High and low rearing subgroups of rats selected in the open field differ in the activity of K+-stimulated p-nitrophenylphosphatase in the hippocampus

Na+/K+-adenosinetriphosphatase (Na+/K+-ATPase) is of paramount importance for the proper functioning of the organism. The enzyme is involved in several aspects of brain function, such as the repolarization of the neuronal membranes and neurotransmitters uptake/release. Therefore, individual differences in the activity of brain Na+/K+-ATPase may result in differences in the functioning of the brain, which, in consequence, could lead to behavioral divergences. Individual differences in rearing, an exploratory behavior, have been shown to be genetically determined. In rats, the inhibition of the activity of Na+/K+-ATPase was reported to induce changes in the exploratory behavior. The aim of this work was to verify if subgroups of rats selected according to the number of rearings (high and low rearing subgroups) in the open field test differ in the activity of Na+/K+-ATPase in brain regions. Adult, male outbred Wistar rats were selected in the open field test according to the number of rearings in subgroups of high (HR) and low (LR) rearing responders. After a rest of about 20 days after the open field session, HR and LR rats were sacrificed. In the first experiment, frontal cortex, striatum, brainstem, hippocampus and the amygdala (including the overlying limbic cortex) were dissected. The reaction of dephosphorylation of Na+/IC+-ATPase (K+ stimulated p-nitrophenylphosphatase) was assayed in homogenates rich in synaptosomes. The results obtained showed a statistically significant higher activity of K+ p-nitrophenylphosphatase only in the hippocampus of HR subgroup of rats. This result was replicated in two other subsequent experiments with different HR and LR subgroups of rats selected at different times of the year. Our data suggest that the difference in the activity of Na+/K+-ATPase in the hippocampus is innate and is involved in the expression of the rearing behavior. (c) 2005 Elsevier B.V. All rights reserved.