Coupling of astroglial alpha 2-adrenoreceptors to second messenger pathways

We have investigated which alpha 2-receptor subtypes are expressed in cultured cortical astroglia, and their coupling to second messengers. Binding assays using [3H]rauwolscine showed a very low number of alpha 2 receptors in the astrocytic cultures. Treatment of cultures with dibutyryl cyclic AMP (dBcAMP) increased significantly the number of receptors. The RNase protection assay was used to investigate which receptor subtype the cells express. The alpha 2B message was expressed at a low level in both treated and untreated cells, the levels of mRNA for the alpha 2A/D subtype were up-regulated significantly in cells treated with dBcAMP and no expression of mRNA for the alpha 2C subtype was detected. The alpha 2 agonist dexmedetomidine inhibited forskolin-induced increases in cyclic AMP both in treated and untreated cultures in a pertussis toxin-dependent manner. This effect was abolished by the alpha 2-receptor antagonist rauwolscine. Selective alpha 2-receptor agonists dexmedetomidine, clonidine, and UK14,304 all increased intracellular calcium only in dBcAMP-treated cells. The antagonist rauwolscine abolished this effect. Ca2+ responses were also seen in the absence of extracellular Ca2+ and they were inhibited by the phospholipase C inhibitor U-73122, suggesting that astroglial alpha 2 receptors are coupled to the inositol phospholipid pathway. We therefore also tested the effect of dexmedetomidine directly on inositol 1,4,5-trisphosphate accumulation. A significant increase was seen that was blocked by the antagonist rauwolscine and, as expected, by U-73122. In short, the results demonstrate that the alpha 2 receptors in astroglia are coupled to multiple second messenger pathways. They are up-regulated in cells treated with dBcAMP, which simultaneously assume a process-bearing morphology. If this morphological change reflects some in vivo process such as reactive gliosis, the up-regulation of alpha 2-receptor expression could mean an adaptive change in astrocytic responses to a common neurotransmitter, noradrenaline.