Limbic system dopamine pathway

The nucleus accumbens is part of the limbic system. It receives dopaminergic input through the mesolimbic system that originates from cell bodies in the ventral segmental area (A 10 cell group). This mesolimbic dopaminergic pathway is part of the reward pathways. Drugs of abuse (cocaine, amphetamine, opiates or nicotine) have been shown to increase the level of dopamine release in these neurons. 

The pleasure derived from using tobacco is linked to the stimulation of dopamine-dependent neurotransmitter pathways in the brain, particularly in the meso-limbic system. The precise nature of this link remains controversial, but many of the neurophysiological processes underlying nicotine addiction are common to other addictive drugs with diverse pharmacological actions such as opiates, cannabis, alcohol and cocaine. 

Dopaminergic neuromodulatory system. The neurons that synthesize dopamine (structural formula in box) are found in the midbrain, from which they project to the limbic system (the mesolimbic pathway), the cerebral cortex (the mesocortical pathway), as well as to the extrapyramidal motor system (the nigrostriatal pathway). 

Schizophrenia appears to be caused by an overactivity of dopamine pathways in certain parts of the brain such as the limbic system.2,23 This idea is based primarily on the fact that most antipsychotics block dopamine receptors, thereby reducing dopaminergic hyperactivity in mesolimbic pathways and other limbic structures (see the next section of this chapter). The increased dopamine influence underlying psychosis could be caused by excessive dopamine synthesis and release by the presynaptic neuron, decreased dopamine breakdown at the synapse, increased postsy-naptic dopamine receptor sensitivity, or a combination of these and other factors. 

The distribution of dopamine in the brain is very non-uniform. There is some in the limbic system, and a large proportion is found in the corpus striatum - a part of the extrapyramidal motor system which is concerned with the coordination of movement. Dopamine-containing nerves are found in three main pathways in the brain. The nigrostriatal pathway contains about 75% Of the dopamine in the brain, and the cell bodies lie in the substantia nigra and the nerves terminate in the corpus striatum. The second important pathway is the mesolimbic pathway, the cell bodies of which lie in the mid-brain and project to parts of the limbic system, particularly the nucleus accumbens. The third, the tubero-infundibular system, consists of short neurons that run from the arcuate nucleus of the hypothalamus to the median eminence and the pituitary gland, the secretions of which they regulate. 

The nigrostriatal pathway and the limbic system also seem to be involved in behavioural effects, and there is evidence that schizophrenia in humans is associated with dopaminergic hyperactivity, and dopamine receptor antagonists are used as antipsychotic agents see dopamine... 

The mesolimbic and mesocortical pathways are dopamine-containing neurones that run from an area in the midbrain called the ventral tegmentum to the limbic system and the prefrontal area of the cerebral cortex respectively. These are areas of the brain that are normally involved in behavioural and emotional functions. 

Fig. 2. A. Forebrain dopamine projection system in rodents and primates. The nigrostriatal pathway projects from the A8 and A9 groups of the substantia nigra (SN) via the medial forebrain bundle (mfb) to the neostriatum (NS). The mesocorticolimbic pathway projects from the more medially located A10 cell group of the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) and olfactory tubercle (OT) of the ventral striatum (VS) and limbic forebrain areas including prefrontal cortex (Ctx), septum (Se) and amygdala (A). B. Striatal projection areas in the rodent brain are divided into the more dorsal neostriatum, and ventral striatum. C. In the primate brain, including human and illustrated for the marmoset, the neostriatum is divided by the fibers of the internal capsule into caudate nucleus (CN) and putamen (Pu). Correspondingly, the neostriatum of rats is sometimes designated the caudate-putamen (CPu) complex.

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