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

Abstract

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.