Inositol 1,4,5-trisphosphate receptor localized to endoplasmic reticulum in cerebellar Purkinje neurons

INOSITOL 1,4,5-trisphosphate (InsP₃) mediates the effects of several neurotransmitters, hormones and growth factors by mobilizing Ca²⁺ from a vesicular, non-mitochondrial intracellular store¹. Many studies have indirectly suggested the endoplasmic reticulum (ER) to be the site of InsP₃ action^2-6, though some have implicated the plasma membrane^7,8 or a newly described smooth surfaced structure, termed the calciosome⁹. Using antibodies directed against a purified InsP₃-receptor glycoprotein¹⁰, of relative molecular mass 260,000, in electron microscope immunocytochemical studies of rat cerebellar Purkinje cells, we have now localized the InsP₃ receptor to ER, including portions of the rough endoplasmic reticulum, a population of smooth-membrane-bound organelles (smooth ER), a portion of subplas-malemmal cisternae and the nuclear membrane, but not to mitochondria or the cell membrane. These results suggest that in cerebellar Purkinje cells, InsP₃-induced intracellular calcium release is not the property of a single organelle, but is effected by specialized portions of both rough and smooth ER, and possibly by other smooth surfaced structures. The present findings are the first immunocytochemical demonstration of an InsP₃ receptor within a cell.