α2-Adrenoceptors activate noradrenaline-mediated glycogen turnover in chick astrocytes.

In the brain, glycogen is primarily stored in astrocytes where it is regulated by several hormones/neurotransmitters, including noradrenaline that controls glycogen breakdown (in the short term) and synthesis. Here, we have examined the adrenoceptor (AR) subtype that mediates the glycogenic effect of noradrenaline in chick primary astrocytes by the measurement of glycogen turnover (total (14) C incorporation of glucose into glycogen) following noradrenergic activation. Noradrenaline and insulin increased glycogen turnover in a concentration-dependent manner. The effect of noradrenaline was mimicked by stimulation of α(2) -ARs (and to a lesser degree by ß(3) -ARs), but not by stimulation of α(1) -, ß(1) -, or ß(2) -ARs, and occurred only in astrocytes and not neurons. In chick astrocytes, studies using RT-PCR and radioligand binding showed that α(2A) - and α(2C) -AR mRNA and protein were present. α(2) -AR- or insulin-mediated glycogen turnover was inhibited by phosphatidylinositol-3 kinase inhibitors, and both insulin and clonidine caused phosphorylation of Akt and glycogen synthase kinase-3 in chick astrocytes. α(2) -AR but not insulin-mediated glycogen turnover was inhibited by pertussis toxin pre-treatment indicating involvement of Gi/o proteins. These results show that the increase in glycogen turnover caused by noradrenaline is because of activation of α(2) -ARs that increase glycogen turnover in astrocytes utilizing a Gi/o-PI3K pathway.

Role of ß-adrenoceptors in glucose uptake in astrocytes using ß-adrenoceptor knockout mice.

BACKGROUND AND PURPOSE: ß(1) -, ß(2) - and ß(3) -adrenoceptors determined by functional, binding and reverse transcription polymerase chain reaction (RT-PCR) studies are present in chick astrocytes and activation of ß(2) - or ß(3) -adrenoceptors increase glucose uptake. The aims of the present study are to identify which ß-adrenoceptor subtypes are present in mouse astrocytes, the signal transduction mechanisms involved and whether ß-adrenoceptor stimulation regulates glucose uptake. EXPERIMENTAL APPROACH: Astrocytes were prepared from four mouse strains: FVB/N, DBA/1 crossed with C57BL/6J, ß(3) -adrenoceptor knockout and ß(1) ß(2) -adrenoceptor knockout mice. RT-PCR and radioligand binding studies were used to determine ß-adrenoceptor expression. Glucose uptake and cAMP were assayed to elucidate the signalling pathways involved. KEY RESULTS: mRNAs for all three ß-adrenoceptors were identified in astrocytes from wild-type mice. Radioligand binding studies identified that ß(1) - and ß(3) -adrenoceptors were predominant. cAMP studies showed that ß(1) - and ß(2) -adrenoceptors coupled to G(s) whereas ß(3) -adrenoceptors coupled to both G(s) and G(i) . However, activation of any of the three ß-adrenoceptors increased glucose uptake in mouse astrocytes. Interestingly, there was no functional compensation for receptor subtype loss in knockout animals. CONCLUSIONS AND IMPLICATIONS: This study demonstrates that although ß(1) -adrenoceptors are the predominant ß-adrenoceptor in mouse astrocytes and are primarily responsible for cAMP production in response to ß-adrenoceptor stimulation, ß(3) -adrenoceptors are also present in mouse astrocytes and activation of ß(2) - and ß(3) -adrenoceptors increases glucose uptake in mouse astrocytes.