Summary

The Molecular Biology of Memory Storage:
A Dialogue Between Genes and Synapses
Eric R. Kandel

The main focus of Eric Kandel's work has been to study the changes in the brain that occur during learning and how this information is retained. After attempting to tackle this question on one of the most complex levels by studying the cellular properties of the hippocampus, two problems arose. The first is that all nerve cells have similar signaling properties, so no concrete conclusions could be reached by examining the signaling properties of specific neurons (Kandel & Spencer 1968, as cited in Kandel 2000) . The second problem Spencer and Kandel encountered was that the function of the hippocampus was determined by the connections between the neurons. Since the hippocampus is such a complex part of the brain which many connections, it would be close to impossible to study every pathway.

This led Kandel to simplify his area of study and take a reductionist approach through focusing on Aplysia. Aplysia offers a small number of large nerve cells that have many desirable qualities when studying neural pathways. First, the large nerve cells are easily identifiable. Second, it is easy to inject them with various compounds to study signal transduction in individual nerve cells, and third, these cells are easily dissected for biochemical studies.

Tom Carew, Robert Hawkins, Irving Kupfermann, Harold Pinsker, and Kandel found, through a series of studies, that the withdrawal reflex of the gill when the siphon was stimulated, a defensive reflex in Aplysia, could be modified by habituation, sensitization, and classical conditioning (Pinsker, Kupfermann, Castellucci, & Kandel 1970; Pinsker, Hening, Carew, & Kandel 1973; Carew, Pinsker, & Kandel 1972, as cited in Kandel 2000). "Each type of learning had two phases: a transient memory that lasts minutes and an enduring memory that lasts days. Conversion of short-term memory storage requires spaced-repetition" (Kandel, 2000). The initial type of learning that was focused on was sensitization. When Aplysia received a tail shock, it recognized it as harmful and increased it's defensive reflex responses toward other stimuli it encountered. (Castellucci, Blumenfeld, Goelet, & Kandel 1973, as cited in Kandel 2000). The amount of time Aplysia reacted in this way was dependent upon the amount of shocks it received. One shock only had an effect for a few minutes and did not require the synthesis of new proteins. However, if Aplysia was shocked several times through a series of spaced shocks, it reacted toward other stimuli in this defensive manner for several days. This reaction does require the synthesis of new proteins.

Upon studying the neural circuit of the gill withdrawal reflex, Kupfermann, Castellucci, Carew, Hawkins, and Kandel found that individual cells have large effects on behavior. They found that each cell was connected only to certain target cells. This discovery led them to question how learning could occur in such an exact neural circuit. Their findings supported the theory proposed by Santiago Ramon y Cajal (1894) (as cited by Kandel, 2000) which states " memory is stored in the growth of new connections" and also "synaptic plasticity [is a] fundamental mechanism for information storage by the nervous system".

Kandel then went on to research which chemicals were involved in the formation and storage of these new memories. In short-term sensitization, Cedar and Schwartz (1972) (as cited by Kandel, 2000) found that "neurotransmitter candidates serotonin and dopamine could simulate the action of electrical stimulation and increase levels of cAMP". "[These] increased levels of presynpatic cAMP activates PKA and leads to synaptic strengthening through enhanced transmitter release produced by a combination of mechanisms" (Kandel, 2000). In long-term sensitization, the repeated shocks causes the level of cAMP to rise for several minutes. The cAMP translocates to the nucleus, activations the CREB protein, and removes CREB-2, which represses CREB-1. Once CREB-1 is released, it activates several immediate-response genes, including a subunit of PKA. The splitting of these PKA subunits results in the constant activity of PKA. Another immediate-response gene activated by CREB-1 is C/EBP which activates genes that results in the growth of synaptic connections (Kandel, 2000). A study done by Martin, Andrea Casadio, Bailey, and Kandel found that only those terminals that have been marked by serotonin can effectively use the CREB for synaptic growth. Another study done by Casadio et. al. (1999) (as cited by Kandel, 2000) found that "in order for structural changes to persist, local protein synthesis is required".

After studying implicit memory, Kandel turned his attention to explicit memory. Bacskai et. al. (1993) (as cited by Kandel, 2000) showed "explicit memory, like implicit memory, has a short-term phase that does not require protein synthesis and a long-term phase that does". Terje Lomo and Time Bliss (1972) (as cited by Kandel, 2000) discovered " the perforant path, a major pathway in the hippocampus, exhibits activity-dependent plasticity, a change now called long-term potential (LTP)". Building on this research, Richard Morris found that "blocking the NMDA receptor pharmacologically not only interfered with LTP, but also blocked memory storage" (Bliss & Lomo, 1973; Morris, Anderson, Lynch, & Baudry, 1986, as cited by Kandel, 2000). Once Kandel found that long-term memory in Aplysia was formed with repeated stimuli, he too turned his attention to the hippocampus. Following the example set by Aplysia, Uwe Frey, Yan-You Huang, Peter Nguyen, and Kandel focused on whether LTP changes with repeated stimulation. They found that LTP in the hippocampus has phases that are similar to the phases found in Aplysia. "The early phase of LTP, produced by a single train of stimuli, lasts only 1 to 3 hours and does require new protein synthesis" (Nguyen, Abel, & Kandel, 1994, as cited by Kandel, 2000).. This process involved changing proteins that already existed in order to strengthen preexisting neural connections. They also found that the late phase of LTP, which is produced by repeated trains of electrical stimuli, lasts for at least a day and requires both translation and transcription in order to produce new synaptic connections.   

Overall, Kandel summarizes his research in two groups. The first found "with both implicit and explicit memory, there are stages in memory that are encoded as changes in the synaptic strength and that correlate with the behavorial phases of short- and long-term memory" (Kandel, 2000). The second area that Kandel's research focused on was the study of learning. This revealed that "different forms of learning recruit different modulatory transmitters, which then act in one of three ways: (i) they activate second-messenger kinases that are transported to the nucleus  where they initiate processes required for neuronal growth and long-term memory; (ii) they mark the specific synapses for capture of the long-term process and regulate local protein synthesis for stabilization; and (iii) the mediate, in ways we are just beginning to understand, attentional processes required for memory formation and recall"(Kandel, 2000). Overall, Kandel's study of long-term memory has turned attention toward the connection between the synapse and the nucleus in nerve cells.