Figure 6.

A model depicting the CM-induced plasticity of synapse formation between soma–soma paired Lymnaea neurons. In the presence of trophic factors [(+) trophic factors] identified Lymnaea neurons form appropriate excitatory synapses. In the absence of trophic factors [(−) trophic factors], however, inappropriate inhibitory synapses develop in vitro. Because these synapses are novel and inhibitory, we have termed them default synapses. These inappropriate inhibitory synapses are reduced in incidence by the addition of CM-derived trophic molecules (thick black arrow), which also induce appropriate excitatory synapse formation between the paired cells. This trophic factor-induced plasticity requires de novo protein synthesis and is mediated via receptor tyrosine kinases (RTKs).