Certain classes of neuroactive steroids (NASs) are positive allosteric modulators (PAM) of synaptic and extrasynaptic GABA receptors. Herein, we report new SAR insights in a series of 5β-nor-19-pregnan-20-one analogues bearing substituted pyrazoles and triazoles at C-21, culminating in the discovery of 3α-hydroxy-3β-methyl-21-(4-cyano-1H-pyrazol-1'-yl)-19-nor-5β-pregnan-20-one (SAGE-217, 3), a potent GABA receptor modulator at both synaptic and extrasynaptic receptor subtypes, with excellent oral DMPK properties. Compound 3 has completed a phase 1 single ascending dose (SAD) and multiple ascending dose (MAD) clinical trial and is currently being studied in parallel phase 2 clinical trials for the treatment of postpartum depression (PPD), major depressive disorder (MDD), and essential tremor (ET).

Zuranolone (SAGE-217) is a novel, synthetic, clinical stage neuroactive steroid GABA A receptor positive allosteric modulator designed with the pharmacokinetic properties to support oral daily dosing. In vitro , zuranolone enhanced GABA A receptor current at nine unique human recombinant receptor subtypes, including representative receptors for both synaptic (γ subunit-containing) and extrasynaptic (δ subunit-containing) configurations. At a representative synaptic subunit configuration, α 1 β 2 γ 2 , zuranolone potentiated GABA currents synergistically with the benzodiazepine diazepam, consistent with the non-competitive activity and distinct binding sites of the two classes of compounds at synaptic receptors. In a brain slice preparation, zuranolone produced a sustained increase in GABA currents consistent with metabotropic trafficking of GABA A receptors to the cell surface. In vivo , zuranolone exhibited potent activity, indicating its ability to modulate GABA A receptors in the central nervous system after oral dosing by protecting against chemo-convulsant seizures in a mouse model and enhancing electroencephalogram β-frequency power in rats. Together, these data establish zuranolone as a potent and efficacious neuroactive steroid GABA A receptor positive allosteric modulator with drug-like properties and CNS exposure in preclinical models. Recent clinical data support the therapeutic promise of neuroactive steroid GABA A receptor positive modulators for treating mood disorders; brexanolone is the first therapeutic approved specifically for the treatment of postpartum depression. Zuranolone is currently under clinical investigation for the treatment of major depressive episodes in major depressive disorder, postpartum depression, and bipolar depression.

Dentate granule cell (DGC) mossy fiber sprouting (MFS) in mesial temporal lobe epilepsy (mTLE) is thought to underlie the creation of aberrant circuitry which promotes the generation or spread of spontaneous seizure activity. Understanding the extent to which populations of DGCs participate in this circuitry could help determine how it develops and potentially identify therapeutic targets for regulating aberrant network activity. In this study, we investigated how DGC birthdate influences participation in MFS and other aspects of axonal plasticity using the rat pilocarpine-induced status epilepticus (SE) model of mTLE. We injected a retrovirus (RV) carrying a synaptophysin-yellow fluorescent protein (syp-YFP) fusion construct to birthdate DGCs and brightly label their axon terminals, and compared DGCs born during the neonatal period with those generated in adulthood. We found that both neonatal and adult-born DGC populations participate, to a similar extent, in SE-induced MFS within the dentate gyrus inner molecular layer (IML). SE did not alter hilar MF bouton density compared to sham-treated controls, but adult-born DGC bouton density was greater in the IML than in the hilus after SE. Interestingly, we also observed MF axonal reorganization in area CA2 in epileptic rats, and these changes arose from DGCs generated both neonatally and in adulthood. These data indicate that both neonatal and adult-generated DGCs contribute to axonal reorganization in the rat pilocarpine mTLE model, and indicate a more complex relationship between DGC age and participation in seizure-related plasticity than was previously thought.

Hilar ectopic dentate granule cells (DGCs) are a salient feature of aberrant plasticity in human temporal lobe epilepsy (TLE) and most rodent models of the disease. Recent evidence from rodent TLE models suggests that hilar ectopic DGCs contribute to hyperexcitability within the epileptic hippocampal network. Here we investigate the intrinsic excitability of DGCs from humans with TLE and the rat pilocarpine TLE model with the objective of comparing the neurophysiology of hilar ectopic DGCs to their normotopic counterparts in the granule cell layer (GCL). We recorded from 36 GCL and 7 hilar DGCs from human TLE tissue. Compared with GCL DGCs, hilar DGCs in patient tissue exhibited lower action potential (AP) firing rates, more depolarized AP threshold, and differed in single AP waveform, consistent with an overall decrease in excitability. To evaluate the intrinsic neurophysiology of hilar ectopic DGCs, we made recordings from retrovirus-birthdated, adult-born DGCs 2-4 mo after pilocarpine-induced status epilepticus or sham treatment in rats. Hilar DGCs from epileptic rats exhibited higher AP firing rates than normotopic DGCs from epileptic or control animals. They also displayed more depolarized resting membrane potential and wider AP waveforms, indicating an overall increase in excitability. The contrasting findings between disease and disease model may reflect differences between the late-stage disease tissue available from human surgical specimens and the earlier disease stage examined in the rat TLE model. These data represent the first neurophysiological characterization of ectopic DGCs from human hippocampus and prospectively birthdated ectopic DGCs in a rodent TLE model.

One of the most common types of epilepsy in adults is temporal lobe epilepsy. Temporal lobe epilepsy is often resistant to pharmacological treatment, requiring urgent understanding of its molecular and cellular mechanisms. It is generally accepted that an imbalance between excitatory and inhibitory inputs is related to epileptogenesis. We have recently identified that fibroblast growth factor (FGF) 7 is critical for inhibitory synapse formation in the developing hippocampus. Remarkably, FGF7 knockout mice are prone to epileptic seizures induced by chemical kindling (Terauchi et al., 2010). Here we show that FGF7 knockout mice exhibit epileptogenesis-related changes in the hippocampus even without kindling induction. FGF7 knockout mice show mossy fiber sprouting and enhanced dentate neurogenesis by 2 months of age, without apparent spontaneous seizures. These results suggest that FGF7-deficiency impairs inhibitory synapse formation, which results in mossy fiber sprouting and enhanced neurogenesis during development, making FGF7 knockout mice vulnerable to epilepsy.