In vivo binding of 3H-etorphine in morphine-dependent rats
Abstract
The opiate agonist 3H-etorphine was used to search for potential changes in opiate receptor binding in rats following chronic exposure to morphine sulfate. A rapid filtration method was employed to allow assessment of binding; receptor dissociation following labeling was also measured. No significant differences in total binding were seen with addicted animals, naive controls and naive animals pre-injected with morphine, at two different 3H-etorphine doses. dissociation under several conditions also yielded no differences. However, the rate of dissociation in the presence of both Na+ and a guanine nucleotide showed a small but significant decrease in dependent animals, suggesting a possible impairment of receptor effector coupling with morphine addiction.
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Cited by (11)
Prenatal morphine exposure differentially alters expression of opioid peptides in striatum of newborns
1995, Molecular Brain ResearchThe biochemical and cellular mechanisms involved in the development and/or maintenance of morphine tolerance remain unclear. In the adult central nervous system (CNS) results are contradictory. For the neonate, a variety of drug induced deficits have been observed following prenatal addiction to opioids, although very little work on the biochemical and molecular level has been done. Therefore, the present study was carried out to investigate the effects of prenatal morphine treatment on the levels and expression of endogenous opioid peptides in brain regions of newborns. Dams were implanted with one morphine pellet (75 mg each) 1 week prior to the birth of pups. Changes in mRNA levels for the opioid peptides were determined by Northern blot analysis. Alterations in opioid peptide levels were determined by radioimmunoassays. Prenatal morphine treatment significantly increased proenkephalin mRNA levels and decreased met-enkephalin levels in striatum of newborns. These data are in contrast to what is observed in the adult CNS. These data indicate that prenatal morphine treatment may increase met-enkephalin release and/or cause inhibition at the level of translation. In addition, increased transcription may be necessary to maintain equilibrium in the system when there is an increase in met-enkephalin release.
Chronic prenatal methadone exposure alters central opioid μ-receptor affinity in both fetal and maternal brain
1992, Neurotoxicology and TeratologyThe effects of chronic prenatal methadone exposure (6.3–9.0 mg/kg/day) via osmotic minipumps to pregnant dams on fetal and maternal brain opioid μ-receptors were assessed on gestation day 20 and day 7 postnatally. By using the 3H-DAMGO binding assay, it was shown that chronic methadone treatment (gestation days 7–20) did not affect μ-receptor capacity in both fetal and maternal brains during gestation day 20, nor when tested 7 days after delivery. However, this chronic exposure decreased μ-receptor affinity in both fetal and maternal brain homogenates when determined on day 20 of pregnancy. Scatchard analysis of binding data in both tissues indicated that the methadone-induced increase in KD returned to control values when tested 7 days after delivery. The change in μ-receptor affinity was not due to competition between 3H-DAMGO and residual methadone. Extensive washing of the brain homogenates failed to alter the affinity of the receptor but decreased the concentration of the residual methadone. This decrease in receptor affinity was also observed in extensively washed brain tissue from female adult rats treated acutely with methadone (9.0 mg/kg, IP) or when brain homogenates were exposed to methadone (50 ng/ml) in vitro. Thus, these data suggest that methadone alters μ-receptor affinity by some unknown mechanism.
Simultaneous development of opioid tolerance and opioid antagonist-induced receptor upregulation
1990, Brain ResearchMice treated chronically with opioid antagonists have increased receptor density in brain and are supersensitive to the pharmacodynamic action of morphine. In the present study mice were implanted subcutaneously with naltrexone or placebo pellets for 8 days. During implantation mice received daily injections of morphine (100 or 250 mg/kg) or saline. Morphine analgesia was completely blocked in mice that were implanted with naltrexone at the low dose of morphine; while some analgesic action was observed at the higher dose. Mice implanted with placebo were analgesic following the daily morphine treatment. At the end of 8 days the pellets were removed and 24 h later some mice were tested for morphine analgesia while others were examined in binding studies. Naltrexone treatment increased [3H]naloxone,3H[d-Ala2-d-Leu5]enkephalin (DADLE) and3H[d-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) binding compared to controls and increased the analgesic potency of morphine. Daily treatment with morphine did not alter brain opioid binding or naltrexone-induced receptor upregulation. Mice injected daily with morphine were significantly less sensitive to morphine (tolerant) than their respective saline control group for both the placebo and the naltrexone-treated groups. However, naltrexone-treated mice were more sensitive to morphine than placebo controls regardless of whether they were injected daily with morphine or not. These results indicate that if naltrexone-induced opioid receptor upregulation occurs in the presence of repeated agonist administration, the new binding sites mediate tolerance via desensitization to morphine.
Opioid receptor subtypes in the supraoptic nucleus and posterior pituitary gland of morphine-tolerant rats
1990, NeuroscienceMorphine, given acutely, inhibits oxytocin secretion in adult female rats, but chronic intracerebroventricular infusion for five to six days induces tolerance and dependence in the mechanisms regulating oxytocin secretion. One explanation for tolerance could be that there is a loss of opioid receptors.
To test this hypothesis cryostat sections of selected brain regions and the pituitary, from six control and six intracerebroventricular morphine-infused rats, were processed for quantitative in vitro receptor autoradiography. [3H]Etorphine or [3H](−)-bremazocine were used as ligands, and DAGO, DPDPE and U50,488H as selective displacers from μ-, δ-, and κ-receptors, respectively. Control incubations had naloxone determined specificity. The supraoptic nucleus (site of oxytocin-secreting magnocellular perikarya) contained both μ- and κ-receptors in control rats (mean ± S.E.M. binding of μ-selective [3H]etorphine was 91.8 ± 25.4 fmol/mg of tissue, and of κ-selective [3H](−)-bremazocine was 130.4 ± 25.6 fmol/mg). Chronic morphine treatment caused a 83.9% decrease in binding in μ-selective conditions (P < 0.05), but no significant change in κ-selective binding. In the median preoptic nucleus (which projects to the supraoptic nucleus) mean ± S.E.M. binding of [3H]etorphine decreased by 77.0% (P < 0.01) in chronic morphine-treated rats, from the control value of 76.2 ± 9.8 fmol/mg of tissue. In the posterior pituitary gland (site of the terminals of the oxytocin-secreting magnocellular perikarya) binding with [3H](−)-bremazocine in controls was over 90% lower than in the supraoptic nucleus. No changes followed chronic morphine treatment.
Thus chronic morphine exposure reduces the numbers of available μ-receptors in the supraoptic nucleus, and of opioid receptors in the median preoptic nucleus, perhaps accounting for morphine-tolerance in relation to oxytocin secretion.
Differential down-regulation of delta opioid binding sites during physical dependence on methionine enkephalin in the rat
1989, Life SciencesPost-synaptic receptor modulation is thought to be one important mechanism involved in the adaptation of a neuronal system during chronic exposure to a drug. However, initial studies of opioid receptor regulation following chronic in vivo administration of narcotic agonists, such as morphine, reported no down-regulation in the number of opioid receptors in the brain. Subsequent studies, employing in vitro preparations, have reported evidence of opioid receptor down-regulation under specific conditions. It remains to be determined whether the in vitro phenomena of opioid receptor plasticity is relevant to the intact mammalian central nervous system. The data in this report shows that chronic in vivo administration the opioid peptide methionine enkephalin, results in a significant, regionally specific down-regulation of delta opioid receptors in rat brain: 30% decrease in receptor density in the striatum; no change in hypothalamus.
Morphine-induced downregulation of μ-opioid receptors in neonatal rat brain
1988, Developmental Brain ResearchChronic administration of morphine to pre- and postnatal rats produced a marked decrease in brain μ-opioid receptor density without a change in receptor affinity. No significant changes in δ- or κ-receptors were observed. This downregulation was accompanied by tolerance to the analgesic actions of morphine. In neonates exposed to morphine from postnatal day one, μ-receptor number was significantly depressed until postnatal day 8, then increased gradually to control levels by day 14 of treatment. Longer treatment produced no further change in opioid receptors. These data represent the first demonstration of in vivo downregulation of brain μ-opioid receptors following morphine administration and provide evidence for a unique plasticity of the immature opioid receptor system.