Cell Biological Studies of Memory 
Eric R. Kandel, M.D. — Senior Investigator 
Dr. Kandel is also University Professor of Physiology and Psychiatry at the Center for Neurobiology and 
Behavior of the Columbia University College of Physicians and Surgeons. He was born in Vienna, Austria. 
He graduated from Harvard College, having majored in history and literature, and received his medical 
degree from the New York University School of Medicine. He took postdoctoral training with Wade 
Marshall in the Laboratory of Neurophysiology at NIH and with Ladislav Tauc at the Institut Morey in 
Paris. Dr. Kandel was the founding director of the Center for Neurobiology and Behavior at Columbia. 
He is a member of the National Academy of Sciences and counts among his honors the Lasker Award, the 
Gairdner Award, and the National Medal of Science. 
LEARNING is commonly divided into two ma- 
jor types: declarative and reflexive. Declara- 
tive learning refers to the acquisition of informa- 
tion about people, places, or things. Reflexive 
learning refers to the acquisition of motor skills 
and strategies. Our laboratory has been studying 
elementary forms of reflexive learning in the gill- 
withdrawal reflex of the marine snail Aplysia. In 
an attempt to compare these mechanisms with 
those underlying declarative forms of learning, 
we are also conducting a study of long-term po- 
tentiation in the mammalian hippocampus. The 
present discussion is limited to reflexive studies 
in Aplysia. 
We have shown that the gill-withdrawal reflex 
can be modified by nonassociative and associa- 
tive forms of reflexive learning, giving rise to 
both short- and long-term memory, whose dura- 
tion is a function of the number of training trials. 
We have recently focused on sensitization, a non- 
associative form of learning in response to a 
noxious stimulus. To analyze the relationship be- 
tween the memory for short- and long-term sensi- 
tization, we have studied in particular detail one 
component of the neural circuit of this reflex: the 
connections between the siphon sensory neuron 
and the gill motor neurons. These connections 
can be studied both in the intact animal and in 
dissociated cell culture. 
With sensitization the connections undergo an 
increase in synaptic effectiveness (facilitation), 
whose duration is a function of the number of 
behavioral reinforcing stimuli to the tail. Simi- 
larly, in culture, the duration of the facilitation is 
a function of the number of applications of seroto- 
nin (5-HT), a modulatory transmitter released by 
tail stimuli . A single tail stimulus or a single pulse 
of 5-HT produces short-term facilitation lasting 
minutes, whereas continuous application of 5-HT 
for 1 .5 h or four or five pulses over a 1 .5-h period 
elicit long-term facilitation lasting one or more 
days. 
In both the behavioral reflex and the monosyn- 
aptic facilitation, there is a parallel requirement 
for protein and mRNA synthesis for long-term but 
not for short-term effectiveness. The short-term 
process reflects enhanced transmitter release 
from preexisting synaptic connections due to co- 
valent modification of preexisting proteins. The 
long-term process requires new protein synthesis 
and leads to the grovvT:h of new synaptic 
connections. 
Proteins and mRNAs necessary for long-term 
memory must either be induced during the brief 
(1 .5-h) time window of training or be constituti- 
vely expressed and transiently accessible to cova- 
lent modification during this period. The experi- 
ments described below were designed to 
distinguish between these two possibilities. 
In earlier experiments, Ari Barzilai, Tim Ken- 
nedy, David Sweatt, and I found that repeated 
pulses of 5-HT induced changes in the synthesis 
of specific proteins in the sensory neurons. Simi- 
lar changes in protein synthesis can be produced 
by cAMP, a second messenger activated in the sen- 
sory neurons by 5-HT. These proteins could there- 
fore reflect the transcription of cAMP-inducible 
genes. In mammals, genes induced by cAMP share 
a control element called the cAMP recognition 
element (CRE), which binds transcriptional acti- 
vators called the CRE-binding proteins (CREBs) . 
Aplysia neurons contain proteins homologous to 
mammalian CREBs. 
These findings raise three questions that Bong- 
Kiun Kaang, Seth Grant, and I attempted to ad- 
dress: Can the facilitating transmitter 5-HT in- 
duce transcriptional activation of reporter genes 
in the sensory neurons that are driven by the CRE? 
Does this transcriptional activation correlate 
with the graded induction of long-term facilita- 
tion? and How does 5-HT activate transcription 
by CREB? 
Because the CRE will also confer cAMP induc- 
ibility when placed upstream of reporter genes 
such as (8-galactosidase, these gene constructs can 
serve as an assay system for transcriptional induc- 
tion by cAMP. Kaang, Grant, and I therefore mi- 
croinjected a CKE-lacZ reporter plasmid into 
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