Differential activation of the μ-opioid receptor by oxycodone and morphine in pain-related brain regions in a bone cancer pain model

Background and Purpose

Bone cancer pain is chronic and often difficult to control with opioids. However, recent studies have shown that several opioids have distinct analgesic profiles in chronic pain.

Experimental Approach

To clarify the mechanisms underlying these distinct analgesic profiles, functional changes in the μ-opioid receptor were examined using a mouse femur bone cancer (FBC) model.

Key Results

In the FBC model, the B_max of [³H]-DAMGO binding was reduced by 15–45% in the periaqueductal grey matter (PAG), region ventral to the PAG (vPAG), mediodorsal thalamus (mTH), ventral thalamus and spinal cord. Oxycodone (10⁻⁸–10⁻⁵ M) and morphine (10⁻⁸–10⁻⁵ M) activated [³⁵S]-GTPγS binding, but the activation was significantly attenuated in the PAG, vPAG, mTH and spinal cord in the FBC model. Interestingly, the attenuation of oxycodone-induced [³⁵S]-GTPγS binding was quite limited (9–26%) in comparison with that of morphine (46–65%) in the PAG, vPAG and mTH, but not in the spinal cord. Furthermore, i.c.v. oxycodone at doses of 0.02–1.0 μg per mouse clearly inhibited pain-related behaviours, such as guarding, limb-use abnormalities and allodynia-like behaviour in the FBC model mice, while i.c.v. morphine (0.05–2.0 μg per mouse) had only partial or little analgesic effect on limb-use abnormalities and allodynia-like behaviour.

Conclusion and Implications

These results show that μ-opioid receptor functions are attenuated in several pain-related regions in bone cancer in an agonist-dependent manner, and suggest that modification of the μ-opioid receptor is responsible for the distinct analgesic effect of oxycodone and morphine.     

Involvement of PKCα and G-protein-coupled receptor kinase 2 in agonist-selective desensitization of μ-opioid receptors in mature brain neurons

Background and purpose:

The ability of an agonist to induce desensitization of the µ-opioid receptor (MOR) depends upon the agonist used. Furthermore, previous data suggest that the intracellular mechanisms underlying desensitization may be agonist-specific. We investigated the mechanisms underlying MOR desensitization, in adult mammalian neurons, caused by morphine (a partial agonist in this system) and DAMGO (a high-efficacy agonist).

Experimental approach:

MOR function was measured in locus coeruleus neurons, by using whole-cell patch-clamp electrophysiology, in rat and mouse brain slices (both wild-type and protein kinase C (PKC)α knockout mice). Specific isoforms of PKC were inhibited by using inhibitors of the receptors for activated C-kinase (RACK), and in vivo viral-mediated gene-transfer was used to transfect neurons with dominant negative mutants (DNMs) of specific G-protein-coupled receptor kinases (GRKs).

Key results:

Morphine-induced desensitization was attenuated by using RACK inhibitors that inhibit PKCα, but not by other isoform-specific inhibitors. Further, the PKC component of morphine-induced desensitization was absent in locus coeruleus neurons from PKCα knockout mice. The PKC-enhanced morphine-induced desensitization was not affected by over-expression of a GRK2 dominant negative mutant (GRK2 DNM). In contrast, DAMGO-induced MOR desensitization was independent of PKC activity but was reduced by over-expression of the GRK2 DNM but not by that of a GRK6 DNM.

Conclusions and implications:

In mature mammalian neurons, different MOR agonists can induce MOR desensitization by different mechanisms, morphine by a PKCα-mediated, heterologous mechanism and DAMGO by a GRK-mediated, homologous mechanism. These data represent functional selectivity at the level of receptor desensitization.     

μ-Opioid receptor and α2-adrenoceptor agonist binding sites in the postmortem brain of heroin addicts

The biochemical status of human brain -opioid receptors and ₂-adrenoceptors during opiate dependence was studied by means of the binding of [³H] [D-Ala², MePhe⁴, Gly-ol⁵] enkephalin (DAGO) and [³H]clonidine, respectively, in postmortem brains of heroin addicts who had died by opiate overdose or other causes. In the frontal cortex, thalamus and caudate of heroin addicts the density (B_max) and affinity (K_D) of -opioid receptors were similar to those in controls. In contrast, the density of ₂-adrenoceptors in heroin addicts was found to be significantly decreased in frontal cortex (B_max 31% lower), hypothalamus (B_max 40% lower) and caudate (B_max 32% lower) without changes in K_D values. When heroin addicts were divided into two subgroups according to the presence or absence of morphine in body fluids, only the group with positive screening for morphine showed relevant decreases in brain ₂-adrenoceptor density (B_max 36–48% lower), whereas the decreases in receptor density observed in the subgroup with negative screening for morphine did not reach statistical significance. The results suggest that desensitization of brain _2A-adrenceptors is a relevant adaptative receptor mechanism during opiate addiction in humans.     

A6V polymorphism of the human μ-opioid receptor decreases signalling of morphine and endogenous opioids in vitro

Background and Purpose

Polymorphisms of the μ opioid receptor (MOPr) may contribute to the variation in responses to opioid drugs in clinical and unregulated situations. The A6V variant of MOPr (MOPr-A6V) is present in up to 20% of individuals in some populations, and may be associated with heightened susceptibility to drug abuse. There are no functional studies examining the acute signalling of MOPr-A6V in vitro, so we investigated potential functional differences between MOPr and MOPr-A6V at several signalling pathways using structurally distinct opioid ligands.

Experimental Approach

CHO and AtT-20 cells stably expressing MOPr and MOPr-A6V were used. AC inhibition and ERK1/2 phosphorylation were assayed in CHO cells; K channel activation was assayed in AtT-20 cells.

Key Results

Buprenorphine did not inhibit AC or stimulate ERK1/2 phosphorylation in CHO cells expressing MOPr-A6V, but buprenorphine activation of K channels in AtT-20 cells was preserved. [D-Ala2, N-MePhe4, Gly-ol]-enkephalin, morphine and β-endorphin inhibition of AC was significantly reduced via MOPr-A6V, as was signalling of all opioids to ERK1/2. However, there was little effect of the A6V variant on K channel activation.

Conclusions and Implications

Signalling to AC and ERK via the mutant MOPr-A6V was decreased for many opioids, including the clinically significant drugs morphine, buprenorphine and fentanyl, as well endogenous opioids. The MOPr-A6V variant is common and this compromised signalling may affect individual responses to opioid therapy, while the possible disruption of the endogenous opioid system may contribute to susceptibility to substance abuse.     

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.     

Respiratory and cardiovascular effects of the μ-opioid receptor agonist [Lys7]dermorphin in awake rats

1. Changes in respiratory variables, arterial blood pressure and heart rate were studied in awake rats after injection of the opioid peptide [Lys⁷]dermorphin and its main metabolites, [1-5]dermorphin and [1-4]dermorphin.
2. Fifteen minutes after injection, doses of [Lys⁷]dermorphin producing antinociception (i.c.v., 36–120 nmol; s.c., 0.12–4.7 μmol kg⁻¹) significantly increased respiratory frequency and minute volume of rats breathing air or hypoxic inspirates. This respiratory stimulation was reversed to depression by the 5-HT receptor antagonist ritanserin (2 mg kg⁻¹, s.c.), was blocked by naloxone (0.1 mg kg⁻¹, s.c.), significantly reduced by the μ₁ opioid receptor antagonist naloxonazine (10 mg kg⁻¹, s.c., 24 h before) but unaffected by peripherally acting opioid antagonist naloxone methyl bromide (3 mg kg⁻¹, s.c.). Forty five minutes after injection, doses of the peptide producing catalepsy (s.c., 8.3–14.2 μmol kg⁻¹, i.c.v., 360 nmol) significantly reduced respiratory frequency and volume of rats breathing air and blocked the hypercapnic ventilator response of rats breathing from 4% to 10% CO₂. I.c.v. administration of [1-5]dermorphin and [1-4]dermorphin (from 36 to 360 nmol) never stimulated respiration but significantly reduced basal and CO₂-stimulated ventilation. Opioid respiratory depression was only antagonized by naloxone.
3. In awake rats, [Lys⁷]dermorphin (0.1–1 mg kg⁻¹, s.c.) decreased blood pressure. This hypotensive response was abolished by naloxone, reduced by naloxone methyl bromide and unaffected by naloxonazine.
4. In conclusion, the present study indicates that analgesic doses of [Lys⁷]dermorphin stimulate respiration by activating central μ₁ opioid receptors and this respiratory stimulation involves a forebrain 5-hydroxytryptaminergic excitatory pathway.

     

Different mechanisms of homologous and heterologous μ-opioid receptor phosphorylation

The efficiency of μ-opioid receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation of intracellular serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific μ-opioid receptor antibodies, which have proved to be excellent tools for monitoring the spatial and temporal dynamics of receptor phosphorylation and dephosphorylation. Agonist-induced phosphorylation of μ-opioid receptors occurs at a conserved 10 residue sequence ³⁷⁰TREHPSTANT³⁷⁹ in the receptor''s carboxyl-terminal cytoplasmic tail. Diverse opioids induce receptor phosphorylation at S375, present in the middle of this sequence, but only high-efficacy opioids have the ability to drive higher order phosphorylation on flanking residues (T370, T376 and T379). S375 is the initiating residue in a hierarchical phosphorylation cascade. In contrast, agonist-independent heterologous μ-opioid receptor phosphorylation occurs primarily at T370. The combination of phosphosite-specific antibodies and siRNA knockdown screening also facilitated the identification of relevant kinases and phosphatases. In fact, morphine induces a selective S375 phosphorylation that is predominantly catalysed by GPCR kinase 5 (GRK5), whereas multisite phosphorylation induced by high-efficacy opioids specifically requires GRK2/3. By contrast, T370 phosphorylation stimulated by phorbol esters or heterologous activation of G_q-coupled receptors is mediated by PKCα. Rapid μ-opioid receptor dephosphorylation occurs at or near the plasma membrane and is catalysed by protein phosphatase 1γ (PP1γ). These findings suggest that there are distinct phosphorylation motifs for homologous and heterologous regulation of μ-opioid receptor phosphorylation. However, it remains to be seen to what extent different μ-opioid receptor phosphorylation patterns contribute to the development of tolerance and dependence in vivo.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Preferential antagonism by diltiazem of α2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats

Summary Vasoconstrictor responses mediated by the ₂-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the ₁-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (>300 nmol/l) diltiazem became an effective antagonist of all -adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent againts vasoconstrictor responses to TL99 than in SHR. The blockade of ₁-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries.The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/l–3 mol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow SHR-tail arteries after labelling the tissue with³H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations.The tail artery preparation of SHR contains a population of postsynaptic ₂-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular ₂-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.     

Activation of presynaptic α2-adrenoceptors attenuates the inhibitory effect of μ-opioid receptor agonists on noradrenaline release from brain slices

Summary ³H-noradrenaline release from rat neocortical slices induced by 15 mM K⁺ was concentration-dependently inhibited by morphine, [D-Ala²-D-Leu⁵] enkephalin (DADLE) and the calcium entry blocker Cd²⁺. Blockade of presynaptic ₂-adrenoceptors with phentolamine, almost doubling K⁺-induced ³H-noradrenaline release, slightly enhanced the relative inhibitory effects of morphine and DADLE, whereas that of Cd²⁺ remained unaffected. In contrast, activation of presynaptic ₂-adrenoceptors with clonidine (1 M) or TL-99 (1 M), inhibiting release by about 50%, completely abolished the inhibitory effects of morphine and DADLE without affecting that of Cd²⁺. When in the presence of 1 M clonidine adenylate cyclase was activated with forskolin (10 M), which restored release to the drug-free control level, the opioids still did not display their inhibitory effects. Therefore, -opioid receptor efficacy appears to be dependent on the degree of activation of ₂-adrenoceptors in central noradrenergic nerve terminals, probably through a local receptor interaction within the nerve terminal membrane.     

Desensitization of δ-opioid receptors in nucleus accumbens during nicotine withdrawal

Rationale  

The synthesis and release of met-enkephalin and β-endorphin, endogenous ligands for δ-opioid peptide receptors (DOPrs), are altered following nicotine administration and may play a role in nicotine addiction.     

Levels of immunoreactive dynorphin A_(1-13) during development of morphine dependence in rats

目的：研究吗啡依赖大鼠组织内免疫活性强啡肽Ａ１－１３含量的动态变化及其与依赖程度的关系．方法：用纳洛酮催促的戒断症状评分测定吗啡依赖程度，用放射免疫测定组织内强啡肽Ａ１－１３水平．结果：在３－６天给药期内，吗啡可进行性降低脊髓、垂体、血浆内免疫活性强啡肽Ａ１－１３水平，升高海马及下丘脑的强啡肽Ａ１－１３水平，继续给药至１２天，各组织内免疫活性强啡肽Ａ１－１３水平不再有显著性变化．结论：吗啡依赖形成过程中脊髓、垂体及血浆内强啡肽Ａ１－１３呈进行性降低，其趋势与吗啡依赖程度一致     

The attenuation by Δ 9-Tetrahydrocannabinol and morphine of the quasi-morphine withdrawal syndrome in rats

The effect of ⁹-tetrahydrocannabinol (THC), morphine, haloperidol and chlordiazepoxide on the exhibition of the signs of the quasi-morphine withdrawal syndrome was studied in rats. In preliminary studies approximately equi-sedative doses of these drugs were chosen. Morphine and THC produced a very similar degree of suppression of the signs of the quasi-morphine withdrawal, but unlike morphine, the effects of THC were not reversed by the narcotic antagonist, naloxone. The dopamine receptor antagonist, haloperidol, produced a moderate suppression of the withdrawal syndrome and chlordiazepoxide was without significant effect. It is concluded that THC is of very similar potency to morphine in suppressing the quasi-morphine withdrawal syndrome, but its activity in this regard does not appear to be dependent upon the availability of opiate or dopamine receptors, nor is it due to sedation alone.To whom offprint requests should be sent     

Effect of α2-adrenoceptor agonists on the expression of morphine-withdrawal in rats

Summary Previous studies using clonidine indicate that ₂-adrenoceptors are involved in suppressing opiate-withdrawal symptoms. However, clonidine may act as a partial agonist at ₂-adrenoceptors and it also possesses significant ₁-receptor agonist activity.The aim of this study was to determine the role of ₂-adrenoceptors in the expression of opiate withdrawal signs using morphine-dependent rats. A range of agonists were selected for study on the basis of their differential preferences for -adrenoceptors.Hooded Wistar rats were made physically dependent on morphine (s.c. injection of an emulsion releasing a total of 250 mg/kg of morphine base over 48 h). Test drugs were injected s.c. followed by naloxone (10 mg/kg i.p.) 20 min later. The incidence of 5 selected withdrawal signs was recorded during the following 20 min. The ₂-adrenoceptor agonists displayed different profiles of activity. Azepexole (1–10 mg/kg) reduced all signs. Clonidine (80–800 g/kg) reduced all signs except paw shakes while guanfacine (25–250 g/kg) reduced all except jumping and diarrhoea. Talipexole (0.1–1 mg/kg) reduced all signs except diarrhoea which was not affected and jumping which was markedly enhanced. UK 14,304 (80–800 g/kg) reduced jumps, potentiated paw shakes but did not affect body shakes, teeth chattering or diarrhoea. The results suggest that there are subpopulations of ₂-adrenoceptors that modulate the expression of opiate withdrawal signs and/or that some of the drugs used affect receptors other than ₂-adrenoceptors.     

Involvement of μ-opioid receptors and α-adrenoceptors in the immunomodulatory effects of dihydroetorphine

The present study investigated the effects of acutely administered dihydroetorphine on mitogen-stimulated lymphocytes proliferation and lyrnphokine production in mice.These immune functions were significantly suppressed by dihydroetorphine at 24μg·kg~(-1) and 128μ·g-kg~(-1) in a dose-dependent fashion.This study further examined the involvement of μ-opioid receptors and     

The α2-adrenoceptor antagonists idazoxan and yohimbine increase rates of DRL responding in rats

Previous studies have reported that antidepressant drugs exert specific effects on responding maintained by DRL schedules of reinforcement, giving rise to increased frequencies of reinforcement. In order to investigate whether the ₂-adrenoceptor antagonist idazoxan would produce similar effects, the actions of this compound were compared with those of yohimbine, imipramine, mianserin and d-amphetamine in rats trained to lever press for food reinforcement on a DRL 60-s schedule. Neither imipramine nor mianserin produced any effects on response rate or reinforcement frequency, except at the highest doses. In contrast, both idazoxan and yohimbine gave rise to dose-related increases in rates of responding and consequent decreases in reinforcement frequencies. Amphetamine also increased responding, but higher doses of this drug produced marked hyperactivity and stereotyped movements which were not observed after idazoxan and yohimbine. Although the present behavioural baseline was not sensitive to antidepressants, it demonstrated an unexpected activity of two ₂-adrenoceptor antagonists which deserves further investigation.     

Deciphering μ-opioid receptor phosphorylation and dephosphorylation in HEK293 cells

BACKGROUND AND PURPOSE

The molecular basis of agonist-selective signalling at the µ-opioid receptor is poorly understood. We have recently shown that full agonists such as [D-Ala²-MePhe⁴-Gly-ol]enkephalin (DAMGO) stimulate the phosphorylation of a number of carboxyl-terminal phosphate acceptor sites including threonine 370 (Thr³⁷⁰) and serine 375 (Ser³⁷⁵), and that is followed by a robust receptor internalization. In contrast, morphine promotes a selective phosphorylation of Ser³⁷⁵ without causing rapid receptor internalization.

EXPERIMENTAL APPROACH

Here, we identify kinases and phosphatases that mediate agonist-dependent phosphorylation and dephosphorylation of the µ-opioid receptor using a combination of phosphosite-specific antibodies and siRNA knock-down screening in HEK293 cells.

KEY RESULTS

We found that DAMGO-driven phosphorylation of Thr³⁷⁰ and Ser³⁷⁵ was preferentially catalysed by G-protein-coupled receptor kinases (GRKs) 2 and 3, whereas morphine-driven Ser³⁷⁵ phosphorylation was preferentially catalysed by GRK5. On the functional level, inhibition of GRK expression resulted in enhanced µ-opioid receptor signalling and reduced receptor internalization. Analysis of GRK5-deficient mice revealed that GRK5 selectively contributes to morphine-induced Ser³⁷⁵ phosphorylation in brain tissue. We also identified protein phosphatase 1γ as a µ-opioid receptor phosphatase that catalysed Thr³⁷⁰ and Ser³⁷⁵ dephosphorylation at or near the plasma membrane within minutes after agonist removal, which in turn facilitates receptor recycling.

CONCLUSIONS AND IMPLICATIONS

Together, the morphine-activated µ-opioid receptor is a good substrate for phosphorylation by GRK5 but a poor substrate for GRK2/3. GRK5 phosphorylates µ-opioid receptors selectively on Ser³⁷⁵, which is not sufficient to drive significant receptor internalization.     

Renal aging change of α1-adrenoceptor in wistar rats

• 1.1. The aging changes of density of the α₁-adrenoceptors in the kidney were evaluated with Wistar rats of several ages (8, 52 and 104 weeks old).
• 2.2. [³H]prazosin and [³H]YM617 (newly synthesized α₁-blocker) were used for the ligand. The B_max of [³H]prazosin was 74.0 ± 9.5 fmol/mg/protein in 8 week, 52.1 ± 7.3 fmol/mg protein in 52 week, and 31.3 ± 4.2 fmol/mg/protein in 104 week rats, and that of [³H]YM617 was 45.0 ± 6.6 fmol/mg/protein in 8 week, 32.4 ± 5.7 fmol/mg/protein in 52 week, and 19.3 ± 5.5 fmol/mg/protein in 104 week rats.
• 3.3. The B_max of both ligands for 104 week rats was significantly decreased compared to 8 week rats, however, 52 week rats showed no decrease of B_max for both ligands.
• 4.4. The K_d values showed no difference in these three age groups for both ligands.
• 5.5. Autoradiographic study supported the result above mentioned. Furthermore, the binding sites of α₁-adrenoceptors were mainly in the cortex (vascular wall and peritubular area) and that α₁-adrenoceptors were chiefly chlorethylclonidine dihydrochloride (CEC) insensitive.

     

Effects of κ-opioid receptor ligands on intracranial self-stimulation in rats

Rationale Elevations in cAMP response element binding protein (CREB) function within the mesolimbic system of rats reduce cocaine reward in place conditioning studies and increase immobility in the forced swim test. Each of these behavioral adaptations can be interpreted as a depressive-like effect (i.e., anhedonia, despair) that may reflect reduced activity of brain reward systems. Furthermore, each effect appears due to increases in CREB-mediated expression of dynorphin, since each is attenuated by intracranial injections of the -opioid receptor antagonist norBNI.Objectives Intracranial self-stimulation (ICSS) studies were conducted in rats to determine whether administration of a -agonist would have depressive-like effects on brain stimulation reward, and whether pretreatment with a -antagonist would attenuate any such effects. Conditions that have depressive effects in people (e.g., drug withdrawal) increase the threshold amounts of stimulation required to sustain ICSS in rats.Methods Sprague-Dawley rats with lateral hypothalamic stimulating electrodes were tested in a curve-shift variant of the ICSS procedure after systemic administration of the -agonist U-69593 alone, the novel -antagonist 5-acetamidinoethylnaltrindole (ANTI) alone, or co-administration of both drugs.Results U-69593 dose dependently increased ICSS thresholds, suggesting that activation of -receptors reduced the rewarding impact of the brain stimulation. ANTI had no effects on its own, but it attenuated increases in ICSS thresholds caused by the agonist.Conclusions These data provide further evidence that stimulation of brain -receptors may trigger certain depressive-like signs, and that antagonists may have efficacy as antidepressants without having reward-related actions of their own.     

Development of mechanical hypersensitivity in rats during heroin and ethanol dependence: alleviation by CRF₁ receptor antagonism

Animal models of drug dependence have described both reductions in brain reward processes and potentiation of stress-like (or anti-reward) mechanisms, including a recruitment of corticotropin-releasing factor (CRF) signaling. Accordingly, chronic exposure to opiates often leads to the development of mechanical hypersensitivity. We measured paw withdrawal thresholds (PWTs) in male Wistar rats allowed limited (short access group: ShA) or extended (long access group: LgA) access to heroin or cocaine self-administration, or in rats made dependent on ethanol via ethanol vapor exposure (ethanol-dependent group). In heroin self-administering animals, after transition to LgA conditions, thresholds were reduced to around 50% of levels observed at baseline, and were also significantly lower than thresholds measured in animals remaining on the ShA schedule. In contrast, thresholds in animals self-administering cocaine under either ShA (1 h) or LgA (6 h) conditions were unaltered. Similar to heroin LgA rats, ethanol-dependent rats also developed mechanical hypersensitivity after eight weeks of ethanol vapor exposure compared to non-dependent animals. Systemic administration of the CRF1R antagonist MPZP significantly alleviated the hypersensitivity observed in rats dependent on heroin or ethanol. The emergence of mechanical hypersensitivity with heroin and ethanol dependence may thus represent one critical drug-associated negative emotional state driving dependence on these substances. These results also suggest a recruitment of CRF-regulated nociceptive pathways associated with escalation of intake and dependence. A greater understanding of relationships between chronic drug exposure and pain-related states may provide insight into mechanisms underlying the transition to drug addiction, as well as reveal new treatment opportunities. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

The effects of ethanol in combination with the α2-adrenoceptor agonist dexmedetomidine and the α2-adrenoceptor antagonist atipamezole on brain monoamine metabolites and motor performance of mice

The time course of the effects of ethanol alone and in combination with the selective α₂-adrenoceptor agonist dexmedetomidine and the α-adrenoceptor antagonist atipamezole was studied in NIH-Swiss mice. Core body temperature, rotarod performance, motility and changes in the noradrenaline, dopamine, and 5-hydroxytryptamine (5-HT) metabolite contents of different brain parts (limbic forebrain, striatum, lower brainstem, the rest of the forebrain + midbrain and hypothalamus) were measured. Atipamezole (3 mg/kg) attenuated the hypothermia induced by either ethanol (3 g/kg) alone or ethanol in combination with dexmedetomidine (0.3 mg/kg). Atipamezole shortened the duration of the ethanol-impaired and ethanol + dexmedetomidine-impaired rotarod performance. Further, atipamezole prevented the decreased motility due to the combined treatment with ethanol and dexmedetomidine. Ethanol increased 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) values. Dexmedetomidine alone decreased MHPG and 5-hydroxyindoleacetic acid (5-HIAA) concentrations and increased DOPAC and HVA values. Dexmedetomidine combined with ethanol resulted in a further increase in DOPAC and HVA values. Pharmacokinetic parameters did not contribute to this antagonism of ethanol's effects by atipamezole, nor did the antagonism observed in rotarod performance or hypothermia seem to correlate with the changes seen in the brain noradrenaline and dopamine or 5-HT metabolism. In conclusion, these findings suggest that several ethanol effects are not mediated via direct activation of α₂-adrenoceptors, even though some of ethanol's behavioral and physiological effects may be antagonized by coadministration of α₂-adrenoceptor antagonists.