humans, REM sleep time decreases from ~8 hours (50% of sleep time) in
the newborn to ~1 hour (15% of sleep time) in the adult. A similar
decrease occurs in the rat at postnatal days 10-30. We hypothesize that
if this developmental decrease in REM sleep does not occur, a number of
disorders may occur, which all have a common symptom of increased REM
sleep drive and hypervigilance. These disorders include schizophrenia,
anxiety disorders, depression, and many sleep disorders. Sensory inputs
and changes in arousal activate the pedunculopontine (PPN) nucleus,
which is part of the reticular activating system (RAS) and responsible
for modulating arousal states, such as waking and REM sleep. The PPN
contains cholinergic, glutamatergic and GABAergic cells. The PPN
receives glutamatergic input from the mesopontine tegmentum. In turn,
glutamatergic and cholinergic efferents from the PPN ascend to the
parafasicular (Pf) nucleus of the thalamus, which then sends
glutamatergic efferents to the cortex. Furthermore, cholinergic (and
probably glutamatergic) efferents from the PPN descend to the
Subcoeruleus (SubC). The SubC also receives glutamatergic input from
other nuclei in the mesopontine tegmentum. This glutamatergic input
modulates the cholinergic input from the PPN and affects the generation
of PGO waves and REM sleep atonia. My hypothesis is that there is a
change in the response of PPN and SubC neurons to glutamate during the
developmental decrease in REM sleep. Disturbance of these developmental
changes may lead to the disturbances of vigilance in the disorders
My preliminary results
reveal two novel findings, a) there may be a developmental change in the
responses of PPN neurons to glutamate receptor agonists, and b) cells in
the PPN appear to fire maximally at gamma band frequency and to exhibit
gamma band subthreshold oscillations. I will explore these new findings
using whole-cell patch clamp and population activity recordings in the
PPN and the SubC. I hypothesize the following: 1) PPN cells will
respond differentially to glutamate receptor agonists, indicating
distinct populations of cells in the PPN are modulated by NMDA vs.
kainic acid receptors. Furthermore, there will be developmental changes
in the responses of PPN and SubC neurons to glutamate receptor
agonists. 2) Application of glutamate receptor agonists will induce
gamma band activity in at least some PPN and SubC neurons and this
activity will be correlated with the response of the population as a
whole in these nuclei.
The information gathered
from these experiments is critical for further understanding the role of
glutamatergic inputs to the PPN and descending inputs to SubC, and how
these inputs modulate signs of waking and REM sleep. These studies
represent novel ideas for sleep-wake control and may revolutionize how
we develop new therapeutic strategies for the treatment of a number of
devastating disorders that have as a common symptom the manifestation of
increased REM sleep drive and hypervigilance.
Study objectives: The pedunculopontine
nucleus (PPN) is involved in the activated states of waking and
paradoxical sleep, forming part of the reticular activating system
(RAS). The studies described tested the hypothesis that PPN neurons are
capable of generating gamma frequency activity.
Whole-cell patch clamp recordings (immersion chamber) were conducted on
9-17 days old rat brainstem slices.
Measurements and Results:
Regardless of cell type (I, II or III) or type of response to carbachol
(excitation, inhibition, biphasic), almost all PPN neurons fired at
gamma frequency when subjected to depolarizing steps (50 +/- 16 Hz, mean
Conclusion: Gamma band activity
appears to be a part of the intrinsic membrane properties of PPN
neurons. Given sufficient excitation, the PPN may impart gamma band
activation on its targets.
Figure 1. Gamma band activity in whole-cell recorded PPN cells.
A) Increasing steps
of current (increase of 30 pA per step, each step was 500 ms in
duration) induced cells to fire action potentials at higher frequencies.
This cell fired maximally at 54 Hz, which is within the gamma range. B)
Average first (■), middle (●) and end (▲)
interspike interval during each current step. The average maximal
firing frequency was during the first interspike interval of the 180 pA
current step, when cells fired at an average rate of 50 +/- 16 Hz.
During the middle and end interspike intervals, the cell firing rate
decreased to low gamma frequency. C) Graph showing the first, middle,
and end firing frequencies of each cell type at the 180 pA current
step. Each cell type fired fastest during the beginning of the
stimulus; type I (n = 17) cells fired significantly faster than type II
(n = 16) or type III (n = 16) [p > 0.05]. The average (+/- SD) maximal
firing frequency for type I, II, and III neurons were: 58 +/- 15 Hz, 45
+/- 15 Hz, and 46 +/- 16 Hz, respectively. There was no significant
difference in the firing frequencies of type I, II, or III PPN neurons
during the middle and end interspike intervals.