Gonadal hormones, but not sex, affect the acquisition and maintenance of a Go/No-Go odor discrimination task in mice
Introduction
Numerous mammalian species ranging from rodents to primates rely on the main olfactory system to detect and discriminate between different volatile environmental odorants that provide critical information about the presence of food as well as dangerous, toxic chemicals in the environment. Volatile chemosignals from conspecifics that are detected by the main olfactory system also complement the action of pheromones detected by a parallel, vomeronasal-accessory olfactory system in signaling the sex and social status of conspecifics. Over the past several decades a large literature has examined the existence of sex differences and related effects of circulating sex hormones on aspects of olfactory function (Dorries, 1992; Kass et al., 2017). For example, early studies by Carr et al. (1962) used a thirst-motivated operant task to determine that prepubertal castration of male rats failed to disrupt their later ability to detect diminishing concentrations of urinary volatiles from estrous female rats or to discriminate between urinary volatiles from estrous vs anestrous females (Carr and Caul, 1962). In another early study using rats in a thirst motivated operant task (Pietras and Moulton, 1974) the ability of females to detect several non-social volatile odorants (e.g., eugenol) was maximal when subjects were in vaginal estrus and was much diminished at vaginal diestrus or after ovariectomy. In a pioneering series of studies Dorries and co-workers extended these early findings to the domestic pig by using a thirst-motivated operant task to show that gonadally intact (GI) females were significantly more sensitive than males to diminishing concentrations of the putative volatile boar pheromone, androstenone (Dorries, 1991; Dorries et al., 1995). A similar sex difference (female > male) in the capacity to detect very low concentrations of male as well as female urinary volatiles was seen in gonadectomized (GDX) mice that were tested using a simple habituation/dishabituation paradigm (Baum and Keverne, 2002). A similar sex difference (female > male) was also seen in a food motivated operant sand digging task in which ovariectomized female mice were better able than castrated males to detect diminishing concentrations of male urinary volatiles when estradiol was administered to both sexes (Sorwell et al., 2008). In several instances, these animal results have been extended to humans. Thus prepubertal children of both sexes were able to detect the volatile human male axillary secretion, androstenone, whereas after puberty women were significantly more likely than men to detect this odorant (Dorries et al., 1989). More recently, Dalton et al. (2002) reported that the ability of diminishing concentrations of several odorants (e.g., benzaldehyde) to be detected after repeated exposure trials was significantly greater in young adult women than in men. No such sex differences were seen in prepubertal children or when post-menopausal women were compared to men, suggesting that circulating ovarian sex hormones may augment odorant detection.
Nearly all of the above-mentioned animal and human studies concerning the effects of subjects' sex and/or circulating sex hormones on main olfactory system function have assessed odorant detection thresholds instead of subjects' capacity to discriminate different odorants. An exception is a study (Wesson et al., 2006) that used a hunger motivated, Go/No-Go (GNG) task to compare odorant discrimination among male and female wild type mice as well as in mice with a null mutation of the CYP19 gene (aromatase knockout; ArKO), which encodes aromatase, the enzyme that synthesizes estradiol from testosterone. In that study all mice were GDX and treated daily with estradiol throughout the experiment. The main behavioral findings were that wild type and ArKO males as well as ArKO females were significantly better than wild type females in discriminating pairs (male vs estrous female; testes-intact vs castrated male) of urinary odors as well as a pair of non-social odorants (amyl acetate vs butyl acetate). This outcome points to a possible early developmental role of estradiol, acting in the female, to disrupt brain mechanisms controlling olfactory discrimination. However, these behavioral results are surprising given earlier studies (reviewed above) showing that the capacity for odorant detection is normally greater in female than in male rodents. Also, the study of Wesson et al. (2006) did not assess the potential role of circulating sex steroids in modulating olfactory discrimination. We conducted the present experiments to assess more thoroughly the possible activational role of sex hormones in both male and female mice, first in the acquisition of a GNG task for assessing olfactory discrimination capacity and second in maintaining accurate discrimination of socially relevant as well as non-social pairs of volatile odorants.
We first compared the ability of adult GI vs GDX male and female mice to acquire a thirst-motivated GNG odor discrimination task. We then trained GI males and females to discriminate between pairs of urinary volatiles (male vs estrous female) followed by a pair of non-social odorants (amyl acetate vs peppermint) using the same GNG procedure. We subsequently assessed the ability of these same mice to discriminate these two types of odorants several weeks after GDX and then again several weeks after replacement hormones were given (males received testosterone propionate; females received estradiol benzoate).
Section snippets
Subjects
Male (n = 19) and female (n = 20) CFW mice were purchased at 5–7 weeks of age from Charles River Laboratories (Wilmington, MA, USA). All the procedures involving animals were approved by the Boston University Institutional Animal Care Use Committee (IACUC). Animals were group-housed (4 per cage) in same-sex cages under a 12:12 h reversed light: dark cycle (lights off at 9 am). All behavioral tests were carried out during the dark phase of the cycle. All mice were sexually naïve and had no
Results
During training phase 1, GI and GDX mice of both sexes equivalently learned to locate the water reward port and steadily increased their water intake over the 3 days of testing [day: F(2,54) = 32.9, p < 0.0001, η2p = 0.55; sex: F(1,27) = 0.59, p > 0.05] (Fig. 2; panels A and B). There were no effects of GDX on water intake [F(1,27) = 0.70, p > 0.05], although there was a statistically significant gonadal status X test day interaction [F(2,54) = 3.59, p = 0.03, η2p = 0.12], which reflected
Discussion
The observed failure of GDX mice of both sexes to learn the GNG task is consistent with three previous studies suggesting that gonadal hormones facilitate thirst- as well as hunger-motivated operant learning in which either a urinary odor or a non-social odorant served as a discriminative stimulus. Thus, in an early study (Doty and Ferguson-Segall, 1989) castrated male rats were less capable than testes-intact males in learning to use ethyl acetate odor as a discriminative stimulus for
Acknowledgements
We thank David Giese for help in programming the apparatus used in GNG testing and Alberto Cruz-Martin for comments on an early version of the manuscript. This work was supported by NIDCD grant DC008962 to JAC.
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