Genetic relationship between central beta-endorphin and novelty-induced locomotor activity

The goal of research presented in this report was to investigate the possibility that differences in beta-ED levels could account for some portion of genetically mediated variation in locomotor activity. Subjects representing six of the LSXSS RI mouse strains were activity tested in a novel environment. Significant effects of strain and time as well as a significant strain by time interaction were detected. Beta-ED levels were estimated from several brain regions and the pituitary of naive and activity-tested mice. Significant strain effects were detected in all brain regions but not in the pituitary. There was no overall effect of exposure to novelty, but some strains showed either a decrease or an increase in beta-ED levels in the septum, amygdala, and midbrain. A significant genetic correlation between adaptation to the novel environment (locomotor activity at 30 min subtracted from locomotor activity at 5 min) and beta-ED levels in the septum was observed. Estimates of hypothalamic mRNA for the beta-ED precursor POMC revealed no effect of strain. Finally, locomotor activity was tested following doses of the mu-opioid antagonist naltrexone. Out of six strains tested (naive to the apparatus), naltrexone dose dependently attenuated locomotor activity in only the strain that showed the highest control level of activity. The results suggest that beta-ED levels influence novelty-induced locomotor activity, but that other important genotype-dependent factors are involved.