Neurosteroid modulation of the presynaptic NMDA receptors regulating hippocampal noradrenaline release in normal rats and those exposed prenatally to diazepam

Abstract

Prenatal exposure to diazepam (DZ), a positive allosteric modulator of the γ-aminobutyric acid A (GABA A) receptor complex, exerts profound effects that become more evident during puberty and in many cases are sex-specific, suggesting that such exposure interferes with the activity of steroid hormones. Apart from their well known effects on the genome, the reduced metabolites of many steroid hormones also interact directly with membrane receptors, including those for N-methyl- d-aspartate (NMDA). In this study, we compared the effects of several neurosteroids on NMDA receptors from normal rats and those exposed in utero to DZ (1.25 mg/kg per day) from the 14th through the 20th day of gestation. In superfused rat hippocampal synaptosomes, activation of the NMDA receptor stimulates the basal release of [ 3 H ]noradrenaline ([ 3 H ]NA), which was used in our study as an index of receptor function. [ 3 H ]NA release was evoked in a concentration-dependent manner by NMDA (100 μM) plus glycine (GLY). The maximal increase (68.23±3.86%) with respect to basal release was achieved with a GLY concentration of 10 μM, and the EC 50 for GLY was 0.1 μM. Release stimulated by 100 μM NMDA+0.1 μM GLY was not modified by any of the neurosteroids tested, with the exception of pregnenolone sulfate (PREG-S), which produced a 78.57±3.94% reduction in release at the maximal concentration used (0.3 μM). In synaptosomes from animals exposed in utero to DZ, the inhibitory effect of PREG-S was reduced by 46.55±2.33%. Given the important roles played by NMDA receptors in physiological and pathological processes within the central nervous system (CNS), characterization of NMDA receptor modulation is an important objective. The fact that this modulation can be altered by exposure in utero to DZ indicates that the behavioral abnormalities observed in exposed animals might be partially attributed to an altered sensitivity of NMDA receptors to the modulatory effects of neurosteroids.