An autoradiographic survey of mouse brain nicotinic acetylcholine receptors defined by null mutants

Abstract

Nine nicotinic receptor subunits are expressed in the central nervous system indicating that a variety of nicotinic acetylcholine receptors (nAChR) may be assembled. A useful method with which to identify putative nAChR is radioligand binding. In the current study the binding of [(125)I]α-bungarotoxin, [(125)I]α-conotoxinMII, 5[(125)I]-3-((2S)-azetidinylmethoxy)pyridine (A-85380), and [(125)I]epibatidine has been measured autoradiographically to provide data on many nAChR binding sites. Each binding site was evaluated semi-quantitatively for samples prepared from wild-type and α2, α4, α6, α7, β2, β4, α5 and β3 null mutant mice. Deletion of the α7 subunit completely and selectively eliminated [(125)I]α-bungarotoxin binding. The binding of [(125)I]α-conotoxinMII was eliminated in most brain regions by deletion of either the α6 or β2 subunit and is reduced by deletion of either the α4 or β3 subunit. The binding of 5[(125)I]A-85380 was completely eliminated by deletion of the β2 subunit and significantly reduced by deletion of the α4 subunit. Most, but not all, α4-independent sites require expression of the α6 subunit. The effect of gene deletion on total [(125)I]epibatidine binding was very similar to that on [(125)I]A-85380 binding. [(125)I]Epibatidine also labels β4* nAChR, which was readily apparent for incubations conducted in the presence of 100nM cytisine. The effects of α3 gene deletion could not be evaluated, but persistence of residual sites implies the expression of α3* nAChR. Taken together these results confirm and extend previously published evaluations of the effect of nAChR gene deletion and help to define the nAChR subtypes measurable by ligand binding.

Figure 1

Autoradiographic images of coronal mouse brain sections at approximately −3.5 mm Bregma. Autoradiograms for each of the ligands [¹²⁵I]epibatidine (three conditions: total binding, binding in the presence of 100 nM cytisine and binding in the presence of 100 nM cytisine and 100 nM α-conotoxinMII), [¹²⁵I]A-85380, [¹²⁵I]α-conotoxinMII and [¹²⁵I]α-bungarotoxin are shown for wild-type mice and for each of the nAChR null mutants for sections at a level approximately −3.5 mm Bregma.