EFFECTS OF ELECTRICAL-STIMULATION ON GAP-43 EXPRESSION IN MOUSE SENSORY NEURONS

Effects of electrical activity on GAP-43 expression were tested in mou se dorsal root ganglion (DRG) neurons subjected to electrical stimulat ion in culture. Patterned electrical stimulation was provided through extracellular electrodes placed in multicompartment cell culture chamb ers. 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 immunocytochemistr y, two ELISA methods, and Northern blot analysis within three experime ntal protocols: (1) prior to synaptogenesis, (2) after synaptogenesis with spinal cord neurons, and (3) within the context of activity-depen dent synaptic competition, in which synapses from active and inactive DRG neurons converge on the same postsynaptic neurons. None of the sti mulation treatments produced a measurable change in GAP-43 or RNA mess age for the protein, although this electrical stimulus induces persist ent changes in synaptic strength, and alters neurite outgrowth in thes e cultures. The decline in GAP-43 levels between 1 and 3 weeks in cult ure, 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 motil ity, synaptogenesis and synapse elimination, or changes in synaptic st rength. Instead, post-translational modification, such as phosphorylat ion, may be the primary means of regulating any GAP-43 functions assoc iated with these activity-dependent processes.