METABOTROPIC GLUTAMATE-RECEPTOR MGLUR5 IN ASTROCYTES - PHARMACOLOGICAL PROPERTIES AND AGONIST REGULATION

Metabotropic glutamate receptor (mGluR) agonists induce extensive phos phoinositide (PI) hydrolysis in astrocytes grown in a chemically defin ed medium with select growth factors, These astrocytes express mGluR5 transcripts, but none of the splice variants of mGluR1, thus permittin g the characterization of mGluR5 in a native CNS cell without interfer ence from mGluR1 activity. mGluR5 activation (I) was not associated wi th stimulation of cyclic AMP formation, (2) showed high sensitivity to the removal of extracellular versus intracellular Ca2+, (3) displayed high coupling efficiency relative to receptor density, and (4) induce d PI hydrolysis that was suppressed by phorbol esters with low potency . The rank order of agonist potency was similar to that observed in mG luR1 and mGluR5 transfected cells. The phenylglycine antagonists teste d were effective in blocking responses to 1-aminocyclopentane-1 S,3R-d icarboxylic acid, but not to glutamate, Prolonged exposure to agonists induced a two-phase desensitization of mGluR5 function, an initial ph ase (completed by 1 h and plateaus for another 3 h) and a late phase ( progressive decrease to similar to 30% of control levels by 24 h), Onl y the latter phase was associated with receptor down-regulation, Desen sitization of mGluR5 function did not involve receptor internalization or phosphorylation mediated by protein kinase C or A; it was purely h omologous, and reversible, Resensitization after short agonist treatme nt did not require prior receptor sequestration. Recovery after prolon ged agonist exposure required new protein synthesis, but the restorati on of function was more rapid than normalization of receptor protein l evels, indicating that regulation also involves other components of th e transduction system, The protracted desensitization of mGluR5 in ast rocytes suggests that the functions mediated by this receptor are main tained under a variety of conditions ranging from repetitive stimulati on to injury responses.