Desensitization contributes to the synaptic response of gain-of-function mutants of the muscle nicotinic receptor.
Although the muscle nicotinic receptor (AChR) desensitizes almost completely in the steady presence of high concentrations of acetylcholine (ACh), it is well established that AChRs do not accumulate in desensitized states under normal physiological conditions of neurotransmitter release and clearance. Quantitative considerations in the framework of plausible kinetic schemes, however, lead us to predict that mutations that speed up channel opening, slow down channel closure, and/or slow down the dissociation of neurotransmitter (i.e., gain-of-function mutations) increase the extent to which AChRs desensitize upon ACh removal. In this paper, we confirm this prediction by applying high-frequency trains of brief ( approximately 1 ms) ACh pulses to outside-out membrane patches expressing either lab-engineered or naturally occurring (disease-causing) gain-of-function mutants. Entry into desensitization was evident in our experiments as a frequency-dependent depression in the peak value of succesive macroscopic current responses, in a manner that is remarkably consistent with the theoretical expectation. We conclude that the comparatively small depression of the macroscopic currents observed upon repetitive stimulation of the wild-type AChR is due, not to desensitization being exceedingly slow but, rather, to the particular balance between gating, entry into desensitization, and ACh dissociation rate constants. Disruption of this fine balance by, for example, mutations can lead to enhanced desensitization even if the kinetics of entry into, and recovery from, desensitization themselves are not affected. It follows that accounting for the (usually overlooked) desensitization phenomenon is essential for the correct interpretation of mutagenesis-driven structure-function relationships and for the understanding of pathological synaptic transmission at the vertebrate neuromuscular junction.
Elenes
Sergio
S
Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Ni
Ying
Y
Cymes
Gisela D
GD
Grosman
Claudio
C
D000818:Animals; D023902:Chromosome Pairing / Q000235:genetics* / Q000502:physiology*; D004558:Electric Stimulation; D004594:Electrophysiology; D006801:Humans; D007668:Kidney / Q000166:cytology / Q000196:embryology / Q000502:physiology; D008433:Mathematics; D051379:Mice; D009132:Muscles / Q000502:physiology*; D009154:Mutation / Q000235:genetics*; D018377:Neurotransmitter Agents / Q000502:physiology; D018408:Patch-Clamp Techniques; D011978:Receptors, Nicotinic / Q000235:genetics* / Q000502:physiology*; D009435:Synaptic Transmission / Q000235:genetics / Q000502:physiology; D013997:Time Factors; D014162:Transfection
D018377:Neurotransmitter Agents; D011978:Receptors, Nicotinic