1Institute of Neurology, UCL, London,UK
During gamma oscillations, firing rates of principal cells and interneurons in a network are periodically modulated at 30 100Hz, resulting in field potential oscillations.
The coherent periodic firing of interneurons imposes alternating epochs of high and low excitability on the principal cells in the network. If input to a network arrives at a random phase relative to ongoing oscillation this variable inhibitory tone may compromise the reliability of the network response. Alternatively, periodic input may be able to entrain local oscillatory circuitry, thereby controlling the relative phase of input and inhibition.
The dentate gyrus and CA3 region both act as gamma generators in vivo and the CA3 region receives strong input from dentate gyrus via the mossy fibres. Entrainment of CA3 gamma by the dentate gyrus could significantly affect signal propagation between these regions.
One approach to studying coupling between oscillators is to experimentally measure their phase response curves (PRCs) and then consider the entrainment properties using the theory of pulse coupled oscillators.
Although it is an oversimplification to consider the spatially extended CA3 region as a single oscillator or the output from the dentate gyrus on a gamma cycle as a single pulse, these methods provide a tool for experimentally investigating coupling between the dentate and CA3 gamma oscillators.
We have measured the phase response of carbachol induced gamma oscillations in the CA3 region in vitro to mossy fiber stimulation.
At low stimulation intensities the rephasing is type 1 by winding number; the new phase after stimulation varies through a whole cycle as the phase of stimulation is varied over a whole cycle. At stronger stimulation intensities the rephasing becomes type 0 by winding number with the new phase after stimulation only varying over a small range as the phase of stimulation is varied over a full cycle.
In the weak stimulation regime, the phase response curve has a region of stimulation phase in which mossy fiber input delays the oscillation and a region in which it advances the oscillation. This shape of PRC allows a pulse coupled oscillator to entrain 1:1 with a periodic input of either higher or lower frequency. Our experimentally measured PRCs predict that for inputs at similar frequency to unforced CA3 gamma, entrainment will occur at a phase such that mossy fiber input arrives while local inhibition in CA3 is weak. Entrainment of gamma oscillations in CA3 by mossy fiber input may therefore facilitate signal propagation between dentate gyrus and CA3.