Sex differences in conditioned taste aversion and in the effects of exposure to a specific pulsed magnetic field in deer mice Peromyscus maniculatus

Differential effects of thirst and satiety on conditioned taste aversion acquisition, retrieval, and memory extinction

Thirst is an essential motivational component that could modulate the strength of conditioning; pioneer studies show that the rats' sexual dimorphism observed in the rate of aversive memory extinction of conditioned taste aversion (CTA) is affected by the state of fluid deprivation. On the other hand, previous evidence suggests that fluid intake volume and temporal context before and during conditioning may influence CTA. Furthermore, although CTA has been demonstrated using various types of stimuli, neural processing and homeostatic regulation of water and nutritional balance may differ depending on the stimulus used and the conditioning stages. Therefore, this study explored the effects of state motivated by thirst and satiation, using saccharin, as a non-caloric sweet stimulus, during CTA and the aversive memory extinction process under similar contextual and temporal conditions. First, we implemented an ad libitum water protocol in male and female adult rats to evaluate saccharin aversive memory formation; we compared this with a traditional CTA with liquid deprivation in the same context and temporal consumption conditions. Furthermore, we evaluated whether liquid satiety affects the acquisition or the aversive memory retrieval differentially. Our results show that the ad libitum liquid regimen allows reliable quantifications of basal water consumption, monitored every hour for more than five days. We observed a reliable CTA, where the magnitude of aversive memory and its extinction is significantly higher in both male and female rats; the strong CTA observed is substantially due to the satiety state during taste aversion memory retrieval. Our data show that although liquid deprivation does not affect CTA acquisition, it does induce weakness in the magnitude of aversive retrieval expression and fast aversive memory extinction, similarly in male and females. Overall, the results indicate that the need to satiate the demand for liquids during retrieval prevails over the conditioned aversion learned, suggesting, that thirst is a source of temporary variables dominating the aversive responses during CTA retrieval.

Early exposure to intermediate-frequency magnetic fields alters brain biomarkers without histopathological changes in adult mice

Recently we have reported that intermediate-frequency magnetic field (IF-MF) exposure transiently altered the mRNA expression levels of memory function-related genes in the hippocampi of adult male mice. However, the effects of IF-MF exposure during brain development on neurological biomarkers have not yet been clarified. In the present study, we investigated the effect of IF-MF exposure during development on neurological and immunological markers in the mouse hippocampus in 3- and 7-week-old male mice. Pregnant C57BL/6J mice were exposed to IF-MF (21 kHz, 3.8 mT) for one hour per day from organogenesis period day 7 to 17. At adolescence, some IF-MF-exposed mice were further divided into exposure, recovery, and sham-exposure groups. The adolescent-exposure groups were exposed again to IF-MF from postnatal day 27 to 48. The expression of mRNA in the hippocampi was examined using a real-time RT-PCR method, and microglia activation was examined by immunohistochemical analysis. The expression levels of NR1 and NR2B as well as transcription factors (CaMKIV, CREB1), inflammatory mediators (COX2, IL-1 β,TNF-α), and the oxidative stress marker heme-oxygenase (HO)-1 were significantly increased in the IF-MF-exposed mice, compared with the control group, in the 7-week-old mice, but not in the 3-week-old mice. Microglia activation was not different between the control and other groups. This study provides the first evidence that early exposure to IF-MF reversibly affects the NMDA receptor, its related signaling pathways, and inflammatory mediators in the hippocampus of young adult mice; these changes are transient and recover after termination of exposure without histopathological changes.

Stronger geomagnetic fields may be a risk factor of male suicides

AIM

Some previous studies have shown a positive relation between geomagnetic disturbances and an increased incidence of suicide. If such a relation exists, stronger geomagnetic fields may affect the number of suicides, because stronger geomagnetic fields generally cause larger geomagnetic field disturbances. Therefore, we here investigated the relation between local geomagnetic field magnetic flux density and the standardized morbidity ratios (SMR) for suicide by each prefecture in Japan.

METHODS

Monthly suicide data for each prefecture in the period January 1999 to December 2008 was obtained, and it was found that a total of 216 171 male individuals and 85 154 female individuals committed suicide during this period. A multiple linear regression analysis was carried out with a backward elimination procedure. The SMR for suicide by each prefecture was taken as the response variable and the explanatory variables were each prefecture's local geomagnetic field magnetic flux density (nT), north latitude (°), monthly mean unemployment rate (%), monthly mean air pressure (hPa), monthly mean air temperature (°C), monthly mean humidity (%), and monthly total day length (hours). Analyses were carried out separately for each sex.

RESULTS

In the multiple linear regression analysis for male subjects, the local geomagnetic field magnetic flux density (nT), monthly mean unemployment rate (%), and monthly mean humidity (%) were associated with the incidence of suicide, but in the multiple linear regression analysis of female subjects, only north latitude was associated with that.

CONCLUSION

In this study, we generated a hypothesis that stronger geomagnetic fields affect the number of cases of male suicide.

Effect of sex on ethanol consumption and conditioned taste aversion in adolescent and adult rats

RATIONALE

Vulnerability to alcoholism is determined by many factors, including the balance of pleasurable vs. aversive alcohol-induced sensations: pleasurable sensations increase intake, while aversive sensations decrease it. Female sex and adolescent age are associated with lower sensitivity to intake-reducing effects and more rapid development of alcohol abuse.

OBJECTIVES

This study assessed voluntary drinking and the aversive effects of alcohol to determine whether these measures are inversely related across the sexes and development.

METHODS

Voluntary drinking of 20 % ethanol in an every-other-day (EOD) availability pattern and the dose-response relationship of ethanol conditioned taste aversion (CTA) were assessed in male and female adolescent and adult rats.

RESULTS

CTA was sex specific in adult but not adolescent rats, with adult females exhibiting less aversion. Voluntary ethanol consumption varied according to age and individual differences but was not sex specific. Adolescents initially drank more than adults, exhibited greater day-to-day variation in consumption, were more susceptible to the alcohol deprivation effect, and took longer to establish individual differences in consumption patterns.

CONCLUSIONS

These results show that the emergence of intake patterns differs between adolescents and adults. Adolescents as a group initiate drinking at high levels but decrease intake as they mature. A subset of adolescents maintained high drinking levels into adulthood. In contrast, most adults consumed at steady, low levels, but a small subset quickly established and maintained high-consumption patterns. Adolescents also showed marked deprivation-induced increases. Sex differences were not observed in EOD drinking during either adolescence or adulthood.

Association of geomagnetic disturbances and suicides in Japan, 1999-2010

OBJECTIVES

Previous studies have shown a positive relationship between geomagnetic disturbances and an increased incidence of suicide. The Japanese suicide rate is the ninth highest in the world, but there have been no reports examining the relationship between geomagnetic disturbance and the number of suicides, and, therefore, this paper examines this relationship.

METHODS

The number of Japanese suicides per month from January 1999 to December 2010 was obtained, and it was found that a total of 262,596 males and 102,539 females committed suicide during this period. To adjust the other factors which affect the number of suicides, a multiple linear regression analysis with backward elimination was carried out, with the monthly number of suicides as the response variable and the monthly mean K index value, monthly mean number of sunspots, monthly mean unemployment rate, proportion of elderly people (%), monthly mean air pressure (hPa), monthly mean air temperature (°C), monthly mean humidity (%), and monthly mean day length (h) as the explanatory variables.

RESULTS

In the multiple linear regression analysis for males, the monthly mean K index value was associated with the monthly number of suicides, but in females, the monthly mean K index value was not associated with the monthly number of suicides.

CONCLUSION

In this study, we generated a hypothesis that geomagnetic disturbances may trigger male suicides.

Lateral gradients significantly enhance static magnetic field-induced inhibition of pain responses in mice--a double blind experimental study

Recent research demonstrated that exposure of mice to both inhomogeneous (3-477 mT) and homogeneous (145 mT) static magnetic fields (SMF) generated an analgesic effect toward visceral pain elicited by the intraperitoneal injection of 0.6% acetic acid. In the present work, we investigated behavioral responses such as writhing, entry avoidance, and site preference with the help of a specially designed cage that partially protruded into either the homogeneous (ho) or inhomogeneous (inh) SMF. Aversive effects, cognitive recognition of analgesia, and social behavior governed mice in their free locomotion between SMF and sham sides. The inhibition of pain response (I) for the 0-5, 6-20, and 21-30 min periods following the challenge was calculated by the formula I = 100 (1 - x/y) in %, where x and y represent the number of writhings in the SMF and sham sides, respectively. In accordance with previous measurements, an analgesic effect was induced in exposed mice (Iho  = 64%, P < 0.0002 and Iinh  = 62%, P < 0.002). No significant difference was found in the site preference (SMFho, inh vs. sham) indicating that SMF is neither aversive nor favorable. Comparison of writhings observed in the sham versus SMF side of the cage revealed that SMF exposure resulted in significantly fewer writhings than sham (Iho  = 64%, P < 0.004 and Iinh  = 81%, P < 0.03). Deeper statistical analysis clarified that the lateral SMF gradient between SMF and sham sides could be responsible for most of the analgesic effect (Iho  = 91%, P < 0.02 and Iinh  = 54%, P < 0.02).

Neuroendocrinology of social information processing in rats and mice

We reviewed oxytocin (OT), arginine-vasopressin (AVP) and gonadal hormone involvement in various modes of social information processing in mice and rats. Gonadal hormones regulate OT and AVP mediation of social recognition and social learning. Estrogens foster OT-mediated social recognition and the recognition and avoidance of parasitized conspecifics via estrogen receptor (ER) alpha (ERalpha) and ERbeta. Testosterone and its metabolites, including estrogens, regulate social recognition in males predominantly via the AVP V1a receptor. Both OT and AVP are involved in the social transmission of food preferences and ERalpha has inhibitory, while ERbeta has enhancing, roles. OT also enhances mate copying by females. ERalpha mediates the sexual, and ERbeta the recognition, aspects of the risk-taking enhancing effects of females on males. Thus, androgens and estrogens control social information processing by regulating OT and AVP. This control is finely tuned for different forms of social information processing.

Sex differences in learning processes of classical and operant conditioning

Males and females learn and remember differently at different times in their lives. These differences occur in most species, from invertebrates to humans. We review here sex differences as they occur in laboratory rodent species. We focus on classical and operant conditioning paradigms, including classical eyeblink conditioning, fear-conditioning, active avoidance and conditioned taste aversion. Sex differences have been reported during acquisition, retention and extinction in most of these paradigms. In general, females perform better than males in the classical eyeblink conditioning, in fear-potentiated startle and in most operant conditioning tasks, such as the active avoidance test. However, in the classical fear-conditioning paradigm, in certain lever-pressing paradigms and in the conditioned taste aversion, males outperform females or are more resistant to extinction. Most sex differences in conditioning are dependent on organizational effects of gonadal hormones during early development of the brain, in addition to modulation by activational effects during puberty and adulthood. Critically, sex differences in performance account for some of the reported effects on learning and these are discussed throughout the review. Because so many mental disorders are more prevalent in one sex than the other, it is important to consider sex differences in learning when applying animal models of learning for these disorders. Finally, we discuss how sex differences in learning continue to alter the brain throughout the lifespan. Thus, sex differences in learning are not only mediated by sex differences in the brain, but also contribute to them.

Conditioned flavor aversions: assessment of drug-induced suppression of food intake

Administration of a drug following ingestion of a novel food or solution often suppresses subsequent intake of the new food or solution. This suppression is associative, in that consumption is not suppressed when there is no temporal relationship between consumption and drug administration. The robust nature of aversion learning has made this procedure a sensitive and widely used behavioral index of drug side effects. The procedures described in this unit are suitable for work with rodents, and may require modifications, e.g., in presentation of the ingesta and drug for other species. Familiar and novel foods may be used instead of solutions, with similar results.

Pain perception and electromagnetic fields

A substantial body of evidence has accumulated showing that exposure to electromagnetic fields (EMFs) affects pain sensitivity (nociception) and pain inhibition (analgesia). Consistent inhibitory effects of acute exposures to various EMFs on analgesia have been demonstrated in most studies. This renders examinations of changes in the expression of analgesia and nociception a particularly valuable means of addressing the biological effects of and mechanisms underlying the actions of EMFs. Here we provide an overview of the effects of various EMFs on nociceptive sensitivity and analgesia, with particular emphasis on opioid-mediated responses. We also describe the analgesic effects of particular specific EMFs, the effects of repeated exposures to EMFs and magnetic shielding, along with the dependence of EMF effects on lighting conditions. We further consider some of the underlying cellular and biophysical mechanisms along with the clinical implications of these effects of various EMFs.

Involvement of estrogen receptor ?, ? and oxytocin in social discrimination: a detailed behavioral analysis with knockout female mice

Social recognition, processing, and retaining information about conspecific individuals is crucial for the development of normal social relationships. The neuropeptide oxytocin (OT) is necessary for social recognition in male and female mice, with its effects being modulated by estrogens in females. In previous studies, mice whose genes for the estrogen receptor-alpha (alpha-ERKO) and estrogen receptor-beta (beta-ERKO) as well as OTKO were knocked out failed to habituate to a repeatedly presented conspecific and to dishabituate when the familiar mouse is replaced by a novel animal (Choleris et al. 2003, Proc Natl Acad Sci USA 100, 6192-6197). However, a binary social discrimination assay, where animals are given a simultaneous choice between a familiar and a previously unknown individual, offers a more direct test of social recognition. Here, we used alpha-ERKO, beta-ERKO, and OTKO female mice in the binary social discrimination paradigm. Differently from their wild-type controls, when given a choice, the KO mice showed either reduced (beta-ERKO) or completely impaired (OTKO and alpha-ERKO) social discrimination. Detailed behavioral analyses indicate that all of the KO mice have reduced anxiety-related stretched approaches to the social stimulus with no overall impairment in horizontal and vertical activity, non-social investigation, and various other behaviors such as, self-grooming, digging, and inactivity. Therefore, the OT, ER-alpha, and ER-beta genes are necessary, to different degrees, for social discrimination and, thus, for the modulation of social behavior (e.g. aggression, affiliation).

Zingiber officinale exhibits behavioral radioprotection against radiation-induced CTA in a gender-specific manner

At the organismic level, exposure to radiation can produce taste aversion (CTA) learning and emesis, which have been proposed as behavioral endpoints that are mediated by harmful effects of radiations on peripheral systems, primarily the gastrointestinal system. Thus, the aim of the present investigation was to study the gastroprotective action of hydroalcoholic extract of zingiber rhizome (Zingiber officinale Rosc.) against radiation-induced conditioned taste aversion (CTA) in both male and female species of animals, for testing its potential as a behavioral radioprotector. Administration of zingiber extract 1 h before 2-Gy gamma-radiation was significantly effective in blocking the saccharin avoidance response, with 200 and 250 mg/kg b.wt. i.p., being the most effective doses for male and female rats, respectively. A comparison of the efficacy of zingiber extract with two antiemetic drugs, ondansteron and dexamethasone, revealed that the extract rendered comparable protection against radiation-induced CTA. Our experiments also confirmed the existence of sex dichotomy (i.e., the sex of animal greatly influenced response towards radiation exposure) in relation to behavioral responses (CTA) or differential metabolism. The observed gender variations were hypothesized to be a result of hormonal fluctuations and differences in pharmacological parameters in male and female rats. To correlate the mechanism of action, the free-radical-scavenging potential of zingiber extract to scavenge hydroxyl ion and nitric oxide was also tested, in cell-free system and a concentration of 1000 microg/ml, was found to be the most potent, which has been proposed as one the many activities assisting in its overall ability to modulate radiation-induced taste aversion. The results demonstrate that Z. officinale possesses antioxidant, radioprotective and neuromodulatory properties that can be effectively utilized for behavioral radioprotection and for efficiently mitigating radiation-induced CTA in both males and females species.

Sex and estrous cycle differences in the behavioral effects of high-strength static magnetic fields: role of ovarian steroids

Advances in magnetic resonance imaging are driving the development of higher-resolution machines equipped with high-strength static magnetic fields (MFs). The behavioral effects of high-strength MFs are largely uncharacterized, although in male rats, exposure to 7 T or above induces locomotor circling and leads to a conditioned taste avoidance (CTA) if paired with a novel taste. Here, the effects of MFs on male and female rats were compared to determine whether there are sex differences in behavioral responses and whether these can be explained by ovarian steroid status. Rats were given 10-min access to a novel saccharin solution and then restrained within a 14-T magnet for 30 min. Locomotor activity after exposure was scored for circling and rearing. CTA extinction was measured with two-bottle preference tests. In experiment 1, males were compared with females across the estrous cycle after a single MF exposure. Females circled more and acquired a more persistent CTA than males; circling was highest on the day of estrus. In experiment 2, the effects of three MF exposures were compared among intact rats, ovariectomized females, and ovariectomized females with steroid replacement. Compared with intact rats, ovariectomy increased circling; estrogen replacement blocked the increase. Males acquired a stronger initial CTA but extinguished faster than intact or ovariectomized females. Thus the locomotor circling induced by MF exposure was increased in females and modulated by ovarian steroids across the estrous cycle and by hormone replacement. Furthermore, female rats acquired a more persistent CTA than male rats, which was not dependent on estrous phase or endogenous ovarian steroids.

Morphine-induced conditioned taste aversions: assessment of sexual dimorphism

Although sex differences in taste aversions have been reported with emetics such as lithium chloride (LiCl), little is known whether such findings generalize to other aversion-inducing drugs, including recreational compounds. One particular class of recreational compounds that induces taste aversions but that has not been examined for sex differences in its aversive properties is the opioids. To assess sex differences in the aversive properties of the opioids, Experiment 1 examined the acquisition and extinction of morphine-induced taste aversions in male and female rats. To determine whether the specific parametric conditions used in Experiment 1 would support sex differences in general, Experiment 2 examined possible sex differences in the acquisition and extinction of LiCl-induced taste aversions, a compound for which sex differences have been previously reported. During acquisition, male and female rats were given 20-min access to a novel saccharin solution and injected with either morphine (0, 10, 18 and 32 mg/kg s.c.; Experiment 1) or LiCl (0, 0.3, 0.6 and 1.2 mEq s.c.; Experiment 2) every fourth day for a total of four conditioning trials. During extinction, subjects were allowed access to saccharin but were not injected (for a total of eight trials). There were no sex differences in acquisition with either morphine or LiCl. There were also no sex differences in extinction with morphine; however, sex differences were found with LiCl, an effect consistent with prior assessments with this drug. The basis for and implications of the differences in the effects of sex on morphine- and LiCl-induced taste aversions were discussed.

Conditioned taste aversion is not disrupted in rats exposed to weak, complex magnetic fields during the CS-UCS interval

40 normal male Wistar rats were trained for 8 successive days to consume water ad libitum during once-daily 20-min. sessions. On the following day (training day) the rats were presented with a novel solution of 10% sucrose for 20 min. followed by a single exposure for 2 hr. to one of two weak (200 to 500 nanoTesla) complex magnetic fields or to sham-field conditions. The patterns of the two magnetic fields and the durations of their repeated presentations (interstimulus interval) were designed to be resonant with the intrinsic firing of hippocampal pyramidal and solitary neurons, respectively. Immediately after the applications of the fields one-half the number of rats were injected with lithium to evoke gastrointestinal malaise. Although on the test day, three days later, rats previously injected with the lithium exhibited the usual robust reduction in the consumption of sucrose compared to the training day, there were no statistically significant differences between field-exposed and sham-field groups for these ratios. We conclude that a 2-hr. exposure to weak magnetic fields designed to simulate the pattern of two structures likely involved with conditioned taste aversion between the conditioned stimulus and the unconditioned stimulus did not affect this behavior.

Behavioral effects of high-strength static magnetic fields on rats

Advances in magnetic resonance imaging are driving the development of more powerful and higher-resolution machines with high-strength static magnetic fields. The behavioral effects of high-strength magnetic fields are largely uncharacterized, although restraint within a 9.4 T magnetic field is sufficient to induce a conditioned taste aversion (CTA) and induce brainstem expression of c-Fos in rats. To determine whether the behavioral effects of static magnetic fields are dependent on field strength, duration of exposure, and orientation with the field, rats were restrained within the bore of 7 or 14 T superconducting magnets for variable durations. Behavioral effects were assessed by scoring locomotor activity after release from the magnetic field and measuring CTA acquisition after pairing intake of a palatable glucose and saccharin (G+S) solution with magnetic field exposure. Magnetic field exposure at either 7 or 14 T suppressed rearing and induced tight circling. The direction of the circling was dependent on the rat's orientation within the magnetic field: if exposed head-up, rats circled counterclockwise; if exposed head-down, rats circled clockwise. CTA was induced after three pairings of taste and 30 min of 7 T exposure or after a single pairing of G+S and 1 min of 14 T exposure. These results suggest that magnetic field exposure has graded effects on rat behavior. We hypothesize that restraint with high-strength magnetic fields causes vestibular stimulation resulting in locomotor circling and CTA acquisition.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents--as well as many other species including man--learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS-US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS-US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras-MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.