Calcium signaling components of oscillating invertebrate neurons in vitro

We have studied the Ca2+ dynamics of bursting–spiking neurons in the lobster stomatogastric ganglion. Neurons in this ganglion undergo spontaneous oscillations in membrane voltage with a period of 1–10 s in situ. We found that neurons isolated from the ganglion and filled with the fluorescent calcium indicator Fluo-4 show simultaneous changes of membrane potential and cytoplasmic Ca2+ concentration ([Ca2+]I). These Ca2+ signals are highly heterogeneous both in terms of amplitude and time constants. They showed variable spatial distributions with the soma exhibiting low and slow signals, and a region in the process with large and fast signals.Ca2+ transients in the processes are dependent on external Ca2+ and can be blocked by Co2+, but not other, more specific Ca2+ current blockers. Rather, nifedipine a known Ca2+ current blocker, affects the distribution of the Ca2+ signal, which suggests a specific localization of Ca2+ channels. Although the signal is not absolutely dependent on action potentials, it is greatly reduced when action potentials are blocked by tetrodotoxin. Termination of the signal depends only slightly on Ca2+ buffering mechanisms such as mitochondria, Ca2+/Na+ and Ca2+/H+ exchangers.We also demonstrate the presence of caffeine-sensitive internal stores in stomatogastric ganglion cells. The store distribution is different but overlaps with the voltage-dependent distribution. The maximal caffeine-activated Ca2+ signal is in the soma and it is smaller in the processes. Unlike the voltage-activated Ca2+ signal this signal is not blocked by Co2+. Nevertheless, the two types of signal interact during caffeine application. This unique spatial separation of two Ca2+ sources may have important functional implication. [Copyright &y& Elsevier]