Figure 1.

Excitatory synapse formation between Lymnaea neurons requires brain CM. When paired overnight in DM (does not contain trophic factors), the identified neuron VD4 (presynaptic) and LPeD1 (postsynaptic) developed inappropriate inhibitory synapses (not seen in vivo), which switched to appropriate excitatory synapses after the addition of CM. Cells were paired in DM overnight and synapses tested by direct intracellular recordings. All pairs cultured in DM exhibited inhibitory synapses (n = 7). DM was then replaced with CM and synapses were sampled electrophysiologically at hour 1, 4 and 12–18. We found that a 1 h exposure to CM resulted in 14% (1 out 7) of cell pairs exhibiting appropriate excitatory synapses. The incidence of excitatory synapse formation increased with increasing time points and 86% (6 of 7) pairs exhibited excitatory synapses after 4 h of CM addition, whereas by 12–18 h, all (7 of 7) pairs examined had established appropriate excitatory synapses.