Darling RA, Zhao H, Kinch D, Li A-J, Simasko SM, Ritter S. Mercaptoacetate and fatty acids exert direct and antagonistic effects on nodose neurons via GPR40 fatty acid receptors. Am J Physiol Regul Integr Comp Physiol 307: R35-R43, 2014. First published April 23, 2014 doi:10.1152 doi:10. /ajpregu.00536.2013 is a drug known to block mitochondrial oxidation of medium-and long-chain fatty acids (FAs) and to stimulate feeding. Because MA-induced feeding is vagally dependent, it has been assumed that the feeding response is mediated by MA's antimetabolic action at a peripheral, vagally innervated site. However, MA's site of action has not yet been identified. Therefore, we used fluorescent calcium measurements in isolated neurons from rat nodose ganglia to determine whether MA has direct effects on vagal sensory neurons. We found that MA alone did not alter cytosolic calcium concentrations in nodose neurons. However, MA (60 M to 6 mM) significantly decreased calcium responses to both linoleic acid (LA; 10 M) and caprylic acid (C8; 10 M) in all neurons responsive to LA and C8. GW9508 (40 M), an agonist of the FA receptor, G proteincoupled receptor 40 (GPR40), also increased calcium levels almost exclusively in FA-responsive neurons. MA significantly inhibited this response to GW9508. MA did not inhibit calcium responses to serotonin, high K ϩ , or capsaicin, which do not utilize GPRs, or to CCK, which acts on a different GPR. GPR40 was detected in nodose ganglia by RT-PCR. Results suggest that FAs directly activate vagal sensory neurons via GPR40 and that MA antagonizes this effect. Thus, we propose that MA's nonmetabolic actions on GPR40 membrane receptors, expressed by multiple peripheral tissues in addition to the vagus nerve, may contribute to or mediate MA-induced stimulation of feeding. calcium imaging; nodose ganglion; G-protein coupled receptor 40; fatty acids; -mercaptoacetate AGENTS THAT REDUCE METABOLISM of fatty acids (FA), a major metabolic fuel for peripheral tissues, induce robust feeding responses (12, 64). These agents include -mercaptoacetate (MA), which antagonizes fat oxidation by blocking mitochondrial acyl-CoA-dehydrogenase (2, 3), and methyl-palmoxirate, which inhibits fat oxidation by antagonizing the mitochondrial long-chain fatty acid transporter, carnitine palmitoyltransferase 1 (CPT1). Etomoxir, which also blocks CPT1 (72), has been shown to stimulate feeding in humans (33), as well as in rats (23). Moreover, MA-induced feeding is antagonized by infusion of nutrients, including lipids, glucose, and fructose, further suggesting a metabolic mechanism of action (67). These and other observations have led to the concept of a "lipoprivic" control of food intake (41, 43, 62), which has been widely accepted as an important control of ingestive behavior.Feeding elicited by MA is dependent on vagal sensory neurons (5, 62, 63) and on higher-order neurons comprising a visceral afferent pathway in the brain that includes the nucleus of the solitary tract (63), lateral parabrachial nucleus (9), and ...