Journal of Neuroscience, Vol 14, 1011-1029, Copyright © 1994 by Society for Neuroscience
Synaptic properties of serotonergic growth cones in developing rat brain
N Ivgy-May, H Tamir and MD Gershon
Department of Anatomy and Cell Biology, Columbia University College of Physicians and Surgeons, New York, New York 10032.
In order to gain insight into the events that take place when serotonergic
growth cones are remodeled into synapses, we tested the hypothesis that
neurotransmitter-related properties of presynaptic terminals are already
present in these growth cones before synaptogenesis begins. The ontogeny of
markers for the specific reuptake of 5-HT and for 5-HT-storing synaptic
vesicles was studied in isolated growth cone (IGC) fractions from
developing rat brain. High- affinity 3H-imipramine binding (a marker for
the plasma membrane 5-HT transporter) was significantly enriched in IGC
fractions prepared before the beginning of cortical synaptogenesis
[embryonic day 15 (E15) and E20]. Radioautography with 3H-imipramine or
3H-paroxetine (another marker for the transporter) confirmed that the 5-HT
transporter is present in the cerebral cortex when it contains serotonergic
growth cones, but not serotonergic synapses. Specific uptake of 3H-5-HT was
found in IGC fractions as early as E15; this uptake was inhibited by
fluoxetine. Electron microscopic radioautography demonstrated directly that
growth cones were the structures in IGC fractions that took up 3H- 5-HT.
The synaptic vesicle protein synaptophysin and a 45 kDa protein found
specifically in serotonergic synaptic vesicles, serotonin-binding protein
(SBP), were each enriched in IGC fractions from E15 to postnatal day 5; SBP
immunoreactivity increased approximately 10-fold between E15 and E20.
Endogenous 5-HT was detected in IGC fractions at E15 and increased in
amount as development proceeded. The ratio of 5-HT to 5-hydroxyindole
acetic acid suggested that 5-HT within growth cones is protected from
catabolism by monoamine oxidase. Reserpine-induced depletion of 5-HT, a
marker for the vesicular carrier of 5-HT, was apparent in IGC fractions at
E20, but not at E15. These data suggest that properties that characterize
the presynaptic components of mature serotonergic synapses develop in
growth cones before synapses are formed. The early development of these
properties may permit neurotransmission to be established rapidly during
synaptogenesis or, alternatively, enable 5-HT to play a role in ontogeny.