Morphine Dose-effect Curve Research Articles

Mu opioid receptor (MOR) agonist‐induced antihyperalgesic effects following spared nerve injury in male and female rats

In most preclinical models of neuropathic pain, hypersensitivity to pain often resolves after a few weeks. However, spared nerve injury (SNI) produces a persistent pain state that lasts at least 18 months. The goal of the current study was to evaluate antihyperalgesic effects of mu‐opioid receptor (MOR) agonists varying in potency and efficacy in the SNI model. We hypothesized that the MOR agonists, fentanyl, morphine, and nalbuphine, would produce antihyperalgesic effects in both male and female rats. Mechanical hypersensitivity was evaluated by measuring responses to increasing pressure (g) applied to each paw (paw pressure test or Randall‐Selitto test) with a maximum cut‐off of 300 g. Using a within‐subjects design, responses were evaluated before and after SNI or sham surgery in male and female rats, and cumulative dose effects curves for fentanyl (0.01‐0.1 mg/kg), morphine (0.3‐10 mg/kg), and nalbuphine (0.3‐10 mg/kg) were determined. Both male and female sham rats demonstrated an initial hypersensitivity on their ipsilateral paw following surgery that dissipated within 7 d. Following SNI surgery, paw pressure thresholds on the injured paw were lower as compared with the pre‐surgical response and with the contralateral paw for at least 20 wks in male and female rats. In male rats, fentanyl, morphine, and nalbuphine dose‐dependently alleviated SNI‐induced hypersensitivity, with EC50 values of 0.03 (± 0.007), 3.9 (± 0.8), and 5.3 (± 0.8) mg/kg, respectively. In female rats, fentanyl, morphine, and nalbuphine also alleviated SNI‐induced hypersensitivity in a dose‐dependent manner, with EC50 values of 0.01 (± 0.002), 4.1 (± 0.8), and 5.5 (± 0.8) mg/kg, respectively. Naltrexone (0.3 mg/kg) produced rightward shifts in the fentanyl and morphine dose effect curves. Interestingly, fentanyl and morphine also increased paw pressure thresholds in the uninjured, contralateral paws and sham‐treated ipsilateral paws in male and female rats. In contrast, gabapentin produced anti‐hyperalgesic effects in both male and female rats at 180 mg/kg as demonstrated by a return to pre‐surgical, paw pressure thresholds without altering thresholds in the non‐injured paw. These results demonstrate MOR agonists produce dose‐dependent increases in paw pressure thresholds following SNI surgery in both male and female rats. As expected, MOR agonists demonstrated a rank order in potency with fentanyl > morphine > nalbuphine in both sexes, and only fentanyl was slightly more potent in females. Fentanyl and morphine also increased paw pressure thresholds in non‐injured paws, suggesting that large doses inhibited responses to mechanical stimulation perhaps due to sedation and/or behavioral suppression. Overall, these findings demonstrate that MOR agonists produce antihyperalgesic effects in male and female rats potentially with narrow therapeutic indices. Future studies will investigate other behavioral effects of MOR agonists, such as reinforcing and interoceptive effects, in this chronic neuropathic pain model.

The Role of Peripheral Opioid Receptors in the Development of Tolerance to Mu‐Opioid Receptor Agonists

Opioids are regarded as the most effective drugs for pain therapy. However, adverse effects, such as tolerance development, hinder the effectiveness of opioids during chronic treatment. Therefore, elucidating mechanisms involved in tolerance may aid in the design of novel treatments of chronic pain. This study evaluates the role of peripheral opioid receptors in tolerance development to mu-opioid receptor (MOR) agonists. All animals used in this study were male, C57BL/6N, and weighed between 20-30 g at 7-15 weeks old. Antinociceptive effects of MOR agonists were evaluated in a state of inflammatory pain (CFA-induced mechanical allodynia) and a centrally-mediated, thermal reflex assay (50°C warm water tail withdrawal (WWTW)). Following five days of 3x daily administration of morphine (10mg/kg), the ED50 of the morphine dose-effect curve was shifted 2.5-fold to the right in both assays. Interestingly, in the same two assays, naloxone-methiodide (10mg/kg) pretreatments before each chronic morphine injection prevented the rightward shift in the morphine dose-effect curve without altering the acute antinociceptive effects of morphine in mice treated with chronic saline. In the WWTW, five days of 4x daily administration of loperamide (3.2mg/kg, an ineffective dose in the WWTW) was sufficient to produce cross-tolerance to the antinociceptive effects of morphine, as displayed by a rightward shift in the morphine dose-effect curve. Pretreatments of naloxone-methiodide (10mg/kg) completely attenuated loperamide-induced cross-tolerance to the antinociceptive effects of morphine. These results suggest that repeated activation of peripheral opioid receptors may be involved in developing tolerance to the central antinociceptive effects of MOR agonists.

Potential Involvement of Peripheral Opioid Receptors in the Development of Tolerance to Mu‐Opioid Receptor Agonists

Clinically used opioids are effective analgesics but are highly associated with many adverse effects including physical dependence, tolerance development, rewarding properties and respiratory depression, the cause of opioid‐related overdoses and deaths. The goal of this study is to evaluate tolerance development to centrally and peripherally‐acting mu‐opioid receptor (MOR) agonists. In this study, we used a model of acute, peripherally‐mediated visceral pain, the acetic acid stretch assay (AASA) and an inflammatory model of pain measuring mechanical allodynia using von Frey monofilaments. In the AASA, loperamide and morphine displayed acute antinociceptive effects with ED50 values of 0.62 and 0.15 mg/kg, respectively. Naloxone pretreatment attenuated the acute antinociceptive effects of both loperamide and morphine. Following 5 days of 2x daily drug administration, the ED50 for the loperamide and morphine dose effect curves were shifted to the right. In the model of inflammatory pain, acute administration of loperamide was ineffective at doses up to 3.2mg/kg. Morphine produced dose‐dependent acute antinociceptive effects in this assay with an ED50 of 4.2mg/kg. This effect was attenuated by naloxone but was not altered by naloxone‐methiodide. Following 5 days of 3x daily administration of morphine, the ED50 of the morphine dose effect curve was shifted 3.5 fold to the right. Interestingly, pretreatments of naloxone‐methiodide prior to each chronic morphine injection prevented the rightward shift in the morphine dose effect curve following chronic morphine treatment. These results suggest that repeated activation of peripheral opioid receptors produce tolerance and that peripheral opioid receptors may be involved in the development of tolerance to morphine.

S38NEUROANATOMICAL CHARACTERIZATION OF GENES GENETICALLY ASSOCIATED WITH DEPRESSION

Accumulating evidence indicates that phosphorylated serum- and glucocorticoid-regulated kinase 1 (SGK1) is associated with spinal nociceptive sensitization by modulating glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined whether spinal SGK1 signaling contributes to the development of morphine analgesic tolerance. Chronic morphine administration markedly induced phosphorylation of SGK1 in the spinal dorsal horn neurons. Intrathecal injection of SGK1 inhibitor GSK-650394 reduced the development of morphine tolerance with a significant leftward shift in morphine dose–effect curve. Furthermore, spinal inhibition of SGK1 suppressed morphine-induced phosphorylation of nuclear factor kappa B (NF-κB) p65 and upregulation of NMDAR NR1 and NR2B expression in the spinal dorsal horn. In contrast, intrathecal administration of NMDAR antagonist MK-801 had no effect on the phosphorylation of SGK1 in morphine-treated rats. In addition, morphine-induced upregulation of NR2B, but not NR1, was significantly abolished by intrathecal pretreatment with PDTC, a specific NF-κB activation inhibitor. Finally, spinal delivery of SGK1 small interfering RNA exhibited similar inhibitory effects on morphine-induced tolerance, phosphorylation of NF-κB p65, as well as upregulation of NR1 and NR2B expression. Our findings demonstrate that spinal SGK1 contributes to the development of morphine tolerance by enhancing NF-κB p65/NMDAR signaling. Interfering spinal SGK1 signaling pathway could be a potential strategy for prevention of morphine tolerance in chronic pain management.

Spinal Serum- and Glucocorticoid-Regulated Kinase 1 (SGK1) Signaling Contributes to Morphine-Induced Analgesic Tolerance in Rats

Accumulating evidence indicates that phosphorylated serum- and glucocorticoid-regulated kinase 1 (SGK1) is associated with spinal nociceptive sensitization by modulating glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined whether spinal SGK1 signaling contributes to the development of morphine analgesic tolerance. Chronic morphine administration markedly induced phosphorylation of SGK1 in the spinal dorsal horn neurons. Intrathecal injection of SGK1 inhibitor GSK-650394 reduced the development of morphine tolerance with a significant leftward shift in morphine dose–effect curve. Furthermore, spinal inhibition of SGK1 suppressed morphine-induced phosphorylation of nuclear factor kappa B (NF-κB) p65 and upregulation of NMDAR NR1 and NR2B expression in the spinal dorsal horn. In contrast, intrathecal administration of NMDAR antagonist MK-801 had no effect on the phosphorylation of SGK1 in morphine-treated rats. In addition, morphine-induced upregulation of NR2B, but not NR1, was significantly abolished by intrathecal pretreatment with PDTC, a specific NF-κB activation inhibitor. Finally, spinal delivery of SGK1 small interfering RNA exhibited similar inhibitory effects on morphine-induced tolerance, phosphorylation of NF-κB p65, as well as upregulation of NR1 and NR2B expression. Our findings demonstrate that spinal SGK1 contributes to the development of morphine tolerance by enhancing NF-κB p65/NMDAR signaling. Interfering spinal SGK1 signaling pathway could be a potential strategy for prevention of morphine tolerance in chronic pain management.

Ventilatory-depressant effects of opioids alone and in combination with cannabinoids in rhesus monkeys

Pain is a serious health problem that is commonly treated with opioids, although the doses of opioids needed to treat pain are often similar to those that decrease respiration. Combining opioids with drugs that relieve pain through non-opioid mechanisms can decrease the doses of opioids needed for analgesia, resulting in an improved therapeutic window, but only if the doses of opioids that decrease respiration are not similarly decreased. Using small doses of opioids to treat pain has the potential to reduce the number of overdoses and deaths. This study investigated whether the cannabinoid receptor agonists Δ9-tetrahydrocannabinol (Δ9-THC) and CP 55,940 modify the ventilatory-depressant effects of morphine and fentanyl in three monkeys. Ventilatory parameters, including minute volume (VE), were monitored with a head plethysmograph. When given alone, morphine (0.032 – 10 mg/kg) and fentanyl (0.00032 – 0.1 mg/kg) dose dependently decreased VE. Doses of Δ9-THC (1 mg/kg) and CP 55,940 (0.01 mg/kg) that enhance the potency of opioids to produce antinociception modestly decreased ventilation when given alone but did not significantly change morphine or fentanyl dose-effect curves. A larger dose of CP 55,940 (0.032 mg/kg) shifted the fentanyl dose-effect curve downward in two monkeys, without significantly changing the morphine dose-effect curve. In summary, cannabinoid receptor agonists, which increase the potency of opioids to produce antinociception, did not increase their potency to depress ventilation. Thus, the therapeutic window is greater for opioids when they are combined with cannabinoid receptor agonists, indicating a possible advantage for these drug mixtures in treating pain.

Repeated Morphine Produces Sensitization to Reward and Tolerance to Antiallodynia in Male and Female Rats with Chemotherapy-Induced Neuropathy.

Paclitaxel is a cancer chemotherapy drug with adverse effects that include chemotherapy-induced neuropathic pain (CINP) as well as depression of behavior and mood. In the clinical setting, opioids are often used concurrently with or after chemotherapy to treat pain related to the cancer or CINP, but repeated opioid exposure can also increase the risk of opioid abuse. In this study, male and female Sprague-Dawley rats were used to test the hypothesis that repeated 3.2-mg/kg doses of morphine would induce tolerance to its antinociceptive effects in a mechanical sensitivity assay and increased expression of its abuse-related rewarding effects in an assay of intracranial self-stimulation (ICSS). Three weeks after four injections of vehicle or 2.0 mg/kg of paclitaxel, the initial morphine dose-effect curves were determined in both assays. Subsequently, rats were treated with 3.2 mg/kg per day morphine for 6 days. On the final day of testing, morphine dose-effect curves were redetermined in both assays. On initial exposure, morphine produced dose-dependent antiallodynia in the assay of mechanical sensitivity, but it produced little or no rewarding effects in the assay of ICSS. After 6 days of repeated treatment, morphine antiallodynia decreased, and morphine reward increased. Females exhibited greater morphine reward on initial exposure than males, but repeated morphine eliminated this sex difference. These results suggest that repeated morphine may produce tolerance to therapeutically beneficial analgesic effects of morphine but increased sensitivity to abuse-related rewarding effects of morphine in subjects treated with paclitaxel.

Impact of efficacy at the μ-opioid receptor on antinociceptive effects of combinations of μ-opioid receptor agonists and cannabinoid receptor agonists.

Cannabinoid receptor agonists, such as Δ(9)-tetrahydrocannabinol (Δ(9)-THC), enhance the antinociceptive effects of μ-opioid receptor agonists, which suggests that combining cannabinoids with opioids would improve pain treatment. Combinations with lower efficacy agonists might be preferred and could avoid adverse effects associated with large doses; however, it is unclear whether interactions between opioids and cannabinoids vary across drugs with different efficacy. The antinociceptive effects of μ-opioid receptor agonists alone and in combination with cannabinoid receptor agonists were studied in rhesus monkeys (n = 4) using a warm water tail withdrawal procedure. Etorphine, fentanyl, morphine, buprenorphine, nalbuphine, Δ(9)-THC, and CP 55,940 (2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol) each increased tail withdrawal latency. Pretreatment with doses of Δ(9)-THC (1.0 mg/kg) or CP 55,940 (0.032 mg/kg) that were ineffective alone shifted the fentanyl dose-effect curve leftward 20.6- and 52.9-fold, respectively, and the etorphine dose-effect curve leftward 12.4- and 19.6-fold, respectively. Δ(9)-THC and CP 55,940 shifted the morphine dose-effect curve leftward only 3.4- and 7.9-fold, respectively, and the buprenorphine curve only 5.4- and 4.1-fold, respectively. Neither Δ(9)-THC nor CP 55,940 significantly altered the effects of nalbuphine. Cannabinoid receptor agonists increase the antinociceptive potency of higher efficacy opioid receptor agonists more than lower efficacy agonists; however, because much smaller doses of each drug can be administered in combinations while achieving adequate pain relief and that other (e.g., abuse-related) effects of opioids do not appear to be enhanced by cannabinoids, these results provide additional support for combining opioids with cannabinoids to treat pain.

Interactions between mu opioid receptor agonists and cannabinoid receptor agonists CP55940 and WIN55212–2 in rhesus monkeys: evaluation of treatment‐ and abuse‐related effects

Mu opioid receptor agonists and cannabinoid receptor agonists can be combined to possibly enhance therapeutic effectiveness (e.g., pain treatment); however, it is unclear whether combinations of these drugs enhance abuse liability. These studies examined the effects of cannabinoid receptor agonists CP55940 and WIN55212‐2 on the antinociceptive, discriminative stimulus, and positive reinforcing effects of mu opioid receptor agonists in rhesus monkeys. In one group (n=3), CP55940 (0.001–0.056 mg/kg), and WIN55212‐2 (0.1–1.78 mg/kg) shifted morphine dose‐effect curves leftward up to 8 fold. In monkeys discriminating 3.2 mg/kg morphine from saline (n=3), CP55940 and WIN55212‐2 shifted the morphine discriminative stimulus dose‐effect curves rightward up to 7 fold. In monkeys (n=4) self‐administering heroin (0.32–32.0 μg/kg/infusion), CP55940 and WIN55212‐2 shifted the heroin dose‐effect curves rightward (up to 15 fold) and downward. Cannabinoid receptor agonists CP55940 and WIN55212‐2 enhanced the antinociceptive effects but not the discriminative stimulus or positive reinforcing effects of mu opioid receptor agonists. These data support the view that combining mu opioid receptor agonists and cannabinoid receptor agonists might enhance treatment effectiveness without enhancing abuse liability. Supported by USPHS Grants R01DA05018, K05DA017918 (CPF) and T32DA031115 (DRM).

Interactions betweenμ-Opioid Receptor Agonists and Cannabinoid Receptor Agonists in Rhesus Monkeys: Antinociception, Drug Discrimination, and Drug Self-Administration

Cannabinoid receptor agonists enhance the antinociceptive effects of μ-opioid receptor agonists, which suggests that combinations of these drugs might enhance therapeutic effectiveness (e.g., analgesia). However, it is not clear whether combinations of these drugs also enhance abuse or dependence liability. This experiment examined whether combinations of cannabinoids and opioids that enhance antinociception also increase abuse-related effects by studying the effects of the cannabinoid receptor agonists 2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]-5-(2-methyloctan-2-yl)phenol (CP 55,940) and (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212) on the antinociceptive, discriminative stimulus, and positive reinforcing effects of μ-opioid receptor agonists in rhesus monkeys. In one group of monkeys (n = 3), morphine (0.1-5.6 mg/kg s.c.), CP 55,940 (0.0032-0.032 mg/kg s.c.), and WIN 55,212 (0.1-1.0 mg/kg s.c.) dose-dependently increased tail withdrawal latency from 50°C water, and pretreatment with small, otherwise ineffective, doses of CP 55,940 and WIN 55,212 shifted the morphine dose-effect curve to the left. In monkeys (n = 3) discriminating 3.2 mg/kg morphine, CP 55,940 (0.01-0.032 mg/kg s.c.) and WIN 55,212 (0.1-1.78 mg/kg s.c.) attenuated the discriminative stimulus effects of morphine, shifting the dose-effect curve to the right. In monkeys (n = 4) self-administering heroin (0.32-32.0 µg/kg/infusion i.v.), CP 55,940 (0.001-0.032 mg/kg s.c.), and WIN 55,212 (0.1-1.0 mg/kg s.c.) shifted the heroin dose-effect curve rightward and downward. Cannabinoid receptor agonists CP 55,940 and WIN 55,212 enhanced the antinociceptive effects but not the discriminative stimulus or positive reinforcing effects of μ-opioid receptor agonists in rhesus monkeys, supporting the view that combining cannabinoid and opioid receptor agonists might result in enhanced treatment effectiveness for pain without similarly enhancing abuse and dependence liability.

Effects of imidazoline I2 receptor agonists and morphine on schedule-controlled responding in rats

Accumulating evidence indicates that imidazoline I2 receptor agonists enhance the antinociceptive effects of opioids and therefore may be suitable for combination therapy with opioids for pain treatment. However, little is known of the effects of I2 receptor agonists on other behavioral effects of opioids. This study used schedule-controlled responding and dose-addition analyses to examine interactions between the μ opioid receptor agonist morphine and two imidazoline I2 receptor agonists, 2-BFI and BU224. In 8 rats responding under a fixed ratio 10 schedule of food presentation, morphine (3.2–17.8mg/kg), 2-BFI (3.2–17.8mg/kg), and BU224 (5.6–17.8mg/kg) each dose-dependently decreased responding. The addition of fixed proportions of 2-BFI or BU224 shifted the morphine dose–effect curves leftward. The interactions between morphine and 2-BFI or BU224 were infra-additive when the same proportions of morphine and I2 receptor agonists were mixed; however, the interaction between morphine and I2 receptor agonists was additive when the drugs were mixed at other proportions. These results provide quantitative evidence that I2 receptor agonists do not enhance the response rate-decreasing effect of morphine and suggest that the enhancement of morphine antinociception is selective. Together, these results further support the therapeutic potential of combining I2 receptor agonists and opioids for pain control.

Opioid antinociception, tolerance and dependence

This study explored the involvement of N-methyl-D-aspartate (NMDA) in the effects of μ-opioid agonists. A hot-plate procedure was used to assess antinociception and tolerance in mice in which the NR1 subunit of the NMDA receptor was reduced [knockdown (KD)] to approximately 10%, and in mice treated with the NMDA antagonist, (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959). The μ opioid agonists, morphine, l-methadone and fentanyl, were approximately three-fold less potent in the NR1 KD mice than in wild-type (WT) controls; however, the development of morphine tolerance and dependence did not differ markedly in the NR1 KD and the WT mice. Acute administration of the NMDA antagonist, LY235959, produced dose-dependent, leftward shifts in the morphine dose-effect curve in the WT mice, but not in the NR1 KD mice. Chronic administration of LY235959 during the morphine tolerance regimen did not attenuate the development of tolerance in the NR1 KD or the WT mice. These results indicate that the NR1 subunit of the NMDA receptor does not play a prominent role in μ opioid tolerance.

Attenuation of morphine antinociceptive tolerance by a CB1 receptor agonist and an NMDA receptor antagonist: Interactive effects

CB(1) cannabinoid (CB(1)) receptor agonists and N-Methyl-d-Aspartate (NMDA) receptor antagonists attenuate the development of morphine antinociceptive tolerance. The present study used dose-addition analysis to evaluate CB(1)/NMDA receptor interactions on this endpoint. Chronic morphine administration (5 days, 100 mg/kg, twice daily) resulted in a 2.8-fold rightward shift in the morphine dose-effect curve. Co-administration of either the CB(1) receptor agonist CP-55940 (5-(1,1-Dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol; 0.32-1.0 mg/kg) or the NMDA receptor antagonist (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959; 1.0-3.2 mg/kg) with morphine dose-dependently attenuated morphine tolerance. The relative potency of each drug alone was quantified using a defined level of effect (one-quarter log shift in the morphine dose-effect curve), resulting in equieffective doses of 0.42 mg/kg and 1.1 mg/kg for CP-55940 and LY235959, respectively. Subsequent experiments assessed CP-55940/LY235959 interactions using a fixed-proportion design. Co-administration of CP-55940/LY235959 mixtures (1:1, 1:3.2, or 1:10 CP-55940/LY235959) with morphine dose-dependently attenuated morphine tolerance. Isobolographic and dose-addition analysis were used to statistically compare the experimentally determined potency for each mixture (z(mix)) with predicted additive potency (z(add)). Mixtures of 1:1 and 1:3.2 CP-55940/LY235959 produced additive effects (z(add) = z(mix)), while the mixture of 1:10 CP-55940/LY235959 produced a supra-additive effect (z(add) > z(mix)). These results suggest that CP-55940 and LY235959 produce additive or supra-additive attenuation of morphine antinociceptive tolerance after repeated morphine administration, depending on their relative concentrations.

Time-dependent decreases in apparent pA2 values for naltrexone studied in combination with morphine in rhesus monkeys

One assumption of Schild analyses, which have been used extensively to characterize opioids in vitro and in vivo, is that the concentration of antagonist is at equilibrium; violation of this assumption would yield slopes of Schild plots that deviate from unity. For in vivo studies, the concentration of antagonist changes as it is eliminated; however, if studies are conducted at a time when the antagonist is maximally effective, the assumption of equilibrium is not violated. The goal of these studies was to determine how increasing the delay between antagonist administration and determination of dose-effect curves alters results of Schild analyses. Monkeys received 3.2 mg/kg/day of morphine and discriminated naltrexone while responding under a fixed-ratio 5 schedule of stimulus-shock termination. In monkeys acutely deprived of morphine (27 h), naltrexone was administered 1, 2, 4, or 6 h before determination of morphine dose-effect curves. Morphine-deprived monkeys responded on the naltrexone-appropriate lever, and this effect was reversed by morphine. Naltrexone dose- and time-dependently antagonized morphine. Schild analyses yielded slopes that did not deviate from unity; however, as the delay between naltrexone administration and determination of morphine dose-effect curves increased, apparent pA(2) values decreased. The assumption of equilibrium of antagonist at receptor sites does not appear to be violated, regardless of when it is administered, and changes in naltrexone concentration as it is eliminated are reflected in orderly decreases in potency. These results further indicate the strength of Schild analyses for describing interactions between drugs and receptors in vivo.

PAG mu opioid receptor activation underlies sex differences in morphine antinociception

Given the findings that (1) systemic opioid antinociception varies by estrous stage in females and (2) the magnitude of sex differences in opioid antinociception is negatively correlated with opioid agonist efficacy, we hypothesized that sex differences in the function of the descending pain modulatory system are likely influenced by estrous stage in females and by the number of available opioid receptors therein. The present study tested these hypotheses by (1) comparing antinociception produced by morphine microinjection to the ventral periaqueductal gray (vPAG) in females at different stages of the estrous cycle and (2) examining systemic morphine antinociception in males versus females under conditions of reduced vPAG mu opioid receptor availability. When estrous stage of females was not controlled for (Experiment 1), there was no significant sex difference in tail withdrawal antinociception following morphine microinjection (0.3–10 μg), although morphine was more potent in males than females in producing immobility. Experiment 2 showed that intra-vPAG morphine produced less antinociception and immobility in estrus than in diestrus females; that is, only estrus females’ response to morphine was lower than that of males. Experiment 3 showed that microinjection of the irreversible mu opioid antagonist β-funaltrexamine (β-FNA) into the vPAG shifted the systemic morphine dose–effect curve farther to the right in females than in males. That is, a reduction in available vPAG mu opioid receptors had a greater impact on opioid antinociception in females than in males, suggesting that females have fewer vPAG mu opioid receptors than males. Overall, these data suggest that ovarian hormones and PAG mu opioid receptor density contribute to sex differences in antinociception produced by morphine.

P.2.131 Trends in antidepressants use in different european union countries

The cold water tail-flick test in the rat is somewhat unique in that it is sensitive to the analgesic effects of δ- and k- in addition to μ-opioid agonists. The present study was designed to test whether a component of morphine-induced analgesia in this test might be mediated by δ- or k-opioid receptors. Morphine was administered icv in combination with the non-selective opioid antagonist naloxone (NLX), as well as the μ-, δ- and k-selective antagonists. d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr (CTAP), naltrindole (NTI) and norbinaltorphimine (norBNI), respectively. Morphine induced analgesia in a dose related manner. Administration of NLX (1–10 μg) or CTAP (1μg) antagonized morphine in a competitive fashion. Neither NTI (1–10 μg) nor norBNI (0.1 μg) had any effect on the morphine dose-effect curve. Thus, morphine appeared to be a selective μ agonist in the cold water tail-flick test, at least by the icv route.

Short Communication

Rats given long-term access to a palatable sucrose solution are more sensitive to (1) the analgesic potency of morphine and (2) the anorectic actions of naltrexone. In the present experiment, the ability of naltrexone to antagonize the antinociceptive properties of morphine was examined in sucrose- and chow-fed rats. Twenty adult male Long-Evans rats were maintained on chow and water. Ten rats were also given ad libitum access to a 32% sucrose solution for three weeks. Baseline and post-drug nociceptive thresholds were determined with a warm water (52°C) tail-withdrawal procedure. Complete morphine dose-effect curves (0.32-100.0 mg/kg) were determined alone and in the presence of naltrexone (0.32-0.32 mg/kg). Morphine produced significant increases in tail-withdrawal latencies in both groups of animals. Rats drinking sucrose had significantly longer tail-withdrawal latencies than rats given only chow and water. Naltrexone produced significant and surmountable dose-dependent rightward shifts in the morphine dose-response curves for both groups, but did not differ in potency across diets. The present data support and extend our earlier findings showing sucrose enhancement of morphine-induced antinociception in rats given a palatable sucrose solution to drink.

Activation of kappa-opioid receptors inhibits pruritus evoked by subcutaneous or intrathecal administration of morphine in monkeys.

Pruritus (itch sensation) is the most common side effect associated with spinal administration of morphine given to humans for analgesia. A variety of agents have been proposed as antipruritics with poorly understood mechanisms and they are effective with variable success. kappa-Opioid agonists possess several actions that are opposite to micro -opioid agonists. We proposed to investigate the role of kappa-opioid receptors (KORs) in morphine-induced scratching and antinociception in monkeys. Scratching responses were counted by observers blinded to treatment. Antinociception was measured by a warm water (50 degrees C) tail-withdrawal assay. Pretreatment with low doses of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide (U-50488H) (0.032-0.18 mg/kg s.c.), a selective KOR agonist, dose dependently suppressed the s.c. morphine dose-effect curve for scratching and potentiated s.c. morphine-induced antinociception. In addition, s.c. U-50488H attenuated i.t. morphine (10 and 32 micro g)-induced scratching while maintaining or enhancing i.t. morphine-induced antinociception. The combination of s.c. or i.t. morphine with low doses of U-50488H did not cause sedation. More importantly, pretreatment with 3.2 mg/kg nor-binaltorphimine, a selective KOR antagonist, blocked the effects of s.c. U-50488H on both s.c. and i.t. morphine-induced scratching. These results indicate that activation of KOR attenuates morphine-induced scratching without interfering with antinociception in monkeys. This mechanism-based finding provides functional evidence in support of the clinical potential of KOR agonists as antipruritics in the presence of MOR agonist-induced pruritus.

Role of morphine maintenance dose in the development of tolerance and its attenuation by an NMDA receptor antagonist.

Current research shows that N-methyl-d-aspartate (NMDA) receptor antagonists attenuate the development of morphine tolerance in rodent antinociceptive assays. The purpose of this study was to determine the role of morphine maintenance dose in the attenuation of morphine tolerance by the competitive NMDA receptor antagonist, LY235959. A rat warm-water tail-withdrawal procedure was used to measure the antinociceptive effects of morphine and LY235959. In this procedure, the distal 8 cm of each rat's tail is immersed in 40 degrees (non-noxious) and 55 degrees C (noxious) water, and the latency to remove the tail is recorded. Morphine (0.3-10 mg/kg, SC) produced dose-dependent increases in tail-withdrawal latencies from the 55 degrees C water. Following determination of the morphine dose-effect curves, rats were administered chronically one of three doses of morphine (10, 20, or 40 mg/kg) either alone or in combination with LY235959 (1.0, 3.0, or 5.6 mg/kg, SC) twice daily for 7 days. Chronic administration of 10, 20, and 40 mg/kg morphine produced rightward shifts in the morphine dose-effect curves of approximately 3-, 6-, and 12-fold, respectively. When LY235959 (1.0-5.6 mg/kg) was co-administered with 10 mg/kg morphine, the development of morphine tolerance was attenuated in a dose-dependent manner, with complete prevention observed following 3.0 mg/kg LY235959. LY235959 (1.0, 3.0 mg/kg) also attenuated the development of tolerance to 20 and 40 mg/kg morphine; however, tolerance was not completely prevented. Administering 3.0 mg/kg LY235959 along with 20 and 40 mg/kg morphine was functionally equivalent to treating rats with half the amount of morphine. These data suggest that the maintenance dose of morphine, and thus the magnitude of tolerance, can determine the effectiveness of an NMDA receptor antagonist to attenuate morphine tolerance.

Selective attenuation of the antinociceptive effects of mu opioids by the putative dopamine D3 agonist 7-OH-DPAT.

The purpose of the present investigation was to evaluate the effects of the D3 agonist (+/-)-7-hydroxy-dipropylaminotetralin (7-OH-DPAT), various dopamine (DA) agonists and DA antagonists on the antinociceptive effects of mu opioids. Antinociception was assessed using a warm-water tail-withdrawal procedure in rats. The mu opioids morphine (0.3-10 mg/kg) and dezocine (0.03-3.0 mg/kg) produced dose-dependent increases in antinociception with maximal effects obtained at the higher doses tested. Pretreatment with the putative D3 agonist 7-OH-DPAT (1.0-10 mg/kg) produced a dose-dependent attenuation of the antinociceptive effects of morphine and dezocine. At the highest dose of 7-OH-DPAT tested, the morphine dose-effect curve was shifted rightward by approximately 1.5 log units and the dezocine curve by greater than 2.3 log units. The (+)-isomer of 7-OH-DPAT (1.0 and 3.0 mg/kg) also shifted the morphine dose-effect curve to the right in a dose-dependent manner. The DA D3/D2 agonist (-)-quinpirole (0.1-10mg/kg) attenuated the effects of morphine, but these effects were small in magnitude, not dose-dependent and observed only under a limited set of conditions. The DA D2/D3 antagonist spiperone failed to alter the morphine dose-effect curve, but reversed the effects of 7-OH-DPAT on morphine antinociception. Pretreatment with the DA D1 agonist (+/-)-SKF38393 (1.0 and 10 mg/kg) and the D1 antagonist (+)-SCH23390 (0.1 and 1.0 mg/kg) failed to alter the morphine dose-effect curve. The finding that 7-OH-DPAT markedly attenuated the effects of morphine and that these effects were reversed with spiperone suggests that activity at the D3, and possibly the D2, receptor can modulate mu agonist-induced antinociception.