CHRONIC ACTIVATION OF MUSCARINIC AND METABOTROPIC GLUTAMATE RECEPTORSDOWN-REGULATES TYPE-I INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR EXPRESSION IN CEREBELLAR GRANULE CELLS

The ability of receptors coupled to phosphoinositide turnover to evoke accumulation of inositol 1,4,5-trisphosphate (InsP(3)) over extended incubation periods, and consequently to affect the level of InsP(3) re ceptor expression, was studied in cultured cerebellar granule cells. T he cholinergic agonist carbachol (CCh; 1 mM) evoked a biphasic accumul ation of InsP(3), a rapid three- to fourfold peak increase over contro l levels at similar to 10 s, decreasing within 1 min to a long-lasting plateau elevation. Using an antibody against the type I InsP(3) recep tor, it was demonstrated that > 50% down-regulation of type I InsP(3) receptor expression in cerebellar granule cells occurred within 1 h of incubation with 1 mM CCh. Over 24 h, 1 mM CCh caused an similar to 85 % decrease in type I InsP(3) receptor levels, and significant decrease s in immunoreactivity were evident at much lower concentrations of CCh . Direct assessment of total InsP(3) receptor expression using a radio ligand binding method also detected down-regulation, but to an apparen tly lesser extent. 1-Aminocyclopentane-1 S,3R-dicarboxylic acid (200 m u M), an agonist of metabotropic glutamate receptors, evoked a marked decrease in type I InsP(3) receptors after 24 h of incubation. These f indings demonstrate that a functional consequence of maintained InsP(3 ) production in cerebellar granule cells is the down-regulation of Ins P(3) receptor expression and that this down-regulation may be a common mechanism of action of phosphoinositide-linked receptors during prolo nged stimulation.