Abstract
The growth cone is responsible for axonal elongation and pathfinding by responding to various modulators for neurite growth, including neurotransmitters. We demonstrated an outline of the gamma aminobutyric acid (GABA)(A)-dependent signaling in growth cones. Here, we examined the effects of the modulators of GABA(A) receptor on the signaling in growth cones. Phenobarbital or propofol, acting on beta-subunit, enhanced the [Cl(-)]infi change and [Ca(2+)](i) elevation by the GABA stimulation to isolated growth cones. Besides, propofol enhanced GABA-dependent phosphorylation of growth-associated protein of 43 kDa (GAP-43) by protein kinase C. In contrast, an alpha-subunit acting agent diazepam did not modulate any of the above signals. Next, we examined the effect of the developmental change of alpha-subunit on the outline of the GABA(A)-dependent signaling in growth cones. We also found that the amounts of several different alpha-subunit isoforms developmentally increased or decreased in growth cone membrane (GCM), but that the affinity and density of the [(3)H]diazepam binding sites were similar to those in adult synaptic membrane. Taken together, our results strongly suggest that each step of GABA(A)-dependent signaling in GCM is not modified by diazepam, indicating that the signaling pathway mediated by GABA(A) receptor in growth cones is applicable to any compositional change of alpha-subunit isoforms.
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