ALTERED SYNAPTIC TRANSMISSION IN THE HIPPOCAMPUS OF THE CASTRATED MALE MOUSE IS REVERSED BY TESTOSTERONE REPLACEMENT

Interplay between hippocampal TACR3 and systemic testosterone in regulating anxiety-associated synaptic plasticity

Tachykinin receptor 3 (TACR3) is a member of the tachykinin receptor family and falls within the rhodopsin subfamily. As a G protein-coupled receptor, it responds to neurokinin B (NKB), its high-affinity ligand. Dysfunctional TACR3 has been associated with pubertal failure and anxiety, yet the mechanisms underlying this remain unclear. Hence, we have investigated the relationship between TACR3 expression, anxiety, sex hormones, and synaptic plasticity in a rat model, which indicated that severe anxiety is linked to dampened TACR3 expression in the ventral hippocampus. TACR3 expression in female rats fluctuates during the estrous cycle, reflecting sensitivity to sex hormones. Indeed, in males, sexual development is associated with a substantial increase in hippocampal TACR3 expression, coinciding with elevated serum testosterone and a significant reduction in anxiety. TACR3 is predominantly expressed in the cell membrane, including the presynaptic compartment, and its modulation significantly influences synaptic activity. Inhibition of TACR3 activity provokes hyperactivation of CaMKII and enhanced AMPA receptor phosphorylation, associated with an increase in spine density. Using a multielectrode array, stronger cross-correlation of firing was evident among neurons following TACR3 inhibition, indicating enhanced connectivity. Deficient TACR3 activity in rats led to lower serum testosterone levels, as well as increased spine density and impaired long-term potentiation (LTP) in the dentate gyrus. Remarkably, aberrant expression of functional TACR3 in spines results in spine shrinkage and pruning, while expression of defective TACR3 increases spine density, size, and the magnitude of cross-correlation. The firing pattern in response to LTP induction was inadequate in neurons expressing defective TACR3, which could be rectified by treatment with testosterone. In conclusion, our study provides valuable insights into the intricate interplay between TACR3, sex hormones, anxiety, and synaptic plasticity. These findings highlight potential targets for therapeutic interventions to alleviate anxiety in individuals with TACR3 dysfunction and the implications of TACR3 in anxiety-related neural changes provide an avenue for future research in the field.

The role of puberty on physical and brain development: A longitudinal study in male Rhesus Macaques

This study examined the role of male pubertal maturation on physical growth and development of neurocircuits that regulate stress, emotional and cognitive control using a translational nonhuman primate model. We collected longitudinal data from male macaques between pre- and peri-puberty, including measures of physical growth, pubertal maturation (testicular volume, blood testosterone -T- concentrations) and brain structural and resting-state functional MRI scans to examine developmental changes in amygdala (AMY), hippocampus (HIPPO), prefrontal cortex (PFC), as well as functional connectivity (FC) between those regions. Physical growth and pubertal measures increased from pre- to peri-puberty. The indexes of pubertal maturation -testicular size and T- were correlated at peri-puberty, but not at pre-puberty (23 months). Our findings also showed ICV, AMY, HIPPO and total PFC volumetric growth, but with region-specific changes in PFC. Surprisingly, FC in these neural circuits only showed developmental changes from pre- to peri-puberty for HIPPO-orbitofrontal FC. Finally, testicular size was a better predictor of brain structural maturation than T levels -suggesting gonadal hormones-independent mechanisms-, whereas T was a strong predictor of functional connectivity development. We expect that these neural circuits will show more drastic pubertal-dependent maturation, including stronger associations with pubertal measures later, during and after male puberty.

Gender and Neurosteroids: Implications for Brain Function, Neuroplasticity and Rehabilitation

Neurosteroids are synthesized de novo in the nervous system; they mainly moderate neuronal excitability, and reach target cells via the extracellular pathway. The synthesis of neurosteroids occurs in peripheral tissues such as gonads tissues, liver, and skin; then, because of their high lipophilia, they cross the blood-brain barrier and are stored in the brain structure. Neurosteroidogenesis occurs in brain regions such as the cortex, hippocampus, and amygdala by enzymes necessary for the in situ synthesis of progesterone from cholesterol. Neurosteroids could be considered the main players in both sexual steroid-induced hippocampal synaptic plasticity and normal transmission in the hippocampus. Moreover, they show a double function of increasing spine density and enhancing long term potentiation, and have been related to the memory-enhancing effects of sexual steroids. Estrogen and progesterone affect neuronal plasticity differently in males and females, especially regarding changes in the structure and function of neurons in different regions of the brain. Estradiol administration in postmenopausal women allowed for improving cognitive performance, and the combination with aerobic motor exercise seems to enhance this effect. The paired association between rehabilitation and neurosteroids treatment could provide a boosting effect in order to promote neuroplasticity and therefore functional recovery in neurological patients. The aim of this review is to investigate the mechanisms of action of neurosteroids as well as their sex-dependent differences in brain function and their role in neuroplasticity and rehabilitation.

Effects of prenatal testosterone exposure on the development of autism-like behaviours in offspring of Wistar rats

BACKGROUND

A neurodevelopmental disease, autism spectrum disorder (ASD) occurs in males three times more commonly than girls. Higher prenatal testosterone exposure may result in autistic-like behaviour in boys, according to earlier research. It is unclear how fetal testosterone affects the development of autism. In this study, we tested the hypothesis that prenatal testosterone exposure in an animal model may result in autistic behaviours by modifying serotonin, dopamine, IGF-1 and oxytocin levels.

MATERIALS AND METHODS

Group 1 (control, n = 6) and Group 2 (testosterone undecanoate, n = 6) of female rats were randomly assigned. For 2-3 days during the oestrus cycle, female rats were housed with a reproductive male (three females/one male). On the 10th day of gestation, rats in Group 1 received 1 ml/kg% 0.9 NaCl saline, whereas rats in Group 2 received 250 mg/kg testosterone undecanoate. Until weaning on postnatal day 21 (P21), the mothers were permitted to care for their pups. On P21, 40 littermates-10 male and female for control and 10 male and female from mothers that exposed to testosterone-were arbitrarily split up and housed. On P50, these mature rats were tested for their behaviour. The rats were then sacrificed. The brain tissue was subjected to histological examinations as well as biochemical tests for homovanillic acid (HVA), 5-Hydroxyindoleacetic acid (5-HIAA), oxytocin and insulin-like growth factor-1 (IGF-1).

RESULTS

The groups differed significantly in the behavioural examinations (three-chamber social test, passive avoidance learning analysis, open field test), with the testosterone-exposed groups exhibiting autistic symptoms to a higher extent. When compared with the control groups, testosterone exposure caused significant histological changes in the hippocampus CA1 and CA3 areas, including gliosis and cell death of neurons. In the testosterone-exposed groups, HVA, 5-HIAA and IGF-1 tissue expressions in the brain elevated, whereas oxytocin levels reduced. These findings point to a potential connection between neurodevelopmental disorders like ASD and exposure to testosterone during gestation.

CONCLUSION

Overall, we revealed that prenatal testosterone exposure led to autistic traits by elevating serotonin, dopamine and IGF-1 levels while lowering oxytocin levels.

Androgen Effects on Neural Plasticity

Androgens are synthesized in the brain, gonads, and adrenal glands, in both sexes, exerting physiologically important effects on the structure and function of the central nervous system. These effects may contribute to the incidence and progression of neurological disorders such as autism spectrum disorder, schizophrenia, and Alzheimer's disease, which occur at different rates in males and females. This review briefly summarizes the current state of knowledge with respect to the neuroplastic effects of androgens, with particular emphasis on the hippocampus, which has been the focus of much of the research in this field.

Associations among gonadal hormone, triglycerides and cognitive decline in female patients with major depressive disorders

BACKGROUND

Cognitive impairment has been identified as a core feature of depression. Serum triglycerides (TG), gonadal hormone and sex difference were shown to influence cognitive performance. The purpose of this study was to investigate the associations among serum TG, gonadal hormone, sex difference and cognitive performance in patients with major depressive disorders (MDD).

METHODS

The enrolled 183 patients (male/female = 80/103) meeting DSM-IV criteria for MDD were divided into high TG group (patients-HTG) and normal TG group (patients-NTG) according to TG level. Serum TG, estradiol (E2) and testosterone (T) levels were measured by the glycerokinase peroxidase-peroxidase and chemiluminescence methods. Cognition was assessed by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). The study was conducted between August 2016 and January 2020.

RESULTS

In female, patients-HTG had lower immediate memory, language, attention, delayed memory and RBANS total scores than patients-NTG after adjusting for covariates. There were significant differences in serum E2 and T levels between patients-HTG and patients-NTG in female after controlling for covariates. In female patients-HTG, serum E2 level was positively associated with immediate memory, delayed memory and RBANS total scores, and serum T level was positively related to immediate memory, language and RBANS total scores. These findings were not seen in male patients.

CONCLUSIONS

Our data suggested that patients-HTG exhibited poorer cognitive function compared with patients-NTG in female. Moreover, the decline in serum gonadal hormone level might contribute to the high TG development of female MDD, and was further implicated in their cognitive decline.

Testosterone deprivation intensifies cognitive decline in obese male rats via glial hyperactivity, increased oxidative stress, and apoptosis in both hippocampus and cortex

AIM

The study hypothesized that testosterone deprivation aggravates cognitive decline in obesity through increasing oxidative stress, glial activation, and apoptosis.

METHODS

Male Wistar rats (n = 24) were fed with either normal-diet (ND) or high-fat diet (HFD) for 24 weeks. At week 13, ND-fed rats and HFD-fed rats were randomly assigned to two subgroups to receive either a sham-operation or bilateral-orchiectomy (ORX). Rats were evaluated for metabolic parameters and cognition at 4, 8, and 12 weeks after the operation. At the end of protocol, the reactive oxygen species (ROS), glial morphology, and cell apoptosis were determined in hippocampus and cortex.

RESULTS

Both HFD-fed groups developed obese-insulin resistance, but ND-fed rats did not. HFD-fed rats with sham-operation showed cognitive decline, when compared to ND-fed rats with sham-operation at all time points. At 4- and 8-week after ORX, the cognitive impairment of ND-fed rats and both HFD-fed groups was not different. However, 12-week after ORX, cognitive decline and of glial hyperactivity of HFD-fed rats had the greatest increase among all groups. Hippocampal ROS levels and apoptotic cells in both HFD-fed groups were equally increased, but the cortical ROS levels and apoptotic cells of HFD-fed rats with ORX were the highest ones.

CONCLUSIONS

These findings suggest that testosterone deprivation aggravates cognitive decline in obesity via increasing oxidative stress, glial activity and apoptosis.

Comparative effects of sex hormone deprivation on the brain of insulin-resistant rats

Obese-insulin resistance following chronic high-fat diet consumption led to cognitive decline through several mechanisms. Moreover, sex hormone deprivation, including estrogen and testosterone, could be a causative factor in inducing cognitive decline. However, comparative studies on the effects of hormone-deprivation on the brain are still lacking. Adult Wistar rats from both genders were conducted sham operations or orchiectomies/ovariectomies and given a normal diet or high-fat diet for 4, 8, and 12 weeks. Blood was collected to determine the metabolic parameters. At the end of the experiments, rats were decapitated and their brains were collected to determine brain mitochondrial function, brain oxidative stress, hippocampal plasticity, insulin-induced long-term depression, dendritic spine density, and cognition. We found that male and female rats fed a high-fat diet developed obese-insulin resistance by week 8 and brain defects via elevated brain oxidative stress, brain mitochondrial dysfunction, impaired insulin-induced long-term depression, hippocampal dysplasticity, reduced dendritic spine density, and cognitive decline by week 12. In normal diet-fed rats, estrogen-deprivation, not testosterone-deprivation, induced obese-insulin resistance, oxidative stress, brain mitochondrial dysfunction, impaired insulin-induced long-term depression, hippocampal dysplasticity, and reduced dendritic spine density. In high-fat-diet-fed rats, estrogen deprivation, not testosterone-deprivation, accelerated and aggravated obese-insulin resistance and brain defects at week 8. In conclusion, estrogen deprivation aggravates brain dysfunction more than testosterone deprivation through increased oxidative stress, brain mitochondrial dysfunction, impaired insulin-induced long-term depression, and dendritic spine reduction. These findings may explain clinical reports which show more severe cognitive decline in aging females than males with obese-insulin resistance.

Prepubertal castration-associated developmental changes in sigma-1 receptor gene expression levels regulate hippocampus area CA1 activity during adolescence

The functional relevance of sigma-1 (σ1 ) receptor expression in the rat hippocampal CA1 during adolescence (i.e., 35-60 days old) was explored. A selective antagonist for the σ1 receptor subtype, BD-1047, was applied to study hippocampal long-term potentiation (LTP) and spatial learning performance. Changes in the expression of the σ1 receptor subtype and its function were compared between castrated and sham-castrated rats. Castration reduced the magnitude of both field excitatory postsynaptic potential (fEPSP)-LTP and population spike (PS)-LTP at 35 days (d). BD-1047 decreased PS-LTP in sham-castrated rats, whereas BD-1047 reversed the effect of castration on fEPSP-LTP at 35 d. In addition, BD1047 impaired spatial learning and augmented σ1 receptor mRNA levels in castrated rats at 35 d. Surprisingly, neither castration nor BD1047 had an effect on fEPSP-LTP and PS-LTP, spatial learning ability or gene expression levels at 45 d. Castration had no effect on fEPSP-LTP but reduced PS-LTP at 60 d. BD1047 increased the magnitude of fEPSP-LTP, but had no effect on PS-LTP in castrated rats at 60 d. However, BD1047 reduced spatial learning ability, and σ1 receptor mRNA levels were decreased in castrated rats at 60 d. This study shows that σ1 receptors play a role in the regulation of both CA1 synaptic efficacy and spatial learning performance. The regulatory role of σ1 receptors in activity-dependent CA1-LTP is locality- and age-dependent, whereas its role in spatial learning ability is only age-dependent. Prepubertal castration-associated changes in the expression and function of the σ1 receptor during adolescence may play a developmental role in the regulation of hippocampal area CA1 activity and plasticity. © 2016 Wiley Periodicals, Inc.

Testosterone deprivation has neither additive nor synergistic effects with obesity on the cognitive impairment in orchiectomized and/or obese male rats

OBJECTIVE

Previous studies demonstrated a correlation between cognitive decline and either testosterone deprivation or obesity. However, the effect of obesity combined with testosterone deprivation on cognitive function has not been investigated. This study investigated the effects of obesity on brain insulin sensitivity, brain mitochondrial function, hippocampal synaptic plasticity and cognitive function in testosterone-deprived male rats.

MATERIALS/METHODS

Male Wistar rats were divided into sham-operated (control) and bilateral orchiectomized (ORX) groups. Rats in each group were further divided into two subgroups to receive either a normal diet (ND) or a high fat diet (HFD) for 4, 8 or 12weeks. Blood samples were collected to determine metabolic parameters. Cognitive function was tested using the Morris Water Maze Test. At the end of the study, brains were removed to investigate brain insulin sensitivity, brain mitochondrial function and hippocampal synaptic plasticity.

RESULTS

Both control-obese and ORX-obese rats developed peripheral insulin resistance at week eight, and brain insulin resistance as well as brain mitochondrial dysfunction at week 12. However, the ORX-obese rats developed cognitive impairment and decreased hippocampal synaptic plasticity beginning at week eight, whereas the control-obese rats developed these impairments later at week 12. Although both peripheral and brain insulin resistance were not observed in both the control-lean and ORX-lean rats, impaired cognition and decreased hippocampal synaptic plasticity were found in the ORX-lean rats beginning at week eight.

CONCLUSION

These findings suggest that testosterone deprivation has neither additive nor synergistic effects over obesity in the development of cognitive dysfunction in orchiectomized-obese male rats.

Testosterone replacement attenuates cognitive decline in testosterone-deprived lean rats, but not in obese rats, by mitigating brain oxidative stress

Testosterone replacement improves metabolic parameters and cognitive function in hypogonadism. However, the effects of testosterone therapy on cognition in obese condition with testosterone deprivation have not been investigated. We hypothesized that testosterone replacement improves cognitive function in testosterone-deprived obese rats by restoring brain insulin sensitivity, brain mitochondrial function, and hippocampal synaptic plasticity. Thirty male Wistar rats had either a bilateral orchiectomy (ORX: O, n = 24) or a sham operation (S, n = 6). ORX rats were further divided into two groups fed with either a normal diet (NDO) or a high-fat diet (HFO) for 12 weeks. Then, ORX rats in each dietary group were divided into two subgroups (n = 6/subgroup) and were given either castor oil or testosterone (2 mg/kg/day, s.c.) for 4 weeks. At the end of this protocol, cognitive function, metabolic parameters, brain insulin sensitivity, hippocampal synaptic plasticity, and brain mitochondrial function were determined. We found that testosterone replacement increased peripheral insulin sensitivity, decreased circulation and brain oxidative stress levels, and attenuated brain mitochondrial ROS production in HFO rats. However, testosterone failed to restore hippocampal synaptic plasticity and cognitive function in HFO rats. In contrast, in NDO rats, testosterone decreased circulation and brain oxidative stress levels, attenuated brain mitochondrial ROS production, and restored hippocampal synaptic plasticity as well as cognitive function. These findings suggest that testosterone replacement improved peripheral insulin sensitivity and decreased oxidative stress levels, but failed to restore hippocampal synaptic plasticity and cognitive function in testosterone-deprived obese rats. However, it provided beneficial effects in reversing cognitive impairment in testosterone-deprived non-obese rats.

Testosterone deficiency, insulin-resistant obesity and cognitive function

Testosterone is an androgenic steroid hormone, which plays an important role in the regulation of male reproduction and behaviors, as well as in the maintenance of insulin sensitivity. Several studies showed that testosterone exerted beneficial effects in brain function, including preventing neuronal cell death, balancing brain oxidative stress and antioxidant activity, improving synaptic plasticity and involving cognitive formation. Although previous studies showed that testosterone deficiency is positively correlated with cognitive impairment and insulin-resistant obesity, several studies demonstrated contradictory findings. Thus, this review comprehensively summarizes the current evidence from in vitro, in vivo and clinical studies of the relationship between testosterone deficiency and insulin-resistant obesity as well as the correlation between either insulin-resistant obesity or testosterone deficiency and cognitive impairment. Controversial reports and the mechanistic insights regarding the roles of testosterone in insulin-resistant obesity and cognitive function are also presented and discussed.

Dipeptidyl peptidase 4 inhibitor improves brain insulin sensitivity, but fails to prevent cognitive impairment in orchiectomy obese rats

It is unclear whether the dipeptidyl peptidase 4 (DPP4) inhibitor can counteract brain insulin resistance, brain mitochondrial dysfunction, impairment of hippocampal synaptic plasticity and cognitive decline in testosterone-deprived obese rats. We hypothesized that DPP4 inhibitor vildagliptin improves cognitive function in testosterone-deprived obese rats by restoring brain insulin sensitivity, brain mitochondrial function and hippocampal synaptic plasticity. Thirty male Wistar rats received either a sham-operated (S, n=6) or bilateral orchiectomy (ORX, n=24). ORX rats were divided into two groups and fed with either a normal diet (ND (NDO)) or a high-fat diet (HFO) for 12 weeks. Then, ORX rats in each dietary group were divided into two subgroups (n=6/subgroup) to receive either a vehicle or vildagliptin (3 mg/kg per day, p.o.) for 4 weeks. After treatment, cognitive function, metabolic parameters, brain insulin sensitivity, hippocampal synaptic plasticity and brain mitochondrial function were determined in each rat. We found that HFO rats exhibited peripheral and brain insulin resistance, brain mitochondrial dysfunction, impaired hippocampal synaptic plasticity and cognitive decline. NDO rats did not develop peripheral and brain insulin resistance. However, impaired hippocampal synaptic plasticity and cognitive decline occurred. Vildagliptin significantly improved peripheral insulin sensitivity, restored brain insulin sensitivity and decreased brain mitochondrial reactive oxygen species production in HFO rats. However, vildagliptin did not restore hippocampal synaptic plasticity and cognitive function in both NDO and HFO rats. These findings suggest that vildagliptin could not counteract the impairment of hippocampal synaptic plasticity and cognitive decline in testosterone-deprived subjects, despite its effects on improved peripheral and brain insulin sensitivity as well as brain mitochondrial function.

Sexual neurosteroids and synaptic plasticity in the hippocampus

Sexual neurosteroids (SN), namely 17β-estradiol (E2) and 5α-dehydrotestosterone (DHT), are synthesized in the hippocampus, where they induce circuit modifications by changing the number of excitatory spine synapses in a paracrine and sex-specific manner. The mechanisms of this sex-specific synapse turnover, which are likely to affect cognitive functions, are poorly understood. We found that hippocampal neurons synthesize estradiol, which maintains LTP and synapses in females but not in males. In females, inhibition of estradiol synthesis results in impairment of LTP and synapse loss. These effects were not seen in males. The essential role of local estrogen on the stability and maintenance of connectivity in the hippocampus is consistent with age-related cognitive decline in women after menopause. In male animals the regulation of synaptic stability and plasticity by locally synthesized sexual steroids remains to be clarified. This article is part of a Special Issue entitled SI: Brain and Memory.

Pathway based analysis of genes and interactions influencing porcine testis samples from boars with divergent androstenone content in back fat

One of the primary factors contributing to boar taint is the level of androstenone in porcine adipose tissues. A majority of the studies performed to identify candidate biomarkers for the synthesis of androstenone in testis tissues follow a reductionist approach, identifying and studying the effect of biomarkers individually. Although these studies provide detailed information on individual biomarkers, a global picture of changes in metabolic pathways that lead to the difference in androstenone synthesis is still missing. The aim of this work was to identify major pathways and interactions influencing steroid hormone synthesis and androstenone biosynthesis using an integrative approach to provide a bird's eye view of the factors causing difference in steroidogenesis and androstenone biosynthesis. For this purpose, we followed an analysis procedure merging together gene expression data from boars with divergent levels of androstenone and pathway mapping and interaction network retrieved from KEGG database. The interaction networks were weighted with Pearson correlation coefficients calculated from gene expression data and significant interactions and enriched pathways were identified based on these networks. The results show that 1,023 interactions were significant for high and low androstenone animals and that a total of 92 pathways were enriched for significant interactions. Although published articles show that a number of these enriched pathways were activated as a result of downstream signaling of steroid hormones, we speculate that the significant interactions in pathways such as glutathione metabolism, sphingolipid metabolism, fatty acid metabolism and significant interactions in cAMP-PKA/PKC signaling might be the key factors determining the difference in steroidogenesis and androstenone biosynthesis between boars with divergent androstenone levels in our study. The results and assumptions presented in this study are from an in-silico analysis done at the gene expression level and further laboratory experiments at genomic, proteomic or metabolomic level are necessary to validate these findings.

Pre-pubertal castration improves spatial learning during mid-adolescence in rats

Hippocampus functions, including spatial cognition and stress responses, mature during adolescence. In addition, hippocampus neuronal structures are modified by circulating sex steroids, which dramatically increase during adolescence. Therefore, the effects of castration and the circulating levels of the main sex steroid testosterone on spatial learning and memory were examined across postnatal ages to test whether pre-pubertal castration affected rats' spatial ability in the Morris Water maze (MWM). Male rats were either castrated or sham-castrated at 22d (days of age), or left gonadally intact. They were then trained and tested in the MWM beginning at 28d, 35d, 45d or 60d. We found that all of the intact rats learned the spatial task; however, the males at 22d and 28d required more trials to acquire the task than the males at older ages. The males castrated at 22d and tested at 35d had significantly lower escape latency and traveled distance during training than the sham-castrated males trained at the same age. No differences were observed in mean values of escape latency and traveled distance at 45d even though they had comparable levels of testosterone. We conclude that adult-typical performance for male spatial memory emerges during mid-adolescence and that pre-pubertal castration appears to improve spatial learning during this time.

Mice overexpressing type 1 adenylyl cyclase show enhanced spatial memory flexibility in the absence of intact synaptic long-term depression

There is significant interest in understanding the contribution of intracellular signaling and synaptic substrates to memory flexibility, which involves new learning and suppression of obsolete memory. Here, we report that enhancement of Ca²⁺-stimulated cAMP signaling by overexpressing type 1 adenylyl cyclase (AC1) facilitated long-term potentiation (LTP) but impaired long-term depression (LTD) at the hippocampal Shaffer collateral-CA1 synapses. However, following the induction of LTP, low-frequency stimulation caused comparable synaptic depotentiation in both wild type and AC1 transgenic (AC1 TG) mice. Although previous studies have suggested the function of LTD in spatial memory flexibility, AC1 TG mice showed not only better initial learning in the Morris water maze, but also faster acquisition and increased ratio of new memory formation to old memory retention during the reversal platform training. In the memory extinction test, which requires suppression of old memory without involving the acquisition of the new platform information, AC1 TG and wild type mice showed comparable performance. Our results demonstrate new functions of Ca²⁺-stimulated AC1, and also suggest that certain aspects of hippocampus-dependent behavioral flexibility may not require intact LTD.

Testosterone modulates spatial recognition memory in male rats

A growing body of research indicates that testosterone influences spatial cognition in male rats; however, the overwhelming majority of studies have been conducted on tasks motivated by either food deprivation or water escape. The hippocampus-dependent version of the Y-maze task, which characterizes spatial recognition memory, capitalizes on the propensity of rats to gravitate toward novel spatial environments and is not contingent upon either appetite or the stress associated with water escape, two factors also affected by testosterone. Accordingly, the aim of the current study was to examine the effects of orchidectomy and subsequent testosterone treatment on spatial recognition memory. Orchidectomy did not impact spatial recognition memory when the delay between the information and retention trials of the Y-maze task was 24h. Alternatively, on the second Y-maze task, which featured a 48-h delay between trials, orchidectomy reduced, and treatments that produced higher levels of testosterone restored, preference for the arm associated with the novel spatial environment. Importantly, there were no differences in activity levels as a function of orchidectomy or testosterone treatment on either of the two tasks. Consistent with previous findings, orchidectomy attenuated, and testosterone treatment restored, both body weight gain and the relative weight of the androgen-sensitive ischiocavernosus muscle, which confirmed the efficacy of orchidectomy and testosterone treatments on physiological outcomes. Therefore, testosterone influenced spatial cognition on a task that minimized the influence of non-mnemonic factors and took advantage of the innate preference of rodents to seek out novel spatial environments.

Photoperiod-mediated impairment of long-term potention and learning and memory in male white-footed mice

Adult mammalian brains are capable of some structural plasticity. Although the basic cellular mechanisms underlying learning and memory are being revealed, extrinsic factors contributing to this plasticity remain unspecified. White-footed mice (Peromyscus leucopus) are particularly well suited to investigate brain plasticity because they show marked seasonal changes in structure and function of the hippocampus induced by a distinct environmental signal, viz., photoperiod (i.e. the number of hours of light/day). Compared to animals maintained in 16 h of light/day, exposure to 8 h of light/day for 10 weeks induces several phenotypic changes in P. leucopus, including reduction in brain mass and hippocampal volume. To investigate the functional consequences of reduced hippocampal size, we examined the effects of photoperiod on spatial learning and memory in the Barnes maze, and on long-term potentiation (LTP) in the hippocampus, a leading candidate for a synaptic mechanism underlying spatial learning and memory in rodents. Exposure to short days for 10 weeks decreased LTP in the Schaffer collateral-CA1 pathway of the hippocampus and impaired spatial learning and memory ability in the Barnes maze. Taken together, these results demonstrate a functional change in the hippocampus in male white-footed mice induced by day length.

Influence of testosterone on synaptic transmission in the rat medial vestibular nuclei: estrogenic and androgenic effects

In brainstem slices of young male rat, we investigated the influence of the neuroactive steroid testosterone (T) on the synaptic responses by analyzing the field potential evoked in the medial vestibular nucleus (MVN) by vestibular afferent stimulation. T induced three distinct and independent long-term synaptic changes: fast long-lasting potentiation (fLP), slow long-lasting potentiation (sLP) and long-lasting depression (LD). The fLP was mediated by 17β-estradiol (E(2)) since it was abolished by blocking the estrogen receptors (ERs) or the enzyme converting T to E(2). Conversely, sLP and LD were mediated by 5α-dihydrotestosterone (DHT) since they were prevented by blocking the androgen receptors (ARs) or the enzyme converting T to DHT. Therefore, the synaptic effects of T were mediated by its androgenic or estrogenic metabolites. The pathways leading to estrogenic and androgenic conversion of T might be co-localized since, the occurrence of fLP under block of androgenic pathway, and that of sLP and LD under estrogenic block, were higher than those observed without blocks. In case of co-localization, the effect on synaptic transmission should depend on the prevailing enzymatic activity. We conclude that circulating and neuronal T can remarkably influence synaptic responses of the vestibular neurons in different and opposite ways, depending on its conversion to estrogenic or androgenic metabolites.

A sexual dimorphism in intrathymic sialylation survey is revealed by the trans-sialidase from Trypanosoma cruzi

Sialylation is emerging as an important issue in developing thymocytes and is considered among the most significant cell surface modifications, although its physiologic relevance is far from being completely understood. It is regulated by the concerted expression of sialyl transferases along thymocyte development. After in vivo administration of trans-sialidase, a virulence factor from the American trypanosomatid Trypanosoma cruzi that directly transfers the sialyl residue among macromolecules, we found that the alteration of the sialylation pattern induces thymocyte apoptosis inside the "nurse cell complex." This suggests a glycosylation survey in the development of the T cell compartment. In this study, we report that this thymocyte apoptosis mechanism requires the presence of androgens. No increment in apoptosis was recorded after trans-sialidase administration in females or in antiandrogen-treated, gonadectomized, or androgen receptor mutant male mice. The androgen receptor presence was required only in the thymic epithelial cells as determined by bone marrow chimeric mouse approaches. The presence of the CD43 surface mucin, a molecule with a still undefined function in thymocytes, was another absolute requirement. The trans-sialidase-induced apoptosis proceeds through the TNF-alpha receptor 1 deathly signaling leading to the activation of the caspase 3. Accordingly, the production of the cytokine was increased in thymocytes. The ability of males to delete thymocytes altered in their sialylation pattern reveals a sexual dimorphism in the glycosylation survey during the development of the T cell compartment that might be related to the known differences in the immune response among sexes.

Genomic and nongenomic effects of intrahippocampal microinjection of testosterone on long-term memory in male adult rats

In addition to their well-known genomic effects via intracellular receptors, androgens rapidly alter neuronal excitability through a nongenomic pathway. The nongenomic effect of testosterone, as the main androgen, apart from its traditional effects, was assessed in one of the fundamental centers of learning and memory, the hippocampus, on long-term memory (LTM) in passive avoidance conditioning. Different doses of testosterone enanthate (T) or testosterone-BSA (T-BSA) bilaterally were injected into the CA1 region of the hippocampus 15 min before shock delivery (1 mA during 5 s) in a two-compartment passive avoidance apparatus. After 24 h, animals were tested for passive avoidance retrieval. Bilateral injection of 20 microg T or 55 microg T-BSA into the CA1 significantly decreases step-through latency. Therefore, it seems that testosterone can impair LTM in passive avoidance conditioning both via intracellular receptors and through nongenomic pathway.

Heterotopic graft of infant rat brain as an ischemic model for prolonged whole-brain ischemia

By using a heterotopic brain graft model, we have made histological and electrophysiological studies of the infant rat brain after prolonged ischemia. An infant rat head which had undergone ischemia for more than 90 min, was grafted onto an adult rat by anastomosing the thoracic vessels to the femoral vessels of the host rat. Histological and histochemical studies carried out 10 days after the operation showed that the development of the hippocampus and cerebellum in the grafted brain appeared to be normal. Interneuron growth in the hippocampus and migration of the granule cells in the cerebellum had occurred to a similar extent as in control rats. Extracellular recordings in the hippocampus showed normal characteristics of the postsynaptic potentials including long-term potentiation. This heterotopic graft model would be useful for studying brain function after long periods of ischemia.

N-cadherin is regulated by gonadal steroids in the adult hippocampus

In the adult hippocampus, gonadal steroids induce neural remodeling through cellular and molecular mechanisms that are largely unknown. The calcium-dependent cell adhesion molecule N-cadherin, which participates in the developmental organization of the nervous system, has recently been localized to hippocampal synapses and is suspected to participate in adult synaptic physiology. Little is currently known about the regulation of cadherins in the adult central nervous system, although posttranslational modifications are thought to account for variability in N-cadherin expression levels. To evaluate the possibility that gonadal steroids regulate N-cadherin in the adult hippocampus, we examined hippocampal N-cadherin mRNA levels and protein expression in castrated adult male rats treated with testosterone, or its metabolites 17beta-estradiol or dihydrotestosterone. Northern blot analysis indicated increased hippocampal N-cadherin mRNA levels in the adult rat hippocampus after treatment with 17beta-estradiol but not testosterone or dihydrotestosterone. Increased N-cadherin immunoreactivity was observed in CA1 and CA3 pyramidal cells after 17beta-estradiol treatment. Additionally, both 17beta-estradiol and testosterone treatment increased N-cadherin immunoreactivity in the neuropil of the stratum lacunosum-moleculare, which includes apical dendrites from pyramidal cells. In contrast, dihydrotestosterone treatment had no effect on levels of N-cadherin protein expression in CA1 or CA3 pyramidal cells or in the stratum lacunosum-moleculare. These results demonstrate that, in the hippocampus, expression levels of N-cadherin are dynamic in adulthood. To our knowledge, there have been no other demonstrations of steroid regulation of cadherin expression in neural populations. These results suggest a possible adhesive mechanism for steroid-induced plasticity of the adult nervous system.