Abstracts

Rationale: Seizure threshold fluctuates during the estrus cycle. Steroid hormones and derived neurosteroids modulate the GABAA receptor-mediated inhibitory neurotransmission. However it is not known whether progesterone also regulates the AMPA receptor (AMPAR)-mediated neurotransmission. We tested whether elevating progesterone levels increase hippocampal AMPA receptor expression. Methods: Estrus cycle stages were assessed by cytological analysis of vaginal swabs. Progesterone levels were raised in adult female rats by treatment with PMSG and β-HCG (PMSG 20 IU, followed 48 hrs later by β-HCG 10 IU) or progesterone (50 mg/kg). Serum progesterone levels were determined by HPLC. The expression of GluA1 and GluA2 subunits of AMPARs was determined by standard Western blotting techniques. Results: The expression of GluA1 subunit-containing AMPARs fluctuated during the estrus cycle stages- proestrus, estrus, metestrus and diestrus. GluA1 subunit expression in the hippocampi at various stages was normalized to that in proestrus. It peaked in estrus (179 ± 29%, n=5, p<0.05), then declined in metestrus (136 ± 9%, n=4) and returned to baseline in diestrus stage (119 ± 13%, n=4). The GluA2 subunit expression did not change. To test whether high progesterone levels during proestrus stage triggered the increase in GluA1 subunit expression, animals were treated with progesterone for 48 hrs; and GluA1 and GluA2 subunit protein levels increased (GluA1: 134 ± 8% and GluA2: 149 ± 17%, n=6, p<0.05). In contrast 24 hr progesterone treatment did not alter AMPAR subunit expression (GluA1: 100 ± 19% and GluA2: 131 ± 38% respectively, n=5, p>0.05). To determine whether prolonged elevations in progesterone levels further raised the subunit expression, animals were treated with PMSG and β-HCG. Progesterone levels were 4 ± 1.4-fold of the baseline levels (n=6, p<0.05) 4-7 days following β-HCG administration). GluA1 and GluA2 subunit expression 7 days following β-HCG administration was 174 ± 39% and 174 ± 24% of controls respectively (n=9, p<0.05). The putative progesterone-induced increase in GluA1 and GluA2 subunit expression could occur via activation of progesterone receptors (PRs) or through homeostatic alterations. The role of PRs was studied in the animals treated with PR blocker Ru486 (10 m/kg). Preliminary studies found that treatment with Ru486 blocked the increase in GluA1 and GluA2 subunit expression in progesterone-treated animals (86 ± 7% and 93 ± 18% respectively, n=2). Conclusions: There is cyclical elevation of GluA1 subunit levels during estrus cycle. Exogenous progesterone administration elevates the expression of GluA1 and GluA2 subunits.