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J. Fraser Mustard
genetically vulnerable strain was characterized as hyperreactors to stress or challenge. If not raised by a nurturing mother, these monkeys have an overstimulated CRH-HPA system with exaggerated cortisol responses and poor return to resting levels. These vulnerable animals, poorly nurtured when young, show avoidance of novel stimuli as well as anxious and depressive reactions to maternal separation. As adults, they show increased anxiety and depressive behavior, excessive alcohol consumption when given access to alcohol, impulse aggression and violent behavior, and high circulating sterol levels. The females tend to be poor mothers. Researchers have studied in detail the biological pathways in these animals, which show high sterol levels in response to mild stress, high resting sterol levels, low brain serotonin levels, and a disrupted circadian rhythm for sterols. When offspring from “poor� mothers in the vulnerable strain are taken and reared by highly nurturing mothers (Suomi 1997, 2000), the high-risk infants become secure and precocious in their exploratory patterns. As adults, they rise to the top of the social hierarchy; have a robust immune response, a better regulated sterol pathway, and normal brain serotonin levels; and the females become very nurturing mothers. Researchers have found a genetic characteristic related to serotonin metabolism in the vulnerable monkeys (Bennett and others 2000; Suomi 2000). If poorly nurtured, the animals with the short allele have excess sterol responses to stimuli and low brain serotonin levels; if well nurtured, the animals do not show these adverse responses (Suomi 2000). This finding exemplifies the effect of environment on gene expression. Immune Effects
One of the important effects of the HPA system is on the immune system. Excess activity of the CRH-HPA system with increased sterol levels in early life can produce permanent and marked reductions in immune competence (Coe 1999; Suomi 2000). The effect of sterols on the immune system is complex; for example, a strong CRH-HPA response can be beneficial in bringing an acute illness