Analgesic effects of morphine and loperamide in the rat formalin test: interactions with NMDA receptor antagonists

To reveal peripheral components of opiate analgesia, effects of loperamide, opioid agonist which does not penetrate the blood-brain barrier, were examined in formalin and acute thermal pain tests in comparison with morphine. Formalin administration induces pain behaviour such licking/biting of injected paw expressed as two phases. The first phase is caused by C-fibre activation due to peripheral stimulation, the second phase attributed to ongoing input from peripheral site, leading to spinal hyperexcitability, which is dependent on N-methyl-D-aspartate (NMDA) receptor activation. Loperamide (3-10 mg/kg) and morphine (6 mg/kg) reduced formalin-induced nociceptive behaviours and these effects were reversed by naloxone methiodide (0.03-10 mg/kg), opioid receptor antagonist which poorly penetrates the blood-brain barrier. Loperamide action was enhanced only by centrally active NMDA receptor antagonists memantine (3 mg/kg) and CGP 37849 (3 mg/kg), but not by NMDA/glycineB receptor antagonists showing weak or no central nervous system (CNS) activity. Present results suggest that central NMDA receptor blockade may be necessary to enhance analgesia induced through peripheral opioid mechanisms in formalin-evoked nociception.     

Opioid and nicotinic medullary hyperalgesic influences in the decerebrated rat

The effects of ethylketazocine (EKC) administered intraperitoneally and the nicotinic ligands (-)- and (+)-nicotine, (-)-cytisine, (-)-lobeline, and (+)-2-methylpiperidine administered into the 4th ventricle on the latency of the thermally evoked withdrawal reflex of the decerebrate rat were investigated. EKC administered intraperitoneally produced both hyperalgesia and analgesia. (-)-Nicotine administered into the 4th ventricle produced a biphasic dose related effect on the latency of the withdrawal reflex; low doses produced a dose related analgesia while higher doses produced hyperalgesia. (-)-Cytisine and (-)-lobeline administered into the 4th ventricle produced biphasic effects. (+)-2-Methylpiperidine administered into the 4th ventricle produced a significant degree of hyperalgesia. Both the analgesic and hyperalgesic effects of (-)-nicotine were antagonized by mecamylamine (1 mg/kg) and naltrexone (5 mg/kg). The hyperalgesic action of (+)-2-methylpiperidine was antagonized by naltrexone but not by mecamylamine. These observations suggest that there are both medullary opioidergic and nicotinic cholinergic mechanisms for modulating both analgesic and hyperalgesic processes and that nicotinic ligands have multiple mechanisms of action in the brain.     

Naloxone-sensitive hyperalgesia follows analgesia induced by morphine and environmental stimulation

Manipulations of attack parameters in murine agonistic encounters have shown that moderate attack terminating at the first unambiguous display of the upright submissive posture results in nonopioid analgesia. By contrast, attack continuing significantly beyond this behavioural marker results in decreases in nociception mediated by opiodergic mechanisms. However, these effects have only been demonstrated immediately upon termination of encounters. As such, little is known of the influences of agonistic encounters on nociceptive responding beyond this. Tail-flick latencies of male DBA/2 mice that had been exposed to opioid activating attack parameters were established at 15-minute intervals up to 90 min postattack. Data suggest the existence of at least two distinct phases in nociceptive responding with 1) analgesia being evident in the early phase (0-45 min) and 2) short-lasting (detectable only at 75 min postattack) hyperalgesia in the late phase. Further studies revealed both of these effects to be reversed by low (1-10 mg/kg) doses of naloxone. Interestingly, alterations in responsivity to noxious stimulation postmorphine (1-20 mg/kg) administration followed a similar pattern, with analgesia being detectable 0-2 hr and hyperalgesia being evident at 4 hr postinjection of either 10 or 20 mg/kg morphine. However, only the hyperalgesia induced by 20 mg/kg morphine was reversed by 10 but not 1 mg/kg naloxone. These data together suggest a relationship between the dose of morphine required to induce hyperalgesia and the amount of naloxone needed to reverse this response. The naloxone-reversibility of postencounter or morphine-induced hyperalgesia suggests that these effects are not a consequence of the absence of opiates/opioids per se.(ABSTRACT TRUNCATED AT 250 WORDS)     

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.     

Involvement of kappa opioid receptors in formalin-induced inhibition of analgesic tolerance to morphine in mice

This study examined the role of kappa opioid receptors (KOR) in the mechanism underlying tolerance to the analgesic effects of morphine induced by chronic pain. The analgesic effect of morphine (10 mg kg(-1)), estimated by the tail-flick test in mice, gradually decreased during repeated daily morphine treatment. A significant decrease in the analgesic effect of morphine was seen on the fifth day of repeated morphine treatment compared with the first day. Chronic pain was induced by subcutaneous administration of 2% formalin into the dorsal part of the left hind paw, which significantly inhibited development of tolerance to morphine analgesia. The effect of formalin-induced pain on inhibition of morphine tolerance was reversed by the KOR antagonist nor-binaltorphimine. Furthermore, an antisense oligodeoxynucleotide, but not a missense oligodeoxynucleotide, against KOR completely suppressed the inhibitory effect of formalin-induced pain on morphine tolerance. Naltrindole, an antagonist of delta opioid receptor, did not affect chronic-pain-induced tolerance to morphine. Our findings show that the inhibitory effect of chronic pain on analgesic tolerance to morphine is mediated by KOR rather than delta opioid receptors.     

Reductions in pain thresholds and morphine analgesia following intracerebroventricular parachlorophenylalanine

The selective decreases in both basal and analgesic pain thresholds following systemic administration of parachlorophenylalanine (PCPA) has been attributed to the inhibition of tryptophan hydroxylase and subsequent depletion of brain serotonin. These effects only occur at high systemic doses which have other general debilitating effects. The present study examined the relationship between PCPA's nociceptive and serotonin-depleting effects following intracerebroventricular (ICV) administration. The first experiment determined that an ICV dose of 3 mg, but not 1 mg, of PCPA significantly decreased jump thresholds at 0.5, 48 and 120 hr after injection. These effects were not due to osmolarity shifts since hypertonic saline injections failed to alter thresholds. The second experiment demonstrated a time-dependent reduction of morphine (5 mg/kg) analgesia as a function of the interval between ICV PCPA and the systemic morphine injection. PCPA reduced morphine analgesia if it was administered 24 hr prior to the opiate and eliminated morphine analgesia if it was administered 48 hr prior to the opiate. Pretreatment with ICV PCPA either 0.5 or 72 hr prior to the opiate failed to alter morphine analgesia. The third and fourth experiments indicated that hippocampal and spinal levels of either serotonin or 5-hydroxyindoleacetic acid were not significantly affected by ICV PCPA pretreatment. These data indicate that the hyperalgesia and morphine analgesia impairments noted following ICV PCPA do not correspond with changes in serotonin from hippocampal or spinal tissue and such effects are discussed in terms of alternative modes of action.     

C-fragment of lipotropin—an endogenous potent analgesic peptide

1 A series of peptides derived from porcine lipotropin was examined for analgesic and other morphine-like properties on infusion into the cannulated third ventricle of cats.2 Lipotropin (LPH 1-91) itself produced no analgesia or other morphine-like effects when infused in a dose of 150 mug.3 C-fragment (LPH 61-91) produced strong long-lasting analgesia when infused in a dose of 10 or 20 mug; on a molar basis the potency was between 90 and 180 times that of morphine. The following morphine-like effects were also produced: shivering leading to fever, vasodilatation of the pinnae, mydriasis, opening of the palpebral fissures, tachypnoea with bouts of panting, vocalization, hyperexcitability, restlessness and catalepsy. All the effects, including analgesia, were abolished by an intraperitoneal injection of naloxone (1 mg/kg).4 Hyperglycaemia, another central effect produced by morphine, was obtained with C-fragment infused in a dose of 60 mug.5 On intravenous injection, C-fragment produced analgesia with a dose of about 200 mug/kg. Administered by this route, C-fragment was again more potent than morphine.6 C'-fragment (LPH 61-87), LPH 61-78 and LPH 61-69, either had no analgesic effect or produced weak short-lasting analgesia when infused in doses up to 100 mug.7 Methionine enkephalin (LPH 61-65) either produced very weak short-lasting analgesia or had no analgesic effect when infused in doses of between 30 and 400 mug.8N-methyl methionine enkephalin amide in which both termini of methionine enkephalin were protected against degradation by exopeptidases produced long-lasting analgesia when infused in doses of 150 to 180 mug; its analgesic potency was approximately 100 times less than that of C-fragment. Blocking only one terminus of methionine enkephalin did not appear to endow the peptide with analgesic properties. The N-methyl pentapeptide amide produced other morphine-like effects of which the most striking was catalepsy. All the effects were abolished by intraperitoneal naloxone (1 mg/kg).     

胍丁胺对炎性疼痛的镇痛作用及对吗啡镇痛作用的影响

目的观察胍丁胺对炎性疼痛的镇痛作用及其对吗啡镇痛作用的影响,研究胍丁胺的镇痛作用是否与激动咪唑啉受体或影响受体前谷氨酸和γ-氨基丁酸(gamma-aminobutyr-icacid,GABA)释放有关。方法应用福尔马林致大鼠炎性疼痛模型,观察胍丁胺镇痛和增强吗啡镇痛的作用。应用高效液相色谱技术测定胍丁胺对脊髓切片孵育液中谷氨酸和GABA基础释放量及对高钾诱发神经元去极化引起神经递质释放的影响。结果单侧足底注射5%福尔马林使大鼠出现明显的双相伤害性行为反应。胍丁胺抑制福尔马林引起的第二相疼痛行为反应及痛觉过敏,并增强吗啡对第二相疼痛的镇痛作用,但在第一相疼痛过程中,无明显镇痛和增强吗啡镇痛的作用。咪唑啉受体拮抗剂咪唑克生不能拮抗胍丁胺镇痛及增强吗啡镇痛的作用。1~1000μmol.L-1胍丁胺对脊髓谷氨酸和GABA的基础释放量和高钾诱发谷氨酸和GABA释放量的升高均没有影响。结论胍丁胺对炎性疼痛具有明确的镇痛作用,并明显增强吗啡的镇痛效果,其镇痛机制可能与咪唑啉受体无关,也不是通过在受体前水平抑制谷氨酸或促进GABA释放来实现的。     

Analgesic actions of the N-type voltage-dependent calcium channel blockers

OBJECTIVE The participation of N-type voltage-dependence calcium channel(N-VDCC) in pain transmission has been proved,and the analgesia of N-VDCC blokcers has also been confirmed.The aim of this study was to obtain the small molecular N-VDCC blockers with oral analgesic activity and tho investigate the characteristics of its analgesia.METHODS Taken NMED-160 and ZC88 as leading compounds,a series of candidates of non-peptide N-VDCC blockers were synthesized.The inhibition of these chemicals to N-VDCC currents was tseted by using two-electrode voltage-clamp technique,and the oral analgesic activity was detected by using the model of chronic construction injury of sciatic nerve(CCI) in rats,a classic model of neuropathic pain.Among these compounds,D304 was selected to further investigate its analgesic action by using a variety of acute and chronic pain models.RESULTS Among these compounds we synthesized,eight compounds displayed high activity of inhibiting N-VDCC currents,and D214 and D304,showed strong oral analgesic activity in the rat CCI model.According to these results,D304 was selected to further study its analgesic action.In the mouse 55℃ hot-plate model,D304 given by oral route(10,30,100 mg·kg-1) failed to observed significant analgesia,suggesting that D304 had no effect on the thermal-stimulated acute pain.In the rat formalin-induced persistent pain model,D304 administered orally(10,30,100 mg·kg-1) dose-dependently inhibited formalin-induced pain in phaseⅡ,rather than in phaseⅠ,with equivalent intensity of analgesia to positive control of NMED-160.In the rat complete Freund adjuvant-induced chronic inflammatory pain model,D304(10,30,100 mg·kg-1,po) dose-dependently inhibited mechanical-stimulated allodynia and thermal-stimulated hyperalgesia,and its analgesic strength was equal to or even slightly stronger than that of NMED-160.In the two classical peripheral neuropathic pain models-the rat CCI model and the rat diabetic peripheral neuropathic pain model,D304(10,30,100 mg·kg-1,po) dose-dependently inhibited mechanical-stimulated allodynia and thermal-stimulated hyperalgesia,and its analgesic strength was slightly stronger than that of NMED-160 or gabapentin.The preliminary safety evaluation showed that the LD50 of D304(po) was 570 mg·kg-1,and therapeutic index was about 10.CONCLUSIONTaken together,we designed and synthesized a series of samll molecular N-VDCC blockers with oral analgesic activity.Among these compounds,D304 had no significant analgesia to acute pain,but showed strong oral analgesic activity to persistent and chronic pain,especially to chronic inflammatory pain and neuropathic pain.This findings set up a foundation for the further research and development of samll molecular N-VDCC blockers with oral analgesic activity.     

Acute exposure to saccharin reduces morphine analgesia in the rat: evidence for involvement of N-methyl-d-aspartate and peripheral opioid receptors

Rationale: Pairings of a sweet taste and injection of morphine result in a learned avoidance of that taste and learned analgesic tolerance. This avoidance is mediated by the drug’s peripheral effect, while learned tolerance involves activation of N-methyl-d-aspartate (NMDA) receptors. Exposure to a sweet taste also reduces morphine analgesia. We studied whether this taste-mediated reduction was reversed by an NMDA or peripheral opioid receptor antagonist. Objectives: To determine whether an intraoral infusion of saccharin would modulate morphine analgesia in rats, and to study the contribution of NMDA as well as peripheral opioid receptors to this modulation. Methods: Six experiments used the rat’s tail-flick response to study the effect of an intraoral infusion of a sodium saccharin solution on morphine analgesia, and the effects of the quaternary opioid receptor antagonist methylnaltrexone as well as the non-competitive NMDA receptor antagonist MK-801 on this modulation of analgesia. Results: An intraoral infusion of saccharin reduced the analgesic effects of an intraperitoneal (i.p.) injection of morphine across a range of doses (experiment 1a), which was not attributable to an influence on tail-skin temperature (experiment 1b). This reduction was mediated by opioid receptors in the periphery and activation of NMDA receptors because morphine analgesia was reinstated by an i.p. injection of either methylnaltrexone (experiment 2a) or MK-801 (experiment 3a), which was not due to the effect of methylnaltrexone (experiment 2b) or MK-801 (experiment 3b) on morphine analgesia in the absence of saccharin. Conclusions: These results document evidence for an antagonism of morphine analgesia by actions of the drug at peripheral opioid receptors and excitatory amino-acid activity at NMDA receptors. They are discussed with reference to the aversive motivational effects of peripheral opioid receptors and pain facilitatory circuits. Received: 12 June 1999 / Final version: 1 November 1999     

Local peripheral antinociceptive effects of 14-O-methyloxymorphone derivatives in inflammatory and neuropathic pain in the rat

Antinociception achieved after peripheral administration of opioids has opened a new approach to the treatment of severe and chronic pain. Additionally, opioid analgesics with restricted access to the central nervous system could improve safety of opioid drugs used in clinical practice. In the present study, peripheral components of antinociceptive actions of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone were investigated after local intraplantar (i.pl.) administration in rat models of inflammatory and neuropathic pain. Their antinociceptive activities were compared with those of morphine, the classical mu-opioid receptor agonist. Intraplantar administration of morphine and the 6-amino acid derivatives produced dose-dependent reduction of formalin-induced flinching of the inflamed paw, without significant effect on the paw edema. Local i.pl. administration of the new derivatives in rats with neuropathic pain induced by sciatic nerve ligation produced antiallodynic and antihyperalgesic effects; however, the antinociceptive activity was lower than that observed in inflammatory pain. In both models, the 6-amino acid derivatives and morphine at doses that produced analgesia after i.pl. administration were systemically (s.c.) much less active indicating that the antinociceptive action is due to a local effect. Moreover, the local opioid antinociceptive effects were significantly attenuated by naloxone methiodide, a peripherally acting opioid receptor antagonist, demonstrating that the effect was mediated by peripheral opioid receptors. The present data indicate that the peripherally restricted 6-amino acid conjugates of 14-O-methyloxymorphone elicit antinociception after local administration, being more potent in inflammatory than in neuropathic pain. Opioid drugs with peripheral site of action can be an important target for the treatment of long lasting pain.     

Studies of inflammatory pain response: related pain producing substance and endogenous opioid system

A method for assessing inflammatory pain response was developed by modification of the formalin test. Formalin (0.5%, 25 microliters) was injected into the hindpaw of the mouse, and the durations spent in licking or biting response were measured as an indicator of pain response. The response curve was biphasic, having two peaks, from 0 to 5 min (first phase) and from 15 to 20 min (second phase). Morphine, ethylketocyclazocine, ketocyclazocine and pentazocine inhibited the response dose-dependently at the first and the second phases. Aspirin, oxyphenbutazone and dexamethasone inhibited only the second phase. Aminopyrine and mefenamic acid which acted at both central and peripheral sites inhibited both phases; however, the inhibition of the second phase was stronger than that of the first phase. Substance P (SP) antagonist inhibited only the first phase. Bradykinin (BK) inhibitor caused a inhibition of both first and second phases, and pretreatment of compound 48/80 and indomethacin inhibited only the second phase. From these facts, it was suggested that SP and BK played a role in the pain response at the first phase, and histamine, BK and PG were involved at the second phase. Naloxone produced hyperalgesia and bestatin produced analgesia at the second phase; then, it seems that the endogenous opioid system is activated by formalin stimulation and modulates the pain perception. Based on these findings, it is presumed that the pain of the first phase is evoked by the direct stimulation of the nerve fibers, and that of the second phase is due to the inflammatory reaction.     

Pharmacological validation of an automated method of pain scoring in the formalin test in rats

In 1997, we described a new automated method of scoring the pain behaviors in the formalin test. The algic behavior was automatically measured with the help of a video-analysis system. The time during which the animal grooms, licks, or bites itself was used as the parameter of pain. In the present study, we tested various analgesics to realize a pharmacological validation of the system. The effect of opiate analgesic (morphine, i.v.), nonsteroidal anti-inflammatory drugs (paracetamol, i.v., piroxicam, i.v., indomethacin, i.v.), antidepressant drugs (clomipramine, desipramine, nortryptyline, and paroxetine, i.p.), and serotonin (i.t.) were analyzed. A dose of 1.25 mg/kg of morphine induced a decrease in the scores of phases 1 and 2. Naloxone (0.25 mg/kg) reversed the effect of morphine (2.5 mg/kg). A 20-mg/kg dose of indomethacin induced a decrease in the second phase, and paracetamol induced a decrease in both phases (analgesic doses were 400 mg/kg and 200 mg/kg for first and second phases, respectively). Piroxicam had no effect on the pain scores. Clomipramine, desipramine, and paroxetine at a dose of 5 mg/kg induced a significant decrease in the second phase. Nortriptyline had no effect on the pain scores. A dose of 75 microg of serotonin induced a decrease in both phases 1 and 2. This study demonstrated that this system shows a good pharmacological sensitivity, although it is lower than that of manual assessment.     

Analgesic potency of TRIMU-5: a mixed mu 2 opioid receptor agonist/mu 1 opioid receptor antagonist.

TRIMU-5 (Tyr-D-Ala-Gly-NH-(CH2)2CH(CH3)2) is a potent enkephalin analog with analgesic actions. Detailed studies show high affinity for both mu 1 and mu 2 sites, with poor affinity for delta, kappa 1 and kappa 3 receptors. Of all the mu ligands examined in binding assays, TRIMU-5 was the most mu-selective. In mice, TRIMU-5 administered either intracerebroventricularly (i.c.v.) or intrathecally elicited analgesia which was readily reversed by the mu-selective antagonist beta-funaltrexamine (beta-FNA). However, the analgesia observed following i.c.v. injections differed from traditional mu ligands: (1) the dose of drug required for analgesic activity i.c.v. was 100-fold greater than those following intrathecal administration; (2) although sensitive to beta-FNA, the analgesia was not antagonized by naloxonazine; and (3) the analgesia was reversed by an opioid antagonist given intrathecally (i.t.) but not i.c.v. Thus, TRIMU-5 analgesia appeared to be mediated spinally through mu 2 receptors. TRIMU-5 did have supraspinal actions, inhibiting gastrointestinal transit, another mu 2 action. In binding studies TRIMU-5 had high affinity for mu 1 sites, but pharmacological studies revealed antagonist actions at this receptor. In mice, the analgesia produced by morphine given i.c.v. was antagonized by coinjection of a low TRIMU-5 dose which was inactive alone. Similarly, TRIMU-5 coadministered with morphine into the periaqueductal gray of rats reversed the analgesia seen with morphine alone. Thus, TRIMU-5 is a highly selective mixed mu 2 opioid receptor agonist/mu 1 opioid receptor antagonist.     

Antinociceptive properties of extracts and two flavonoids isolated from leaves of<Emphasis Type="Italic">Danae racemosa</Emphasis>

The antinociceptive properties of the hydro-methanolic extract (HME) and two flavonoids isolated from Danae racemosa have been investigated in several nociceptive rat models. The HME from D. racemosa (100-400 mgkg(-1), i.p.) produced significant dose-related inhibition of acetic acid-induced abdominal constriction. In the same dose range, the HME produced dose-related inhibition in both phases of a formalin-test. Treatment of animals with naloxone (5 mgkg(-1), i.p.) completely reversed the antinociceptive effect caused by morphine (5 mgkg(-1), s.c.) and the HME (200 mgkg(-1), i.p.) when assessed against the first phase of the formalin-test, but this effect was less significant for the HME in the second phase. Furthermore, when assessed via a hot-plate test, the HME (100-400 mgkg(-1), i.p.) caused a significant increase in response latency. The HME, given daily for to 7 consecutive days, develop tolerance, but did not induce cross-tolerance to morphine. These data demonstrate that the HME elicites pronounced antinociception against several pain models. The actions of the HME involve, at least in part, an interaction with the opioid system, but does not seem to be related with non-specific peripheral or central depressant actions. Finally, the active principle(s) responsible for the antinociceptive action of D. racemosa is likely to be partially related to the presence of quercetin and kaempferol.     

Analgesia induced by morphine microinjected into the nucleus raphe magnus: effects on tonic pain

One of the possible sites of action of the analgesic effect of morphine is the Nucleus Raphe Magnus, as morphine injected into this structure induces analgesia in transient pain models. In order to test if morphine in the Nucleus Raphe Magnus is also analgesic in a tonic pain model, 5 microg of morphine or saline (control) were microinjected into the Nucleus Raphe Magnus of the rat. Analgesic effects were assessed following nociceptive stimulation using transient heating of the tail (phasic pain) and subcutaneous orofacial injection of 1.5 % formalin (tonic pain). While morphine was strongly analgesic for the tail-flick response (p <0.0001 compared to control), analgesia on the response to formalin was also observed for both early (p = 0.007) and late responses (p = 0.02). However, the response to formalin was not completely blunted. These results suggest that the Nucleus Raphe Magnus is not the exclusive site of action of morphine-induced analgesia in clinical conditions.     

Lactoferrin enhances peripheral opioid-mediated antinociception via nitric oxide in rats

Lactoferrin (LF) is a multifunctional protein found in various biological fluids. However, the peripheral action of lactoferrin remains unknown. In this study, peripherally applied bovine lactoferrin showed antinociceptive effect that was reversed by a mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH(2) (CTOP), or by a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), but not by an inactive enantiomer of L-NAME, N(G)-nitro-D-arginine methyl ester (D-NAME), during phase 1 and phase 2 in the rat formalin test. Peripheral coadministration of a micro-opioid receptor agonist, morphine, with subeffective dose of bovine lactoferrin produced a potentiated antinociceptive effect compared to that of morphine alone during both phases in the formalin test. This potentiated antinociception by morphine with bovine lactoferrin was reversed by CTOP or by L-NAME. These results suggest that bovine lactoferrin exerts an antinociceptive activity via potentiation of the peripheral micro-opioidergic system, and that nitric oxide (NO) is involved in this potentiation.     

Lack of effect of levallorphan on analgesia induced by intraventricular application of porcine calcitonin in mice.

Intraventricular administration to mice porcine calcitonin (10 U/kg) as well as of morphine (3 microgram/kg) elevated the threshold pressure of stimuli applied to the base of the tail as assessed by squeaking, struggling or biting, all of which were regarded as manifestations of pain sensation in the animals. Pretreatment with an opiate antagonist, levallorphan (30 mg/kg i.p.) showed no influence upon the analgesic effect of calcitonin, though it completely antagonized the effect of morphine. The results suggested that a peptide hormone, calcitonin, exerted its analgesic action in a manner distinct from the narcotic analgesic.     

Activation of spinal anti-analgesic system following electroacupuncture stimulation in rats

We evaluated the interaction between electroacupuncture (EA)-induced antinociception and an endogenous anti-analgesic system. EA was applied to the ST-36 acupoint for 45 min in male Sprague-Dawley rats, and pain thresholds were assessed by the hind-paw pressure test. EA produced a marked increase in pain thresholds and its antinociceptive action was completely reversed by naloxone (5 mg/kg). The analgesic effects of subcutaneous morphine (7 mg/kg) following EA stimulation were significantly attenuated. The attenuation of morphine analgesia was inversely proportional to the time intervals between EA termination and morphine injection, and the effect was not observed 120 min after EA stimulation. The analgesic effects of i.t. morphine (10 microg), but not i.c.v. morphine (25 microg), following EA were also attenuated. On the other hand, systemic morphine (7 mg/kg)-induced hyperthermia was not affected by EA. Moreover, i.c.v. morphine, but not i.t. morphine, produced hyperthermia. The i.c.v. morphine-induced hyperthermia was not affected by EA, similar to i.c.v. morphine analgesia. These results suggest that the attenuation of morphine analgesia following EA, that is, the activation of an endogenous anti-analgesic system, is closely related to the activation of an analgesic system by EA and that the spinal cord plays a critical role in the activation of the endogenous anti-analgesic systems.     

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.