RAPID COMMUNICATION Trophic and Contact Conditions Modulate Synapse Formation Between Identified Neurons

Magoski, Neil S. and Andrew G. M. Bulloch. Trophic and conissue yet to be thoroughly explored is whether or not contact tact conditions modulate synapse formation between identified neubetween neurites is necessary for synapse formation or if rons. J. Neurophysiol. 79: 3279–3283, 1998. We tested the ability the factors present in hemolymph or conditioned medium of an identified interneuron from the mollusk, Lymnaea stagnalis , are more important requirements. For example, the cytokine, to reestablish appropriate synapses in vitro. In the CNS, the giant ciliary neurotrophic factor (CNTF) induces neurite outdopaminergic neuron, designated as right pedal dorsal one growth and contact but not synapse formation in Lymnaea (RPeD1), makes an excitatory, chemical synapse with a pair of neurons (Syed et al. 1996). This suggests that neuritic conessentially identical postsynaptic cells known as visceral dorsal two tact alone is not necessarily sufficient for synapse formation. and three (VD2/3). When the somata of the preand postsynaptic In fact, certain neurons from both Aplysia and Helisoma can neurons were juxtaposed and cultured in vitro in defined medium, i.e., a soma-soma synapse, only an inappropriate electrical synapse form synapses between apposed cell somata (Haydon 1988; was observed. The postsynaptic cell still responded to applied doKlein 1994). pamine, the presynaptic transmitter, indicating that the lack of The present study addresses these issues by examining chemical synapse formation was not due to lack of dopamine recepsynapse formation between identified neurons from the moltors. When the somata were cultured apart in conditioned medium lusk, L. stagnalis . The presynaptic neuron is right pedal (medium previously incubated with Lymnaea CNS, thereby derivdorsal one (RPeD1), a respiratory interneuron (Syed and ing trophic factors) , the cells exhibited overlapping neurite outWinlow 1991; Syed et al. 1990), and the postsynaptic neugrowth that resulted in an appropriate excitatory, chemical synapse rons are the giant neurons, visceral dorsal two and three from RPeD1 to VD2/3. On the other hand, when the cell pair was (VD2/3) (Benjamin and Winlow 1981). In the intact CNS, cultured in a soma-soma configuration, but in conditioned medium, RPeD1 makes an excitatory, dopaminergic connection with a mixed chemical-electrical synapse was observed. Because conditioned medium could partially overcome the limitations of the VD2/3 (Magoski and Bulloch 1997; Magoski et al. 1995). soma-soma configuration and initiate chemical synapse formation, The ability of this connection to reestablish itself in vitro this data suggests that conditioned medium contains a factor(s) under a variety of trophic and contact conditions, including that supports synaptogenesis. defined medium versus brain-conditioned medium and soma-soma versus neurite-neurite configurations, is tested.