Mice that do not have the gastrin-releasing peptide (GRP) gene show enhanced learning of fear.
The researchers next explored whether eliminating GRP's activity could affect the ability to learn fear by studying a strain of knockout mice that lacked the receptor for GRP in the brain.
In behavioral experiments, they first trained both the knockout mice and normal mice to associate an initially neutral tone with a subsequent unpleasant electric shock. As a result of the training, the mouse learns that the neutral tone now predicts danger. After the training, the researchers compared the degree to which the two strains of mice showed fear when exposed to the same tone alone — by measuring the duration of a characteristic freezing response that the animals exhibit when fearful.
"When we compared the mouse strains, we saw a powerful enhancement of learned fear in the knockout mice," said Kandel. Also, he said, the knockout mice showed an enhancement in the learning-related cellular process known as long-term potentiation.
"It is interesting that we saw no other disturbances in these mice," he said. "They showed no increased pain sensitivity; nor did they exhibit increased instinctive fear in other behavioral studies. So, their defect seemed to be quite specific for the learned aspect of fear," he said. Tests of instinctive fear included comparing how both normal and knockout mice behaved in mazes that exposed them to anxiety-provoking environments such as open or lighted areas.
"These findings reveal a biological basis for what had only been previously inferred from psychological studies — that instinctive fear, chronic anxiety, is different from acquired fear," said Kandel.
In additional behavioral studies, the researchers found that the normal and knockout mice did not differ in spatial learning abilities involving the hippocampus, but not the amygdala, thus genetically demonstrating that these two anatomical structures are different in their function.
The regulation of the expression of a large variety of genes in the brain varies from person to person because there are genetic variations in genes and regulatory areas that govern how much each gene makes its resulting protein product(s). Personality types will eventually be shown to have genetic causes. This will of course lead to a desire on the part of prospective parents to exercise some control over which personality-related genetic variations their offspring will get. Genetic engineering of personality will become a hotly debated topic when it becomes clear to the general public that this will be technically possible.
|Share |||Randall Parker, 2002 December 12 02:57 PM Brain Genetics|