Serodolin,a β-arrestin–biased ligand of 5-HT7 receptor,attenuates pain-related behaviors

G protein–coupled receptors (GPCRs) are involved in regulation of manifold physiological processes through coupling to heterotrimeric G proteins upon ligand stimulation. Classical therapeutically active drugs simultaneously initiate several downstream signaling pathways, whereas biased ligands, which stabilize subsets of receptor conformations, elicit more selective signaling. This concept of functional selectivity of a ligand has emerged as an interesting property for the development of new therapeutic molecules. Biased ligands are expected to have superior efficacy and/or reduced side effects by regulating biological functions of GPCRs in a more precise way. In the last decade, 5-HT₇ receptor (5-HT₇R) has become a promising target for the treatment of neuropsychiatric disorders, sleep and circadian rhythm disorders, and pathological pain. In this study, we showed that Serodolin is unique among a number of agonists and antagonists tested: it behaves as an antagonist/inverse agonist on G_s signaling while inducing ERK activation through a β-arrestin–dependent signaling mechanism that requires c-SRC activation. Moreover, we showed that Serodolin clearly decreases hyperalgesia and pain sensation in response to inflammatory, thermal, and mechanical stimulation. This antinociceptive effect could not be observed in 5-HT₇R knockout (KO) mice and was fully blocked by administration of SB269-970, a specific 5-HT₇R antagonist, demonstrating the specificity of action of Serodolin. Physiological effects of 5-HT₇R stimulation have been classically shown to result from G_s-dependent adenylyl cyclase activation. In this study, using a β-arrestin–biased agonist, we provided insight into the molecular mechanism triggered by 5-HT₇R and revealed its therapeutic potential in the modulation of pain response.

Among 14 serotonin receptor subtypes, 5-hydroxytryptamine 7receptors (5-HT₇Rs) belong to the G protein–coupled receptor (GPCR) family or the so-called seven transmembrane-spanning receptor. It is the last identified member and has been cloned from several animal species, including human (1). 5-HT₇R couples to the heterotrimeric G protein G_s, which in turn stimulates adenylate cyclase (AC), leading to an increase of 3′-5′-cyclic adenosine monophosphate (cAMP) production in both recombinant and native systems (1). This allows the activation of cAMP-dependent protein kinase (PKA), which acts on the MAPK cascade in a cell type–specific manner (2–4).In HEK-293 cells, the observed agonist-induced ERK1/2 activation requires PKA, Ras, and Raf activation independently of Rap-1 (4). In neurons, the 5-HT₇R–induced ERK activation is mediated through a PKA-independent pathway that utilizes cAMP-guanine nucleotide exchange factor (cAMP-GEF) (3). It was shown that 5-HT₇R signaling also depends on the activation of Gα₁₂ protein that in turn triggers activation of multiple signaling pathways through the family of small Rho GTPases, Cdc42 and RhoA (5).The 5-HT₇R is expressed in the peripheral and central nervous system with highest densities in thalamus, hypothalamus, cerebral cortex, amygdala, and striatal complex (1). Numerous data have established 5-HT₇R implication in the control of circadian rhythms and thermoregulation, learning, and memory as well as in central nervous system (CNS) disorders such as depression, Alzheimer’s disease, and schizophrenia (6). There is mounting evidence that 5-HT₇R is an important modulator of nociceptive transmission (7). It was also reported that 5-HT₇R is involved in the antinociceptive effects of morphine (8), antidepressants, and nonopioid analgesics (9). Collectively, these observations underscore the interest of developing new 5-HT₇R ligands for the treatment of pain. To date, many 5-HT₇R potent agonists (5-CT, E55888, AS-19, and LP-211) and antagonists (SB269-970 and DR4004) against 5-HT₇R have been described (10); these are all ligands that commit the receptor to a G protein–dependent pathway, although few 5-HT₇R–biased ligands have been described (11, 12).In contrast to standard agonists and antagonists, which activate or inactivate the entirety of a receptor’s signaling network, some ligands termed biased ligands are capable of stabilizing subsets of receptor conformations, hence eliciting selective modulation within the network. From data obtained in the last two decades, the concept of functional selectivity of a ligand has emerged as an interesting property in modern drug discovery. Increasing preclinical data highlight the value of using such ligands, which exhibit a unique spectrum of pharmacological responses, for instance, by specifically targeting G protein– or β-arrestin–dependent signaling. Biased ligands, by selectively modulating a subset of receptor functions, may optimize therapeutic action and generate less pronounced side effects than compounds globally affecting receptor activity. Although binding of β-arrestins to GPCR has primarily been involved in the termination of G protein signaling by inducing desensitization and internalization of the receptor, numerous studies have indicated that β-arrestins can be intimately involved in additional signaling events through mechanisms dependent on or independent of G protein coupling (13, 14). Several β-arrestin–biased ligands have been identified and show therapeutic interest in other receptor classes (15). In particular, several studies demonstrate the role of β-arrestin signaling on opioid analgesia and tolerance (16, 17). In the present study, we used a combination of pharmacological, genetic, and behavioral approaches to identify a β-arrestin–biased 5-HT₇R ligand and evaluate its therapeutical potential for the treatment of pain.