Reply to Arduini et al.: Acetyl-l-carnitine and the brain: Epigenetics, energetics, and stress

Arduini et al. raise interesting issues related to mechanisms involving carnitine (1). The authors ask whether there is a free-carnitine deficiency in Flinders Sensitive Line rats (FSL) (2) and, more broadly, raise the question of whether the deficiency of acetyl-l-carnitine (LAC) occurs systemically or in the brain in FSL (1). We found that carnitine acetyltransferase (CrAT) mRNA levels in the ventral dentate gyrus (vDG) were not different between FSL and Flinders Resistant Line animals; thus, the source of the deficiency is likely to be systemic. We are measuring blood levels of LAC and carnitine in FSL as well as other animal models and in human subjects.

Arduini et al. (1) ask about LAC deficiency in other brain regions besides the vDG. This was not the goal of our study (2), which focused on the vDG because of its importance for depressive-like features of animal models (3). We know from previous and current work that all FSL rats respond to LAC. However, a point that Arduini et al. (1) may have missed is that it was an acute stress episode that triggered treatment resistance in a subset of FSL animals, which we show have certain gene-expression characteristics in the vDG (2).

Arduini et al. (1) suggest treating FSL rats with carnitine to elevate LAC levels. In elderly men, oral LAC significantly increased both plasma and CSF LAC concentration (4). Whatever the source, increased LAC appears to be beneficial for the brain (5).

A most surprising finding of our study is that LAC treatment reduced insulin, glucose, and triglyceride levels in FSL rats (2), implying some kind of insulin-resistant state that responds to LAC. Therefore, as Arduini et al. (1) suggest, glitazone treatment is warranted as an acknowledged way to treat insulin resistance in depression (6). Moreover, because type 2 diabetes apparently raises blood levels of LAC, at least in African American women (7), there may be a treatment resistance in type 2 diabetics, like that reported in our paper (2), resulting from acute stress, which prevents beneficial actions of increased LAC. It is important to determine if the finding of elevated LAC levels in diabetes holds in men.

Finally, regarding the epigenetic mechanism for mGlu2 regulation, Arduini et al. (1) question the need for a pathway leading from extracellular LAC to nuclear acetylation of H3K27 because there is a nuclear CrAT that could generate LAC within the cell from carnitine and catalyze H3K27 acetylation. We note that in cultured mouse ganglia neurons, LAC—and not carnitine—treatment increases lysine acetylation, an epigenetic mechanism shared by HDAC inhibitors to regulate mGlu2 transcription (8). LAC, given orally, to old rats significantly restored CrAT activity and counteracted mitochondrial structural decay in hippocampal neurons (9), indicating that LAC enters the cells. However, we note that there are other possible mechanisms of LAC action: namely, acetylation of cytoskeleton to enhance structural plasticity and facilitation of mitochondrial function (10), including fatty-acid oxidation for energy production. Clearly, there is much to be explored that will increase our understanding of LAC in brain plasticity and lead to more rapid treatment of major depression.