We have found the heterogeneous distribution of the endogenous (basal) acetate in brain regions. It supports the idea of the local origin (at least partially) of acetate in the brain. The higher basal level of acetate in frontal cortex of wSS rats can be explained by the inborn specificities of acetaldehyde and acetate metabolism, in particular, higher activity of ALDH in the brain cortex structures when compared with wLS (Zimatkin, 2008
). The cause of alcohol-induced increase of acetate concentration in the brain can be its transport with the blood from the periphery, mainly from the liver, the major ethanol-oxidative organ, as well as the local oxidation in the brain itself (Zimatkin et al., 2006
We have found significantly lower level of acetyl-CoA in brain cortex of wSS, when compared with wLS rats. Acute ethanol administration increased the concentration of acetyl-CoA in brain cortex of wSS, but not of wLS rats and eliminated the comparative deficiency of acetyl-СoA in brain cortex of wSS rats. It indicates the inborn acetyl-CoA deficiency in the brain cortex of wSS rats and its compensation from ethanol-derived acetate. Acetate-derived acetyl-CoA is the main point of entrance of two-carbonic residue of ethanol into a basic metabolism. Probably wSS rat brains can utilize acetate for acetyl-CoA production better then wLS and it may be one of the reasons of their higher tolerance to hypnotic effect of ethanol.
Oxidation of pyruvate is the main source of acetyl-CoA in brain tissue. It is confirmed by high positive correlation between PDG activity and acetyl-CoA content in rat brain regions in our animals (r = 0.845; P = 0.002). wSS rats are characterized by the initially low activity of PDG when compared with wLS rats. It can be the reason of the initial comparative deficiency of acetyl-СCoA in brain cortex of wSS rats. The similar decreased activity of PDG has been found in the brain cortex of LAS when compared with HAS rats.
On the contrary, both wSS and LAS rats have higher initial activity of acetyl-CoA synthetase in the brain cortex. But that enzyme can be the alternative source of acetyl-CoA. In our experiments with acute alcohol administration, the % of increase of acetyl-CoA level in the brain regions positively correlated with the activity of acetyl-CoA synthetase, which catalyses the acetyl-CoA synthesis directly from acetate. Therefore, the higher initial activity of acetyl-CoA synthetase in wSS rat's brain cortex can be the reason of the higher increase of acetyl-CoA after an acute alcohol administration in these animals. Therefore, the acute alcohol administration eliminates the initial comparative deficiency of acetyl-CoA in brain cortex of wSS rats. It seems to be that the wSS rats are better adapted to utilize acetate instead of pyruvate for acetyl-CoA formation for general and energy metabolism in brain cortex, when compared with wLS rats.
The data obtained in our study demonstrate a reduced capability to utilize pyruvate and much lower level of acetyl-CoA in wSS rat brain cortex synaptosomes. This indicates a less efficient system of synthesis of acetyl-CoA from pyruvate, and, as a result, the lower initial AcH synthesis in wSS rats.
Our data demonstrate that the addition of acetate to the incubation medium of wLS brain cortex synaptosomes inhibits both the basal and calcium-stimulated release of AcH. On the contrary, in wSS rats, acetate significantly activates these processes. This indicates that in wSS rats acetyl-CoA, formed from acetate, can be a source of acetyl residues for biosynthesis of AcH under the conditions of insufficiency of the major, PDG, pathway.
It is known that acute ethanol administration even in low doses decreases glucose utilization in the brain. The absence of cognitive performance in those experiments is explained by the possible shift in the substrate of energy metabolism from glucose to acetate (Volkow et al., 2006
). Our data have confirmed that suggestion and explain possible neurochemical mechanisms of that phenomenon.
The different response of AcH synaptosomal release to calcium in the brain cortex in wSS and wLS rats in the presence of acetate can be mediated by adenosine, which is one of the best known modulators of cholinergic activity (Dunwiddie and Masino, 2001
). The stimulation of A1
receptors inhibits, while stimulating of A2
receptors activates cholinergic neurotransmission (Dunwiddie and Fredholm, 1997
). Effect of adenosine as an inhibiting neuromodulator is implemented through the A1
-receptors that are localized on the presynaptic membrane. These receptors are known to be excessively present on axonal projections of cholinergic neurons in rat brain cortex (Olah and Stiles, 1995
It has been demonstrated that acetate increases the level of adenosine in blood plasma and in other tissues of the body, including brain (Puig and Fox, 1984
). We have found that acetate significantly (2-fold) increases the release of adenosine from rat brain cortex synaptosomes (Kiselevski et al., 2003
). One of the reasons for that is the increase of adenosine production in the course of acetate activation in acetyl-CoA synthetase reaction (Yamamoto et al., 2005
). Therefore, it may be suggested that the addition of acetate into the incubation medium can increase the level of extrasynaptosomal adenosine, which by means of modulating effect in wLS rats upon A1
receptors in the synaptosomes, leads to inhibition of calcium-stimulated release of AcH. The inhibitory effect of adenosine upon the cholinergic neurotransmission has been well documented and is known to be a physiologic one (Bouron, 2001
; Materi et al., 2000
The sensitivity of mice with genetically high tolerance to hypnotic effect of ethanol (SS) to agonists of A1
receptors is much lower when compared with LS mice (Proctor et al., 1985
; Smolen and Smolen, 1991
). It can be explained by the lower density of A1
receptors in the brain of SS rats (Fredholm et al., 1985
). It also may be the reason of the lower initial inhibitory effect of acetate in wSS rats, but it has not been proved yet.