Received for publication March 5, 2004.
Revised May 25, 2004.
Accepted for publication June 17, 2004.

THE 7 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 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 bp cassette exon in the N-terminus of the receptor and preserves the reading frame of the transcript. This 7 isoform was detected using RT-PCR 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, mAb 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 oocytes. This 7 isoform exhibited a slow rate of desensitization, and inhibition of these channels by aBgt reversed rapidly following washout. Taken together, these data indicate that the 7-2 subunit is capable of forming functional Bgt-sensitive AChRs that resemble the a7-AChRs previously identified in rat autonomic neurons. Furthermore, the distribution of the 7-2 isoform is not limited to peripheral neurons.

Key words: Nicotinic cholinergic, Ion channel regulation