Progesterone Withdrawal Reduces Paired-Pulse Inhibition in Rat Hippocampus: Dependence on GABAA Receptor 4 Subunit Upregulation

Hsu, Fu-Chun and Sheryl S. Smith. Progesterone withdrawal reduces paired-pulse inhibition in rat hippocampus: dependence on GABAA receptor 4 subunit upregulation. J Neurophysiol 89: 186–198, 2003; 10.1152/jn.00195.2002. Withdrawal from the endogenous steroid progesterone (P) after chronic administration increases anxiety and seizure susceptibility via declining levels of its potent GABA-modulatory metabolite 3 -OH-5 -pregnan-20-one (3 ,5 THP). This 3 ,5 -THP withdrawal also results in a decreased decay time constant for GABA-gated current assessed using whole cell patch-clamp techniques on pyramidal cells acutely dissociated from CA1 hippocampus. The purpose of this study was to test the hypothesis that the decreases in total integrated GABA-gated current observed at the level of the isolated pyramidal cell would be manifested as a reduced GABA inhibition at the circuit level following hormone withdrawal. Toward this end, adult, female rats were administered P via subcutaneous capsule for 3 wk using a multiple withdrawal paradigm. We then evaluated paired-pulse inhibition (PPI) of pyramidal neurons in CA1 hippocampus using extracellular recording techniques in hippocampal slices from rats 24 h after removal of the capsule (P withdrawal, P Wd). The population spike (PS) was recorded at the stratum pyramidale following homosynaptic orthodromic stimulation in the nearby stratum radiatum. The threshold for eliciting a response was decreased after P Wd, and the mean PS amplitude was significantly increased compared with control values at this time. Paired pulses with 10-ms inter-pulse intervals were then applied across an intensity range from 2 to 20 times threshold. Evaluation of paired-pulse responses showed a significant 40–50% reduction in PPI for PS recorded in the hippocampal CA1 region after P Wd, suggesting an increase in circuit excitability. At this time, enhancement of PPI by the benzodiazepine lorazepam (LZM; 10 M) was prevented, while pentobarbital (10 M) potentiation of PPI was comparable to control levels of response. These data are consistent with upregulation of the 4 subunit of the GABAA receptor (GABAR) as we have previously shown. Moreover, the reduced PPI caused by P Wd was prevented by suppression of GABAR 4-subunit expression following intraventricular administration of specific antisense oligonucleotides (1 g/h for 72 h). These results demonstrating a reduction in PPI following P Wd suggest that GABAergic-mediated recurrent or feed-forward inhibition occurring at the circuit level were decreased following P Wd in female rats, an effect at least partially attributable to alterations in the GABAR subunit gene expression. I N T R O D U C T I O N