A role played by dopamine and opioid neuronal systems in stress-induced motor suppression (conditioned suppression of motility) in mice.

Mice exhibited a marked suppression of motility (conditioned suppression of motility, which is one of stress-induced motor suppressions) when placed in the same chamber where they had previously received an electric footshock. In the present study the role played by dopamine and opioid neuronal systems in the conditioned suppression of motility has been investigated. In conditioned suppression group, the contents of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and methionine-enkephalin (Met-enk) did not change in the hypothalamus, cerebral cortex and medulla oblongata/pons, while a decrease in the contents of striatal DOPAC were induced compared to that in the control group, which was accompanied by reduction in the contents of Met-enk. In addition, the contents of DA were increased in the midbrain. The ratio of DA metabolites/DA contents in the striatum of the conditioned suppression group was decreased. In the striatum, moreover, the depletion of DA induced by alpha-methyl-p-tyrosine (alpha-MT) in the conditioned suppression group was significantly lower than that in the control group but not in the midbrain. These results suggest that the DA turnover rate and Met-enkergic activity decrease in the striatum of conditioned suppression group.