stimulation of subtypes of mu-opioid receptor, which is different from that of mu-opioid receptor subtype stimulation by endomorphin-1 and DAMGO.

Blockade of the antinociceptive effect of spinally administered kyotorphin by naltrindole in mice

We investigated the role of spinal opioid receptors in the antinociceptive effect of kyotorphin (Tyr-Arg, KTP) by using an in vivo mice tail-pinch test and an in vitro opioid receptor binding assays. Intrathecal administration of KTP produced a dose-dependent antinociceptive effect with an ED(50) value of 24 microg/mouse. This antinociception, which was reversed by the KTP antagonist Leu-Arg, was completely blocked by naltrindole but not by naloxonazine, beta-funaltrexamine, or nor-binaltorphimine. The results from the binding study in vitro indicated that KTP bound to spinal KTP receptors but not to any opioid receptors in the mouse spinal cord. These results suggest that KTP-induced antinociception is mediated by binding to KTP receptors followed by an indirect activation of the delta-opioid receptors in the spinal cord.

Identification and characterization of novel mammalian neuropeptide FF-like peptides that attenuate morphine-induced antinociception

The two mammalian neuropeptides NPFF and NPAF have been shown to have important roles in nociception, anxiety, learning and memory, and cardiovascular reflex. Two receptors (FF1 and FF2) have been molecularly identified for NPFF and NPAF. We have now characterized a novel gene designated NPVF that encodes two neuropeptides highly similar to NPFF. NPVF mRNA was detected specifically in a region between the dorsomedial and ventromedial hypothalamic nuclei. NPVF-derived peptides displayed higher affinity for FF1 than NPFF-derived peptides, but showed poor agonist activity for FF2. Following intracerebral ventricular administration, a NPVF-derived peptide blocked morphine-induced analgesia more potently than NPFF in both acute and inflammatory models of pain. In situ hybridization analysis revealed distinct expression patterns of FF1 and FF2 in the rat central nervous system. FF1 was broadly distributed, with the highest levels found in specific regions of the limbic system and the brainstem where NPVF-producing neurons were shown to project. FF2, in contrast, was mostly expressed in the spinal cord and some regions of the thalamus. These results indicate that the endogenous ligands for FF1 and FF2 are NPVF- and NPFF-derived peptides, respectively, and suggest that the NPVF/FF1 system may be an important part of endogenous anti-opioid mechanism.

Involvement of beta2-adrenergic and mu-opioid receptors in antinociception produced by intracerebroventricular administration of alpha,beta-methylene-ATP

The present study examined what kind of receptors are involved in the antinociception produced by intracerebroventricular (i.c.v.) administration of a,beta-methylene-ATP using antagonists at adrenergic, serotonin or opioid receptors. Antinociceptive effect of alpha,beta-methylene-ATP (10 nmol/rat) was significantly attenuated by subcutaneous pretreatment with propranolol and naloxone, but not phentolamine or methysergide, at a dose of 10 mg/kg. I.c.v. pretreatment with propranolol (100 nmol/rat), butoxamine (100 nmol/rat), ICI-I 18,551 (100 nmol/rat) and naloxone (30 nmol/rat) significantly attenuated the antinociceptive effect of alpha,beta-methylene-ATP. However, i.c.v. pretreatment with atenolol (100 nmol/rat), naltrindole (30 nmol/rat) or nor-binaltorphimine (30 nmol/rat) did not show any significant effects. These results suggest that supraspinal beta2-adrenergic and mu-opioid receptors are involved in the antinociceptive effect of i.c.v. administered alpha,beta-methylene-ATP.

The effects of chronic morphine treatment on neurotensin-induced antinociception

Previous studies have shown that the opioid antagonist naloxone does not alter neurotensin (NT)-induced antinociception. In the present studies, tolerance to morphine in mice significantly attenuated NT-induced antinociception, but not NT-induced hypothermia. In addition, centrally administered NT inhibited naloxone-precipitated jumping in morphine-dependent mice. These results indicate complex interactions between NT-induced antinociception and opioid systems.

Designing of an orally active complement C3a agonist peptide with anti-analgesic and anti-amnesic activity

Complement C3a is an anti-opioid peptide, having anti-analgesic and anti-amnesic effects after intracerebroventricular administration. However, the peptide is inactive after oral administration. Orally active C3a agonist peptide was designed based on the structure of oryzatensin, a C3a agonist peptide derived from rice albumin. Tyr-Pro-Leu-Pro-Arg, a pentapeptide at the carboxyl terminus of oryzatensin is the minimally essential structure for exerting C3a activity. Due to the affinity for mu-opioid receptor, both oryzatensin and Tyr-Pro-Leu-Pro-Arg showed analgesia after intracerebroventricular administration in mice which was blocked by the opioid antagonist naloxone. Tyr-Pro-Leu-Pro-Arg lost opioid activity by substitution the amino terminus tyrosine with other hydrophobic residues. Among the newly designed peptides, Trp-Pro-Leu-Pro-Arg was found to possess the strongest C3a activity. The peptide antagonized morphine-induced analgesia at 300 mg/kg after oral administration and also improved scopolamine- and ischemia-induced amnesia in a step-through passive avoidance test.

Preliminary studies on the analgesic activity of latex of Calotropris procera

In this study we have evaluated the analgesic activity of dry latex (DL) of Calotropis procera. A single oral dose of DL ranging from 165 to 830 mg/kg produced a significant dose dependent analgesic effect against acetic acid induced writhings. The effect of DL at a dose of 415 mg/kg was more pronounced as compared to a 100 mg/kg oral dose of aspirin. On the other hand DL (830 mg/kg) produced marginal analgesia in a tail-flick model which was comparable to aspirin. The analgesic effect of DL was delayed by 1 h by naloxone at a dose of 0. 5 mg/kg, i.p., which completely blocked the analgesic effect of morphine (10 mg/kg, i.p.). However, the effect of aspirin was not blocked by naloxone. The 830 mg/kg oral dose of DL did not produce toxic effects in mice and the LD(50) was found to be 3 g/kg.

Selective and segmentally restricted antinociception induced by MPV-2426, a novel alpha-2-adrenoceptor agonist, following intrathecal administration in the rat

BACKGROUND

MPV-2426 (radolmidine) is a novel alpha-2-adrenoceptor agonist developed for spinal pain therapy. In the present study we determined the segmental distribution and selectivity of the antinociceptive effect induced by MPV-2426 following i.t. administration in rats.

METHODS

The experiments were performed in lightly anesthetized rats with an i.t. catheter for administration of drugs into the lumbar spinal cord level. To determine segmental distribution of antinociception, the withdrawal latency of the tail and forepaw from a hot water bath was measured. To determine selectivity of reflex modulation, the effect of i.t. MPV-2426 on the innocuous H-reflex was determined.

RESULTS

In the hot water immersion test MPV-2426 produced a dose-dependent (0.3-3.0 microg) prolongation of tail withdrawal latency whereas the effect on forepaw withdrawal latency was short of significance. Dexmedetomidine, the reference alpha-2-adrenoceptor agonist, produced a significant dose-related prolongation of both the tail and the forepaw withdrawal (0.3 and 1.0 microg). MPV-2426 (1.0 and 3.0 microg) produced no significant change in the amplitude of the H-reflex or M-response induced by electrical stimulation of the tibial nerve, nor any change in the modulation of the H-reflex amplitude induced by conditioning sural nerve stimulation. The antinociception induced by MPV-2426 was completely reversed by atipamezole (1 mg/kg s.c.), an alpha-2-adrenoceptor antagonist.

CONCLUSION

MPV-2426 produces a selective and segmentally more restricted antinociceptive effect than dexmedetomidine following i.t. administration. The antinoception induced by MPV-2426 is due to action on spinal alpha-2-adrenoceptors.

Distinct domains of IFNalpha mediate immune and analgesic effects respectively

Interferon-alpha (IFNalpha) is not only an immunoregulatory factor, but is also an analgesic molecule. The analgesic effect of IFNalpha was mediated by mu opioid receptor. After the 129th Tyr residue of human IFNalpha was mutated to Ser, the antiviral activity almost disappeared, but there still remained a strong analgesic activity that could be blocked by naloxone. These results indicate that there exist distinct domains in the IFNalpha molecule, which mediate immune and analgesic effects respectively, and suggest that there are different receptor mechanisms inducing immune and analgesic effects of IFNalpha. However, although the antiviral activity of IFNalpha decreased to 34.1% of wild type IFNalpha after the 122nd Tyr residue was changed to Ser, the analgesic activity of this mutant was lost completely. There were significant cross reactivities between INFalpha and anti-opioid sera. These studies show strong structural and functional similarities between INFalpha and opioid peptides, and inferred that the analgesic domain locates around the 122nd Tyr residue of IFNalpha molecule in tertiary structure.

Antinociceptive effect of the essential oil from Cymbopogon citratus in mice

The essential oil (EO) from leaves of Cymbopogon citratus increased the reaction time to thermal stimuli both after oral (25 mg/kg) and intraperitoneal (25-100 mg/kg) administration. EO (50-200 mg/kg, p.o. or i.p.) strongly inhibited the acetic acid-induced writhings in mice. In the formalin test, EO (50 and 200 mg/kg, i.p.) inhibited preferentially the second phase of the response, causing inhibitions of 100 and 48% at 200 mg/kg, i.p. and 100 mg/kg, p.o., respectively. On the other hand, the opioid antagonist naloxone blocked the central antinociceptive effect of EO, suggesting that EO acts both at peripheral and central levels.

Dissociable effects of the kappa-opioid receptor agonists bremazocine, U69593, and U50488H on locomotor activity and long-term behavioral sensitization induced by amphetamine and cocaine

RATIONALE

Mesolimbic dopaminergic neurotransmission plays a critical role in the locomotor effects of psychostimulant drugs, but a general involvement in the induction of long-term psychostimulant sensitization is questionable. By influencing dopaminergic neurotransmission, opioid drugs can alter the behavioral effects of psychostimulants.

OBJECTIVES

The effects of the kappa-opioid receptor agonists bremazocine, U69593, and U50488H on the locomotor stimulant and the long-term sensitizing effects of amphetamine and cocaine were investigated in rats. Unlike U69593 and U50488H, bremazocine is also an antagonist at mu- and delta-opioid receptors, as well as an agonist at a subtype of delta-opioid receptors inhibiting dopamine D1 receptor-stimulated adenylate cyclase.

METHODS

Bremazocine, U69593, and U50488H were administered prior to amphetamine and cocaine, and locomotor activity was measured. In separate studies, the opioids were co-administered with amphetamine and cocaine for 5 days, and locomotor sensitization was assessed 3 weeks post-treatment.

RESULTS

Bremazocine and U69593 attenuated the psychomotor stimulant effects of amphetamine and cocaine. U50488H attenuated the locomotor effect of cocaine and biphasically affected amphetamine-induced locomotion, i.e., suppression followed by stimulation. Bremazocine prevented the development of amphetamine-induced but not cocaine-induced long-term sensitization. Neither U69593 nor U50448H affected the induction of long-term amphetamine or cocaine sensitization.

CONCLUSIONS

In agreement with previous studies, the present data suggest that differential mechanisms underlie the acute stimulant versus the long-term sensitizing effects of psychostimulants, and the induction of long-term sensitization by amphetamine versus cocaine. Stimulation of kappa-opioid receptors does not seem to block the induction of long-term psychostimulant sensitization. Thus, bremazocine is likely to block the induction of amphetamine sensitization through a non-kappa-opioid receptor mechanism. We suggest that this effect of bremazocine is the result of its unique agonist action at a subtype of delta-opioid receptors, thereby acting as a functional dopamine D1 receptor antagonist. This would be consistent with the literature showing that the induction of long-term amphetamine sensitization depends on the activation of dopamine D1 receptors. In addition, the present data are in keeping with studies showing that dopamine neurotransmission is not critical for the induction of long-term cocaine sensitization.

Antiallodynic effect of intrathecal gabapentin and its interaction with clonidine in a rat model of postoperative pain

BACKGROUND

Systemic administration of gabapentin was shown previously to attenuate mechanical allodynia in a rat model of postoperative pain. Because intrathecal administration of gabapentin is effective in other hypersensitivity states, the authors tested its effect in the postoperative model, its interaction with another antiallodynic agent (clonidine), and a possible mechanism of gabapentin action (entry into sites of action via an L-amino acid transporter).

METHODS

Male Sprague-Dawley rats were anesthetized with halothane, and an incision of the plantaris muscle of right hind paw induced punctate mechanical allodynia. Withdrawal threshold to von Frey filament application near the incision site was determined before and 2 h after surgery. Then, an intrathecal injection was performed and thresholds were determined every 30 min for 3 h thereafter.

RESULTS

Paw incision induced a mechanical hypersensitivity (mechanical threshold > 25 g before incision and < 5 g after). Intrathecal gabapentin dose-dependently (10-100 microg) reduced mechanical allodynia. Intrathecal injection of an inhibitor of L-amino acid transporters or a competitor for this transporter, L-leucine, did not reverse the intrathecal effect of gabapentin. The ED50 of intrathecal gabapentin, clonidine, and their combination were 51, 31, and 9 microg, respectively, and isobolographic analysis showed synergy between gabapentin and clonidine.

CONCLUSIONS

Intrathecal gabapentin is effective against tactile allodynia that occurs after paw incision, and interacts synergistically with clonidine. Unlike results in vitro, gabapentin does not obligatorily need to enter cells via the L-amino acid transporter mechanism to achieve its effects in vivo.

Postnatal naltrindole treatments induce behavioural modifications in preweanling rats

To investigate the physiological role of the delta-opioid receptor during the preweanling period, we have studied the effects of chronic (daily injections from birth to postnatal day 19) and acute treatments with the selective delta-antagonist naltrindole (1 mg/kg), on behavioural and nociceptive responses in 20-day old male rats. Behavioural testing was performed using an open field paradigm. Acute naltrindole induced significant decreases in external and total ambulation (horizontal activity) and rearing behaviour (vertical activity), as well as a significant increase in grooming frequency. In animals chronically treated with naltrindole there was an increase in total ambulation one day after the discontinuation of the treatment. In a test of nociception (tail immersion) no significant effect of chronic naltrindole treatment on baseline latencies or of acute naltrindole on latency quotients (post-treatment latency/pre-treatment latency) were found. However, chronic naltrindole administration significantly decreased the latency quotients. The results show that the delta-opioid receptor participates in the tonic regulation of motor activity during the preweanling period and might be involved in certain aspects of stress responsiveness.

Effects of 2-arachidonylglycerol, an endogenous cannabinoid, on neuronal activity in rat hippocampal slices

The monoacylglycerol 2-arachidonylglycerol is an endogenous ligand of cannabinoid receptors. We examined whether 2-arachidonylglycerol can influence excessive neuronal activity by investigating stimulation-induced population spikes and epileptiform activity in rat hippocampal slices. For this purpose, the effects of 2-arachidonylglycerol were compared with those of the synthetic cannabinoid agonist WIN 55,212-2. At concentrations of 10-50 microM, 2-arachidonylglycerol attenuated the amplitude of the orthodromic population spike and the slope of the field excitatory postsynaptic potential (field EPSP). However, the effect of the synthetic cannabinoid WIN 55,212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolol[ 1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone; 0.1 microM and 1 microM) was significantly higher than that of the endogenous ligand. At a concentration of 1 microM, WIN 55,212-2 completely suppressed the field EPSP. However, none of the investigated compounds did affect the presynaptic fiber spike of the afferents. The CB1 receptor antagonist SR 141716 (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorphenyl)-4-methyl-3- pyrazole-carboxamide) blocked the inhibition evoked by the cannabinoids. Both 2-arachidonylglycerol (30 microM) and WIN 55,212-2 (100 nM) shifted the input-output curve of the postsynaptic spike and the field EPSP to the right and increased the magnitude of paired-pulse facilitation, indicating a presynaptic mechanism of action. 2-Arachidonylglycerol and WIN 55,212-2 attenuated the frequency of spontaneously occurring epileptiform burst discharges in CA3 elicited by omission of Mg2+ and elevation of K+ to 8 mM. The antiepileptiform effect of these cannabinoids was blocked by SR 141716. In conclusion, 2-arachidonylglycerol seems to limit neuronal excitability via cannabinoid receptors of the CB1 type. By acting predominantly at a presynaptic site, it is capable of reducing excitatory neurotransmission, a mechanism which might be involved in the prevention of excessive excitability leading to epileptiform activity.

Analgesic effects of adenosine in syndrome X are counteracted by theophylline: a double-blind placebo-controlled study

It has been proposed that adenosine mediates ischaemic pain in humans. Patients with cardiac Syndrome X are hypersensitive to potential pain stimuli, including adenosine. On the other hand, recent findings suggest that low-dose adenosine infusion may have analgesic effects. Our aim was to test two hypotheses: (1) that the analgesic effect of adenosine is peripheral in origin, and (2) that part of the hypersensitivity to pain of patients with cardiac Syndrome X results from a disturbed mechanism of adenosine analgesia. A total of 12 female Syndrome X patients and eight healthy age-matched female controls were studied in a randomized, double-blind and placebo-controlled study. Adenosine (70 microg/min) or placebo was infused into the forearm via an intra-arterial catheter. After 15 min of infusion, a tourniquet on the upper arm was inflated to 225 mmHg to ensure arterial occlusion. The patient then carried out dynamic handgrip work at 60 Hz. Pain or discomfort in the forearm was estimated continuously according to the Borg CR-10 scale. After the first test, theophylline was infused for 10 min intravenously at a dose of 5 mg/kg body weight. The ischaemic forearm test was then repeated. On a second occasion, the procedure was repeated with the opposite treatment (adenosine/placebo). Only six of 12 Syndrome X patients completed the protocol because of pain during the catheterization procedure or an inability to establish an intra-arterial line. The time to onset of pain in the working, ischaemic forearm was greater for subjects treated with adenosine than for those treated with placebo, both in those Syndrome X patients who tolerated catheterization (49+/-27 s compared with 32+/-18 s; P<0.03) and in healthy controls (40+/-19 s compared with 16+/-8 s; P<0.02). The time to maximum pain, limiting ischaemic work, was also greater with adenosine pretreatment both in Syndrome X patients (137+/-28 s compared with 106+/-28 s; P<0.03) and in healthy controls (109+/-31 compared with 82+/-18 s; P<0.01). After infusion of theophylline there was no difference between adenosine and placebo in either group. Intra-arterially infused adenosine had similar peripheral analgesic effects on experimentally induced muscular ischaemia in those female Syndrome X patients who tolerated intra-arterial catheterization and in healthy controls. Thus adenosine analgesia is counteracted by theophylline, suggesting that the effect is mediated by membrane-bound peripheral adenosine receptors.

Antisense oligodeoxynucleotide targeting distinct exons of the cloned mu-opioid receptor distinguish between endomorphin-1 and morphine supraspinal antinociception in mice

Antisense oligodeoxynucleotides (ODN) were used to investigate the supraspinal antinociceptive effects of endomorphin-1, an endogenous peptide whose analgesic profile suggests that it is a ligand at the mu-opioid receptor. To selectively restrict the expression of this receptor, five ODN targeting distinct exons of the gene sequence were injected subchronically by the intracerebroventricular route (i.c.v.) into mice. The antinociception induced by endomorphin-1 was greatly reduced in animals receiving the ODN directed to nucleotides 677-697, which code for a sequence located on the second extracellular loop of the mu receptor. ODN-mu(un), one of the two antisense ODN directed to exon 1, also impaired endomorphin-1 antinociception. ODN targeting exons 2 and 4 were totally inactive. In contrast, all five ODN blocked the antinociception induced by morphine and beta-casomorphin. The analgesic potency of endomorphin-1, morphine, and beta-casomorphin remained unaltered by administration of an ODN to nucleotides 29-46 of the murine delta-opioid receptor gene sequence of a random-sequence ODN. This suggest the existence of diverse molecular forms for the mu-opioid receptor that mediate the antinociceptive effects of endomorphin-1 and morphine/beta-casomorphin.

Activation of peripheral kappa opioid receptors inhibits capsaicin-induced thermal nociception in rhesus monkeys

8-Methyl-N-vanillyl-6-nonenamide (capsaicin) was locally applied in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, which was manifested as reduced tail-withdrawal latencies in normally innocuous 46 degrees C water. Coadministration of three kappa opioid ligands, U50,488 (3.2-100 microgram), bremazocine (0.1-3.2 microgram), and dynorphin A(1-13) (3.2-100 microgram), with capsaicin in the tail dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by a small dose of an opioid antagonist, quadazocine; (0.32 mg), applied in the tail; however, this dose of quadazocine injected s.c. in the back did not antagonize local U50,488. Comparing the relative potency of either agonist or antagonist after local and systemic administration confirmed that the site of action of locally applied kappa opioid agonists is in the tail. In addition, local nor-binaltorphimine (0.32 mg) and oxilorphan (0.1-10 microgram) antagonist studies raised the possibility of kappa opioid receptor subtypes in the periphery, which indicated that U50,488 produced local antinociception by acting on kappa1 receptors, but bremazocine acted probably on non-kappa1 receptors. These results provide functional evidence that activation of peripheral kappa opioid receptors can diminish capsaicin-induced allodynia in primates. This experimental pain model is a useful tool for evaluating peripherally antinociceptive actions of kappa agonists without central side effects and suggests new approaches for opioid pain management.

Spinal administration of selective opioid antagonists in amphibians: evidence for an opioid unireceptor

In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.

Opioid activity of sendide, a tachykinin NK1 receptor antagonist

Sendide, a tachykinin NK1 receptor antagonist, was tested for antagonism against scratching, biting and licking responses elicited by intrathecal (i.t.) injections of various tachykinin receptor agonists, N-methyl-D-aspartate (NMDA), somatostatin and bombesin, in mice. Tachykinin NK1 receptor agonists, substance P, physalaemin and septide, produced a characteristic behavioural response, consisting of scratching, biting and licking. The substance P-induced response was reduced by small doses (0.0625-1.0 pmol) of sendide in a dose-dependent manner. The behavioural response elicited by other tachykinin NK1 receptor agonists, physalaemin and septide, was also reduced significantly by a small dose (1.0 pmol) of sendide. The inhibitory effect of sendide (1.0 pmol) was not affected by pretreatment with the opioid receptor antagonist, naloxone, at doses up to 4.0 mg/kg. Higher doses of sendide were needed to reduce the behavioural response to neurokinin A, a tachykinin NK2 receptor agonist, neurokinin B, a tachykinin NK3 receptor agonist and eledoisin, a tachykinin NK2/NK3 receptor agonist. Pretreatment with naloxone (2.0 mg/kg, i.p.) significantly antagonized sendide (1024 pmol)-induced inhibition of the behavioural responses to neurokinin A, neurokinin B and eledoisin. The behaviours elicited by i.t. injection of NMDA, somatostatin or bombesin were also reduced by a higher dose (1024 pmol) of sendide and this sendide effect was reversed by naloxone. These findings suggest that sendide at higher doses may possess opioid activity in addition to an antagonistic action at tachykinin NK1 receptors in the spinal cord.