27th Annual Meeting of the Society for Neuroscience,
October 25-30, 1997, New Orleanse, La.
Intradendritic Ca2+ transients of hippocampal CA1 neurons
induced by local high frequency stimulation
T. Jäger, K.G. Reymann and T. Behnisch*
The importance of changes in the intracellular Ca2+ concentration ([Ca2+]i) for the induction of several forms of synaptic plasticity has been long established. However, knowledge about the kinetics of Ca2+ transients and the sources involved in response to different tetanizations is very limited. Thus, we used a high-speed confocal imaging system (Noran Odyssey XL) for acquisition of intracellular Ca2+ changes, induced by a set of different tetanizations (100 Hz tetanizations with several durations and stimulation strengths, theta-bursts) to each of 8 single cells. Neurons were filled with the Ca2+ fluorescent indicator Calcium Green-1 (2 mM). Weak tetanizations evoked small and short increases in [Ca2+]i which decayed rapidly, whereas strong tetanizations were responsible for a huge release of Ca2+, and showed a complex behavior, indicating the involvement of different Ca2+ sources. In comparison to a single 200 ms / 100 Hz tetanization, a triple 200 ms / 100 Hz tetanization evoked a 8-fold increase of the fluorescence maximum which also occured with tripple 400 ms and 1 s trains. A train duration of 200 ms, 400 ms and 1 s using triple pulse width caused a prolonged fluorescence signal which reached baseline level 2 s, 4 s and 6 s after tetanization, respectively. The area of induced fluorescent changes correlated with the depolarization area, which was recorded simultaneously. Blocking Na+ spikes with the lidocaine derivate QX-314 (30 mM; n=6) indirectly minimized the influx of Ca2+ into the neuron through voltage dependent calcium channels and unmasked Ca2+ signals from other sources. Thus we were able to observe transients, consisting of two peaks, where the latter was probably mediated through internal Ca2+ stores.
We conclude, that the increases in [Ca2+]i in dendrites depend on the size and time- course of the applied tetanization, and on the activated Ca2+ sources.
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 426).
