Neurosteroid-based intervention using Ganaxolone and Emapunil for improving stress-induced myelination deficits and neurobehavioural disorders

How is prenatal stress transmitted from the mother to the fetus?

Prenatal stress programmes long-lasting neuroendocrine and behavioural changes in the offspring. Often this programming is maladaptive and sex specific. For example, using a rat model of maternal social stress in late pregnancy, we have demonstrated that adult prenatally stressed male, but not prenatally stressed female offspring display heightened anxiety-like behaviour, whereas both sexes show hyperactive hypothalamo-pituitary-adrenal (HPA) axis responses to stress. Here, we review the current knowledge of the mechanisms underpinning dysregulated HPA axis responses, including evidence supporting a role for reduced neurosteroid-mediated GABAergic inhibitory signalling in the brains of prenatally stressed offspring. How maternal psychosocial stress is signalled from the mother to the fetuses is unclear. Direct transfer of maternal glucocorticoids to the fetuses is often considered to mediate the programming effects of maternal stress on the offspring. However, protective mechanisms including attenuated maternal stress responses and placental 11β-hydroxysteroid dehydrogenase-2 (which inactivates glucocorticoids) should limit materno-fetal glucocorticoid transfer during pregnancy. Moreover, a lack of correlation between maternal stress, circulating maternal glucocorticoid levels and circulating fetal glucocorticoid levels is reported in several studies and across different species. Therefore, here we interrogate the evidence for a role for maternal glucocorticoids in mediating the effects of maternal stress on the offspring and consider the evidence for alternative mechanisms, including an indirect role for glucocorticoids and the contribution of changes in the placenta in signalling the stress status of the mother to the fetus.

Optimization of overhead enclosure monitoring in juvenile male Dunkin Hartley guinea pigs (Cavia porcellus)

Overhead enclosure monitoring provides objective quantitative mobility measurements for animals undergoing open-field testing. Notably, protocols for testing optimization have been minimally established for the guinea pig. It is unknown whether (a) repeated exposure, (b) time-of-day, or (c) length of testing duration influence outcome parameters. We hypothesized that guinea pigs would display decreased activity following repeated exposure to the open field; heightened activity during the earliest testing period; and that 10 min would be adequate for data collection. The study was conducted in two separate phases to distinguish between enclosure habituation and time-of-day effects, respectively. Two cohorts of male Dunkin Hartley guinea pigs were allowed voluntary movement in an open-field enclosure for 14 min to quantify mobility outcomes, including total distance traveled, total time mobile, average speed while mobile, and total time spent in the shelter. For both phases, testing occurred at four different times of day, and overhead monitoring software was programmed to divide the total testing duration into 2-min bins. Habituation phase results showed time mobile and distance traveled were influenced significantly by repeat exposure, as animals were most active during the first testing event. Time-of-day phase animals spent significantly more time mobile during the earliest testing period. Interestingly, significant differences were observed across 2-min bins for the time-of-day phase but not during the habituation phase. Specifically, progressively decreased ambulatory activity was observed as testing duration increased. Thus, habituation and time-of-day should be accounted for when possible. Finally, a trial period greater than 10 min may not yield additional data.

Human pharmacokinetics of XBD173 and etifoxine distinguish their potential for pharmacodynamic effects mediated by translocator protein

XBD173 and etifoxine are translocator protein (TSPO) ligands that modulate inflammatory responses in preclinical models. Limited human pharmacokinetic data is available for either molecule, and the binding affinity of etifoxine for human TSPO is unknown. To allow for design of human challenge experiments, we derived pharmacokinetic data for orally administered etifoxine (50 mg 3 times daily) and XBD173 (90 mg once daily) and determined the binding affinity of etifoxine for TSPO. For XBD173, maximum plasma concentration and free fraction measurements predicted a maximal free concentration of 1.0 nM, which is similar to XBD173 binding affinity. For etifoxine, maximum plasma concentration and free fraction measurements predicted a maximal free concentration of 0.31 nM, substantially lower than the Ki for etifoxine in human brain derived here (7.8 μM, 95% CI 4.5-14.6 μM). We conclude that oral XBD173 dosing at 90 mg once daily will achieve pharmacologically relevant TSPO occupancy. However, the occupancy is too low for TSPO mediated effects after oral dosing of etifoxine at 50 mg 3 times daily.

Evaluating changes in GABAergic and glutamatergic pathways in early life following prenatal stress and postnatal neurosteroid supplementation

BACKGROUND

A correct balance of activity of the GABA and glutamate systems is vital for optimal neurodevelopment and general CNS function, and the dysregulation of this balance has been implicated in a number of neurological conditions. Maternal exposure to stressors is known to have long lasting, deleterious impacts on neurobehaviour, and similarly, results in dysregulation of inhibitory and excitatory pathways in the offspring. The current study aimed to examine effects on these pathways in a guinea pig model of prenatal stress and to elucidate whether increased neuroprotective support by postnatal neurosteroid supplementation would ameliorate adverse outcomes.

METHODS

Prenatal stress was achieved by exposing pregnant guinea pigs dams to a strobe light for 2hrs/day on gestational age (GA) 50, 55, 60 and 65. Dams were allowed to spontaneously deliver (~GA70) and pups were orally administered either allopregnanolone analogue, ganaxolone (5 mg/kg/day in 45% cyclodextrin), the translocator protein (TSPO) agonist, emapunil (XBD173; 0.3 mg/kg/day in 1% tragacanth gum) or vehicle on postnatal days (PND) 1-7. Hippocampal samples were collected at PND30 to measure relative mRNA expression of components involved in the inhibitory GABAergic pathway and exctitatory glutamatergic pathway by real-time PCR. GABAergic interneurons were quantified by assessing immunohistochemical protein expression of markers parvalbumin, calbindin and calretinin.

RESULTS

mRNA expression of GABAergic pathway components at one week of age indicated immature expression profiles of the GABA_A receptors as well as decreased GABA synthesis and transport suggesting reduced extrasynaptically-mediated tonic inhibition. Expression profiles of the pathways examined evolved between one week and one month of age but an imbalance in inhibitory/excitatory components persisted. The allopregnanolone analogue ganaxolone offered some protection against excitotoxicity in female hippocampus, however neurosteroid supplementation with ganaxolone or emapunil were unable to fully correct the GABAergic/glutamatergic imbalance observed following prenatal stress.

CONCLUSION

Prenatal stress leads to programmed lasting effects on the major inhibitory and excitatory pathways in the guinea pig brain that continue evolving between the equivalent of early and late childhood. Neurosteroid therapies particularly improved outcomes in females. Further studies are required to identify additional therapeutic targets that are able to fully restore imbalances in the excitatory and inhibitory systems, which may act to prevent development of childhood behavioural disorders.