Pauline Yahr

c-Fos immunoreactivity in the sexually dimorphic area of the hypothalamus and related brain regions of male gerbils after exposure to sex-related stimuli or performance of specific sexual behaviors.

The sexually dimorphic area of the gerbil hypothalamus is essential for male sex behavior. To determine which aspects of mating activate its cells, or cells near or connected to it, we visualized c-Fos in the brains of male gerbils that had been exposed to various types of sex-related stimuli or that had displayed various aspects of sex behavior. Five groups of males were placed in familiar arenas containing sex-related odors. All subjects had previously mated in these arenas. For four groups, a female was introduced and remained with the male until he ejaculated, intromitted, mounted or sniffed her. Males in the fifth group remained in the arena alone. Males in a sixth group were placed in a clean arena in another room. These males were also familiar with this arena but had never encountered a female there. The seventh group remained in their home cages. The posterodorsal preoptic nucleus, the lateral part of the posterodorsal medial amygdala, the medial part of the sexually dimorphic area and the parvicellular part of the subparafascicular nucleus of the thalamus expressed c-Fos after ejaculation. Whether these cells triggered ejaculation or responded to it is not clear. The latter two areas also expressed c-Fos whenever males were exposed to the sex arena, but the sexually dimorphic area pars compacta did not express c-Fos under any condition. The medial core of the nucleus accumbens, the ventrolateral septum, the caudomedial bed nucleus of the stria terminalis, the medial/central part of the posterodorsal medial amygdala and the lateral part of the sexually dimorphic area also expressed c-Fos when males entered the sex arena. The ventrolateral part of the ventromedial nucleus of the hypothalamus expressed c-Fos whenever males were with females. None of the 31 areas studied responded to mounting or intromission, but the zona incerta, the amygdalohippocampal area, the lateral part of the sexually dimorphic area and the area lateral to the medial part of the sexually dimorphic area showed progressive increases in c-Fos expression as mating progressed. The area dorsal to the medial part of the sexually dimorphic area, the paraventricular nucleus of the hypothalamus, the ventral premammillary nucleus and the retrorubral field showed the same level of c-Fos expression when males were exposed to the non-sexual context as when they were exposed to the sexual one. While a projection to the retrorubral field from the sexually dimorphic area is critical for male sex behavior, the retrorubral field did not show a sex-related c-Fos response. The data suggest that brain regions involved in male sex behavior are involved in different aspects of it and that this can also apply to different subsets of cells in each area. The data also indicate that cells involved in mating do not necessarily show mating-related patterns of c-Fos expression. Thus, while c-Fos is useful for identifying areas involved in mating, or other behaviors, its characteristics could cause relevant areas to be overlooked.

Lesions of the sexually dimorphic area disrupt mating and marking in male gerbils

At the border between the medial preoptic area (MPOA) and the anterior hypothalamus (AH), gerbils have a sexually dimorphic area (SDA) that accumulates and responds to gonadal steroids. To assess the role of the SDA in the hormonal control of two sexually dimorphic behaviors, masculine sexual behavior and ventral scent marking, we studied changes in these behaviors in male gerbils after lesioning the lateral SDA or the entire SDA. Lateral SDA lesions disrupt openfield scent marking, at least temporarily, and produce long-lasting deficits in, but do not abolish, mating behavior. Lesioning both the medial and the lateral SDA produces more profound deficits in mating and marking. Similar lesions anterior or posterior to the SDA also impair these behaviors, but their effects are less severe. Thus the SDA is more important than other parts of the MPOA-AH in the control of mating and marking in male gerbils.

Cell-body lesions of the posterodorsal preoptic nucleus or posterodorsal medial amygdala, but not the parvicellular subparafascicular thalamus, disrupt mating in male gerbils

In gerbils, the posterodorsal preoptic nucleus (PdPN) and the lateral part of the posterodorsal medial amygdala (MeApd) express Fos with ejaculation. In contrast, the medial/central part of the MeApd expresses Fos when a sexually experienced male reenters the environment associated with mating. The parvicellular part of the subparafascicular thalamic nucleus (SPFp) of gerbils expresses Fos under both conditions. To study the role of the PdPN and MeApd in male sex behavior, male gerbils were tested for mating before and after these areas were bilaterally lesioned by infusions of N-methyl-D-aspartate (NMDA). Controls received the vehicle or inactive isomer, NMLA. Lesions in either area reduced mounting, but MeApd lesions, which were more complete than PdPN lesions, also delayed ejaculation when males intromitted. To determine if the MeApd and PdPN affect mating via a common pathway, they were bilaterally disconnected by lesioning them unilaterally, contralateral to each other. Other groups received ipsilateral lesions, NMLA, or bilateral lesions of the PdPN or MeApd. In addition, the SPFp was studied using bilateral lesions. MeApd and PdPN lesions again decreased mounting, and this time both lesions, which were quite complete, delayed ejaculation when males intromitted. Contralateral lesions that bilaterally disconnected these cell groups from each other mimicked both effects. Thus, the MeApd and PdPN affect mounting and ejaculation, at least in part, via their connections with each other. In contrast, SPFp lesions did not affect mating. Thus, SPFp cells activated at ejaculation may react to ejaculation rather than trigger it, possibly initiating preparations for paternity.

Anatomical and functional connections among cell groups in the gerbil brain that are activated with ejaculation

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. J. Comp. Neurol. 439:248–258, 2001.

The medial and lateral cell groups of the sexually dimorphic area of the gerbil hypothalamus are essential for male sex behavior and act via separate pathways

Male reptiles, birds and mammals do not copulate if the medial preoptic area (MPOA) is destroyed but the MPOA cell groups necessary for male sexual behavior were not known. Here, two cell groups essential for copulation are identified in the sexually dimorphic area (SDA) of the gerbil (Meriones unguiculatus) MPOA. Bilateral cell-body lesions of either the medial or lateral SDA eliminated mating in sexually experienced male gerbils given testosterone. Nearby MPOA lesions did not. The medial and lateral SDA affect sex behavior via separate pathways since lesioning the medial SDA on one side of the brain and the lateral SDA on the other did not stop sexual behavior.

Hormonal control of sexual and scent marking behaviors of male gerbils in relation to the sexually dimorphic area of the hypothalamus

The sexual and scent marking behaviors of male gerbils are stimulated by testosterone (T) action in the preoptic area (POA) of the hypothalamus. The sexually dimorphic area (SDA) in the posterior POA, which also responds to T, is implicated in this process. This research studied the sensitivities of mating, marking, and the SDA to T metabolites and other steroids. Experiment 1 focused on mating. Male gerbils were implanted at castration with 2-mm Silastic capsules containing T, dihydrotestosterone (DHT), 19-nortestosterone (19-nor T), estradiol (E), or no hormone and were tested 3-7 weeks later. T, E, and 19-nor T maintained intromissions, but E-treated males rarely ejaculated. Controls and DHT-treated males stopped mounting. Experiment 2 compared the ability of these steroids to reinstate marking and mating using the same dose and a larger one (5 mm). Androstenedione, 19-hydroxytestosterone (19-OHT), and E plus DHT were studied as well. Volumes of the SDA and SDA pars compacta (SDApc) were also measured. Only T, 19-nor T, E, and E + DHT reinstated sexual behavior, but all steroids except 19-OHT stimulated marking. E and DHT synergized to elicit mating. For marking, they were no more effective together than alone. Steroid-treated males had larger SDAs than controls. Moreover, steroids that stimulated sexual activity produced larger SDAs than steroids that did not. SDA size correlated with copulatory rate, but not with copulatory efficiency. SDApc size correlated with copulatory efficiency, but not with copulatory rate. Like copulatory rate and efficiency, sizes of the SDA and SDApc did not correlate with each other.

Aromatization and androgen stimulation of sexual behavior in male and female rats

Abstract 6α-Fluorotestosterone, an androgen that is not aromatized in a standard assay system, stimulated sexual behavior in both male and female rats. In males, it was as effective as testosterone. 19-Nortestosterone also stimulated more male sexual behavior than would be expected on the basis of its aromatizability in standard assays. In other tests of the aromatization hypo we used the anti-estrogen, CI-628. This drug inhibited androgen-induced sexual receptivity in female rats, but did not inhibit androgen-induced sexual behavior in male rats. In females, CI-628 antagonized testosterone and 6α-fluorotestosterone equally. These data suggest that the structures of androgens, rather than their abilities to be aromatized, determine behavioral effectiveness.

Independent control of sexual and scent marking behaviors of male gerbils by cells in or near the medial preoptic area

This research studied the role of the medial preoptic area and adjacent cell populations in androgen control of scent marking and sexual behavior in male gerbils (Meriones unguiculatus). Experiment 1 replicated previous research showing that implants of testosterone propionate in or near the medial preoptic area reinstate marking behavior in castrates. Implant sites near the diagonal band of Broca or in the posterior part of the medial preoptic area, near the anterior hypothalamus, are more effective than other sites. Experiment 2 showed that medial preoptic area lesions permanently impair sexual behavior despite testosterone stimulation. Experiments 2–4 showed that lesions in or near the medial preoptic area can also disrupt scent marking; however, this behavior gradually recovered in many lesioned males, especially if they received testosterone. The data suggest that both scent marking and sexual behavior are controlled by androgens acting on cells in or near the medial preoptic area, but the cell populations involved in these two behaviors are probably not the same.

Role of neonatal androgens in sexual differentiation of brain structure, scent marking, and gonadotropin secretion in gerbils

Gerbils display a sexually dimorphic scent marking behavior that responds to testosterone (T) in adulthood and develops under the influence of testosterone perinatally. A complex of cell groups between the preoptic area and anterior hypothalamus of the gerbil brain is also sexually dimorphic and responsive to testosterone. One of these cell groups, the sexually dimorphic area pars compacta (SDApc), usually exists only in males. Even when given testosterone, adult female gerbils rarely have an SDApc. To determine if the SDApc develops under the influence of testosterone, male gerbils were castrated or given sham operations on the day they were born or 1 day later, or were not manipulated. Female gerbils were injected subcutaneously with 0, 50, or 100 micrograms testosterone propionate (TP) on the day after birth. When given ovarian transplants as adults, neonatally castrated males scent marked at low levels typical of females. Neonatally androgenized females given testosterone as adults scent marked at high levels typical of males. Neonatal castration did not affect the probability that the SDApc would develop, but neonatal androgenization did. Half the females given either dose of TP as neonates had SDApcs bilaterally. The sizes of the SDApcs present in females depended on the dose of testosterone given neonatally. The larger dose produced larger SDApcs. The 100-micrograms dose of TP also defeminized gonadotropin secretion, but the 50-micrograms dose did not. The castration of males neonatally prevented the defeminization normally caused by endogenous testosterone. Both groups of neonatally castrated males formed corpora lutea in their ovarian transplants, but control males did not.

GABA and glutamate in mating-activated cells in the preoptic area and medial amygdala of male gerbils

The posterodorsal medial amygdala (MeApd), the posterodorsal preoptic nucleus (PdPN), and the medial cell group of the sexually dimorphic preoptic area (mSDA) contain cells that are activated specifically at ejaculation as assessed by Fos expression. The mSDA also expresses Fos early in the mating context. Because little is known about the neurotransmitters of these activated cells, the possibility that they use γ‐aminobutyric acid (GABA) or glutamate was assessed. Putative glutamatergic cells were visualized with immunocytochemistry (ICC) for glutamate and its neuron‐specific transporter. Their distributions were compared with those of GABAergic cells visualized with ICC for the 67‐kDa form of glutamic acid decarboxylase (GAD67) and in situ hybridization for GAD67 messenger RNA (mRNA). Colocalization of Fos and GAD67 mRNA in recently mated males indicated that half of the activated cells in the PdPN, mSDA, and lateral MeApd are GABAergic. Colocalization of Fos and glutamate suggested that a quarter of the activated mSDA and lateral MeApd cells are glutamatergic. The PdPN does not appear to have glutamatergic cells. In the lateral MeApd, the percentage of activated cells that are GABAergic (45%) matches the percentage that project to the principal part of the bed nucleus of the stria terminalis (BST; 43%), and the percentage likely to be glutamatergic (27%) matches the percentage projecting to the mSDA (27%). The latter could help to trigger ejaculation. The distribution of GABAergic and putative glutamatergic cells in the caudal preoptic area, caudal BST, and medial amygdala of male gerbils is also described. J. Comp. Neurol. 459:290–300, 2003.