Potassium depolarization-induced cAMP stimulates somatostatin mRNA levels in cultured diencephalic neurons

In the nervous system, signals transmitted across synapses are known to regulate gene expression in the postsynaptic cells. This process often involves membrane depolarization and subsequent elevation of intracellular Ca(2+). We have previously demonstrated in fetal cerebrocortical cells, that somatostatin (SS) mRNA levels can be induced by depolarizing agents such as high potassium concentrations and veratridine (VTD), and that these effects are calcium dependent. SS expression is regulated by cAMP, and in the cerebral cortex adenylate cyclase activity is regulated through fluctuations in intracellular Ca(2+) concentrations. The present experiments were undertaken to determine the mechanism by which calcium upregulates the levels of SS mRNA. Cerebrocortical cells from 17-day-old fetuses were exposed to the different agents for 24 h and the levels of SS mRNA were measured by Northern blot. Incubation of cells with the calcium channel antagonist nifedipine (Nf), the calcium chelating agent EGTA, calcium free KRB and the calcium calmodulin inhibitors trifluoroperazine (TFP) and the napthelene sulfonamide, W7, resulted in the inhibition of K(+)-induced SS mRNA levels. K(+)-evoked depolarization increased the intracellular concentration of cAMP and this effect was antagonized by verapamil (VPM). Forskolin (Fk) provoked a higher increment in cAMP concentration than potassium, although the induction of SS mRNA was more evident following K(+) depolarization indicating a lack of correlation between levels of cAMP and induction of SS mRNA. The role of K(+)-induced cAMP on the increment of SS mRNA that occurred upon membrane depolarization was further explored with the inhibitor of protein kinase A (PKA), Rp cAMP whose presence significantly reduced depolarization-induced SS mRNA levels. This study confirms that Ca(2+) influx is required for K(+)depolarization-induced stimulation of cAMP whereby the increment of SS mRNA is partly produced.