Estrous cycle influences on sexual diergism of HPA axis responses to cholinergic stimulation in rats

Estrogen receptor GPR30 in the anterior cingulate cortex mediates exacerbated neuropathic pain in ovariectomized mice

Menopausal women experience neuropathic pain 63% more frequently than men do, which may attribute to the estrogen withdrawal. However, the underlying mechanisms remain unclear. Here, the role of estrogen receptors (ERs) in ovariectomized (OVX) female mice following chronic constriction injury (CCI) was investigated. With 17β-estradiol (E2) supplemented, aggravated mechanical allodynia in OVX mice could be significantly alleviated, particularly after intra-anterior cingulate cortex (ACC) E2 delivery. Pharmacological interventions further demonstrated that the agonist of G-protein-coupled estrogen receptor 30 (GPR30), rather than ERα or ERβ in the ACC, exhibited the similar analgesic effect as E2, whereas antagonist of GPR30 exacerbated allodynia. Furthermore, OVX surgery reduced GPR30 expression in the ACC, which could be restored with estrogen supplementation. Selective downregulation of GPR30 in the ACC of naïve female mice induces mechanical allodynia, whereas GPR30 overexpression in the ACC remarkedly alleviated OVX-exacerbated allodynia. Collectively, estrogen withdrawal could downregulate the ACC GPR30 expression, resulting in exacerbated neuropathic pain. Our findings highlight the importance of GPR30 in the ACC in aggravated neuropathic pain during menopause, and offer a potential therapeutic candidate for neuropathic pain management in menopausal women.

Estrous cycle modulates the anxiogenic effects of caffeine in the elevated plus maze and light/dark box in female rats

Caffeine is a commonly used stimulant of the central nervous system that reduces fatigue, increases alertness, and exerts positive effects on emotion through actions on various brain structures. High doses of caffeine can cause headaches, heart palpitations, hyperactivity, and anxiety symptoms. Consequently, reducing the consumption of stimulant substances, such as sugar and caffeine, is proposed to ameliorate symptoms of premenstrual syndrome in women. The administration of steroid hormones has been suggested to modulate the effects of caffeine, but unknown is whether endogenous hormone variations during the estrous cycle modulate the pharmacological effects of caffeine. The present study evaluated the effects of caffeine (10, 20, and 40 mg/kg) during metestrus-diestrus and proestrus-estrus of the ovarian cycle in rats on anxiety-like behavior using the elevated plus maze and light/dark box. During metestrus-diestrus, all doses of caffeine increased anxiety-like behavior, indicated by the main variables in both behavioral tests (i.e., higher Anxiety Index and lower percent time spent on the open arms in the elevated plus maze and less time spent in the light compartment in the light/dark box). During proestrus-estrus, only 20 and 40 mg/kg caffeine increased these parameters of anxiety-like behavior, albeit only slightly. In conclusion, caffeine increased anxiety-like behaviors in metestrus-diestrus, with an attenuation of these effects of lower doses of caffeine in proestrus-estrus. These effects that were observed in metestrus-diestrus and proestrus-estrus may be associated with low and high concentrations of steroid hormones, respectively, that naturally occur during these phases of the ovarian cycle.

Endocrine and Electrolyte Balances during Periovulatory Period in Cycling Mares

Simple Summary

This study provides new evidence on the physiological mechanisms involved in the electrolyte balance during periovulatory period in cycling mares. The interrelationships among adrenocorticotropic hormone (ACTH), cortisol (CORT), aldosterone (ALD) and electrolytes (sodium—Na⁺, potassium—K⁺ and chloride—Cl⁻) were evaluated. The simultaneous increase in ACTH, CORT and ALD toward the time of ovulation could suggest the involvement of the adrenocortical pituitary axis in the ovulatory mechanisms, contributing at the same time to the maintenance of electrolyte homeostasis.

Abstract

In cycling females, the periovulatory period is characterized by stimulation of the hypothalamic pituitary adrenal (HPA) axis. The aim of present study was to analyze the pattern and interrelationships among adrenocorticotropic hormone (ACTH), cortisol (CORT), aldosterone (ALD) and electrolytes (sodium—Na⁺, potassium—K⁺ and chloride—Cl⁻) during periovulatory period in cycling mares. Venous blood samples were obtained daily from a total of 23 Purebred Spanish broodmares, aged 7.09 ± 2.5 years, from day −5 to day +5 of estrous cycle, considering day 0, the day of ovulation. Plasma ACTH was measured by a fluorescent immunoassay kit, serum CORT and ALD by means of a competitive ELISA immunoassay, and plasma Na⁺, K⁺ and Cl⁻ were quantified by an analyzer with selective electrodes for the three ions. ACTH showed higher concentrations at day 0 compared to days −5 to −1 and +1 to +3 (p < 0.05). CORT showed higher concentrations at day 0 compared to days −5 to −2 and +1 to +5 (p < 0.05). ALD showed higher concentrations at day 0 compared to days −5 to −2 (p < 0.05) and +2 (p < 0.05). Na⁺ and Cl⁻ showed higher concentrations at day 0, compared to day −5 and +5. K⁺ showed lower concentrations at day 0 compared to day +1 (p < 0.05). The significant correlations obtained between ACTH and CORT (r = 0.20) and between ACTH and ALD (r = 0.32) suggest that although ACTH may have an effect both on CORT and ALD, there are other very important determinants that could be considered. Hence, it is possible to presume that the pituitary adrenocortical response and ALD may be involved in the ovulatory mechanisms without a direct relation with electrolyte pattern.

Chrysin reduces anxiety-like behavior through actions on GABAA receptors during metestrus-diestrus in the rat

Low concentrations of ovarian hormones, among other factors, are associated with greater vulnerability to negative effects of environmental stressors and may trigger anxiety symptoms in females. The flavonoid chrysin (5,7-dihydroxyflavone) exerts anxiolytic-like effects in male and ovariectomized female rats, but it is unknown if chrysin could reduce anxiety-like behavior that naturally occurs through the ovarian cycle phases. The present study evaluated the effect of chrysin on anxiety-like behavior associated with the ovarian cycle phases in rats and the participation of γ-aminobutyric acid-A (GABA_A) receptors in these actions. The acute effects of chrysin (2 mg/kg) were investigated in female cycling Wistar rats in the elevated plus maze, locomotor activity test, and light/dark test. Diazepam (2 mg/kg) was used as reference anxiolytic drug. The participation of GABA_A receptor in the anxiolytic actions of chrysin was explored by pretreating the rats with the noncompetitive GABA_A chloride ion channel antagonist picrotoxin (1 mg/kg). Chrysin and diazepam prevented anxiety-like behavior that was associated with the metestrus-diestrus phase in both the elevated plus maze and light/dark test, and these effects were reversed by picrotoxin, with no significant changes in spontaneous locomotor activity. No significant motor effects of chrysin were detected in either behavioral test during proestrus-estrus or metestrus-diestrus phases, whereas diazepam produced motor hypoactivity in the locomotor activity test during proestrus-estrus phase. These results indicate that the flavonoid chrysin prevents anxiety-like behavior that naturally occurs during metestrus-diestrus in two unconditioned models that are used to evaluate anxiety-like behavior, and these effects were mediated by actions on GABA_A receptors.

Betaglycan (TGFBR3) Functions as an Inhibin A, but Not Inhibin B, Coreceptor in Pituitary Gonadotrope Cells in Mice

Inhibins are gonadal hormones that act on pituitary gonadotrope cells to suppress FSH synthesis and secretion. Inhibin A and B are heterodimers of the inhibin ⍺-subunit disulfide-linked to one of two inhibin β-subunits. Homodimers or heterodimers of the inhibin β-subunits form the activins, which stimulate FSH production. Activins signal through complexes of type I and II receptor serine/threonine kinases to increase transcription of the FSHβ subunit gene. According to in vitro observations, inhibins impair FSH synthesis by competitively binding to activin type II receptors, particularly in the presence of the TGFβ type III receptor (TGFBR3, or betaglycan). The role of TGFBR3 in inhibin action in vivo has not been determined. Here, we ablated Tgfbr3 specifically in murine gonadotropes. Conditional knockout females were supra-fertile, exhibiting enhanced folliculogenesis, numbers of ovulated eggs per cycle, and litter sizes relative to control mice. Despite these phenotypes, FSH levels appeared to be unaltered in knockout mice, and the mechanisms underlying their enhanced fertility remain unexplained. Inhibin B is the predominant form of the hormone in males and in females during most stages of the estrous cycle. Remarkably, inhibin A, but not inhibin B, suppression of FSH synthesis was impaired in cultured pituitaries of knockout mice, which may explain the absence of discernible changes in FSH levels in vivo. Collectively, these data challenge current dogma by demonstrating that TGFBR3 (betaglycan) functions as an inhibin A, but not an inhibin B, coreceptor in gonadotrope cells in vivo. Mechanisms of inhibin B action merit further investigation.

Effects of inducing exercise on growing mice by means of three-dimensional structure in rearing environment

We reared ICR mice during a growth period (3 to 10 weeks of age) and examined the effect of exercise induction, by enriching the rearing environment with obstacles such as ladders, compared to the standard environment. Environmental enrichment significantly increased the amount of exercise in both sexes (P<0.01). Enriched exercise mice had higher body weight than control mice at 6 to 9 weeks of age in males and 8 weeks of age in females (P<0.05). The sexual maturation of female enriched exercise mice was significantly advanced compared to the control (P<0.001). Enriched exercise mice showed decreased anxiety-like behavior in the open field test and lower plasma corticosterone levels in both sexes compared to the control, and differences were statistically significant in males (P<0.05). In both sexes, enriched exercise appeared to increase natural killer cells in blood compared to the control, but no statistical differences was detected. In conclusion, we confirmed that daily low-stress exercise could be induced using a three-dimensional rearing environment in growing mice. In addition, we suggest that exercise has beneficial effects on physical growth, sexual maturation and anxiety-like behavior. Furthermore, environmental enrichment might be more effective in male than female in group-housed mice.

Central insulin signaling modulates hypothalamus-pituitary-adrenal axis responsiveness

Objective

Obesity is often accompanied by hyperactivity of the neuroendocrine stress axis and has been linked to an increased risk of psychiatric disorders. Insulin is reciprocally regulated with the stress hormone corticosterone (CORT), raising the possibility that insulin normally provides inhibitory tone to the hypothalamus-adrenal-pituitary (HPA) axis. Here we examined whether disrupting signaling via the insulin receptor (InsR) in hypothalamic subpopulations impacts the neuroendocrine response to acute psychological stress.

Methods

We used Nkx2.1-Cre, Sim1-Cre and Agrp-Cre transgenic driver lines to generate conditional knockouts of InsR signaling throughout the hypothalamus, paraventricular nucleus of the hypothalamus (PVH) and in neurons expressing Agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus (ARH), respectively. We used a combination of molecular, behavioral and neuroendocrine criteria to evaluate the consequences on HPA axis responsiveness.

Results

Endpoints related to body weight and glucose homeostasis were not altered in any of the conditional mutant lines. Consistent with observations in the neuronal Insr knockout mice (NIRKO), baseline levels of serum CORT were similar to controls in all three lines. In male mice with broad disruptions of InsR signals in Nkx2.1-expressing regions of the hypothalamus (IR^Nkx2.1 KO), we observed elevated arginine vasopressin (AVP) levels at baseline and heightened neuroendocrine responses to restraint stress. IR^Nkx2.1 KO males also exhibited increased anxiety-like behaviors in open field, marble burying, and stress-induced hyperthermia testing paradigms. HPA axis responsivity was not altered in IR^Sim1 KO males, in which InsR was disrupted in the PVH. In contrast to observations in the IR^Nkx2.1 KO males, disrupting InsR signals in ARH neurons expressing Agrp (IR^Agrp KO) led to reduced AVP release in the median eminence (ME).

Conclusions

We find that central InsR signals modulate HPA responsivity to restraint stress. InsR signaling in AgRP/NPY neurons appears to promote AVP release, while signaling in other hypothalamic neuron(s) likely acts in an opposing fashion. Alterations in InsR signals in neurons that integrate metabolic and psychiatric information could contribute to the high co-morbidity of obesity and mental disorders.

Sex differences in the behavioural and hypothalamic-pituitary-adrenal response to contextual fear conditioning in rats

In recent years, special attention is being paid to sex differences in susceptibility to disease. In this regard, there is evidence that male rats present higher levels of both cued and contextual fear conditioning than females. However, little is known about the concomitant hypothalamic-pituitary-adrenal (HPA) axis response to those situations which are critical in emotional memories. Here, we studied the behavioural and HPA responses of male and female Wistar rats to context fear conditioning using electric footshock as the aversive stimulus. Fear-conditioned rats showed a much greater ACTH and corticosterone response than those merely exposed to the fear conditioning chamber without receiving shocks. Moreover, males presented higher levels of freezing whereas HPA axis response was greater in females. Accordingly, during the fear extinction tests, female rats consistently showed less freezing and higher extinction rate, but greater HPA activation than males. Exposure to an open-field resulted in lower activity/exploration in fear-conditioned males, but not in females, suggesting greater conditioned cognitive generalization in males than females. It can be concluded that important sex differences in fear conditioning are observed in both freezing and HPA activation, but the two sets of variables are affected in the opposite direction: enhanced behavioural impact in males, but enhanced HPA responsiveness in females. Thus, the role of sex differences on fear-related stimuli may depend on the variables chosen to evaluate it, the greater responsiveness of the HPA axis in females perhaps being an important factor to be further explored.

Disturbance of the gut microbiota in early-life selectively affects visceral pain in adulthood without impacting cognitive or anxiety-related behaviors in male rats

Disruption of bacterial colonization during the early postnatal period is increasingly being linked to adverse health outcomes. Indeed, there is a growing appreciation that the gut microbiota plays a role in neurodevelopment. However, there is a paucity of information on the consequences of early-life manipulations of the gut microbiota on behavior. To this end we administered an antibiotic (vancomycin) from postnatal days 4-13 to male rat pups and assessed behavioral and physiological measures across all aspects of the brain-gut axis. In addition, we sought to confirm and expand the effects of early-life antibiotic treatment using a different antibiotic strategy (a cocktail of pimaricin, bacitracin, neomycin; orally) during the same time period in both female and male rat pups. Vancomycin significantly altered the microbiota, which was restored to control levels by 8 weeks of age. Notably, vancomycin-treated animals displayed visceral hypersensitivity in adulthood without any significant effect on anxiety responses as assessed in the elevated plus maze or open field tests. Moreover, cognitive performance in the Morris water maze was not affected by early-life dysbiosis. Immune and stress-related physiological responses were equally unaffected. The early-life antibiotic-induced visceral hypersensitivity was also observed in male rats given the antibiotic cocktail. Both treatments did not alter visceral pain perception in female rats. Changes in visceral pain perception in males were paralleled by distinct decreases in the transient receptor potential cation channel subfamily V member 1, the α-2A adrenergic receptor and cholecystokinin B receptor. In conclusion, a temporary disruption of the gut microbiota in early-life results in very specific and long-lasting changes in visceral sensitivity in male rats, a hallmark of stress-related functional disorders of the brain-gut axis such as irritable bowel disorder.

Montanoa frutescens and Montanoa grandiflora extracts reduce anxiety-like behavior during the metestrus-diestrus phase of the ovarian cycle in Wistar rats

In previous studies, the anxiolytic-like effects of Montanoa tomentosa and Montanoa frutescens were reported in male rats, but the potential anxiolytic-like effects of Montanoa plants during the different phases of the ovarian cycle in rats remain to be explored. The anxiolytic-like effects of the aqueous crude extracts of M. frutescens (25 and 50 mg/kg) and M. grandiflora (25 and 50 mg/kg) in the elevated plus maze were investigated in Wistar rats during the estrous cycle and compared with 2 mg/kg diazepam as a reference anxiolytic drug. To investigate any motor effect (i.e., hyperactivity, no changes, or hypoactivity) associated with the treatments, the rats were evaluated in the open field test. The M. frutescens (25 and 50 mg/kg) and M. grandiflora (50 mg/kg) extracts exerted anxiolytic-like effects during the metestrus-diestrus phase, similar to diazepam, without disrupting spontaneous motor activity. No significant effects of the extracts were detected in either behavioral test during the proestrus-estrus phase, whereas diazepam produced motor hypoactivity in the open field test. These results indicate that the M. frutescens and M. grandiflora extracts possess anxiolytic-like effects that depend on the ovarian cycle phase, supporting the Mexican ancient medicinal use of these plants to ameliorate anxiety disorders.

Sex differences in activated corticotropin-releasing factor neurons within stress-related neurocircuitry and hypothalamic-pituitary-adrenocortical axis hormones following restraint in rats

Women may be more vulnerable to certain stress-related psychiatric illnesses than men due to differences in hypothalamic-pituitary-adrenocortical (HPA) axis function. To investigate potential sex differences in forebrain regions associated with HPA axis activation in rats, these experiments utilized acute exposure to a psychological stressor. Male and female rats in various stages of the estrous cycle were exposed to 30min of restraint, producing a robust HPA axis hormonal response in all animals, the magnitude of which was significantly higher in female rats. Although both male and female animals displayed equivalent c-fos expression in many brain regions known to be involved in the detection of threatening stimuli, three regions had significantly higher expression in females: the paraventricular nucleus of the hypothalamus (PVN), the anteroventral division of the bed nucleus of the stria terminalis (BSTav), and the medial preoptic area (MPOA). Dual fluorescence in situ hybridization analysis of neurons containing c-fos and corticotropin-releasing factor (CRF) mRNA in these regions revealed significantly more c-fos and CRF single-labeled neurons, as well as significantly more double-labeled neurons in females. Surprisingly, there was no effect of the estrous cycle on any measure analyzed, and an additional experiment revealed no demonstrable effect of estradiol replacement following ovariectomy on HPA axis hormone induction following stress. Taken together, these data suggest sex differences in HPA axis activation in response to perceived threat may be influenced by specific populations of CRF neurons in key stress-related brain regions, the BSTav, MPOA, and PVN, which may be independent of circulating sex steroids.

Sexually diergic, dose-dependent hypothalamic-pituitary-adrenal axis responses to nicotine in a dynamic in vitro perfusion system

INTRODUCTION

The hypothalamic-pituitary-adrenal cortical (HPA) axis modulates physiological responses to stress. We previously reported sexually diergic, dose-dependent HPA responses in vivo following nicotine administration: Male rats had greater arginine vasopressin (AVP) responses than females, and female rats had greater adrenocorticotropic hormone (ACTH) and corticosterone (CORT) responses than males. The goal of the present study was to further investigate sexually diergic, dose-dependent HPA responses following nicotine addition to an in vitro model of the HPA axis, so that hormone output could be determined at each level of the axis.

METHODS

Hypothalami, pituitaries, and adrenal glands were harvested from male and female rats. One-half hypothalamus, one-half pituitary, and one adrenal gland were placed individually into three jacketed tissue baths connected by tubing and perfused in series with physiological medium. Sampling ports between tissue baths were used to collect buffer before and after addition of various doses of nicotine, for measurement of AVP and corticotropin-releasing hormone (CRH) from the hypothalamus bath, ACTH from the pituitary bath, and CORT from the adrenal bath. Hormones were measured by highly specific immunoassays.

RESULTS

Stable temperatures, flow rates, pH, and hormone baselines were achieved in the in vitro system. Consistent with our in vivo and earlier in vitro studies, nicotine added to the hypothalamus tissue bath significantly increased HPA responses in a sex- and dose-dependent manner: Males had greater AVP responses than did females, and females had greater CRH responses than did males. Sexually diergic ACTH and CORT responses were less apparent and were higher in females.

DISCUSSION

Our in vitro system accurately models in vivo HPA responses to nicotine in both sexes and thus represents a reliable method for investigating the effects of nicotine on components of the HPA axis. These studies may be pertinent to understanding the biological differences to nicotine between men and women smokers.

Enduring effects of environmental enrichment from weaning to adulthood on pituitary-adrenal function, pre-pulse inhibition and learning in male and female rats

Environmental enrichment (EE) increases stimulation and provides richer sensory, cognitive and motor opportunities through the interaction with the social and physical environment. EE produces a wide range of neuroanatomical, neurochemical and behavioural effects in several animal species. However, the effects of EE have mainly been studied shortly after the treatment, so its long-lasting effects remain to be elucidated. Thus, we studied in male and female Sprague-Dawley rats the enduring effects of EE on tasks that measured emotional reactivity, social exploration and memory, sensorimotor gating and learning. After weaning, rats reared in EE were housed in single-sex groups of 12-14 in enriched cages during 12 weeks, whereas control rats were housed in single-sex groups of 2-3 animals in standard cages. Then, all rats were housed in pairs and successively exposed to different tests between 4 and 60 weeks post-EE. The results indicated that animals of both sexes reared in EE gained less weight during the enrichment period; differences disappeared in females during the post-EE period, but were maintained intact in males. Rats reared in EE showed an altered daily pattern of corticosterone and a lower hormone response to a novel environment (hole board, HB), although no differences in ACTH were found. EE resulted in more exploratory behaviour in the HB and higher number of entries in the open arms of the elevated plus maze (with no changes in the time spent in the open arms), suggesting a greater motivation to explore. Unexpectedly, rats reared in EE showed reduced pre-pulse inhibition (PPI), a measure of sensorimotor gating, suggesting lower capability to filter non-relevant information compared with control rats. EE increased social exploratory behaviour towards juvenile rats and social discrimination in males, but decreased social discrimination in females. Finally, in the Hebb-Williams maze, rats reared in EE showed better performance in terms of reduced number of errors and shorter distances travelled in the mazes. It is concluded that EE exposure from weaning to adulthood has important and long-lasting consequences on physiological and behavioural variables, most of them similar in both sexes, although sex differences in response to the EE are also reported.

Testosterone reduces cumulative burying in female Wistar rats with minimal participation of estradiol

Testosterone exerts anxiolytic effects, but the participation of its aromatase metabolic product estradiol is controversial. Therefore, we used the defensive burying paradigm in female Wistar rats to explore testosterone's (1.0 mg/rat, s.c.) interactions with picrotoxin (a noncompetitive gamma-aminobutyric acid-A receptor [GABA(A)] antagonist; 1.0 mg/kg, i.p.), formestane (an aromatase inhibitor; 3.0 mg/rat, s.c.), and tamoxifen (an estrogen receptor-beta antagonist; 1.0 mg/kg, s.c.). Serum levels of testosterone, estradiol, and progesterone were determined in the same rats. Burying latency and locomotion did not significantly change. Systemic testosterone administration enhanced serum testosterone and estradiol levels and reduced defensive burying. This reduction in total burying was blocked by pretreatment with picrotoxin and tamoxifen, but not formestane. We conclude that testosterone produced anxiolytic-like effects in female rats that were mediated by actions at the GABA(A) receptor, with participation of the estradiol receptor-beta, rather than estradiol aromatization.

The rodent estrous cycle: characterization of vaginal cytology and its utility in toxicological studies

While an evaluation of the estrous cycle in laboratory rodents can be a useful measure of the integrity of the hypothalamic-pituitary-ovarian reproductive axis, it can also serve as a way of insuring that animals exhibiting abnormal cycling patterns are disincluded from a study prior to exposure to a test compound. Assessment of vaginal cytology in regularly cycling animals also provides a means to establish a comparable endocrine milieu for animals at necropsy. The procedure for obtaining a vaginal smear is relatively non-invasive and is one to which animals can become readily accustomed. It requires few supplies, and with some experience the assessments can be easily performed in fresh, unstained smears, or in fixed, stained ones. When incorporated as an adjunct to other endpoint measures, a determination of a female's cycling status can contribute important information about the nature of a toxicant insult to the reproductive system. In doing so, it can help to integrate the data into a more comprehensive mechanistic portrait of the effect, and in terms of risk assessment, may provide some indication of a toxicant's impact on human reproductive physiology.

Locomotor activity to nicotine and Fos immunoreactivity in the paraventricular nucleus of the hypothalamus in adolescent socially-stressed rats

We reported previously that social stressors in adolescence (SS: one-hour isolation and new cage partners daily for 16 days) increased locomotor activity to nicotine and to amphetamine in females, but not in males, when tested as adults. Here, we investigated whether effects of stressors in adolescence on locomotor responses to nicotine would be observed in both sexes if tested closer in time to the stressor exposure. We also tested whether social instability was necessary to alter nicotine's effects on locomotor activity by including a group that underwent daily isolation but was housed with the same partner (ISO). The locomotor-activating effects of nicotine were lower in SS rats compared to ISO and non-stressed control rats. In males, but not in females, there were effects of nicotine treatment and of stress condition on Fos immunoreactive (Fos-ir) cell counts in the paraventricular nucleus (PVN) of the hypothalamus: SS males had higher Fos-ir counts than did ISO and non-stressed control males, and higher Fos-ir counts in the PVN were found in repeated-nicotine groups than in acute-nicotine and saline groups. These results add to evidence that adolescents are uniquely vulnerable to stressors due to ongoing brain development, and also indicate that effects are sex- and stressor-specific.

Estradiol replacement in ovariectomized rats is antihyperalgesic in the formalin test

A subcutaneous implant of 17beta-estradiol or progesterone provides steady-state serum hormone levels from 7 to 24 days after implantation and allows the evaluation of the effects of the replacement with these hormones on phase 1 and phase 2 formalin-induced behaviors in ovariectomized (OVX) rats. Graded doses of 17beta-estradiol (5% to 40%) reduce formalin-induced behavior by 35% to 49% during phase 2 but not during phase 1, as measured with an automated formalin apparatus. The maximal response is seen with 20% 17beta-estradiol. The antihyperalgesic effect of 20% 17beta-estradiol is significant at 8 days after implantation and persists at 21 days. In contrast, graded doses of progesterone have no effect on either phase of formalin. The estrogen receptor antagonist tamoxifen completely prevents the antihyperalgesic effect of the 20% 17beta-estradiol implant. Formalin-induced behaviors during phase 2 are significantly less in proestrus females and OVX rats given 20% 17beta-estradiol compared with OVX control rats. Also, the formalin-induced increase in serum corticosterone is attenuated in OVX control rats compared with proestrus females and OVX rats given 20% 17beta-estradiol. These results indicate that estrogen replacement in OVX rats restores the maximal corticosterone response to tonic pain and, by an estrogen receptor-mediated process, inhibits tonic pain.

PERSPECTIVE

Hormone replacement (HR) therapy remains a widely used modality. We used a pharmacokinetically based rat HR model that results in continuous physiological levels of 17beta-estradiol to demonstrate the analgesic (antihyperalgesic) effects of estrogen replacement in an inflammatory pain model (formalin). These results suggest a potentially important consequence of HR therapy.

Sequence of pituitary–adrenal cortical hormone responses to low-dose physostigmine administration in young adult women and men

We previously demonstrated greater HPA axis activation in adult men compared to adult women following low-dose administration of the anticholinesterase inhibitor, physostigmine (PHYSO). Because blood sampling was done infrequently following PHYSO, the rise times of AVP, ACTH1-39, and cortisol could not be determined. In the present study, we determined the sequence of hormone increases by frequent blood sampling following PHYSO. Twelve adult women and 12 adult men underwent three test sessions 5-7 days apart: PHYSO, saline control, and repeat PHYSO. As in the earlier study, PHYSO produced no side effects in half the subjects and mild side effects in the other half, with no significant female-male differences. None of the hormone responses was significantly correlated with the presence or absence of side effects. In both women and men, the AVP increase preceded the ACTH1-39 increase, which in turn preceded the cortisol increase. The AVP and ACTH AUCs were significantly positively correlated in both women and men, supporting AVP as an acute stimulus to ACTH secretion. Also as in the earlier study, the AVP response to PHYSO was more than twice as great in men as in women, but the difference was not statistically significant. We therefore analyzed the results of both studies combined (N=26 women and 26 men). The men had a significantly greater AVP response and a trend toward a greater ACTH1-39 response compared to the women. These findings further support the concept of sexual diergism (functional sex difference) in the influence of CNS cholinergic systems on HPA hormone secretion.

Pituitary-adrenal responses to cholinergic stimulation and acute mild stress are differentially elevated in male and female M(2) muscarinic receptor knockout mice

Adrenocorticotrophic hormone (ACTH) and corticosterone responses to cholinergic stimulation are greater in male rats and mice than in females. To explore the role of M(2) muscarinic receptors in this sex difference, we administered the nonselective muscarinic agonist, oxotremorine, the acetylcholinesterase inhibitor, physostigmine, and saline (a mild stressor) to male and female M(2) receptor knockout (KO) and wild-type (WT) mice of the same genetic background. Because M(2) receptors function primarily as presynaptic autoreceptors, we hypothesized that their absence in M(2) KO mice would increase the sensitivity of hormone responses to cholinergic stimulation in these groups. Both male and female M(2) KO mice were significantly more responsive to the stress of saline injection than were their WT counterparts. Oxotremorine and physostigmine increased ACTH and corticosterone in all four groups, but to a significantly greater degree in KO males compared to WT males, KO females, and WT females. The increase in ACTH also was significantly greater in WT males compared to WT females. By contrast, the increase in corticosterone was significantly more in females compared to males, independent of genotype. Following pretreatment with the nonselective muscarinic antagonist, scopolamine, ACTH and corticosterone responses to oxotremorine and to saline in the M(2) KO mice were comparable with those of their WT counterparts. These findings suggest that the M(2) muscarinic receptor subtype influences male and female pituitary-adrenal responses following stimulation by both mild stress and cholinergic drugs. The M(2) receptor appears to regulate ACTH responses to cholinergic stimulation in males but not in females; however, other muscarinic receptors may be involved because corticosterone responses were higher in females compared to males. Because ACTH and corticosterone responses were greater in male and female M(2) KO mice, the M(2) receptor appears to dampen the stress response.

Novel in vitro perfusion system for the determination of hypothalamic-pituitary-adrenal axis responses

INTRODUCTION

The hypothalamic-pituitary-adrenal (HPA) axis is a three-gland component of the endocrine system and a key modulator of the stress response. We have developed a novel in vitro perfusion system to enable the study of pharmacological and hormonal challenges to tissue components of the HPA axis. In vivo studies have shown functional sex differences (sexual diergism) in HPA responses to cholinergic drugs, and in the present in vitro study, we examine these differences at several levels of the HPA axis.

METHODS

Hypothalami, pituitaries, and adrenal glands were collected from male and female rats (n=3 per sex). One-half hypothalamus, one-half pituitary, and one adrenal gland were placed individually into three Erlenmeyer flasks connected by tubing. Flasks were perfused with medium (pH 7.4) at 37 degrees C. Sampling ports between the flasks were used to collect buffer for determination of corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and corticosterone (CORT) release from the hypothalamus, pituitary, and adrenal flasks, respectively, over an extended baseline period, to determine stability of the system, and after nicotine administration.

RESULTS

The perfusion system produced steady CRH, ACTH, and CORT baselines, the ACTH and CORT values being comparable to in vivo basal ACTH and CORT values in jugular-vein-cannulated rats. In vitro CRH, ACTH, and CORT responses to nicotine were significantly increased at 10 min and returned to baseline by 30 min, the CRH and ACTH responses from female tissues being greater than responses from male tissues. These sex differences were similar to those following nicotine administration in vivo.

DISCUSSION

The ability of this novel, dynamic in vitro system to replicate in vivo HPA axis responses supports its potential as a new method for pharmacological and toxicological studies.

Rat estrous cycle influences the sexual diergism of HPA axis stimulation by nicotine

We previously reported that female rats had significantly greater hypothalamic-pituitary-adrenal (HPA) axis responses to cholinergic stimulation by nicotine (NIC) than did male rats. Females in defined estrous cycle stages, however, were not studied because of sample size limitations. We further explored this finding by determining HPA axis responses to two doses of NIC in female rats (N = 101) during different estrous cycle stages, and in males (N = 69). NIC doses were: 0.3 mg/kg, which provided the greatest female-male difference in the earlier study, and 0.5 mg/kg, which stimulated the HPA axis similarly in the two sexes. Plasma AVP, ACTH, and corticosterone were measured. Proestrous and estrous females had higher ACTH responses to NIC (0.3 mg/kg) compared to metestrous and diestrous females, and compared to males. ACTH responses to NIC (0.5 mg/kg) were similar, regardless of estrous cycle stage or sex. Males had higher AVP responses to both NIC doses compared to females in all estrous cycle stages. Corticosterone responses followed the ACTH responses, except that females in all estrous stages started from a higher corticosterone baseline compared to males. These results are similar to our earlier findings across the estrous cycle with non-specific cholinergic stimulation by physostigmine and suggest that the nicotinic system contributes to the differential HPA axis responses to cholinergic challenge across the estrous cycle.

Environmental enrichment lowers stress-responsive hormones in singly housed male and female rats

Structural and social aspects of an environmental system can influence the physiology and behavior of animals occupying that system. This study examined the physiological effects of environmental enrichment (EE) with Kong Toys and Nestlets on stress-responsive hormones of the hypothalamic-pituitary-adrenal (HPA) axis under basal and mild stress conditions in singly housed, jugular vein-cannulated, male and female rats. Animals of both sexes housed with EE had significantly lower baseline adrenocorticotropic hormone (ACTH) and corticosterone (CORT) concentrations compared to those housed without EE. ACTH responses to the mild stress of saline injection were significantly lower in female rats housed with EE. Interaction with the Kong Toys and Nestlets appears to have provided the rats with a diversion from monotonous cage life, resulting in lower HPA axis activity before and after mild stress. These results are important because low, stable baselines are essential for accurately discerning pharmacological and other influences on the HPA axis.