Intracellular messengers in the generation and degeneration of hippocampal neuroarchitecture

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Abstract
The actions and interactions of the neurotransmitter glutamate and the intracellular messengers calcium, cyclic AMP, and protein kinase C (PKC) in the regulation of neurite outgrowth and cell survival were examined in hippocampal pyramidal‐like neurons in isolated cell culture. Low, subtoxic levels of glutamate (10–100 μM) caused the regression of dendrites but not axons; millimolar levels caused cell death. Calcium ionophore A23187 (50–100 nM) and the PKC activator phorbol‐12‐myristate‐13‐acetate (PMA; 10–50 nM) caused the regression of both axons and dendrites, whereas the adenylate cyclase activator forskolin enhanced outgrowth rates in both axons and dendrites. The effects of glutamate, A23187, PMA, and forskolin on outgrowth were mediated locally at the growth cones; dendrites were more sensitive than axons to each of these agents. High levels of A23187 (1 μM) or PMA (100 nM) significantly reduced cell survival. Co2+ and trifluoperazine each significantly reduced glutamate‐induced dendritic regression and neurotoxicity suggesting that calcium influx and/or PKC activation mediated glutamate's actions. Fura‐2 measurements showed that glutamate caused a rapid rise in intracellular calcium levels; this rise was prevented by Co2+. PMA and forskolin did not alter intracellular calcium levels, nor did these agents affect glutamate‐induced calcium rises. Taken together, the results indicate that parallel intracellular messenger pathways that influence neurite outgrowth and cell survival are operative in hippocampal neurons; these messengers may play roles in the formation and modification of neuronal circuitry.