Dopamine: a key regulator to adapt action, emotion, motivation and cognition.

Dopamine is expressed in restricted brain areas involved in numerous integrative functions contributing to automated behaviours that are highly adaptive. During ontogenesis, dopamine can have a trophic action, which influences cortical specification directly, especially in prefrontal areas. Such a role is attested by the close relationships existing between the development of dopamine cortical innervation and cognitive abilities. Interestingly, such a proposal is reinforced by phylogenetic data. During the last stages of evolution in mammals, the characteristic extension of dopamine cortical innervation is also correlated with the development of cognitive capacities. More generally, the contribution of dopamine will be to increase the processing of cortical information through basal ganglia, either during the course of evolution or development. In this respect, dysmaturation suspected in schizophrenia and attention deficit-hyperactivity disorder in children can emphasize such a defect in basal ganglia processing. Remarkably, these diseases are improved by dopamine antagonists and agonists, respectively. In this line of evidence, experimental studies showed that selective lesions of the dopamine neurones in rats or primates can actually provide motor, limbic and cognitive deficits, in later cases especially when the mesocorticolimbic dopamine pathways are altered. Data resulting from lesion studies also showed significant alteration in attentional processes, thus raising the question of the direct involvement of dopamine in regulating attention. Dopamine can act as a powerful regulator and integrator of different aspects of brain functions. For example, in Parkinson's disease, besides motor impairment, dopamine degeneration is also expressed by alterations of both limbic, executive and cognitive functions, both improved by dopamine receptor agonists and dopa therapy. Dopamine has thus to be considered as a key regulator that contributes to behavioural adaptation and to the anticipatory processes necessary for preparing voluntary action consequent upon intention. In this respect, the alteration of dopamine transmission with age could contribute to cognitive impairment. Therefore, to normalize dopamine transmission pharmacologically could actually improve the cognitive and limbic deficits during normal aging, as it does in psychiatric and neurological disorders.