Journal of Neuroscience, Vol 7, 3612-3622, Copyright © 1987 by Society for Neuroscience
The properties and regulation of functional acetylcholine receptors on chick ciliary ganglion neurons
JF Margiotta, DK Berg and VE Dionne
Department of Pharmacology, University of California, San Diego, La Jolla 92093.
The properties of acetylcholine receptor (AChR) channels on chick ciliary
ganglion neurons in culture were examined using patch-clamp recording
techniques. Acetylcholine (ACh) was applied by rapid microperfusion.
Whole-cell current noise analysis revealed a single class of functional
receptors on the neurons. Dose-response studies indicated a Kd of about 36
microM and a Hill coefficient of 1.5-1.7, predicting 2 ACh binding sites
per receptor. Both fast and slow components of receptor desensitization
were observed. Single-channel recordings from excised outside-out patches
of soma membrane exposed to 2-5 microM ACh indicated a single-channel
conductance of 40 pS, a reversal potential of -9 mV, a mean open duration
of 1 msec, and an opening probability of 0.34. The kinetic behavior of the
channels was provisionally described by a 3-closed, 1-open state model for
receptor activation. In all of these properties, AChRs of ciliary ganglion
neurons resemble those on skeletal muscle fibers. Growing the neurons in an
elevated K+ concentration produced a 2-3-fold decrease in peak whole-cell
currents induced by ACh under standard test conditions, without altering
any of the single-channel properties described above. Neither changes in
cholinesterase activity nor receptor distribution accounted for the
decrease. Instead, calculations indicated that elevated K+ reduced the ACh
response by decreasing the number of functional AChRs on the neurons. No
K+-dependent decrease is observed, however, in the number of total
receptors on the neurons detected either by a monoclonal antibody specific
for the receptor or by an alpha-neurotoxin that binds to the receptor and
blocks its function. Moreover, the number of receptors detected by the 2
probes is at least 10-fold greater than the calculated number of functional
receptors. The findings suggest that only a small fraction of the AChRs on
the neuronal surface is functional and that the cell can alter the ratio of
functional and nonfunctional receptors in response to growth conditions.
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