The pedunculopontine nucleus (PPN), part of the reticular activating
system, modulates the 'activated states' of waking and paradoxical
sleep. We previously reported (see Christen Simon's page) that gamma
band activity appears to be intrinsically generated by the membrane
electroresponsiveness of PPN neurons, and that the neuronal ensemble
generates different patterns of gamma activity in response to specific
transmitters. Regardless of type (I, II or III), PPN neurons fired at
gamma frequency, but no higher, during depolarizing steps. The present
studies tested the hypothesis that P/Q-type calcium channels are
responsible for generating gamma band frequency activity in PPN
neurons. We found that all PPN neuron types showed gamma oscillations
in the presence of TTX and synaptic blockers when the membrane potential
was depolarized using current ramps. PPN neurons showed gamma band
current oscillations when voltage-clamped at potentials above -30 mV,
indicating their origin is spatially located beyond voltage clamp
control. Such gamma oscillations were present at higher amplitudes in
type I and III compared to type II cells, but at similar frequencies.
The N-type calcium channel blocker w-conotoxin-GVIA
partially reduced gamma oscillations, while the P/Q-type blocker
w-agatoxin-IVA abolished them.
and w-Aga blocked
voltage-dependent calcium currents by 56% and 52%, respectively.
Carbachol-induced PPN population responses typically show low theta and
gamma activity, and w-agatoxin-IVA
blocked gamma but not low theta activity. These results strongly
suggest that voltage-dependent P/Q-, and to a lesser extent N-, type
calcium channels mediate gamma oscillations in the PPN. Moreover,
P/Q-type channels appear to be essential for gamma oscillation genesis.
All PPN neuron types exhibited gamma band
oscillations when depolarized.
A) Representative membrane potential responses to
depolarizing 2-sec square pulses obtained in the presence of synaptic
blockers and TTX in type I (green records), type II (blue records) and
type III (red records) PPN neurons. B) Representative membrane
potential responses to depolarizing 2-sec long ramps for the same
neurons shown in A. C) Overlapping curves comparing power spectrum
amplitudes for oscillations obtained using square pulses vs. ramps. All
PPN neurons were recorded in current-clamp configuration combining
high-K+ intracellular solution and synaptic blockers + TTX.
The respective resting membrane potentials were -48 mV, -49 mV and -51
mV for type I, II and II neurons.