Presence of α7 nicotinic acetylcholine receptors on dorsal root ganglion neurons proved using knockout mice and selective α-neurotoxins in histochemistry

In complex tissues where multiple subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed, immunohistochemistry has been the most popular tool for investigation of nAChR subunit distribution. However, recent studies with nAChR subunit knockout mice demonstrated that a large panel of antibodies is unsuitable. Thus, we aimed to develop a histochemical method for selective labeling of alpha 7 nAChR with neurotoxins, utilizing alpha 7 nAChR-transfected cells, dorsal root ganglia (DRG) and spinal cord from wild-type and knockout mouse. The specificity of Alexa Fluor 488-conjugated alpha-bungarotoxin (Alexa-alpha Bgt) was demonstrated in binding to alpha 7-transfected cells inhibited by long-chain alpha-cobratoxin (CTX), but not short-chain alpha-neurotoxin II (NTII). In contrast, binding to Torpedo muscle-type nAChRs and to motor end plates in mouse tongue sections was prevented by both CTX and NTII. In tissue sections of DRG, expressing all neuronal nAChR subunits, only CTX precluded Alexa-alpha Bgt labeling of neurons, with no staining for alpha 7 nAChR knockout tissue. It proved that alpha 7 nAChRs are the major alpha Bgt-binding sites in mouse DRG. Corresponding results were obtained for terminals in the spinal cord. Thus, we present a protocol utilizing Alexa-alpha Bgt and non-labeled CTX/NTII that allows specific histochemical detection of alpha 7 nAChR with a spatial resolution at the level of single axon terminals.