In vivo down-regulation of rat striatal opioid receptors by chronic enkephalin

Hypoxia decreases opioid delta receptor expression in mouse brain

Delta opioid receptor activation is protective during hypoxic injury. Many adaptive responses occur during exposure to hypoxia to facilitate survival. It is possible that increased activity of the delta opioid receptor system is one such adaptation. We tested the hypothesis that mice exposed to prolonged hypoxia have increased expression of the delta opioid receptor in brain tissue. Prolonged exposure to hypoxia (9% oxygen, balance nitrogen) continuously for seven days selectively decreased delta opioid receptor expression in mouse brain homogenate. The same hypoxic treatment had no effects on either mu or kappa opioid receptor expression, indicating that this response was not due to non-selective degradation of protein. Shorter term hypoxic treatments (one day and three days) did not induce changes in delta opioid receptor expression in whole brain homogenate. Binding assays were also conducted in grossly dissected brain regions (cortex, midbrain, hindbrain) to determine whether the shorter term treatments would induce changes in receptor expression in more discrete areas. No consistent changes in delta opioid receptor expression were detected in these brain regions. These data demonstrate that opioid delta receptors are hypoxia sensitive and may be a part of an adaptive process to increase survival in the organism. One possible cause for the decrease in delta opioid receptor expression following seven days of hypoxic exposure may be receptor down-regulation caused by an increased release of endogenous substances acting at delta receptors. As delta opioid receptor agonists appear promising for therapeutic potential in management of hypoxic injury, changes in delta receptor expression in response to long-term hypoxia could impact potential utilization of delta agonists in patients suffering chronic hypoxia.

Prenatal morphine exposure differentially alters expression of opioid peptides in striatum of newborns

The 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 morphine treatment decreases G alpha s mRNA levels in neonatal frontal cortex

G alpha s mRNA levels were measured in brain regions of newborn pups following prenatal morphine treatment. A significant decrease (24%) in G alpha s mRNA levels was observed in the frontal cortex. No changes were observed in other regions. This report demonstrates the first in vivo study of opiate effects on G-protein gene expression in neonates. The development of tolerance in vivo may involve complex interactions between several neurotransmitter systems having opposing actions on the G-protein system.

Reduction of hibernation bout duration by intraventricular infusion of met-enkephalin

The potential of brain met-enkephalin (met-enk) systems to modulate central nervous system (CNS) activity during periods of general depression (modeled by the mammalian hibernation state) was studied in the ground squirrel (Citellus lateralis). Following entrance into hibernation, continuous met-enk infusion into the lateral ventricle (1 microliter/h; 0.2, 1 and 5 micrograms/microliter) produced a dose-dependent reduction in bout duration ranging from 1.2 to 3.9 days (13.8-44.6% of baseline bout duration). We suggest that the activity of met-enk-releasing neurons may serve to increase the excitability of the depressed CNS, thus accelerating the termination of the hibernation bout.

Enhanced development of morphine tolerance in rats treated with 2-deoxy-D-galactose

Rats treated subcutaneously for 6 days with morphine developed a weak tolerance which was characterized by a decrease in the analgesic action of the opioid. Under those experimental conditions a simultaneous intracerebroventricular application of 2-deoxy-D-galactose enhanced development of morphine tolerance, while other deoxy-sugars like 2-deoxy-D-glucose and 6-deoxy-D-galactose were ineffective. In contrast, no influence of 2-deoxy-D-galactose on a more enhanced morphine tolerance after a 11-day pretreatment with morphine was found. The results are discussed in the light of a rather specific interference of 2-deoxy-D-galactose with neuronal glycoprotein processing and related cellular mechanism underlying adaptive processes involved in the development of morphine tolerance.

Is intracellular sodium involved in the mechanism of tolerance to opioid drugs?

Recent studies indicate that one of the more likely mechanisms of opioid tolerance could involve a decrease in the efficiency with which agonists can induce coupling of their specific binding sites in neuronal membranes to the activation (or deactivation) of an effector system. Reports of sodium-induced decreases in opioid receptor agonist binding and in the size of ligand/receptor complexes, as well as modulation of opioid activity by manipulation of sodium in vivo, indicate that sodium might play a physiological role in modulating opioid receptor function. Reports of morphine-induced systemic sodium retention in animals, as well as morphine-induced increases in brain intracellular sodium and decreases in brain Na+, K(+)-ATPase activity, indicate that the development of tolerance may be accompanied by changes in the disposition of sodium. The direction of these sodium- and morphine-induced changes is consistent with the hypothesis that an increase in intracellular sodium could participate in the mechanism(s) of opioid tolerance development.

Effects of acute and chronic administration of Leu-enkephalin on cultured serotonergic neurons: evidence for opioids as inhibitory neuronal growth factors

Leu-enkephalin, at concentrations between 18 microM and 1.8 pM, was administered in a single or daily dose to dissociated mesencephalic raphe cell cultures maintained for 3 or 5 days. Daily administration of Leu-enkephalin produced an inhibition of high affinity uptake of [3H]5-HT, a measure of serotonergic process outgrowth in cultures of fetal neurons. This inhibition was maximal at a dose of 18 nM in both 3 (59%, P less than 0.05)- and 5 (38%, P less than 0.05)-day cultures. The expression of uptake was consistently lower in 5-day cultures than in 3-day cultures at all concentrations tested. In marked contrast, a single dose of Leu-enkephalin at the time of plating stimulated uptake in 3- and 5-day cultures. Maximal stimulation was observed at 180 nM for both 3 (191%, P less than 0.05)- and 5 (140%, P less than 0.05)-day cultures. The results obtained after a single dose of the opioid may reflect a paradoxical stimulation probably due to a rebound mechanism of receptors since co-administration of bacitracin (0.5 mg/ml), an aminopeptidase inhibitor, resulted in inhibition of the uptake expression. Together these results indicate that Leu-enkephalin can function as an inhibitory regulatory growth factor for neuronal cultures when constant exposure to this opioid is maintained over time.

Differential down-regulation of delta opioid binding sites during physical dependence on methionine enkephalin in the rat

Post-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 mu-opioid receptors in neonatal rat brain

Chronic administration of morphine to pre- and postnatal rats produced a marked decrease in brain mu-opioid receptor density with-out change in receptor affinity. No significant changes in delta- or kappa-receptors were observed. This downregulation was accompanied by tolerance to the analgesic actions of morphine. In neonates exposed to morphine from postnatal day one, mu-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 mu-opioid receptors following morphine administration and provide evidence for a unique plasticity of the immature opioid receptor system.

Neutral endopeptidase-24.11, mu and delta opioid receptors after selective brain lesions: an autoradiographic study

The cellular localization of the rat brain neutral endopeptidase (NEP, EC 3.4.24.11) was investigated by quantitative autoradiography of the enzyme inhibitor [3H]N-[(2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl]glycine ([3H]HACBO-Gly) after lesions of the striatum, nigrostriatal and corticostriatal pathways. The effect of these lesions on NEP levels was compared with that on delta and mu opioid receptors, selectively labeled with [3H]Tyr-D-Thr-Gly-Leu-Thr ([3H]DTLET) and [3H]Tyr-D-Ala-Gly-MePhe-Glycinol ([3H]DAGO), respectively. Twenty-one days after injection of kainate in the caudate putamen (CP), the NEP level was locally decreased (52%) but the time course of this decrease was different from that of mu and delta opioid receptors: [3H]DAGO binding was diminished by 40% from day 2 whereas that of [3H]DTLET was reduced by 51% from day 7. Kainic acid injection in the CP induced in the globus pallidus (GP) and substantia nigra (SN) a distant reduction of the 3 opioid markers. Likewise after injection of colchicine in the CP, [3H]HACBO-Gly binding was decreased in the GP (60%) and SN (58%), [3H]DTLET binding was reduced by 54 and 55% in the GP and SN, respectively and [3H]DAGO labeling was diminished by 49% in the GP, and 58% in the SN. Finally, lesion of the nigrostriatal dopamine pathway by 6-hydroxydopamine did not induce any change of NEP level in the CP and GP whereas delta and mu opioid receptor levels were diminished respectively by 25 and 29% in the CP, and 45 and 39% in the GP, a new finding of the present study. Taken together these data suggest that NEP is in part associated with striatal intrinsic neurons. In the GP and SN, a large part of NEP seems to be presynaptically associated with nerve terminals endowed with mu and delta opioid receptors, which originate from efferent striatal neurons. In contrast to opioid receptors in the CP, the NEP appears not to be associated with dopaminergic nerve terminals originating from the SN. Cortical ablation did not affect any of the opioid markers.

Temporal changes in the activity of endogenous opioid mechanisms regulating oxytocin secretion in saline-loaded rats

The effect of the opiate antagonist naloxone on neurohypophyseal hormone secretion has been investigated in rats given 2% NaCl to drink for 0-9 days. Naloxone elicited a rise in plasma levels of oxytocin (OT), but not arginine-vasopressin (AVP), on days 1-3 only, the greatest response being on day 2. In a further experiment, the OT response to an acute stimulus (1.5 M NaCl, i.p.) was found to be similar in controls and rats drinking 2% NaCl for 7 days, indicating that depletion of readily releasable hormone was not the cause of the loss of sensitivity to naloxone; rather a down-regulation of opiate receptor mechanisms may be responsible. The temporal changes in opiate antagonist-sensitivity in saline-loaded rats may provide a functional paradigm for the investigation of regulatory opiate mechanisms in neural tissue.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.