Morphine effects on mouse locomotor/exploratory activity: test dependency, test reliability, uni- and multi-variate analyses

Morphine reduces the interest for natural rewards

RATIONALE

Alongside a pathological, excessive, motivation for substances of abuse, substance use disorder (SUD) patients often show a dramatic loss of interest for naturally rewarding activities, such as positive peer social interaction and food intake. Yet, pre-clinical evidence of the latter SUD features remains scarce and inconsistent.

OBJECTIVES

In the current study, we investigated the effect of non-rewarding and rewarding doses of morphine upon social behaviour, motivation for and intake of palatable food, in male and female C57BL/6J mice.

METHODS

First, the rewarding effects of two relatively low morphine doses (1.25 and 2.5 mg/kg) were assessed using a newly established single substance administration/conditioning trial conditioned place preference (CPP) paradigm. Then, morphine (1.25 and 2.5 mg/kg) effects upon social behaviour, motivation for and intake of palatable food were examined by the three-chamber (3-CH), an operant behaviour and a palatable food preference test, respectively.

RESULTS

Morphine (2.5 mg/kg) induced CPP in both male and female mice, whereas morphine (1.25 mg/kg) induced CPP only in female mice. Both morphine doses (1.25 and 2.5 mg/kg) reduced sociability, motivation for and intake of palatable food in male and female mice, independently of cognitive function or locomotor activity.

CONCLUSIONS

Female mice were more sensitive than male mice to the rewarding effects of morphine. Moreover, both a non-rewarding and a rewarding dose of morphine impaired the interest for naturally rewarding activities, indicating that brain reward systems might be more sensitive to the deleterious than to the rewarding effects of substances of abuse.

Role of Efficacy as a Determinant of Locomotor Activation by Mu Opioid Receptor Ligands in Female and Male Mice

Mu opioid receptor (MOR) agonists produce locomotor hyperactivity in mice as one sign of opioid-induced motor disruption. The goal of this study was to evaluate the degree of MOR efficacy required to produce this hyperactivity. Full dose-effect curves were determined for locomotor activation produced in male and female ICR mice by (1) eight different single-molecule opioids with high to low MOR efficacy, and (2) a series of fixed-proportion fentanyl/naltrexone mixtures with high to low fentanyl proportions. Data from the mixtures were used to quantify the efficacy requirement for MOR agonist-induced hyperactivity relative to efficacy requirements determined previously for other MOR agonist effects. Specifically, efficacy requirement was quantified as the EP₅₀ value, which is the "Effective Proportion" of fentanyl in a fentanyl/naltrexone mixture that produces a maximal effect equal to 50% of the maximal effect of fentanyl alone. Maximal hyperactivity produced by each drug and mixture in the present study correlated with previously published data for maximal stimulation of GTPɣS binding in MOR-expressing Chinese hamster ovary cells as an in vitro measure of relative efficacy. Additionally, the EP₅₀ value for hyperactivity induced by fentanyl/naltrexone mixtures indicated that opioid-induced hyperactivity in mice has a relatively high efficacy requirement in comparison to some other MOR agonist effects, and in particular is higher than the efficacy requirement for thermal antinociception in mice or fentanyl discrimination in rats. Taken together, these data show that MOR agonist-induced hyperactivity in mice is efficacy dependent and requires relatively high levels of MOR agonist efficacy for its full expression. Significance Statement Mu opioid receptor (MOR) agonist-induced hyperlocomotion in mice is dependent on the MOR efficacy of the agonist and requires a relatively high degree of efficacy for its full expression.

Neurobehavioral and Cardiovascular Effects of Potassium Cyanide Administered Orally to Mice

The Food and Drug Administration Animal Rule requires evaluation of cardiovascular and central nervous system (CNS) effects of new therapeutics. To characterize an adult and juvenile mouse model, neurobehavioral and cardiovascular effects and pathology of a single sublethal but toxic, 8 mg/kg, oral dose of potassium cyanide (KCN) for up to 41 days postdosing were investigated. This study describes the short- and long-term sensory, motor, cognitive, and behavioral changes associated with oral dosing of a sublethal but toxic dose of KCN utilizing functional observation battery and Tier II CNS testing in adult and juvenile mice of both sexes. Selected tissues (histopathology) were evaluated for changes associated with KCN exposure with special attention to brain regions. Telemetry (adult mice only) was used to evaluate cardiovascular and temperature changes. Neurobehavioral capacity, sensorimotor responsivity or spontaneous locomotor activity, and rectal temperature were significantly reduced in adult and juvenile mice at 30 minutes post-8 mg/kg KCN dose. Immediate effects of cyanide included bradycardia, adverse electrocardiogram arrhythmic events, hypotension, and hypothermia with recovery by approximately 1 hour for blood pressure and heart rate effects and by 2 hours for body temperature. Lesions consistent with hypoxia, such as mild acute tubular necrosis in the kidneys corticomedullary junction, were the only histopathological findings and occurred at a very low incidence. The mouse KCN intoxication model indicates rapid and completely reversible effects in adult and juvenile mice following a single oral 8 mg/kg dose. Neurobehavioral and cardiovascular measurements can be used in this animal model as a trigger for treatment.

Local anti-inflammatory effect and behavioral studies on new PDE4 inhibitors

Phosphodiesterase 4 (PDE4) inhibitors are effective anti-inflammatory drugs, although some adverse effects are observed in animals and humans. These effects have forced researchers to find new PDE4 inhibitors with less adverse effects. We recently reported the synthesis of novel heterocyclic-fused pyridazinones that inhibit PDE4. As a first step in the study of the anti-inflammatory properties of these compounds, we studied the effects of local administration of these pyridazinone derivatives in a mouse model of acute inflammation. We found that 6-Benzyl-3-methyl-4-phenylpyrazolo[3,4-d]pyridazin-7(6H)-one (CC4), ethyl 6,7-dihydro-6-ethyl-3-methyl-7-oxo-4-phenyl-thieno[2,3-d]pyridazine-2-carboxylate (CC6) and ethyl 6,7-dihydro-6-ethyl-3-methyl-4-phenyl-1H-pyrrolo[2,3-d]pyridazine-2-carboxylate (CC12) reduced the paw edema induced by zymosan in mice as rolipram (the PDE4 inhibitor prototype with anti-inflammatory activity) and indomethacin did. It is well known that rolipram locally administered induces some adverse effects such as hyperalgesia. Thus, we studied this effect after local administration of CC4, CC6 and CC12 in the formalin test. We found that CC6 induced hyperalgesic effects, whereas CC4 and CC12 did not change the nociceptive threshold. Furthermore, we found that rolipram and CC6 reduced locomotor activity, whereas CC4 and CC12 did not change locomotor performance of the mice. Since CC4 and CC12 neither affected the nociceptive threshold nor changed the locomotor performance of mice, they appear more suitable than CC6 for future studies on animals and could be developed as an anti-inflammatory drug for humans.

Effects of prior apparatus experience and novelty of testing environment on locomotor activity following MK-801

Robust increases in locomotor activity are observed following administration of dizocilpine maleate (MK-801). The present study investigated the effects of prior apparatus experience and manipulation of the testing environment on locomotor activity following peripheral MK-801. Gerbils were given zero or nine sessions of apparatus exposure prior to testing with MK-801 (0.1 mg/kg ip) or saline. Sessions were 10 min in duration and separated by 24 h. As previously reported, naive animals treated with MK-801 were significantly more active relative to controls. Exposure to the apparatus for nine sessions resulted in a significant reduction in MK-801-induced activity, but did not alter the activity levels of control animals. To evaluate the effect of changes to the testing environment, animals previously evaluated in the familiar condition were retested in the identical apparatus relocated to a novel experimental room. MK-801-treated animals exhibited a significant increase in activity when tested in this novel environment while the locomotor activity of control gerbils was not significantly altered. The results illustrate the importance of repetitive testing and environmental changes as moderating variables in studies that evaluate locomotor activity. These data also indicate that the effects of MK-801 on activity are sensitive to prior experience with the apparatus and the novelty of the testing environment.

Postnatal growth and behavioral development of mice cloned from adult cumulus cells

Since the first successful cloning of mammals from adult somatic cells, there has been no examination of the learning or behavior of cloned offspring. The possibility of adverse effects on animals produced through adult somatic cell cloning is high because many natural biological processes are bypassed and DNA from adult cells, which presumably contain mutations, are used. In this study, we compared cloned mice produced by microinjection transfer of cumulus cell nuclei into enucleated oocytes, to control mice that were specifically generated to eliminate confounding factors that are unique to our cloning procedure. Postnatal weight gain of clones was significantly greater than that of controls. Preweaning development observations revealed that first appearance or performance of 3 out of 10 measures was delayed in cloned mice; however, results of subsequent tests of learning and memory, activity level, and motor skills were comparable for both groups. Together, these data suggest that nuclear transfer of adult somatic cell nuclei to produce cloned mice may delay the appearance of a few developmental milestones but it does not adversely affect the overall postnatal behavior of mice. In addition, this procedure may cause late onset of significantly increased body weight in cloned offspring, the cause or causes of which are being further examined.

Behavioural effects of endocrine disrupting chemicals on laboratory rodents: statistical methodologies and an application concerning developmental PCB exposure

Appropriate behavioural tests and adequate statistical tools may help to establish the ED properties of a given compound by pointing out the alterations of selected behavioural endpoints. Frequently, laboratory collected data consist of frequencies and/or durations of specific items, and the analysis of variance (ANOVA) technique is performed to assess whether the investigated factors affect these behavioural endpoints. Moreover, when numerous aspects of behaviour are investigated simultaneously, Principal Component Analysis (PCA), a multivariate technique, may be very useful to reduce the overwhelming number of correlated original variables to a few orthogonal artificial variables (factors). Continuous Time Markov Chain (CTMC) models may be applied to analyse the time structure of a behavioural pattern when data consist of sequences of events and the time points at which they occur. Moreover, the Cox Proportional Hazard Model, a methodology originally developed for the analysis of failure time data, may help to evidence the effects of a given treatment on behavioural sequences when the assumptions of CTMC models are not fully satisfied. Analyses on data from mice of the outbred CD-1 strain (controls in a study of toxicity and exposed to PCB during development) are presented as examples to show how adequate statistical analyses and appropriate behavioural tests may reveal relevant effect of treatments otherwise not easily detected.

A comparison of behavioural effects of prenatally administered oxazepam in mice exposed to open-fields in the laboratory and the real world

Prenatal benzodiazepine exposure has been reported to result in abnormal neurobehavioural development in laboratory animals but little is known about the behavioural relevance of this effect ina naturalistic environment. In this study, outbred CD-1 male mice were prenatally exposed to oxazepam (15 mg/kg per os, twice daily) on days 12-16 of fetal life and fostered at birth to untreated dams. At adulthood, each mouse was fitted with a radio collar and its first reactions assessed. Three hours later, behavioural and exploratory activities were recorded in a laboratory open field, and 24 h later in a natural setting. Immediate reactions to the radio collar were higher in the oxazepam-treated mice than in controls consisting of more attempts to remove it and an increase of push-digging. The attempts to remove the collar were still evident in oxazepam treated mice tested in the laboratory open-field 3 h later. Moreover, oxazepam increased the frequency of grooming and reduced walking in both the laboratory and the natural settings. In the natural settings running was increased during the initial 30-min test, while a pronounced level of grooming and a lower frequency of eating were observed 140 min after release. Frequency of sniffing, grooming, and rearing behaviours were higher in the laboratory test when compared to the natural settings. On the other hand, prolonged bouts of sniffing were recorded in the natural environment. These findings permit separation of robust drug effects (increased grooming, reduced walking) from situation-dependent effects, the natural environment revealing, in addition, more subtle effects.

Prenatal cocaine potentiates the effects of morphine in adult mice

Prenatal cocaine exposure has been reported to result in abnormal neurobehavioral development, both in animals and humans. In this study, outbred CD-1 mice were exposed in utero to cocaine hydrochloride administered daily as i.p. injections to dams from day 10 of gestation to day 16, at the dose 0, 5 or 50 mg/kg. Cocaine did not alter duration of pregnancy while it decreased the difference in maternal body weight from days 10 to 16 in the dams receiving the higher dose of cocaine. The body weight of the offspring from birth to 15 days of age and the physical maturation were not affected by prenatal cocaine exposure. The development of the response to strong tactile stimulation was either slightly delayed in the 5 mg/kg group or markedly accelerated in the 50 mg/kg group. At adulthood, animals were assessed for behavioral responses to a novel environment, for response to painful stimulation (hot-plate test set at 55 +/- 1 degree C), and for the effects of a single morphine injection (30 mg/kg, i.p.). Data showed that in the absence of prenatal cocaine exposure effects, morphine increased the time spent in inactivity, while it decreased rearing, grooming and bar-holding behaviors. In the case of sniffing, morphine increased this behavior, except in the 5 mg/kg cocaine group. Moreover, morphine administration induced the expected increase of locomotion, irrespective of prenatal condition. With respect to pain reactivity, prenatal cocaine exposure resulted in an increase of licking latency in the 5 mg/kg group.(ABSTRACT TRUNCATED AT 250 WORDS)

Different capability of N-methyl-D-aspartate antagonists to affect locomotor/exploratory activity of mice in a computerized on-line open field test

The effects of the competitive N-methyl-D-aspartate (NMDA) antagonists CGS 19755 and CPP, and of the noncompetitive NMDA antagonists PCP, MK 801, and dextromethorphan (DM) have been studied on the locomotor/exploratory activity of mice in a computerized on-line open field test. CGS 19755 (12.5-25 mg/kg, IP) induced a dose-dependent decrease in the locomotor/exploratory activity of mice; CPP (25-50 mg/kg, IP) did not present such an effect. PCP (1.25-10 mg/kg, IP) induced a dose-dependent increase/decrease in the locomotor/exploratory activity of mice, and DM (25-50 mg/kg, IP) and MK 801 (0.125-0.250 mg/kg, IP) increased it. The data show that NMDA antagonists affect locomotor/exploratory activity of mice in different ways, inducing both potentiating and inhibitory effects.

Non-opioid antitussives potentiate some behavioural and EEG effects of N-methyl-D-aspartate channel blockers

The effects of the non-opioid oral antitussives dextromethorphan (DM) and caramiphen (CP) were tested against the behavioural and EEG effects elicited by the N-methyl-D-aspartate (NMDA) antagonists dizocilpine (MK 801) and phencyclidine (PCP) in rats and mice. PCP (1.25-10 mg/kg i.p.) induced a dose-dependent increase/decrease of the locomotor/exploratory activity of mice. DM (25-50 mg/kg i.p.) and MK 801 (0.125-0.250 mg/kg i.p.) induced an increase of the locomotor/exploratory activity of mice, while CP (25-50 mg/kg i.p.) did not produce such an effect. CP (12.5 mg/kg i.p.) and DM (12.5 mg/kg i.p.) significantly potentiated the effects of PCP (1.25 mg/kg i.p.) and MK 801 (0.062 mg/kg i.p.) in the open field test in mice. In rats, PCP (1.25-10 mg/kg i.p.) induced three dose-dependent EEG stages: 1) increase of the cortical desynchronization periods; 2) increase of the amplitude of cortical background activity; 3) appearance of cortical slow wave-spike complexes. Even though DM (up to 100 mg/kg i.p.) only induced PCP-like EEG stage 1 by itself, and CP (up to 50 mg/kg i.p.) did not affect basal cortical EEG activity, these drugs, at the doses of 30-50 mg/kg i.p., potentiated all the EEG effects induced by PCP. These data support the view of an interaction between non-opioid antitussives and non-competitive NMDA antagonists.

Selective changes in mouse behavioral development after prenatal benzodiazepine exposure: a progress report

1. Animal studies of the effects of early exposure to CNS agents devoid of a major teratogenic potential must assess possible deviations from normal behavioral development in both a stage-specific and a behavior-specific fashion; several experiments on prenatal benzodiazepine (BDZ) exposure are reviewed, illustrating such an assessment strategy and discussing caveats on experimental designs and statistical analysis. 2. The offspring of mouse dams treated in late pregnancy with oxazepam (15 mg/kg p.o. twice daily on days 12-16) show a mild and reversible impairment in somatic and neurobehavioral development which is unlikely to be responsible for a series of other more specific changes. 3. The treatment produces a selective reduction of locomotor activity and amphetamine hyperactivity at the end of the second postnatal week, as well as a selective impairment of active avoidance at the young adult stage, in the absence of similar changes in scopolamine hyperactivity and passive avoidance. 4. The treatment also prevents the appearance at 28 days of morphine hyperactivity and of rebound hyperactivity after muscimol depression, without modifying the developmental profile of pain reactivity and of morphine and muscimol analgesia. 5. Young adult females previously exposed to oxazepam in utero show a marked enhancement of maternal aggression towards male intruders; mother-pup interactions are also modified, leading either to reduced or to exaggerated maternal care as a function of fostering procedures. 6. Overall, several effects of prenatal BDZ exposure appear to be amenable to modifications in monoaminergic system functions and/or to an accelerated development of GABAergic mechanisms; some of the changes in social and parental interactions, however, point to subtle modifications in the balance between different components of the fear-defensive repertoire, possibly due to an altered stimulus reactivity by mechanisms which are still poorly understood.