The TAAR1 inhibitor EPPTB suppresses neuronal excitability and seizure activity in mice

Highlights

• The expression of TAAR1 increased significantly in TLE patients compared with that in control group.

• EPPTB suppressed hyperexcitability of pyramidal neurons in mouse hippocamus.

• EPPTB suppressed electrographic seizures.

• EPPTB had anticonvulsant effects.

Abstract

Epilepsy is a common neurological disease. G protein-coupled receptors (GPCRs) are extensively distributed and play an important role in human health by serving as therapeutic targets for various diseases. As one of the GPCRs, trace amine-associated receptor 1 (TAAR1) has recently aroused increasing interest as a potential therapeutic target for psychiatric disorders. However, the effect of TAAR1 on epileptic seizures remains unclear. We hypothesized that TAAR1 plays an important role in epilepsy and might represent a potential therapeutic target. In this study, we analyzed a mouse epilepsy model and patients with temporal lobe epilepsy (TLE) and observed substantially increased TAAR1 expression compared with the control group. In recordings of hippocampal slices, the TAAR1-specific inhibitor N-(3-ethoxyphenyl)-4-(pyrrolidin-1-yl)-3-(trifluoromethyl) benzamide (EPPTB) suppressed the excitability of hippocampal pyramidal neurons. EPPTB also reduced seizure-like events (SLEs) and seizure activity. Our results suggest that EPPTB attenuates seizure activity and that TAAR1 might be a potential drug target for individuals with epilepsy.

Introduction

G protein-coupled receptors (GPCRs) are crucial to human health. A previous study found that approximately 19 % of human drug targets are GPCRs (Rask-Andersen et al., 2011). Trace amine-associated receptors (TAARs) are members of a family of vertebrate rhodopsin-like type A GPCRs (Borowsky et al., 2001; Bunzow et al., 2001) that includes 9 members in humans (TAAR1−9). TAAR family members have many subfamilies, and only TAAR1 and TAAR4 have been found to exhibit high affinity for trace amines (TAs) (Lindemann et al., 2005). TAAR1 is expressed in various brain areas and in the periphery, especially in the limbic and monoaminergic systems (Borowsky et al., 2001; Bunzow et al., 2001; Xie and Miller, 2007). TAAR1 has many agonists in vivo, including p-tyramine, b-phenylethylamine, tryptamine, 3-iodothyronamine and octopamine (Borowsky et al., 2001; Bunzow et al., 2001). TAAR1 has been shown to modulate neuronal activity and to play a role in neurological diseases, including schizophrenia (John et al., 2017; Revel et al., 2013), cocaine-seeking behaviors (Asif-Malik et al., 2017; Liu et al., 2017; Pei et al., 2014), narcolepsy (Black et al., 2017; Schwartz et al., 2017; Siemian et al., 2017), parkinsonism (Alvarsson et al., 2015) and mood disorders (Revel et al., 2012, 2013). Moreover, TAAR1 agonists block compulsive-like eating (Ferragud et al., 2017) and have therapeutic relevance to leukemia (Wasik et al., 2012) and metabolic disorders (Raab et al., 2016; Revel et al., 2013). N-(3-ethoxyphenyl)-4-(pyrrolidin-1-yl)-3-(trifluoromethyl) benzamide (EPPTB) is a drug that acts as a potent and selective antagonist for TAAR1, with no significant effects on other TAARs. Studies of TAAR1 provide new insights for drug research, especially for neuropsychiatric disorders characterized by the aberrant regulation of brain monoamines or neurotransmitters.

Epilepsy is a brain disorder characterized by spontaneous recurrent seizures (SRSs). Although most patients are able to control their epilepsy with currently available therapies, approximately 30 % of patients are drug-resistant (French, 2007). Aberrant excitatory transmission results in the neuronal hyperexcitability that induces recurrent seizures (Scharfman, 2007). The cAMP-PKA pathway is well-known to play roles in neuronal excitability and epilepsy. TAAR1 activation increases intracellular cAMP levels (Harmeier et al., 2015). Although TAs do not appear to alter neuronal excitability at physiological levels (Berry, 2007), the administration of high doses of beta-phenylethylamine (a nonselective endogenous TAAR1 agonist) has been shown to induce seizure activity in mice (Dourish and Cooper, 1983). Based on the evidence mentioned above, we hypothesized that TAAR1 may be involved in epilepsy.

We assessed the expression of TAAR1 in patients with temporal lobe epilepsy (TLE) and in a mouse epilepsy model to test this hypothesis. Whole cell patch-clamp recording was performed to evaluate the role of EPPTB in hippocampal pyramidal neurons. We then explored the role of EPPTB in modulating the local field potentials (LFPs) in vivo and the effect of EPPTB on seizure activity.