Changes in properties and neurosteroid regulation of GABAergic synapses in the supraoptic nucleus during the mammalian female reproductive cycle

1. GABA(A) receptor-mediated synaptic innervation of oxytocin neurones in the supraoptic nucleus (SON) was analysed in adult female rats going through their first reproductive cycle by recording the spontaneous inhibitory postsynaptic currents (sIPSCs) at six stages of female reproduction. 2. During pregnancy we observed a reduction in the interval between monoquantal sIPSCs. The synaptic current amplitude, current decay and neurosteroid sensitivity of postsynaptic GABA(A) receptors observed at this stage were not distinguishable from those measured in virgin stage SON. 3. Upon parturition an increase in monoquantal synaptic current decay occurred, whereas potentiation by the progesterone metabolite allopregnanolone (3α-OH-DHP) was suppressed. 4. Throughout a substantial part of the lactation period the decay of synaptic currents remained attenuated, whilst the potentiation by 3α-OH-DHP remained suppressed. 5. Several weeks after the end of lactation sIPSC intervals, their current decay velocity as well as the potentiation by 3α-OH-DHP were restored to pre-pregnancy levels, which is indicative of the cyclical nature of synaptic plasticity in the adult SON. 6. Competitive polymerase chain reaction (PCR) analysis showed that virgin animals expressed α1 and α2 GABA(A) receptor subunit mRNA at a relative ratio of 2:1 compared with p-actin. After pregnancy both α1 and α2 subunit mRNA levels were transiently increased, although at a relative ratio of 1:4, in line with the hypothesis that α2 plays a large role in postsynaptic receptor functioning. During post-lactation both α subunits were downregulated. 7. We propose that synaptic remodelling in the SON during pregnancy includes changes in the putative number of GABA release sites per neurone. At parturition, and during the two consecutive weeks of lactation, a subtype of postsynaptic GABA(A) receptors was observed, distinct from the one being expressed before and during pregnancy. Synaptic current densities, calculated in order to compare the impact of synaptic inhibition, showed that, in particular, the differences in 3α-OH-DHP potentiation of these two distinct GABA(A) receptor subtypes produce robust shifts in the impact of synaptic inhibition of oxytocin neurones at the different stages of female reproduction.