Neuronal cues regulate uptake in cultured astrocytes

Abstract

Astrocytes of the vertebrate central nervous system are postulated to nourish neurons4,~0,13, to electrically insulate cortical neuronal columns 15, to cue patterns of developmental migrations of neurons 14, and to regulate the ionic environment of brain 5, 17,~9. Despite persistent investigation, the function or functions of this phylogenetically ubiquitous cell type remain largely hypothetical. Neurons and their attendant glia are considered to be a functional metabolic unit. For this to be the case, at least two criteria must be met: (1) some mechanism must exist for communication of relevant information about the metabolic state of one cell-type to the other; and (2) either cell-type must be able to alter its metabolism in response to cues emitted by the other. Although the spectrum of neuronally emitted cues is large, ideally, such cues should reflect some aspect of neuronal activity. In the present study we have selected extracellular K + and 3 putative neurotransmitters as neuronal cues, since it is known that the cerebral levels of both reflect neuronal activity and that each also affects glial membrane potentiala, 18, oxygen uptake 1 and cyclic nucleotide levels 3,9,11. One manner in which neurons may affect glial metabolism is by inducing alterations in the glial uptake of key metabolites, such as glucose, possibly a rate-limiting step in cerebral glycolysis 16 and selected amino acids, the uptake of which by the glial cells may, in part, reflect transfer from the extracellular space of brain tissue 7,12. Recently, we developed a technique for the cultivation of a purified population of astrocytes, derived from neonatal rat forebrain 2 and in the present study we investigate the effect of neuronal cues (see above) on the astrocytic uptake of the glucose analogue 2-deoxy-D-glucose (2-DOG) and of L-methionine (MET), the precursor amino acid essential for the cerebral synthesis of the universal methyl donor molecule, S-adenosylL-methionine. Under the conditions of the experiment described in Fig. 1, the uptake of 2-DOG remained linear for at least 15 min. To determine the effects of extracellular K + on the uptake of 2-DOG, KC1 replaced NaCI on a molar basis. Plates of confluent cells were incubated with 0.25, 0.50, and 1.0 mM 2-DOG at the indicated concentrations of extra-