Induction of Fos in the accessory olfactory system by male odors persists in female mice with a null mutation of the aromatase (cyp19) gene

The androgen receptor is selectively involved in organization of sexually dimorphic social behaviors in mice

It is well established that sexually dimorphic neural regions are organized by steroid hormones during development. In many species, neonatal males are exposed to more testosterone than their female littermates, and ultimately it is the estradiol, produced by aromatization of testosterone, that affects sexual differentiation. However, the androgen receptor also plays an important role in the masculinization of brain and behavior. Here we tested the hypothesis that sexually dimorphic social and odor preference behaviors can be differentiated by a nonaromatizable androgen during development by treating female mice on the day of birth (PN0) with dihydrotestosterone (DHT). Control mice received a single vehicle injection on PN0. Adults were gonadectomized, treated with estradiol, and tested for social behaviors. In contrast with control females, females treated on PN0 with DHT, like male controls, exhibited a preference for female-soiled vs. male-soiled bedding, a preference to investigate a female vs. a male and reduced c-Fos-immunoreactivity (ir) in several neural areas after exposure to male-soiled bedding. However, females treated with DHT on PN0 had normal female-typical sexual behavior. The number of calbindin-ir cells in the preoptic area is sexually dimorphic (males more than females), but females given DHT on PN0 had intermediate numbers of calbindin-ir neurons, not significantly different from control males or females. Our data demonstrate that organization of social and olfactory preferences in mice can be affected by perinatal DHT and lends support to the role of androgen receptor in organization of sexual differentiation of brain and behaviors.

Sex difference in Fos induced by male urine in medial amygdala-projecting accessory olfactory bulb mitral cells of mice

We previously reported that exposure to soiled male bedding induced Fos protein immunoreactivity (Fos-IR) in significantly more neurons of the vomeronasal organ (VNO) and medial amygdala of gonadectomized, estradiol-treated female than male mice whereas no such sex difference was seen in the intervening mitral cells of the accessory olfactory bulb (AOB). We asked whether a sexually dimorphic functional response to male urinary pheromones might be revealed in AOB mitral cells that project specifically to the medial amygdala. Gonadectomized mice of both sexes were treated with estradiol and 3 days later received bilateral injections of the retrograde tracer, Cholera toxin-B (CTB) into the medial amygdala. Five days later male urine or saline was applied nasally to each subject 90 min prior to sacrifice, and sections of the AOB were processed for double-label fluorescent immunocytochemistry for Fos protein and CTB. In both the rostral and caudal AOB, there were significantly more double-labeled mitral cells in female than in male subjects following exposure to male urine. A sex difference in the responsiveness of VNO sensory neurons seen previously to male soiled bedding is reflected in a parallel sex difference in the responsiveness of AOB mitral cells when only AOB cells that project to the amygdala are examined and when male urine as opposed to soiled male bedding is used as the activating stimulus.

Use of c-fos to identify activity-dependent spinal neurons after stepping in intact adult rats

STUDY DESIGN

An investigation of c-fos activation pattern in spinal neurons of intact adult rats after acute bouts of treadmill locomotion.

OBJECTIVES

To map spinal neurons that are involved in quadrupedal treadmill stepping of intact adult rats by using c-fos as a marker.

SETTINGS

Los Angeles, CA, USA.

METHODS

Spinal cord sections of rats that were not stepped (n = 4) were used to map the FOS-positive (+) neurons under basal conditions. The stepped group (n = 16) was placed on a treadmill to step quadrupedally for varying durations to induce c-fos activity. Spinal cord sections of thoracic and lumbar segments of Stp and Nstp rats were processed using a c-fos antibody, choline acetyl transferase and heat shock protein 27 for identifying motoneurons.

RESULTS

Stepping induced a greater number of FOS+ neurons than was observed in rats that did not step on the treadmill. There was a rostrocaudal and a dorsoventral gradient of FOS labeled neurons. The number of FOS+ neurons increased with the duration of treadmill stepping. Significant increases in FOS+ neurons were in the most medial parts of laminae IV, V, and VII. FOS+ motoneurons increased with treadmill stepping, particularly in large motoneurons (> or = 700 microm2).

CONCLUSION

These data suggest that FOS can be used to identify activity-dependent neuronal pathways in the spinal cord that are associated with treadmill stepping, specifically in lamina VII and in alpha motoneurons.

SPONSORSHIP

NIH NS16333, NS40917, and the Christopher Reeve Paralysis Foundation (CRPF VEC 2002).

Localization and characterization of glutathione-s-transferase isozymes alpha, mu, and pi within the mouse vomeronasal organ

The nasal cavity of vertebrates contains a variety of xenobiotic metabolizing enzymes that possess a broad range of substrate specificity ranging from metabolism of drugs, carcinogens, and steroid hormones, to dietary components and environmental pollutants. This investigation sought to localize the cellular expression and distribution of glutathione-s-transferase (GST) alpha, mu, and pi detoxifying enzymes, and to study GST activity toward different substrates in the mouse vomeronasal organ (VNO). Immunohistochemistry was used to identify GST alpha, mu and pi in the non-sensory and sensory layer of the VNO. Western blot analysis of cytosolic proteins revealed a qualitatively higher enzyme expression of GST alpha and mu in the main olfactory tissue (OE) in comparison to VNO tissue, whereas the GST pi isozyme was equally expressed in both. Total GST metabolism of 1-chloro-2, 4-dinitrobenzene (CDNB) revealed a higher activity level in the OE when compared to the VNO. In contrast, thin-layer chromatographic analysis of GST conjugation of the odorant, trans-2-hexenal (t-hex) (10 mM) showed more conjugate formed per unit protein in the VNO than the OE. The analysis of GST expression and enzyme activity within the VNO parallels the reported localization of phase I metabolizing enzymes and suggests that GST isozymes play independent roles that characterize multiple processes within VNO chemosensitivity.

The aromatase knockout (ArKO) mouse provides new evidence that estrogens are required for the development of the female brain

The classic view of sexual differentiation is that the male brain develops under the influence of testicular secretions, whereas the female brain develops in the absence of any hormonal stimulation. However, several studies have suggested a possible role of estradiol in female neural development, although they did not provide unequivocal evidence that estradiol is indispensable for the development of the female brain and behavior. As a result, the hypothesis subsequently languished because of the lack of a suitable animal model to test estrogen's possible contribution to female differentiation. The recent introduction of the aromatase knockout (ArKO) mouse, which is deficient in aromatase activity because of a targeted mutation in the CYP19 gene and therefore cannot aromatize androgen to estrogen, has provided a new opportunity to reopen the debate of whether estradiol contributes to the development of the female brain. Female ArKO mice showed reduced levels of lordosis behavior after adult treatment with estradiol and progesterone, suggesting that estradiol is required for the development of the neural mechanisms controlling this behavior in female mice. The neural systems affected may include the olfactory systems in that ArKO females also showed impairments in olfactory investigation of odors from conspecifics. Thus, the classic view of sexual differentiation, that is, the female brain develops in the absence of any hormonal secretion, needs to be re-examined.

Sexual differentiation of the neuroendocrine mechanisms regulating mate recognition in mammals

When in breeding condition, male and female mammals seek out and mate with opposite-sex conspecifics. The neural mechanisms controlling mate recognition and heterosexual partner preference are sexually differentiated by the perinatal actions of sex steroid hormones. Many mammalian species use odours to identify potential mates. Thus, sex differences in partner preference may actually reflect sex differences in how male and female mammals perceive socially relevant odours. Two olfactory systems have evolved in vertebrates that differ considerably in their anatomy and function. It is generally believed that the main olfactory system is used to detect a wide variety of volatile odours derived from food prey among many sources, whereas the accessory olfactory system has evolved to detect and process primarily nonvolatile odours shown to influence reproductive behaviours and neuroendocrine functions. Some recent results obtained in oestradiol-deficient aromatase knockout (ArKO) mice that provide evidence for a developmental role of oestradiol in olfactory investigation of volatile body odours are discussed, suggesting that: (i) oestrogens contribute to the development of the main olfactory system and (ii) mate recognition is mediated by the main as opposed to the accessory olfactory system. Thus, sex differences in mate recognition and sexual partner preference may reflect sex differences in the perception of odours by the main olfactory system.