THE EFFECT OF METHIONINE ON THE REGIONAL AND INTRACELLULAR DISPOSITION OF [ 3 H]-METHIONINE SULPHOXIMINE IN RAT BRAIN 1

Abstract

—The intracellular disposition of the convulsant agent, methionine sulphoximine (MSO), administered as methyl-labelled [ 3 H]MSO, was examined in rat brain. Intraperitoneal (i.p.) and intrathecal (i.th.) routes were compared. The effect of simultaneous administration of methionine on the uptake, the regional distribution and the intracellular disposition of [ 3 H]MSO was also assessed: (1) The peak uptake of i.p. [ 3 H]MSO was at 2 h and amounted to about 1 per cent of the dose; the peak uptake of i.th. [ 3 H]MSO was at 30 min post-injection and amounted to 40 per cent of the administered dose. The uptake was effectively reduced when methionine was simultaneously administered. (2) The regional distribution of [ 3 H]MSO as a function of time after injection revealed a rather uniform penetration of the entire brain by the drug. A maximum of 43 per cent of the tissue radioactivity was found in the cerebellum 2 h after i.p. injection, while 49 per cent accumulated in the extracortical portion of the brain 3·5 h after i.th. administration. Methionine did not affect the regional distribution of [ 3 H]MSO. (3) Differential centrifugation of samples of cortex and cerebellum revealed an association of [ 3 H]MSO with intracellular particulate fractions. Since closely similar proportions of MSO occurred in the crude mitochondrial and the microsomal fractions, these fractions were analysed further: (a) [ 3 H]MSO was bound to nerve endings sedimenting at the 1·0 m–1·2 m-sucrose interface; this binding was not abolished by prior increase of the endogenous cerebral methionine pool; and (b) [ 3 H]MSO was released by subjecting the nerve endings to osmotic shock. However, the striking finding was that [ 3 H]MSO could not be released from the nerve endings of the cerebellum from animals pre-treated with methionine. (4) An association of [ 3 H]MSO was observed with the membranes of the endoplasmic reticulum and specifically with its agranular component. (5)The results implicate the cerebellum as the primary target for MSO, in confirmation of the original observations of Lodin (1958).