Characterization of the glycogenolysis elicited by vasoactive intestinal peptide, noradrenaline and adenosine in primary cultures of mouse cerebral cortical astrocytes
In recent years evidence has accumulated indicating the presence of functional receptors for most neurotransmitters on astrocytes. In particular, receptors coupled to adenylate cyclase have been demonstrated, in primary astrocyte cultures, for vasoactive intestinal peptide (VIP), noradrenaline (NA) and adenosine. Here we provide, in primary cultures of cerebral cortical astrocytes prepared from neonatal mice, a detailed characterization of a cAMP-dependent process elicited by VIP, NA and adenosine, i.e. the hydrolysis of glycogen. The EC50s for the glycogenolytic effect of VIP, NA and adenosine are 3, 20 and 800 nM, respectively. The initial rate of glycogen hydrolysis is, in nmol/mg prot/min, 9.1 for VIP and 7.5 for NA. The effect of NA is predominantly mediated by beta-adrenoceptors, although an alpha 1-adrenergic component, acting most likely through protein kinase C activation, is also present. The action of VIP is mimicked by peptides sharing sequence homologies such as PHI and secretin. Glutamate, GABA, carbachol and the peptides NPY and somatostatin do not influence glycogen levels. The glycogen content of the cultures can be markedly increased by anabolic factors present in fetal calf serum, by high (e.g. 25 mM) glucose in the medium and by 48-h pretreatment of the cultures with dibutyryl cAMP. These results indicate that the glycogen content of astrocytes is under the dynamic control of various factors, including certain neurotransmitters. They also further stress the notion of a functional interaction between neurons and glial cells aimed at maintaining local energy metabolism homeostasis.