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.
Read full abstract