An Introduction to Dopamine
Dopamine is a monoaminergic neurotransmitter. Monoamines are a class of neurotransmitters which also include Serotonin and Norepinephrine (or noradrenaline). A further classification of monoamines is catecholamines. Both norepinephrine and dopamine are catecholamines. Both these catecholamines are produced within the body by chemically converting Tyrosine, an essential amino acid, into L-Dopa and then to dopamine. Dopamine is then chemically converted to norepinephrine. This chemical process is outlined simply in the following figure.
Dopamine can have either an excitatory or inhibitory effect on the postsynaptic potential. In other words, when dopamine leaves the presynaptic neuron and goes into the synapse, it can then bind to receptors on the postsynaptic neuron. After dopamine is bound to the postsynaptic cell, it can either facilitate an action potential or inhibit it. Neurotransmitters are the messengers within the central nervous system. They are responsible for communicating with other neurons either through chemical or electrical signals. A neuron that classically releases dopamine is called a dopaminergic neuron. A series of neurons that are connected by synapses forms a pathway called a dopaminergic pathway. (Kind of makes sense, huh?)
There are three dopaminergic pathways:
Before I explain what each system is responsible for, I would like to explain that systems or tracts in the Central Nervous System (CNS) usually have names that tell you where the neurons begin, (where the cell bodies are located) and where the axons terminate (and therefore what structures it serves). With that information in mind, the nigrostriatal system begins in the Substantia Nigra and projects it's axons to the striatum which is made up of two structures: the caudate and the putamen. This system is concerned with the intiation and maintenance of motor behaviors. The striatum and the Substantia Nigra are both key players in a group of structures called the basal ganglia. The basal ganglia, as well as the cerebellum, is the key area of motor coordination, movement and initiation of movement. The Substantia Nigra are a group of pigmented cells in the midbrain that are damaged in Parkinson's Disease, an illness that impairs the initiation of movement.
The mesolimbic system will be referred to a lot in this site, as it is the main focus of dopamine's involvement with drugs. The mesolimbic system begins in the ventral tegmental area (in the midbrain) and projects to parts of the limbic system including the nucleus accumbens, the amygdala, the septal area and the hippocampus. The nucleus accumbens is the site in this system where there is a reinforcing effect. In other words, the nucleus accumbens is thought of as the reward center. When one partakes in alcohol, marijuana, and even chocolate, dopaminergic neurons in the nucleus accumbens are releasing dopamine.
The mesocortical system has cell bodies located in the ventral tegmental area as well, but this pathway terminates on the prefrontal cortex. These neurons have an excitatory effect on the frontal cortex and thus affect such functions as formation of short term memories, planning, strategy and preparations for problem solving.
The mesolimbic and mesocortical systems appear to be important in the initiation and maintenance of goal directed and reward mediated behaviors. This would include the maintaining of cognitive sets, or logical thought. A disruption of this system alters the normal association process and leads to a breakdown in the proper perceptual functioning of certain areas of the frontal lobe. This results in the inability to ignore or screen nonmeaningful stimuli. There are a number of possible consequences including delusional perceptions and a looseneing of associations. There is possibly a link between the dopamine system and the regulation of affective expression.