Dopamine-independent psychostimulant activity of a delta-agonist

Onge CM, Lee YK, Li M, Rice KC, Zhang Y. Effects of Selective and Mixed-Action Kappa and Delta Opioid Receptor Agonists on Pain-Related Behavioral Depression in Mice

We recently developed a series of nalfurafine analogs (TK10, TK33, and TK35) that may serve as non-addictive candidate analgesics. These compounds are mixed-action agonists at the kappa and delta opioid receptors (KOR and DOR, respectively) and produce antinociception in a mouse warm-water tail-immersion test while failing to produce typical mu opioid receptor (MOR)-mediated side effects. The warm-water tail-immersion test is an assay of pain-stimulated behavior vulnerable to false-positive analgesic-like effects by drugs that produce motor impairment. Accordingly, this study evaluated TK10, TK33, and TK35 in a recently validated assay of pain-related behavioral depression in mice that are less vulnerable to false-positive effects. For comparison, we also evaluated the effects of the MOR agonist/analgesic hydrocodone (positive control), the neurokinin 1 receptor (NK1R) antagonist aprepitant (negative control), nalfurafine as a selective KOR agonist, SNC80 as a selective DOR agonist, and a nalfurafine/SNC80 mixture. Intraperitoneal injection of dilute lactic acid (IP lactic acid) served as a noxious stimulus to depress vertical and horizontal locomotor activity in male and female ICR mice. IP lactic acid-induced locomotor depression was alleviated by hydrocodone but not by aprepitant, nalfurafine, SNC80, the nalfurafine/SNC80 mixture, or the KOR/DOR agonists. These results suggest that caution is warranted in advancing mixed-action KOR/DOR agonists as candidate analgesics.

Critical Role for Gi/o-Protein Activity in the Dorsal Striatum in the Reduction of Voluntary Alcohol Intake in C57Bl/6 Mice

The transition from non-dependent alcohol use to alcohol dependence involves increased activity of the dorsal striatum. Interestingly, the dorsal striatum expresses a large number of inhibitory G-protein-coupled receptors (GPCRs), which when activated may inhibit alcohol-induced increased activity and can decrease alcohol consumption. Here, we explore the hypothesis that dorsal striatal G_i/o-protein activation is sufficient to reduce voluntary alcohol intake. Using a voluntary, limited-access, two-bottle choice, drink-in-the-dark model of alcohol (10%) consumption, we validated the importance of G_i/o signaling in this region by locally expressing neuron-specific, adeno-associated-virus encoded G_i/o-coupled muscarinic M₄ designer receptors exclusively activated by designer drugs (DREADD) in the dorsal striatum and observed a decrease in alcohol intake upon DREADD activation. We validated our findings by activating G_i/o-coupled delta-opioid receptors (DORs), which are natively expressed in the dorsal striatum, using either a G-protein biased agonist or a β-arrestin-biased agonist. Local infusion of TAN-67, an in vitro-determined G_i/o-protein biased DOR agonist, decreased voluntary alcohol intake in wild-type and β-arrestin-2 knockout (KO) mice. SNC80, a β-arrestin-2 biased DOR agonist, increased alcohol intake in wild-type mice; however, SNC80 decreased alcohol intake in β-arrestin-2 KO mice, thus resulting in a behavioral outcome generally observed for G_i/o-biased agonists and suggesting that β-arrestin recruitment is required for SNC80-increased alcohol intake. Overall, these results suggest that activation G_i/o-coupled GPCRs expressed in the dorsal striatum, such as the DOR, by G-protein biased agonists may be a potential strategy to decrease voluntary alcohol consumption and β-arrestin recruitment is to be avoided.

Involvement of μ- and δ-opioid receptor function in the rewarding effect of (±)-pentazocine

Most opioid receptor agonists have abuse potential, and the rewarding effects of opioids can be reduced in the presence of pain. While each of the enantiomers of pentazocine has a differential pharmacologic profile, (±)-pentazocine has been used clinically for the treatment of pain. However, little information is available regarding which components of pentazocine are associated with its rewarding effects, and whether the (±)-pentazocine-induced rewarding effects can be suppressed under pain. Therefore, the present study was performed to investigate the effects of pain on the acquisition of the rewarding effects of (±)-pentazocine, and to examine the mechanism of the rewarding effects of (±)-pentazocine using the conditioned place preference paradigm. (±)-Pentazocine and (-)-pentazocine, but not (+)-pentazocine, produced significant rewarding effects. Even though the rewarding effects induced by (±)-pentazocine were significantly suppressed under pain induced by formalin, accompanied by increase of preprodynorphin mRNA levels in the nucleus accumbens, a high dose of (±)-pentazocine produced significant rewarding effects under pain. In the normal condition, (±)-pentazocine-induced rewarding effects were blocked by a low dose of naloxone, whereas the rewarding effects induced by high doses of pentazocine under pain were suppressed by naltrindole (a δ-opioid receptor antagonist). Interestingly, (±)-pentazocine did not significantly affect dopamine levels in the nucleus accumbens. These findings suggest that the rewarding effects of (-)-pentazocine may contribute to the abuse potential of (±)-pentazocine through μ- as well as δ-opioid receptors, without robust activation of the mesolimbic dopaminergic system. We also found that neural adaptations can reduce the abuse potential of (±)-pentazocine under pain.

δ-Opioid mechanisms for ADL5747 and ADL5859 effects in mice: analgesia, locomotion, and receptor internalization

N,N-diethyl-4-(5-hydroxyspiro[chromene-2,4'-piperidine]-4-yl) benzamide (ADL5859) and N,N-diethyl-3-hydroxy-4-(spiro[chromene-2,4'-piperidine]-4-yl)benzamide (ADL5747) are novel δ-opioid agonists that show good oral bioavailability and analgesic and antidepressive effects in the rat and represent potential drugs for chronic pain treatment. Here, we used genetic approaches to investigate molecular mechanisms underlying their analgesic effects in the mouse. We tested analgesic effects of ADL5859 and ADL5747 in mice by using mechanical sensitivity measures in both complete Freund's adjuvant and sciatic nerve ligation pain models. We examined their analgesic effects in δ-opioid receptor constitutive knockout (KO) mice and mice with a conditional deletion of δ-receptor in peripheral voltage-gated sodium channel (Nav)1.8-expressing neurons (cKO mice). Both ADL5859 and ADL5747, and the prototypical δ agonist 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethyl-piperazin-1-yl]-(3-methoxyphenyl)methyl]-N,N-diethyl-benzamide (SNC80) as a control, significantly reduced inflammatory and neuropathic pain. The antiallodynic effects of all three δ-opioid agonists were abolished in constitutive δ-receptor KO mice and strongly diminished in δ-receptor cKO mice. We also measured two other well described effects of δ agonists, increase in locomotor activity and agonist-induced receptor internalization by using knock-in mice expressing enhanced green fluorescence protein-tagged δ receptors. In contrast to SNC80, ADL5859 and ADL5747 did not induce either hyperlocomotion or receptor internalization in vivo. In conclusion, both ADL5859 and ADL5747 showed efficient pain-reducing properties in the two models of chronic pain. Their effects were mediated by δ-opioid receptors, with a main contribution of receptors expressed on peripheral Nav1.8-positive neurons. The lack of in vivo receptor internalization and locomotor activation, typically induced by SNC80, suggests agonist-biased activity at the receptor for the two drugs.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The co-application effects of fullerene and ascorbic acid on UV-B irradiated mouse skin

The role of fullerene as a pro-oxidant or anti-oxidant in Ultraviolet B ray (UV-B)-induced disorders in mouse skin was investigated. Fullerene gave no photo-toxic effect to UV-B-irradiated mouse skin. Since erythema was concentrated at the pore circumference in a UV-B irradiation experiment in mouse skin, the sebaceous gland pairs was strongly implicated as a site for the generation of reactive oxygen species (ROS). In a histological evaluation of the skin stained with CH(3)MDFDA (ROS index) and YO-Pro-1 (apoptosis index), the fluorescence intensity of a sebaceous gland significantly increased with UV-B irradiation. With the application of fullerene to UV-irradiated mouse skin, no toxicity was recognized in comparison with the control, and erythema, the ROS index, and the apoptosis index decrease with the application of fullerene. Ascorbyl radical (AA*) increased with the application of ascorbate (AA) to UV-B-irradiated mouse skin, and AA* decreased with the application of fullerene. The co-application of AA and fullerene, which suppressed AA* in vitro, significantly suppressed erythema, and also suppressed both the ROS index and apoptosis index in mouse skin after UV-B irradiation. In both mouse skin at 48 h after UV-B irradiation and in an attempt to reproduce this phenomenon artificially in vitro, a similar high AA* peak (AA*/H*>4) was observed in electron spin resonance (ESR) charts. The binding of fullerene with AA impairs the Fenton reaction between AA and Fe-protein based on the observation of ascorbate-specific UV absorption and a linear equation for the calibration curve. Therefore, fullerene may impair the intercalation of AA to a heme pocket by binding with AA. These results suggest that the co-application of AA and fullerene is effective against oxidative skin damage caused by UV-B irradiation, and the development of an AA* inhibitor such as fullerene should be useful for reducing organ damage associated with Fe-protein oxidation.

Pain-inhibiting inhomogeneous static magnetic field fails to influence locomotor activity and anxiety behavior in mice: no interference between magnetic field- and morphine-treatment

OBJECTIVES

We wanted to demonstrate (i) in the writhing test in mice, whether there was a prolonged analgesic effect induced by an inhomogeneous static magnetic field (SMF) exposure; (ii) whether SMF had an effect on the analgesic effect induced by 0.5mg/kgs.c. administered morphine, on the behavioral patterns, and on the hyperlocomotion-inducing effect of morphine.

DESIGN

A magnetic exposure system developed by the present authors was used with peak-to-peak flux densities in the 2-754mT range. The writhing test was used for the assessment of pain. An elevated plus maze and a Conducta System was used for studying the anxiogenic or anxyolitic effect in mice, and the locomotor activity, respectively.

OUTCOME MEASURES

We looked for the difference in the number of writhings and in the behavioral patterns between treated (s.c. morphine and/or SMF exposure) and control animals, respectively.

RESULTS

(i) The antinociceptive effect could be identified 10-30min following SMF exposition in the writhing test in mice. (ii) SMF failed to affect the morphine-induced antinociception, the behavioral patterns in either type of tests, and the hyperlocomotion-inducing effect of morphine.

CONCLUSIONS

(i) The long-lasting antinociceptive effect of SMF allows experiments under conditions, when in situ application of the SMF-producing device would be technically difficult or impossible; or where it would disturb the experiments. (ii) The results of behavioral tests with freely moving mice in or in the vicinity of inhomogeneous SMFs are not affected by the SMF in the applied flux density range. (iii) Morphine in treated subjects is not interacting with the inhomogeneous SMFs in the applied flux density range.

M5 muscarinic receptor knockout mice show reduced morphine-induced locomotion but increased locomotion after cholinergic antagonism in the ventral tegmental area

M(5) muscarinic receptors are the only muscarinic receptor subtype expressed by mesencephalic dopamine neurons and provide an important excitatory input to mesolimbic and nigrostriatal dopamine systems. Here, we studied locomotion induced by systemic morphine (3, 10, and 30 mg/kg i.p.) in M(5) knockout mice of the C57BL/6 (B6) and CD1 x 129SvJ background strains. M(5) knockout mice of both strains showed reduced locomotion in response to 30 mg/kg morphine. B6 M(5) knockout mice were less sensitive to naltrexone in either the antagonism of morphine-induced locomotion or in the reduction of locomotion by naltrexone alone. This suggests that M(5) knockout mice are less sensitive to the effects of either exogenous or endogenous opiates on locomotion and that spontaneous locomotion in B6 mice is sustained by endogenous opiates. In B6 wild-type mice, ventral tegmental area (VTA) pretreatment with the muscarinic receptor antagonist atropine (3 microg bilateral), but not the nicotinic receptor antagonist mecamylamine (5 microg bilateral), reduced locomotion in response to 30 mg/kg morphine to a similar extent as systemic M(5) knockout, suggesting that reduced morphine-induced locomotion in M(5) knockout mice is due to the loss of M(5) receptors on VTA dopamine neurons. In contrast, in M(5) knockout mice, but not in wild-type mice, either intra-VTA atropine or mecamylamine alone increased locomotion by almost 3 times relative to saline and potentiated morphine-induced locomotion. Therefore, in M(5) knockout mice, blockade of either VTA muscarinic or nicotinic receptors increased locomotion, suggesting that in the absence of VTA M(5) receptors, VTA cholinergic inputs inhibit locomotion.