Effects of testosterone dose on spatial memory among castrated adult male rats

Effects of testosterone dose on depression-like behavior among castrated adult male rats

Previous research has shown a decrease in serum testosterone levels in male patients with depression. In recent years, the results of testosterone replacement therapy (TRT) to improve depression have been mixed. Using the classic CUMS model, we induced depressive-like behaviors in rats and observed a decrease in their serum testosterone levels along with an increase in androgen receptor expression in the hippocampus. We then performed castration and sham surgery on male rats and found that testosterone deprivation led to the manifestation of depressive-like behavior that could be ameliorated by TRT. Through a repeated measures experiment consisting of five blocks over a period of 25 days, we discovered that the reduction in depressive-like behavior in testosterone-deprived rats began 22 days after drug administration (0.5 and 0.25 mg/rat). Furthermore, rats in 0.5mgT group showed the most significant improvements. Subsequently, this dose was used in CUMS rats and reduced the occurrence of depressive-like behaviors. Our study has demonstrated the complex interplay between depression and testosterone, as well as the intricate dose-response relationship between TRT and reduction in depression. Our research supports the use of TRT to alleviate depression, but dosage and duration of treatment are critical factors in determining efficacy.

The effects of peripheral hormone responses to exercise on adult hippocampal neurogenesis

Over the last decade, a considerable amount of new data have revealed the beneficial effects of exercise on hippocampal neurogenesis and the maintenance or improvement of cognitive function. Investigations with animal models, as well as human studies, have yielded novel understanding of the mechanisms through which endocrine signaling can stimulate neurogenesis, as well as the effects of exercise on acute and/or chronic levels of these circulating hormones. Considering the effects of aging on the decline of specific endocrine factors that affect brain health, insights in this area of research are particularly important. In this review, we discuss how different forms of exercise influence the peripheral production of specific endocrine factors, with particular emphasis on brain-derived neurotrophic factor, growth hormone, insulin-like growth factor-1, ghrelin, estrogen, testosterone, irisin, vascular endothelial growth factor, erythropoietin, and cortisol. We also describe mechanisms through which these endocrine responses to exercise induce cellular changes that increase hippocampal neurogenesis and improve cognitive function.

Effects of testosterone replacement on lipid profile, hepatotoxicity, oxidative stress, and cognitive performance in castrated wistar rats

OBJECTIVE

Androgen deficiency is associated with multiple biochemical and behavioral disorders. This study investigated the effects of testosterone replacement and Spirulina Platensis association on testosterone deficiency-induced metabolic disorders and memory impairment.

METHODS

Adult male rats were randomly and equally divided into four groups and received the following treatments for 20 consecutive days.

CONTROL GROUP

non-castrated rats received distilled water. Castrated group received distilled water. Testosterone treated group: castrated rats received 0.20 mg of testosterone dissolved in corn oil by subcutaneous injection (i.p.). Spirulina co-treated group: castrated rats received 0.20 mg of testosterone (i.p.) dissolved in corn oil followed by 1000 mg/kg of Spirulina per os.

RESULTS

Data showed that castration induced an increase in plasma ALT, AST, alkaline phosphatase (PAL), cholesterol, and triglycerides level. Castrated rats showed a great elevation in SOD and CAT activities and MDA and H2O2 levels in the prostate, seminal vesicles, and brain. Testosterone deficiency was also associated with alteration of the spatial memory and exploratory behaviour. Testosterone replacement either alone or with Spirulina combination efficiently improved most of these biochemical parameters and ameliorated cognitive abilities in castrated rats.

CONCLUSIONS

Testosterone replacement either alone or in combination with Spirulina improved castration-induced metabolic, oxidative, and cognitive alterations.

Sex-dependent differences in animal cognition

The differences in cognitive processes driven by biological sex are the issues that have gotten growing attention recently. Considering the increasing population suffering from various cognitive impairments and the development of therapeutic strategies, it is essential that we recognize the mechanisms responsible for discrepancies observed in male and female learning and memory functions. In this review, we discuss recent reports from preclinical studies on rodents regarding selected cognitive domains to explore the state of knowledge on sex-dependent differences and point to challenges encountered during such research. We focus on spatial, recognition, and emotional memory, as well as on executive functions, such as attention, cognitive flexibility, and working memory. This review will help to acknowledge sex-related differences in cognition and indicate some fields that lack sufficient data.

Testosterone ameliorated the behavioural deficits of gonadectomised rats and counteracted free radicals in a dosage-dependent manner

Testosterone deficiency may induce behavioural changes in individuals. Oxidative stress resulting from a redox imbalance may be implicated in the initiation and progression of neurobehavioural disorders. However, whether exogenous testosterone intervention in male gonadectomised (GDX) rats ameliorates oxidative stress and plays a neuroprotective role remains unknown. Therefore, we examined this hypothesis by performing sham or gonadectomy surgeries on Sprague-Dawley rats with or without supplementation with different doses of testosterone propionate (TP). Open field and Morris water maze tests were performed, the serum and brain testosterone levels, and oxidative stress markers were analysed. GDX and lower TP doses (0.5mg/kg) induced reduced exploratory and motor behaviours, but impaired spatial learning and memory compared to Sham rats. Administration of physiological TP levels (0.75-1.25mg/kg) to the GDX rats restored the behaviour observed in the intact rats. However, higher TP doses (1.5-3.0mg/kg) induced increased exploratory and motor behaviours but impaired spatial learning and memory. These behavioural impairments were accompanied by a marked decrease in levels of antioxidant enzymes (superoxide dismutase and catalase) and an increase in lipid peroxidation levels in the substantia nigra and hippocampus. These findings indicate that TP administration can alter behavioural performance and induce memory and learning impairment, which may result from changes in redox homeostasis in male GDX animals.

Combined effects of the contraceptive hormones, ethinyl estradiol and levonorgestrel, on the use of place and response memory in gonadally-intact female rats

During maze navigation rats can rely on hippocampus-mediated place memory or striatum-mediated response memory. Ovarian hormones bias whether females use place or response memory to reach a reward. Here, we investigated the impact of the contraceptive hormones, ethinyl estradiol (EE) and levonorgestrel (LNG), on memory bias. A total of 63 gonadally-intact female rats were treated with either 10 μg/kg of EE alone, 20 μg/kg of LNG alone, both 10 μg/kg of EE and 20 μg/kg of LNG together, or a sesame oil injection with 5% ethanol as a vehicle control. Rats in the control condition were tested during the diestrus phase of the estrous cycle in order to control for the low circulating levels of gonadotropin and ovarian hormones that occur with oral contraceptive administration. Rats treated with LNG alone had a bias towards the use of place memory compared to diestrus phase control rats. This bias was not observed if LNG was administered in combination with EE. Rats treated with EE or EE+LNG did not have a statistically significant difference in memory bias compared to rats in the control group. These data show that synthetic hormones contained in oral contraceptives administered to females influence which cognitive strategy is predominantly used during navigation.

Hippocampal synaptic dysfunction and spatial memory impairment in omeprazole-treated rats

Although the association of prolonged use of proton pump inhibitors, such as omeprazole, with memory impairment has been reported more than two decades ago, its underlying molecular mechanism is yet to be determined. Thus, in this study, we aimed to determine the mechanisms underlying the effect of prolonged omeprazole treatment on hippocampal synaptic function and spatial memory in male rats. Adult rats were subcutaneously administered with omeprazole for 12 or 24 weeks. Spatial memory was assessed using the Morris water maze (MWM) test. We examined the hippocampal protein expression of synaptic plasticity proteins, including the AMPA receptor subunit GluA1, postsynaptic density-95 (PSD-95), and activity-regulated cytoskeleton-associated protein (Arc), and the hippocampal expression and localization of androgen receptor (AR). In the MWM test, the escape latency was found to be significantly higher, and the number of platform crossings and the time spent in the target quadrant were significantly lower in the rats treated with omeprazole compared to the control rats. Hypomagnesemia and lower bone and brain Mg²⁺ content were also detected in the omeprazole-treated groups compared with the control group. The expression of GluA1, PSD-95, and Arc in the hippocampus and the expression of AR in the dentate gyrus and CA1 of the hippocampus were significantly lower in the omeprazole-treated groups than in the control group. These results suggest that prolonged omeprazole treatment might lead to memory deficit by impairing glutamate receptor trafficking or synaptic anchoring. Hypomagnesemia and brain Mg²⁺ deficiency may be, at least in part, involved in omeprazole-induced memory impairment.

Effect of Testosterone on the Extinction Period of Morphine-induced CPP in Male Rats

Background: Sex-related differences in the incidence, prevalence, symptoms, and side effects of drug use, especially narcotics, have been previously shown in animal models and human studies. Objectives: In the presents study, the effects of different doses of testosterone on morphine extinction period were investigated in a rat model. Methods: Forty mature male Wistar rats were randomly allocated to four categories (10 in each group), including control (received intramuscular injection of vehicle) and testosterone (received intramuscular injection of testosterone at 1, 2.5 and 5 mg/kg) during the extinction period. Conditioned place preference (CPP) test was done to assess the psychological phenomena of drug craving and relapse. The CPP score was calculated in four stages, including the baseline (preconditioning), expression (postconditioning), extinction, and reinstatement. Results: Our results demonstrated that testosterone (1, 2.5 and 5 mg/kg) significantly extended morphine extinction duration compared to the control group. Conclution: It has been shown that dopamine neurotransmission in mesocorticolimbic system is affected by testosterone through androgen receptors in adolescence and alteration in testosterone level could affect drug use vulnerability. It seems that normalization of testosterone levels reduces the symptoms of opioid withdrawal syndrome and have important clinical implication for clinicians to understand the effects of testosterone dysregulation on the extinction and withdrawal periods.

Seasonal variation in gonadal hormones, spatial cognition, and hippocampal attributes: More questions than answers

A large body of research has been dedicated to understanding the factors that modulate spatial cognition and attributes of the hippocampus, a highly plastic brain region that underlies spatial processing abilities. Variation in gonadal hormones impacts spatial memory and hippocampal attributes in vertebrates, although the direction of the effect has not been entirely consistent. To add complexity, individuals in the field must optimize fitness by coordinating activities with the appropriate environmental cues, and many of these behaviors are correlated tightly with seasonal variation in gonadal hormone release. As such, it remains unclear if the relationship among systemic gonadal hormones, spatial cognition, and the hippocampus also exhibits seasonal variation. This review presents an overview of the relationship among gonadal hormones, the hippocampus, and spatial cognition, and how the seasonal release of gonadal hormones correlates with seasonal variation in spatial cognition and hippocampal attributes. Additionally, this review presents other neuroendocrine mechanisms that may be involved in modulating the relationship among seasonality, gonadal hormone release, and the hippocampus and spatial cognition, including seasonal rhythms of steroid hormone binding globulins, neurosteroids, sex steroid hormone receptor expression, and hormone interactions. Here, endocrinology, ecology, and behavioral neuroscience are brought together to present an overview of the research demonstrating the mechanistic effects of systemic gonadal hormones on spatial cognition and the hippocampus, while, at a functional level, superimposing seasonal effects to examine ecologically-relevant circannual changes in gonadal hormones and spatial behaviors.

B6D2F1 mice that retain sexual behavior long term after castration outperform those that cease in the radial arm maze

In rodents, gonadal steroids play a critical yet variable role in behaviors such as social interaction and cognitive performance. Gonadal steroids organize sex differences observed in spatial working memory, while the absence of activational effects induced by castration generally impedes spatial learning and memory. Although male sexual behavior is typically inhibited following castration, a significant proportion of gonadectomized B6D2F1 hybrid males retains the complete repertoire of male reproductive behavior. In a prior study, amyloid precursor protein and tau, proteins involved in cognitive behavior, facilitated steroid-independent male sex behavior in B6D2F1 hybrid male mice. We used this strain to investigate the relationship between gonadal steroid-independent male sexual behavior and cognition. After identifying "maters" (animals retaining steroid-independent male sex behavior) and "non-maters," we tested spatial memory in an 8-arm radial arm maze. Although neither group demonstrated a decrease in errors as a function of time, maters committed fewer errors compared to non-maters overall (p < 0.05). Maters also completed the maze more quickly than non-maters (p < 0.05). We measured mRNA expression of APP and MAPT as well as LEPR and D2R to probe potential roles of metabolism and motivation. Uniquely among maters, increased relative expression of D2R and LEPR in the hippocampus was associated with a longer latency to complete the maze during the last 3 or across all trials, respectively. These data demonstrate that maters outperform non-maters in the radial arm maze, warranting further study of potential differences in acquisition of spatial memory tasks or learning strategy between these groups.

Testosterone Influence on Microtubule-Associated Proteins and Spine Density in Hippocampus: Implications on Learning and Memory

The thorny protrusions or spines increase the neuronal surface area, facilitate synaptic interconnections among neurons, and play an essential role in the hippocampus. Increasing evidence suggests that testosterone, the gonadal hormone, plays an important role in neurogenesis and synaptic plasticity. The role of testosterone on microtubule-associated proteins on dendritic neurite stability in the hippocampus and its impact on learning disability is not elucidated. Adult male Wistar albino rats were randomly selected for the control, castrated, castrated + testosterone, and control + testosterone groups. Bilateral orchidectomy was done, and the testosterone propionate was administered during the entire trial period, i.e., 14 days. The learning assessments were done using working/reference memory versions of the 8-arm radial maze and hippocampal tissues processed for histological and protein expressions. There were reduced expressions of microtubule-associated protein 2 (MAP2), postsynaptic density protein 95 (PSD95), and androgen receptor (AR) and increased expression of pTau in the castrated group. Conversely, the expression of MAP2, PSD95, and AR was increased, and the pTau expression was reduced in the hippocampus of the castrated rat administrated with testosterone. Androgen-depleted rats showed impaired synaptic plasticity in the hippocampus associated with contracted microtubule dynamics. Along with learning disability, there was an increased number of reference memory errors and working memory errors in castrated rats. Observations suggest that androgen regulates expression of neural tissue-specific MAPs and plays a vital role in hippocampus synaptic plasticity and that a similar mechanism may underlie neurological disorders in aging and hypogonadal men.

Testosterone ameliorates age-related brain mitochondrial dysfunction

Brain mitochondrial dysfunction and reduced testosterone levels are common features of aging in men. Although evidence suggests that the two phenomena are interrelated, it is unclear whether testosterone supplementation ameliorates mitochondrial dysfunction in the aging male brain. Here, we show that testosterone supplementation significantly alleviates exploratory behavioral deficits and oxidative damage in the substantia nigra and hippocampus of aging male rats. These effects were consistent with improved mitochondrial function, reflected by testosterone-induced increases in mitochondrial membrane potential (MMP), antioxidant enzyme (GSH-PX, catalase, and Mn-SOD) expression/activity, and mitochondrial respiratory complex activities in both brain regions. Furthermore, elevated PGC-1α, NRF-1, and TFAM expression (suggestive of enhanced mitochondrial biogenesis), increased citrate synthase activity, mtDNA copy number, and ND1, COX1, and ATP6 expression (indicative of increased mitochondrial content), as well as increased PINK1/Parkin and decreased P62 expression (suggesting mitophagy activation), were detected in the substantial nigra and hippocampus of aged male rats after testosterone supplementation. These findings suggest that testosterone supplementation may be a viable approach to ameliorating brain mitochondrial dysfunction and thus prevent or treat cognitive-behavioral deficits and neurodegenerative conditions associated with aging.

Effects of Long-Acting Testosterone Undecanoate on Behavioral Parameters and Na + , K+-ATPase mRNA Expression in Mice with Alzheimer`s Disease

Previous studies have shown that testosterone attenuates stress-induced mood dysfunction and memory deterioration. However, the exact mechanism is still unknown. This study was conducted to investigate the role of long-term testosterone undecanoate on the behavioral responses in AD induced by AlCl3 + D-galactose administration and the possible alteration of the gene expression level of the Na/K ATPase pump. Adult male mice received AlCl3 in drinking water (10 mg/kg/day) and (D-gal 200 mg/kg/day), subcutaneously for 90 consecutive days, then received a single intramuscular (I.M) injection of castor oil (vehicle) on day 91, while treated groups received a single I.M injection of either low (100 mg/kg/45 days) or high dose (500 mg/kg/45 days) respectively of long-acting testosterone undecanoate on day 91. The time spent in the interaction zone during the open field test, preference index to novel objects in the novel object recognition test, spontaneous alternation percentage (SAP) in Y-maze test, and escape latency time in the Morris water maze test were used to measure the locomotor activity, long-term memory, and spatial memory in mice, respectively. The results showed that testosterone undecanoate treatment improved locomotor activity, improved preference to novel objects, improved spatial memory, and reversed anxiety and depression induced by AlCl3 + D-galactose administration in male mice, suggesting the enhancement of behavioral and memory functions brought by testosterone treatment. Moreover, testosterone undecanoate treatment did alter gene expression levels of Na/K ATPase isoforms in the brain hippocampus. In most cases, altered gene expression was significant and correlated with the observed behavioral changes. Taken together, our findings provide new insight into the effects of long-acting testosterone undecanoate administration on locomotor activity, long-term memory, anxiety, and spatial memory in male mice with Alzheimer's disease.

Regulation of risky decision making by gonadal hormones in males and females

Psychiatric diseases characterized by dysregulated risky decision making are differentially represented in males and females. The factors that govern such sex differences, however, remain poorly understood. Using a task in which rats make discrete trial choices between a small, "safe" food reward and a large food reward accompanied by varying probabilities of footshock punishment, we recently showed that females are more risk averse than males. The objective of the current experiments was to test the extent to which these sex differences in risky decision making are mediated by gonadal hormones. Male and female rats were trained in the risky decision-making task, followed by ovariectomy (OVX), orchiectomy (ORX), or sham surgery. Rats were then retested in the task, under both baseline conditions and following administration of estradiol and/or testosterone. OVX increased choice of the large, risky reward (increased risky choice), an effect that was attenuated by estradiol administration. In contrast, ORX decreased risky choice, but testosterone administration was without effect in either ORX or sham males. Estradiol, however, decreased risky choice in both groups of males. Importantly, none of the effects of hormonal manipulation on risky choice were due to altered shock sensitivity or food motivation. These data show that gonadal hormones are required for maintaining sex-typical profiles of risk-taking behavior in both males and females, and that estradiol is sufficient to promote risk aversion in both sexes. The findings provide novel information about the mechanisms supporting sex differences in risk taking and may prove useful in understanding sex differences in the prevalence of psychiatric diseases associated with altered risk taking.

Sexual Motivation and Sexual Reward in Male Rats are Attenuated by the Gonadotropin-Releasing Hormone Receptor Antagonist Degarelix

BACKGROUND

Preclinical studies that have examined the effects of androgen deprivation therapies (ADTs) on sexual outcomes have either relied on a surgical castration model of ADTs or have largely focused on consummatory sexual behaviors.

AIM

The aim of this study was to examine the effects of a single administration of the gonadotropin-releasing hormone receptor antagonist, degarelix, on sexual incentive motivation (SIM), sexual reward, consummatory sexual behaviors, anxiety-like behavior, and androgen receptor signaling in male rats, and to determine if sexual stimulation attenuates the effects of degarelix on SIM.

METHODS

Male rats were treated with degarelix, or vehicle, and half of the rats in each condition were briefly exposed to a sexually receptive female immediately before SIM trials (experiment 1). Rats treated with degarelix or vehicle were also given a sex-conditioned place preference test (experiment 2A), weekly mating tests (experiment 2B), and an elevated zero maze test (experiment 3). Androgen-sensitive tissues were excised upon completion of testing.

OUTCOMES

SIM was indicated by the percentage of time spent near a sexually receptive female on the SIM tests. The percentage of time spent in the chamber of a conditioned place preference maze associated with sexual experience was indicative of sexual reward. The percentage of trials in which a mount, intromission, and ejaculation occurred was indicative of copulatory ability. Sexual performance was characterized by the average latencies to first exhibit these behaviors, as well as the average frequency of these behaviors. Anxiety-like behavior was indicated by the percentage of time in the open zones of an elevated zero maze. Relative weights of the seminal vesicles and bulbourethral glands were used to quantify androgen activity.

RESULTS

Rats treated with degarelix exhibited lower levels of SIM. In rats treated with degarelix, contact with a female immediately before SIM testing increased activity, but not SIM. Treatment with degarelix reduced the rewarding aspects of sexual behavior, as well as most aspects of copulatory ability and sexual performance. Degarelix treatment reduced androgen signaling, but did not impact anxiety-like behavior.

CLINICAL IMPLICATIONS

The behavioral side effects associated with the use of degarelix may be restricted to sexual behaviors.

STRENGTHS & LIMITATIONS

Strengths include the objective measurement of sexual behaviors. The study is limited in that only one ADT was examined.

CONCLUSION

These findings serve as an extension of previous preclinical studies as they indicate that gonadotropin-releasing hormone receptor antagonism in male rats also attenuates sexual motivation and sexual reward, in addition to copulatory ability and sexual performance. Hawley WR, Kapp LE, Green PA, et al. Sexual Motivation and Reward in Male Rats are Attenuated by the Gonadotropin-Releasing Hormone Receptor Antagonist Degarelix. J Sex Med 2021;18:240-255.

Apoptosis in hippocampal tissue induced by oxidative stress in testosterone deprived male rats

The testosterone decline is one of the potential causes of oxidative stress-induced anxiety and depressive behaviors, and cognitive impairment induces irreversible neuronal damage, which is not clearly understood. The orchidectomized rat model was used; the hippocampal neurons and anxiety behavior were analyzed. Adult male albino rats were divided into control and orchidectomy (ORX) groups, orchidectomy (ORX + T), and normal (Cont + T) groups. Testosterone propionate was used as a testosterone supplement. The anxiety and depressive-like behavior observed in ORX animals in the open field (OF) and elevated plus-maze experiments were effectively overturned in the ORX + T group. Studies on isolated hippocampus showed reduced antioxidant enzymes (SOD, CAT, and glutathione (GSH) compounds), increased lipid peroxidation (LPO), elevated caspase3, and reduced anti-apoptotic protein Bcl-2, and increased apoptotic nuclei in TUNEL staining of the hippocampus in the ORX rats. These observations indicate free radical-mediated neural damage. Testosterone presence promoted the antioxidant defense system and restored normal pyramidal neuron morphology in ORX + T. This study confirms that testosterone is indispensable in the normal adult hippocampus and deficiency seems to be a potential risk factor for neurodegenerative disorders. Besides, androgen appears to be a possible therapeutic strategy for treating depression/neurodegenerative diseases in aging men.

Dose-dependent effects of testosterone on spatial learning strategies and brain-derived neurotrophic factor in male rats

Studies suggest that males outperform females on some spatial tasks. This may be due to the effects of sex steroids on spatial strategy preferences. Past experiments with male rats have demonstrated that low doses of testosterone bias them toward a response strategy, whereas high doses of testosterone bias them toward a place strategy. We investigated the effect of different testosterone doses on the ability of male rats to effectively employ these two spatial learning strategies. Furthermore, we quantified concentrations of brain-derived neurotrophic factor (pro-, mature-, and total BDNF) in the prefrontal cortex, hippocampus, and striatum. All rats were bilaterally castrated and assigned to one of three daily injection doses of testosterone propionate (0.125, 0.250, or 0.500 mg/rat) or a control injection of the drug vehicle. Using a plus-maze protocol, we found that a lower testosterone dose (0.125 mg) significantly improved rats' performance on a response task, whereas a higher testosterone dose (0.500 mg) significantly improved rats' performance on a place task. In addition, we found that a low dose of testosterone (0.125 mg) increased total BDNF in the striatum, while a high dose (0.500 mg) increased total BDNF in the hippocampus. Taken altogether, these results suggest that high and low levels of testosterone enhance performance on place and response spatial tasks, respectively, and this effect is associated with changes in BDNF levels within relevant brain regions.

Rapid Estrogenic and Androgenic Neurosteroids Effects in the Induction of Long-Term Synaptic Changes: Implication for Early Memory Formation

Mounting experimental evidence demonstrate that sex neuroactive steroids (neurosteroids) are essential for memory formation. Neurosteroids have a profound impact on the function and structure of neural circuits and their local synthesis is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission and for neural spine formation in different areas of the central nervous system (CNS). Several studies demonstrated that in the hippocampus, 17β-estradiol (E2) is necessary for inducing LTP, while 5α-dihydrotestosterone (DHT) is necessary for inducing LTD. This contribution has been proven by administering sex neurosteroids in rodent models and by using blocking agents of their synthesis or of their specific receptors. The general opposite role of sex neurosteroids in synaptic plasticity appears to be dependent on their different local availability in response to low or high frequency of synaptic stimulation, allowing the induction of bidirectional synaptic plasticity. The relevant contribution of these neurosteroids to synaptic plasticity has also been described in other brain regions involved in memory processes such as motor learning, as in the case of the vestibular nuclei, the cerebellum, and the basal ganglia, or as the emotional circuit of the amygdala. The rapid effects of sex neurosteroids on neural synaptic plasticity need the maintenance of a tonic or phasic local steroid synthesis determined by neural activity but might also be influenced by circulating hormones, age, and gender. To disclose the exact mechanisms how sex neurosteroids participate in finely tuning long-term synaptic changes and spine remodeling, further investigation is required.

Testicular hormones mediate robust sex differences in impulsive choice in rats

Impairments in choosing optimally between immediate and delayed rewards are associated with numerous psychiatric disorders. Such ‘intertemporal’ choice is influenced by genetic and experiential factors; however, the contributions of biological sex are understudied and data to date are largely inconclusive. Rats were used to determine how sex and gonadal hormones influence choices between small, immediate and large, delayed rewards. Females showed markedly greater preference than males for small, immediate over large, delayed rewards (greater impulsive choice). This difference was neither due to differences in food motivation or reward magnitude perception, nor was it affected by estrous cycle. Ovariectomies did not affect choice in females, whereas orchiectomies increased impulsive choice in males. These data show that male rats exhibit less impulsive choice than females and that this difference is at least partly maintained by testicular hormones. These differences in impulsive choice could be linked to gender differences across multiple neuropsychiatric conditions.

Risk-based decision making in rats: Modulation by sex and amphetamine

Decision-making is a complex process essential to daily adaptation in many species. Risk is an inherent aspect of decision-making and it is influenced by gonadal hormones. Testosterone and 17β-estradiol may modulate decision making and impact the mesocorticolimbic dopamine pathway. Here, we explored sex differences, the effect of gonadal hormones and the dopamine agonist amphetamine on risk-based decision making. Intact or gonadectomised (GDX) male and female rats underwent to a probabilistic discounting task. High and low doses of testosterone propionate (1.0 or 0.2 mg) and 17β-estradiol benzoate (0.3 μg) were administered to assess acute effects on risk-based decision making. After 3-days of washout period, intact and GDX rats received high or low (0.5 or 0.125 mg/kg) doses of amphetamine and re-tested in the probabilistic discounting task. Under baseline conditions, males made more risky choices during probability discounting compared to female rats, particularly in the lower probability blocks, but GDX did not influence risky choice. The high, but not the low dose, of testosterone modestly reduced risky decision making in GDX male rats. Conversely, 17β-estradiol had no significant effect on risky choice regardless of GDX status in either sex. Lastly, a higher dose of amphetamine increased risky decision making in both intact males and females, but had no effect in GDX rats. These findings demonstrated sex differences in risk-based decision making, with males showing a stronger bias toward larger, uncertain rewards. GDX status influenced the effects of amphetamine, suggesting different dopaminergic regulation in risk-based choices among males and females.

The challenge hypothesis revisited: Focus on reproductive experience and neural mechanisms

Our review focuses on findings from mammals as part of a Special Issue "30th Anniversary of the Challenge Hypothesis". Here we put forth an integration of the mechanisms through which testosterone controls territorial behavior and consider how reproductive experience may alter these mechanisms. The emphasis is placed on the function of socially induced increases in testosterone (T) pulses, which occur in response to social interactions, as elegantly developed by Wingfield and colleagues. We focus on findings from the monogamous California mouse, as data from this species shows that reproductive status is a key factor influencing social interactions, site fidelity, and vigilance for offspring defense. Specifically, we examine differences in T pulses in sexually naïve versus sexually experienced pair bonded males. Testosterone pulses influence processes such as social decision making, the winner-challenge effect, and location preferences through rewarding effects of T. We also consider how social and predatory vigilance contribute to T pulses and how these interactions contribute to a territory centered around maximizing reproduction. Possible underlying mechanisms for these effects include the nucleus accumbens (rewarding effects of testosterone), hippocampus (spatial memories for territories), and the bed nucleus of the stria terminalis (social vigilance). The development of the challenge effect has provided an ideal framework for understanding the complex network of behavioral, environmental, physiological and neural mechanisms that ultimately relates to competition and territoriality across taxa. The opportunity to merge research on the challenge effect using both laboratory and field research to understand social behavior is unparalleled.

Age-dependent effects of testosterone on spatial memory in male rats

Decreased spatial memory is common in aging populations and reduces their quality of life. Although its role is still controversial, low testosterone (T) may contribute to impaired cognition in aged men. This study aimed to identify the role of T in age-related deficiencies in spatial memory among male rats. Young adult (3 months old) and aged (21 months old) Wistar rats were assigned to independent groups: intact, orchidectomized, or orchidectomized + subcutaneous pellets of T propionate. The phases of spatial memory acquisition (4 daily trials/4 days) and spatial memory retention (1 trial/day, 3 and 12 days after acquisition) were evaluated using the Barnes maze. Compared with young adults, aged intact rats took longer to find the goal, made more mistakes, and showed only slight improvements in goal sector exploration across the acquisition period. The young orchidectomized rats showed no improvement in performance over the days during the acquisition phase. T treatment in hormonally deprived old rats produced a small improvement in goal sector exploration and number of errors during the acquisition phase. Meanwhile, in young adults, this treatment improved the goal sector searching in the retention phase (12 days after acquisition training). Our results suggested that age-related spatial memory deficits cannot be entirely explained by the decline in T levels; however, this androgen produced subtle and mild beneficial effects on spatial memory in young and old males. Taken together, our findings suggest age differences in the role of T on spatial memory in males.

Testosterone enhances mitochondrial complex V function in the substantia nigra of aged male rats

Deficits in coordinated motor behavior and mitochondrial complex V activity have been observed in aged males. Testosterone supplementation can improve coordinated motor behavior in aged males. We investigated the effects of testosterone supplementation on mitochondrial complex V function in the substantia nigra (a brain region that regulates motor activity) in aged male rats. These rats exhibited diminished ATP levels, attenuated mitochondrial complex V activity, and reduced expression of 3 of the 17 mitochondrial complex V subunits (ATP6, ATP8 and ATP5C1) in the substantia nigra. Testosterone supplementation increased ATP levels, mitochondrial complex V activity, and ATP6, ATP8 and ATP5C1 expression in the substantia nigra of the rats. Conversely, orchiectomy reduced mitochondrial complex V activity, downregulated ATP6 and ATP8 expression, and upregulated ATP5C1, ATP5I and ATP5L expression in the substantia nigra. Testosterone replacement reversed those effects. Thus, testosterone enhanced mitochondrial complex V function in the substantia nigra of aged male rats by upregulating ATP6 and ATP8. As potential testosterone targets, these two subunits may to some degree maintain nigrostriatal dopaminergic function in aged males.

Testosterone replacement causes dose-dependent improvements in spatial memory among aged male rats

Testosterone has been shown to have dose-dependent effects on spatial memory in males, but the effects of aging upon this relationship remain unclear. Additionally, the mechanism by which testosterone regulates memory is unknown, but may involve changes in brain-derived neurotrophic factor (BDNF) within specific brain regions. We tested the effects of age and testosterone on spatial memory among male rats using two spatial memory tasks: an object-location memory task (OLMT) and the radial-arm maze (RAM). Castration had minimal effect on performance on the RAM, but young rats (2 months) performed significantly fewer working memory errors than aged rats (20 months), and aged rats performed significantly fewer reference memory errors. Both age and castration impaired performance on the OLMT, with only the young rats with intact gonads successfully performing the task. Subsequent experiments involved daily injections of either drug vehicle or one of four doses of testosterone propionate (0.125, 0.250, 0.500, and 1.00 mg/rat) given to castrated aged males. On the RAM, a low physiological dose (0.125 mg) and high doses (0.500-1.000 mg) of testosterone improved working memory, while an intermediate dose (0.250 mg) did not. On the OLMT, only the 0.250 mg T group showed a significant increase in exploration ratios from the exposure trials to the testing trials, indicating that this group remembered the position of the objects. Brain tissue (prefrontal cortex, hippocampus, and striatum) was collected from all subjects to assay BDNF. We found no evidence that testosterone influenced BDNF, indicating that it is unlikely that testosterone regulates spatial memory through changes in BDNF levels.

Testosterone and Adult Neurogenesis

It is now well established that neurogenesis occurs throughout adulthood in select brain regions, but the functional significance of adult neurogenesis remains unclear. There is considerable evidence that steroid hormones modulate various stages of adult neurogenesis, and this review provides a focused summary of the effects of testosterone on adult neurogenesis. Initial evidence came from field studies with birds and wild rodent populations. Subsequent experiments with laboratory rodents have tested the effects of testosterone and its steroid metabolites upon adult neurogenesis, as well as the functional consequences of induced changes in neurogenesis. These experiments have provided clear evidence that testosterone increases adult neurogenesis within the dentate gyrus region of the hippocampus through an androgen-dependent pathway. Most evidence indicates that androgens selectively enhance the survival of newly generated neurons, while having little effect on cell proliferation. Whether this is a result of androgens acting directly on receptors of new neurons remains unclear, and indirect routes involving brain-derived neurotrophic factor (BDNF) and glucocorticoids may be involved. In vitro experiments suggest that testosterone has broad-ranging neuroprotective effects, which will be briefly reviewed. A better understanding of the effects of testosterone upon adult neurogenesis could shed light on neurological diseases that show sex differences.

Neutering in dogs and cats: current scientific evidence and importance of adequate nutritional management

Neutering or spaying is a commonly recommended veterinary procedure. However, veterinarians are often confronted with conflicting findings and differences in concepts regarding practice and proper nutritional management after the procedure. The objective of the present review was to bring to light the most recent literature, summarise it and discuss the findings focusing on the risks and benefits of neutering in dogs and cats, and to determine the appropriate nutritional management for these animals.

Activation of androgen receptors protects intact male mice from memory impairments caused by aromatase inhibition

Although 17β-estradiol (E₂) is known to regulate hippocampal function, the specific contributions of hippocampally-synthesized E₂ remain unclear. Infusion of the aromatase inhibitor letrozole into the dorsal hippocampus (DH) of ovariectomized mice disrupts object recognition and object placement memory consolidation, suggesting that DH-synthesized E₂ is essential for memory. However, the role of DH-synthesized E₂ in memory among male rodents is unknown. Here, we examined effects of aromatase inhibition on memory consolidation in male mice. Intact and gonadectomized mice were infused with vehicle or letrozole into the DH immediately post-training in object placement and object recognition tasks. Letrozole blocked memory in both tasks among gonadectomized males only, suggesting that circulating androgens, or a rise in hippocampal androgens due to aromatase inhibition, may support memory consolidation in intact males. To test this hypothesis, intact males were infused with the androgen receptor antagonist flutamide into the DH after object training. A dose-dependent impairment was observed in both tasks, indicating that blocking androgen signaling can impair memory consolidation. To test if hippocampal androgen receptor activation protected intact males from the impairing effects of letrozole, a non-impairing dose of flutamide was co-infused with letrozole. Co-administration of both drugs blocked object placement and object recognition memory consolidation, demonstrating that letrozole impairs memory in intact males only if androgen receptors are blocked. Together, these data suggest that DH-synthesized E₂ and androgen receptor activation may work in concert to mediate memory consolidation in intact males, such that androgen receptor activation protects against memory impairments caused by aromatase inhibition.

Sex differences in sleep and sleep loss-induced cognitive deficits: The influence of gonadal hormones

Males and females can respond differentially to the same environmental stimuli and experimental conditions. Chronic sleep loss is a frequent and growing problem in many modern societies and has a broad variety of negative outcomes for health and well-being. While much has been done to explore the deleterious effects of sleep deprivation (SD) on cognition in both human and animal studies over the last few decades, very little attention has been paid to the part played by sex differences and gonadal steroids in respect of changes in cognitive functions caused by sleep loss. The effects of gonadal hormones on sleep regulation and cognitive performances are well established. Reduced gonadal function in menopausal women and elderly men is associated with sleep disturbances and cognitive decline as well as dementia, which suggests that sex steroids play a key role in modulating these conditions. Finding out whether there are sex differences in respect of the effect of insufficient sleep on cognition, and how neuroendocrine mediators influence cognitive impairment induced by SD could provide valuable insights into the best therapies for each sex. In this review, we aim to highlight the involvement of sex differences and gonadal hormone status on the severity of cognitive deficits induced by sleep deficiency in both human and animal studies.