Ractopamine, a Livestock Feed Additive, Is a Full Agonist at Trace Amine–Associated Receptor 1

Abstract

Ractopamine (RAC) is fed to an estimated 80% of all beef, swine, and turkey raised in the United States. It promotes muscle mass development, limits fat deposition, and reduces feed consumption. However, it has several undesirable behavioral side effects in livestock, especially pigs, including restlessness, agitation, excessive oral-facial movements, and aggressive behavior. Numerous in vitro and in vivo studies suggest RAC's physiological actions begin with its stimulation of b 1 -and b 2 -adrenergic receptor-mediated signaling in skeletal muscle and adipose tissue; however, the molecular pharmacology of RAC's psychoactive effects is poorly understood. Using human cystic fibrosis transmembrane conductance regulator (hCFTR) chloride channels as a sensor for intracellular cAMP, we found that RAC and p-tyramine (TYR) produced concentrationdependent increases in chloride conductance in oocytes coexpressing hCFTR and mouse trace amine-associated receptor 1 (mTAAR1), which was completely reversed by the trace amine-Oocytes coexpressing hCFTR and the human b 2 -adrenergic receptor showed no response to RAC or TYR. These studies demonstrate that, contrary to expectations, RAC is not an agonist of the human b 2 -adrenergic receptor but rather a full agonist for mTAAR1. Since TAAR1-mediated signaling can influence cardiovascular tone and behavior in several animal models, our finding that RAC is a full mTAAR1 agonist supports the idea that this novel mechanism of action influences the physiology and behavior of pigs and other species. These findings should stimulate future studies to characterize the pharmacological, physiological, and behavioral actions of RAC in humans and other species exposed to this drug.

Trace Amines and Their Receptors

“…As a G s -coupled receptor, TAAR1 promotes cAMP production via stimulation of adenylyl cyclase (Borowsky et al, 2001;Bunzow et al, 2001;. This has been confirmed after expression of TAAR1 in a variety of cell types and with various approaches to analyze cAMP concentrations used (Reese et al, 2007;Wainscott et al, 2007;Barak et al, 2008;Hu et al, 2009;Espinoza et al, 2011;Revel et al, 2011;Liu et al, 2014). In fact, cAMP assays are now a central component of TAAR1 ligand screening programs (Bradaia et al, 2009;Revel et al, 2011Revel et al, , 2012aRevel et al, , 2013Stalder et al, 2011;Galley et al, 2012Galley et al, , 2015.…”

“…It should be stressed, however, that such effects often show pronounced species dependence, and this is perhaps most notable with the endogenous hallucinogen DMT and lysergic acid diethylamide, both of which show limited, if any, activity at the human isoform (Table 7). In addition to psychotropic agents, a variety of other synthetic compounds have been reported to exhibit agonistic activity at TAAR1, including apomorphine (Sukhanov et al, 2014), ractopamine (Liu et al, 2014), and the imidazoline ligands clonidine, guanabenz, and idazoxan (Hu et al, 2009) (Fig. 2; Table 7).…”

Trace Amine-Associated Receptor 1 Regulation of Methamphetamine Intake and Related Traits

“…All injections were given intraperitoneally at a volume of 10 ml/kg. N-(3-Ethoxy-phenyl)-4-pyrrolidin-1-yl-3-trifluoromethyl-benzamide (EPPTB) (Liu et al, 2014) was first diluted in DMSO, and subsequently diluted into cAMP assay buffer for a final DMSO concentration of 0.1%.…”

“TAARgeting Addiction”—The Alamo Bears Witness to Another Revolution

“…TAAR1 has a wide agonist spectrum that includes endogenous amines - both the common biogenic amines (DA; norepinephrine, NE and serotonin, 5HT) and trace amines (e.g., PEA, tyramine, octopamine, synephrine and tryptamine) as well as structurally-related AMPH-like compounds (AMPH, METH and MDMA; Bunzow et al, 2001; ractopamine, Liu et al, 2014), and thyronamines (Scanlan et al, 2004; Hart et al, 2006; Wainscott et al, 2007; Xie et al, 2007b). The receptor’s wide-spread expression in many different types of neurons (Borowsky et al, 2001; Bunzow et al, 2001; Lindemann et al, 2008; Espinoza et al, 2015), astrocytes (Cisneros and Ghorpade, 2014), immune cells (D’Andrea et al, 2003; Nelson et al, 2007; Wasik et al, 2012; Panas et al, 2012; Sriram et al, 2015) and cells of the cardiovascular system (Scanlan et al, 2004; Frascarelli et al, 2008; Broadley et al, 2009) suggests TAAR1-mediated signaling subserves one or more fundamental roles in cellular physiology (Grandy, 2007; Xie et al, 2007b; Lindemann et al, 2008).…”