Abstracts

Synaptogenesis in the hippocampus is an intricate physiological process which forms the structural basis of learning and memory. Reorganization of synapses after neural injury may cause recurrent excitatory networks and the development of spontaneous recurrent seizures. We have identified one putative determinant of hippocampal synaptogenesis, semaphorin 3F signaling, whose regulation during pathologic synaptic reorganization after kainic acid status epilepticus may lead to maladaptive synaptic circuitry. It is unknown how upstream regulatory pathways influence semaphorin signaling and its subsequent physiologic effects on epileptogenesis. While neurotrophins, cyclic AMP, and MAP kinase pathways are identified as putative regulators of neuropilin 2 (NPN2, a subunit of the sema 3F receptor) gene expression, we have now investigated the specificities of the hippocampal cell populations, glutamate receptors, and subcellular localization of NPN2 regulatory pathways in hippocampal cultures via RT-PCR and fluorescence immunocytochemistry. NPN2+ immunoreactive punctate in cultured rat hippocampal excitatory neurons and interneurons are located on neurites, growth cones, [amp] cell bodies. 50% of NPN2+ punctata are positive for the synaptic proteins synaptophysin and NSF. NPN2+ punctata on cell bodies of excitatory neurons are positive for synaptophysin, NSF, and GABA receptor beta2/3, suggesting NPN2 in axosomatic inhibitory synapses. Bath application of NMDA and AMPA suggest that physiologic neurotransmission stimulates NPN2 mRNA production (40%, p[lt]0.05). In contrast, kainic acid (KA) receptor stimulation, even at low concentrations (100 nM), reduces NPN2 mRNA levels (by 80%, p[lt] 0.01) and reduces the levels of cell surface NPN2 containing sema receptors (90%, p[lt]0.001) on neurites in CA3 and CA1. While there is a linear correlation between NGF levels and increased production of NPN2 mRNA, rat hippocampal neurons cultured with 200 ng/ml BDNF or NT3 results in reduced 1) NPN2 mRNA production (50%, p[lt]0.05) and 2) reduced cell surface expression of NPN2 with 3) a relative increased NPN2 expression on intracellular perinuclear and nuclear membranes. Taken together, these data suggest neuropilin 2 signaling present at the synapse may stabilize synaptic connections including inhibitory connections. Similar to neurotrophins, KA but not AMPA or NMDA receptors may signal back to the nucleus to affect NPN2 gene expression and subsequent subcellular localization of NPN2 containing sema receptors. In summary, both synaptic and intracellular expression of sema 3F receptors may act at hippocampal synapses to suppress aberrant axonal sprouting and synaptogenesis after intense depolarization and neural injury. (Supported by KSCHIRT [amp] Vanderbilt.)