Role of COMT in catecholamines metabolism during morphine withdrawal

P Almela, E Martínez-Laorden, L Mora, MV Milanés, ML Laorden. University of Murcia, Pharmacology 30100, Spain

Introduction

The enzyme catechol-O-methyltransferase (COMT) is a 5-adenosylmethionine-dependent methyl transferase enzyme that methylates catechol substrates. It exists in two forms, soluble (S-COMT) and membrane-bound (MB-COMT), arising from different translation start sites. In mammals, COMT is widely distributed throughout the organs of the body, including the heart. Exposure to a stressful situation, such as morphine withdrawal, leads to the activation of the catecholaminergic system which results in enhanced circulating catecholamines levels. Therefore, the present study was designed to investigate the possible involvement of COMT enzyme in noradrenaline (NA) metabolism during morphine withdrawal in cardiac tissue.

Material and Methods

Male Sprague-Dawley rats (220-240 g at the beginning of the experiments) were rendered dependent on morphine by a 6-days s.c. implantation of morphine pellets. Morphine withdrawal was precipitated on day 7 by naloxone injection (2 mg/kg, s.c.) and rats were killed 60 minutes after the antagonist injection. COMT expression was determined by western blot and the noradrenergic turnover in left ventricle was studied by high-performance liquid chromatography (HPLC). Results are expressed as the mean ± SEM. Data were analyzed by analysis of variance (ANOVA) followed pendencndered the beginning of the experiments)by the Newman–Keuls post-hoc test.

Results and Conclusions

Our results show that 60 min after naloxone administration to morphine dependent rats there was an increase (P<0.001) of NMN content (58.85 ± 8.70, n = 7) and NMN/NA ratio (as index of NA turnover) (0.3957 ± 0.03, n = 7) concomitantly with a decrease in NA content (180 ± 32.7, n = 7) when compared with morphine dependent rats receiving saline (27.20 ± 4.86, n = 7; 0.064 ± 0.01, n = 7; 489.36 ± 80.3, n = 7, respectively) or naïve rats injected with naloxone (39.55 ± 6.34, n = 5; 0.07274 ± 0.02, n = 5; 427.9 ± 30.2, n = 5, respectively). In parallel to the enhancement of NMN, we observed an increase (P<0.05) in MB-COMT (255.7 ± 53.06, n = 3) and S-COMT (295.8 ± 25.29, n = 3) expression in the left ventricle after naloxone-induced morphine withdrawal. The enhancement of S-COMT was significant versus the morphine dependent group injected with saline (185.6 ± 28.62, n = 3) and the placebo group receiving naloxone (151.1 ± 39.39, n = 3) whereas the increase of MB-COMT expression was only significant versus rats dependent on morphine and injected with saline (144 ± 27.84, n = 4). These data demonstrate that both forms of COMT are implicated in the degradation of NA and suggest that COMT plays an important role in NA metabolism and in the enhancement of NA turnover observed after naloxone-induced morphine withdrawal.