Genetically determined differences in noradrenergic input to the brain cortex: a histochemical and biochemical study in two inbred strains of mice.

The C57 BL/6 and Balb/c strains of mice are characterized by their opposite behavioral responses in several tests. Since several central catecholaminergic systems could be involved, we have analyzed particularly the noradrenergic input to the frontal, parietal, hippocampal and cerebellar cortex in the two strains, using three different approaches. The fluorescence histochemical study revealed the presence of more numerous noradrenergic terminals in all the areas examined (except in the deep layers of the parietal cortex) and a higher number of catecholaminergic cell bodies in the locus coeruleus of the C57 BL/6 mice. Higher levels of norepinephrine were also found in the same cortical areas, except the parietal cortex. However, as indicated by the estimations of the activity of the isoproterenol-sensitive adenylate cyclase and the specific binding of [³H]dihydroalprenolol, there was no difference in the number and the affinity of β-adrenergic receptors in the cerebral cortex of the two strains. The higher density of the cortical noradrenergic innervation observed in the C57 BL/6 mice, which persisted in older animals, seems to be genetically determined.The marked difference in the extent of the dorsal noradrenergic pathway seen between the C57 BL/6 and Balb/c strains could thus make them a valuable model to further define the functional role of the dorsal noradrenergic system.