ADRENOCEPTOR PHARMACOLOGY Emma Robinson and Alan Hudson within each group have now been cloned and Psychopharmacology Unit pharmacologically characterised. Thea1- adrenoceptor Department of Pharmacology subtypes have been classified as theaa1A , 1B and a 1D - School of Medical Sciences adrenoceptor and thea2 -adrenoceptors have been University of Bristol, University Walk classified as thea2A (a2D species variation of the human BS8 1TD, UK aa2A),2B anda 2C -adrenoceptor. Dr Alan Hudson is Head of Preclinical Research b-Adrenoceptors are also heterogeneous in nature and for the Psychopharmacology Unit, Emma were again initially subdivided intobb - and - Robinson is a Teaching Assistant in the 12 adrenoceptors, on the basis of the relative potencies of Department of Pharmacology a series of catecholamines inin vitro and in vivo systems.3 Subsequently theb -adrenoceptors have Introduction been classified using functional studies, receptor binding and genetic techniques. Theb -adrenoceptor Adrenoceptors are membrane bound receptors located family is subdivided into three distinct subtypes, the b1- throughout the body on neuronal and non-neuronal andb2- adrenoceptors and the atypicalb3 - tissues where they mediate a diverse range of adrenoceptor.4 There is an additionalb -adrenoceptor responses to the endogenous catecholamines subtype which has been identified in cardiac tissue and noradrenaline and adrenaline. The adrenoceptor family is a putative, atypical subtype classified as theb4 - was first divided into two subtypes, theab - and - adrenoceptor.5 adrenoceptors as determined by pharmacological studies in isolated tissue.1 A quarter of a century later, a -Adrenoceptors thea -adrenoceptors were further subdivided based on 1 their anatomical location, witha -adrenoceptors located a1-Adrenoceptor subtypes on peripheral sympathetic nerve terminals designated Subdivision of thea1- adrenoceptors has been a2-adrenoceptors and those located post-synaptically facilitated by both pharmacological and molecular 2 designateda1 -adrenoceptors. This anatomical biological techniques.6 The initial classification of the classification rapidly gave way to the identification of aa11B-adrenoceptors asa1A and -subtypes was pharmacological differences between thea - determined from differences in the binding adrenoceptors, notably the ability of yohimbine and characteristics of the competitive antagonist WB 4101 rauwolscine to act asa2- adrenoceptor antagonists. and the site-directed alkylating agent Subsequent studies using pharmacological and chloroethylclonidine (CEC). From radioligand binding molecular biological techniques have further studies it was determined that thea1A -subtype has a subdivided thea -adrenoceptor family; three subtypes high affinity for WB 4101 and is CEC-insensitive, whilst Table 1.a1 -Adrenoceptor characteristics Receptor typea1A -Adrenoceptoraa1B -Adrenoceptor 1D -Adrenoceptor Selective agonistsoxymetazoline, A61603 none none Non-subtype selectivecirazoline , M-6434 , cirazoline, M-6434 , cirazoline, M-6434, agonists methoxamine, phenylephrine, methoxamine, phenylephrine, methoxamine, phenylephrine, Selective antagonistsRS 17053, WB 4101, CEC (irreversible) BMY 7378 , SKF105854, (S)-(+)-niguldipine, CEC (irreversible) 5-methylurapidil, SNAP5089, Rec152739, SB216469, Ro700004, KMD3213 Non-subtype selective corynanthine, prazosin corynanthine, prazosin corynanthine, prazosin antagonists 2+ 2+ Transduction activates Gp/q ,­­ PI turnover, [Ca ]i.c. , activates voltage-gated Ca channels mechanism Physiological function smooth muscle contraction, smooth muscle contraction smooth muscle contraction myocardial contraction (bold text denotes compounds available from Tocris) Tocris Cookson Ltd., UK Tocris Cookson Inc., USA Tel: + 44 (0)117 982 6551 Toll Free Tel: (800) 421-3701 Tel: (636) 207-7651 Fax: + 44 (0)117 982 6552 Toll Free Fax: (800) 483-1993 Fax: (636) 207-7683 e-mail: [email protected] e-mail: [email protected] e-mail: [email protected] 16 thea1B -subtype exhibits a low affinity for WB 4101 but is adrenoceptor mRNA in the hippocampus and cortex. sensitive to CEC (Table 1). These characteristics were Peripherala1 -adrenoceptors are located on both also shown in a variety of tissue preparations vascular and non-vascular smooth muscle where confirming the presence of functional receptor activation of the receptor results in contraction.17 On 7 subtypes. In addition, three different cDNAs which vascular smooth muscle thea1 -adrenoceptors are coded fora1 -subtypes were isolated. These have since located intrasynaptically where they mediate the been characterised and are believed to code for three response to endogenous neurotransmitter release. functionalaaa11A1B -adrenoceptors: the and -subtypes They are also located on the heart where they mediate a positive inotropic effect and on the liver where they as described above and a third subtype, thea1D which has a low affinity for oxymetazoline in contrast to the activate glycogen phosphorylation. With the availability of subtype selective compounds thea -subtype has a1A-subtype, and is only partially sensitive to CEC, thus 1A been shown to be responsible for the contraction of vas making it pharmacologically distinct from the a1B receptor.8 The putativea -adrenoceptor shows similar deferens smooth muscle. Vasoconstriction in some 1L 18 characteristics to theaa - and -adrenoceptor but blood vessels has been shown to bea1B -mediated 1A 1D 16, 19 exhibits a low affinity for prazosin and, as yet the gene and contraction of the rat aorta isa1D -mediated. has not been identified.9 Compounds displaying a high a -Adrenoceptors affinity for thea1 -adrenoceptor subtype compared to 2 other receptors include the agonists cirazoline and M- 6434 and antagonists such as prazosin and HEAT. a2-Adrenoceptor subtypes Thea -adrenoceptors are located on both pre and post These twoa1 -selective antagonists are also used as 2 radioligands for investigating the binding synaptic neurones where they mediate an inhibitory role in the central and peripheral nervous system.20 The characteristics ofa1 -adrenoceptors. However, heterogeneous nature of thea -adrenoceptor was first following the initial characterisation of thea1 - 2 adrenoceptor subtypes subtype selective agonists and determined from the different pharmacological profiles antagonists are now being developed which include the of the receptor between species and subsequent a -selective agonists A6160310 and oxymetazoline, 4 studies have revealed the presence of different 1A subtypes within the same tissue. Thus, on the basis of and theaa -selective antagonist RS17053.11 The - 1A 1B radioligand binding profiles, amino acid sequence and adrenoceptor subtype has a higher affinity for CEC than chromosomal location, four distinct subtypes of thea - thea -subtype but CEC also exhibits a similar affinity 2 1A adrenoceptor have been characterised.21 Thesea - for thea -subtype. However, antagonists selective for 2 1D adrenoceptor subtypes,aaa , , and a are found thea -subtype such as BMY7378 and SKF105854 are 2A 2B 2C 2D 1D in a variety of species and tissues and have been now available.12, 13 characterised using tissue and cell lines expressing Signal transduction mechanism only one subtype. Thea2D -subtype exhibits a distinct pharmacological profile but, from the sequence Thea -adrenoceptors mediate their response via G- 1 homology is believed to be a species variation of the protein coupled receptors through a G /G mechanism. pq 22 All the subtypes are coupled to phospholipase C and a2A-subtype and is not recognised as separate. Although selective compounds are now being activation of the receptor results in the production of IP3 and DAG. The production of these second messengers developed to differentiate between thea2 -adrenoceptor results in an activation of both voltage dependent and subtypes there is no ligand available that is highly independent Ca2+ channels as well as stimulation of selective in functional studies for thea2 -adrenoceptor subtypes. Characterisation is based on the affinity of a protein kinase C, phospholipase A2 and D, arachidonic acid release and cyclic AMP formation.14, 15 range of compounds which exhibit different affinities for the subtypes (Table 2). Oxymetazoline displays a Location and function higher affinity for thea2A/D -subtype whilst prazosin Thea1 -adrenoceptors are located in the central and exhibits a higher affinity for theaa2B -subtype. The 2C - peripheral nervous system. In the CNS they are subtype has a high affinity for prazosin and low affinity predominantly located post-synaptically where they for oxymetazoline, characteristic of thea2B - mediate an excitatory role. Following cloning of thea1 - adrenoceptor subtype but the affinity for yohimbine is adrenoceptor subtypes, mRNA studies have showna1 - characteristic of thea2A -subtype. The antagonist MK Table 2.a2 -Adrenoceptor characteristics Receptor typea2A -Adrenoceptoraa2B -Adrenoceptor 2C -Adrenoceptor (previous name) (aaa2A , 2D , 2- C10, RG20) ( a 2 -C2, RNG) ( a 2 -C4) Selective agonistsoxymetazoline (partial agonist), none none guanfacine Non-subtype selective UK 14,304 UK 14,304 UK 14,304 agonists Clonidine Clonidine Clonidine Selective antagonistsBRL 44408 , BRL 48962 ARC 239 , imiloxan rauwolscine , MK 912 Non-subtype selectiveRS 79948, yohimbine , RS 79948, yohimbine , RS 79948, yohimbine , antagonists RS 15385, RX821002, RS 15385, RX821002, RS 15385, RX821002, SKF 86466, MK-912, SKF 86466, MK-912, SKF