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A method combining steroid autoradiography and fluorescent dye retrograde neuroanatomical tracing has been devised. This method makes it possible to demonstrate that some estrogen-concentrating cells in the ventrolateral subdivision of the ventromedial nucleus of the rat hypothalamus are neurons that send axons to the dorsal midbrain. Other cells only concentrate estrogen or only project to the midbrain. Estrogen-concentrating neurons in the ventromedial hypothalamus that project to the dorsal midbrain are likely to transmit hormone-influenced signals that regulate circuits for reproductive or other behaviors or autonomic functions.

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Autoradiographic methods were used to study the location of estrogen-concentrating cells in the brain of the female hamster. In the hypothalamus, well-labelled cells were reliably found in the posterior medial preoptic area (MPOA), the anterior hypothalamus (AHA), and the ventromedial (VM), arcuate (ARC) and ventral premammillary nuclei (VPM). In the limbic system, well-labelled cells were found in the ventro-lateral septum, bed nucleus of the stria terminalis, and the medial and cortical nuclei of the amygdala. Labelled cells, in small numbers, were also detected in the mesencephalic central gray (CG), lateral hypothalamus, subiculum and entorhinal cortex. The neuro-anatomical pattern of estrogen-concentrating cells in the hamster supports the concept of a generalized vertebrate pattern. Furthermore, a comparison of hamster and rat patterns of cellular ³H-estradiol (³H-E₂) concentration appears to suggest that species differences in their responsiveness to estrogen may be paralleled by differences in estrogen binding.

This report describes the distribution of neurophysin-containing, estradiol-concentrating neurons in a strain of rat which is congenitally unable to produce vasopressin and its associated neurophysin (the Brattleboro rat). In this strain of rat, all of the neurophysin-containing cells are oxytocin producing. The magnocellular neurons which produce vasopressin in the normal rat are present in their normal numbers and normal locations (Rhodes, C. H., J. I. Morrell, and D. W. Pfaff (1981) J. ) and can be identified as the neurophysin-negative magnocellular neurons.Estradiol-concentrating cell nuclei were observed in magnocellular neurons with neurophysincontaining cytoplasm as well as in magnocellular neurons lacking immunocytochemically detectable neurophysin. The majority of these neurons were found in the paraventricular nucleus (PVN), ventral and medial to its lateral subnucleus, and in the posterior subnucleus of the PVN. There were, in addition, many neurophysin-containing and neurophysin-lacking magnocellular neurons with nuclei which did not concentrate estradiol. Within the PVN, the majority of the neurophysinnegative, non-estradiol-concentrating neurons were in the lateral subnucleus, while the majority of the neurophysin-positive, non-estradiol-concentrating neurons were in the medial subnucleus.Comparison of the results of experiments using homozygous Brattleboro rats with the results of similar experiments using the (normal) parent strain Long-Evans rat suggests that, in the normal animals, there are both oxytocin-and vasopressin-producing neurons which concentrate estradiol. Comparison of these observations with published descriptions of the anatomical distribution of neurons which project to the medulla or spinal cord suggests that many of the oxytocin-.or vasopressin-containing, estrogen-concentrating neurons in the PVN send axons to regions regulating autonomic functions.Estrogen treatment is a stimulus for the release of oxytocin (Yamaguchi et al., 1979) and vasopressin (Skowsky et al., 1979). A decrease in immunocytochem-' This paper is dedicated to the Rev. Dr. Winthrop Brainerd,