Mechanism behind gamma band activity in developing parafascicular nucleus

KEZUNOVIC N; HYDE JR; URBANO FJ; GARCIA-RILL E

Washington DC, USA

Congreso; 41th Society for Neuroscience Annual Meeting; 2011

Society for Neuroscience

The parafascicular nucleus (Pf) is considered to be part of the ?non-specific? intralaminar thalamus and sends widespread projections to the cortex. Pf neurons receive cholinergic input from the pedunculopontine nucleus (PPN), part of the reticular activating system (RAS) that is active during waking and REM sleep. These states are characterized in the EEG by low amplitude, high frequency (20-80 Hz) activity in the gamma range. This study tested the hypothesis that Pf neurons develop the ability to oscillate at gamma frequencies during the 8-25 day postnatal period, and that the cholinergic agonist carbachol (CAR) can further increase the oscillatory frequency. Furthermore, we tested the role of high voltage-activated calcium channels (P/Q- and N-type) as a possible mechanism in the generation of gamma band oscillations in Pf cells. Whole-cell patch-clamp responses were recorded in 8-25 day old rat parasagittal slices containing the Pf nucleus. Slices were recorded at 37oC while perfused with oxygenated aCSF in an immersion chamber containing synaptic blockers: APV to block NMDA, CNQX to block AMPA/KA, Gabazine to block GABA-A, strychnine to block glycine, receptors. Furthermore, tetrodotoxin was used to block sodium channels while the specific calcium channel blockers ω-Agatoxin-IVA, and ω-Conotoxin-GVIA were used to block P/Q- and N-type calcium channels, respectively. CAR was used to study cholinergic effects on oscillating Pf neurons. Intrinsic membrane properties were measured using current clamp step and ramp protocols. Our studies showed that Pf neurons have the ability to oscillate at gamma band frequency when maximally activated, even at early stages of development, and that the frequency of the oscillations increased with age. Furthermore, pre-treatment with CAR induced significantly higher oscillatory frequencies compared to those not treated with CAR (p<0.001). Also, ω-Agatoxin_IVA completely blocked gamma frequency oscillations (n=9), while ω-Conotoxin-GVIA (n=20) partially decreased the amplitude of gamma frequency oscillations. Gamma band activity appears to be part of the intrinsic membrane properties of Pf neurons, and P/Q-type, and to some extent N-type, calcium channels mediate these gamma band oscillations. Moreover, the frequency of the oscillations is age-dependent and can be enhanced by CAR. We hypothesize that the cholinergic input to the Pf could activate these cells to oscillate at gamma frequency, and perhaps relay these rhythms to cortical areas, thus providing a stable high frequency state for non-specific thalamocortical processing.