Aluminum fluoride reveals a phosphoinositide system within the suprachiasmatic region of rat hypothalamus

Abstract

The phosphoinositide (PI) transduction system has proven to be of major importance in several regions of mammalian brain. In this report, we examined in rats whether a PI system is present in the hypothalamic suprachiasmatic nuclei (SCN), the site of a biological clock that generate circadian rhythms. Autoradiographic localization of phorbol ester binding revealed moderate levels of protein kinase C, a component of the PI system, in the SCN. Hypothalamic explants containing SCN showed substantial incorporation of [ 3H]myoinositol into lipids. A AlF 4 −, a non-specific activator of G proteins, produced a dose-dependent increase in inositol monophosphate (IP 1) levels in the explants in calcium-free medium, with a maximum increase of 216% of control at 50 mM NaF. Medium containing 1.8 mM calcium stimulated a similar increase in IP 1 levels, but the stimulatory effects of AlF 4 − and calcium were not additive, so that the effect of AlF 4 − was obscured in medium containing calcium. AlF 4 − stimulated accumulation of IP 1, as well as inositol bis-, and triphosphate, over a 40-min time course in the presence and absence of lithium (10 mM LiCl). Lithium, a known inhibitor of phosphatases in the inositol phosphate recycling pathway, raised levels of all 3 inositol phosphates in SCN explants both at baseline (without AlF 4 −) and after 30 minAlF 4 − stimulation. The results show the existence of a lithium-sensitive PI system within the suprachiasmatic region of the rat hypothalamus.