Asymmetric modulation of cytosolic cAMP activity induces growth cone turning

The possible role of cyclic nucleotides as second messengers mediating growth cone turning was studied by producing an asymmetric distribution of cyclic nucleotides across the growth cone. A repetitive pulse application method was developed to produce microscopic chemical gradients near the growth cone of embryonic Xenopus neurons in cell culture. When picoliters of a solution containing 20 mM dibutyryl cAMP (dB-cAMP), a membrane-permeable analog of cAMP, were repetitively ejected from a micropipette near the growth cone, neurite growth was consistently directed toward the pipette. Theoretical analysis of the diffusion gradient showed that the neurite is capable of detecting a 10% difference in dB-cAMP concentration across the growth cone. Similar responses were also observed using gradients of the phosphodiesterase inhibitor isobutylmethylxanthine, or of forskolin, which activates adenylate cyclase. Dibutyryl cGMP, however, produced no significant turning. These results suggest that a cytoplasmic gradient of cAMP across the growth cone is sufficient to initiate its turning response, and that cAMP in the growth cone could serve as a second messenger in mediating the action of extracellular guidance cues.