To explore how gene products, required for the initiation of synaptic growth, move from the cell body of the sensory neuron to its presynaptic terminals, and from the cell body of the motor neuron to its postsynaptic dendritic spines, we have investigated the anterograde transport machinery in both the sensory and motor neurons of the gill-withdrawal reflex of Aplysia. We found that the induction of long-term facilitation (LTF) by repeated applications of serotonin, a modulatory transmitter released during learning in Aplysia, requires upregulation of kinesin heavy chain (KHC) in both pre- and postsynaptic neurons. Indeed, upregulation of KHC in the presynaptic neurons alone is sufficient for the induction of LTF. However, KHC is not required for the persistence of LTF. Thus, in addition to transcriptional activation in the nucleus and local protein synthesis at the synapse, our studies have identified a third component critical for long-term learning-related plasticity: the coordinated upregulation of kinesin-mediated transport.
