Chicks hatch chemical clues to memory.

In the 24 hours after hatching, baby chicks learn to recognize and follow their mothers by a process known as imprinting. In fact, scientists have known for decades that chicks and some other newborn fowl will seek out any conspicuous object they have been exclusively exposed to during that critical period.

A new study finds that a key chemical reaction in a particular spot in the chick's brain fosters imprinting. The same mechanism may help promote long-term memory in humans and other mammals, the researchers contend in the April 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

"This chemical process occurs in many animals, not just chicks who are imprinting," says Aryeh Routtenberg, a psychologist at Northwestern University in Evanston, Ill.

For the past decade, Routtenberg and his colleagues have studied an enzyme, protein kinase C (PKC), that plays a critical role in memory formation in rats and other animals. In a process called phosphorylation, PKC adds phosphates to proteins, thereby changing their shape and function. Two such proteins, MARCKS (myristoylated alanine-rich C-kinase substrate) and GAP-43 (growth-associated protein-43), amass in the spaces, or synapses, between brain cells and apparently affect the maturation and shape of the synapses, Routtenberg says.

The two proteins congregate in the areas of monkey and human brains associated with memory formation, he adds.

To determine whether these proteins play a direct role in imprinting, Routtenberg and Northwestern colleague Fwu-Shan Sheu teamed with Gabriel Horn and Brian J. McCabe, zoologists at the University of Cambridge in England. The British group had previously found that the left-brain portion of a specific structure, the intermediate and roedial hyperstriatum ventrale (IMHV), supports imprinting in chicks.

Horn and McCabe exposed groups of chicks, hatched and raised in darkness for 15 to 30 hours, to a rotating, illuminated red box for two 15-minute periods. Shortly afterwards, the researchers compared the tendency of chicks to approach the red box or a stuffed animal they had never seen before. Control chicks got no training and remained in darkness.

The U.S. scientists then chemically analyzed brain samples from the chicks. Only the left IMHV displayed marked jumps in phosphorylation of the MARCKS protein following imprinting, particularly in chicks who showed the strongest memory for the red box. This alteration did not extend to the GAP-43 protein, although studies of adult rats have linked phosphorylation of both proteins to learning, Routtenberg says.

Such findings suggest that the chemistry of memory may operate differently in young and old animals, he theorizes.

Phosphorylation of the MARCKS protein alters its interaction with another protein that regulates synapse structure, suggesting that specific changes in the shape of synapses foster imprinting, Routtenberg holds.