Opioidergic Mechanisms Research Articles

Androgen dependence in hamsters: Overdose, tolerance, and potential opioidergic mechanisms

Anabolic steroids are drugs of abuse. However, the potential for steroid reward and addiction remains largely unexplored. This study used i.c.v. testosterone self-administration and controlled infusions of testosterone or vehicle in hamsters to explore central mechanisms of androgen overdose. Forty-two hamsters used nose-pokes to self-administer 1 μg/μl testosterone i.c.v. 4 h/day in an operant chamber. During 1–56 days of androgen self-administration, 10 (24%) hamsters died. Deaths correlated with peak daily intake of testosterone. Of the hamsters that self-administered a peak intake of <20 μg/day, there was 100% survival (10/10). Survival decreased to 86% (19/22) when daily testosterone intake peaked at 20–60 μg/day. Only 30% (three of 10) survived when daily testosterone intake exceeded 60 μg/day. Deaths are not due to volume or vehicle because i.c.v. infusions of 80 μl vehicle had no effect. Testosterone overdose resembles opiate intoxication. When male hamsters received infusions of 40 μg testosterone, locomotion (25.1±18.8 grid-crossings/10 min), respiration (72.7±5.4 breaths/min) and body temperature (33.5±0.4 °C) were significantly reduced, compared with males receiving vehicle infusions (186.1±8.1 crossings/10 min, 117.6±1.0 breaths/min, 35.9±0.1 °C, P<0.05). However, males developed tolerance to continued daily testosterone infusion. After 15 days, locomotion (170.2±6.3 crossings), respiration (118.4±1.3 breaths/min), and body temperature (35.3±0.3 °C) in testosterone-infused males were equivalent to that in vehicle controls ( P>0.05). The depressive effects of testosterone infusion are blocked by the opioid antagonist, naltrexone. With naltrexone pre-treatment (10 mg/kg s.c.), locomotion (183.7±1.8 crossings/10 min), respiration (116.9±0.3 breaths/min), and body temperature (36.1±0.4 °C) during testosterone infusion were equivalent to vehicle controls. Likewise, naltrexone prevents the reinforcing effects of i.c.v. testosterone self-administration. These results indicate that testosterone at high doses causes central autonomic depression, which may be a factor in deaths during self-administration. As well, the depressive effects of large quantities of testosterone may be mediated, at least in part, by an opioidergic mechanism.

Induction of opioid tolerance by lysine-acetylsalicylate in rats

The analgesic effect of non-steroidal antiinflammatory drugs (NSAIDs) is due to their action upon the peripheral damaged tissues, the spinal cord, and brain stem structures of the ‘descending pain-control system’ such as the periaqueductal gray matter (PAG) and the nucleus raphe magnus (NRM). The NSAID dipyrone (metamizol) has been shown to engage opioidergic circuits at the PAG, the NRM and the spinal cord, but it is unknown whether this can be generalized to typical NSAIDs and to systemic administration. In the present study lysine-acetylsalicylate (LASA), an injectable form of the prototypical NSAID aspirin, was microinjected into the PAG (100 μg/0.5 μl) in freely moving rats to induce inhibition of tail flick and hot plate responses. This antinociception was reverted by naloxone (1 mg/kg i.p.). PAG microinjection of LASA twice daily for three days induced tolerance to LASA (i.e. a progressive loss of effectiveness) and cross-tolerance to PAG-microinjected morphine (5 μg/0.5 μl). The antinociceptive effect of systemically administered LASA (300 mg/kg i.p., equivalent to the 1000 mg analgesic dose for humans) was also abolished by naloxone. Intraperitoneal injection of LASA twice daily induced tolerance to LASA and cross-tolerance to i.p. morphine (1 or 5 mg/kg). LASA-tolerant rats showed opioid withdrawal signs when injected with naloxone. These findings support the notion that the contribution of the PAG and downstream pain-control structures to the analgesic effect of NSAIDs involves opioidergic mechanisms, and suggest that repeated therapeutic administration of NSAIDs may induce tolerance, cross-tolerance to opiates, and susceptibility to a withdrawal syndrome.

Antinociceptive effect of trans-resveratrol in rats: Involvement of an opioidergic mechanism

trans-Resveratrol, a polyphenolic compound with potent antioxidant activity has recently been shown to be effective against carrageenan-induced hyperalgesia. In the present study, the effect of graded doses of trans-resveratrol was studied using a hot plate analgesiometer in rats. trans-Resveratrol at graded doses of 5, 10, 20 and 40 mg/kg i.p. produced dose-dependent analgesia. Pretreatment (20 min) with naloxone (1 mg/kg i.p.) blocked the analgesic effect. When the submaximal dose of trans-resveratrol (5 mg/kg i.p.) was combined with a submaximal dose of morphine (2 mg/kg i.p.), a potentiation effect was observed. The effect of trans-resveratrol (20 mg/kg i.p.) was also studied on morphine tolerance. Rats were divided into different groups: Group 1: morphine (10 mg/kg i.p.); Group 2: trans-resveratrol (5 mg/kg i.p.) administered 10 min before morphine (2 mg/kg i.p.); Group 3: trans-resveratrol (20 mg/kg i.p.) per se. Vehicle treated groups were run parallel. The treatment continued for 7 days. The occurrence of tolerance was estimated by comparing the antinociceptive effect of morphine with trans-resveratrol on day 1 and day 8. Both morphine and trans-resveratrol produced tolerance. However, in the group that received the combination of submaximal doses of trans-resveratrol and morphine, there was insignificant tolerance. These findings suggest that trans-resveratrol analgesia is mediated via an opioidergic mechanism and produces tolerance to its analgesic effect similar to morphine.

Ethanol Palatability and Consumption by High Ethanol-Drinking Rats: Manipulation of the Opioid System With Naltrexone.

Three experiments examined the effect of acute naltrexone treatment on both taste reactivity and consumption of ethanol in high ethanol-preferring rat lines: Alko Alcohol-Accepting (AA) rats (Experiments 1 and 2) and Alcohol-Preferring (P) rats (Experiment 3). A 3.0 mg/kg naltrexone dose was ineffective at altering ethanol palatability for either line, whereas 7.5 mg/kg was effective at reducing palatability of 10% ethanol for AA, but not P, rats, as reflected by both a decrease in ingestive responding and an increase in aversive responding. The effects of naltrexone on ethanol consumption were quite consistent: At both dosages, acute naltrexone treatment significantly decreased consumption of 10% ethanol. Termination of naltrexone resulted in an immediate increase in ethanol consumption to control levels. Results show that ethanol palatability and consumption can be dissociated in the rat and that the organization of opioidergic mechanisms that mediate ethanol responses may vary between rat lines.

Possible Involvement of Opioidergic and Serotonergic Mechanisms in Antinociceptive Effect of Paroxetine in Acute Pain

Antidepressant drugs, especially tricyclics have been widely used in the treatment of chronic pain, but not in acute pain. Because of numerous undesirable side effects, the selective serotonin reuptake inhibitors (SSRIs), with their favorable side effect profile, are preferred nowadays. An activation of the endogenous opioid mechanisms or potentiation of the analgesic effect mediated by serotonergic and/or noradrenergic pathways are thought to be involved in the antinociceptive action of SSRIs. In this study, the potential antinociceptive effect of paroxetine and its interaction with opioidergic system and serotonin receptors were evaluated. The antinociceptive effect of paroxetine was tested using a hot plate test in mice. Paroxetine, a SSRI antidepressant drug, induced an antinociceptive effect following i.p. administration. This antinociception was significantly inhibited by naloxone, an opioid receptor antagonist, suggesting the involvement of opioidergic mechanisms. While ondansetron (a 5-HT3-receptor antagonist) inhibited the effect of paroxetine, ketanserin (a 5-HT2-receptor antagonist) could not. In conclusion, paroxetine-induced antinociception, similar to morphine, suggests an involvement of direct or indirect action (via an increase in release of endogenous opioid peptide(s)) at opioid receptor sites and an involvement of serotonergic mechanisms mainly at the receptor level.

Quercetin, a bioflavonoid, attenuates thermal hyperalgesia in a mouse model of diabetic neuropathic pain

Diabetic neuropathic pain, an important microvascular complication in diabetes mellitus, has been recognised as one of the most difficult types of pain to treat. Lack of understanding of etiology involved, inadequate relief, development of tolerance and potential toxicity of classical antinociceptives warrant the investigation of newer agents to relieve pain. The aim of the present study was to explore the antinociceptive effect of a bioflavonoid, quercetin, both in control and streptozotocin (STZ)-induced diabetic mice. After 4 weeks of a single intraperitoneal injection of STZ (200 mg/kg), both control and diabetic mice were subjected to test thermal hyperalgesia by tail-immersion assay (warm water). Diabetic mice exhibited a significant hyperalgesia as compared with control mice. Quercetin (100 but not 50 mg/kg po) produced a marked increase in tail-flick latencies in both diabetic and nondiabetic mice. Quercetin-induced increase in nociceptive threshold was reversed by naloxone (2 mg/kg ip), an opioid receptor antagonist. These preliminary results indicate an antinociceptive activity of quercetin, probably through modulation of opioidergic mechanism and point towards its potential to attenuate diabetic neuropathic pain.

Contribution of crotoxin for the inhibitory effect of Crotalus durissus terrificus snake venom on macrophage function

Previous work of our group demonstrated that Crotalus durissus terrificus venom has a dual effect on macrophage function: it inhibits spreading and phagocytosis and stimulates hydrogen peroxide and nitric oxide production, antimicrobial activity and glucose and glutamine metabolism of these cells. Crotalid venom also induces analgesia and this effect is mediated by opioid receptors. The involvement of opioidergic mechanism and the determination of the active component responsible for the inhibitory effect of crotalid venom on macrophage function were investigated. The venom reduced the spreading and phagocytic activities of peritoneal macrophages. This effect was observed in vitro, 2 h after incubation of resident peritoneal macrophages with the venom, and in vivo, 2 h after subcutaneous injection of the venom. The inhibition of phagocytosis was not modified by naloxone, an antagonist of opioid receptors. Venom neutralization with crotalid antivenom abolished the inhibitory effect of the venom, indicating that venom toxins are involved in this effect. Crotoxin, the main toxin of crotalid venom, s.c. injected to rats or added to the medium of peritoneal cell incubation, inhibited macrophage function in a similar manner to that observed for crude venom. The present results suggest that crotoxin causes a direct inhibition of macrophage spreading and phagocytic activities and may contribute to the inhibitory effect of crotalid venom on macrophage function.

The antihyperalgesic effect of venlafaxine in diabetic rats does not involve the opioid system

Venlafaxine (VFX) is a structurally novel antidepressant that inhibits reuptake of serotonin and norepinephrine but, unlike tricyclic antidepressants, has few side effects. The present work studies the antihyperalgesic effect of repeated administrations of VFX (five successive injections of 2.5, 5 or 10 mg/kg, s.c., every half-life) in diabetic rats with the paw pressure test and the effect of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) because an opioidergic mechanism is usually considered to be involved in the analgesic effect of antidepressants. VFX induced a significant dose-dependent increase in vocalization thresholds. This effect was not reversed by naloxone. Thus, we demonstrate a clear antinociceptive effect of VFX which, unlike that of most mixed tricyclic antidepressants, does not involve the endogenous opioid system.

Somatostatin treatment affects testicular function in stallions

This study investigated the regulation of growth hormone (GH) release in stallions and tested the hypothesis that the somatotrophic axis influences testicular function. Basal plasma GH concentrations, effects of an experimental decrease of GH release on testicular function and an opioidergic regulation of GH release were investigated in Shetland stallions ( n=6). No seasonal variations in plasma GH concentrations were found over a 12-month period. Treatment with the somatostatin analogue octreotid (100 mg twice daily over 10 days) caused a decrease in semen motility from 38.7±8.4% progressively motile spermatozoa before treatment to 18.3±5.4% on day 3 after end of treatment ( P<0.05). Values returned to 35.0±8.5% on day 5 after treatment. On the last day of octreotid treatment, a hCG stimulation test was performed (3000 IU hCG i.v.). The hCG-induced testosterone release was significantly higher in saline treated than in octreotid pretreated animals ( P<0.05). Neither plasma GH concentrations nor volume and density of ejaculates, total sperm count, or semen morphology were different between saline and octreotid treatments. Injection of the opioid antagonist naloxone (0.5 mg/kg) significantly increased GH release in June (from 1.1±0.3 ng/ml before to 3.7±2.2), while a minor and not significant increase occurred in January. In conclusion, our results indicate a non-seasonal basal GH release with a fine-modulation by season-dependent opioidergic mechanisms in the male horse. A transient decrease in semen motility and hCG-induced testosterone release following ocreotid treatment indicate a role of GH in the regulation of testicular function in stallions.

Central nucleus of the amygdala and the control of tonic immobility in guinea pigs

Tonic immobility (TI) is a temporary state of profound motor inhibition induced by situations that supposedly generate intense fear, with the objective to protect the animal from attacks by predators. A previous study by our group demonstrated that cholinergic stimulation of the central, basolateral, and lateral posterior nuclei of the amygdala decreases the duration of TI in guinea pigs. In the current study, we attempted to investigate the effects of cholinergic, opioidergic, and γ-aminobutyric acid (GABA)ergic stimulation of the central amygdala (CEA) on TI modulation. We observed that both cholinergic (carbachol, 2.7 nmol/0.2 μl) and opioidergic (morphine, 2.2 and 4.4 nmol/0.2 μl) stimulation of the CEA decreased the duration of TI and that these effects could be reversed by pretreatment with naloxone (1.3 and 2.7 nmol/0.2 μl). Our results also showed that microinjection of the GABAergic agonist muscimol (0.26 nmol/0.2 μl) reduced the duration of TI episodes, whereas microinjection of the GABAergic antagonist bicuculline (BIC, 1 nmol/0.2 μl) increased it. Thus, the present experiments demonstrated that cholinergic, opioidergic, and GABAergic systems of the CEA have an inhibitory action on the duration of TI in guinea pigs. Furthermore, the current study suggests an interaction of cholinergic and opioidergic mechanisms. In addition, the GABAergic circuit of the CEA has a tonic inhibitory influence on the duration of TI and is mediated by GABA A receptors.

Possible role of mast cells in cineole-induced scratching behavior in mice

We examined the scratch (itch) inducing effect of 1,8-cineole (cineole), a monoterpene oxide present in many plant essential oils and the possible role of mast cells in the response. Subcutaneous injection of cineole (10, 20 and 40 μl/site) or the mast cell degranulating agent, compound 48/80 (25, 50 and 100 μg/site) into the rostral back of mice induced a scratching behavior. This response of cineole as well as that of 48/80 was markedly suppressed in mice subjected to mast cell desensitization by repeated injections of 48/80. The cineole-induced scratching was also significantly diminished in animals pretreated with diphenhydramine, the histamine H 1-receptor antagonist or cyproheptadine, the dual histamine/serotonin-receptor antagonist. Furthermore, the scratch-inducing effect of cineole was greatly reduced in mice that received the opioid antagonist naloxone or the selective adenosine A 1-receptor agonist, N 6-cyclopentyladenosine (CPA), but not the more selective adenosine A 2-receptor agonist, 5′- N-ethylcarboxamidoadenosine (NECA). The data suggest a likely role for mast cells in cineole-induced scratching behavior of mice, possibly involving adenosinergic and opioidergic mechanisms.

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle.

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Opioidergic modulation of ventilatory response to sustained hypoxia in obese Zucker rats.

To determine whether altered central and/or peripheral opioidergic mechanisms contribute to the altered ventilatory response to sustained hypoxia in obese Zucker rats. Eight lean (176 +/- 8 [SEM] g) and eight obese (225 +/- 12 g) Zucker rats were studied at 6 weeks of age. Pulmonary ventilation ((E)), tidal volume (V(T)), and breathing frequency (f) at rest and in response to sustained (30 minutes) hypoxic (10% O(2)) challenges were measured on three separate occasions by the barometric method after the randomized, blinded administration of equal volumes of saline (control), naloxone methiodide (N(M); 5 mg/kg, peripheral opioid antagonist), or naloxone hydrochloride (N(HCl); 5 mg/kg, peripheral and central opioid antagonist). Administration of N(M) and N(HCl) in lean animals had no effect on (E) either at rest or during 30 minutes of sustained exposure to hypoxia. Similarly, N(M) failed to alter (E) in obese rats. In contrast, N(HCl) significantly (p < 0.05) increased (E) and V(T) both at rest and during 2 to 10 minutes of hypoxic exposure in obese rats. After 20 to 30 minutes of hypoxic exposure, V(T) remained elevated with N(HCl), but the earlier elevation of (E) seemed to be attenuated due to a decrease in f at 20 minutes of exposure to hypoxia. Thus, endogenous opioids modulate both resting (E) and the ventilatory response to sustained hypoxia in obese, but not in lean, Zucker rats by acting specifically on opioid receptors located within the central nervous system.

Enteric co-innervation of striated muscle fibers in the esophagus: just a "hangover"?

Striated muscle of the esophagus was until recently considered to consist of "classical" skeletal muscle fibers innervated by cholinergic vagal motoneurons. The recently described co-innervation originating from enteric neurons expressing nNOS, VIP, NPY, and galanin added a new dimension of complexity. The aim of this study was to summarize current knowledge about, and to get further hints as to the possible function of enteric co-innervation of striated esophageal muscle fibers. Aldehyde fixed rat esophagi were processed for immunocytochemistry for CGRP or VAChT (to demonstrate vagal motor terminals), nNOS/NADPH-d, VIP, NPY, and galanin (to demonstrate enteric terminals), met-enkephalin, mu opiate receptor, muscarinic receptors m1-3, soluble guanylyl cyclase, and cGMP dependent kinase type I and II. Motor endplates were visualized using fluorochrome tagged alpha-bungarotoxin to label nicotinic receptors, or with AChE histochemistry. Besides light and confocal laser scanning microscopy, immuno electron microscopy was also employed. Up to 80% of motor endplates were co-innervated. In addition to nNOS, VIP, NPY, and galanin, many enteric terminals in esophageal motor endplates expressed met-enkephalin. Some appeared to stain for the muscarinic m(2) receptor. There was prominent immunostaining for the micro opioid receptor in the sarcolemma at both junctional and extrajunctional sites. Immunostaining for soluble guanylyl cyclase was prominent immediately beneath the clusters of nicotinic receptors. Enteric varicosities and vagal terminals intermingled in motor endplates often without intervening teloglial processes. During ontogeny, initially high co-innervation rates were reduced to adult levels in a cranio-caudally progressing manner. We conclude that, in addition to a possible nitrergic, VIP-, NPY-, and galaninergic modulation of neuromuscular transmission by enteric neurons, opioidergic mechanisms could play a role. On the other hand, cholinergic influence on enteric neurons may be exerted also by the nucleus ambiguus via motor endplates, in addition to the input from the dorsal motor nucleus. The observations that enteric nerve fibers contact striated muscle fibers at specialized sites, i.e., motor endplates, and that these contacts appear in an ordered cranio-caudal sequence after cholinergic motor endplates have been established point to a specific function in neuronal control of esophageal muscle rather than to be an unspecific "hangover" from the smooth muscle past of this organ.

The opioidergic system in the combined regulation of pain and immunity

Opioidergic mechanisms are involved in responses to nociceptive and antigenic stimuli at all levels and stages (from peripheral nociceptors to the cerebral cortex and from the precursors of immunocompetent cells to mature effector cells). In most experimental and clinical studies, opioid-mediated analgesia proved to be accompanied by immunosuppression. Opioid receptors of mu, delta, and kappa types are involved in the mechanisms of combined regulation of pain and immunity, with mu and delta receptors suppressing the immune response and kappa receptors enhancing it. By modifying the chemical structure of opioid ligands, it is possible to preserve the analgesic effect and avoid the development of immunosuppression. The opioidergic mechanisms are coupled with nonopioid peptidergic and nonpeptide systems of pain and immunity regulation.

Mechanisms of Chloroquine-Induced Body-Scratching Behavior in Rats: Evidence of Involvement of Endogenous Opioid Peptides

Chloroquine is commonly used in the chemotherapy of malaria fever, and as an antiinflammatory disease-modifying agent in patients with rheumatoid arthritis or systemic lupus erythematosus. Administration of chloroquine (20.0 mg/kg IP) significantly ( p < 0.05) increased the frequency of body scratching in rats to 29.5 ± 9 in 30 min, compared to saline control animals (6.5 ± 2/30 min). Morphine, a μ-opiate receptor agonist (1.0 mg/kg IP), potentiated the chloroquine-induced rat body scratching to 40 ± 6.6, while the μ-opiate receptor antagonist, naltrexone (0.25 mg/kg, IP, given15 min prior) blocked the chloroquine induced body scratching to 4.5 ± 2 ( p < 0.05 ANOVA). In addition, the frequency of chloroquine (20.0 mg/kg IP)-induced body scratching was significantly reduced to 9.1 ± 3 in 30 min in rats rendered tolerant to morphine ( p < 0.05 ANOVA) compared to the scratching frequency of 40 ± 6.6 in morphine-naive rats. These suggests an involvement of μ-opioid receptors and/or endogenous opioid peptides in chloroquine induced body scratching in rats. Promethazine, a histamine- receptor antagonist (1.0 mg/kg IP, given 15 min prior to chloroquine) and the corticosteroid, dexamethasone (1.0 mg/kg, IP, given 15 min prior) separately and significantly ( p < 0.01) inhibited the chloroquine-induced scratching in rats, in a similar manner to clinical studies in malaria. Collectively, the novel results implicate opioidergic mechanisms, and confirm the efficacy of antihistamine and corticosteroids in chloroquine body scratching in rats. It also strongly suggests that the chloroquine-induced body-scratching behavior in the rat may be a useful experimental model for chloroquine-induced pruritus in humans.

The antinociceptive effect of PAG-microinjected dipyrone in rats is mediated by endogenous opioids of the rostral ventromedial medulla

Microinjection of non-opioid analgesics, such as dipyrone (DIP), into the periaqueductal gray matter (PAG) in rats causes an inhibition of nociceptive circuits in the spinal cord. We have herein investigated whether this effect is mediated by opioidergic mechanisms in the rostral ventromedial medulla (RVM), which is an important relay between the PAG and the spinal cord. The responses of spinal wide-dynamic-range neurons to noxious stimulation of their receptive field (RF) were inhibited by microinjection of DIP (100 μg/0.5 μl) into PAG. Subsequent microinjection of naloxone (NAL; 0.5 μg/0.5 μl) into RVM reversed this inhibition. The present and previous results suggest that non-opioid analgesics, as well as opiates, inhibit nociception by activating descending opioidergic mechanisms in PAG and RVM.

Enkephalinergic Neurons in the Periaqueductal Gray and Morphine Withdrawal

The effects of opioid (e.g., morphine) withdrawal on levels of endogenous opioid peptides and their mRNA in the various brain regions have been studied. However, the role of this opioidergic mechanism in the mediation of opioid withdrawal is not fully understood. Preproenkephalin (PPE) mRNA in the caudal periaqueductal gray (cPAG), an important brain region in opioid withdrawal, is increased by both opioid antagonist (naloxone)-precipitated and spontaneous morphine withdrawal, but not by various other stresses in rats, indicating a role of endogenous enkephalins in the cPAG in morphine withdrawal. In addition, PPE mRNA levels in the cPAG increase in the course of the dissipation of morphine withdrawal, and they are returned to the control levels after disappearance of morphine withdrawal signs. Local administration of an enkephalin analog or peptidase inhibitors into the cPAG suppresses morphine withdrawal signs. These facts suggest that enkephalinergic neurons in the PAG may have a critical role in the recovery phase of morphine withdrawal. Recently, an involvement of transcription factors in morphine withdrawal has been suggested. Thus, the possible role of transcription factors in the regulation of PPE gene expression in the cPAG during morphine withdrawal is also discussed.

The antinociceptive effect of leukotriene D 4 receptor antagonist, MK-571, in mice: possible involvement of opioidergic mechanism

The effect of a leukotriene D 4 receptor antagonist, (3-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl(3-dimethyl amino-3-oxo propyl)thio)methyl)thio) propanoic acid (L-660,711; MK-571), was investigated on nociceptive responses in mice using three different assays: acetic-acid-induced abdominal constrictions, formalin response and tail-flick test. MK-571 (8–32 mg/kg, i.v.) produced dose-dependent protection against acetic-acid-induced abdominal constriction (ED 50=30 mg/kg). The compound (10–80 mg/kg, i.p.) was also effective, in a dose-dependent manner, on the second phase of the formalin response (ED 50=26 mg/kg). However, it had no effect on the first phase of the formalin response and in the tail-flick test. Naloxone (1 mg/kg, i.v.), an opioid antagonist, almost completely blocked the antinociceptive effect of MK-571 in both acetic-acid-induced abdominal constriction and the second phase of the formalin test. These results provide evidence for an antinociceptive action of MK-571 at peripheral sites and suggest that opioid mechanisms are involved.

Chimeric peptide of Met-enkephalin and FMRFa induces antinociception and attenuates development of tolerance to morphine antinociception

A synthetic chimeric peptide of Met-enkephalin and FMRFamide (YGGFMKKKFMRFa), based on MERF was synthesized. This peptide was tested for possible antinociceptive effects using the tail flick test in mice. The effect of the chimeric peptide on morphine antinociception and development of tolerance to the antinociceptive action of morphine was also investigated. The chimeric peptide produced significant, dose-dependent antinociception (40, 60 and 90 mg/kg) in the tail flick test. Pretreatment with naloxone (5 mg/kg, IP) significantly attenuated the antinociceptive effect induced by the chimeric peptide (90 mg/kg, IP), indicating involvement of an opioidergic mechanism. In combination experiments with morphine, the antinociceptive dose of the chimeric peptide (60 mg/kg, IP) potentiated morphine (7 mg/kg, IP) antinociception. A low dose of the chimeric peptide (10 mg/kg, IP), that did not produce significant antinociception on its own, also potentiated morphine antinociception. In the tolerance studies, male albino mice received twice daily injections of morphine (20 mg/kg, IP) followed by either saline (0.1 ml) or chimeric peptide (80 mg/kg, IP) for a period of 4 days. A control group received twice daily injections of saline (0.1 ml) for the same period. When tested on Day 5, tolerance to antinociceptive action of morphine (15 mg/kg, IP) was evidenced by decreased response in chronic morphine plus saline treated mice compared to control group. Concurrent administration of chimeric peptide (80 mg/kg, IP) with morphine significantly attenuated the development of tolerance to the antinociceptive action of morphine. The preliminary results of this study demonstrate that peripherally administered chimeric peptide can produce dose dependent, naloxone reversible, antinociception; potentiate morphine antinociception and attenuate morphine tolerance, indicating a possible role of these type of amphiactive sequences in antinociception and its modulation. These chimeric peptides may also prove to be useful tools for further ascertaining the role of FMRFa family of peptides in mechanisms leading to opiate tolerance and dependence.