The Soma Soma Synapse Model

Even in simpler model systems such as mollusks, synapses that develop between neurites are located at some distance from the somata and are thus inaccessible for direct morphological and electrophysiological analysis. Moreover, the precise timing and the numbers of synapses in these preparations are also difficult to define fully. Therefore, molluscan preparations were explored further to develop synapses between the somata, in the absence of outgrowth - an approach that was pioneered in leech21,22 and subsequently refined in the snail Helisoma23. Specifically, Haydon23 used identified neurons from Helisoma and juxtaposed their somata in culture. Indeed, chemical synapses developed between the somata in the absence of neurite outgrowth. Although the synapses developed between soma-soma paired Helisoma neurons did not exist in the intact brain, this model did nevertheless provide important insights into mechanisms of synapse formation, at a resolution that had not been attained before. Feng and colleagues24 paired functionally well-defined, pre-and postsynaptic neurons from Lymnaea in a soma-soma configuration. The soma-soma paired Lymnaea neurons (Figure 2.2A; Color-plate 3) developed appropriate inhibitory, cholinergic connections overnight, and these synapses were target cell specific and both structurally (Figure 2.2B) and electrophysiological^ similar to those seen in vivo24. In addition, voltage-induced, Ca2+ hotspots were shown to develop between soma-soma paired cells during synapse formation and these Ca2+ specializations were both target cell and contact site specific25. Using FM1-43 dye, which is taken up at the active synaptic site through endocytosis, the presynaptic secretory machinery was demonstrated to be specialized at the contact site between the paired cells26,27. Together, these data show that soma-soma synapses are both structurally and functionally similar to their neurite-neurite and in vivo counterparts, and are thus suitable for further studies on synapse formation.

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