Study of the endogenous cortical activity : the role of GABAergic and cholinergic neurotransmission in the physiology and pathophysiology of the brain’s neuronal networks

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Kaplanian, Egis Ani

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Slow oscillations, the hallmark of non-REM sleep, and their cellular counterpart, Up and Down states, are considered a signature of cortical dynamics that reflect the intrinsic network organization. Although previous studies have explored the role of inhibition in regulating Up and Down state activity, little is known about whether this role changes with maturation. This is surprising since both slow oscillations and Up and Down states exhibit significant age-dependent alterations. Furthermore, despite the fact that cholinergic neuromodulation is a crucial regulator of cortical microcircuits’ properties and excitation-inhibition balance, the implication of cholinergic wiring in Up and Down states modulation is poorly studied. To elucidate the developmental impact of GABAb and GABAa receptors on Up and Down state activity, we conducted simultaneous extracellular and intracellular recordings ex vivo, in brain slices of young and adult male mice, using selective blockers, CGP and non-saturating concentration of gabazine, respectively. Blockade of both GABAb- and GABAa- signaling showed age-differentiated functions. CGP caused an increase in Down state duration in young animals, but a decrease in adults. Gabazine evoked Spike-and-Wave-Discharges in both ages; however, while young networks became completely epileptic, adults maintained the ability to generate Up and Down states. Furthermore, voltage clamp recordings of mIPSCs revealed that gabazine selectively blocks phasic currents, particularly involving postsynaptic mechanisms. The latter exhibit clear maturational changes, suggesting a different subunit composition of GABAa receptors between young and adult animals. Indeed, subsequent LFP recordings under diazepam (nanomolar or micromolar concentrations) revealed that mechanisms engaging the drug’s classical-binding-site, mediated by α1-subunit containing GABAa receptors, have a bigger contribution in Up state initiation in young networks compared to adults. In the continuum of our study, we strived to illuminate the role of α5-containing nicotinic acetylcholine receptors (α5* nAChRs) in the generation and maintenance mechanisms of Up and Down states. For this purpose we conducted simultaneous extracellular and intracellular recordings ex vivo in brain slices of young mice from two genotypes (wild type and a5-nACh receptor subunit knockout-ACNA5 Chrna5 or α5-/-) and observed a significant genotype dependent effect on Up and Down state activity: ACNA5 networks turned sluggish with alternating periods of shorter and less frequent Up states and longer Down states compared to wild type. To further elucidate the physiological properties of the ACNA5 microcircuit, we conducted voltage-clamp recordings from layer II/III pyramidal neurons. We showed that these cells receive significantly elevated levels of action-potential dependent inhibitory input which has possibly led to up regulation of synaptic GABAa receptor expression. Subsequent application of CGP and gabazine shed light on an interesting interplay only between α5-nACh receptors and GABAa receptors: blockade of synaptic fast inhibition did not affect ACNA5 networks which kept transiting in physiological Up and Down states, while wild type turned epileptic. Blockade of GABAb receptors had the same effects in the two genotypes, elongation of Up as well as Down states. Taken together, these findings help clarify the mechanisms that underlie the maturation of cortical network activity and enhance our understanding regarding the cortical circuits’ properties in the synchronized slow oscillatory state of the brain.

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Neural circuits, Up and down states, Endogenous activity, Somatosensory cortex, GABAa receptors, GABAb receptors, Nicotinic receptors

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