Ultrastructural characteristics of estrogen receptor-containing neurons of the ventrolateral nucleus of the Guinea-pig hypothalamus.

Abstract The ventrolateral nucleus of the hypothalamus (VL) of the guinea-pig is a key cell group in the neural circuitry underlying the estrogen-dependent lordosis reflex. The extent to which neurons in the VL are responsive to estrogen or to synaptic inputs depends in part on the presence of specific estrogen and neurotransmitter receptors within the target cells. It also depends on the number, type and location of synaptic inputs. In addition, both sensitivity to circulating hormones and transmitter responsiveness show estrogen-inducible alterations in the VL. To understand more about the cell types that are directly modulated by estrogens via the nuclear steroid binding protein and the synaptic connectivity of these neurons, we have carried out an ultrastructural study of estrogen receptor-containing cells in the VL of the female guinea-pig. Estrogen receptor was localized for both light and electron microscopy using a specific monoclonal antibody, H-222, directed against the human estrogen receptor. Numerous immunoreactive neurons were found in the VL. These cells had simple, relatively smooth dendritic processes that were generally unbranched. Reaction product was most intense in the nucleus; lighter deposits were seen in some but not all somata and proximal dendrites. No cell was observed with only cytoplasmic staining. At the ultrastructural level, this distribution of reaction product within cells was confirmed. Gold deposits were associated with euchromatin and excluded from the nucleolus, nucleolar-associated heterochromatin and Barr body. In the cytoplasm, the small aggregates of gold particles were randomly distributed. Two types of cytologically distinct immunoreactive neurons were characterized. The most numerous category was of large cells with extensive rough endoplasmic reticulum, frequently organized as whorls or ribbons, several stacks of Golgi cisterna, numerous mitochondria and multivesicular bodies. A smaller population, representing approximately 5% of the total, was of much smaller cells which had only a thin rim of cytoplasm around the nucleus, scattered elements of the rough endoplasmic reticulum and a single small Golgi saccule. Based on size, we suggest that the larger neurons are projection neurons and that the smaller ones form local circuits. The larger cells received a dense axo-somatic and axo-dendritic innervation. Most of the presynaptic terminals contained small, clear round vesicles; synaptic densities on either pre- or postsyn- aptic side were absent though a well defined synaptic cleft was evident. Very few synapses were found on the small cells even when serial sections were examined. It is clear that the larger, estrogen receptor-containing neurons are in a position to integrate both hormonal and neuronal signals and to transmit this information to other regions of the central nervous system involved in the outflow of reproductive behaviors.