A model of delta frequency neuronal network activity and theta-gamma interactions in rat sensorimotor cortex in vitro

Abstract

In recent decades, advances in electrophysiological techniques have enabled understanding of neuronal network activity, with in vitro brain slices providing insights into the mechanisms underlying oscillations at various frequency ranges. Understanding the electrical and neuro-pharmacological properties of brain networks using selective receptor modulators in native tissue allows to compare such properties with those in disease models (e.g. epilepsy and Parkinson’s). In vivo and in vitro studies have implicated M1 in execution of voluntary movements and, from both local network in vitro and whole brain in vivo perspectives. M1 has been shown to generate oscillatory activity at various frequencies, including beta frequency and nested theta and gamma oscillations similar to those of rat hippocampus. In vivo studies also confirmed slow wave oscillations in somatosensory cortex including delta and theta band activity. However, despite these findings, non-thalamic mechanisms underlying cortical delta oscillations remain almost unexplored. Therefore, we determined to explore these oscillations in vitro in M1 and S1. Using a modified sagittal plane slice preparation with aCSF containing neuroprotectants, we have greatly improved brain slice viability, enabling the generation and study of dual rhythms (theta and gamma oscillations) in deep layers (LV) of the in vitro sensorimotor slice (M1 and S1) in the presence of KA and CCh. We found that theta-gamma activity in M1 is led by S1 and that the amplitude of gamma oscillations was (phase-amplitude) coupled to theta phase in both regions. Oscillations were dependent on GABAAR, AMPAR and NMDAR and were augmented by DAR activation. Experiments using cut/reduced slices showed both M1 and S1 could be intrinsic generators of oscillatory activity. Delta oscillations were induced in M1 and S1 by maintaining a neuromodulatory state mimicking deep sleep, characterised by low dopaminergic and low cholinergic tone, achieved using DAR blockade and low CCh. Delta activity depends on GABAAR, GABABR and AMPAR but not NMDAR, and once induced was not reversible. Unlike theta-gamma activity, delta was led by M1, and activity took >20mins to develop in S1 after establishement of peak power in M1. Unlike M1, S1 alone was unable to support delta activity. Dopamine modulates network activity in M1 and it is known that fast-spiking interneurons are the pacemakers of network rhythmogenesis. Recent studies reported that dopamine (DA) controled Itonic in medium spiny, ventrobasal thalamus and nucleus accumbens neurons by modulation of GABARs or cation channels. In the current study, voltage-clamp whole cell recordings were performed in fast spiking interneurons (FS cells) in Layer V of M1. These recordings revealed tonic and phasic GABAAR inhibition and when DA was bath applied, a slow inward current (IDA) was induced. IDA was mediated by non-specific cationic TRPC channels following D2R-like receptor activation. Overall, my studies show the strong interdependence of theta-gamma rhythmogenesis between M1 and S1, dominanace of M1 at delta frequency and the crucial role of dopamine in controlling FS cell activity. Further exploration of these rhythms in models of pathological conditions such as Parkinson`s disease and Epilepsy may provide insights into network changes underlying these disease conditions.