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Gq G Protein Coupled Receptor Mediated Potentiation of Astrocytic Glutamate Uptake

  • Author(s): Devaraju, Prakash
  • Advisor(s): Fiacco, Todd A.
  • et al.
Abstract

Astrocytic glutamate transporters, GLAST and GLT-1 (rodent analogs of EAAT1 and EAAT2, respectively) constitute the major pathway of glutamate uptake in the central nervous system. Uptake of synaptically released glutamate by astrocytes is essential for maintaining a healthy level of excitatory activity in the brain. It is also known that astrocytes in situ possess a myriad of G-protein coupled receptors (GPCRs) of the Gi, Gs, and Gq families. Stimulation of glutamatergic afferents not only leads to astrocytic glutamate uptake but also has been shown to stimulate group I (Gq type) metabotropic glutamate receptor (mGluR) mediated intracellular Ca2+ elevations in astrocytes both in vivo and in situ. Activation of mGluRs has been associated with short and long-term plastic changes of glutamate transport in cultured astroglia and in Purkinje neurons in cerebellar slices. This led us to hypothesize that astrocytic Gq GPCRs modulate glutamate transport. This hypothesis was tested in acute mouse hippocampal slices using two different approaches: (1) activation of astrocytic mGluRs in slices from wild-type mice using a tetanic high frequency stimulus (200Hz, 1s) applied to the Schaffer collaterals (SCs); (2) activation of a transgenic Gq GPCR (the MrgA1R) which is selectively expressed by astrocytes. Synaptically evoked glutamate transporter currents (STCs) were isolated from the total evoked astrocytic currents and analyzed for changes in amplitude, rise time, rise slope and decay time constant. High frequency stimulation (HFS) of SCs led to potentiation of the amplitude of the STCs without changes in kinetics. Similar potentiation was not observed in the presence of group I mGluRs or the PKC inhibitor, PKC 19-36, suggesting that HFS induced potentiation of glutamate uptake is group I mGluR - PKC dependent. Activation of the MrgA1Rs selectively expressed by astrocytes also potentiated the STC amplitude indicating the sufficiency of astrocytic Gq GPCR activation for potentiation of glutamate uptake. Surprisingly, the amplitude of the slow inward astrocytic current due to potassium uptake is also enhanced following activation of either the endogenous mGluRs or the astrocyte-specific MrgA1Rs. These findings collectively suggest that astrocytic Gq GPCR activation has a synergistic modulatory effect on the uptake of both glutamate and potassium.

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