Target cell contact suppresses neurite outgrowth from soma-soma paired Lymnaea neurons

Neurite extension from developing and/or regenerating neurons is terminated on contact with their specific synaptic partner cells. However, a direct r elationship between the effects of target cell contact on neurite outgrowth suppression and synapse formation has not yet been demonstrated. To determ ine whether physical/synaptic contacts affect neurite extension from cultur ed cells, we utilized soma-soma synapses between the identified Lymnaea neu rons. A presynaptic cell (right pedal dorsal 1, RPeD1) was paired either wi th its postsynaptic partner cells (visceral dorsal 4, VD4, and Visceral dor sal 2, VD2) or with a non-target cell (visceral dorsal 1, VD1), and the int eractions between their neurite outgrowth patterns and synapse formation we re examined. Specifically, when cultured in brain conditioned medium (CM, c ontains growth-promoting factors), RPeD1, VD4, and VD2 exhibited robust neu rite outgrowth within 12-24 h of their isolation. Synapses, similar to thos e seen in vivo, developed between the neurites of these cells. RPeD1 did no t, however, synapse with its non-target cell VD1, despite extensive neuriti c overlap between the cells. When placed in a soma-soma configuration (soma ta juxtaposed against each other), appropriate synapses developed between t he somata of RPeD1 and VD4 (inhibitory) and between RPeD1 and VD2 (excitato ry), Interestingly, pairing RPeD1 with either of its synaptic partner (VD4 or VD2) resulted in a complete suppression of neurite outgrowth from both p re- and postsynaptic neurons, even though the cells were cultured in CM. A single cell in the same dish, however, extended elaborate neurites, Similar ly, a postsynaptic cell (VD4) contact suppressed the rate of neurite extens ion from a previously sprouted RPeD1. This suppression of the presynaptic g rowth cone motility was also target cell contact specific. The neurite supp ression from soma-soma paired cells was transient, and neuronal sprouting b egan after a delay of 48-72 h, In contrast, when paired with VD1, both RPeD 1 and this non-target cell exhibited robust neurite outgrowth. We demonstra te that this neurite suppression from soma-soma paired cells was target cel l contact/synapse specific and Ca2+ dependent. Specifically, soma-soma pair ing in CM containing either Lower external Ca2+ concentration (50 % of its control level) or Cd2+ resulted in robust neurite outgrowth from both cells ; however, the incidence of synapse formation between the paired cells was significantly reduced. Taken together, our data show that contact (physical and/or synaptic) between synaptic partners strongly influence neurite outg rowth patterns of both pre- and postsynaptic neurons in a time-dependent an d cell-specific manner. Moreover, our data also suggest that neurite outgro wth and synapse formation are differentially regulated by external Ca2+ con centration. (C) 2000 John Wiley & Sons, Inc.