Mammalian Nicotinic Receptors with α7 Subunits That Slowly Desensitize and Rapidly Recover from α-Bungarotoxin Blockade

Abstract

One of the most abundant nicotinic receptors in the nervous system is a species that contains the α7 gene product, rapidly desensitizes, and binds α-bungarotoxin with great affinity. The receptor has a high relative permeability to calcium and performs a variety of functions including presynaptic modulation of transmitter release and postsynaptic generation of synaptic currents. Fast excitatory transmission in mammalian intracardiac ganglia is mediated primarily by nicotinic receptors, and although intracardiac ganglion neurons express the α7 gene, no toxin-sensitive response has been detected previously in them. We report here that whole-cell patch-clamp recordings from freshly dissociated intracardiac ganglion neurons reveal a nicotinic response that desensitizes slowly and is blocked by α-bungarotoxin in a rapidly reversible manner. The only rat gene previously thought capable of forming such receptors was α9, but no evidence suggests that the α9 gene is expressed in neurons. We find that reverse transcription (RT)–PCR detects α7 but not α9 mRNA in the ganglia. In addition, the pharmacology of the nicotinic response is typical of α7-containing receptors but differs in several respects from that expected for α9. Binding experiments with immunotethered receptors identifies a ganglionic species that contains the α7 gene product. Moreover, intracellular perfusion of the cells with an anti-α7 monoclonal antibody specifically reduces the amplitude of the toxin-sensitive response. The results indicate that α7-containing receptors are responsible for the slowly desensitizing, toxin-reversible response and suggest that the receptors are modified in cell-specific ways to influence their functional properties.