Phosphorylation influences neurosteroid modulation of synaptic GABA _A receptors in rat CA1 and dentate gyrus neurones

The neurosteroid 5β-pregnan-3α-ol-20-one (5β3α) is a potent, endogenous, positive allosteric modulator of the GABA_A receptor. Relatively low concentrations of 5β3α (10-100 nM), thought to occur physiologically, caused a concentration-dependent slowing of the decay of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) recorded from hippocampal CA1 pyramidal neurones. However, much greater concentrations of this neurosteroid (≥300 nM) were required to similarly influence dentate granule cell mIPSCs. By contrast, the allosteric modulators pentobarbitone and flunitrazepam were equi-effective in prolonging mIPSCs in both neuronal types. Hence, the neurosteroid selectively differentiates between the synaptic GABA_A receptors of these hippocampal neurones. Inhibition of either protein kinase A, or C, greatly reduced the sensitivity of CA1 synaptic GABA_A receptors to 5β3α, but not pentobarbitone, whereas stimulation of PKC had no effect on steroid sensitivity. However, in dentate gyrus granule cells, activation of PKC made mIPSCs sensitive to a previously ineffective concentration of 5β3α. Collectively, these results suggest that the GABA-modulatory effects of physiological levels of the neurosteroid will not be uniformly experienced throughout the central nervous system, or even within the same brain region such as the hippocampus, but will be neurone-specific and will be dependent on the phosphorylation status of the GABA_A receptor, or associated proteins.