Mechanism behind gamma band activity in the pedunculopontine nucleus

Abstract

The pedunculopontine nucleus (PPN), part of the reticular activating system, modulates waking and paradoxical sleep. During waking and paradoxical sleep, EEG responses are characterized by low amplitude, high frequency oscillatory activity in the beta/gamma band range (~20–80 Hz). We reported that gamma band activity may 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. This study attempted to identify the voltage-gated calcium and potassium channels involved in the rising and falling phases of gamma oscillations in PPN neurons. We found that all rat (8–14 day) PPN cell types showed gamma oscillations in the presence of TTX and synaptic blockers when membrane potential was depolarized using current ramps. PPN neurons showed gamma oscillations when voltage-clamped at holding potentials above −30 mV, indicating their origin, we speculate, is spatially located beyond voltage clamp control. The average frequency for all PPN cell types was 23 +/− 1 Hz, which increased under carbachol (47 +/− 2 Hz; ANOVA df= 64, t= 12.5, p<0.001). The N-type calcium channel blocker ω-conotoxin-GVIA partially reduced gamma oscillations, while the P/Q-type blocker ω-agatoxin-IVA abolished them. Both ω-CgTX and ω-Aga blocked voltage-dependent calcium currents by 56% and 52%, respectively. The delayed rectifier-like potassium channel blocker α-dendrotoxin also abolished gamma oscillations. In carbachol-induced PPN population responses, ω-agatoxin-IVA reduced higher, and ω-CgTx mostly lower, frequencies. These results suggest that voltage-dependent P/Q-, and to a lesser extent N-, type calcium channels mediate gamma oscillations in PPN.

Modulation of gamma oscillations in the pedunculopontine nucleus by neuronal calcium sensor protein-1: relevance to schizophrenia and bipolar disorder

“…These findings revealed one of the main characteristics of PPN neuronal gamma-frequency oscillations, showing that the membrane potential at the soma had to be gradually depolarized up to approximately Ϫ20 mV (using 2-s current ramps) to activate high-threshold, voltage-dependent P/Q-type calcium channels, and to a lesser extent, N-type calcium channels, that mediate high-frequency oscillations. That is, we found that P/Q-type channels were essential for eliciting the oscillations, but N-type channels were only permissive (Kezunovic et al 2011). The membrane potential at the dendrites, where these calcium channels are located (Hyde et al 2013a,b), was closer to physiological levels.…”

“…The role of gamma activity in the PPN was proposed to underlie the process of preconscious awareness : in other words, to participate in the essential mechanism that allows the uninterrupted flow of afferent sensory information necessary for the "stream of consciousness" (James 2007). We also discovered that gamma oscillations in the PPN and ILT are based on high-threshold, voltage-dependent N-and P/Q-type calcium channels (Kezunovic et al 2011. We localized P/Q-type calcium channel-mediated oscillations to the dendrites of PPN and Pf cells (Hyde et al 2013a,b).…”

“…Based on our previous findings, PPN neurons could not be depolarized beyond Ϫ25 mV using square steps due to the activation of potassium channels during rapid depolarization, instead requiring ramps to reach highly depolarized levels (Kezunovic et al 2011. These findings revealed one of the main characteristics of PPN neuronal gamma-frequency oscillations, showing that the membrane potential at the soma had to be gradually depolarized up to approximately Ϫ20 mV (using 2-s current ramps) to activate high-threshold, voltage-dependent P/Q-type calcium channels, and to a lesser extent, N-type calcium channels, that mediate high-frequency oscillations.…”

“…Sagittal sections (400 m) containing the PPN were cut, and slices were allowed to equilibrate in normal aCSF at room temperature for 1 h. The aCSF was composed of the following (in mM): 117 NaCl, 4.7 KCl, 1.2 MgCl 2 , 2.5 CaCl 2 , 1.2 NaH 2 PO 4 , 24.9 NaHCO 3 , and 11.5 glucose. Slices were recorded at 37°C while perfused (1.5 ml/min) with oxygenated (95% O 2 -5% CO 2 ) aCSF in an immersion chamber for patch-clamp studies as previously described (Kezunovic et al 2011. The superfusate contained the following synaptic receptor antagonists: the selective NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV; 40 M), the competitive AMPA/kainate glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 M), the glycine receptor antagonist strychnine (STR; 10 M), the specific GABA A receptor antagonist gabazine (GBZ; 10 M), and the nicotinic receptor antagonist mecamylamine (MEC; 10 M) collectively referred to here as synaptic blockers (SB).…”

“…Gamma activity in the motor cortex lags behind coherent activity in subcortical structures (Lalo et al 2008;Litvak et al 2012), suggesting that gamma synchronization reflects an arousal-related event for enabling initiation of movement (Brucke et al 2012;Cheyne and Ferrari 2013;Jenkinson et al 2013). In addition, we showed that every cell in the reticular activating system (RAS), especially the pedunculopontine nucleus (PPN), plateaus at gamma frequencies (Kezunovic et al 2011;Simon et al 2010;Urbano et al 2012). We made similar observations in the intralaminar thalamus (ILT), specifically the parafascicular nucleus (Pf), which relays PPN activity to the cortex .…”