Outgrowth-Regulating Pyramidal Neurons Actions Glutamate in Isolated Hippocampal

The present study examined the effects of glutamate on the outgrowth of dendrites and axons in isolated hippocampal pyramidal-like neurons in cell culture. During the first day of culture the survival and outgrowth of these neurons was unaffected by high concentrations (up to 1 mu) of glutamate, quisqualic acid (QA), kainic acid (KA), and Kmethyl-D-aspartic acid. Beginning on day 2 of culture high levels of glutamate, KA and QA were toxic to the majority of pyramidal neurons, while subtoxic levels of these agents caused a welldefined, dose-dependent, sequence of effects on dendritic outgrowth. At increasing concentrations of glutamate, QA, and KA, the following events were observed: (1) dendritic outgrowth rates were reduced, while axonal elongation rates were unaffected; (2) dendritic length was reduced, while axons continued to grow; (3) dendrites regressed dramatically, and axonal outgrowth rate was reduced. These dendritespecific effects of glutamate were apparently mediated at the growth cones since focal application of glutamate to individual dendritic growth cones resulted in suppression of growth cone activity and a regression of the dendrite; axons were unaffected by focal glutamate application. Pharmacological tests using glutamate receptor agonists and antagonists demonstrated that receptors of the KAIQA type mediated the glutamate effects on outgrowth and survival. The calcium channel blocker Co2+ prevented both glutamate neurotoxicity and glutamate-induced dendritic regression. lonophore A23187 and elevations in extracellular K+ levels each caused a dose-dependent series of outgrowth and survival responses similar to those caused by glutamate. Taken together, these results indicate that activation of glutamate receptors leads to the opening of voltage-dependent calcium channels; the resulting increases in calcium influx lead to the observed alterations in dendritic outgrowth and neuronal survival.