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Rodrigo Pena,
Horacio RotsteinThe neocortex is a brain region responsible for many higher-order functions. Sensory signals arriving from different areas are integrated into the neocortex. Oscillations at certain frequency bands are believed to coordinate activity in many areas [1]. Resonance refers to the ability of a system to generate an amplified response if the input oscillation is tuned in a specific frequency band. Recent work showed the role of inhibition on the control of theta (4-11 Hz) oscillations through resonance [2]. By using optogenetic activation interneurons (inhibitory) induced theta-band-limited spiking in pyramidal (excitatory) neurons. On the other side, direct optogenetic activation of pyramidal cells did not generate any resonance pattern. Although it is clear that this phenomenon is neuron-specific, the network architecture responsible for the observed resonance and how this is related to the correct gating of the signals in such a network is currently unknown.
We address these issues by constructing a microcircuit biophysical minimal model of the neocortex using the Hodgkin-Huxley formalism [3]. We consider two pyramidal cells (PYR), one parvalbumin-positive (PV) interneuron, and one somatostatin-expressing (SOM) interneuron. These cells are interconnected with exponential decaying event-driven synapses where short-term depression/facilitation is present when appropriate [4]. Every cell spontaneously fires while receiving a noise input process to simulate _in vivo_ synaptic barrage [5]. We apply periodic currents with different frequencies into PV cells and evaluate the PYR firing rate.
By applying oscillatory activation in PV, theta-band resonance was induced in PYRs whereas direct activation of PYRs did not show resonance, as experimentally reported. First, our results highlight the importance of post- inhibitory rebound in order to transfer signals from PV to PYR cells. Secondly, our results show that SOMs, adaptation, depression, and facilitation regulate these resonance effects. We explain these effects in terms of additional frequency filters that are added to the system: adaptation and facilitation act as a high-pass filter while depression acts as a low-pass filter. SOM cells regulate the low frequencies since they connect to other neurons through facilitation. In summary, when a current with a specific frequency is applied to the PV cells, this input signal is processed by a combination of filters, in the form of synapses and ionic currents, until a final output is produced from PYR cells. Our results highlight the importance of the combined activity of different neocortical cells in flexibly selecting inputs.
AcknowledgmentThis work was supported by the National Science Foundation grant DMS-1608077 (HGR).
References[1] Buzsaki, G. Rhythms of the Brain. Oxford University Press 2006.
[2] Stark, E., Eichler, R., Roux, et al. Inhibition-induced theta resonance in cortical circuits. Neuron. 2013, 80, 1263-1276.
[3] Pospischil, M., Toledo-Rodriguez, M., Monier, C., et al. Minimal Hodgkin–Huxley type models for different classes of cortical and thalamic neurons. Biol Cybern. 2008, 99, 427-441.
[4] Tsodyks, M., Pawelzik, K., Markram, H. Neural networks with dynamic synapses. Neural Comput. 1998, 10, 821-835
[5] Destexhe, A., Rudolph, M., Fellous, J. M., et al. Fluctuating synaptic conductances recreate in vivo-like activity in neocortical neurons. Neurosci. 2001, 107, 13-24.
