The molecular mechanisms of the analgesic action of melatonin

Objective To analyse the potential involvement of the opioid receptor gene expression in the mechanisms of the analgesic action of melatonin.Methods A trauma-pain model was established in Wistar rats by combining right-hind limb amputation with 50 ℃ tail-flick test.Antinociception was determined by tail-flick latency to hot waster at 50 ℃.RT-PCR was used to observe the the expression of the M1OR and KOR gene.Results Melatonin produced the antinociceptive effect in dose-dependent manner after i.p or i.c.v.administration.Injected i.c.v.to rats,naloxone(10 μg)obviously antagonized the antinociceptive effect induced by i.p.melatonin.The expression of the M1OR gene in the rat hypothalamus and the KOR gene in the hippocampus was both significantly reduced at day 3 after injury,which was parallel to the reduction of the rat pain thresholds.However,the expression of the M1OR gene in the hippocampus and the KOR gene in the hypothalamus was not changed.Treatment of trauma-pain rats with melatonin(30-120 mg·kg-1)i.p.administrated induced the up-regulation of M1OR mRNA in the hypothalamus and the KOR mRNA in the hippocampus in a concentration-dependent manner.Conclusions The present observations suggest that Melatonin-induced antinociceptive effect may partially contribute to the up-regulation of M1OR mRNA level in the hypothalamus and the KOR mRNA level in the hippocampus.     

Melatonin reverses the expression of morphine-induced conditioned place preference through its receptors within central nervous system in mice

A growing body of evidence indicates the prominent actions of melatonin on the opioidergic system. Nevertheless, effect of melatonin on rewarding properties of morphine is still obscure. In particular, effect of melatonin on the expression of morphine reward is unknown. We evaluated the effect of exogenous administration of melatonin on the expression of morphine reward in mice using a conditioned place preference (CPP) paradigm. The conditioned place preference was induced by morphine (s.c., 3 mg/kg, once each day for 5 consecutive days) in mice. Our data showed that the intraperitoneal (i.p.) administration of melatonin (12.5-50 mg/kg) reversed the expression of morphine-induced conditioned place preference in a dose-dependent manner. Furthermore, the intracerebroventricular (i.c.v.) administration of melatonin (0.125-0.5 mg/kg) also resulted in dose-dependent reversal effect on the expression of morphine-induced conditioned place preference. We further investigated which of melatonin receptor subtypes within the central nervous system was mediating this reversal action in mice using luzindole (2-benzyl-N-acetyltryptamine, a non-selective antagonist for melatonin MT(1) and MT(2) receptors) and K185 {N-butanoyl-2-(5,6,7-trihydro-11-methoxybenzo[3,4]cyclohept[2,1-alpha]indol-13-yl)ethanamine, a selective antagonist for melatonin MT(2) receptor}. It was shown that the i.c.v. administration of either K185 (5, 20 mug) or luzindole (6.25, 12.5 mug) significantly antagonized the reversal effect of melatonin (50 mg/kg, i.p) on the expression of morphine-induced conditioned place preference, while the i.c.v. administration of 20 mug of K185 or 12.5 mug of luzindole by itself did not alter the expression of morphine-induced conditioned place preference. These results suggest that melatonin reverses the expression of morphine-induced rewarding effect, and this action is mediated by the activation of melatonin MT(2) receptor subtype within the central nervous system.     

Mechanisms of L-NG-nitro arginine methyl ester-induced antinociception in mice: a role for serotonergic and adrenergic neurons.

1. The mechanisms involved in the antinociceptive action of L-NG-nitro arginine methyl ester (L-NAME) were investigated in mice. 2. Intraperitoneal administration of L-NAME produced a dose-dependent antinociception in the tail-flick, hot-plate and phenyl-p-quinone-induced writhing tests. 3. Pretreatment with the catecholamine depletors 6-hydroxydopamine (5 micrograms i.c.v.) or reserpine (5 mg/kg i.p.) or the serotonin synthesis inhibitor, p-chlorophenylalanine methyl ester (200 mg/kg i.p. on 2 consecutive days) resulted in a significant decrease in the antinociceptive effect of L-NAME. 4. Similarly, pretreatment with the selective alpha 1-adrenoceptor antagonist prazonin (2.5 mg/kg, i.p.), or the non-selective alpha-adrenoceptor blocker, phentolamine (5 mg/kg, i.p.) antagonized the antinociceptive effect of L-NAME. 5. However, the administration of the selective alpha 2-adrenoceptor antagonist, idazoxan (2.5 mg/kg i.p.) was without effect. 6. Likewise, pretreatment with the serotonin 5-HT2 receptor blocker, ketanserin (1 mg/kg, i.p.), the D2 dopamine receptor antagonist (+/-) sulpiride (30 mg/kg, i.p.) or the opioid antagonist naloxone (5 mg/kg, i.p.) did not inhibit the antinociceptive effect of L-NAME. 7. These results suggest that L-NAME produces antinociception in the mouse probably by an action on adrenergic and serotonergic synapses.     

Paradoxical effects of opioid antagonist naloxone on SSRI-induced analgesia and tolerance in mice

Selective serotonin reuptake inhibitors (SSRIs) have been used clinically as co-analgesics in various devastating painful conditions. Upon chronic treatment tolerance develops to their analgesic effect, which is often refractory to increasing dose. Although modulation of serotonergic pathways considerably explains their clinical efficacy, numerous reports nevertheless indicate the direct/indirect role of the opioidergic pathway in SSRI-induced analgesia. The present study was designed to investigate the effect, if any, of the opioid antagonist naloxone on SSRIs-induced analgesia and tolerance employing acetic acid-induced writhing assay. Two SSRIs, fluoxetine (FLX), and citalopram (CTP) were used in the study. Acute systemic (5-40 mg kg(-1) i.p.), or intrathecal (5-40 microg per mouse, i.t.) administration of fluoxetine or citalopram exhibited a dose-dependent and significant (p < 0.05) antinociceptive effect. Single systemic (2-5 mg kg(-1) i.p.) or intrathecal (1 microg per mouse, i.t.) administration of opioid antagonist naloxone blocked where as systemic ultra-low dose (10 ng/kg) or intrathecal (0.05 ng) naloxone potentiated the acute antinociceptive effect of both SSRIs (10 mg kg(-1) i.p. and 10 microg i.t.). Animals treated chronically over a 7-day period with SSRIs developed tolerance to their antinociceptive effect. Further, chronic administration of ultra-low dose of naloxone intrathecal (0.05 ng per mouse, i.t.) or systemic (10 ng kg(-1) i.p.) with fluoxetine or citalopram (10 microg i.t.; 5 mg kg(-1) i.p.) over a 7-day period reversed the tolerance to the antinociceptive effect of SSRIs. Thus, in ultra-low doses, naloxone paradoxically enhances SSRIs-induced analgesia and reverse tolerance through spinal and peripheral action. These effects of opioid antagonist naloxone on SSRIs-induced antinociception may have an implication in refractory cases upon chronic use of SSRIs as co-analgesics.     

Baclofen analgesia: involvement of the GABAergic system.

The effect of baclofen, a GABA(B) agonist, has been studied in the hot plate test in mice, to analyze the possible involvement of the GABAergic system in baclofen analgesia. Baclofen (1-3 mg kg(-1) intraperitoneal (i.p.)) was found to elicit a dose-dependent antinociceptive effect. The antinociceptive effect of baclofen cannot be due to motor incoordination or sedation as the doses of baclofen which produce analgesia did not induce these effects during the rota-rod test. The antinociceptive effect of baclofen was reversed by 2-hydroxysaclofen, a GABA(B) antagonist by both systemic (3 mg kg(-1)) and intra cisterna magna (intracisternal (i.c.)) (0.3 mg kg(-1)) administration. The antagonist dose administered via i.c. produced a complete blockade and was 10-fold lower than the dose employed in i.p. administration. The data suggest that the antinociceptive effect of baclofen is GABA(B) receptor-mediated and reveal a central location of the analgesic effect of baclofen.     

Opiate and acetylcholine-independent analgesic actions of crotoxin isolated from crotalus durissus terrificus venom.

The venom of Crotalus durissus terrificus is reported to have analgesic activity and the administration of Crotoxin (Cro) to cancer patients is reported to reduce the consumption of analgesics. This study investigated the analgesia induced by Cro and the effects of atropine and naloxone on the antinociceptive activity of Cro in mice and rats. The results showed that Cro at 66.5, 44.3 and 29.5microg/kg (ip) exhibited a dose-dependent analgesic action in mice using the hotplate and acetic acid writhing tests. Cro at 44.3microg/kg (ip) had significant analgesic action in the rat tail-flick test. In the mouse acetic acid-writhing test, intracerebral ventricular administration of Cro 0.3microg/kg produced marked analgesic effects. Microinjection of Cro (0.15microg/kg) into the periaqueductal gray area also elicited a robust analgesic action in rat hotplate test. Atropine at 0.5mg/kg (im) or 10mg/kg (ip) or naloxone at 3mg/kg (ip) failed to block the analgesic effects of Cro. These results suggest that Cro has analgesic effects mediated by an action on the central nervous system. The muscarinic and opioid receptors are not involved in the antinociceptive effects of Cro.     

A long-form α-neurotoxin from cobra venom produces potent opioidindependent analgesia

AIM: In light of the antinociceptive activity of the short-chain neurotoxin, cobrotoxin, and other acetylcholine antagonists, the antinociceptive activity and mechanisms of cobratoxin (CTX), a long-chain postsynaptic alpha-neurotoxin, was investigated in rodent pain models. METHODS: CTX was administered intraperitoneally (30, 45, 68 microg/kg), intra-cerebral ventricularly (4.5 microg/kg) or microinjected into periaqueductal gray (PAG; 4.5 microg/kg). The antinociceptive action was tested using the hot-plate and acetic acid writhing tests in mice and rats. The involvement of the cholinergic system and opioid system in CTX-induced analgesia was examined by pretreatment of animals with atropine (0.5 mg/kg, im; or 10 mg/kg, ip) or naloxone (1 and 5 mg/kg, ip). The effect of CTX on motor activity was tested using the Animex test. RESULTS: CTX exhibited a dose-dependent analgesic action in mice as determined by both the hot-plate and acetic acid writhing tests. The peak effect of analgesia was seen 3 h after administration. In the mouse acetic acid writhing test, the intra-cerebral ventricular administration of CTX at 4.5 microg/kg (1/12th of a systemic dose) produced marked analgesic effects. Microinjection of CTX (4.5 microg/kg) into the PAG region did not elicit an analgesic action in rats in the hot-plate test. Atropine at 0.5 mg/kg (im) and naloxone at 1 and 5 mg/kg (ip) both failed to block the analgesic effects of CTX, but atropine at 10 mg/kg (ip) did antagonize the analgesia mediated by CTX in the mouse acetic acid writhing test. Acetylsalicylic acid (300 mg/kg) did not enhance the analgesic effects of CTX. At the highest effective dose of 68 microg/kg the neurotoxin did not change the spontaneous mobility of mice. CONCLUSION: CTX has analgesic effects, which are mediated in the central nervous system though not through the PAG. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CTX.     

Effects of the analgesic agent tramadol in normal and arthritic rats: comparison with the effects of different opioids, including tolerance and cross-tolerance to morphine

The effects of the analgesic agent tramadol (0.1-1 mg/kg i.v.) were compared to those of the mixed agonist-antagonist analgesics nalbuphine (1 mg/kg i.v.) and buprenorphine (3 micrograms/kg i.v.) in the vocalization threshold to paw pressure test. Normal and Freund's adjuvant-induced arthritic rats were used. We have shown previously that these animals used as a model of clinical pain exhibit an enhanced sensitivity to morphine (0.1-1 mg/kg i.v.), with a rapid development of tolerance after repetitive low doses, a response not observed in normal rats. In the present study, the antinociceptive effects of tramadol, buprenorphine and nalbuphine were enhanced (by 2- to 5-fold) in arthritic compared to normal rats. In this model, these effects were significantly reduced by a dose of naloxone (0.1 mg/kg i.v.) that completely antagonized the effect of morphine. In this model, the antinociceptive effect of tramadol (1 mg/kg i.v.) was comparable to that of nalbuphine (1 mg/kg i.v.), buprenorphine (3 micrograms/kg i.v.) and morphine (1 mg/kg i.v.). Repeated administration of low doses of tramadol twice daily for 4 days to arthritic rats did not induce tolerance, in contrast to nalbuphine, buprenorphine, and morphine. In addition, no cross-tolerance between tramadol and morphine was observed in these animals.     

Influence of omega-conotoxin on morphine analgesia and withdrawal syndrome in rats.

The effect of omega-conotoxin on opiate analgesia and withdrawal syndrome was investigated in rats. omega-Conotoxin given i.c.v. and i.p. caused weak analgesia in the tail-flick test. When the toxin (20 ng/rat) was given i.c.v. immediately before morphine (1.5 micrograms/rat i.c.v.) the resultant analgesic effect was additive. In contrast, the analgesia elicited by morphine (3 micrograms/rat i.c.v.) was greatly reduced after 24-h pretreatment with the toxin (20 ng/rat i.c.v.). The systemic administration of the toxin (10 micrograms/kg i.p.) did not affect morphine analgesia whether omega-conotoxin was coadministered with morphine (2.5 mg/kg i.p.) or was given 24 h before the opiate (5 mg/kg i.p.). omega-Conotoxin i.c.v. injected in morphine-dependent rats 15 min before naloxone challenge significantly attenuated the abstinence syndrome. On the contrary systemic administration of omega-conotoxin failed to suppress the morphine withdrawal syndrome. The present results suggest that omega-conotoxin affects both acute and chronic effects of morphine.     

Antinociceptive activity of Vitex-negundo Linn leaf extract

Tail flick test in rats and acetic acid induced writhing in mice were employed to study the antinociceptive activity of ethanolic leaf extract of Vitex-negundo (VN) (100, 250 and 500 mg/kg, p.o). The effect was compared with meperidine (40 mg/kg, sc) in tail flick method and aspirin (50 mg/kg, p.o) in writhing test as a standard control respectively. An interaction with naloxone hydrochloride was also studied in tail flick method for its mechanism of central analgesic action. The test drug showed significant analgesic activity in dose dependant manner in both the experimental models. In comparison to standard drug (meperidine), more than ten times dose of VN extract was required to produce comparable significant antinociceptive activity. The sub-effective dose (5 mg/kg, po) of VN potentiated the analgesic activity of meperidine (4 mg/kg, sc) and aspirin (25 mg/kg, po). Naloxone (1 mg/kg, sc) did not reverse the analgesic effect of VN extract. Our observations suggest that VN possesses both central and peripheral analgesic activity. The central analgesic action does not seem to be mediated through opioid receptors. It, may prove to be a useful adjuvant therapy along with standard analgesic drug.     

The involvement of central cholinergic system in (+)-matrine-induced antinociception in mice

The antinociceptive effect of (+)-matrine was examined in mice by writhing, tail-pressure and hot-plate tests. (+)-Matrine (5, 10 and 20 mg/kg s.c.) produced antinociception in a dose-dependent manner. In hot-plate test, the antinociception produced by (+)-matrine (10 mg/kg s.c.) was attenuated by muscarinic receptor antagonists atropine (5 mg/kg i.p.) and pirenzepine (0.1 mug/mouse i.c.v.) and acetylcholine depletor hemicholinium-3 (HC-3) (1 mug/mouse i.c.v.), but not by opioid receptor antagonist naloxone (2 mg/kg i.p.), dopamine D(2) receptor agonist (-)-quinpirole (0.1 mg/kg i.p.) or catecholamine depletor reserpine (2.5 mg/kg i.p.). Radioligand binding assay demonstrated that (+)-matrine had no affinity for mu-, kappa- or delta-opioid receptors in a wide concentration range (1 x 10(-11)-1 x 10(-3) M). The results suggest that (+)-matrine exerts its antinociceptive effect through multiple mechanism(s) such as increasing cholinergic activation in the CNS rather than acting on opioid receptors directly.     

Corticosteroid effects on morphine-induced antinociception as a function of two types of corticosteroid receptors in brain

The antinociceptive effect of parenterally and intracerebroventricularly injected morphine and beta-endorphin in adrenalectomized rats and in adrenalectomized rats treated with adrenal steroids was examined employing the hot-plate method. (1) Adrenalectomy sensitized the rats to an analgesic effect of morphine and beta-endorphin. (2) Replacement therapy (chronic and acute) with corticosterone, dexamethasone or RU 28362 (glucocorticoid receptor agonist) effectively reversed the increase in the sensitivity to the analgesic effect of peripherally injected morphine (5 mg/kg i.p.) induced by adrenalectomy to the level of sham-operated animals. Glucocorticosteroids administered to non-adrenalectomized rats did not change the sensitivity to morphine. (3) Corticosterone had a biphasic, dose-dependent effect; the most significant attenuation of the hypersensitivity to morphine-induced antinociception in adrenalectomized rats was achieved after 0.01 mg and after 10 mg (per kg b.w.). Doses of corticosterone of 0.005 mg/kg and in a range of 0.05-0.30 mg/kg were ineffective. (4) Corticosterone in a dose of 0.01 mg/kg (s.c.) had suppressant effects on the adrenalectomy-induced increase in the sensitivity to antinociception induced by morphine when given prior to morphine (60, 30 and 5 min) as well as after the injection of morphine (before the first and the second testing on the hot-plate, 15 and 5 min, respectively). (5) Intracerebroventricularly (i.c.v.) injected morphine and beta-endorphin also displayed the hypersensitivity to the analgesic effect in adrenalectomized rats which in both cases could be suppressed by 0.01 mg/kg of corticosterone given subcutaneously 5 min prior to administration of the opiate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Evidence for a noradrenergic component in the antinociceptive effect of the analgesic agent tramadol in an animal model of clinical pain, the arthritic rat.

The analgesic agent tramadol has a potent antinociceptive effect in arthritic rats. In the present study, the actions of the selective alpha 2-adrenoceptor antagonists yohimbine and idazoxan on this antinociceptive effect were tested in arthritic rats, using vocalization thresholds to paw pressure as a nociceptive test. The antagonists were administered 30 min before tramadol, at doses (0.5 and 1 mg/kg i.v.) without action per se, but which prevented the antinociceptive action of the prototypic alpha 2-adrenoceptor agonist clonidine (0.1 mg/kg i.v.) in these animals. The potent antinociceptive effect of tramadol (1 mg/kg i.v.) was significantly decreased (mean total effect reduced about 2-fold) by yohimbine and idazoxan. In alpha 2-adrenoceptor antagonists-pretreated arthritic rats, the effect of tramadol was almost abolished when tramadol was coinjected with the opioid antagonist naloxone. In addition to the involvement of opioid receptors, these results provide evidence for a noradrenergic component to the antinociceptive action of tramadol in this model of clinical pain.     

Ghrelin inhibits inflammatory pain in rats: involvement of the opioid system

This study examined the effects of intracerebroventricular (i.c.v.), intraperitoneal (i.p.) or intraplantar (i.pl.) administration of ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, in the development of hyperalgesia and edema induced by intraplantar injection of carrageenan in rats. Central ghrelin (4 ng to 4 microg/rat) given 5 min before carrageenan produced a dose-related reversal of carrageenan-induced mechanical hyperalgesia measured by Randall-Selitto test with an ED50 of 81.7 ng/rat. Ghrelin at the dose of 4 microg/rat i.c.v. was also effective in inhibiting edema. When ghrelin (4 microg/rat i.c.v.) was administered 150 min after carrageenan, it failed to modify either hyperalgesia or the paw volume. Given i.p., 30 min before carrageenan, ghrelin (20-160 microg/kg) induced a significant dose-dependent inhibition of hyperalgesia with an ED50 of 77 microg/kg and a slight reduction of edema. Intraplantar ghrelin (40 ng to 12 microg/rat) did not significantly modify both the hyperalgesic and edematous activities of carrageenan. The anti-hyperalgesic and anti-edematous effects of ghrelin (4 microg/rat i.c.v.) were reversed by naloxone (10 microg/rat i.c.v.). Systemic administration of the peripheral selective opioid antagonist, naloxone methiodide (3 mg/kg s.c.), did not antagonize antinociception elicited by i.p. ghrelin. Overall these data indicate that ghrelin exerts an inhibitory role on inflammatory pain through an interaction with the central opioid system.     

Interactions of ketamine, naloxone and morphine in the rat

The effect of naloxone on the ketamine-induced anesthesia and analgesia, and the development of tolerance to ketamine and the cross-tolerance to morphine (measured by an analgesic effect) were investigated in the rat. Ketamine produced a dose-dependent analgesia. Naloxone, 1 mg/kg, significantly inhibited analgesia induced by ketamine, 100 mg/kg, but even in a dose of 4 mg/kg it did not affect the duration of anesthesia. A chronic administration of ketamine (100 mg/kg twice a day (b.i.d.) for 7 days) resulted in the development of tolerance to analgesic effects of ketamine. The analgesic action of morphine was attenuated in rats receiving ketamine chronically, while the analgesic effects of ketamine were significantly potentiated in morphine-dependent rats. Ketamine, 25 mg/kg, significantly attenuated the withdrawal signs evoked by naloxone in morphine-dependent rats. The results corroborate the suggestion about the participation of the central opioid neurotransmission in the mechanism of ketamine action.     

Central endothelin-B receptor stimulation does not affect morphine analgesia in rats

Several neurotransmitter mechanisms have been proposed to play a role in the actions of morphine. We reported that centrally administered endothelin A (ETA) receptor antagonists potentiate morphine analgesia in rats. It has also been reported that ETB agonist, IRL1620, has antinociceptive action mediated through opiate receptors in the periphery. The present study was conducted to determine if central ETB receptors are involved in analgesic actions of morphine. The effect of intracerebroventricular (i.c.v.) administration of ETB receptor agonist, IRL1620, on morphine-induced analgesia and hyperthermia was determined in the rat. Morphine (4 mg/kg, s.c.) produced a significant increase (84%) in tail-flick latency compared to the control group and the analgesic response lasted for 4 h. IRL1620 (30 microg, i.c.v.) did not produce any increase (16%) in tail-flick latency over the 5-hour observation period in vehicle-treated rats. Pretreatment with IRL1620 (3, 10, and 30 microg, i.c.v.) did not have any significant effect on the intensity and duration of morphine (4 mg/kg, s.c.)-induced analgesia. Morphine (4 mg/kg, s.c.) administration produced an increase in body temperature compared to the control group. In vehicle-pretreated rats, IRL1620 (30 microg, i.c.v.) did not produce any change in body temperature. The morphine-induced hyperthermic effect was not altered in IRL1620-pretreated rats. These studies demonstrate that IRL1620, a specific ETB receptor agonist, did not affect the morphine-induced analgesic and hyperthermic effect in rats. It can be concluded that central ETB receptors are not involved in modulation of pharmacological actions of morphine.     

Antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, in mice

The antinociceptive effects of Nigella sativa oil and its major component, thymoquinone, were examined in mice. The p.o. administration of N. sativa oil (50-400 mg/kg) dose-dependently suppressed the nociceptive response in the hot-plate test, tail-pinch test, acetic acid-induced writhing test and in the early phase of the formalin test. The systemic administration (2.5-10 mg/kg, p.o. and 1-6 mg/kg, i.p.) and the i.c.v. injection (1-4 microgram/mouse) of thymoquinone attenuated the nociceptive response in not only the early phase but also the late phase of the formalin test. Naloxone injected s.c. (1 mg/kg) significantly blocked N. sativa oil- and thymoquinone-induced antinociception in the early phase of the formalin test. Moreover, the i.c.v. injection of naloxone (10 microgram/mouse), the mu(1)-opioid receptor antagonist, naloxonazine (1-5 microgram/mouse), or the kappa-opioid receptor antagonist, nor-binaltorphimine (1-5 microgram/mouse), significantly reversed thymoquinone-induced antinociception in the early phase but not the late phase of the formalin test, whereas the delta-opioid receptor antagonist, naltrindole (1-5 ng/mouse, i.c.v.), had no effect on either phase. The antinociceptive effect of morphine was significantly reduced in thymoquinone- and N. sativa oil-tolerant mice, but not vice versa. These results suggest that N. sativa oil and thymoquinone produce antinociceptive effects through indirect activation of the supraspinal mu(1)- and kappa-opioid receptor subtypes.     

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.     

Evidence for the involvement of central opioidergic systems in L-tyrosine methyl ester-induced analgesia in the rat

Intraperitoneal administration of L-tyrosine (used as methyl ester HCl) produced dose-dependent analgesia in male Sprague-Dawley rats as measured by the tail-flick test. The maximal analgesic response was obtained with 200 mg/kg dose of tyrosine. Administration of morphine also produced a dose-dependent analgesic response. Tyrosine in doses of 50 mg/kg or higher potentiated morphine-induced analgesia. The analgesic response of tyrosine (200 mg/kg) was antagonized by naloxone (1 mg/kg), an opiate antagonist. Subcutaneous administration of methyl naltrexone bromide (MRZ 2663 BR, 1 and 10 mg/kg) had no effect on tyrosine-induced analgesia. Intracerebroventricular injection of MRZ 2663 BR (1 and 10 micrograms/rat) effectively blocked tyrosine-induced analgesia. It is concluded that tyrosine-induced analgesia and its potentiation of analgesic response to morphine may be mediated via either the opiate receptors or activation of endogenous opioidergic systems of central origin.