Male and female C57BL/6 mice respond differently to diazepam challenge in avoidance learning tasks

Greater resistance to footshock punishment in female C57BL/6J mice responding for ethanol

BACKGROUND

One characteristic of alcohol use disorder is compulsive drinking or drinking despite negative consequences. When quinine is used to model such aversion-resistant drinking, female rodents typically are more resistant to punishment than males. Using an operant response task where C57BL/6J responded for ethanol mixed with quinine, we previously demonstrated that female mice tolerate higher concentrations of quinine in ethanol than males. Here, we aimed to determine whether this female vulnerability to aversion-resistant drinking behavior is similarly observed with footshock punishment.

METHODS

Male and female C57BL/6J mice were trained to respond for 10% ethanol in an operant task on a fixed-ratio three schedule. After consistent responding, mice were tested in a punishment session using either a 0.25 mA or 0.35 milliamp (mA) footshock. To assess footshock sensitivity, a subset of mice underwent a flinch, jump, and vocalize test in which behavioral responses to increasing amplitudes of footshock (0.05 to 0.95 mA) were assessed. In a separate cohort of mice, males and females were trained to respond for 2.5% sucrose and responses were punished using a 0.25 mA footshock.

RESULTS

Males and females continued to respond for 10% ethanol when paired with a 0.25 mA footshock. Females alone continued to respond for ethanol when a 0.35 mA footshock was delivered. Both males and females reduced responding for 2.5% sucrose when punished with a 0.25 mA footshock. Footshock sensitivity in the flinch, jump, and vocalize test did not differ by sex.

CONCLUSIONS

Females continue to respond for 10% ethanol despite a 0.35 mA footshock, and this behavior is not due to differences in footshock sensitivity between males and females. These results show that female C57BL/6J mice are generally more resistant to punishment in an operant self-administration paradigm. The findings add to the literature characterizing aversion-resistant alcohol-drinking behaviors in females.

Crossed high alcohol preferring mice exhibit aversion-resistant responding for alcohol with quinine but not footshock punishment

A symptom of alcohol use disorder (AUD) is compulsive drinking, or drinking that persists despite negative consequences. In mice, aversion-resistant models are used to model compulsive-like drinking by pairing the response for alcohol with a footshock or by adding quinine, a bitter tastant, to the alcohol solution. crossed High Alcohol Preferring (cHAP) mice, a selectively bred line of mice that consumes pharmacologically relevant levels of alcohol, demonstrate a high level of aversion-resistance to quinine-adulterated alcohol. The current study investigated quinine-resistant and footshock-resistant responding for 10% ethanol in male and female cHAP mice with vs. without a history of alcohol exposure. cHAP mice were first trained to respond for 10% ethanol in an operant-response task. Next, mice were exposed to water or 10% ethanol for twelve 24-h sessions using a two-bottle choice procedure. Footshock-resistant ethanol responding was then tested in the operant chamber by pairing a footshock (0.35 mA) with the nose-poke response during one session. Quinine-resistant responding for alcohol was tested over five sessions (500-2500 μM quinine). Finally, footshock sensitivity was assessed using a flinch, jump, vocalize test. Alcohol exposure history did not influence responses for 10% ethanol or either measure of aversion-resistance. Further, cHAP mice were sensitive to footshock punishment but continued to respond for alcohol at all quinine concentrations. No sex differences were observed in any measure of alcohol responding, but female cHAP mice were less sensitive to footshock than males. These results replicate and extend the previous demonstration of a robust, innate resistance to quinine aversion in cHAP mice and further suggest that this tendency is not observed when footshock is used to punish drinking.

On the basis of sex: Differences in safety discrimination vs. conditioned inhibition

Inaccurate discrimination between threat and safety cues is a common symptom of anxiety disorders such as Post-Traumatic Stress Disorder (PTSD). Although females experience higher rates of these disorders than males, the body of literature examining sex differences in safety learning is still growing. Learning to discriminate safety cues from threat cues requires downregulating fear to the safety cue while continuing to express fear to the threat cue. However, successful discrimination between safety and threat cues does not necessarily guarantee that the safety cue can effectively reduce fear to the threat cue when they are presented together. The conditioned inhibitory ability of a safety cue to reduce fear in the presence of both safety and threat is most likely dependent on the ability to discriminate between the two. There are relatively few studies exploring conditioned inhibition as a method of safety learning. Adding to this knowledge gap is the general lack of inclusion of female subjects within these studies. In this review, we provide a qualitative review of our current knowledge of sex differences in safety discrimination versus conditioned inhibition in both humans and rodents. Overall, the literature suggests that while females and males perform similarly in discrimination learning, females show deficits in conditioned inhibition compared to males. Furthermore, while estrogen appears to have a protective effect on safety learning in humans, increased estrogen in female rodents appears to be correlated with impaired safety learning performance.

Sex and Time-of-Day Impact on Anxiety and Passive Avoidance Memory Strategies in Mice

In humans, anxiety and cognitive processes are age, gender, and time of day dependent. The purpose of the present study was to assess whether the time of day and sex have an influence on anxiety and emotional memory in adult mice. Light-dark and passive avoidance (PA) tests were performed at the beginning and at the end of the light cycle, defined as Zeitgeber time (ZT) ZT0–2.5 and ZT9.5–12, respectively. A baseline difference in anxiety was not found, but on the 24 h retention trial of the PA test, females presented longer latencies to enter into the dark compartment at the ZT0–2.5 time point of the day. The data from the second test day (PA reversal trial) indicated that some animals associated the dark compartment with an aversive stimulus (shock), while others associated the aversive stimulus with crossing from one compartment to another. At the ZT9.5–12, female mice mainly related the aversive stimulus to transferring from one compartment to another, while male mice associated darkness with the aversive stimulus. There was a negative correlation between the frequency of light-dark transitions in the light-dark test and the PA latency on the 24 h retention trial in males tested at ZT0–2.5. The PA latency on the reversal and 24 h retention trials negatively correlated with a risk assessment behavior in male mice tested on ZT0–2.5 and ZT9.5–12, respectively. In conclusion, our data reveal that the impact of motor activity and risk assessment behavior on PA memory formation and applied behavioral strategies are time of day and sex dependent.

Sex-specific differences in adult cognition and neuroplasticity following repeated combinatory stress and TrkB receptor antagonism in adolescence

Evidence supports brain-derived neurotrophic factor (BDNF) and its primary receptor tyrosine-related kinase B (TrkB) as targets in the treatment of mood disorders. This study characterized the impact of a 10-day combinatory stress paradigm (alternating days of restraint stress and forced swim) and administration of the selective TrkB antagonist ANA-12 (0.5 mg/kg, i.p.) during adolescence in male and female Wistar rats on adulthood behavioral and neurochemical responses. The social interaction/preference (SIT/SP), and Y maze conditioned place preference (YMCPP) and passive avoidance tests (YMPAT), initiated on PND 62, served to determine sex-related behavioral responses. Results support reduced sociability in females in the SIT/SP, but no impact of ANA-12 to regulate sociability or social memory. Blockade of TrkB during adolescence facilitated YMCPP-related reward behavior in both sexes, and reduced YMPAT fear conditioning in females. Following behavioral testing, rats were exposed to 5-min acute forced swim and brains collected 2 h post swim to determine effects of adolescent TrkB blockade and stress exposure on neurochemical regulators of stress and plasticity. Findings show elevated glucocorticoid receptor (GR-) and TrkB-immunoreactivity (ir) in the amygdalar central nucleus, and GR-ir in the hypothalamic paraventricular nucleus of females compared to males. In the hippocampal CA1, BDNF-ir was lower in females versus males, and GR-ir was elevated in stress versus non-stress males. Together, we demonstrate that inherent sex-specific differences, which may modulate impact of adolescence stress exposure and TrkB inhibition, differentially affect male and female adulthood behavior and biochemical response profiles, suggesting that these responses are in part conditioned by prior experience.

Sex differences in mouse models of fear inhibition: Fear extinction, safety learning, and fear-safety discrimination

BACKGROUND AND PURPOSE

Women are overrepresented in post-traumatic stress disorder (PTSD), a mental disorder characterised by ineffective inhibition of fear. The use of male animals dominates preclinical studies, which may contribute to a lack of understanding as to why this disparity exists. Thus, the current study explores sex differences in three mouse models of fear inhibition.

EXPERIMENTAL APPROACH

All experiments tested male and female C57Bl/6J mice. Experiment 1 employed two fear conditioning protocols, in which tones were paired with footshocks of differing intensity (moderate or intense). Fear recall and extinction were tested subsequently. In Experiment 2, safety learning was investigated. Tones were explicitly unpaired with footshocks during safety conditioning. Recall of safety learning was tested 24 hr later. Experiment 3 assessed a model of fear-safety discrimination. Cued stimuli were paired or never paired with footshocks during fear and safety conditioning, respectively. Discrimination between stimuli was assessed 24 hr later.

KEY RESULTS

In fear extinction, males, compared to females, responded with greater fear in sessions most proximal to conditioning but subsequently showed a more rapid fear extinction over time. Sex differences were not observed during safety learning. During fear-safety discrimination, both males and females discriminated between stimuli; however, males revealed a greater level of freezing to stimuli.

CONCLUSION AND IMPLICATIONS

The current study provides evidence that sex differences influence fear but not safety-based behaviour in C57Bl/6J mice. These findings indicate that processing of fear, but not safety, may play a greater role in sex differences observed for PTSD.

LINKED ARTICLES

This article is part of a themed section on The Importance of Sex Differences in Pharmacology Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.21/issuetoc.

Human umbilical cord plasma proteins revitalize hippocampal function in aged mice

Ageing drives changes in neuronal and cognitive function, the decline of which is a major feature of many neurological disorders. The hippocampus, a brain region subserving roles of spatial and episodic memory and learning, is sensitive to the detrimental effects of ageing at morphological and molecular levels. With advancing age, synapses in various hippocampal subfields exhibit impaired long-term potentiation, an electrophysiological correlate of learning and memory. At the molecular level, immediate early genes are among the synaptic plasticity genes that are both induced by long-term potentiation and downregulated in the aged brain. In addition to revitalizing other aged tissues, exposure to factors in young blood counteracts age-related changes in these central nervous system parameters, although the identities of specific cognition-promoting factors or whether such activity exists in human plasma remains unknown. We hypothesized that plasma of an early developmental stage, namely umbilical cord plasma, provides a reservoir of such plasticity-promoting proteins. Here we show that human cord plasma treatment revitalizes the hippocampus and improves cognitive function in aged mice. Tissue inhibitor of metalloproteinases 2 (TIMP2), a blood-borne factor enriched in human cord plasma, young mouse plasma, and young mouse hippocampi, appears in the brain after systemic administration and increases synaptic plasticity and hippocampal-dependent cognition in aged mice. Depletion experiments in aged mice revealed TIMP2 to be necessary for the cognitive benefits conferred by cord plasma. We find that systemic pools of TIMP2 are necessary for spatial memory in young mice, while treatment of brain slices with TIMP2 antibody prevents long-term potentiation, arguing for previously unknown roles for TIMP2 in normal hippocampal function. Our findings reveal that human cord plasma contains plasticity-enhancing proteins of high translational value for targeting ageing- or disease-associated hippocampal dysfunction.

Sex and estrous cycle influence diazepam effects on anxiety and memory: Possible role of progesterone

Studies with rodents and humans show the relationship between female sex hormones and cognitive/emotional tasks. However, despite the greater incidence of anxiety disorders in women, the data are still inconclusive regarding the mechanisms related to this phenomenon. We evaluated the effects of a classical anxiolytic/amnestic drug (diazepam; DZP) on female (at different estrous cycle phases) and male rats tested in the plus-maze discriminative avoidance task (PMDAT), that allows the concomitant evaluation of memory and anxiety-like behavior. Further, in order to investigate the role of progesterone and its metabolites in the effects of DZP in the PMDAT, female rats were pre-treated with the progesterone receptor antagonist mifepristone or the 5-alpha-reductase inhibitor finasteride. The main findings were: (1) DZP caused memory impairment and anxiolysis in both sexes, but only the highest dose induced the anxiolytic effect in females; (2) females in proestrus did not present the amnestic and anxiolytic effects of DZP (at 2.0 and 4.0mg/kg, respectively) and (3) the co-administration of mifepristone reestablished both amnestic and anxiolytic effects of DZP, while finasteride reinstated the amnestic effect in proestrus female rats. These results suggest that changes in the endogenous levels of progesterone and its metabolites are important in the modulation of emotional/cognitive behavior in female rats. Based on the influence on different aspects of DZP action, the mechanisms related to this modulation are probably linked to GABAergic transmission, but this point remains to be investigated. Further, the variation in therapeutic and adverse effects of DZP depending on sex and hormonal state is of great relevance considering the higher prevalence of anxiety disorders in women.

A Compact Blast-Induced Traumatic Brain Injury Model in Mice

Blast-induced traumatic brain injury (bTBI) is a common injury on the battlefield and often results in permanent cognitive and neurological abnormalities. We report a novel compact device that creates graded bTBI in mice. The injury severity can be controlled by precise pressures that mimic Friedlander shockwave curves. The mouse head was stabilized with a head fixator, and the body was protected with a metal shield; shockwave durations were 3 to 4 milliseconds. Reflective shockwave peak readings at the position of the mouse head were 12 6 2.6 psi, 50 6 20.3 psi, and 100 6 33.1 psi at 100, 200, and 250 psi predetermined driver chamber pressures, respectively. The bTBIs of 250 psi caused 80% mortality, which decreased to 27% with the metal shield. Brain and lung damage depended on the shockwave duration and amplitude. Cognitive deficits were assessed using the Morris water maze, Y-maze, and open-field tests. Pathological changes in the brain included disruption of the blood-brain barrier, multifocal neuronal and axonal degeneration, and reactive gliosis assessed by Evans Blue dye extravasation, silver and Fluoro-Jade B staining, and glial fibrillary acidic protein immunohistochemistry, respectively. Behavioral and pathological changes were injury severity-dependent. This mouse bTBI model may be useful for investigating injury mechanisms and therapeutic strategies associated with bTBI.

Endocrine-disrupting chemicals: elucidating our understanding of their role in sex and gender-relevant end points

Endocrine-disrupting chemicals (EDCs) are diverse and pervasive and may have significant consequence for health, including reproductive development and expression of sex-/gender-sensitive parameters. This review chapter discusses what is known about common EDCs and their effects on reproductively relevant end points. It is proposed that one way that EDCs may exert such effects is by altering steroid levels (androgens or 17-estradiol, E₂) and/or intracellular E₂ receptors (ERs) in the hypothalamus and/or hippocampus. Basic research findings that demonstrate developmentally sensitive end points to androgens and E₂ are provided. Furthermore, an approach is suggested to examine differences in EDCs that diverge in their actions at ERs to elucidate their role in sex-/gender-sensitive parameters.

Anesthetic effects of a mixture of medetomidine, midazolam and butorphanol in two strains of mice

The combination of ketamine and xylazine is a widely used anesthetic for laboratory animals. However, due to an abuse problem in Japan, ketamine has been specified as a narcotic since 2007. Instead of using ketamine, Kawai et al. reported an injectable formula with an equivalent effect to the mixture of ketamine and xylazine [11]. The mixture of 0.3 mg/kg body weight (b.w.) medetomidine (Med.), 4.0 mg/kg b.w. midazoram (Mid.), and 5.0 mg/kg b.w. butorphanol (But.) produced an anesthetic duration of around 40 min in outbred ICR mice. However, the anesthetic effect of the mixture for inbred mice strains remains unknown. Therefore, we examined anesthetic effects of the mixture of Med., Mid., and But. in the BALB/c and C57BL/6J strains. After intraperitoneal injection into mice, right front paw, left hind paw, and tail pinch reflexes as well as corneal and righting reflexes were observed. Every 5 min, we scored each reflex category as 0 for reaction or 1 for no reaction. As long as the total score was at least 4 out of 5, we considered the mixture as putting a mouse in a surgical anesthetic state. The mixture produced an anesthetic duration of more than 45 min in both strains of mice. These results indicate that the mixture of Med., Mid., and But. can be a useful and effective anesthesia for the BALB/c and C57BL/6J strains of inbred mice as well as outbred ICR mice.

Sex differences in contextual fear conditioning are associated with differential ventral hippocampal extracellular signal-regulated kinase activation

Although sex differences have been reported in hippocampal-dependent learning and memory, including contextual fear memories, the underlying molecular mechanisms contributing to such differences are not well understood. The present study examined the extent to which sex differences in contextual fear conditioning are related to differential activation of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK), a protein kinase critically involved in memory formation. We first show that male rats exhibit more long-term retention of contextual fear conditioning than female rats. During a tone test, females spent more time freezing than males, although both sexes exhibited robust retention of auditory fear learning. Using Western blot analysis, we then show that phosphorylated ERK levels in ventral, but not dorsal, hippocampus are higher in males than females, relative to same-sex controls, 60 minutes after fear conditioning. Post-conditioning increases in ERK activation were observed in the amygdala in both males and females, suggesting a selective effect of sex on hippocampal ERK activation. Together, these findings suggest that differential activation of the ERK signal transduction pathway in male and female rats, particularly in the ventral hippocampus, is associated with sex differences in contextual fear.

Pitfalls in the interpretation of genetic and pharmacological effects on anxiety-like behaviour in rodents

Over the last 15 years, genetically modified mice have added important data to our knowledge on psychiatric diseases including anxiety. This has produced many behavioural publications, partially by non-behaviourists, in which differences between mutants and normal wild-type animals were described. The popularity of these novel tools allowing the study of new mechanisms also, however, led to observations that could not be confirmed. This review attempts to summarize various factors that can lead to difficult and partially incorrect interpretation of data collected in anxiety-related paradigms. These pitfalls are explained by using virtual data. Our analysis illustrates that determining anxiety in rodents is more complicated than measuring a single parameter in a particular paradigm. It is important to use proper controls such as additional measures in the same or other procedures, as well as a conservative estimation of the chance of finding an actual effect. In this way, it is possible to enhance confidence in the findings. Alternative explanations for findings, like side effects or main effects in a different domain, such as cognition, should always be taken into account. Finally, several examples from the literature are presented as illustrations of the theoretical issues discussed. We believe that considering the pitfalls presented here will help researchers to design optimized experiments that can be more readily interpreted and replicated across laboratories.

Do rats really express neophobia towards novel objects? Experimental evidence from exposure to novelty and to an object recognition task in an open space and an enclosed space

Three set of experiments were performed in an enclosed space (open-field) and in an open space (elevated platform). The surface of the open-field and the elevated platform were divided in nine equal squares. Rats were exposed (without previous habituation) in a unique session (experiment 1) or three consecutive sessions (experiment 2) either to an open-field (enclosed space) or to an elevated platform (open space) with and without an object on the centre of the field. In experiment 3, rats were exposed (without previous habituation) either to an enclosed or an open space on five consecutive sessions, one session a day. They were tested in an object recognition test in sessions 1, 3 and 5. In sessions 2 and 4, no objects were present. In experiment 1, we recorded the latency, frequency and duration of entries into different areas of the field. In experiment 3, we recorded the latency, frequency and duration of contacts with objects in addition to entries into different areas of the field. The first experiment demonstrates that rats exposed for the first time to an enclosed or an open space do not express neophobia toward novel objects in the absence of walls that surround an open-field. They crossed frequently into and spent more time in areas occupied with an object than in unoccupied areas. After two sessions of habituation to an empty open space or an empty enclosed space, the latency of first approach to a novel object is reduced while the frequency and duration of approaches are increased. The third experiment on object recognition confirmed that rats do not avoid novel objects; they made frequent visit and spent more time in the corner of the field occupied with an object than in empty corners. Recording of crossings provided detailed information about the patterns of exploratory behavior of rats but failed to reveal discrimination between novel and familiar objects which was evident in both open and enclosed space with recording of contacts with objects on the fifth exposure.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Are the effects of the antidepressants amitriptyline, maprotiline, and fluoxetine on inhibitory avoidance state-dependent?

State-dependent learning (SDL) is a phenomenon in which the retrieval of newly acquired information is possible if the subject is in the same physiological state as during the encoding phase. SDL makes it possible to separate the effects of drugs per se on learning from the effects due to changes in drug state during the task. The present work was designed to investigate whether the antidepressants amitriptyline (30 mg/kg), maprotiline (25 mg/kg), and fluoxetine (15 mg/kg) produce SDL of the inhibitory avoidance conditioning in male and female CD1 mice. In three separate experiments, independent groups were used for each pharmacological treatment and for each sex using a 2 x 2 experimental design. The results do not show SDL in any of the drugs. In the case of amitriptilyline, the data can be attributed to a memorization deficit, while the maprotiline results are interpreted as simultaneously influenced by memorization deficit and performance facilitation due to motor impairment. Fluoxetine treatment did not produce any deteriorating effect on the conditioning. Drugs had some different effects on the performance of males and females, males showing a slightly higher deterioration than females with administration of amitriptyline and maprotiline. This study shows that these antidepressants affect the acquisition/consolidation but not the retrieval process in the inhibitory avoidance learning.

Diisopropylfluorophosphate administration in the pre-weanling period induces long-term changes in anxiety behavior and passive avoidance in adult mice

The developing brain may be particularly vulnerable to exposure to acetylcholinesterase (AChE) inhibitors because of the role of AChE on neuronal development and the effects of cholinergic pathways in mediating behavioral and hormonal responses to stress. C57BL/65 mice of both sexes were injected with 1 mg/kg s.c. diisopropylfluorophosphate (DFP) or saline in three separate experiments, on postnatal days (PNDs) 4-10, 14-20, or 30-36. Anxiety and conditioned avoidance were assessed on the elevated-plus maze (EPM) and step-down passive avoidance (PA) paradigms, respectively, at age 4-5 months. In addition, locomotion and reactivity to pain on the hot plate were assessed. Mice treated on PNDs 4-10 or PNDs 14-20 spent relatively more time and made more entries to the open arms on the first, but not second, exposure to the EPM. Females, but not males, treated with DFP showed deficits in PA retention after 24 h when treated on PNDs 4-10 and on PNDs 14-20. Mice treated on PNDs 30-36 were not impaired in either behavior. Administration of DFP in the preweanling period did not affect locomotor activity or pain reactivity. The results suggest that preweanling exposure to DFP results in anxiolysis in novel conflict situations but exacerbated context-enhanced anxiety.

Piracetam counteracts the effects of amitriptyline on inhibitory avoidance in CD1 mice

The purpose of the present work was to study the effects of amitriptyline on animal cognition in relation to some characteristics of its therapeutic effects. The modulation of acute and chronic effects of amitriptyline on inhibitory avoidance in male and female mice by piracetam was investigated. In Experiment 1, mice were subjected to the training phase of inhibitory avoidance conditioning 60 min after acute piracetam (100 mg/kg) or physiological saline administration. Immediately after the behavioural task, they received a single injection of the tricyclic antidepressant amitriptyline (30 mg/kg) or physiological saline. Twenty-four hours later, subjects were tested for avoidance. In Experiment 2, the same doses of amitriptyline and piracetam were chronically administered. Mice were subjected to the training phase of inhibitory avoidance on the 22nd day, and to the test phase 24 h later. Forty-five minutes after test, subjects explored the elevated plus-maze for 5 min in order to assess whether the effects of amitriptyline on avoidance performance may reflect general behavioural changes. Results obtained were that: (a) acute and chronic amitriptyline impaired inhibitory avoidance of male and female mice, (b) piracetam counteracted the effect of acutely administered amitriptyline on inhibitory avoidance, and (c) piracetam counteracted the effects of chronically administered amitriptyline in males but not females in the same learning task. These effects do not seem to be mediated by non-specific drug effects on spontaneous motor activity or anxiety.

The influence of midazolam on active avoidance retrieval and acquisition rate in rats

The purpose of the present study was to examine the influence of midazolam on the retrieval and acquisition rate of two-way active avoidance in rats. In the schedule 2 x 100 trials, the effects of midazolam (0.5-5.0 mg/kg), benzodiazepine binding site antagonist flumazenil (2.5-10.0 mg/kg), specific antagonist of GABA(A) receptor, bicuculline (0.5-4.0 mg/kg), and the blocker of GABA(A) receptor containing Cl(-) channels, picrotoxin (1.0-5.0 mg/kg), on the second day retrieval of avoidance performance were investigated, as well as the influence of the used blockers of GABA(A) receptor function on midazolam effects. Furthermore, the effect of midazolam (1.0 mg/kg) on acquisition rate in the 5 x 50 schedule, as well as the effects of third day treatment changing in that paradigm, was examined. Throughout the study, drugs were given intraperitoneally, 30 min before testing. Midazolam at the dose of 1.0 mg/kg facilitated avoidance retrieval, whereas flumazenil and bicuculline did not significantly change behavior. Picrotoxin (5.0 mg/kg) diminished performance. All three kinds of blockers used abolished facilitatory action of midazolam, confirming GABAergic mediation of the effect of benzodiazepine. Midazolam (1.0 mg/kg) increased acquisition rate during five consecutive days relative to saline, but without significant effect on the first day acquisition. In the case of third day changing of treatments, the intersection of regression rate lines was detected. Results from active avoidance paradigm experimentally support the findings from human studies that in certain circumstances, benzodiazepines, potentiating GABAergic neurotransmission, could produce retrieval-enhancing effects in memory tasks.