dendritic shaft itself. After initial contact, however, 
the growth cone may migrate to a final position on 
the dendritic shaft or cell body before stabilizing 
and continuing to later stages of synapse formation. 
Drs. Waxman and Cooper are also studying intra- 
cellular calcium transients associated with early 
stages of synapse formation in cell culture. They are 
using time-lapse video recording to image cultured 
hippocampal neurons loaded with the fluorescent 
calcium indicator Fluo-3. They have observed three 
different types of calcium signal at around the time 
of initial intercellular contact: 1) spontaneous cal- 
cium oscillations occurring prior to any distinguish- 
able cell contact, 2) calcium elevations occurring at 
the very moment of initial contact, and 3) synchro- 
nized calcium oscillations in groups of cells that 
have already established contact. They are now in- 
vestigating the mechanisms of these calcium signals 
and their significance in the processes of partner 
selection and synapse formation. 
V. Localization of Calcium Channels at the Presyn- 
aptic Active Zone. 
In studies of the squid giant synapse over the 
last three years. Dr. Smith and JoAnn Buchanan 
have been collaborating with Drs. George Au- 
gustine and Milton Charlton (Woods Hole) to study 
the molecular organization of the active zone at 
the squid giant synapse. The main thrust of this 
work has been to test earlier indications that the 
presynaptic calcium channels are highly localized 
to the active zone region. Two studies completed 
this year provide strong support for this channel lo- 
calization hypothesis. A study correlating fluores- 
cent measurements of the presynaptic calcium tran- 
sient with detailed anatomical reconstructions 
PUBLICATIONS 
found an excellent agreement between loci of cal- 
cium influx and ultrastructurally identified active 
zones; high-resolution observations with a laser 
confocal microscope have confirmed and extended 
earlier results from conventional fluorescence mi- 
croscopy. 
VI. Propagation of Calcium Signals Across Intercel- 
lular Gap Junctions. 
Studies completed this year on two different sys- 
tems have led to similar observations: intercellular 
propagation of a physiological calcium signal 
through a gap junction. In the first study, a collabo- 
ration with Drs. Paul Brehm, James Lechleiter, and 
Kathleen Dunlap (Woods Hole), digital imaging of 
endogenous, calcium-dependent bioluminescence 
in the hydrozoan coelenterate Obelia provided the 
evidence for intercellular propagation of voltage- 
induced calcium signals. The second study, con- 
ducted in Dr. Smith's laboratory by Steven Fink- 
beiner and Drs. Ann Cornell-Bell and Mark Cooper, 
grew from a discovery of glutamate-receptor-acti- 
vated calcium signals in cultured hippocampal 
astrocytes. This team found that, under many con- 
ditions, glutamate-induced calcium signals propa- 
gated from cell to cell in confluent cultures and 
that this propagation occurs through gap junctions. 
For both Obelia and the astrocyte cultures, major 
questions remain as to whether the gap junctional 
signal is calcium itself or some other intermedi- 
ate chemical message, such as inositol trisphos- 
phate (IPj). 
Dr. Smith is also Associate Professor in the Sec- 
tion of Molecular Neurobiology at the Yale Univer- 
sity School of Medicine. 
Books and Chapters of Books 
Augustine, GJ., Buchanan, J., Charlton, M.R, Osses, L.R., and Smith, S.J. 1989. Fingering the trigger for neu- 
rotransmitter secretion: studies on the calcium channels of squid giant presynaptic terminals. In Secretion 
and Its Control (Oxford, G., and Armstrong, C, Eds.). New York: Rockefeller University Press, pp 204- 
223. 
Articles 
Brehm, R, Lechleiter, J., Smith, S J., and Dunlap, K. 1989. Intercellular signalling as visualized by endogenous 
calcium-dependent bioluminescence. A^ewron 3:191-198. 
Forscher, R , and Smith, SJ. 1988. Actions of cytochalasins on the organization of actin filaments and microtu- 
bules in a neuronal growth cone. J Cell Biol 107:1505-1516. 
Smith, S J. 1988. Neuronal cytomechanics: the actin-based motility of growth cones. Science 242:708-715. 
Continued 
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