Inverse agonism of cannabinoid CB1 receptors potentiates LiCl-induced nausea in the conditioned gaping model in rats

BACKGROUND AND PURPOSE

Cannabinoid CB(1) receptor antagonists/inverse agonists, potentiate toxin-induced nausea and vomiting in animal models. Here, we sought to determine if this potentiated nausea was mediated by inverse agonism or neutral antagonism of the CB(1) receptor, and if the potentiated nausea would be produced by intracerebroventricular (icv) administration of an inverse agonist.

EXPERIMENTAL APPROACH

The conditioned gaping model of nausea in rats was used to compare the CB(1) receptor antagonist/inverse agonist, AM251, and the CB(1) receptor neutral antagonists, AM6527 (centrally and peripherally active) and AM6545 (peripherally active), in potentiating conditioned gaping produced by lithium chloride (LiCl) solution. The effect of icv (lateral ventricle and 4th ventricle) administration of AM251 on LiCl-induced gaping in this model was also evaluated.

KEY RESULTS

At a dose that did not produce conditioned gaping on its own, systemically administered AM251 (1.25 mg.kg(-1)) potentiated LiCl-induced conditioned gaping and reduced sucrose palatability; however, even doses as high as 8 mg.kg(-1) of AM6545 and AM6527 neither potentiated LiCl-induced conditioned gaping nor reduced sucrose palatability. Infusions of AM251 into the lateral ventricles (1.25, 12.5 and 125 microg) or the 4th ventricle (2.5, 12.5 and 125 microg) did not potentiate LiCl-induced conditioned gaping reactions, but all doses attenuated saccharin palatability during the subsequent test.

CONCLUSIONS AND IMPLICATIONS

Inverse agonism, but not neutral antagonism, of CB(1) receptors potentiated toxin-induced nausea. This effect may be peripherally mediated or may be mediated centrally by action on CB(1) receptors, located distal to the cerebral ventricles.

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.

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.

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.

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.

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.

Rotation-induced conditioned rejection in the taste reactivity test

The taste reactivity test was used to evaluate the ability of motion sickness to produce conditioned rejection reactions, a putative measure of nausea in rats. Following three conditioning trials, rats displayed conditioned rejection reactions during an intraoral infusion of a rotation-paired saccharin solution. This is the first demonstration of conditioned rejection produced with a non-pharmacological emetic agent and provides support that the conditioned rejection reaction may serve as a rat model of nausea.

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.

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.

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.

Taste reactivity responses in rats: influence of sex and the estrous cycle

Gonadal hormones (e.g., estradiol) may regulate feeding by producing a shift in the taste or palatability of food items. This study examined the impact of endogenous gonadal hormones on palatability by investigating sex differences in taste responsivity, as well as the effect of the estrous cycle on taste responsivity, in a rodent model. In the taste reactivity test, male and female Long-Evans rats received a brief (1 min) intraoral infusion of one of three tastants: sucrose (0.3 M), quinine (0.0003 M), and a sucrose-quinine mixture (0.3 M sucrose and 0.0003 M quinine). Statistical analyses indicated that female rats tested during diestrus or proestrus produced significantly more ingestive responses than did male rats and fewer aversive responses than did both male rats and female rats tested during estrus or metestrus (P < 0.05). These results indicate a sex difference in taste responsivity in the rat that is modulated by the reproductive status of female rats. This finding implies a role of gonadal hormones in the regulation of taste responsivity in the rat.

Hormone replacement modifies cholecystokinin-induced changes in sucrose palatability in ovariectomized rats

The taste reactivity test was used to examine the effect of CCK-octapeptide (CCK-8) on the palatability of a sucrose solution in ovariectomized rats either receiving hormonal replacement (estradiol and progesterone; OVX + HRT), or treated with vehicle only (OVX + VEH). Statistical analyses revealed that the OVX + HRT rats treated with CCK-8 exhibited a robust decrease in ingestive responses, and an increase in aversive responses and passive drips to the intraoral sucrose infusions, relative to treatment with the NaCl vehicle. In contrast, a weak effect of CCK-8 on ingestive responses, no significant effect on the frequency of aversive responses, and a reduced effect on passive drips was observed in the OVX + VEH rats. These results show that CCK-8 modifies sucrose palatability, and that this effect is modulated by gonadal hormone levels.

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.

Forced-choice discrimination of equimolar NaCl and LiCl solutions in rats: effects of ablating the chemosensitive area postrema on acquisition and retention

The area postrema (AP), a chemosensitive organ located in the fourth ventricle, has been shown to mediate the formation of a lithium-induced conditioned taste avoidance (CTA) in rats. The present experiments examined the role of the AP in the discrimination between two equimolar solutions of sodium chloride (NaCl) and lithium chloride (LiCl). In the first experiment adult male rats were trained to discriminate between equimolar (0.12 M) solutions of NaCl and LiCl in a forced-choice procedure over a 10-day acquisition phase. Subsequently half of the rats (n = 7) received AP lesions (APX) and the other half (n = 7) were given sham lesions (SHAM). In the retention phase all animals were again exposed to the same salt solutions over a 10-day period. Good discrimination (P < 0.001) between the two salt solutions was demonstrated by the end of the acquisition phase and both the APX and SHAM groups exhibited robust retention (P < 0.01) of this discrimination in the second phase. However, when only a LiCl solution was available the APX group ingested significantly more (P < 0.01) than the SHAM rats. No significant group difference emerged when only NaCl was available. In the second experiment rats received ablations of AP or sham lesions and were then trained to discriminate between 0.12 M NaCl and LiCl solutions in a forced-choice procedure over a 10-day period. Both groups exhibited a clear discrimination (P < 0.01) between the two solutions by the end of the acquisition phase. APX rats ingested significantly more LiCl (P < 0.01) than did the SHAM group when this was the only type of fluid available. Again, no such difference was evident when only NaCl was available. These experiments demonstrate that the AP is not necessary for either the acquisition or retention of a discrimination between equimolar solutions of NaCl and LiCl in a forced-choice procedure and that this discrimination is not mediated by a conditioned taste aversion to the LiCl solution.

Sexually dimorphic aspects of spontaneous activity in meadow voles (Microtus pennsylvanicus): effects of exposure to fox odor

In this study, a multivariate analysis of the locomotor activity of adult, breeding male and female meadow voles (Microtus pennsylvanicus) was conducted. Overall, male voles made more movements and spent more time in the center of the activity chambers than did female voles. The authors further investigated the effects of brief exposure (3 min) to predator (red fox [Vulpes vulpes]) odor and various control odors (butyric acid, extract of orange) on subsequent activity. Control odors had no effects. Immediately following exposure to the fox odor, male voles exhibited significantly lower levels of activity and decreased center time. No significant changes in any activity variable were observed in the female voles following exposure to fox odor. This study provides evidence for sex differences in both basal activity levels of meadow voles and activity following exposure to a predator odor.

Antinociceptive effects of a pulsed magnetic field in the land snail, Cepaea nemoralis

Pulsed magnetic fields (patent pending) consisting of approximately 100 microT (peak), frequency modulated, extremely low frequency magnetic fields (ELFMF) were shown to induce a significant degree of antinociception ('analgesia') in the land snail Cepaea nemoralis. Fifteen minute exposures to a specific magnetic field both increased enkephalinase inhibitor induced opioid analgesia and induced analgesia in untreated snails. Injection of the prototypic opioid antagonist naloxone, attenuated, but did not completely block, the pulsed magnetic field induced analgesia. Two other pulsed waveform designs failed to induce analgesia in untreated snails. These findings suggest that specific magnetic field exposure designs may be tailored to produce significant behavioral effects including, but certainly not limited to, the induction of analgesia.

Pulsed magnetic field induced "analgesia" in the land snail, Cepaea nemoralis, and the effects of mu, delta, and kappa opioid receptor agonists/antagonists

A brief exposure to a pulsed magnetic field (Cnp: patent pending) had significant antinociceptive or "analgesic" effects in the land snail, Cepaea nemoralis, as evidenced by an increase in the latency of response to a warmed (40 degrees C) surface. This analgesia was in part opioid mediated being significantly reduced, but not eliminated: by the prototypic opiate antagonist, naloxone; the mu (mu) opioid receptor directed antagonists, naloxazine or beta-funaltrexamine, and the delta (delta) opioid receptor directed antagonists, naltrindole-5'-isothiocyanate or ICI 174,864. However the Cnp induced analgesia was unaffected by the kappa (kappa) opioid receptor directed antagonist, nor-binaltorphimine. The delta 1 and delta 2 opioid receptor directed agonists, (DPDPE, [D-Pen2,D-Pen5]enkephalin), (deltorphin, [D-Ala2,Glu4]), respectively, also had significant differential analgesic effects, supporting a functional delta opioid receptor mediated enkephalinergic mechanism in Cepaea. These results suggest that this specific pulsed magnetic field (Cnp) elicits significant analgesic effects through mechanisms that, in part, involve delta and, to a lesser extent mu opioid receptors.

Sex differences in performance in the Morris water maze and the effects of initial nonstationary hidden platform training

Sex differences in rats' performance on a stationary hidden-platform task (spatial task) in the Morris water maze and the effects of initial nonstationary hidden platform training (NSP training) were examined. The NSP training was designed to familiarize rats with the general requirements of the water-maze task without providing spatial information. NSP training led to faster acquisition and improved retention of the subsequent spatial task in both males and females. There was a sex difference favoring males on acquisition and retention of the spatial task only in rats that had not received previous NSP training. Moreover, there was an apparent reversed sex difference favoring females on some measures of spatial performance in NSP-trained rats. These results suggest that performance on the water-maze task, including the expression of sex differences, can be altered by previous familiarization with nonspatial aspects of the task.

Oestradiol-induced taste avoidance is the result of a conditioned palatability shift

Taste avoidances were conditioned in male rats by pairing ingestion of a novel sucrose (0.3 M solution) taste with injections of 17 beta oestradiol (100 micrograms kg-1, s.c.). Following conditioning and prior to a two-bottle choice test (sucrose vs water), taste reactivity responses to three 30 s intraoral sucrose infusions were quantified. A robust conditioned shift in palatability, consisting of reduced ingestive and increased aversive taste reactivity responses, was obtained in the oestradiol group but not the vehicle control group. This conditioned palatability shift was also reflected in subsequent strong avoidance of the sucrose solution in the two-bottle choice test. The findings that oestradiol can condition taste aversions and shift food preferences support a role for oestrogens in the production of anorexia.

Spatial learning in deer mice: sex differences and the effects of endogenous opioids and 60 Hz magnetic fields

We examined the effects of brief exposure to weak 60 Hz extremely low frequency (ELF) magnetic fields and opioid systems on spatial behavior and learning in reproductive adult male and female deer mice, Peromyscus maniculatus. Sex differences were evident in spatial performance, with male deer mice displaying significantly better performance than female mice in the Morris water maze, whereby animals had to acquire and retain the location of a submerged hidden platform. Brief (maximum 5 min) exposure to weak (100 microT) 60 Hz magnetic fields during task acquisition significantly improved female performance, eliminating the sex differences in acquisition. The opiate antagonist, naltrexone, also improved female acquisition, though significantly less than the magnetic fields. These facilitatory effects involved alterations of "non-spatial" (task familiarization and reduction of related anxiety/aversive related behaviors) and possibly "spatial" aspects of the task. Enhancement of enkephalin activity with the enkephalinase inhibitor, SCH 34826, significantly reduced task performance by male deer mice. Both naltrexone and the 60 Hz magnetic fields attenuated the enkephalin mediated reductions of spatial performance. These findings indicate that brief exposure to 60 Hz magnetic fields can enhance water maze task acquisition by deer mice and suggest that these facilitatory effects on spatial performance involve alterations in opioid activity.

Sexually dimorphic aspects of spontaneous activity in meadow voles (Microtus pennsylvanicus): effects of exposure to fox odor

In this study, a multivariate analysis of the locomotor activity of adult, breeding male and female meadow voles (Microtus pennsylvanicus) was conducted. Overall, male voles made more movements and spent more time in the center of the activity chambers than did female voles. The authors further investigated the effects of brief exposure (3 min) to predator (red fox [Vulpes vulpes]) odor and various control odors (butyric acid, extract of orange) on subsequent activity. Control odors had no effects. Immediately following exposure to the fox odor, male voles exhibited significantly lower levels of activity and decreased center time. No significant changes in any activity variable were observed in the female voles following exposure to fox odor. This study provides evidence for sex differences in both basal activity levels of meadow voles and activity following exposure to a predator odor.

Area postrema mediates the formation of rapid, conditioned palatability shifts in lithium-treated rats

The rapid acquisition and subsequent retention of lithium-induced conditioned changes in taste reactivity responses to sucrose were examined in rats with the area postrema (AP) either ablated or intact. On 2 conditioning days, a series of brief intraoral sucrose infusions was paired with the effects of LiCl or NaCl injections. Repeated associations of the sucrose taste with the effects of lithium significantly reduced ingestive responses and increased aversive responses only in the AP-intact group. AP-ablated rats treated with LiCl and rats injected with NaCl displayed an ingestive pattern of responses. Only the AP-intact rats, previously injected with LiCl, subsequently displayed evidence of a conditioned taste aversion. We conclude that toxin activation of the AP is required to produce the conditioned shift in taste reactivity responses and subsequent expression of a taste aversion in rats treated with lithium.

Sexually dimorphic spatial learning in meadow voles Microtus pennsylvanicus and deer mice Peromyscus maniculatus

A number of studies examining developmental, neural and hormonal aspects of sexually dimorphic spatial learning (Morris water-maze) in meadow voles (Microtus pennsylvanicus) and deer mice (Peromyscus maniculatus) are described. We found that, in adult deer mice, female spatial performance decreased during the breeding season relative to the non-breeding season, whereas the reverse pattern was observed in male performance. There was a sex difference favouring males in spatial learning during the breeding season, but not during the non-breeding season. In adult meadow voles, females with low levels of oestradiol and males performed better in the water-maze than females with high levels of oestradiol. Postweaning voles (20 and 25 days after birth) acquired the water-maze task more quickly than preweaning voles (day 10). No sex difference in water-maze performance was evident at any of these juvenile ages. When these same voles were tested again as adults to investigate retention and re-acquisition of the water-maze, both males and females from male-biased litters re-acquired the task better than males and females from female-biased litters. Together, the results of these studies indicate that sexually dimorphic spatial ability is dependent on the organization (in utero) and activational effects of gonadal hormones. These studies provide the first demonstration of the influence of natural changes in reproductive status on spatial learning of deer mice and meadow voles. The results also demonstrate that spatial performance of males and females is differentially affected by changes in reproductive status and that group differences in the laboratory are associated with group differences in space utilization in the wild. These findings help to clarify previous apparently contradictory findings about sex differences in spatial ability.

Possible mechanisms by which extremely low frequency magnetic fields affect opioid function

Although extremely low frequency (ELF, < 300 Hz) magnetic fields exert a variety of biological effects, the magnetic field sensing/transduction mechanism (or mechanisms) remain to be identified. Using the well-defined inhibitory effects that magnetic fields have on opioid peptide mediated antinociception or "analgesia" in the land snail Cepaea nemoralis, we show that these actions only occur for certain frequency and amplitude combinations of time-varying sinusoidal magnetic fields in a manner consistent with a direct influence of these fields. We exposed snails with augmented opioid activity to ELF magnetic fields, which were varied in both amplitude and frequency, along with a parallel static magnetic field. When the peak amplitude (0-547 microT) of a magnetic field of 60 Hz was varied systematically, we observed a nonlinear response, i.e., a nonlinear reduction in analgesia as measured by the latency of a defined response by the snails to a thermal stimulus. When frequency (10-240 Hz) was varied, keeping the amplitude constant (141 microT), we saw significant inhibitory effects between 30 and 35 Hz, 60 and 90 Hz and at 120 and 240 Hz. Finally, when the static field was varied but the amplitude and frequency of the time-varying field were held constant, we observed significant inhibition at almost all amplitudes. This amplitude/frequency "resonance-like" dependence of the magnetic field effects suggests that the mechanism (or mechanisms) of response to weak ELF fields likely involves a direct magnetic field detection mechanism rather than an induced current phenomenon. We examined the implications of our findings for several models proposed for the direct sensing of ELF magnetic fields.

A multivariate assessment of spontaneous locomotor activity in the Mongolian gerbil (Meriones unguiculatus): influences of age and sex

A multivariate assessment of the spontaneous locomotor activity of male and female Mongolian gerbils (Meriones unguiculatus) was obtained using a Digiscan automated animal activity monitoring system. Spontaneous motor activity data were collected over 1 h (5-min samples) for groups of male and female gerbils ranging from 26-341 days of age (26, 38, 62, 116, 151, 172, 196, 247, and 341). Variables examined included: total distance travelled, average distance per movement, average speed, number of horizontal movements, time in horizontal movement, time per horizontal movement, number of vertical movements, time in vertical movement, and time per vertical movement. Age had a significant effect on spontaneous activity; all measures of horizontal activity increased from preadulthood (26 and 38 days) and remained relatively constant thereafter for adults (62+ days). Vertical activity (rearing) measures were found to increase from the 62-day-old group to the 151- and 172-day-old groups and then decrease among the older groups (196+ days). Across the 12 samples, within sessions, all horizontal and vertical activity measures (except average speed) declined for both males and females. Habituation was more rapid for the preadults than for the adults on all horizontal measures except average distance per movement. No consistent sex differences in locomotor activity were found.

Gonadal hormone levels and spatial learning performance in the Morris water maze in male and female meadow voles, Microtus pennsylvanicus

The present study examined the relationships between spatial learning and circulating levels of plasma estradiol and testosterone in adult male and female meadow vole, Microtus pennsylvanicus. Meadow voles are induced ovulators and most females that are housed with females or in isolation are in constant diestrus, whereas most females that are housed with males are in constant behavioral estrus. In this study sexually mature, adult male and female meadow voles housed with either females (constant diestrus) or males (constant behavioral estrus) were required to learn the spatial position of a hidden, submerged platform in the Morris water maze. Individual voles were tested using two blocks of four trials twice a day for 3 days for a total of six blocks. Task retention was examined with a probe trial 1 day after the last acquisition trial. Females were divided into two groups based on the median level of plasma estradiol [High Estradiol (15.79 +/- 1.20 pg/ml) and Low Estradiol (6.22 +/- 2.79 pg/ml) Females]. Males were similarly divided on the basis of median plasma testosterone levels [High (2.53 +/- 0.96 ng/ml) and Low Testosterone (0.45 +/- 0.08 ng/ml) Males]. High Estradiol females exhibited significantly longer latencies to reach the hidden platform, indicating poorer acquisition, than did either males (P = 0.025) or Low Estradiol females (for Blocks, 2, 3, 4, and 6, P = 0.037). Male superiority in spatial learning performance was evident only when High Estradiol females were compared to males. There were no significant performance differences between High and Low Testosterone males. There were also no group differences in retention, with all voles displaying significant retention of the spatial task. There was, however, a significant correlation between plasma estradiol levels in females and retention, with higher estradiol levels being associated with poorer retention. These results suggest that levels of estradiol in adult female meadow voles are significantly related to spatial learning, with low levels of estradiol being associated with better spatial learning. There was no evidence that levels of testosterone were related to spatial performance in adult male meadow voles. The results suggest that estradiol may have activational effects on spatial learning in the adult meadow vole and that sex differences in spatial learning are evident only when High Estradiol females are compared to adult males.

Toxin-induced conditioned changes in taste reactivity and the role of the chemosensitive area postrema

Conditioned taste avoidances (CTAs) are an important component of behavioral regulation of ingestion. In the laboratory CTAs can be produced by pairing a novel taste stimulus with the physiological feedback produced by a toxin, such as lithium. Such toxins putatively activate a chemosensitive brainstem structure, the area postrema, which ultimately results in the production of a CTA. The present review describes a series of studies which examined conditioned changes in taste reactivity responses (TRRs) when a novel intraoral sucrose taste was paired with the effects of an intraperitoneal (IP) injection of LiCl, and the role of the area postrema in the formation of conditioned palatability shifts. It was first of all necessary to examine the effects of area postrema ablations on TRRs to a range of intraoral sucrose and quinine stimulus intensities. In the first study area postrema lesioned rats exhibited concentration dependent changes in TRRs to these taste stimuli that were very similar to those exhibited by sham lesioned rats. The second study demonstrated that 30 s intraoral infusions of sucrose (0.3 M), presented at 5 or 10 min intervals following an IP injection of LiCl (3.0 meq), resulted in conditioned changes in TRRs. These were characterized by orderly, gradual reductions in ingestive responses and increases in aversive responses. Finally, when area postrema lesioned rats (Study 3) were subjected to this conditioning procedure (brief sucrose presentations paired with the effects of LiCl) no evidence for conditioned or unconditioned changes in TRRs to sucrose were obtained. Lesioned rats injected with LiCl behaved similarly to sham lesioned rats injected with NaCl. These series of studies provide evidence indicating that the chemosensitive area postrema mediates the formation of conditioned palatability shifts induced by treatment with a toxin such as lithium.

Cholecystokinin reduces ingestive taste reactivity responses to water in fluid-replete but not fluid-deprived rats

The effects of hydrational factors on the ability of cholecystokinin (CCK) to alter taste reactivity responses to intraoral water infusions was assessed in fluid-replete and fluid-deprived rats. Naive male rats were injected with CCK (8 micrograms/kg, IP), or physiological saline (1 ml/kg, IP) and taste reactivity responses elicited by brief (30 s) intraoral water infusions at 2, 4, 6, 8, and 10 min postinjection were measured. One week later and following a 24 h water deprivation period, taste reactivity responses to intraoral water infusions were measured again. Exogenous administration of CCK was found to produce a significant decrease in the frequency of ingestive responses accompanied by a significant increase in passive drip during oral water infusions in fluid-replete rats. In contrast, CCK was found to have no effect on the frequency of taste reactivity responses when rats were subjected to a period of water deprivation. These results demonstrate that the rat's state of hydration interacts with the ability of CCK to alter taste reactivity responses to intraoral water infusions.

Cholecystokinin reduces sucrose palatability in rats: evidence in support of a satiety effect

To investigate the hypothesis that peripherally administered cholecystokinin (CCK) reduces food intake by the production of aversive internal cues, we examined the effects of the sulfated, octapeptide form of CCK on taste reactivity responses to oral sucrose infusions in male rats implanted with intraoral cannulas. After injection of CCK (4, 8, or 16 micrograms/kg ip) or 0.15 M saline (1 ml/kg ip), a series of brief (30 s) intraoral infusions of a 0.30 M sucrose solution was administered at 2-min intervals for 10 min. All doses of CCK were found to significantly decrease ingestive responding during the first and subsequent sucrose infusions without promoting a significant increase in aversive responses relative to controls. The lack of a gradual, conditioned shift in taste reactivity responses, from an ingestive to an aversive pattern (which is typically observed after LiCl administration), suggests that the production of nausea-like aversive internal cues was likely not responsible for the observed CCK-induced alterations in taste reactivity responses. It appears that the unconditioned, satiogenic effects of CCK contributed to the selective reduction in ingestive responses observed in the present study.

Alcohol-induced conditioned taste aversions in chemically labyrinthectomized rats

Male rats were chemically labyrinthectomized (n = 22) by intratympanic injections of sodium arsanilate, and control rats (n = 15) received intratympanic injections of isotonic saline. All rats were tested for labyrinthine integrity and then adjusted to a 23 h.d-1 water deprivation schedule. Both labyrinthectomized and control rats were exposed to a conditioned taste aversion (CTA) procedure or a control procedure. The CTA technique involved pairing a novel saccharin taste with subsequent intraperitoneal injection of ethanol (1.5 g.kg-1; 15% solution). The control CTA procedure paired a novel saccharin taste with injections of isotonic saline. Following two conditioning trials and 3 d of water only, saccharin preference ratios were obtained in two-bottle choice tests (saccharin vs. water) over 4 consecutive days. Control rats conditioned with ethanol exhibited a strong CTA (p < 0.01) relative to control rats injected with saline. Labyrinthectomized rats drinking saccharin followed by ethanol injections showed a strong CTA (p < 0.01) if conditioning occurred 29-30 d post-labyrinthectomy. However, CTA's were not apparent in labyrinthectomized rats conditioned with ethanol 19 d post-labyrinthectomy. Thus, ethanol-induced CTA formation varied across the post-labyrinthectomy time period.

Performance (re-acquisition) of a water-maze task by adult meadow voles: effects of age of initial task acquisition and in utero environment (litter sex-ratio)

Previous research in this laboratory has shown that preweaning and postweaning juvenile meadow voles, Microtus pennsylvanicus, can acquire a spatial task, the Morris water-maze task. The present study examined the influence of age of juvenile acquisition ("before weaning" (BW; Day 10 and 15 after birth) and "after weaning" (AW; Day 20 and 25 after birth)) of a spatial task on subsequent re-acquisition of the same hidden-platform spatial water-maze task. This study also compared sex differences and litter sex-ratio effects on reacquisition performance. Fifteen litters of adults were re-tested in the same water maze 6 weeks after being initially tested as juveniles. All analyses were conducted using a covariate that removed the group differences in the original task performance. Adult voles from female-biased litters, that had previously learned the task at an older juvenile age (AW), reacquired the same task faster than adults that had previously learned the task at a younger juvenile age (BW). In the adult BW group there was also a significant litter sex-ratio effect such that voles born into a female-biased litter re-acquired the task more slowly than did voles born into a male-biased litter. There were no significant sex or litter sex-ratio effects on spatial learning in the AW group. These results show that adult meadow voles can require a spatial task more quickly if they initially learned the task at an older juvenile age, suggestive of a period of infantile amnesia. In addition, these results indicate that the litter sex-ratio can affect adult spatial performance, suggesting that the relative amount of androgens in utero may influence the development of sexually-dimorphic spatial ability in adulthood.

Reductions in body temperature and spontaneous activity in rats exposed to horizontal rotation: abolition following chemical labyrinthectomy

The effect of horizontal rotation of male rats (70 rpm) on core temperature and spontaneous motor activity levels was examined. In Experiment 1, subjects were chemically labyrinthectomized (VNX) by intratympanic (IT) injections of sodium arsanilate and control rats (VNS) received IT injections of saline. Half of the rats in each group were subsequently rotated and the other half sham rotated. Measurement of body temperature prior to, immediately after, and 20 min following rotation revealed significant (all p < 0.01) reductions in temperature immediately after treatment, and 20 min later, in VNS rats. Sham-rotated VNS and all VNX rats failed to exhibit any significant changes in temperature following treatment. In Experiment 2, motor activity level was monitored in chemically labyrinthectomized (VNX) and control (VNS) rats prior to, and following, horizontal rotation. The VNS rats exhibited large (all p < 0.01) depressions in measures of horizontal and vertical spontaneous motor activity following rotation treatment, whereas VNX rats exhibited similar levels of activity in the pre- and postrotation period. These experiments show that, as in humans, exposing rats to horizontal rotation results in reduction of body temperature and motor activity, and that these physiological and behavioral changes require a functional vestibular system.

Naloxone facilitates spatial learning in a water-maze task in female, but not male, adult nonbreeding meadow voles

The present study examined the effects of the opiate antagonist naloxone on spatial acquisition and retention in a water-maze task by adult, nonbreeding, male and female meadow voles (Microtus pennsylvanicus). Voles were required to learn the position of a hidden, submerged platform using distal visual cues. There were four trials per day for 6 days. Daily pretraining (15 min before first trial) systemic administrations of naloxone (1.0 mg/kg, IP) significantly facilitated spatial acquisition in female, but not in male, voles in a water-maze task on days 2, 3, and 4. There were two probe tasks given 1 day and 1 week after the last training trial. All groups acquired the spatial task by the end of the fifth day with no significant effects of naloxone on retention of the spatial task. There were also no significant sex differences in acquisition of the spatial task and task retention in control, nonbreeding adult voles. It is suggested that the lack of sex differences in basal spatial performance may be related to the low levels of testosterone in male nonbreeding voles. The obtained sex differences in the effects of naloxone on spatial acquisition are considered in relation to sex differences in stress, opiate responses, and gonadal steroid levels.

Deoxynivalenol (vomitoxin)-induced conditioned taste aversions in rats are mediated by the chemosensitive area postrema

The present experiments used a conditioned aversion to a novel saccharin taste to assess the aversive effects of deoxynivalenol (vomitoxin) administration, and to examine the putative mediating role of the chemosensitive area postrema (AP). In experiment 1 adult male rats drank a novel 0.15% saccharin solution followed by injection of deoxynivalenol (n = 7; 0.125 mg/kg, IP) or vehicle (n = 7; propylene glycol, 0.5 ml/kg). In subsequent two-bottle preference tests the rats conditioned with deoxynivalenol displayed significantly (p < 0.01) lower absolute and relative saccharin intake levels in comparison to control rats which exhibited a strong preference for saccharin solution. In experiment 2 adult male rats received area postrema ablations (n = 6) or sham lesions (n = 6). On two conditioning days all rats drank a novel 0.15% saccharin solution followed by injections of deoxynivalenol (0.125 mg/kg, IP). In subsequent two-bottle preference tests the sham-lesioned rats displayed a significant (p < 0.01) aversion to the saccharin stimulus, relative to the area postrema-ablated rats which exhibited a preference for the saccharin solution. Thus, systemic administration of deoxynivalenol, following a novel taste, induced conditioned taste aversions which were mediated by the area postrema.

Developmental changes in spatial learning in the Morris water-maze in young meadow voles, Microtus pennsylvanicus

Spatial learning in pre- and postweaning meadow voles, (Microtus pennsylvanicus) was examined in a Morris water-maze task. The learning performance of 10-day-old (preweaning) and 15-, 20- and 25-day-old (postweaning) male and female voles was assessed by measuring the latency to reach a hidden platform by each animal twice a day for 5 days. Voles of all age groups were able to learn the spatial task with Day 10 and Day 15 voles acquiring the task more slowly than did Day 20 and Day 25 voles. There were no significant sex differences in task acquisition in any of the four age groups. In addition, although swimming speed was related to age, with older animals swimming faster than younger ones, differences in swim speed did not account for the faster acquisition by the older animals. These results show that both preweaning and postweaning voles can successfully learn a spatial task. This is in contrast to preweaning laboratory rats which cannot successfully acquire a similar spatial task. These findings indicate that there are species differences in the ontogeny of spatial learning, which are likely related to the ecological and behavioural developmental characteristics of the species. Furthermore, in contrast to the sex difference in water-maze performance obtained in adult, breeding meadow voles who demonstrate a sex difference, there were no significant sex differences in the spatial performance of the juvenile voles. This suggests that sex differences in spatial learning in the meadow vole do not appear until voles reach reproductive adulthood.

Sexually dimorphic spatial learning varies seasonally in two populations of deer mice

Spatial learning in photoperiodically induced breeding (reproductive) and non-breeding (non-reproductive) adult male and female deer mice (Peromyscus maniculatus) was examined in a Morris water-maze task. Sexually mature, adult male and female deer mice that were derived from either a mainland population (P. m. artemisiae) or an island population (P. m. angustus) were required to learn the spatial position of a hidden, submerged platform in a water maze. Deer mice were tested either during the breeding season (summer; long day photoperiod) or during the non-breeding season (winter; short day photoperiod) with a total of six blocks of four trials conducted in a single day. Retention was tested with two probe trials which occurred one and three days after acquisition. During the breeding season male spatial task acquisition was superior to female spatial task acquisition for both populations. In contrast, during the non-breeding season there were no significant sex differences in spatial acquisition for either population. This change in sexually dimorphic spatial learning was due to female spatial-performance decreasing from non-breeding season to the breeding season and male spatial-performance increasing over the same period. Both populations displayed similar seasonal variations in sexually dimorphic water-maze task performance. There were, however, overall population differences in water-maze task performance that were related to the ecology of the mice, with the insular mice displaying shorter latencies to reach the hidden platform than did the mainland deer mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Repeated naloxone treatments and exposures to weak 60-Hz magnetic fields have 'analgesic' effects in snails

Results of studies with rodents have shown that animals repeatedly injected with the opioid antagonist, naloxone, acquire a hypoalgesic response to thermal nociceptive stimuli. The present study revealed a similar response in the terrestrial pulmonate snail, Cepaea nemoralis. Snails receiving daily injections of naloxone followed by measurements of thermal nociceptive sensitivity also developed hypoalgesia. Daily brief (30-min) exposures to a weak 60-Hz magnetic field (1.0 gauss or 0.1 mT), which acutely antagonize opioid-mediated nociception and antinociception in a manner comparable to that of naloxone, also led to the expression of a hypoalgesic responses. This suggests that opioid antagonist-induced thermal hypoalgesia may be a basic feature of opioid systems. This naloxone- and magnetic field-induced 'analgesia' is consistent with either a facilitation of aversive thermal conditioning and or antagonism of the excitatory, hyperalgesic effects of low levels of endogenous opioids.

Spatial learning in an enclosed eight-arm radial maze in rats with sodium arsanilate-induced labyrinthectomies

Bilateral vestibular dysfunction was induced in Long-Evans male rats (n = 7) by intratympanic injections of sodium arsanilate (30 mg/side). Control rats (n = 6) received isotonic saline. Animals were tested for labyrinthine integrity by measuring air-righting and contact-righting reflexes. Rats were reduced to 85% of free-feeding body weight and tested in an enclosed 8-arm radial maze (1 trial/day over 10 days). Labyrinthectomized animals made significantly more errors (p < .001) and, unlike the controls, showed no significant improvement on this measure over acquisition training. These rats also made significantly more (p = 0.018) sequential same arm reentries and fewer sequential adjacent arm entries (p < .01). These findings demonstrate that information obtained from the vestibular system is very important in spatial learning in the rat.

Novel diet consumption and body weight gain are reduced in rats chronically infused with lithium chloride: mediation by the chemosensitive area postrema

The effects of chronic lithium chloride infusions on consumption of, and subsequent preferences for, a novel diet were examined in rats with ablations of the area postrema (AP) and sham-lesioned control rats. Osmotic minipumps (Alza), filled with a saturated aqueous solution of LiCl (63 g/100 ml), were implanted in the peritoneal cavity of half of the lesioned rats (n = 9) and half of the control rats (n = 8). The remaining rats received empty pumps (n = 9 and n = 7 for lesioned and controls, respectively). The LiCl or sham drug phase was paired with free access to a highly palatable novel diet (AIN diet) during a 7-day conditioning period. Subsequent preferences for the novel diet relative to a familiar diet (ground Purina lab pellets) were determined using a two-food choice procedure. The only group to show a persistent and significant reduction in novel food consumption during the conditioning phase was the sham-lesioned group infused with LiCl (p < 0.01). This group also exhibited a marked aversion for the novel diet, indicative of a conditioned food aversion (CFA), during the preference tests. No significant differences in novel diet consumption or in novel diet preference were found between the two AP-lesioned groups. This study provides evidence that anorexia and CFAs to a novel diet, induced with chronic infusions of lithium, are abolished by destruction of the chemosensitive area postrema.

Effects of central administration of kynurenic acid on spontaneous locomotor activity in the kindled rat: a multivariate approach using the automated Digiscan monitoring system

Changes in spontaneous motor activity in kindled hooded rats were measured following intracerebroventricular administration of three doses of kynurenic acid (65, 39, and 6.5 micrograms, dissolved in 3.3 microliters isotonic saline). Behavior was measured in the automated Digiscan system on every third day during 13 days of drug administration to assess initial behavioral impairment and the development of tolerance. Activity data were collected beginning 5 min after drug administration for six consecutive 5-min samples. The results revealed a suppressive effect of central administration of kynurenic acid on the pattern of spontaneous locomotor activity and showed the development of behavioral tolerance. Initially, the degree of suppression was dose related, but as tolerance developed group differences were minimized. Most measures returned to predrug levels by day 13 except vertical movement, which remained suppressed in the 65-micrograms group throughout testing. This measure may have been more sensitive to the subtle and long-lasting motor impairments resulting from kynurenic acid.