Identification of structurally distinct alpha 2-adrenergic receptors

IL-1 Signaling in Tumor Microenvironment

Interleukin 1 (IL-1) has long been known for its pleiotropic effects on inflammation that plays a complex, and sometimes contrasting, role in different stages of cancer development. As a major proinflammatory cytokine, IL-1β is mainly expressed by innate immune cells. IL-1α, however, is expressed by various cell types under physiological and pathological conditions. IL-1R1 is the main receptor for both ligands and is expressed by various cell types, including innate and adaptive immune cell types, epithelial cells, endothelial cells, adipocytes, chondrocytes, fibroblasts, etc. IL-1 and IL-1R1 receptor interaction leads to a set of common signaling pathways, mainly the NF-kB and MAP kinase pathways, as a result of complex positive and negative regulations. The variety of cell types with IL-1R1 expression dictates the role of IL-1 signaling at different stages of cancer, which under certain circumstances leads to contrasting roles in tumor development. Recent availability of IL-1R1 conditional knockout mouse model has made it possible to dissect the role of IL-1/IL-1R1 signaling transduction in different cell types within the tumor microenvironment. This chapter will focus on the role of IL-1/IL-1R1 in different cell types within the tumor microenvironment and discuss the potential of targeting this pathway in cancer therapy.

Adrenergic regulation of salt and fluid secretion in human medullary collecting duct cells

Transepithelial salt and fluid secretion mediated by cAMP in initial inner medullary collecting ducts (IMCDi) may be important for making final adjustments to urine composition. We examined in primary cultures of human IMCDi cells the effects of adrenergic receptor (AR) agonists and antagonists on intracellular cAMP levels, short-circuit current (I(SC)), and fluid secretion. Epinephrine (1 microM), norepinephrine (1 microM), and isoproterenol (10 nM) individually increased intracellular cAMP levels 57-, 2-, and 25-fold, respectively, and stimulated I(SC) 3.3-, 2.9-, and 3.4-fold, respectively. beta-AR activation increased net fluid secretion by cultured human IMCDi cell monolayers from 0.09 +/- 0.04 to 0.26 +/- 0.05 microl x h(-1) x cm(-2) and freshly isolated rat IMCDi from 0.02 +/- 0.01 to 0.09 +/- 0.02 nl x h(-1) x mm(-1). In monolayers, these effects were eliminated by blocking beta2-AR, but not beta1-AR. Activation of alpha2-AR with guanabenz inhibited isoproterenol-induced I(SC) by 37% in human IMCDi monolayers and fluid secretion by 91% in rat IMCDi. Immunohistochemistry of human medullary tissue sections revealed greater expression of beta2-AR than beta1-AR; beta2-AR was localized to the basolateral membranes of human IMCDi. Immunoblots identified alpha2A-AR and alpha2B-AR in cultured human IMCDi cell monolayers. We conclude that 1) catecholamines stimulate cAMP-dependent anion and fluid secretion by IMCDi cells primarily through beta2-AR activation and 2) alpha2-AR activation attenuates cAMP-dependent anion secretion.

Molecular and pharmacological characterization of the canine brainstem alpha-2A adrenergic receptor

This study characterizes the alpha2-adrenergic receptors present in canine brainstem. Radioligand binding and reverse transcriptase-polymerase chain reaction (RT-PCR) experiments were performed in canine brainstem to identify the receptors present and determine the pharmacological properties of these receptors. The pKi values derived from radioligand competition curves for a number of adrenergic receptor agents at the four alpha2-adrenergic receptor subtypes were compared to the canine brainstem. The pKi values at the canine brainstem alpha2-adrenergic receptor were consistent with the presence of the alpha2A-adrenergic receptor. To determine whether the canine brainstem expressed the message for the alpha2A-adrenergic receptor, RT-PCR was performed with specific primers for the four subtypes of alpha2-adrenergic receptors. In the canine brainstem, only the primers corresponding to a region in the human alpha2A-adrenergic receptor produced a PCR product. No bands were detected in the canine brainstem lanes with the alpha2B-, alpha2C-, or alpha2D-receptor primers. These data suggest that the canine brainstem contains the alpha2A-adrenergic receptor.

alpha2A-adrenergic receptors in the rat nucleus locus coeruleus: subcellular localization in catecholaminergic dendrites, astrocytes, and presynaptic axon terminals

To define the anatomic substrates subserving the inhibitory actions of alpha2-adrenergic receptors (alpha2-ARs) in the locus coeruleus (LC), we used dual-label immunoelectron microscopy with antibodies directed against the A-subtype of alpha2-AR (alpha2A-AR) and the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Of the profiles containing peroxidase labeling for alpha2A-AR (alpha2A-AR-IR) in the LC (n=735), most were dendrites ( approximately 50%), glial processes ( approximately 30%), and axon terminals ( approximately 15%). alpha2A-AR-IR was also observed in unmyelinated axons and perikarya. Within dendrites, alpha2A-AR-IR was associated with nonsynaptic regions of the plasma membrane and subsurface cisternae. Approximately 60% of dendrites with alpha2A-AR-IR were dually labeled for TH. Fifty percent of the axon terminals contacting alpha2A-AR-immunoreactive dendrites formed asymmetric (excitatory) synaptic contacts. Axon terminals with alpha2A-AR-IR were not dually labeled for TH and generally formed asymmetric synapses with TH-immunoreactive dendrites that contained or lacked alpha2A-AR-IR. Astrocytic processes exhibiting alpha2A-AR-IR were closely apposed to TH-labeled dendrites. These results extend previous ultrastructural observations of alpha2A-ARs in the LC and suggest that the inhibitory actions of norepinephrine and epinephrine in this region may be mediated by postsynaptic alpha2A-ARs on catecholaminergic dendrites and presynaptic alpha2A-ARs on excitatory inputs to catecholaminergic dendrites. In addition, the localization of alpha2A-AR-IR in astrocytic processes apposed to TH-immunoreactive dendrites suggests a role for alpha2A-ARs in functional interactions between catecholaminergic dendrites and neighboring astrocytes.

Affinity of detomidine, medetomidine and xylazine for alpha-2 adrenergic receptor subtypes

alpha 2-Adrenergic receptor agonists are widely used in veterinary medicine as sedative/hypnotic agents. Four pharmacological subtypes of the alpha 2-adrenergic receptor (A, B, C and D) have been identified based primarily on differences in affinity for several drugs. The purpose of this study was to examine the affinities of the sedative agents, xylazine, detomidine and medetomidine at the four alpha 2-adrenergic receptor subtypes. Saturation and inhibition binding curves were performed in membranes of tissues containing only one subtype of alpha 2-adrenergic receptor. The KD for the alpha 2-adrenergic receptor radioligand, [3H]-MK-912, in HT29 cells (alpha 2A-), neonatal rat lung (alpha 2B-), OK cells (alpha 2C-) and PC12 cells transfected with RG20 (alpha 2D-) were 0.38 +/- 0.08 nM, 0.70 +/- 0.5 nM. 0.07 +/- 0.02 nM and 0.87 +/- 0.03 nM, respectively. Detomidine and medetomidine had approximately a 100 fold higher affinity for all the alpha 2-adrenergic receptors compared to xylazine but neither agonist displayed selectivity for the alpha 2-adrenergic receptor subtypes. These data suggest that available sedative/hypnotic alpha 2-adrenergic receptor agonists can not discriminate between the four known alpha 2-adrenergic receptor subtypes.

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.

Further biochemical characterization of imidazoline binding sites from the human brainstem

Biochemical characteristics of imidazoline specific binding sites from the human brainstem were further investigated using [3H]idazoxan as radiolabeled ligand. The study of the interaction of [3H]idazoxan binding sites with heparin and lectins (soybean and lentil lectin) confirm the heterogeneity of these sites in the human brain. In fact, about 10-15% of [3H]idazoxan binding sites were retained by each of the three supports used, leading to the hypothesis that two populations of sites, with different biochemical characteristics, coexist in this tissue. A small proportion of [3H]idazoxan binding sites was retained on an affinity chromatography support consisting of a clonidine-derived Pharmalink column. The binding activity of these clonidine-eluted sites was markedly and dose-dependently improved by the addition of 'treated fall-through' fraction from the same column. On the other hand, this 'treated fall-through' fraction inhibited the binding activity detected in the solubilized human brainstem membranes. These results also suggest the existence of heterogeneous imidazoline specific binding sites in the human brainstem and the existence of endogenous factors able to discriminate between them.

Distribution of alpha 2C-adrenergic receptor-like immunoreactivity in the rat central nervous system

The distribution of alpha 2C-adrenergic receptors (ARs) in rat brain and spinal cord was examined immunohistochemically by using an affinity purified polyclonal antibody. The antibody was directed against a recombinant fusion protein consisting of a 70-amino-acid polypeptide portion of the third intracellular loop of the alpha 2C-AR fused to glutathione-S-transferase. Selectivity and subtype specificity of the antibody were demonstrated by immunoprecipitation of [125I]-photoaffinity-labeled alpha 2-AR and by immunohistochemical labeling of COS cells expressing the individual rat alpha 2-AR subtypes. In both cases the antibody recognized only the alpha 2C-AR subtype, and immunoreactivity was eliminated by preadsorption of the antibody with excess antigen. In rat brain, alpha 2C-AR-like immunoreactivity (alpha 2C-AR-LI) was found primarily in neuronal perikarya, with some labeling of proximal dendrites; analysis by confocal microscopy revealed the intracellular localization of some of the immunoreactivity. Areas of dense immunoreactivity include anterior olfactory nucleus, piriform cortex, septum, diagonal band, pallidum, preoptic areas, supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, amygdala, hippocampus (CA1 and dentate gyrus), substantia nigra, ventral tegmental area, raphe (pontine and medullary), motor trigeminal nucleus, facial nucleus, vestibular nucleus, dorsal motor nucleus of the vagus, and hypoglossal nucleus. Labeling was found in specific laminae throughout the cortex, and a sparse distribution of very darkly labeled cells was observed in the striatum. At all levels of the spinal cord there were small numbers of large, darkly labeled cells in layer IX and much smaller cells in layer X. In general, the pattern of alpha 2C-LI throughout the neuraxis is consistent with previously published reports of the distribution of receptor mRNA detected by hybridization histochemistry.

Cloning of a melatonin-related receptor from human pituitary

We have cloned an orphan G protein-coupled receptor from a human pituitary cDNA library using a probe generated by PCR. The cDNA, designated H9, encodes a protein of 613 amino acids that is 45% identical at the amino acid level to the recently cloned human Mel(1a) and Mel(1b) melatonin receptors. Structural analyses of the encoded protein and its gene, along with phylogenetic analysis, further show that H9 is closely related to the G protein-coupled melatonin receptor family. Unusual features of the protein encoded by H9 include a lack of N-linked glycosylation sites and a carboxyl tail >300 amino acids long. H9 transiently expressed in COS-1 cells did not bind [125I]melatonin or [3H]melatonin. H9 mRNA is expressed in hypothalamus and pituitary, suggesting that the encoded receptor and its natural ligand are involved in neuroendocrine function.

The three alpha 2-adrenergic receptor subtypes achieve basolateral localization in Madin-Darby canine kidney II cells via different targeting mechanisms

The present studies examined the localization of the alpha2B- and alpha2C-adrenergic receptor (AR) subtypes in polarized Madin-Darby canine kidney cells (MDCK II) and the mechanisms by which this is achieved. Previously we demonstrated that the alpha2AAR subtype is directly delivered to lateral subdomain of MDCK II cells. Surface biotinylation strategies demonstrated that the alpha2BAR, like the alpha2AAR, achieves 85-90% basolateral localization at steady-state. However, in contrast to the alpha2AAR, this polarization occurs after initial random insertion of the alpha2BAR into both apical and basolateral surfaces followed by selective retention on the lateral subdomain (t1/2 the apical surface is 15-30 min; t1/2 the basolateral surface is 8-10 h). The alpha2CAR also is enriched on the basolateral surface at steady-state and, like the alpha2AAR, is directly delivered there. Morphological evaluation of the epitope-tagged alpha2AAR, alpha2BAR, and alpha2CAR subtypes by laser confocal microscopy not only corroborated the biochemically-defined basolateral localization of all three alpha2AR subtypes but also revealed that the alpha2CAR uniquely exists in an intracellular compartment(s) as well. Immunofluorescence due to intracellular alpha2CAR partially overlaps that due to calnexin, a marker for endoplasmic reticulum, as well as that due to mannosidase II, a marker for the trans-Golgi network. Taken together, the present findings demonstrate that the alpha2AAR, alpha2BAR, and alpha2CAR subtypes, which possess highly homologous structures and ultimately achieve similar polarization to the lateral surface of MDCK II cells, nonetheless manifest distinct trafficking itineraries.

Vascular alpha-adrenoceptors: from the gene to the human

Adrenoceptors can be subdivided into three major types, the alpha 1-, alpha 2-, and beta-adrenoceptors. Each of these types can be further subdivided into three subtypes, based on pharmacological characteristics. Molecular cloning techniques have supported this subclassification. Recent data now suggest that alpha-adrenoceptor subtypes identified by pharmacological and molecular techniques correspond well, although species orthologs of several adrenoceptor subtypes have been identified. The secondary structure of the adrenoceptors has been elucidated and correlated with their interaction with second messenger molecules. alpha 1-Adrenoceptors, beta-adrenoceptors, and alpha 2-adrenoceptors mediate their actions through stimulation of inositol phosphate release, stimulation of adenylate cyclase, and inhibition of adenylate cyclase, respectively. Site-directed mutagenesis and the preparation of chimeric receptors have located the site of receptor--second messenger interaction to the third intracellular loop for each of these adrenoceptors. While subtypes of each of these classes all interact with the same second messenger, studies with recombinant alpha 2-adrenoceptors show subtype-related differences in receptor--second messenger interaction. Multiple alpha-adrenoceptor subtypes are expressed in vascular smooth muscle and are involved in various aspects of blood vessel function, including contraction, cellular growth, and proliferation. Various physiological factors can selectively influence responses to a particular subtype, and the relative roles of each subtype can vary between vascular beds and along an individual blood vessel as its caliber changes. Functional studies in blood vessels suggest the presence of additional alpha-adrenoceptor subtypes not yet identified via molecular techniques. Optimization of the therapeutic profile of an alpha-adrenoceptor antagonist may be possible via enhancement of selectivity for a particular subtype or by design of a specific profile of affinity for the individual subtypes.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Cloning and expression of a fish alpha 2-adrenoceptor

1. Pigment granule aggregation in specialized cells (melanophores) from the skin of teleost fishes has been shown to be mediated by receptors with an alpha 2-adrenoceptor pharmacology. We now report the cloning of the alpha 2-F, a fish skin alpha 2-receptor from the cuckoo wrasse (Labrus ossifagus). 2. Degenerate oligonucleotides corresponding to conserved regions of the human alpha 2-adrenoceptor subtypes were used in a polymerase chain reaction (PCR) with cDNA prepared from mRNA isolated subtypes were used in a polymerase chain reaction (PCR) with cDNA prepared from mRNA isolated from the skin of the cuckoo wrasse. An 876 base pair (bp) product was obtained that was homologous with that of the human alpha 2-adrenoceptor and was used to screen a genomic library from the cuckoo wrasse. 3. A clone (pTB17BS) consisting of approximately 5 kb of genomic DNA was obtained which contained the nucleotide sequence of the initial PCR product. In addition, it contained an open reading frame that encoded a protein of 432 amino acids and approximately 2 kb of 5'untranslated sequence. The deduced amino acid sequence of this protein showed 47-57% identity with the human alpha 2-adrenoceptors and thus appeared to encode a fish alpha 2-adrenoceptor. 4. In the 5'-untranslated region of the gene, nucleotide sequences were present suggesting that transcription of the alpha 2-F might be regulated by cyclic AMP, calcium and/or steroids. 5. The alpha 2-F was expressed in COS-7 cells and radioligand binding studies were performed with [3H]-rauwolscine. The binding was of high affinity and it was saturable with a KD of 0.8 +/- 0.1 nM and a Bmax of 5.7 +/- 1.0 pmol mg-1 of protein.6. Competition curves for the displacement of specific [3H]-rauwolscine binding showed the following order of potency: for agonists, medetomidine > clonidine >p-aminoclonidine> B-HT 920> (- )-noradrenaline;for antagonists, rauwolscine > atipamezole > yohimbine > phentolamine > prazosin.7. These results show that alpha2-F has characteristics of both the human alpha2-CIO and alpha2-C4 and that it might represent an ancestral alpha2-adrenoceptor subtype.

Immunohistochemical localization of alpha 2A-adrenergic receptors in catecholaminergic and other brainstem neurons in the rat

alpha 2-Adrenergic receptors mediate a large portion of the known inhibitory effects of catecholamines on central and peripheral neurons. Molecular cloning studies have established the identity of three alpha 2-adrenergic receptor genes from several species that encode the A, B and C subtypes of the receptor. The rat alpha 2A-adrenergic receptor, as defined by sequence similarity, is the orthologue of the human alpha 2A-adrenergic receptor. In this paper, we report the development of rabbit antisera directed against a portion of the third intracellular loop of the rat alpha 2A-adrenergic receptor and the histochemical localization of alpha 2A-adrenergic receptor-like immunoreactive material in the brainstem and spinal cord of the adult rat. Our antisera detected alpha 2A-adrenergic receptor-specific punctate staining associated with neuronal perikarya. alpha 2A-adrenergic receptor-like immunoreactivity was widely, but heterogeneously, distributed in the brainstem and spinal cord, predominantly in areas involved in the control of autonomic function. Double labelling with antisera to tyrosine hydroxylase or phenylethanolamine-N-methyl-transferase revealed that alpha 2A-adrenergic receptor-like immunoreactivity is present in most, perhaps all, noradrenergic and adrenergic cells of the brainstem. alpha 2A-Adrenergic receptor-like immunoreactivity was detected in a small percentage of the dopaminergic cells of the A9 and A10 groups. This study provides the first description of the specific immunohistochemical localization of alpha 2A-adrenergic receptors using a subtype-specific polyclonal antibody. The results support the view that alpha 2-adrenergic receptors are involved in central cardiovascular control and suggest that the catecholaminergic autoreceptors of central noradrenergic and adrenergic neurons are the A subtype of the alpha 2-adrenergic receptors.

Distributions of mRNAs for alpha-2 adrenergic receptor subtypes in rat brain: an in situ hybridization study

Selective 35S-labeled oligonucleotide probes were designed to sequences of the rat alpha-2A (RG20), alpha-2B (RNG), and alpha-2C (RG10) adrenoreceptor mRNAs for use in in situ hybridization experiments on sections of unfixed rat brain, spinal cord and kidney. After hybridized sections were exposed to film or dipped in autoradiographic emulsion, specific and selective labeling patterns characteristic for each probe and region of the central nervous system were observed. Alpha-2A mRNA labeling was most pronounced in neurons in layer six of the cerebral cortex, hypothalamic paraventricular nucleus, reticular thalamic nucleus, pontine nuclei, locus coeruleus, vestibular nuclei, trapezoid nuclei, deep cerebellar nuclei, nucleus tractus solitarii, ventrolateral medullary reticular formation, and the intermediolateral cell column of the thoracic spinal cord. In some of these locations, the receptor mRNA, in all probability, is present in noradrenaline and perhaps adrenaline neurons. The alpha-2B probe, which primarily labels the kidney, gave only a very light signal in the thalamus in the central nervous system after extended exposure times. Alpha-2C mRNA labeling was primarily observed in the olfactory bulb, cerebral cortex, islands of Calleja, striatum, hippocampal formation, cerebellar cortex, and dorsal root ganglia. Labeling patterns disappeared when excess unlabeled probes were added to their respective radiolabeled probes, or when sense probes were employed. When a hybrid antisense probe homologous to all three alpha-2 probes was used, labeling patterns also disappeared. The present study therefore justifies the pharmacological subclassification of alpha-2 receptors by providing anatomical evidence for specific and selective cell groups in the rat central nervous system containing mRNA for three alpha-2 receptor subtypes.

Platelet α2-adrenoreceptors in depression: a critical examination

During the past decade, results from radioligand studies comparing platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers have been inconsistent, especially when related to the known functional characteristics of these receptors. Despite the availability of radioligands for α(2)-adrenoreceptors, inherent methodological problems exist which make data from these studies difficult to interpret. The authors review the overall data from radioligand studies using [(3)H] clonidine and [(3)H] yohimbine of platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers. Theoretical and methodological issues are critically examined in the light of recent findings. Finally, alternative strategies for studying α( 2)-adrenoreceptors in clinical populations are considered.

α- and β-Adrenoceptors in Hypertension: Molecular Biology and Pharmacological Studies

Recent years have witnessed astonishing progress in our understanding of the molecular basis of adrenoceptor structure, function and regulation and revealed an unexpected heterogeneity of adrenoceptors demonstrating the existence of at least 11 subtypes. This paper discusses the implications of these advances on studies regarding a specific role of adrenoceptors in the development of genetic hypertension. The available data indicate that among the alpha-adrenoceptor subtypes the alpha 2A-adrenoceptor is the most likely candidate for an alteration specifically linked to genetic hypertension in the animal model of the spontaneously hypertensive rat and possibly in some patients. Alterations of other alpha-adrenoceptor subtypes may be specific for some forms of genetic hypertension but are unlikely to play an important role for blood pressure regulation. Most beta-adrenoceptor alterations appear to occur secondary to blood pressure elevation independently of whether hypertension has occurred on a genetic basis or not. Moreover, the mechanisms regulating alpha- and beta-adrenoceptor responsiveness upon prolonged agonist exposure may be altered in hypertension and thereby contribute to the pathophysiology of this disease.

Intrathecal coadministration of clonidine with serotonin receptor agonists produces supra-additive visceral antinociception in the rat

The intrathecal (i.t.) coadministration of sub-antinociceptive doses of clonidine, an alpha 2-adrenoceptor agonist, with DOI or RU-24969 (5-HT2 or 5-HT1B receptor agonists, respectively) produced dose-dependent supra-additive antinociceptive effects in a model of visceral pain. The enhanced attenuation of responses to noxious colorectal distension produced by the coadministration of these drugs is evidenced by significant leftward shifts in the dose-response curves as compared to those of each drug alone and by isobolographic analysis. The supra-additive antinociceptive effects produced following the i.t. coadministration of clonidine with RU-24969 were antagonized by i.t. pretreatment with phentolamine; the coadministration of phentolamine with methysergide produced no greater antagonism of effects. The supra-additive antinociceptive effects produced by i.t. coadministration of clonidine with DOI were antagonized by i.t. pretreatment with methysergide; the coadministration of methysergide with yohimbine produced no greater antagonism of effects. These data suggest that receptors acted upon by descending bulbospinal neurons interact to modulate the rostrad transmission of visceral nociceptive transmission.

Distribution of alpha 2-adrenergic receptor mRNAs in the rat CNS

alpha 2-Adrenergic receptors (ARs) are involved in central nervous system (CNS) control of blood pressure. It is now known that there are three human genes that encode subtypes of alpha 2-ARs, but little is known regarding the distribution of these subtypes throughout the CNS. The availability of receptor clones allows the mapping of mRNAs encoding the individual alpha 2-AR subtypes in the CNS. In this communication, we report that there are three, closely related rat alpha 2-AR genes. We have developed subtype-specific hybridization probes from each of these genes and have used these reagents to measure alpha 2-AR subtype mRNA accumulation in extracts of discrete regions of the rat CNS. We found that mRNAs encoding the alpha 2A-AR and alpha 2C-AR subtypes are distributed widely, but unevenly, throughout the rat CNS. The A subtype is prominent in the midbrain, brainstem, spinal cord, pituitary and diencephalon while the C subtype predominates in basal ganglia and cerebellum. The cortex, olfactory bulb and hippocampus contain roughly equal amounts of the alpha 2A- and alpha 2C-AR mRNAs. A third subtype's (alpha 2B-AR) mRNA is far less abundant in brain tissues, and is only found in the diencephalon.

Molecular characterization of alpha 1- and alpha 2-adrenoceptors

Three 'alpha 1-adrenoceptors' and three 'alpha 2-adrenoceptors' have now been cloned. How closely do these receptors match the native receptors that have been identified pharmacologically? What are the properties of these receptors, and how do they relate to other members of the cationic amine receptor family? Kevin Lynch and his colleagues discuss these questions in this review.

Distribution of alpha-1 and alpha-2 binding sites in the rat locus coeruleus

Precise anatomical distribution of alpha-1 and alpha-2 adrenergic binding sites has been investigated in the rat locus coeruleus (LC) using quantitative radioautography of brain sections incubated with 3H-prazosin or 3H-idazoxan. Distribution patterns of 3H-prazosin (alpha-1 sites) and 3H-idazoxan (alpha-2 sites) were heterogeneous and different along a postero-anterior axis in the LC. Comparison between distribution of alpha-2 binding sites and noradrenergic (NA) cellular density suggests that at least a fraction of these sites might be localized on NA perikarya or dendrites in this structure. Quantitative estimations of the binding parameters along this postero-anterior axis in the LC have revealed that the heterogeneous distributions of alpha-1 and alpha-2 binding sites are due not only to variations in the maximal densities of sites but also to variations in the affinities of these sites for their respective ligand.

Identification of an alpha 2-adrenoceptor in human coronary arteries by radioligand binding assay

A single high affinity binding site for an alpha 2-adrenoceptor in human coronary arteries was identified by radioligand binding assay. Human coronary arteries were obtained at autopsy within 6 hours of death. A crude membrane solution was incubated with (3H)-rauwolscine at 25 degrees C for 30 min. The binding of (3H)-rauwolscine was rapidly saturable and reversible. Kd was 1.2 +/- 0.2 (SE) nM and Bmax 22 +/- 3 fmol/mg protein. This is the first study which has shown the presence of an alpha 2-adrenoceptor in human coronary arteries using a radioligand binding assay method.

Autoradiography of adrenoceptors in rat and human brain: alpha-adrenoceptor and idazoxan binding sites

This chapter reviews the current classification of adrenoceptors, and notes the difficulties of combining the molecular biological and pharmacological classifications of adrenoceptors. Possibilities for mapping the distribution of the proposed subtypes of adrenoceptors using currently available ligands are discussed, and the autoradiographic visualisation of the broad subtypes of alpha 1-, alpha 2-, beta 1-, and beta 2-adrenoceptors in the rat, monkey and human brain described and illustrated. The non-selectivity of ligands currently being used to label alpha-adrenoceptors is shown; we compare the distribution of [3H]idazoxan binding sites with the distribution of alpha 2-adrenoceptors visualised using other ligands. Resolution limitations of current autoradiographic approaches are considered and we shown how in situ hybridisation can complement data from receptor labelling studies used to localise receptors to pre- or postsynaptic sites.

Expression of multiple alpha 2-adrenergic receptor messenger RNA species in rat tissues

We used a human platelet alpha 2-adrenergic receptor probe to study the tissue distribution and messenger RNA (mRNA) forms of the rat alpha 2-adrenergic receptor. Under stringent conditions of hybridization and washing, we detected an mRNA species of 3.8 kb. The abundance of this form follows the order spleen, kidney, brain stem and cortex, and skeletal muscle and lung and is consistent with the reported abundance and tissue distribution of the alpha 2 receptor activity. A 3.0 kb mRNA form was also detected in cerebral cortex and brain stem and a 4.1 kb mRNA form was observed in kidney under less stringent hybridization conditions. The tissue distribution of the 3.0 kb form is different from that of alpha 1- and beta-adrenergic receptors and the D2 dopaminergic receptor. The mRNA analysis combined with Southern blot analysis of rat and human genomic DNA indicate that: 1) in addition to a 3.8 kb rat alpha 2-adrenergic receptor transcript, there are other mRNA forms in the rat that do not correspond to previously described adrenergic receptor mRNA species and 2) more than one alpha 2-adrenergic receptor gene in the rat is expressed in a tissue-specific manner.

High level of expression of functional human platelet alpha 2-adrenergic receptors in a stable mouse C127 cell line

We have generated, by transfection and proper selection, a stable mouse C127 cell line which expresses the human alpha 2-adrenergic receptor gene. The size of the mRNA produced by the cloned gene is 1.8 kb. Electrophoretic analysis and autoradiography of cell membrane proteins photoaffinity labeled with p-[3H]azidoclonidine gave a broad protein band of molecular mass of approx. 64 kDa. Saturation binding with [3H]rauwolscine as ligand gave an equilibrium dissociation constant of 1.29 +/- 0.46 nM (mean +/- S.D.) and binding capacity range of 18-35 pmol/mg membrane protein, with (3-6) x 10(6) receptors per cell. Antagonist competition experiments displayed the order of potency: yohimbine greater than rauwolscine greater than phentolamine much greater than prazosin. Agonist competitions demonstrated the order of potency: p-aminoclonidine greater than (-)epinephrine much greater than (+)epinephrine much greater than (-)isoproterenol. This pharmacological profile is characteristic of the human platelet alpha 2-adrenergic receptor. The expressed receptor is able to couple to the Gi protein. Thus, when epinephrine competition for specific binding of [3H]rauwolscine was performed in the presence of 1 mM MgCl2, 1 mM Gpp[NH]p increased the Ki for epinephrine from 164 to 315 nM. Following preincubation of cultures with 1 mM isobutylmethylxanthine, 1 microM epinephrine decreased forskolin-stimulated cellular cyclic AMP accumulation by 72%. The response was biphasic, and the attenuation effect disappeared at 100 microM epinephrine. A transfected clone which did not demonstrate detectable alpha 2-adrenergic receptor mRNA displayed low levels of alpha 2-adrenergic receptor, (less than 50 fmol/mg membrane protein), similar to those found in the parent C127 cell line. In this clone, epinephrine did not attenuate but, rather, enhanced forskolin-stimulated cyclic AMP accumulation. This new C127 cell line expressing high levels of alpha 2-adrenergic receptor provides an abundant source of a single human adrenergic receptor subtype in membrane-bound conformation which is able to couple to the Gi protein and inhibit forskolin-stimulated adenylate cyclase activity. This cell line will facilitate studies of the structure: function relationship of the alpha 2-adrenergic receptor and should aid in separating the components of various signal transduction mechanisms putatively attributed to this receptor.

Heterogeneity of alpha 2-adrenoceptors in rat cortex but not human platelets can be defined by 8-OH-DPAT, RU 24969 and methysergide

1. Saturation experiments indicated that [3H]-yohimbine binding was specific, saturable and labelled a single population of sites in rat cerebral cortex (Kd 5.3 +/- 0.9 nM, Bmax 121 +/- 10 fmol mg-1 protein) and human platelets (Kd 0.7 +/- 0.1 nM, Bmax 152 +/- 10 fmol mg-1 protein). 2. The alpha 2-adrenoceptor antagonists, yohimbine, rauwolscine, WY 26703, idazoxan and BDF 6143 displaced [3H]-yohimbine binding to each tissue in a simple manner, with high affinity and Hill slopes close to unity. 3. The alpha 1-adrenoceptor agonist, oxymetazoline and the antagonist prazosin inhibited the binding of [3H]-yohimbine to rat in a complex manner consistent with an interaction at more than one site. However, indoramin and WB 4101 only appeared to interact with one site. In contrast, in human platelets, all antagonists gave rise to monophasic displacement curves with Hill slopes close to unity suggesting a single site of interaction. 4. The 5-hydroxytryptamine (5-HT) receptor ligands, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), RU 24969, and methysergide inhibited the binding of [3H]-yohimbine to rat cortex with high and low affinity, consistent with an interaction with two populations of binding sites. However, inhibition of [3H]-yohimbine binding to human platelets suggested a single site of interaction. The low affinity of 5-HT, 5-carboxyamidotryptamine (5-CT) and dipropyl-5-CT indicated that [3H]-yohimbine was not labelling a 5-HT1-like site in rat cortex. 5. The ability of 8-OH-DPAT, RU 24969 and methysergide in addition to prazosin and oxymetazoline to differentiate [3H]-yohimbine binding provides additional pharmacological evidence for heterogeneity within rat cortical alpha 2-adrenoceptors. However, if the two sites in rat cortex that are differentiated by the 5-HT ligands represent (alpha 2A- and alpha 2B-adrenoceptor subtypes as defined by prazosin and oxymetazoline, then they do not correspond to the population of sites in human platelets. As receptor classification should be linked to affinity of drugs rather than tissue distribution, the current classification of alpha 2-adrenoceptor subtypes does not appear to be satisfactory.

Adrenoceptors in airway smooth muscle

This review examines the roles and functional significance of alpha and beta-adrenoceptor subtypes in airway smooth muscle, with emphasis on human airway function and the influence of asthma. Specifically, we have examined the distribution of beta-adrenoceptors in lung and the influence of age, the epithelium, respiratory viruses and inflammation associated with asthma on airway smooth muscle beta-adrenoceptor function. Sites of action, beta 2-selectivity, efficacy and tolerance are also examined in relation to the use of beta 2-agonists in man. In addition, alpha-adrenoceptor function in airway smooth muscle has been reviewed, with some emphasis on comparing observations made in airway smooth muscle with those in animal models.

Imidazolinic radioligands for the identification of hamster adipocyte alpha 2-adrenoceptors

Imidazolinic radioligands ([3H]UK 14304, [3H]idazoxan and [3H]RX 821002) were used for the identification of alpha 2-adrenoceptors on hamster fat cell membranes since there are limitations to the use of [3H]yohimbine and [3H]clonidine, which suggest alpha 2-adrenoceptor heterogeneity. Biological assays (lipolysis measurements) were performed on isolated fat cells and binding studies were carried out on fat cell membranes. The imidazolinic derivative, UK 14304, was a full agonist as compared to clonidine. Idazoxan and RX 821002 (2-(2-methoxy-1,4-benzodioxan-2yl)-2-imidazoline), a recently developed alpha 2-antagonist, were more potent alpha 2-antagonists than yohimbine in this fat cell model. [3H]UK 14304 was the most suitable agent for the quantification of the 'high-affinity state' alpha 2-adrenoceptors in binding studies since it did not exhibit the sensitivity to the composition of the buffer shown by [3H]clonidine. Although it is a potent alpha 2-antagonist, [3H]idazoxan had major limitations for use in the identification of alpha 2-adrenoceptors in this cell model since it also bound to 'non-adrenaline displaceable' binding sites which were revealed when imidazolinic derivatives (phentolamine) were used instead of adrenaline to determine the non-specific binding. We demonstrated that [3H]RX 821002 was a more suitable radioligand than [3H]yohimbine for labelling hamster fat cell alpha 2-adrenoceptors (KD = 1.0 +/- 0.1 nM, Bmax = 776 +/- 60 fmol/mg protein). Moreover, since it exhibited low affinity for 'imidazoline-preferring sites', it represents a valuable ligand even in tissues possessing such binding sites. We suggest that [3H]RX 821002 can be used to identify alpha 2-adrenoceptors in various tissues when these sites cannot be labelled with [3H]yohimbine.

Structure and function of alpha-adrenergic receptors

Alpha 1- and alpha 2-adrenergic receptors are the initial recognition sites on a wide variety of catecholamine-responsive target cells. This article addresses several major questions related to subtypes, structure, signal transduction mechanisms, and regulation of alpha 1- and alpha 2-adrenergic receptors. The application of biochemical and cell and molecular biologic techniques has provided many new insights regarding alpha-adrenergic receptors. Two (and perhaps three) distinct alpha 2-adrenergic receptor subtypes have been identified, and subtypes may exist for alpha 1-adrenergic receptors as well. These multiple subtypes imply much greater diversity among alpha-adrenergic receptors than among beta-adrenergic receptors. Alpha-adrenergic receptors are membrane glycoproteins with several common structural features (including seven membrane-spanning domains with extracellular amino terminus and intracellular carboxyl terminus) that are shared with other types of membrane receptors linked to guanine nucleotide-binding regulatory (G) proteins. These G proteins appear to link alpha-adrenergic receptors to multiple effector systems, including enzymes such as adenylate cyclase and phospholipases, and ion channels. The receptors themselves are dynamic entities, the number of which is regulated as a consequence of a poorly understood life cycle. Although unproven, it seems likely that several important clinical disorders represent alterations in alpha-adrenergic receptors themselves or in the G proteins or effector systems to which these receptors couple. New tools for studying receptor structure and function should help clarify the numerous, inadequately understood issues regarding alpha-adrenergic receptors and their possible alteration in disease states.

Antibodies to the GTP binding protein, Go, antagonize noradrenaline-induced calcium current inhibition in NG108-15 hybrid cells

The voltage-dependent calcium current in chemically differentiated NG108-15 cells is depressed by noradrenaline acting on alpha-adrenoreceptors. The response is absent in cells pretreated with pertussis toxin, implicating the involvement of a G-protein. To identify this G-protein, we have studied the response to noradrenaline in cells preinjected with antibodies specific for two G-proteins, Gi and Go. Cells injected with the Gi antibody responded normally to noradrenaline. In contrast, the response to noradrenaline in cells injected with the Go antibody was markedly attenuated. We conclude that Go is employed in coupling alpha-adrenoreceptors to the calcium channels in NG108-15 cells.

Selective amplification and cloning of four new members of the G protein-coupled receptor family

An approach based on the polymerase chain reaction has been devised to clone new members of the family of genes encoding guanosine triphosphate-binding protein (G protein)-coupled receptors. Degenerate primers corresponding to consensus sequences of the third and sixth transmembrane segments of available receptors were used to selectively amplify and clone members of this gene family from thyroid complementary DNA. Clones encoding three known receptors and four new putative receptors were obtained. Sequence comparisons established that the new genes belong to the G protein-coupled receptor family. Close structural similarity was observed between one of the putative receptors and the 5HT1a receptor. Two other molecules displayed common sequence characteristics, suggesting that they are members of a new subfamily of receptors with a very short nonglycosylated (extracellular) amino-terminal extension.

The imidazoline-preferring receptor

Evidence gathered over the past ten years supports the existence of subtypes of alpha 2-adrenoceptors. A receptor which resembles the alpha 2-adrenoceptor, called the imidazoline-preferring receptor (IPR), is virtually insensitive to catecholamines but binds selectively imidazolines and oxazolines such as idazoxan and rilmenidine. In contrast, the catecholamine-preferring alpha 2-adrenoceptor is preferentially activated by catecholamines including alpha-methylnorepinephrine and epinephrine and is antagonized selectively by rauwolscine. In addition to different pharmacological profiles to agonists and antagonists, the IPR and alpha 2-adrenoceptors show differences in anatomical distribution and molecular properties. The evidence has been drawn primarily from in vitro physiological and radioligand binding studies, but is gradually extending into in vivo and even clinical studies.