Potential role for nonesterified fatty acids in β-adrenoceptor-induced increases in brain tryptophan

Evidence for fatty acids mediating CL 316,243-induced reductions in blood glucose in mice

CL 316,243, a β3-adrenergic agonist, was developed as an antiobesity and diabetes drug and causes rapid decreases in blood glucose levels in mice. The mechanisms mediating this effect have not been fully elucidated; thus, the purpose of the current study was to examine the role of fatty acids and interleukin-6, reputed mediators of insulin secretion, in this process. To address this question, we used physiological and pharmacological approaches in combination with knockout mouse models. CL 316,243 treatment in male C57BL6 mice increased plasma fatty acids, glycerol, interleukin-6, and insulin and reduced blood glucose concentrations 2 h following injections. The ability of CL 316,243 to increase insulin and fatty acids and reduce glucose was preserved in interleukin-6-deficient mice. CL 316,243-induced drops in blood glucose occurred in parallel with increases in circulating fatty acids but prior to increases in plasma interleukin-6. CL 316,243-mediated increases in plasma insulin levels and reductions in blood glucose were attenuated when mice were pretreated with the lipase inhibitor nicotinic acid or in whole body adipose tissue triglyceride lipase knockout mice. Collectively, our findings demonstrate an important role for fatty acids in mediating the effects of CL 316,243 in mice. Not only do our results provide new insight into the mechanisms of action of CL 316,243, but they also hint at an unappreciated aspect of adipose tissue -pancreas cross-talk.

Alterations in kynurenine precursor and product levels in schizophrenia and bipolar disorder

Increased concentrations of kynurenine pathway metabolites have been reported by several groups for disorders involving psychosis, including schizophrenia and bipolar disorder. To identify components of the pathway that may be relevant as biomarkers or may underlie the etiology of psychosis, it is essential to characterize the extent of kynurenine pathway activation and to investigate known regulators of one of the key kynurenine-producing enzymes, tryptophan 2,3-dioxygenase (TDO2), previously shown in this laboratory to be increased commensurate with kynurenine in postmortem anterior cingulate brain tissue from individuals with schizophrenia. Using this same anterior cingulate sample set from individuals with schizophrenia, bipolar disorder, depression and controls (N=12-14 per group), we measured the precursor of kynurenine and two downstream products. The precursor, tryptophan, was significantly increased only in the schizophrenia group (1.54-fold the mean control value, p=0.02), and through substrate-induced activation, may be one cause of the increased kynurenine and kynurenine metabolites. This finding for tryptophan differs from some, but not all, previous reports and methodological reasons for the discrepancies are discussed. A product of kynurenine metabolism, 3-OH-anthranilic acid was also significantly increased only in the schizophrenia group (1.68-fold the mean control value, p=0.03). 3-OH-anthranilic acid is a reactive species with cytotoxic properties, although the threshold for such effects is not known for neurons. Analysis of major pre- and post-mortem variables showed that none were confounding for these between-group experimental comparisons. Nicotinamide, a pathway end product, did not differ between groups but was associated with cause of death (suicide) within the bipolar group (p=0.03).

Exercise, serum free tryptophan, and central fatigue

Brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis and release increase during running. This increase in 5HT function may promote central fatigue and contribute to suboptimal physical performance. The rise in brain TRP is reputed to result from exercise-induced elevations in serum nonesterified fatty acid (NEFA) concentrations, which dissociate TRP from albumin in blood and increase the serum free TRP pool. But, as discussed in this article, ample evidence exists that the serum free TRP pool does not control brain TRP uptake. The clearest data are dietary, but pharmacologic data in exercising rats also support this conclusion. Changes in the serum levels of amino acids that compete with TRP for brain uptake appear also not to explain the rise in brain TRP. The mechanism is therefore not presently known. The link between the rise in brain TRP and 5HT synthesis/release is not simple: a rise in brain TRP stimulates 5HT synthesis/release in actively firing neurons. Hence, during exercise, only 5HT neurons that are firing should increase 5HT production/release when brain TRP rises. It is not known which 5HT neurons fire during exercise; the 5HT neurons that respond to exercise-induced increases in brain TRP are therefore not known. Hence, it is not possible to conclude which 5HT neurons contribute to the generation of central fatigue. Because some 5HT neurons control specific functions important to physical performance (e.g., respiration), the current understanding of 5HT neuronal function in central fatigue might benefit from the study of specific 5HT pathways during exercise.

Mechanisms and Significance of the Increased Brain Uptake of Tryptophan

Changes in brain tryptophan concentrations may affect the synthesis of brain serotonin (5-hydroxytryptamine, 5-HT). Concentrations of tryptophan are regulated more than those of any other amino acid. Such stimuli as acute stress, carbohydrate ingestion, and treatment with various drugs increase the brain content of tryptophan. Treatment of rats and mice with interleukin-1 (IL-1), interleukin-6 (IL-6), lipopolysaccharide (LPS), and beta-adrenoceptor agonists, as well as a variety of stressors, such as footshock and restraint, all increase brain concentrations of tryptophan. The peak effect following both acute stress and beta-adrenoceptor agonist administration occurs within 30-60 min, whereas the peak effect following LPS and the cytokines occurs much later at around 4-8 h. Experiments using the ganglionic blocker chlorisondamine, and beta-adrenoceptor antagonists suggest that the sympathetic nervous system plays an important role in the modulation of brain tryptophan concentrations. The mechanisms involved in the increases observed in brain tryptophan are discussed, as well as their possible biological significance.