Increase of in vitro brain [3H]muscimol binding after L-homocysteine administration to rat

S-adenosyl-L-homocysteine and 5'-methylthioadenosine inhibit binding of [3H]flunitrazepam to rat brain membranes

The effect of inhibitors of transmethylation reactions, S-adenosylhomocysteine and 5'-methylthioadenosine, on [3H]flunitrazepam-specific binding to the rat brain membranes has been investigated. Both S-adenosylhomocysteine and 5'-methylthioadenosine are able to inhibit binding with Ki values of 7.9 microM and 15.8 microM respectively. These compounds therefore may be candidate endogenous benzodiazepine-receptor ligands. In light of these and other data possible correlations between phospholipid methylation and gamma-aminobutyric acid (GABA) receptor function are discussed.

S-adenosyl-l-methionine inhibits phosphoinositide metabolism in the rat brain synaptosomal suspensions

S-adenosyl-l-methionine (AdoMet) has been reported to affect events linked to noradrenergic neurotransmission. In the present work, we studied the effect of AdoMet on norepinephrine (NE)-stimulated inositol phosphate production in 3H-inositol-labelled crude synaptosomal suspensions of rat brain. AdoMet (50-1000 microM) decreased both the synthesis of labelled polyphosphoinositide (30-50%) and the release of inositol mono- and bisphosphate (40-50%). The AdoMet effect was not dependent on NE concentration (10-1000 microM), suggesting that the inhibition of inositol phosphate release was not the result of a modification of the norepinephrine binding to its receptor sites. S-adenosyl-L-homocysteine (AdoHcy) (1 mM) an inhibitor of methyltransferase activities, partially inhibited (70%) the AdoMet (0.1 mM) effect, indicating that the methylation processes cannot explain all the effects observed. We conclude that, in addition to previously reported effects of AdoMet on NE transport, AdoMet may reduce NE-linked intracellular signalling.

Increase of GABA-stimulated diazepam binding after lipid methylation in membrane preparations from rat brain

Incubation of membrane preparations from rat brain with S-adenosyl-L-methionine resulted in methylation of the lipid fraction. Neither [3H]diazepam nor [3H]muscimol (a gamma-aminobutyric acid (GABA) agonist) binding was affected by this incubation. In contrast, when [3H]diazepam binding was stimulated by GABA, the methylation caused both a decrease of the minimal GABA concentration needed to produce its effect (10(-9) M instead of 10(-7) M) and an enhancement (from 36 to 66% over basal) of the GABA-stimulated [3H]diazepam binding.