Dose and Time Effects of Estrogen on Expression of Neuron-Specific Protein and Cyclic AMP Response Element-Binding Protein and Brain Region Volume in the Medial Amygdala of Ovariectomized Rats

Abstract

Although estrogen has been shown to be neuroprotective, studies concerning its effect on some behaviors are contradictory, reporting both ameliorative and detrimental effects. A factor involved in hormone efficacy is the estrogen regimen. We reported an effect of 10 µg estrogen for 14 days on the cyclic AMP response element-binding protein (CREB) pathway, including brain-derived neurotrophic factor, in rat medial amygdala (MeA). To determine the effects of estrogen on neuronal numbers and brain region volume in MeA and central nucleus of the amygdala (CeA), we used stereology to test the effect of various estrogen regimens on the number of neuron-specific protein (NeuN)-labeled neurons and brain region volume of MeA and CeA. Ovariectomized rats were injected with vehicle for 14 days, 2.5 µg estradiol benzoate (E2) for 4 or 14 days, or 10 µg estrogen for 14 days. Because NeuN-labeled neuronal number may be related to neuronal survival and upregulation of CREB signaling, we tested the effect of these regimens on levels of phosphorylated CREB (pCREB) labeling in the MeA and CeA. The 2.5 µg estrogen for 14 days regimen increased the mean number of NeuN-labeled neurons and pCREB-labeled cells in the MeA compared to vehicle or 2.5 µg for 4 days. There was an increase in volume of the MeA with 2.5 µg estrogen for 14 days compared to vehicle or 2.5 µg for 4 days. No differences in these parameters were seen in CeA. These data indicate a neuroanatomical heterogeneity of a time effect of estrogen on cells expressing NeuN and pCREB in the MeA versus CeA.