Naloxone-precipitated Morphine Withdrawal Syndrome Research Articles

Antagonism of orexin type-1 receptors (OX1Rs) attenuates naloxone-precipitated morphine withdrawal syndrome in rat dorsal hippocampus

Herein the effect of hippocampal orexin type-1 receptors (OX1Rs) blockade on morphine withdrawal syndrome was studied. Animals were made dependent by subcutaneous (s.c.) administration of morphine sulfate (10mg/kg) at an interval of 12h for 9 consecutive days. Thereafter, on day 10, naloxone hydrochloride (1.5mg/kg, i.p.) was injected and the somatic signs of withdrawal syndrome were monitored during a 25-min period. Two groups of animals received bilateral microinjection of either SB-334867, a selective OX1Rs antagonist, (0.5μg/0.5μl), or its vehicle into the dorsal hippocampus immediately before each morphine injection. Other groups of animals were made dependent at first and only received a single microinjection of SB-334867 or vehicle on day 10 before naloxone injection. The results showed that intra-hippocampal microinjection of SB-334867 before each morphine treatment, significantly decreased the signs of morphine withdrawal, including teeth chattering (dependent: 18.5±2.3, SB treated: 5±1, p<0.001), diarrhea (dependent: 8.7±0.6, SB treated: 4.1±0.6, p<0.001), ptosis (dependent: 33.8±3.7, SB treated: 11.6±1.1, p<0.001), and chewing (dependent: 40±2.3, SB treated: 29±2.4, p<0.01). SB-334867 did not attenuate withdrawal syndrome, when it was microinjected as a single dose immediately before naloxone injection. The present results suggest a role for orexin in naloxone-precipitated withdrawal and thus possibly morphine dependence and this effect is, at least in part, via OX1Rs in the dorsal hippocampus.

Antinociceptive and antidepressant-like effects of the crude extract of Vitex megapotamica in rats

Ethnopharmacological relevanceVitex megapotamica (Spreng) Moldenke has been used in South American folk medicine to treat inflammatory diseases. However, the effects of V. megapotamica on animal models of nociception and depression have not been evaluated. Aim of the studyThis study investigated whether the crude leaf extract of V. megapotamica exhibits antinociceptive and antidepressant-like effects in a Freund’s adjuvant-induced chronic inflammation and depression model. Materials and methodsChronic inflammation was induced in rats by the intraplantar administration of complete Freund's adjuvant (CFA; 100μl). The effect of oral crude extract of V. megapotamica (VmE; 3–30mg/kg, p.o.) on nociception (thermal hyperalgesia, mechanical allodynia and arthritis score), inflammation (edema, myeloperoxidase activity), immobility (forced swimming test), locomotor activity (open field), gastrointestinal transit, hyperalgesia and naloxone-precipitated morphine withdrawal syndrome was evaluated. Naloxone (0.4mg/kg, i.p.) was used to investigate the involvement of opioid system in the currently described effects of VmE. ResultsCrude extract caused antinociceptive/antidepressant-like effects in the CFA-induced chronic inflammation model, which was prevented by naloxone. The VmE extract (10mg/kg, p.o.) did not alter the locomotor activity, gastrointestinal function and inflammatory parameters and did not cause hyperalgesia. ConclusionV. megapotamica induces opioid-dependent antinociception and antidepressant-like effect, without anti-inflammatory activity. The results support the use of VmE as analgesic and antidepressant.

Baclofen prevents the elevated plus maze behavior and BDNF expression during naloxone precipitated morphine withdrawal in male and female mice

In previous studies we have shown that baclofen, a selective GABAB receptor agonist, prevents the somatic expression and reestablishes the dopamine and μ-opioid receptors levels, modified during naloxone-precipitated morphine withdrawal syndrome in male and female mice. There are no previous reports regarding sex differences in the elevated plus maze (EPM) and the expression of BDNF in morphine-withdrawn mice. The present study analyses the behavioral and biochemical variations during morphine withdrawal in mice of both sexes, and whether these variations are prevented with baclofen. Swiss-Webster albino prepubertal mice received morphine (2 mg/kg, i.p.) twice daily, for 9 consecutive days. On the 10th day, one group of morphine-treated mice received naloxone (opioid receptor antagonist; 6 mg/kg, i.p.) 1 h after the last dose of morphine to precipitate withdrawal. A second group received baclofen (2 mg/kg, i.p.) before naloxone administration. The EPM behavior was measured during 15 min after naloxone injection. The expression of BDNF-positive cells was determined by immunohistochemistry. Withdrawn male mice showed a higher percentage of time spent and number of entries to the open arms compared to withdrawn female mice. Baclofen prevented this behavior in both sexes. BDNF expression decreased in the AcbC, BNST, CeC, and CA3 of the hippocampus while increased in the BLA of morphine withdrawn male. Baclofen pretreatment prevented the BDNF expression observed in morphine withdrawn male mice in all the brain areas studied except in the CeC. Baclofen prevention of the EPM behavior associated to morphine withdrawal could be partially related to changes in BDNF expression.

In vivo and in vitro attenuation of naloxone-precipitated experimental opioid withdrawal syndrome by insulin and selective KATP channel modulator.

Opiate exposure for longer duration develops state of dependence in humans and animals, which is revealed by signs and symptoms of withdrawal precipitated by opioid receptor antagonists. The sudden withdrawal of opioids produces a withdrawal syndrome in opioid-dependent subjects. Insulin and ATP-sensitive potassium (KATP) channel-mediated glucose homeostasis have been shown to modulate morphine withdrawal. Present study has been structured to investigate the role of insulin and pharmacological modulator of KATP channel (gliclazide) in experimental morphine withdrawal syndrome, both invivo and invitro. In this study, naloxone-precipitated morphine withdrawal syndrome in mice (invivo) as well as in rat ileum (invitro) were utilized to assess opioid withdrawal phenomenon. Morphine withdrawal syndromes like jumping and rearing frequency, forepaw licking, circling, fore paw tremor, wet dog shake, sneezing, overall morphine withdrawal severity (OMWS), serum glucose, brain malondialdehyde (MDA), glutathione (GSH), nitrite/nitrate, and calcium (Ca(+2)) were assessed. Naloxone has significantly increased morphine withdrawal syndrome, both invivo and invitro. Insulin and gliclazide have significantly attenuated, naloxone induced behavioral changes like jumping and rearing frequency, forepaw licking, wet dog shake, sneezing, straightening, circling, OMWS, and various biochemical impairments such as serum glucose, brain MDA, GSH, nitrite/nitrate, and Ca(+2) in morphine-dependent animals (invivo). In vitro, insulin and gliclazide have significantly reduced naloxone-induced contraction in morphine-withdrawn rat ileum preparation. Insulin and gliclazide (KATP channel blocker) have attenuated naloxone-precipitated morphine withdrawal syndrome, both invivo and invitro. Thus, insulin and KATP channel modulation may provide new avenues for research in morphine withdrawal.

A comparison of the effects of digoxin, ouabain and milrinone on naloxone-precipitated withdrawal syndrome in mice

Modulation of Na+, K+-ATPase activity by acute and chronic opiates has been established for many years. However, the effects of digoxin, a putative inhibitor of Na+, K+-ATPase, on naloxone-precipitated morphine withdrawal syndrome are unknown. In the present study, a digoxin dose-response curve was conducted to observe the effects on naloxone-precipitated withdrawal and locomotor activity in mice. Higher doses of digoxin (1.0 and 2.5mg/kg) inhibited locomotor activity and naloxone-precipitated withdrawal jumping and weight loss, while lower doses of digoxin (0.1 and 0.25mg/kg) inhibited withdrawal weight loss precipitated by naloxone without affecting locomotor activity and naloxone-precipitated withdrawal jumping. To explore the possible mechanisms underlying this behavior, another Na+, K+-ATPase inhibitor ouabain, which does not cross the blood brain barrier, and another cardiotonic drug milrinone, a non-inhibitor of Na+, K+-ATPase, were also included in the present study. Both milrinone and ouabain inhibited, in a dose-dependent manner, naloxone-precipitated weight loss while neither affected naloxone-precipitated withdrawal jumping nor locomotor activity in mice. These results indicate that both the cardiotonic effects and central inhibition of Na+, K+-ATPase contribute to the inhibitory effects of digoxin on morphine withdrawal syndrome in mice.

The prolactin-releasing peptide antagonizes the opioid system through its receptor GPR10

Prolactin-releasing peptide (PrRP) and its receptor G protein-coupled receptor 10 (GPR10) are expressed in brain areas involved in the processing of nociceptive signals. We investigated the role of this new neuropeptidergic system in GPR10-knockout mice. These mice had higher nociceptive thresholds and stronger stress-induced analgesia than wild-type mice, differences that were suppressed by naloxone treatment. In addition, potentiation of morphine-induced antinociception and reduction of morphine tolerance were observed in mutants. Intracerebroventricular administration of PrRP in wild-type mice promoted hyperalgesia and reversed morphine-induced antinociception. PrRP administration had no effect on GPR10-mutant mice, showing that its effects are mediated by GPR10. Anti-opioid effects of neuropeptide FF were found to require a functional PrRP-GPR10 system. Finally, GPR10 deficiency enhanced the acquisition of morphine-induced conditioned place preference and decreased the severity of naloxone-precipitated morphine withdrawal syndrome. Altogether, our data identify the PrRP-GPR10 system as a new and potent negative modulator of the opioid system.

Serotonin reuptake inhibitors attenuate morphine withdrawal syndrome in neonatal rats passively exposed to morphine

Previous investigations had shown that inhibitor of serotonin reuptake transporter (SERT) could attenuate morphine withdrawal syndrome in adult animals. In the present study, we determined whether postnatal injection of serotonin reuptake inhibitors, fluoxetine, clomipramine, or citalopram, is able to attenuate the expression of the naloxone-precipitated morphine withdrawal syndrome in 5-day-old neonatal Sprauge–Dawley rats born to dams rat that received morphine injection since a week before mating till 5 days after delivery. Withdrawal syndrome of morphine, manifested as frequent abdominal stretching and yawning, was generated by injection of naloxone on postnatal day 5. Pre-injection with fluoxetine, clomipramine, or citalopram, significantly attenuated the naloxone-precipitated syndrome in a dose-dependent manner without apparent side effect. The rank order of inhibitory potency is citalopram=clomipramine>fluoxetine. This result suggests that inhibitor of SERT may be of potential in treating neonatal morphine withdrawal syndrome.

Effects of harman and harmine on naloxone-precipitated withdrawal syndrome in morphine-dependent rats

The effects of the beta-carbolines, harman and harmine, on naloxone-precipitated withdrawal syndrome in morphine-dependent rats were investigated. Two morphine pellets containing 75 mg morphine base were implanted subcutaneously in the scapular area of adult male Wistar rats (200–250 g) under light ether anesthesia. Rats were then assigned to several groups (n = 12 for each group). Seventy-two hours after morphine implantation, harman (5 and 10 mg/kg), harmine (5 and 10 mg/kg) or saline was injected to rats intraperitoneally (ip). After 45 min, a morphine withdrawal syndrome was precipitated by naloxone (2 mg/kg, ip), and morphine withdrawal signs were observed and evaluated for 15 min. Harmine (5 and 10 mg/kg) attenuated significantly the intensity of all signs of morphine withdrawal except for jumping. While jumping behaviour appearing in morphine withdrawal was intensified by harman (5 and 10 mg/kg) treatment, harmine administration did not produce any significant change in the intensity of this sign. Harman attenuated significantly the intensity of wet dog shakes, writhing, defecation, tremor and ptosis. However, it produced no significant changes in the intensity of teeth chattering and diarrhea. Our results suggest that harman and harmine, β-carbolines, have some beneficial effects on naloxone-precipitated morphine withdrawal syndrome in rats. Findings from the present study also indicated that harmine was more effective than harman on morphine abstinence syndrome.

Increase of morphine withdrawal in mice lacking A2a receptors and no changes in CB1/A2a double knockout mice

CB1 cannabinoid and A2a adenosine receptors are highly expressed in the central nervous system where they modulate numerous physiological processes including emotional behaviour and the responses of several drugs of abuse. To investigate the contribution of these receptors in emotional-like responses and opioid dependence we have generated CB1/A2a double deficient mice (CB1-/-/A2a-/-). The spontaneous locomotor activity was reduced in double knockout as compared to wild-type animals. Emotional-like responses of CB1-/-/A2a-/- mice were investigated using the elevated plus-maze and the lit-dark box. Mutant mice exhibited an increased level of anxiety in both behavioural models. The specific involvement of CB1 and A2a receptors in morphine dependence was evaluated by using A2a knockout mice and CB1/A2a double mutant mice. The severity of naloxone-precipitated morphine withdrawal syndrome was significantly increased in the absence of A2a adenosine receptors whereas no modifications were observed in the double knockout mice. These results suggest that both receptors participate in the control of emotional behaviour and seem to play an opposite role in the expression of opioid physical dependence.

Dextromethorphan attenuates morphine withdrawal syndrome in neonatal rats passively exposed to morphine

We had previously found that co-injection of dextromethorphan, an antitussive drug and a non-competitive NMDA receptor antagonist, with morphine into dam rats throughout the pregnancy period could attenuate the naloxone-precipitated morphine withdrawal syndrome in their offspring. In the present study, we further tested whether postnatal injection of dextromethorphan into the neonatal rats or a 3-day co-injection of dextromethorphan with morphine into the dam rats before delivery is also effective. Female Sprague–Dawley rats were bi-daily injected with escalating doses of morphine from a week before mating till the first postnatal week. Withdrawal syndrome of morphine in the offspring, manifested mainly as abdominal stretching, was generated by injection of naloxone on postnatal day 5. Direct injection of dextromethorphan into the offspring effectively reduced the severity of naloxone-precipitated abdominal stretching in a dose-dependent manner. A 3-day co-treatment with dextromethorphan given to the dam rat before delivery also had a similar attenuating effect, but the efficacy was lower than that produced by postnatal injection. Thus, the results from the present study support that dextromethorphan is of potential in treating or preventing neonatal morphine withdrawal syndrome.

Involvement of cyclic AMP systems in morphine physical dependence in mice: prevention of development of morphine dependence by rolipram, a phosphodiesterase 4 inhibitor.

In this study, we examined whether morphine dependence was inhibited by rolipram, a cyclic AMP selective phosphodiesterase inhibitor in mice, since a role for the cyclic AMP systems in the development of morphine dependence has been reported. Mice, which received morphine (10 mg kg(-1) s.c.) twice a day for 5 days showed withdrawal syndromes such as jumping, rearing and forepaw tremor following naloxone challenge (5 mg kg(-1) i.p.) on the 6th day. Such mice exhibited a significant elevation of cyclic AMP levels in the thalamus compared to control mice. However, co-administration of rolipram (1 mg kg(-1) i.p.) with morphine for 5 days significantly attenuated the severity of the withdrawal syndrome and the increase in the cyclic AMP levels after the administration of naloxone. In naïve mice, acute morphine treatment (10 mg kg(-1) s.c.) decreased cyclic AMP levels in the thalamus and cerebral cortex 10 min later. The decrease of cyclic AMP levels induced by acute morphine treatment was blocked by co-administration of rolipram (1 mg kg(-1) i.p.). However, acute rolipram did not affect the naloxone-precipitated morphine withdrawal syndrome. These results suggest that the elevation of the cyclic AMP levels is involved in the development of morphine withdrawal syndrome and that blockade of the morphine-induced reduction of cyclic AMP levels by chronic rolipram inhibits the development of dependence and the behavioural and biochemical changes induced by naloxone. Furthermore, rolipram may be a useful drug for attenuating the development of morphine dependence.

Possible involvement of the locus coeruleus in inhibition by prostanoid EP 3 receptor-selective agonists of morphine withdrawal syndrome in rats

We examined the mechanism of the inhibitory effect of prostanoid EP 3 receptor agonists on naloxone-precipitated withdrawal syndrome in morphine-dependent rats. Rats were rendered morphine dependent by subcutaneous (s.c.) implantation of two pellets containing 75 mg morphine for 5 days. Morphine withdrawal syndrome was precipitated by i.p. injection of naloxone (3 mg/kg). Intracerebroventricular (i.c.v.) administration of (±)-15α-hydroxy-9-oxo-16-phenoxy-17,18,19,20-tetranorprost-13- trans-enoic acid (M&B28,767: prostanoid EP 3 receptor agonist) or sulprostone (prostanoid EP 1/EP 3 receptor agonist) significantly suppressed many withdrawal signs. Northern blotting and in situ hybridization studies revealed that i.c.v. administration of M&B28,767 (1 pg/rat) attenuated the elevation of c- fos mRNA during naloxone-precipitated withdrawal in many brain regions, including the cerebral cortex, thalamus, hypothalamus and locus coeruleus. Double in situ hybridization analysis revealed that in the locus coeruleus most of the tyrosine hydroxylase mRNA-positive neurons expressed μ-opioid receptor mRNA and more than half of these neurons were positive for prostanoid EP 3 receptor mRNA. These results indicate that the suppression by prostanoid EP 3 receptor agonists of naloxone-precipitated morphine withdrawal syndrome can be attributed to the inhibition of neuronal activity in several brain regions, including the locus coeruleus, the largest source of central noradrenergic neurons.

Effect of alprazolam on opiate withdrawal: a combined behavioural and microdialysis study

Benzodiazepines are commonly abused concurrently with opioids. The pharmacological rationale for this remains unknown. The present study has addressed behaviourally and neurochemically the action of alprazolam on the naloxone-precipitated morphine withdrawal syndrome in male rats. In naloxone (1 mg kg −1 i.p.)-precipitated morphine withdrawn rats, alprazolam (2.5 mg kg −1 s.c.) reduced the severity of the affective component, as measured by squeal on touch hostility, and the physical sequelae of opioid withdrawal. The microdialysis study in anaesthetized rats identified an increase in noradrenaline levels in hippocampal dialysates in rats undergoing naloxone-precipitated opioid withdrawal. Acute treatment with alprazolam (2.5 mg kg −1 s.c.) 20 min before administration of naloxone prevented the previously identified increase in noradrenaline in hippocampal dialysates. The only observable effect alprazolam induced in non-morphine-dependent rats was a 15% reduction in spontaneous locomotor activity. In conclusion, one interpretation of the data suggests that alprazolam decreases the withdrawal syndrome in rats through dampening down the previously identified hyperactivity of the locus coeruleus.

Neurosteroids and the naloxone-precipitated withdrawal syndrome in morphine-dependent mice

The effects of some neurosteroids on behavioral and hormonal markers of the naloxone-precipitated withdrawal syndrome in morphine-dependent mice were examined. 5a-Pregnane-3,20-dione inhibited dose-dependently the jumping behavior and the number of forepaw tremors, at a dose of 5 mg/kg it attenuated body weight loss; deoxycorticosterone acetate inhibited dose-dependently the jumping behavior and body weight loss, whereas dehydroepiandrosterone in a dose of 1 mg/kg prevented body weight loss, and in a dose of 20 mg/kg forepaw tremor. The other steroids tested had no significant effect on the morphine withdrawal syndrome. None of the neurosteroids influenced the increased plasma corticosterone level in the naloxone-precipitated morphine withdrawal syndrome. The above results indicate that 5α-pregnane-3,20-dione, deoxycorticosterone acetate and dehydroepiandrosterone, but not other neurosteroids tested, alleviate some signs of the morphine abstinence syndrome in mice.

Ethopharmacological analysis of naloxone-precipitated morphine withdrawal syndrome in rats: a newly-developed “etho-score”

The intensity of opiate withdrawal syndrome in rats is usually quantified on the basis of selected physical signs or global scores. However, the selection criteria of signs and scores have not been subjected to an ethological discussion, hence they appear to be somewhat arbitrary. The objectives of this study were thus: i) to analyse the rat's behaviour during the naloxone-precipitated morphine withdrawal syndrome, ii) to evaluate the validity of classic methods, and iii) to design a new "etho-score". Ten rats were implanted with morphine pellets (75 mg x 2, SC), all receiving different naloxone doses following a within-subject design (0, 0.01, 0.05, 0.1, 0.5, 1 mg/kg SC). Twenty unexperienced rats and 20 with placebo pellets were injected with either saline or naloxone. Behaviour was videotaped and later analysed by computer-based ethological techniques. The ethogram encompassed 16 patterns displayed by rats during morphine withdrawal. Frequency, duration and latency of each pattern was measured, and a cluster analysis allowed discerning the structure of behaviour. Several physical signs and the Gellert-Holtzman score were also evaluated. The data revealed that writhing responses linearly changed in a dose-related fashion, and mastication was also enhanced after naloxone. Wet-dog shakes and jumping changed following an U-shaped curve. Significant changes in weight loss were found to be dose-dependent, and highly correlated to diarrhea. Learning effects were found to reliably affect exploration, writhing responses and some physical signs. The Gellert-Holtzman score was gradually enhanced after naloxone, being affected by learning as well. Naloxone affected lying and self-care responses in placebo rats. To sum up, the data indicated that: i) classic signs are useful, although most of them are disrupted by high naloxone or affected by learning effects, ii) the Gellert-Holtzman score was validated in this study, and iii) mastication and weight loss are good indicators of naloxone-precipitated morphine withdrawal, representing the basis of an "etho-score" which is herein proposed.

Anandamide decreases naloxone-precipitated withdrawal signs in mice chronically treated with morphine

The effect of anandamide, a putative endogenous ligand of the cannabinoid receptor, has been studied in a naloxone-precipitated morphine withdrawal syndrome in mice. Animals were chronically treated with increasing doses of morphine (from 8 to 45 mg/kg) over 5 days or implanted with morphine pellets (72 hr). Typical signs of withdrawal (jumping and body weight loss) were examined after naloxone administration (1 mg/kg). In these conditions, anandamide (5 mg/kg, i.v.) decreased both the number of jumps, measured over 30 min (81.2% ± 3.15 and 92.2% ± 3.5 decrease in chronically administered morphine and pellet implanted mice, respectively), and the body weight loss at 30 and 60 min (30 min: 2.6% ± 0.4 vs 4.4% ± 0.2 and 3.7% ± 0.4 vs 5.3% ± 0.4; 60 min: 3.2% ± 0.5 vs 5.0% ± 0.4 and 4.1% ± 0.5 vs 6.0% ± 0.5 in chronically treated morphine and pellet implanted mice respectively) after naloxone administration. This suggests, as shown in the case of Δ 9-tetrahydrocannabinol, a modulation by anandamide of pathways involved in the expression of physical signs of opioid dependence and support its role as an endogenous cannabinoid agonist.

Inhibition of morphine withdrawal by the association of RB 101, an inhibitor of enkephalin catabolism, and the CCKB antagonist PD-134,308.

1. The effects induced in rats on naloxone-precipitated morphine withdrawal syndrome by the new mixed inhibitor of enkephalin catabolism able to cross the blood-brain barrier RB 101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl-L-phenylalanine benzyl ester) given alone or associated with the selective CCKB antagonist, PD-134,308, were investigated. 2. The systemic administration of RB 101 (5, 10 and 20 mg kg-1, i.v.) elicited a significant decrease in 8 of the 14 withdrawal signs evaluated. PD-134,308 (3 mg kg-1, i.p.) did not modify the expression of morphine abstinence when given alone, but induced a strong facilitation of RB 101 responses (12 of 14 withdrawal signs were decreased). This potentiation was particularly intense in peripherally mediated withdrawal signs. 3. In order to clarify the biochemical mechanisms implicated in these responses, the effects induced by the association of RB 101 and PD-134,308 on the occupation of brain opioid receptors by endogenous enkephalins were also investigated in mice. PD-134,308, as well as RB 101, inhibited [3H]-diprenorphine binding to opioid receptors. These results suggest that an increase in endogenous enkephalin levels induced by PD-134,308 could participate in the facilitation of RB 101 behavioural responses. 4. RB 101 has a promising potential role in the management of the opiate withdrawal syndrome. CCKB antagonists, such as PD-134,308 may be useful in potentiating this anti-withdrawal effect.

Effects induced by BC 264, a selective agonist of CCK-B receptors, on morphine-dependent rats

The aim of this study was to investigate the possible interaction between neuronal cholecystokinin (CCK) and opiate dependence. Rats were made dependent to morphine and the ability of cholecystokinin-octapeptide (CCK-8) and Tyr(SO 3H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH 2 (BC 264), a selective agonist of CCK-B receptors, to induce signs of morphine withdrawal after ICV injection was tested. Behavioral responses were compared to those occuring during the naloxone-precipitated morphine withdrawal syndrome. In contrast tonaloxone, CCK-8 (0.1, 1, and 10 μg, ICV) did not precipitate any sign of withdrawal. BC 264 (0.1, 1, and 10 μg, ICV) induced a strong hyperlocomotion and wet dog shakes in morphine-dependent rats, the latter effect also observed in nondependent animals. In rats receiving acute morphine, BC 264 induced an opposite effect (i.e., blockade of morphine-induced hyperactivity). Taken together, these results suggest that CCK plays only a minor role in the expression of morphine physical dependence.

Attenuation of the morphine withdrawal syndrome by inhibition of catabolism of endogenous enkephalins in the periaqueductal gray matter.

We have investigated the effects of the local administration into the periaqueductal gray matter of thiorphan, a selective inhibitor of endopeptidase 24.11 "enkephalinase", kelatorphan, (R)-3-(N-hydroxy-carboxamido-2-benzylpropanoyl)- L-alanine, and RB 38 A, (R)-3-(N-hydroxy-carboxamido-2-benzylpropanoyl)-L-phenylalanine, two almost complete inhibitors of enkephalin metabolism, on the naloxone-precipitated morphine withdrawal syndrome in rats. Local administration of these inhibitors decreased the severity of the withdrawal syndrome. Jumping, chewing, diarrhea, piloerection, salivation and hypothermia were decreased by all drugs. Lacrimation and weight loss were reduced by kelatorphan and RB 38 A whereas teeth chattering, tremor, eye twitch and rhinorrhea were decreased only by RB 38 A. The rise in plasma corticosterone levels was only slightly reduced by the three inhibitors. Wet dog shakes and ptosis remained unchanged. These results indicate that during the morphine withdrawal syndrome in rats there is a tonic or/and naloxone evoked release of opioid peptides, presumably enkephalins, into the periaqueductal gray matter and that inhibition of their degradation strongly decreases the severity of the withdrawal syndrome.

Comparison of selective and complete inhibitors of enkephalin-degrading enzymes on morphine withdrawal syndrome

We investigated the effects of thiorphan, a selective inhibitor of endopeptidase 24.11 ‘enkephalinase’, kelatorphan ((R)-3-(N-hydroxy-carboxamido-2-benzylpropanoyl)-L-alanine), and RB 38 A ((R)-3-(N-hydroxycarboxamido-2-benzylpropanoyl)-L-phenylalanine) two almost complete inhibitors of enkephalin metabolism, on the naloxone-precipitated morphine withdrawal syndrome in rats. Inhibitors administered intracerebroventricularly reduced several symptoms of the withdrawal syndrome. Jumping, chewing and tooth chattering were decreased by all drugs. The rise in plasma corticosterone and the hypothermia were reduced by kelatorphan and RB 38 A whereas rhinorrhea was blocked by thiorphan, tremor by kelatorphan and diarrhoea by RB 38 A. Other signs remained unchanged. These data suggest that an increase in opioid receptor occupancy by endogenous opioid peptides, protected from biotransformation specially by mixed inhibitors reduced the severity of the morphine abstinence symptoms in rats.