[3H]‐GUANFACINE: A RADIOLIGAND THAT SELECTIVELY LABELS HIGH AFFINITY α2‐ADRENOCEPTOR SITES IN HOMOGENATES OF RAT BRAIN

BEVYN JARROTT, WILLIAM J. LOUIS, ROGER J. SUMMERS

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Abstract

[3H]‐guanfacine (N‐amidino‐2‐(2,6‐dichloro 3[3H] phenyl) acetamide hydrochloride; 24.2 Ci/mmol) has been used as a radioligand in homogenates of rat cerebral cortex. Specific binding of [3H]‐guanfacine was linear with respect to tissue concentration (2.5–15 mg/ml), saturable and not markedly affected in the pH range 6.5–8.0. Analysis of the saturation of [3H]‐guanfacine binding using an iterative least squares fitting procedure gave best fits to a single site model. [3H]‐guanfacine binding was of high affinity (Kd 1.71 ± 0.24 nm; n = 8) to a population of non interacting sites (nH 0.99 ± 0.02; n = 8) with a density of 118.2 ± 8.4 fmol/mg protein (n = 8). Highest levels of binding were achieved in cerebral cortex followed by thalamus > hypothalamus > medulla/pons > spinal cord > striatum > cerebellum. Binding was stereoselective with regard to the (—)‐isomer of noradrenaline and the order of potency for displacement of [3H]‐guanfacine by agonists was naphazoline > clonidine > (—)‐adrenaline > (—)‐α methylnoradrenaline > (—)‐noradrenaline > (±)‐α‐methylnoradrenaline > (+)‐noradrenaline > methoxamine > (+)‐adrenaline > phenylephrine and by antagonists was phentolamine > dihydroergocryptine > piperoxane > yohimbine > prazosin > labetalol > indoramin suggested binding to α2‐adrenoceptors. The monovalent cations Na+ and K+ and also guanosine 5′‐triphosphate (GTP) produced concentration‐dependent inhibition whereas the divalent cations Ca2+, Mg2+, and Mn2+ first enhanced, then inhibited [3H]‐guanfacine binding. Na+ (150 mm) or GTP (100 μm) produced marked reductions and Mn2+ (5 mm) marked increases in the number of receptor sites labelled by [3H]‐guanfacine. It is concluded that [3H]‐guanfacine preferentially labels a high affinity state of the α2‐adrenoceptor in homogenates of rat cerebral cortex. 1982 British Pharmacological Society

Original languageEnglish
Pages (from-to)401-408
Number of pages8
JournalBritish Journal of Pharmacology
Volume75
Issue number2
DOIs
Publication statusPublished - 1 Jan 1982

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