Differential expression of synaptophysin and synaptoporin during pre- and postnatal development of the hippocampal network

The closely related synaptic vesicle membrane proteins
synaptophysin and synaptoporin are abundant in the hippocampal
formation of the adult rat. But the prenatal hippocampal formation
contains only synaptophysin, which is first detected at embryonic
day 17 (E17) in perikarya and axons of the pyramidal neurons. At
E21 synaptophysin immunoreactivity extends into the apical
dendrites of these cells and in newly formed terminals contacting
these dendrites. The transient presence of synaptophysin in axons
and dendrites suggests a functional involvement of synaptophysin in
fibre outgrowth of developing pyramidal neurons. Synaptoporin
expression parallels the formation of dentate granule cell synaptic
contacts with pyramidal neurons: the amount of hippocampal
synaptoporin, determined in immunoblots and by synaptoporin
immunostaining of developing mossy fibre terminals, increases
during the first postnatal week. Moreover, in the adult,
synaptoporin is found exclusively in the mossy fibre terminals
present in the hilar region of the dentate gyrus and the regio
inferior of the cornu ammonis. In contrast, synaptophysin is
present in all synaptic fields of the hippocampal formation,
including the mossy fibre terminals, where it colocalizes with
synaptoporin in the same boutons. Our data indicate that granule
neuron terminals differ from all other terminals of the hippocampal
formation by the presence of both synaptoporin and synaptophysin.
This difference, observed in the earliest synaptic contacts in the
postnatal hippocampus and persisting into adult life, suggests
distinct functions of synaptoporin in these nerve terminals.