Production of 5α-Reduced Neurosteroids Is Developmentally Regulated and Shapes GABAAMiniature IPSCs in Lamina II of the Spinal Cord

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Abstract
In lamina II of the spinal dorsal horn, synaptic inhibition mediated by ionotropic GABAAand glycine receptors contributes to the integration of peripheral nociceptive messages. Whole-cell patch-clamp recordings were performed from lamina II neurons in spinal cord slices to study the properties of miniature IPSCs (mIPSCs) mediated by activation of GABAAand glycine receptors in immature (N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (PK11195), an inhibitor of the peripheral benzodiazepine receptor (PBR), or finasteride, which blocks 5α-reductase, accelerated the decay kinetics of GABAAreceptor-mediated mIPSCs in immature, but not in adult animals. Glycine receptor-mediated mIPSCs remained unaffected under these conditions. These results suggest the presence of a tonic production of 5α-reduced neurosteroids in young rats that confers slow decay kinetics to GABAAmIPSCs. At all of the ages, selective stimulation of PBR by diazepam in the presence of flumazenil prolonged GABAAmIPSCs in a PK11195- and finasteride-sensitive manner. This condition also increased the proportion of mixed GABAA/glycine mIPSCs in the immature animals and led to the reappearance of mixed GABAA/glycine mIPSCs in the adult. Our results might point to an original mechanism by which the strength of synaptic inhibition can be adjusted locally in the CNS during development and under physiological and/or pathological conditions by controlling the synthesis of endogenous 5α-reduced neurosteroids.

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