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

[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 (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). 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