Inositol trisphosphate receptor localization in brain: variable stoichiometry with protein kinase C

Abstract

Many neurotransmitters, hormones and growth factors act at membrane receptors to stimulate the phosphodiesteratic hydrolysis of phosphatidyl-inositol 4,5-bisphosphate generating the co-messengers inositol 1,4,5-trisphosphate (Ins(l,4,5)P₃) and diacylglycerol^1,2. Diacylglycerol stimulates protein kinase C³ while Ins(l,4,5)P₃ is postulated to activate specific receptors leading to release of intracellular calcium^4–9, probably from the endoplasmic reticulum10. In recent preliminary reports, Rubin and associates^11–13 detected ³²P-Ins(l,4,5)P₃ binding to liver and adrenal microsomes and to permeabilized neutrophils and liver cells. We now report the biochemical and autoradiographic demonstration in brain of high affinity, selective binding sites for ³H- and ³²P-labelled Ins(l,4,5)P₃ at levels 100–300 times higher than those observed in peripheral tissues. The potencies of various myo-inositol analogues at the Ins(l,4,5)P₃ binding site correspond to their potencies in releasing calcium from microsomes¹, supporting the physiological relevance of this receptor. Brain autoradiograms demonstrate discrete, heterogeneous localizations of Ins(l,4,5)P₃ receptors. In some regions localizations of Ins(l,4,5)P₃ receptors resemble those of protein kinase C¹⁴, while in others they differ markedly, suggesting a novel mechanism whereby the relative activity of the two limbs of the PI cycle can be differently regulated