THE BIOLOGY OF SYNAPSES 
Stephen J. Smith, Ph.D., Associate Investigator 
Research in Dr. Smith's laboratory addresses the 
biology of synapses, with a special focus on the de- 
velopment of synaptic structure. Closely related ef- 
forts address mechanisms of intracellular signaling 
by the calcium ion, a cytoplasmic messenger that 
plays a major role in synaptic development and 
function. These studies employ cultured mamma- 
lian neurons and glial cells, slices and other forms 
of explanted mammalian brain tissue, and marine 
specimens, including the squid. The central meth- 
ods include digital video light microscopy, confocal 
scanning laser microscopy, electron microscopy, im- 
munohistochemistry, optical measurement of intra- 
cellular calcium, and electrophysiology. 
I. Mechanisms of Growth Cone Motility. 
The growth of nerve fibers is guided by a highly 
motile terminal specialization called the growth 
cone: its motility involves alternating protrusion 
and retraction phases that apparently explore and 
test embryonic surfaces. Recent experiments by Dr. 
Paul Forscher in this laboratory have employed digi- 
tal video microscopy to discriminate between two 
general motility models that might apply to growth 
cones: one model is based on membrane traffic and 
the other on dynamics of the actin cytoskeleton. Dr. 
Forscher's results have strongly favored actin-based 
models and have led to a detailed model for growth 
cone motility that involves a combination of two 
actin-based mechanochemical mechanisms. In this 
model the protrusive phase of crawling is related 
primarily to the actin polymerization cycle, while 
the retraction phase reflects the actin-myosin inter- 
action. 
II. Neuronal Migration in Developing 
Telencephalon. 
The neurons that populate layers of mature 
neocortex arise from mitoses of stem cells lining 
the cerebral ventricle and then migrate in a pre- 
cisely choreographed fashion toward the pial sur- 
face. Cells destined for the most superficial layers 
arise later on and bypass older neurons that remain 
in deeper layers. Experiments carried out in this 
laboratory by Drs. Monica Cooper and Aaron Wax- 
man may have taken a large step toward elucidating 
the mysterious basis for this migratory choreogra- 
phy. Employing the new techniques of laser confo- 
cal microscopy, they have developed the first means 
for live observation of cell migration within intact 
cortical tissue explants. These methods have 
yielded dynamic observations of subcellular details 
(e.g., growth cones, submicron neurites, single or- 
ganelles, and individual filopodia) at depths up to 
100 |Jim within acutely isolated brain tissue. They 
have also recorded intracellular calcium from indi- 
vidual neurons under similar circumstances. 
III. Cell Motility and Long-Term Synaptic Plasticity. 
Synapses in the mammalian hippocampus show 
a long-lasting form of functional plasticity called 
long-term potentiation (LTP). One hypothesis 
about LTP envisions a motility event somehow con- 
solidated into a lasting structural change as the fun- 
damental synaptic modification. Although there 
have been a few observations in support of this hy- 
pothesis, it has proved difficult to test decisively, be- 
cause it has not been possible to visualize living 
synapses during LTR Ongoing studies in this labora- 
tory demonstrate that the new laser scanning con- 
focal microscope may be capable of providing such 
a definitive test. With vital fluorescent staining, indi- 
vidual synaptic varicosities and dendritic spines 
have been resolved in good detail. Dr. Robert Gold- 
man and Alex Chernjavsky have recently succeeded 
in marrying this new instrument to the type of elec- 
trical recording apparatus needed to induce and 
test LTR 
IV Early Stages in Synapse Formation. 
When an axonal growth cone contacts an appro- 
priate target cell, the growth cone begins a trans- 
formation into the secretory presynaptic terminal, 
while the adjacent site on the target cell acquires 
the specialized features of the postsynaptic com- 
plex, including neurotransmitter receptors. Dr. 
Cooper has been working to elucidate details of 
this process as it occurs in cell culture. Her ongoing 
studies have revealed a striking and unexpected 
feature of the synaptogenic contact: the growth 
cone and target cell almost always make their initial 
contact at a site where the target cell dendrite is ac- 
tively growing fine processes of some type. Thus 
the initial contact between an axonal growth cone 
and a target cell dendrite generally occurs at the tip 
of some fine branch of the dendrite, not on the 
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