Researchers probe biochemical basis of learning Neuroscientists are eagerly devel oping a framework for describing the brain structures and chemistry in volved in learning. Some of the more eager represen tatives among neuroscientists now claim that the chemical pieces of that framework are beginning to fit to gether in a coherent pattern. But most such scientists also admit that it's overoptimistic to expect learning either to yield a simple description or to follow any one mechanism. None theless, some hints that vastly dif ferent animal species may learn by relying on surprisingly similar bio chemical mechanisms were reported at the recent meeting of the Society for Neuroscience. Many neuroscientists say that learning depends in part on structural and physiologic changes in and be tween nerve cells leading to some thing called "synaptic facilitation." Synapses are connections between nerve cells, connections that are needed for transmitting nerve im pulses. Presumably, many synapses in the brain are involved in "un learned/' but of course vital, activi ties. Such synapses are part of nerve
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circuits that probably are determined genetically and hence are set for life. Other synapses, however, are con sidered "plastic"—they can form and break or perhaps change in subtler fashion in part of the overall process called learning. Several simple animals (some sci entists studying them say, rather, simple-minded) can learn rudimen tary tasks as part of learning para digms. For example, Aplysia, a sea snail, and Drosophila, a fruit fly, cannot be considered ideal models. But both species exhibit specific be haviors that can be modified to suit particular stimuli—stimuli that are chosen and inflicted by the scientist seeking to establish something of a tutorial role with such experimental subjects. Regarding Aplysia, the nervous system is so simple and ac cessible that it can be dissected out for structural and chemical study. And the fruit fly, if somewhat less easy to dissect literally, has had its genetic system dissected figuratively in great detail. This genetic infor mation can pay rich dividends for anyone seeking biochemical clues about the fly's learning abilities.
One other simple system for studying learning is receiving new attention. The "system" consists of brain slices from animals such as ro dents. Brain tissue slices can be kept alive, so to speak, in the laboratory where particular physiologic activi ties, that are considered part of learning that takes place in intact brains, may be measured. "Synaptic facilitation occurs with as little as 100 milliseconds of stimulation," says Gary S. Lynch of the University of California, Irvine. "This phenomenon has the 'noise' of memory. The effect is as robust as is seen in live ani mals." In short, brain slices can be "taught" a rudimentary trick, based on the nerve cells' response to an electric stimulus. And some of the changes observed in the cells in vitro are like changes seen in cells taken from animals that learned tricks be fore their brains were sacrificed. Whether such tricks represent true learning in a philosophical sense may be arguable, but it is an altogether different subject. Lynch finds that such brain cells undergo subtle structural changes. "We have no evi dence that this isn't just an inter esting correlation," he admits, adding, however, "the data stand on their
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own. Brief events can produce struc digestion of specific proteins in [brain tural changes that are used by the cell] membranes," Lynch says. But central nervous system to encode in many loose ends within that putative formation." Specifically, spines jut loop still need to be braided together ting out from dendrites, which before being accepted as a basis for themselves are outgrowths along the describing learning. membranes of nerve cells, change The intellectual station now in shape, according to Lynch. habited by fruit flies is certainly more Such structural changes are ac whimsical but also in some ways more companied by biochemical changes, convincing than that occupied by he continues. For example, there's a mere brain slices. Fruit flies can be change in binding sites for glutamic taught definite tasks, can remember acid (an amino acid that can serve as and forget, but most important can be a neurotransmitter) with what seems turned into mutants having defective like an increase in the binding of intellectual faculties compared to glutamic acid to these cells. That their normal progenitors. Seymour change might depend on the exposure Benzer and his collaborators at Cali of occult binding sites when a nerve fornia Institute of Technology in cell's dendrites change shape. Pasadena have championed the fruit All of this might come about when fly as learner. And one of Benzer's calcium ion concentrations in differ former students, W·. G. "Chip" Quinn, ent parts of the nerve cells fluctuate and his collaborators at Princeton in response to changes in the mem University now are delving into the brane—changes that depend, for ex fruit fly biochemistry that goes hay ample, on the electric stimulation of wire when the creatures lose their nerve cells during learning. It is pos learning abilities. sible to build a whole biochemical For instance, one such mutant, loop describing what might be hap called "duns," is genetically defective pening, working around from electric in its enzyme for digesting (hydrostimuli, exposed binding sites and lyzing) cyclic adenosine monophos structural changes, dependence on phate. That particular mutant, first protein-dissolving enzymes to tailor described several years ago, could be some of those changes, and so forth. classified as a "slow" learner, in an "Calcium does stimulate proteolytic thropomorphic terms. Other mutants have defective memories, meaning they can learn a task adeptly enough, but they forget the lesson faster than do normal flies. The Institute of Gas "We're looking for other biochem Technology (IGT) presents ical deficiencies," says Quinn. One group of candidate flies with such a its deficiency are unable to metabolize Fifth Annual Conference on properly the neurotransmitter do Energy from Biomass and Wastes pamine. This molecule (whose chemical name bears no etymologic January 26-30, 1981 link to the fruit fly's intellectual at the Hotel Royal Plaza, problems) is one of several monoam Lake Buena Vista, Florida ines that act as neurotransmitters in (Walt Disney Resort Community) the brain. One mutant fly can me A comprehensive technical program on re tabolize dopamine properly, however, search and commercial projects for the pro only if grown at relatively low tem duction of energy and synthetic fuels in peratures. When temperatures are cluding— raised, Quinn says, the flies' learning • Liquefaction • Biomass ability "is zero," corresponding to the Production • Environment specific metabolic derangement • Government • Combustion • Gasification Programs caused by the loss of a single enzyme, World Energy • Tour of Walt Disney namely dopamine decarboxylase. Recovery Systems Such observations begin to fall in Registration fees are $480 U.S. for IGT with what Quinn calls "Eric Kandel's Members and International Associates and dogma." Kandel, a physiologist at $560 U.S. for non-Members. Columbia University medical school Send or phone reservations, and address in quiries to: in New York, has been studying Aplysia as a learner for more than a Ms. Kathy Fisher decade. Aplysia depends on mo Institute of Gas Technology noamines, such as serotonin, to 3224 South State Street achieve synaptic facilitation. "We Chicago, Illinois 60616 may be beginning to see convergence 312/567-3881 to a common mechanism of learning," Special reduced room rates are available Kandel says. But he cautions, "I don't through IGT for one to four people until think monoamines will account for all December 19, 1980, so register early. of learning by any means.*' Jeffrey Fox, Washington
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