Sexual dimorphism in neuronal number of the posterodorsal medial amygdala is independent of circulating androgens and regional volume in adult rats

Menstrual cycle-related changes in amygdala morphology are associated with changes in stress sensitivity

Premenstrual increases in negative mood are thought to arise from changes in gonadal hormone levels, presumably by influencing mood regulation and stress sensitivity. The amygdala plays a major role in this context, and animal studies suggest that gonadal hormones influence its morphology. Here, we investigated whether amygdala morphology changes over the menstrual cycle and whether this change explains differences in stress sensitivity. Twenty-eight young healthy women were investigated once during the premenstrual phase and once during the late follicular phase. T1-weighted anatomical images of the brain were acquired using magnetic resonance imaging and analyzed with optimized voxel-based morphometry. To measure mood regulation and stress sensitivity, negative affect was assessed after viewing strongly aversive as well as neutral movie clips. Our results show increased gray matter volume in the dorsal part of the left amygdala during the premenstrual phase when compared with the late follicular phase. This volume increase was positively correlated with the premenstrual increase in stress-induced negative affect. This is the first study showing structural plasticity of the amygdala in humans at the macroscopic level that is associated with both endogenous gonadal hormone fluctuations and stress sensitivity. These results correspond with animal findings of gonadal hormone-mediated neural plasticity in the amygdala and have implications for understanding the pathogenesis of specific mood disorders associated with hormonal fluctuations.

Stereological analysis of the rat and monkey amygdala

The amygdala is part of a neural network that contributes to the regulation of emotional behaviors. Rodents, especially rats, are used extensively as model organisms to decipher the functions of specific amygdala nuclei, in particular in relation to fear and emotional learning. Analysis of the role of the nonhuman primate amygdala in these functions has lagged work in the rodent but provides evidence for conservation of basic functions across species. Here we provide quantitative information regarding the morphological characteristics of the main amygdala nuclei in rats and monkeys, including neuron and glial cell numbers, neuronal soma size, and individual nuclei volumes. The volumes of the lateral, basal, and accessory basal nuclei were, respectively, 32, 39, and 39 times larger in monkeys than in rats. In contrast, the central and medial nuclei were only 8 and 4 times larger in monkeys than in rats. The numbers of neurons in the lateral, basal, and accessory basal nuclei were 14, 11, and 16 times greater in monkeys than in rats, whereas the numbers of neurons in the central and medial nuclei were only 2.3 and 1.5 times greater in monkeys than in rats. Neuron density was between 2.4 and 3.7 times lower in monkeys than in rats, whereas glial density was only between 1.1 and 1.7 times lower in monkeys than in rats. We compare our data in rats and monkeys with those previously published in humans and discuss the theoretical and functional implications that derive from our quantitative structural findings.

Aromatase mRNA in the brain of adult green anole lizards: effects of sex and season

Neural testosterone metabolism, particularly the synthesis of oestradiol (E(2)) via the aromatase enzyme, is important for sexual behaviours in many vertebrates. In green anole lizards, E(2) metabolised from testosterone facilitates female receptivity and increases sexual motivation in males. Testosterone treatment increases aromatase activity in the whole brain homogenates of gonadectomised male, but not female, anoles, which is an effect limited to the breeding season (BS). To investigate the potential for local effects of this enzyme in reproductive behaviour, we used in situ hybridisation for aromatase mRNA to examine expression during the BS and nonbreeding season (NBS) in areas of the brain that control male sexual behaviours [preoptic area (POA) and amygdala (AMY)], as well as one regulating female reproductive behaviours ventromedial hypothalamus (VMH). Males had a greater total number of aromatase-expressing cells in the POA than females, and the density of aromatase-expressing cells (number per unit volume) was greater in the VMH and AMY of females. This density was also higher during the BS than NBS in the POA. Expression of aromatase in the AMY appeared to be lateralised because trends were detected for the left side to have more total cells and more cells per unit volume than the right. These results suggest that, similar to other vertebrates, regional aromatisation of testosterone may be important for the control of sex-specific reproductive behaviours.

Effects of aromatase or estrogen receptor gene deletion on masculinization of the principal nucleus of the bed nucleus of the stria terminalis of mice

The principal nucleus of the bed nucleus of the stria terminalis (BNSTp) is a sexually dimorphic nucleus, and the male BNSTp is larger and has more neurons than the female BNSTp. To assess the roles of neuroestrogen synthesized from testicular androgen by brain aromatase in masculinization of the BNSTp, we performed morphometrical analyses of the adult BNSTp in aromatase knockout (ArKO), estrogen receptor-α knockout (αERKO), and estrogen receptor-β knockout (βERKO) mice and their respective wild-type littermates. In wild-type littermates, the BNSTp of males had a larger volume and greater numbers of neuronal and glial cells than did that of females. The volume and neuron number of the BNSTp in ArKO and αERKO males and glial cell number of the BNSTp in αERKO males were significantly smaller than those of wild-type male littermates, and they were not significantly different from those in female mice with either gene knockout. In contrast, there was no significant morphological difference in the BNSTp between βERKO and wild-type mice. Next, we examined the BNSTp of ArKO males subcutaneously injected with estradiol benzoate (EB) on postnatal days 1, 2, and 3 (1.5 μg/day). EB-treated ArKO males had a significantly greater number of BNSTp neurons than did oil-treated ArKO males. The number of BNSTp neurons in EB-treated ArKO males was comparable to that in wild-type males. These findings suggested that masculinization of the BNSTp in mice involves the actions of neuroestrogen that was synthesized by aromatase and that this estrogen mostly binds to ERα during the postnatal period.

Detection of conspecific pheromones elicits fos expression in GABA and calcium-binding cells of the rat vomeronasal system-medial extended amygdala

The olfactory accessory system is specialized in the detection of pheromones, being an afferent to medial extended amygdala. In spite of the fact that numerous phenotypes are found in these structures, in the current literature, there are no detailed descriptions about the phenotype of neurons in the vomeronasal system-medial extended amygdala after their activation by pheromonal stimuli. Using immunohistochemistry for fos and dual immunohistochemistry for fos and phenotypes, here we show that females have a greater number of activated neurons by the pheromonal stimulus. Likewise, a great colocalization of fos with GABA, calretinin, and calbindin was observed in the vomeronasal system-medial extended amygdala. These data suggest that in amygdaloid areas, neuronal excitability is controlled by GABAergic neurons that contain different calcium-binding proteins, indicating the important role of inhibitory control on the incoming sensory pheromonal and olfactory inputs controlled and processed by the vomeronasal system.

Astrocytes in the amygdala

The amygdala has received considerable attention because of its established role in specific behaviors and disorders such as anxiety, depression, and autism. Studies have revealed that the amygdala is a complex and dynamic brain region that is highly connected with other areas of the brain. Previous works have focused on neurons, demonstrating that the amygdala in rodents is highly plastic and sexually dimorphic. However, our more recent work explores sex differences in nonneuronal cells, joining a rich literature concerning glia in the amygdala. Prior investigation of glia in the amygdala can generally be divided into disease-related and hormone-related categories, with both areas of research producing interesting findings concerning glia in this important brain region. Despite a wide range of research topics, the collected findings make it clear that glia in the amygdala are sensitive and plastic cells that respond and develop in a highly region specific manner.

Influence of substitutive ovarian steroids in the nuclear and cell body volumes of neurons in the posterodorsal medial amygdala of adult ovariectomized female rats

The volumes of the neuronal nucleus and the cell body in the left posterodorsal medial amygdala (MePD) of adult ovariectomized (OVX) female rats submitted to different hormonal therapies were studied here, aiming to reveal possible influence of substitutive sex steroids in these morphological parameters. One week following ovariectomy and at the end of treatments, brains were cut to semi-thin sections (1 microm) and stained with 1% toluidine blue for stereological estimations, carried out using the Cavalieri method and the technique of point counting. Both the volume of the neuronal nucleus and the soma showed a statistically significant difference when comparing the data among OVX females treated with vehicle (V), estradiol (EB) alone, EB plus progesterone (EB+P) or P alone [n=5 rats in each group; one-way ANOVA test, P<0.01 in both cases]. The Tukey test showed that OVX and EB+P treated females had higher mean neuronal nucleus and somatic volumes when compared to V (P<0.01) or EB alone (P<0.01). Also, OVX females treated with P alone showed larger mean neuronal nucleus and somatic volumes when compared to V (P<0.05). These results suggest that the neuronal nucleus and the somatic volumes can be modulated by substitutive ovarian hormones administered to OVX females, for which P can lead to higher results. These findings reveal additional epigenetic actions of the sex steroids in the MePD and new neuronal morphological features in adult female rats.

Estrogen masculinizes neural pathways and sex-specific behaviors

Sex hormones are essential for neural circuit development and sex-specific behaviors. Male behaviors require both testosterone and estrogen, but it is unclear how the two hormonal pathways intersect. Circulating testosterone activates the androgen receptor (AR) and is also converted into estrogen in the brain via aromatase. We demonstrate extensive sexual dimorphism in the number and projections of aromatase-expressing neurons. The masculinization of these cells is independent of AR but can be induced in females by either testosterone or estrogen, indicating a role for aromatase in sexual differentiation of these neurons. We provide evidence suggesting that aromatase is also important in activating male-specific aggression and urine marking because these behaviors can be elicited by testosterone in males mutant for AR and in females subjected to neonatal estrogen exposure. Our results suggest that aromatization of testosterone into estrogen is important for the development and activation of neural circuits that control male territorial behaviors.

Testosterone and dihydrotestosterone differentially improve cognition in aged female mice

Compared with age-matched male mice, female mice experience a more severe age-related cognitive decline (ACD). Since androgens are less abundant in aged female mice compared with aged male mice, androgen supplementation may enhance cognition in aged female mice. To test this, we assessed behavioral performance on a variety of tasks in 22- to 24-mo-old gonadally intact female mice treated for 6 wk with silastic capsules containing either testosterone (T) or dihydrotestosterone (DHT) or empty capsules (placebo). Compared with placebo-treated mice, spatial memory retention in the water maze was enhanced by testosterone treatment, but not DHT treatment. In contrast, DHT treatment improved passive avoidance (PA) retention, while T treatment only did so marginally. These data support that androgen supplementation in old female mice improves cognitive performance differentially depending upon the type of hormone treatment and cognitive task.

Effects of prepubertal gonadectomy on a male-typical behavior and excitatory synaptic transmission in the amygdala

Mammalian puberty entails the emergence of behaviors such as courtship, coitus, and territorial aggressiveness. In adult rodents, the medial amygdala (MeA) is an important site for gonadal steroid hormone regulation of social behaviors and is sensitive to changes in the level of gonadal steroids. Here we show that prepubertal gonadectomy of male rats reduces the expression of a sexually dimorphic behavior, juvenile rough-and-tumble play, as well as the level of excitatory synaptic transmission assayed in adulthood. Behavioral observations in juveniles showed that gonadectomy reduced the initiation of playful attacks, particularly between postnatal days 31-35. Whole-cell voltage clamp recordings made in slices from adults showed that gonadectomy also reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) in MeA neurons without affecting paired pulse facilitation, an index of vesicle release probability. As mEPSC frequency can reflect the number of excitatory synapses per neuron, we also compared the dendritic morphology of Lucifer Yellow filled neurons from intact and gonadectomized adults. This showed that gonadectomy significantly reduced the density of dendritic spines without affecting overall dendritic length or branching of MeA neurons, which is consistent with a gonadectomy-induced reduction in the number of excitatory synapses. These findings suggest that peripubertal androgens activate rough-and-tumble play and promote the maintenance and/or development of new excitatory synapses in the MeA.

Regulation of progestin receptors in medial amygdala: estradiol, phytoestrogens and sex

Exposure to estrogens during critical developmental periods and in adulthood affects sex differences in the brain. We examined the roles of estradiol (E2) and phytoestrogens, and their interactions, on potential sex differences in brain. We used aromatase knockout (ArKO) mice, which cannot produce endogenous estrogens, along with wild type (WT) littermates. Mice were gestated, raised and maintained on a diet either rich in phytoestrogens or a diet virtually void of soy-derived phytoestrogens. Adult males and females were gonadectomized and received implants filled with 17-beta-estradiol to induce progestin receptors (PR), while controls received empty implants. Mice were sacrificed five days later and brain sections containing the posterodorsal medial amygdala (MePD) were processed for PR immunoreactivity. Activation of sex differences in PR required adult E2 treatment. A diet high in phytoestrogens was required for expression of sex differences in PR after E2 treatment. Our data underscore the important contribution of dietary phytoestrogens for the development of sex differences in PR-ir in the adult mouse medial amygdala. We hypothesize that both aromatization of androgens to estrogens and dietary sources of additional estrogens are part of the normal requirement for sex differences in the rodent brain.

Neuron and glia numbers in the basolateral nucleus of the amygdala from preweaning through old age in male and female rats: a stereological study

The rat basolateral nucleus of the amygdala continues to develop connectivity with the frontal cortex through the periadolescent period and even into young adulthood. Although neuronal loss in the prefrontal cortex has been found during the periadolescent period, prior literature has not examined whether neuron number in the basolateral amygdala is stable through this period. In addition, aging of the rat basolateral nucleus is accompanied by significant increases in the dendritic tree of its principal neurons, but whether this occurs in the context of neuronal death has not been previously explored. In the present study, a stereological examination of neuron and glia numbers in the rat basolateral amygdalar nucleus was undertaken in male and female hooded rats at four ages across the lifespan. Our findings indicate 1) a significant decrease in the number of neurons and glia in the basolateral nucleus between adolescence and adulthood; and 2) the number of glia, as well as the volume of the basolateral nucleus, increases between adulthood and old age, whereas neuron number remains stable. These findings provide an important cellular context for interpretation of the neurochemical and other alterations documented in developmental and age-related literature on the rat basolateral amygdala, and underline the substantial development of this brain area during adolescence, as well as its comparative preservation during aging.

Sex differences and laterality in astrocyte number and complexity in the adult rat medial amygdala

The posterodorsal portion of the medial amygdala (MePD) is sexually dimorphic in several rodent species. In several other brain nuclei, astrocytes change morphology in response to steroid hormones. We visualized MePD astrocytes using glial-fibrillary acidic protein (GFAP) immunocytochemistry. We compared the number and process complexity of MePD astrocytes in adult wildtype male and female rats and testicular feminized mutant (TFM) male rats that lack functional androgen receptors (ARs) to determine whether MePD astrocytes are sexually differentiated and whether ARs have a role. Unbiased stereological methods revealed laterality and sex differences in MePD astrocyte number and complexity. The right MePD contained more astrocytes than the left in all three genotypes, and the number of astrocytes was also sexually differentiated in the right MePD, with males having more astrocytes than females. In contrast, the left MePD contained more complex astrocytes than did the right MePD in all three genotypes, and males had more complex astrocytes than females in this hemisphere. TFM males were comparable to wildtype females, having fewer astrocytes on the right and simpler astrocytes on the left than do wildtype males. Taken together, these results demonstrate that astrocytes are sexually dimorphic in the adult MePD and that the nature of the sex difference is hemisphere-dependent: a sex difference in astrocyte number in the right MePD and a sex difference in astrocyte complexity in the left MePD. Moreover, functional ARs appear to be critical in establishing these sex differences in MePD astrocyte morphology.

Histamine in the posterodorsal medial amygdala modulates cardiovascular reflex responses in awake rats

Centrally injected histamine (HA) affects heart rate (HR), arterial blood pressure (BP), and sympathetic activity in rats. The posterodorsal medial amygdala (MePD) has high levels of histidine decarboxylase, connections with brain areas involved with the modulation of cardiovascular responses, and is relevant for the pathogenesis of hypertension. However, there is no report demonstrating the role of the MePD histaminergic activity on the cardiovascular function in awake rats. The aims of the present work were: 1) to study the effects of two doses (10-100 nM) of HA microinjected in the MePD on basal cardiovascular recordings and on baroreflex- and chemoreflex-mediated responses; 2) to reveal whether cardiovascular reflex responses could be affected by MePD microinjections of (R)-alpha-methylhistamine (AH3), an agonist of the inhibitory autoreceptor H3; and, 3) to carry out a power spectral analysis to evaluate the contribution of the sympathetic and parasympathetic components in the variability of the HR and BP recordings. When compared with the control group (microinjected with saline, 0.3 microl), HA (10 nM) promoted an increase in the MAP50, i.e. the mean value of BP at half of the HR range evoked by the baroreflex response. Histamine (100 nM) did not affect the baroreflex activity, but significantly decreased the parasympathetic component of the HR variability, increased the sympathetic/parasympathetic balance at basal conditions (these two latter evaluated by the power spectral analysis), and promoted an impairment in the chemoreflex bradycardic response. Microinjection of AH3 (10 microM) led to mixed results, which resembled the effects of both doses of HA employed here. Present data suggest that cardiovascular changes induced by baroreceptors and chemoreceptors involve the histaminergic activity in the MePD. This neural regulation of reflex cardiovascular responses can have important implications for homeostatic and allostatic conditions and possibly for the behavioral displays modulated by the rat MePD.

Neonatal gonadectomy and adult testosterone replacement suggest an involvement of limbic arginine vasopressin and androgen receptors in the organization of the hypothalamic-pituitary-adrenal axis

Testosterone exposure during critical periods of development exerts major organizing effects on the hypothalamic-pituitary-adrenal (HPA) axis. Here we examined how neonatal gonadectomy (GDX) with or without testosterone treatment during the first week of life alters the HPA response to adult testosterone replacement in 65-d-old male rats. As adults, neonatal GDX rats showed higher levels of plasma corticosterone and Fos activation in medial parvocellular part of the paraventricular nucleus of the hypothalamus under basal conditions and during 30 min of restraint exposure. These responses were normalized with testosterone treatment on postnatal d 1-5 but were not restored with adult testosterone replacement. As adults, neonatal GDX rats also showed a decrease in the number of androgen receptor and arginine vasopressin-positive cells in the bed nucleus of the stria terminalis and in the medial nucleus of the amygdala, and both of these responses were reversed with postnatal testosterone treatment. In stressed and unstressed animals, the number of androgen receptors and arginine vasopressin-expressing neurons in both of these nuclei correlated negatively with corticosterone concentrations in plasma and Fos levels in the paraventricular nucleus. Taken together, our findings suggest that testosterone exposure during the neonatal period primes the adult HPA response to testosterone by altering androgen receptor levels and function within afferent mediators of basal and stress-related input to the HPA axis.

Sexual dimorphism and steroid responsiveness of the posterodorsal medial amygdala in adult mice

The posterodorsal aspect of the medial amygdala (MePD) is sexually dimorphic in regional volume, rostrocaudal extent, and neuronal soma size in rats. These dimorphisms are maintained by circulating gonadal hormones, as castration of adult male rats reduces MePD measures, while testosterone treatment of females increases them. We now report that the MePD is also sexually dimorphic in volume, rostrocaudal extent, and somal area in BALB/c mice. Four weeks after castration of adult male mice, MePD regional volume and soma size are reduced, but rostrocaudal extent is not, compared to sham-castrated males. Treatment of adult ovariectomized females with an aromatized metabolite of testosterone, estradiol, for 8 weeks increased MePD volume and soma size, but not rostrocaudal extent. To probe the possible role of afferents in the steroid-induced plasticity of the MePD, we examined the effect of removing the olfactory bulbs in gonadally intact males and in estrogen-treated females. Bulbectomy had no effect on MePD morphology with one exception: among gonadally intact males, neuronal soma size was slightly smaller in the right MePD of bulbectomized males compared to males with intact bulbs. These results indicate that the sexual dimorphism and hormone responsiveness of the MePD that has been extensively studied in rats is also present in mice, which offers genetic tools for future research. We detected little or no evidence that olfactory bulb afferents play a role in maintaining MePD morphology in adult mice.