Behavioral effects of low and high acute doses of morphine in solitary mice

HIV-1 Tat and morphine interactions dynamically shift striatal monoamine levels and exploratory behaviors over time

Despite the advent of combination anti-retroviral therapy (cART), nearly half of people infected with HIV treated with cART still exhibit HIV-associated neurocognitive disorders (HAND). HAND can be worsened by co-morbid opioid use disorder. The basal ganglia are particularly vulnerable to HIV-1 and exhibit higher viral loads and more severe pathology, which can be exacerbated by co-exposure to opioids. Evidence suggests that dopaminergic neurotransmission is disrupted by HIV exposure, however, little is known about whether co-exposure to opioids may alter neurotransmitter levels in the striatum and if this in turn influences behavior. Therefore, we assayed motor, anxiety-like, novelty-seeking, exploratory, and social behaviors, and levels of monoamines and their metabolites following 2 weeks and 2 months of Tat and/or morphine exposure in transgenic mice. Morphine decreased dopamine levels, but significantly elevated norepinephrine, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid, which typically correlated with increased locomotor behavior. The combination of Tat and morphine altered dopamine, DOPAC, and HVA concentrations differently depending on the neurotransmitter/metabolite and duration of exposure but did not affect the numbers of tyrosine hydroxylase-positive neurons in the mesencephalon. Tat exposure increased the latency to interact with novel conspecifics, but not other novel objects, suggesting the viral protein inhibits exploratory behavior initiation in a context-dependent manner. By contrast, and consistent with prior findings that opioid misuse can increase novelty-seeking behavior, morphine exposure increased the time spent exploring a novel environment. Finally, Tat and morphine interacted to affect locomotor activity in a time-dependent manner, while grip strength and rotarod performance were unaffected. Together, our results provide novel insight into the unique effects of HIV-1 Tat and morphine on monoamine neurochemistry that may underlie their divergent effects on motor and exploratory behavior.

Depressive-like Behavior Is Accompanied by Prefrontal Cortical Innate Immune Fatigue and Dendritic Spine Losses after HIV-1 Tat and Morphine Exposure

Opioid use disorder (OUD) and HIV are comorbid epidemics that can increase depression. HIV and the viral protein Tat can directly induce neuronal injury within reward and emotionality brain circuitry, including the prefrontal cortex (PFC). Such damage involves both excitotoxic mechanisms and more indirect pathways through neuroinflammation, both of which can be worsened by opioid co-exposure. To assess whether excitotoxicity and/or neuroinflammation might drive depressive behaviors in persons infected with HIV (PWH) and those who use opioids, male mice were exposed to HIV-1 Tat for eight weeks, given escalating doses of morphine during the last two weeks, and assessed for depressive-like behavior. Tat expression decreased sucrose consumption and adaptability, whereas morphine administration increased chow consumption and exacerbated Tat-induced decreases in nesting and burrowing—activities associated with well-being. Across all treatment groups, depressive-like behavior correlated with increased proinflammatory cytokines in the PFC. Nevertheless, supporting the theory that innate immune responses adapt to chronic Tat exposure, most proinflammatory cytokines were unaffected by Tat or morphine. Further, Tat increased PFC levels of the anti-inflammatory cytokine IL-10, which were exacerbated by morphine administration. Tat, but not morphine, decreased dendritic spine density on layer V pyramidal neurons in the anterior cingulate. Together, our findings suggest that HIV-1 Tat and morphine differentially induce depressive-like behaviors associated with increased neuroinflammation, synaptic losses, and immune fatigue within the PFC.

µ-Opioid Receptors Expressed by Intrinsically Photosensitive Retinal Ganglion Cells Contribute to Morphine-Induced Behavioral Sensitization

Opioid drugs are the most effective tools for treating moderate to severe pain. Despite their analgesic efficacy, long-term opioid use can lead to drug tolerance, addiction, and sleep/wake disturbances. While the link between opioids and sleep/wake problems is well-documented, the mechanism underlying opioid-related sleep/wake problems remains largely unresolved. Importantly, intrinsically photosensitive retinal ganglion cells (ipRGCs), the cells that transmit environmental light/dark information to the brain's sleep/circadian centers to regulate sleep/wake behavior, express μ-opioid receptors (MORs). In this study, we explored the potential contribution of ipRGCs to opioid-related sleep/circadian disruptions. Using implanted telemetry transmitters, we measured changes in horizontal locomotor activity and body temperature in mice over the course of a chronic morphine paradigm. Mice lacking MORs expressed by ipRGCs (McKO) exhibited reduced morphine-induced behavioral activation/sensitization compared with control littermates with normal patterns of MOR expression. Contrastingly, mice lacking MORs globally (MKO) did not acquire morphine-induced locomotor activation/sensitization. Control mice also showed morphine-induced hypothermia in both the light and dark phases, while McKO littermates only exhibited morphine-induced hypothermia in the dark. Interestingly, only control animals appeared to acquire tolerance to morphine's hypothermic effect. Morphine, however, did not acutely decrease the body temperature of MKO mice. These findings support the idea that MORs expressed by ipRGCs could contribute to opioid-related sleep/wake problems and thermoregulatory changes.

A limited access oral oxycodone paradigm produces physical dependence and mesocorticolimbic region-dependent increases in DeltaFosB expression without preference

The abuse of oral formulations of prescription opioids has precipitated the current opioid epidemic. We developed an oral oxycodone consumption model consisting of a limited access (4 h) two-bottle choice drinking in the dark (TBC-DID) paradigm and quantified dependence with naloxone challenge using mice of both sexes. We also assessed neurobiological correlates of withdrawal and dependence elicited via oral oxycodone consumption using immunohistochemistry for DeltaFosB (ΔFosB), a transcription factor described as a molecular marker for drug addiction. Neither sex developed a preference for the oxycodone bottle, irrespective of oxycodone concentration, bottle position or prior water restriction. Mice that volitionally consumed oxycodone exhibited hyperlocomotion in an open field test and supraspinal but not spinally-mediated antinociception. Both sexes also developed robust, dose-dependent levels of opioid withdrawal that was precipitated by the opioid antagonist naloxone. Oral oxycodone consumption followed by naloxone challenge led to mesocorticolimbic region-dependent increases in the number of ΔFosB expressing cells. Naloxone-precipitated withdrawal jumps, but not the oxycodone bottle % preference, was positively correlated with the number of ΔFosB expressing cells specifically in the nucleus accumbens shell. Thus, limited access oral consumption of oxycodone produced physical dependence and increased ΔFosB expression despite the absence of opioid preference. Our TBC-DID paradigm allows for the study of oral opioid consumption in a simple, high-throughput manner and elucidates the underlying neurobiological substrates that accompany opioid-induced physical dependence.

HIV-1 Tat and morphine decrease murine inter-male social interactions and associated oxytocin levels in the prefrontal cortex, amygdala, and hypothalamic paraventricular nucleus

Many persons infected with HIV-1 (PWH) and opioid-dependent individuals experience deficits in sociability that interfere with daily living. Sociability is regulated by the prefrontal cortico-hippocampal-amygdalar circuit. Within this circuit HIV-1 trans-activator of transcription (HIV-1 Tat) and opioids can increase dendritic pathology and alter neuronal firing. Changes in sociability are also associated with dysregulation of hypothalamic neuropeptides such as oxytocin or corticotropin releasing factor (CRF) in the prefrontal cortico-hippocampal-amygdalar circuit. Accordingly, we hypothesized that the interaction of HIV-1 Tat and morphine would impair inter-male social interactions and disrupt oxytocin and CRF within the PFC and associated circuitry. Male mice were exposed to HIV-1 Tat for 8 weeks and administered saline or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of HIV-1 Tat exposure. Tat attenuated aggressive interactions with an unknown intruder, whereas morphine decreased both non-aggressive and aggressive social interactions in the resident-intruder test. However, there was no effect of Tat or morphine on non-reciprocal interactions in the social interaction and novelty tests. Tat, but not morphine, decreased oxytocin levels in the PFC and amygdala, whereas both Tat and morphine decreased the percentage of oxytocin-immunoreactive neurons in the hypothalamic paraventricular nucleus (PVN). In Tat(+) or morphine-exposed mice, regional levels of CRF and oxytocin correlated with alterations in behavior in the social interaction and novelty tests. Overall, decreased expression of oxytocin in the prefrontal cortico-hippocampal-amygdalar circuit is associated with morphine- and HIV-Tat-induced deficits in social behavior.

Effects of Three Consecutive Days of Morphine or Methadone Administration on Analgesia and Open-Field Activity in Mice with Ehrlich Carcinoma

This study assessed the exploratory behavioral responses in BALB/c mice inoculated with Ehrlich ascitic carcinoma after 3 consecutive days of treatment with morphine or methadone. Fifty-three female mice, 60 ± 10 d old, were used. Seven days after intraperitoneal tumor inoculation (2 × 10⁶ cells), the animals were randomized into 7 groups: morphine 5 mg/kg (MO₅), morphine 7.5 mg/kg (MO_7.5), morphine 10 mg/kg (MO₁₀), methadone 2.85 mg/kg (ME_2.85), methadone 4.3 mg/kg (ME_4.3), methadone 5.7 mg/kg (ME_5.7), and 0.9% NaCl (Saline) (n = 7). Drug treatments were administered subcutaneously every 6 h for 3 d. The animals were evaluated for analgesia using the mouse grimace scale (MGS) and for general activity using the open field test. The MGS was performed before tumor inoculation (day 0), on day 7 at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 at 40, 150, 240, and 360 min after drug injection. The open field test was performed before tumor inoculation (day 0), on day 7 after inoculation at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 after inoculation at 40, 150, and 360 min after drug injection. MGS results indicated that administration of morphine promoted analgesia for up to 240 min. Conversely, methadone reduced MGS scores only at 40 min. All tested doses promoted a significant dose-dependent increase in the total distance traveled and the average speed, and increase that was markedly pronounced on days 8 and 9 as compared with day 7. The frequencies of rearing and self-grooming decreased significantly after morphine or methadone administration. Despite the difference in analgesia, both drugs increased locomotion and reduced the frequency of rearing and self-grooming as compared with the untreated control animals.

Behavioral and Auditory Electrophysiological Rebound as a Compensatory Response to the Reinforcing Effects of Morphine

Auditory-evoked potentials (AEPs) can be modified by associative learning, where the appearance of learned compensatory responses (CCRs) may result in the emergence of drug withdrawal symptoms and relapse. Although CCRs' influence on later attentive and cognitive domains has been extensively examined, contextual conditioned tolerance occurring in preattentive mechanisms operating at earlier stages of information processing has remained largely unexplored. To extend our knowledge on this subject, compensatory changes on the motor and emotional aspects of behavior evoked by contextual cues were investigated with an electronic open field in morphine-pretreated rats challenged with two morphine overdoses (40 and 80 mg/kg). CCRs influence on the AEPs recorded in the central nucleus of the inferior colliculus (CIC) was analyzed with the help of a field potential recording device and a two-chamber shuttle box placed inside a Faraday cage system. The emergence of electrophysiological CCRs was analyzed by recording AEP latency and amplitude elicited in the central nucleus of the IC (CIC) with the aid of a field potential recording device and a two-chamber shuttle box placed inside a Faraday cage system. Behavioral analysis indicated that CCRs ensue in non-familiar contexts. Electrophysiological data revealed increases in the amplitude of AEPs evoked in a non-familiar context. Our results indicate that behavioral learning responses emerge following Pavlovian conditioning even with the use of low and regular doses of morphine over a short-term treatment. Changes in the CIC electrophysiology may indicate that the development of drug dependence occurs covertly in the early stages of sensory information processing.

Oxycodone physical dependence and its oral self-administration in C57BL/6J mice

Abuse of prescription opioids, such as oxycodone, has markedly increased in recent decades. While oxycodone's antinociceptive effects have been detailed in several preclinical reports, surprisingly few preclinical reports have elaborated its abuse-related effects. This is particularly surprising given that oxycodone has been in clinical use since 1917. In a novel oral operant self-administration procedure, C57BL/6J mice were trained to self-administer water before introducing increasing concentrations of oxycodone (0.056-1.0mg/ml) under post-prandial conditions during daily, 3-h test sessions. As the concentration of oxycodone increased, the numbers of deliveries first increased, then decreased in an inverted U-shape fashion characteristic of the patterns of other drugs self-administered during limited access conditions. After post-prandial conditions were removed, self-administration at the highest concentration was maintained suggesting oral oxycodone served as a positive reinforcer. In other mice, using a novel regimen of physical dependence, mice were administered increasing doses of oxycodone (9.0-33.0mg/kg, s.c.) over 9 days, challenged with naloxone (0.1-10.0mg/kg, s.c.), and then observed for 30min. Naloxone dose-dependently increased the observed number of somatic signs of withdrawal, suggesting physical dependence of oxycodone was induced under this regimen. This is the first report demonstrating induction of oral operant self-administration of oxycodone and dose-dependent precipitations of oxycodone withdrawal in C57BL/6J mice. The use of oral operant self-administration as well as the novel physical dependence regimen provides useful approaches to further examine the abuse- and dependence-related effects of this highly abused prescription opioid.

Placebo-induced analgesia in an operant pain model in rats

Analgesia is particularly susceptible to placebo responses. Recent studies in humans have provided important insights into the neurobiology underlying placebo-induced analgesia. However, human studies provide incomplete mechanistic explanations of placebo analgesia because of limited capacity to use cellular, molecular, and genetic manipulations. To address this shortcoming, this article describes the development of a rat model of conditioned analgesia in an operant pain assay. Specifically, rats were conditioned to associate a placebo manipulation with the analgesic effect of 1mg/kg morphine (subcutaneously) on facial thermal pain. We found that conditioned (placebo) responding bore 3 of the hallmarks of placebo-induced analgesia: (1) strong interanimal variability in the response, (2) suppression by the opiate antagonist naloxone (5mg/kg subcutaneously), and (3) a positive predictive relationship between the unconditioned analgesic effect and the conditioned (placebo) effect. Because of the operant nature of the assay and the use of only a mild noxious thermal stimulus, we suggest that these results provide evidence of placebo-induced analgesia in a preclinical model that utilizes an affective behavioral end point. This finding may provide opportunities for invasive preclinical studies allowing greater understanding of placebo-induced analgesia, thus paving the way for avenues to harness its benefits.

Morphine-induced stereotyped thigmotaxis could appear as enhanced fear and anxiety in some behavioural tests

This study questions whether the classical interpretation for unconditional fear/anxiety tests is valid when animals are under the influence of some drugs of abuse. We used a modified version of the trimethylthiazoline (TMT)-avoidance task, a measure of unconditional fear. Halfway into a corridor maze we placed a 3-cm-high barrier. This provided a wall in the middle of the corridor, one that the mice can easily climb over. Saline- and morphine-treated mice were randomly placed in the 'safe' or 'unsafe' (TMT) side and observed for 10 min. As expected, saline-injected mice spent only about 25% of the time in the TMT side, regardless of the side they were initially placed into. In contrast, morphine-treated mice did not cross the barrier even once, regardless of their initial placement. Specifically, morphine-treated mice initially placed in the TMT side appeared to exhibit the expected reduction in unconditional fear, that is, spending the entire time in the TMT side, a significant increase over the controls. Yet, morphine-treated mice placed in the safe side never even entered the TMT side; thus, these mice appeared to exhibit a behavioural response that is classically interpreted as increased fear, that is, spending significantly less time in the TMT side versus the controls. In summary, this study demonstrates that the classical interpretation of some unconditioned fear or anxiety tests could be misleading when animals are under the influence of drugs that might induce other competing behaviours.

Alpha2-adrenoceptor antagonist SL 84.0418 reduces the expression of neocortical spike-and-wave spindling episodes in DBA/2J mice

The present work was undertaken to study the effects of alpha(2)-adrenoceptor antagonist 2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2-propyl-pyrrolo[3,2,1-hi]-indole hydrochloride (SL 84.0418) on the neocortical spike-and-wave spindling episodes (S and W) in the cortical electroencephalogram (ECoG) of DBA/2J mice. Our data indicate that SL 84.0418 (0.1-1.0-10 mg/kg ip) dose-dependently reduces the S and W of DBA/2J mice. This effect appears 30 min after drug administration and lasts for the duration of the recording period (240 min). The reducing effect of the SL 84.0418 on the S and W of mice was comparable to that induced by tolazoline (10-20-40 mg/kg ip), a well-known alpha(2)-adrenoceptor antagonist, whereas the alpha(2)-adrenoceptor agonist clonidine (0.02-0.1-0.5 mg/kg ip) significantly increased it. The present results indicate that SL 84.0418 administration induces significant reduction on S and W of DBA/2J mice suggesting a possible involvement of noradrenergic system in the development of S and W of DBA/2J mice.

Pharmacological studies and synthesis of morpholino alkyl derivatives

Seven morpholin derivatives were synthesized (compounds 1-7) and their behavioural effects were evaluated; the elements considered were locomotor activity, motor coordination, catalepsy, stereotyped behaviour and antinociception. All the compounds, at doses of 10-20-40 mg/kg/i.p., induced a reduction of all behavioural elements without a significant antinociceptive effect. These results indicate that these morpholin derivatives affect the central nervous system.

Anticonvulsive activity of a new GABA mimetic drug

This study examines the pharmacological profile of a new GABA mimetic drug, 4-[(2H)-1,3-benzoxazine-2,4(3H)-dione]-butyric acid (BXDBA), using both a behavioral and an anticonvulsive study. The behavior elements considered were locomotor activity, motor coordination, catalepsy, behavior and antinociception. The anticonvulsive study was performed using the convulsive agent bicuculline. BXDBA [10, 20 and 40 mg/kg, intraperitoneally (i.p.)] did not significantly modify animal behavior or the nociceptive threshold of the animals. The anticonvulsive study indicated that BXDBA (10, 20 and 40 mg/kg, i.p.), injected 60 min before bicuculline (10 micrograms/intracerebroventricular (i.c.v.)/mouse) induced a dose-dependent and significant reduction of the convulsive activity of bicuculline whereas it was ineffective if injected immediately before the convulsive agent. Our data indicate that this new GABA mimetic drug possesses good anticonvulsive activity and its ability to block bicuculline-induced convulsions suggests that it could be a GABAA mimetic drug. Furthermore, since BXDBA is able to act after systemic administration, our data suggest that this new GABA mimetic drug crosses the blood-brain barrier.

Dexamethasone modifies the behavioral effects induced by clonidine in mice

The present study examines the influence of dexamethasone on behavioral effects induced by clonidine in mice. 2. The behavior elements considered were locomoter activity, rota rod, catalepsy and stereotyped behavior (rearing, grooming, social response test, crossing, smelling, washing face, scratching and bar holding). 3. Clonidine (0.1-0.5-1.0 mg/kg, IP) induced a significant reduction of all behavioral elements studied when compared to the saline treated group: the behavioral reduction was significant 10 min after administration and lasted for the entire recording period (120 min). 4. Dexamethasone (0.1-0.5-1.0 mg/kg, IP) per se did not induce significant changes in the behavior elements recorded. 5. Dexamethasone (0.1-0.5 mg/kg, IP) dod not affect behavioral effects induced by the 3 doses of clonidine, whereas the high dose (1 mg/kg) of the steroid significantly reduced its behavioral inhibition. 6. The results of the present study suggest that dexamethasone induces significant effects on clonidine-induced behavioral effects and that this may be related to an interference with the monoaminergic system.

Reductions in locomotor activity following central opioid receptor subtype antagonists in rats

Opioid agonists produce biphasic (decreases then increases) effects upon activity in rats. General opioid antagonists typically suppress activity. Selective opioid antagonists reduce weight and food intake. However, the latter effects cannot fully account for the former effects. To assess the possibility that selective opioid antagonists might decrease weight by increasing activity, the present study examined whether central administration of either mu (beta-funaltrexamine: 20 micrograms), mu1 (naloxonazine: 50 micrograms), delta1 ([D-Ala2,Leu5,Cys6]enkephalin: 40 micrograms), delta2 (naltrindole isothiocyanate: 20 micrograms), or kappa1 (nor-binaltorphamine: 20 micrograms) opioid antagonists altered total, ambulatory, or stereotypic activity. Each of the antagonists significantly reduced total (mu: 18%, mu1: 31%, delta1: 42%, delta2: 37%, kappa1: 31%), ambulatory (mu: 17%, mu1: 27%, delta1: 34%, delta2: 37%, kappa1: 31%), and stereotypic (mu: 19%, mu1: 34%, delta1: 49%, delta2: 37%, kappa1: 31%) activity on the first day. All three activity measures were reduced by delta1 and delta2 antagonism on the second day, whereas mu antagonism reduced total and stereotypic activity on the second day. The activity reductions induced by selective opioid receptor subtype antagonists parallel effects induced by general opioid antagonism, and suggest that antagonist-induced weight loss effects independent of intake reductions are not due to antagonist-induced hyperactivity.

Neuropharmacological effects of extracts from Sickingia williamsii

Sickingia williamsii Standl. (Rubiaceae) is used in Peruvian folk medicine for its analgesic and anti-inflammatory activity. In this study we have examined the pharmacological profiles of petroleum ether, chloroform, chloroform-methanol (9:1) and methanol extracts of the tree. Both chloroform-methanol (250, 500 or 750 mg kg-1, p.o.) and methanol (125, 250 or 500 mg kg-1, p.o.) extracts significantly reduced both the locomotor activity and motor co-ordination of mice; they also prolonged sleep induced by pentobarbital, although no significant cataleptic response was observed. These extracts did not have a significant impact on the nociceptive threshold in the hot-plate and tail-flick tests. Petroleum ether and chloroform extracts did not, furthermore, induce any significant pharmacological effects. The results of the study suggest that these extracts possess neurosedative-like properties.

Dexamethasone reduces the behavioural effects induced by baclofen in mice

The present study examines the influence of dexamethasone on the behavioural effects induced by baclofen in mice. The behaviour elements considered were locomotor activity, motor co-ordination, catalepsy, stereotyped behaviour and antinociception. Baclofen (1.0-4.0-6.0 mg kg-1, i.p.) induced a significant reduction of all behavioural elements studied and an antinociceptive effect was recorded. Dexamethasone alone (0.1-0.5-1.0 mg kg-1, i.p.) did not induce significant changes in the behaviour elements considered. On the other hand, when the steroid was injected immediately before baclofen a significant reduction of baclofen's behavioural effects was found. Our results suggest a possible link between glucocorticoid and the GABA-ergic system.

Biphasic effects of intraaccumbens mu-opioid peptide agonist DAMGO on locomotor activities

The effects of bilateral microinjections of mu-opioid receptor agonist DAMGO (0.00, 0.01, 0.1, or 1.0 microgram/side) were tested in rats for 120 min in activity monitors. The horizontal movement, rearing, and stereotypy times in seconds were measured during 12 consecutive 10-min time blocks. DAMGO (0.01, 0.1, and 1.0 microgram) resulted in biphasic effects, inhibition followed by activation for each of the three measures. These data replicate the behavioral effects of ICV DAMGO except that the duration of the behavioral effects were longer with Acb injections.

Behavioral effects of the mu-opioid peptide agonists DAMGO, DALDA, and PL017 on locomotor activities

The relative role of central mu-opioid receptor agonists Tyr-D-Ala-Gly-N-Methyl-Phe-Gly-ol (DAMGO), Tyr-D-Arg-Phe-Lys (DALDA), and Tyr-Pro-MePhe-D-Pro (PL017) (0.00, 0.01, 01, or 1.0 micrograms, ICV) on behavior was investigated in rats for 60 min in activity monitors. DAMGO (0.1 and 1.0 micrograms) and PL017 (1.0 micrograms) resulted in biphasic effects, inhibition followed by hyperactivity for linear locomotor, whereas the 0.01-micrograms dosage was associated with hyperactivity. On the other hand, DALDA (0.1 and 1.0 micrograms) suppressed locomotor activity over the 60-min session.

Behavioral effects of opioid peptide agonists DAMGO, DPDPE, and DAKLI on locomotor activities

The effects of the mu-selective agonist DAMGO (ICV doses of 0.00, 0.01, 0.1, or 1.0 micrograms), the delta-selective agonist DPDPE (ICV doses of 0.00, 0.1, 1.0, or 10.0 micrograms), and the kappa-selective agonist DAKLI (ICV doses of 0.00, 0.01, 0.1, or 1.0 micrograms) were tested in rats for 60 min in an activity monitor. The durations in seconds of linear locomotor time, rearing time, stereotypy time, and margin time (thigmotaxis) were measured during six 10-min time blocks. DAMGO (0.1 and 1.0 micrograms) resulted in biphasic effects, inhibition followed by hyperactivity for linear locomotor, rearing, and stereotypy times, and an inhibition of thigmotaxis. DPDPE (10.0 micrograms) was associated with monophasic potentiation of linear locomotor activity and mixed effects in stereotypy times. DAKLI did not effect horizontal, rearing, or margin times; only stereotypy times resulted in mixed effects of DAKLI. The differential behavioral profiles were discussed in reference to the three opioid receptor subtypes.

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

Drug and toxicant effects on locomotor/exploratory activity can be quite variable depending on the test and the schedule of exposure. In neurobehavioral toxicology and teratology, these interactions can affect the inferences based on the use of selected drugs as probes to assess which regulatory mechanisms are affected by one or the other treatment. The present experiments were aimed at comparing morphine effects in CD-1 mice under three conditions, namely, Varimex apparatus (VAR), toggle floor box (TOGGLE), videotape recording (VIDEO) in a home cage environment. Morphine HCI (0, 10, 33, or 100 mg/kg) was given IP 20 min before the start of a 30-min test session. The same procedure was repeated 24 h later. Results of VAR and TOGGLE tests were: dose 10 was largely ineffective; dose 33 induced depression in VAR and hyperactivity in TOGGLE; dose 100 enhanced activity in TOGGLE. There were no differences between session 1 and 2. VIDEO: Univariate analysis results showed that morphine produced a dose-dependent depression of Rearing and Grooming, and an enhancement of Crossing, again without changes due to repeated exposure. Results of Principal Component Analysis supported a response competition model of the changes observed in the mouse behavioral profile. The videorecording (VIDEO) procedure is the one providing the most accurate picture of changes in locomotor/exploratory activity and drug effects thereon, also allowing a more comprehensive statistical analysis of the relationships between various types of response changes.

Inhibitory and stimulatory effects of morphine on locomotor activity in mice: biochemical and behavioral studies

A possible interaction between the opposite effect (inhibition and stimulation) of morphine on locomotor activity in mice and monoaminergic systems in the striatum was studied. Ten minutes after systemic administration, morphine at 1.25 mg/kg decreased locomotor activity and the levels of 3-methoxytyramine (3-MT), whereas at 20 mg/kg locomotor activity and 3-MT levels increased. At the same time, no change in the other monoamine metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels was observed. Sixty minutes after administration, morphine at 1.25 mg/kg did not induce any change in locomotor activity or in all the monoamine metabolite levels measured. On the other hand, morphine at 20 mg/kg maintained an initial increase in locomotor activity and increased not only 3-MT levels, but also other metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels. These results suggest that, at low dosages, the inhibitory effect of morphine on locomotor activity in mice may be related to a decrease of the presynaptic dopamine release in the striatum and that the stimulatory effect of morphine, at high dosages, may be related to an increase of the presynaptic dopamine release in the striatum.

Effects of methamphetamine and morphine on the vertical and horizontal motor activities in mice

The effects of methamphetamine and morphine on the vertical (VMA) and the horizontal motor activities (HMA) in male ddY mice (six weeks of age) was investigated between 9:00 and 13:00, using an apparatus which can differentiate spontaneous motor activity into VMA and HMA, measuring their activities simultaneously. VMA and HMA were evaluated by counting the number of times that an infrared ray was blocked by the mouse in the activity cage. Nine infrared photo-couplers were used to measure the VMA and one to measure the HMA. All measurements were taken at 10 min intervals during the 180 min period after subcutaneous injection of methamphetamine (0.1, 1 and 10 mg/kg) or morphine (2, 10 and 50 mg/kg). A small dose of methamphetamine (0.1 mg/kg) did not exert influence on the counts of the VMA and the HMA, whereas a large dose enhanced both activities, especially at 10 mg/kg, where each activity showed qualitatively different biphasic patterns. On the other hand, three doses of morphine significantly inhibited the VMA for 20 min after administration, while morphine at 2 mg/kg depressed the HMA for 10 min after administration and at 10 mg/kg or more markedly enhanced it during the 180 min observation period. These results show that different doses of methamphetamine and morphine exert different effects on the VMA and the HMA in mice.

A comparison of the antinociceptive and behavioral effects of D-Arg substituted dipeptides and tetrapeptides in mice

Intracerebroventricular administration of D-Arg substituted dipeptides, H-Tyr-D-Arg-OMe and H-Tyr(Et)-D-Arg-OMe, and D-Arg2 substituted N-terminal tetrapeptides of dermorphin, H-Tyr-D-Arg-Phe-Gly-OEt and H-Tyr(Et)-D-Arg-Phe-Gly-OEt resulted in dose-related and naloxone-reversible antinociceptive effects. Among them, tetrapeptides not only exhibited much more potent and prolonged activities than dipeptides but also were significantly antagonized even by a low dose of naloxone. Spontaneous motor activity was lowered by dipeptides throughout the observation period, which was scarcely antagonized by naloxone. Tetrapeptides elicited locomotor hyperactivity following an initial locomotor suppression. Only the locomotor hyperactivity was significantly antagonized by naloxone. These results suggest that tetrapeptides induce the effects via opioid receptors, whereas the effects of dipeptides are involved in various systems non-specifically.

Opiate effects on social investigatory behavior of male mice

The effects of morphine and naloxone, alone and in combination, on social investigatory behavior and motor activity was examined in CD-1 male mice. Tests were conducted in a Plexiglas apparatus in which a center area was separated from two adjacent stimulus compartments by wire mesh screens. One compartment housed a female conspecific while the other remained empty and served as a control for non-specific investigatory responses. A photocell bisected the center compartment and recorded motor activity. Male mice were placed individually into the center area and the time spent investigating each screen was recorded using contact circuits during the 15-min test. In Experiment 1, males (N = 11) received saline, 0.1, 1.0 and 10.0 mg/kg morphine sulfate IP 20 min prior to testing. The high dose significantly decreased investigation of the female compartment while investigation of the uninhibited chamber and motor activity were not significantly affected. In a second experiment (N = 16), 3, 10 and 30 mg/kg naloxone administered 30 min prior to testing had no significant effect on any of the measures recorded. In a third group of subjects (N = 16), 3 mg/kg naloxone reversed the decrease in female investigation time observed with 10 mg/kg morphine, indicating an opiate mechanism for these results. These data provide further evidence that an animal model can be used to study the disruption of socio-sexual behavior produced by opiates.

Behavioural effects of hashish in mice in comparison with other psychoactive drugs

1. The behavioural effects of hashish extract (10 mg delta 9-THC/kg) were compared to those of morphine (20 mg/kg), diazepam (10 mg/kg), imipramine (10/kg), amphetamine (10 mg/kg) and chlorpromazine (10 mg/kg) by testing male mice in a resident-intruder situation. 2. The drugs were given either to the resident or to the intruder male. 3. Hashish extract similar to diazepam and chlorpromazine reduced social activities in resident and intruder males. 4. Intruder males treated with hashish or diazepam were more frequently attacked and submission and light was increased. 5. Amphetamine and morphine stimulated locomotion and non-social activities but impaired social behaviour especially in residents. 6. Imipramine increased aggressive behaviour in resident and intruder males.

Nonspecific excitatory effects of morphine: reverse-order precipitated withdrawal and dose-dose interactions

We recently reported that, in naive mice pretreated with naloxone, morphine can cause withdrawal-like signs that seemingly are not mediated by "opiate" receptors. Such results were confirmed and extended here with another mouse strain. Repetitive vertical jumping could occur respective of injection sequence and depended on dose and dose ratio of the two drugs.

Biphasic effects of a potent enkephalin analogue (D-Met2,Pro5)-enkephalinamide and morphine on locomotor activity in mice

The effects of morphine and a potent enkephalin analogue on spontaneous locomotion have been compared in mice. In doses of 3-10 mg/kg SC both compounds induced a brief reduction of motor activity followed by a period of behavioral hyperactivity. Similar receptorial mechanisms are suggested in mediation of their motor effects.