GABAB - and GABAA -receptor-mediated regulation of Up and Down states across development.

Slow oscillations, the hallmark of non-REM sleep, and their cellular counterpart, Up and Down states (UDSs), are considered a signature of cortical dynamics that reflect the intrinsic network organization. Although previous studies have explored the role of inhibition in regulating UDSs, little is known about whether this role changes with maturation. This is surprising since both slow oscillations and UDSs exhibit significant age-dependent alterations. To elucidate the developmental impact of GABAB and GABAA receptors on UDS activity, we conducted simultaneous local field potentials and intracellular recordings ex vivo, in brain slices of young and adult male mice, using selective blockers, CGP55845 and a non-saturating concentration of gabazine, respectively. Blockade of both GABAB and GABAA signalling showed age-differentiated functions. CGP55845 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 UDSs. Furthermore, voltage clamp recordings of miniature inhibitory postsynaptic currents 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 in young vs. adult animals. Indeed, subsequent local field potential recordings under diazepam (nanomolar or micromolar concentrations) revealed that mechanisms engaging the drug's classical binding site, mediated by α1-subunit-containing GABAA receptors, make a bigger contribution to Up state initiation in young networks compared to adults. Taken together, these findings help clarify the mechanisms that underlie the maturation of cortical network activity and enhance our understanding regarding the emergence of neurodevelopmental disorders. KEY POINTS: Slow oscillations, the EEG hallmark of non-REM sleep, and their cellular counterpart, Up and Down states (UDSs), are considered the default activity of the cerebral cortex and reflect the underlying neural connectivity. GABAB - and GABAA -receptor-mediated inhibition play a major role in regulating UDS activity. Although slow oscillations and UDSs exhibit significant alterations as a function of age, it is unknown how developmental changes in inhibition contribute to the developmental profile of this activity. In this study, we reveal for the first time age-dependent effects of GABAB and GABAA signalling on UDSs. We also document the differential subunit composition of postsynaptic GABAA receptors in young and adult animals, highlighting the α1-subunit as a major component of the age-differentiated regulation of UDSs. These findings help clarify the mechanisms that underlie the maturation of cortical network activity, and enhance our understanding regarding the emergence of neurodevelopmental disorders.