TROPHIC AND CONTACT CONDITIONS MODULATE SYNAPSE FORMATION BETWEEN IDENTIFIED NEURONS

We tested the ability of an identified interneuron from the mollusk, L ymnaea stagnalis, to reestablish appropriate synapses in vitro. In the CNS, the giant dopaminergic neuron, designated as right pedal dorsal one (RPeD1), makes an excitatory, chemical synapse with a pair of esse ntially identical postsynaptic cells known as visceral dorsal two and three (VD2/3). When the somata of the pre- and postsynaptic neurons we re juxtaposed and cultured in vitro in defined medium, i.e., a soma-so ma synapse, only an inappropriate electrical. synapse was observed. Th e postsynaptic cell still responded to applied dopamine, the presynapt ic transmitter, indicating that the lack of chemical synapse formation was not due to lack of dopamine receptors. When the somata were cultu red apart in conditioned medium (medium previously incubated with Lymn aea CNS, thereby deriving trophic factors), the cells exhibited overla pping neurite outgrowth that resulted in an appropriate excitatory, ch emical synapse from RPeD1 to VD2/3. On the other hand, when the cell p air was cultured in a soma-soma configuration, but in conditioned medi um, a mixed chemical-electrical synapse was observed. Because conditio ned medium could partially overcome the limitations of the soma-soma c onfiguration and initiate chemical synapse formation, this data sugges ts that conditioned medium contains a factor(s) that supports synaptog enesis.