Time-Dependent Changes in Rat Brain Neuroactive Steroid Concentrations and GABA _A Receptor Function after Acute Stress

Abstract

The time courses of changes in rat brain neuroactive steroid concentrations and γ-aminobutyric acid type A (GABA_A) receptor function elicited by acute stress were investigated in animals exposed to CO₂ for 1 min, a treatment known to induce stress in rats and panic attacks in humans. Inhalation of CO₂ induced increases in cerebral cortical steroid concentrations, the time dependence of which varied with the steroid examined. Thus, progesterone and deoxycorticosterone showed maximal increases (10- and 4-fold, respectively) 10 min after CO₂ inhalation and had returned to basal values by 30 and 60 min, respectively. In contrast, pregnenolone and 3_α-hydroxy-5_α-pregnan-20-one (allopregnanolone) concentrations showed maximal increases (+174 and +200%, respectively) at 30 min, were still higher than control at 60 min and returned to control values 120 min after stress. Inhalation of CO₂ also resulted in increases in plasma steroid concentrations, most of which peaked at 30 min and had returned to control values by 60 min. A parallel analysis of the stress-induced changes in GABA_A receptor function, assessed either biochemically by t-[³⁵S]butylbicyclophosphorothionate ([³⁵S]TBPS) binding to cerebral cortical membranes or behaviorally by the punished responding score in Vogel’s test, showed that the effects of CO₂ inhalation on both parameters were maximal (+51 and –40%, respectively) after 10 min; the behavioral reaction returned to normal after 60 min, whereas [³⁵S]TBPS binding had returned to control values 120 min after stress. The results show that: (a) the maximal increase in the brain concentrations of allopregnanolone, a potent and efficacious positive modulator of GABA_A receptors, occurred at a time (30 min) when both conflict behavior and [³⁵S]TBPS binding begun to decrease, and (b) both allopregnanolone concentrations and [³⁵S]TBPS binding had returned to control values 120 min after CO₂ inhalation. The data are thus consistent with a physiological role of neuroactive steroids in restoring GABAergic tone after stress.