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Investigation of the role of intracellular Ca 2+ stores in generation of the muscarinic agonist‐induced slow afterdepolarization (sADP) in guinea‐pig olfactory cortical neurones in vitro
Author(s) -
Postlethwaite Michael,
Constanti Andrew,
Libri Vincenzo
Publication year - 2000
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1038/sj.bjp.0703236
Subject(s) - afterdepolarization , ryanodine receptor , muscarinic acetylcholine receptor , chemistry , cyclopiazonic acid , muscarinic agonist , biophysics , caffeine , thapsigargin , carbachol , medicine , endocrinology , depolarization , intracellular , electrophysiology , biology , biochemistry , repolarization , receptor
Intracellular recordings were made from guinea‐pig olfactory cortical brain slice neurones to assess the possible role of intracellular Ca 2+ stores in the generation of the slow post‐stimulus afterdepolarization (sADP) and its underlying tail current ( I ADP ), induced by muscarinic receptor activation. Caffeine or theophylline (0.5–3 m M ) reduced the amplitude of the I ADP (measured under ‘hybrid’ voltage clamp) induced in the presence of the muscarinic agonist oxotremorine‐M (OXO‐M, 10 μ M ) by up to 96%, without affecting membrane properties or muscarinic depolarization of these neurones. The L‐type Ca 2+ channel blocker nifedipine (1, 10 μ M ) also inhibited I ADP (by up to 46%), while ryanodine (10 μ M ) (a blocker of Ca 2+ release from internal stores) produced a small (∼10%) reduction in I ADP amplitude; however, neither 10 μ M dantrolene (another internal Ca 2+ release blocker) nor the intracellular Ca 2+ store re‐uptake inhibitors thapsigargin (3 μ M ) or cyclopiazonic acid (CPA, 15 μ M ) affected I ADP amplitude. IBMX (100 μ M ), a phosphodiesterase inhibitor, also had no effect on I ADP . Furthermore, inhibition of I ADP by caffeine was not reversed by co‐application of 100 μ M adenosine. Caffeine (3 m M ) or nifedipine (10 μ M ) reduced the duration of presumed Ca 2+ spikes revealed by intracellular Cs + loading. When applied in combination, nifedipine and caffeine effects were occlusive, rather than additive, suggesting a common site of action on L‐type calcium channels. We conclude that Ca 2+ ‐induced Ca 2+ release (CICR) from internal stores does not contribute significantly to muscarinic I ADP generation in olfactory cortical neurones. However caffeine and theophylline, which enhance CICR in other systems, blocked I ADP induction. We suggest that this action might involve a combination of L‐type voltage‐gated Ca 2+ channel blockade, and a direct inhibitory action on the putative I ADP K + conductance.British Journal of Pharmacology (2000) 129 , 1447–1457; doi: 10.1038/sj.bjp.0703236