Research ReportHigh and low rearing subgroups of rats selected in the open field differ in the activity of K+-stimulated p-nitrophenylphosphatase in the hippocampus
Introduction
Sodium/potassium-adenosinetriphosphatase (Na+/K+-ATPase) is of paramount importance for the proper functioning of the organism; for instance, mice lacking one of the enzyme subunits die just after birth [14], [24]. In the nervous tissue, Na+/K+-ATPase moves Na+ ions out and K+ ions into the neurons, re-establishing the differences in ions' concentrations between intra and extracellular medium, thus repolarizing the neurons' membranes and preparing them to fire upon stimulation [13]. Besides its role on ions' translocation, the enzyme is also involved in other aspects of brain functioning, such as neurotransmitters release and uptake [16], [23], [35], [37]. On the other hand, neurotransmitters regulate the activity of Na+/K+-ATPase [12]. Therefore, differences in the activity of Na+/K+-ATPase could be reflected in differences in the brain electrical excitability leading to divergences in the behavior of the individuals.
Open field is one of the most common behavioral tests used in studies with rodents [28]. Behaviors assumed to reflect distinct brain functions can be scored in an open field session. Rearing, for instance, is a behavior considered to be an indication of exploratory behavior [4], whereas larger time and higher number of visits to the center of the open field are considered to be measures related to a lower fear/anxiety [25], [26], [28]. As already mentioned, differences in brain excitability due to changes in Na+/K+-ATPase activity may lead to changes in behavior. For instance, inhibition of Na+/K+-ATPase with ouabain, a specific inhibitor of the enzyme [13], induced seizures [1], [3] and caused an increase in the activity of rats in the open field [30]. There is a paucity of data regarding the role of Na+/K+-ATPase in behavior. Therefore, the aim of this study was to verify if high (HR) and low (LR) rearing subgroups of rats selected in the open field test would differ in the activity of K+-stimulated p-nitrophenylphosphatase, which reflects K+-dependent dephosphorylation of phosphorylated Na+/K+-ATPase, in brain regions.
Section snippets
Subjects and reagents
Naive adult (3 months old) outbred male Wistar rats from our own colony were used to select subgroups of rats showing high (HR) and low (LR) rearings number in the open field test. The rats after weaning at 21 days old were kept in polypropylene cages (60 × 50 × 22 cm) large enough to support 5–6 rats. The rats were kept in a room with lights (lights on from 07:00 a.m. to 07:00 p. m.) and temperature (22 ± 2 °C)-controlled environment until they were tested in the open field and sacrificed.
Results
As it can be seen in Table 1, the highly statistically significant difference in the mean number of rearings between HR and LR subgroups of rats selected in the open field was kept constant in the 3 selections (Exp. 1 P = 0.0002, Exp. 2 P = 0.0002, Exp. 3 P = 0.0003, Mann–Whitney U test, two-tailed), and the number of rearings in each subgroup was similar in the 3 selections carried out months apart. In the 3 selections, a difference in ambulation was also statistically significant between HR
Discussion
The data obtained in this study showed a consistent higher activity of K+-stimulated dephosphorylation of phosphorylated Na+/K+-ATPase in the hippocampus of HR subgroup of rats selected in the open field test. The higher activity of Na+/K+-ATPase in the hippocampus of HR rats suggests, among other possibilities, a more efficient repolarization of the cell membrane in HR subgroup of rats. This result was obtained in 3 different experiments carried out at different times of the year and using
Acknowledgments
This work was supported by Associação Fundo de Incentivo à Psicofarmacologia (AFIP). Rosana Alves is the recipient of a fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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