Masahiko Funada

Regulations of opioid dependence by opioid receptor types

Three major types of opioid receptors, designated mu, delta, and kappa, are widely expressed in the CNS. Development of selective receptor ligands and recent cloning of each receptor have contributed greatly to our increasing knowledge of the neuropharmacological profile of each opioid receptor type. It is of interest to note that they include noncompetitive and allosteric interactions among their types. This review focuses on the functional interaction among these opioid receptor types that contribute to opioid dependence. Various studies provide arguments to support substantial roles for mu-opioid receptors and the possible involvement of delta-opioid receptors in the development of physical and psychological dependence on morphine. Noradrenergic transmission originating in the locus coeruleus is most likely to play the primary causal role in the expression of physical dependence on morphine. In contrast, many studies have pointed to the mesolimbic dopaminergic pathway projecting from the ventral tegmental area to the nucleus accumbens as a critical site for the initiation of psychological dependence on opioids. It is noteworthy as the broad existence of opposing interactions between mu/delta- and kappa-receptors in the brain. The activation of kappa-receptors leads to the suppression of unpleasant mu/delta-mediated side effects such as the rewarding effect. Considering the functional interaction among opioid receptor types, the co-administration of morphine-like compounds with kappa-receptor agonists may constitute a preferable and superior approach to the treatment of pain with fewer side effects.

Blockade of morphine reward through the activation of κ-opioid receptors in mice

Abstract The effects of systemic (s.c.) treatment with the κ-agonists U-50,488H and E-2078 (a stable dynorphin analog) on the morphine-induced place preference were examined in mice. Morphine (s.c.) caused a dose-related preference for the drug-associated place; the effects at doses of 3 and 5 mg/kg were significant. On the other hand, U-50,488H or E-2078 produced a dose-related conditioned place aversion. Both U-50,488H (1 mg/kg, s.c.) and E-2078 (0.1 mg/kg, s.c.) induced a slight, nonsignificant place aversion. Pretreatment with U-50,488H (1 mg/kg) abolished the morphine (3 mg/kg)-induced place preference. The morphine-induced place preference was also significantly decreased by pretreatment with E-2078 (0.1 mg/kg). The inhibitory effects of the κ-agonists were antagonized by the κ-antagonist nor-binaltorphimine (nor-BNI; 3 mg/kg, s.c.). In contrast, pretreatment with U-50,488H did not affect the place preference induced by the dopamine (DA) receptor agonist apomorphine (1 mg/kg, s.c.). In addition, morphine (3 mg/kg, s.c.) significantly increased the levels of the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the limbic forebrain (nucleus accumbens and olfactory tubercle) but not in the striatum, implying that activation of the mesolimbic DA system may play an important role in the morphine-induced place preference in mice. Pretreatment with U-50,488H significantly reduced the morphine-induced elevation of DA metabolites in the limbic forebrain. These results suggest that κ-agonists suppress the morphine-induced place preference, and that activation of κ-opioid receptors could suppress the reinforcing effects of morphine which may be induced by enhanced DA transmission in the mesolimbic DA system.

Morphine-induced place preference in the CXBK mouse : characteristics of μ opioid receptor subtypes

The role of mu opioid receptor subtypes, mu 1 and mu 2, in morphine-conditioned place preference was examined using ddY and mu 1 opioid receptor-deficient CXBK mice. In ddY mice, the mu receptor agonist morphine caused a dose-related preference for the drug-associated place, but the kappa agonist U-50,488H produced a dose-related place aversion. These results demonstrated that the mouse is available for place preference conditioning using opioids. Under this condition, the influence of pretreatment with the selective mu 1 opioid receptor antagonist naloxonazine on morphine-induced place preference was investigated in ddY mice. Although pretreatment with the selective mu 1 antagonist naloxonazine (35 mg/kg, s.c.) did not modify the morphine-induced place preference, pretreatment with the selective mu antagonist beta-funaltrexamine (beta-FNA 10 mg/kg, s.c.) eliminated the appetitive effect of morphine. Furthermore, morphine (1-5 mg/kg, s.c.) produced a dose-related preference for the drug-associated place in CXBK mice. These findings suggest that the morphine-induced conditioned place preference may be mediated by naloxonazine-insensitive sites (mu 2 opioid receptors). In addition, chronic infusion of the dopamine D1 antagonist SCH23390 (1.0 mg/kg/day) during the conditioning sessions eliminated the morphine-induced place preference in CXBK mice. Similarly, morphine combined with naloxonazine failed to produce the place preference in ddY mice chronically treated with SCH23390. The blocking effect of SCH23390 on the morphine-conditioned place preference suggests that mu 2 receptors may regulate the dopaminergic system, especially dopamine D1 receptors, and are also involved in the reinforcing effects of morphine.

Differential effects of psychological stress on activation of the 5-hydroxytryptamine- and dopamine-containing neurons in the brain of freely moving rats

We investigated the effects of psychological stress, lacking direct physical stimulus, on the release of 5-hydroxytryptamine (5-HT) and dopamine (DA) in the basolateral nucleus of the amygdala (BLA) and the dorsal raphe nuclei (DRN) in the rat using the in vivo microdialysis technique with dual probes, one in each region of the same animals. Psychological stress was employed using the communication box paradigm. Psychological stress for 1 h significantly increased dialysate 5-HT levels in the DRN and the BLA. Psychological stress-induced 5-HT release in the BLA was significantly greater than those in the DRN, indicating that modifications of the serotonergic neurons in the BLA are more sensitive to psychological stress than are those of the DRN. Psychological stress also increased DA release in the BLA, while the dialysate DA levels in the DRN were unchanged. These results suggest that psychological stress preferentially activates ascending serotonergic neurons from the DRN to the BLA but not those of dopaminergic neurons. Furthermore, our findings indicate that both the serotonergic neurons and the dopaminergic neurons in the BLA may have a distinct role to play in the neuronal responses to psychological stress.

Involvement of δ-opioid receptors in the effects of morphine on locomotor activity and the mesolimbic dopaminergic system in mice

Naltrindole (NTI) and naltriben (NTB), a benzofuran derivative of NTI, were recently synthesized as highly selective δ-opioid receptor antagonists. Both NTI and NTB failed to suppress the antinociceptive effect induced by morphine. In contrast, both NTI and NTB significantly suppressed the morphine-induced hyperlocomotion and increase in turnover of dopamine (DA) in the mouse limbic forebrain. These results suggest that δ-opioid receptors play, at least in part, a role in the morphine-induced hyperlocomotion and excitation of mesolimbic DA systems, but not antinociception.

Evaluation of rewarding effect of toluene by the conditioned place preference procedure in mice.

Toluene and many toluene-containing products are abused via inhalation. Previous investigations have used the place preference paradigm to evaluate the rewarding effects of commonly abused drugs such as morphine, cocaine, and amphetamine. A conditioning paradigm of toluene inhalation was developed in order to estimate the rewarding effect in mice. Conditioning sessions (five for toluene, five for air) were conducted twice daily for 5 days using a newly developed airtight inhalation shuttlebox (15x30x15 cm: wxlxh), which was divided into two compartments of equal size. One compartment was white with a textured floor, and the other was black with a smooth floor. All conditioning sessions were 20 min in duration, with a minimum of 7 h between sessions. Test sessions were carried out 1 day after the final training session with mice in a drug-free state. The time spent in each compartment during a 20-min session was measured using a digital video camera. Exposure to toluene vapors (700-3200 ppm) produced a significant conditioned place preference in mice. These results suggest that the conditioned place preference procedure using the newly developed airtight inhalation shuttlebox constitutes an important tool for studying the rewarding effect of abused solvents.

Diazepam pretreatement suppresses morphine withdrawal signs in the mouse

The effect of diazepam on the development of physical dependence on morphine and on the naloxone-precipitated increase in cortical NA turnover were investigated in mice. Co-administration of diazepam (1-4 mg/kg, i.p.) during chronic morphine treatment suppressed the expression of naloxone (3 mg/kg, s.c.)-precipitated withdrawal signs (jumping, exploratory rearing and weight loss). However, a single injection of diazepam (4 mg/kg, i.p.) in morphine-dependent mice did not affect the expression of naloxone-precipitated withdrawal signs. The 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) level and noradrenaline (NA) turnover (MHPG/NA) in the cerebral cortex were increased by naloxone (3 mg/kg) challenge. These increases in the cortical MHPG level and NA turnover were significantly prevented by co-administration of diazepam (4 mg/kg, i.p.) during chronic morphine treatment. These findings suggest that the co-administration of diazepam during chronic morphine treatment may prevent some neurochemical changes in the central noradrenergic system during chronic morphine treatment, and may suppress the development of physical dependence on morphine. Therefore, the inhibitory action of GABA via benzodiazepine binding sites may play an important role in the development of physical dependence on morphine.

Blockade of the morphine-induced increase in turnover of dopamine on the mesolimbic dopaminergic system by κ -opioid receptor activation in mice

Activation of central mu opioid receptors by treatment with systemic morphine elevates 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) levels without changing dopamine (DA) and 5-hydroxytryptamine (5-HT: serotonin) steady-state levels in the mouse limbic forebrain (including nucleus accumbens and olfactory tubercle). Pretreatment with systemic U-50,488H, a selective kappa agonist, could dose-dependently block the morphine-induced increase in turnover of DA. This blocking action by treatment with U-50, 488H was completely reversed by nor-binaltorphimine (nor-BNI), a selective kappa antagonist. On the other hand, U-50,488H did not affect the enhancement of 5-HT turnover induced by morphine. These findings suggest that kappa receptor activation can inhibit the mu agonist-induced activation of mesolimbic DA pathway but not ascending 5-HT pathway.

The role of dopamine D1-receptors in morphine-induced hyperlocomotion in mice

The effects of treatment with dopamine (DA) D1-agonist SKF38393 and D2-agonist quinpirole on morphine-induced hyperlocomotion were investigated in mice. Morphine-induced hyperlocomotion was increased by approximately 2.0-fold in SKF38393 (10 nmol, i.c.v.)-treated mice. Pretreatment with SCH23390 antagonized the enhancing effect of SKF38393. In contrast, pretreatment with quinpirole (10 nmol, i.c.v.) reduced morphine-induced hyperlocomotion. Morphine significantly increased DA metabolite levels, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the limbic forebrain (nucleus accumbens and olfactory tubercle). This elevation of DA metabolites by treatment with morphine was not modified by the co-administration of SKF38393. These results suggest that the activation of D1-receptors in the limbic forebrain may enhance the expression of morphine-induced hyperlocomotion.

Involvement of corticotropin-releasing factor receptor subtype 1 in morphine withdrawal regulation of the brain noradrenergic system.

Effects of pretreatment with the selective corticotropin-releasing factor (CRF) subtype 1 (CRF(1)) receptor antagonist, 2-(N-(2-methylthio-4-isopropylphenyl)-N-ethyl-amino-4-(4-(3-fluorophenyl)-1,2,3,6-tetrahydropyridin-1-yl)-6-methylpyrimidine (CRA1000) on the behavioral and biochemical changes after naloxone-precipitated morphine withdrawal were examined in ICR mice. Mice were chronically treated with morphine (8-45 mg/kg) for 5 days. Naloxone (3 mg/kg, s.c.) precipitated jumping, diarrhea, and body weight loss in morphine-dependent mice. In addition, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) and noradrenaline turnover (MHPG/noradrenaline) levels in the cerebral cortex were increased following naloxone challenge in morphine-dependent mice. However, 5-hydroxytriptamine turnover did not alter the increase following naloxone challenge in morphine-dependent mice. Pretreatment with CRA1000 (20 mg/kg, i.p.) attenuated the incidence of withdrawal signs and naloxone-precipitated increases in noradrenaline turnover. These results suggest that the activation of CRF(1) receptor may play an important role in the elevation of noradrenaline transmission, but not in 5-hydroxytriptamine transmission, in the cerebral cortex, which projects from the locus coeruleus during morphine withdrawal.