Meijun Ye
MYE@uams.edu
Study objectives: Recent evidence suggests that certain
anesthetics decrease electrical coupling while the stimulant modafinil
appears to increase electrical coupling. Meijun is investigating the
potential role of electrical coupling in the cholinergic arm of the
reticular activating system (RAS), the pedunculopontine nucleus (PPN),
which has been implicated in the modulation of arousal via ascending
cholinergic activation of intralaminar thalamus and descending
activation of pontine regions to generate some of the signs of rapid eye
movement (REM) sleep.
Design: Meijun uses 6-30 day brainstem
slices to perform whole cell patch-clamp recordings.
Measurements and Results: Recordings from
single cells revealed the presence of spikelets, manifestations of
action potentials in coupled cells, and of dye coupling of neurons in
the PPN. Recordings in pairs of PPN revealed that some of these were
electrically coupled with coupling coefficients ~2%. After blockade of
fast synaptic transmission, the cholinergic agonist carbachol (CAR) was
found to induce rhythmic activity in PPN neurons, an effect eliminated
by the gap junction blockers carbenoxolone or mefloquine. The stimulant
modafinil was found to decrease resistance in PPN neurons after fast
synaptic blockade, indicating that the effect may be due to increased
coupling.
Conclusions: The finding of electrical
coupling in specific RAS cell groups supports the concept that this
underlying process behind specific neurotransmitter interactions
modulates ensemble activity across cell populations to promote changes
in sleep-wake state.
Modulation of electrical couplings in the PPN.
A. Whole cell patch-clamp recordings from a pair of electrical
coupled PPN neurons under voltage-clamp. Hyperpolarizing pulses (top
two records show current pulses) injected to one cell induced a current
in the other cell in the presence of 1uM TTX to block sodium channels
and thus action potential generation. The coupling ratio (CR) was
calculated using the current amplitude in the injected cell divided by
the response current in the coupled cell. For this pair, the CR of cell
1 to cell 2 was 1.6%, and of cell 2 to cell 1 was 2%. The gray line
represents the average of 20 sweeps after 3 min superfusion of TTX.
B. No significant activity was present in this cell during
superfusion of fast inhibitory and excitatory synaptic blockers [CAGM=CNQX
10uM, APV 10uM, Gabazine 10uM and mecamylamine (MEC) 10uM] (top
record). Carbachol (CAR, 50 uM) induced oscillations in this PPN cell
in the presence of fast inhibitory and excitatory synaptic blockers (CAGM)
(second record), which was blocked by 300uM carbenexolone (CBX), a
putative gap junction blocker (bottom record). C. Power
spectrum histogram of the oscillations induced by fast synaptic blockers
(no discernible synchronization), CAR in the presence of fast synaptic
blockers (theta frequency oscillations), and their blockade by CBX in
the same cell shown in B. Each histogram was obtained from a 1 min
recording sample. D. An example of a PPN cell whose input
resistance was decreased by fast synaptic blockers (CAGM), then
decreased further by the superfusion of modafinil (MOD, 150 uM) in the
presence of fast synaptic blockers (CAGM). The decrease in resistance
was partially reversed by the putative gap junction blockers mefloquine
(MEF, 25 uM). The cell was recorded under voltage-clamp mode. A ramp
protocol was applied in order to test the change of membrane resistance,
such that a higher the current was required to compensate for the
voltage change in the presence of MOD, indicating a decrease in
resistance. E. The ramp protocol used in the recording shown in
D. The voltage was held at -60 mV during baseline and then was held at
-105 mV for 500 msec to test the compensatory current. A 1000 msec ramp
from -105 mV to -35 mV was then applied. F. The membrane
resistance change during 50 min recording from the same cell shown in
D. The bars indicate the period when drugs were applied (black: 1 uM
TTX + 10 uM CAGM; maroon: 150 uM MOD + TTX + CAGM; green: 25
|
