Nebosja Kezunovic NKezunovic@uams.edu

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.  Both w-CgTX  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.