Memories might be made of this: closing in on the biochemistry of learning

Science News, May 25, 1991 by Carol Ezzell

A blue-and-yellow sea snail sits complacently in its dark test chamber -- a plastic trough atop an electronic shaker-table at the bottom of a jury-rigged refrigerator. Suddenly, bright light shines down on the snail and the chamber floor shakes mildly in a simulation of ocean turbulence. The snail instinctively anchors itself in place by tensing the muscular "foot" running along the underside of its body. Seconds later, the cycle of light and shaking repeats, once again prompting the snail to contract its foot.

After 150 such "training" cycles, the snail finds itself in a second refrigerated chamber, now with the eye of a video camera staring at it from below. This time, when bursts of light flash, an interesting thing happens behind the closed refrigerator door: The animal tenses its foot -- without being shaken.

"Ordinarily, light alone would never cause that response," says Daniel L. Alkon, chief of neural systems at the National Institute of Neurological Disorders and Stroke (NINDS) in Bethesda, Md. But if the light appears repeatedly just before and during shaking, this snail, called Hermissenda crassicornis, eventually learns to contract its foot when light flashes, "just as Pavlov's dog would salivate when the bell occurred, as if the smell of meat were there," Alkon says.

For the past 20 years, Alkon and a number of other researchers have been studying the nervous systems of marine snails -- and those of rats and rabbits -- in a quest for the molecular mechanisms of memory. Their searches have led them to a molecule called protein kinase C (PKC) in the surface membranes of nerve cells.

PKC exists in all animal cells, where it plays a role in such diverse physiological processes as growth, blood clothing and the action of hormones. The molecule was first discovered in the early 1970s by a Japanese scientist; in 1979, researchers found that it acts by tacking a phosphate group onto specific sites on other molecules. The added phosphate changes the function of those molecules, increasing or decreasing their level of activity.

One of the first direct clues that PKC might underlie learning and memory came in 1986. Joseph Farley of Princeton (N.J.) University observed that injections of PKC, or of a chemical known to activate PKC, excite light-receptor nerve cells in the eyes of H. crassicornis. This excitation mimics that induced by the light-and-shake regimen: The nerve cells open pores in their membranes that absorb calcium, and close other pores that expel potassium. Using tiny electrodes to track this process, Farley observed that it reversed the normal negative charge inside the cells.

Six years earlier, Alkon had shown that the electrochemical current in neurons changes as an animal learns. He and Joseph Neary, now at the University of Miami, went on to demonstrate that a protein requiring calcium is involved in learning. Because chemicals like PKC mimic the cellular changes of learning, Alkon and Farley launched separate studies investigating PKC as the agent behind those learning-induced current alterations. They proposed that PKC contributes to learning by somehow closing potassium pores, priming the neurons to react more strongly to a new stimulus.

Alkon and Farley reasoned that if a single molecule was responsible for learning and memory, its appearance, disappearance and reappearance should coincide with learning, forgetting and remembering. The molecule might also bring about structural changes in neurons so that they branched to communicate with other neurons in different ways. Moreover, the learning agent would likely prove active in only one region of a neuron at a time, so that one nueron would have the capacity to hold multiple memories.

Over the past 18 months, evidence has piled up in support of the theory that PKC orchestrates neuronal functions necessary for learning and memory. "I have no doubt PKC is central to learning and memory in the models we have looked at," says Alkon, because "we've used so many different measures, and have so many different pieces of evidence that are consistent" with PKC's important role.

Farley, now at Indiana University in Bloomington, agrees. The most compelling evidence, he says, comes from experiments with marine snails.

"I think it's reasonably clear you can mimic learning [in these snails] using PKC," Farley says, noting that "inhibitors of PKC will block those changes." Moreover, his data suggest that "ongoing PKC activation is also necessary for the maintenance of memory in H. crassicornis."

Terry J. Crow, who also works with the colorful marine snail, says the link between PKC and learning is gaining acceptance among other neuroscientists. "All of the things we have done here suggest that PKC is sufficient to get the neural changes involved in learning going," says Crow, a neurobiologist at the University of Texas Medical School at Houston.

A year ago, he reported studies demonstrating that a chemical that inhibits protein synthesis also prevents sea snails from remembering a training cycle for more than one hour. Crow's experiments differed from Alkon's because these snails received only one flash of light, immediately followed by an injection of serotonin -- a neurotransmitter that Crow believes is also crucial to memory.