Heterogeneity of alpha-2 adrenergic receptors

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Introduction: A Short History of Adrenoceptor Research

This chapter provides a short history of adrenoceptor research starting from the initial discovery of adrenaline. It covers the evolving classification of adrenoceptor subtypes, the cloning of these subtypes from multiple species, and factors such as adrenoceptor regulation, inverse agonism and biased agonism. More details on many of these aspects are provided in other chapters of this volume of Handbook of Experimental Pharmacology.

Noradrenaline and Movement Initiation Disorders in Parkinson’s Disease: A Pharmacological Functional MRI Study with Clonidine

Slowness of movement initiation is a cardinal motor feature of Parkinson’s disease (PD) and is not fully reverted by current dopaminergic treatments. This trouble could be due to the dysfunction of executive processes and, in particular, of inhibitory control of response initiation, a function possibly associated with the noradrenergic (NA) system. The implication of NA in the network supporting proactive inhibition remains to be elucidated using pharmacological protocols. For that purpose, we administered 150 μg of clonidine to 15 healthy subjects and 12 parkinsonian patients in a double-blind, randomized, placebo-controlled design. Proactive inhibition was assessed by means of a Go/noGo task, while pre-stimulus brain activity was measured by event-related functional MRI. Acute reduction in noradrenergic transmission induced by clonidine enhanced difficulties initiating movements reflected by an increase in omission errors and modulated the activity of the anterior node of the proactive inhibitory network (dorsomedial prefrontal and anterior cingulate cortices) in PD patients. We conclude that NA contributes to movement initiation by acting on proactive inhibitory control via the α2-adrenoceptor. We suggest that targeting noradrenergic dysfunction may represent a new treatment approach in some of the movement initiation disorders seen in Parkinson’s disease.

Alpha1-adrenergic antagonists block 6-nitrodopamine contractions on the rat isolated epididymal vas deferens

6-nitrodopamine (6-ND) is released from rat isolated vas deferens and modulates electrical-field stimulation (EFS) contractions of the rat isolated epididymal vas deferens (RIEVD) via a specific receptor which is blocked by tricyclic antidepressants. Here, the effects of selective α1-adrenergic receptor antagonists on RIEVD contractions induced by 6-ND, dopamine, noradrenaline, adrenaline and EFS were investigated. Doxazosin and tamsulosin (3-10 nM) caused significant rightward shifts of the concentration-response curve to 6-ND, but had no effect on dopamine-, noradrenaline- and adrenaline-induced contractions. Alfuzosin (10 nM) produced rightward shifts on concentration-response curves to all catecholamines. Silodosin (10 nM) and terazosin (100 nM) displaced to the right the noradrenaline, dopamine and adrenaline curves, but higher concentrations of both antagonists (100 and 300 nM, respectively) were required to displace the 6-ND curves. The EFS-induced contractions were significantly inhibited only at the concentrations that the α1-adrenergic receptor antagonists caused rightward shifts on the 6-ND concentration-response curves. The inhibition of EFS-induced contractions by doxazosin (10 nM), tamsulosin (10 nM), alfuzosin (10 nM), silodosin (100 nM) and terazosin (300 nM), were not observed in RIEVD obtained from animals chronically treated with L-NAME. This work demonstrates that α1-adrenoceptor antagonists act as 6-ND receptor antagonists in RIEVD, opening the possibility that many actions previously attributed to noradrenaline could be due to 6-ND antagonism. In addition, blockade of the 6-ND receptors by both tricyclic antidepressants and α1-adrenergic receptor antagonists may represent the common mechanism of action responsible for their therapeutic use in the treatment of premature ejaculation.

Medication Safety in Intravenous Therapy: A Compatibility Study of Clonidine with Drugs Frequently Used in Intensive Care

The intravenous pharmacotherapy of critically ill patients is extremely challenging due to the high number of drugs administered. We therefore evaluated the physicochemical compatibility of combinations of clonidine with drugs frequently used in an intensive care unit setting. Amiodarone, dihydralazine, furosemide, levosimendan, metamizole, milrinone, urapidil, and verapamil were each prepared as binary combinations with clonidine at the standard low and high administration concentrations. Selected ternary combinations were also analyzed. Samples were examined for physical compatibility. To verify chemical compatibility in samples deemed either physically compatible or to exhibit uncertain results, the drug content was quantified using high-performance liquid chromatography. Admixtures of clonidine with amiodarone or furosemide proved to be physically incompatible, whereas mixtures with levosimendan and metamizole exhibited results, which were not clearly meeting the specification criteria for physical compatibility. Binary combinations of clonidine with dihydralazine, milrinone, urapidil, and verapamil were found to be physically compatible. Combinations with dihydralazine, levosimendan, metamizole, milrinon, urapidil, or verapamil were chemically compatible for the analyzed concentrations. Ternary admixtures of clonidine, metamizole, and urapidil; clonidine, metamizole, and verapamil; clonidine, urapidil, and verapamil were shown to be physicochemically compatible for the analyzed concentrations. These data suggest that clonidine can be coadministered with dihydralazine, levosimendan, metamizole, milrinone, urapidil, and verapamil. However, the concomitant administration of clonidine with amiodarone or furosemide is not recommended.

Dexmedetomidine protects the heart against ischemia-reperfusion injury by an endothelial eNOS/NO dependent mechanism

The alpha2-adrenergic receptor agonist Dexmedetomidine (Dex) is a sedative medication used by anesthesiologists. Dex protects the heart against ischemia-reperfusion (IR) and can also act as a preconditioning mimetic. The mechanisms involved in Dex-dependent cardiac preconditioning, and whether this action occurs directly or indirectly on cardiomyocytes, still remain unclear. The endothelial nitric oxide synthase (eNOS)/nitric oxide (NO) signaling pathway and endothelial cells are known to play key roles in cardioprotection against IR injury. Therefore, the aims of this work were to evaluate whether the eNOS/NO pathway mediates the pharmacological cardiac effect of Dex, and whether endothelial cells are required in this cardioprotective action. Isolated adult rat hearts were treated with Dex (10nM) for 25min and the dimerization of eNOS and production of NO were measured. Hearts were then subjected to global IR (30/120min) and the role of the eNOS/NO pathway was evaluated. Dex promoted the activation of eNOS and production of NO. Dex reduced the infarct size and improved the left ventricle function recovery, but this effect was reversed when Dex was co-administered with inhibitors of the eNOS/NO/PKG pathway. In addition, Dex was unable to reduce cell death in isolated adult rat cardiomyocytes subjected to simulated IR. Cardiomyocyte death was attenuated by co-culturing them with endothelial cells pre-treated with Dex. In summary, our results show that Dex triggers cardiac protection by activating the eNOS/NO signaling pathway. This pharmacological effect of Dex requires its interaction with the endothelium.

Effects of modafinil and prazosin on cognitive and physiological functions in healthy volunteers

Previous research has demonstrated cognitive-enhancing effects of modafinil in humans and generated evidence for its therapeutic potential in psychiatric disorders. The neurochemical basis of these effects remains unresolved although a role for α1-adrenoceptors has been hypothesised. In this within-subject, double-blind, placebo-controlled study, 12 healthy male adults received modafinil (300 mg), the α1-adrenoceptor antagonist prazosin (3 mg), both together and placebo on separate occasions at least 5 days apart. Cognitive effects were assessed using a well-validated testing battery focusing on executive and working memory functions. Blood pressure, heart rate and salivary α-amylase (sAA) were measured at hourly intervals. Cognitive effects of modafinil and prazosin were identified at the difficult levels of the One-Touch Stockings of Cambridge (OTSOC) planning task. Prazosin antagonized the error-reducing effect of modafinil when the agents were given together. In contrast, the combined agents acted synergistically to increase time taken to complete OTSOC problems compared with placebo. The tachycardic and sAA-elevating effects of prazosin were also potentiated by concurrent modafinil administration. The current data suggest that the cognitive effects of modafinil on performance accuracy and latency are dissociable in terms of their neurochemical mechanisms. Our findings support the hypothesised involvement of α1-adrenoceptors in some of the cognitive-enhancing effects of modafinil and warrant further investigation.

Pharmacological evidence for heterogeneity of ocular $aL2adrenoceptors

Previous studies have shown that ocular alpha 2 adrenoceptors are located prejunctionally on sympathetic neurons and postjunctionally on cells in the iris/ciliary body. While the activation of alpha 2 adrenoceptors at each site has been postulated to alter aqueous humor dynamics, little is known about the pharmacological characteristics of these receptors or their role in the modulation of anterior segment function. The purpose of the current study was to determine the possible heterogeneity of ocular alpha 2 adrenoceptors using relatively selective alpha 2 adrenoceptor agonists and antagonists to examine ocular pre- and postjunctional alpha 2 adrenoceptors. Prejunctional alpha 2 effects were evaluated by means of the cat nictitating membrane (CNM) preparation. Postjunctional alpha 2 effects were evaluated by means of the cAMP assay in rabbit iris root/ciliary body. In the CNM, the administration of UK-14, 304 (UK) produced a dose-related inhibition of neuronally mediated contractions. Pretreatment with the alpha 2 antagonist rauwolscine caused a 1 to 2 log unit right shift in the dose-response curve of UK in the CNM. However, pretreatment with alpha 2 antagonist SKF 104078 had no demonstrable effect on UK-induced inhibition of neuronally mediated contractions of the CNM. In the rabbit iris root/ciliary body, UK produced a concentration-dependent inhibition of cAMP accumulation on isoproterenol- and VIP-induced cAMP production. Pretreatment of iris root/ciliary bodies with SKF 104078 or rauwolscine reversed the inhibitory effect of UK on isoproterenol- and VIP-induced accumulation of cAMP. These data provide the first evidence that the pre- and postjunctional alpha 2 adrenoceptors represent pharmacologically distinct subpopulations of receptors in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Fibrinogen−Catecholamine Interaction as Observed by NMR and Fourier Transform Infrared Spectroscopy

In this work, the interactions between the main catecholamines-epinephrine and norepinephrine-and fibrinogen were investigated by NMR and Fourier transform infrared spectroscopies. The two hormones were found to interact with fibrinogen and to affect the protein secondary structure to a different extent. In particular, the protein selectively binds epinephrine at both the basal and stress concentrations, while it shows a weak nonspecific interaction with norepinephrine. The interaction with the stress level of epinephrine leads to drastic protein conformational changes, whereas norepinephrine does not affect fibrinogen secondary structure, even at stress concentration.

Characterisation of alpha2-adrenoceptor subtypes involved in gastric emptying, gastric motility and gastric mucosal defence

The effect of clonidine on ethanol-induced gastric mucosal damage, gastric emptying and gastric motility was compared. The clonidine-induced gastroprotective effect (0.03-0.09 micromol/kg, s.c.) was antagonised by yohimbine (5 micromol/kg, s.c.), prazosin (0.23 micromol/kg; alpha2B-adrenoceptor antagonist) and naloxone (1.3 micromol/kg, s.c.). Clonidine also inhibited the gastric emptying of liquid meal (0.75-3.75 micromol/kg, s.c.) and gastric motor activity (0.75 micromol/kg, i.v.) stimulated by 2-deoxy-D-glucose (300 mg/kg, i.v.). Inhibition of gastric emptying and motility was reversed by yohimbine (5 and 10 micromol/kg, s.c., respectively), but not by prazosin (0.23 micromol/kg, s.c.) or naloxone (1.3 micromol/kg, s.c.). Oxymetazoline-an alpha2A-adrenoceptor agonist-inhibited both gastric emptying (0.67-6.8 micromol/kg, s.c.) and motility (0.185-3.4 micromol/kg, i.v.), whereas it failed to affect gastric mucosal lesions. The results indicate that in contrast to the gastroprotective effect, which is mediated by alpha2B-adrenoceptor subtype, alpha2A-adrenoceptor subtype may be responsible for inhibition of gastric emptying and motility. However, the site of action (central, peripheral, both) remains to be established.

Are so many adrenergic receptor subtypes really present in domestic animal tissues? A pharmacological perspective

Adrenergic receptors (ARs) are the cellular membrane binding sites through which natural catecholamines and sympathomimetic drugs exert their physiological and pharmacological effects. In recent decades, studies to clarify the distribution and function of ARs have been performed mostly on cultured cells, laboratory animals and human target tissues, but little is known about these aspects in domestic animals. This review focuses on AR structure, classification and signalling pathways and on AR subtype distribution in target tissues of some domestic animals, namely dogs, horses and bovines. In these species, different alpha- and beta-AR subtypes have been characterized and the functions controlled by the adrenergic systems have been studied. In the dog, the role played by the adrenergic system in the pathogenesis of cardiovascular disorders and in the modulation of canine aggression has roused particular interest. In dogs affected by dilated cardiomyopathy a significant down-regulation of beta-ARs has been observed both in the heart and circulating lymphocytes. This finding confirms the involvement of the adrenergic system in the pathogenesis and progression of the disorder and suggests new therapeutic strategies. In the horse, AR distribution has been studied in the cardiac, respiratory and gastrointestinal systems as well as in digital veins and arteries. The cardiac beta-ARs in healthy horses seem to be predominantly represented by the beta(1) subtype. In this species, heart failure may increase the expression of the beta(2) subtype, rather than causing AR down-regulation. Different beta- and alpha-AR subtypes have been characterized in the smooth muscle of equine ileum. The sympathetic relaxation of equine ileum smooth muscle seems to depend mainly on beta(3)-AR subtype activation, with minor involvement of the beta(2) subtype. In the respiratory tract, regional differences have been evidenced in the functionality of beta-AR subtype. The beta(2) subtype predominates in all segments but the beta(2) subtype-mediated adenyl cyclase response is tissue-dependent, with higher activity in tracheal membranes than bronchial or pulmonary ones. Both alpha- and beta-AR subtypes are present in the genital tract of cows. Bovine ovarian and myometrial cell membranes express higher concentrations of beta(2)-ARs than the beta(1) subtype, whereas as far as alpha-ARs are concerned, a single class of alpha(1)-ARs and two distinct classes of alpha(2)-AR binding sites have been discriminated. Interestingly, it has been observed that the activation of the sympathetic system could play an important role in the pathogenesis of bovine ovarian cysts as suggested by the modifications in beta-AR levels in the hypophysis and ovary of cows affected by ovarian cysts. In this species, the phenomenon of down-regulation has been well studied in different organs of veal calves treated with clenbuterol as a "partitioning agent". Since differences exist in AR distribution among species, data obtained in laboratory animals or in human beings cannot be extrapolated to domestic animals and further investigation on AR subtypes in domestic animal tissues is necessary.

Medetomidine and dexmedetomidine: a review of cardiovascular effects and antinociceptive properties in the dog

Alpha(2)-adrenoreceptor agonists (alpha(2)-agonists) are commonly used in small animal anaesthesia for their potent sedative and analgesic properties, although concerns about their cardiovascular effects have prevented their full adoption into veterinary practice. Research into alpha(2) adrenoreceptor agonists and their clinical use is extensive, therefore this review focuses on the use of dexmedetomidine and medetomidine in dogs. Emphasis is given to the cardiovascular effects and antinociceptive action of these agents.

Characterization of the regulatory region of Adra2c, the gene encoding the murine alpha2C adrenoceptor subtype

The 5' flanking sequence (3,227 base pairs, bp) of the mouse Adra2c subtype gene was determined and characterized. The transcription start site was mapped to nucleotide 'A' of two initiator motifs in tandem array, i.e. 1,159 and 1,153 bp upstream from the initiation codon of the open reading frame (ORF) of Adra2c, respectively. One structural feature salient to the 5' regulatory region of Adra2c is present in the sequence 1 kb immediately upstream from the receptor ORF, which is highly enriched in GC content (76%) and CpG island counts (i.e. CpG/GpC, 146:177), and thus rich in Sp1-binding motifs. At the 3' flanking region, the polyadenylation signal was mapped to 481 bp downstream from the termination codon. The transcript defined by sequence data thereby is consistent with a size of 3 kb (brain form) determined by Northern blot analysis. The transgene, Adra2c-NN- lacZ, which links the promoter region of Adra2c to the lacZ reporter gene, was constructed in order to evaluate the functional capacity of the promoter and the putative motifs residing within the defined regulatory region (1.9 kb upstream from the ORF) in directing the reporter gene expression in vitro in transiently transfected cells and in vivo in transgenic (Tg) mice. Permissive cell types to Adra2c-NN include those derived from neural and kidney lineages. Significant Adra2c-NN-driven reporter expression in Tg mice established suggests that alpha2C adrenoceptor expression is permissive under Adra2c-NN in central (cerebral cortex, hippocampus, subthalamus, hypothalamus, superior colliculus, cerebellum, and brain stem) and peripheral (pancreatic beta-islets) tissues.

The role of α2-agonists in neurosurgery

Alpha2-agonists have been extensively used and studied in anaesthesia and intensive care medicine. A list of benefits includes anxiolysis, blood pressure stabilization, analgesia, anaesthetic sparing effects and sedation without respiratory depression or significant cognitive impairment. Fear of inadvertent hypotension, bradycardia or post-operative sedation, and the variability of the haemodynamic response to different doses or rates of administration, have meant that universal acceptance in clinical practice has not yet been seen. Recently, the actions of these agents on the alpha2-adrenoreceptor and the imidazoline receptor have been more accurately identified, helping to explain the differences between the actions of various agents. The two readily available agents, clonidine and dexmedetomidine have already been used in many different anaesthetic situations, for a wide variety of surgical procedures. We believe that both of these alpha2-agonists are useful adjuncts for the management of the neurosurgical patient during surgery, and in the intensive care unit.

Alpha-2 agonist-induced memory impairment is mediated by the alpha-2A-adrenoceptor subtype

The activation of alpha2-adrenoceptors has been reported to impair memory functions in both rats and humans. The alpha2-adrenoceptor subtype responsible for this detrimental effect is still unknown. The effect of the alpha2-agonists clonidine and guanabenz on memory processes, in dependence to the time of administration, was evaluated in the mouse passive avoidance test. Clonidine (0.02-0.2 mg kg(-1) i.p.) and guanabenz (0.1-0.3 mg kg(-1) i.p.) induced amnesia in a dose-dependent manner. From time-course experiments emerged that the impairment of memory function was detectable only when clonidine and guanabenz were administered 60 min before or immediately after the training test, respectively. This detrimental effect was prevented by pretreatment with the alpha2-antagonist yohimbine (1-3 mg kg(-1) i.p.) and by the alpha2A-antagonist BRL-44408 (0.3-1 mg kg(-1) i.p.). By contrast, the alpha(2B,C) antagonists ARC-239 (10 mg kg(-1) i.p.) and prazosin (1 mg kg(-1) i.p.) did not revert the amnesia induced by both clonidine and guanabenz. At the highest effective doses, clonidine and guanabenz were devoid of behavioral side-effects as well as maintained unaltered the motor coordination, as revealed by the rota-rod test. Furthermore, none of the compounds used modified the spontaneous motility as indicated by the Animex apparatus. These results indicate that clonidine and guanabenz impaired memory processes in a mouse passive avoidance paradigm through the selective activation of the alpha2A-adrenoceptor subtype.

Effect of alpha 2-adrenoceptor blockade on lithium action in the rat brain

The inhibitory effect of different concentrations of lithium (0.15-10 x 10(-3) M) on cAMP production induced by isoprenaline (1 x 10(-4) M) after the blockade of alpha(2)-adrenoceptors in the rat cerebral cortex was investigated. Low lithium concentrations (0.3-0.6 x 10(-3) M) exerted a significant inhibitory effect after yohimbine (1 x 10(-5) M) addition, but had no effect when isoprenaline alone or prazosin (1 x 10(-7) M) was added. The recovery of [3H]yohimbine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was evaluated in cortical membranes to study how alpha(2)-adrenoceptor repopulation affects the action of lithium on the adenylyl cyclase-cAMP system. When the density of alpha(2)-adrenoceptors was lower than 21%, lithium showed a significant inhibitory effect at all concentrations tested. However, at higher densities, increased concentrations of lithium were required to inhibit cAMP production. Our results suggest that the inhibitory effect of lithium on cAMP levels in the rat brain is conditioned by alpha(2D)-adrenoceptors.

Medetomidine analogs as selective agonists for the human alpha2-adrenoceptors

alpha(2)-Adrenoceptor (AR) agonists have therapeutic applications in a variety of diseases. Medetomidine, an alpha(2)-AR agonist, belongs to 4-substituted imidazole class of compounds and is highly selective for the alpha(2)-AR. The benzylic methyl group of medetomidine and naphthalene imidazole was proposed to interact with rat brain alpha(2)-ARs via a methyl binding pocket in a manner analogous to its presence in alpha-methyl norepinephrine. A series of derivatives containing hydrophilic and hydrophobic substituents, as well as chiral and conformationally rigid analogs were used. In current binding and functional studies using human alpha(2)-AR subtypes expressed in Chinese hamster ovary cells, optimal interactions were observed with the presence of the methyl group on the benzylic carbon atom of naphthyl imidazole. Data obtained with various analogs have demonstrated that size, electronegativity, lipophilicity, chirality and conformational flexibility of the substituents at the carbon bridge of naphthyl imidazole are important factors for interaction of the imidazole class of ligands with these alpha(2)-AR subtypes. Taken collectively, the results obtained support the existence of the methyl binding pocket for optimal ligand receptor binding interactions in human alpha(2)-AR subtypes. Further, the results also suggest that, additional modifications of medetomidine and naphthyl methyl imidazole at the benzylic carbon atom, and/or on the aromatic and imidazole ring systems could provide insights into the chemical requirements for optimizing alpha(2)-AR subtype selectivity. This could eventually lead to the discovery of promising compounds for the evaluation of the physiological importance of the three alpha(2)-AR subtypes.

Yohimbine dimers exhibiting selectivity for the human alpha 2C-adrenoceptor subtype

Yohimbine is a potent and selective alpha2- versus alpha1-adrenoceptor antagonist. To date, drugs with high specificity for the alpha2-adrenoceptor show marginal selectivity among the three alpha2-adrenoceptor subtypes. Initial studies showed that yohimbine was about 4- and 15-fold more selective for the human alpha2C-adrenoceptor in comparison with the alpha2A- and alpha2B-adrenoceptors, respectively. To improve on this alpha2-adrenoceptor subtype selectivity, a series of yohimbine dimers (varying from n = 2 to 24 spacer atoms) were prepared and evaluated for receptor binding on human alpha2-adrenoceptor subtypes expressed in Chinese hamster ovary cells. Each dimeric analog showed higher affinities for alpha2A- and alpha2C-adrenoceptor versus the alpha2B-adrenoceptor; and yohimbine dimers with spacers of n = 2, 3, 4, 18, and 24 exhibited selectivity for the alpha2C-adrenoceptor. The yohimbine dimers n = 3 and n = 24 showed the highest potency and selectivity (32- and 82-fold. respectively) for the alpha2C-adrenoceptor in receptor binding and in functional studies (42- and 29-fold, respectively) measuring cAMP changes using a cell-based luciferase reporter gene assay. The dimers (n = 3 and n = 24) had high selectivity (>1000-fold) for the alpha2C-adrenoceptor compared with the three alpha1-adrenoceptor subtypes. These findings demonstrate that the addition of spacer linkages to bivalent yohimbine molecules provides a successful approach to the development of ligands that are potent and highly selective for the alpha2C-adrenoceptor.

Regulated expression of ?2B adrenoceptor during development

There are three subtypes of alpha2 adrenoceptor, i.e., alpha2A, alpha2B, and alpha2C, mediating the specific effect of epinephrine and norepinephrine in various tissues by means of G protein-coupled signal transduction pathways. In an attempt to delineate the regulatory mechanism of the alpha2B receptor subtype (encoded by subtype gene Adra2b) expression in the central nervous system (CNS), we have established transgenic (Tg) mice lines in which the transgene (NN-lacZ) was composed of the promoter region of Adra2b (NcoI fragment, 4.7 kb immediately upstream from receptor coding region) and a reporter gene lacZ (encoding beta-galactosidase). The selective expression of alpha2B in brain as indexed by beta-galactosidase, under the direction of this promoter region, may be traced in situ by using X-gal staining. The expression pattern of Adra2b-NN-lacZ in CNS of Tg mice during development was examined. The temporal course of examination was from gestation day 9.5 (E9.5) to postnatal day 28 (P28). Significant X-gal staining was detected in the dorsal root ganglion and cranial nerves V and VII at E12.5. By E18.5, expression was noted in the cerebral cortex, anterior olfactory nucleus, hypothalamus, brainstem, and cerebellar Purkinje cells, among others, and persisted through postnatal development. Adra2b-NN-directed reporter expression was detected in the hippocampal dentate gyrus first at P4. The temporal course of expression up to P28 in this area is in accordance with the developmental profiles of granule neurons of dentate gyrus. From P7 on, transgene expression was detected in additional brain areas, including the septum and thalamus. The expression correlates well with the noradrenergic innervations as evidenced by colocalization by using tyrosine hydroxylase or dopamine-beta-hydroxylase immunocytochemistry.

Novel concepts for analgesia in pediatric surgical patients. Cyclo-oxygenase-2 inhibitors, alpha 2-agonists, and opioids

Pain has been an understated concern in infants and children. Failure to recognize pain in the past has resulted in undue suffering by infants and children of all ages, but with the introduction of instruments to measure pain and a widespread appreciation of the severity of this problem in this age group, pain is rapidly coming under control. Novel concepts in both old and new analgesics have created safe and effective pain management strategies for infants and children. This review examines three analgesics and their potential roles in infants and children: cyclo-oxygenase (COX)-2 inhibitors, alpha 2-agonists and opioids.

Alpha-adrenoceptor subtypes

Different studies have led to our present knowledge of the membrane receptors responsible for mediating the responses to the endogenous catecholamines. These receptors were initially differentiated into alpha - and beta-adrenoceptors. Alpha-adrenoceptors mediate most excitatory functions, and were in turn differentiated in the 1970s into alpha(1)- and alpha(2)-adrenoceptors. The alpha(1)-adrenoceptor type usually mediates responses in the effector organ. The alpha(2)-adrenoceptor type is located presynaptically and regulates the release of the neurotransmitter but it is also present in postsynaptical locations. Both alpha-adrenoceptors are important for the control of vascular tone, but we now know that neither alpha(1)- nor alpha(2)-adrenoceptors constitute homogeneous groups. Each alpha-adrenoceptor type can be subdivided into different subtypes and in this review we have turned our attention to these. The alpha(1)- and the alpha(2)-adrenoceptor subtypes were previously defined pharmacologically by functional and binding studies, and later they were also isolated and identified using cloning methods. In fact, the study of alpha-adrenoceptors was revolutionized by the techniques of molecular biology which permitted us to establish the present classification. The present classification of alpha(1)-adrenoceptors stands as follows: alpha(1A)-adrenoceptor subtype (cloned alpha(1c) and redesignated alpha(1a/c)), alpha(1B)-adrenoceptor subtype (cloned alpha(1b)) and alpha(1D)-adrenoceptor subtype (cloned alpha(1d) and redesignated alpha(1a/d)). It has not been easy to establish the distribution of these alpha(1)-adrenoceptor subtypes in the various organs and tissues, or to define the functional response mediated by each one in the different species studied. Nevertheless it seems that the alpha(1A)-adrenoceptor subtype is more implicated in the maintenance of vascular basal tone and of arterial blood pressure in conscious animals, and the alpha(1B)-adrenoceptor subtype participates more in responses to exogenous agonists. It has also been observed that the expression of the alpha(1B)-adrenoceptor subtype can be modified in pathological situations and particular attention has been paid to the regulation of expression of this receptor. The present classification of alpha(2)-adrenoceptors stands as follows: alpha(2A/D)-adrenoceptor subtype (today it is accepted that the alpha(2A)-adrenoceptor subtype and the alpha(2D)-adrenoceptor subtype are the same receptor but they were identified in different species: the alpha(2A) in human and the alpha(2D) in rat); alpha(2B)-adrenoceptor subtype (cloned alpha(2b)) and alpha(2C)-adrenoceptor subtype (cloned alpha(2c)). Today we know that the alpha(2A/D)- and alpha(2B)-adrenoceptor subtypes in particular control arterial contraction, and that the alpha(2C)-adrenoceptor subtype is responsible above all for venous vasoconstriction. We also know that the alpha(2 A/D)-adrenoceptor subtype fundamentally mediates the central effects of the alpha(2)-adrenoceptor agonists. Despite the validity of the above-mentioned classification of the alpha(1)- and alpha(2)-adrenoceptors, it seems clear that the contractions of a large number of tissues including smooth muscle are mediated by more than one alpha-adrenoceptor subtype. Moreover, few ligands recognise only one alpha-adrenoceptor subtype and the lack of specifity in the different drugs for each one limits their administration in vivo and their therapeutic use.

Characterization of alpha-adrenoceptor subtypes in smooth muscle of equine ileum

OBJECTIVE

To determine the concentration and binding characteristics of alpha-adrenoceptor subtypes in smooth muscle cell membranes of equine ileum.

SAMPLE POPULATION

Segments of longitudinal and circular smooth muscle from the ileum of 8 male and 8 female adult horses.

PROCEDURE

Distribution of alpha-adrenoceptor subtypes was assessed by use of radioligand binding assays incorporating [3H]-prazosin and [3H]-rauwolscine, highly selective alpha1- and alpha2-adrenoceptor antagonists, respectively. Characterization of adrenoceptor subtypes was performed by use of binding inhibition assays.

RESULTS

On the basis of binding affinity for specific radioligands, low- and high-affinity alpha1- and alpha2-adrenoceptors were detected. Concentration of low-affinity alpha2-adrenoceptors was significantly greater in male horses, compared with females. Competition studies confirmed the specificity of the radioligands used in the binding assays. Alpha1-adrenoceptors of both subtypes in male and female horses had a higher affinity for prazosin than phentolamine, whereas yohimbine did not compete with the radioligand for binding. For alpha2-adrenoceptors regardless of subtype, potency of inhibition elicited by each drug varied between sexes. In males, yohimbine was a more potent inhibitor than phentolamine, which was more potent than prazosin. In females, yohimbine was more potent than prazosin, which was more potent than phentolamine.

CONCLUSIONS AND CLINICAL RELEVANCE

High- and low-affinity alpha1- and alpha2-adrenoceptors were detected in smooth muscle of equine ileum. Because alpha-adrenoceptor subtypes, particularly alpha2-adrenoceptors, are involved in the regulation of gastrointestinal tract function, characterization of these receptors may represent the basis for development of new therapeutic strategies for the control of gastrointestinal disturbances in horses.

Interaction of formamidine pesticides with the presynaptic alpha(2)-adrenoceptor regulating

The effects of the formamidine pesticides amitraz and chlordimeform on the alpha(2)-adrenergic receptor subtype that mediates the release of [(3)H]noradrenaline by synaptosomes from rat hypothalami were studied. We initially characterized the presynaptic autoreceptor on noradrenergic nerve endings using selective antagonists. Yohimbine (a nonselective alpha(2) antagonist) and BRL 44408 (selective for subtypes alpha(2A)/alpha(2D)) diminished the inhibitory effect of xylazine on K(+)-evoked release of [(3)H]noradrenaline; the K(B) values were 481 and 154 nM, respectively. In contrast, prazosin (a selective alpha(2B)/alpha(2C) antagonist) did not modify the inhibitory effect of xylazine. These results indicate that the release of noradrenaline by noradrenergic nerve endings in the rat hypothalamus is regulated by alpha(2D)-adrenoceptors, a species variation of the human alpha(2A) subtype. We then assessed the effects of the two pesticides on the K(+)-evoked release of [(3)H]noradrenaline. Amitraz reduced release in a dose-dependent manner; the effect observed at the maximal concentration tested (10 microM) was 13.0 +/- 2.0% and it was reversed by yohimbine. Amitraz also diminished the inhibitory effects of the alpha(2)-adrenergic agonists clonidine and xylazine. Chlordimeform displayed no effects, possibly because the true active compound of this insecticide is its demethylated metabolite. Based on these findings we conclude that the formamidine pesticides act as partial agonists of presynaptic alpha(2D)-adrenergic receptors in the rat hypothalamus. This interaction may be responsible for the in vivo alterations in catecholaminergic regulation of cyclic variations in gonadotropin-releasing hormone (GnRH) secretion, which can have grave functional repercussions on the reproductive system of mammals exposed to these xenobiotics.

Involvement of the opioid system in the central antisecretory action of alpha-2 adrenoceptor agonists in rat

The aim of the present study was to analyse the role of the central alpha-2 adrenoceptors in the regulation of gastric acid secretion in pylorus ligated rats. It was found that the intracerebroventricularly (icv.) injected presynaptic alpha-2 adrenoceptor agonist clonidine and the alpha-2A adrenoceptor subtype selective stimulant oxymetazoline exerted a dose dependent inhibition on gastric acid secretion. The antisecretory ED(50) values for clonidine and oxymetazoline were 20 and 7.5 nmol/rat icv., respectively. The antisecretory effect of these compounds was antagonised by the presynaptic adrenoceptor antagonist yohimbine (50 nmol/rat icv.) indicating that the action is mediated through central presynaptic alpha-2 adrenoceptors. Moreover, naloxone (50 nmol/rat icv.)--non-selective opioid antagonist--and naltrindole (0.5 nmol/rat icv.)--delta-opioid receptor selective antagonist--also decreased the antisecretory effect of clonidine and oxymetazoline suggesting that the endogenous opioid system is likely to be involved in the central antisecretory action of alpha-2 adrenoceptor stimulants.

Adrenoceptor subclassification: an approach to improved cardiovascular therapeutics

The subdivision of alpha adrenoceptors into the alpha 1 and alpha 2 classes was the impetus for the design of the selective alpha 1-adrenoceptor antagonists, which remain useful antihypertensives. alpha 2-Adrenoceptor agonists also have application as antihypertensive drugs, based on their ability to reduce sympathetic outflow. Likewise, subdivision of the beta adrenoceptors has lead to the development of selective beta 1-adrenoceptor antagonists as antihypertensive and selective beta 2 agonists as bronchodilators. In the past decade, both the alpha 1 and alpha 2 adrenoceptors have been further subdivided, each into three subclasses. In addition, there is strong functional evidence to suggest the presence of additional adrenoceptor subtypes, such as the "alpha 1L" adrenoceptor and "beta 4" adrenoceptor. alpha 1A (or alpha 1L)-Adrenoceptor antagonists have been evaluated for benign prostatic hyperplasia (BPH), and selective alpha 1A agonists for stress incontinence. Gene knockout experiments in mice suggest an important role for the alpha 1B adrenoceptor in the control of vascular tone. Hence, selective alpha 1B antagonists may offer a new approach toward hypertension. Although targeting of specific adrenoceptors can be used to optimize the therapeutic profile of a drug, there are also cases where blockade of multiple adrenoceptors is desirable, as with the alpha/beta-adrenoceptor antagonist carvedilol in congestive heart failure. It is possible that combination of affinities for selected adrenoceptor subtypes within a single molecule may be desirable for certain applications.

Antinociception by epidural and systemic alpha(2)-adrenoceptor agonists and their binding affinity in rat spinal cord and brain

This study was designed primarily to relate the antinociceptive and hemodynamic effects of clinically available alpha(2)-adrenoceptor agonists to their binding affinity for alpha(2)-adrenoceptors in the spinal cord and brain. In rats with chronic indwelling epidural catheters, the percentage maximal possible effect on tail-flick latency was measured after epidural or IM dexmedetomidine (DXM), clonidine (CL), or tizanidine (TZ) administration. To examine their binding affinities, isolated spinal cord and brain membranes with an alpha(2) agonist were incubated with (3)H-UK14304, a selective alpha(2) agonist, and the radioactivity in the reaction mixtures was measured by liquid scintillation spectrometry. Epidural DXM (0.5-10 microg), CL (10-500 microg), and TZ (5-500 microg) all produced dose-dependent antinociceptive effects; the rank order of potencies was DXM > CL > TZ, the same as for their systemic administration. The antinociceptive effects were blocked by epidural yohimbine. The receptor binding affinities expressed as the concentration that inhibits 50% for spinal cord and brain, respectively, were 0.25 and 1.3 nM (DXM), 10.8 and 12.5 nM (CL), and 48.2 and 96.8 nM (TZ). The changes in arterial blood pressure and heart rate evoked by antinociceptive doses did not correlate with the rank order of antinociceptive potencies. The relative antinociceptive potencies of epidural alpha(2) agonists may depend on their binding affinities to alpha(2)-adrenoceptors in the spinal cord, but their cardiovascular effects may result from actions both inside and outside the central nervous system.

IMPLICATIONS

Spinal antinociception caused by the epidural administration of alpha(2) agonists is well correlated with their binding affinity to spinal alpha(2)-adrenoceptors.

Alpha(2) adrenoceptor agonists as potential analgesic agents. 1. (Imidazolylmethyl)oxazoles and -thiazoles

A series of (imidazolylmethyl)oxazoles and -thiazoles were prepared and evaluated as alpha(2) adrenoceptor agonists. These compounds were also tested in in vivo paradigms that are predictive of analgesic activity. Variations in both the imidazole and thiazole portions of the molecule were investigated. Some of the more potent compounds such as 22, 26, 45, and 53 displayed alpha(2) receptor binding in the 10-20 nM range and also had significant antinociceptive activity in the mouse abdominal irritant test (MAIT).

Peripheral and central imidazoline receptor-mediated natriuresis in the rat

On the basis of both radioligand and functional studies, the existence of a novel receptor that was unique from the alpha 2-adrenoceptor has become evident. Our initial studies contrasted the function of I1 imidazoline receptor agonists with that of purported alpha 2-adrenoceptor agonists in the kidney. The mechanism by which urine flow increased (osmolar vs free water clearance) as well as the effects of idazoxan, rauwolscine, a V2 vasopressin receptor antagonist, indomethacin pretreatment, and one-kidney one clip hypertension in rats were different following moxonidine when compared to an alpha 2-adrenoceptor agonist. This indicated two separate receptor systems. Subsequent studies determined that i.c.v. administration of moxonidine would also increase the urine flow rate by increasing osmolar clearance. This response to i.c.v. moxonidine differed from the response of an alpha 2-adrenoceptor agonist administered i.c.v.. Moreover, this effect of i.c.v. moxonidine was unique from that observed following the intrarenal infusion of moxonidine (Fig. 2). Denervation, intravenous prazosin, and i.c.v. idazoxan selectively blocked the effects of i.c.v. moxonidine. Intravenous idazoxan selectively blocked the response to intrarenal infusion of moxonidine. On the basis of the response to i.c.v. moxonidine in SH rats, the site(s) and/or receptor(s) responsible for blood pressure lowering were altered and those for increasing sodium excretion appear to be inactive. The significance of the findings in long-term regulation of blood pressure remain to be determined.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Alpha-noradrenergic receptor modulation of the phencyclidine- and delta9-tetrahydrocannabinol-induced increases in dopamine utilization in rat prefrontal cortex

The noncompetitive NMDA receptor antagonist phencyclidine (PCP) and the neuronal cannabinoid receptor agonist delta9-tetrahydrocannabinol (THC) are two agents shown to have psychotomimetic properties in humans. Both drugs increase dopamine release and utilization in the prefrontal cortex, a brain region thought to be dysfunctional in schizophrenia. In the present series of studies, the effects of drugs acting at alpha-noradrenergic receptors on PCP- and THC-induced increases in prefrontal cortical and nucleus accumbens dopamine utilization in the rat were examined. Clonidine, an alpha2 noradrenergic receptor agonist, completely blocked the activation of mesoprefrontal dopamine system by THC or PCP. In addition, the alpha1 noradrenergic receptor antagonist prazosin blocked the PCP-induced increase in prefrontal cortical dopamine utilization. These data may provide new insights concerning pharmacological therapies for acute drug-induced psychoses and behavioral abnormalities in human PCP and THC abusers.

Alpha2C-adrenoceptors mediate inhibition of forskolin-stimulated cAMP production in rat striatum

The potential role of alpha2-adrenoceptors in modulating the activity of adenylyl cyclase in the rat striatum was examined. The selective alpha2-adrenoceptor agonist, UK14,304, produced a concentration-dependent inhibition of forskolin-stimulated accumulation of cAMP in striatal slices. The effect of UK14,304 was reversed by pre-incubation of striatal slices with the selective alpha2-adrenoceptor antagonist, RX821002. To determine whether alpha2C-adrenoceptors contribute to the alpha2-adrenoceptor-induced inhibition of forskolin-stimulated cAMP accumulation, an antisense oligodeoxynucleotide directed against alpha2C-adrenoceptor mRNA (alpha(2C)AS) or a random sequence (RS) was infused directly into the striatum. The ability of alpha(2C)AS to reduce the expression of alpha2C-adrenoceptors has been previously demonstrated. Alpha2C(AS) infusions did not reduce the ability of UK14,304 to inhibit forskolin-stimulated cAMP accumulation. Instead, alpha(2C)AS significantly enhanced forskolin-stimulated cAMP accumulation on the infusion side compared to the contralateral striatum. In contrast to the effects of alpha(2C)AS, infusions of RS had no effects on forskolin-stimulated cAMP accumulation or on the ability of UK14,304 to inhibit this effect. Incubation of striatal slices from untreated rats with RX821002 could mimic the ability of alpha(2C)AS infusion to enhance forskolin-stimulated cAMP accumulation, and did so in a concentration-dependent manner. Alpha2-adrenoceptors are negatively coupled to adenylyl cyclase in the rat striatum and alpha2C-adrenoceptors appear to be under tonic activation by an endogenous ligand in striatal slices.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Determination of dexmedetomidine in rat plasma by a sensitive [3H]clonidine radioreceptor assay

This paper describes the development and implementation of a sensitive radioreceptor assay (RRA) for determining concentrations of dexmedetomidine, an alpha-2 adrenergic agonist with anesthetic properties, in rat plasma. Calf retina membranes were selected as the alpha-2 adrenergic receptor source, and the alpha-2 antagonist [3H]RX821002 and the alpha-2 agonist [3H]clonidine were evaluated as radioligands. We optimized the binding conditions for both radioligands and chose a radioligand for implementation in the RRA based on the characteristics of the inhibition binding curves with dexmedetomidine. The final method is based on competition between the radioligand [3H]clonidine and dexmedetomidine for high-affinity binding sites present in calf retina membranes. The assay has a coefficient of variation of 8% in the range 23.7-592 pg for 0.2 mL of plasma. This assay can be applied to pharmacokinetic-pharmacodynamic studies of dexmedetomidine.

Reduced expression of alpha2C-adrenoceptors in rat striatum following antisense oligodeoxynucleotide infusion

The predominate subtypes of alpha2-adrenoceptors in the brain are alpha2A and alpha2C. The lack of selective ligands for these receptors hampers their functional characterization. We exploited an antisense strategy as an alternative pharmacological tool to study alpha2C-adrenoceptors. In rat striatum (caudate-putamen), alpha2-adrenoceptors were characterized using the subtype-non-selective antagonist [3H]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002). Specific [3H]RX821002 binding was saturable and to a single class of high-affinity sites. Curves for the inhibition of [3H]RX821002 binding by the alpha2C-selective compound, prazosin, were fit best by a model assuming binding to two sites, presumably reflecting binding to alpha2A- and alpha2C-adrenoceptors. A 15-mer phosphorothioate oligodeoxynucleotide (alpha2C AS) complementary to the alpha2C-adrenoceptor mRNA, or a random sequence (RS) was administered to rats continuously for 4.5 days directly into the striatum. Compared to RS infusions, alpha2C AS infusions induced a 35% reduction in the Bmax of [3H]RX821002 in striatal homogenates (P < 0.05). Curves for the inhibition of [3H]RX821002 binding by prazosin were fit best by a model assuming a single interaction in alpha2C AS-infused rats and to a model assuming two sites in RS-infused rats. These results are consistent with the conjecture that both alpha2A- and alpha2C-adrenoceptors occur in the rat striatum and also demonstrate the feasibility of an antisense approach to examine the functional role of subtypes of alpha2C-adrenoceptors in the brain.

[3H]R-terazosin binds selectively to alpha1-adrenoceptors over alpha2-adrenoceptors - comparison with racemic [3H]terazosin and [3H]prazosin

Most tissue sources for adrenoceptors contain a mixed population of alpha1- and/or alpha2-adrenoceptor subtypes; thus studies using non-specific radioligands are complicated by receptor heterogeneity. The examination of alpha1-adrenoceptor radioligand binding by radiolabeled terazosin and its enantiomers was simplified by using mouse fibroblast cells, which are thymidine kinase mutant (LTK-), transfected with cloned alpha1a-, alpha1b-, and alpha1d-adrenoceptor subtypes. [3H]Terazosin and its enantiomers were equipotent at the alpha1b-adrenoceptor. [3H]R-Terazosin was significantly less potent than [3H]terazosin and [3H]S-terazosin at the alpha1a- and the alpha1d-adrenoceptors. Using tissue derived alpha-adrenoceptors prepared in cold 25 mM glycyl-glycine buffer, [3H]prazosin, [3H]terazosin and [3H]S-terazosin bound to two sites in the rat neonatal lung preparation consistent with the presence of both alpha1- and alpha2B-adrenoceptors. The relative binding potencies of these radioligands at these two sites correlated with low affinity binding to the alpha2B-adrenoceptor and high affinity binding to an alpha1-adrenoceptor. [3H]R-Terazosin, on the other hand, bound to a single site in the rat neonatal lung membrane preparation, most likely an alpha1-adrenoceptor. Thus, [3H]R-terazosin may be useful as a selective alpha1-adrenoceptor radioligand for establishing the functional role of adrenoceptors in tissues expressing multiple subtypes.

Role of different subtypes of adrenoceptors in pressor responses to catecholamines released from sympathetic nerve endings

The role of alpha 1- and alpha 2-adrenoceptors in the vascular effects of catecholamines, either released locally from sympathetic nerve endings (e.g., in vascular smooth muscle) or derived from the adrenal medulla or administered intravenously, was studied using selective antagonists of these adrenoceptors. The ganglionic stimulant dimethylphenyl-piperazinium-iodide (DMPP) exerted dual actions on blood pressure: a rapid and short-term pressor reaction (phase I) resulting from catecholamine release elicited by ganglion stimulation, followed by a more sustained blood pressure elevation (phase II) resulting from the circulating catecholamines released from the adrenal medulla. The selective alpha 2-adrenoceptor, but a not subtype selective, antagonist 7,8-(methylenedioxi)-14-alpha-alloberbane HCl (CH-38083) (50-100 micrograms/kg, IV) significantly (p < 0.05) inhibited the pressor effects of epinephrine and norepinephrine given intravenously and phase II of the DMPP-induced pressor reaction. Idazoxan exerted similar effects, but at higher doses (400-600 micrograms/kg, IV). WB-4101 (50-100 micrograms/kg, IV) and BRL-44408 (2-3 mg/kg, IV), two selective alpha 2A-adrenoceptor antagonists, had the same activity as CH-38083, except did not inhibit the pressor effect of intravenously administered norepinephrine. The alpha 2B-adrenoceptor selective antagonist, ARC-239 (150 micrograms/kg, IV) did not influence phase II of DMPP-induced pressor reaction. Prazosin (200 micrograms/kg, IV), an antagonist of alpha 1 and alpha 2B-adrenoceptors, reduced blood pressure, the pressor response to intravenously administered epinephrine, and phase I of the DMPP-induced pressor effect. In addition, it completely inhibited the pressor responses to DMPP remaining after administration of CH-38083. These results suggest that the postsynaptically located alpha 1- and alpha 2(A and B)-adrenoceptors are involved in pressor response to norepinephrine and epinephrine, and are sensitive and accessible to catecholamines released locally from the axon terminals, and from the circulation to a different extent. These results may have great therapeutical importance in hypertension, for which the involvement of both a high level of circulating and locally released catecholamines may be indicative of the usefullness of a combination (alpha 1- and alpha 2-adrenoceptors- and Ca-channel-blocking agents) therapy.

Expression of alpha 2 adrenoceptors during rat brain development--I. Alpha 2A messenger RNA expression

The distribution of alpha 2A adrenoceptor messenger RNA expression in developing rat brain was characterized using in situ hybridization with 35S-labeled riboprobes. Intense hybridization signal was detected as early as embryonic day 14 in several areas adjacent to the forebrain and hindbrain germinal zones and in central noradrenergic neurons. A marked increase in messenger RNA expression was observed throughout the brain during late prenatal development, consistent with the migration and maturation of neurons in developing brain structures. In embryonic brain, there was a temporal and spatial correspondence in the appearance of alpha 2A messenger RNA expression and binding sites labeled with [3H]idazoxan or p-[125I]iodoclonidine, indicating translation into receptor protein at an early stage of development. Whereas the presynaptic expression remained constant throughout development, there was an early postnatal decline of alpha 2A receptor expression in many brain regions, including the olfactory bulb, cortex, caudate-putamen, hippocampus, thalamus, hypothalamus and medulla. Thereafter, messenger RNA expression increased, establishing an adult-like pattern during the second postnatal week, but remained low in areas such as the caudate-putamen, thalamus and hippocampus, which do not exhibit extensive expression in the adult. The transient perinatal expression of this alpha 2 adrenoceptor type, which coincides with a period of hyperreactivity to sensory stimuli in the locus coeruleus, may indicate a specific functional role for the alpha 2A receptor in the developing rat brain. The early and intense expression in olfactory structures suggests an involvement in early olfactory learning. The pattern of widespread, transient expression of alpha 2A receptors in the fetal brain is in marked contrast to the postnatal development of the alpha 2C receptor type.

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

Expression of alpha 2 adrenoceptors during rat brain development--II. Alpha 2C messenger RNA expression and [3H]rauwolscine binding

The distributions of alpha 2C adrenoceptor messenger RNA and high-affinity [3H]rauwolscine binding sites were characterized in developing rat brain. Using in situ hybridization with 35S-labeled riboprobes directed against the third intracellular loop, alpha 2C messenger RNA expression appeared in an adult-like pattern during the first and second postnatal weeks, in the anterior olfactory nucleus, caudate-putamen, olfactory tubercles, islands of Calleja and hippocampus, following the time-course of maturation of these structures. Only in the cerebellum was alpha 2C messenger RNA transiently expressed during the critical period of granule cell development. High-affinity [3H]rauwolscine binding sites were detected using receptor autoradiography and revealed a similar spatial and temporal time-course of appearance during rat brain development. The highest numbers of binding sites were detected in the olfactory tubercles and islands of Calleja, and moderate numbers in the anterior olfactory nucleus, caudate-putamen and hippocampus. Like alpha 2C messenger RNA expression, high-affinity [3H]rauwolscine binding sites were transiently expressed in the cerebellum. In some areas (e.g., the substantia nigra), [3H]rauwolscine binding sites were detected even though alpha C2 messenger RNA expression was absent. The strong spatial and temporal correspondence between messenger RNA expression and radioligand binding supports the conclusion that [3H]rauwolscine selectively labels alpha 2C adrenoceptors in the rat brain. The developmental pattern which was observed is in marked contrast to the early, transient expression of the alpha 2A adrenoceptor. Thus, the alpha 2A and alpha 2C receptor types may serve distinct functional roles in the developing brain.

Catecholamine regulation of the prefrontal cortex

The catecholamines dopamine (DA) and norepinephrine provide an essential modulatory influence on the working memory and attentional functions of the prefrontal cortex (PFC). The following critique reviews evidence that (1) either insufficient or excessive DA D1 receptor stimulation is detrimental to PFC function, while DA stimulation of the D2 family of receptors may contribute to detrimental actions in PFC and (2) that norepinephrine has an important beneficial influence on PFC function through its actions at post-synaptic, alpha 2A adrenergic receptors, but impairs PFC function through actions at alpha 1 adrenergic receptors. Critical levels of catecholamine stimulation may be needed to optimize PFC cognitive function; high levels of catecholamine release during stress may serve to take the PFC 'off-line' to allow faster, more habitual responses mediated by the posterior and/or subcortical structures to regulate behavior. These studies have relevance to our understanding and treatment of disorders with prominent symptoms of PFC dysfunction.

Clonidine-induced increase in osmolar clearance and free water clearance via activation of two distinct alpha 2-adrenoceptor sites

1. Clonidine, an alpha 2-adrenoceptor agonist, will increase urine flow rate in the anaesthetized rat by increasing both free water and osmolar clearance. In the present study, we investigated whether these effects of clonidine were mediated at two sites which could be distinguished pharmacologically in uninephrectomized male Sprague-Dawley rats. 2. Clonidine (1.0 nmol kg-1 min-1) infused into the renal artery increased osmolar and free water clearance. Following pretreatment with prazosin (0.15 mg kg-1, i.v.), an antagonist with reported selectivity for the alpha 2b-adrenoceptor subtype, the increase in free water but not osmolar clearance was decreased. Pretreatment with the opioid receptor antagonist, naltrexone (3.0 mg kg-1, i.v.) attenuated the increase in osmolar but not free water clearance. This disparate antagonism of clonidine by prazosin and naltrexone was consistent with two distinct sites. 3. We submit the hypothesis that the alpha 2a- and alpha 2b-adrenoceptor subtypes mediated the clonidine-induced osmolar and free water clearance. The blockade in free water clearance by prazosin indicated a possible role of the alpha 2b-adrenoceptor subtype whereas the alpha 2a-adrenoceptor subtype was considered as the site mediating the clonidine-induced increase in osmolar clearance. UK-14,304 (1.0 nmol kg-1 min-1), a mixed alpha 2-adrenoceptor/imidazoline receptor agonist with selectivity for the alpha 2a-subtype increased only osmolar clearance. This increase was blocked by naltrexone but not prazosin pretreatment. The imidazoline receptor was not involved, as naltrexone failed to alter the moxonidine (3.0 nmol kg-1-min-1) induced increase in osmolar clearance. These data suggested to us that the alpha 2a-/alpha 26-subtype hypothesis should be investigated more closely in future studies. 4. These findings indicate that the increase in osmolar and free water clearance following clonidine can be distinguished pharmacologically indicating that two sites were involved. Furthermore, we propose the hypothesis that the alpha 2a-adrenoceptor subtype mediated osmolar clearance whereas the alpha 2b-subtype mediated free water clearance. The prazosin-sensitive increase in free water clearance following clonidine suggested a possible role for the alpha 2b-subtype. The naltrexone-sensitive increase in osmolar clearance following clonidine and UK-14,304 (but not moxonidine) suggested a possible role of the alpha 2a-subtype. Clearly, this postulate requires further study.

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.

Prejunctional actions of K+ channel blockers in rat vas deferens

In studies of isometric contractions in prostatic portions of rat vas deferens evoked by single pulse electrical stimulation, the K+ channel blockers 4-aminopyridine, tetraethylammonium and charybdotoxin, but not apamin, significantly reduced the prejunctional inhibitory potency and the maximum inhibitory effect of the alpha 2-adrenoceptor agonist xylazine. The protein kinase C activator phorbol dibutyrate had similar effects to 4-aminopyridine against xylazine. However, 4-aminopyridine, tetraethylammonium, charybdotoxin and phorbol dibutyrate, but not apamin, significantly increased the magnitude of the isometric contraction to a single stimulus. 4-Aminopyridine and phorbol dibutyrate significantly reduced, while tetraethylammonium did not affect, isometric contractions to noradrenaline, and 4-aminopyridine failed to affect contractions to alpha,beta-methylene-ATP, so that the effects of these agents on the isometric contraction to a single stimulus were presumably by a prejunctional action. The Ca2+ entry facilitator Bay K 8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyrid ine carboxylic acid methylester) increased stimulation-evoked contractions by a postjunctional action and reduced the inhibitory effects of xylazine. When the isometric contraction following 4-aminopyridine was reduced by decreasing the stimulation voltage or by reducing the Ca2+ concentration from 2.5 to 0.9 mM, 4-aminopyridine significantly reduced the potency of xylazine. However, tetraethylammonium and Bay K 8644 failed to affect the inhibitory potency of xylazine in low Ca2+. It is concluded that the K+ channel blocker 4-aminopyridine reduces the prejunctional inhibitory potency of xylazine, and this action is independent of increased neurotransmitter release. These results suggest that prejunctional alpha 2-adrenoceptor-mediated inhibition in rat vas deferens involves K+ channels sensitive to block by 4-aminopyridine.