Dextromethorphan attenuated the higher vulnerability to inflammatory thermal hyperalgesia caused by prenatal morphine exposure in rat offspring

<p>Abstract</p> <p>Background</p> <p>Co-administration of dextromethorphan (DM) with morphine during pregnancy and throughout lactation has been found to reduce morphine physical dependence and tolerance in rat offspring. No evidence was presented, however, for the effect of DM co-administered with morphine during pregnancy on inflammatory hyperalgesia in morphine-exposed offspring. Therefore, we attempt to investigate the possible effect of prenatal morphine exposure on the vulnerability to hyperalgesia and the possible therapeutic effect of DM in the present study.</p> <p>Methods</p> <p>Fifty μl of carrageenan (20 mg/ml) was injected subcutaneously into the plantar surface of the right hind paw in p18 rats to induce hyperalgesia. Mean paw withdrawal latency was measured in the plantar test to index the severity of hyperalgesia. Using Western blotting and RT-PCR, the quantitative analyses of NMDA receptor NR1 and NR2B subunits were performed in spinal cords from different groups of animals.</p> <p>Results</p> <p>In the carrageenan-induced hyperalgesia model, rat offspring passively exposed to morphine developed a severe hyperalgesia on postnatal day 18 (p18), which also had a more rapid time course than those in the controls. Co-administration of DM with morphine in the dams prevented this adverse effect of morphine in the offspring rats. Western blot and RT-PCR analysis showed that the levels of protein and mRNA of NMDA receptor NR1 and NR2B subunits were significantly higher in the lumbar spinal cords of rats (p14) exposed to prenatal morphine; the co-administration of DM could reverse the effect of morphine on NR1 and attenuate the effect on NR2B.</p> <p>Conclusions</p> <p>Thus, DM may have a great potential in the prevention of higher vulnerability to inflammatory thermal hyperalgesia in the offspring of morphine-addicted mothers.</p

Attenuation by dextromethorphan on the higher liability to morphine-induced reward, caused by prenatal exposure of morphine in rat offspring

Co-administration of dextromethorphan (DM) with morphine during pregnancy and throughout lactation has been found to reduce morphine physical dependence and tolerance in rat offspring. No evidence was presented, however, for the effect of DM co-administered with morphine during pregnancy on morphine-induced reward and behavioral sensitization (possibly related to the potential to induce morphine addiction) in morphine-exposed offspring. Conditioned place preference and locomotor activity tests revealed that the p60 male offspring of chronic morphine-treated female rats were more vulnerable to morphine-induced reward and behavioral sensitization. The administration of a low dose of morphine (1 mg/kg, i.p.) in these male offspring also increased the dopamine and serotonin turnover rates in the nucleus accumbens, which implied that they were more sensitive to morphine. Co-administration of DM with morphine in the dams prevented this adverse effect of morphine in the offspring rats. Thus, DM may possibly have a great potential in the prevention of higher vulnerability to psychological dependence of morphine in the offspring of morphine-addicted mothers

Antinociceptive effects of morphine and naloxone in mu-opioid receptor knockout mice transfected with the MORS196A gene

<p>Abstract</p> <p>Background</p> <p>Opioid analgesics such as morphine and meperidine have been used to control moderate to severe pain for many years. However, these opioids have many side effects, including the development of tolerance and dependence after long-term use, which has limited their clinical use. We previously reported that mutations in the mu-opioid receptors (MOR) S196L and S196A rendered them responsive to the opioid antagonist naloxone without altering the agonist phenotype. In MORS196A knock-in mice, naloxone and naltrexone were antinociceptive but did not cause tolerance or physical dependence. In this study we delivery this mutated MOR gene into pain related pathway to confirm the possibility of <it>in vivo </it>transfecting MORS196A gene and using naloxone as a new analgesic agent.</p> <p>Methods</p> <p>The MOR-knockout (MOR-KO) mice were used to investigate whether morphine and naloxone could show antinociceptive effects when MORS196A gene was transfected into the spinal cords of MOR-KO mice. Double-stranded adeno-associated virus type 2 (dsAAV2) was used to deliver the MORS196A-enhanced green fluorescence protein (EGFP) gene by microinjected the virus into the spinal cord (S2/S3) dorsal horn region. Tail-flick test was used to measure the antinociceptive effect of drugs.</p> <p>Results</p> <p>Morphine (10 mg/kg, s.c.) and naloxone (10 mg/kg, s.c.) had no antinociceptive effects in MOR-KO mice before gene transfection. However, two or three weeks after the MOR-S196A gene had been injected locally into the spinal cord of MOR-KO mice, significant antinociceptive effects could be induced by naloxone or morphine. On the other hand, only morphine but not naloxone induced significant tolerance after sub-chronic treatment.</p> <p>Conclusion</p> <p>Transfecting the MORS196A gene into the spinal cord and systemically administering naloxone in MOR-KO mice activated the exogenously delivered mutant MOR and provided antinociceptive effect without causing tolerance. Since naloxone will not activate natural MOR in normal animals or humans, it is expected to produce fewer side effects and less tolerance and dependence than traditional opioid agonists do.</p

Chronic intracerebroventricular administration of morphine down-regulates spinal adenosine A1 receptors in rats

[[abstract]]Previous studies from our laboratory have shown that systemic chronic morphine treatment causes down-regulation of spinal adenosine A1 receptors in rats. In this study, we further investigated whether supraspinal morphine treatment causes this effect. Adult male Sprague-Dawley rats were rendered tolerant to morphine by multiple intracerebroventricular (i.c.v.) injections for 2 or 4 days. Adenosine A1 receptor binding activities were measured with [3H]cyclohexyladenosine in the spinal cord and midbrain. A significant decrease in [3H]cyclohexyladenosine binding was found in the spinal cord but not in the midbrain region after 2 or 4 days of chronic i.c.v. morphine treatment. A decrease in the number of binding sites (Bmax) with no change in the affinity (Kd) of the ligand for the adenosine A1 receptor was observed. These results suggest that supraspinal morphine administration could cause the down-regulation of spinal adenosine A1 receptors and this may play a role in the mechanism of morphine tolerance

Chronic morphine treatment causes down-regulation of spinal adenosine A1 receptors in rats.

[[abstract]]Recent studies suggest that the release of adenosine in the spinal cord may be a significant component of the morphine antinociceptive action. We wanted to know whether the spinal adenosine system is involved in morphine tolerance. Animals were rendered tolerant to morphine, and A1 adenosine receptor binding activity was measured. Treating Sprague-Dawley rats with multiple, increasing doses of morphine i.p. for 6 days resulted in an about 10-fold increase in the median antinociceptive dose (AD50) of morphine to elicit an antinociceptive response. On the other hand, this treatment also caused a 4 to 5-fold increase in the AD50 of cyclopentyladenosine (CPA). When A1 adenosine receptor binding was determined by using [3H]cyclohexyladenosine ([3H]CHA) a significant decrease in binding (P less than 0.05) in the spinal cord but not in the cortex was observed. Scatchard analysis of the [3H]CHA saturation binding data revealed a decrease in Bmax values (from 185.5 fmol/mg to 110.2 fmol/mg) and no significant change in Kd values

Comparisons of stress-related neuronal activation induced by restraint in adult male rat offspring with prenatal exposure to buprenorphine, methadone, or morphine

Prenatal opioid exposure may impede the development of adaptive responses to environmental stimuli by altering the stress-sensitive brain circuitry located at the paraventricular nucleus of the hypothalamus (PVH) and locus coeruleus (LC). Corticotropin-releasing factor (CRF) released from neurons in the PVH has emerged as a key molecule to initiate and integrate the stress response. Methadone (Meth) and buprenorphine (Bu) are two major types of synthetic opioid agonists for first-line medication-assisted treatment of opioid (e.g., morphine, Mor) use disorder in pregnant women. No studies have compared the detrimental effects of prenatal exposure to Meth versus Bu on the stress response of their offspring upon reaching adulthood. In this study, we aimed to compare stress-related neuronal activation in the PVH and LC induced by restraint (RST) stress in adult male rat offspring with prenatal exposure to the vehicle (Veh), Bu, Meth, or Mor. CFos-immunoreactive cells were used as an indicator for neuronal activation. We found that RST induced less neuronal activation in the Meth or Mor exposure groups compared with that in the Bu or Veh groups; no significant difference was detected between the Bu and Veh exposure groups. RST-induced neuronal activation was completely prevented by central administration of a CRF receptor antagonist (α-helical CRF9-41, 10 μg/3 μL) in all exposure groups, suggesting the crucial role of CRF in this stress response. In offspring without RST, central administration of CRF (0.5 μg/3 μL)-induced neuronal activation in the PVH and LC. CRF-induced neuronal activation was lessened in the Meth or Mor exposure groups compared with that in the Bu or Veh groups; no significant difference was detected between the Bu and Veh exposure groups. Moreover, RST- or CRF-induced neuronal activation in the Meth exposure group was comparable with that in the Mor exposure group. Further immunohistochemical analysis revealed that the Meth and Mor exposure groups displayed less CRF neurons in the PVH of offspring with or without RST compared with the Bu or Veh groups. Thus, stress-induced neuronal activation in the PVH and LC was well preserved in adult male rat offspring with prenatal exposure to Bu, but it was substantially lessened in those with prenatal exposure to Meth or Mor. Lowered neuronal activation found in the Meth or Mor exposure groups may be, at least in part, due to the reduction in the density of CRF neurons in the PVH