ELSEVIER Molecular Brain Research 23 (1994) 333-337 MOLECULAR BRAIN RESEARCH Research Report Effect of chronic exposure to naltrexone and opioid selective agonists on G protein mRNA levels in the rat nervous system Tiziana Rubino, Paola Massi, Gabriela Patrini, Ida Venier, Gabriella Giagnoni, Daniela Parolaro * Institute of Pharmacology, Faculty of Sciences, University of Milan, L'ia Vanvitelli 3 2 / A , Milan, Italy (Accepted 23 November 1993) Abstract The in situ hybridization technique was used to investigate the effect on G protein a subunit expression throughout the brain of rats chronically infused with naltrexone (70 tzg//~l, 1 ~l/h), DAGO (0.5 izg//.d, 1 ~l/h), DADLE (11.4 p.g/p.l, 1 ~l/h), DPDPE (3.4/zg//zl, 1 /zl/h) and U-50,488H (4/zg/~l, 1 /~l/h). Prolonged exposure to naltrexone did not modify G protein a subunit mRNA expression, whereas DADLE and U-50,488H, respectively, increased the levels of as and ao mRNA in specific brain regions. In particular, a 15% increase in as expression was only observed in the dorsomedial hypothalamic nucleus of rats undergoing chronic DADLE infusion: a 15% increase in ao levels was detected in the claustrum and endopiriform nucleus of rats chronically treated with U-50,488H. These are the first in vivo data to demonstrate that only chronic stimulation with an opioid agonist (morphine and/or DADLE and U-50,488H) is capable of modifying G protein a subunit mRNA. The regional selectivity of these modifications is discussed, together with the receptor specificity of the opioid effects. Key words: G protein expression; In situ hybridization histochemistry; Chronic naltrexone; Chronic selective agonist; Brain region 1. Introduction • Because of their intermediary role between receptor and effector systems, G proteins represent likely sites at which modulation of drug activity following sus- tained drug treatment might occur. Over recent years, research involving neuronal cultures and different cell lines have demonstrated that chronic exposure to mor- phine or selective opioid agonists induces changes in the level of G proteins and/or their mRNA [3,4,8,14,15,16]. However, it still remains to be estab- lished whether or not these changes in signal transduc- tion mechanisms can be demonstrated in whole ani- mals, where the occurrence of interactions between different neuronal pathways could represent an impor- tant difference. Using the in situ hybridization tech- nique, we have recently demonstrated that changes in the contents of the mRNA coding for some G protein a subunits occur in rat brain upon the development of morphine tolerance [10]: specifically, the expression of as and ao messages was significantly increased in * Corresponding author. Fax: (39) 2-7000 2270. 0169-328X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-328X(93)E0222-L discrete brain regions of animals chronically treated with morphine, whereas ai mRNA appeared to be unaffected. This increase in as and ao was character- ized by striking regional specificity: the as message was significantly increased (30%) only in the hypothalamic paraventricular nucleus; ao mRNA was increased in the claustrum and the endopiriform nucleus (20%). Moreover, the as and ao mRNA changes correlated with equivalent changes in protein levels in the same areas. On the basis of these data, the purpose of the present study was to determine whether alterations in G protein a subunit mRNA could be induced by chronic exposure to opioid antagonists, and secondly to investigate the involvement of a specific receptor sub- type in morphine-induced mRNA changes. 2. Materials and methods 2.1. Animals and treatment Male Sprague-Dawleyrats (Charles River, Calco, Italy) weighing 175-200 g were used. The animals were housed individually in