PerspectiveBehavior

The Importance of a Well-Groomed Child

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Science  12 Sep 1997:
Vol. 277, Issue 5332, pp. 1620-1621
DOI: 10.1126/science.277.5332.1620

It is a rare parent of a newborn who does not feel a panic built around the consequences that her or his actions now have. Developmental studies have indicated that the quality, quantity, and timing of infant stimulation can have long-lasting effects—and soon the anxious parent is convinced that one lullaby sung off-key ensures that a child will not only one day be a sociopath, but will also never use dental floss. If mothers of newborn rats harbor similar anxieties, a report by Liu and colleagues (1) affirms their worries: The authors show that subtle stimulation in a rat's infancy has marked consequences that are probably life-long.

More than 40 years ago, Seymour Levine and his colleagues uncovered the effects of “neonatal handling” (2). Human handling of newborn rats for 15 min daily during the first few weeks of life produced salutary neuroendocrine, neurochemical, and behavioral changes in the adult. However, rats handled at later points in time did not produce the same changes (3). One of the most interesting changes was the decreased secretion of glucocorticoids. These hormones are released by the adrenals during stress, and glucocorticoid excess can have numerous deleterious effects. These include damage to the nervous system, where chronic glucocorticoid excess can accelerate the loss of certain classes of neurons during aging. And indeed, handled rats do have less neurodegeneration and fewer cognitive deficits in old age (4).

Some mechanisms underlying the handling effect on glucocorticoid secretion have been uncovered (3). Handling triggers thyroid hormone release, which appears to activate ascending serotonergic projections into the hippocampal region of the brain. Serotonin can cause long-lasting increases in glucocorticoid receptor number in hippocampal neurons. The hippocampus helps to mediate negative-feedback inhibition of subsequent glucocorticoid secretion, and the higher receptor density enhances the sensitivity with which the hippocampus detects circulating glucocorticoid levels, resulting in more tightly regulated feedback inhibition.

Early care counts.

An obvious question is whether handling is relevant to humans. When we reported the decelerated brain aging in handled rats many masseuses wanted to know if they could claim that massages prevented Alzheimer's disease. A less obvious question is what handling means for rats. Levine speculated that it was not the handling that caused the changes, but the behavior of the mother. Specifically, he theorized that the burst of licking and grooming of her pups upon their return to the fold caused the subsequent changes (5). The current report supports this idea.

The authors showed that handling approximately doubled a mother's rate of licking and grooming. They then examined mothers of nonhandled controls and identified approximately a third of the mothers who naturally licked at the same high rate as that induced by handling. Pups of such high lickers-groomers were followed into adulthood and, when compared with offspring of low lickers-groomers, had the same changes induced by handling—less glucocorticoid secretion during a stressor and a faster recovery to baseline afterward, more hippocampal glucocorticoid receptors, and lower hypothalamic corticosteroid releasing factor (CRF) mRNA levels (CRF being the hormone that ultimately controls the release of glucocorticoids). Other changes occurred that were identical to those seen in neonatally handled rats: more open-field exploration (an indication of less anxiety in animals) and more receptors throughout the brain for benzodiazepines (anxiety-reducing tranquilizers) (6).

Obviously, mothering style can influence an offspring's development, but few have shown differences this subtle to be associated with so global a change, or uncovered intervening physiologic steps in the process. Far more remains to be done. First it is to determine whether licking and grooming are indeed critical to the changes that occur in handled rats, or are correlates of even subtler individual differences in mothering style. Next one must show how mothering differences initiate neurotransmitter and receptor changes in the hippocampus. Of potential relevance, tactile stimulation by mother rats maintains growth hormone and ornithine decarboxylase levels in pups (7). One must also test whether some intrinsic feature of a rat that predisposes it to the “handled” profile in adulthood also elicits more maternal behavior when it is a pup. And finally, it must be determined whether the offspring of high lickers-groomers are spared some neurodegenerative ravages of senescence.

The authors suggest that their finding may be ecologically relevant. Various rodent species differ in their storage patterns: a single cache in their burrow or in scattered and distant caches. Although it has been assumed that a genetic difference underlies this behavior, it could also be driven by the organizational effects. Rodents in the latter category have larger hippocampi and better spatial skills (8). The assumption has always been that this represents a genetic difference; instead, it might be driven by the organizational effects of different foraging patterns (for example, how often an animal leaves and then returns to the burrow) on mothering style.

Finally, these findings should have relevance even closer to home. Although the specifics of licking and grooming do not extend to humans, the broader point emphasizing the importance of early experience certainly does. We are in an era filled with parental quandaries such as the type of day-care to provide, the inner-city specter of the dissolution of the family, teen pregnancy, and low government spending on child-related social services during critical periods of brain development. This current study must spur on work examining how early experience alters the trajectory of our own development.

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