Social and sexual incentive properties of estrogen receptor alpha, estrogen receptor beta, or oxytocin knockout mice

Social and sexual incentive motivation, defined as the intensity of approach to a social and a sexual incentive, respectively, were studied in female Swiss Webster mice. In the first experiment, the social incentive was a castrated mouse of the same strain as the females, whereas the sexual incentive was an intact male mouse of the same strain. Ovariectomized females were first tested after oil treatment and then after administration of estradiol benzoate + progesterone in doses sufficient to induce full receptivity. The hormones increased sexual incentive motivation while leaving social incentive motivation unaffected. This suggests that sexual incentive motivation in the female mouse is dependent on ovarian hormones. In the next experiment, ovariectomized females were tested with an intact, male estrogen receptor alpha knockout and its wild type as incentives, first without hormones and then when fully receptive. There were no differences in incentive properties between the wild type and the knockout. In a similar experiment, we used an intact male estrogen receptor beta knockout and its corresponding wild type as incentives. The wild type turned out to be a more attractive social incentive than the knockout, while they were equivalent as sexual incentives. Finally, an intact male oxytocin knockout and its wild type were used as incentives. The knockout turned out to be a superior incentive, particularly a superior sexual incentive. The fact that the estrogen receptor beta and oxytocin knockouts have incentive properties different from their wild types may be important to consider in studies of these knockouts' sociosexual behaviors.

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).

Oxytocin and estrogen receptor alpha and beta knockout mice provide discriminably different odor cues in behavioral assays

Social behavior involves both the recognition and pro-duction of social cues. Mice with selective deletion(knockout) of either the gene for oxytocin (OT) or genes for the estrogen receptor (ER) -c or -B display impaired social recognition. In this study we demonstrate that these gene knockout mice also provide discriminably different social stimuli in behavioral assays. In an odor choice test, which is a measure of social interest and discrimination, outbred female Swiss-Webster mice discriminated the urine odors of male knock-outs IKO: OTKO, alphaERKO, betaERKO) from the odors of their wildtype littermates (WT: OTWT, alphaERWT, betaERWT). Females showed marked initial choices of the urine odors of OTWT and betaERWT males over those of OTKOand PERKO males, and alphaERKO males over alphaERWT males. The odors of OTKO and betaERKO males also induced aversive, analgesic responses, with the odors of WTs having no significant effects. Odors of both the alphaERWT andalphaERKO males induced aversive, analgesic responses,with the odors of the WT inducing significantly greater analgesia. The odors of restraint stressed WT and KO males also elicited analgesia with, again, females dis-playing significantly greater responses to the odors of stressed OTKO and betaERKO males than their WTs, and significantly lower analgesia to the odors of stressedalphaERKO than alphaERWT males. These findings show that the KO mice are discriminated from their WTs on the basis of odor and that the various KOs differ in the relative attractiveness/aversiveness of their odors. Therefore, in behavioral assays one causal route by which gene inactivation alters the social behavior of knockout mice may be mediated through the partners'modified responses to their odors.

Functional genomics of social recognition

Although various types of group living are widespread in mammals, including humans, the study of the hormonal and genetic underpinnings of nonsexual social behaviour, is in its infancy compared to the analysis of sexual behaviour mechanisms. Oxytocin, vasopressin and gonadal hormones certainly play an important role. Social recognition, where animals identify and recognize other individual conspecifics, is a crucial prerequisite for the occurrence of a wide range of social behaviours. Social recognition is also important for coping with one major cost of life in a group: the increased risk of exposure to parasites and infection. We review recent functional genomic studies on the involvement of oxytocin and oestrogen-receptor genes in the regulation of social recognition in mice and in the ecologically relevant context of parasite recognition and avoidance. Based on quantitative studies of social recognition with gene-knockout mice and with antisense DNA, we propose a four-gene micronet contributing to social recognition. This micronet involves the genes coding for oestrogen receptors alpha (ER-alpha), beta (ER-beta), oxytocin and the oxytocin receptor. In this model, circulating oestrogens promote transcription of (i) oxytocin in the paraventricular nucleus of the hypothalamus through ER-beta and (ii) oxytocin receptor in the amygdala through ER-alpha. This model forms the core around which increasingly complex genetic, hormonal and neural interactions associated with social behaviours and recognition can be organized.

Impaired discrimination of and aversion to parasitized male odors by female oxytocin knockout mice

A major cost of social behavior is the increased risk of exposure to parasites, with animals utilizing social information to recognize and avoid infected conspecifics. In mice, females can discriminate between infected and uninfected males on the basis of social cues, displaying aversive responses to the odors of infected males. In the present study, using female mice whose gene for oxytocin (OT) has been selectively deleted (OT knockout mice (OTKO)), we show that at least one normal allele for OT is required for the mediation of the recognition and avoidance of parasitized males. Female wild type (OTWT) and heterozygous (OTHZ) mice distinguished between the odors of individual males infected with the louse, Polyplax serrata, and uninfected males while the KO mice did not. Exposure to the odors of infected males induced analgesia in OTWT and OTHZ females, with OTKO females displaying attenuated analgesia. OTWT and OTHZ females, but not the OTKO females, also distinguished between the odors of novel and familiar infected males and modulated their analgesic responses on the basis of prior familiarity. In an odor choice test, OTWT and OTHZ females displayed a marked initial choice for the odors of uninfected males, whereas the OTKO females showed no consistent choice. This impairment was specific to the odors of infected males. OTKO females displayed normal analgesic responses to another aversive social odor, that of a stressed male, and an aversive non-social odor, that of a cat. The OTKOs had normal non-social olfactory memory, but were impaired in their social odor memory. These findings indicate that a normal OT gene comprises an essential part of the central recognition mechanism whereby females can both reduce the transmission of parasites to themselves and select for parasite-free males.

Learning to fear and cope with a natural stressor: individually and socially acquired corticosterone and avoidance responses to biting flies

Animals learn to recognize and respond to a variety of dangerous factors, with biting and blood-feeding flies being among the most prevalent of natural stressors. Here we describe the behavioral avoidance and hormonal (corticosterone) stress responses to biting fly exposure and the roles of individual and social learning in the acquisition of these fear-associated responses. Male mice exposed to a single 30-min session of attack by intact biting flies (stable fly, Stomoxys calcitrans L.) exhibited increased plasma corticosterone levels and active self-burying responses to avoid the flies. When exposed 24 h later to altered flies whose biting mouth parts were removed and were incapable of biting, the mice displayed conditioned increases in corticosterone and avoidance responses. This conditioned increase in corticosterone and self-burying was also acquired through social learning without direct individual experience with the intact biting flies. Fly naive "observer" mice that witnessed other "demonstrator" mice being attacked by biting flies, but were not exposed to intact flies themselves, displayed increases in corticosterone levels and self-burying to avoid flies when exposed 24 h later to altered flies. The social learning was not due to social facilitation or sensitization. Observers had to witness the self-burying avoidance responses of the demonstrator to the biting flies in order to subsequently recognize a potential threat to themselves and display the appropriate responses. These individually and socially acquired conditioned fear responses are likely part of the mechanisms that allow animals to defend themselves from biting and blood-feeding arthropods.

Brief exposure to female odors "emboldens" male mice by reducing predator-induced behavioral and hormonal responses

In rodents, where chemical signals play a particularly important role in determining intersexual interactions, various studies have shown that male behavior and physiology is sensitive to female odor cues. Here we examined the effects of brief (1 min) and more prolonged (60 min) preexposure to the odors of a novel estrous female on the behavioral and hormonal responses of sexually experienced and inexperienced male mice, Mus musculus, to subsequent predator (cat and weasel) odor exposure and potential predator risk. Brief, but not prolonged, preexposure to the odors of an estrous female decreased the aversion and avoidance responses of male mice to cat odor in a Y-maze preference test, with the extent of responses being affected by a males prior sexual experience. Similarly, brief, but not prolonged, preexposure to female odors markedly attenuated the analgesic responses elicited in male mice by weasel odor. Brief exposure to a novel estrous female by itself had no significant immediate effects on either corticosterone or testosterone levels in the males. However, brief, but not prolonged, preexposure to the odors of an estrous female attenuated the marked increase in corticosterone and decrease in testosterone that were induced in males by exposure to weasel odor. The decreases in aversive responses to, and effects of, predator odor exposure that are induced by brief exposure to a novel estrous female may reflect a greater risk taking and boldness in males that could directly facilitate access to an immediately, and possibly transiently, available novel sexually receptive female.

Antipredator responses and defensive behavior: ecological and ethological approaches for the neurosciences

In nature, animals are exposed to a wide range of threats and dangers with predators being amongst the more prominent and intensely studied of these. The responses of prey to predators and various predator avoidance and antipredator behaviors have been extensively evaluated from ecological and ethological perspectives and more recent ethopharmacological and neuroscience approaches. Unfortunately, there has been relatively little interchange between the ecological-ethological and neuroscience areas with the latter often using responses to predators just simply as another 'model' system. There is, however, now a growing realization that integrative approaches incorporating ecological, evolutionary and neurobiological explanations are required for the understanding of behavior and its functions. This necessitates an incorporation of ecological and ethological concepts and validity with neuroscience approaches to the analysis of antipredator responses and defensive behavior. A number of selected ecological approaches that are used for the investigation of predator avoidance mechanisms and antipredator defensive behavior patterns are briefly reviewed here. These include examinations of how predation risk and its variation affect decision making in animals and how learning affects these responses. The trade-offs that are involved, how the risk of predation affects decisions concerning foraging behavior, mating and reproduction, as well as how varying levels of risk affect decisions relative to the type of defensive mechanisms utilized are briefly outlined. The utility of these approaches and their relevance to the design and interpretation of various neuroscience studies is addressed here.

Corticosterone rapidly reduces male odor preferences in female mice

There is accumulating evidence for rapid, non-genomic behavioral effects of various steroids including that of the glucocorticoid, corticosterone. Using an odor preference test, the responses of which are indicative of mate preferences and sexual interest, we examined the effects of acute corticosterone on the responses of oestrous female mice to male odors. Control female mice displayed an overwhelming preference for the odors of male mice. Peripheral administration of corticosterone elicited a significant dose-related (1.0-5.0 mg/kg) decrease in female preference for male odors at 10 min, but not at 60 min, after administration. These inhibitory effects of corticosterone on odor preferences were significantly reduced by the competitive NMDA antagonist, NPC 12626, and enhanced by the GABA antagonist bicuculline. This indicates that corticosterone has rapid inhibitory effects on olfactory mediated female mate preferences and responses to male odor that in part involve interactions with NMDA and GABA receptor mechanisms.

A detailed ethological analysis of the mouse open field test: effects of diazepam, chlordiazepoxide and an extremely low frequency pulsed magnetic field

The open field test (OFT) is a widely used procedure for examining the behavioral effects of drugs and anxiety. Detailed ethological assessments of animal behavior are lacking. Here we present a detailed ethological assessment of the effects of acute treatment with the benzodiazepines, diazepam (DZ, 1.5mg/kg) and chlordiazepoxide (CDP, 5.0 and 10.0mg/kg), as well as exposure to a non-pharmacological agent, a specific pulsed extremely low frequency magnetic field (MAG) on open field behavior. We examined the duration, frequency and time course of various behaviors (i.e. exploration, walk, rear, stretch attend, return, groom, sit, spin turn, jump and sleep) exhibited by male mice in different regions of a novel open field. Both DZ and CDP consistently reduced the typical anxiety-like behaviors of stretch attend and wall-following (thigmotaxis), along with that of an additional new measure: 'returns', without producing any overall effects on total locomotion. The drugs also differed in their effects. CDP elicited a shift in the locomotor pattern from a 'high explore' to a 'high walk', while DZ mainly elicited alterations in sit and groom. The MAG treatment was repeated twice with both exposures reducing horizontal and vertical (rearing) activity and increasing grooming and spin turns. However, the anxiety-like behaviors of stretch attend and return were marginally reduced by only the first exposure. We conclude that a detailed ethological analysis of the OFT allows not only the detection of specific effects of drugs and non-pharmacological agents (i.e. pulsed magnetic field) on anxiety-like behaviors, but also permits the examination of non-specific effects, in particular those on general activity.

Learning from others to cope with biting flies: social learning of fear-induced conditioned analgesia and active avoidance

Although fear conditioning has received extensive attention, little is known about the roles of social learning whereby an individual may learn and acquire the fear responses of another. The authors examined individually and socially mediated acquisition of fear and analgesia to the natural aversive stimulus of biting flies. Exposure to biting flies elicited in individual naive mice analgesia and active self-burying to avoid the flies. When exposed 24 hr later to flies whose biting parts were removed, but not to nonbiting house flies, these mice displayed conditioned analgesia and self-burying. This "one-trial" conditioned analgesia and avoidance was also acquired through social learning without direct individual experience with biting flies. Naive "observer" mice that witnessed other "demonstrator" mice being attacked by biting flies exhibited analgesia and self-burying 24 hr later to altered flies.

NMDA-mediated social learning of fear-induced conditioned analgesia to biting flies

Although fear conditioning has received extensive neurobiological attention little is known about social learning whereby one individual may learn and acquire the fear responses of another. A 30 min exposure to intact biting flies (stable fly, Stomoxys colcitrans L.) elicits in individual fly-naive mice analgesia and active self burying responses to avoid the flies. Fly-naive observer mice that witnessed other demonstrator mice being attacked by biting flies exhibited analgesia and self-burying to avoid flies when exposed 24 h later to altered flies whose biting mouth parts were removed. The opiate antagonist naloxone, while reducing the analgesic responses elicited by exposure to a fly-stressed demonstrator, did not affect either the subsequent conditioned analgesia or self-burying. However, the specific NMDA receptor antagonist NPC 12626, given to observers prior to, but not after, presentation of fly attacked demonstrators blocked the socially determined conditioned analgesia and self burying avoidance. This supports NMDA involvement in the mediation of the social transmission and long-term (24h) retention of conditioned analgesia and fear.

Locomotor activity changes following lipopolysaccharide treatment in mice: a multivariate assessment of behavioral tolerance

To determine the effects of repeated, acute endotoxin exposure on locomotor behavior, male laboratory mice were injected intraperitoneally with lipopolysaccharide (LPS: 50, 100 or 200 microg/kg) or saline vehicle on experimental Days 1, 4 and 7. At 2 h after each treatment, locomotor activity was assessed in a nonnovel, automated open-field apparatus (Digiscan) for 30 min. On Day 1, all horizontal and vertical activity measures were significantly reduced to near zero values by each dose of LPS. Behavioral tolerance to LPS formed rapidly, as locomotor activity of the treated groups did not differ from the control group on Days 4 or 7. In a second study, mice were given LPS (50, 100 or 150 microg/kg ip) or saline vehicle on two test days, 28 days apart. Activity was assessed, 1 h after injection, in a novel open field on the first test day and in a nonnovel open field on the second test day. Significant locomotor activity decrements were readily apparent in LPS-treated mice only in the nonnovel open field. This latter finding indicates that environmental novelty mediates, at least partially, the locomotor-reducing effects of LPS in mice.

Acute effects of corticosterone on LiCl-induced rapid gustatory conditioning in rats: a taste reactivity analysis

Previous research has shown that acute corticosterone treatment can have rapid effects on learning and memory. Using the taste reactivity test (TRT), the present study examined the effect of acute administration of corticosterone on sucrose palatability and the development of LiCl-induced rapid gustatory conditioning. On each of two conditioning days rats were injected with either a low dose of lithium chloride (LiCl; 0.75 mEq, i.p.) or saline (NaCl; 0.9%, i.p.) and 10 min later, received a second injection of either corticosterone (5 mg/kg, i.p.) or cyclodextrin vehicle. Rats were then placed in the TRT chamber, where 1 min intraoral infusions of sucrose (0.3 M) were delivered every 10 min. Taste reactivity responses were videotaped and later analyzed for frequency of occurrence. Rats treated with both LiCl and corticosterone showed enhanced aversive responding and reduced ingestive responding relative to control rats treated with LiCl and vehicle. The implication that corticosterone may have a rapid enhancing effect on gustatory conditioning is discussed.

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

Although conditioned taste aversion (CTA) has been investigated and described in laboratory rodents and domestic animals, less is known regarding wild rodents. Here, we describe CTA in males and females of a "wild" species of rodent, the deer mouse (Peromyscus maniculatus). In addition, as CTA has often been induced by exposure to intense electromagnetic, X or gamma, radiation, in a second study, we also investigated the effects of a specifically designed, pulsed extremely low-frequency and low-intensity magnetic field on the flavor preferences of male and female deer mice. The results of these investigations showed that: (i) deer mice quickly developed a marked CTA for a novel flavor after a single pairing with LiCl; (ii) although the intensity of the CTA was the same in males and females, there was a sex difference in the duration of the flavor aversion, with males displaying it for a longer period (4 days) than females (3 days); (iii) both males and females showed a rapid and complete extinction of the aversion, in contrast to what has been reported for laboratory rodents; (iv) there was no recovery of CTA on re-test 10 days after extinction; (v) neither male or female deer mice developed a taste aversion as a consequence of exposure to a weak electromagnetic field; and (vi) there was a sex difference in response to the magnetic field, with exposure to the magnetic field significantly enhancing novel taste preference in male but not in female deer mice. Overall, our results show that there are several sex differences in the behavior of deer mice, both in the characteristics of the CTA and in the response to magnetic field exposure. The sex differences are discussed in terms of a sexually dimorphic sensitivity to experimental manipulation and the induction of stress and/or anxiety.

Parasites and behaviour: an ethopharmacological perspective

Ethopharmacology combines an ethological approach to the understanding of the causes and functions of behaviour with pharmacological analysis of the underlying neuromodulatory mechanisms. Recently, this approach has been applied to the analysis of the responses of animals to parasitized individuals and to the effects of parasites on various host behaviours (eg. host defences, mate responses) and their neurobiological correlates. Martin Kavaliers, Douglas Colwell and Elena Choleris explain here how ethopharmacology can be used to address the mechanisms that underlie the often subtle effects of parasites on host behaviour.

Light-dependent and -independent behavioral effects of extremely low frequency magnetic fields in a land snail are consistent with a parametric resonance mechanism

Exposure to extremely low frequency (ELF) magnetic fields has been shown to attenuate endogenous opioid peptide mediated antinociception or "analgaesia" in the terrestrial pulmonate snail, Cepaea nemoralis. Here we examine the roles of light in determining this effect and address the mechanisms associated with mediating the effects of the ELF magnetic fields in both the presence and absence of light. Specifically, we consider whether the magnetic field effects involve an indirect induced electric current mechanism or a direct effect such as a parametric resonance mechanism (PRM). We exposed snails in both the presence and absence of light at three different frequencies (30, 60, and 120 Hz) with static field values (BDC) and ELF magnetic field amplitude (peak) and direction (BAC) set according to the predictions of the PRM for Ca2+. Analgaesia was induced in snails by injecting them with an enkephalinase inhibitor, which augments endogenous opioid (enkephalin) activity. We found that the magnetic field exposure reduced this opioid-induced analgaesia significantly more if the exposure occurred in the presence rather than the absence of light. However, the percentage reduction in analgaesia in both the presence and absence of light was not dependent on the ELF frequency. This finding suggests that in both the presence and the absence of light the effect of the ELF magnetic field was mediated by a direct magnetic field detection mechanism such as the PRM rather than an induced current mechanism.

Extremely low frequency magnetic fields can either increase or decrease analgaesia in the land snail depending on field and light conditions

Results of prior investigations with opioid peptide mediated antinociception or analgaesia have suggested that these extremely low frequency (ELF) magnetic field effects are described by a resonance mechanism rather than mechanisms based on either induced currents or magnetite. Here we show that ELF magnetic fields (141-414 microT peak) can, in a manner consistent with the predictions of Lednev's parametric resonance model (PRM) for the calcium ion, either (i) reduce, (ii) have no effect on, or (iii) increase endogenous opioid mediated analgaesia in the land snail, Cepaea nemoralis. When the magnetic fields were set to parameters for the predictions of the PRM for the potassium ion, opioid-peptide mediated analgaesia increased and there was evidence of antagonism by the K(+) channel blocker, glibenclamide. Furthermore, these effects were dependent on the presence of light; the effects were absent in the absence of light. These observed increases and decreases in opioid analgaesia are largely consistent with the predictions of Lednev's PRM.

Examining the effects of lipopolysaccharide and cholecystokinin on water ingestion: comparing intake and palatability

Lipopolysaccharide (LPS) and cholecystokinin (CCK) have been shown to have anorectic properties in a variety of species. The present study examined the effects of LPS and CCK, both alone and in combination, on two different aspects of water ingestion, water intake and palatability. On test days, animals were first injected intraperitoneally (i.p.) with either LPS (200 microg/kg) or NaCl vehicle, and 2 h later received a second injection of either CCK (8 microg/kg) or NaCl vehicle. In Experiment 1, water intake was monitored for 1 h on 3 separate test days 72 h apart; while in Experiment 2, water palatability was assessed using the taste reactivity test (TRT), on two separate test days 72 h apart. Both LPS and CCK significantly (p<0.05) reduced water intake, with the effects of combined LPS with CCK being more pronounced than either agent injected alone. Rats developed a rapid tolerance to the effects of LPS on water intake on subsequent exposures to LPS. Results from the TRT indicated that LPS enhanced water palatability (p<0.05), as evidenced by a high level of ingestive responding, whereas CCK produced a pattern of responding indicative of satiety. LPS plus CCK reduced ingestive responding on the first test day, but these responses were significantly increased on the second test day (p<0.05). These results demonstrate that although LPS reduces water intake, it enhances water palatability. The results further underscore the necessity for examining palatability changes in addition to intake measures when studying the regulation of feeding and drinking.

Influence of a natural stressor (predator odor) on locomotor activity in the meadow vole (Microtus pennsylvanicus): modulation by sex, reproductive condition and gonadal hormones

Sex differences in a variety of non-reproductive behaviors have been indicated to occur in seasonally breeding polygynous promiscuous rodents such as the meadow vole, Microtus pennsylvanicus. The present study was designed to assess the effects of reproductive and hormonal status on the locomotor responses of meadow voles following brief exposure to the odors of a natural predator, the Red fox (Vulpes vulpes). Adult male and female meadow voles, which are seasonal photoperiodically-induced breeders, were housed in either mixed sex pairs under a long, reproductively stimulatory photoperiod (simulating breeding: long light cycle, paired: LLC + P) or in same-sex pairs under a short, reproductively inhibitory photoperiod (simulated non-breeding: short light cycle, non-paired: SLC-NP). On 2 consecutive days following 1 day of baseline activity monitoring, voles were exposed individually for 3 min to fox odor and a novel pungent control odor (extract of almond). The levels of various measures of activity that were displayed by the voles were assessed by an automated Digiscan activity monitoring system. LLC + P (simulated breeding) voles displayed higher basal levels of activity relative to SLC + NP (simulated non-breeding) voles, with males displaying greater activity than females. LLC + P (simulated breeding) males displayed a significant reduction in activity levels following exposure to fox odor relative to control odor. The reductions in activity following fox odor exposure were related to plasma testosterone levels such that a larger behavioral response (i.e. greater reduction) was associated with higher levels of testosterone. Furthermore, dividing males into high and low testosterone groups based on the median levels of testosterone revealed that high but not low testosterone males displayed reductions in activity following exposure to fox odor relative to control odor. No changes in activity levels following exposure to fox odor were noted in SLC-NP males, and either SLC-NP or LLC + P females. These results show that this sexually dimorphic non-reproductive behavior is significantly influenced by reproductive condition and gonadal hormone levels.

Antinociceptive effects of the neuroactive steroid, 3alpha-hydroxy-5alpha-pregnan-20-one and progesterone in the land snail, Cepaea nemoralis

Results of investigations with vertebrates have implicated neuroactive steroids and in particular 5alpha-reduced metabolites of progesterone such as 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP/3A5P and originally allopregnanolone) in the rapid modulation of diverse functions including that of nociceptive sensitivity. These effects have been indicated to involve modulation of GABA receptors. Results of recent phylogenetic studies have revealed the presence of GABA receptors in invertebrates that may also be subject to modulation by steroids and neuroactive steroids. The present study examined the effects of the neuroactive steroid, 3alpha-hydroxy-5alpha-pregnan-20-one, as well as progesterone on aversive thermal (nociceptive) responses in a mollusc, the land snail, Cepaea nemoralis. 3alpha-Hydroxy-5alpha-pregnan-20-one had significant dose-related (0.01-1.0 microg) antinociceptive effects in Cepaea increasing the latency of response to a 40 degrees C surface, with maximum effects being evident 15-30 min after administration. These effects of 3alpha-hydroxy-5alpha-pregnan-20-one were stereospecific, with the stereoisomer 3beta-hydroxy-5alpha-pregnan-20-one (3B5P) failing to affect nociceptive responses. Progesterone also had significant dose-related (0.10-10 microg) antinociceptive effects that, however, were delayed in onset and relatively prolonged (60-120 min), suggestive of the formation of active metabolites. The presence of endogenous progesterone (12.36+/-0.17 ng/g tissue) was ascertained by a radioimmunoassay further supporting a functional role for steroids in Cepaea. The antinociceptive effects of 3alpha-hydroxy-5alpha-pregnan-20-one and progesterone were blocked by the GABA antagonists, bicuculline and picrotoxin, while being relatively insensitive to opioid and N-methyl-D-aspartate antagonists. These results suggest an early evolutionary development and phylogenetic continuity of neuroactive steroid and GABA involvement in the mediation of nociception.

Repeated injections of lipopolysaccharide attenuate the satiety effects of cholecystokinin

The present study examined the effects of repeated injections of lipopolysaccharide (LPS) and cholecystokinin (CCK) on both sucrose palatability and body weight. Rats received repeated injections of LPS (200 microg/kg), CCK (8 microg/kg) and/or NaCl on days 1, 4, 7 and 10. Body weight was monitored on each test day and sucrose palatability was assessed using the taste reactivity test (TRT) on days 7 and 10. Rats treated with LPS developed a rapid tolerance to the reductions in body weight; however, this tolerance did not affect sucrose palatability. Furthermore it was found that repeated exposures to LPS and CCK attenuated the typical satiety related behaviors produced in the TRT by administration of CCK. This desensitization to CCK with repeated exposures to LPS may be one of the mechanisms that could account for the rapid development of tolerance to LPS-induced hypophagia.

Social learning in animals: sex differences and neurobiological analysis

Social learning where an "individual's behavior is influenced by observation of, or interaction with, another animal or its products" has been extensively documented in a broad variety of species, including humans. Social learning occurs within the complex framework of an animal's social interactions that are markedly affected by factors such as dominance hierarchies, family bonds, age, and sex of the interacting individuals. Moreover, it is clear that social learning is influenced not only by important sexually dimorphic social constraints but also that it involves attention, motivational, and perceptual mechanisms, all of which exhibit substantial male-female differences. Although sex differences have been demonstrated in a wide range of cognitive and behavioral processes, investigations of male-female differences in social learning and its neurobiological substrates have been largely neglected. As such, sex differences in social learning and its neurobiological substrates merit increased attention. This review briefly considers various aspects of the study of social learning in mammals, and indicates where male-female differences have either been described, neglected and, or could have a potential impact. It also describes the results of neurobiological investigations of social learning and considers the relevance of these findings to other sexually dimorphic cognitive processes.

Parasites and behavior: an ethopharmacological analysis and biomedical implications

Parasites and disease are increasingly recognized as agents of behavioral, ecological and evolutionary importance having a variety of influences on their hosts other than the more obvious pathological and immunological changes. Parasites can have significant behavioral effects even when parasitism is sub-clinical with these effects proposed to either benefit the parasite (parasite 'manipulation'), benefit the host, or to simply arise as side-effects of the infection (parasitic 'constraints'). However, until relatively recently little attention has been paid to the neuromodulatory substrates that mediate these behavioral changes. Ethopharmacology incorporates an evolutionary approach to the study of behavior with pharmacological analysis of neuromodulatory mechanisms. As such, this approach is appropriate for, and has been applied to, the analysis of the effects of ectoparasites (e.g. biting and blood-feeding flies) and endoparasites (e.g. protozoa, nematodes) on a number of behaviors (e.g. pain inhibition, learning and memory, responses to predators and anxiety, mate selection) in selected host-parasite systems. Ethopharmacology suggests a promising direction by which neuromodulatory mechanisms that underlie the effects of parasites on behavior, including that of humans, can be addressed.

Differential effects of lipopolysaccharide and cholecystokinin on sucrose intake and palatability

The differential effects of CCK and lipopolysaccharide (LPS) on sucrose intake and palatability were examined. Rats were injected with LPS (200 microg/kg ip) or NaCl (0.9%, vehicle) and 2 h later received a second injection of either CCK (8 microg/kg ip) or NaCl. In experiment 1, sucrose (0.3 M) intake was monitored for 1 h on three different test days 72 h apart, while in experiment 2, palatability was assessed by means of the taste reactivity test (TRT) on two separate days (72 h apart). In the TRT, orofacial and somatic responses to brief (30 s) intraoral infusions of sucrose were recorded and analyzed for response frequency. Singly, LPS and CCK reduced sucrose intake, with a more pronounced effect from combined LPS and CCK. LPS by itself did not alter sucrose palatability, as evidenced by continuous high levels of ingestive responding. In contrast, CCK-treated rats displayed a pattern of responding indicative of satiety, as did the combined LPS-CCK-treated rats. These results suggest that LPS does not induce hypophagia by altering palatability.

Light-dependent effects of magnetic fields on nitric oxide activation in the land snail

The attenuation of opioid analgaesia is a well established effect of extremely low frequency (ELF) magnetic fields with light having a modulatory role in these effects such that when the ELF exposures are carried out in the dark, the inhibitory effects on analgesia are reduced. Here, we considered the light dependency of the effects of exposure to an ELF magnetic field prior to the induction of opioid analgesia. Brief exposure to a 60 Hz field potentiated in a light dependent manner the inhibitory effects of the nitric oxide (NO) precursor L-arginine on the level of subsequent opioid-induced analgesia in the land snail Cepaea nemoralis. This suggests a general light dependency of the actions of magnetic fields and the involvement of NO in the mediation of the effects of magnetic fields.

Brief predator odour exposure activates the HPA axis independent of locomotor changes

Although predator exposure has been proposed to elicit both behavioural responses and neuroendocrine effects in rodents, results of an increasing number studies have failed to consistently detect both of these alterations. We provide a detailed multivariate quantitative assessment of locomotor activity levels and plasma levels of adrenocorticotrophic hormone (ACTH) and corticosterone of male and female laboratory rats following brief (5 min) exposure to a predator odour, (2-propylthietane, the main constituent of weasel anal gland secretion). We show that there is a clear distinction between the behavioural and neuroendocrine responses, with predator odour inducing significant increases in corticosterone and ACTH levels without any significant accompanying changes in various locomotor activity measures.

Relations of hippocampal volume and dentate gyrus width to gonadal hormone levels in male and female meadow voles

The present study examined hippocampal volume and dentate gyrus width and their relations to gonadal hormone levels in adult male and female meadow voles, Microtus pennsylvanicus. Females were split into High and Low Estradiol groups based on the median estradiol level. Males were similarly split into High and Low Testosterone groups. Contrary to previous reports in wild meadow voles, there was no evidence of an overall sex difference in hippocampal volume. However, when male-female comparisons were limited to High Testosterone males and Low Estradiol females a significant sex difference in hippocampal volume favouring males did emerge. Hippocampal volume in males was related to testosterone level, with High Testosterone males having significantly larger hippocampi than Low Testosterone males. Similarly, there was a significant influence of plasma estradiol level on hippocampal volume and left dentate gyrus width, with High Estradiol females having larger hippocampi and dentate gyrus width than Low Estradiol females. In addition, consistent with previous findings in the laboratory rat, there were sex differences favouring males in right dentate gyrus width. These findings show that there is a complex relationship between hippocampal volume, dentate gyrus width and gonadal hormone levels in male and female meadow voles.

Learning to cope with biting flies: rapid NMDA-mediated acquisition of conditioned analgesia

A 30-min exposure to intact biting flies (stable flies) induced an opioid-mediated analgesia in fly-naive male deer mice, whereas exposure to either altered biting flies whose biting mouthparts were removed or nonbiting house flies had no significant effects. However, mice that were previously exposed to intact stable flies for 30 min exhibited significant analgesia when exposed 24-168 hr later to stable flies whose biting parts were removed, but not to nonbiting house flies. Administration of the specific N-methyl-D-aspartate (NMDA) antagonist NPC 12626 to fly-naive mice before exposure to intact flies, although not significantly reducing the analgesic response, blocked the subsequent conditioned analgesia. Naloxone, which blocked the intact biting fly-induced analgesia, did not alter the acquisition of the conditioned analgesic response to the altered stable flies. This demonstrates an NMDA-mediated acquisition of conditioned analgesia to a natural aversive stimulus.

Brief exposure of mice to 60 Hz magnetic fields reduces the analgesic effects of the neuroactive steroid, 3alpha-hydroxy-4-pregnen-20-one

Relatively weak, extremely low frequency (ELF), magnetic fields have been shown to exert a variety of biological effects, although the modes of action remain to be established. Neuroactive steroids and neurosteroids have been shown to produce a diverse range of rapid centrally mediated behavioral and physiological effects that are reported to be sensitive to magnetic fields. Here we show that brief exposure of male mice to an ELF magnetic field (30 min, 60 Hz, 141 microT peak) significantly reduces the analgesic effects arising from intracerebroventricular (i.c.v.) administration of the centrally produced allylic neuroactive steroid, 3alpha-hydroxy-4-pregnen-20-one (3alphaHP) and that the dihydropyridine (DHP) calcium channel antagonists, diltiazem and nifedipine, block the inhibitory effects of the 60 Hz ELF on 3alphaHP-induced analgesia. These results indicate that exposure to 60 Hz ELF affects the analgesic effects of neuroactive steroids such as 3alphaHP through alterations in calcium channel function. These findings raise the possibility that ELF magnetic fields may, in part, exert their actions through effects on diverse neuroactive steroid modulated processes.

Individual differences in radial maze performance and locomotor activity in the meadow vole, Microtus pennsylvanicus

Individual differences in the radial maze performance and locomotor activity of wild-caught and first-generation laboratory-born meadow voles are described. Based on their patterns of response in an eight-arm radial maze the essentially wild voles fell into three behavioral categories: 1) strict algorithmic (i.e., they systematically chose the next adjacent arm to their previous choice); 2) nonalgorithmic (i.e., they ran the maze without any consistent or definable pattern); and 3) nonrunners (i.e., nonperformers of the task who remained relatively immobile in the arms of the maze). The algorithmic and nonalgorithmic voles further differed in their responses to an interference manipulation of the radial maze task. Algorithmic individuals displayed a marked performance deficit, while the nonalgorithmic individuals showed minimal disruption to a 1-min delay interruption of the maze task. Measurements of several aspects of locomotor activity using the automated Digiscan activity monitoring system revealed that the algorithmic individuals also displayed significantly greater levels of activity than the nonalgorithmic or nonrunners, with no significant difference in activity between the latter two groups. These findings suggest that the algorithmic voles were relatively inflexible in their behavior, while the nonalgorithmic individuals were more flexible in their maze performance and likely in their use of spatial and nonspatial information. These individual differences in laboratory measures of learning behavior and locomotor activity in meadow voles are consistent with the polymorphism that is proposed to occur in the wild.

Sex differences in spontaneous locomotor activity and rotational behavior in meadow voles

Sex differences in rotational behavior have been most clearly established in laboratory rats with females exhibiting a turning bias. Here, using an automated open-field apparatus, locomotor activity and spontaneous rotational behavior were examined in diurnally crepuscularly active reproductive male and female meadow voles (Microtus pennsylvanicus). Meadow voles, being induced ovulators, permitted analysis of females in constant behavioral estrous. Males displayed significantly greater levels of activity and also significantly greater levels of clockwise but not counterclockwise rotational behavior relative to the females. Rotational behavior was less strongly related to activity levels in female as compared to male voles. In addition, females displayed an overall turning bias. These results contrast with findings from laboratory rats in which females are reported to display greater levels of both locomotor activity and rotational behavior. They are, however, consistent with the rotational bias evident in female rats. The present findings confirm the presence of sex differences in rotational behavior and indicate that factors other than activity levels are involved in the generation and/or expression of these sex differences. Sex differences in anxiety and routine-like behavior (i.e., asymmetry in movement) are discussed as possible factors contributing to these male-female differences in rotational behavior.

Sex differences in spatial learning and prefrontal and parietal cortical dendritic morphology in the meadow vole, Microtus pennsylvanicus

The prefrontal and parietal cortex has been implicated in the mediation of spatially related behaviors in male and female laboratory rats. Meadow voles, Microtus pennsylvanicus, are diurnally-crepuscularly active microtine rodents that exhibit a variety of sexually dimorphic spatially associated behaviors in both the laboratory and wild. In the present study we examined both the spatial Morris water maze performance and dendritic architecture and branching of neuronal cells in the prefrontal and parietal cortex of reproductive male and female meadow voles. Males learned the location of the hidden platform in the water task faster than estrous females and on probe trials they spent more time in the previously correct quadrant than females. Dendritic analysis with Golgi-Cox stained sections showed that male voles had significantly more dendritic arborization in the medial prefrontal and parietal cortex than females. These sex differences in both spatial navigation ability and in neural structures related to spatial navigation in meadow voles suggest that the size of neural areas might be shaped by ecological pressures associated with sexually dimorphic spatial behaviors.

Evidence for the involvement of nitric oxide and nitric oxide synthase in the modulation of opioid-induced antinociception and the inhibitory effects of exposure to 60-Hz magnetic fields in the land snail

The attenuation of opioid peptide-mediated antinociception is a well-established effect of extremely low frequency (ELF) electromagnetic fields with alterations in calcium channel function and/or calcium ion flux and protein kinase C activity being implicated in the mediation of these effects. The present study was designed to examine the effects of nitric oxide (NO) and calcium ion/calmodulin-dependent nitric oxide synthase (NOS) on opioid-induced antinociception and their involvement in mediating the inhibitory effects of exposure to ELF magnetic fields. We observed that enkephalinase (SCH 34826)-induced, and likely enkephalin-mediated, antinociception in the land snail, Cepaea nemoralis, as measured by the enhanced latency of a foot withdrawal response to a thermal (40 degreesC) stimulus, was reduced by the NO releasing agent, S-nitro-N-acetylpenicillamide (SNP), and enhanced by the NO synthase inhibitor, NG-nitro-l-arginine methyl ester (l-NAME). Exposure of snails to an ELF magnetic field (15 min, 60 Hz, 141 microT peak) also reduced the enkephalinase-induced antinociception. The inhibitory effects of the 60-Hz magnetic field were significantly reduced by the NO synthase inhibitor, l-NAME, and significantly enhanced by the NO releasing agent, SNP, at dosages which by themselves had no evident effects on nociceptive sensitivity. These results suggest that: (1) NO and NO synthase have antagonistic effects on opioid-induced analgesia in the snail, Cepaea and (2) the inhibitory effects of ELF magnetic fields on opioid analgesia involve alteration in NO and NO synthase activity.

Spatial learning and hippocampal volume in male deer mice: relations to age, testosterone and adrenal gland weight

Spatial learning and various physiological parameters were examined in old (57 month), middle aged (38 month), adult (18 month) and young (3-3.5 month) male deer mice (Peromyscus maniculatus). Performance during acquisition of a water maze task was not significantly reduced for middle aged and adult mice relative to young reproductively active (breeding) mice. Performance was deteriorated in old mice relative to young breeding mice on block 4 of training. Retention of this spatial task, however, was reduced in all three older groups relative to young breeding mice. Corrected hippocampal volume (corrected for brain weight) was reduced only in old mice relative to young breeding mice although absolute volumes of hippocampus were lower in all groups relative to young breeding mice. Old mice also were shown to have lower levels of plasma testosterone and lighter brains relative to young breeding mice. Spatial retention was not deficient in old, middle aged and adult mice relative to a group of young reproductively quiescent male (non-breeding) mice. Young breeding mice displayed better spatial performance and had significantly higher plasma testosterone levels, corrected hippocampal volume and brain weight relative to young non-breeding mice. These results indicate that retention of a spatial task is more sensitive to the age of male deer mice than acquisition of the task. Hippocampal volume, although a gross morphological feature, appears to be sensitive to the effects of ageing in male deer mice. Plasma testosterone levels do not appear to be a crucial factor underlying age-related deficits in retention of a spatial task.

Analgesic responses of male mice exposed to the odors of parasitized females: effects of male sexual experience and infection status

The present study shows that parasites influence both the responses of males to infected females and the responses of male hosts to females. Male mice exposed for 30 min to the odors of females infected with the nematode parasite Heligmosomoides polygyrus displayed a naloxone-sensitive, opioid-mediated analgesia, whereas males exposed for 1 min showed a shorter duration and lower amplitude naloxone-insensitive "nonopioid" analgesia that involved serotoninergic (5-HT) and excitatory amino acid (N-methyl-D-aspartate [NMDA] receptor) systems. The male mice distinguished between the odors of infected and physically stressed females, displaying greater analgesia after exposure to the odors of infected than stressed females. The analgesic responses to the odors of infected females were also affected by the males' prior sexual experience; sexually experienced males exhibited significantly greater analgesia than sexually naive males. In contrast, male mice infected with H. polygyrus failed to show a nonopioid analgesia after exposure to the odors of infected females and displayed a markedly lower level of opioid analgesia than uninfected mice. These results show that male mice can discriminate between the odors of parasitized and nonparasitized females and find the odors of parasitized estrous females aversive.

Parasitized female mice display reduced aversive responses to the odours of infected males

The present study showed that parasites influence both the responses of uninfected females to males and the responses of female hosts to infected males. In female laboratory mice one of the consequences of exposure to the olfactory cues associated with an infected male was a reduction of the reactivity to a thermal surface, i.e. pain inhibition or analgaesia. Uninfected oestrous and non-oestrous female mice displayed marked analgaesic responses after exposure to the odours of males infected with either the enteric single-host nematode parasite, Heligmosomoides polygyrus, or the protozoan parasite, Eimeria vermiformis. The uninfected oestrous females distinguished between infected and physically stressed males, displaying a greater analgaesic response to the odours of infected males. These analgaesic responses and their anxiety/ fearfulness-associated behavioural correlates could elicit either a reduced interest in, or avoidance of, parasitized males by females. Oestrous female mice infected with H. polygyrus displayed a reduced analgaesic response to the odours of the infected males and differentially responded to the odours of males infected with either the same (H. polygyrus) or a different parasite (E. vermiformis). An exposure time of 1 min elicited minimal responses to the odours of males infected with the same parasite, H. polygyrus, and an attenuated, though significant, non-opioid peptide-mediated analgaesic response to males infected with E. vermiformis. An exposure time of 30 min elicited similar markedly reduced endogenous opioid peptide-mediated analgaesic responses to the odours of both of the categories of infected males. The responses to the odours of a stressed male were, however, unaffected by the parasitic infection. The reduced analgaesic responses of the parasitized females to the odours of infected males may involve either enhanced odour familiarity and responses to group odour templates and/or neuromodulatory shifts resulting in reduced fearfulness and potentially greater interest in the infected males.

Social learning of a food preference in male and female Mongolian gerbils is facilitated by the anxiolytic, chlordiazepoxide

Social transmission of a food preference in Mongolian gerbils (Meriones unguiculatus) depends on the presence of a social bond between the interacting animals. An "observer" gerbil can acquire a preference for a novel food item from a familiar and, or related "demonstrator" animal. However, exposure to an unfamiliar and unrelated demonstrator gerbil does not lead to acquisition of a food preference, even though the extent of social interaction and likelihood of transmission of food information is unaffected. Likewise, individual preexposure to a novel food does not affect diet preference in individual animals. Here we show that oral, nongavage, administration of the benzodiazepine anxiolytic, chlordiazcpoxide (CDP, 2.5, 5, and 10 mg/kg) has significant dose-associated differential facilitatory effects on social learning in male and female gerbils, while having no significant effects on either individual learning or total food consumption. These results suggest that the CDP mediated reduction of the anxiety associated with the interactions between unfamiliar/unrelated gerbils facilitates social learning. These findings also rise the possibility of sex differences in socially related anxiety and the effects of CDP on social learning in gerbils.

Plasma testosterone levels are related to various aspects of locomotor activity in wild-caught male meadow voles (Microtus pennsylvanicus)

The relationship between plasma testosterone levels and locomotor activity in wild-caught sexually mature male meadow voles (Microtus pennsylvanicus) was assessed in the laboratory. Several aspects of locomotor activity were monitored for 1 h on two consecutive days using the automated Digiscan activity monitoring system. Plasma testosterone levels were determined immediately following the second day of activity monitoring. Significant Pearson correlations were obtained between plasma testosterone levels and total distance traveled [r(10) = 0.55, p < 0.05] and amount of time spent in movement [r(10) - 0.55, p < 0.05] on the second day. The wild voles showed a reduction in activity levels from the first to the second day of activity monitoring, which is indicative of habituation to a novel environment. This study provides direct evidence for a significant correlation between laboratory measures of behavioral activity and plasma testosterone levels in a wild-caught rodent. These findings indicate that previous assessments of hormone-behavior relationships in laboratory-bred rodents are consistent with the relationship between hormones and behavior in wild rodents.

Analgesic effects of a specific pulsed magnetic field in the land snail, Cepaea nemoralis: consequences of repeated exposures, relations to tolerance and cross-tolerance with DPDPE

It has been demonstrated previously that a short acute exposure to a specific extremely low frequency pulsed magnetic field (Cnp) can induce significant partly opioid-mediated analgesia in the land snail, Cepaea nemoralis. Here, this Cnp-induced analgesia is examined for the development of tolerance to daily repeated acute exposures of 15 or 30 min duration. Acute cross-tolerance to the delta opioid receptor directed agonist DPDPE, [D-Pen2, D-Pen5]enkephalin, was also found. Before (pre-exposure) and after (0, 15, 30 and 60 min) exposure to either a sham or Cnp magnetic field, snails were tested for an aversive reaction to a warmed surface (40 degrees C), and the latency time to the aversive reaction was recorded. Snails that were exposed to the Cnp showed a significant increase in the latency time (F1.55 = 2856.4; p < 0.001; Eta2 = 0.95), which may be interpreted as an induction of analgesia. During the daily (9 day) repeated acute exposures, the induction of analgesic response was significantly reduced, but not ablated. Altering the environmental conditions of the Cnp exposure restored a significant proportion of the partly developed tolerance, consistent with previous reports of environmental specificity in the development of opioid tolerance. These findings suggest that the partial development of tolerance to the opioid-mediated portion of Cnp-induced analgesia may be countered by altering the specific environmental Cnp exposure conditions.

Stable fly, Stomoxys calcitrans, mouthpart removal influences stress and anticipatory responses in mice

Biting fly attack induces a variety of stress and anxiety related changes in the physiology and behaviour of the target animals. Significant reductions in pain, or more appropriately, nociceptive sensitivity (latency of a foot-lifting response to an aversive thermal stimulus), are evident in laboratory mice after a 1 h exposure to stable flies, Stomoxys calcitrans. The role of the various components of biting fly attack in the development of this stress-induced reduction in pain sensitivity (analgesia) is, however, unclear. This study demonstrates that fly-naive mice do not exhibit a stress-induced analgesia when exposed to stable flies whose biting mouthparts have been removed. In contrast, mice that have been previously exposed to intact stable flies exhibit significant analgesia when exposed to flies that are incapable of biting. However, the level of analgesia induced is lower than that elicited by exposure to intact stable flies. Exposure to nonbiting house flies, Musca domestica, has no effect on nociceptive sensitivity. It appears that the actual bite of the stable fly is necessary for the induction of analgesia and probably other stress and anxiety associated responses in fly naive mice. However, mice rapidly learn to recognize biting flies and exhibit significant, possibly anticipatory analgesic responses to the mere presence of biting flies.

Sex differences in N-methyl-D-aspartate involvement in kappa opioid and non-opioid predator-induced analgesia in mice

There are suggestions of sex differences in N-methyl-D-aspartate (NMDA) receptor system involvement in the mediation of analgesia. The present study examined the effects of the specific, competitive NMDA antagonist, NPC 12626, on the nociceptive (50 degrees C hot plate) responses of reproductive male and female laboratory mice exposed to (i) an ethologically relevant aversive stimulus, the odor of a predator and (ii) administration of the kappa opiate agonist, U69,593. A 30-s exposure to 2-propylithietane, the major component of weasel odor, elicited a 'non-opioid' analgesia that was in both sexes insensitive to naloxone and the kappa opiate antagonist nor-binaltorphimine. In male mice this non-opioid analgesia was antagonized by NPC 1262, while in reproductive females the predator-induced analgesia was insensitive to NPC 12626. Similarly, NPC 12626 attenuated the analgesic effects of the kappa opiate agonist, U69,593, in male mice while having no significant effects on the equivalent levels of kappa opiate analgesia in females. These results show that there are sex differences in NMDA involvement in the expression and, or mediation of both non-opioid stress-induced and kappa opiate-mediated analgesia.