This work was supported by Sniss National Science Foundation Grants 31-39426.93 (A4.T.) and 31-40565.94 (P.J.M.). We thank Drs. S. Poitry, C. Poitry-Yamate, and A.-L. Veuthey, and L. Pellerin and 0. Sorg for their contribution to the experimental work described i n this review: we also thank P. Perrottet and M. Maillard for technical assistance.

Several isoenzymes of the Na(+),K(+)-ATPase are expressed in brain but their specific roles are poorly understood. Recently, it was suggested that an isoenzyme of the Na(+),K(+)-ATPase containing the alpha(2) subunit, together with the glutamate transporters GLAST and GLT-1, participate in a coupling mechanism between neuronal activity and energy metabolism taking place in astrocytes. To substantiate this hypothesis, we compared the distribution of alpha(2), GLAST and/or GLT-1 in the rat cerebral cortex using double immunofluorescence and confocal microscopy, and immunocytochemistry at the electron microscopic level. We also investigated the relationship between alpha(2), GLAST or GLT-1 and asymmetrical synaptic junctions (largely glutamatergic) and GABAergic nerve terminals. Results show that the alpha(2) subunit has an exclusive astroglial localization, and that it is almost completely co-distributed with GLAST and GLT-1 when evaluated by confocal microscopy. This similar distribution was confirmed at the ultrastructural level, which further showed that the vast majority of the alpha(2) staining (73% of all labelled elements), like that of GLAST and GLT-1, was located in glial leaflets surrounding dendritic spines and the dendritic and/or axonal elements of asymmetrical (glutamatergic) axo-dendritic synapses. Synapses ensheathed by alpha(2), GLAST or GLT-1 virtually never included (

The effect of increasing [K+]0 on 3H-glycogen levels was examined in mouse cerebral cortical slices. K+ stimulates in a time- and concentration-dependent manner the hydrolysis of 3H-glycogen. Over 70% of the maximal effect is reached within 30 sec and the EC50 for the glycogenolytic action of K+ is 11 mM. Significant 3H-glycogen hydrolysis occurs at 5-12 mM [K+]0, concentrations reached by the ion in the extracellular space during neuronal activity. The K+-evoked glycogenolysis is Ca2+-dependent, and is inhibited by Ca2+-channel blockers such as Ni2+ and Mn2+, but not by Cd2+, nifedipine, and omega-conotoxin. Furthermore, the effect of K+ is not enhanced by the Ca2+-channel agonist Bay K 8644. This type of pharmacological profile suggests that the activation of voltage-sensitive Ca2+ channels of the T subtype mediates the glycogenolytic action of K+. This set of observations suggests that K+ released in the extracellular space by active neurons may promote the mobilization of energy substrates and therefore play a role in the coupling between neuronal activity and energy metabolism.

We have described previously a transcription-dependent induction of glycogen resynthesis by the vasoactive intestinal peptide (VIP) or noradrenaline (NA) in astrocytes, which is mediated by cAMP. Because it has been postulated that the cAMP-mediated regulation of energy balance in hepatocytes and adipocytes is channeled at least in part through the CCAAT/enhancer binding protein (C/EBP) family of transcription factors, we tested the hypothesis that C/EBP isoforms could be expressed in mouse cortical astrocytes and that their level of expression could be regulated by VIP, by the VIP-related neuropeptide pituitary adenylate cyclase-activating peptide (PACAP), or by NA. We report in this study that in these cells, C/EBP beta and C/EBP delta are induced by VIP, PACAP, or NA via the cAMP second-messenger pathway. Induction of C/EBP beta and -delta mRNA by VIP occurs in the presence of a protein synthesis inhibitor. Thus, c/ebp beta and c/ebp delta behave as cAMP-inducible immediate-early genes in astrocytes. Moreover, transfection of astrocytes with expression vectors selectively producing the transcriptionally active form of C/EBP beta, termed liver-enriched transcriptional activator protein, or C/EBP delta enhance the glycogen resynthesis elicited by NA, whereas an expression vector producing the transcriptionally inactive form of C/EBP beta, termed liver-enriched transcriptional inhibitory protein, reduces this resynthesis. These results support the idea that C/EBP beta and -delta regulate gene expression of energy metabolism-related enzymes in astrocytes.

Adenosine promotes a concentration-dependent hydrolysis of 3H-glycogen newly synthesized from 3H-glucose by mouse cerebral cortical slices. The EC50 for this effect is 7 microM. Theophylline antagonizes the glycogenolysis induced by adenosine with an EC50 of 80 microM. The rank-order of potencies of adenosine agonists is adenosine 5'-cyclopropyl-carboxamide greater than 2-chloroadenosine much greater than N6-cyclohexyladenosine = adenosine, suggesting that adenosine promotes glycogenolysis via receptors of the A2 type. This contention is substantiated by the weak stereospecificity observed for the glycogenolytic action of D- and L-(phenylisopropyl)-adenosine. The glycogenolysis elicited by adenosine at 10 and 100 microM is inhibited by ouabain at 10 microM, a concentration of the cardiac glycoside not significantly affecting 3H-glycogen levels per se. Interestingly, the previously demonstrated glycogenolytic action of vasoactive intestinal peptide (Magistretti et al., 1981, 1984) and of norepinephrine (Quach et al., 1978) is also antagonized by ouabain. These results demonstrate the existence of a metabolic action of adenosine, which is sensitive to ouabain and appears to be mediated by A2 receptors. The concentrations at which adenosine promotes glycogenolysis are of the same order of magnitude as those reached extracellularly by the nucleoside during neuronal depolarization (Pull and McIlwain, 1972). This set of observations therefore supports the notion that adenosine plays a modulatory role in the coupling between neuronal activity and energy metabolism in the CNS.