Hippocampal neurogenesis is a process in which new nerve growth occurs in an area of the brain called the hippocampus. In most parts of the brain, new nerves are not created, but in the hippocampus, nerve creation, or neurogenesis, takes place throughout a person's lifetime. Conditions such as depression and stress seem to be associated with a decrease in hippocampal neurogenesis, while antidepressant drug treatment may increase new nerve formation.
The hippocampus is found in a part of the brain called the limbic system, which is located in the medial temporal lobe. Functionally, the limbic system is concerned with emotions and memories. The hippocampus is positioned just beneath the brain's outer layer of gray matter, known as the cerebral cortex, and it is thought to be involved in the conversion of short-term memory into long-term memory. It may also have a part to play in the process of spatial navigation, where people have to find their way by noting and remembering their current and past positions in the environment.
Hippocampal neurogenesis was first linked with depression when researchers noticed that the hippocampus was smaller in people who had suffered from clinical depression for a long time. Researchers also found that, after people had been treated for a number of weeks with antidepressant medication, hippocampal neurogenesis seemed to improve, with the result that the size of the hippocampus began to approach its former state. From studies involving mice, it has been suggested that the way antidepressant drugs work may depend on hippocampal neurogenesis occurring. This is thought to be the case even though the action of antidepressants also causes an increase in levels of chemicals in the brain known as neurotransmitters. The true picture of the way in which drugs help to relieve depression may well involve a combination of chemical and structural brain changes.
Drug abuse is also thought to have an effect on hippocampal neurogenesis. In studies carried out with rats which were given alcohol, cocaine and nicotine, all of these substances were associated with decreased hippocampal neurogenesis. Some researchers found that long-term administration of synthetic cannabinoid drugs could have an opposite effect, with an increase seen in hippocampal nerve growth, together with reductions in anxiety and depression which are similar to the effects of antidepressant medication. Hippocampal neurogenesis research provides useful insights for the development of more effective drugs to treat depression.