Neurotrophins act at presynaptic terminals to activate synapses among cultured hippocampal neurons

We have recently demonstrated that embryonic E16 hippocampal neurons grown in cultures are unable to form fast synaptic connections unless treated wit h BDNF or NT-3, This experimental system offers an opportunity to define th e roles of neurotrophins in processes leading to formation of functional sy naptic connections. We have used ultrastructural and electrophysiological m ethods to explore the cellular locations underlying neurotrophin action on synaptic maturation. The rate of spontaneous miniature excitatory postsynap tic currents (mEPSCs) evoked by hyperosmotic stimulation was 7-16-fold high er in neurotrophin-treated cells than in controls. In addition, the potent neurotransmitter-releasing drug alpha -latrotoxin was virtually ineffective in the control cells while it stimulated synaptic events in neurotrophin-t reated cells. Likewise, the membrane-bound dye FM1-43 was taken up by termi nals in neurotrophin-treated cultures five-fold more than in controls. Both the total number and the number of docked synaptic vesicles were increased by neurotrophin treatment. Activation of synaptic responses by neurotrophi ns occurred even when postsynaptic glutamate receptors and action potential discharges were pharmacologically blocked. These results are consistent wi th a presynaptic locus of action of neurotrophins to increase synaptic vesi cle density which is critical for rapid synaptic transmission. They also su ggest that neurotrophins can activate synapses in the absence of pre- and p ostsynaptic neuronal activity.