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. 2010 Oct;335(1):133-9.
doi: 10.1124/jpet.110.170969. Epub 2010 Jul 6.

Dual efficacy of delta opioid receptor-selective ligands for ethanol drinking and anxiety

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Dual efficacy of delta opioid receptor-selective ligands for ethanol drinking and anxiety

Richard M van Rijn et al. J Pharmacol Exp Ther. 2010 Oct.

Abstract

Alcoholism and anxiety disorders have a huge impact on society and afflict 17.6 million and 40 million people in the United States, respectively. A strong comorbidity exists between alcoholism and anxiety disorders. Indeed, alcohol withdrawal-induced anxiety is a primary contributing factor for relapse, and anxiolytics are a common adjuvant therapy prescribed for treatment-seeking alcoholics. It is thought that the use of alcohol to self-medicate and relieve anxiety contributes to the development of addiction. Treatment for anxiety disorders and alcoholism exist but are not universally effective. The delta opioid receptor (DOR) plays a role in both alcohol consumption and anxiety, making it a very interesting clinical target. Two pharmacologically distinct DORs have been described: DOR1 and DOR2. We find here that the relative specificity of DOR agonists for DOR1 or DOR2 can greatly affect the effects they exert on ethanol consumption and anxiety. The DOR1 agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydro-quinolino[2,3,30g]isoquinoline (TAN-67), although not effective in decreasing anxiety-like behavior in naive mice, has anxiolytic-like properties in ethanol-withdrawn mice. In contrast, a less subtype-selective agonist, (+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), while also reducing anxiety-like behavior, increases ethanol consumption. In addition, we found that the conical anxiolytic diazepam [DZ; 7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2(1H)-one] is a less effective anxiolytic in ethanol-withdrawn mice than in naive mice. Together, our findings suggest that selective DOR agonists can decrease anxiety-like behavior and are more effective than diazepam at reducing ethanol consumption. We believe the dual efficacy of DOR1 agonists makes these receptors an interesting therapeutic target for treatment-seeking alcoholics.

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Figures

Fig. 1.
Fig. 1.
DOR KO mice show a higher degree of anxiety-like behavior compared with WT C57BL/6 mice. A to D, anxiety-like behaviors were measured in WT and DOR KO C57BL/6 mice by using the elevated-plus maze (A–C) and dark–light transition box (D). For the elevated-plus maze relative time spent in the open arms and center was measured (A) and the number of entries into the open arm was counted (C) for 5 min. For the dark–light transition box, relative time spent in the light chambers (D) was measured for 5 min. E, locomotor activity (distance traveled) was assessed in the dark–light box. The number of animals used in each group is indicated in each bar of the histogram. *, p < 0.05; ***, p < 0.001.
Fig. 2.
Fig. 2.
DOR subtype-selective agonists have opposing actions in ethanol consumption. Wild-type C57BL/6 mice (n = 9), trained to drink in a limited-access, two-bottle choice paradigm, were injected subcutaneously with saline, 25 mg/kg of the DOR1 agonist TAN-67, or 20 mg/kg of the DOR agonist SNC80 (A and B) or 1.5 mg/kg of the nonselective opioid antagonist NTX or 1 mg/kg of the benzodiazepine diazepam (C and D). Thirty minutes after injection ethanol, water consumption were measured over a 4-h period. Ethanol preference = ethanol consumption/(ethanol consumption + water consumption). **, p < 0.01; ***, p < 0.001.
Fig. 3.
Fig. 3.
The DOR-selective agonist SNC80 decreases anxiety-like behavior in naive mice. A, naive wild-type C57BL/6 mice were injected subcutaneously with saline, 25 mg/kg of the DOR1 agonist TAN-67, or 20 mg/kg of the DOR agonist SNC80 (A and B) or 1.5 mg/kg of the nonselective opioid antagonist NTX or 1 mg/kg of the benzodiazepine diazepam (C and D). Thirty minutes after injection, anxiety-like behavior was measured by using the dark–light transition box (A and C) and the elevated-plus maze (B and D). For the dark–light transition box relative time spent in the light chambers was measured for 5 min. For the elevated-plus maze relative time spent in the open arms was measured for 5 min. The number of animals used in each group is indicated in each bar of the histogram. *, p < 0.05; **, p < 0.01.
Fig. 4.
Fig. 4.
Mice trained to voluntarily consume ethanol display ethanol withdrawal-induced anxiety-like behavior. A, C57BL/6 mice (n = 9) were trained to consume either ethanol or sucrose (see Materials and Methods). B and C, anxiety-like behavior was measured by using the elevated plus maze (B) and dark–light transition box (C). For the elevated-plus maze relative time spent in the open arms was measured for 5 min. For the dark–light transition box relative time spent in the light chambers was measured for 5 min. D, locomotor activity (distance traveled) was assessed in the dark–light box. The number of animals used in each group is indicated in each bar of the histogram. *, p < 0.05; **, p < 0.01.
Fig. 5.
Fig. 5.
DOR-selective agonists can abolish ethanol withdrawal-induced anxiety. Ethanol-withdrawn (24 h after last exposure) C57BL/6 mice were injected subcutaneously with saline, 25 mg/kg of the DOR1 agonist TAN-67, and 20 mg/kg of the DOR agonist SNC80 (A and B) or 1.5 mg/kg of the nonselective opioid antagonist NTX or 1 mg/kg of the benzodiazepine diazepam (C and D). Thirty minutes after injection, anxiety-like behavior was measured by using the dark–light transition box (A and C) and the elevated-plus maze (B and D). For the dark–light transition box relative time spent in the light chambers was measured for 5 min. For the elevated-plus maze relative time spent in the open arms was measured for 5 min. The number of animals used in each group is indicated in each bar of the histogram. *, p < 0.05; **, p < 0.01; ***, p < 0.001 versus ethanol; †, p < 0.05 versus naive.

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