Emory Report

 September 22, 1997

 Volume 50, No. 5

Studies on rats hold clues
to responses to stress

The more newborn rat pups are licked and groomed by their mothers, the better equipped they are to handle acute stress in adulthood, reported Paul Plotsky, a professor in the Department of Psychiatry and Behaviorial Sciences, and his McGill University colleagues in this week's issue of Science.

Plotsky, who directs the Stress Neurobiology Laboratory at the School of Medicine, and his Emory colleagues are applying the current findings and those of their earlier work to observational human studies of maternal support (or lack thereof) of infants born prematurely or born to mothers diagnosed with significant postpartum depression.

Those rats in the current investigation who received the most infantile stimulation handled stress well both externally-by exhibiting appropriate behaviors-and internally, as evidenced by an appropriate and balanced response by the animal's endocrine system and brain chemistry.

Researchers have long suspected that early experiences affect the long-term development of a complex aspect of the central nervous system known as the hypothalamic-pituitary-adrenal (HPA) response. The basis for this hypothesis can be attributed in great part to earlier work that showed "handling" during infancy improved behavioral and endrocrine stress responses. Handling involves separating, as a group, a litter of rat pups from their mother for a few minutes each day for the first 10 days after birth. The action is seen not as maternal deprivation, since mother rats routinely leave the nest for varying periods of time after birth, but as a form of infantile stimulation.

The current study shows a high correlation in handled rat pups between optimum maternal care-as measured by the amount of licking and grooming, and an arched-back form of nursing-and better stress responses later in life compared to animals experiencing moderate maternal deprivation.

"These findings provide support for the Levine hypothesis that the effect of postnatal handling on HPA development is mediated by effects on mother-pup interaction," the authors reported. "Thus handling increases the frequency of licking/grooming, and these maternal behaviors are, in turn, associated with dampened HPA responsivity to stress."

Several physiologic measurements noted by the McGill-Emory team verified the association in adult rats between the desirable trait of HPA inhibition and maternal care.

"We believe that the effects of early environment on the development of HPA responses to stress reflect a naturally occurring plasticity (adaptability) whereby factors such as maternal care are able to program rudimentary, biological responses to threatening stimuli," the authors wrote.

"Like humans, Norway rats inhabit a tremendous variety of ecological niches, each with varied sets of environmental demands. Such plasticity could allow animals to adapt defensive systems to the unique demands of the environment. Since most mammals usually spend their adult lives in an environment that is either the same or quite similar to that in which they were born, developmental 'programming' of [central nervous system] responses to stress in early life is likely to be of adaptive value to the adult."

Long-range goals of this line of research are to develop interventions for human infants in whom the HPA and behavioral responses to stress are at risk of being compromised-and to design drugs for adults in whom the stress response has been adversely affected and who are thus at risk for affective disorders such as addiction, anxiety disorders and depression, said Plotsky.

-Lorri Preston

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