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

Abstract

1 [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. 2 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. 3 Analysis of the saturation of [³H]-guanfacine binding using an iterative least squares fitting procedure gave best fits to a single site model. 4 [³H]-guanfacine binding was of high affinity (K_d 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). 5 Highest levels of binding were achieved in cerebral cortex followed by thalamus > hypothalamus > medulla/pons > spinal cord > striatum > cerebellum. 6 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. 7 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. 8 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. 9 It is concluded that [³H]-guanfacine preferentially labels a high affinity state of the α₂-adrenoceptor in homogenates of rat cerebral cortex.