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The neuroactive steroid allopregnanolone (ALLO) is an endogenous allosteric modulator of γ-aminobutyric acid type A (GABAA) receptors. There is evidence that ALLO, at physiologically relevant concentrations, modulates GABAA receptor function in the cerebral cortex. The widely used anesthetic agent propofol and ALLO share a similar mode of molecular action. Here, we ask how GABAA receptor-mediated synaptic inhibition and action potential firing of neurons in cultured cortical slices are altered by either ALLO or propofol or by coapplying both agents.
We explored the effects of ALLO and propofol on spontaneous action potential activity of neocortical neurons in organotypic slices cultured from C57BL6 mice by performing extracellular multiunit recordings. Furthermore, we carried out whole-cell voltage-clamp experiments to quantify the drug effects on GABAA receptor-mediated tonic and phasic currents.
We found that ALLO (100 nM) decreased multiunit action potential firing of neocortical neurons by approximately 21%. Moreover, the duration of GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) was prolonged (mean Δdecay time prolongation: 12.9 ± 2.2 milliseconds; n = 23), and a bicuculline-sensitive tonic current was induced (mean Δbaseline shift: -24.6 ± 13.6 pA; P = .002; n = 6). A subanesthetic concentration of propofol (250 nM) decreased the discharge rates of cortical neurons to a similar degree as ALLO (100 nM). ALLO and propofol administered in combination acted in an additive manner to reduce action potential firing. However, during ALLO administration, propofol was significantly more effective in enhancing GABAergic synaptic transmission. Propofol (250 nM) prolonged the inhibitory postsynaptic currents decay times by 10.4 ± 6.1 milliseconds (n = 9) with ALLO added to the bathing solution; in the absence of ALLO, however, propofol prolonged the decay time by only 3.8 ± 2 milliseconds (n = 13).
In cortical neurons, GABAA receptor-mediated synaptic transmission is potentiated by ALLO and propofol in a synergistic manner, whereas the effects on spontaneous action potential activity appear additive. A coapplication of neurosteroids and propofol in general anesthesia and intensive care medicine may open new ways to reduce anesthetic dose requirements and, thus, avoid undesired anesthetic-induced side effects.
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