A characterization of muscarinic receptor‐mediated intracellular Ca2+ mobilization in cultured rat hippocampal neurones

Publisher Website
Google Scholar
Abstract
The properties of muscarinic receptor‐mediated Ca2+ mobilization were investigated in hippocampal cultures using fluorescent imaging techniques. Somatic responses to carbachol (1‐10 μm) were observed in 21 % of neurones under control conditions (5.4 mM K+, 1.8 mM Ca2+, 0.5‐1 μm tetrodotoxin). Smaller responses were observed in Ca2+‐free medium. In cells where responses to carbachol were absent under control conditions, responses were often observed following depolarization with high extracellular K+ (16.2‐25 mM). These responses decreased in magnitude with time after the depolarizing episode. Mobilization of Ca2+ from stores using caffeine (50 mM) exhibited similar properties. Carbachol responses were greatly facilitated in the presence of moderate elevations in extracellular K+ or Ca2+ levels (2‐ or 3‐fold, respectively). These conditions were usually, but not always, associated with a small increase in cytosolic Ca2+ levels (< 50 nM). Muscarinic responses in 10.8 mM K+ were inhibited by 80–95 % in the presence of the L‐type voltage‐gated Ca2+ channel antagonists nitrendipine (2‐5 μm) or nifedipine (10 μm). Depletion of intracellular Ca2+ stores with thapsigargin (2‐10 μm) blocked responses. Oscillatory Ca2+ mobilizing responses were observed in some cells. Their expression was facilitated by moderate cytosolic Ca2+ elevations and by increasing the duration of carbachol exposure. Ca2+ mobilizing responses were also observed in dendritic regions. These were smaller than somatic responses, but had faster decay kinetics. In conclusion, muscarinic receptor‐mediated Ca2+ mobilization in cultured hippocampal neurones shows a strong Ca2+ dependence. Moderate intracellular Ca2+ rises greatly facilitate muscarinic responses and uncover, in some cells, oscillatory Ca2+ mobilization. These effects appear to reflect the loading state of intracellular Ca2+ stores.