Changes in Catechol-O-Methyltransferase Gene Expression Associated with Agonistic Behavior in Male Mice

Abstract

Catechol-O-methyltransferase (COMT) is anenzyme catalyzing the transfer of the methyl groupfrom S-adenosine-methionine to the hydroxyl group ofcatecholamines, which are thereby inactivated. COMTfunctions in the central nervous system (CNS) andperipheral tissues plays an important role in catabolismof brain dopamine and noradrenaline, which areinvolved in regulation of human and animal behavior,both normal and pathological. We suggested that thelevel of the COMT gene expression may play a certainrole in basic behavioral processes, such as agonistic(competitive) behavior characteristic of animals at var-ious levels of evolutionary hierarchy. This behavior isused to defend the dominated territory or to establishthe domination–submission relationships. In this study,we used the midbrain of male C57BL/6J mice witheither aggressive or submissive behavioral type to ana-lyze the possible changes in the COMT gene expres-sion (the level of mRNA) with the use of quantitativepolymerase chain reaction (PCR).To obtain alternative types of behavior in mice, weused the sensory contact model [1]. The individualsocial victory or defeat experienced in the first pairwisesocial confrontation was subsequently reinforced bydaily contacts with the animal exhibiting the oppositetype of behavior. After aggressive interactions during10 to 20 days (tests), we obtained groups of animalswith experience of several consecutive victories (win-ners, or aggressors) or defeats (losers, or victims). Thewinners always displayed pronounced aggression andattacked the opponents, whereas the losers submitted orran away, thus displaying submissive behavior. Intactmice served as a control group; they were kept individ-ually for five days to reduce the effect of the group rela-tionships.On day after the last agonistic interaction, bothexperimental and control animals were decapitated,their brain was withdrawn in the cold, and the midbrainand pons were isolated (these structures contain mostof the monaminergic, i.e., noradrenergic, dopaminer-gic, and serotonergic neurons). Tissues were pooled(two midbrains per sample) and stored at t = –70 ° Cuntil biochemical analysis. Three measurements weremade with simultaneous analysis of brain tissue sam-ples from all animal groups.Total RNA was isolated [2], dissolved in water, andtreated with DNAse. The qualitative pattern of the prep-aration was obtained by electrophoresis in 1.5% agar-ose gel. The amount of RNA was measured spectropho-tometrically in each sample. RNA samples were storedat – 70i C before use. cDNA was obtained by means ofreverse transcription of 300 ng of the total RNA for 1 hat 37 i C using 5 µ M d (ð T )