Ca²⁺ signals regulate numerous cellular functions, and are generated by Ca²⁺ ions entering the cytosol from extracellular sources (e.g. entry through voltage gated channels), and by Ca²⁺ ions liberated from intracellular stores through inositol triphosphate (IP₃) receptors and/or ryanodine (RyR) receptors. Ca²⁺ ions subserve particularly complex signaling roles in neurons, regulating functions ranging from gene transcription to modulation of membrane excitability. Although the roles of extracellular Ca²⁺ entry are well characterized, the roles of intracellular Ca²⁺ liberation are less well understood. We therefore used a combination of techniques including whole cell electrophysiological recordings, photolysis of caged IP₃ and 2photon microscopy to image IP₃-evoked Ca²⁺ signals in pyramidal cortical neurons in mouse brain slices. We describe here both the physiological functioning of the IP₃-signaling pathway in neurons, and the involvement of disruptions of this pathway in the pathogenesis of Alzheimer’s disease (AD). Physiological Ca²⁺ responses to photoreleased IP₃ in wild-type mice varied greatly between different neurons; however, within IP₃-responsive neurons, the soma invariably showed the highest sensitivity, and Ca²⁺ signals increased nonlinearly with [IP₃]. IP₃-evoked Ca²⁺ release was potentiated by Ca²⁺ entry during action potentials and vice versa, indicating bidirectional facilitation between intra- and extracellular Ca²⁺ sources. In particular, IP₃-evoked Ca²⁺ signals strongly inhibited spike firing through activation of an outward hyperpolarizing membrane conductance. Thus, the IP₃/Ca²⁺ signaling pathway serves as a powerful and sustained modulator of excitability in cortical neurons and may mediate complex reciprocal interactions between electrical signals and chemical signals arising through metabotropic synaptic inputs (Stutzmann et al., 2003). The dark side of the IP₃/Ca²⁺ signaling pathway, however, is that perturbations outside its normal operating range can lead to pathological changes, including necrotic and apoptotic cell death. For example, disruptions in intracellular Ca²⁺ signaling have been proposed to underlie the pathophysiology of Alzheimer’s disease (AD), and it has recently been shown that AD-linked mutations in the presenilin 1 gene (PS1) enhance IP₃-mediated Ca²⁺ liberation in nonexcitable cells (Leissring et al., 2001). However, little is known of these actions in neurons, which are the principal locus of AD pathology. We therefore examined how PS1 mutations affect Ca²⁺ signals and their subsequent downstream effector functions in cortical neurons. We found that IP₃-evoked Ca²⁺ responses are more than threefold greater in PS1 knock-in mice relative to age-matched non transgenic controls, and electrical excitability was concomitantly reduced via enhanced Ca²⁺ activation of outward hyperpolarizing K⁺ conductances. Moreover, IP₃ receptor levels in cortical homogenates were not different between knock-in and control mice, suggesting that the exaggerated cytosolic Ca²⁺ signals likely result from increased store filling and not from increased flux through additional IP₃-gated channels. Notably, action potential-evoked Ca²⁺ signals were unchanged, indicating that PS1 mutations specifically disrupt intracellular Ca²⁺ liberation rather than impairing cytosolic Ca²⁺ buffering or clearance. A limitation of the PS1 knock-in model is that these mice do not go on to develop the plaques and tangles characteristic of AD. We have thus begun to investigate Ca²⁺ signaling in a novel triple transgenic model of AD, where mice expresses mutant PS1, APP and tau genes simultaneously, and display the histopathological markers of AD at later ages (Oddo et al., 2003). These studies in transgenic AD mouse models suggest that PS1 mutations are predominantly responsible for the IP₃mediated Ca²⁺ dysregulation in neurons, and that Ca²⁺ dysregulation precedes the histological and cognitive impairments observed in AD. The profound effects of these mutations on neuronal Ca²⁺ and electrical signaling patterns may contribute to the long-term pathophysiology of AD.