Stimulation-dependent calcium binding sites in the guinea pig hippocampal slice: an electrophysiological and electron microscopic study.

Transverse slices of the hippocampus of guinea pigs were prepared in order to investigate Ca2+ binding sites in CA1. Electrical stimulation (Schaffer collaterals and stratum oriens) combined with different aminopyridine compounds (AP) were used for neuronal activation. With histochemical methods Ca2+ binding sites were identified and localized at the electron microscopic level as electron dense deposits of granular or elongated shape. After electrical stimulation, electron dense deposits of 30-50 nm diameter were spread at low density over all layers of CA1. Electrical stimulation combined with application of aminopyridine compounds led to electron dense deposits of 60-400 nm diameter, mainly restricted to the activated input layers. Deposits were predominantly found at presynaptic sides, with few at dendrites and glial cells. Application of aminopyridine alone led to very few deposits, spread over the total CA1 area. The results indicate that aminopyridines, if combined with electrical stimulation, display a strong presynaptic action, which results in a remarkable Ca2+-translocation at the preterminal and terminal level. On the dendritic side aminopyridines in the concentrations used for the study weakly activate Ca2+ movements.