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Background and purpose: Pharmacological validation of novel functions for the a 2A -, a 2B -, and a 2C -adrenoceptor (AR) subtypes has been hampered by the limited specificity and subtype-selectivity of available ligands. The current study describes a novel highly selective a 2C -adrenoceptor antagonist, JP-1302 (acridin-9-yl-[4-(4-methylpiperazin-1-yl)-phenyl]amine). Experimental approach: Standard in vitro binding and antagonism assays were employed to demonstrate the a 2C -AR specificity of JP-1302. In addition, JP-1302 was tested in the forced swimming test (FST) and the prepulse-inhibition of startle reflex (PPI) model because mice with genetically altered a 2C -adrenoceptors have previously been shown to exhibit different reactivity in these tests when compared to wild-type controls. Key results: JP-1302 displayed antagonism potencies (K B values) of 1,500, 2,200 and 16 nM at the human a 2A -, a 2B -, and a 2C -adrenoceptor subtypes, respectively. JP-1302 produced antidepressant and antipsychotic-like effects, i.e. it effectively reduced immobility in the FST and reversed the phencyclidine-induced PPI deficit. Unlike the a 2 -subtype non-selective antagonist atipamezole, JP-1302 was not able to antagonize a 2 -agonist-induced sedation (measured as inhibition of spontaneous locomotor activity), hypothermia, a 2 -agonist-induced mydriasis or inhibition of vas deferens contractions, effects that have been generally attributed to the a 2A -adrenoceptor subtype. In contrast to JP-1302, atipamezole did not antagonize the PCPinduced prepulse-inhibition deficit. Conclusions and implications:The results provide further support for the hypothesis that specific antagonism of the a 2C -adrenoceptor may have therapeutic potential as a novel mechanism for the treatment of neuropsychiatric disorders.

Atipamezole is an alpha2-adrenoceptor antagonist with an imidazole structure. Receptor binding studies indicate that its affinity for alpha2-adrenoceptors and its alpha2/alpha1 selectivity ratio are considerably higher than those of yohimbine, the prototype alpha2-adrenoceptor antagonist. Atipamezole is not selective for subtypes of alpha2-adrenoceptors. Unlike many other alpha2-adrenoceptor antagonists, it has negligible affinity for 5-HT1A and I2 binding sites. Atipamezole is rapidly absorbed and distributed from the periphery to the central nervous system. In humans, atipamezole at doses up to 30 mg/subject produced no cardiovascular or subjective side effects, while at a high dose (100 mg/subject) it produced subjective symptoms, such as motor restlessness, and an increase in blood pressure. Atipamezole rapidly reverses sedation/anesthesia induced by alpha2-adrenoceptor agonists. Due to this property, atipamezole is commonly used by veterinarians to awaken animals from sedation/anesthesia induced by alpha2-adrenoceptor agonists alone or in combination with various anesthetics. Atipamezole increased sexual activity in rats and monkeys. In animals with sustained nociception, atipamezole increased pain-related responses by blocking the noradrenergic feedback inhibition of pain. In tests assessing cognitive functions, atipamezole at low doses has beneficial effects on alertness, selective attention, planning, learning, and recall in experimental animals, but not necessarily on short-term working memory. At higher doses atipamezole impaired performance in tests of cognitive functions, probably due to noradrenergic overactivity. Recent experimental animal studies suggest that atipamezole might have beneficial effects in the recovery from brain damage and might potentiate the anti-Parkinsonian effects of dopaminergic drugs. In phase I studies atipamezole has been well tolerated by human subjects.

In the present study we evaluated the alpha 1- and alpha 2-adrenoceptor subtype binding, central alpha 2-adrenoceptor antagonist potency, as well as effects on brain neurochemistry and behavioural pharmacology of two alpha 2-adrenoceptor antagonists, atipamezole and yohimbine. Atipamezole had higher selectivity for alpha 2- vs. alpha 1-adrenoceptors than yohimbine regardless of the subtypes studied. Both compounds had comparable affinity for the alpha 2A-, alpha 2C- and alpha 2B-adrenoceptors, but yohimbine had significantly lower affinity for the alpha 2D-subtype. This may account for the fact that significantly higher doses of yohimbine than atipamezole were needed for reversal of alpha 2-agonist (medetomidine)-induced effects in rats (mydriasis) and mice (sedation and hypothermia). The effect on central monoaminergic activity was estimated by measuring the concentrations of transmitters and their main metabolites in whole brain homogenate. At equally effective alpha 2-antagonising doses in the rat mydriasis model, both drugs stimulated central noradrenaline turnover (as reflected by increase in metabolite levels) to the same extent. Atipamezole increased dopaminergic activity only slightly, whereas yohimbine elevated central dopamine but decreased central 5-hydroxytryptamine turnover rates. In behavioural tests, atipamezole (0.1-10 mg/kg) did not affect motor activity but stimulated food rewarded operant (FR-10) responding (0.03-3 mg/kg) whereas yohimbine both stimulated (1 mg/kg) and decreased (> or = 3 mg/kg) behaviour in a narrow dose range in these tests. In the staircase test, both antagonists increased neophobia, but in the two compartment test only yohimbine (> or = 3 mg/kg) decreased exploratory behaviour. The dissimilar effects of the antagonists on neurochemistry and behaviour are thought to be caused by non alpha 2-adrenoceptor properties of yohimbine. In conclusion, the alpha 2-antagonist atipamezole blocked all alpha 2-adrenoceptor subtypes at low doses, stimulated central noradrenergic activity and had only slight effects on behaviour under familiar conditions, but increased neophobia. The low affinity for the alpha 2D-adrenoceptor combined with its unspecific effects complicates the use of yohimbine as pharmacological tool to study alpha 2-adrenoceptor physiology and pharmacology.

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Abstract:The a 2 -adrenoceptors (ARs) are important modulators of a wide array of physiological responses. As only a few selective compounds for the three a 2 -AR subtypes (a 2A , a 2B and a 2C ) have been available, the pharmacological profile of a new a 2C -selective AR antagonist ORM-10921 is reported. Standard in vitro receptor assays and antagonism of a 2 , and a 1 -AR agonist -evoked responses in vivo were used to demonstrate the a 2C -AR selectivity for ORM-10921 which was tested in established behavioural models related to schizophrenia and cognitive dysfunction with an emphasis on pharmacologically induced hypoglutamatergic state by phencyclidine or MK-801. The Kb values of in vitro a 2C -AR antagonism for ORM-10921 varied between 0.078-1.2 nM depending on the applied method. The selectivity ratios compared to a 2A -AR subtype and other relevant receptors were 10-100 times in vitro. The in vivo experiments supported its potent a 2C -antagonism combined with only a weak a 2A -antagonism. In the pharmacodynamic microdialysis study, ORM-10921 was found to increase extracellular dopamine levels in prefrontal cortex in the baseline conditions. In the behavioural tests, ORM-10921 displayed potent antidepressant and antipsychotic-like effects in the forced swimming test and prepulse-inhibition models analogously with the previously reported results with structurally different a 2C -selective AR antagonist JP-1302. Our new results also indicate that ORM-10921 alleviated the NMDA-antagonist-induced impairments in social behaviour and watermaze navigation. This study extends and further validates the concept that a 2C -AR is a potential therapeutic target in CNS disorders such as schizophrenia or Alzheimer's disease and suggests the potential of a 2C -antagonism to treat such disorders. a 2 -Adrenoceptors (a 2 -ARs) consist of three distinct a 2 -adrenergic receptor subtypes in mammals: a 2A , a 2B and a 2C . They are distributed widely, yet not homogenously, regulating a diverse range of physiological processes including sedation/ vigilance, anxiety, pain/analgesia and cardiovascular function [1][2][3]. In the CNS, the a 2C -ARs are expressed most extensively in the striatum and hippocampus [4][5][6][7]. Valuable information on the discrete functions of each a 2 -AR subtype has been gained by using gene-targeted mice [8,9], but the limited availability of suitable subtype selective ligands has hindered their pharmacological exploration [10].The discovery and pre-clinical profile of a chemically novel and exceptionally selective antagonist at the a 2C -AR, JP-1302, has been described [11]. Compared to other available a 2 -ARs, this agent displays striking (>100 times) selectivity. In the behavioural profiling, JP-1302 showed potent antidepressantand antipsychotic-like properties in the forced swimming test and prepulse inhibition (PPI) paradigms [9,12]. Although part of these results are consistent with those obtained with a 2 -AR knockout mice [9], the antipsychotic-like effect, that is, the reversal of impairme...

Inhibition of steroidogenesis may be produced perioperatively by imidazole compounds, such as the hypnotic agent etomidate, with potentially serious consequences for patient morbidity and mortality. Dexmedetomidine, ([+]4-[1-[2,3-dimethylphenyl]-ethyl]-1H-imidazole), another imidazole compound with anesthetic like properties, is now being used perioperatively. Therefore, we investigated the effects of dexmedetomidine on steroidogenesis as well as on binding to glucocorticoid receptors in a series of in vitro and in vivo animal studies. The effect of dexmedetomidine, 10(-8)-10(-3) M, on adrenocorticotrophic hormone (ACTH) stimulated release of corticosterone was assessed in isolated rat adrenal cells. To characterize dexmedetomidine interactions with the glucocorticoid receptor, dexmedetomidine's ability to compete for [3H]dexmethasone binding sites was studied in renal tubular cells. The effect of dexmedetomidine, 80 micrograms/kg subcutaneously, on ACTH-stimulated release of cortisol was studied in separate cohorts of dogs at various time intervals during and after anesthesia was given. To compare the inhibitory effects of etomidate and dexmedetomidine on steroidogenesis, ACTH-stimulated release of cortisol was studied in dogs treated with anesthetic doses of either dexmedetomidine (80 micrograms/kg IV) or etomidate (1 mg/kg IV). Finally, dogs were given dexmedetomidine by continuous subcutaneous infusion for 7 days at sedative doses after which their cortisol response to ACTH was determined. At dexmedetomidine concentrations greater than 10(-7) M, a dose-dependent inhibition of corticosterone release was detected in response to ACTH stimulation in vitro. At these high dexmedetomidine concentrations, [3H]dexamethasone binding was not affected. In the in vivo dog experiments, basal cortisol levels decreased and the cortisol response to ACTH was blunted 3 h after dexmedetomidine administration.(ABSTRACT TRUNCATED AT 250 WORDS)