• S Scheggi, A Crociani, M G De Montis, A Tagliamonte, and C Gambarana.
• Department of Neuroscience, Pharmacology Unit, University of Siena, Italy.
• Neuroscience. 2009 Oct 6; 163 (2): 627-39.

AbstractMorphine sensitization is a model of latent, functionally inducible increase in dopamine D(1) receptor-mediated transmission, which may be unmasked by an external stimulus. Morphine-sensitized rats present dopamine D(1) receptor-dependent stereotypies upon morphine challenge and resilience to unavoidable stress-induced behavioral deficits. This tonic increase in dopamine D(1) dopaminergic transmission is counter-adaptive to an enhanced mu-opioid receptor-dependent signaling in striatal areas. Control and sensitized rats show a similar dopamine and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) phosphorylation pattern in striatal areas. Acute morphine administration induced an early increase and delayed decrease in phospho-threonine (Thr)34 DARPP-32 levels accompanied by a delayed increase in phospho-Thr75 DARPP-32 levels in the nucleus accumbens and caudate-putamen of sensitized rats, while it had no effects in control animals. The administration of a selective dopamine D(1) receptor antagonist (SCH 23390) before morphine challenge prevented the behavioral and neurochemical modifications in sensitized rats. 6-Methyl-2-(phenylethynyl)-pyridine, a selective metabotropic glutamate receptor 5 (mGluR(5)) antagonist, administered 1 h after morphine challenge, prevented the delayed phosphorylation changes, but it had no effect when administered before challenge. Moreover, the DARPP-32 phosphorylation pattern in the caudate-putamen of sensitized rats after unavoidable stress exposure was studied. The stress-induced neurochemical modifications and their sensitivity to receptor antagonists were similar to those observed after acute morphine administration. In conclusion, these results suggest that in the experimental conditions used an increase in dopamine output in striatal areas is followed by a complex neurochemical pattern, in which the initial stimulation of dopamine D(1) receptors triggers a sequence of signaling events that lead to an mGluR(5)-mediated increase in phospho-Thr75 DARPP-32 levels. Since DARPP-32 phosphorylated in Thr75 inhibits cAMP-dependent protein kinase (PKA) activity, the final result is a decrease in the dopamine D(1) receptor-dependent phosphorylation events.

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