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Notes: Abstract: The accumulation of inositol polyphosphates in the cerebellum in response to agonists has not been demonstrated. Guinea pig cerebellar slices prelabeled with [3H]inositol showed the following increases in response to 1 mM serotonin: At 15 s, there was a peak in 3H label in the second messenger inositol 1,4,5-trisphosphate [Ins(l,4,5)P3], decreasing to a lower level in about 1 min. The level of 3H label in the putative second-messenger inositol 1,3,4,5-tetra-kisphosphate [Ins(l,3,4,5)P4] increased rapidly up to 60 s and increased slowly thereafter. The accumulation of 3H label in various inositol phosphate isomers at 10 min, when steady state was obtained, showed the following increases due to serotonin: inositol 1,3,4-trisphosphate [Ins(l,3,4)P3], eightfold; Ins(l,3,4,5)P4, 6.4-fold; Ins(l,4,5)P3, 75%; inositol 1,4-bisphosphate [Ins(1,4)P2, 0%; inositol 3,4-bisphosphate, 100%; inositol 1-phosphate/inositol 3-phosphate, 30%; and inositol 4-phosphate, 40%. [3H]Inositol 1,3-bisphosphate was not detected in controls, but it accounted for 7.2% of the total inositol bisphosphates formed in the serotonin-stimulated samples. The fact that serotonin did not increase the formation of Ins(1,4)P2 could be due to the fact that Ins(1,4)P2 is rapidly degraded or that Ins(l,4,5)P3 is metabolized primarily by Ins(1,4,5)P3-3’ kinase to form Ins(1,3,4,5)P4. In the presence of pargyline (10 nM), [3H]Ins(l,3,4,5)P4 and [3H]Ins(l,3,4)P3 levels were increased, even at 1 μM serotonin. Ketanserin (7 μM) completely inhibited the serotonin effect, indicating stimulation of serotonin2 receptors. Quisqualic acid (100 μM) also increased the levels of [3H]Ins(l,4,5)P3, [3H]Ins(l,3,4,5)P4, and [3H]Ins(l,3,4)P3, but the profile of these increases was different. The quisqualic acid-stimulated formation of inositol phosphate isomers was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that it was not due to the ionotropic properties of the quisqualate receptor. Similar results were obtained on stimulation of the labeled slices with glutamate, but the magnitudes were less. The data show that in the guinea pig cerebellum, stimulation of the serotonin2 and metabotropic quisqualic acid receptors leads to the initial formation of Ins(l,4,5)P3, but its subsequent metabolism varies, presumably owing to two kinds of receptors, localized on different cell types in the cerebellum with varying levels of inositol phosphate-metabolizing enzymes.

Notes: Abstract: We previously reported that lithium, in the presence of acetylcholine, increased accumulations of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in brain cortex slices from the guinea pig, rabbit, rat, and mouse. In the mouse and rat, the Li+-induced increases required supplementation of the medium with inositol. This probably relates to the following facts: (a) Brain cortices of the mouse and rat contain in vivo concentrations of inositol half of that of the guinea pig. (b) Incubated rat brain cortex slices are depleted of inositol by 80%. (c) The slices require 10 mM inositol supplementation to restore in vivo concentrations. We now show that in monkey brain cortex slices, therapeutic concentrations of Li+ increase accumulation of inositol 1,4,5-trisphosphate. The inositol 1,3,4,5-tetrakisphosphate level is not increased. Neither inositol nor an agonist is required. The same effects are seen whether inositol 1,4,5-trisphosphate is quantified by the [3H]inositol prelabeling technique or by mass assay, although mass includes a pool of inositol 1,4,5-trisphosphate that is metabolically inactive. Thus, in a therapeutically relevant model for humans, Li+ increases inositol 1,4,5-trisphosphate levels in brain cortex slices, as was previously seen in lower mammals at non-rate-limiting concentrations of inositol.