SUMMARY␣ 2 -Adrenergic receptors (␣ 2 -ARs) regulate many physiological functions and are targets for clinically important antihypertensive and anesthetic agents. Three human and mouse genes encoding ␣ 2 -AR subtypes (␣ 2A , ␣ 2B , and ␣ 2C ) have been cloned. We investigated the involvement of the ␣ 2C -AR in ␣ 2 -adrenergic pharmacology by applying molecular genetic techniques to alter the expression of ␣ 2C -AR in mice. The effects of dexmedetomidine, a subtype-nonselective ␣ 2 -AR agonist, on monoamine turnover in brain and on locomotor activity were similar in mice with targeted inactivation of the ␣ 2C -AR gene and in their controls, but the hypothermic effect of the ␣ 2 -AR agonist was significantly attenuated by the receptor gene inactivation. Correspondingly, another strain of transgenic mice with 3-fold overexpression of ␣ 2C -AR in striatum and other brain regions expressing ␣ 2C -AR showed normal reductions in brain monoamine metabolism and locomotor activity after dexmedetomidine, but their hypothermic response to the ␣ 2 -AR agonist was significantly accentuated. The hypothermic effect of ␣ 2 -AR agonists thus seems to be mediated in part by ␣ 2C -AR. Some small but statistically significant differences between the strains were also noted in brain dopamine metabolism. Lack of ␣ 2C -AR expression was linked with reduced levels of homovanillic acid in brain, and mice with increased ␣ 2C -AR expression had elevated concentrations of the dopamine metabolite compared with their controls.␣ 2 -ARs mediate many physiological functions and pharmacological effects in the central nervous system, mainly by inhibiting neuronal firing and release of NE and other neurotransmitters. ␣ 2 -ARs are also involved in a wide range of functions in peripheral tissues (e.g., in the regulation of NE release from sympathetic nerves, smooth muscle contraction, platelet aggregation, insulin secretion, glomerular filtration, and energy metabolism) (1). Activation of ␣ 2 -ARs with the highly specific ␣ 2 -AR agonist dexmedetomidine results in bradycardia, hypotension, hypothermia, locomotor inhibition, anxiolysis, analgesia, sedation, and, with higher doses, anesthesia. Dexmedetomidine also reduces the turnover of the monoamine neurotransmitters NE, DA, and 5-HT (serotonin) in brain (2).Recent pharmacological and biochemical research has led to a subdivision of ␣ 2 -ARs into three distinct subtypes: ␣ 2A -, ␣ 2B -, and ␣ 2C -ARs. This classification was first based on the pharmacological properties of the receptors and was confirmed through the cloning of three distinct ␣ 2 -AR genes in humans, rats, mice, and other species (3). Each receptor has a distinct tissue distribution. In the central nervous system of the rat, ␣ 2A -ARs are widely expressed, whereas the other ␣ 2 -AR subtypes have more limited distributions. ␣ 2C -ARs are present in the basal ganglia, olfactory tubercle, hippocam-ABBREVIATIONS: AR, adrenergic receptor; DA, dopamine; NE, norepinephrine; MHPG, 3-methoxy-4-hydroxyphenylglycol; HVA, homovanillic acid; 5-HT,...

Studies on animal models of stress, anxiety, aggression, and sensorimotor gating have linked specific monoamine neurotransmitter abnormalities to the cognitive and behavioral disturbances associated with many affective neuropsychiatric disorders. Although alpha2-adrenoceptors (alpha2-ARs) have been suggested to have a modulatory role in these disorders, the specific roles of each alpha2-AR subtype (alpha2A, alpha2B, and alpha2C) are largely unknown. The restricted availability of relevant animal models and the lack of subtype-selective alpha2-AR drugs have precluded detailed studies in this area. Therefore, transgenic mice were used to study the possible role of the alpha2C-AR subtype in two well established behavioral paradigms: prepulse inhibition (PPI) of the startle reflex and isolation-induced aggression. The alpha2C-AR-altered mice appear grossly normal, but subtle changes have been observed in their brain dopamine (DA) and serotonin (5-HT) metabolism. In this study, the mice with targeted inactivation of the gene encoding alpha2C-ARs (alpha2C-KO) had enhanced startle responses, diminished PPI, and shortened attack latency in the isolation-aggression test, whereas tissue-specific overexpression of alpha2C-ARs (alpha2C-OE) was associated with opposite effects. Correlation analyses suggested that both the magnitude of the startle response and its relative PPI (PPI%) were modulated by the mutations. In addition, the differences in PPI, observed between drug-naive alpha2C-OE mice and their wild-type controls, were abolished by treatment with a subtype nonselective alpha2-agonist and antagonist. Thus, drugs acting via alpha2C-ARs might have therapeutic value in disorders associated with enhanced startle responses and sensorimotor gating deficits, such as schizophrenia, attention deficit disorder, post-traumatic stress disorder, and drug withdrawal.

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.

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␣ 2 -Adrenoceptors (␣ 2 -AR) modulate many central nervous system functions, such as regulation of sympathetic tone, vigilance, attention, and reactivity to environmental stressors. Three ␣ 2 -AR subtypes (␣ 2A , ␣ 2B , and ␣ 2C ) with distinct tissue-distribution patterns are known to exist, but the functional significance of each subtype is not clear. Since specific, ␣ 2 -AR subtype-selective pharmacological probes are not available, mice with genetically altered ␣ 2C -AR expression were studied in order to investigate the possible involvement of the ␣ 2C -AR in physiological and behavioral responses to acute and repeated stress. A modified version of Porsolt's forced swimming test was used to assess the possible effects of altered ␣ 2C -AR expression on the development of behavioral despair. ␣ 2C -Overexpression increased and the lack of ␣ 2C -AR (␣ 2C -KO) decreased the immobility of mice in the forced swimming test, ie ␣ 2C -AR expression appeared to promote the development of behavioral despair. In addition, ␣ 2C -KO was associated with attenuated elevation of plasma corticosterone after different stressors, and overexpression of ␣ 2C -ARs was linked with increased corticosterone levels after repeated stress. Moreover, the brain dopamine and serotonin balance, but not norepinephrine turnover, was dependent on ␣ 2C -AR expression, and the expression of c-fos and junB mRNA was increased in ␣ 2C -KO mice. Since ␣ 2C -KO produced stress-protective effects, and ␣ 2C -AR overexpression seemed to promote the development of changes related to depression, it is suggested that a yet-to-be developed subtype-selective ␣ 2C -AR antagonist might have therapeutic value in the treatment of stress-related neuropsychiatric disorders.

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RationaleThis review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.ObjectivesAlthough PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) “Novel Methods leading to New Medications in Depression and Schizophrenia” (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, “Cross-species and neurochemical imaging (PET) methods for drug discovery”, commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.ResultsSharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.ConclusionsPET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

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