Retraction and degeneration of sympathetic neurites in response to locally elevated potassium.

Sympathetic neurons from superior cervical ganglia of newborn rats were plated into center compartments of 3-compartment culture dishes, allowing exposure of distal neurites to media of different composition than provided to cell bodies and proximal neurites. Cultures were maintained initially with an external potassium concentration ([K+]o) of either 5 mM in all compartments or 50 mM in all compartments. After neurites had elongated into distal compartments, the culture medium was changed such that: the cell bodies and proximal neurites were exposed to 5 mM [K+]o; the distal neurites in one side compartment of each culture were also exposed to 5 mM [K+]o; but the distal neurites in the opposite side compartment were exposed to 50 mM [K+]o. During the next 7-10 days, the distal neurites locally exposed to 50 mM [K+]o degenerated. Many neurites developed a stretched appearance before degenerating, and detailed observations suggest that the neurites retracted to the point where mechanical tension exceeded their strength and then abruptly disintegrated. Neurites in opposite side compartments exposed to 5 mM [K+]o were normal in appearance and did not degenerate. These results suggest that a proximo-distal increase in [K+]o causes an extreme retraction of neurites distal to the increase. These results raise the possibility that K+ released by active nerve endings might cause the retraction of inactive nerve endings, thus providing a possible mechanism for the influence of activity on competition for synaptic sites, a pervasive phenomenon in the developing nervous system.