Agonist-evoked Ca2+ mobilization from stores expressing inositol 1,4,5-trisphosphate receptors and ryanodine receptors in cerebellar granule neurones.
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 control), and, similarly, a much reduced caffeine response was detectable after prior stimulation with CCh (to 27 +/- 6% of the control). CCh-evoked [Ca2+]i responses were inhibited by preincubation with thapsigargin (10 microM), 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ; 25 microM), ryanodine (10 microM), or dantrolene (25 microM). BHQ pretreatment was 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-dicarboxylic acid (ACPD; 200 microM) evoked a much diminished increase in [Ca2+]i in granule cells pretreated with CCh for 24 h compared with vehicle-treated control cells (CCh, 23 +/- 14%; ACPD, 27 +/- 1% of respective control values). In contrast, a 24-h CCh pretreatment decreased the subsequent inositol 1,4,5-trisphosphate (InsP3) response to CCh to a much greater extent compared with responses evoked by metabotropic glutamate receptor (mGluR) agonists; this suggests that the former effect on Ca2+ mobilization represents a heterologous desensitization of 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 receptors, as well as InsP3 receptors, appear to be crucial to the mAChR-mediated [Ca2+]i response in granule cells. As BHQ apparently differentiates between the CCh- and NMDA-evoked responses, it is possible that the directly InsP3-sensitive pool is physically different from the ryanodine receptor pool. Also, activation of InsP3 receptors may not contribute significantly to NMDA-evoked elevation of [Ca2+]i in cerebellar granule cells. A model for the topographic organization of cerebellar granule cell Ca2+ stores is proposed.
Simpson
P B
PB
Department of Cell Physiology, University of Leicester, Leicester, England.
Nahorski
S R
SR
Challiss
R A
RA
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects*; D002135:Calcium-Binding Proteins / Q000187:drug effects; D002217:Carbachol / Q000494:pharmacology; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D002531:Cerebellum / Q000166:cytology*; D003515:Cycloleucine / Q000031:analogs & derivatives / Q000494:pharmacology; D015536:Down-Regulation / Q000187:drug effects; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D005260:Female; D015544:Inositol 1,4,5-Trisphosphate / Q000819:agonists; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008297:Male; D009124:Muscle Proteins / Q000187:drug effects*; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009498:Neurotoxins / Q000494:pharmacology; D018722:Nicotinic Agonists / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists*; D019837:Ryanodine Receptor Calcium Release Channel; D015398:Signal Transduction / Q000187:drug effects
D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D000697:Central Nervous System Stimulants; D018690:Excitatory Amino Acid Agonists; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009124:Muscle Proteins; D009498:Neurotoxins; D018722:Nicotinic Agonists; D018160:Receptors, Cytoplasmic and Nuclear; D019837:Ryanodine Receptor Calcium Release Channel; D003515:Cycloleucine; C036361:1-amino-1,3-dicarboxycyclopentane; D002110:Caffeine; D016202:N-Methylaspartate; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D002118:Calcium