Role of activity in the selection of new electrical synapses between adult Helisoma neurons.

Our aim was to determine whether neural activity in the form of sodium-dependent action potentials play a role in the formation, maintenance and specificity of electrical synapses between regenerating neurons. We axotomized buccal neurons of the mollusc, Helisoma trivolvis, and placed ganglia into organ culture in the absence or presence of tetrodotoxin (TTX), a specific sodium channel blocker. Electrical coupling was measured using intracellular microelectrodes positioned within the soma of identified neurons. Neurite outgrowth was assessed by epifluorescence microscopy after filling neurons by iontophoresis with Lucifer yellow. Previous studies found that two days after axotomy transient electrical synapses form between heterologous neurons (e.g. buccal neurons 4 and 5). Five days after axotomy these transient connections disappeared and a new electrical synapse was stabilized between the paired buccal neurons 5. To determine whether blocking neural activity with TTX affected the specificity and formation of new electrical synapses, we examined electrical coupling between the heterologous neurons 4 and 5 two days after axotomy, and the paired buccal neurons 5 five days after axotomy. Our electrophysiological recordings indicated that different neurons in the buccal ganglion varied in their sensitivity to TTX (i.e. sensitivity of buccal neurons 19 greater than 5 greater than 4), but spontaneous activity was abolished in all 3 neurons by 2 x 10(-5) M TTX. Furthermore, the inhibitory effects of TTX occurred within seconds of superfusion and persisted for at least 6 days. Inhibition of activity by TTX could be reversed after superfusion with normal saline. Neurite outgrowth from axotomized neurons was not appreciably altered in the presence of TTX. Furthermore, no differences in the incidence of electrical coupling or the coupling resistance were detected between neurons 4 and 5 two days after axotomy and organ culture in the presence of TTX. However, electrical coupling between the symmetrically paired neurons 5 was elevated in the presence of TTX after 5 days. We conclude from these results that neural activity in the form of sodium-dependent action potentials does not play an important role in the formation or breaking of transient electrical synapses during neuronal regeneration in the mollusc Helisoma trivolvis.