High abundance of testosterone and salivary androgen-binding protein in the lateral nasal gland of male mice

Sex steroid hormone synthesis, metabolism, and the effects on the mammalian olfactory system

Sex steroid hormones influence olfactory-mediated social behaviors, and it is generally hypothesized that these effects result from circulating hormones and/or neurosteroids synthesized in the brain. However, it is unclear whether sex steroid hormones are synthesized in the olfactory epithelium or the olfactory bulb, and if they can modulate the activity of the olfactory sensory neurons. Here, we review important discoveries related to the metabolism of sex steroids in the mouse olfactory epithelium and olfactory bulb, along with potential areas of future research. We summarize current knowledge regarding the expression, neuroanatomical distribution, and biological activity of the steroidogenic enzymes, sex steroid receptors, and proteins that are important to the metabolism of these hormones and reflect on their potential to influence early olfactory processing. We also review evidence related to the effects of sex steroid hormones on the development and activity of olfactory sensory neurons. By better understanding how these hormones are metabolized and how they act both at the periphery and olfactory bulb level, we can better appreciate the complexity of the olfactory system and discover potential similarities and differences in early olfactory processing between sexes.

Respiratory syncytial virus tropism for olfactory sensory neurons in mice

The olfactory mucosa, where the first step of odor detection occurs, is a privileged pathway for environmental toxicants and pathogens toward the central nervous system. Indeed, some pathogens can infect olfactory sensory neurons including their axons projecting to the olfactory bulb allowing them to bypass the blood-brain barrier and reach the central nervous system (CNS) through the so-called olfactory pathway. The respiratory syncytial virus (RSV) is a major respiratory tract pathogen but there is growing evidence that RSV may lead to CNS impairments. However, the mechanisms involved in RSV entering into the CNS have been poorly described. In this study, we wanted to explore the capacity of RSV to reach the CNS via the olfactory pathway and to better characterize RSV cellular tropism in the nasal cavity. We first explored the distribution of RSV infectious sites in the nasal cavity by in vivo bioluminescence imaging and a tissue clearing protocol combined with deep-tissue imaging and 3D image analyses. This whole tissue characterization was confirmed with immunohistochemistry and molecular biology approaches. Together, our results provide a novel 3D atlas of mouse nasal cavity anatomy and show that RSV can infect olfactory sensory neurons giving access to the central nervous system by entering the olfactory bulb. Cover Image for this issue: doi: 10.1111/jnc.14765.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.

Poor levels of agreement between serum and saliva testosterone measurement following exercise training in aging men

Testosterone (T) is a biologically important androgen that demonstrates a widely-known natural decline with advancing age. The use of salivary T (sal-T), as a determinant of systemic T, has shown promising results in recent years. However, the strength of the salivary-serum T relationship may be affected by measurement method and binding capacity with salivary proteins. The potential influence exercise may impact on this relationship is unstudied in aging men. Therefore, the aim of the present investigation was to examine the relationship of the delta change (Δ) in sal-T with Δserum T following six weeks exercise training. Fifteen sedentary (SED) males (aged 60.4 ± 5.0 years of age) and 20 lifelong exercising (LE) males (60.4 ± 4.7 years of age) were participated. Pearson's correlation coefficient revealed sal-T did not correlate with total testosterone (TT), sex hormone binding globulin (SHBG), bioactive T (bio-T), or free T (free-T) at week 0 or week 6. Δsal-T did not correlate with ΔTT, ΔSHBG, Δbio-T or Δfree-T (r = 0.271, p = 0.180; r = 0.197, p = 0.335; r = 0.258, p = 0.205; and r = 0.257, p = 0.205, respectively). In conclusion, poor levels of agreement existed between saliva and serum measurements of T in response to exercise amongst aging men.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Generation and characterization of a novel Cyp2a(4/5)bgs-null mouse model

Knockout mouse models targeting various cytochrome P450 (P450 or CYP) genes are valuable for determining P450's biologic functions, including roles in drug metabolism and chemical toxicity. In this study, a novel Cyp2a(4/5)bgs-null mouse model was generated, in which a 1.2-megabase pair genomic fragment containing nine Cyp genes in mouse chromosome 7 (including, sequentially, Cyp2a5, 2g1, 2b19, 2b23, 2a4, 2b9, 2b13, 2b10, and 2s1) are deleted, through Cre-mediated recombination in vivo. The resultant mouse strain was viable and fertile, without any developmental deficits or morphologic abnormalities. Deletion of the constitutive genes in the cluster was confirmed by polymerase chain reaction analysis of the genes and the mRNAs in tissues known to express each gene. The loss of this gene cluster led to significant decreases in microsomal activities toward testosterone hydroxylation in various tissues examined, including olfactory mucosa (OM), lung, liver, and brain. In addition, systemic clearance of pentobarbital was decreased in Cyp2a(4/5)bgs-null mice, as indicated by >60% increases in pentobarbital-induced sleeping time, compared with wild-type (WT) mice. This novel Cyp2a(4/5)bgs-null mouse model will be valuable for in vivo studies of drug metabolism and chemical toxicities in various tissues, including the liver, lung, brain, intestine, kidney, skin, and OM, where one or more of the targeted Cyp genes are known to be expressed in WT mice. The model will also be valuable for preparation of humanized mice that express human CYP2A6, CYP2A13, CYP2B6, or CYP2S1, and as a knockout mouse model for five non-P450 genes (Vmn1r184, Nalp9c, Nalp4a, Nalp9a, and Vmn1r185) that were also deleted.

Respective roles of CYP2A5 and CYP2F2 in the bioactivation of 3-methylindole in mouse olfactory mucosa and lung: studies using Cyp2a5-null and Cyp2f2-null mouse models

The aim of this study was to determine whether mouse CYP2A5 and CYP2F2 play critical roles in the bioactivation of 3-methylindole (3MI), a tissue-selective toxicant, in the target tissues, the nasal olfactory mucosa (OM) and lung. Five metabolites of 3MI were identified in NADPH- and GSH-fortified microsomal reactions, including 3-glutathionyl-S-methylindole (GS-A1), 3-methyl-2-glutathionyl-S-indole (GS-A2), 3-hydroxy-3-methyleneindolenine (HMI), indole-3-carbinol (I-3-C), and 3-methyloxindole (MOI). The metabolite profiles and enzyme kinetics of the reactions were compared between OM and lung, and among wild-type, Cyp2a5-null, and Cyp2f2-null mice. In lung reactions, GS-A1, GS-A2, and HMI were detected as major products, and I-3-C and MOI, as minor metabolites. In OM reactions, all five metabolites were detected in ample amounts. The loss of CYP2F2 affected formation of all 3MI metabolites in the lung and formation of HMI, GS-A1, and GS-A2 in the OM. In contrast, loss of CYP2A5 did not affect formation of 3MI metabolites in the lung but caused substantial decreases in I-3-C and MOI formation in the OM. Thus, whereas CYP2F2 plays a critical role in the 3MI metabolism in the lung, both CYP2A5 and CYP2F2 play important roles in 3MI metabolism in the OM. Furthermore, the fate of the reactive metabolites produced by the two enzymes through common dehydrogenation and epoxidation pathways seemed to differ with CYP2A5 supporting direct conversion to stable metabolites and CYP2F2 supporting further formation of reactive iminium ions. These results provide the basis for understanding the respective roles of CYP2A5 and CYP2F2 in 3MI's toxicity in the respiratory tract.

A novel defensive mechanism against acetaminophen toxicity in the mouse lateral nasal gland: role of CYP2A5-mediated regulation of testosterone homeostasis and salivary androgen-binding protein expression

To identify novel factors or mechanisms that are important for the resistance of tissues to chemical toxicity, we have determined the mechanisms underlying the previously observed increases in resistance to acetaminophen (APAP) toxicity in the lateral nasal gland (LNG) of the male Cyp2g1-null/Cyp2a5-low mouse. Initial studies established that Cyp2a5-null mice, but not a newly generated strain of Cyp2g1-null mice, were resistant to APAP toxicity in the LNG; therefore, subsequent studies were focused on the Cyp2a5-null mice. Compared with the wild-type (WT) male mouse, the Cyp2a5-null male mouse had intact capability to metabolize APAP to reactive intermediates in the LNG, as well as unaltered circulating levels of APAP, APAP-GSH, APAP-glucuronide, and APAP-sulfate. However, it displayed reduced tissue levels of APAP and APAP-GSH and increased tissue levels of testosterone and salivary androgen-binding protein (ABP) in the LNG. Furthermore, we found that ABP was able to compete with GSH and cellular proteins for adduction with reactive metabolites of APAP in vitro. The amounts of APAP-ABP adducts formed in vivo were greater, whereas the amounts of APAP adducts formed with other cellular proteins were substantially lower, in the LNG of APAP-treated male Cyp2a5-null mice compared with the LNG of APAP-treated male WT mice. We propose that through its critical role in testosterone metabolism, CYP2A5 regulates 1) the bioavailability of APAP and APAP-GSH (presumably through modulation of the rates of xenobiotic excretion from the LNG) and 2) the expression of ABP, which can quench reactive APAP metabolites and thereby spare critical cellular proteins from inactivation.

Generation of a mouse model with a reversible hypomorphic cytochrome P450 reductase gene: utility for tissue-specific rescue of the reductase expression, and insights from a resultant mouse model with global suppression of P450 reductase expression in extrahepatic tissues

A mouse model termed Cpr-low (CL) was recently generated, in which the expression of the cytochrome P450 reductase (Cpr) gene was globally down-regulated. The decreased CPR expression was accompanied by phenotypical changes, including reduced embryonic survival, decreases in circulating cholesterol, increases in hepatic P450 expression, and female infertility (accompanied by elevated serum testosterone and progesterone levels). In the present study, a complementary mouse model [named reversible-CL (r-CL)] was generated, in which the reduced CPR expression can be reversed in an organ-specific fashion. The neo cassette, which was inserted into the last Cpr intron in r-CL mice, can be deleted by Cre recombinase, thus returning the structure of the Cpr gene (and hence CPR expression) to normal in Cre-expressing cells. All previously identified phenotypes of the CL mice were preserved in the r-CL mice. As a first application of the r-CL model, we have generated an extrahepatic-CL (xh-CL) mouse for testing of the functions of CPR-dependent enzymes in all extrahepatic tissues. The xh-CL mice, generated by mating of r-CL mice with albumin-Cre mice, had normal CPR expression in hepatocytes but down-regulated CPR expression elsewhere. They were indistinguishable from wild-type mice in body and liver weights, circulating cholesterol levels, and hepatic microsomal P450 expression and activities; however, they still showed elevated serum testosterone and progesterone levels and sterility in females. Embryonic lethality was prevented in males, but apparently not in females, indicating a critical role for fetal hepatic CPR-dependent enzymes in embryonic development, at least in males.

Role of CYP2A5 in the clearance of nicotine and cotinine: insights from studies on a Cyp2a5-null mouse model

CYP2A5, a mouse cytochrome P450 monooxygenase that shows high similarities to human CYP2A6 and CYP2A13 in protein sequence and substrate specificity, is expressed in multiple tissues, including the liver, kidney, lung, and nasal mucosa. Heterologously expressed CYP2A5 is active in the metabolism of both endogenous substrates, such as testosterone, and xenobiotic compounds, such as nicotine and cotinine. To determine the biological and pharmacological functions of CYP2A5 in vivo, we have generated a Cyp2a5-null mouse. Homozygous Cyp2a5-null mice are viable and fertile; they show no evidence of embryonic lethality or developmental deficits; and they have normal circulating levels of testosterone and progesterone. The Cyp2a5-null mouse and wild-type mouse were then used for determination of the roles of CYP2A5 in the metabolism of nicotine and its major circulating metabolite, cotinine. The results indicated that the Cyp2a5-null mouse has lower hepatic nicotine 5'-hydroxylation activity in vitro, and slower systemic clearance of both nicotine and cotinine in vivo. For both compounds, a substantially longer plasma half-life and a greater area under the concentration-time curve were observed for the Cyp2a5-null mice, compared with wild-type mice. Further pharmacokinetics analysis confirmed that the brain levels of nicotine and cotinine are also influenced by the Cyp2a5 deletion. These findings provide direct evidence that CYP2A5 is the major nicotine and cotinine oxidase in mouse liver. The Cyp2a5-null mouse will be valuable for in vivo studies on the role of CYP2A5 in drug metabolism and chemical toxicity, and for future production of CYP2A6- and CYP2A13-humanized mouse models.