Short-term exposure to a neuroactive steroid increases α4 GABA_A receptor subunit levels in association with increased anxiety in the female rat

Previous work from this laboratory has demonstrated that withdrawal from the neuroactive steroid 3α,5α-THP (3α-hydroxy-5α-pregnan-20-one) after 3-week exposure to its parent compound, progesterone (P), increases anxiety and produces benzodiazepine (BDZ) insensitivity in female rats. These events were linked to upregulation of the α4 subunit of the GABA_A receptor (GABAR) in the hippocampus [Brain Res. 507 (1998) 91; Nature 392 (1998) 926; J. Neurosci. 18 (1998) 5275]. The present study investigates the role of shorter term hormone treatment on α4 subunit levels as well as relevant behavioral and pharmacological end-points related to GABAR function. After 2-3 days of P exposure, two- to threefold increases in α4 protein levels were observed, which declined to control values after 5-6 days of hormone exposure. This effect was due to the GABA-modulatory metabolite of P, 3α,5α-THP. α4 upregulation was inversely correlated with BDZ potentiation of GABA-gated current, assessed using whole cell patch clamp techniques on acutely isolated hippocampal pyramidal cells. A near total BDZ insensitivity was observed by 2-3 days of hormone exposure in association with the maximal increase in α4 levels. Up-regulation of the α4 GABAR subunit was also reflected by an increase in anxiety in the elevated plus maze. A significant decrease in open arm entries was observed after 72-h exposure to P, an effect which recovered by 6 days of P treatment. As demonstrated in vitro, α4 upregulation also resulted in a relative insensitivity to the anxiolytic actions of BDZ. These results suggest that short-term exposure to 3α,5α-THP produces changes in GABAR subunit composition similar to those that occur after chronic exposure and withdrawal from the steroid.