Differential distribution of TRP Ca²⁺ channel isoforms in mouse brain

MAMMALIAN homologues of the Drosophila TRP proteins, which are essential for light-activated, phosphatidyl-inositide (PI)-dependent Ca²⁺ conductance in Drosophila photoreceptors, were molecularly identified, to investigate receptor-activated Ca²⁺ influx in the mammalian nervous system. Two cloned mouse TRP homologues, TRP3 and TRP4, structurally related to the voltage- dependent Na⁺ channel, were expressed predominantly in the brain, where a sharp contrast in the distribution of the RNA transcripts for TRP isoforms was demonstrated by in situ hybridization analysis. TRP3 mRNA was concentrated in cerebellar Purkinje cells and sparsely localized in the cerebellar granule layer, pontine nuclei, and thalamus, whereas TRP4 mRNA was abundantly expressed in hippocampal CA1 pyramidal neurons, dentate gyrus granule cells, and cerebral cortical neurons, and in the septal nuclei and the mitral layer of olfactory bulb. The distinct special patterns of TRP isoforms implicate that neurons are highly heterogeneous in receptor- activated Ca²⁺ influx responsible for the second phase of PI-mediated rise in intracellular Ca²⁺.