Mechanisms underlying down-regulation of mGluR5 in astrocytes with ages

[Speaker] Masayoshi Tanaka:1
[Co-author] Eiji Shigetomi:1, Bijay Parajuli:1, Youichi Shinozaki:1, Yuri Hirayama:1, Kozo Saito:2, Junichi Nabekura:3,4, Schuichi Koizumi:1
1:Neuropharmacology, Interdisciplinary Graduate School of Medicine, University of Yamanashi, Japan, 2:Department of Neurology, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto, Japan, 3:Division of Homeostatic Development, National Institute for Physiological Sciences, Okazaki, Aichi, Japan, 4:Department of Physiological Sciences, The Graduate School for Advanced Study, Hayama, Kanagawa, Japan

Metabotropic glutamate receptor 5 (mGluR5) is highly expressed in immature astrocytes, but is gradually decreased with ages, and it is no longer observed in adult astrocytes in the cortex. Interestingly, expression of mGluR5 is up-regulated in several diseases such as neuropathic pain, even in adult astrocytes. However, mechanisms underlying such down- or up-regulation of mGluR5 expression remains totally unknown. Here, we sow its down-regulation mechanisms focusing on ATP (adenosine)-mediated signals, one of the main gliotransmitters in astrocytes.

Calcium Imaging: Calcium imaging was performed using Fluo-4 in the acute slices of cortex (300 micrometers) including S1 hind-limb area from P7-P28 mice.
quantitative PCR (qPCR): qPCR was performed using RNAs isolated from primary astrocytes from cerebral cortices of P0-P1 mice.

First, we investigated functional mGluR5 responses by Ca2+ imaging of acute S1 cortical slices. When stimulated with either (S)-3, 5-dihydroxyphenylglycine (DHPG), a mGluR5 agonist, at the ages from P7 to P28, the amplitude was highest at P7, which was gradually decreased with ages, and at P28 the responses were 27% of those at P7. On the contrary, the amplitude of ATP-evoked Ca2+ responses was not affected throughout ages (P7-P28). ATP is known to be degraded into adenosine by ecto-nucleotidases, and thus, we further tested adenosine-evoked responses. Unlike DHPG, the amplitude of the adenosine -evoked Ca2+responses in S1 astrocytes was increased with ages, and peaked at P21. There seems to be an inverse correlation between DHPG- and adenosine-evoked Ca2+ responses, suggesting that adenosine could negatively regulate expression of mGluR5 in astrocytes. To clarify this, we used cultured astrocytes, and tested whether adenosine could decreased the expression of mRNA for for Grm5 by qPCR. Adenosine acting mainly at A2a or A2breceptors, decreased expression of Grm5. Effect of adenosine-mediated signals on mGluR5 expression in astrocytes in vivo is now under investigation.
Adenosine negatively regulates mGluR5 expression in cortical astrocytes. An increase in the adenosine-mediated signals with ages in cortical astrocytes could be a mechanism that underlies a decrease in mGluR5 with ages.
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