Selective activation of the mesocortical DA system by stress

IT is now well established that an important system of dopaminergic (DA) neurones innervates various parts of the cerebral cortex in the rat and other species1–3. In contrast to noradrenergic (NA) terminals which are widely distributed in this structure, the DA terminals are mainly confined to deep layers, particularly in the frontal, the cingular and the entorhinal areas4,5. The results of lesion studies demonstrated that the terminal endings in the frontal cortex originate from the A10 group of DA cell bodies localised in the mesencephalon6–8. This group also contains the cell bodies of the classical mesolimbic DA system projecting to the tuberculum olfactorium, the nucleus accumbens, the nucleus of the stria terminalis, and the amydgala9. The DA terminals found in the cingulate and entorhinal areas of cortex may originate mainly from the A9 group of DA neurones6–8. This group gives rise to the well known nigrostriatal DA system which is implicated in extrapyramidal processes. Its degeneration is in part responsible for some of the symptoms seen in Parkinsonian patients. Little is yet known about the functions of the mesocortical and mesolimbic DA pathways. Electrocoagulation or 6-hydroxydopamine (6-OHDA)-induced lesions of the ventral tegmental area, containing the A10 group, produce a syndrome characterised by “locomotor hyperactivity, serious impairment in tests requiring inhibition of a previously learned response, facilitation of approach learning and of active avoidance and hypoemotivity”10,11. Various workers have suggested that the antipsychotic effects of neuroleptics are in part related to the blockade of postsynaptic DA receptors localised in areas innervated by the mesolimbic and mesocortical DA systems12,13. It seems important to establish whether neurones of these two DA systems correspond to an homogeneous population of cells with similar functional characteristics. We have therefore explored this problem in the rat by examining the reactivity of the mesocortical and mesolimbic DA pathways as well as that of nigrostriatal DA system to stress induced by electric foot shocks. Our results suggest that the mesocortical DA system is selectively activated by this stress.