Part 7 (1/2)

A significant amount of the work which has been conducted in the area of biological information and communication systems is easily cla.s.sified as ”basic research” (refs. [ref.106]-[ref.109]). This discussion will be limited to those aspects closely related to the fields of molecular biology and experimental psychology, which seem to have universal application to all known animal life forms. Studies involving the basic principles of acquisition, processing, storage, and retrieval of information in living systems are emphasized.

Early Work

Early speculations on the operational nature of memory have been based upon relatively little experimental evidence. Charles Darwin observed that domestic rabbits had smaller brains than their wild counterparts, and attributed this to lack of exercise of their intellect, senses, and voluntary movements. Unfortunately, subsequent studies of the brains of men with greatly differing intellectual capability did not substantiate the hypothesis. Idiots sometimes had larger brains than geniuses. Later, an idea proposed by Ramon y Cajal came into favor. Since brain cells did not increase in number after birth, he proposed that memory involved the establishment of new and more extended intercortical connections.

Unfortunately, methods were not available to test this hypothesis adequately and it has remained until quite recently in the realm of conjecture.

Another major hypothesis was that there were two or more stages in the information storage process. The final form the information took in the brain was called a brain engram, or memory trace. However, prior to the formation of the engram, a transitory process denoted as ”reverberational memory” was postulated to exist for a relatively short time (minutes to hours) (refs. [ref.106] and [ref.107]). This hypothesis was used by Pauling to explain why an elderly chairman of a board could brilliantly summarize a complex 8-hour meeting and yet, after its conclusion and his return to his office, not even remember having attended the meeting. Thus, this individual's reverberational memory functioned well, but advanced years had seriously impaired his brain's ability to form a permanent engram. Similar, although less dramatic, observations in other situations are not uncommon. A wide variety of experiments have been conducted to study this aspect of memory and to relate it to the process whereby the information is transformed to a more stable form (refs. [ref.110]-[ref.112]).

More recently, the concept of a specific biochemical activity during the process of long-term storage of information has gained considerable favor. Initially, neither the site nor the nature of the change was well defined. Quite recent studies by Krech et al. (refs. [ref.113] and [ref.114]), Bennett et al. ([ref.115]), Rosenzweig et al. (refs.

[ref.116] and [ref.117]) support the view that alteration of the levels of acetylcholinesterase at cortical synapses play an important role in information storage. These studies will be discussed in a later section.

However, these authors do not claim that the changes observed are unambiguously related to the storage of memory. It may well be that the alterations observed are in some way related to this process but are still secondary to some other, more basic, process.

An alternative hypothesis is that the information resides in its ultimate form in some more central structure of the neurone than the synapse. (It has even been postulated that the basic information is stored in nonneuronocortical material.) Perhaps Halstead was the first to postulate the involvement of nucleoprotein in this process ([ref.107]). From the biochemist's point of view, this is an extremely attractive hypothesis. Both proteins and nucleic acids possess sufficient possible permutations of structure to permit storage of a lifetime's acc.u.mulation of information in an organ the size of the brain. From the previously known ability of the nucleic acids to code genetic information, they are the prime suspects. However, from the known regulatory ability of nucleic acids in specific protein synthesis, it is possible that the final repository is protein.

Recent Biochemical Studies

Among the foremost investigators of the chemistry and biochemistry of the central nervous system is Holger Hyden at the University of Goteborg, Sweden. He and others (refs. [ref.118]-[ref.120]) have for many years performed elegant microa.n.a.lytical studies of single nerve cells. The evidence which Hyden has obtained is consistent with the hypothesis that the initial electrical reverberations in the brain induce a change in the molecular structure of the ribonucleic acid (RNA) of the neurones which, in turn, leads to a subsequent deposition of specific proteins. It is well known from other investigations that a major role of RNA in any type of cell is to specify and mediate synthesis of the protein enzymes of the cells. Thus, in this hypothesis, it is only necessary to postulate the modification of brain RNA by the activities a.s.sociated with reverberational memory. Particularly pertinent to this hypothesis are observations that-

(1) Large nerve cells have a very high rate of metabolism of RNA and proteins, and, of the somatic cells, are the largest producers of RNA.

(2) Vestibular stimulation by pa.s.sive means leads to an increase in the RNA content of the Deiters nerve cells of rabbits ([ref.121]).

The protein content of these cells is also increased.

(3) Changes in the RNA composition of neurones and glia of the brainstem occur during a learning situation. Animals were trained over a period of 4 to 5 days to climb a steeply inclined wire to obtain food. The big nerve cells and the glia of their lateral vestibular apparatus were a.n.a.lyzed, since the Deiters neurones present in this structure are directly connected to the middle ear. The amount of RNA was found to be increased in the nerve cells; and, more significantly, the adenine-to-uracil ratio of both the nuclear RNA of nerve cells and glia cells became significantly increased ([ref.119]). A variety of control experiments were conducted. Although there was an increase in RNA content of these cells in animals exposed to pa.s.sive stimulation, there was no change in the ratio of adenine to uracil. Nerve cells from the reticular formation, another portion of the brain, had only an increased content of RNA with no base-ratio change.

Animals subjected to a stress experiment involving the vestibular nucleus showed only an increase in content of RNA. Littermates living in cages on the same diet as learning animals showed no change in content of RNA. Thus, it would appear that the change in the base ratio of the RNA synthesized is not due to increased neurone function per se, but is more directly related to the learning process. The fact that this was nuclear RNA implies that it was immediately related to chromosomal DNA.

(4) Neuronal RNA with changed cytosine-guanine ratios synthesized during a short period of induced protein synthesis could be blocked by actinomycin D. It was concluded, therefore, that the RNA was immediately DNA dependent and directly related to the genetic apparatus.

Rats which were normally right handed were forced to modify their handedness in order to obtain food. The RNA of nerve cells in that part of the cortex, whose destruction destroys the ability to transfer handedness, was a.n.a.lyzed. A significant increase in RNA of nerve cells of the fifth to sixth cortical layers on the right side of the brain was observed. The corresponding nerve cells on the opposite side of the same brain served as controls. There was an increase in RNA and a significant increase in the purine bases relative to the pyrimidine bases in the learning side of the cortex. When the animals were not forced to learn a new procedure, only an increase of RNA was observed, with no change in base ratio.

Frank Morrell, head of the Neurology Department at Stanford Medical School, has also been active in this field during the past 6 years. He has found that if a primary epileptic lesion is induced on one side of the cortex, a secondary mirror lesion eventually develops in the contralateral h.o.m.ologous cortex. This secondary lesion, which showed self-sustaining epileptiform discharge, could be isolated, whereupon the epileptiform discharge disappeared. This was interpreted as learned behavior of the secondary lesion. From changes in the staining properties of the secondary lesion, Morrell concluded that changes in RNA had occurred in the cell. Changes in the composition of the RNA could not be shown by these techniques.

At the University of California at Berkeley, Drs. Rosenzweig, Bennett, and Krech have conducted extensive studies related to this topic. These investigators have directed their efforts toward demonstrating alterations in the cerebral cortex of animals exposed to continuing learning situations or continuously deprived of sensory stimulation. In a recent publication ([ref.116]), which also summarizes a considerable amount of previous work, they report studies which demonstrate the following:

(1) Rats given enriched experience develop, in comparison with their restricted littermates, greater weight and thickness of cortical tissue and an a.s.sociated proportional increase in total acetylcholinesterase activity of the cortex.

(2) The gain in weight of cortical tissue is relatively larger than the increase in enzymatic activity. Acetylcholinesterase activity increases in other portions of the brain even though tissue weight decreases.

(3) The changes appear in a variety of lines of rats, although differing in amount between strains.

(4) The changes are observed in both the young and adult animals.

The previous studies were comparisons between experience-enriched animals and animals maintained in isolation. Animals which were housed in colonies, but given no special treatment, showed intermediate effects in those situations studied.

The Berkeley group emphasized that the finding of changes in the brain subsequent to experience does not prove that the changes have anything to do with memory storage, but do establish the fact that the brain can respond to environmental pressure. However, the results are compatible with the hypothesis that long-term memory storage involves the formation of new somatic connections among neurones. Calculations of the amount of additional material required to permit this to exist are compatible with the increases observed.

A number of investigators have studied the effects of antimetabolites and drugs on the learning process. Since their specific metabolic effects are known in other tissues, the rationale is that if these materials do interfere with memory, then specific types of metabolic activities may be implicated in the deposition of the engram.

One of the initial studies of this type was conducted by Dingman and Sp.o.r.n ([ref.122]), presently at the National Inst.i.tute of Mental Health.

They showed that 8-azaguanine, a purine antagonist, injected intra-cisternally was incorporated into the RNA of the brains of rats.