The α7 Nicotinic Acetylcholine Receptor Subunit Exists in Two Isoforms that Contribute to Functional Ligand-Gated Ion Channels

Abstract

Fast synaptic transmission in mammalian autonomic ganglia is mediated primarily by nicotinic receptors, and one of the most abundant nicotinic acetylcholine receptor subtypes in these neurons contains the ␣7 subunit (␣7-nAChRs). Unlike ␣7-nAChRs expressed in other cells, the predominant ␣7-nAChR subtype found in rat intracardiac and superior cervical ganglion neurons exhibits a slow rate of desensitization and is reversibly blocked by ␣-bungarotoxin (␣Bgt). We report here the identification of an ␣7 subunit sequence variant in rat autonomic neurons that incorporates a novel 87-base pair cassette exon in the N terminus of the receptor and preserves the reading frame of the transcript. This ␣7 isoform was detected using reverse transcriptase-polymerase chain reaction techniques in neonatal rat brain and intracardiac and superior cervical ganglion neurons. Immunoblot experiments using a polyclonal antibody directed against the deduced amino acid sequence of the ␣7-2 insert showed a pattern of expression consistent with ␣7-2 subunit mRNA distribution. Moreover, the ␣7-2 subunit could be immunodepleted from protein extracts by solid-phase immunoprecipitation techniques using the anti-␣7 monoclonal antibody 319. The ␣7-2 subunit was shown to form functional homomeric ion channels that were activated by acetylcholine and blocked by ␣-bungarotoxin when expressed in Xenopus laevis oocytes. This ␣7 isoform exhibited a slow rate of desensitization, and inhibition of these channels by ␣Bgt reversed rapidly after washout. Taken together, these data indicate that the ␣7-2 subunit is capable of forming functional ␣Bgt-sensitive acetylcholine receptors that resemble the ␣7-nAChRs previously identified in rat autonomic neurons. Furthermore, the distribution of the ␣7-2 isoform is not limited to peripheral neurons.

Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

“…Although we have immunocytochemical data confirming expression of α7-and β2-subunits in the same enteric neurons (not shown), we have no independent evidence of co-localization in the same nAChR molecule. Furthermore, the presence of α7-isoforms 1 and 2 (Severance et al, 2004) in the ENS remains an open question. Since they differ in their extracellular domain, we could not have distinguished them with mAb 306, which recognizes an intracellular epitope; and since we did not use α-Bgt for functional studies, we could not assess their individual contributions to the overall α7-nAChR pool on the basis of their differential affinities for this toxin.…”

Random Assembly of GABA ρ1 and ρ2 Subunits in the Formation of Heteromeric GABAC Receptors

“…On the other hand, homomeric ligand-gated channels are also thought to exist in certain cases. Thus, the α7 nicotinic acetylcholine receptor subunit was shown to form functional homomeric AChRs in Xenopus ooctyes (Seguela et al, 1993), and there is now good evidence that native α7-nAChRs are homopentameric structures (Chen and Patrick, 1997;Severance et al, 2004).…”

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

“…The role of ␣7 subtype is supported by the finding that ␣7 nAChRmediated currents can also be measured in cultured superior cervical ganglion neurons (97). In addition, two different splice variants of ␣7 nAChRs have been detected in the superior cervical ganglion with distinct desensitization properties (23,147). In the case NE release, ␣3␤4 nAChRs were assumed for both TTX-sensitive and TTXinsensitive release forms in cultured sympathetic neurons (85).…”