Stability of new electrical connections between adult Helisoma neurons is influenced by preexisting neuronal interactions.

The factors responsible for the selective stability of new electrical connections between regenerating, adult Helisoma neurons (5, 6) were examined. In particular the alternative hypotheses that the normal reduction in strength of a specific connection is dependent on the formation of a new stable connection or the maintenance of a preexisting connection were tested. To this end, single neurons were eliminated either by microdissection or photoinactivation. Formation of stable 5-5 [i.e., left neuron 5 (L5) to right neuron 5 (R5)] coupling was prevented by removal of a single neuron 5 soma. The time course of the normally transient neuron 5-4 connection in these ganglia, however, was the same as that of control preparations. Thus the 5-4 connection is transient even when stable 5-5 coupling is unable to develop. After removal of a single neuron 4 soma, however, the 5-4 connection was stabilized. This was despite the concurrent development of stable 5-5 coupling in these preparations. Similar results were obtained when neurons were eliminated by photoinactivation, a procedure that kills axonal processes in addition to the neuronal cell body. Morphological examination of untreated neurons by Lucifer Yellow CH stain showed that neurite outgrowth was not prevented by either of the techniques employed to eliminate single neurons. It is concluded that the development of the new, stable 5-5 electrical connection does not influence the transient nature of the 5-4 connection. The presence of both neurons 4, however, which have a preexisting connection, is required for reduction of 5-4 coupling. Thus preexisting neurons and/or their connections appear to play a role in the stability of new connections in this adult nervous system.