AGONIST-EVOKED CA2-TRISPHOSPHATE RECEPTORS AND RYANODINE RECEPTORS INCEREBELLAR GRANULE NEURONS( MOBILIZATION FROM STORES EXPRESSING INOSITOL 1,4,5)

The mechanisms involved in Ca2+ mobilization evoked by the muscarinic cholinoceptor (mAChR) agonist carbachol (CCh) and N-methyl-D-aspartate (NMDA) in cerebellar granule cells have been investigated. An initial challenge with caffeine greatly reduced the subsequent intracellular Ca2+ concentration ([Ca2+](i)) response to CCh (to 45 +/- 19% of the c ontrol), and, similarly, a much reduced caffeine response was detectab le after prior stimulation with CCh (to 27 +/- 6% of the control). CCh -evoked [Ca2+](i) responses were inhibited by preincubation with thaps igargin (10 mu M), 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ; 25 m u M), ryanodine (10 mu M), or dantrolene (25 mu M). BHQ pretreatment w as found to have no effect on the sustained phase of the NMDA-evoked [ Ca2+](i) response, Both CCh (1 mM) and 1-aminocyclopentane-1S,3R-dicar boxylic acid (ACPD; 200 mu M) evoked a much diminished increase in [Ca 2+](i) in granule cells. pretreated with CCh for 24 h compared with ve hicle-treated control cells (CCh, 23 +/- 14%; ACPD, 27 +/- 1% of respe ctive control values), in contrast, a 24-h CCh pretreatment decreased the subsequent inositol 1,4,5-trisphosphate (InsP(3)) response to CCh to a much greater extent compared with responses evoked by metabotropi c glutamate receptor (mGluR) agonists; this suggests that the former e ffect on Ca2+ mobilization represents a heterologous desensitization o f the mGluR-mediated response distal to the pathway second messenger, Furthermore, [Ca2+](i) responses to caffeine and NMDA were unaffected by a 24-h pretreatment with CCh. This study indicates that ryanodine r eceptors, as well as InsP(3) receptors, appear to be crucial to the mA ChR-mediated [Ca2+](i) response in granule cells. As BHQ apparently di fferentiates between the CCh- and NMDA-evoked responses, it is possibl e that ti-ie-directly InsP(3)-sensitive pool is physically different f rom the ryanodine receptor pool, Also, activation of InsP(3) receptors may not contribute significantly to NMDA-evoked elevation of [Ca2+](i ) in cerebellar granule cells, A model for the topographic organizatio n of cerebellar granule cell Ca2+ stores is proposed.