Coherence and frequency in the reticular activating system (RAS)

Abstract

This review considers recent evidence showing that cells in the reticular activating system (RAS) exhibit (1) electrical coupling mainly in GABAergic cells, and (2) gamma band activity in virtually all of the cells. Specifically, cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine dorsal subcoeruleus nucleus dorsalis (SubCD) (1) show electrical coupling, and (2) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanism behind electrical coupling is important because the stimulant modafinil was shown to increase electrical coupling. We also provide recent findings demonstrating that all cells in the PPN and Pf have high threshold, voltage-dependent P/Q-type calcium channels that are essential to gamma band activity. On the other hand, all SubCD, and some PPN, cells manifested sodium-dependent subthreshold oscillations. A novel mechanism for sleep-wake control based on transmitter interactions, electrical coupling, and gamma band activity is described. We speculate that continuous sensory input will modulate coupling and induce gamma band activity in the RAS that could participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Copyright © 2012 Elsevier Ltd. All rights reserved.

Figures

Fig. 1

Electrical coupling in the reticular activating system (RAS). A) Fluorescence labeling for neurobiotin injected into one pedunculopontine nucleus (PPN) cell that manifested spikelets. Multiple (four) other PPN cells were dye coupled by the injection (see arrowheads). B) Protocol for testing electrical coupling and coupling ratio. Under tetrodotoxin (TTX) and fast synaptic blockers, intracellular stimulation of one cell produced a response in the other, suggesting that these subcoeruleus nucleus dorsalis (SubCD) cells were coupled via gap junctions. The ratio of the responses was in the 2–3% range, although not identical in both directions. C) Shows the ramp protocol used to test membrane resistance of coupled neurons. The effects of modafinil (MOD) on a PPN neuron was a decrease in input resistance by the superfusion of MOD (300 μM) in the presence of fast synaptic blockers (CAGM: CNQX, APV, gabazine, and mecamylamine all at 10 μM). The cell was recorded under voltage-clamp mode. A ramp protocol was applied in order to test the change of membrane resistance under TTX and CAGM (black line), such that a higher current was required to compensate for the voltage change in the presence of MOD, indicating a decrease in resistance that could only occur if the neuron was electrically coupled (red line). The decrease in resistance with MOD that was partially blocked by the gap junction blocker mefloquine (MEF, 25 μM) (blue line). D) Shows a ramp protocol conducted on a SubCD neuron revealing a decrease in resistance by MOD (red line), that was partially reversed by MEF (25 μM) application (black line). E) Results from the recordings shown in C, in which testing of input resistance was carried out every 10 s (black squares) in the presence of TTX and CAGM (black arrow denotes black record shown in C). MOD decreased input resistance (red arrow denotes red record shown in C), while MEF partially blocked the effects of MOD and increased input resistance (blue arrow denotes blue record shown in C). F) Results from the recordings shown in D, in which testing of input resistance was carried out every 10 s (black squares) in the presence of TTX and CAGM (black arrow denotes black record shown in D). MOD decreased input resistance (red arrow denotes red record shown in D), while MEF partially blocked the effects of MOD and increased input resistance.

Fig. 2

Gamma band activity in the reticular activating system (RAS). A) Representative membrane potential responses of the same pedunculopontine nucleus (PPN) neuron to depolarizing 1 s square step (gray record, left), and to 1 s long ramp (black record, right) obtained in the presence of fast synaptic blockers and tetrodotoxin (TTX). B) Overlapping curves comparing power spectrum amplitudes for oscillations obtained in A, pulses vs ramps. Note the higher amplitude of the oscillations obtained in the same neuron using ramps vs square steps. C) Representative membrane potential responses of the same parafascicular nucleus (Pf) neuron to depolarizing 1 s square step (gray record, left), and to 1 s long ramp (black record, right) obtained in the presence of fast synaptic blockers and TTX. D) Overlapping curves comparing power spectrum amplitudes for oscillations obtained in C, pulses vs ramps. Again, note the higher amplitude of the oscillations obtained using ramps vs square steps. E) Representative subthreshold oscillations at membrane potentials above action potential (AP) threshold (–43 mV). The dotted boxes include 1 s of recordings (upper records) that are also shown at higher resolution (lower records), revealing gamma frequency oscillations at –43 mV holding potential. F) Power spectrum confirming that the subthreshold oscillations were at beta/gamma frequencies, but no higher.

Fig. 3

Coherent activation of the pedunculopontine nucleus (PPN). Upper panel. Local stimulation of the PPN was applied as 10 pulses at 1 Hz, 10 Hz or 40 Hz, in the presence of carbachol (CAR, 50 μM). The poststimulus event related spectral perturbation (ERSP) of the local field potential recorded in the PPN was calculated. Note that increasing frequency of stimulation induced greater and greater coherence in the response. An ERSP represents a measure of event related brain dynamics induced in the EEG or local field potential spectrum by a stimulus or event. It basically measures average dynamic changes in amplitude of the broad band frequency spectrum as a function of time relative to an experimental event. These analyses use MatLab to generate power spectra for continuous points in time, e.g., during and after stimulation or application of an agent or washout. These graphs plot frequency of activity over time, and the amplitude of the frequency shown is color-coded such that background (control) appears blue, and higher amplitudes appear progressively more yellow, then red. A convenient way of reading these graphs is as a running power spectrum over time., Lower panel. Hypothesized organization of response to afferent input (green arrow) impinging on PPN dendrites bearing P/Q-type high threshold calcium channels. GABAergic neurons are shown in red and as electrically coupled. Cholinergic and glutamatergic output neurons are shown in blue. Higher frequency stimulation induced coherent peaks of activation that were separated by recurrent gaps. This pattern would be expected following activation of a system which activates output neurons (blue) as well as local inhibitory, electrically coupled neurons (red).