Use of optical imaging in brain slices from developing rats to evaluate spatio-temporal dynamics of large-scale neuronal coalitions: effects of physiological and pharmacological manipulations

Voltage-sensitive dye imaging (VSDI), is a relatively novel technique which enables the recording and observation of neuronal population activity at the meso-scale of brain organisation. This optical imaging technique is used here in combination with extracellular electrophysiology (field potentials) in order to investigate ‘Neuronal Assemblies’, i.e. the phenomenon of synchronous and dynamic activation of millions of inter-connected neurons, giving rise to detectable patterns of large-scale population activity. For the experiments presented here, VSDI and field potentials were used to characterise and compare assemblies evoked in 2 cortical and 2 sub-cortical preparations: for the former, the well-studied primary somatosensory cortex (S1BF) was chosen, in addition to the agranular insular cortex (AIC) which has been less extensively studied. For the latter sub-cortical areas, responses were evoked in ventroposteromedial (VPM) thalamic nucleus and basal forebrain (BF). These investigations suggested cortical tissue produced similar response dynamics regardless of location, meaning large neuronal assemblies may be a inherent property of cortical tissue, since these response types were not found in sub-cortical structures. In addition to this, comparisons of evoked assemblies highlighted clear differences between cortical, thalamocortical and sub-cortical activity patterns, with evoked responses appearing geared for their biological functions, and allowed for the selection of 3 preparations of interest, i.e. S1BF LIV, AIC LII/III and thalamocortical stimulations, for characterising more fully the impact of anaesthetics on the parameters of neuronal assemblies. All anaesthetics tested here induced dose-dependent inhibitions of cortical activity, which were significantly more pronounced when assemblies were triggered via thalamocortical (remote) activation and in BF. Comparison of anaesthetic effects seen here with those induced on target proteins (from the literature) highlighted GABAA and nicotinic acetylcholine (nACh) receptors as key effectors in anaesthetic-induced inhibition by group 1 anaesthetics, while experiments with ketamine highlighted NMDA and nACh receptors in addition to HCN channels. These data describe more fully the manner in which anaesthetics rely on multi-target mechanisms, as well as exemplifying the practicality of combining VSDI with field potentials in the investigation of drug action on neuronal assembly functioning and cerebral function.