The mitogen-activated protein kinases (MAPKs), such as the extracellular signal-regulated kinases ERK1 and ERK2, are activated by a wide variety of G protein-coupled receptors (GPCRs), including the opioid receptors (Ortiz et al. 1995). GPCR-mediated activation of MAPKs has been studied extensively in vitro, although little is known about GPCR-mediated activation of MAPKs in vivo (Bhat et al. 1998). Recently, we described that acute sufentanil administration (a potent µ-opioid agonist) in rats increased the phosphorylated form (active) of MEK (ERK kinase) by a naloxone-sensitive mechanism. The aim of this work was to compare the effect of sufentanil administration on MEK1/2 and ERK1/2 in various regions of the rat brain.

Male Sprague-Dawley rats (250-300 g) received a single 0.9 % saline vehicle S.C. (control rats, n = 7-9) or sufentanil (1, 2.5, 5, 15 or 30 µg kg^-1, S.C., n = 4) and were killed by decapitation 30 min later. The cerebral frontal cortex, caudoputamen and hippocampus were dissected, immediately frozen in liquid nitrogen, and stored at -80 °C. The experiments were performed according to the guidelines of the University of the Balearic Islands. The phospho-MEK1/2, MEK1/2, phospho-ERK1/2, and ERK1/2 proteins were immunodetected with specific antibodies by Western blot. The immunoreactivity was detected with an enhanced chemiluminescence (ECL) Western blot detection system and quantified by densitometric analysis. Results are given as means ± S.E.M.; ANOVA followed by Scheffé’s test was used for the statistical evaluations.

High doses of sufentanil increased the density of p-MEK1/2 in both frontal cortex (15 µg kg^-1; 206 ± 21 %, P < 0.001, and 30 µg kg^-1; 207 ± 7 %, P < 0.001) and caudoputamen (15 µg kg^-1; 213 ± 32 %, P < 0.05, and 30 µg kg^-1; 277 ± 28 %, P < 0.001), and decreased the density of p-MEK1/2 in the hippcampus (15 µg kg^-1; 78 ± 0.5 %, P < 0.05, and 30 µg kg^-1; 77 ± 5 %, P < 0.05). The density of p-ERK1/2 was decreased in hippocampus (p-ERK1: 15 µg kg^-1; 77 ± 6 %, P < 0.05, and 30 µg kg^-1; 69 ± 2 %, P < 0.01; p-ERK2: 68 ± 6 %, P < 0.01), whereas only p-ERK1 decreased in frontal cortex (30 µg kg^-1; 77 ± 3 %, P < 0.01), and no changes were observed in caudoputamen. No changes were observed in the density of total MEK1/2 and total ERK1/2 in any brain region studied after sufentanil treatment. The results indicate the existence of a region-specific regulation of the ERK pathway in rat brain after sufentanil treatments.

This work was supported by grant BFI2000-306 (MCYT, Spain). V.J.A. was supported by a FPU fellowship (MECD).