Clinical and pharmacological significance of α2-adrenoceptor polymorphisms in cardiovascular diseases

Designing Chromane Derivatives as α2A-Adrenoceptor Selective Agonists via Conformation Constraint

Enhancing the selectivity of alpha2-adrenoceptor (α2A-AR) agonists remains an unresolved issue. Herein, we reported the design of an α2A-AR agonist using the conformation constraint method, beginning with medetomidine. The structure-activity relationship indicated that the 8-substituent of chromane derivatives exerted the most pronounced effect on α2A-AR agonistic activity. Compounds A9 and B9 were identified as the most promising, exhibiting EC50 values of 0.78 and 0.23 nM, respectively. Their selectivity indexes surpassed dexmedetomidine (DMED) by 10-80 fold. In vivo studies demonstrated that both A9 and B9 dose-dependently increased the loss of righting reflex in mice, with ED50 values of 1.54 and 0.138 mg/kg, respectively. Binding mode calculations and mutation studies suggested the indispensability of the hydrogen bond between ASP1283.32 and α2A-AR agonist. In particular, A9 and B9 showed no dual reverse pharmacological effect, a characteristic exhibited by DMED in α2A-AR activation.

Influence of Receptor Polymorphisms on the Response to α-Adrenergic Receptor Blockers in Pheochromocytoma Patients

Background: Presurgical treatment with an α-adrenergic receptor blocker is recommended to antagonize the catecholamine-induced α-adrenergic receptor mediated vasoconstriction in patients with pheochromocytoma or sympathetic paraganglioma (PPGL). There is, however, a considerable interindividual variation in the dose-response relationship regarding the magnitude of blood pressure reduction or the occurrence of side effects. We hypothesized that genetically determined differences in α-adrenergic receptor activity contribute to this variability in dose-response relationship. Methods: Thirty-one single-nucleotide polymorphisms (SNPs) of the α1A, α1B, α1D adrenoreceptor (ADRA1A, ADRA1B, ADRA1D) and α2A, α2B adrenoreceptor (ADRA2A, ADRA2B) genes were genotyped in a group of 116 participants of the PRESCRIPT study. Haplotypes were constructed after determining linkage disequilibrium blocks. Results: The ADRA1B SNP rs10515807 and the ADRA2A SNPs rs553668/rs521674 were associated with higher dosages of α-adrenergic receptor blocker (p < 0.05) and with a higher occurrence of side effects (rs10515807) (p = 0.005). Similar associations were found for haplotype block 6, which is predominantly defined by rs10515807. Conclusions: This study suggests that genetic variability of α-adrenergic receptor genes might be associated with the clinically observed variation in beneficial and adverse therapeutic drug responses to α-adrenergic receptor blockers. Further studies in larger cohorts are needed to confirm our observations.

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Antidepressant Drugs Effects on Blood Pressure

Individuals suffering from depressive disorders display a greater incidence of hypertension compared with the general population, despite reports of the association between depression and hypotension. This phenomenon may depend, at least in part, on the use of antidepressant drugs, which may influence blood pressure through different effects on adrenergic and serotoninergic pathways, as well as on histaminergic, dopaminergic, and cholinergic systems. This review summarizes extant literature on the effect of antidepressant drugs on blood pressure. Selective serotonin reuptake inhibitors are characterized by limited effects on autonomic system activity and a lower impact on blood pressure. Thus, they represent the safest class—particularly among elderly and cardiovascular patients. Serotonin–norepinephrine reuptake inhibitors, particularly venlafaxine, carry a greater risk of hypertension, possibly related to greater effects on the sympathetic nervous system. The norepinephrine reuptake inhibitor reboxetine is considered a safe option because of its neutral effects on blood pressure in long-term studies, even if both hypotensive and hypertensive effects are reported. The dopamine–norepinephrine reuptake inhibitor bupropion can lead to blood pressure increases, usually at high doses, but may also cause orthostatic hypotension, especially in patients with cardiovascular diseases. The norepinephrine–serotonin modulators, mirtazapine and mianserin, have minimal effects on blood pressure but may rarely lead to orthostatic hypotension and falls. These adverse effects are also observed with the serotonin-reuptake modulators, nefazodone and trazodone, but seldomly with vortioxetine and vilazodone. Agomelatine, the only melatonergic antidepressant drug, may also have limited effects on blood pressure. Tricyclic antidepressants have been associated with increases in blood pressure, as well as orthostatic hypotension, particularly imipramine. Oral monoamine–oxidase inhibitors, less frequently skin patch formulations, have been associated with orthostatic hypotension or, conversely, with hypertensive crisis due to ingestion of tyramine-containing food (i.e., cheese reaction). Lastly, a hypertensive crisis may complicate antidepressant treatment as a part of the serotonin syndrome, also including neuromuscular, cognitive, and autonomic dysfunctions. Clinicians treating depressive patients should carefully consider their blood pressure status and cardiovascular comorbidities because of the effects of antidepressant drugs on blood pressure profiles and potential interactions with antihypertensive treatments.

Relationship between hemodynamic alteration and sympathetic nerve activation following a single oral dose of cinnamtannin A2

We previously found that a single dose of B-type procyanidin mixture increase in skeletal muscle blood flow (BF). We compared BF changes following administration of (-)-epicatechin (EC, monomer) and the B-type procyanidins procyanidin B2 (B2, dimer), procyanidin C1 (C1, trimer), and cinnamtannin A2 (A2, tetramer). Each chemical was administered orally to rats, followed by BF measurement in cremaster arteriole for 60 min. About 10 and 100 µg/kg of B2 and C1 elicited BF increase, the effect was potent at 100 µg/kg. BF also increased significantly after administration of 10 µg/kg A2, but not with the administration at 100 µg/kg. EC yielded no BF changes. Co-treatment with the nonselective adrenaline blocker carvedilol attenuated the BF increase seen with 10 µg/kg A2 treatment. This outcome suggested the involvement of sympathetic nerve activation in the BF increase by this dose of A2. Co-treatment of 100 µg/kg A2 with the α2 blocker yohimbine exhibited an increase of BF significantly. The α2 adrenaline receptor in the vasomotor centre is an inhibitory receptor and it regulates hemodynamics. This result suggested that high doses of A2 did not alter BF because of activating the α2 adrenergic receptor. Phosphorylation of aortic endothelial nitric oxide synthase (eNOS) increased with 10 µg/kg A2 alone or co-treatment with 100 µg/kg A2 and yohimbine, but not with co-treatment of 10 µg/kg A2 and carvedilol or 100 µg/kg A2 alone. These results imply that A2 does not directly activate eNOS, but that shear stress from the increased BF might be associated with eNOS phosphorylation.

Circulating microRNA-30a-5p, microRNA-101-3p, microRNA-140-3p and microRNA-141-3p as potential biomarkers for dexmedetomidine response in pediatric patients

PURPOSE

The aim of this study was to investigate the expression levels of plasma miR-30a-5p, miR-101-3p, miR-140-3p and miR-141-3p and their relationship to dexmedetomidine efficacy and adverse effects in pediatric patients.

METHODS

The expression levels of miR-30a-5p, miR-101-3p, miR-140-3p and miR-141-3p were measured by qRT-PCR in plasma of 133 pediatric patients receiving dexmedetomidine for preoperative sedation. We analyzed the relationship between miRNA abundance and dexmedetomidine response, including sedative effect and adverse effects, and assessed the predictive power of miRNAs for drug response.

RESULTS

Among 133 pediatric patients, 111 patients were dexmedetomidine responders (UMSS ≥ 2) and 22 patients were non-responders (UMSS < 2). We observed higher expression levels of miR-101-3p and miR-140-3p in dexmedetomidine responders compared with non-responders (P < 0.05, P < 0.0001). In contrast, there was no significant difference in the expression levels of miR-30a-5p and miR-141-3p between responders and non-responders (P > 0.05). The plasma levels of miR-101-3p and miR-30a-5p were markedly downregulated in patients who experienced hypotension and bradycardia, respectively (P < 0.05). MiR-101-3p and miR-140-3p demonstrated a potential discriminatory ability between dexmedetomidine responders and non-responders, with AUC of 0.64 (P < 0.05) and 0.77 (P < 0.0001), respectively. The AUC of miR-101-3p in distinguishing patients without hypotension was 0.63 (P < 0.05). The AUC of miR-30a-5p in distinguishing patients without bradycardia was 0.74 (P < 0.05).

CONCLUSION

Our study demonstrated that circulating miR-101-3p, miR-140-3p and miR-30a-5p might be used as a blood-based marker for dexmedetomidine efficacy and safety in pediatric patients.

Pharmacodynamic Gene Testing in Prader-Willi Syndrome

Prader-Willi syndrome (PWS) is a rare genetic disorder with a complex neurobehavioral phenotype associated with considerable psychiatric co-morbidity. This clinical case series, for the first time, describes the distribution and frequency of polymorphisms of pharmacodynamic genes (serotonin transporter, serotonin 2A and 2C receptors, catechol-o-methyltransferase, adrenergic receptor 2A, methylene tetrahydrofolate reductase, and human leucocytic antigens) across the two major molecular classes of PWS in a cohort of 33 referred patients who met medical criteria for testing. When results were pooled across PWS genetic subtypes, genotypic and allelic frequencies did not differ from normative population data. However, when the genetic subtype of PWS was examined, there were differences observed across all genes tested that may affect response to psychotropic medication. Due to small sample size, no statistical significance was found, but results suggest that pharmacodynamic gene testing should be considered before initiating pharmacotherapy in PWS. Larger scale studies are warranted.

Emergence Delirium With Post-traumatic Stress Disorder Among Military Veterans

The clinical characteristics of emergence delirium (ED) associated with post-traumatic stress disorder (PTSD) among military veterans encompass transient agitation, restlessness, disorientation, and violent verbal and physical behaviors due to re-experiencing of PTSD-related incidents. Two cases of ED after general anesthesia associated with PTSD are presented. Different anesthesia methods were applied for the two cases. A traditional medical approach appeared not to prevent the incidence of ED. In the second case, dexmedetomidine infusion along with verbal coaching was effective in preventing ED for a veteran known to have “wild wake-ups” with prior anesthetics. Further clinical studies in effectively utilizing dexmedetomidine in this population with PTSD at high risk for ED are warranted.

Genetic association of adrenergic receptor alpha 2A with obesity and type 2 diabetes

OBJECTIVE

The sympathetic nervous system (SNS) is linked to glucose, lipid, and protein metabolism. The α2A -adrenergic receptor (ADRA2A) is involved in the SNS and mediates inhibition of insulin secretion and lipolysis. The association of ADRA2A single-nucleotide polymorphisms (SNPs) with obesity and/or type 2 diabetes (T2D) was investigated.

DESIGN AND METHODS

Genotyping was performed in a case-control study of 1,177 Swedish individuals, including lean and obese subjects with normal glucose tolerance (NGT) and T2D patients. ADRA2A mRNA expression was measured in pancreatic islets isolated from T2D patients and nondiabetic subjects.

RESULTS

SNP rs553668 was associated with T2D in men (odds ratio [OR] = 1.47; 95% confidence interval [CI] = 1.08-2.01; P = 0.015) but this association was lost after adjusting for age and for body mass index (BMI). Associations were also detected when comparing obese NGT and lean NGT subjects (OR = 1.49; 95% CI = 1.07-2.07; P = 0.017), and in obese (OR = 1.62; 95% CI = 1.06-2.49; P = 0.026), but not in lean T2D. In women, multiple logistic regression regarding SNP rs521674 demonstrated an increased OR of 7.61 (95% CI = 1.70-34.17; P = 0.008) for T2D when including age as a covariant. Correcting for BMI removed the significant association. When age was included in the model, association also found when obese T2D patients were compared with lean NGT subjects (P = 0.041). ADRA2A mRNA expression in human pancreatic islets was detectable, but with no statistically significant difference between the diabetic and the control groups.

CONCLUSIONS

ADRA2A genetic polymorphisms are mainly associated with obesity and possibly with T2D in a Swedish population.

Constitutive coupling of a naturally occurring human alpha1a-adrenergic receptor genetic variant to EGFR transactivation pathway

We previously identified a naturally occurring human SNP, G247R, in the third intracellular loop of the α(1a)-adrenergic receptor (α(1a)-247R) and demonstrated that constitutive expression of α(1a)-247R results in twofold increased cell proliferation compared with WT. In the present study we elucidate molecular mechanisms and signal transduction pathways responsible for increased cell proliferation unique to α(1a)-247R, but not α(1a)-WT, α(1b), or α(1d)AR subtypes. We show that elevated levels of matrix metalloproteinase-7 (MMP7) and a disintegrin and metalloproteinase-12 (ADAM12) in α(1a)-247R-expressing cells are responsible for EGF receptor (EGFR) transactivation, downstream ERK activation, and increased cell proliferation; this pathway is confirmed using MMP, EGFR, and ERK inhibitors. We demonstrate that EGFR transactivation and downstream ERK activation depends on increased shedding of heparin-binding EGF. Finally, we demonstrate that knockdown of MMP7 or β-arrestin1 by shRNAs results in attenuation of proliferation of cells expressing α(1a)-247R. Importantly, accelerated cell proliferation triggered by the α(1a)-247R is serum- and agonist-independent, providing unique evidence for constitutive active coupling to the β-arrestin1/MMP/EGFR transactivation pathway by any G protein-coupled receptor. These findings raise the possibility of a previously unexplored mechanism for sympathetically mediated human hypertension triggered by a naturally occurring human genetic variant.

Alpha-adrenergic receptor gene polymorphisms and cardiovascular reactivity to stress in Black adolescents and young adults

Cardiovascular reactivity to stress and α-adrenergic receptor (α-AR) function may contribute to the development of hypertension. As Black Americans have an increased risk of hypertension, we evaluated associations between α(1A) -AR (Arg492Cys), α(2A) -AR (-1291C/G), and α(2B) -AR (Ins/Del301-303) gene variants and cardiovascular reactivity in 500 normotensive Black youth. Heart rate, preejection period, total peripheral resistance, and blood pressure were measured during cold and psychological stress. The Arg492Cys polymorphism in the α(1A) -AR gene was associated with heart rate reactivity to stress, but the association depended on sex. The -1291C/G promoter polymorphism in the α(2A) -AR gene was associated with vascular reactivity to stress; vasoconstriction increased as a linear function of the number of copies of the variant G allele. Thus, specific associations emerged between genetic variations in α-Ars and cardiovascular reactivity in young Blacks.

Resting sympathetic nerve activity is related to age, sex and arterial pressure but not to α2-adrenergic receptor subtype

OBJECTIVE

Sympathetic nerve hyperactivity has been associated with hypertension and heart failure and their cardiovascular complications. The α2-adrenergic receptors have been proposed to play a prominent role in the control of sympathetic neural output, and their malfunction to constitute a potential central mechanism for sympathetic hyperactivity of essential hypertension. Reports on the relationship between variant alleles of α2-adrenergic receptor subtypes and sympathetic drive or its effects, however, have not been consistent. Therefore, this study was planned to test the hypothesis that variant alleles of subtypes of α2-adrenergic receptors are associated with raised muscle sympathetic nerve activity (MSNA) in man.

METHODS

One hundred and seventy-two individuals, with a wide range of arterial pressure, were prospectively examined. Resting MSNA was quantified from multiunit bursts and from single units, and α2-adrenergic receptor subtypes were genotyped from DNA extracted from leucocytes and quantified by spectrophotometry.

RESULTS

No significant relationships between variant alleles of any of the α2A, α2B or α2C subtypes and raised muscle sympathetic activity were found. In contrast, MSNA showed a marked significant curvilinear relationship with age and systolic pressure; sex had a small but statistically significant effect. The α2-adrenergic receptor variants had a similar frequency when hypertensive and normotensive individuals were compared.

CONCLUSION

Variant alleles of three α2-adrenergic receptor subtypes were not related to resting muscle sympathetic nerve hyperactivity, indicating that their functional differences shown in vitro are not reflected in sympathetic activity in man. Age had a marked effect likely influencing arterial pressure through sympathetic activity.

Genetic variants in the alpha2C-adrenoceptor and G-protein contribute to ethnic differences in cardiovascular stress responses

OBJECTIVES

Cardiovascular responses to stressors are regulated by sympathetic activity, increased in black Americans, and associated with future cardiovascular morbidity. Our aim was to determine whether two functional variants in genes regulating sympathetic activity, a deletion in the alpha2C-adrenergic receptor (ADRA2C del322-325) and a G-protein beta3-subunit variant (GNB3 G825T), affect cardiovascular responses to physiologic stressors and contribute to their ethnic differences.

METHODS

We measured heart rate and blood pressure responses to a cold pressor test (CPT) in 79 healthy participants (40 blacks, 39 whites), aged 25.7+/-5.3 years, and determined genotypes for the ADRA2C and GNB3 variants. We examined the response variables (increase in heart rate and blood pressure) in multiple linear regression analyses adjusting first for baseline measures, ethnicity, and other covariates, and then additionally for genotypes.

RESULTS

Black participants had a greater heart rate response to CPT than whites [mean difference, 9.9 bpm; 95% confidence interval (CI), 4.1 to 15.6; P=0.001]. Both the ADRA2C del/del (15.8 bpm; 95% CI, 8.0-23.7; P<0.001) and GNB3 T/T genotypes (6.8 bpm; 95% CI, 0.9-12.7; P=0.026) were associated with greater heart rate response. After adjusting for genotypes, the ethnic difference was abrogated (1.3 bpm; 95% CI, -5.4-8.0; P=0.70), suggesting that the genetic variants contributed substantially to ethnic differences.

CONCLUSION

Variation in genes that regulate sympathetic activity affects hemodynamic stress responses and contributes to their ethnic differences. This study elucidates how genetic factors may in part explain ethnic differences in cardiovascular regulation.

Hypotension caused by therapeutic doses of venlafaxine: case report and proposed pathophysiological mechanisms

Although venlafaxine is usually associated with modest increases in blood pressure and not so often clinical hypertension, there are a few reported cases of hypotension related to overdoses of this specific antidepressant. The case study of a young female patient with a history of Major Depressive Disorder who initiated treatment with venlafaxine 75 mg/day and developed hypotension when the dosage was titrated up to 225 mg/day is described. The patient did not present comorbid diseases nor use other medication. A temporal association and a dose-dependent relationship between the hypotension and the use of venlafaxine is shown. To the best of the knowledge of the authors,this is the first case report that specifically associates regular doses of venlafaxine with the presence of hypotension. A pathophysiological mechanism is proposed, involving the participation of presynaptic alpha2-adrenergic receptors and the presence of a possible genetic polymorphism of cytochrome P4502D6, which is associated with lower drug metabolization, to explain the relationship between venlafaxine in regular dosage and development of hypotension.

Effect of the alpha2C-adrenoreceptor deletion322-325 variant on sympathetic activity and cardiovascular measures in healthy subjects

BACKGROUND

The alpha2C-adrenergic receptor plays an important role in the regulation of the sympathetic nervous system and, therefore, blood pressure and heart rate. A deletion polymorphism in its gene (ADRA2C del322-325), ten times more common in black than white Americans, has been associated with a loss of function in vitro and, under controlled study conditions, raised blood pressure and catecholamine secretion. We therefore examined the hypothesis that the ADRA2C deletion variant would alter sympathetic activity and contribute to ethnic differences in blood pressure.

METHODS

We measured resting plasma norepinephrine and epinephrine concentrations, blood pressure and heart rate in 224 healthy subjects (127 whites), and determined their ADRA2C del322-325 genotype. Additionally, we analyzed heart rate variability (HRV) in a subgroup of 50 black subjects.

RESULTS

Systolic (SBP) and diastolic blood pressure (DBP) were higher in blacks than whites [difference (95% confidence interval), 4.4 (1.5-7.4) mmHg, P = 0.003; and 2.7 (0.7-4.6) mmHg, P = 0.01, respectively]. Norepinephrine concentrations did not differ among subjects with 0, 1 and 2 copies of the deletion variant [median (interquartile range), 185.0 (147.5-269.8), 200.0 (154.9-257.0) and 173.8 (158.5-235.8) pg/ml, respectively; P = 0.54]. Similarly, none of the HRV parameters differed among the genotype groups. In multiple linear regression analyses adjusting for multiple covariates, the deletion genotype was not associated with SBP or DBP. In contrast, black ethnicity was associated with higher SBP (P = 0.001) and DBP (P = 0.005).

CONCLUSION

The ADRA2C deletion polymorphism had no effect on markers of resting sympathetic activity and cardiovascular measures, and did not account for ethnic differences in blood pressure.

Adrenergic receptor polymorphisms and autonomic nervous system function in human obesity

Adrenergic receptors (ARs) are cell-surface G-protein-coupled receptors for catecholamines. They are essential components of the sympathetic nervous system, organized within the autonomic nervous system (ANS), which controls various physiological functions, including energy homeostasis and metabolism of glucose and lipids. An impairment of ANS function in metabolism is considered to be one of the pathological states associated with human obesity and related metabolic diseases; thus, alterations in AR function might be implicated in the pathophysiology of these diseases. Several studies have suggested an association between obesity phenotypes and some AR polymorphisms. In vitro and human clinical studies indicate that some of these polymorphisms have functional and pathophysiological significance, including the linkage to ANS function. This review summarizes present knowledge of AR polymorphisms related to human obesity, and their association with ANS function.

Perioperative use of α2-adrenoceptor agonists and the cardiac patient

The centrally acting alpha2-adrenoceptor agonists clonidine and dexmedetomidine have been used with success to provide haemodynamic stability for patients undergoing surgery. Particularly in the case of patients with overt or underlying cardiac disease the actions of alpha2-adrenoceptor agonists, which include maintenance of stable systemic blood pressure and low heart rate and a reduction in overall oxygen consumption, can be expected to reduce the risk of procedure-related cardiac events. This expectation has been corroborated in clinical trials with clonidine, dexmedetomidine and mivazerol and meta-analyses; additional large controlled trials would be instructive in establishing a robust estimate of the scale of the benefit. In addition, alpha2-adrenoceptor agonists used as premedication have been shown to substantially reduce anaesthetic requirements among surgical patients, and the use of these agents has been associated with a reduced risk of postoperative delirium, which may be expected to improve considerably the postoperative course for at-risk patients. Dexmedetomidine is the only alpha2-adrenoceptor agonist currently approved for use in the intensive care unit. A distinctive feature of dexmedetomidine in that setting is that in addition to haemodynamic stability it confers a distinctive and advantageous quality of sedation: patients are tranquil but responsive to requests from attending staff. This review examines the pharmacological principles underlying the use of alpha2-adrenoceptor agonists as adjuncts to surgery and clinical experience in that indication.

Expression and function of alpha-adrenoceptors in zebrafish: drug effects, mRNA and receptor distributions

The alpha2-adrenoceptors are G-protein-coupled receptors that mediate many of the physiological effects of norepinephrine and epinephrine. Mammals have three subtypes of alpha2-adrenoceptors, alpha2A, alpha2B and alpha2C. Zebrafish, a teleost fish used widely as a model organism, has five distinct alpha2-adrenoceptor genes. The zebrafish has emerged as a powerful tool to study development and genetics, with many mutations causing diseases reminiscent of human diseases. Three of the zebrafish adra2 genes code for orthologues of the mammalian alpha2-adrenoceptors, while two genes code for alpha2Da- and alpha2Db- adrenoceptors, representing a duplicated, fourth alpha2-adrenoceptor subtype. The three different mammalian alpha2-adrenoceptor subtypes have distinct expression patterns in different organs and tissues, and mediate different physiological functions. The zebrafish alpha2-adrenergic system, with five different alpha2-adrenoceptors, appears more complicated. In order to deduce the physiological functions of the zebrafish alpha2-adrenoceptors, we localized the expression of the five different alpha2-adrenoceptor subtypes using RT-PCR, mRNA in situ hybridization, and receptor autoradiography using the radiolabelled alpha2-adrenoceptor antagonist [ethyl-3H]RS-79948-197. Localization of the alpha2A-, alpha2B- and alpha2C-adrenoceptors in zebrafish shows marked conservation when compared with mammals. The zebrafish alpha2A, alpha2Da, and alpha2Db each partially follow the distribution pattern of the mammalian alpha2A: a possible indication of subfunction partitioning between these subtypes. The alpha2-adrenergic system is functional in zebrafish also in vivo, as demonstrated by marked locomotor inhibition, similarly to mammals, and lightening of skin colour induced by the specific alpha2-adrenoceptor agonist, dexmedetomidine. Both effects were antagonized by the specific alpha2-adrenoceptor antagonist atipamezole.