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Differentially Distributed IP₃ Receptors and Ca²⁺ Signaling in Rod Bipolar Cells

From the Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas.

PURPOSE. Inositol (1,4,5)-trisphosphate receptors (IP₃Rs) contribute substantially to cytosolic free calcium ion (Ca²⁺) concentration transients and thereby modulate neuronal function. The present study was undertaken to determine the contribution of IP₃Rs to the function of rod bipolar cells in the retina.

METHODS. Immunoreactivity for IP₃Rs in rod bipolar cells from mouse retinas was detected by immunocytochemical methods. Intracellular Ca²⁺ concentrations were optically recorded in acutely isolated rod bipolar cells, and biophysical properties of IP₃Rs were analyzed with single channel electrophysiology.

RESULTS. The distribution of IP₃R isoforms was correlated with cytosolic Ca²⁺ transients induced by activation of group I metabotropic glutamate receptors (mGluRs) and with biophysical properties of differentially expressed IP₃Rs.

CONCLUSIONS. The differential distribution of IP₃Rs is used by rod bipolar cells to convey Ca²⁺ signals that are distinct in their duration, amplitude, and kinetics at the subcellular level, and that serve the functions of individual subcellular compartments. IP₃R-mediated Ca²⁺ signaling indicates a potential mechanism for the adaptation of the ON-pathway of vision and for coincidence and threshold detection in retinal neurons.

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