The hippocampus, epigenetics, and PTSD:
In "Determining Nature vs. Nurture," Scientific American Mind (Oct/Nov 2006, p12), Douglas Steinberg provides a great explanation of how the environment can affect neurons, including those in the hippocampi. Steinberg writes:
A field called epigenetics has finally begun to address some of these issues. Its practitioners study how tiny molecules stick to, or become unstuck from, two main targets in a cell's nucleus: the DNA in and around a gene and the histones—the proteins around which chromosomes spool. These tiny molecules are known as methyl and acetyl groups and their presence or absence at target sites controls whether particular genes can generate proteins, the workhorses of most physiological processes.
Until a couple of years ago, the conventional wisdom in biology held that such molecular changes occur in primitive cells, usually during embryonic and fetal development, not in mature cells such as a child's or adult's neurons. Then researchers proved that epigenetic changes are indeed at work in mature cells. Now studies are starting to show how environmental cues can stimulate epigenetic changes that could contribute to several psychiatric diseases. Systematic measurement of those changes could eventually indicate how the environment influences the genetic chemistry underlying many human behaviors.
Steinberg points to the work of Eric J. Nestler, psychiatry department chair at the University of Texas Southwestern Medical Center at Dallas. Nestler has proposed a model of depression that includes "epigenetic changes in the hippocampus, a memory-storing brain region that actually shrinks in some cases of human depression."
The image to the right links to a BBC News story about the work of Michael J. Meaney, a psychiatry professor at McGill University. Steinberg explains that Meaney "has found that when a rat pup receives less licking and grooming from its mother, it is more fearful and more reactive to stressors as it matures." Steinberg reports:
The team found that a hippocampal gene sheds methyl-group molecules during the first week of a [rat] pup's life if its mother is a 'high licker.' Pups of low lickers do not prune the molecules. An adoption experiment proved that licking triggers these events: when the team entrusted pups born to mothers of one licking type to mothers of the other type, the genes' methyl status reflected the licking type of the adoptive parent. Licking is believed to exert its effect by raising the pups' thyroid-hormone production and activity of the neurotransmitter serotonin. … The findings suggest that a mother's parenting style can have very different effect on the activity of a child's genes. … Epigenetics may indeed unveil what is happening at the intersection of genes and environment.
Steinberg calls attention to the work of Dr. J. Douglas Bremner who does research on Post-traumatic Stress Disorder (PTSD). In an on-line article Bremner writes: "Recent studies have shown that victims of childhood abuse and combat veterans actually experience physical changes to the hippocampus, a part of the brain involved in learning and memory, as well as in the handling of stress. The hippocampus also works closely with the medial prefrontal cortex, an area of the brain that regulates our emotional response to fear and stress. PTSD sufferers often have impairments in one or both of these brain regions. Studies of children have found that these impairments can lead to problems with learning and academic achievement." Bremner goes on to say: "Memory problems play a large part in PTSD." He explains that PTSD patients report deficits in declarative memory, such as being able to remember facts or lists, fragmentation of memory, and dissociative amnesia, which involves gaps in memory lasting from minutes to days.
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