Neurosteroid effects at α4βδ GABAA receptors alter spatial learning and synaptic plasticity in CA1 hippocampus across the estrous cycle of the mouse

Fluctuations in circulating levels of ovarian hormones have been shown to regulate cognition (Sherwin and Grigorova, 2011. Fertil. Steril. 96, 399-403; Shumaker et al., 2004. JAMA. 291, 2947-2958), but increases in estradiol on the day of proestrus yield diverse outcomes: In vivo induction of long-term potentiation (LTP), a model of learning, is reduced in the morning, but optimal in the afternoon (Warren et al., 1995. Brain Res. 703, 26-30). The mechanism underlying this discrepancy is not known. Here, we show that impairments in both CA1 hippocampal LTP and spatial learning observed on the morning of proestrus are due to increased dendritic expression of alpha 4 beta delta GABA(A) receptors (GABARs) on CA1 pyramidal cells, as assessed by electron microscopic (EM) techniques, compared with estrus and diestrus. LTP induction and spatial learning were robust, however, when assessed on the morning of proestrus in alpha 4-/- mice, implicating these receptors in mediating impaired plasticity. Although alpha 4 beta delta expression remained elevated on the afternoon of proestrus, increases in 3 alpha-OH-THP (3 alpha-OH-5 alpha-pregnan-20-one) decreased inhibition by reducing outward current through alpha 4 beta delta GABARs (Shen et al., 2007. Nat. Neurosci. 10, 469-477), in contrast to the usual effect of this steroid to enhance inhibition. Proestrous levels of 3 alpha-OH-THP reversed the deficits in LTP and spatial learning, an effect prevented by the inactive metabolite 3 beta-OH-THP (10 mg/kg, i.p.), which antagonizes actions of 3 alpha-OH-THP. In contrast, administration of 3 alpha-OH-THP (10 mg/kg, i.p.) on the morning of proestrus improved spatial learning scores 150-300%. These findings suggest that cyclic fluctuations in ovarian steroids can induce changes in cognition via alpha 4 beta delta GABARs that are dependent upon 3 alpha-OH-THP. This article is part of a Special Issue entitled SI: Brain and Memory. (C) 2014 Elsevier B.V. All rights reserved.