Functional properties of ryanodine receptors in hippocampal neurons change during early differentiation in culture.

6-((4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionyl)amino)hexanoic acid ryanodine (BODIPY-ryanodine) binding and Ca(2+) imaging were used to study the properties of ryanodine receptors (RyRs) and cytoplasmic Ca(2+) (Ca) changes in neurons cultured from the embryonic rat hippocampus during the earliest stages of differentiation. Baseline Ca levels declined from 164 +/- 5 (SD) nM at early stages to 70 +/- 4 nM in differentiated neurons. Fluorescent BODIPY-ryanodine binding signals identified activated RyRs in somata, which were eliminated by removal of external Ca(2+) or by blockage of Ca(2+) entry through L-type but not N-type Ca(2+) channels. The GABA synthesis inhibitor 3-mercaptopropionic acid completely abolished ryanodine binding. Caffeine or K(+)-depolarization inhibited the activity of RyRs at very early stages of differentiation but had stimulatory effects at later stages after a network of processes had formed. BayK-8644 stimulated RyRs throughout all regions of all differentiating cells. The results suggest that in differentiating embryonic hippocampal neurons the activity of RyRs is maintained via Ca(2+) entering through L-type Ca(2+) channels. The mode of activation of L-type voltage-gated Ca(2+) channels with either membrane depolarization or specific pharmacological agents affects the coupled activity of RyRs differently as neurons differentiate processes and networks.