Our knowledge of inositol trisphosphate (IP₃) as a calcium mobilizing second messenger derives primarily from studies in non-excitable cells. In contrast, less is known regarding its functions in neurons, despite the important roles played by cytosolic Ca²⁺ ions in regulating electrical excitability and synaptic plasticity (Berridge, 1998). Most studies of neuronal IP₃ signalling have focused on the cerebellum and hippocampus, and surprisingly little is known about its functions in the cerebral cortex. In this study, we analysed neurons in the prefrontal cortex, where IP₃-evoked Ca²⁺ signals are likely to exert widespread influences on sensory, motor, limbic and memory systems.

We combined in vitro visualized whole-cell electrophysiological recordings with 2-photon, video-rate Ca²⁺ imaging (Nguyen et al. 2001). Layer V pyramidal neurons from mouse brain slices were filled with the Ca²⁺ indicator fura-2 (50 mM) and caged IP₃ (50 mM). Mice were deeply anaesthetized with halothane before decapitation, in compliance with UCI IACUC regulations. Ca²⁺ responses following photorelease of intracellular IP₃ by flashes of UV light could be categorized into the following groups: (1) non-responders (no Ca²⁺ release even with strong UV flashes, 31 %, 33/106 neurons); (2) slow and weak responders (17 %); (3) brisk responders (rapid and large Ca²⁺ increase with short UV flashes, 56 %). In all responding cells, the soma demonstrated the highest sensitivity to IP₃, while release in the dendrites required either much stronger flashes, or was absent altogether (Fig. 1A). Interestingly, IP₃-evoked Ca²⁺ signals in many non- and weakly responding cells could be ‘rescued’ by evoking a train of action potentials prior to uncaging IP₃. In contrast, paired UV flashes in responders resulted in suppression of the second Ca²⁺ response. Voltage-clamp studies revealed the activation of a slow outward current following photorelease of IP₃, and UV flashes during a train of action potentials evoked by injection of inward current resulted in suppression of spikes (Fig. 1B).

These data demonstrate bi-directional interactions between intracellular IP₃-evoked Ca²⁺ release and influx of extracellular Ca²⁺ through voltage-gated channels in cortical pyramidal neurons. Ca²⁺ influx potentiates IP₃-mediated signals via mechanisms involving both a co-agonist effect on the IP₃ receptor and enhanced filling of IP₃-sensitive stores. Conversely, cytosolic Ca²⁺ elevations evoked in the soma by liberation from IP₃-sensitive stores powerfully reduce neuronal excitability by activating Ca²⁺-dependent K⁺ conductances.

This work was supported by the NIH. G.E.S. is supported by an NIA training grant.

All procedures accord with current local guidelines