Influence of Acetylcholine Levels on the Binding of a SPECT Nicotinic Acetylcholine Receptor Ligand [ 123 I]5-I-A-85380 MASAHIRO FUJITA, 1 * MOHAMMED S. AL-TIKRITI, 1 GILLES TAMAGNAN, 1 SAMI S. ZOGHBI, 2 ALI BOZKURT, 1 RONALD M. BALDWIN, 1 AND ROBERT B. INNIS 1,3 1 Department of Psychiatry, Yale University and VA Connecticut HCS, West Haven, Connecticut 2 Department of Diagnostic Radiology, Yale University and VA Connecticut HCS, West Haven, Connecticut 3 Department of Pharmacology, Yale University and VA Connecticut HCS, West Haven, Connecticut KEY WORDS physostigmine; baboon; equilibrium; 3-[2(S)-2-azetidinylmethoxy]- pyridine ABSTRACT Although in vitro theory indicates that ligand binding is sensitive to competition with neurotransmitters, only some imaging ligands have shown such com- petition in vivo. The purpose of this study was to determine whether increases in acetylcholine (ACh) levels induced by an acetylcholinesterase inhibitor, physostigmine, inhibit in vivo binding of [ 123 I]5-iodo-3-(2(S)-2-azetidinyl-methoxy) pyridine (5-I-A- 85380), a single photon emission computed tomography ligand for the high-affinity type nicotinic ACh receptor (nAChR). Baboons were used for seven studies with a bolus plus constant infusion equilibrium paradigm. After achieving equilibrium at 5 h, physostig- mine (0.02 (n = 1), 0.067 (n = 3), and 0.2 (n = 3) mg/kg) was administered intravenously and data were acquired for up to 8 h. To confirm equilibrium conditions, [ 123 I]5-I-A- 85380 plasma levels were measured in four studies, including all studies with 0.2 mg/kg physostigmine. Prior to physostigmine administration, thalamic activities were stable, with changes of 1.1%/h or less, except in one study with a gradual increase of 4.2%/h. Thalamic activities were decreased by 15% in one study with 0.067 mg/kg and 14 –17% in all studies with 0.2 mg/kg physostigmine administration (P = 0.009). In these studies with 0.2 mg/kg physostigmine administration, [ 123 I]5-I-A-85380 plasma levels showed a transient or a sustained increase after physostigmine administration that would have increased thalamic activities. These results suggest that elevated ACh levels induced by physostigmine can effectively compete in vivo with [ 123 I]5-I-A-85380 binding at nAChRs. However, decreased thalamic activities could have been caused by other mechanisms, including internalization of the receptor with an associated decreased affinity for radio- ligand. Synapse 48:116 –122, 2003. © 2003 Wiley-Liss, Inc. INTRODUCTION Nicotinic acetylcholine (ACh) receptors (nAChRs) be- long to the superfamily of ligand-gated ion channels and mediate neurotransmission in both the central and peripheral nervous systems. These receptors are of great interest because they play an important role in tobacco dependence and because drugs active at these receptors may have therapeutic utility in various neu- ropsychiatric disorders such as Alzheimer’s disease (Lindstrom, 1995; Holladay et al., 1997). A newly developed nAChR ligand, 3-[2(S)-2-azetidi- nylmethoxy]pyridine (A-85380) (Abreo et al., 1996), demonstrates high receptor affinity comparable to that of epibatidine (K i approximately 10 pM) (Mukhin et al., 2000), which was previously used to label high-affinity type nAChRs. Epibatidine and its analogs bind to nAChRs in both the brain (predominant high-affinity type is the 42;McGehee and Role, 1995; Paterson and Nordberg, 2000) and sympathetic ganglia (the 34 type; Cimino et al., 1992) showing significant Contract grant sponsor: the Yale Transdisciplinary Tobacco Use Research Center; Contract grant number: NIDA & NCI P50 DA84733). *Correspondence to: Masahiro Fujita, M.D., Ph.D., Molecular Imaging Branch, National Institute of Mental Health, Building 1, Room B3-06B, 1 Center Drive, MSC-0135, Bethesda, MD 20892-0135. E-mail: FujitaM@intra.nimh.nih.gov Received 13 December 2002; Accepted 20 January 2003 DOI 10.1002/syn.10194 SYNAPSE 48:116 –122 (2003) © 2003 WILEY-LISS, INC.