Effects of electrical stimulation on GAP-43 expression in mouse sensory neurons

Effects of electrical activity on GAP-43 expression were tested in mouse dorsal root ganglion (DRG) neurons subjected to electrical stimulation in culture. Patterned electrical stimulation was provided through extracellular electrodes placed in multicompartment cell culture chambers. Stimulation was delivered at 10 Hz, in 0.5 s bursts every 2 s for up to 3 days. Expression of GAP-43 was assessed by immunocytochemistry, two ELISA methods, and Northern blot analysis within three experimental protocols: (1) prior to synaptogenesis, (2) after synaptogenesis with spinal cord neurons, and (3) within the context of activity-dependent synaptic competition, in which synapses from active and inactive DRG neurons converge on the same postsynaptic neurons. None of the stimulation treatments produced a measurable change in GAP-43 or RNA message for the protein, although this electrical stimulus induces persistent changes in synaptic strength, and alters neurite outgrowth in these cultures. The decline in GAP-43 levels between 1 and 3 weeks in culture, which has been reported in other studies, was readily detectable by our measurements. We conclude that regulation of GAP-43 expression is not required for activity-dependent regulation of growth cone motility, synaptogenesis and synapse elimination, or changes in synaptic strength. Instead, post-translational modification, such as phosphorylation, may be the primary means of regulating any GAP-43 functions associated with these activity-dependent processes.