Morphine-induced opioid receptor down-regulation detected in intact adult rat brain cells

[Evaluation of spinal anesthesia blockade time with 0.5% hyperbaric bupivacaine, with or without sufentanil, in chronic opioid users: a randomized clinical trial]

OBJECTIVE

The primary outcome of this study was to evaluate the effect of adding sufentanil to hyperbaric bupivacaine on duration of sensory blockade of spinal anesthesia in chronic opioid users in comparison with non-addicts.

METHODS

Sixty patients scheduled for orthopedic surgery under spinal anesthesia were allocated into four groups: group 1 (no history of opium use who received intrathecal hyperbaric bupivacaine along with 1mL saline as placebo); group 2 (no history of opium use who received intrathecal bupivacaine along with 1mL sufentanil [5μg]); group 3 (positive history of opium use who received intrathecal bupivacaine along with 1mL saline as placebo) and group 4 (positive history of opium use who received intrathecal bupivacaine along with 1mL sufentanil [5μg]). The onset time and duration of sensory and motor blockade were measured.

RESULTS

The duration of sensory blockade in group 3 was 120±23.1min which was significantly less than other groups (G1=148±28.7, G2=144±26.4, G4=139±24.7, p=0.007). The duration of motor blockade in group 3 was 145±30.0min which was significantly less than other groups (G1=164±36.0, G2=174±26.8, G4=174±24.9, p=0.03).

CONCLUSIONS

Addition of 5μg intrathecal sufentanil to hyperbaric bupivacaine in chronic opioid users lengthened the sensory and motor duration of blockade to be equivalent to blockade measured in non-addicts.

Evaluation of spinal anesthesia blockade time with 0.5% hyperbaric bupivacaine, with or without sufentanil, in chronic opioid users: a randomized clinical trial

OBJECTIVE

The primary outcome of this study was to evaluate the effect of adding sufentanil to hyperbaric bupivacaine on duration of sensory blockade of spinal anesthesia in chronic opioid users in comparison with non-addicts.

METHODS

Sixty patients scheduled for orthopedic surgery under spinal anesthesia were allocated into four groups: group 1 (no history of opium use who received intrathecal hyperbaric bupivacaine along with 1mL saline as placebo); group 2 (no history of opium use who received intrathecal bupivacaine along with 1mL sufentanil [5μg]); group 3 (positive history of opium use who received intrathecal bupivacaine along with 1mL saline as placebo) and group 4 (positive history of opium use who received intrathecal bupivacaine along with 1mL sufentanil [5μg]). The onset time and duration of sensory and motor blockade were measured.

RESULTS

The duration of sensory blockade in group 3 was 120±23.1min which was significantly less than other groups (G1=148±28.7, G2=144±26.4, G4=139±24.7, p=0.007). The duration of motor blockade in group 3 was 145±30.0min which was significantly less than other groups (G1=164±36.0, G2=174±26.8, G4=174±24.9, p=0.03).

CONCLUSIONS

Addition of 5μg intrathecal sufentanil to hyperbaric bupivacaine in chronic opioid users lengthened the sensory and motor duration of blockade to be equivalent to blockade measured in non-addicts.

The Effect of Adding Sufentanil to 0.5% Hyperbaric Bupivacaine on Duration of Brachial Plexus Blockade in Chronic Opium Abusers: a Randomized Clinical Trial

Background:

Anesthesia induction in patients with current substance abuse can be a challenge for anesthesiologists.

Objectives:

This study aimed to evaluate the effect of adding Sufentanil to Bupivacaine on duration of brachial plexus nerve block.

Patients and Methods:

One hundred and twenty patients with (Groups C and D) and without (Groups A and B) a history of opium abuse (60 in each group) scheduled for elective upper extremity procedures were randomly assigned to either receive 30 mL bupivacaine alone (Groups A and C) or in combination with additional 10 µg sufentanil (Groups B and D). An ultrasound-guided technique was applied to perform upper extremity brachial plexus blockade. The onset and duration of sensory and motor blocks were recorded and compared between the four groups.

Results:

The duration of sensory and motor block were significantly less in Group C (537.0 ± 40.1 minutes, 479.0 ± 34.8 minutes) and the longest duration of sensory and motor block was observed in group B (705.0 ± 43.8 minutes, 640.0 ± 32.5 minutes). The duration of sensory and motor block in Group B (705.0 ± 43.8 minutes, 640.0 ± 32.5 minutes) was longer and statistically higher than group A (619.5 ± 48.0 minutes, 573.2 ± 31.5 minutes), the same trend was observed in group D (598.6 ± 53.2 minutes, 569.3 ± 39.9 minutes) over group C (537.0 ± 40.1 minutes, 479.0 ± 34.8 minutes) (P < 0.001, one-way ANOVA).

Conclusions:

The length of sensory and motor blockade is shorter in chronic opioid abusers. Adding 10 µg sufentanil to hyperbaric bupivacaine in opium abusers lengthened the sensory and motor block duration.

Perioperative management of the opioid tolerant patient for orthopedic surgery

The prevalence of opioid use in the North America and some countries of the European Union has resulted in an increase in the number of patients who may exhibit opioid tolerance when requiring postoperative pain management. The approach to postoperative pain control in these patients is different from the strategies used in opioid-naïve patients. Better understanding of the cellular mechanisms of opioid tolerance in animals has resulted in the transfer of these concepts from the basic research to the clinical arena. This article presents new developments in opioid tolerance and how this knowledge can be applied to clinical practice.

A review of intrathecal morphine therapy related granulomas

The development of catheter associated granulomatous masses in intrathecal morphine therapy is an uncommon, but potentially serious problem. While these systems have historically been used in patients with short life expectancies, more recently patients with pain from a benign source have benefited from this therapy, and new complications are being encountered secondary to the patients' longer life spans. Morphine is the most commonly used intrathecal opioid and evidence exists that the formation of granulomatous masses are related to the use of higher doses. When the patients' requirement of morphine increases significantly, the physician should be alert for signs of spinal cord compression, such as new neurological deficits, myelopathy, or radiculopathy. Patients that require these higher doses should be properly informed of the association with granulomas and their associated risks. Indolent infection may also be the etiology of granulomatous masses, and the presence of organisms, both aerobic and anaerobic, should be routinely investigated. Patients with catheter-associated granulomas appear to share several features. They exhibit the onset of symptoms several months following the initiation of intraspinal opioids and commonly present with an increase in pain that precedes signs and symptoms of neurological deterioration. While MRI might be the preferred method of detection of intrathecal granulomas, its cost and availability are prohibitive for routine screening. CT myelogram via pump side port injection of contrast can also be performed to detect catheter tip related granulomas/obstructions. Serial neurological examinations for new deficits may be performed and recorded during pump refill visits to recognize a granulomatous mass in its early stages. If an abnormality is identified, imaging studies are appropriate. Awareness of the condition and vigilance are the keys to successful management of this complication.

Effects of Opioid and α2-Adrenoceptor Agonists on the Isolated Ileum of Morphine-dependent Guinea-pigs During Withdrawal and After Clonidine Treatment

The present study was undertaken to investigate the effect of clonidine administration to opiate-dependent guinea-pigs after morphine withdrawal on subsequent twitch responses of the longitudinal muscle-myenteric plexus preparations to electrical field stimulation. The results indicate that clonidine, administered immediately after morphine removal, causes tolerance to the inhibition exerted by opioid and α2-adrenoceptor agonists on the electrically-evoked twitches. Such a finding suggests that the mechanism of action of clonidine involves not only its well-known effects on locus coeruleus neurons but also that it has specific actions on the myenteric plexus. This work shows the existence of interactions between opioid and α-adrenoceptor on the cholinergic neurons present in the isolated ileum.

Sensitivity to the effects of a kappa opioid in rats with free access to exercise wheels: differential effects across behavioral measures

It is well established that chronic exercise decreases sensitivity to mu opioid agonists; however, it is less clear what effects it has on kappa opioids. The purpose of the present study was to examine sensitivity to the effects of the selective, kappa opioid spiradoline in rats with free access to exercise wheels. Rats were obtained at weaning and randomly assigned to either standard polycarbonate cages (sedentary) or modified cages equipped with exercise wheels (exercise). After approximately 7 weeks under these conditions, sensitivity to the effects of spiradoline on tests of antinociception, locomotor activity, conditioned place preference, and diuresis were examined in both groups of rats. Sedentary rats were more sensitive than exercising rats to the antinociceptive effects of spiradoline, and this effect was observed at both low and high nociceptive intensities. In contrast, exercising rats were more sensitive than sedentary rats to the diuretic effects of spiradoline, and slightly more sensitive to spiradoline's effects in the conditioned place preference procedure. No differences in sensitivity were observed to the effects of spiradoline on locomotor activity. Sensitivity to the antinociceptive effects of spiradoline nonsignificantly increased in exercising rats that were reassigned to sedentary housing conditions, and changes in spiradoline sensitivity were correlated with exercise output in individual subjects. Collectively, these data suggest that exercise alters sensitivity to the behavioral effects of kappa opioids, but that the direction and magnitude of this effect depends on the behavioral measure examined.

Long-term intrathecal opioid therapy with a patient-activated, implanted delivery system for the treatment of refractory cancer pain

The present study evaluated the safety and efficacy of patient-activated delivery of intrathecal morphine sulfate boluses delivered by way of a novel internalized intrathecal delivery system. Patients with refractory cancer pain or uncontrollable side effects were enrolled at 17 US and international sites in this prospective, open-label study. Pain relief, reduction in systemic opioid use, and reduction in opioid-related complications were analyzed both individually and together as a measure of overall success. One hundred forty-nine patients were enrolled and 119 were implanted. Average numeric analog scale pain decreased from 6.1 to 4.2 at 1 month and was maintained through month 7 (P <.01) and through month 13 (P <.05). Systemic opioid use was significantly decreased throughout the study (P <.01). Significant reduction in the opioid complication severity index was demonstrated at all 4 follow-up visits (P <.01). Overall success (>/=50% reduction in numeric analog scale pain, use of systemic opioids, or opioid complication severity index) was reported in 83%, 90%, 85%, and 91% of patients at months 1, 2, 3, and 4, respectively. This study demonstrated that patients with refractory cancer pain or intolerable side effects achieved better analgesia when managed with patient-activated intrathecal delivery of morphine sulfate via an implanted delivery system.

Cellular mechanisms of opioid tolerance and the clinical approach to the opioid tolerant patient in the post-operative period

The high prevalence of opioid use for recreational purposes in the USA and the European Union, as well as the use of opioids for the treatment of chronic non-malignant pain, has resulted in an increase in the number of patients with opioid tolerance who undergo surgery and require post-operative pain management. The approach to post-operative pain control in these patients is significantly different to the strategies used in opioid naïve patients. Fortunately, better understanding of the cellular mechanisms of opioid tolerance in animals has resulted in the transfer of concepts from the 'bench' to the clinical arena. This chapter describes the new developments in opioid tolerance and how this knowledge can be applied to clinical practice.

Morphine tolerance in spinal cord is due to interaction between mu- and delta-receptors

When the opioid agonist morphine is given chronically and systemically to mice by pellet implantation for 3 days, the animals develop substantial tolerance to the antinociceptive effect of a test dose of morphine given systemically. When the test dose is administered to the spinal cord, however, very little tolerance is observed. We tested six strains of mice differing in the degree to which they develop systemic tolerance to morphine and found that none of them developed significant tolerance to spinal morphine. However, most of these strains did develop substantial spinal tolerance to antinociception induced by the selective mu-agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) and by the selective delta-agonist [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE). Moreover, in naïve animals, the antinociceptive effect of both DAMGO and DPDPE was blocked by D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2), a selective mu-antagonist, indicating that both agonists mediate antinociception in the spinal cord through mu-receptors. In addition to directly mediating antinociception, however, DPDPE potentiated the antinociceptive activity of DAMGO in the spinal cord of naïve animals, and this antinociception was blocked by the delta-antagonist H-TyrTicPsi[CH(2)NH]Phe-Thr-OH (TIPPpsi), indicating mediation through delta-receptors. In contrast, in tolerant animals, TIPPpsi enhanced the antinociception of DAMGO. These results thus demonstrate not only that mu- and delta-opioid receptors interact in naïve animals, but that the nature of this interaction changes during tolerance, from a potentiation to an inhibition. The lack of tolerance to spinal morphine may result from the ability of morphine to act as a partial antagonist at delta-receptors.

Pathophysiology of opioid tolerance and clinical approach to the opioid-tolerant patient

The physiologic basis for opioid tolerance has been elucidated, resulting in a better understanding of this problem. As a result of this ongoing effort, patients exhibiting opioid tolerance can now expect better pain management both in the postoperative period and during the course of their treatment. This article outlines the pathophysiology of opioid tolerance and a practical clinical approach to this problem.

Morphine tolerance in mice is independent of polymorphisms in opioid receptor sequences

Pharmacogenomics links individual drug response variation to genetic differences, such as single nucleotide polymorphisms (SNPs). In particular, pharmacogenomics will allow clinicians to use genetic diagnostics to predict the response of a patient to a drug. We investigated whether SNPs in opioid receptors correlated with the development of morphine tolerance in mouse strains that showed either high or low tolerance to morphine. Sequencing identified five silent SNPs in the delta opioid receptor that varied from the published sequence in some strains, but which were found in both high and low tolerance strains. The mu and kappa opioid receptor sequences had no SNPs. Taken together, these data definitively demonstrate that morphine tolerance development in mice is independent of opioid receptor sequence.

Numerical density of mu opioid receptor expressing neurons in the frontal cortex of drug related fatalities

In animal and cell culture experiments, chronic morphine treatment has been followed by 'up'- as well as 'down-regulation' of the mu opioid receptor (mu OR) number. The present postmortem morphometric study of morphine-related fatalities of drug addicts (n=12, and 22-35 years old, with blood unconjugated morphine levels from 27.1 to 458 ng/ml, m.v. 198.5 ng/ml) versus a non-addicted control group (n=13 and 10-44 years old) was intended to examine whether chronic opiate exposure affects the numerical density of mu OR expressing neurons in the human neocortex (area 10 according to Brodmann). For the immunohistochemical procedure, thick (100 microm) vibratome sections were incubated with a monoclonal antibody against the mu OR [Arvidsson et al., J. Neurosci. 15 (1995) 3328] and immunoreactive sites were visualized using an immunoperoxidase protocol. The numerical densities of mu OR-expressing and Nissl-stained neurons were assessed morphometrically (camera lucida-drawings). In both collectives, the anti-mu OR immunoreactivity was mainly found in pyramidal neurons of layers (L) II/III and V and in multiform neurons of L VI. In the drug-related fatalities and the control group, the density of neurons expressing mu OR protein was similar, amounting for 2698 +/- 153 and 2688 +/- 172/mm(3), respectively. These findings extend the binding studies of opioid ligands in postmortem brains of heroin addicts [Gabilondo et al., Psychopharmacology 115 (1994) 135] revealing similar receptor densities and affinities by showing no difference in the density of mu OR-positive neurons.

Chronic morphine treatment and withdrawal increase extracellular levels of norepinephrine in the rat bed nucleus of the stria terminalis

Extracellular levels of norepinephrine (NE) and glutamate (Glu) in the ventral bed nucleus of the stria terminalis (vBNST) of saline- and chronic morphine-treated rats, with or without withdrawal, were studied by means of the in vivo microdialysis technique in anesthetized rats. In addition, the tissue concentration of NE was studied at different rostrocaudal levels of the vBNST. Chronic morphine treatment significantly increased extracellular levels of NE, but not Glu, in vBNST. At 48 h after naloxone-induced morphine withdrawal there was a further significant increase in the extracellular levels of NE, but not Glu, in vBNST. The presence of UK 14304, an alpha(2)-adrenergic agonist, induced a significant decrease in NE extracellular levels in all experimental groups. In contrast, UK 14304 induced a significant decrease in Glu extracellular levels only in saline-treated rats. The results also show that the vBNST presents a rostrocaudal gradient of NE and contains 9.4% of total brain NE. The increase in NE extracellular levels in vBNST induced by chronic morphine treatment and the further increase in NE levels 48 h after naloxone-induced morphine withdrawal suggest that NE in vBNST may be involved in the pharmacological effects of chronic morphine and withdrawal.

Influence of continuous levels of fentanyl in rats on the mu-opioid receptor in the central nervous system

The highly potent and efficacious mu-opioid agonist fentanyl was SC infused into rats with submaximal analgesic doses (0-1.14 mumol/kg/day) continuously for 8 days, checked by the constant daily urinary recovery of intact drug (0.43 +/- 0.031% of the daily dose). Tail-flick latencies measured at 24 (day 1) and 48 h (day 2) after starting the infusion were increased in a dose-dependent fashion compared with those before the infusion (day 0). However, at day 8, the latencies were increased only weakly, not significantly, revealing tolerance to the antinociceptive activity of fentanyl. Fentanyl at all doses showed no significant effect on the capacity (Bmax) and affinity (Kd) of the mu-opioid receptor binding of DAMGO to whole brain (Bmax 126.2 +/- 3.00 fmol/mg protein, Kd 1.00 +/- 0.04 nM) and spinal cord (Bmax 48.24 +/- 2.71 fmol/mg protein, Kd 1.93 +/- 0.13 nM) membranes gained from the rats after killing them at day 8. Gpp(NH)p increased the Kd for brain and spinal cord sites by 3.09 and 2.65, respectively, independent of the fentanyl dose. The infusion with fentanyl did not after the basal and forskolin-stimulated adenylate cyclase activity in the whole brain membranes, nor did it change the inhibition of the forskolin-stimulated activity by DAMGO. It is concluded that, in rats, constant long-term body levels of highly potent mu-agonists result in a tolerant state that, however, does not produce overall changes in the parameters of their specific receptor sites in the CNS, i.e., receptor capacity and affinity, and in the events closely related to them, i.e., their regulation by GTP and of adenylate cyclase. This does not exclude such possible changes to be restricted to specific regions in the CNS.

Chronic morphine and naltrexone fail to modify mu-opioid receptor mRNA levels in the rat brain

Previous radioligand-binding studies have reported conflicting results concerning the effect of chronic morphine administration on the regulation of mu-opioid receptor (MOR) density. On the other hand, chronic administration of an opioid antagonist, such as naltrexone, has been shown to increase the density of the MOR. In order to determine if the changes in the MOR are associated with alterations in receptor mRNA levels, we investigated MOR gene expression following chronic treatment with morphine and/or naltrexone. MOR mRNA levels, determined by the ribonuclease protection assay (RPA), were unchanged with respect to control during chronic morphine treatment and morphine withdrawal in each of the analysed brain areas. Furthermore, chronic administration of naltrexone did not result in changes of MOR mRNA levels in rat striatum of naive and morphine-dependent rats, suggesting that the up-regulation of the MOR density, at least in this tissue, is not regulated at transcriptional level.

Postoperative epidural opioid analgesia: what are the choices?

The administration of hydrophilic opioids via a continuous infusion results in selective spinal analgesia with a low incidence of side effects. Lipophilic opioids may also be associated with spinal effects. However, the doses required to produce postoperative analgesia also produce plasma concentrations within the MEAC. Thus, in clinical practice it may not be possible to limit epidural doses of lipophilic opioids to those associated with spinal analgesia. Regardless of the mechanism of action, epidural administration of lipophilic opioids may offer no clinical advantages over the IV route. Notwithstanding, epidural administration of small doses of lipophilic opioids in combination with local anesthetics may offer significant clinical advantages over systemic administration of opioids alone. Dose-ranging studies will be necessary to determine the ideal concentrations of opioids and local anesthetics, as well as the ratios of the two drugs to obtain optimal analgesia with minimal incidence of side effects.

Downregulation of mu-opioid binding sites following chronic administration of neuropeptide FF (NPFF) and morphine

The effect of continuous ICV infusion of NPFF (10 micrograms/microliter) and morphine (40 micrograms/microliter) on mu-opioid binding sites was examined in rats using the in vitro radiolabeled techniques of whole-brain homogenate receptor binding and quantitative autoradiography. Mu receptors were labeled with [3H][D-Ala2-MePhe4,Glyol5] enkephalin in the homogenate binding experiments and with [125I][D-Ala2-MePhe4,Gly-ol5]enkephalin in autoradiographic studies. In homogenate binding studies, chronic administration of NPFF or morphine significantly downregulated mu receptors by 20% and 44%, respectively. Quantitative autoradiographic experiments demonstrated downregulation of mu opioid receptors in specific brain nuclei for both NPFF- and morphine-treated animals. Within the striatum and several nuclei of the thalamus, the mu receptors of the NPFF- and morphine-treated animals were decreased by 20-30% and 38-73%, respectively. These results suggest that NPFF may modulate opioid-mediated responses in part by altering the density of mu-opioid receptors.

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 microM. The morphine-induced release of NPFF was abolished by naloxone (1 microM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c..) 25 min following that of heroin (2.5 mg/kg, s.c.) not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.

Production of antinociception by peripheral administration of [Lys7]dermorphin, a naturally occurring peptide with high affinity for mu-opioid receptors

1. The opioid activity of the amphibian peptide, [Lys7]dermorphin, was studied in rats and mice. When administered intracerebroventricularly (i.c.v.), intravenously (i.v.) or subcutaneously (s.c.) it produced a long lasting analgesia. Its antinociceptive potency exceeded that of morphine 290 times by i.c.v. injection, and 25-30 times by peripheral administration. 2. The dose-response curves of [Lys7]dermorphin antinociception were shifted to the right by the pretreatment with naloxone (0.1 mg kg-1, s.c.) or with the mu 1-selective antagonist, naloxonazine (10 mg kg-1, i.v. 24 h before peptide injection). 3. The peptide also displayed potent antinociceptive effects in a chronic inflammatory pain model (rat Freund's adjuvant arthritis). In this pain model, systemic administration of the peptide raised the nociceptive threshold more in inflamed than in healthy paw. 4. High central and peripheral doses of [Lys7]dermorphin in rats produced catalepsy. The cataleptic response was antagonized by naloxone but left unchanged by naloxonazine pretreatment. 5. In rats and mice, central or peripheral administration of [Lys7]dermorphin induced a significantly slower development of tolerance to the antinociceptive effect than did morphine. 6. Upon naloxone precipitation of the withdrawal syndrome, [Lys7]dermorphin-dependent mice made fewer jumps and lost less weight than the morphine-dependent animals. Withdrawal hyperalgesia did not develop in [Lys7]dermorphin-dependent mice. 7. In conclusion, [Lys7]dermorphin seems to be a unique opioid peptide having a high penetration into the blood-brain barrier despite its low lipid solubility. This peptide causes fewer side-effects than other opioids and appears less likely than morphine to cause physical dependence in rats and mice.

mu-Opioid receptor and alpha 2-adrenoceptor agonist binding sites in the postmortem brain of heroin addicts

The biochemical status of human brain mu-opioid receptors and alpha 2-adrenoceptors during opiate dependence was studied by means of the binding of [3H] [D-Ala2, MePhe4, Gly-ol5] enkephalin (DAGO) and [3H]clonidine, respectively, in postmortem brains of heroin addicts who had died by opiate overdose or other causes. In the frontal cortex, thalamus and caudate of heroin addicts the density (Bmax) and affinity (KD) of mu-opioid receptors were similar to those in controls. In contrast, the density of alpha 2-adrenoceptors in heroin addicts was found to be significantly decreased in frontal cortex (Bmax 31% lower), hypothalamus (Bmax 40% lower) and caudate (Bmax 32% lower) without changes in KD values. When heroin addicts were divided into two subgroups according to the presence or absence of morphine in body fluids, only the group with positive screening for morphine showed relevant decreases in brain alpha 2-adrenoceptor density (Bmax 36-48% lower), whereas the decreases in receptor density observed in the subgroup with negative screening for morphine did not reach statistical significance. The results suggest that desensitization of brain alpha 2A-adrenceptors is a relevant adaptative receptor mechanism during opiate addiction in humans.

Effect of morphine tolerance and abstinence on the binding of [3H]naltrexone to discrete brain regions and spinal cord of the rat

1. The effect of morphine tolerance and abstinence on the binding of [3H]naltrexone to discrete brain regions and spinal cord of the rat was determined. 2. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with six morphine pellets for a 7-day period. Each pellet contained 75 mg of morphine base. Rats implanted with six placebo pellets each served as controls. 3. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to various doses of morphine. 4. The binding characteristics (Bmax or Kd values) of [3H]naltrexone, an opiate receptor antagonist, were determined in various tissues of morphine tolerant and abstinent rats. In the tolerant rats, the pellets were left in place at the time of sacrificing, whereas in the abstinent rats, the pellets were removed 18 hr prior to sacrificing. 5. The binding of [3H]naltrexone to opiate receptors on membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups was determined. 6. [3H]Naltrexone bound to tissue membranes at a single high affinity binding sites. The Bmax values of [3H]naltrexone to bind to opiate receptors on the membranes of amygdala and striatum were increased significantly in morphine tolerant rats when compared to the placebo controls, but the Kd values did not differ. 7. The Bmax and Kd values of [3H]naltrexone did not differ in any other brain region or spinal cord of morphine tolerant rats and their placebo controls. The binding constants of [3H]naltrexone were unaffected in morphine abstinent rats. 8. Previously we had shown that the binding of [3H]D-Ala2, MePhe4, Gly-ol5 enkephalin (DAMGO), a highly specific agonist for mu-opiate receptors was decreased in cortex, pons and medulla and spinal cord of morphine tolerant but not in the abstinent rats. In addition, delta and kappa receptors are unaffected in morphine tolerant and abstinent rats. 9. The results suggest that direction of change, as well as, the brain areas for mu-agonist and -antagonist opiate binding sites are affected differentially in morphine tolerant rats.

Chronic exposure to morphine and naltrexone induces changes in catecholaminergic neurotransmission in rat brain without altering mu-opioid receptor sensitivity

In order to investigate the role of mu-opioid receptor regulation in catecholaminergic neurotransmission during morphine tolerance/dependence and supersensitivity, we measured changes in number and functional properties of two distinct types of mu receptors in the brain of rats chronically treated with morphine and naltrexone. In membranes of striatum and cortex of morphine treated rats the binding of mu ligand [3H]DAMGO was unaltered, whereas an increase in mu binding sites was found in these brain regions of naltrexone treated rats. The ability of the mu agonist DAMGO to inhibit the dopamine D-1 receptor stimulated cAMP production in striatal slices and the electrically evoked release of [3H]noradrenaline from cortical slices was unaffected in both experimental groups. The major changes found were an increased D-1 receptor stimulated cAMP production and an enhanced release of noradrenaline in morphine treated rats and a decreased D-1 receptor stimulated cAMP production in naltrexone treated rats. These data support the hypothesis that tolerance and supersensitivity to morphine and other mu-opioids may be caused by up- and down-regulated neuronal second messenger systems linked to mu-opioid receptors, rather than by changes in the sensitivity of the mu-opioid receptor itself.

Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine

The potent analgesic responses elicited by systemic administration of RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopro pyl]- 1-oxopropyl]-L-phenylalanine benzyl ester, a prodrug able to inhibit enkephalin-degrading enzymes completely after in vivo bioactivation, has made it possible to investigate the development of antinociceptive tolerance after chronic potentiation of endogenous enkephalins. The ED50 values of RB101 obtained 10 min after i.v. injection were not significantly different in mice treated for 4 days with i.p. administered vehicle (ED50 = 9.50 (6.37-14.15) mg/kg), or with 80 mg/kg of RB101 twice daily (ED50 = 9.50 (5.86-15.39) mg/kg). In contrast, a parallel rightwards shift of the dose-response curves, corresponding to a significant 1.92 (1.49-2.52)-fold decrease in analgesic potency, was observed after i.v. administration of morphine in mice chronically treated with morphine (3 mg/kg, twice daily for 4 days) (ED50 = 3.10 (2.52-3.81) mg/kg) vs. saline (ED50 = 1.60 (1.22-2.09) mg/kg). No tolerance to RB101 was observed even after a longer period (8 days) of chronic treatment with the prodrug. Moreover, no cross-tolerance between morphine and RB101 appeared to occur since the ED50 values obtained after i.v. administration of RB101 were not significantly different in mice chronically pretreated with vehicle (ED50 = 9.50 (6.37-14.15) mg/kg) or with morphine (ED50 = 10.00 (6.62-15.10) mg/kg). The analgesic effect of RB101 observed in morphine-tolerant mice was antagonized by prior injection of naloxone, but not naltrindole (delta-selective antagonist), supporting a preferential involvement of mu-opioid receptors in the antinociceptive effect of RB101, at least in mice in the hot-plate test.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic prenatal methadone exposure alters central opioid mu-receptor affinity in both fetal and maternal brain

The 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 mu-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 mu-receptor capacity in both fetal and maternal brains during gestation day 20, nor when tested 7 days after delivery. However, this chronic exposure decreased mu-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 mu-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 mu-receptor affinity by some unknown mechanism.

Up-regulation of [3H]DAMGO and [3H]DTLET opioid binding sites in laminae I-II of the spinal cord in intact and deafferented morphine-tolerant rats

Using quantitative autoradiography and selective opioid ligands, we have measured the effects of morphine-induced tolerance on [3H]DAMGO and [3H]DTLET binding sites in the superficial spinal dorsal horn (laminae I-II) of intact and deafferented rats (unilateral C4-T2 dorsal rhizotomy). In intact rats, the treatment induced an up-regulation of 26% and 39% for [3H]DAMGO and [3H]DTLET binding sites, respectively, without modification of receptor affinity. In deafferented rats, the treatment similarly induced an up-regulation of 31% and 29% for [3H]DAMGO and [3H]DTLET binding sites, respectively, on the contralateral side, and of 21% and 25%, respectively, on the ipsilateral side. These data demonstrate that the up-regulation induced by morphine tolerance is of similar magnitude for both presynaptic (on primary afferent fibers) and postsynaptic (on spinal neurons) opioid binding sites in the rat dorsal horn.

Apparent pA2 value of naltrexone is not changed in rats following continuous exposure to morphine or naloxone

Chronic opioid antagonist administration increases opioid binding sites and potentiates behavioral responses to morphine. Conversely, chronic opioid agonist administration attenuates behavioral responses to morphine, though this is not necessarily accompanied by a parallel loss of binding sites. We examined the possibility that the in vivo affinity of the mu receptors might be altered as a consequence of the continuous administration of either naloxone or morphine. Rats were implanted sc with naloxone- or morphine-filled osmotic pumps; control animals were implanted with sham pumps. One week later, 24 hr after removing the osmotic pumps, cumulative dose-response curves for fentanyl analgesia were generated in the presence of 0.0, 0.03, 0.1, or 0.3 mg/kg naltrexone, using a tail-flick procedure. The analgesic ED50 (with 95% C. L.) of fentanyl in sham implanted animals, following saline pretreatment was 0.027 mg/kg (0.019, 0.039). The potency of fentanyl was decreased in rats infused with morphine, ED50 = 0.051 mg/kg (0.028, 0.093), and increased in rats that received naloxone, ED50 = 0.018 mg/kg (0.015, 0.022). The mean apparent pA2 value for naltrexone (with 95% C.L.) in the control group was 7.7 (7.5, 7.9). No differences were detected in animals that had received either naloxone or morphine for 7 days, pA2 = 7.8 (7.5, 8.1) and 7.4 (7.3, 7.6), respectively. Our results indicate that there is no change in the apparent affinity of the mu-receptor following continuous exposure to either an opioid agonist or antagonist, at a time when the analgesic potency of the agonist is decreased or increased, respectively.

Effect of chronic prenatal morphine treatment of mu-opioid receptor-regulated adenylate cyclase activity and neurotransmitter release in rat brain slices

Timed-pregnant rats received a semisynthetic diet with or without morphine (0.5-1 mg/g) for 2 weeks. After 21 days of gestation the morphine-dependent dams were decapitated and the foetal brains were dissected. Chronic morphine administration caused a profound increase of adenylate cyclase activity stimulated by postsynaptic D1 dopamine receptors in striatal slices. The relative inhibitory effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on D1-stimulated cyclic AMP (cAMP) production was unaffected. In contrast, cAMP production induced via direct activation of the catalytic unit of adenylate cyclase with forskolin was not changed upon long-term morphine treatment, although DAGO strongly inhibited the effect of forskolin. The electrically evoked release of [3H]noradrenaline (NA) from superfused neocortical slices was strongly enhanced upon morphine treatment, whereas release induced by the calcium ionophore A23187, bypassing voltage-sensitive calcium channels, was unchanged. Again, the inhibitory effect of the mu receptor agonist DAGO was unaffected in neocortical slices from morphine-treated rats. It is suggested that tolerance to morphine may be caused by the fact that the opiate is acting against up-regulated signal transduction mechanisms, rather than by desensitization of central mu-opioid receptors. The pre- and postsynaptic changes may include an enhanced expression and/or biochemical modification of D1 receptors, Gs proteins and calcium channels in central neurons on which mu-opioid receptors are present. At the same time, these adaptive changes may underlie morphine withdrawal phenomena.

Brain-associated autoimmune features in heroin addicts: correlation to HIV infection and dementia

We report here on brain associated autoimmune features in opiate-dependent subjects. This study includes 107 (37 HIV + and 70 HIV -) hospitalized heroin-addicted subjects on a methadone maintenance program, and 45 healthy individuals. Human brain S100 protein, neuron specific enolase (NSE), myelin basic protein (MBF), and old tuberculin (OT) were used as antigens in the study. Serum autoantibodies to brain antigens S100, NSE and MBP were detected by ELISA, whereas delayed hypersensitivity skin reactions were evaluated after intradermal injection of S100, NSE, MBP and OT (control brain-irrelevant antigen). In drug-dependent subjects, 68.2% produced anti-S100, 56.1% anti-NSE and 20.5% anti-MBP autoantibodies, while the incidence of autoantibodies in control healthy individuals was 4.4%, 2.2% and 0%, respectively. Occurrence and amount of anti-S100 and anti-NSE autoantibodies were much higher in HIV + than in HIV - heroin-abusing adults. In drug abusers, the incidence of positive delayed hypersensitivity skin reactions were as follows: 67.2% to S100, 51.4% to NSE, 14.9% to MBP, and 94.3% to OT. In control subjects, the occurrence of hypersensitivity reactions to brain antigens was insignificant. Cutaneous reactions were more frequent in HIV - addicts. The incidence of both autoantibodies and delayed skin responses was positively related to the duration of drug abuse, worsening of HIV infection, and dementia. The high incidence of autoantibodies and delayed hypersensitivity skin reactions to S100 and NSE human brain antigens in heroin-abusers indicates that heroin dependence, as well as HIV infection, are associated with a hyperergy towards brain-related autoimmune phenomena. It has been suggested that the brain-associated autoimmune phenomena in HIV + heroin-addicts represent a hyperimmune phase which precedes immunodeficiency that occurs in the further development of HIV infection.

Modifications of social conflict-induced analgesic and activity responses in male mice receiving chronic opioid agonist and antagonist treatments

This study examined the effects of chronic (7 day) administrations of opioid agonists, via osmotic minipumps (20 micrograms/microliters/h, or 2 mg/kg/h for each agent) on: 1) nociception and activity, and 2) the analgesic and locomotor responses of subordinate male mice experiencing social conflict (aggression without defeat) and defeat in a "resident-intruder" paradigm. Chronic infusion of the mu opioid antagonist, naltrexone, resulted in a hypoanalgesic response and a decrease in basal locomotor activity on days 3-7 postimplantation which returned to the basal levels of saline-implanted control mice after termination of the infusions on day 9. Naltrexone reduced defeat-induced analgesia on the second day after implantation, but had no consistent effects on analgesia on test days 6 and 9 or on the aggression-induced (nondefeat) analgesia and increases in activity. The delta opioid antagonist ICI-154, 129, while having no significant effects on basal nociception or locomotor activity, augmented nondefeat-induced analgesia (day 2) and reduced the defeat-induced increases in activity (days 2 and 6). The mu agonist, levorphanol, resulted in a significant analgesia on the first two days after infusion, followed by the development of tolerance to the analgesic effects over days 3-7. On day 9, a hypoanalgesic response indicative of withdrawal was evident. Levorphanol also induced a marked decrease in locomotor activity over days 3-7 postimplantation, with no evidence of the development of tolerance or withdrawal following termination of infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple opiate receptors of brain and spinal cord in opiate addiction

1. Chronic administration of opiates to rodents results in the development of tolerance to their pharmacological effects. Physical dependence also develops and is shown by the appearance of abstinence syndrome. 2. Opiates produce their effects by acting on three types of opiate receptors, namely mu, delta and kappa. The qualitative and quantitative aspects of the tolerance-dependence and abstinence symptoms observed after chronic administration of agonists acting at mu-, delta- and kappa-opiate receptors appear to differ. 3. Tolerance-dependence on mu-opiate agonists, such as morphine, is associated with down-regulation of mu-opiate receptors in spinal cord and specific areas of the brain but delta- and kappa-opiate receptors are unchanged. During abstinence from mu-opiate agonists, brain and spinal cord mu-, delta- and kappa-opiate receptors are unaffected. 4. Chronic administration of kappa-opiate agonists, such as U-50,488H, results in the development of tolerance to its pharmacological effects and a mild degree of physical dependence. Such changes are associated not only with alterations of delta and kappa opiate receptors in brain and spinal cord, but also primarily with a down-regulation of kappa-opiate receptors in spinal cord and specific brain regions. mu-Opiate receptors are unaffected. 5. Chronic administration of delta-opiate agonists results in down-regulation of brain delta-opiate receptors. 6. It is concluded that tolerance-dependence on mu-, delta- and kappa-opiate receptors is associated with down-regulation of their own type of receptors in the spinal and supraspinal areas. Abstinence, on the other hand, does not alter brain and spinal cord opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of opioid actions on neurons of the locus coeruleus

The locus coeruleus (LC) has provided a useful model for pioneering studies of the mechanisms underlying the acute and chronic actions of opioid drugs. Acutely, opioids inhibit the electrical activity of single neurons in the rat and guinea pig LC. Inhibition is due to a membrane hyperpolarisation. In these cells, opioids act on mu-receptors to increase the opening of inwardly rectifying potassium channels, thus leading to hyperpolarisation. The mu-receptors are coupled to potassium channels via G-proteins which are sensitive to inactivation by pertussis toxin. This coupling process is quite direct, in that it does not involve freely diffusible intracellular second messengers. Agonists specific for other receptors, such as alpha 2- and somatostatin-receptors, are capable of opening the same population of potassium channels on LC neurons. Following chronic treatment of animals with morphine, a specific deficit develops in the ability of mu-receptors to open potassium channels, producing reduced sensitivity of LC neurons to inhibition by opioids.

Upregulation of the opioid receptor complex by the chronic administration of morphine: a biochemical marker related to the development of tolerance and dependence

Studies conducted after the development of the rapid filtration assay for opiate receptors, and before the recognition of multiple opioid receptors, failed to detect changes in opioid receptors induced by chronic morphine. Recent experiments conducted in our laboratories were designed to examine the hypothesis that only one of several opioid receptor types might be altered by chronic morphine. Using binding surface analysis and irreversible ligands to increase the "resolving power" of the ligand binding assay, the results indicated that chronic morphine increased both the Bmax and Kd of the opioid receptor complex, labeled with either [3H][D-Ala2,D-Leu5]enkephalin, [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin or [3H]6-desoxy-6 beta-fluoronaltreone. In the present study rats were pretreated with drugs known to attenuate the development of tolerance and dependence [the irreversible mu-receptor antagonist, beta-funaltrexamine (beta-FNA), and the inhibitor of tryptophan hydroxylase, para-chlorophenylalanine], prior to subcutaneous implantation of morphine pellets. The results demonstrated that 1) unlike chronic naltrexone, beta-FNA failed to upregulate opioid receptors and 2) both beta-funaltrexamine and PCPA pretreatment attenuated the chronic morphine-induced increase in the Bmax, but not the Kd, of the opioid receptor complex. These results provide evidence that naltrex-one-induced upregulation of the opioid receptor complex might occur indirectly as a consequence of interactions at beta-funaltrexamine-insensitive opioid receptors and that morphine-induced upregulation (increased Bmax) of the opioid receptor complex is a relevant in vitro marker related to the development of tolerance and dependence. These data collectively support the hypothesis that endogenous antiopiate peptides play an important role in the development of tolerance and dependence to morphine.

Spinal administration of receptor-selective drugs as analgesics: new horizons

The processing at the spinal cord levels of sensory information is subject to modulation by a number of local receptor systems, including opioids: alpha 2 adrenergic; and to a lesser extent serotonin, GABAB, neuropeptide Y, cholinergic, adenosine, and the NMDA-glutamate site. The functional utility of these multiple systems are only partially understood, but it appears that (a) they may act individually to alter different aspects of the nociceptive sensory message (b) they could be used synergistically to reduce the incidence of side effects by reducing the dose of agents required to yield analgesic effects, and (c) they may function variably in animals made tolerant to classes of receptor agonists.

Affinity states of rat brain opioid receptors in different tissue preparations

The binding of [3H]Tyr-D-Ala-Gly-(N-Me)Phe-Gly-ol ([3H]DAGO) and [3H]Tyr-D-Thr-Gly-Phe-Leu-Thr ([3H]DTLET), selective agonists for mu- and delta-opioid binding sites, respectively, has been investigated using different rat brain tissue preparations and buffer systems. The results were compared with the binding of the ligands to crude membrane fractions in Tris-HCl, the most commonly used preparation for binding studies. In both rat brain membranes and intact cells, Krebs-HEPES induced a decrease in the affinities of [3H]DAGO and [3H]DTLET, but little modification was observed when 20-microns tissue slices were used, whatever the brain area studied. The dissociation rate of [3H]DTLET was clearly dependent on the tissue preparation used, because the koff value of this ligand in Krebs-HEPES was 2.5-fold higher in membrane fractions than that measured in intact cells. The kinetic dissociation constant of [3H]DTLET in membrane fractions in Krebs-HEPES was 6.5-fold greater than that measured in Tris-HCl. In intact cells, the koff value for [3H]DTLET was lower than that found in membrane fractions in Krebs-HEPES and similar to that observed in membrane preparations in Tris-HCl supplemented with 30 mM NaCl. These data suggest (a) that the koff constant of [3H]DTLET was regulated by the ionic environment of the delta-opioid receptor, which is clearly dependent on the preservation of cellular structure, and (b) that opioid receptors could exist under different states that are regulated, in part, by the intracellular Na+ concentration.

Long-term pain relief produced by intrathecal morphine infusion in 53 patients

The present report details the characteristics of the analgesic effects of morphine administered chronically by infusion pumps implanted in 53 patients suffering from terminal metastatic disease. The median postimplant survival time in these patients was 4 months. Patients (mean age 58 years) were characterized according to the duration of pain before pump implantation (mean 16 months), prior consumption of systemic opioids (mean one to six daily analgesic equivalents of morphine), and their response to a trial intrathecal dose of morphine (1 to 2 mg). The median infusion dose at 2 weeks was 3.8 mg/day. The analgesic index, calculated as (quality of pain relief x duration of pain relief in hours)/morphine dose in mg, that was observed after the trial dose of morphine was determined for each patient. A close correlation was observed between the acute (2-week) infusion dose necessary to produce pain relief and the analgesic index such that the infusion dose = -8.0 x log (analgesic index) + 17.1. By 16 weeks, the mean spinal morphine dose for the group had increased by a factor of about 2.5; however, significant variation in the dose incrementation was documented. The maximum increase was observed in patients with a low analgesic index, and this rapid incrementation was usually correlated with an unsatisfactory overall outcome. Evidence that long-term infusion continues to yield analgesia was evidenced in six cases where there was an unanticipated loss of drug infusion and a corresponding increase in parenteral narcotic consumption. These data indicate the long-term efficacy and safety of spinal opioid infusion in patients with terminal cancer, and emphasize the advantage of assessing the sensitivity of the patient to spinal opioids by a standardized trial injection prior to pump placement as a prognostic indication of outcome.

Opioid receptor subtypes in the supraoptic nucleus and posterior pituitary gland of morphine-tolerant rats

Morphine, 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 mu-, delta-, and kappa-receptors, respectively. Control incubations had naloxone determined specificity. The supraoptic nucleus (site of oxytocin-secreting magnocellular perikarya) contained both mu- and kappa-receptors in control rats (mean +/- S.E.M. binding of mu-selective [3H]etorphine was 91.8 +/- 25.4 fmol/mg of tissue, and of kappa-selective [3H](-)-bremazocine was 130.4 +/- 25.6 fmol/mg). Chronic morphine treatment caused a 83.9% decrease in binding in mu-selective conditions (P less than 0.05), but no significant change in kappa-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 less than 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 mu-receptors in the supraoptic nucleus, and of opioid receptors in the median preoptic nucleus, perhaps accounting for morphine-tolerance in relation to oxytocin secretion.

Alpha 1-adrenoreceptor-mediated increase in acetylcholine release in brain slices during morphine tolerance

Norepinephrine, clonidine, and phenylephrine increased the electrically evoked release of endogenous acetylcholine in cortical slices taken from morphine-tolerant guinea pigs. This effect was alpha 1-adrenoreceptor mediated and was opposite to the alpha 2-adrenoreceptor-mediated inhibition of acetylcholine release, normally elicited by norepinephrine and clonidine. In the presence of prazosin, clonidine recovered its normal inhibitory properties, suggesting that morphine tolerance induced the appearance of an alpha 1-adrenoreceptor-mediated response that overshadowed, but did not cancel, the still present alpha 2-adrenoreceptor inhibitory control. The attempt to prove the presence of alpha-adrenoreceptors on the nerve endings by testing the effect of norepinephrine in synaptosomal preparations (preloaded with [3H]choline and depolarized with KCl and veratridine) was unsuccessful. Therefore the problem of the exact location of this excitatory input remains to be solved. These results confirm previous findings reporting the increase in cortical acetylcholine release induced by the alpha-adrenoreceptor agonists in morphine-tolerant, freely moving guinea pigs and demonstrate that opiate tolerance inverts the direction of the noradrenergic modulation even in the isolated intracortical cholinergic structures.

Selective changes in mu opioid receptor properties induced by chronic morphine exposure

Chronic infusion of morphine to guinea pigs produced selective changes in mu agonist binding properties in cerebrocortical membrane preparations. Employing the mu-selective opioid agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) in direct binding studies and in competition of labeled antagonist binding, we found that the major changes were a decrease in the number of sites with high affinity for agonist, a small reduction in total receptor number, and a loss in the ability of guanosine 5'-[gamma-thio]triphosphate to regulate binding. A fraction of high-affinity mu receptors appeared to retain their high affinity for agonist and their sensitivity to guanine nucleotide analogue after the induction of morphine tolerance, possibly because the morphine concentrations achieved in brain were insufficient to uncouple all mu receptors from associated guanine nucleotide-binding regulatory proteins. Some membrane preparations were treated with pertussis toxin, which has been shown to functionally uncouple mu opioid receptors from their effector systems. In these preparations, a single agonist-affinity state of the receptor was observed. The apparent dissociation constant for this affinity state in pertussis toxin-treated membranes was similar to the lower-affinity state observed in preparations from morphine-tolerant animals. In contrast to the changes observed at mu opioid binding sites, no significant changes in agonist affinity or binding density were observed for selective delta or kappa agonists, consistent with the development of selective tolerance at mu receptors.

Neurophysiology and neurochemistry of drug dependence: a review

To understand the neurophysiological and neurochemical mechanisms of drug dependence, the functional significance of dopamine, noradrenaline and endogenous opioid peptides in the mediation of natural, self-stimulation and pharmacological reinforcement are discussed. Data on search of system(s), mediator(s) and neurons of reinforcement as well as my own notions on reinforcement as a critical element in organization and regulation of the organism's adaptive activity in variable environments are presented. The role of chronic drug-induced stable modification of central neurochemical systems' functioning as a basis for the alteration of endogenous reinforcement processes and raising drug dependence are examined in detail for main addictive drugs, opiates and psychomotor stimulants.

Control of opiate receptor number in vivo: simultaneous kappa-receptor down-regulation and mu-receptor up-regulation following chronic agonist/antagonist treatment

While it is well established that opiate receptors up-regulate following chronic antagonist treatment in vivo, possible down-regulation following chronic agonist treatment remains controversial. In this study, rats received a continuous seven-day infusion of bremazocine, an opioid drug suggested to be a potent agonist at kappa receptors and an antagonist at mu and delta receptors. Opiate receptor binding was assessed in both cryostat sections and homogenates of rat brain, under conditions selective for mu, delta and kappa sites. Data from both sections and homogenates showed an increase in the capacity of mu binding sites following chronic bremazocine treatment, suggesting that up-regulation of mu receptors had occurred, and that residual ligand from the in vivo treatment had largely been removed. A significant decrease in kappa binding was observed in sections, and experiments using homogenates demonstrated a dramatic loss of high-affinity kappa binding, with an increase in low-affinity binding. There was no apparent alteration in binding to delta receptors. No significant changes were observed following acute injection of bremazocine. Quantitative light-microscopic autoradiography confirmed the results of the binding experiments, and showed that the magnitude of these effects varied between different brain regions. No decrease in kappa binding was seen following chronic administration of the partial kappa agonist nalorphine, indicating that high agonist intrinsic activity is necessary for down-regulation to occur. In addition, chronic co-administration of bremazocine with the partial agonist/antagonist diprenorphine did not cause a significant decrease in kappa binding, implying that diprenorphine can antagonize the down-regulatory effect. These results provide evidence that bremazocine possesses different degrees of intrinsic activity at mu, delta and kappa receptors. They demonstrate that, at least in the case of kappa sites, opiate receptors do show down-regulation following chronic agonist treatment in vivo.

Down-regulation of 3H-lofentanil binding to opiate receptors in different cultured neuronal cells

There was stereospecific binding of 3H-lofentanil (KD value = 1.53 nM) to membranes of neuroblastoma-glioma NG 108-15 cells which are known to bear high affinity binding sites for enkephalin derivatives (delta-opiate receptor subtype). There was no high affinity specific binding of the mu-opiate specific ligand 3H-sufentanil. The specific binding of 3H-lofentanil to delta-opiate receptor subtype was down-regulated (decrease in Bmax value without change in the KD value) after prolonged incubation of the cells in the presence of leu- and met- enkephalin (0.1 microM). There was no down-regulation of the opiate receptors (3H-lofentanil and 3H-D-ala-D-leu-enkephalin specific binding) after incubation of NG 108-15 cells with drugs from the fentanyl series (alfentanil or sufentanil). In cultured neurones from rat forebrain (15 day old embryos), the 3H-lofentanil binding was specific with high affinity (KD: 0.048 nM) and a slow dissociation rate similar to that in adult rat cortex. Drugs of the fentanyl series (4-anilino-piperidines) were potent displacers whereas agonists of the delta- (enkephalin derivatives), sigma- (phencyclidine, haloperidol, 3-hydroxyphenyl-propylpiperidine) or K- (U 50488) opiate sites had a low affinity (Ki greater than 0.5 microM) for 3H-lofentanil specific binding sites. Since there was also specific binding of 3H-sufentanil, the opiate receptors in cultured neurones seem to be mainly of the mu-subtype and this is consistent with the ontogeny of opiate receptors subtypes. These receptors were down-regulated after incubation in the presence of etorphine, sufentanil and alfentanil but not enkephalin derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of morphine and naltrexone up-regulate[3H][D-Ala2,D-leu5]enkephalin binding sites by different mechanisms

Previous studies have demonstrated that chronic administration of morphine up-regulated the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin, without producing a detectable alteration in the higher affinity binding site for [3H][D-ala2,D-leu5]enkephalin (Rothman et al., Eur. J. Pharmac. 124: 113-119, 1986). The experiments reported in this paper tested the hypothesis that chronic administration of morphine and naltrexone up-regulated the binding sites for [3H][D-ala2,D-leu5]enkephalin by different mechanisms. Rats were given either morphine or naltrexone chronically. Chronic administration of morphine up-regulated the lower affinity site, while chronic administration of naltrexone up-regulated both the higher and lower affinity binding sites for [3H][D-ala2,D-leu5]enkephalin. Unlike the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin present in membranes prepared from rats treated with placebo pellets, the lower affinity binding sites which were up-regulated by naltrexone and morphine were partially (naltrexone) or completely (morphine) labile to preincubation for 60 min at 25 degrees C in 50 mM Tris-HCl, pH 7.4, containing 0.4 M NaCl. These data suggest that chronic administration of morphine and naltrexone up-regulate binding sites for [3H][D-ala2,D-leu5]enkephalin through different mechanisms, and that the lower affinity binding sites for [3H][D-ala2, D-leu5]enkephalin which are up-regulated by chronic administration of morphine and naltrexone might differ biochemically from the lower affinity binding sites present in membranes treated with placebo.

Sodium channel toxins veratrine and veratridine modify opioid and muscarinic but not beta-adrenergic binding sites in brain slices

We have examined the influence of the sodium channel toxins veratrine and veratridine on mu-opioid ([3H]-DAGO), muscarinic ([3H] NMS) and beta-adrenergic ([3H] CGP) receptors in rat brain slices. These drugs reduce opioid and muscarinic binding while leaving beta-receptors unaffected. Veratrine is inhibitory at 0 degree or at 30 degrees C whereas veratridine is without effect at 0 degree C. These data suggest that some factor contained in the mixture of drugs (veratrine) can block opioid and muscarinic receptors independently of depolarization. Veratridine does not affect muscarinic receptors at ice temperature. Similar observations were made in thin sections of cat brain at 0 degree C. The concentrations of the toxins which cause 50% inhibition of binding are well within the range (5 x 10(-5) M-10(-4) M) routinely used for depolarization experiments. We suggest that caution be used in the interpretation of results obtained from veratrum alkaloid-induced depolarizations. It would not be surprising if the binding of other ligands to their receptors was also affected.

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.

Evidence for crosstolerance to the analgesic effects between morphine and selective alpha 2-adrenoceptor agonists

Rats were injected subcutaneously (s.c.) with morphine (5 mg/kg) until tolerance developed to its antinociceptive action, or with 0.9% saline which was used as vehicle for morphine. Subsequently, both groups of animals were given an intrathecal (i.th.) dose of either noradrenaline (2 micrograms), clonidine (12.5 micrograms) or guanfacine (12.5 micrograms), that had been found previously to be reliably antinociceptive. In the saline-treated animals, these doses of noradrenaline, clonidine or guanfacine induced clear antinociceptive effects, but not in the morphine-group. It is therefore concluded that cross-tolerance to the antinociceptive effects of systemic morphine and the alpha-adrenoceptor agonists was obtained. The cross-tolerance between morphine on one hand, and noradrenaline, clonidine and guanfacine on the other, implies that a substantial opiate-adrenoceptor interaction exists in antinociceptive processes.

Role of receptor regulation in opioid tolerance mechanisms

The molecular basis of opioid tolerance/dependence has long eluded researchers, but recent advances in receptor regulation have suggested a useful conceptual approach to the problem. In NG108-15 neuroblastoma x glioma hybrid (NG) cells, opioid agonists inhibit adenylate cyclase in a dose-dependent, naloxone-antagonizable fashion. Chronic treatment with opioid agonists results in a series of molecular processes that, in a tolerance-like fashion, counteract this inhibition. These processes include desensitization and down-regulation of receptors and an increase in adenylate cyclase activity. Opioid inhibition of adenylate cyclase and opioid receptor down-regulation also have been observed in the brain. However, most studies have found that the receptors coupled to adenylate cyclase are not of the mu type, which are thought to be the primary mediators of opioid analgesia. Down-regulation has been observed for both mu and delta opioid receptors in the brain. However, in most cases, the time course of down-regulation is not correlated with that for tolerance development, and chronic morphine treatment does not result in down-regulation. Thus, opioid receptors in the brain, like those in NG cells, are subject to dynamic regulation by agonists, which probably has an important role in their function. However, it remains to be established that opioid receptor regulation is the basis of opioid tolerance and dependence.

Chronic morphine upregulates a mu-opiate binding site labeled by [3H]cycloFOXY: a novel opiate antagonist suitable for positron emission tomography

CycloFOXY (17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta- fluoromorphinan) is a novel opiate antagonist synthesized as a ligand suitable for in vivo visualization of opiate receptors using positron emission transaxial tomography. In this paper we report that [3H]cycloFOXY labels two distinct opiate binding sites in rat brain membranes, tentatively identified as mu and kappa. Furthermore, chronic administration of morphine results in a selective up-regulation of the mu binding site. The implications of this finding for models of the opioid receptors and the mechanism of the sodium effect are discussed.