An increase in intracellular levels of cyclic AMP produces trophic effects on striatal neurons developing in culture.

Cyclic AMP-dependent kinases have been suggested to constitute signal transduction pathways involved in the regulation of neuronal development and survival. The present study examined whether elevated levels of cyclic AMP exhibit trophic activities on rat striatal neurons grown under serum-free culture conditions. Treatment with dibutyryl cyclic AMP, a permeable cyclic AMP, increased GABA uptake and immunocytochemically detectable levels of proteins such as c-Fos and calbindin-D28k. Neuronal survival was promoted by dibutyryl cyclic AMP only in lower density cultures. Chronic exposure of neurons to dibutyryl cyclic AMP enhanced the morphological development of calbindin-D28k-positive neurons. Furthermore, pretreatment with dibutyryl cyclic AMP afforded neuroprotection against N-methyl-D-aspartate-induced excitotoxicity. The dibutyryl cyclic AMP-induced trophic effects above were blocked by adenosine 3',5'-cyclic monophosphothioate, a specific inhibitor of cyclic AMP-dependent kinases. We also examined whether cyclic AMP is involved in trophic effects provided by membrane depolarization induced by high K+ and growth factors such as basic fibroblast growth factor and insulin-like growth factor-1. Depolarization, but not the growth factors, increased intracellular levels of cyclic AMP. Adenosine 3',5'-cyclic monophosphothioate diminished depolarization increases in GABA uptake, whereas it did not affect the trophic effect of the growth factors. Co-treatment with the growth factors and dibutyryl cyclic AMP produced additive effects on both increases in GABA uptake and neuroprotection against excitotoxicity. The present results indicate that cyclic AMP-dependent kinases play roles in mediating differentiation and survival of developing striatal neurons. Signalling pathways activated by either basic fibroblast growth factor or insulin-like growth factor-1 are independent of those involving cyclic AMP. In contrast, depolarization-induced trophic effects are mediated, at least in part, by cyclic AMP-dependent pathways. Protective actions of dibutyryl cyclic AMP against excitotoxic injury as well as the additive effects with the growth factors are of potential interest in the experimental therapy of acute or chronic neurodegenerative diseases.