Cellular Neuroscience 207 Ian Parker Lecture

1
Cellular Neuroscience (207)Ian ParkerLecture
4 - Ligand-gated channels
2
The synapse that has taught us most of what we
know about synaptic transmission
The frog neuromuscular junction (endplate)
Advantages the postsynaptic cell (muscle) is big
(100 mm x 2 cm) so easy to impale with
microelectrode although the presynaptic axon is
very thin, the nerve is easily dissected for
extracellular stimulation
Could electrical transmission work at an
endplate? Membrane area of nerve terminal 2 x
10-5 cm2 Assuming Na current density of 1 mA
cm-2, then total current 2 x 10-8 A Input
resistance of muscle fiber 105 Ohm So, nerve
terminal could depolarize the muscle by only
2mV NOT ENOUGH
3
Presynaptic release of neurotransmitter
(acetylcholine ACh) causes opening of
ligand-gated channels (AChR), leading to the
generation of the endplate potential
4
Kinetic scheme for agonist-gating exemplified
by nicotinic ACh receptor
Receptor/channel molecule comprised from total of
5 subunits 2x a, 1each b,g,d. Channel opening
requires that 2 ACh molecules be bound
simultaneously to the 2 a subunits. Channel
closes when one ACh dissociates. Mean channel
lifetime is thus a function of mean time for
which agonist stays bound. This is a function
both of the receptor and the agonist e.g.
carbachol gives longer mean open time than ACh.
5
Requirement for binding of 2 ACh molecules means
that channel opening increases as square of ACh
Low agonist concentration
Double agonist concentration
Channel openings become much more frequent with
increasing agonist.
Mean open time does not change with agonist
Mean channel closed time becomes much shorter
i.e. frequency of openings increases
6
Hill coefficient reveals degree of cooperativity
i.e. number of agonist molecules required to
cause channel opening
Mean open lifetime does not change with agonist
it depends on agonist unbinding, not binding.
agonist at which lines cross (i.e. when mean
open time closed time) gives measure of
apparent affinity of agonist
Closed time shortens with slope of 2 on log/log
plot (i.e. as square of agonist concentration)
Log reciprocal mean closed / open time
A double log plot causes power functions (square,
cube etc.) to appear as straight lines. The slope
of the line (Hill coefficient) indicates the
power e.g. square slope of 2, cube 3 , etc.
Agonist concentration
7
Other kinetic features

• Nachschlags brief closings
• during chanel openings

2. Desensitization bursts whole-cell current
declines even in sustained presence of agonist
Agonist application
Whole cell current declines
Individual channels show bursts of openings,
interrupted by long silent intervals when channel
is desensitized. Whole cell current declines as
more channels enter desensitized state.
8
A (simplified) kinetic model of channel gating
Agonist (ACh)
A R
AR A
A2R
A2 R
A2 D
Receptor (channel shut)
Receptor (channel open)
Desensitized receptor (channel shut)
Receptor can exist in 5 states each with a
characteristic mean lifetime Only 1 open state
(A2R) so distribution of open times shows
single exponential. But 4 closed states so
closed time distribution is actually made up of 4
exponential components. Of these A2R (flickers)
and A2D (silent intervals during desensitization)
are independent of agonist Lifetimes of R and
AR shorten with increasing agonist