Neurosteroid modulation of dendrodendritic inhibition in the mouse olfactory bulb

Neurosteroids; potential underpinning roles in maintaining homeostasis

The neuroactive steroids which are synthesized in the brain and nervous system are known as "Neurosteroids". These steroids have crucial functions such as contributing to the myelination and organization of the brain connectivity. Under the stressful circumstances, the concentrations of neurosteroid products such as allopregnanolone (ALLO) and allotetrahydrodeoxycorticosterone (THDOC) alter. It has been suggested that these stress-derived neurosteroids modulate the physiological response to stress. Moreover, it has been demonstrated that the hypothalamic-pituitary-adrenal (HPA) axis mediates the physiological adaptation following stress in order to maintain homeostasis. Although several regulatory pathways have been introduced, the exact role of neurosteroids in controlling HPA axis is not clear to date. In this review, we intend to discern specific pathways associated with regulation of HPA axis in which neuroactive steroids have the main role. In this respect, we propose pathways that may be initiated after neurosteroidogenesis in different brain subregions following acute stress which are potentially capable of activating or inhibiting the HPA axis.

Dehydroepiandrosterone regulates astroglia reaction to denervation of olfactory glomeruli

Effects of dehydroepiandrosterone (DHEA) on glial reactions of the peripherally denervated olfactory bulb were studied in adult male rats. Denervation was achieved by destroying the olfactory mucosa with ZnSO(4) (0.17 M) irrigation of the nasal cavities. In one series of experiments, chronic DHEA treatment was applied (daily injections for 7 days, i.p., 10 mg/kg b.w. and 25 mg/kg b.w.); in the other series of experiments, animals received a single injection of DHEA (i.p., 10 mg/kg b.w., 25 mg/kg b.w. and 50 mg/kg b.w.) 2 h following ZnSO(4) treatment. To determine whether DHEA conversion to estradiol was involved in the mechanism of DHEA action on glia, a third series of experiments was carried out in which the aromatase inhibitor fadrozole (4.16 mg/ml) was administered using subcutaneously implanted osmotic minipumps. Rats were killed on day 7 after chemical denervation, and the reaction of glial cells was monitored within the olfactory bulb, using GFAP and vimentin immunohistochemistry. Qualitative changes in GFAP expression were analyzed by Western blot. Chronic DHEA treatment with both doses (10 mg/kg b.w. and 25 mg/kg b.w.) and acute DHEA treatment with the highest dose applied (50 mg/kg b.w.), inhibited the increase in GFAP expression induced by the denervation of the olfactory bulb. Furthermore, GFAP and vimentin immunostaining in the glomerular layer of the olfactory bulb were diminished in the denervated and DHEA treated groups. However, when DHEA treatment was combined with fadrozole administration, such a decrease in GFAP expression could not be detected in the chemically denervated olfactory bulb. These findings indicate that DHEA, depending on the dose applied and the mode of administration, attenuates glial reaction to denervation and may regulate glial plasticity in the olfactory glomeruli. These effects are likely to be mediated at least in part by the conversion of DHEA to estradiol.

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.