Anti-nociception following exposure to trimethylthiazoline, peripheral or intra-amygdala estrogen and/or progesterone

Absent LH signaling rescues the anxiety phenotype in aging female mice

Clinical studies and experimental data together support a role for pituitary gonadotropins, including luteinizing hormone (LH), otherwise considered solely as fertility hormones, in age-related cognitive decline. Furthermore, rising levels of LH in post-menopausal women have been implicated in the high prevalence of mood disorders. This study was designed to examine the effect of deficient LH signaling on both cognitive and emotional behavior in 12-month-old Lhcgr-/- mice. For this, we established and validated a battery of five tests, including Dark-Light Box (DLB), Y-Maze Spontaneous Alternation, Novel Object Recognition (NOR), and contextual and cued Fear Conditioning (FCT) tests. We found that 12-month-old female wild type mice display a prominent anxiety phenotype on DLB and FCT. This phenotype was not seen in 12-month-old female Lhcgr-/- mice, indicating full phenotypic rescue. Furthermore, there was no effect of LHCGR depletion on recognition memory or working spatial memory on NOR and Y-maze testing, respectively, in 12-month-old mice, notwithstanding the absence of a basal phenotype in wild type littermates. The latter data do not exclude an effect of LH on cognition documented in previous studies. Finally, 12-month-old male mice and 3-month-old male and female mice did not consistently display deficits on any test. The data collectively document, for the first time, that loss of LH signaling reverses age-related emotional disturbances, a prelude to future targeted therapies that block LH action.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Pharmacological mechanisms involved in the analgesia induced by ethanol extract of Hybanthus enneaspermus leaves

Background

Hybanthus enneaspermus (HE) leaves are being used traditionally to relieve pain, and scientific studies have demonstrated their analgesic potential. This study attempted to elucidate the pharmacological mechanism(s) involved in the analgesic action of ethanol extract of H. enneaspermus leaves (EEHE).

Materials and methods

Forty-two male Wistar rats were separately randomized into seven groups (n=6 rats in each group) for tail immersion and formalin tests. Group I (control) received distilled water (10 mL/kg) while groups II and III received acetaminophen (the reference drug, 100 mg/kg ip) and EEHE (1000 mg/kg po), respectively. Groups IV–VII were pretreated with cimetidine (50 mg/kg ip), naloxone (5 mg/kg ip), propranolol (0.15 mg/kg ip), and prazosin (0.15 mg/kg ip), respectively, 1 hour before EEHE (1000 mg/kg po) treatment.

Results

The EEHE-induced increase in tail-flick latency was reduced by blockade of histamine and adrenergic receptors but prevented by blockade of opiate receptor in the tail-flick test. However, the EEHE-induced decrease in paw licking time was prevented only by blockade of opiate receptor but unaffected by histamine and adrenergic receptors blockers.

Conclusion

These findings suggest that the analgesic effect of EEHE in different pain types may involve different neural mechanisms and that the opioidergic pathway contributes more to EEHE-induced analgesia than the other pathways.

Mild Social Stress in Mice Produces Opioid-Mediated Analgesia in Visceral but Not Somatic Pain States

Visceral pain has a greater emotional component than somatic pain. To determine if the stress-induced analgesic response is differentially expressed in visceral versus somatic pain states, we studied the effects of a mild social stressor in either acute visceral or somatic pain states in mice. We show that the presence of an unfamiliar conspecific mouse (stranger) in an adjacent cubicle of a standard transparent observation box produced elevated plasma corticosterone levels compared with mice tested alone, suggesting that the mere presence of a stranger is stressful. We then observed noxious visceral or somatic stimulation-induced nociceptive behavior in mice tested alone or in mildly stressful conditions (ie, beside an unfamiliar stranger). Compared with mice tested alone, the presence of a stranger produced a dramatic opioid-dependent reduction in pain behavior associated with visceral but not somatic pain. This social stress-induced reduction of visceral pain behavior relied on visual but not auditory/olfactory cues. These findings suggest that visceral pain states may provoke heightened responsiveness to mild stressors, an effect that could interfere with testing outcomes during simultaneous behavioral testing of multiple rodents.

PERSPECTIVE

In mice, mild social stress due to the presence of an unfamiliar conspecific mouse reduces pain behavior associated with noxious visceral but not somatic stimulation, suggesting that stress responsiveness may be enhanced in visceral pain versus somatic pain states.

Odour-induced analgesia mediated by hypothalamic orexin neurons in mice

Various folk remedies employ certain odorous compounds with analgesic effects. In fact, linalool, a monoterpene alcohol found in lavender extracts, has been found to attenuate pain responses via subcutaneous, intraperitoneal, intrathecal, and oral administration. However, the analgesic effects of odorous compounds mediated by olfaction have not been thoroughly examined. We performed behavioural pain tests under odourant vapour exposure in mice. Among six odourant molecules examined, linalool significantly increased the pain threshold and attenuated pain behaviours. Olfactory bulb or epithelium lesion removed these effects, indicating that olfactory sensory input triggered the effects. Furthermore, immunohistochemical analysis revealed that linalool activated hypothalamic orexin neurons, one of the key mediators for pain processing. Formalin tests in orexin neuron-ablated and orexin peptide-deficient mice showed orexinergic transmission was essential for linalool odour-induced analgesia. Together, these findings reveal central analgesic circuits triggered by olfactory input in the mammalian brain and support a potential therapeutic approach for treating pain with linalool odour stimulation.

Relationship of female sex hormones with pain perception: focus on estrogens

SUMMARY The role of gonadal hormones has slowly gathered the right attention in the study of chronic pain mechanisms. The clear presence of sex differences in chronic pain and the number of studies showing the power of gonadal hormones to modify pain-induced behavioral responses appear to have convinced clinicians and researchers. Indeed, available data strongly indicate that more studies on gonadal hormones would certainly enhance the possibility of greatly increasing the knowledge of pain mechanisms and, thus, treatments. In the present article, old and new literature are summarized to evaluate data on pain and its modulation by gonadal hormones, particularly estrogens. Peripheral and central targets of these hormones are discussed with the aim of renewing interest in important aspects of estrogenic functions and their interactions with pain processes.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

Effects of salt-loading hypertension on nociception in rats

Background

There is on going controversy on the effect of experimentally induced hypertension on nociception. The effect of salt-loading-induced hypertension on pain was studied in male rats.

Method

Twenty-four male Sprague-Dawley rats (160–280 g) were divided into two groups. Group A (n = 12) was treated with normal-feed diet (control), while group B (n = 12) was treated with 8% salt-loaded diet for 10 weeks. After 10 weeks of the treatment, six rats each from groups A and B were used for blood pressure measurement, while the remaining six rats were used for both the tail-flick and formalin tests. Thermal and chemical pain test were assessed using tail immersion test (tail flick) and formalin test pain paradigms at onset of salt-loading diet and after 10 weeks of salt loading.

Results

Chronic administration of salt-loading diet caused significant increases (P < 0.001) in systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure. Moreover, salt-loading-induced hypertension was found to significantly reduce pain sensitivity in the tail-immersion test (P < 0.001) and in the early and late phase of the formalin test (P < 0.01). However, the hypoalgesia was higher in the late phase (94.8%) than in the early phase (56.8%) of the formalin test.

Conclusion

The present study suggests that high salt-loading-induced hypertension causes hypoalgesia in rats, which might be due more to reduction in inflammatory response.

Altered anxiety and defensive behaviors in Bax knockout mice

Developmental neuronal cell death is critically regulated by the pro-death protein Bax. Bax-/- mice exhibit increased neuron number, the elimination of several neural sex differences, and altered socio-sexual behaviors. Here we examined the effects of Bax gene deletion on anxiety and defensive behaviors by comparing the responses of male and female wildtype and Bax-/- mice to two different tests. On the elevated plus maze, Bax-/- mice of both sexes made more entries into and spent more time in the outer portion of open arms, indicating decreased anxiety compared to wildtype animals. Next, we exposed mice to two odors: trimethylthiazoline (TMT), an olfactory component of fox feces that rodents find aversive, and butyric acid (BA), an aversive odor without ecological significance. Each odor was presented individually and all animals were tested with both odors in a counterbalanced design. TMT was consistently more aversive than BA across a variety of behaviors (e.g., mice spent less time close to the odor source). Overall, Bax -/- mice showed fewer stretch approaches to both TMT and BA than wildtypes, but they avoided the odor source more (e.g., fewer contacts and less time spent in proximity). Finally, no effect of genotype was seen in baseline olfactory behavior; all mice were able to locate a buried food item, demonstrating that Bax-/- mice do not have impaired olfaction per se. Collectively, these data suggest a change in strategy with anxiety and defensive behaviors in Bax-/- mice, indicating that alterations in cell number affect more general mechanisms of fear and anxiety in addition to behaviors directly related to reproduction.

II. Cognitive performance of middle-aged female rats is influenced by capacity to metabolize progesterone in the prefrontal cortex and hippocampus

Cognitive decline can occur with aging; however, some individuals experience less cognitive decline than do others. Secretion of ovarian hormones is reduced post-menopause and may contribute to cognitive function. The extent to which hormonal effects may be parsed out from other age-related factors to influence cognition is of interest. Middle-aged (12-month-old) female rats that were retired breeders were categorized as maintaining or declining reproductive function based upon their estrous cyclicity (regular 4-5 day cycles), fertility (> 60 % successful pregnancy), and fecundity (>10 pups/litter). Performance in object recognition, Y-maze, water maze, inhibitory avoidance, and contextual-cued fear conditioning was evaluated. Estradiol, progesterone (P(4)), dihydroprogesterone, and 5α-pregnan-3α-ol-20-one (3α,5α-THP) were assessed in medial prefrontal cortex (mPFC) and hippocampus; corticosterone was assessed in plasma. Rats maintaining reproductive function performed significantly better on the object recognition, Y-maze, water maze, inhibitory avoidance, and cued fear conditioning tasks than did rats with declining reproductive function. Steroid concentrations varied greatly within groups. Higher levels of P(4) in mPFC and hippocampus were associated with better Y-maze performance. In mPFC, higher levels of P(4) were associated with poorer inhibitory avoidance performance; greater levels of 3α,5α-THP were associated with better object memory. Neither estradiol nor corticosterone levels significantly contributed to cognitive performance. Thus, the capacity for cortico-limbic P(4) utilization may influence cognitive performance in aging.

Nociceptive and anxiety-like behavior in reproductively competent and reproductively senescent middle-aged rats

BACKGROUND

Changes in levels of estradiol and progesterone that occur with the transition to reproductive senescence may influence nociception or affect.

OBJECTIVE

To ascertain whether nociceptive and affective processes change with reproductive senescence, this study examined pain and anxiety-like behaviors in middle-aged female rats that were reproductively competent, transitioning to reproductive senescence, or reproductively senescent.

METHODS

Middle-aged (12-14 months old) female rats (N = 46) were tested in the following tasks to assess pain and anxiety-like behavior: tail flick, elevated plus maze, elevated zero maze, mirror maze, Vogel punished drinking, and defensive burying. For the tail-flick task, the latency for rats to move their tail from a heat source, as an indication of pain sensitivity, was determined. In the elevated plus and elevated zero mazes, the time spent on the open arms or quadrants, respectively, were determined as measures of reduced anxiety behavior. In the mirror maze, the time spent in the mirrored portion of the chamber was used as an indicator of anxiety-like responding. In the Vogel task, the number of punished licks made was determined as a measure of reduced anxiety-like behavior. In the defensive burying task, the duration spent by rats burying an electrified prod postfootshock was utilized as an index of anxietylike responding. All rats were experimentally naive, retired breeders from our colony and had not had a litter or been lactating for 1 to 4 weeks before behavioral testing.

RESULTS

Although tail-flick latencies were not significantly different among rats that were reproductively competent or senescent, reproductively competent rats had less anxiety-like behavior in the elevated plus maze (more time spent on the open arms: F(2,43) = 5.93; P < 0.01), elevated zero maze (more time spent on the open quadrants: F(2,43) = 4.62; P = 0.01), and Vogel punished drinking task (more punished licks made: F(2,43) = 3.76; P = 0.03). There were no statistically significant differences in the mirror maze and defensive burying task.

CONCLUSION

In this study of adult female rats, nociceptive behavior did not vary significantly with reproductive senescence, but anxiety-like behavior of rats did.

Morphine-induced stereotyped thigmotaxis could appear as enhanced fear and anxiety in some behavioural tests

This study questions whether the classical interpretation for unconditional fear/anxiety tests is valid when animals are under the influence of some drugs of abuse. We used a modified version of the trimethylthiazoline (TMT)-avoidance task, a measure of unconditional fear. Halfway into a corridor maze we placed a 3-cm-high barrier. This provided a wall in the middle of the corridor, one that the mice can easily climb over. Saline- and morphine-treated mice were randomly placed in the 'safe' or 'unsafe' (TMT) side and observed for 10 min. As expected, saline-injected mice spent only about 25% of the time in the TMT side, regardless of the side they were initially placed into. In contrast, morphine-treated mice did not cross the barrier even once, regardless of their initial placement. Specifically, morphine-treated mice initially placed in the TMT side appeared to exhibit the expected reduction in unconditional fear, that is, spending the entire time in the TMT side, a significant increase over the controls. Yet, morphine-treated mice placed in the safe side never even entered the TMT side; thus, these mice appeared to exhibit a behavioural response that is classically interpreted as increased fear, that is, spending significantly less time in the TMT side versus the controls. In summary, this study demonstrates that the classical interpretation of some unconditioned fear or anxiety tests could be misleading when animals are under the influence of drugs that might induce other competing behaviours.

Differential activation of the periaqueductal gray by mild anxiogenic stress at different stages of the estrous cycle in female rats

The effect of acute exposure to mild anxiogenic stress on cutaneous nociceptive threshold was investigated in female Wistar rats at different stages of the estrous cycle. Baseline tail flick latencies did not change significantly during the cycle. However after brief exposure to vibration stress (4 Hz for 5 min), rats in late diestrus, but not at other cycle stages, developed a hyperalgesia (decrease in tail flick latency). Animals in late diestrus revealed a more than fivefold increase in the density of Fos-like immunoreactive nuclei in the dorsolateral, lateral, and ventrolateral columns in the caudal half of the periaqueductal gray matter (PAG). There was no change in the density of Fos-like immunoreactive nuclei in the PAG in rats in estrus and early diestrus, although rats in proestrus showed a smaller (50%) but significant increase. Rats undergoing withdrawal from a progesterone dosing regimen (5 mg/kg i.p. twice daily for 6 days) designed to mimic the fall in progesterone that occurs naturally during late diestrus, exhibited a stress-induced hyperalgesia that was similar to animals in late diestrus and a significant increase in Fos-positive cells in the PAG. We suggest that falling levels of progesterone during late diestrus may be a predisposing factor for the development of stress-induced hyperalgesia, which is linked to differential activation of descending pain control circuits in the PAG. Similar changes in women, when progesterone levels fall during the late luteal phase of the menstrual cycle, may contribute to the development of premenstrual symptoms that include increased anxiety and hyperalgesia.

Acute neurosteroids inhibit the spinal reflex potentiation via GABAergic neurotransmission

Recently, we demonstrated a chronic neurosteroid-dependent inhibition of activity-dependent spinal reflex potentiation (SRP), but it remains unclear whether neurosteroids acutely modulate SRP induction. This study shows progesterone as well as two of its 3alpha,5alpha-derivatives, allopregnalonone and 3alpha,5alpha-tetrahydrodeoxycorticosterone (THDOC), to be capable of producing acute GABA(A) receptor (GABA(A)R)-dependent inhibition of SRP. When compared with test simulation (1 stimulation/30 s) of pelvic afferent nerves that evoked a baseline reflex activity in an external urethra sphincter electromyogram, repetitive stimulation (RS; 1 stimulation/1 s) induced SRP characterized by an increase in the evoked activity. Intrathecal progesterone (3-30 muM, 10 microl) at 10 min before stimulation onset dose dependently prevented RS induction. Intrathecal allopregnalonone (10 muM, 10 microl it) and THDOC (10 microM, 10 microl it) also prevented the SRP caused by RS. Pretreatment with the GABA(A)R antagonist bicuculline (10 microM, 10 microl it) at 1 min before progesterone/neurosteroid injection attenuated the inhibition of SRP caused by progesterone, allopregnanolone, and THDOC. Results suggest that progesterone and its neurosteroid metabolites may be crucial to the development of pelvic visceral neuropathic/postinflammatory pain and imply clinical use of neurosteroids, such as allopregnanolone and THDOC, for visceral pain treatment.

Neuroactive steroids inhibit spinal reflex potentiation by selectively enhancing specific spinal GABA(A) receptor subtypes

Recently, we demonstrated a spinal GABA(A) receptor (GABA(A)R)-dependent inhibition on the induction of repetitive stimulation-induced spinal reflex potentiation. However, it remains unclear whether steroid hormones modulate such an inhibition. Here, we show that progesterone is capable of producing GABA(A)Rs-dependent inhibition of the induction of spinal reflex potentiation by actions through neurosteroid metabolites. Progesterone (5mg/kg, twice daily for 4 days) up-regulates the expression of GABA(A)R alpha2, alpha3, alpha4 and delta subunits, and is associated with attenuated repetitive stimulation-induced spinal reflex activity in ovariectomized rats. These changes were blocked by finasteride (50mg/kg, twice daily), an antagonist of neurosteroid synthesis from progesterone, but not by the progesterone receptor antagonist, RU486 (100mg/kg, twice daily). The induction of spinal reflex potentiation was attenuated after a short (30 min) intrathecal treatment with the neurosteroids, allopregnanolone (ALLOP, 10 microM, 10 microL) and 3 alpha,5 alpha-tetrahydrodeoxycorticosterone (THDOC, 10 microM, 10 microL). Acute intrathecal administration of the GABA(A)R antagonist, bicuculline (10 microM, 10 microL) reversed the inhibition produced by progesterone, THDOC and allopregnanolone. These results imply that progesterone-mediated effects on GABA(A)R expression and neural inhibition are regulated by neurosteroids synthesis rather than progesterone receptor activation.

Increasing 3alpha,5alpha-THP following inhibition of neurosteroid biosynthesis in the ventral tegmental area reinstates anti-anxiety, social, and sexual behavior of naturally receptive rats

The progesterone metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), has actions in the midbrain ventral tegmental area (VTA) to modulate lordosis, but its effects on other reproductively relevant behaviors are not well understood. Effects on exploration, anxiety, and social behavior resulting from inhibition of 3alpha,5alpha-THP formation, as well as 3alpha,5alpha-THP enhancement, were investigated in the midbrain VTA. Naturally sexually receptive, female rats (n=8-10/group) received infusions aimed at the midbrain VTA of vehicle, PK11195 (an inhibitor of neurosteroidogenesis), and/or indomethacin (an inhibitor of 3alpha,5alpha-THP formation from prohormones), and were subsequently infused with vehicle or FGIN 1-27 (a neurosteroidogenesis enhancer). The rats were then assessed in a behavioral battery that examined exploration (open field), anxiety (elevated plus maze), social (social interaction), and sexual (paced mating) behavior. Inhibition of 3alpha,5alpha-THP formation decreased exploratory, anti-anxiety, social, and sexual behavior, as well as midbrain 3alpha,5alpha-THP levels. Infusions of FGIN 1-27 following 3alpha,5alpha-THP inhibition restored these behaviors and midbrain 3alpha,5alpha-THP levels to those commensurate with control rats that had not been administered inhibitors. These findings suggest that 3alpha,5alpha-THP formation in the midbrain VTA may influence appetitive, as well as consummatory, aspects of mating behavior.

Exploratory, anti-anxiety, social, and sexual behaviors of rats in behavioral estrus is attenuated with inhibition of 3alpha,5alpha-THP formation in the midbrain ventral tegmental area

The progesterone (P(4)) metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) acts in the midbrain ventral tegmental area (VTA) to modulate lordosis of female rats. 3alpha,5alpha-THP also mediates exploratory, affective, and social behaviors; whether actions of 3alpha,5alpha-THP in the VTA mediate these behaviors is of interest. To elucidate the role of the VTA in mediating exploratory, affective, and social behaviors, the present study examined effects of inhibiting 3alpha,5alpha-THP formation in the VTA. Rats received intra-VTA infusions of either PK11195 (400ng/mul, which inhibits de novo 3alpha,5alpha-THP production), indomethacin (10mug/mul, which blocks metabolism of P(4) to 3alpha,5alpha-THP), PK11195 and indomethacin together, or beta-cyclodextrin vehicle and tested on a battery of anxiety (open field and elevated plus maze), social (partner preference and social interaction), and sexual (paced mating) tasks. Compared to rats infused with vehicle to the VTA, rats infused with inhibitor(s) demonstrated significant reductions in central entries in the open field, time on open arms of an elevated plus maze, time spent interacting with a conspecific, initiation and intensity of lordosis, sexual solicitations, and midbrain 3alpha,5alpha-THP levels. These findings suggest that actions of 3alpha,5alpha-THP in the VTA are important for mediating aspects of exploration, anxiety, and social behavior related to mating.

Modulation by female sex hormones of the cannabinoid-induced catalepsy and analgesia in ovariectomized mice

Cannabinoids are psychoactive compounds with many pharmacological properties such as analgesia, sedation and catalepsy most of which are mediated by cannabinoid CB1 receptors. In the present study, we evaluated whether the ovarian sex hormones are involved in the cannabinoid-induced catalepsy and analgesia in ovariectomized female mice. Female NMRI mice (weighing 25-30 g) were divided into 3 main groups: unoperated, sham-operated and ovariectomized. Both the catalepsy and analgesia induced by different doses of the synthetic cannabinoid WIN 55,212-2 (2 and 4 mg/kg, i.p.) were examined in the groups in the presence or absence of the cannabinoid CB1 antagonist AM251 (0.5 mg/kg). We also evaluated effects of estradiol valerate (10 mg/kg) and progesterone (25 mg/kg) on catalepsy and analgesia induced by WIN 55,212-2 in ovariectomized mice. The antinociceptive effect of WIN 55,212-2 was significantly (P<0.01) enhanced in ovariectomized mice, which was prevented by pretreatment with estradiol but not by progesterone. There was no significant difference in the cannabinoid-induced catalepsy between control and ovariectomized mice. However, pretreatment with progesterone but not estradiol potentiated the cataleptic effect of low dose of WIN 55,212-2 (2 mg/kg) in ovariectomized mice (P<0.01). The present data demonstrated for the first time that ovarian sex steroids could modulate both cannabinoid-induced catalepsy and analgesia in female ovariectomized mice.

Antisense oligodeoxynucleotides for estrogen receptor-beta and alpha attenuate estradiol's modulation of affective and sexual behavior, respectively

Estradiol (E(2)) modulates affective and socio-sexual behavior of female rodents. E(2)'s functional effects may involve actions through alpha and beta isoforms of estrogen receptor (ERs). The importance of E(2)'s actions at these isoforms for anxiety (open field, elevated plus maze), depression (forced swim test), and sexual behavior (lordosis) was investigated using an antisense oligonucleotide (AS-ODN) strategy. If ERbeta is required for anti-anxiety and antidepressant-like effects, and ERalpha is required for sexual receptivity, of E(2), then intracerebroventricular administration of AS-ODNs against these ERs should attenuate these effects and reduce immunoreactivity of ERs in brain regions that mediate these behaviors, such as the hippocampus and ventral medial hypothalamus (VMH). Ovariectomized rats were primed with 17beta-E(2) (10 microg) 48 h before testing (hour 0). At hours 0, 24, and 47.5, rats were infused with saline vehicle, scrambled control AS-ODNs, or AS-ODNs targeted against ERalpha and/or ERbeta, and were tested at hour 48. Rats infused with ERbeta AS-ODNs, alone, or with ERalpha AS-ODNs had significantly decreased open field central entries, decreased plus maze open arm time and entries, increased time spent immobile, and decreased time spent swimming in the forced swim test, and decreased ERbeta immunoreactivity in the brain than did rats administered ERalpha AS-ODNs, vehicle, or scrambled AS-ODNs. Rats that were administered ERalpha AS-ODNs, alone, or with ERbeta AS-ODNs had significantly decreased lordosis and decreased ERalpha immunoreactivity in the brain compared to rats administered ERbeta AS-ODNs, vehicle, or scrambled AS-ODNs. Thus, ERbeta and ERalpha may be required for E(2)'s modulation of affective and sexual behavior, respectively.

Modulation of pain by estrogens

It has become increasingly apparent that women suffer a disproportionate amount of pain during their lifetime compared to men. Over the past 15 years, a growing number of studies have suggested a variety of causes for this sex difference, from cellular to psychosocial levels of analysis. From a biological perspective, sexual differentiation of pain appears to occur similarly to sexual differentiation of other phenomena: it results in large part from organizational and activational effects of gonadal steroid hormones. The focus of this review is the activational effects of a single group of ovarian hormones, the estrogens, on pain in humans and animals. The effects of estrogens (estradiol being the most commonly examined) on experimentally induced acute pain vs. clinical pain are summarized. For clinical pain, the review is limited to a few syndromes for which there is considerable evidence for estrogenic involvement: migraine, temporomandibular disorder (TMD) and arthritis. Because estrogens can modulate the function of the nervous, immune, skeletal, and cardiovascular systems, estrogenic modulation of pain is an exceedingly complex, multi-faceted phenomenon, with estrogens producing both pro- and antinociceptive effects that depend on the extent to which each of these systems of the body is involved in a particular type of pain. Forging a more complete understanding of the myriad ways that estrogens can ameliorate vs. facilitate pain will enable us to better prevent and treat pain in both women and men.

Dose- and time-dependent estradiol modulation of morphine antinociception in adult female rats

To clarify the activational role of ovarian hormones on pain and analgesia, the present study determined whether estradiol (E2) modulation of nociception and morphine antinociception in adult female rats depends on (1) the dose of E2 and (2) the interval between E2 treatment and nociceptive testing. Female rats were ovariectomized (OvX) and either oil vehicle (0), or E2 (0.25, 2.5 or 25 microg/0.1 ml vehicle) was injected s.c. two consecutive days of every four days for five cycles before testing. Either 4, 24, 48 or 96 h after the last injection, nociception was evaluated on the 50 degrees C hotplate and warm water tail withdrawal tests before and after escalating doses of s.c. morphine. Lordosis behavior and uterine weight were assessed in other rats at the same E2 doses and time points. E2 significantly lengthened latency to respond on the hotplate test at 24 h after the last injection, but had no significant effect on tail withdrawal latencies. The lower doses of E2 significantly increased morphine antinociceptive potency at 4-24 h on one or both tests, but the intermediate E2 dose significantly decreased morphine potency at 48 h on the hotplate test. Thus, E2 modulation of morphine antinociception in the adult female rat is bidirectional, and occurs at E2 doses producing cyclic changes in sexual behavior, uterine weight and vaginal cytology that are similar to those observed in gonadally intact, cycling females.

Genetic influences on behavioral and neuroendocrine responses to predator-odor stress in rats

The exposure of animals to a variety of stressful events can induce behavioral and physiological responses, which can be modulated by anxiety levels. It is well recognized that genetic factors play a substantial role in both anxiety and stress reactivity. The present study examined the effect of exposure to 2,4,5-trimethylthiazoline (TMT), a component of fox feces, on nociception and corticosterone levels in Lewis (LEW) and Spontaneously Hypertensive (SHR) inbred rat strains (which display genetic differences in anxiety models such as the elevated plus-maze and open-field). The influence of two quantitative trait loci (QTL), named Ofil1 and Ofil2, which are known to affect emotionality in LEW versus SHR intercrosses on the responses to TMT was also investigated. LEW and SHR rats of both sexes displayed similar levels of behavioral and neuroendocrine responses after TMT exposure. As expected, TMT odor stress produced analgesia and enhanced corticosterone levels. Ofil1 on chromosome 4 affected stress-induced analgesia in males only. Ofil2 on chromosome 7 had no effect. The results suggest that behaviors measured in classical models of generalized anxiety and reactivity to stress produced by predator odors can be genetically dissociated. Finding a locus with an effect on the behavioral responses to stress represents the starting point in the search for genes responsible for stress-related traits.

Inhibiting 5alpha-reductase in the amygdala attenuates antianxiety and antidepressive behavior of naturally receptive and hormone-primed ovariectomized rats

RATIONALE

Greater incidence of anxiety and depressive disorders of women compared to men may be due in part to progesterone (P) and its neuroactive metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), acting in limbic regions, such as the amygdala.

OBJECTIVE

If P's metabolism via 5alpha-reduction to 3alpha,5alpha-THP in the amygdala is critical for antianxiety and antidepressive behavior, then blocking 5alpha-reductase in the amygdala of female rats is likely to attenuate the antianxiety and antidepressive effects of high progestin levels from both endogenous and exogenous sources.

METHODS

Naturally receptive female rats with high endogenous estrogen (E2) and P and ovariectomized (ovx) rats administered E2 (10 microg) and P (500 microg) subcutaneously were administered finasteride (10 microg/microl), a Type II 5alpha-reductase inhibitor, or vehicle to the amygdala. Anxiety behavior (open field, elevated plus maze, defensive freezing) and depressive behavior (Porsolt forced swim test) were assessed.

RESULTS

There were similar effects of finasteride administration to the amygdala to attenuate antianxiety behavior in naturally receptive and ovx, hormone-primed rats. Finasteride administration significantly decreased central entries in the open field, decreased open arm time in the elevated plus maze, increased defensive freezing in response to footshock, and increased time spent immobile compared to vehicle.

CONCLUSIONS

Thus, formation and subsequent actions of 3alpha,5alpha-THP in the amygdala may be important for antianxiety and antidepressive effects.

A review and update of mechanisms of estrogen in the hippocampus and amygdala for anxiety and depression behavior

Estrogen (E2) has many effects in the central nervous system, including effects on anxiety and depression behavior. This review will address effects of E2 on behaviors related to anxiety and depression in women and animal models and include recent findings from our laboratory related to this topic. E2's antianxiety and antidepressant-like effects may depend upon many factors, including the regimen of E2 utilized and interactions with the hypothalamic-pituitary-adrenal axis. Brain targets for E2's effects on anxiety and depression include the hippocampus and amygdala. Administration of E2, compared to vehicle, subcutaneously or to the hippocampus or amygdala of ovariectomized rats decreases anxiety and depressive behavior. Intracellular estrogen receptors (ERs) may be important for E2's anxiolytic and antidepressant-like effects. Administration of an ER antagonist to the hippocampus, but not amygdala, increases anxiety and depression behavior of naturally receptive female rats. Studies utilizing ER knockout mice or selective ER modulators suggest that ER-mediated effects of E2 on anxiety and depressive behavior may require ERbeta. In addition, the behavioral effects of E2 may involve membrane actions and/or changes in cell cycle processes involved in energy expenditure. Elucidating the mechanisms by which E2 affects anxiety and depression is important in order to enhance its therapeutic potential. It is particularly important to investigate the putative receptor mechanisms and brain targets for E2 to determine whether mood-enhancing effects of E2 can occur without deleterious proliferative effects in reproductive tissues.

3alpha-hydroxy-5alpha-pregnan-20-one in the midbrain ventral tegmental area mediates social, sexual, and affective behaviors

Progestins mediate the onset and duration of lordosis, the mating posture of female rodents, through actions in the hypothalamus and ventral tegmental area. In the hypothalamus, progesterone has traditional, "genomic" actions via intracellular progestin receptors. In the ventral tegmental area, 3alpha-hydroxy-5alpha-pregnan-20-one has "non-genomic" actions independent of progestin receptors to facilitate lordosis that involve GABA(A)/benzodiazepine receptors, NMDA type glutamate receptors, and/or dopamine receptors. 3alpha-Hydroxy-5alpha-pregnan-20-one levels also change with behavioral and/or environmental stimuli and may have a role in other reproductively-relevant behaviors, such as affiliation, exploration, and anxiety (socio-sexual behaviors). Data are reviewed that support the notion that: 1) effects of 3alpha-hydroxy-5alpha-pregnan-20-one in the midbrain ventral tegmental area facilitate lordosis and other reproductively-relevant behaviors. 2) 3alpha-Hydroxy-5alpha-pregnan-20-one, formed in the ventral tegmental area from metabolism of progestins, produced peripherally by endocrine glands, or centrally from biosynthesis in glial cells mediates socio-sexual behaviors. 3) 3alpha-Hydroxy-5alpha-pregnan-20-one's actions at GABA(A)/benzodiazepine receptors, NMDA type glutamate receptors, and dopamine receptors in the ventral tegmental area are important for lordosis; however, effects at these substrates on socio-sexual behaviors have not been elucidated. Given 3alpha-hydroxy-5alpha-pregnan-20-one's involvement in stress responses, its putative role as a homeostatic regulator and in the pathophysiology and treatment of neuropsychiatric disorders is discussed.

TMT-induced autonomic and behavioral changes and the neural basis of its processing

One of the main interests in the field of neuroscience is the investigation of the neural basis of fear. During recent years, an increasing number of studies have used trimethylthiazoline (TMT), a component of red fox feces, as a stimulus to induce fear in predator naive rats, mice, and voles. The aim of the present review is to summarize these studies. We present an overview to the autonomic and behavioral changes that are induced by TMT exposure. Then, we summarize the small number of studies that have examined the neural processing of the TMT stimulus. Finally, we compare these studies with those using a natural predator or predator odor to induce fear and discuss the possible use of TMT exposure in rodents as an animal model of unconditioned fear in humans.

Recognition and avoidance of the odors of parasitized conspecifics and predators: differential genomic correlates

In many species of animals chemical stimuli are an important source of information about the threats and dangers present in the social and non-social world. Olfactory cues play a fundamental role in modulating social recognition and interactions in a wide variety of mammals. Rodents, in particular, utilize chemical signals, to recognize and avoid conspecifics infected with parasites and other pathogens. Animals also respond to, and utilize, predator odor related information to assess and minimize their risk of predation. In this review, we briefly focus on the relations between odors, parasite recognition and avoidance and consider some of the associated hormonal, neural and genomic mechanisms. We describe how both male and female rodents distinguish between infected and uninfected males on the basis of odors, displaying aversive response to, and avoidance of, the urine odors of infected individuals. We further describe how the recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin, (OT), and estrogenic mechanisms. We show that mice with deletions of the oxytocin gene (OT knockout mice (OTKO)) and mice whose genes for estrogen receptor (ER)-alpha or ER-beta [ER knockout mice (ERKO), alphaERKO and betaERKO] have been disrupted are specifically impaired in their recognition, avoidance, and memory of the odors of infected individuals. We contrast this with the recognition and display of aversive responses to predator (cat) odor that are insensitive to these genetic manipulations. These findings reveal some of the mechanisms associated with the olfactory mediated recognition of parasitized individuals and predators.

Antianxiety and antidepressive behavior produced by physiological estradiol regimen may be modulated by hypothalamic-pituitary-adrenal axis activity

Variations in estradiol (E(2)) may influence expression of stress-related anxiety and depression symptoms among women. Effects of E(2) and stress on anxiety and depressive behavior were investigated using an animal model. E(2) was administered subcutaneously (0, 2, 5, 10, 20, 50 mug/rat) to ovariectomized rats 2 days before testing. In experiment 1, open field (anxiety), elevated plus maze (anxiety), or forced swim test (depressive) behavior was evaluated following 20 min of restraint or no such stressor. Rats administered 5 or 10 mug E(2), which produced physiological plasma E(2) concentrations, showed significantly less anxiety and depressive behavior and lower corticosterone levels compared to vehicle, lower, or higher E(2) dosages. Restraint stress prior to behavioral testing attenuated the antianxiety and antidepressive effects of 5 or 10 mug E(2). In experiment 2, effects of adrenalectomy or sham surgery and vehicle or corticosterone replacement in their drinking water on behavior and neuroendocrine measures of rats administered 0, 10, or 50 mug E(2) were examined. E(2), 10 mug, compared to vehicle or 50 mug, reduced anxiety and depressive behavior of sham and adrenalectomized rats administered the low dosage of corticosterone, but not vehicle or the high dosage of corticosterone, suggesting that there may be an optimal level of corticosterone necessary for E(2) to exert these effects. Together, these data suggest that E(2) may have dose-dependent effects on anxiety and depressive behavior of female rodents, which may depend on the tone of the hypothalamic-pituitary-adrenal axis.

Gonadal hormone modulation of mu, kappa, and delta opioid antinociception in male and female rats

Previous studies suggest that sex differences in morphine antinociception in rodents might be attributed to the activational effects of gonadal hormones. The present study determined whether hormonal modulation of opioid antinociception in adult rats extends to opioids other than the prototypic mu agonist morphine. Male and female rats were sham-gonadectomized (sham-GDX) or gonadectomized (GDX) and replaced with no hormone, estradiol (E2, females), progesterone (P4, females), E2+P4 (females), or testosterone (males). Approximately 28 days later, nociception was evaluated on the 50 degrees C hot plate and warm water tail withdrawal tests before and after subcutaneous administration of hydromorphone, buprenorphine, U50,488, or SNC 80. In sham-GDX (gonadally intact) rats, the mu agonists and U50,488 were less effective in females than in males in at least one nociceptive test, and the delta agonist SNC 80 was less effective in males than in females. In males, gonadectomy tended to decrease, and testosterone tended to increase antinociception produced by 3 of the 4 agonists. In females, gonadectomy and hormone treatment had more variable effects, although E2 tended to decrease mu opioid antinociception. The present results suggest that activational effects of gonadal hormones are relatively modest and somewhat inconsistent on antinociception produced by various opioid agonists in the adult rat.

PERSPECTIVE

This study demonstrates that reproductive hormones such as testosterone in males and estradiol in females do not consistently modulate sensitivity to the analgesic effects of opioids in the adult organism.