The central dopaminergic systems

The central dopaminergic systems

These are considerably more complex than the noradrenergic system. This may reflect the greater density of dopamine-containing cells, which have been estimated to be 30–40 000 in number compared with 10 000 noradrenaline-containing cells. There are several dopamine-containing nuclei as well as specialized dopaminergic neurons localized within the retina and the olfactory bulb. The dopaminergic system within the mammalian brain can be divided according to the length of the efferent fibres into the intermediate and long length systems. The intermediate length systems include the tuberoinfundibular system, which projects from the arcuate and periventricular nuclei into the intermediate lobe of the pituitary and the median eminence. This system is responsible for the regulation of such hormones as prolactin. The interhypothalamic neurons send projections to the dorsal and posterior hypothalamus, the lateral septal nuclei and the medullary periventricular group, which are linked to the dorsal motor nucleus of the vagus; such projections may play a role in the effects of dopamine on the autonomic nervous system. The long length fibres link the ventral tegmental and substantia nigra dopamine-containing cells with the neostriatum (mainly the caudate and the putamen), the limbic cortex (the medial prefrontal, cingulate and entorhinal areas) and with limbic structures such as the septum, nucleus accumbens, amygdaloid complex and piriform cortex. These projections areusually called the mesocortical and mesolimbic dopaminergic systems, respectively, and are functionally important in psychotic disorders and in the therapeutic effects of neuroleptic drugs. Conversely, changes in the functional activity of the dopaminergic cells in the neostriatum are primarily responsible for movement disorders such as Parkinsonism and Huntington’s chorea.
The central adrenergic system. It is only recently that immunohistochemical methods have been developed to show that adrenaline-containing cells occur in the brain. Some of these cells are located in the lateral tegmental area, while others are found in the dorsal medulla. Axons from these cells innervate the hypothalamus, the locus coeruleus and the dorsal motor nucleus of the vagus nerve. While the precise function of adrenergic system within the brain is uncertain, it may be surmized that adrenaline could play a role in endocrine regulation and in the central control of blood pressure. There is evidence that the concentration of this amine in cerebrospinal fluidis reduced in depression, which might imply that it is also concerned in the control of mood.