SYNTHESIS AND RECEPTOR-BINDING OF 5-AMINO[H-3](2)METHYL-3-ISOTHIAZOLOL ([H-3]THIOMUSCIMOL), A SPECIFIC GABA(A) AGONIST PHOTOAFFINITY LABEL

The synthesis of tritium labelled thiomuscimol (5-amino[H-3](2)methyl- 3-isothiazolol) (7c), a specific and high-affinity agonist photoaffini ty label for GABA(A) receptors, is described. The synthesis of 7c is b ased on a procedure for the preparation of 5-amino[H-2](2)methyl-3-iso thiazolol (7b). Methyl 3-ethoxyisothiazole-5-carboxylate (3), synthesi zed from 3-hydroxyisothiazole-5-carboxamide (1) via the corresponding methyl ester (2), was reduced with sodium borotritide to give 3-ethoxy -5-hydroxy[H-3](2)-methyl-3-isothiazole (4c). 3-Ethoxy-5-phthalimido[H -3](2)- methylisothiazole (6c), synthesized from the 5-chloro[H-3](2)m ethyl analogue (5c) of (4c), was deprotected by treatment with concent rated hydrochloric acid to give 7c with a specific radioactivity of 29 Ci/mmol. In pilot binding assays, 7c was shown to bind to membranes f rom rat forebrain in a saturable manner and with K-D and B-max values of 28 +/- 6 nM and 50 +/- 4 fmol/mg original tissue, respectively.