Autism's cell off: neural losses appear in boys, men with disorder.

The brains of males with autism contain unusually few neurons in the amygdala, an inner-brain structure involved in emotion and memory, a new study finds.

Although previous research had suggested that wayward amygdala development contributes to autism, the new investigation shows for the first time that the disorder features low numbers of neurons in that part of the brain, say neuroscientists Cynthia M. Schumann of the University of California, San Diego and David Amaral of the University of California, Davis.

Schumann and Amaral used a computer-aided microscopic device to count neurons from representative sections of the amygdala in the preserved brains of 9 males who had been diagnosed with autism and of 10 males who had no psychiatric or developmental disorders. The individuals ranged in age from 10 to 44. In both groups, brains were obtained within 2 days after death.

Autism, which usually becomes apparent by around age 3, interferes with a person's ability to communicate and to interact with others. Autism and related disorders affect an estimated 0.6 percent of children, primarily boys.

The brains of those with and without autism displayed comparable amygdala volume and brain-cell size, Schumann and Amaral report in the July 19 Journal of Neuroscience. However, fewer amygdala neurons appeared in the group with autism.

This new evidence "demonstrates that the structure of the amygdala is abnormal in autism," remarks psychiatrist Thomas R. Insel, director of the National Institute of Mental Health in Bethesda, Md.

Previous studies of six preserved brains of individuals with autism had hinted at a lack of amygdala neurons, compared with the brains of people without autism. However, that work identified amygdala areas with low cell density but didn't count individual neurons. Moreover, most of those brains came from people who had both autism and epilepsy, which also damages the amygdala.

The new findings raise a key question, Schumann notes. "Are there lower numbers of amygdala neurons in autism because fewer have developed in the first place, or have some been lost in a degenerative process?" she asks.

Schumann suspects that in autism, the number of neurons in the amygdala rises precipitously during early childhood and declines sharply later in life. Earlier brainscan studies showed amygdala enlargement in youngsters with autism by around age 6.

One intriguing hypothesis holds that precocious amygdala growth in autism causes children to become preoccupied with potential dangers and thus to feel overly anxious and fearful. After several years, a constant rush of stress hormones into the brain kills many amygdala neurons.

Further research needs to test this hypothesis by examining amygdala-neuron numbers in more brains and in brains that represent a wide age range, Schumann says. Other brain areas, such as the frontal lobe, may also incur cell losses that contribute to autism, she adds.