Prenatal Diazepam Exposure Functionally Alters the GABAA Receptor That Modulates [3H]Noradrenaline Release from Rat Hippocampal Synaptosomes

Abstract

In rats, exposure to diazepam (DZ) during the last week of gestation is associated with behavioral alterations (in some cases sexually dimorphic) that appear when the animals reach adulthood. This study was conducted to evaluate the effects of prenatal DZ exposure on the function of the γ-aminobutyric (GABA)_A receptor complex. The method used – perfusion of rat hippocampal nerve terminals labeled with [³H]noradrenaline (NA) – allowed us to evaluate the effects of DZ on a specific native GABA_A receptor subtype which is located on hippocampal noradrenergic nerve endings and mediates the release of NA. Muscimol stimulated synaptosomal release of [³H]NA in a concentration-dependent manner; maximal stimulation (50%) was achieved with a concentration of 30 µM, and the ED₅₀ was 1.7 µM. The effect of muscimol was potentiated by the positive allosteric modulators DZ and 3α-pregnan-5α-ol-20-one (3α,5α-P; allopregnanolone), which displayed similar maximal effects and affinities. In the presence of DZ (0.1 µM), muscimol stimulated the release of [³H]NA with an ED₅₀ of 0.5 µM; in the presence of 3α,5α-P (0.1 µM), the ED₅₀ of muscimol was 0.3 µM. Prenatal DZ exposure did not modify the concentration-effect curve for muscimol, but it did abolish the potentiating effects of DZ and 3α,5α-P. These findings demonstrate that prenatal exposure to DZ produces functional modifications of the GABA_A receptor subtype we investigated. This effect may be related to the relative contributions of the various protein subunits that compose the GABA_A receptor complex. Exposure to DZ while the GABA_A receptors are developing might influence the expression of these subunits, giving rise to a receptor that can be activated by muscimol but is not susceptible to allosteric modulation by DZ or 3α,5α-P.