Dopamine pathways

Dopaminergic pathways, sometimes called dopaminergic projections, are the sets of projection neurons in the brain that synthesize and release the neurotransmitter dopamine. Individual neurons in these pathways are referred to as dopamine neurons. Dopamine neurons have axons that run the entire length of the pathway. The neurons' somata produce the enzymes that synthesize dopamine, and they are then transmitted via the projecting axons to their synaptic destinations, where most of the dopamine is produced. Dopaminergic nerve cell bodies in such areas as the substantia nigra tend to be pigmented due to the presence of the black pigment melanin. Dopaminergic pathways are involved in many functions such as executive function, learning, reward, motivation, and neuroendocrine control. Dysfunction of these pathways and nuclei may be involved in multiple diseases and disorders such as Parkinson's disease,attention deficit hyperactivity disorder,obsessive compulsive disorder, and addiction.

There are eight dopaminergic pathways. The four major ones are listed in the table below.

Named pathways (same as above)

Minor pathways

The mesocortical and mesolimbic pathways are sometimes referred to simultaneously as the mesocorticolimbic projection, system, or pathway.

The dopaminergic pathways that project from the substantia nigra pars compacta and ventral tegmental area into the striatum (i.e., the nigrostriatal and mesolimbic pathways, respectively) form one component of a sequence of pathways known as the cortico-basal ganglia-thalamo-cortical loop. This method of classification is used in the study of many psychiatric illness. The nigrostriatal component of the loop consists of the SNc, giving rise to both inhibitory and excitatory pathways that run from the striatum into the globus pallidus, before carrying on to the thalamus, or into the subthalamic nucleus before heading into the thalamus. The dopaminergic neurons in this circuit increase the magnitude of phasic firing in response to positive reward error, that is when the reward exceeds the expected reward. These neurons do not decrease phasic firing during a negative reward prediction (less reward than expected), leading to hypothesis that serotonergic, rather than dopaminergic neurons encode reward loss. Dopamine phasic activity also increases during cues that signal negative events, however dopaminergic neuron stimulation still induces place preference, indicating its main role in evaluating a positive stimulus. From these findings, two hypothesis have developed, as to the role of the basal ganglia and nigrostiatal dopamine circuits in action selection. The first model suggests a "critic" which encodes value, and an actor which encodes responses to stimuli based on perceived value. However, the second model proposes that the actions do not originate in the basal ganglia, and instead originate in the cortex and are selected by the basal ganglia. This model proposes that the direct pathway controls appropriate behavior and the indirect suppresses actions not suitable for the situation. This model proposes that tonic dopaminergic firing increases the activity of the direct pathway, causing a bias towards executing actions faster.

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