Function of brain α2B-adrenergic receptor characterized with subtype-selective α2B antagonist and KO mice.

Noradrenergic signaling, through the α2A and α2C adrenergic receptors modulates the cognitive and behavioral symptoms of disorders such as schizophrenia, attention deficit hyperactivity disorder (ADHD), and addiction. However, it is unknown whether the α2B receptor has any significant role in CNS function. The present study elucidates the potential role of the α2B receptor in CNS function via the discovery and use of the first subtype-selective α2B antagonist (AGN-209419), and behavioral analyses of α-receptor knockout (KO) mice. Using AGN-209419 as radioligand, α2B receptor binding sites were identified within the olfactory bulb, cortex, thalamus, cerebellum, and striatum. Based on the observed expression patterns of α2 subtypes in the brain, we compared α2B KO, α2A KO and α2C KO mice behavioral phenotypes with their respective wild-type lines in anxiety (plus maze), compulsive (marble burying), and sensorimotor (prepulse inhibition) tasks. α2B KO mice exhibited increased marble burying and α2C KO mice exhibited an increased startle response to a pulse stimulus, but otherwise intact prepulse inhibition. To further explore compulsive behavior, we evaluated novelty-induced locomotor hyperactivity and found that α2B KO and α2C KO mice exhibited increased locomotion in the open field. Interestingly, when challenged with amphetamine, α2C KO mice increased activity at lower doses relative to either α2A KO or WT mice. However, α2B KO mice exhibited stereotypy at doses of amphetamine that were only locomotor stimulatory to all other genotypes. Following co-administration of AGN-209419 with low-dose amphetamine in WT mice, stereotypy was observed, mimicking the α2B KO phenotype. These findings suggest that the α2B receptor is involved in CNS behaviors associated with sensorimotor gating and compulsivity, and may be therapeutically relevant for disorders such as schizophrenia, ADHD, post-traumatic stress disorder, addiction, and obsessive compulsive disorder.

Flavin-containing monooxygenase S-oxygenation of a series of thioureas and thiones.

Mammalian flavin-containing monooxygenase (FMO) is active towards many drugs with a heteroatom having the properties of a soft nucleophile. Thiocarbamides and thiones are S-oxygenated to the sulfenic acid which can either react with glutathione and initiate a redox-cycle or be oxygenated a second time to the unstable sulfinic acid. In this study, we utilized LC-MS/MS to demonstrate that the oxygenation by hFMO of the thioureas under test terminated at the sulfenic acid. With thiones, hFMO catalyzed the second reaction and the sulfinic acid rapidly lost sulfite to form the corresponding imidazole. Thioureas are often pulmonary toxicants in mammals and, as previously reported by our laboratory, are excellent substrates for hFMO2. This isoform is expressed at high levels in the lung of most mammals, including non-human primates. Genotyping to date indicates that individuals of African (up to 49%) or Hispanic (2-7%) ancestry have at least one allele for functional hFMO2 in lung, but not Caucasians nor Asians. In this study the major metabolite formed by hFMO2 with thioureas from Allergan, Inc. was the sulfenic acid that reacted with glutathione. The majority of thiones were poor substrates for hFMO3, the major form in adult human liver. However, hFMO1, the major isoform expressed in infant and neonatal liver and adult kidney and intestine, readily S-oxygenated thiones under test, with Kms ranging from 7 to 160 µM and turnover numbers of 30-40 min(-1). The product formed was identified by LC-MS/MS as the imidazole. The activities of the mouse and human FMO1 and FMO3 orthologs were in good agreement with the exception of some thiones for which activity was much greater with hFMO1 than mFMO1.

Alpha-1-adrenergic receptor agonist activity of clinical alpha-adrenergic receptor agonists interferes with alpha-2-mediated analgesia.

BACKGROUND: The use of alpha-2 adrenergic agonists for analgesia is limited due to a narrow therapeutic window. Definition of the role of alpha receptor subtypes in alpha agonist mediated analgesia may identify strategies to separate the analgesic from sedative and cardiovascular effects. METHODS: Analgesic activity of brimonidine, clonidine, and tizanidine was investigated in wild-type C57B/6, alpha-2A, and alpha-2C knockout mice with allodynia induced by N-methyl-D-aspartate or sulprostone. The alpha receptor selectivity of the alpha agonists was assessed using functional in vitro recombinant assays. RESULTS: Brimonidine, clonidine, and tizanidine reduced N-methyl-D-aspartate- and sulprostone-induced allodynia in wild-type mice, but not alpha-2A knockout mice. In alpha-2C knockout mice, brimonidine and tizanidine reduced allodynia in both models, whereas clonidine only reduced N-methyl-D-aspartate-induced allodynia. In vitro, clonidine and tizanidine activated alpha-1 and alpha-2 receptors with similar potencies, whereas brimonidine was selective for alpha-2 receptors. In alpha-2C knockout mice with sulprostone-induced allodynia, blockade of clonidine's alpha-1 receptor agonist activity restored clonidine's analgesic efficacy. In wild-type mice, the analgesic potency of intrathecal clonidine and tizanidine was increased 3- to 10-fold by coadministration with the alpha-1A-selective antagonist 5-methylurapidil without affecting sedation. Following intraperitoneal administration, the therapeutic window was negligible for clonidine and tizanidine, but greater for brimonidine. 5-Methylurapidil enhanced the therapeutic window of intraperitoneal clonidine and tizanidine approximately 10-fold. CONCLUSIONS: Alpha-1A receptor agonist activity can counterbalance alpha-2 receptor agonist-induced analgesia. Greater alpha-2 selectivity may enhance the therapeutic window of alpha-2 agonists in the treatment of pain.