Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder. Familial AD (FAD) mutations in presenilins have been linked to calcium (Ca²⁺) signaling abnormalities. To explain these results, we previously proposed that presenilins function as endoplasmic reticulum (ER) passive Ca²⁺ leak channels. To directly investigate the role of presenilins in neuronal ER Ca²⁺ homeostasis, we here performed a series of Ca²⁺ imaging experiments with primary neuronal cultures from conditional presenilin double-knock-out mice (PS1^dTAG/dTAG , PS2 ^−/−) and from triple-transgenic AD mice (KI–PS1_M146V , Thy1–APP_KM670/671NL , Thy1–tau_P301L ). Obtained results provided additional support to the hypothesis that presenilins function as ER Ca²⁺ leak channels in neurons. Interestingly, we discovered that presenilins play a major role in ER Ca²⁺ leak function in hippocampal but not in striatal neurons. We further discovered that, in hippocampal neurons, loss of presenilin-mediated ER Ca²⁺ leak function was compensated by an increase in expression and function of ryanodine receptors (RyanRs). Long-term feeding of the RyanR inhibitor dantrolene to amyloid precursor protein–presenilin-1 mice (Thy1–APP_KM670/671NL , Thy1–PS1_L166P ) resulted in an increased amyloid load, loss of synaptic markers, and neuronal atrophy in hippocampal and cortical regions. These results indicate that disruption of ER Ca²⁺ leak function of presenilins may play an important role in AD pathogenesis.