Roger A. Gorski

Evidence for a morphological sex difference within the medial preoptic area of the rat brain.

The present report demonstrates the existence of a marked sexual difference in the volume of an intensely staining cellular component of the medial preoptic nucleus (MPON) of the rat. Moreover, this sexual dimorphism is shown to be independent of several specific hormonal conditions in the adult, but significantly influenced, perhaps determined, by the perinatal hormone environment. Adult rats were gonadectomized and sacrificed 2 or 5-6 weeks later, or sacrificed after gonadectomy and priming with estradiol benzoate (2 microgram/day x 3) and 500 microgram progesterone, or testosterone propionate (TP, 500 microgram/day x 14), or the ingestion of propylthiouracil (0.15% of the diet) for one month, or following water deprivation for 24 h. These treatments did not affect the sexual dimorphism in the MPON and, in all groups, nuclear volume in the male animals was significantly greater than that of females whether nuclear volume was expressed in absolute terms or relative to brain weight. On the other hand, the volume of the MPON of the adult male castrated neonatally was significantly reduced when compared to that of the male castrated at the time of weaning, i.e. after the period of sexual differentiation of the brain. Consistent with the view that this nuclear region undergoes sexual differentiation is the fact that the volume of the MPON was significantly greater in female rats injected with 1 mg TP on day 4 of life than in oil-treated females. More subtle sex differences in the volume of the suprachiasmatic nucleus were also detected, as were several treatment effects. Although these differences may fall within the error of the analytical procedure, it is possible that hormone- or sex-dependent morphological differences exist elsewhere in the brain. Nevertheless, the gross sexual dimorphism in the MPON clearly demonstrates a possible morphological basis for the sexual differentiation of brain function.

The role of apoptosis in sexual differentiation of the rat sexually dimorphic nucleus of the preoptic area.

The sexually dimorphic nucleus of the preoptic area (SDN-POA) in the rat hypothalamus is larger in volume in males than in females due to a larger number of cells in the nucleus. Although the SDN-POA, and its development, have been extensively studied, the actual mechanism of its sexual differentiation has not been established. The results of previous studies have not supported a role for gonadal steroids in the regulation of neurogenesis or the determination of the migratory pathway perinatally. In this study, the role of cell death in the development of the sexual dimorphism in the SDN-POA was investigated using in situ end-labeling to visualize fragmented DNA in apoptotic cells. In the experiments described here, the incidence of apoptosis was determined in part of the SDN-POA, the central division of the medial preoptic nucleus (MPNc), over the first 13 days postnatally in male and female rats. There was a sex difference in the incidence of apoptosis in the MPNc between postnatal days 7 and 10; the incidence was higher in females. The role of testosterone (T) in regulating the incidence of apoptosis in the developing MPNc was examined in neonatally castrated males following T or vehicle injection. Testosterone had a profound inhibitory effect on the incidence of apoptosis between days 6 and 10. In a control region within the lateral preoptic area, there was no sex difference in the incidence of apoptosis, nor was there an effect of T. Thus, the data indicate that the regulation of apoptosis by T is one mechanism involved in the sexual differentiation of the SDN-POA.

Sex differences in subregions of the medial nucleus of the amygdala and the bed nucleus of the stria terminalis of the rat

Sex differences are described in subregions of two nuclei of the rat brain: the medical nucleus of the amygdala (MA) and the bed nucleus of the stria terminalis (BNST). The volume of the posterodorsal region of the medial nucleus of the amygdala (MApd) is approximately 85% greater and the volume of the encapsulated region of the bed nucleus of the stria terminalis (BNSTenc) is approximately 97% greater in males than in females. The MApd and BNSTenc are distinct subregions of the MA and BNST. They exhibit intense uptake of gonadal hormones and are anatomically connected to each other and to other sexually dimorphic nuclei. The MA and BNST in general are involved in regulation of several sexually dimorphic functions, including aggression, sexual behavior, gonadotropin secretion and integration of olfactory information. Precise localization of sex differences in subregions of the MA and BNST, such as the MApd and BNSTenc, may facilitate understanding of the neural basis of such functions.

Effects of discrete lesions of the sexually dimorphic nucleus of the preoptic area or other medial preoptic regions on the sexual behavior of male rats

The sexually dimorphic nucleus of the rat medial preoptic area (SDN-POA) has a volume five times larger in the adult male compared with that of the adult female. In the present study, the effects of discrete electrolytic destruction of the SDN-POA or other specific medial preoptic (MPOA) regions on masculine sexual behavior were determined in adult, sexually experienced male rats. Small lesions encompassing the SDN-POA had no effect on the maintenance of copulatory behavior. Lesions of similar size placed within the ventral or anterio-dorsal MPOA also did not consistently affect the display of masculine sexual behavior. However, animals that received small lesions within their dorsal MPOA showed a substantial, long-term decrease in number of mounts, intromissions, and ejaculations compared to these parameters in sham-lesioned control rats, thus indicating a lesion-induced disruption of those neural mechanisms mediating these behaviors. Collectively these data suggest that the SDN-POA is not critical for a full expression of male sexual behavior and that the dorsal MPOA may be more important than other MPOA regions for copulatory behavior.

Pre- and postnatal influence of testosterone propionate and diethylstilbestrol on differentiation of the sexually dimorphic nucleus of the preoptic area in male and female rats

The volume of the sexually dimorphic nucleus in the preoptic area (SDN-POA) of the rat brain is several fold larger in males than in females. When female rats were treated pre- and postnatally with testosterone propionate (TP) or with diethylstilbestrol (DES) they became anovulatory and their SDN-POA developed equivalent in size to that of normal males. Identical treatment of male rats resulted in deficient testicular development, but had no influence on SDN-POA volume. The results indicate that the gross morphological sex difference in SDN-POA volume can exclusively be controlled by the hormonal environment during the critical period of sexual brain differentiation, and that non-steroidal estrogens are just as effective as convertible androgens in stimulating SDN-POA differentiation.

The distribution of monoaminergic cells and fibers in a periventricular preoptic nucleus involved in the control of gonadotropin release: immunohistochemical evidence for a dopaminergic sexual dimorphism.

A small, discrete nucleus at the rostral end of the third ventricle, the anteroventral periventricular nucleus (AVPv), has been reported to be involved in the control of gonadotropin release. Since monoaminergic neurotransmitter systems have also been implicated in this function we used an indirect immunohistochemical approach to examine the distribution of 3 monoaminergic neurotransmitter systems in this nucleus. Sections through the AVPv of both colchicine and non-colchicine-treated adult male and female Sprague-Dawley rats were processed for immunohistofluorescence with antisera directed against tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), or serotonin (5-HT), and were subsequently counterstained with the fluorescent Nissl stain ethidium bromide. The distributions of TH-, DBH- and 5-HT-immunoreactive neural elements within the AVPv were evaluated and a comparison was made between males and females. In both sexes, few 5-HT-stained fibers were seen within the borders of the AVPv, in contrast to the relatively high 5-HT-stained fiber density of the surrounding region. A dramatic sexual dimorphism was found in the distribution of TH-immunoreactive fibers and cell bodies. Compared to males, the AVPv in the female contained 3-4 times as many TH-stained perikarya, and a 2- to 3-fold greater density of TH-stained fibers. A low to moderate density of DBH-immunoreactive fibers, and no DBH-stained cell bodies, were seen in the nucleus. A clear sex difference was not found in the density of DBH-stained fibers in the AVPv, indicating that the sexual dimorphism in TH-immunoreactive neural elements in this nucleus is due to a greater density of dopaminergic fibers and a greater number of dopaminergic cell bodies in the female. These results suggest that dopamine may participate in the control of gonadotropin secretion at the level of the AVPv.

Influence of Perinatal Androgen on the Sexually Dimorphic Distribution of Tyrosine Hydroxylase-Immunoreactive Cells and Fibers in the Anteroventral Periventricular Nucleus of the Rat

Recently we reported that the anteroventral periventricular nucleus (AVPv) of the preoptic region contains a dramatic sexual dimorphism in the distribution of tyrosine hydroxylase-(TH-)immunoreactive cells and fibers in the rat. This sexual dimorphism appears to be due to a greater density of dopaminergic fibers, and a larger number of dopaminergic cell bodies, in the AVPv of the female. In the present study we used an indirect immunohistochemical method to evaluate the distribution of TH-immunoreactive cell bodies and fibers, and of dopamine-beta-hydroxylase-(DBH-)stained fibers, in the AVPv of female rats that were treated with testosterone propionate (TP) perinatally or postnatally, or were left untreated. All of the postnatally TP-treated animals failed to show a phasic pattern of luteinizing hormone (LH) secretion in response to estrogen and progesterone injections. Both the perinatally and postnatally TP-treated females had polyfollicular ovaries that lacked corpora lutea at the time of gonadectomy. Perinatal TP exposure resulted in what appears to be a complete masculinization of both the number of TH-stained cells and the density of TH-stained fibers in the AVPv. The number of TH-stained cells in the postnatally TP-treated females was also completely masculinized, although the density of TH-stained fibers did not appear to be altered significantly. Gonadectomy resulted in a moderate increase in the density of TH-stained fibers in adult males, and did not significantly affect the fiber density in females, or the number of TH-stained cells in either sex. The distribution of DBH-stained fibers in the AVPv did not appear to be altered in any of the treatment groups. These results suggest that the distribution of dopaminergic fibers in the AVPv may be sensitive to testosterone levels in the adult male, and that although the critical period for the development of this fiber distribution may begin in the prenatal period, the number of dopaminergic cells in the AVPv can be completely sex reversed by a single postnatal dose of TP that appears to correlate with a permanent disruption of the normal pattern of gonadotropin secretion.

Variations in food and water intake in the normal and acyclic female rat

Abstract Normal intact adult Sprague-Dawley rats were housed in small metabolic cages kept in an isolated constant environment room. Daily measurements were made of voluntary ad lib food and water intake (FWI), urine and fecal output (UFO), body weight and stage of estrous cycle for several months. A significant depression in FWI and UFO was seen at estrus. These parameters were studied during two anestrous states: (a) during presumed pseudopregnancy (diestrous vaginal smear for 14–21 days) the FWI and UFO increased and remained relatively stable until spontaneous estrous cycles resumed when again they showed significant depressions at the time of estrous; (b) following ovariectomy FWI and UFO increased, reached a plateau in about 20 days and showed no periods of depression. Measurements of food intake showed that the rats ate 84 per cent of their daily consumption during the dark period. A sample of rats kept in the same environment were tested with virile males and found to be sexually receptive during that dark period when the vaginal smear was fully cornified and the FWI and UFO depressed. The cyclic suppression in food intake appears to be associated with the neuroendocrine events related to ovulation, ovarian estrogen may be the key factor.

Effects of small medial preoptic area lesions on maternal behavior: Retreiving and nest building in the rat

Previous studies in rats have demonstrated that large lesions in the medial preoptic area (MPOA) disrupt all aspects of maternal behavior. In the present study, small bilateral electrolyte lesions in the MPOA of lactating females abolished nest building and retrieving components of maternal behavior while crouching and nursing were unaffected. Animals which failed to show retrieval and nest building behaviors tended to have a greater area of lesion within the more dorsal part of the MPOA. Although the dorsal MPOA may play a role in the maintenance of the active components of maternal behavior, i.e. nest building and retrieving, another critical factor in determining which components of maternal behavior are disrupted may be the size of the lesion. As one increases the area of damaged tissue there is also an accompanying increase in the components of maternal behavior which are disrupted. No correlation was found between damage to the Sexually Dimorphic Nucleus of the preoptic area and retrieving and nest building.

Termination of the hormone-sensitive period for differentiation of the sexually dimorphic nucleus of the preoptic area in male and female rats

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) in the rat brain is several-fold larger in males than in females. The volume of the SDN-POA can be influenced significantly by the hormone milieu during early postnatal life. The purpose of the present study was to identify when termination of the sensitive period occurs during which exogenous androgen administration influences SDN-POA volume in males gonadectomized on the first day of postnatal life (fales) or intact females. Analysis of the SDN-POA in fales showed that testosterone propionate (TP, 500 micrograms) treatment on days 2, 3, 4, or 5, significantly increased its volume over values from oil-treated fales. In contrast, TP treatment in fales on days 6, 7, or 8, failed to increase SDN-POA volume. A similar pattern was observed in females treated with TP. Females treated with TP (500 micrograms) on days 2, 3, 4, or 5, showed a significant increase in SDN-POA volume compared to the values from oil-injected animals, while the same TP treatment in females on days 6, 7, or 8, resulted in no such enhancement. The absolute and relative change in SDN-POA volume following postnatal androgen treatment is greater in males than in females. We conclude that (1) SDN-POA development is sensitive to hormone action through postnatal day 5 and then abruptly becomes insensitive to this dosage of TP, and (2) although the temporal pattern of the response is similar in males and females, androgen exposure postnatally results in a consistently greater increase in the male SDN-POA volume than in the females. This greater response may be due to exposure prenatally to endogenous androgen in males.