Prostaglandin hyperalgesia, V: A peripheral analgesic receptor for opiates

Prostaglandin E₂ injected in the rat paw causes hyperalgesia which is antagonized by local injections of opiate and opiate antagonists. In the present investigation in rats it i shown at naloxone has an analgesic effect at doses as low as 2 μg/site, injected into the rat hind paw. At a dose that has no analgesic effect (1 μg/site) naloxone antagonized the analgesia produced by either local or systematic administration of morphine. Local administration of levorphanol (50 μg/site) caused a 50% reduction in the intensity of the hyperalgesia induced by prostaglandin E₂. A dose four times greater of its isomer, dextrorphan, had little analgesic effect. The present results support the suggestion that this peripheral analgesia is the result of an action of opiates in receptors located at the nociceptors.     

Prostaglandin hyperalgesia, V: a peripheral analgesic receptor for opiates

Prostaglandin E2 injected in the rat paw causes hyperalgesia which is antagonized by local injections of opiate and opiate antagonists. In the present investigation in rats it is shown that naloxone has an analgesic effect at doses as low as 2 micrograms/site, injected into the rat hind paw. At a dose that has no analgesic effect (1 microgram/site) naloxone antagonized the analgesia produced by either local or systemic administration of morphine. Local administration of levorphanol (50 micrograms/site) caused a 50% reduction in the intensity of the hyperalgesia induced by prostaglandin E2. A dose four times greater of its isomer, dextrorphan, had little analgesic effect. The present results support the suggestion that this peripheral analgesia is the result of an action of opiates in receptors located at the nociceptors.     

A comparison of the analgesic and behavioral effects of [D-Ala2] Met-enkepha-linamide and morphine in the mesencephalic reticular formation of rats.

[D-Ala²]Met-enkephalinamide (DALA) injected intracerebrally (IC) at low doses into specific sites of the mesencephalic reticular formation (MRF), produced a profound, long-lasting analgesia that was blocked by naloxone, a specific opiate antagonist. Morphine was only half as potent as DALA because morphine, injected IC at similar sites in the MRF, yielded a comparable analgesia only when injected at twice the dose. The analgesic effects of morphine were also antagonized by naloxene. Both DALA and morphine produced specific behavioral effects. Naloxone blocked the behavioral effects of DALA, but not those produced by morphine.     

家兔伏核—杏仁核神经通路在吗啡镇痛中的作用

用辐射热照射家兔鼻嘴侧部皮肤,测量其躲避反应潜伏期作为痛反应阈,简称痛阈。通过预先埋植的慢性套管向伏核或杏仁核内进行注射,结果表明:(1)在家兔的伏核内微量注射吗啡可产生镇痛作用,该作用可被杏仁核内注射纳洛酮所削弱,并有量效依从关系;在杏仁核内注射甲啡肽抗血清(ME AS)或β-內啡肽抗血清(β-EP AS)亦可削弱上述镇痛作用;(2)在杏仁核内微量注射吗啡可产生镇痛作用,此作用不能被伏核内注射纳洛酮所阻断;(3)在伏核内注射吗啡所产生的镇痛作用可被同一部位注射γ-氨基丁酸(GAEA)受体阻断剂氯甲基荷包牡丹碱所增强,被 GABA 受体激动剂异鹅羔胺所削弱。上述结果提示:在家兔脑内从伏核到杏仁核可能存在一条与镇痛有关的神经通路,伏核内的阿片样物质及杏仁核内的甲啡肽,β-内啡肽可能参与镇痛信息的传递,而伏核内的 GABA 可能有对抗吗啡镇痛的作用。     

Possible participation of endogenous opioid peptides on the mechanism involved in analgesia induced by vouacapan.

The involvement of opioid peptides in the mechanism of action of vouacapan, a new experimental compound extracted from seeds of Pterodon poligalaeflorus Benth, was investigated both in mice utilizing acetic acid writhing response and in rats utilizing inflammatory hyperalgesia induced by carrageenan and modified Randall-Selitto method. Vouacapan, in both models, caused a dose-dependent analgesia when injected p.o., s.c. and i.p. The analgesic effect was partially blocked by naloxone, nalorphine and n-methyl-nalorphine. Significant tolerance to analgesic effect was observed following repeated administration of vouacapan or morphine. On the last day of treatment, cross administration revealed symmetrical and asymmetrical cross-tolerance between vouacapan and morphine, in rats and mice, respectively. We conclude that a release of endorphins could be involved in the analgesic mechanism of vouacapan in both models tudied.     

Modification of the effects of naloxone in morphine-dependent mice

Mice were rendered dependent on morphine by mixing morphine with their food (2 mg/g) for three days. Increasing doses of naloxone precipitated dose-dependent withdrawal reactions such as weight loss and jumping. These withdrawal reactions were antagonized by morphine pretreatment. Effects of morphine, such as increased locomotor activity, inhibition of intestinal transport, and analgesia were antagonized by naloxone in both non-dependent and dependent subjects. The antagonist actions of naloxone were increased in dependent subjects; lower doses of naloxone were sufficient to antagonize effects of morphine. The present results confirm earlier studies indicating that precipitation of withdrawal can be antagonized by morphine pretreatment suggesting that withdrawal reactions are due to actions of naloxone at the same receptor at which opioid agonists act. The increased antagonist potency of naloxone in dependent subjects extends earlier results obtained with analgesic effects to several other agonist effects of morphine and is consistent with the interpretation that exposure to an opioid agonist induces a change in the conformation of opioid receptors.     

Supraspinal antinociceptive effect of apelin-13 in a mouse visceral pain model

Apelin, as the endogenous ligand of the APJ receptor, is a novel identified neuropeptide whose biological functions are not fully understood. APJ receptor mRNA was found in several brain regions related to descending control system of pain, such as amygdala, hypothalamus and dorsal raphe nucleus (DRN). The present study was designed to determine whether supraspinal apelin-13 may produce antinociceptive effect observed in the acetic acid-induced writhing test, a model of visceral pain. Apelin-13 not only significantly produced preemptive antinociception at the dose of 0.3, 0.5, 1 and 3μg/mouse when injected intracerebroventricularly (i.c.v.) before acetic acid, but also significantly induced antinociception at a dose of 0.5, 1 and 3μg/mouse when injected i.c.v. after acetic acid. And i.c.v. apelin-13 did not influence 30-min locomotor activity counts in mice. Intrathecal (i.t.) administration of apelin-13 (1 and 3μg/mouse) significantly decreased the number of writhes, however, intraperitoneal (i.p.) injection of apelin-13 (10-100μg/mouse) had no effect on the number of writhes in the writhing test. The specific APJ receptor antagonist apelin-13(F13A), no-specific opioid receptor antagonist naloxone and μ-opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) could significantly antagonize the antinociceptive effect of i.c.v. apelin-13, suggesting APJ receptor and μ-opioid receptor are involved in this process. Central low dose of apelin-13 (0.3μg/mouse, i.c.v.) could significantly potentiate the analgesic potencies of modest and even relatively ineffective doses of morphine administrated at supraspinal level. This enhanced antinociceptive effect was reversed by naloxone, suggesting that the potentiated analgesic response is mediated by opioid-responsive neurons.     

III - Prostaglandin hyperalgesia: relevance of the peripheral effect for the analgesic action of opioid-antagonists

Morphine injected into the rat cerebral ventricles had a marked analgesic effect, while no effect was observed with pentazocine and naloxone or nalorphine caused a strong hyperalgesia. Administered systemically (IP) naloxone and nalorphine caused a transitory analgesia followed by a long lasting hyperalgesic effect; morphine and pentazocine showed only an analgesic effect. It was concluded that the site of analgesic action of opioid-antagonists is peripheral rather than central. The peptidase-resistant enkephalin-analog, BW 180c, which does not cross the blood brain barrier, caused a marked analgesia by IP administration to paws made hyperalgesic by PGE2 or carrageenin. It is suggested that agents derived from morphine, morphine-antagonists, enkephalins or cGMP devoid of central effect but having a strong peripheral effect may constitute a new class of safer analgesics.     

III - Prostaglandin hyperalgesia: Relevance of the peripheral effect for the analgesic action of opioid-antagonists

Morphine injected into the rat cerebral ventricles had a marked analgesic effect, while no effect was observed with pentazocine and naloxone or nalorphine caused a strong hyperalgesia. Administered systemically (IP) naloxone and nalorphine caused a transitory analgesia followed by a long lasting hyperalgesic effect; morphine and pentazocine showed only an analgesic effect. It was concluded that the site of analgesic action of opioid-antagonists is peripheral rather than central. The peptidase-resistant enkephalin-analog, BW 180c, which does not cross the blood brain barrier, caused a marked analgesia by IP administration to paws made hyperalgesic by PGE₂ or carrageenin. It is suggested that agents derived from morphine, morphine-antagonists, enkephalins or cGMP devoid of central effect but having a strong peripheral effect may constitute a new class of safer analgesics.     

Central and peripheral components of dermorphin's effect on rat intestinal propulsion in comparison to morphine

Dermorphin, injected intracerebroventricularly (ICV) to rats, provokes, like to morphine, an inhibition of intestinal propulsion linearly related to the log of the administered doses (in the range from 0.06 to 0.56 μg/rat), but it is 143 times more active than morphine. Naloxone, ICV or IP, antagonizes dermorphin less effectively than morphine. Quaternary naloxone ICV administered antagonizes the intestinal effect of ICV dermorphin, while IP administered it is not effective until 8 mg/kg. The dose of dermorphin maximally active by the ICV route (0.56 μg/rat) is completely inactive when injected IP. Increasing doses of dermorphin IP (from 12 to 6400 μg/kg) inhibit intestinal propulsion to the same extent irrespectively of the doses employed, but never by more than 50%. Only a high dose of naloxone (30 mg/kg/IP) antagonizes this IP effect. The central and peripheral components of this intestinal effect of dermorphin are discussed.     

Changes in morphine self-administration in rats induced by prostaglandin E1 and naloxone

Interactions of prostaglandin E₁ (PGE₁) with morphine have been reported in several test systems and an hypothesis has been advanced for a role of prostaglandins in morphine analgesia and physical dependence. In rats self-administering morphine intravenously, a simultaneous and continuous infusion of naloxone hydrochloride at 56 to 560 μg/kg/day caused the expected increase in injection rate for morphine. Infusion of PGE₁ by itself at 56 or 180 μg/kg/day had no effect on the rate of morphine intake. Likewise the addition of PGE₁ at 180 μg/kg/day did not potentiate the increase caused by naloxone (56 or 180 μg/kg/day) when it was added to the naloxone infusion. These results do not support a role for prostaglandins in the behavioral aspects of morphine addiction. However, larger doses of PGE₁ (1 and 1.8 mg/kg/day), which were without overt effects in normal rats, caused severe and incapacitating prostration in morphinized rats.     

Analgesic effects of ethylketocyclazocine and morphine in rat and toad

We have previously found rat and toad (Bufo marinus) brain to contain inverse ratios of benzomorphan-preferring (kappa/sigma) and morphine-preferring (mu) opioid receptor types. The aim of the present study was to compare in vivo pharmacologic activity of a benzomorphan, ethylketocyclazocine (EKC) and morphine sulfate (MS) in rat and toad. Footshock intensity thresholds for eliciting locomotion were determined and dose-response curves for EKC and MS analgesia were obtained. Drugs were injected subcutaneously. In rats (high mu, low kappa in brain), both compounds produced analgesia and displayed similar sensitivity to naloxone antagonism. The analgesic effects of EKC and MS may, therefore, be mediated by a common receptor type (mu) in this pain test in rats. In toads (high kappa, low mu in brain), MS produced naloxone-reversible analgesia at doses 20-fold higher than were effective in rats. Toads did not display EKC analgesia at doses below those producing motor impairment. Moreover, 50-fold higher doses were required to produce such impairment in toads. Thirty minutes following subcutaneous injection of 3H-EKC, similar concentrations were found in rat and toad brain. Uptake into brain is probably not a factor in the behavioral resistance of toads to EKC.     

Analgesia induced by intrathecal injection of dynorphin B in the rat

A dose-dependent analgesic effect of intrathecally injected dynorphin B was observed in rats using the tail flick as nociceptive test. Intrathecal injection of 20 nmol of dynorphin B increased the tail flick latency by 90 +/- 23%, an effect that lasted about 90 min. For the same degree of analgesia, dynorphin B was 50% more potent than morphine on a molar basis. The analgesic effect of this dose of dynorphin B was partially blocked by 10 mg/kg, but not by 1 mg/kg, of subcutaneous naloxone, showing a relative resistance to naloxone reversal as compared with morphine analgesia. The analgesia produced by dynorphin B was unchanged in morphine-tolerant rats but was significantly decreased in rats tolerant to ethylketazocine. These results suggest that dynorphin B produces its potent analgesic effect by activation of kappa rather than mu opioid receptors in the rat spinal cord.     

Effects of oxymorphazone in frogs: long lasting antinociception in vivo, and apparently irreversible binding in vitro

Oxymorphazone (at doses of 50-200 mg/kg) was found to be a relatively weak antinociceptive drug in intact frog (Rana esculenta) when acetic acid was used as pain stimulus. Frogs remained analgesic for at least 48 hrs following oxymorphazone (200 mg/kg) administration. The ligand increased the latency of wiping reflex in spinal frogs too. These effects were blocked by naloxone. In equilibrium binding studies (3H)oxymorphazone had high affinity to the opioid receptors of frog brain and spinal cord as well (apparent Kd values were 8.9 and 10.6 nM, respectively). Kinetic experiments show that only 25% of the bound (3H)oxymorphazone is readily dissociable. Preincubation of the membranes with labeled oxymorphazone results in a washing resistant inhibition of the opioid binding sites. At least 70% of the (3H)oxymorphazone specific binding is apparently irreversible after reaction at 5 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand.     

Differential effects on morphine analgesia and naloxone antagonism by biogenic amine modifiers.

The stimulation of dopaminergic receptors, inhibition of serotonin synthesis or blockade of muscurinic receptors by various modifiers led to inhibition of morphine analgesia in mice. Blockade of dopaminergic receptors or the increase in serotonergic or cholinergic activity resulted in the enhancement of morphine analgesia. Serotonergic and cholinergic systems are proposed as positive and the dopaminergic system as negative modulators of morphine analgesia. The modulation of naloxone antagonism was much more complicated than that of morphine analgesia and often the effect of biogenic amine-modifiers on antagonism differed from that on analgesia. The fact that biogenic amine-modifiers do not affect morphine analgesia and naloxone antagonism by a similar pattern suggest that interaction of narcotics and narcotic antagonists with analgesic receptors may not be exactly the same.     

Disruptive effects of naloxone on development of morphine tolerance

In rats the development of one-trial tolerance to the analgesic actions of morphine is disrupted by the post-administration of naloxone at 5 min, 3 hrs, or 24 hrs. Naloxone injections alone 24 or 48 hrs prior to the analgesic test failed to counteract morphine induced analgesia. It is suggested that naloxone initiates long term biochemical changes that oppose those produced by morphine.     

Dynorphin-A-(1–13) antagonizes morphine analgesia in the brain and potentiates morphine analgesia in the spinal cord

Intrathecal injection of subanalgesic doses of morphine (7.5 nmol) and dynorphin-A-(1–13) (1.25 nmol) in combination resulted in a marked analgesic effect as assessed by tail flick latency in the rat. The analgesic effect of the composite dynorphin/morphine was dose-dependent in serial dilutions so that a composition of 1/8 of the analgesic dose of dynorphin and 1/3 that of morphine produced an analgesic effect equipotent to full dose of either drug applied separately. The analgesic effect induced by dynorphin/morphine mixture was not accompanied by motor dysfunction and was easily reversed by a small dose (0.5 mg/kg) of naloxone. Contrary to the augmentatory effect of dynorphin on morphine analgesia in the spinal cord, intracerevroventricular (ICV) injection of 20 nmol of dynorphin-A-(1–13) exhibited a marked antagonistic effect on the analgesia produced by morphine (120 nmol, ICV). The theoretical considerations and practical implications of the differential interactions between dynorphin-A-(1–13) and morphine in the brain versus spinal cord are discussed.     

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.     

Analgesic effects of dynorphin-A and morphine in mice

To investigate whether or not dynorphin-A is analgesic, the effect of this peptide was tested in comparison with that of morphine in mice. Dynorphin-A produced a potent analgesic effect in the acetic acid writhing and tail pinch tests, but a weak effect in the tail flick test when given by intracerebroventricular injection. In contrast, morphine caused a potent analgesia in all the tests. Dynorphin-A was more effective when given by intrathecal injection than by intracerebroventricular injection, whereas morphine was equipotent by both injection routes. The results suggest that dynorphin-A is analgesic and that its analgesia may be differentiated from that of morphine.     

中脑导水管周围灰质在侧脑室注入微量吗啡或纳洛酮所引起的镇痛或拮抗镇痛中的作用

本工作进一步探索中脑导水管周围灰质(PAG)在吗啡镇痛与纳洛酮拮抗吗啡镇痛中的作用。实验在清醒受限制的大鼠上进行,以电刺激鼠尾出现的甩尾和嘶叫为痛反应指标。结果表明:(1)侧脑室注射微量纳洛酮后,可使电刺激 PAG 或注射微量吗啡于 PAG 所引起的镇痛效应受到明显拮抗;(2)损毀 PAG 或注射微量纳洛酮于 PAG 后,可使由侧脑室注入微量吗啡所引起的镇痛效应显著减弱。由此可见 PAG 既是侧脑室注射吗啡镇痛作用的重要中枢部位,又是侧脑室注射纳洛酮拮抗吗啡镇痛的重要中枢部位。