On again, off again effects of gonadectomy on the acoustic startle reflex in adult male rats

Task-specific effects of biological sex and sex hormones on object recognition memories in a 6-hydroxydopamine-lesion model of Parkinson's disease in adult male and female rats

Many patients with Parkinson's disease (PD) experience cognitive or memory impairments with few therapeutic options available to mitigate them. This has fueled interest in determining how factors including sex and sex hormones modulate higher order function in this disease. The objective of this study was to use the Novel Object Recognition (NOR) and Object-in-Place (OiP) paradigms to compare the effects of a bilateral neostriatal 6-hydroxydopamine (6-OHDA) lesion model of PD in gonadally intact male and female rats, in orchidectomized male rats and in orchidectomized males supplemented with 17β-estradiol or testosterone propionate on measures of recognition memory similar to those at risk in PD. These studies showed that 6-ODHA lesions impaired discrimination in both tasks in males but not females. Further, 6-OHDA lesions disrupted NOR performance similarly in all males regardless of whether they were gonadally intact, orchidectomized or hormone-supplemented. In contrast, OiP performance was disrupted in males that were orchidectomized or 6-OHDA-lesioned but was spared in orchidectomized and orchidectomized, 6-OHDA lesioned males supplemented with 17β-estradiol. The distinct effects that sex and/or sex hormones have on 6-OHDA lesion-induced NOR vs. OiP deficits identified here also differ from corresponding impacts recently described for 6-OHDA lesion-induced deficits in spatial working memory and episodic memory. Together, the collective data provide strong evidence for effects of sex and sex hormones on cognition and memory in PD as being behavioral task and behavioral domain specific. This specificity could explain why a cohesive clinical picture of endocrine impacts on higher order function in PD has remained elusive.

Biological Sex and Sex Hormone Impacts on Deficits in Episodic-Like Memory in a Rat Model of Early, Pre-motor Stages of Parkinson's Disease

Episodic memory deficits are among the earliest appearing and most commonly occurring examples of cognitive impairment in Parkinson's disease (PD). These enduring features can also predict a clinical course of rapid motor decline, significant cognitive deterioration, and the development of PD-related dementia. The lack of effective means to treat these deficits underscores the need to better understand their neurobiological bases. The prominent sex differences that characterize episodic memory in health, aging and in schizophrenia and Alzheimer's disease suggest that neuroendocrine factors may also influence episodic memory dysfunction in PD. However, while sex differences have been well-documented for many facets of PD, sex differences in, and sex hormone influences on associated episodic memory impairments have been less extensively studied and have never been examined in preclinical PD models. Accordingly, we paired bilateral neostriatal 6-hydroxydopamine (6-OHDA) lesions with behavioral testing using the What-Where-When Episodic-Like Memory (ELM) Task in adult rats to first determine whether episodic-like memory is impaired in this model. We further compared outcomes in gonadally intact female and male subjects, and in male rats that had undergone gonadectomy—with and without hormone replacement, to determine whether biological sex and/or sex hormones influenced the expression of dopamine lesioned-induced memory deficits. These studies showed that 6-OHDA lesions profoundly impaired recall for all memory domains in male and female rats. They also showed that in males, circulating gonadal hormones powerfully modulated the negative impacts of 6-OHDA lesions on What, Where, and When discriminations in domain-specific ways. Specifically, the absence of androgens was shown to fully attenuate 6-OHDA lesion-induced deficits in ELM for “Where” and to partially protect against lesion-induced deficits in ELM for “What.” In sum, these findings show that 6-OHDA lesions in rats recapitulate the vulnerability of episodic memory seen in early PD. Together with similar evidence recently obtained for spatial working memory, the present findings also showed that diminished androgen levels provide powerful, highly selective protections against the harmful effects that 6-OHDA lesions have on memory functions in male rats.

Domain-specific contributions of biological sex and sex hormones to what, where and when components of episodic-like memory in adult rats

Episodic memory involves the integration and recall of discrete events that include information about what happened, where it happened and when it occurred. Episodic memory function is critical to daily life, and its dysfunction is both a first identifiable indicator and an enduring core feature of cognitive decline in ageing and in neuropsychiatric disorders including Alzheimer's disease and schizophrenia. Available evidence from human studies suggests that biological sex and sex hormones modulate episodic memory function in health and disease. However, knowledge of how this occurs is constrained by the limited availability and underutilization of validated animal models in investigating hormone impacts on episodic-like memory function. Here, adult female, adult male and gonadally manipulated adult male rats were tested on the what-where-when episodic-like memory task to determine whether rats model human sex differences in episodic memory and how the hormonal milieu impacts episodic-like memory processes in this species. These studies revealed salient ways in which rats model human sex differences in episodic memory, including a male advantage in spatial episodic memory performance. They also identified domain-specific roles for oestrogens and androgens in modulating what, where and when discriminations in male rats that were unlike those engaged in corresponding novel object recognition and novel object location tasks. These studies thus identify rats and the what-where-when task as suitable for investigating the neuroendocrine bases of episodic-like memory, and provide new information about the unique contributions that sex and sex hormones make to this complex mnemonic process.

Startling Differences: Using the Acoustic Startle Response to Study Sex Differences and Neurosteroids in Affective Disorders

PURPOSE OF REVIEW

Neuroactive steroid hormones, such as estradiol and progesterone, likely play a role in the pathophysiology of female-specific psychiatric disorders such as premenstrual dysphoric disorder (PMDD) and postpartum depression and may contribute to the marked sex differences observed in the incidence and presentation of affective disorders. However, few tools are available to study the precise contributions of these neuroactive steroids (NSs). In this review, we propose that the acoustic startle response (ASR), an objective measure of an organism's response to an emotional context or stressor, is sensitive to NSs. As such, the ASR represents a unique translational tool that may help to elucidate the contribution of NSs to sex differences in psychiatric disorders.

RECENT FINDINGS

Findings suggest that anxiety-potentiated startle (APS) and prepulse inhibition of startle (PPI) are the most robust ASR paradigms for assessing contribution of NSs to affective disorders, while affective startle response modulation (ASRM) appears less diagnostic of sex or menstrual cycle (MC) effects. However, few studies have appropriately used ASR to test a priori hypotheses about sex or MC differences. We recommend that ASR studies account for sex as a biological variable (SABV) and hormonal status to further knowledge of NS contribution to affective disorders.

The effect of opiodergic system and testosterone on anxiety behavior in gonadectomized rats

BACKGROUND AND AIM

Removal of the testes (gonadectomy; GDX), the primary source of androgens, increases anxiety behavior in several tasks. Opioids are known to play a role in mediating the effects of androgen. In the present study, the effect of testosterone and opioidergic system on anxiety behavior was investigated.

METHODS

Adult male Wistar rats were bilaterally castrated. The elevated plus maze which is a useful test to investigate the effects of anxiogenic or anxiolytic drugs in rodents was used.

RESULTS

The data indicated that there is a decrease, 10 days after castration, in the percentage of OAT (the ratio of time spent in the open arms to total times spent in any arms × 100) and OAE (the ratio of entries into open arms to total entries × 100) but not locomotor activity, showing anxiogenic-like effects of gonadectomy. Intraperitoneal injection of testosterone (200, 300 and 450 mg/kg) and morphine (2.5, 5 and 7.5mg/kg), before testing 10 days after castration, showed an increase in OAT and OAE. Furthermore, injection of naloxone (5 and 7.5mg/kg, i.p.), 5 min before testing 10 days after castration, decreased OAT and OAE. Also, injection of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the injection of different doses of morphine (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, did not significantly alter OAT, OAE and locomotor activity. While, administration of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the infusion of different doses of naloxone (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, decreased OAT and OAE.

CONCLUSION

The results show the involvement of testosterone and opioidergic system in anxiogenic-like behaviors induced by gonadectomy.

Exposure to bisphenol A exacerbates migraine-like behaviors in a multibehavior model of rat migraine

Migraine is a common and debilitating neurological disorder suffered worldwide. Women experience this condition 3 times more frequently than men, with estrogen strongly implicated to play a role. Bisphenol A (BPA), a highly prevalent xenoestrogen, is known to have estrogenic activity and may have an effect in migraine onset, intensity, and duration through estrogen receptor signaling. It was hypothesized that BPA exposure exacerbates migraine symptoms through estrogen signaling and downstream activation of nociception related pathways. Utilizing a multibehavior model of migraine in ovariectomized female rats, changes in locomotion, light and sound sensitivity, grooming, and acoustic startle were examined. Furthermore, changes in the expression of genes related to estrogen (ERα, GPR30), and nociception (extracellular signal regulated kinase, ERK, sodium gated channel, Nav1.8, and fatty acid amide hydrolase, FAAH) were studied following behavioral experiments. The following results were obtained: BPA treatment significantly exacerbated migraine-like behaviors in rats. Rats exposed to BPA demonstrated decreased locomotion, exacerbated light and sound aversion, altered grooming habits, and enhanced startle reflexes. Furthermore, BPA exposure increased mRNA expression of estrogen receptors, total ERK mRNA and ERK activation, as well as Nav1.8, and FAAH mRNA, indicative of altered estrogen signaling and altered nociception. These results show that BPA, an environmentally pervasive xenoestrogen, exacerbates migraine-like behavior in a rat model and alters expression of estrogen and nociception-related genes.

Inhibition of 5α-reductase in the nucleus accumbens counters sensorimotor gating deficits induced by dopaminergic activation

Cogent evidence highlights a key role of neurosteroids and androgens in schizophrenia. We recently reported that inhibition of steroid 5α-reductase (5αR), the rate-limiting enzyme in neurosteroid synthesis and androgen metabolism, elicits antipsychotic-like effects in humans and animal models, without inducing extrapyramidal side effects. To elucidate the anatomical substrates mediating these effects, we investigated the contribution of peripheral and neural structures to the behavioral effects of the 5αR inhibitor finasteride (FIN) on the prepulse inhibition (PPI) of the acoustic startle reflex (ASR), a rat paradigm that dependably simulates the sensorimotor gating impairments observed in schizophrenia and other neuropsychiatric disorders. The potential effect of drug-induced ASR modifications on PPI was excluded by measuring this index both as percent (%PPI) and absolute values (ΔPPI). In both orchidectomized and sham-operated rats, FIN prevented the %PPI deficits induced by the dopamine (DA) receptor agonists apomorphine (APO, 0.25mg/kg, SC) and d-amphetamine (AMPH, 2.5mg/kg, SC), although the latter effect was not corroborated by ΔPPI analysis. Conversely, APO-induced PPI deficits were countered by FIN infusions in the brain ventricles (10μg/1μl) and in the nucleus accumbens (NAc) shell and core (0.5μg/0.5μl/side). No significant PPI-ameliorating effect was observed following FIN injections in other brain regions, including dorsal caudate, basolateral amygdala, ventral hippocampus and medial prefrontal cortex, although a statistical trend was observed for the latter region. The efflux of DA in NAc was increased by systemic, but not intracerebral FIN administration. Taken together, these findings suggest that the role of 5αR in gating regulation is based on post-synaptic mechanisms in the NAc, and is not directly related to alterations in DA efflux in this region.

Male rats with the testicular feminization mutation of the androgen receptor display elevated anxiety-related behavior and corticosterone response to mild stress

Testosterone influences the hypothalamic-pituitary-adrenal axis, anxiety-related behavior, and sensorimotor gating in rodents, but little is known about the role of the androgen receptor (AR) in mediating these influences. We compared levels of the stress hormone corticosterone at baseline and following exposure to a novel object in an open field in wild type (wt) male and female rats, and male rats with the testicular feminization mutation (Tfm) of the AR, which disables its function. Basal corticosterone was equivalent in all groups, but exposure to a novel object in an open field elicited a greater increase in corticosterone in Tfm males and wt females than in wt males. Tfm males also showed increased behavioral indices of anxiety compared to wt males and females in the test. Analysis of the immediate early gene c-Fos expression after exposure to a novel object revealed greater activation in Tfm males than wt males in some regions (medial preoptic area) and lesser activation in others (dentate gyrus, posterodorsal medial amygdala). No differences were found in a measure of sensorimotor gating (prepulse inhibition of the acoustic startle response), although Tfm males had an increased acoustic startle response compared to wt males and females. These findings demonstrate that ARs play a role in regulating anxiety-related behaviors, as well as corticosterone responses and neural activation following exposure to a mild stressor in rats.

Repeated elicitation of the acoustic startle reflex leads to sensitisation in subsequent avoidance behaviour and induces fear conditioning

BACKGROUND

Autonomous reflexes enable animals to respond quickly to potential threats, prevent injury and mediate fight or flight responses. Intense acoustic stimuli with sudden onsets elicit a startle reflex while stimuli of similar intensity but with longer rise times only cause a cardiac defence response. In laboratory settings, habituation appears to affect all of these reflexes so that the response amplitude generally decreases with repeated exposure to the stimulus. The startle reflex has become a model system for the study of the neural basis of simple learning processes and emotional processing and is often used as a diagnostic tool in medical applications. However, previous studies did not allow animals to avoid the stimulus and the evolutionary function and long-term behavioural consequences of repeated startling remain speculative. In this study we investigate the follow-up behaviour associated with the startle reflex in wild-captured animals using an experimental setup that allows individuals to exhibit avoidance behaviour.

RESULTS

We present evidence that repeated elicitation of the acoustic startle reflex leads to rapid and pronounced sensitisation of sustained spatial avoidance behaviour in grey seals (Halichoerus grypus). Animals developed rapid flight responses, left the exposure pool and showed clear signs of fear conditioning. Once sensitised, seals even avoided a known food source that was close to the sound source. In contrast, animals exposed to non-startling (long rise time) stimuli of the same maximum sound pressure habituated and flight responses waned or were absent from the beginning. The startle threshold of grey seals expressed in units of sensation levels was comparable to thresholds reported for other mammals (93 dB).

CONCLUSIONS

Our results demonstrate that the acoustic startle reflex plays a crucial role in mediating flight responses and strongly influences the motivational state of an animal beyond a short-term muscular response by mediating long-term avoidance. The reflex is therefore not only a measure of emotional state but also influences emotional processing. The biological function of the startle reflex is most likely associated with mediating rapid flight responses. The data indicate that repeated startling by anthropogenic noise sources might have severe effects on long-term behaviour. Future, studies are needed to investigate whether such effects can be associated with reduced individual fitness or even longevity of individuals.

Gonadectomy and hormone replacement exert region- and enzyme isoform-specific effects on monoamine oxidase and catechol-O-methyltransferase activity in prefrontal cortex and neostriatum of adult male rats

Sex differences and gonadal hormone influences are well known for diverse aspects of forebrain amine and indolamine neurotransmitter systems, the cognitive and affective functions they govern and their malfunction in mental illness. This study explored whether hormone regulation/dysregulation of these systems could be related to gonadal steroid effects on catechol-O-methyltransferase and monoamine oxidase which are principal enzymatic controllers of forebrain dopamine, serotonin and norepinephrine levels. Driven by male over female differences in cortical enzyme activities, by male-specific associations between monoamine oxidase and catechol-O-methyltransferase gene polymorphisms and cognitive and dysfunction in disease and by male-specific consequences of gene knockouts in mice, the question of hormone sensitivity was addressed here using a male rat model where prefrontal dopamine levels and related behaviors are also known to be affected. Specifically, quantitative O-methylation and oxidative deamination assays were used to compare the activities of catechol-O-methyltransferase's soluble and membrane-bound isoforms and of monoamine oxidase's A and B isoforms in the pregenual medial prefrontal cortex and dorsal striatum of male rats that were sham operated, gonadectomized or gonadectomized and supplemented with testosterone propionate or with estradiol for 28 days. These studies revealed significant effects of hormone replacement but not gonadectomy on the soluble but not the membrane-bound isorfom of catechol-O-methyltransferase in both striatum and cortex. A significant, cortex-specific testosterone-but not estradiol-attenuated effect (increase) of gonadectomy on monoamine oxidase's A but not B isoform was also observed. Although none of these actions suggest potential roles in the regulation/dysregulation of prefrontal dopamine, the suppressive effects of testosterone on cortical monoamine oxidase-A that were observed could have bearing on the increased incidence of cognitive deficits and symptoms of depression and anxiety that are repeatedly observed in males in conditions of hypogonadalism related to aging, other biological factors or in prostate cancer where androgen deprivation is used as a neoadjuvant treatment.

In vitro binding assays using (3)H nisoxetine and (3)H WIN 35,428 reveal selective effects of gonadectomy and hormone replacement in adult male rats on norepinephrine but not dopamine transporter sites in the cerebral cortex

The prefrontal cortices mediate cognitive functions that critically depend on local dopamine levels. In male rats, many prefrontal tasks where performance is disrupted by changes in dopamine signaling are also impaired by gonadectomy, a manipulation that increases cortical dopamine concentration, prefrontal dopamine axon density and possibly extracellular prefrontal dopamine levels as well. Because these actions could be responsible for the impairing effects of gonadectomy on prefrontal function, the question of how they might arise comes to the fore. Accordingly, the present studies asked whether dopamine levels might be increased via a hormone sensitivity of transporter-mediated dopamine uptake. Specifically, (3)H WIN 35,428 and (3)H nisoxetine, ligands selective for the dopamine (DAT)- and norepinephrine transporter (NET) respectively, were used in in vitro binding assays to ask whether gonadectomy altered transporter affinity (Kd) and/or binding site number (Bmax) in prefrontal cortex, sensorimotor cortex and/or caudate. Assays performed on tissues dissected from sham-operated, gonadectomized and gonadectomized rats supplemented with testosterone propionate or estradiol for 4 or 28 days revealed no significant group differences or obvious trends in Kd or Bmax for DAT binding or in measures of Bmax for NET binding. However, affinity constants for (3)H nisoxetine were found to be significantly higher in sensorimotor and/or prefrontal cortex of rats gonadectomized and gonadectomized and supplemented with estradiol for 4 or 28 days but similar to control in gonadectomized rats given testosterone. Because the NET contributes substantially to extracellular prefrontal dopamine clearance, these androgen-mediated effects could influence prefrontal dopamine levels and might thus be relevant for observed effects of gonadectomy on dopamine-dependent prefrontal behaviors. A hormone sensitivity of the NET could also have bearing on the prefrontal dopamine dysfunction seen in disorders like schizophrenia that disproportionately affect males, whose severity correlates with abnormal testosterone levels, and for which the NET is among suspected sites of pathology.

Effects of gonadectomy and hormone replacement on a spontaneous novel object recognition task in adult male rats

Recent studies in adult male rats have shown that gonadal hormones influence performance on certain working memory and other types of cognitive tasks that are sensitive to lesions of the medial and/or orbital prefrontal cortices. This study asked whether gonadal hormone modulation of prefrontal cortical function in males also extends to the perirhinal division of the rat prefrontal cortex. Specifically, sham-operated control, gonadectomized, and gonadectomized rats supplemented with testosterone propionate or estradiol were tested on a spontaneous novel object recognition task, a paradigm where performance has been shown to be impaired by perirhinal cortical lesions. Using analyses of variance, regression analyses and post-hoc testing to evaluate group differences, it was found that during both the sample and test trials of the task all four groups spent similar absolute and proportional amounts of time ambulating, rearing, stationary, and exploring the two objects present. All groups also explored each of the two identical objects present during sample trials equally. However, during the test trials, only the control and gonadectomized rats given testosterone showed the expected increase in exploration of the novel objects presented, whereas the gonadectomized and gonadectomized, estradiol-supplemental groups continued to explore the novel and familiar objects equally. That regression analyses also identified significant correlations between low bulbospongiosus muscle weight and impaired novel vs. familiar object discrimination further indicates that gonadectomy in adult male rats adversely affects spontaneous novel object recognition in an androgen-sensitive, estrogen-insensitive manner.