P2 purinoceptors in rat cortical astrocytes: expression, calcium-imaging and signalling studies

Extracellular ATP is known to activate intracellular enzymes in astrocytes via P2 purinoceptors that appear to play important physiological and pathological roles in these supporting brain cells. In this study, major P2 purinoceptor subtypes on astrocytes of neonatal rat cerebral cortices were identified in receptor expression experiments, when astrocytic messenger RNA was injected into Xenopus oocytes and recombinant P2 purinoceptors were characterized pharmacologically. In messenger RNA-injected oocytes, ATP evoked inward chloride currents (ICl,Ca) typical of stimulating metabotropic receptors that release intracellular Ca2+. Half-maximal activation with ATP occurred at 40 nM: the Hill coefficient was 0.5, which indicated that ATP stimulated two subtypes of P2 purinoceptor. UTP and 2-methylthioATP were the most active (and equipotent) of a series of nucleotides activating recombinant P2 purinoceptors. These results indicated that the two P2 purinoceptors expressed by astrocytic messenger RNA were of P2U and P2Y subtypes. Responses to ATP were antagonized by the P2 purinoceptor antagonist (suramin) but not by the P1 purinoceptor blocker (sulphophenyltheophylline). Findings in expression studies were confirmed in assays of intracellular signalling systems using primary cultures of rat astrocytes. UTP and 2-methylthioATP stimulated mitogen-activated protein kinase to the same extent as ATP, although UTP was less potent than either ATP or 2-methylthioATP. Both UTP and ATP increased intracellular Ca2+ (as measured by fura-2/AM luminescence) which, in cross-desensitization experiments, indicated the involvement of two subtypes of P2 purinoceptors. In conclusion, rat cortical astrocytes express two major subtypes (P2U and P2Y) of metabotropic ATP receptor which, when activated, raise intracellular Ca2+ and also stimulate mitogen-activated protein kinase.