No abstract

Chronic administration of cocaine has been shown to attenuate the functional capacity of delta opioid receptors to inhibit adenylyl cyclase activity. Abuse and withdrawal from cocaine in humans is associated with increases in anxiety and depression. Since recent research supports the role of delta opioid receptors in anxiety-and depression-like behaviors in rodents, we hypothesized that functional desensitization of delta opioid receptors contributes to anxiety-and depression-like behavioral phenotypes following short-term withdrawal from chronic administration of cocaine. To test this hypothesis, delta opioid receptor signaling and behaviors were evaluated 24 h after 14 days of bingepattern cocaine administration (15 mg/kg three times daily at 1 h intervals) in male Sprague Dawley rats. Results showed that the inhibition of adenylyl cyclase by delta opioid receptor agonists was attenuated in the frontal cortex, nucleus accumbens and caudate putamen 24 h after cessation of cocaine administration. One day withdrawal from chronic administration of cocaine resulted in increased anxiety-and depression-like behaviors as measured by the elevated plus maze and the force swim test respectively, and no change in locomotor activity. The anxiety-and depression-like behaviors were dose-dependently reduced by acute administration of the selective delta opioid receptor agonist, SNC80. These results demonstrate that early withdrawal from cocaine resulted in increased anxiety and depression, which accompanies the desensitization of delta opioid receptor function. Furthermore, cocaine-induced anxiety-and depression-like behaviors were reversible by the delta opioid receptor agonist SNC80.

No abstract

Chronic cocaine administration produces alterations in mu and kappa opioid receptor density as well as striatal and accumbens opioid-regulated adenylyl cyclase activity, suggesting a psychostimulant responsive interaction between opioidergic and dopaminergic systems. Stimulation of G-protein-coupled opioid receptors inhibits adenylyl cyclase production of cyclic AMP. The present study employed in situ [(35)S]GTPgammaS binding to measure opioid receptor-stimulated activation of G-proteins in response to acute and chronic cocaine exposure. Male Fischer rats received acute (1 or 3 days) or chronic (14 days) binge pattern cocaine administration. Three and 14 days of cocaine injections resulted in greater increases in the ability of the mu receptor agonist DAMGO to stimulate [(35)S]GTPgammaS binding in both the core and the shell of the nucleus accumbens, all regions of the caudate putamen and the cingulate cortex compared with saline-matched controls. The greatest increases in DAMGO-stimulated [(35)S]GTPgammaS binding were observed in the dorsal areas of the caudate putamen in animals that received 14 days of cocaine. No significant changes in delta (DPDPE), or kappa (dynorphin A(1-17)) receptor-stimulated [(35)S]GTPgammaS binding were found in any brain region in response to cocaine administration. These results demonstrate that binge pattern cocaine administration induce changes in mu but not delta or kappa opioid receptor-mediated G-protein activity. This study provides support for the hypothesis that the addictive properties of both psychostimulants and opiates may share common neurochemical signaling substrates.

These results suggest that activation of mu opioid receptors by endogenous opioids is an important contributor to cocaine-induced hyperactivity and the development of behavioral sensitization and conditioned reward.