Science
Chemical theory of memory gains support Baylor group discovers three polypeptide "memory" molecules; Michigan work supports fear-producing action of scotophobin Evidence supporting a chemical theory of memory continues to build. Three new "memory molecules" that appear to be related to an animal's ability to avoid different surroundings or perform specific tasks have been discovered by Dr. Georges Ungar and his group at Baylor College of Medicine, Houston. And experimental work at the University of Michigan with fear-producing scotophobin further supports that substance's behavioral effect. At the third annual meeting of the Society for Neuroscience in San Diego, Baylor's Dr. Louis Gal van described how two separate peptides isolated from the brains of trained goldfish, then injected into the brain cavity of untrained fish, induce the untrained fish to avoid either the blue or green ends of a tank. Another brain peptide appears to be associated with the ability of a handicapped goldfish to learn how to swim normally, according to Jo Anne Heltzel, a Baylor graduate student. In the first two cases, the goldfish are trained with electric shocks to avoid either the green end or the blue end of the fish tank. After the goldfish learn to avoid the colored end without being shocked, the brains are removed and the substances corresponding to the two types of avoidance behavior are isolated, Dr. Galvan explains. Preliminary evidence indicates that the polypeptide associated with avoiding the blue end of the box consists of about 12 amino acids and that it is inactivated by the enzyme trypsin. The green-avoidance polypeptide also appears to contain the same number of amino acids but differs from the blueavoidance factor in that it is inactivated by chymotrypsin, according to Dr. Galvan. Before isolating the third "memory molecule," Miss Heltzel first attached a small buoyant polystyrene float to the body of the fish, which was then carried to the top of the tank. Within three to four hours the average fish learned to fight the buoyancy of the float and swim normally, she explains. 20
C&EN Nov. 26, 1973
After the polypeptide believed to be responsible for this behavior was removed from the goldfish brain and chemically isolated, it appeared to consist of 20 to 25 amino acids and was inactivated by trypsin. The polypeptide was injected into the brain cavity of untrained goldfish carrying the polystyrene float. According to Miss Heltzel, they learned to swim normally in two hours, rather than the four hours usually required by goldfish that had not been injected with the material. All of the new peptides were isolated and purified by the same general technique. First the polypeptides were separated from brain ribonucleic acid by dialysis at pH 3.7. The dialyzate was then concentrated and filtered through Sephadex. Further purification employed thin-layer chromatography with two solvent systems. In the case of the factor associated with learned normal swimming behavior, 1 gram of goldfish brain yielded about 200 ng. of polypeptide. Discovery of the three new molecules is a continuation of the still controversial work begun by Dr. Ungar and his colleagues in 1964. In 1970 Dr. Ungar reported the isolation, purification, and synthesis of a pentadecapeptide isolated from the brains of rats trained to avoid the dark end of a box. Called scotophobin, for fear of the dark, the same material as well as a synthetic version have been used to produce the same response in untrained rats, mice, goldfish, and, most recently, cockroaches. Recent experiments in various laboratories also indicate that goldfish produce their own form of scotophobin when they are trained to avoid the dark end of a fish tank. Critics of the Ungar work suggest that if scotophobin is indeed produced in the brains of trained animals, it may be due to a generalized state of fear brought on by the experimental conditions rather than by the specific fear of the dark. In response to that objection, Dr. David H. Malin and Glen J. Radcliffe, Jr., of the Mental Health Research Institute of the University of Michigan compared the behavior patterns of mice injected with synthetic scotophobin to the behavior of noninjected mice. They told the San Diego meeting that scotophobin recipients showed more emotionality, as measured by defecation rate, when locked in a black box, and less emotionality than the control mice when locked in a white box. To Dr. Malin the experi-
Ungar: controversial work ment suggests that the scotophobininjected mice are exhibiting a specific fear of the dark rather than a more generalized response. Dr. Malin's work also indicates that animals injected with scotophobin do not avoid the dark end of the box because their vision has somehow been weakened by the injection, as other critics suggest. In a test designed to measure the rate at which a mouse escapes through an electrically charged container, he found that reduced illumination had little effect on the escape behavior of the scotophobin-treated mice. Dr. Ungar and Dr. Malin agree that the long-term memory of a specific experience is associated with the production of a single polypeptide in the brain that mediates the formation of a circuit from sense to brain. Short-term memory, however, probably is physical rather than chemical in nature, resulting from the persistence of electrical circuits in the brain for brief periods of time, they suggest. They also agree that the memory code probably is the same throughout the animal world: The same polypeptide, or a close variant, is responsible for establishing the same memory in a man and other animals. Admitting that it is difficult for some to accept the fact that every long-term memory generates its own polypeptide, Dr. Ungar notes that scotophobin is only one of 1019 possible amino acid arrangements for a peptide containing 15 amino acids. In addition, says Dr. Ungar, the fact that the polypeptides responsible for green and blue avoidance in goldfish both contain the same number of amino acids suggests that they form part of a broader color avoidance code. "However," he tells C&EN, "there are so many proteins to be isolated, I doubt that we will break the chemical code for memory in my lifetime."
