No abstract

This research shows that the medial preoptic area-anterior hypothalamus (MPOA-AH) of the gerbil contains a sexually dimorphic area (SDA) whose morphology is influenced by adult gonadal steroids. The characteristics of the SDA were studied initially in gonadally intact males and females and later in males and females that had been gonadectomized or gonadectomized and given testosterone. Coronal sections through the MPOA-AH were stained with thionin to visualize the SDA. In male gerbils, the SDA is a darkly staining, hook-shaped structure on either side of the third ventricle, above the suprachiasmatic nuclei and below the anterior commissure. Within the male SDA lies a small, dense cluster of cells, the SDA pars compacta (SDApc). In female gerbils, the SDA is more ovoid and diffuse and the SDApc is absent. Camera lucida tracings and planimeter measurements of the SDA showed that the total volume of the SDA is similar in the two sexes, but the proportion of the SDA that stains darkly and/or stands out clearly from the surround (dark volume) is larger in males. Gonadectomy decreases both total and dark volume of the SDA in both sexes and decreases the volume of the SDApc in males. Testosterone treatment prevents these changes. This is the first report of changes in the gross morphology of the mammalian brain after steroid hormone manipulations in adulthood. The changes in the SDA may be related to testosterone activation of scent marking behavior.

Outputs of the sexually dimorphic area (SDA) of the gerbil hypothalamus were identified by injecting Phaseolus vulgaris-leucoagglutinin into the medial or lateral SDA (mSDA, lSDA) in males and females. They were verified by injecting Fluoro-Gold or rhodamine-labeled beads into over half the areas that contained labeled fibers. Both anterograde and retrograde tracing showed that the mSDA and lSDA project to many of the same sites but often to differing degrees. The mSDA projects more heavily than the lSDA to many of their forebrain targets including the ventral part of the lateral septal nucleus, the bed nucleus of the stria terminalis, the medial tuberal area, and the anteroventral periventricular, arcuate, ventromedial and ventral premammillary nuclei of the hypothalamus. The lSDA projects more heavily than the mSDA to many of their mid- and hindbrain targets including the caudal, ventrolateral part of the periaqueductal gray, the retrorubral field, the pedunculopontine tegmental nucleus, and the locus coeruleus. In many other areas of the brain, the projections of the mSDA and lSDA are similar in size. These areas include the substantia innominata, the vascular organ of the lamina terminalis, the anterior amygdala, the posterior hypothalamus, the reuniens and paraventricular nuclei of the thalamus, and the pontine periaqueductal gray lateral to the fourth ventricle. The SDA pars compacta (SDApc), a small cell group embedded in the mSDA of males, projects to many fewer areas than the surrounding mSDA. It was strongly labeled when retrograde tracers were injected into the encapsulated part of the bed nucleus of the stria terminalis, the anteroventral periventricular nucleus, or the mSDA. It was also labeled from the vascular organ of the lamina terminalis, the caudal part of the lateral bed nucleus of the stria terminalis, the lSDA, the area lateral to the mSDA, the arcuate nucleus, the ventral premammillary nucleus, and the ventrolateral part of the ventromedial nucleus of the hypothalamus. Nothing resembling an SDApc was identified during retrograde tracing in females.

Based on Fos expression, four areas of the gerbil brain are activated with ejaculation, i.e., the posterodorsal preoptic nucleus (PdPN), the lateral part of the posterodorsal medial amygdala (MeApd), the medial cell group of the sexually dimorphic preoptic area (medial SDA), and the parvicellular part of the subparafascicular thalamus (SPFp). The SPFp and medial SDA also express Fos earlier in the context of mating. To study connections among these areas, we injected one with FluoroGold and assessed the colocalization of FluoroGold and mating-induced Fos in the others. To determine if any of these areas activates the others, we lesioned one unilaterally and measured mating-induced Fos ipsilaterally and contralaterally in the others. Half of the SPFp cells projecting to the medial SDA, PdPN, and MeApd were activated with mating. SPFp lesions also decreased Fos expression in those areas. However, those areas do not project to the SPFp or affect its Fos expression with mating. Projections from the lateral MeApd to the medial SDA and PdPN, and from the medial SDA to the lateral MeApd, were also activated with mating, but lesions in these areas did not affect Fos expression in the others. Because 32-50% of the mating-activated cells in the SPFp participated in each SPFp projection identified, projections may have been identified for all of the mating-activated cells in the SPFp. In contrast, most of the mating-activated cells in the lateral MeApd, PdPN, and medial SDA do not participate in any projection studied, suggesting that they are either interneurons or project elsewhere.

Estrogen pellets were placed in either the right or left hypothalamus of newborn female rats so that only one side of this brain area was exposed to the postnatal masculinizing and defeminizing effects of the hormone. The effects of estrogen on gonadotropin secretion and reproductive behavior depended on both the region and the side of implantation. Exposure of the left hypothalamus to estrogen resulted in defeminized development. Exposure of the right hypothalamus to estrogen resulted in masculinized development. Thus the response of the developing hypothalamus to gonadal steroids may be asymmetric.