Effect of chronic exposure to naltrexone and opioid selective agonists on G protein mRNA levels in the rat nervous system

The in situ hybridization technique was used to investigate the effect on G protein alpha subunit expression throughout the brain of rats chronically infused with naltrexone (70 micrograms/microliters, 1 microliter/h), DAGO (0.5 micrograms/microliter, 1 microliter/h), DADLE (11.4 micrograms/microliters, 1 microliter/h), DPDPE (3.4 micrograms/microliters, 1 microliter/h) and U-50,488H (4 micrograms/microliters, 1 microliter/h). Prolonged exposure to naltrexone did not modify G protein alpha subunit mRNA expression, whereas DADLE and U-50,488H, respectively, increased the levels of alpha s and alpha o mRNA in specific brain regions. In particular, a 15% increase in alpha s expression was only observed in the dorsomedial hypothalamic nucleus of rats undergoing chronic DADLE infusion: a 15% increase in alpha o levels was detected in the claustrum and endopiriform nucleus of rats chronically treated with U-50,488H. These are the first in vivo data to demonstrate that only chronic stimulation with an opioid agonist (morphine and/or DADLE and U-50,488H) is capable of modifying G protein alpha subunit mRNA. The regional selectivity of these modifications is discussed, together with the receptor specificity of the opioid effects.

A 24-h inpatient detoxification treatment for heroin addicts: a preliminary investigation

The present study describes a detoxification method which takes only 4 h to complete. Eleven polydrug users addicted to opiates were detoxified 12 h after their last dose of heroin, in an intensive care unit. The detoxification was carried out using naltrexone, after sedation with midazolan. After detoxification (12 h) patients were discharged without withdrawal symptoms. This procedure offers a rapid detoxification procedure which seems to be widely accepted by the addicts themselves.

The effect of selective opioid antagonists on butorphanol-induced feeding

Butorphanol tartrate (BT) potently stimulates food intake in satiated rats. The opioid receptor profile of BT is complex and is dependent upon the assay and animal species studied. In the present study we utilized three selective opioid antagonists; namely beta-funaltrexamine (beta-FNA), naltrindole (NTI) and norbinaltorphimine (nor-BNI), to probe the opioid receptor profile of BT as an orexigenic agent. Intracerebroventricular administration of nor-BNI (kappa) antagonized the feeding effects of BT (8 mg/kg, s.c.) at doses of 1, 10 and 100 nmol at the 1-2 h time point and decreased feeding at all time points for the 10 nmol dose. After 1 h, the 100 nmol dose of nor-BNI decreased BT-induced feeding by about 72%. In contrast, intraventricular injection of only the highest dose of the selective mu opioid antagonist, beta-FNA (50 nmol), decreased BT-induced feeding. Intraventricular administration of the delta opioid agonist, NTI, failed to alter BT-induced feeding at doses as high as 50 nmol. These data suggest that BT is dependent upon the kappa and perhaps the mu opioid receptors to increase food intake in satiated rats.

Determination of naltrexone dosage for narcotic agonist blockade in detoxified Asian addicts

Thirty-eight, adult, male, detoxified, Malaysian opiate addicts participated in this double-blind clinical evaluation of naltrexone. Addicts from three ethnic groups: Chinese, Malays, and Indians received a weekly regimen of naltrexone (100 mg on Days 1 and 3, and 150 mg on Day 5). Subjects were randomly assigned to receive intravenous challenge with either 25 mg heroin or placebo 12, 24, 48, and 72 h after the third naltrexone dose. Physiological and subjective parameters were measured at each challenge interval. Results indicated that naltrexone was effective in blocking the physiological and psychological effects of heroin for at least 48 and 72 h, respectively, in detoxified Malaysian opiate addicts. The efficacy of naltrexone did not differ among the three ethnic groups evaluated in this study.

Etonitazene-induced antinociception in mu1 opioid receptor deficient CXBK mice: evidence for a role for mu2 receptors in supraspinal antinociception

The prevailing view is that supraspinal mu opioid-mediated antinociception in mice is mediated via the mu 1 subtype. The purpose of the present study was to determine if the highly mu-selective compound etonitazene could produce supraspinal (intracerebroventricular; i.c.v.) antinociception in CXBK mice, which are deficient in brain mu1, but not mu2, opioid receptors. CXBK or normal Crl:CD-1 (ICR)BR mice were administered graded doses of etonitazene i.c.v. and 15 min later antinociception was assessed by a standard radiant-heat or 55 degrees C water tail-flick test. Etonitazene produced dose-related antinociception that was blocked by naloxone and by beta-FNA (demonstrating a mu opioid mechanism), but not by either ICI-174,864 or naltrindole (demonstrating the lack of involvement of delta opioid receptors). These findings suggest that mu2 opioid receptors are important contributors to opioid-induced supraspinal antinociception in mice.

GRF-induced feeding: evidence for protein selectivity and opiate involvement

Growth hormone-releasing factor (GRF) is a hypothalamic peptide named for its ability to induce release of growth hormone from the anterior pituitary. GRF also acts as a neurotransmitter in the suprachiasmatic nucleus/medial preoptic area (SCN/MPOA) to stimulate food intake. The purpose of this series of experiments was to explore the nature of GRF-induced feeding, with a particular emphasis on macronutrient selectivity, and to examine the role of opiate activity in the paraventricular nucleus of the hypothalamus (PVN). Chow intake stimulated by GRF microinjection (1 pmol/0.5 microliters) into the SCN/MPOA was blocked by injection of methyl-naltrexone (3 micrograms/0.5 microliters) into the PVN. In animals habituated to macronutrient diets (Teklad, WI), GRF preferentially stimulated intake of protein at 2 and 4 h postinjection, whereas it had no effect on carbohydrate intake. Further, this effect was blocked by injection of naloxone (40 nmol/0.5 microliters) into the PVN. Microinjection of morphine (0, 1, 10, and 17 micrograms/0.5 microliter) into the PVN also specifically stimulated protein intake at 2 and 4 h postinjection. These results suggest that feeding derived from GRF actions in the SCN/MPOA is macronutrient selective, and is dependent on PVN opiate activity for expression.

The effect of central blockade of kappa-opioid receptors on neuropeptide Y-induced feeding in the rat

Neuropeptide Y (NPY) and the endogenous kappa-opioid receptor ligand, dynorphin (dyn), stimulate feeding when injected centrally in the rat. Norbinaltorphimine (norBNI, 25 nmol) reduced the feeding response to NPY (2.4 nmol) by 67% (P < 0.02). An additive effect of dynorphin and NPY was seen on food intake (saline 0.8 +/- 0.1, dyn 1.9 +/- 0.4, NPY 6.1 +/- 1.4, dyn and NPY 9.7 +/- 2.2). A component of NPY-induced feeding may be mediated by kappa-opioid neuronal systems.

delta-Opioid receptor-mediated regulation of central dopaminergic neurons in the rat

Effects of intraventricular injections of the delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) and antagonist 17-cyclopropylmethyl-6,7-dehydro-4,5-epoxy-3,14-dihydroxy-6,7,2',3'-indo l morphinan (naltrindole) hydrochloride were determined on the activities of mesolimbic, nigrostriatal, tuberoinfundibular and periventricular-hypophysial dopaminergic neurons in brains of male rats. Dopaminergic neuronal activity was estimated by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of the decarboxylase inhibitor 3-hydroxybenzylhydrazine in regions of the brain (nucleus accumbens, striatum, median eminence) and the intermediate lobe of the pituitary which contain terminals of these neurons. DPDPE produced a dose- and time-related increase in concentrations of DOPAC and accumulation of DOPA in nucleus accumbens and median eminence, but had no effect in striatum or intermediate lobe of the pituitary. Naltrindole hydrochloride had no effect per se, but blocked the ability of DPDPE to increase DOPAC concentrations in nucleus accumbens and median eminence. These results reveal that activation of delta-opioid receptors selectively increases the activities of mesolimbic and tuberoinfundibular but not of nigrostriatal or periventricular-hypophysial dopaminergic neurons.

Effects of naltrexone on the binding of [3H]D-Ala2, MePhe4, Gly-ol5-enkephalin to brain regions and spinal cord and pharmacological responses to morphine in the rat

1. The effects of naltrexone pellet implantation and removal on the analgesic and hypothermic effects of morphine and the binding of 3H-D-Ala2, MePhe4, Gly-ol5-enkephalin (DAMGO) to mu-opiate receptors in rat brain regions and spinal cord were determined. 2. Male Sprague-Dawley rats were implanted subcutaneously with a pellet containing 10 mg of naltrexone for 7 days. Placebo pellet implanted rats served as controls. The pellets were removed on day 8, and the analgesic and hyperthermic effects were determined in the rat 24 hr later. Morphine produced a dose-dependent analgesic and hyperthermic responses in rats implanted with placebo pellets. Enhanced analgesic and hyperthermic responses to morphine were produced in rats implanted with naltrexone pellets. 3. The binding constants (Bmax and Kd values) of [3H]DAMGO in regions of the brain (amygdala, hypothalamus, striatum, midbrain, hippocampus, pons + medulla and cortex), and spinal cord of rats with naltrexone pellet left intact or removed were determined. The Bmax values of [3H]DAMGO were increased in all brain regions and spinal cord of rats in which the naltrexone pellets were left in place or removed prior to sacrificing. However, the Kd values of [3H]DAMGO were unaffected by naltrexone treatment. 4. It is concluded that enhanced analgesic and hyperthermic response to morphine is produced in rats implanted with naltrexone pellets and such alterations in the pharmacological responses are due to up-regulation of mu-opiate receptors in all the brain regions and spinal cord. Additionally whether the pellets were left intact (receptors blocked) or removed (receptors not blocked), the mu-opiate receptors were up-regulated in spinal cord and all the regions of the brain.

Pulsatile administration of gonadotrophin releasing hormone and oral administration of naltrexone in hypothalamic amenorrhoea

Between 1979 and 1990, 73 patients suffering from hypothalamic amenorrhoea were treated by pulsatile administration of gonadotrophin releasing hormone (GnRH) in 359 treatment cycles. Seventy-two pregnancies were achieved. In 64 favourable patients in whom hypothalamic amenorrhoea constituted the only reason for infertility, a pregnancy rate of 29% per cycle could be obtained. Patients who conceived during pulsatile GnRH required an average of only 2.4 cycles per conception. Twelve out of 24 patients with hypothalamic amenorrhoea who exhibited an ovulatory response to pulsatile GnRH, ovulated during oral administration of naltrexone; such responsiveness to opioid antagonism was, however, restricted to the less serious grades. In conclusion, pulsatile administration of GnRH continues to be a highly effective mode of treatment of infertility due to hypothalamic amenorrhoea of various aetiologies. A subgroup of these patients may be successfully treated by the oral administration of naltrexone.

Opioid antagonist adjuncts to epidural morphine for postcesarean analgesia: maternal outcomes

This prospective, randomized, controlled investigation compared the effects of three prophylactic mu-opioid antagonists, epidural butorphanol (BU) 3 mg, epidural nalbuphine (NB) 10 mg, and oral naltrexone (NX) 6 mg, on postcesarean epidural morphine analgesia. After randomization, 102 term parturients underwent cesarean delivery with epidural anesthesia, 2% lidocaine and epinephrine 1:200,000. When the umbilical cord was clamped, each patient received one epidural solution (containing morphine 4 mg plus either saline or treatment drug), and one oral capsule (containing either placebo or treatment drug) in a double-blind manner. Maternal outcomes included pain and satisfaction [assessed with 100-mm visual analog scales (VAS)], and the incidence and severity of respiratory depression, somnolence, pruritus, nausea, and emesis. Through the first 12 h postpartum, the BU group achieved significantly greater analgesia than the morphine sulfate (control) (MS), NB, and NX groups, a significantly lower incidence of severe pruritus than the MS group, and significantly greater satisfaction than MS and NX groups. Epidural morphine and BU promoted better analgesia and satisfaction than any previously documented postcesarean regimen.

Opiate antagonist treatment of ovarian failure

One-hundred-and-thirty-eight women suffering from hypothalamic or hyperandrogenic ovarian failure were treated with daily doses of 25-150 mg of the opiate antagonist naltrexone for 4-100 weeks. In patients with hypothalamic ovarian failure, treatment with naltrexone alone was followed by an increase of gonadotrophins and by normalization of the menstrual cycle in approximately 70% of patients. Eight of 10 patients who did not respond to naltrexone and had not previously ovulated in response to clomiphene administration exhibited ovulatory cycles when both compounds were administered. Twenty-four pregnancies were achieved in 22 women, corresponding to an overall pregnancy rate of 26%, with a cumulative pregnancy rate closely resembling that of a normal population. In contrast, in hyperandrogenic insulin-resistant patients, the pattern of gonadotrophin secretion did not seem to change dramatically during naltrexone treatment. However, the rise of insulin in plasma following an oral load of glucose (oGTT) was blunted considerably, resulting in normalization of previously elevated circulating insulin levels. Since the time course of plasma glucose after oGTT did not appear to be affected by treatment, this indicates an increase in insulin sensitivity (or a decrease in insulin resistance) during naltrexone therapy. Side-effects of naltrexone treatment were negligible in patients with hypothalamic ovarian failure. Hyperandrogenic patients, however, did experience more intense and prolonged side-effects, such as nausea and dizziness.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of endorphin blocking on conditioned SCR in humans

In order to test the hypothesis that low levels of endogenous opioids (endorphins) predispose to strong conditioning effects, female Ss (N = 36) were assigned to a placebo group, a low-dose naltrexone group, or a high-dose naltrexone group and then underwent a classical conditioning procedure. This procedure consisted of an acquisition phase in which all Ss received 5 pairings of a CS+ (neutral picture) and a UCS (100 dB white noise). The CS- (neutral picture) was never followed by a UCS. During extinction, Ss received 4 unreinforced presentations of CS+ and CS-. Throughout the experiment, skin conductance responses (SCRs) to the CSs and UCSs were recorded. Acquisition was successful in that CS+ slides elicited stronger SCRs than CS- slides. However, during acquisition, there was no interaction between drug and differential response (CS+ vs CS-). During extinction, there was no overall remaining effect of conditioning. Again, no evidence was found to suggest that (remaining) effects of conditioning were stronger in the naltrexone treated Ss than in the placebo Ss. If anything, the opposite seemed to be true with especially high-dose naltrexone Ss showing relatively weak conditioning effects.

Naltrexone in autistic children: behavioral symptoms and attentional learning

OBJECTIVE

To assess critically the short-term efficacy and safety of naltrexone in autistic children and its effects on discrimination learning in the laboratory.

METHOD

A parallel group design was employed. After a 2-week placebo baseline period, children were randomly assigned either to naltrexone or to placebo for a period of 3 weeks followed by a one-week posttreatment placebo period. Multiple raters and rating scales were employed in a variety of conditions. Forty-one children, all inpatients, ages 2.9 to 7.8 years, completed the study. Naltrexone reduced hyperactivity and had no effect on discrimination learning in the laboratory. There was a suggestion that it had a beneficial effect on decreasing self-injurious behavior. Untoward effects were mild and transient.

CONCLUSION

In the present study, naltrexone significantly reduced only hyperactivity, and no serious untoward effects were observed. The effectiveness of naltrexone in the treatment of autism and self-injurious behavior requires additional assessment in a sample of children with moderate to severe self-injurious behavior.

Mesolimbic dopamine neurotransmission is increased by administration of mu-opioid receptor antagonists

Microdialysis and high pressure liquid chromatography were used to assess the effects of ventral tegmental area microinjections of the mu-opioid receptor antagonists D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and beta-funaltrexamine (beta-FNA) on extracellular ventral striatal dopamine and metabolite concentrations. While CTOP is known to antagonize the increases in extracellular ventral striatal dopamine and dopamine metabolite concentrations induced by ventral tegmental area microinjections of a mu-opioid receptor agonist, it produced dose-orderly increases in ventral striatal dopamine and dopamine metabolite concentrations when administered by itself. beta-FNA also elevated dopamine and metabolite concentrations. These mu-agonist-like effects of the mu-opioid receptor antagonists were unexpected and suggest that a complex local circuitry mediates opioid-dopamine interactions in the ventral tegmental area. Since mu-opioids are known to act on ventral tegmental neurons that contain gamma-aminobutyric acid (GABA), a model of interactions between GABAergic afferents to the ventral tegmental area and ventral tegmental GABAergic interneurons is proposed.

Opioid antagonist affects behavioral effects of exposure in vivo

This study tested the hypothesis that endogenous opioids are involved in the extinction of phobic fear through exposure in vivo. Forty-eight spider phobics participated in a 2-hr therapist-directed exposure in vivo treatment. Sixteen Ss were assigned to placebo, 16 to a low dose of naltrexone, and 16 to a high dose of naltrexone. Before intervention, after treatment, and at a 1-wk follow-up test, self-report, physiological, and behavioral measures of phobic fear were completed. At 1-wk follow-up, naltrexone was significantly related, in a dose-dependent way, to a greater relapse on avoidance measures but not on emotional, cognitive, and physiological measures. Endogenous opioids may be specifically involved in the extinction of avoidance behavior but not in the extinction of all aspects of phobic fear.

Regulation of substance P receptor system in rat striatum by chronic naltrexone treatment

Chronic blockade of opioid receptors by naltrexone increases opioid peptides in the striatum, and up-regulates brain opioid receptors resulting in functional supersensitivity. Striatal SP content was increased 3.5-fold after 8 days of naltrexone treatment relative to control animals. The present study was undertaken to determine whether SP receptors in the striatum and SP receptor-coupled second messenger system are modulated by increased striatal SP content induced by chronic opioid receptor blockade. The binding affinity and capacity of SP receptors, determined using [125I]Bolton-Hunter SP ([125I]BHSP) labeled at Lys3, in striatal synaptosomal membranes were not significantly altered by chronic blockade of opioid receptors. Although the concentrations of [Sar9,Met (O2)11]SP, a NK-1 receptor-specific agonist, and SP(1-7), an aminoterminal major metabolite of SP, required to inhibit half of [125I]BHSP binding (IC50) in striatal synaptosomal membranes were significantly decreased, the IC50s for SP and an NK-2 receptor-specific agonist, [Nle10]NK A (4-10), remained unchanged by chronic naltrexone treatment. The data suggest that naltrexone which has no SP receptor antagonistic action, not only indirectly acts on SP-ergic neurons but also causes a change in the apparent affinity of NK-1 receptor (as reflected by changes in IC50 values) in the striatum. Cellular inositol-1,4,5-trisphosphate [Ins(1,4,5)P3], quantified by a highly sensitive and selective radioreceptor mass assay, was increased in the striatum by 28% relative to control levels. With [3H]Ins(1,4,5)P3 as a ligand, Scatchard analyses of the concentration-dependent saturation curves showed that the density of striatal intracellular Ins(1,4,5)P3 receptors was increased by 53%. The levels of SP and cellular Ins(1,4,5)P3, and the density of Ins(1,4,5)P3 receptors in the cerebellum, used as a positive control, were unchanged by chronic naltrexone treatment. The findings of opiate antagonist-induced increases in SP striatal content and Ins(1,4,5)P3 receptor densities, appear to support the concept of a role of endogenous opioids in the regulation of SP receptor activity. The data also suggest that inter-regulatory mechanisms exist between phospholipase C/phosphoinositide-coupled receptors such as SP receptors, and adenylate cyclase-coupled inhibitory receptors, such as opioid receptors.

Effect of diabetes on the antinociceptive effect of beta-endorphin

We examined whether streptozotocin-induced diabetes can modulate beta-endorphin-induced antinociception in mice. While beta-endorphin administered i.c.v. produced a dose-dependent inhibition of the tail-flick response in both diabetic and non-diabetic mice, the antinociceptive response was greater in diabetic mice than in non-diabetic mice. The ED50 value of beta-endorphin administered i.c.v. in diabetic mice was significantly lower than that in non-diabetic mice. The antinociceptive effects of beta-endorphin administered i.c.v. in both diabetic and non-diabetic mice were significantly antagonized by s.c. administration of naltrindole, a selective delta-opioid receptor antagonist. beta-Endorphin administered i.t. also produced a dose-dependent antinociception in both diabetic and non-diabetic mice. However, the ED50 value of kappa-opioid receptor antagonist. On the other hand, the antinociceptive potency of DPDPE, a selective delta-opioid agonist, administered i.t. is significantly increased in diabetic mice, as compared with non-diabetic mice, whereas, the antinociceptive potency of U-50,488H, a kappa-opioid receptor agonist, administered i.t. is significantly less than in non-diabetic mice. These results suggest that diabetes may modulate beta-endorphin-induced antinociception differently at the spinal and supraspinal levels.

Modification of morphine-induced locomotor activity by pertussis toxin: biochemical and behavioral studies in mice

The effect of pertussis toxin (PTX) on the locomotor-enhancing action of systemic and intracerebroventricular (i.c.v.) morphine was investigated in mice. Mice were i.c.v. injected with either PTX (0.25 and 0.5 micrograms) or saline as a control. The s.c. (5-20 mg/kg) and i.c.v. (7-30 nmol) administration of morphine produced a dose-related locomotor-enhancing action in control mice. The peak effect of morphine (30 nmol, i.c.v.)-induced hyperlocomotion was observed 90 min after the morphine injection. At the same time, morphine significantly increased dopamine (DA) metabolism in the limbic forebrain (nucleus accumbens and olfactory tubercle). Similarly, the selective mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO, 4 nmol, i.c.v.) also significantly increased locomotor activity and DA metabolism in the limbic forebrain. Both morphine- and DAGO-induced hyperlocomotion and elevation of DA turnover were antagonized by pretreatment with the mu antagonist beta-funaltrexamine (beta-FNA). These results suggest that the locomotor-enhancing action of morphine results from the activation of central mu-opioid receptors, and that the activation of the mesolimbic DA system may be involved in the expression of morphine-induced hyperlocomotion in mice. Furthermore, pretreatment with PTX (0.5 micrograms, i.c.v., 6 days prior to the testing) significantly reduced hyperlocomotion and elevation of DA turnover in the limbic forebrain which had been induced by administrations of morphine (30 nmol, i.c.v.) and DAGO (4 nmol, i.c.v.). These findings suggest that the central PTX-sensitive GTP-binding protein (G-protein) mechanism may play an important role in opioids-induced locomotor-enhancing action. Furthermore, the activation of mesolimbic DA transmission by mu-opioid agonists may also be mediated by a PTX-sensitive G-protein mechanism in mice.

Dose-related antagonism of the emetic effect of morphine by methylnaltrexone in dogs

Opioids administered to produce analgesia cause unwanted emesis in patients (incidence 20%-30%, depending on situation). Tests in animals show that quaternary narcotic antagonists like methylnaltrexone (MNTX) do not affect the analgesic potency of morphine, but such compounds have not been examined for their potential to antagonize morphine-induced emesis. To determine the effects of MNTX on emetic response, we assigned 85 dogs to one of 11 groups challenged with morphine alone or morphine and various doses of MNTX IM or i.v. Antagonism of the emetic response was dose related: MNTX, 0.25 mg/kg IM or 0.2 mg/kg i.v., completely blocked the emetic effect of morphine in dogs for approximately 60 minutes. If morphine-induced emesis is mediated by receptors available to a quaternary antagonist (perhaps on the peripheral side of the blood-brain barrier), MNTX may prevent opioid-induced emesis. These data indicate that opioid-induced emesis might be prevented without affecting analgesia.

Cessation of long-term naltrexone therapy and self-injury: a case study

The cessation of long-term Naltrexone administration was investigated through a double-blind, placebo-controlled, withdrawal design in a 28-year-old, profoundly mentally retarded woman with a history of severe self-injurious behavior (SIB). The subject had previously exhibited a dramatic decrease in SIB subsequent to Naltrexone administration. At the end of 1 year of continuous drug treatment, the subject displayed a near-zero rate of SIB episodes. This near-zero rate continued through placebo and no-drug phases of the study and at 6-month (no-drug) follow-up. This trend suggests not only that Naltrexone may be effective in reducing SIB, but that long-term administration may produce durable results after treatment cessation. Findings are discussed in relation to the endogenous opioid system theories of SIB.

Opioid antagonists: indirect antagonism of morphine analgesia by spinal dynorphin A

Naloxone and norbinaltorphimine when given ICV to mice can antagonize IT morphine-induced analgesia indirectly by releasing spinal dynorphin A(1-17) (Dyn A). Dyn A produces an antianalgesic action against IT morphine. In the present study, drugs with varying amounts of opioid antagonist to agonist action (nalbuphine, levallorphan, naltrexone, and naltrindole) were given ICV to determine whether they antagonized IT morphine-induced inhibition of the tail-flick response as an indication of spinal Dyn A release. Additional pharmacological tests were used as criteria for Dyn A release: a) Small doses of the opioid antagonists naloxone and norbinaltorphimine administered IT inhibited the antagonistic action; b) dynorphin antiserum given IT blocked the action of Dyn A; c) desensitization to the effect of Dyn A was produced by 3-h pretreatment with morphine, 10 mg/kg SC, or by pretreatment with the agents themselves. When given ICV, nalbuphine, levallorphan, and naltrexone released Dyn A in the spinal cord to produce an antianalgesic effect. Naltrindole, a delta-receptor antagonist, did not release Dyn A. Dyn A release did not appear to involve delta-receptors. Thus, a number of opioid antagonists inhibit the analgesic action of opioid agonists indirectly through Dyn A release.

Involvement of spinal kappa opioid receptors in a type of footshock induced analgesia in mice

We have studied the effects of several opioid antagonists on a type of footshock stress-induced analgesia (FSIA) measured by the tail-flick test in male mice. Naloxone injected either subcutaneously (0.1-10 mg/kg) or intrathecally (1-20 micrograms) antagonized FSIA at higher doses than those that blocked a similar degree of analgesia induced by morphine. Intracerebroventricular (i.c.v.) naloxone (1-20 micrograms) did not modify the FSIA while antagonizing the i.c.v. morphine-induced analgesia. As a consequence, the antagonism of the FSIA by naloxone probably occurs at the level of the spinal cord and through receptors different than mu. The delta selective antagonist naltrindole (0.1-3 mg/kg s.c.) did not antagonize the analgesic effects of the stress. Nor-binaltorphimine, a kappa selective antagonist, blocked the FSIA when administered systemically (1-4 mg/kg i.p.) or locally (0.1-1 microgram i.t.). These results strongly suggest that spinal kappa opioid receptors are responsible for this type of endogenous analgesia.

Narcotic receptor blockade and its effect on the analgesic response to placebo and ibuprofen after oral surgery

The purpose of this study was to evaluate the contribution of endogenous opiates to the analgesic response after treatment with placebo, codeine, and ibuprofen after oral surgery. Eighty-one patients undergoing complicated dental extractions were pretreated with either a placebo or the narcotic antagonist naltrexone 50 mg, 30 minutes before surgery. After surgery, patients self administered one of three possible postsurgical medications, which included placebo, codeine 60 mg, and ibuprofen 400 mg, when their pain reached a moderate or severe intensity. The study was double-blind with the three postsurgical treatments being randomly allocated within each presurgical treatment block. Pain intensity, pain relief, pain half gone, and overall evaluations were assessed for up to 6 hours. Ibuprofen was significantly more efficacious (p < .05) than codeine or placebo for most analgesic measures. The administration of naltrexone before surgery reduced the analgesic response to both placebo and codeine. Pretreatment with naltrexone did not diminish the peak analgesic response to ibuprofen, but surprisingly prolonged (p < .05) the duration of its action. The results suggest that a blockade of endogenous opiates by naltrexone diminished the placebo response, but that naltrexone may prolong ibuprofen analgesia by some unknown mechanism.

Modulation of morphine-induced sensitization by endogenous kappa opioid systems in the rat

Sensitization to both the motor stimulant and mesolimbic dopamine-releasing effects of morphine were studied in animals chronically treated with morphine and those that had received the kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI) prior to the commencement of morphine treatment. Rats were pretreated with either nor-BNI (30 micrograms; i.c.v.) or its vehicle and then received injections of morphine for 10 days. Locomotor activity and microdialysis studies were then conducted 3 and 30 days after termination of the chronic morphine treatment. In chronic morphine-treated rats, sensitization developed to both the motor stimulatory effects of morphine and the mesolimbic dopamine-releasing effects of this drug. Sensitization was observed 3 and 30 days after termination of morphine treatment. In animals pretreated with nor-BNI, sensitization to both the motoric and dopamine-releasing effects of morphine was significantly greater than that of chronic morphine-treated rats. These results suggest that endogenous kappa opioid systems play an important role in morphine-induced sensitization and that manipulations of these systems can markedly influence both its behavioral and neurochemical expression.

Determination of nalmefene in plasma by high-performance liquid chromatography with electrochemical detection and its application in pharmacokinetic studies

A method for measuring human plasma levels of nalmefene after oral and intravenous administration is presented. The method consists of a solid-phase extraction procedure followed by HPLC analyses. A cyanopropyl column is used for the solid-phase extraction and 60% (v/v) acetonitrile in dilute sodium pentanesulfonic acid solution for elution. The concentrated and filtered eluate is injected into the HPLC system, which is equipped with an electrochemical dual-electrode detector. A phenyl column is used in this HPLC system with a mobile phase containing 30% (v/v) acetonitrile in dilute sodium pentanesulfonic acid solution. A signal-to-noise ratio of 4.5 is obtained when a 1 ng/ml spiked plasma sample is analyzed. To determine the applicability of this method for human pharmacokinetic studies, nalmefene levels in plasma were measured at time points up to 24 h following oral and intravenous administration of 30 mg of nalmefene hydrochloride to two subjects. These studies demonstrated that the proposed method is sufficiently sensitive to study the pharmacokinetic profile of nalmefene in man.

Chronic food restriction and weight loss produce opioid facilitation of perifornical hypothalamic self-stimulation

Electrical stimulation frequency thresholds for lateral hypothalamic (LH) self-stimulation were monitored throughout a 3 week period of food restriction and a subsequent 3 week period of re-feeding. Rats with electrodes placed in the perifornical LH were sensitive to this dietary manipulation as evidenced by a high positive correlation between body weight and self-stimulation threshold. Rats with electrodes in the zona incerta/subincertal region or ventral hypothalamus displayed little or no change in threshold. Lateral ventricular injection of naltrexone (200.0 nM) reversed the decline in threshold that was otherwise present during food restriction in rats with perifornical placements. Naltrexone had no effect on thresholds of rats with placements outside the perifornical region. These findings suggest that food restriction and weight loss activate an opioid mechanism that facilitates perifornical LH self-stimulation. The documented association of perifornical LH with the phenomenon of stimulation-induced feeding, and the reciprocal connections between this region and gustatory structures, supports the hypothesis that facilitation of self-stimulation by food restriction is related to the natural phenomenon of positive alliesthesia (i.e. the hunger-dependency of food reward).

[Opiate hypothesis in infantile autism? Therapeutic trials with naltrexone]

The opioid hypothesis suggests that childhood autism may result from excessive brain opioid activity during neonatal period which may constitutionally inhibit social motivation, yielding autistic isolation and aloofness (Panksepp, 1979). This hypothesis has now received strong support and is currently based on three types of arguments: (1) similarity between autistic symptomatology and abnormal behaviors induced in young animals by injections of exogenous opioids, such as increasing social aloofness and decreasing social vocalization; (2) direct biochemical evidence of abnormalities of peripheral endogenous opioids being reported in autism and (3) therapeutic effects of the long lasting opioid receptor blocking agent naltrexone in autism. In this article, we give description of open and double-blind studies of naltrexone in autism. Naltrexone has been tested in several open studies. We performed an open trial with naltrexone in 2 autistic girls, displaying serious self-injurious behavior, reduced crying and a marked preference for salty and spicy foods, symptoms that could be related to a dysfunction of the opioid system. With dosages of 1 mg/kg/day, we observed an immediate reduction of hyperactivity, self-injurious behavior and aggressiveness, while attention improved. In addition, social behaviors, smiling, social seeking behaviors and play interactions increased (Leboyer, Bouvard et Dugas, 1988). Campbell et al. (1988) has also reported a tranquilizing and a stimulating effect in 6 out of 8 children with autism. We did confirm these preliminary results in a double-blind study performed on 4 children with autism. In a cross-over double-blind study, three dosages of naltrexone (0.5, 1 and 2 mg/kg/day) and placebo were compared.(ABSTRACT TRUNCATED AT 250 WORDS)

Effectiveness and safety of intravenous nalmefene for emergency department patients with suspected narcotic overdose: a pilot study

STUDY OBJECTIVE

To evaluate the efficacy and safety of nalmefene, an investigational narcotic antagonist that has potential advantages over naloxone because of its four- to eight-hour half-life, in emergency department patients with possible narcotic overdose.

DESIGN

Multi-institutional, prospective, phase II, open-label study.

TYPE OF PARTICIPANTS

Complete data were available for 53 cases from two teaching hospitals. Men 18 years old or older who would otherwise receive naloxone were eligible (two women were enrolled inadvertently).

METHODS

Over four hours, one to ten boluses (median, one) of 0.5 or 1.0 mg nalmefene IV were given as often as every two minutes based on clinical need. Respirations, blood pressure, pulse, pupil size, and overall clinical response were monitored. Overall clinical response (1, no change; 2, partial response; 3, complete response), first assessed at two minutes, was analyzed by the Mann-Whitney U test.

RESULTS

Fifteen of 25 (0.5 mg) and nine of 28 (1.0 mg) cases were opiate positive. Twelve of 15 (0.5 mg) and six of nine (1.0 mg) opiate-positive cases had a rapid complete response. Coincident causes of depressed sensorium were identified in the remaining six opiate-positive cases. No difference in initial overall clinical response was seen between 0.5-mg and 1.0-mg opiate-positive cases (P = .59). No deterioration requiring repeat nalmefene occurred in opiate-positive cases, even if methadone (four), codeine (two), or pentazocine (one) was found. No serious adverse events were judged to be related to nalmefene.

CONCLUSION

Nalmefene is effective in the reversal of opiate overdose and appears to be safe in the management of patients with altered sensorium.

Formalin-induced nociceptive responses in diabetic mice

In non-diabetic mice, s.c. injection of formalin to the hindpaw had a biphasic effect: an immediate nociceptive response (first-phase) followed by a tonic response (second-phase). However, only the immediate nociceptive response was observed in diabetic mice. The duration of the first-phase response was significantly longer in diabetic mice than in non-diabetic mice. In diabetic mice, when spantide, an antagonist of substance P, reduced the duration of the nociceptive response in the first-phase to the levels that were observed in non-diabetic mice, the second-phase response appeared. The second phase also became apparent in diabetic mice after pretreatment with naltrindole (3 mg/kg), an antagonist of delta-opioid receptors. These results suggest that a negative control system, which is mediated by delta-opioid receptors and links substance P with somatostatin-mediated nociceptive transmission, may inhibit the formalin-induced second-phase of the nociceptive response in diabetic mice.

Relative involvement of supraspinal and spinal mu opioid receptors in morphine dependence in mice

Involvement of supraspinal and spinal mu opioid receptors in the development degree of morphine dependence was estimated by the ED50 values of naloxone (s.c.) required to precipitate withdrawal jumping and diarrhea 72 hr after morphine-pellet implantation. beta-FNA was administered 4 times (24 hr before, just before, 24 and 48 hr after morphine-pellet implantation) by the i.c.v. or i.t. route. beta-FNA (both i.c.v. and i.t. routes) significantly increased the ED50 values of naloxone for jumping and diarrhea and the increase in the ED50 value of naloxone for jumping was much greater than that for diarrhea. I.c.v. administered beta-FNA was more potent in increasing the ED50 value of naloxone for jumping than i.t. administered beta-FNA. On the other hand, i.c.v. administered beta-FNA was equipotent with i.t. administered beta-FNA in increasing the ED50 value of naloxone for diarrhea. These results suggest that both supraspinal and spinal mu opioid receptors are involved in the development of morphine dependence and that supraspinal mu receptors play a more important role than spinal mu receptors in withdrawal jumping which may reflect an excitation of the central nervous system whereas supraspinal and spinal mu receptors have similar importance in withdrawal diarrhea which may reflect an abnormal function of the autonomic nervous system.

The effect of chronic treatment with naltrindole, a selective delta-opioid antagonist, on mu-opioid receptor-mediated antinociception in diabetic mice

The effects of chronic treatment with naltrindole (NTI), a selective delta-opioid receptor antagonist, on the antinociceptive effects of mu-opioid agonists, such as morphine and [D-Ala2, N-MePhe4, Gly-ol(5)]enkephalin (DAMGO) were examined in diabetic mice. Antinociception induced by morphine (10 micrograms, ICV) and DAMGO (0.5 microgram, ICV) was significantly lower in diabetic mice than in non-diabetic mice. The low sensitivities to the antinociceptive potencies of ICV morphine (10 micrograms) and DAMGO (0.5 micrograms) in diabetic mice were reversed compared with those in saline-treated non-diabetic mice when diabetic mice had been pretreated with NTI (2 mg/kg per day, SC) for 14 days. Naive mice which had been injected with spleen mononuclear cells from saline-treated diabetic mice were less sensitive to DAMGO-induced antinociception. However, adoptive transfer of spleen mononuclear cells from NTI-treated diabetic mice to naive mice had no effect on the recipients' antinociceptive sensitivity to DAMGO. These results suggest that the effect of NTI on the sensitivity to mu-opioid agonists in diabetic mice may be due to the immunosuppressive effects of NTI.

Comparison of bolus and infusion methods for receptor quantitation: application to [18F]cyclofoxy and positron emission tomography

Positron emission tomography studies with the opiate antagonist [18F]cyclofoxy ([18F]CF) were performed in baboons. Bolus injection studies demonstrated initial uptake dependent on blood flow. The late uptake showed highest binding in caudate nuclei, amygdala, thalamus, and brainstem and the least accumulation in cerebellum. By 60 min postinjection, regional brain radioactivity cleared at the same rate as metabolite-corrected plasma, i.e., transient equilibrium was achieved. Compartmental modeling methods were applied to time-activity curves from brain and metabolite-corrected plasma. Individual rate constants were estimated with poor precision. The model estimate of the total volume of distribution (VT), representing the ratio of tissue radioactivity to metabolite-corrected plasma at equilibrium, was reliably determined. The apparent volume of distribution (Va), the concentration ratio of tissue to metabolite-corrected plasma during transient equilibrium, was compared with the fitted VT values to determine if single-scan methods could provide accurate receptor measurements. Va significantly overestimated VT and produced artificially high image contrast. These differences were predicted by compartment model theory and were caused by a plasma clearance rate that was close to the slowest tissue clearance rate. To develop a simple method to measure VT, an infusion protocol consisting of bolus plus continuous infusion (B/I) of CF was designed and applied in a separate set of studies. The Va values from the B/I studies agreed with the VT values from both B/I and bolus studies. This infusion approach can produce accurate receptor measurements and has the potential to shorten scan time and simplify the acquisition and processing of scan and blood data.

A morphine-triggered delivery system useful in the treatment of heroin addiction

The ultimate objective of this work is to develop a device that can be triggered by morphine to release naltrexone. Two device configurations are described. In one configuration, naltrexone is dispersed in cellulose acetate phthalate microspheres which are then spray-coated with trilaurin. In the other configuration, naltrexone is dispersed in an n-octyl half ester of methyl vinyl ether and maleic anhydride copolymer and the mixture fabricated into a disk which is then coated with trilaurin. The microspheres are designed to release naltrexone abruptly while the disks are designed to release naltrexone at a constant rate over a two week period. The microspheres, or the disk along with a reversibly inactivated lipase are placed inside a semipermeable membrane that allows free passage of morphine and naltrexone but excludes the higher molecular weight components of the device. Reversible inactivation of lipase is achieved by covalent attachment of morphine and complexing with morphine antibody. Activation of the device occurs by diffusion of morphine into the device and displacing the lipase-morphine conjugate from the antibody. The activated lipase then removes the trilaurin protective coating, thus triggering naltrexone release.

Effects of chronic nocturnal opiate antagonism on the menstrual cycle

OBJECTIVE

Opioids generally suppress gonadotropin secretion. We investigated whether chronic opiate antagonism would affect the occurrence or timing of menstrual cycle events.

STUDY DESIGN

Opiate blockade was maintained in nine normal volunteers from cycle days 1 through 13 with naltrexone. A double-blind, placebo-controlled crossover design was used. Serum estradiol and progesterone were measured daily. Vaginal ultrasonography was performed daily beginning cycle day 10. Endometrial biopsies were performed 10 to 12 days after ovulation. Nonparametric paired data analysis was used.

RESULTS

No significant differences in the following menstrual cycle parameters were detected between naltrexone and placebo cycles: cycle length (25.4 +/- 0.9 vs 25.7 +/- 0.5 days), maximum follicle size (21.5 +/- 1.0 vs 20.8 +/- 1.1 mm), peak serum progesterone (15.8 +/- 1.8 vs 15.2 +/- 2.2 ng/ml), or integrated serum progesterone (108.1 +/- 12.8 vs 101.2 +/- 14.6 ng/ml/day). Histologic endometrial development was normal in all subjects during naltrexone cycles.

CONCLUSION

These data suggest that naltrexone-suppressible opioid activity does not play a significant role in the regulation of follicular events in normal women.

The delta receptor is involved in sufentanil-induced respiratory depression--opioid subreceptors mediate different effects

It is generally accepted that analgesia induced by central analgesics is mediated through the mu-receptor. However, it still remains open to question as to whether or not the mu- and/or the delta-receptor site is mainly involved in the mediation of opioid-related respiratory impairment. Using a highly selective antagonist, naltrindole (NTI), or its benzofuran analogue naltriben (NTB), the hypothesis that competitive antagonism at the delta-receptor is able to attenuate sufentanil-related respiratory depression was tested in the dog. High dose (20 micrograms kg-1) sufentanil-induced respiratory impairment could be reversed by selective NTI-antagonism in a dose-related fashion (40-80-160 micrograms kg-1) increasing PaO2 from 57 to 81 mmHg and lowering PaCO2 from 52.1 to 49.2 mmHg. NTB-antagonism (40-80-160 micrograms kg-1) increased PaO2 from 48.4 to 91.2 mmHg and reduced PaCO2 from 46.9 to 37.6 mmHg. Simultaneously, somatosensory-evoked potentials (SEP) were used to quantify the opioid-induced attenuation and the reversal of afferent sensory input to pain modulating centres in the CNS. Sufentanil induced a significant depression (P < 0.01) of amplitude height of the SEP (13.9 to 0.9 microV in the NTI- and 8.8 microV to 1.3 microV in the NTB-group) which was only partially reversed by NTI (2.6 microV) and NTB (2.3 microV) respectively. The results suggest that delta-receptors are involved in sufentanil-related respiratory impairment. These receptors play a minor role in opioid-induced attenuation of sensory input to the brain. Highly selective delta-antagonists may be of clinical interest in reversing the respiratory depressant effect of potent opioids while maintaining analgesia.

Discriminative stimulus properties of cocaine, alone and in combination with buprenorphine, morphine and naltrexone

Rats were trained to discriminate a dose of 10.0 mg/kg cocaine from saline. During substitution tests, both cocaine (5.6-10.0 mg/kg) and d-amphetamine (1.0-3.0 mg/kg) produced greater than 80% responding on the cocaine-appropriate level. In contrast, buprenorphine (0.03-0.56 mg/kg), morphine (0.3-10.0 mg/kg) and naltrexone (1.0-10.0 mg/kg) failed to substitute for the cocaine stimulus, up to doses that substantially decreased rate of responding. When the cocaine dose-effect curve was redetermined in the presence of selected doses of buprenorphine, the amount of cocaine-appropriate responding following a low dose of cocaine (1.0 mg/kg) was increased slightly whereas cocaine-appropriate responding following higher doses of cocaine (3.0 and 5.6 mg/kg) was reduced slightly. Responding following the training dose of cocaine (10.0 mg/kg) was not changed. These results indicate that buprenorphine produced only small alterations in cocaine's discriminative stimulus effects and that the nature of these alterations differed depending on the dose of cocaine examined.

Delta opiate receptors mediate tail-shock induced antinociception at supraspinal levels

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The preceding companion paper and the present work were aimed at identifying the neuroanatomical loci at which opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 8 experiments included in the present paper examined the effect of microinjecting either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist) either into the third ventricle or over the frontal cortex. Taken together, these experiments demonstrate that the late (80-100 shock) opiate analgesia is mediated by delta opiate receptors located within subcortical structures rostral to the 4th ventricle. No evidence for supraspinal opiate involvement in the early (2 shock) opiate analgesia was found.

The effect of lipase on the release of naltrexone from triglyceride-coated cellulose acetate phthalate microspheres

The ultimate objective of this work is to develop a device that can be triggered by morphine to release naltrexone. In this device, naltrexone is dispersed in cellulose acetate phthalate microspheres which are then spray-coated with a trilaurin protective coating. The microspheres are contained within a macroporous cylinder which also contains a reversibly inactivated lipase. This enzyme in its inactive state is unable to remove the protective coating but in its active state is able to do so. Inactivation is achieved by the covalent attachment of morphine followed by complexation with a morphine antibody. Triggering is accomplished by the displacement of the lipase-morphine conjugate from the antibody. In this phase we have investigated the effect of lipase on the release of naltrexone from trilaurin-coated microspheres and found that the coated microspheres are stable in a pH 7.4 phosphate buffer at 37 degrees C for at least 1 month, but release 80% of the incorporated naltrexone in one hour when 100 mg of capsules in 5 mL buffer are exposed to 25 micrograms of lipase.

Kappa opiate receptors mediate tail-shock induced antinociception at spinal levels

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The present work and the following companion paper were aimed at identifying the neuroanatomical loci at which endogenous opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 3 experiments included in the present paper focus on the role of spinal opiates in tail-shock induced analgesia. The first experiment demonstrates that the tail-shock parameters used do not directly activate pain suppressive circuitry within the spinal cord, but rather activate centrifugal pain modulation circuitry originating within the brain. The last two experiments examine the effect of intrathecal microinjection of either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist). Taken together, these latter 2 experiments demonstrate that both the early (after 2 shocks) and late (after 80-100 shocks) opiate analgesias are mediated by kappa opiate receptors within the spinal cord.

Pentobarbital attenuates antinociception induced by i.c.v. morphine but not beta-endorphin in the mouse

The effects of pentobarbital anesthesia (45 mg/kg i.p.) on the inhibition of the tail-flick response induced by beta-endorphin and morphine injected intracerebroventricularly (i.c.v.) and intrathecally (i.t.) were studied in male ICR mice. Pentobarbital anesthesia attenuated the inhibition of the tail-flick response induced by morphine but not beta-endorphin given i.c.v. However, the tail-flick inhibition induced by morphine given i.t. was not attenuated by pentobarbital. beta-Endorphin-(1-27) (3 micrograms) given i.c.v. or naloxone (2 micrograms) given i.t. blocked inhibition of the tail-flick response induced by morphine given i.c.v. only in pentobarbital-anesthetized mice but not in conscious mice. beta-Funaltrexamine (beta-FNA, 2.5 micrograms) given i.c.v. or yohimbine (2 micrograms) and methysergide (2 micrograms) injected i.t. blocked the morphine (i.c.v.)-induced inhibition of the tail-flick response in conscious mice but not in pentobarbital-anesthetized mice. The results indicate that pentobarbital attenuates the morphine-induced inhibition of the tail-flick response by inhibiting descending noradrenergic and serotonergic pathways and uncovers a descending opioid system. The tail-flick inhibition induced by supraspinal morphine is mediated by stimulation of mu-opioid receptors in conscious mice and epsilon-opioid receptors in pentobarbital-anesthetized mice. The epsilon-opioid receptor-mediated descending system activated by supraspinally injected beta-endorphin is not attenuated by pentobarbital anesthesia.

Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

The mesolimbic dopaminergic system has been implicated in mediating the motivational effects of opioids and other drugs of abuse. The site of action of opioids within this system and the role of endogenous opioid peptides in modulating dopamine activity therein remain unknown. Employing the technique of in vivo microdialysis and the administration of highly selective opioid ligands, the present study demonstrates the existence of tonically active and functionally opposing mu and kappa opioid systems that regulate dopamine release in the nucleus accumbens, the major terminal area of A10 dopaminergic neurons. Thus, stimulation of mu-type receptors in the ventral tegmental area, the site of origin of A10 dopaminergic neurons, increases dopamine release whereas the selective blockade of this opioid receptor type results in a significant decrease in basal dopamine release. In contrast, stimulation of kappa-type receptors within the nucleus accumbens decreases dopamine release whereas their selective blockade markedly increases basal dopamine release. These data show that tonic activation of mu and kappa receptors is required for the maintenance of basal dopamine release in the nucleus accumbens. In view of the postulated role of the mesolimbic system in the mediation of drug-induced alterations in mood and affect, such findings may have implications for the treatment of opiate dependence and affective disorders.

Naloxone and norbinaltorphimine administered intracerebroventricularly antagonize spinal morphine-induced antinociception in mice through the antianalgesic action of spinal dynorphin A (1-17)

Previously, a number of analgesic agonists, when administered i.c.v. to mice, were shown putatively to activate the release of dynorphin A (1-17) (Dyn A) in the spinal cord. Whether released endogenously or administered i.t., Dyn A produces an antianalgesic action against i.t. administered morphine. In the present study, the opioid antagonists, naloxone and norbinaltorphimine (N-BNI), were shown to activate the Dyn A system. Intracerebro-ventricular administration of both naloxone and N-BNI antagonized the antinociceptive effect of i.t. morphine in the mouse tail-flick test, an effect designated as an antianalgesic action. This antianalgesic action was demonstrated to be mediated by spinal Dyn A in the following ways: 1) the antagonistic effect of i.c.v. naloxone and N-BNI was eliminated by administration of small doses of i.t. naloxone and N-BNI, a unique situation where administration of the opioid antagonists at a second (i.t.) site reversed the antagonistic effect of opioid antagonists administered at the other (i.c.v.) site; 2) i.t. pretreatment with dynorphin antiserum prevented the antianalgesic effect; 3) morphine pretreatment (s.c., 10 mg/kg), which produces desensitization to the effect of spinal Dyn A, eliminated the antianalgesic effect; and 4) pretreatment with i.c.v. naloxone (3 hours) and N-BNI (24 hours) which presumably releases Dyn A produced desensitization to the antagonistic effect of i.c.v. naloxone and N-BNI as well as to the antianalgesic action of i.t. Dyn A. Taken together, the results indicate that both i.c.v. naloxone and N-BNI produced indirect antagonistic actions which were mediated at the spinal cord by the antianalgesic action of Dyn A.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nor-binaltorphimine on the development of analgesic tolerance to and physical dependence on morphine

The effects of a highly selective kappa antagonist, nor-binaltorphimine (nor-BNI), on the development of tolerance to morphine analgesia and physical dependence on morphine were examined. Pretreatment with nor-BNI (5 mg/kg s.c.) 2 h prior to injection of morphine or a selective kappa agonist, U-50,488H, significantly antagonized the analgesic effect of U-50,488H, but not morphine analgesia in mice. The development of tolerance to morphine analgesia was significantly potentiated by pretreatment of mice with nor-BNI 2 h prior to morphine treatment during chronic morphine treatment for 5 days. Additionally, the pretreatment with nor-BNI during chronic treatment with the high dose of morphine for 5 days significantly potentiated the naloxone-induced body weight loss in morphine-dependent mice and rats. These findings suggest that inactivation of the kappa opioid system may potentiate the development of tolerance to morphine analgesia in mice and may aggravate the naloxone-precipitated body weight loss in morphine-dependent mice and rats.

Distinction between the in vitro and in vivo inhibitory effects of morphine on lymphocyte proliferation based on agonist sensitivity and naltrexone reversibility

We have previously reported that administration of a single dose of morphine (25 mg/kg) to rats results in a naltrexone-sensitive suppression of mitogen-stimulated lymphocyte proliferation. To further delineate the site of action of this inhibitory effect, the in vitro and in vivo effects of morphine on mitogen-stimulated lymphocyte proliferation were examined. In vitro, concentrations of morphine exceeding 0.1 mM exhibited a dose-dependent inhibition of Concanavalin A-induced proliferation of both whole blood and splenic lymphocytes. This inhibitory effect of morphine on lymphocyte proliferation was not attenuated by co-incubation with the opioid antagonist naltrexone (0.25 mM). These data indicate that the in vitro inhibitory effects of morphine occur at only high concentrations and are not opioid receptor mediated. In vivo, a dose-dependent inhibition of blood lymphocyte proliferation was also observed 2 h following the subcutaneous injection of morphine. In contrast to these effects, proliferation of splenic lymphocyte cultures was not significantly inhibited by morphine at doses of up to 40 mg/kg. However, following morphine administration, a greater than 90% inhibition of proliferation was obtained in cultures containing either whole blood or Ficoll-separated lymphocytes, indicating that plasma was not a contributory factor in the differential sensitivity of blood and splenic lymphocyte responses to morphine. Moreover, in these experiments, significant inhibition of lymphocyte proliferation occurred at plasma concentrations that were two orders of magnitude less than those required to produce inhibition in vitro. The in vivo inhibition of lymphocyte proliferation by morphine (10 mg/kg) was completely antagonized by pretreatment with naltrexone (5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Naltrexone-induced aversions: assessment by place conditioning, taste reactivity, and taste avoidance paradigms

The reinforcing/aversive properties of various doses of naltrexone (0.01, 1, and 10 mg/kg) were assessed in three experiments that employed place conditioning, taste reactivity, and taste avoidance paradigms. Naltrexone produced a place aversion and a taste aversion, but did not produce aversive taste reactivity responses, even at the highest dose (10 mg/kg) tested. This suggests that drugs that produce a place aversion do not necessarily produce a conditional dislike for a flavored solution with which they are paired.