Receptor subtype involved and mechanism of norepinephrine‐induced stimulation of glutamate uptake into primary cultures of rat brain astrocytes

Glutamate uptake into rat brain astrocytes is potently stimulated by addition of norepinephrine (NE). This effect is mediated by α 1 ‐adrenergic receptors expressed by these cells (Hansson and Rönnbäck: Life Sci 44:27, 1989; Brain Res 548:215, 1991). The present study was undertaken in order to identify the adrenergic receptor subtype involved, and to determine the sequence of events following receptor activation. NE increased glutamate uptake rates in a dose‐ and time‐dependent manner (EC 50 = 6 μM). Both, the selective α 1 ‐receptor antagonist prazosine (IC 50 = 2.5 μM) and the α 1b ‐adrenergic receptor subtype specific alkylating agent chlorethylclonidine (CEC, 100 μM) prevented NE (100 μM) evoked stimulation of glutamate uptake. Furthermore, omission of Ca 2+ from the extracellular medium had no significant influence on NE‐induced increase in glutamate uptake, indicating that the stimulatory effect is mediated by α 1b ‐adrenergic receptors. Treatment of cells with pertussis toxin (PTX) for 24 h or with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) for 30‐45 min prior to NE addition abolished the NE‐mediated effect on glutamate uptake. Addition of TPA alone resulted in a rapid increase of glutamate uptake, which declined to control levels when TPA was applied 30 min prior to uptake initiation by glutamate. The increase in glutamate uptake elicited by TPA and NE added at the same time showed no additivity of the stimulatory effect resulting from treatment with each agent alone. Treatment of cell cultures with 1 mM Ba 2+ , which has been shown to block outwardly directed K + ‐currents in astrocytes (Barres et al.: Glia 1:10, 1988), influenced glutamate transport principally in the same manner as found for TPA and NE, respectively. Again no additivity of Ba 2+ and NE effects on glutamate uptake was observed. Finally, NE; TPA; and Ba 2+ ‐stimulated glutamate uptake exhibited nearly identical time‐response curves.