Homeostatic and stimulus-induced coupling of the L-type Ca2+ channel to the ryanodine receptor in the hippocampal neuron in slices

Activity-dependent increase in cytosolic calcium (Ca²⁺]_i) is a prerequisite for many neuronal functions. We previously reported a strong direct depolarization, independent of glutamate receptors, effectively caused a release of Ca²⁺ from ryanodine-sensitive stores and induced the synthesis of endogenous cannabinoids (eCBs) and eCB-mediated responses. However, the cellular mechanism that initiated the depolarization-induced Ca²⁺-release is not completely understood. In the present study, we optically recorded Ca²⁺]_i from CA1 pyramidal neurons in the hippocampal slice and directly monitored miniature Ca²⁺ activities and depolarization-induced Ca²⁺ signals in order to determine the source(s) and properties of Ca²⁺]_i-dynamics that could lead to a release of Ca²⁺ from the ryanodine receptor. In the absence of depolarizing stimuli, spontaneously occurring miniature Ca²⁺ events were detected from a group of hippocampal neurons. This miniature Ca²⁺ event persisted in the nominal Ca²⁺-containing artificial cerebrospinal fluid (ACSF), and increased in frequency in response to the bath-application of caffeine and KCl. In contrast, nimodipine, the antagonist of the L-type Ca²⁺ channel (LTCC), a high concentration of ryanodine, the antagonist of the ryanodine receptor (RyR), and thapsigargin (TG) reduced the occurrence of the miniature Ca²⁺ events. When a brief puff-application of KCl was given locally to the soma of individual neurons in the presence of glutamate receptor antagonists, these neurons generated a transient increase in the Ca²⁺]_i in the dendrosomal region. This Ca²⁺]_i-transient was sensitive to nimodipine, TG, and ryanodine suggesting that the Ca²⁺]_i-transient was caused primarily by the LTCC-mediated Ca²⁺-influx and a release of Ca²⁺ from RyR. We observed little contribution from N- or P/Q-type Ca²⁺ channels. The coupling between LTCC and RyR was direct and independent of synaptic activities. Immunohistochemical study revealed a cellular localization of LTCC and RyR in a juxtaposed configuration in the proximal dendrites and soma. We conclude in the hippocampal CA1 neuron that: (1) homeostatic fluctuation of the resting membrane potential may be sufficient to initiate functional coupling between LTCC and RyR; (2) the juxtaposed localization of LTCC and RyR has anatomical advantage of synchronizing a Ca²⁺-release from RyR upon the opening of LTCC; and (3) the synchronized Ca²⁺-release from RyR occurs immediately after the activation of LTCC and determines the peak amplitude of depolarization-induced global increase in dendrosomal Ca²⁺]_i.