An autoradiographic study of the distribution of binding sites for the novel α7‐selective nicotinic radioligand [³H]‐methyllycaconitine in the mouse brain

Abstract

[3H]-Methyllycaconitine ([3H]-MLA) is a new radioligand with selectivity for alpha7-type neuronal nicotinic acetylcholine receptors (nAChRs). In our previous study [Davies, A.R.L., Hardick, D.J., Blagbrough, I.S., Potter, B.V.L., Wolstenholme, A.J. & Wonnacott, S. (1999) Neuropharmacology, 38, 679-690], this radioligand labelled a single class of site in rat brain membranes; its pharmacology and distribution in crudely dissected brain regions closely paralleled that of the well-established alpha7-ligand [125I]-alpha-bungarotoxin. However, a small population of [3H]-MLA binding sites was apparently insensitive to alpha-bungarotoxin. Here we have extended the study to mouse brain, using autoradiography to examine the distribution of [3H]-MLA and [125I]-alpha-bungarotoxin binding sites. [3H]-MLA labelled a single class of site in mouse brain membranes with a KD of 2.2 nM and a Bmax of 45.6 fmol/mg protein. Specific binding, defined by unlabelled MLA (Ki = 0.69 nM), was completely inhibited by (-)-nicotine (Ki = 1.62 microM), whereas alpha-bungarotoxin inhibited only 85% of specific binding (Ki = 3.5 nM). The distributions of [125I]-alpha-bungarotoxin and [3H]-MLA binding sites were compared by autoradiography, and binding was quantitated in 72 brain regions. Binding of both radioligands was highly correlated, with highest densities in the dorsal tegmental nucleus of the pons, colliculi and hippocampus. Serial sections labelled with [3H]-MLA in the absence or presence of unlabelled MLA or alpha-bungarotoxin provided no evidence for any alpha-bungarotoxin-resistant binding. The results are discussed in terms of binding sites that are inaccessible to alpha-bungarotoxin in membrane preparations. This study demonstrates the utility of [3H]-MLA for characterization of alpha7-type nicotinic receptors in mammalian brain, and suggests that it labels a population identical to that defined by [125I]-alpha-bungarotoxin.

Altered hippocampal circuit function in C3H α7 null mutant heterozygous mice

“…In addition, hippocampal auditory gating is influenced by stimulation of both the PRF and nucleus of the lateral lemniscus (NLL) (Bickford et al, 1993). All but one of these brain regions, the PRF, appear to express the α7 nicotinic receptor (Arimatsu and Seto, 1982;Azam et al, 2003;Breese et al, 1997;Happe and Morley, 2004;Hellstrom-Lindahl et al, 1998;Henderson et al, 2005;Seguela et al, 1993;Thinschmidt et al, 2005;Whiteaker et al, 1999;Zhang et al, 1998). It is currently unknown whether the α7 receptor modulates auditory gating in any of these extra-hippocampal areas.…”

“…The α7 receptor is expressed in many brain regions, but is especially dense in the hippocampus (Breese et al, 1997;Seguela et al, 1993;Whiteaker et al, 1999). This receptor has been localized to hippocampal neurons containing the inhibitory neurotransmitter γ-aminobutyric acid (GABA) (Breese et al, 1997;Fabian-Fine et al, 2001;Frazier et al, 1998a;Freedman et al, 1993;Khiroug et al, 2003) and activation of the receptor leads to the release of GABA in hippocampal cultures and slices (Alkondon and Albuquerque, 2001;Alkondon et al, 1999;Buhler and Dunwiddie, 2002;Radcliffe et al, 1999).…”

Nicotinic receptor subtypes and cognitive function

“…Septohippocampal cholinergic innervation uses not only muscarinic but also nicotinic receptors. Autoradiographic studies have demonstrated the presence of ␣7 (Whiteaker et al, 1999;Fabian-Fine et al, 2001) and ␣4 (Didier et al, 1995) containing nicotinic receptors in the hippocampus. We have found the hippocampus to be important for chronic nicotine effects on memory function.…”

Development of 18F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

“…[ 3 H]cytisine [109,110] while the α7 nAChR has been characterized using [ 125 I]α-bungarotoxin [111,112] or [ 3 H]methyllycaconitine [113] .…”

“…In particular, homogenate-binding or cell-binding assays allow high-throughput screening if needed. Alternatively, autoradiography on brain slices may be used; this is much more time-consuming but allows additional investigation of the regional distribution of receptors in the brain [100,108] .With regard to nAChRs, the α4β2 subunit distribution has been investigated by in vitro autoradiography using[ 3 H]cytisine [109,110] while the α7 nAChR has been characterized using [ 125 I]α-bungarotoxin [111,112] or [ 3 H]methyllycaconitine [113] .For various reasons, these three ligands are not suitable as lead compounds for PET radiotracer development [6] .However, these highly selective compounds can be used to obtain information on the specifi c receptor binding of new drugs. For example, the highly-selective α7 nAChR ligand NS10743 [114] (for structure see Fig.…”