deprived rats seem to be less responsive to their environment and exhibit dull reactions, similar to 1human mood disorders, he says.

These behavioral syndromes correspond to different changes in the rats' brain chemistry. By inserting microprobes into rat brains and sampling neurotransmitters, Robbins's group found that the isolation-reared rats had higher levels of dopamine—a neurotransmitter thought to go awry in schizophrenia— in certain areas of the brain known to be involved in addiction and motor control. The maternally deprived rats, in contrast, were found in post-mortem tissue analyses to have reduced levels of the mood-mediating neurotransmitter serotonin—malregulated in depressed humans—in parts of the brain that process emotions and memory, Robbins reported at the Madison meeting.

In contrast, researchers have shown that intense mothering—presumably an emotionally positive experience for the infant rat—also has a powerful effect on brain development. Graduate students have long noticed that when rat pups are handled often by people, they grow up to be relatively less anxious and more resilient. The key, Michael Meaney of McGill University in Montreal Paul Plotsky of Emory University in Atlanta, and their colleagues found last year, is the intense attention—licking, grooming, and nursing—that the rat mothers lavish on the pups each time they are resumed to the nest (Science, 12 September 1997, pp. 1620 and 1659). The team found that a certain subset of "good mothers" gave even nonhandled pups this extra attention, and these pups showed similar beneficial effects.

And in the 28 April Proceedings of the Narional Academy of Sciences, researchers report neurochemical changes that correspond to these behavioral differences. Rats with especially attentive mothers have more receptors for neurotransmitters that inhibit the activity of the amygdala and fewer for corticotropinreleasing hormone, a stress hormone. Those changes in receptor numbers could explain why the adult animals display more equanimity in novel environments, says Plotsky.

Even subtle factors in a young animal's environment can color the emotional life of the adult. For example, in a study of 28 lab rhesus monkeys, Kalin and his colleagues identified a birth-order effect. Later born monkeys had lower levels of cortisol—a neurally controlled stress hormone—than first- or early-looms, the researchers report in the Febouary Behavioral -Neuroscience. Noting that experienced mothers have different mothering styles, Kalin suspects that the emotional state of the mother, who might be calmer with later boms somehow affects the offspring's hormone levels.

Although no one is ready to make a direct leap from rats or monkeys to humans, the point is clear: Emotional events in young mammals can have major, long-lasting effects on the neurochemistry of the developing brain—and therefore on mood and behavior. "We're talking about the effects of very early experience on the adult brain, when most of the very early [hormonal] stressor effects have waned," says Robbins.

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