Effects of benzodiazepine receptor antagonist, flumazenil, on antinociceptive and behavioural responses to the elevated plus-maze in mice

No man is an island. A personal tribute to Bob Blanchard and ethoexperimental approaches to the study of behaviour

I first met Bob Blanchard at an international conference in Paris some 40 years ago. We collaborated intensively during the late 1980s/early 1990s on the ethopharmacology of antipredator defence in wild and laboratory rats, and remained good friends until his untimely passing in November 2013. Bob will undoubtedly be remembered as one of the most influential behavioural neuroscientists of the 20th century and, with Caroline, the most eloquent advocate of ethoexperimental approaches to the study of behaviour. In this brief trip down memory lane, I describe when and where Bob and I first met and how, over a lengthy period, he directly and indirectly helped shape my own research career. His profound influence in this regard is illustrated by reference to not only our collaborative research on antipredator behaviour but also my other work on the ethopharmacology of agonistic behaviour, social conflict analgesia, anxiety, and appetite. The element common to all of this work has been ethoexperimental analysis and, for teaching me the true value of this approach, I shall always remain indebted to the big man. Literally and figuratively, Bob was most certainly larger than life.

Cannabidiol increases survival and promotes rescue of cognitive function in a murine model of cerebral malaria

Cerebral malaria (CM) is a severe complication resulting from Plasmodium falciparum infection that might cause permanent neurological deficits. Cannabidiol (CBD) is a nonpsychotomimetic compound of Cannabis sativa with neuroprotective properties. In the present work, we evaluated the effects of CBD in a murine model of CM. Female mice were infected with Plasmodium berghei ANKA (PbA) and treated with CBD (30mg/kg/day - 3 or 7days i.p.) or vehicle. On 5th day-post-infection (dpi), at the peak of the disease), animals were treated with single or repeated doses of Artesunate, an antimalarial drug. All groups were tested for memory impairment (Novel Object Recognition or Morris Water Maze) and anxiety-like behaviors (Open field or elevated plus maze test) in different stages of the disease (at the peak or after the complete clearance of the disease). Th1/Th2 cytokines and neurotrophins (brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF)) were measured in the prefrontal cortex and hippocampus of experimental groups. PbA-infected mice displayed memory deficits and exhibited increase in anxiety-like behaviors on the 5dpi or after the clearance of the parasitemia, effects prevented by CBD treatment. On 5dpi, TNF-α and IL-6 increased in the hippocampus, while only IL-6 increased in the prefrontal cortex. CBD treatment resulted in an increase in BDNF expression in the hippocampus and decreased levels of proinflammatory cytokines in the hippocampus (TNF-α) and prefrontal cortex (IL-6). Our results indicate that CBD exhibits neuroprotective effects in CM model and might be useful as an adjunctive therapy to prevent neurological symptoms following this disease.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Pharmacological alterations of anxious behaviour in mice depending on both strain and the behavioural situation

A previous study comparing non-emotive mice from the strain C57BL/6/ByJ with ABP/Le mice showed ABP/Le to be more anxious in an open-field situation. In the present study, several compounds affecting anxiety were assayed on ABP/Le and C57BL/6/ByJ mice using three behavioural models of anxiety: the elevated plus-maze, the light-dark discrimination test and the free exploratory paradigm. The compounds used were the full benzodiazepine receptor agonist, chlordiazepoxide, and the antagonist, flumazenil, the GABA(A) antagonist, bicuculline, the full 5-HT(1A) agonist 8-OH-DPAT, and the mixed 5-HT(1A)/5-HT(1B) agonist, RU 24969. Results showed the effect of the compounds to be dependent on both the strain and the behavioural task. Several compounds found to be anxiolytic in ABP/Le mice had an anxiogenic effect on C57BL/6/ByJ mice. More behavioural changes were observed for ABP/Le in the elevated plus-maze, but the clearest findings for C57BL/6/ByJ mice were observed in the light-dark discrimination apparatus. These data demonstrate that anxious behaviour is a complex phenomenon which cannot be described by a single behavioural task nor by the action of a single compound.

Similar anxiolytic-like effects following intra-amygdala infusions of benzodiazepine receptor agonist and antagonist: evidence for the release of an endogenous benzodiazepine inverse agonist in mice exposed to elevated plus-maze test

Previous studies have demonstrated that microinjections of midazolam, a benzodiazepine receptor agonist, into the amygdala produce anxiolytic-like effects in elevated plus-maze (EPM)-naïve rodents. However, systemic or intracerebral administration of benzodiazepines (BDZ) fails to alter anxiety in maze-experienced animals, a phenomenon defined as "one trial tolerance" (OTT). This study focused on the effects of intra-amygdala infusion of midazolam in maze-experienced mice. In addition, the effects of flumazenil in the amygdala of maze-naïve and experienced mice were also investigated. To investigate intrinsic effects of intra-amygdala flumazenil on anxiety, animals were systemically treated with the BDZ receptor inverse agonist, DMCM (4-ethyl-6,7-dimethoxy-9H-pyrido[3,4-b]indole-3-carboxylic acid methyl ester hydrochloride). Conventional measures of anxiety (% open arm entries and % open arm time), locomotor activity (frequency of closed arm entries) and a range of ethological measures related to risk assessment were recorded. Intra-amygdala midazolam (3.0 and 30 nmol) attenuated anxiety in maze-experienced mice. A similar behavioral profile was produced by intra-amygdala flumazenil in maze-naïve (4.0 and 16 nmol) and maze-experienced (16 nmol) mice. Intra-amygdala flumazenil (at 2.0 nmol, a dose devoid of any intrinsic effect on anxiety measures in the EPM) selectively and completely blocked the anxiogenic-like effects of systemic administration of DMCM (1.0 mg/kg, i.p.) in maze-naïve mice. Together, these results demonstrate that the GABA(A)-benzodiazepine receptor complex located within the amygdala does not play a role in the OTT phenomenon. Present results also suggest that the release of an endogenous BDZ receptor inverse agonist within the amygdala seems to be an important correlate of the emotional state induced by the plus-maze test.

Possible involvement of GABA A-benzodiazepine receptor in the anxiolytic-like effect induced by Passiflora actinia extracts in mice

Hydroethanol (HE) and methanol (ME) extracts obtained from the leaves of Passiflora actinia Hooker were evaluated for behavioral effects in mice. Single-dose oral administration of HE (300 and 600 mg/kg) or ME (100 and 300 mg/kg) resulted in anxiolytic-like effects in the elevated plus-maze. The anxiolytic-like effects were also seen after the repeated administration of the HE (100 and 300 mg/kg). Flumazenil (10mg/kg, i.p.), a GABA(A)-benzodiazepine receptor antagonist, blocked the effects of ME (300 mg/kg, p.o.) and HE (600 mg/kg). At higher doses, a sedative effect produced by acute administration of HE (600 mg/kg) or ME (300 mg/kg) was indicated by the potentiation of pentobarbital-induced sleep. With regard to memory-disrupting effects of anxiolytics, mice were evaluated by measuring the retest step-down latency 24h after foot-shock in a passive avoidance task. In contrast to diazepam (0.5mg/kg) or piracetam (200mg/kg), ME (30, 100 and 300 mg/kg) or HE (100, 300 and 600 mg/kg) did not influence the step-through latency in the acquisition or retention memory tasks. The present results show an anxiolytic profile for HE and ME of Passiflora actinia. There are also indications of an involvement of GABA(A) system in this effect.

Anxiolytic-like effect of cannabidiol in the rat Vogel conflict test

Cannabidiol (CBD) is a major constituent of the Cannabis sativa plant. It inhibits the anxiogenic activity of high doses of Delta9-tetrahydrocannabinol and induces anxiolytic-like effects. However, the mechanisms underlying the actions of CBD are unknown. Therefore, the aim of the present study was to test the effects of CBD in the Vogel test, a widely used animal model of anxiety. In addition, it was verified if these effects would depend on benzodiazepine-receptor activation. After 24 h of water deprivation, male Wistar rats were subjected to an initial 3-min non-punished (pre-test) drinking session. This was followed by an additional 24-h period of water deprivation followed by a 3-min punished-licking session (test). Diazepam (3 mg/kg) or CBD (2.5, 5 or 10 mg/kg) were intraperitoneally injected 30 min before the test session. CBD (10 mg/kg) and diazepam had similar anticonflict effects, increasing the number of punished licks. The effect of diazepam, but not of CBD, was prevented by the benzodiazepine-receptor antagonist flumazenil (10 mg/kg). To exclude that the anticonflict effects were reflecting non-specific drug effects, we checked the effects of CBD on water consumption and nociceptive response. The drug did not interfere on the former variable in a non-punished test session. Moreover, contrary to morphine (5 mg/kg), CBD was ineffective in the tail-flick test. In conclusion, CBD induced an anticonflict effect not mediated by benzodiazepine receptors or by non-specific drug interference on nociceptive threshold or water consumption. These results reinforce the hypothesis that this cannabinoid has anxiolytic properties.

Bidirectional effects of benzodiazepine binding site ligands on active avoidance acquisition and retention: differential antagonism by flumazenil and beta-CCt

RATIONALE

The pharmacological approach, using subtype selective ligands, complements genetic studies on the specific contribution of individual receptor subtypes to the various effects of benzodiazepines.

OBJECTIVE

The aim of this study was to examine the relative significance of alpha1-containing GABA(A) receptors in the effects of modulators at the benzodiazepine site on anxiety and memory processes.

METHODS

We tested the effects of the nonselective antagonist flumazenil, the preferential alpha1-subunit selective antagonist beta-carboline-3-carboxylate-t-butyl ester (beta-CCt), the nonselective agonist midazolam, the preferential alpha1-subunit selective agonist zolpidem, and the nonselective inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) in a two-way active avoidance task in rats. The influence of flumazenil (10.0 mg/kg) and beta-CCt (30.0 mg/kg) on the effects of the two agonists were also examined. In the schedule 2 x 30 trials, drugs were administered i.p. 20 min before the training session. Avoidance responses in the training session are an anxiety-mediated behavior, whereas performance in the retention session relates to the effects on memory.

RESULTS

Flumazenil and beta-CCt did not affect behavior. Midazolam (2.0 mg/kg) facilitated acquisition performance, while DMCM (1.0 and 2.0 mg/kg) induced the opposite effect. Flumazenil antagonized both effects. Beta-CCt potentiated the effect of midazolam, and partly antagonized the effect of DMCM. Midazolam (0.5 and 1.0 mg/kg) and zolpidem (1.0-3.0 mg/kg) impaired, while DMCM (0.1 mg/kg) facilitated the subjects' performance in the retention test. The amnesic effects were attenuated but not fully reversed, while the effect of DMCM was counteracted by both antagonists.

CONCLUSION

The results indicate the alpha1-subunit interferes with the anxiolytic effect of a benzodiazepine site agonist and may contribute to the DMCM-induced anxiogenic effect. It is also substantially involved in the bidirectional memory processing in the active avoidance paradigm.

A comparative study with two types of elevated plus-maze (transparent vs. opaque walls) on the anxiolytic effects of midazolam, one-trial tolerance and fear-induced analgesia

The phenomenon known as one-trial tolerance (OTT) to the anxiolytic effects of benzodiazepines observed in rats submitted to the elevated plus-maze test (EPM) is considered to be due to the emergence of phobic states across the test/retest sessions. Antinociception is a usual component of the defense reaction. Until now, no study has examined antinociception and OTT together in freely behaving rats in the EPM. This work is a new approach looking at the sensorimotor gatings underlying OTT through the examination of the changes in reactivity to noxious stimuli during OTT development. We used the tail-flick test to assess the reactivity of rats to noxious stimulus during the effects of midazolam in test/retest sessions using two types of EPM, one with opaque (standard EPM) and another one with transparent walls (modified EPM). The authors had previously shown that this modified test caused an overall stressful situation more related to anxiety while the standard test coursed with a mixture of anxiety and high fear levels. In both plus mazes, the study was conducted in two experiments: (i) midazolam before the first trial, and (ii) midazolam before the second trial. In each experimental condition the effects of midazolam were tested under two doses (0.5 and 1.0 mg/kg) against a control group that received injections of saline. The anxiolytic effects of midazolam were more pronounced in animals tested in the modified EPM than in the standard EPM. Stressful stimuli present in both types of maze were able to elicit one-trial tolerance to midazolam on re-exposure. However, anxiolytic-insensitive behaviors in the first and the reduction in exploratory activity in the second trial are more pronounced in the standard EPM indicating that this test is more prone to transfer fear-related states across trials than the modified maze test. Antinociception is not present upon the re-exposure of rats to the EPM. These findings show that animals tested in the modified EPM showed higher sensitivity to the anxiolytic effects of midazolam than the standard EPM. Antinociception was not a concomitant of the shift in the emotional state present in the retest sessions of the EPM. These results are in agreement with the premises that repeated stressful experience leads to anxiolytic-insensitive fear state different from anxiety.

Bidirectional effects of benzodiazepine binding site ligands in the elevated plus-maze: differential antagonism by flumazenil and beta-CCt

Recent research using genetically modified mice has pointed to the specific contribution of individual receptor subtypes to the various effects of benzodiazepines. The aim of this study was to examine the relative significance of alpha(1)-containing GABA(A) receptors in the effects of modulators at the benzodiazepine site in the elevated plus-maze (EPM) under dim red light in rats. We tested the effects of the non-selective antagonist flumazenil (0-20.0 mg/kg), the preferential alpha(1)-subunit selective antagonist beta-carboline-3-carboxylate-t-butyl ester (beta-CCt, 0-30.0 mg/kg), the non-selective agonist midazolam (0-2.0 mg/kg), the preferential alpha(1)-subunit selective agonist zolpidem (0-2.0 mg/kg) and the non-selective inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM, 0-2.0 mg/kg). The influence of flumazenil (10.0 mg/kg) and beta-CCt (30.0 mg/kg) on the effects of both kinds of agonists were also examined. The standard spatio-temporal parameters reflecting anxiety (percentage of open arm entries and time) and locomotion (closed and total arm entries) were analyzed. beta-CCt did not affect behavior, while flumazenil at the highest dose (20.0 mg/kg) decreased indices of open arm activity and total arm entries. Midazolam at the dose of 1.0 mg/kg significantly increased the percentage of open arm time, whereas at 2.0 mg/kg both anxiety-related parameters were increased. In contrast to the open arm entries, the open arm time was independent of the decreased closed arm entries, observed at 2.0 mg/kg. Flumazenil abolished these effects, whereas beta-CCt partially potentiated the anxiolytic actions of midazolam. Zolpidem significantly increased both open-arm indices at 1.0 mg/kg, but the effect was dependent on the decreased closed arm entries. The selectivity of the anxiolytic-like effects of zolpidem was further checked under brighter white illumination. In these settings, the influence on anxiety-related, but not activity-related parameters, was absent. All of the activity-related effects of midazolam and zolpidem were mainly counteracted by both antagonists. DMCM produced significant anxiogenic effects at 1.0 mg/kg (open arm time) and 2.0 mg/kg (both parameters). beta-CCt (30.0 mg/kg) and flumazenil at higher dose (20.0 mg/kg) antagonized the effects of DMCM. The results indicate the anxiolytic effects of a non-selective benzodiazepine site agonist involve a predominant role of subunits other than alpha(1), whereas the behavioral indices of the anxiolytic-like properties of an alpha(1)-selective ligand, if observed, depend on the experimental settings and the changes in locomotor activity, and hence were behaviorally non-specific. The present results generally correspond well to the behavioral findings with the genetically modified mice. On the other hand, the relative significance of the alpha(1)-subunit in the anxiogenic effects of DMCM could not be clearly deduced.

Home cage activity and activity-based measures of anxiety in 129P3/J, 129X1/SvJ and C57BL/6J mice

We investigated the home cage activity and emotional behavior in mouse strains used as background for many studies of altered genes [C57BL/6J (B6, n=20), 129X1/SvJ (X1, n=20) and 129P3/J (P3, n=19)]. In their home cages, X1 and P3 mice exhibited less locomotion than did B6 mice, and the X1 mice showed significantly greater rearing than B6 and P3 mice did. A battery of three tests conducted in an open field (open field, emergence and novel object) revealed strain rankings of B6>X1>P3 or B6>X1=P3 in most activity variables. Significant correlations were found between home cage activity and activity in each of three tests, but not in all observation periods. Strain rankings on the elevated zero maze test were B6=X1>P3 in the number of stretched-attend body postures (SAPS) during the initial 6-min exposure for naive mice. Naive and nonnaive mice showed significantly different behaviors on the elevated zero maze. The results suggest that rankings on anxiety are P3>X1>B6 and that B6 mice have greater exploration in a novel environment compared with X1 and P3 mice. However, anxiety-like behaviors differed among strains in open-field-based tests and in the zero maze, and testing experience impacted performance on the zero maze. The findings illustrate that test variations and experience can influence performance and suggest the need for the consideration of how these factors interact with background strains in assessing gene-altered mice.

Behavioral effects of flumazenil in the social conflict test in mice

RATIONALE

Flumazenil, a competitive antagonist of benzodiazepine receptors (BZRs), has been used as a probe to detect effects of putative endogenous ligands for BZRs in anxiety. Flumazenil is renowned for its highly inconsistent behavioral effects.

OBJECTIVE

To ascertain effects of flumazenil in the social conflict test in mice, which provides complex measures for prediction of anxiolytic and anxiogenic activity of drugs in behaviorally different groups of animals.

METHODS

Singly housed male mice treated with flumazenil (5, 20 or 80 mg/kg i.p.) or vehicle were paired with untreated non-aggressive group-housed male mice in a novel cage. Behavior was analyzed from video tapes of the social interactions in three populations of mice: timid (n=21), aggressive (n=11), and sociable (n=7). Levels of gamma-aminobutyric acid (GABA) were measured in vivo in the prefrontal cortex.

RESULTS

Flumazenil reduced timid (defensive-escape) and increased locomotor activities in timid mice. The drug reduced aggressive and increased sociable (social investigation) activities in aggressive mice. These behavioral changes were produced at the lowest dose of flumazenil tested (5 mg/kg) and were not increased further by higher doses of the drug (20 mg/kg or 80 mg/kg). A tendency to increased timidity was found after flumazenil in sociable mice. Concentrations of GABA were markedly higher in the prefrontal cortex of sociable mice than in timid or aggressive mice.

CONCLUSIONS

Flumazenil produced moderate anxiolytic-like behavioural changes and a slight anxiogenic-like effect. The present data might be reflecting antagonism of corresponding endogenous BZR ligands. However, these putative ligands seem to exert only modest modulatory influence.

Anxiolytic profile of the antiepileptic drug levetiracetam in the Vogel conflict test in the rat

The novel antiepileptic drug levetiracetam has been shown to reverse anxiogenic effects of benzodiazepine withdrawal in mice tested in an elevated plus-maze without altering the behaviour of normal mice in this model. This could suggest that the effect of levetiracetam is dependent upon the level of stress/anxiety of the animals. Levetiracetam was therefore further examined in another widely used animal model of anxiety, the Vogel conflict test. In the first experiment, water-deprived rats were submitted to a free drinking period (habituation) in a chamber equipped with a bottle of water. Twenty-four hours later, animals were returned to the same chamber but the licks to the water bottle were then punished with a foot shock (0.5 mA, 90 ms). In the second experiment, the procedure was modified by administering a foot shock at the end of the habituation period in order to induce a state of stress/anxiety (conditioned fear/ anticipatory anxiety) for subsequent testing. Levetiracetam (17 and 54 mg/kg) and chlordiazepoxide (5 mg/kg) were administered via the intraperitoneal route. The results indicated that in the first experiment only chlordiazepoxide showed a statistically significant anxiolytic effect. In contrast, in the second experiment, where the shock was given at the end of the habituation period, levetiracetam (54 mg/kg) revealed significant anxiolytic activity similar to chlordiazepoxide. This suggests that levetiracetam may have potential anxiolytic effects and may provide therapeutic benefits to individual with anxiety spectrum disorders.

Effects of diazepam and flumazenil on food competition behavior in high- and low-aggression pigeons

The food competition interaction test performed with food-restricted pigeons with previously consolidated dominance is a useful tool for the study of offensive and defensive social aggression. In the present study, we examined the effect of GABA-A-benzodiazepine (BZD) receptor manipulation on aggression, emotion, feeder control, and eating behavior in high- and low-aggression female pigeons maintained at 80% of their normal weight and exposed to food competition interactions. The pigeons were divided into pairs by previously ranked high-aggression females (total time spent in aggression over 60 s/5 min; n=6 pairs) and low-aggression females (time spent in aggression less than 10 s/5 min; n=6 pairs). In Experiment 1, a pigeon in each pair of high- and low-aggression subjects were treated daily with an oral dose of diazepam (DZP, 0.6 mg/kg/0.3 ml) for 8 days. The other animal received the vehicle. On Day 8, food competition trials (10 min) were performed 30 min after treatments. In Experiment 2, pigeons were injected subcutaneously with flumazenil (FZL, 0.1 mg/kg/1 ml) or saline and exposed to a food competition trial 30 min after injections. In Experiment 3, one animal in each pair received DZP for 8 days. The other animal received the vehicle. On Day 8, the DZP-treated subjects were injected subcutaneously with FZL (0.1 mg/ kg/1 ml) 30 min before the oral dose of DZP. Trials were performed 30 min after DZP or vehicle administration. In Experiment 1, it was found that the DZP group of high-aggression pigeons showed lower scores of aggression (P<.05) and emotional responses (P<.05) than controls. The other group-scored behaviors were not affected. The DZP low-aggressions, however, showed scores of aggression eightfold higher than their controls (P<.05) but the other scored behaviors were not changed. In Experiment 2, FZL injection did not induce intrinsic effects on aggression either in the high- or in the low-aggression group. Experiment 3 showed that the emotional and aggressive responses to DZP were neutralized by FZL. This shows that GABA-A-BZD receptor mechanisms are implicated in the DZP responses in high- and low-aggression pigeons.

Home cage activity and behavioral performance in inbred and hybrid mice

Locomotor activity is a key component in many behavioral tests, suggesting that genetic differences in activity levels may be a critical consideration when comparing mouse strains. In order to assess the relationship between activity and performance, we recorded home cage activity, and locomotion and defecation, a non-activity-linked behavior, in tests of anxiety in inbred (C57BL/6J (B6), n = 25; BALB/cJ (C), n = 24; DBA/2J (D2), n = 28) and hybrid (CB6F1/6J (CB6: B6 x C) n = 19) mice. Under our test conditions, the strains showed significant differences in home cage activity levels: C > B6 > D2. The CB6 mice were similar to the B6 mice in horizontal activity and were intermediate between the parental strains in vertical movement. Based on measures of locomotion and defecation in the open field, emergence and novel object tests, and the elevated zero maze, the C mice appeared to be the most anxious and the B6 were the least anxious. The D2 mice were intermediate on some measures but more similar to B6 mice on others, making ranking them more difficult. In addition, the CB6 mice displayed characteristics of both parental strains. They had greater similarity to B6 mice in measures of horizontal movement in the home cage and locomotion in the open field and emergence tests, but exhibited defecation responses similar to those of C mice in the novel object test and elevated zero maze. The results suggest that strain differences in spontaneous locomotion should be considered when interpreting strain differences in behavioral tests, and that home cage activity may be a useful interpretive aid.

Progesterone withdrawal increases the alpha4 subunit of the GABA(A) receptor in male rats in association with anxiety and altered pharmacology - a comparison with female rats

Withdrawal from the neurosteroid 3alpha,5alpha-allopregnanolone after chronic administration of progesterone increases anxiety in female rats and up-regulates the alpha4 subunit of the GABA(A) receptor (GABA(A)-R) in the hippocampus. We investigated if these phenomena would also occur in male rats. Progesterone withdrawal (PWD) induced higher alpha4 subunit expression in the hippocampus of both male and female rats, in association with increased anxiety (assessed in the elevated plus maze) comparable to effects previously reported. Because alpha4-containing GABA(A)-R are insensitive to the benzodiazepine (BDZ) lorazepam (LZM), and are positively modulated by flumazenil (FLU, a BDZ antagonist), we therefore tested the effects of these compounds following PWD. Using whole-cell patch clamp techniques, LZM-potentiation of GABA ((EC20))-gated current was markedly reduced in CA1 pyramidal cells of male rats undergoing PWD compared to controls, whereas FLU had no effect on GABA-gated current in control animals but increased it in PWD animals. Behaviorally, both male and female rats were significantly less sensitive to the anxiolytic effects of LZM. In contrast, FLU demonstrated significant anxiolytic effects following PWD. These data suggest that neurosteroid regulation of the alpha4 GABA(A)-R subunit may be a relevant mechanism underlying anxiety disorders, and that this phenomenon is not sex-specific.

Participation of GABAA receptors in the modulation of experimental anxiety by tachykinin agonists and antagonists in mice

Mice were acutely intraperitoneally treated with diazepam (DZP), pentylenetetrazol (PTZ) or NaCl 0.9% (control group), and 15 min later, the DZP-treated group received substance P (SP), neurokinin A (NKA; NK1 and NK2 natural preferential agonists), [Trp7 beta-Ala8] NKA(4-10) (Trp-7; NK3 antagonist) or vehicle intracerebroventricularly, whereas the PTZ-treated group was intracerebroventricularly administered with FK 888, SR 48968 (NK1 and NK2 antagonists, respectively) or senktide (SENK--[succinil-Asp6, MePhe8] substance P(6-11); NK3 agonist), or vehicle immediately before they were submitted to the elevated plus-maze (EPM) test. Another group of animals was repeatedly treated with increasing doses of DZP or NaCl 0.9% intraperitoneally for 28 days, and 3 days after the last injection (test day), animals received DZP, FK 888, SR 48968, SENK or vehicle intracerebroventricularly, or DZP (NaCl 0.9%) intraperitoneally before the EPM evaluation. The anxiolytic action of the acute treatment with DZP was inhibited by the central administration of NKA and Trp-7 but not by SP. NK1 and NK2 antagonists, but not NK3 agonist, blocked the anxiogenic action of PTZ, as evaluated in the plus-maze test. Flumazenil (FLM), a benzodiazepine antagonist, was not able to inhibit the anxiolytic profile of action induced by the NK2 antagonist. Central administration of FK 888 and SR 48968 promoted anxiolytic effects in both control and DZP-withdrawn animals, suggesting a clear relationship between the GABAergic and the tachykinergic systems, mostly involving NK1 and NK2 receptors, in the modulation of experimental anxiety in mice.

The new antiepileptic drug levetiracetam normalises chlordiazepoxide withdrawal-induced anxiety in mice

Some antiepileptic drugs have been used with success to counteract withdrawal symptoms following chronic use of sedatives, hypnotics or alcohol. We evaluated the potential of levetiracetam (Keppra), a new antiepileptic drug, to prevent benzodiazepine withdrawal in an animal model sensitive to the anxiogenic effect resulting from drug cessation. The effects of levetiracetam (17 and 54 mg/kg) given intraperitoneally (i.p.) were determined on anxiety induced in female NMRI mice by withdrawal from 21 days of chronic administration of chlordiazepoxide. Administration of chlordiazepoxide was i.p. twice daily, in increments of 2 mg/kg, from 10 up to 40 mg/kg. Anxiety was evaluated using an elevated plus-maze test 24-h after chlordiazepoxide withdrawal. Discontinuation of chronic chlordiazepoxide induced a significant anxiogenic profile in the plus-maze test mainly characterised by a decrease in open arm exploration. This effect was dose-dependently prevented by administration of levetiracetam during the withdrawal period. The highest dose tested (54 mg/kg) induced statistically significant effects on all variables recorded but had no effect upon plus-maze exploration in normal mice. This suggests that the observed effects are dependent upon the level of stress or anxiety of the animals. These results support potential efficacy of levetiracetam in the benzodiazepine withdrawal syndrome.

GABAergic and glutamatergic modulation of exploratory behavior in the dorsomedial hypothalamus

Systemic injection of glutamate NMDA receptor antagonists or drugs that facilitate GABA(A)-mediated neurotransmission produces anxiolytic effects. The dorsomedial hypothalamic (DMH) region is proposed to be a possible site of action of these drugs. The objective of the present study was to investigate if facilitation of GABA(A)-mediated neurotransmission or blockade of NMDA receptors in the DMH would produce anxiolytic effects in the elevated plus-maze (EPM). Seven days after surgery, male Wistar rats with unilateral cannulas in the DMH were submitted to the behavioral studies. Results showed that midazolam, a benzodiazepine anxiolytic (30-60 nmol/0.3 microl), produced a dose-dependent increase in open arm exploration without changing the number of enclosed arm entries, indicating an anxiolytic effect. This effect was antagonized by previous treatment with flumazenil, a benzodiazepine receptor antagonist (60 nmol/0.3 microl). Flumazenil alone had an anxiogenic effect, decreasing exploration of the open arms of the EPM. 2-Amino-7-phosphonoheptanoic acid (AP7), an NMDA receptor antagonist (0.2-2 nmol/0.3 microl), did not modify open arm exploration but decreased general exploratory activity. These results indicate that benzodiazepine receptors located in the DMH could modulate anxiety. Interference with NMDA receptor-mediated neurotransmission in this region, however, seems to change general exploratory activity rather than anxiety.

The anxiolytic effect of Sho-ju-sen, a Japanese herbal medicine, assessed by an elevated plus-maze test in mice

Sho-ju-sen (SK), a Japanese herbal medicine with a nourishing tonic action, is composed of a water extract of Kumazasa leaves (Sasa kurinensis Makino et Sibata) (SS), and ethanol extracts of Japanese red pine needles (Pinus densiflora Sieb. et Zucc) (PN) and Ginseng roots (Panax ginseng C. A. Meyer) (PX) in the ratio 8:1:1. In this study, an elevated plus-maze test in mice was carried out to assess whether SK had an anxiolytic effect. No significant change was observed in either the plus-maze or activity test following a single administration of SK (10 and 20 mL/kg p.o.). However, mice allowed a free intake of SK (10% solution) for 5 days and longer showed a significant prolongation of the time spent in the open arms (an anxiolytic effect), as long as that caused by the benzodiazepine anxiolytic diazepam (1 mg/kg p.o.). SK (1%, 3% and 30% solutions for 7 days) tended to develop the anxiolytic effect. Of the constituents of SK, SS (8% solution), but not PN (1% solution) or PX (1% solution), resulted in the anxiolytic effect. Except for a slight acceleration in the motor activity by PN (1% solution), no significant change in the motor activity was produced by any treatment with SK, SS or PX. The combined treatment of SK (10% solution) or SS (8% solution) with 1 mg/kg diazepam enhanced the anxiolytic effect. Flumazenil (0.1 mg/kg s.c.), a benzodiazepine receptor antagonist, alone did not change the time spent in the open arms. However, it completely reversed the anxiolytic effect of SK, SS and diazepam. The present results suggest that: (1) long-term treatment with SK develops an anxiolytic effect, (2) SS is the main constituent for the anxiolytic effect of SK, and (3) benzodiazepine receptors are involved in the anxiolytic effect of SK and SS.

Does dihydrohonokiol, a potent anxiolytic compound, result in the development of benzodiazepine-like side effects?

The aims of this study were to assess whether dihydrohonokiol, 3'-(2-propenyl)-5-propyl-(1,1'-biphenyl)-2,4'-diol (DHH-B), a potent anxiolytic compound, developed benzodiazepine-like side effects. A 1 mg kg(-1) dose of diazepam, almost equivalent to the minimum dose for the anxiolytic effect, disrupted the traction performance, potentiated hexobarbital-induced sleeping and impaired learning and memory performance. DHH-B, even at a dose of 1 mg kg(-1) (i.e. five times higher than the minimum dose for significant anxiolytic effect) neither developed diazepam-like side effects nor enhanced the side effects of diazepam. Rather, the potentiation by diazepam of hexobarbital-induced sleeping was reduced by 1 mg kg(-1) DHH-B. Furthermore, mice treated with 10 daily administrations of 1 and 5 mg kg(-1) diazepam, but not 0.2-5 mg kg(-1) DHH-B, showed precipitated withdrawal symptoms characterized by hyper-reactivity, tremor and tail-flick reaction when they were challenged with flumazenil (10 mg kg(-1) i.p.). These results suggest that, unlike the benzodiazepine anxiolytic diazepam, DHH-B is less likely to induce motor dysfunction, central depression, amnesia or physical dependence at the effective dose required for the anxiolytic effect.

Pontine tegmentum lesions increase anxiety-like behavior in rats: a comparison with anxiety produced by beta-CCE

Electrolytic lesions of the pedunculopontine tegmental nucleus (PPTg) have been previously reported to increase anxiety-like behavior in rats. The aim of the present study was to compare these behavioral changes with those produced by an anxiogenic compound, the partial inverse agonist at benzodiazepine receptors, beta-CCE. Three groups of rats, sham-lesioned treated with vehicle, sham-lesioned treated with 10 mg/kg of beta-CCE, and PPTg-lesioned rats treated with vehicle, were tested in the elevated plus-maze, the social-interaction test, and for spontaneous locomotion. Histology showed that lesions were concentrated on the caudal half of the PPTg. Measures of both the PPTg-lesioned and beta-CCE-treated rats indicated increased anxiety-like behavior in the elevated plus-maze and in the social-interaction test. Spontaneous locomotion, measured in the open- field arena, did not differ between sham controls and PPTg-lesioned rats, but was decreased in rats treated with beta-CCE. Our results confirmed that electrolytic lesions of the caudal PPTg produce increased anxiety-like behavior. This behavior is quantitatively and qualitatively similar to that produced by 10 mg/kg of beta-CCE.

Behavioral effects of diazepam in the murine plus-maze: flumazenil antagonism of enhanced head dipping but not the disinhibition of open-arm avoidance

Although it is widely believed that benzodiazepines reduce anxiety through positive allosteric modulation of the GABA(A)-chloride channel complex, this is not the only mechanism through which agents of this class can modify CNS function. Furthermore, a significant number of reports of apparent flumazenil blockade of diazepam anxiolysis in animal models have paid limited attention to possible intrinsic behavioral actions of the antagonist per se. In the present study, ethological methods were employed to assess in detail the effects of diazepam, flumazenil, and their combination on the behavior of male DBA/2 mice in the elevated plus-maze paradigm. In two experiments, diazepam (1.5 mg/kg) alone reduced open-arm avoidance and increased head dipping, whereas flumazenil (10-40 mg/kg) alone was without significant behavioral effect. However, with the sole exception of head dipping, prior administration of flumazenil (10 and 40 mg/kg) failed to block the behavioral effects of diazepam under present test conditions. These findings imply that the anxiolytic effects of diazepam in the mouse plus-maze are not mediated through flumazenil-sensitive benzodiazepine receptors and that alternate mechanisms must be considered.

Behavioural pharmacological characteristics of honokiol, an anxiolytic agent present in extracts of Magnolia bark, evaluated by an elevated plus-maze test in mice

Honokiol, a neolignane derivative of Magnolia bark, has central depressant action and, at much lower doses, anxiolytic activity. We have investigated the characteristics of the behavioural effects of honokiol by means of an elevated plus-maze test. In the plus-maze test a single oral dose of 20 mg kg(-1) honokiol significantly prolonged the time spent in the open arms of the maze, suggesting anxiolytic effect. Moreover, when honokiol was administered daily for seven days and the plus-maze test was conducted 3 or 24 h after the last administration, significant prolongation of the time in the open arms was manifested even for doses of 0.2 mg kg(-1). The maximum effect was observed for doses of 0.5 mg kg(-1). Honokiol at any dose in both single and repeated administration schedules caused neither change in motor activity nor disruption of traction performance. Orally administered diazepam, 0.5-2 mg kg(-1), caused dose-dependent prolongation of the time spent in the open arms of the maze with a significant increase in motor activity at 1 mg kg(-1), and dose-dependent disruption of traction performance. The changes in the plus-maze performance after treatment for seven days with 0.2 mg kg(-1) honokiol and after a single treatment with 1 mg kg(-1) diazepam were almost equivalent. The effect of honokiol (0.2 mg kg(-1), treatment for seven days) was inhibited by subcutaneous flumazenil (0.3 mg kg(-1)) and (+)-bicuculline (0.1 mg kg(-1)) and by intraperitoneal CCK-4 (50 microg kg(-1)) and caffeine (30 mg kg(-1)). The anxiolytic effect of diazepam (1 mg kg(-1)) was also inhibited by flumazenil and bicuculline. However, the combined administration of diazepam with caffeine enhanced the effect, and diazepam completely reversed the effect of CCK-4. These results suggest that, in contrast with diazepam, honokiol selectively induces an anxiolytic effect with less liability of eliciting motor dysfunction and sedation or disinhibition. The combined effects of the drug also revealed that the mechanism of anxiolytic effect of honokiol is partially different from that of diazepam.

Supraspinal flumazenil inhibits the antianalgesic action of spinal dynorphin A (1-17)

DynorphinA (Dyn) administered intrathecally or released spinally in mice produces antianalgesia, that is, antagonizes morphine analgesia (tail-flick test). Spinal transection eliminates this Dyn antianalgesia. Present results in mice show that intracerebroventricular administration of flumazenil, a benzodiazepine receptor antagonist, also eliminated the antianalgesic action of Dyn; flumazenil in the brain eliminated the suppressant effect of intrathecal Dyn on intrathecal and intracerebroventricular morphine-induced antinociception. Intracerebroventricular clonidine, naloxone, and norbinaltorphimine release spinal Dyn. The latent antinociceptive actions of these compounds were uncovered by intracerebroventricular flumazenil. Thus, Dyn, given intrathecally or released spinally, activates a pathway that is inhibited by intracerebroventricular flumazenil. Dyn antianalgesia is not significantly altered by intracerebroventricular administration of bicuculline and picrotoxin, suggesting that activation of the gamma-aminobutyric acid receptor has little if any involvement in the antianalgesic action of Dyn. The antagonistic effect of Dyn seems to be mimicked by benzodiazepine agonists. Furthermore, administration of a benzodiazepine receptor inverse agonist (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) inhibited Dyn antianalgesia as did flumazenil. Thus, flumazenil, through a benzodiazepine antagonist or inverse agonist action, interrupts, as does spinal transection, the neuronal circuit (cord/brain/cord) necessary for the antianalgesic action of spinal Dyn. Because Dyn antianalgesia is an indirect action, activation of the neuronal circuit must lead to the release of a direct-acting antianalgesic mediator in the spinal cord.

Hippocampal histamine receptors and conflictive exploration in the rat: studies using the elevated asymmetric plus-maze

The possible role of histamine receptors in the hippocampal formation on the exploratory motivation and emotionality of the rat was studied. An elevated asymmetric plus-maze composed of 4 different arms (no walls, single high wall, high and low walls and two high walls) arranged at 90 degrees angles was used. The exploration score, considered to be an index of exploratory motivation, and the permanency score, considered to be an index of emotionality (anxiety), were determined. Histamine was administered locally into the ventral hippocampus at three different doses (9, 45 and 90 nmol). Another group of rats was also microinjected with 45 nmol of pyrilamine (a histamine H1 receptor antagonist) or ranitidine (a histamine H2 receptor antagonist) in addition to 9 nmol of histamine in order to identify the possible type of histamine receptor involved. Histamine administration significantly inhibited the exploration score and increased the permanency score at the doses of 9 and 45 nmol in two of four arms. These effects were completely blocked by the administration of either histamine receptor antagonist. The present results suggest that in the hippocampal formation histamine inhibits exploratory motivation and decreases emotionality by activating both types of histamine receptors. Also, the elevated asymmetric plus-maze appears to be a suitable technique to quantify exploration and possibly "anxiety".

Anxiety, defence and the elevated plus-maze

The elevated plus-maze test has been in use as a rodent model of anxiety for a decade, and is representative of those tests that are based upon the study of spontaneous behaviour patterns and which have high ecological validity. The origins of the test in studies of the relationship between exploration and fear are reviewed, and attention is drawn to the distinct possibility that variation in the pharmacosensitivity of the procedure may be attributable to often extreme methodological variation between laboratories. In considering further this issue, attention is also drawn to the need to collect data under constant test conditions and to provide the minimum database necessary to reach conclusions regarding the behavioural specificity of drug action. Recent research, which has extended the conventional plus-maze scoring technique to include specific behavioural acts and postures (in particular, those relating to defensive behaviour), is described. The value of such an ethological approach to the plus-maze is then exemplified with original data that demonstrate behaviourally selective, anti-anxiety effects of the GABAA receptor agonist, muscimol (0.125-1.0 mg/kg). It is concluded that, when used appropriately, the elevated plus-maze test can be a very valuable tool in drug screening and in the study of the neurobiology of anxiety and defence. More attention to behaviour and somewhat less emphasis on test simplicity and convenience would seem to be warranted.

The effects of cholecystokinin A and B receptor antagonists on exploratory behaviour in the elevated zero-maze in rat

The aim of the present study was to investigate the effects of cholecystokinin (CCK) CCK(A) and CCKB receptor antagonists SR 27897 B, devazepide, L 365260 and PD 135158 (CAM 1028) on exploratory behaviour in the elevated zero-maze in the rat. For further validation of the elevated zero-maze, the effects of a reference anxiolytic diazepam (0.25, 0.5, 1.0, 2.0 mg/kg), a non-benzodiazepine (BDZ) anxiolytic buspirone (0.04, 0.2, 1.0, 5.0 mg/kg), BDZ receptor inverse agonists FG 7142 (5, 10, 15, 20 mg/kg) and DMCM (0.1, 0.5, 1.0, 1.5 mg/kg), and a BDZ receptor antagonist flumazenil (10 mg/kg) were studied. Diazepam decreased the number of stretched-attend postures in all doses used and increased the percentage of time spent exploring in open parts at doses of 0.5 and 1.0 mg/kg. The effects of diazepam were blocked by flumazenil. FG 7142 and DMCM had effects only in subconvulsive doses (20 mg/kg and 1.5 mg/kg). Flumazenil and buspirone failed to show any effect. The CCK(A) receptor antagonists were also without any effect. The CCK(B) receptor antagonists L 365260 (1.0 and 5.0 mg/kg) and PD 135158 (100 microg/kg) had a significant anxiolytic-like effect. The CCK(B) receptor antagonists increased the number of open part entries, the number of head dips, the percentage of time spent exploring in the open part and decreased the number of stretched-attend postures. These data support the hypothesis of the involvement of the CCK(B) receptor subtype in the neurobiological mechanisms of anxiety.

Transgenic and null mutant animals for psychosomatic research

OBJECTIVE

Progress in the use of genetically altered animals for psychosomatic research is reviewed.

METHOD

Analysis of the strengths and weaknesses of these models, particularly from a developmental and behavioral prospective is used to assess the validity of these models.

RESULTS

Genetically altered animals can be used to create models of the estimated 5000 human diseases in which genetic predispositions play a role, as well as models for diseases that do not involve gene defects, such as human immunodeficiency virus (HIV) infection. In addition, these models have already contributed immensely to our understanding of basic biology and the biology of behavior. Replication of human gene defects in mice has provided direct models of human disease, but there are various factors that sometimes prevent the gene defect from producing the human disease in mice. However, even in this case, the models can contribute to understanding the basic biology of the disease.

CONCLUSIONS

While genetically altered animals have revolutionized the understanding of single gene disorders, their promise has not yet been fulfilled for multigenic behavioral disorders. Newer techniques to allow control of the tissue and stage of development at which a gene is expressed are likely to enhance the usefulness of these models for psychosomatic research. New models of disease for testing psychological impacts on illness and specific ways altering neurotransmitter function will be discovered. While these models will be extremely useful to psychosomatic medicine, the nature of this discipline of necessity involves emphasis on individual experience, and thus will never be amenable to exclusively genetic analysis.

The benzodiazepine receptor antagonists flumazenil and CGS8216 block the enhancement of fear conditioning and interference with escape behavior produced by inescapable shock

Prior work has suggested that the mediation of the behavioral effects of inescapable shock (IS) might involve release of an endogenous beta-carboline-like ligand at the dorsal raphe nucleus (DRN) that binds to the benzodiazepine (BZ) recognition site on the GABAA complex, thereby disinhibiting the DRN. This was tested by microinjection of the BZ receptor antagonists flumazenil and CGS8216 in the region of the DRN, either before IS or before later behavioral testing. Both compounds blocked subsequent enhancement of fear conditioning and interference with shuttlebox escape when administered before IS, but had no effect when given before testing. In addition, flumazenil did not alter the behavior of escapably shocked subjects.

The effects of drugs acting at GABA-benzodiazepine-barbiturate receptor complex on the behaviour of sleep-deprived mice

The effects of the benzodiazepine receptor agonist diazepam, the benzodiazepine receptor antagonist flumazenil and the benzodiazepine receptor inverse agonist ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-[1,5-a][1-4] benzodiazepine-3-carboxylate (RO 15-4513) on the locomotor activity and the behaviour of animals in the plus-maze test were studied in sleep-deprived mice. The effects of convulsants acting at GABA-benzodiazepine-barbiturate receptor complex-bicuculline, picrotoxin and pentylenetetrazole, were also studied. Sleep deprivation of mice for 24 hr using the platform technique caused behavioural excitation that was reflected by an increase in the locomotor activity. Administration of diazepam (0.5 and 2.0 mg/kg), flumazenil (5.0 and 10.0 mg/kg) and RO 15-4513 (1.0, 2.0 and 3.0 mg/kg) either did not affect (in low doses) or inhibited (in high doses) locomotions of control animals. The inhibition of locomotor activity by these drugs was greater in sleep-deprived animals. In the plus-maze test, diazepam in a dose of 2.5 mg/kg had an anxiolytic effect in control mice that was reflected by an increase in the percentage of entries onto and the percentage of time spent on the open arms of the plus-maze. In contrast, in sleep-deprived animals, diazepam did not induce anxiolytic action at any dose tested. In the highest dose (2.5 mg/kg) diazepam produced a sedative effect that was reflected by a decrease in the total number of entries made onto the open and into the closed arms of the plus-maze.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioural and pharmacological characterisation of the elevated "zero-maze" as an animal model of anxiety

The elevated "zero-maze" is a modification of the elevated plus-maze model of anxiety in rats which incorporates both traditional and novel ethological measures in the analysis of drug effects. The novel design comprises an elevated annular platform with two opposite enclosed quadrants and two open, removing any ambiguity in interpretation of time spent on the central square of the traditional design and allowing uninterrupted exploration. Using this model, the reference benzodiazepine anxiolytics, diazepam (0.125-0.5 mg/kg) and chlordiazepoxide (0.5-2.0 mg/kg) significantly increased the percentage of time spent in the open quadrants (% TO) and the frequency of head dips over the edge of the platform (HDIPS), and reduced the frequency of stretched attend postures (SAP) from the closed to open quadrants. In contrast, the anxiogenic drug m-chlorophenyl-piperazine (mCPP; 0.25-1.0 mg/kg) induced the opposite effects, decreasing %TO and HDIPS, and increasing SAP. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.001-0.1 mg/kg) had no effects on either %TO or HDIPS, but did decrease SAP at 0.01 mg/kg although not at higher or lower doses. Similarly, the 5-HT3 receptor antagonist, ondansetron (0.0001-1.0 mg/kg) decreased SAP and increased %TO at 0.01 mg/kg, but not at other doses. The present data suggest that a combination of the novel "zero-maze" design and a detailed ethological analysis provides a sensitive model for the detection of anxiolytic/anxiogenic drug action.

Nitrous oxide anxiolytic effect in mice in the elevated plus maze: mediation by benzodiazepine receptors

In earlier research, we have hypothesized that exposure to nitrous oxide (N2O) produces an anxiolytic effect that is mediated by benzodiazepine (BZ) receptors. The present research was conducted to characterize pharmacologically the behavioral effects of N2O in comparison with a BZ standard, chlordiazepoxide (CP), in the mouse elevated plus maze. Exposure to increasing levels of N2O produced a concentration-related increase in the percent of total entries into and the percent of total time spent on the open arms, a pattern of response similar to that induced by CP. These effects on N2O and CP were both antagonized by pretreatment with the BZ receptor blocker flumazenil (FLU). In another experiment, mice made tolerant to CP also exhibited a cross-tolerance to N2O. These results support the hypothesis that the anxiolytic effect of N2O is mediated by BZ receptors.