Ultrastructural evidence that hippocampal alpha estrogen receptors are located at extranuclear sites

Estrogen may mediate some of its effects on hippocampal function through the alpha isoform of the estrogen receptor (ERα). By light microscopy, ERα‐immunoreactivity (‐I) is found in the nuclei of scattered inhibitory γ‐aminobutyric acid (GABA)ergic interneurons. However, several lines of evidence indicate that estrogen also may exert some of its effects through rapid nongenomic mechanisms, possibly by binding to plasma membranes. Thus, to determine whether ERα is found in extranuclear sites in the hippocampal formation (HF), four different antibodies to ERα were localized by immunoelectron microscopy in proestrous rats. Ultrastructural analysis revealed that in addition to interneuronal nuclei, ERα‐I was affiliated with the cytoplasmic plasmalemma of select interneurons and with endosomes of a subset of principal (pyramidal and granule) cells. Moreover, ERα labeling was found in profiles dispersed throughout the HF, but slightly more numerous in CA1 stratum radiatum. Approximately 50% of the ERα‐labeled profiles were unmyelinated axons and axon terminals that contained numerous small, synaptic vesicles. ERα‐labeled terminals formed both asymmetric and symmetric synapses on dendritic shafts and spines, suggesting that ERαs arise from sources in addition to inhibitory interneurons. About 25% of the ERα‐I was found in dendritic spines, many originating from principal cells. Within spines, ERα‐I often was associated with spine apparati and/or polyribosomes, suggesting that estrogen might act locally through the ERα to influence calcium availability, protein translation, or synaptic growth. The remaining 25% of ERα‐labeled profiles were astrocytes, often located near the spines of principal cells. Collectively, these results suggest that ERα may serve as both a genomic and nongenomic transducer of estrogen action in the HF. J. Comp. Neurol. 429:355–371, 2001. © 2000 Wiley‐Liss, Inc.