¹Laboratori de Neurofarmacologia, Institut Universitari d'Investigació en Ciències de la Salut (IUNICS), Universitat de les Illes Balears, Palma de Mallorca, Balears, Spain

Correspondence: Dr JA García-Sevilla, Laboratory of Neuropharmacology, IUNICS, University of the Balearic Islands, Cra. Valldemossa km 7.5, Palma de Mallorca, Balears E-07122, Spain. Tel: +1 34 971 173148; Fax: +1 34 971 173184; E-mail: jesus.garcia-sevilla@uib.es

Received 17 June 2005; Revised 22 November 2005; Accepted 8 December 2005; Published online 8 February 2006.

Abstract

This study was designed to assess the effects of opiate treatment on the expression of Fas-associated protein with death domain (FADD) in the rat brain. FADD is involved in the transmission of Fas-death signals that have been suggested to contribute to the development of opiate tolerance and addiction. Acute treatments with high doses of sufentanil and morphine (-agonists), SNC-80 (-agonist), and U50488H (-agonist) induced significant decreases (30–60%) in FADD immunodensity in the cerebral cortex, through specific opioid receptor mechanisms (effects antagonized by naloxone, naltrindole, or nor-binaltorphimine). The cannabinoid CB₁ receptor agonist WIN 55,212-2 did not alter FADD content in the brain. Chronic (5 days) morphine (10–100 mg/kg), SNC-80 (10 mg/kg), or U50488H (10 mg/kg) was associated with the induction of tachyphylaxis to the acute effects. In morphine- and SNC-80-tolerant rats, antagonist-precipitated (2 h) or spontaneous withdrawal (24–48 h) induced a new and sustained inhibition of FADD (13–50%). None of these treatments altered the densities of caspases 8/3 (including the active cleaved forms) in the brain. Pretreatment of rats with SL 327 (a selective MEK1/2 inhibitor that blocks ERK activation) fully prevented the reduction of FADD content induced by SNC-80 in the cerebral cortex (43%) and corpus striatum (29%), demonstrating the direct involvement of ERK1/2 signaling in the regulation of FADD by the opiate agonist. The results indicate that - and -opioid receptors have a prominent role in the modulation of FADD (opposite to that of Fas) shortly after initiating treatment. Opiate drugs (and specifically the -agonists) could promote survival signals in the brain through inhibition of FADD, which in turn is dependent on the activation of the antiapoptotic ERK1/2 signaling pathway.