During postnatal development endogenous neurosteroids influence GABA-ergic neurotransmission of mouse cortical neurons

Abstract

As neuronal development progresses, GABAergic synaptic transmission undergoes a defined program of reconfiguration. For example, GABAA receptor (GABAAR)-mediated synaptic currents, (miniature inhibitory postsynaptic currents; mIPSCs), which initially exhibit a relatively slow decay phase, become progressively reduced in duration, thereby supporting the temporal resolution required for mature network activity. Here we report that during postnatal development of cortical layer 2/3 pyramidal neurons, GABAAR-mediated phasic inhibition is influenced by a resident neurosteroid tone, which wanes in the second postnatal week, resulting in the brief phasic events characteristic of mature neuronal signalling. Treatment of cortical slices with the immediate precursor of 5α-pregnan-3α-ol-20-one (5α3α), the GABAAR-inactive 5α-dihydroprogesterone, (5α-DHP), greatly prolonged the mIPSCs of P20 pyramidal neurons, demonstrating these more mature neurons retain the capacity to synthesize GABAAR-active neurosteroids, but now lack the endogenous steroid substrate. Previously, such developmental plasticity of phasic inhibition was ascribed to the expression of synaptic GABAARs incorporating the α1 subunit. However, the duration of mIPSCs recorded from L2/3 cortical neurons derived from α1 subunit deleted mice, were similarly under the developmental influence of a neurosteroid tone. In addition to principal cells, synaptic GABAARs of L2/3 interneurons were modulated by native neurosteroids in a development-dependent manner. In summary, local neurosteroids influence synaptic transmission during a crucial period of cortical neurodevelopment, findings which may be of importance for establishing normal network connectivity.

Endogenous neurosteroids influence synaptic GABAA receptors during postnatal development

“…Similar to VB neurones, the mIPSC frequency increased and their duration decreased between P7‐8 and P20‐24. In common with VB neurones, the mIPSC kinetic change did not result from neurosteroid insensitivity of synaptic GABA A R, but resulted from a loss of neurosteroid production because the mIPSC decay time of P7, but not of P20‐24 cortical neurones was substantially reduced by pre‐incubation with γ‐CD, or by finasteride pre‐treatment . By contrast to the thalamic VB neurones, γ‐CD influenced the mIPSC decay time at P10 and at P15 but, as described above, was inert by P20‐24 (Figure ).…”

“…As described above, the GABA A R subunit composition may influence the mIPSC duration, particularly the α1 subunit associated with brief events. We found the mIPSCs recorded from α1 −/− cortical L2/3 pyramidal neurones to be prolonged compared to their wild‐type counterparts at all ages studied (P7‐8, P10, P15 and P20‐24) . In common with WT neurones, treatment with γ‐CD, or finasteride, reduced the mIPSC decay time of P7‐15 α1 −/− neurones, although with no effect on P20‐24 neurones (Figure ).…”

“…In L2/3 cortical pyramidal neurones, the mIPSC profile changed considerably from neonatal (P7-15), to juvenile stages (P20-24). 90 Similar to VB neurones, the mIPSC frequency increased and their duration decreased between P7-8 and P20-24. In common with VB neurones, the mIPSC kinetic change did not result from neurosteroid insensitivity of synaptic GABA A R, but resulted from a loss of neurosteroid production because the mIPSC decay time of P7, but not of P20-24 cortical neurones was substantially reduced by pre-incubation with γ-CD, or by finasteride pre-treatment.…”

“…Finally, the frequency of mIPSCs increased during development in both cortical and thalamic neurones. However, although, in the thalamus, the increased mIPSC frequency was the result of a waning neurosteroid presynaptic influence, the frequency of cortical mIPSCs was not influenced by γ‐CD pre‐treatment and therefore may reflect increased GABAergic innervation.…”

“…In L2/3 cortical pyramidal neurones, the mIPSC profile changed considerably from neonatal (P7‐15), to juvenile stages (P20‐24) . Similar to VB neurones, the mIPSC frequency increased and their duration decreased between P7‐8 and P20‐24.…”