Selective enhancement by serum factors of cyclic AMP accumulation in rat microglial cultures

Using purified microglial cultures obtained from the neonatal rat brain we found that media containing fetal calf serum (as well as human, horse and goat sera) enhanced by about 3-fold the accumulation of cyclic AMP induced by the beta-adrenergic agonist isoproterenol and did not affect in a significant way that induced by the direct adenylyl cyclase stimulator forskolin. The effect of fetal calf serum was (i) dose dependent, and statistically significant also at serum concentrations below 1%; (ii) rapidly lost (half life of about 15 min) when the serum-containing medium was exposed to microglia, astrocytes or neuroblastoma cells; (iii) present also when cyclic AMP accumulation was enhanced by prostaglandin E2 or by cholera toxin; (iv) absent on basal cyclic AMP levels. When media containing fetal calf serum or the other mammalian sera mentioned above were tested on astrocyte cultures, an inhibitory, rather than enhancing activity on cyclic AMP levels was observed, indicating that the facilitatory factor(s) present in serum acts specifically on microglial cells. Moreover, in astrocytes the effect of serum was identical when tested on basal and on isoproterenol or forskolin-stimulated cyclic AMP levels. Thus, the mechanism of cyclic AMP inhibition in astrocytes is unrelated to the mechanism of activation in microglia. Our observations suggest that serum contains factor(s), promptly cleared by different cell types. Such factors may interact with so far unidentified microglial receptors responsible for a facilitation of G protein-mediated activation of adenylyl cyclase. Regulation of the cyclic AMP cascade at this step has not been described previously, and may be important for the modulation of microglial functions controlled by the cyclic nucleotide.