Genetic Alteration of α_2C-Adrenoceptor Expression in Mice: Influence on Locomotor, Hypothermic, and Neurochemical Effects of Dexmedetomidine, a Subtype-Nonselective α₂-Adrenoceptor Agonist

Abstract

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,...

Genetic alteration of the α2-adrenoceptor subtype c in mice affects the development of behavioral despair and stress-induced increases in plasma corticosterone levels

“…21,25 Briefly, the ␣ 2C -AR gene was inactivated in 129/Sv embryonic stem cells 31 which were injected into C57BL/6J blastocysts, and the resulting chimeric mice were bred to F 1 (C57BL/6J × DBA/2J) animals. These animals were back-crossed for several generations to C57BL/6J mice; then intercrossed, and the tested ␣ 2C -KO mice were offspring of closely related F [11][12] pairs, which were combinations of mice with wild-type, heterozygous or homozygous genotypes for the ␣ 2C -AR mutation.…”

“…[16][17][18][19][20] It has also been demonstrated that genetically altered ␣ 2C -AR expression in mice does not significantly change the potent inhibitory effect of ␣ 2 -AR agonists on brain monoamine turnover. 21 However, according to a recent study employing transgenic mice, ␣ 2C -AR may have a subtle modulatory effect on brain DA and 5-HT balance. 21 Furthermore, the ␣ 2C -AR gene mutations in mice have been associated with changes in aggressive behavior, the startle reflex and its prepulse inhibition, 22 damphetamine-stimulated locomotor activity, and l-5-hydroxytryptophan-induced behaviors.…”

“…21 However, according to a recent study employing transgenic mice, ␣ 2C -AR may have a subtle modulatory effect on brain DA and 5-HT balance. 21 Furthermore, the ␣ 2C -AR gene mutations in mice have been associated with changes in aggressive behavior, the startle reflex and its prepulse inhibition, 22 damphetamine-stimulated locomotor activity, and l-5-hydroxytryptophan-induced behaviors. 23 These results raise the possibility that yet-to-be developed drugs selective for the ␣ 2C -AR would have therapeutic potential in neuropsychiatric disorders and that they would perhaps not have the typical sedative and cardiovascular adverse effects associated with current subtypenonselective ␣ 2 -AR drugs.…”

“…4,30 Therefore, and because our previous study hinted at ␣ 2C -AR-dependent modulation of DA and 5-HT balance in the brain, 21 we investigated whether altered ␣ 2C -AR gene expression affects the brain monoaminergic response to swim and restraint stress. To further characterize the brain regions where ␣ 2C -AR gene mutations might have effects on neuronal activity, mRNA levels of the stressresponsive immediate-early genes c-fos and junB were determined.…”

“…Porsolt's forced swimming test (FST) 24 was selected for the evaluation of behavioral stress reactivity in two distinct transgenic mouse strains; one had a targeted inactivation of the gene encoding the ␣ 2C -AR (␣ 2C -knockout, ␣ 2C -KO), and the other had tissuespecific overexpression of ␣ 2C -ARs (␣ 2C -OE). 21,25 In the FST, animals are exposed to swim stress in order to produce a stress reaction that decreases the animals' activity in a subsequent test swim session; this phenomenon is called 'behavioral despair'. 26 Since the FST has predictive validity in the development of antidepressants, [27][28][29] we aimed to predict whether drugs acting via ␣ 2C -AR would possibly have either stressprotective and/or antidepressant-like actions in the FST.…”