Delta opioid receptor ligands modulate anxiety-like behaviors in the rat

The role of the delta opioid receptor in regulating anxiety-like behavior in male Sprague–Dawley rats was examined. Using an elevated plus maze, the effects of the selective delta opioid receptor antagonist naltrindole (1 or 5 mg kg−1) and agonist SNC80 (1, 5 or 20 mg kg−1) on anxiety-like behavior were measured. Anxiety was also measured following administration of diazepam (3 mg kg−1) and amphetamine (1 mg kg−1) and compared to the effects of SNC80. Locomotor activity following administration of naltrindole, SNC80, diazepam, and amphetamine was measured. Finally, the defensive burying paradigm was used to confirm the findings from the elevated plus maze. Results demonstrated that SNC80 produced dose-dependent anxiolytic effects similar to that of the classical antianxiety agent, diazepam. Administration of naltrindole caused anxiogenic behavior in rats further supporting the involvement of the delta opioid receptor system in regulating anxiety. Naltrindole also blocked the anxiolytic effects of SNC80. Amphetamine had no effect on anxiety-like behavior. SNC80 induced hyperactivity similar to amphetamine at the doses tested, while naltrindole and diazepam did not significantly affect locomotor activity. Although SNC80 can increase locomotor activity, control experiments reported herein indicate that hyperlocomotion is not sufficient to produce an anxiolytic response on the elevated plus maze. Together with the results from the defensive burying paradigm, this suggests that the effects of SNC80 on reducing anxiety are independent of its effects on locomotion. Collectively these data show that the delta opioid receptor system can regulate anxiety-like behavior in an anxiolytic (agonist) and anxiogenic (antagonist) manner. Keywords: Delta opioid receptor, SNC80, naltrindole, anxiety, locomotion, elevated plus maze, defensive burying paradigm, rat Introduction Several lines of investigation support the involvement of the opioid receptor system in the regulation of anxiety. The most compelling evidence for the involvement of the delta opioid receptor system in anxiety comes from a study on delta opioid receptor knockout mice. Specifically, delta opioid receptor deficient mice exhibit an anxiogenic-like phenotype measured in a light dark box and an elevated plus maze (Filliol et al., 2000). To explore the hypothesis that delta opioid receptors are involved in tonic and/or active control of anxiety-like behavior in the rat, the present study utilized the selective delta opioid receptor agonist SNC80 and antagonist naltrindole. SNC80 {(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide} is a nonpeptide delta opioid receptor agonist that is active after systemic administration. It is a methyl ether analog of a group of compounds known as diarylmethylpiperazines (Calderon et al., 1994; Bilsky et al., 1995; reviewed by Calderon & Coop, 2004). The calculated Ki ratios for SNC80 at mu/delta and kappa/delta sites are 495- and 248-fold, respectively, exemplifying the high selectivity for delta opioid receptors (Bilsky et al., 1995). SNC80 binds to cloned delta opioid receptors to initiate GTPγS binding (Plobeck et al., 2000) and inhibit adenylyl cyclase activity (Knapp et al., 1996). Furthermore, several studies have characterized the antinociceptive, antidepressant, stimulant and convulsant activities of SNC80 (Bilsky et al., 1995; Broom et al., 2002a, 2002b). Naltrindole {17-cyclopropylmethyl-6,7-dehydro-4,5α-epoxy-3,14-dihydroxy-6,7,2′,3′-indolomophinan} is a highly selective, nonpeptide delta opioid receptor antagonist that crosses the blood–brain barrier. Portoghese et al. (1988) first reported it as a potent antagonist, and it has been used extensively both in vitro and in vivo due to its high selectivity for the delta opioid receptor. Previous studies indicate that SNC80 has effects on depression-like behavior. Unlike typical antidepressants, delta opioid receptor agonists are effective following a single dose in the forced swim test (Broom et al., 2002b). Furthermore, it has been shown that the hyperactivity and convulsant properties of SNC80 are not required for its antidepressant-like effects (Broom et al., 2002a, 2002b), and the antidepressant-like behaviors are independent of SNC80's disruptive effects on learning (Jutkiewicz et al., 2003). The majority of behavioral research that has been done on SNC80 and other nonpeptide delta opioid receptor agonists has focused on antinociception (Bilsky et al., 1995; Pacheco et al., 2005), antidepressant-like effects and convulsant properties (reviewed in Broom et al., 2002c). A focused study examining the effects of delta opioid receptor agonists on anxiety-like behavior has not been done heretofore. Here, the role of delta opioid receptors in anxiety using two behavioral models, the elevated plus maze and the defensive burying paradigm, was examined. Both models have been well characterized and have common appeal due to their ability to detect novel and proven therapeutic agents that reduce anxiety, thus making them pharmacologically robust models with high-predictive validity (Dawson & Tricklebank, 1995; De Boer & Koolhaas, 2003). The elevated plus maze was chosen as the main rodent model because it is a noninvasive test that does not involve a painful stimulus since it measures the tendency for a rat to avoid the open arms (Pellow et al., 1985; Handley & McBlane, 1993; Dawson & Tricklebank, 1995). The defensive burying paradigm measures the instinctual behavior of a rat to bury something that poses a threat or danger (De Boer & Koolhaas, 2003). The goals of the present study were to evaluate the potential anxiolytic effects of the delta opioid receptor agonist SNC80 and to elucidate the role of the endogenous delta opioid receptor system in modulating anxiety. The elevated plus maze was used to thoroughly explore this aim and the defensive burying paradigm was used to confirm results. Finally, the potential confounding effect of hyperactivity on the anxiolytic effects of SNC80 as measured by the elevated plus maze was examined.