Beyond the Reward Pathway
What Is a Brain Pathway?
The Dopamine Pathways
Dopamine is the neurotransmitter used by the reward pathway (also called the mesolimbic pathway, which is closely linked with the mesocortical pathway). But there are two other important pathways in the brain that use dopamine: the nigrostriatal pathway and the tuberoinfundibular pathway. Generally, drugs that affect dopamine levels affect all three of these pathways.
Nigrostriatal pathway: Substantia nigra to striatum
- Motor control
- Death of neurons in this pathway is linked to Parkinson's Disease
Mesolimbic and Mesocortical pathways: Ventral tegmental area to nucleus accumbens, amygdala, hippocampus, and prefrontal cortex
- Memory, motivation, emotion, reward, desire, and addiction
- Dysfunction is connected to hallucinations and schizophrenia
Tuberoinfundibular pathway: Hypothalamus to pituitary gland
- Hormone regulation, nurturing behavior, pregnancy, sensory processes
Dopamine and another neurotransmitter called serotonin are released by just a small number of neurons in the brain. But each of these neurons connects to thousands of other neurons in many areas of the brain, giving them a great deal of influence over complex processes.
The Serotonin Pathways
Serotonin is another neurotransmitter affected by many drugs of abuse, including cocaine, amphetamines, LSD, and alcohol. Serotonin is made by neurons in the Raphe nuclei. These neurons reach and dump serotonin onto almost the entire brain, as well as the spinal cord.
Serotonin plays a role in many brain processes, including body temperature regulation, sleep, mood, appetite, and pain. Problems with the serotonin pathway are linked to obsessive-compulsive disorder, anxiety disorders, and depression. Most prescription drugs used to treat depression today work by increasing serotonin levels in the brain.
Glutamate and GABA: A System in Balance
Glutamate and GABA (gamma-aminobutyric acid) are the brain's most plentiful neurotransmitters. Over half of all brain synapses use glutamate, and 30-40% use GABA.
Since GABA is inhibitory and glutamate is excitatory, both neurotransmitters work together to control many processes, including the brain's overall level of excitation. Many of the drugs of abuse change the balance of glutamate or GABA, exerting tranquilizing or stimulating effects on the brain. Drugs that increase GABA or decrease glutamate are depressants. Those that decrease GABA or increase glutamate are tranquilizers or stimulants.
- Alcohol decreases glutamate activity.
- PCP, or "angel dust," increases glutamate activity.
- Caffeine increases glutamate activity and inhibits GABA release.
- Alcohol increases GABA activity.
- Tranquilizers increase GABA activity.