Emotional behavior and expression patterns of tyrosine hydroxylase and tryptophan hydroxylase in senescence-accelerated mouse (SAM) P6 mice

Senescence-accelerated prone mouse 6 (SAMP6) is a model for senile osteoporosis. It was recently reported that SAMP6 has a memory deficit in the water maze test. Because emotion and cognition are thought to interact, in the present study to examine emotional behavior in SAMP6, we employed a battery of tasks (open field, elevated plus maze, light–dark exploration, marble-burying behavior, tail suspension), using three age groups (1, 4, and 8 months of age) of SAMP6 mice and age-matched control SAMR1 (senescence-accelerated resistant mouse 1) mice. All three age groups of SAMP6 showed higher activity than SAMR1 in the open field test and reduced anxiety as measured in terms of time spent on the open arms in the elevated plus maze, time spent in the light box in the light–dark exploration, and time spent in marble-burying behavior in the marble-burying test. All three age groups of SAMP6 showed reduced immobility time compared with SAMR1 in the tail suspension test. Western blot analyses showed increased expression levels of tyrosine hydroxylase phosphorylated at serine-40 in striatum and nucleus accumbens and of tryptophan hydroxylase phosphorylated at serine-58 in brain stem of 1-month-old SAMP6. These results suggest that one possible reason for the alterations of motor activity and emotional behavior of SAMP6, at least after 1 month of age, is increased dopamine and serotonin levels.