Acute and Repeated Administration of NLX-101, a Selective Serotonin-1A Receptor Biased Agonist, Reduces Audiogenic Seizures in Developing Fmr1 Knockout Mice

Fragile XSyndrome (FXS) is a leading known genetic cause of Autism Spectrum Disorders (ASD) and intellectual disability. A consistent and debilitating phenotype of FXS is sensory hypersensitivity that manifests strongly in the auditory domain and may lead to delayed language and high anxiety. The mouse model of FXS, the Fmr1 KO mouse, also shows auditory hypersensitivity, an extreme form of which is seen as audiogenic seizures (AGS). The midbrain inferior colliculus (IC) is critically involved in generating audiogenic seizures and IC neurons are hyper-responsive to sounds in developing Fmr1 KO mice. Serotonin-1A receptor (5-HT_1A) activation reduces IC activity. Therefore, we tested whether 5-HT_1A activation is sufficient to reduce audiogenic seizures in Fmr1 KO mice. A selective and post-synaptic 5-HT_1A receptor biased agonist, 3-Chloro-4-fluorophenyl-[4-fluoro-4-[[(5-methylpyrimidin-2-ylmethyl)amino]methyl]piperidin-1-yl] methanone (NLX-101, 0.6, 1.2, 1.8 or 2.4 mg/kg, i.p.) was administered to Fmr1 KO mice 15 min before seizure induction. Whereas the 0.6 mg/kg dose was ineffective in reducing seizures, the 1.2, 1.8 and 2.4 mg/kg doses of NLX-101 dramatically reduced seizures and increased mouse survival. Treatment with a combination of NLX-101 and 5-HT_1A receptor antagonists prevented the protective effects of NLX-101, indicating that NLX-101 acts selectively through 5-HT_1A receptors to reduce audiogenic seizures. NLX-101 (1.8 mg/kg) was still strongly effective in reducing seizures even after repeated administration over 5 days, suggesting an absence of tachyphylaxis to the effects of the compound. Together, these studies point to a promising treatment option targeting post-synaptic 5-HT_1A receptors to reduce auditory hypersensitivity in FXS, and potentially across autism spectrum disorders.

Angiotensin II receptor type 1 - An update on structure, expression and pathology

The AT₁ receptor, a major effector of the renin-angiotensin system, has been extensively studied in the context of cardiovascular and renal disease. Moreover, angiotensin receptor blockers, sartans, are among the most frequently prescribed drugs for the treatment of hypertension, chronic heart failure and chronic kidney disease. However, precise molecular insights into the structure of this important drug target have not been available until recently. In this context, seminal studies have now revealed exciting new insights into the structure and biased signaling of the receptor and may thus foster the development of novel therapeutic approaches to enhance the efficacy of pharmacological angiotensin receptor antagonism or to enable therapeutic induction of biased receptor activity. In this review, we will therefore highlight these and other seminal publications to summarize the current understanding of the tertiary structure, ligand binding properties and downstream signal transduction of the AT₁ receptor.

Emerging Insights into Mu Opioid Pharmacology

Opioid analgesics, most of which act through mu opioid receptors, have long represented valuable therapeutic agents to treat severe pain. Concerted drug development efforts for over a 100 years have resulted in a large variety of opioid analgesics used in the clinic, but all of them continue to exhibit the side effects, especially respiratory depression, that have long plagued the use of morphine. The recent explosion in fatalities resulting from overdose of prescription and synthetic opioids has dramatically increased the need for safer analgesics, but recent developments in mu receptor research have provided new strategies to develop such drugs. This chapter reviews recent advances in developing novel opioid analgesics from an understanding of mu receptor structure and function. This includes a summary of the mechanism of agonist binding deduced from the crystal structure of mu receptors. It will also highlight the development of novel agonist mechanisms, including biased agonists, bivalent ligands, and allosteric modulators of mu receptor function, and describe how receptor phosphorylation modulates these pathways. Finally, it will summarize research on the alternative pre-mRNA splicing mechanisms that produces a multiplicity of mu receptor isoforms. Many of these isoforms exhibit different pharmacological specificities and brain circuitry localization, thus providing an opportunity to develop novel drugs with increased therapeutic windows.

The G-protein biased mu-opioid agonist, TRV130, produces reinforcing and antinociceptive effects that are comparable to oxycodone in rats

Mu-opioid agonists (e.g., oxycodone) are highly effective therapeutics for pain. However, they also produce reinforcing effects that increase their likelihood of abuse. Recent strategies in drug development have focused on opioids with biased receptor-signaling profiles that favor activation of specific intracellular pathways over others with the aim of increasing therapeutic selectivity. TRV130, a mu agonist biased towards G-protein signaling, produces antinociceptive effects comparable to the mu agonist, morphine, but exhibits reduced side effects. However, in terms of abuse potential, we know of no published preclinical data investigating the effects of TRV130 as a reinforcer. In the present study, we assessed the relative reinforcing effects of TRV130 and oxycodone, a commonly-prescribed mu agonist, in rats self-administering the drugs under a progressive-ratio (PR) schedule of reinforcement. In addition, we assessed the relative potency and efficacy of TRV130 and oxycodone in rats in a test of thermal antinociception (Hot Plate). For self-administration, male Sprague-Dawley rats (n = 7) self-administered intravenous infusions of TRV130 or oxycodone (0.01-0.32 mg/kg/inj) under a PR schedule of reinforcement. For the Hot-Plate test, male rats (n = 7) received subcutaneous injections of TRV130 (0.1-3.2 mg/kg/inj) or oxycodone (0.1-5.6 mg/kg/inj), and nociceptive response latencies were measured. TRV130 and oxycodone were equi-potent and equi-effective in self-administration and thermal antinociception. This study demonstrates that TRV130 produces reinforcing and antinociceptive effects that are quantitatively similar to oxycodone, and that a biased-signaling profile does not necessarily reduce abuse potential.

TCB-2 [(7R)-3-bromo-2, 5-dimethoxy-bicyclo[4.2.0]octa-1,3,5-trien-7-yl]methanamine]: A hallucinogenic drug, a selective 5-HT2A receptor pharmacological tool, or none of the above?

The development of 5-HT_2A receptor agonists has been considerably marginalized since the demonstration that the tryptaminergic drugs, LSD and psilocybin, or the phenylakylamine drugs, mescaline and DOI, exert their hallucinogenic properties via the stimulation of 5-HT_2A receptors. Nonetheless, the ability of drugs to stimulate 5-HT_2A receptors is not necessarily associated with psychedelic experience and the hallucinogenic properties are still not understood. Several studies have increased interest in stimulating 5-HT_2A receptors in various CNS diseases. (7R)-3-bromo-2, 5-dimethoxy-bicyclo[4.2.0]octa-1,3,5-trien-7-yl]methanamine (TCB-2) which was synthetized in 2006 presents a high affinity with human and rat 5-HT_2A receptors. Its main feature of interest is that it preferentially stimulates the phospholipase C and not phospholipase A2 pathway, which is at variance with several hallucinogenic drugs. Preference for TCB-2 has increased in preclinical studies and it exhibits subtle differences compared to DOI or LSD in some molecular, cellular and behavioral studies. The purpose of this review is to take a position on the use of TCB-2 as a pharmacological tool. A careful reading of the literature has revealed that the suspected hallucinogenic properties of TCB-2 cannot firmly be ascertained while its pharmacological profile is unknown and likely not selective at 5-HT_2A receptors. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Biased μ-opioid receptor agonists diversely regulate lateral mobility and functional coupling of the receptor to its cognate G proteins

There are some indications that biased μ-opioid ligands may diversely affect μ-opioid receptor (MOR) properties. Here, we used confocal fluorescence recovery after photobleaching (FRAP) to study the regulation by different MOR agonists of receptor movement within the plasma membrane of HEK293 cells stably expressing a functional yellow fluorescent protein (YFP)-tagged μ-opioid receptor (MOR-YFP). We found that the lateral mobility of MOR-YFP was increased by (D-Ala²,N-MePhe⁴,Gly⁵-ol)-enkephalin (DAMGO) and to a lesser extent also by morphine but decreased by endomorphin-2. Interestingly, cholesterol depletion strongly enhanced the ability of morphine to elevate receptor mobility but significantly reduced or even eliminated the effect of DAMGO and endomorphin-2, respectively. Moreover, the ability of DAMGO and endomorphin-2 to influence MOR-YFP movement was diminished by pertussis toxin treatment. The results obtained by agonist-stimulated [³⁵S]GTPγS binding assays indicated that DAMGO exhibited higher efficacy than morphine and endomorphin-2 did and that the efficacy of DAMGO, contrary to the latter agonists, was enhanced by cholesterol depletion. Overall, our study provides clear evidence that biased MOR agonists diversely affect receptor mobility in plasma membranes as well as MOR/G protein coupling and that the regulatory effect of different ligands depends on the membrane cholesterol content. These findings help to delineate the fundamental properties of MOR regarding their interaction with biased MOR ligands and cognate G proteins.

Vasopressin receptors and pharmacological chaperones: from functional rescue to promising therapeutic strategies

Conformational diseases result from protein misfolding and/or aggregation and constitute a major public health problem. Congenital Nephrogenic Diabetes Insipidus is a typical conformational disease. In most of the cases, it is associated to inactivating mutations of the renal arginine-vasopressin V2 receptor gene leading to misfolding and intracellular retention of the receptor, causing the inability of patients to concentrate their urine in response to the antidiuretic hormone. Cell-permeable pharmacological chaperones have been successfully challenged to restore plasma membrane localization of the receptor mutants and to rescue their function. Interestingly, different classes of specific ligands such as antagonists (vaptans), agonists as well as biased agonists of the V2 receptor have proven their usefulness as efficient pharmacochaperones. These compounds represent a potential therapeutic treatment of this X-linked genetic pathology.