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

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