Toward a molecular definition of long-term memory storage

Abstract

The storage of long-term memory is associated with a cellular program of gene expression, altered protein synthesis, and the growth of new synaptic connections. Recent studies of a variety of memory processes, ranging in complexity from those produced by simple forms of implicit learning in invertebrates to those produced by more complex forms of explicit learning in mammals, suggest that part of the molecular switch required for consolidation of long-term memory is the activation of a cAMP-inducible cascade of genes and the recruitment of cAMP response element binding protein-related transcription factors. This conservation of steps in the mechanisms for learning-related synaptic plasticity suggests the possibility of a molecular biology of cognition.

Figure 1

Behavioral long-term sensitization. A summary of the effects of long-term sensitization training on the duration of siphon withdrawal in Aplysia californica. The retention of the memory for sensitization is a graded function proportional to the number of training trials. Experimental animals received either four single shocks for 1 day (▴), four trains of shocks for 1 day (▵), or four trains of shocks a day for 4 days (○). Control animals were not shocked (•). A pretest determined the mean duration of siphon withdrawal for all animals before training. Posttraining testing was carried out 1, 4, or 7 days after the last day of training. The asterisks indicate a significant difference between the duration of siphon withdrawal for the trained and control animals (Mann–Whitney U tests, P < 0.01). N represents the number of animals per group. [Reproduced with permission from Frost et al. (5).]

Figure 2

Time course of the effects of injection of ApCREB2 antiserum on short- and long-term facilitation. (A) Time course of excitatory postsynaptic potenial (EPSP) amplitude changes recorded in motor neuron L7 in response to stimulation of the sensory neuron (expressed as percent change in the amplitude of the EPSP) after single and multiple applications of 5-HT to Aplysia sensory–motor neuron cocultures. Changes in EPSP amplitude after application of one 5-min pulse of 5-HT (1× 5-HT, short-term facilitation) and one 5-min pulse of 5-HT paired with injection of anti-ApCREB2 antibodies (1× 5-HT + CREB-2 Ab, both in boldface lines) are compared with changes in EPSP amplitude induced by five pulses of 5-HT (5× 5-HT) at 2 and 24 hr. While the EPSP facilitation decays rapidly after one pulse of 5-HT (with a return to base line after 10 min), pairing one pulse of 5-HT with injection of anti-ApCREB2 antibodies induces a long-term facilitation paralleling that of 5× 5-HT. This long-term facilitation is abolished by the application of the protein synthesis inhibitor anisomycin (1× 5-HT + CREB-2 Ab + ANISO) or the RNA synthesis inhibitor actinomycin D (1× 5-HT + CREB-2 Ab + ACTINO) during the training. The difference in EPSP amplitude at 2 hr between 5× 5-HT and 1× 5-HT + CREB-2 Ab may reflect the transient protein synthesis-dependent, but RNA synthesis-independent, component of long-term facilitation 2 hr after 5-HT stimulation (29). The controls are either untreated (control), or injected with ApCREB2 antiserum without 5-HT administration (CREB-2 Ab). (B) Comparison of the time course of the EPSP amplitude changes in the first 2 hr after application of a single 5-min pulse of 5-HT with or without injection of CREB-2 antibody. The control cells were not exposed to 5-HT. (C) Example of EPSPs recorded in motoneuron L7 after stimulation of the sensory neuron before (0 hr) and 2 and 24 hr after 5-HT treatment. One pulse of 5-HT paired with the injection of an ApCREB2 antiserum induces a significant increase in EPSP amplitude at 2 and 24 hr, but injection the preimmune serum (PRE-CREB-2 Ab) or depleted immune serum does not induce long-term facilitation. (D) Examples of EPSPs recorded at indicated times in cocultures injected with ApCREB2 antiserum paired with one 5-min pulse of 5-HT. [Reproduced with permission from Bartsch et al. (27) (Copyright 1995, Cell Press).]

Figure 3

Long-term functional and structural changes evoked by one pulse of 5-HT paired with injection of ApCREB2 antiserum. (A) Summary of the structural and functional changes induced by one pulse of 5-HT. Injection of the ApCREB2 antiserum paired with one pulse of 5-HT 24 hr later results in a significant enhancement of the EPSP amplitude and a concomitant significant increase in the number of varicosities. (B) Examples of structural changes evident 24 hr after one pulse of 5-HT paired with injection of ApCREB2 antiserum. The fluorescent micrographs taken from the same regions of sensory neurites contacting the axon hillock of L7 before (1 and 3) and 24 hr after treatment (2 and 4). Arrows in 2 illustrate examples of some of the new varicosities present 1 day after one pulse of 5-HT paired with the injection of the ApCREB2 antiserum. The EPSPs, evoked before (0 hr) and after (24 hr) one pulse of 5-HT in the pictured neurons are indicated in the insets. (Bar = 20 μm.) [Reproduced with permission from Bartsch et al. (27) (Copyright 1995, Cell Press).]

Figure 4

Injection Of ERE oligonucleotides blocks 5-HT-induced long- but not short-term facilitation in sensory motor synapses. (A) Examples of EPSPs recorded in motoneuron L7 after stimulation of the sensory neuron before (0 hr) and 24 hr after 5-HT treatment. Injection of the ERE oligonucleotide but not of the corresponding mutant (ERE Mutant) blocks the 5-HT-induced increase in EPSP amplitude at 24 hr. The control culture did not receive 5-HT applications or oligonucleotide injections. (B) Bar graph representing the effects of oligonucleotide injections in long-term facilitation. The height of each bar corresponds to the mean percentage change ± SEM in EPSP amplitude tested 24 hr after 5-HT treatment. Five pulses of 5-HT significantly increase the EPSP amplitude in noninjected cells, as well as in ERE mutant or ApCRE mutant injected cells, relative to the control (not 5-HT-treated and noninjected cells). On the contrary, the EPSP amplitude change in ERE or ApCRE injected cells was not significantly different from that of control cells that were neither injected nor treated. (C) Bar graph representing the mean EPSP amplitude percentage change ± SEM of short-term facilitated cells injected with ERE oligonucleotides, with ApCRE, or with buffer. A single pulse of 5-HT had no significant effect on EPSP amplitude in noninjected cells or cells injected with ERE, or ApCRE. [Reproduced with permission from Alberini et al. (34) (Copyright 1995, Cell Press).]

Figure 5

Molecular pathways underlying short- and long-term presynaptic facilitation in Aplysia. See text for details.

Figure 6

A working model of the molecular mechanisms underlying the early and late phases of LTP in hippocampal regions CA1 And CA3. See text for details.