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

Influence of G protein-biased agonists of μ-opioid receptor on addiction-related behaviors

Opioid analgesics remain a gold standard for the treatment of moderate to severe pain. However, their clinical utility is seriously limited by a range of adverse effects. Among them, their high-addictive potential appears as very important, especially in the context of the opioid epidemic. Therefore, the development of safer opioid analgesics with low abuse potential appears as a challenging problem for opioid research. Among the last few decades, different approaches to the discovery of novel opioid drugs have been assessed. One of the most promising is the development of G protein-biased opioid agonists, which can activate only selected intracellular signaling pathways. To date, discoveries of several biased agonists acting via μ-opioid receptor were reported. According to the experimental data, such ligands may be devoid of at least some of the opioid side effects, such as respiratory depression or constipation. Nevertheless, most data regarding the addictive properties of biased μ-opioid receptor agonists are inconsistent. A global problem connected with opioid abuse also requires the search for effective pharmacotherapy for opioid addiction, which is another potential application of biased compounds. This review discusses the state-of-the-art on addictive properties of G protein-biased μ-opioid receptor agonists as well as we analyze whether these compounds can diminish any symptoms of opioid addiction. Finally, we provide a critical view on recent data connected with biased signaling and its implications to in vivo manifestations of addiction.

Functional characterization of a novel opioid, PZM21, and its effects on the behavioural responses to morphine

BACKGROUND AND PURPOSE

The concept of opioid ligands biased towards the G protein pathway with minimal recruitment of β-arrestin-2 is a promising approach for the development of novel, efficient, and potentially nonaddictive opioid therapeutics. A recently discovered biased μ-opioid receptor agonist, PZM21, showed analgesic effects with reduced side effects. Here, we aimed to further investigate the behavioural and biochemical properties of PZM21.

EXPERIMENT APPROACH

We evaluated antinociceptive effects of systemic and intrathecal PZM21 administration. Its addiction-like properties were determined using several behavioural approaches: conditioned place preference, locomotor sensitization, precipitated withdrawal, and self-administration. Also, effects of PZM21 on morphine-induced antinociception, tolerance, and reward were assessed. Effects of PZM21 on striatal release of monoamines were evaluated using brain microdialysis.

KEY RESULTS

PZM21 caused long-lasting dose-dependent antinociception. It did not induce reward- and reinforcement-related behaviour; however, its repeated administration led to antinociceptive tolerance and naloxone-precipitated withdrawal symptoms. Pretreatment with PZM21 enhanced morphine-induced antinociception and attenuated the expression of morphine reward. In comparison to morphine, PZM21 administration induced a moderate release of dopamine and a robust release of 5-HT in the striatum.

CONCLUSIONS AND IMPLICATIONS

PZM21 exhibited antinociceptive efficacy, without rewarding or reinforcing properties. However, its clinical application may be restricted, as it induces tolerance and withdrawal symptoms. Notably, its ability to diminish morphine reward implies that PZM21 may be useful in treatment of opioid use disorders.

Effect of prenatal restraint stress and morphine co-administration on plasma vasopressin concentration and anxiety behaviors in adult rat offspring

Stressful events and exposure to opiates during gestation have important effects on the later mental health of the offspring. Anxiety is among the most common mental disorders. The present study aimed to identify effects of prenatal restraint stress and morphine co-administration on plasma vasopressin concentration (PVC) and anxiety behaviors in rats. Pregnant rats were divided into four groups (n = 6, each): saline, morphine, stress + saline and stress + morphine treatment. The stress procedure consisted of restraint twice per day, two hours per session, for three consecutive days starting on day 15 of pregnancy. Rats in the saline and morphine groups received either 0.9% saline or morphine intraperitoneally on the same days. In the morphine/saline + stress groups, rats were exposed to restraint stress and received either morphine or saline intraperitoneally. All offspring were tested in an elevated plus maze (EPM) on postnatal day 90 (n = 6, each sex), and anxiety behaviors of each rat were recorded. Finally, blood samples were collected to determine PVC. Prenatal morphine exposure reduced anxiety-like behaviors. Co-administration of prenatal stress and morphine increased locomotor activity (LA) and PVC. PVC was significantly lower in female offspring of the morphine and morphine + stress groups compared with males in the same group, but the opposite was seen in the saline + stress group. These data emphasize the impact of prenatal stress and morphine on fetal neuroendocrine development, with long-term changes in anxiety-like behaviors and vasopressin secretion. These changes are sex specific, indicating differential impact of prenatal stress and morphine on fetal neuroendocrine system development. Lay Summary Pregnant women are sometimes exposed to stressful and painful conditions which may lead to poor outcomes for offspring. Opiates may provide pain and stress relief to these mothers. In this study, we used an experimental model of maternal exposure to stress and morphine in pregnant rats. The findings indicated that maternal stress increased anxiety in offspring while morphine decreased such effects, but had negative effects on the levels of a hormone controlling blood pressure, and activity of offspring. Hence morphine should not be used in pregnancy for pain and stress relief.

Morphine efficacy is altered in conditional HIV-1 Tat transgenic mice

Opiate abuse reportedly can exaggerate complications of human immunodeficiency virus type-1 (HIV-1) infection in the central nervous system (CNS), while opiate drugs are often indicated in the treatment of HIV-1-related neuropathic pain. Despite this quandary, few studies have assessed the relationship between the duration or extent of HIV-1 infection and the intrinsic neurobehavioral responsiveness to opioids. To address this problem, doxycycline (DOX)-inducible HIV-Tat(1-86) transgenic mice were used as a model for HIV-1-associated neurocognitive disorders, which permitted the regulation of Tat exposure and duration. The effects of continuous Tat induction on the activity of morphine were examined at weekly intervals using standard behavioral assays for nociception and motor function. In the spinal cord, Tat mRNA levels did not increase until the second and third weeks following induction, which corresponded to a significant loss of morphine antinociception as assessed in the tail-flick test. Alternatively, in the striatum, sustained increases in Tat mRNA expression during the second week of induction coincided with significant decreases in rotarod performance and interactions with morphine. Importantly, the behavioral effects of morphine differed depending on the timing and location of Tat expression, with increases in Tat transcript levels in the spinal cord and striatum corresponding to significant alterations in morphine-dependent nociception and rotarod performance, respectively. Assuming Tat levels contribute to the clinical manifestations of HIV-1, the results suggest that regional differences in viral load and opioid phenotype might influence the nature and degree that opiate responsiveness is altered in HIV-1-infected individuals.

Histopathological and biochemical changes of morphine sulphate administration on the cerebellum of albino rats

In this study the long-term effects of morphine sulphate treatment (MST) on histopathological and biochemical changes in the cerebellum was assessed in albino rats. Normal saline (5ml) was given orally as placebo in the control group (n=25). Morphine groups received morphine orally at a dose level of 5mg/kg body weight day after day for 10, 20 and 30 days (n=25/group). Light microscopy revealed that the molecular layer showed vacuolation. The Purkinje cells lost their specific shaped appearance, decreased in size and numbers. The granular cells highly degenerated. Electron microscopy revealed fragmentation of the cisterns of the both types of endoplasmic reticulum, resulted in a progressive depletion of total protein contents as well as general carbohydrates in all treated groups as supported by histochemical observation. Obvious destruction of mitochondrial inner membrane and cristae mediate cell death. Also, abnormal nucleus with deformed perforated nuclear membrane and deformation of the plasma membrane with degeneration of the synapses could interpreted as a sign of necrosis. Biochemical analysis revealed that dopamine (DA) and norepinephrine (NE) were significantly decreased in four brain areas (cortex striatum, thalamus/hypothalamus, and cerebellum). In contrast, serotonin (5-HT) level was increased in these brain regions; with an exception of 5-HT on day 10 and neurotransmitter levels in the pons were unaffected. The quantitative analysis showed a significant decrease (P<0.05) in the diameter of Purkinje cells and in the thickness of both molecular and granular layers treated groups. Morphine sulphate induces may be a cell death or necrosis in the rat cerebellum and modulating neurotransmitter system. Our findings pointed out the risk of increased cerebellum damage due to long-term of morphine use.

Enhancing effects of morphine on methamphetamine-induced reinforcing behavior and its association with dopamine release and metabolism in mice

Polydrug abuse has become a significant problem worldwide, and the combined use of methamphetamine (MA) and morphine (M) is now highly prevalent among addicts. In the present study, we investigated the neurobehavioral effects of repeated treatment regimens of these drugs (i.p. administration of 0.75 mg/kg/day MA, 5 mg/kg/day M, and their combination for five consecutive days followed by once weekly for five consecutive weeks) in mice. In addition, we used an in vivo microdialysis technique to study the changes in extracellular concentrations of dopamine (DA) and its metabolites in the mouse striatum after challenge administration of these drugs. The results showed that systemic M increased MA-induced conditioned place preference (CPP), as revealed by higher CPP values which were also maintained for a longer duration compared with those induced by an identical dose of MA or M alone. Subsequent to challenge with combined MA and M, mice exhibited an increase in stereotyped behavior, which appeared to be associated with an elevation of extracellular concentration of DA in the striatum. Our findings suggest that M not only produces synergistic effects on MA-induced CPP, but also interacts with MA to induce stereotyped behavioral sensitization which is mediated by an increase in DA outflow in the striatum. These findings provide insight into the behavioral and neurochemical basis responsible for the combined abuse liability of MA and M.

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.

Methamphetamine-elicited alterations of dopamine- and serotonin-metabolite levels within mu-opioid receptor knockout mice: a microdialysis study

mu-Opioid receptors (mu-ORs) modulate methamphetamine (MA)-induced behavioral responses, increased locomotor activity and stereotyped behavior in the mouse model. We investigated the changes in dopamine (DA) and serotonin (5-HT) metabolism in the striatum following either acute or repeated MA treatment using in vivo microdialysis. We also studied the role of mu-ORs in the modulation of MA-induced DA and 5-HT metabolism within mu-OR knockout mice. Subsequent to either acute or repeated intraperitoneal administration of MA, wild-type mice revealed decreases in extracellular concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in a dose-dependent manner. Moreover, wild-type mice had reductions in basal concentrations of DOPAC and HVA following repeated MA treatment with a higher dose. The effects of acute, repeated or challenge MA administration upon extracellular levels of DOPAC and HVA within mu-OR knockout mice significantly differed from the wild-type controls. The duration of recovery to the basal levels of extracellular DA and 5-HT metabolites induced by MA were much longer in wild-type mice than for mu-OR knockout mice. These findings suggest that mu-ORs play a modulatory role in MA-induced DA and 5-HT metabolism in the mouse striatum. This possible mechanism of MA-induced behavioral change as modulated by mu-OR merits further study.

Comparison of opioid receptor binding in horse, guinea pig, and rat cerebral cortex and cerebellum

OBJECTIVE

To compare the density and binding characteristics of opioid receptor subtypes in horse, rat, and guinea pig cerebral cortex and cerebellum.

STUDY DESIGN

Prospective receptor binding study.

ANIMALS

Whole brains were obtained from four neurologically normal adult horses during necropsy. Rat and guinea pig brains were obtained commercially.

METHODS

The cerebellum and cerebral cortex were dissected from each brain, and tissue homogenates prepared. A radioligand binding technique with the highly selective ligands [(3)H]-DAMGO, [(3)H]-U69593, and [(3)H]-DPDPE was used to identify the mu- (mu), kappa- (kappa) and delta- (delta) opioid receptors, respectively. Competitive binding assays were performed with these ligands and varying concentrations of one of multiple unlabeled ligands.

RESULTS

While there were marked species differences in relative densities of opioid receptors, all radioligands interacted with their binding sites with high, nanomolar affinity in both the cerebral cortex and cerebellum. In the horse cerebral cortex, the percentages of total opioid binding sites for the mu-, kappa- and delta-receptors were 71%, 14% and 15%, respectively. In the rat and guinea pig cerebral cortex, the corresponding values were 56% mu-, 4% kappa- and 40% delta-receptors, and 25% mu-, 37% kappa- and 38% delta-receptors, respectively. In horse and guinea pig cerebellum, the binding was 37% mu-, 59% kappa- and 4% delta-receptors, and 15% mu-, 76% kappa- and 10% delta-receptors, respectively. For competitive analysis, all competitors of the mu-, kappa- and delta-receptors completely displaced [(3)H]-DAMGO, [(3)H]-U69593, and [(3)H]-DPDPE and had inhibitory constants in the nanomolar range.

CONCLUSION AND CLINICAL RELEVANCE

Horses used in this study had a greater density of mu-receptors in the cerebral cortex compared with rats and guinea pigs but without further characterization of the functional role of these receptors it is impossible to determine the clinical significance of these data.

Effects of morphine on the plus-maze discriminative avoidance task: role of state-dependent learning

RATIONALE

The amnesic effects of morphine may be related to its action on nociception, anxiety, or locomotion. This effect is also suggested to be related to state dependency.

OBJECTIVES

The aims of this study were to verify the effects of morphine on mice tested in the plus-maze discriminative avoidance task (DAT) that uses light and noise as aversive stimuli and allows the concomitant evaluation of learning, memory, anxiety, and locomotion and also to verify the possible role of state-dependent learning in the effects of morphine.

METHODS AND RESULTS

The DAT was conducted in a modified elevated plus-maze. In the training, the aversive stimuli were applied when mice entered in one of the enclosed arms, whereas in the test, no stimuli were applied. The main results showed that (1) pretraining morphine (5-20 mg/kg i.p.) induced retrieval deficits (evaluated by the time spent in the aversive arm in the test) but not acquisition deficits (evaluated by the decrease in aversive arm exploration along the training); (2) pretest morphine (5-10 but not 20 mg/kg) counteracted this deficit; (3) morphine induced hypolocomotion (decreased number of entries in the arms), irrespective of memory alterations; and (4) morphine did not alter anxiety-like behavior (evaluated by the time spent in the open arms) during the training.

CONCLUSIONS

Morphine given before training induces retrieval deficits in mice tested in the DAT, and these deficits could be related to morphine-induced state-dependent learning. Neither the memory deficit induced by pretraining morphine nor the reversal of this deficit by pretest morphine seems to be related to anxiety levels or locomotor alterations.

The involvement of dopamine and nitric oxide in the endocrine and behavioural action of endomorphin-1

Previous publications have demonstrated a prominent central and corticotropin releasing hormone-mediated action of the endomorphins (EMs) on both open-field behaviour and the hypothalamo-pituitary-adrenal (HPA) axis. In the present experiments, the direct action of endomorphin-1 (EM1) on pituitary adrenocorticotropic hormone (ACTH) release, adrenal corticosterone secretion and the roles of nitric oxide (NO) and dopamine (DA) in the HPA and behavioural responses elicited by EM1 were investigated in mice. In vitro perifusion studies indicated that the action of EM1 on the HPA system appears to be confined to the hypothalamus, as EM1 did not influence the corticosterone secretion from adrenal slices and moderately attenuated the ACTH release from anterior pituitary slices. In in vivo experiments, NG-nitro-L-arginine (L-NNArg) pretreatment brought about a profound inhibition of both the endocrine and the behavioural responses. On the other hand, haloperidol completely abolished the increases in square crossing and rearing, without affecting corticosterone release. The direct action of EM1 on striatal DA release was therefore also investigated in an in vitro superfusion system. Although EM1 did not influence the basal release of tritiated DA, it significantly enhanced the transmitter release evoked by electric impulses and pretreatment with L-NNArg resulted in a considerable inhibition of the release elicited by EM1. In conclusion, our endocrine studies suggest an important role of NO in the mediation of the EM1-evoked corticosterone secretion. They also indicate that EM1 activates the HPA axis at a hypothalamic level and dopamine is not involved in this process. In contrast, the behavioural experiments reflect that the locomotor activation induced by EM1 is mediated by NO and dopamine, and the superfusion studies demonstrate that NO transmits the dopamine release enhancing effect of EM1.

Alterations within the endogenous opioid system in mice with targeted deletion of the neutral endopeptidase ('enkephalinase') gene

The biological inactivation of enkephalins by neutral endopeptidase (enkephalinase, NEP, EC3.4.24.11) represents a major mechanism for the termination of enkephalinergic signalling in brain. A pharmacological blockade of NEP-activity enhances extracellular enkephalin concentrations and induces opioid-dependent analgesia. Recently, knockout mice lacking the enzyme NEP have been developed [Lu et al., J. Exp. Med. 1995;181:2271-2275]. The present study investigates the functional consequences and biochemical compensatory strategies of a systemic elimination of NEP activity in these knockout mice. Using biochemical and behavioural tests we found that the lack of NEP activity in brain is not compensated by enhanced activities of alternative enkephalin-degrading enzymes. Also no change in enkephalin biosynthesis was detectable by in situ methods quantifying striatal proenkephalin-mRNA levels in NEP-deficient mice compared with wildtype. Only a 21% reduction of mu receptor density in crude brain homogenates of NEP knockout mice was observed, while delta- and kappa-opioid receptor densities were unchanged. This receptor downregulation was also confirmed functionally in the hot-plate paradigm. NEP knockouts developed normally, but showed enhanced aggressive behaviour in the resident-intruder paradigm, and altered locomotor activity as assessed in the photobeam system. Thus, although NEP plays a substantial role in enkephalinergic neurotransmission, the biochemical adaptations within the opioid system of NEP-deficient mice are of only modest nature.

Ultra-wideband electromagnetic pulses and morphine-induced changes in nociception and activity in mice

Mice were exposed to ultra-wideband (UWB) electromagnetic pulses averaging 99-105 kV/m peak amplitude, 0.97-1.03 ns duration, and 155-174 ps rise time, after intraperitoneal administration of saline or morphine sulfate. They were then tested for thermal nociception on a 50 degrees C surface and for spontaneous locomotor activity and its time profile over 5 min. Analysis of results showed no effect of UWB exposure on nociception and activity measures in CF-1 mice after 15-, 30-, or 45-min exposure to pulses at 600/s or after 30-min exposure to UWB pulses at 60/s. Similarly, no effect was seen in C57BL/6 mice after 30-min exposure to pulses at 60/s or 600/s. Although trends in morphine-modified measures seen with UWB pulse repetition frequency could be expected because of increased levels of low-frequency energy, no significant change was seen in normal or morphine-modified nociception or activity after UWB exposure. This indicated lack of effect of the UWB pulses used in these experiments on nervous system components, including endogenous opioids, involved in these behaviors.

Effects of naloxone on amphetamine induced striatal dopamine release in vivo: a microdialysis study

The opiate antagonist naloxone (NX) alters amphetamine (AMPH) induced behaviors including locomotor activity, rearing and stereotypy. However, the exact nature of the NX induced alteration of AMPH induced behaviors is controversial, with some studies using high (5-40 mg/kg) doses of NX reporting an inhibition, and others using low (< or = 1-2 mg/kg) doses observing a potentiation. As these behaviors are mediated by AMPH induced dopamine (DA) release, the effect of NX on the latter was examined by microdialysis in an effort to resolve the controversy. Saline and NX pretreated groups subsequently administered AMPH were compared in vivo across nine separate 10 min intervals as well as by grouped intervals. NX alone (0.8 mg/kg) and saline exerted statistically equivalent effects on striatal DA release with the exception of the fifth interval, where a small but significant increase was seen after NX. On the other hand, the same dose of NX significantly enhanced AMPH induced striatal DA release relative to saline pretreated animals during each of four separate intervals, from 30 to 70 minutes following AMPH (1.5 mg/kg), and across all nine intervals combined. NX pretreatment (0.8 mg/kg) followed by a higher dose of AMPH (3.0 mg/kg) produced a significantly greater cumulative effect on DA release than saline pretreatment over the last six combined intervals (30-90 min) and over two grouped intervals (30-50 min and 40-60 min inclusive). However, a comparison of single rather than paired or grouped intervals revealed no significant differences. Previous studies have also examined the effect of NX on AMPH induced striatal DA release using in vivo microdialysis. However, the doses used were invariably high (5 mg/kg) and the results on striatal DA release always inhibitory. The present results suggest that NX potentiates AMPH induced striatal DA release when lower doses of NX are used. These results combined with those of previous studies also suggest that NX exerts a biphasic effect on AMPH induced DA release, with lower doses potentiating release and higher doses inhibiting release. This is close agreement with behavioral observations and may be due to the effect of low versus high doses of NX on intraterminal calcium influx.

Effect of isolation on morphine-induced running and changes in body temperature

1. The influence of isolation of three durations 8, 15 and 30 days has been examined in mice on the effects of morphine on rectal temperature and on locomotor activity. Isolated mice were compared to non isolated mice with the same age. 2. Morphine (20 mg/kg ip) induced in mice an early hypothermia followed by a late hyperthermia. The hypothermic effect was significantly reduced following isolation, but the duration of isolation (8, 15, 30 days) had no influence. Isolation did not modify the hyperthermic effect of morphine. 3. Morphine (40 mg/kg ip) induced in mice an increase in locomotor activity called "running". The running activity was significantly increased following isolation. The duration of isolation (8, 15, 30 days) did not seem to influence this effect. 4. These results show that isolation does not modify in the same way every effects of morphine, they suggest that isolation alters the mechanism involved in the running activity and in the hypothermic effect. The nature of these mechanisms is discussed.

Regional differences in cerebral noradrenaline turnover in mice withdrawn from repeated morphine treatment and tolerance to the effects of acute morphine

The effects of morphine withdrawal and challenge doses (10 or 30 mg/kg) on the alpha-methyl-p-tyrosine (alpha MT)-induced noradrenaline (NA) depletion as well as on the free 3-methoxy-4-hydroxyphenylethylene glycol (MOPEG) concentration were studied in various brain areas of NMRI mice. Morphine was given subcutaneously 3 times daily for 5 days followed by 1 or 3 days' withdrawal. In morphine withdrawn mice the alpha MT-induced NA depletion and the free MOPEG concentrations were differentially altered. At 1-day withdrawal the alpha MT-induced NA depletion was retarded and the NA concentration was elevated in the forebrain area indicating reduced release of NA. Simultaneously, however, the free MOPEG concentration was significantly elevated in the forebrain area and in the lower brain stem suggesting enhanced NA turnover. No withdrawal-induced changes were found in the hypothalamic NA turnover. Acute morphine elevated the free MOPEG concentration and accelerated the alpha MT-induced NA depletion in all brain areas of control mice but not in mice withdrawn for 1 day from repeated morphine treatment. At 3 days' withdrawal, however, the 30 mg/kg morphine dose slightly accelerated the NA depletion in the forebrain area. These results show that morphine withdrawal differentially alters the alpha MT-induced NA depletion and the free MOPEG concentration in various mouse brain areas. These effects are relatively modest suggesting that in mice the noradrenergic mechanisms play a minor role in morphine withdrawal syndrome. However, in all brain areas of the morphine-withdrawn mice tolerance was found towards the NA turnover and release accelerating effect of acute morphine.

Effects of naloxone on the behaviors evoked by amphetamine and apomorphine in adult cats

1. This work was undertaken in order to study whether the opioid system is involved in the modulation of the behaviors induced by two agonists of the dopaminergic system, amphetamine and apomorphine in adult cats. 2. Naloxone, an antagonist of the mu, delta and kappa opioid receptors was administered to twelve female mongrel cats; 0.5, 1.0 and 2.0 mg/kg s.c. were injected in order to analyse its own effect of naloxone. This drug produced NREMs behavior and accordingly the cat showed an overall decrease of its activities. 3. Amphetamine (2.5 mg/kg s.c.) and apomorphine (2.0 mg/kg s.c.) were injected before and after naloxone administration (2.0 mg/kg s.c.), in separate sessions. 4. The behaviors recorded were compared. Some of the behaviors showed modifications both with amphetamine (inappetence was increased and locomotion decreased) and apomorphine (indifference and inappetence increased; locomotion and olfaction decreased). 5. These changes were considered as consequence of the NREMs behavior induced by naloxone and not as a result of a modulation by the opioid system of the activation of the dopaminergic system elicited by amphetamine and apomorphine. Regarding the mechanism of NREMs induced by naloxone probably the dopaminergic, noradrenergic and GABAergic systems may be involved.

Modification of morphine-induced locomotor activity by pertussis toxin: biochemical and behavioral studies in mice

The effect of pertussis toxin (PTX) on the locomotor-enhancing action of systemic and intracerebroventricular (i.c.v.) morphine was investigated in mice. Mice were i.c.v. injected with either PTX (0.25 and 0.5 micrograms) or saline as a control. The s.c. (5-20 mg/kg) and i.c.v. (7-30 nmol) administration of morphine produced a dose-related locomotor-enhancing action in control mice. The peak effect of morphine (30 nmol, i.c.v.)-induced hyperlocomotion was observed 90 min after the morphine injection. At the same time, morphine significantly increased dopamine (DA) metabolism in the limbic forebrain (nucleus accumbens and olfactory tubercle). Similarly, the selective mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO, 4 nmol, i.c.v.) also significantly increased locomotor activity and DA metabolism in the limbic forebrain. Both morphine- and DAGO-induced hyperlocomotion and elevation of DA turnover were antagonized by pretreatment with the mu antagonist beta-funaltrexamine (beta-FNA). These results suggest that the locomotor-enhancing action of morphine results from the activation of central mu-opioid receptors, and that the activation of the mesolimbic DA system may be involved in the expression of morphine-induced hyperlocomotion in mice. Furthermore, pretreatment with PTX (0.5 micrograms, i.c.v., 6 days prior to the testing) significantly reduced hyperlocomotion and elevation of DA turnover in the limbic forebrain which had been induced by administrations of morphine (30 nmol, i.c.v.) and DAGO (4 nmol, i.c.v.). These findings suggest that the central PTX-sensitive GTP-binding protein (G-protein) mechanism may play an important role in opioids-induced locomotor-enhancing action. Furthermore, the activation of mesolimbic DA transmission by mu-opioid agonists may also be mediated by a PTX-sensitive G-protein mechanism in mice.

Gliquidone, an ATP-dependent K+ channel antagonist, antagonizes morphine-induced hypermotility

The effect of gliquidone, an ATP-dependent K+ (KATP) channel blocker, on morphine-induced hypermotility in mice was studied. Morphine (5-40 mg/kg s.c.) dose dependently increased ambulatory activity. Gliquidone (10 micrograms/mouse i.c.v.) induced a parallel displacement to the right of the morphine dose-response curve. Moreover, gliquidone (10 and 40 micrograms/mouse i.c.v.) produced a dose-dependent antagonism of morphine (20 mg/kg s.c.)-induced hypermotility. These results suggest that KATP channels are involved in morphine-induced hypermotility. The present data, together with those of previous studies showing antagonism by KATP channel blockers of morphine-induced antinociception and hyperthermia, further indicate that the opening of KATP channels plays an important role in the mechanism of action of morphine.

Effects of intracerebroventricularly administered mu-, delta- and kappa-opioid agonists on locomotor activity of the guinea pig and the pharmacology of the locomotor response to U50,488H

The effects of intracerebroventricular administration of morphine, the selective mu-agonist DAMGO, the delta-agonist DPDPE, the kappa-preferring peptide dynorphin A(1-13) and the kappa-agonist U50,488H on locomotor behaviour in the guinea pig were investigated. Morphine (total dose = 0.01, 0.1, 1, 10, 200 nmol), DAMGO and DPDPE (total dose = 0.1, 1, 10, 100 nmol of each) produced piloerection and sedation, indicating that the responses of guinea pigs to mu- and delta-opioid agonists differed from those of rats and mice. In contrast, U50,488H (total dose = 10, 100 nmol) and dynorphin A(1-13) (total dose = 100 nmol) produced increased locomotor activity which was attenuated by pretreatment with naloxone and norbinaltorphimine, thus confirming the involvement of kappa-opioid receptors. Furthermore, pretreatment with spantide, baclofen, muscimol, bicuculline, MK-801, raclopride and atropine also inhibited the U50,488H-induced locomotor activity, suggesting the involvement of GABA, dopamine, excitatory amino acids, substance P and acetylcholine in this response.

Locomotor responses to benzodiazepines, barbiturates and ethanol in diazepam-sensitive (DS) and -resistant (DR) mice

Diazepam-sensitive (DS) and -resistant (DR) mice were selectively bred for increased and reduced sensitivity to the ataxic effects of diazepam (40 mg/kg). Other response differences between DS and DR mice may reflect pleiotropic effects of the genes fixed during their selection. These mice were tested for their sensitivity to the locomotor stimulant effects of several doses of diazepam, flunitrazepam, pentobarbital, phenobarbital, and ethanol. DR mice were more sensitive than DS mice to the locomotor stimulant effects of all drugs except phenobarbital. These results largely support the hypothesis that a common biological mechanism mediates sensitivity to the stimulant effects of sedative-hypnotic drugs. Receptor mediation of the benzodiazepine effects was examined by administering the benzodiazepine receptor antagonist, RO15-1788. Locomotor depression produced by diazepam and flunitrazepam in DS mice was blocked by RO15-1788. However, while the locomotor stimulation produced by diazepam in DR mice was antagonized, the stimulant effect of flunitrazepam was not. This suggests that binding of flunitrazepam to the GABAA-benzodiazepine receptor is not necessary for production of locomotor stimulation.

Antagonism of the morphine-induced locomotor activation of mice by fructose: comparison with other opiates and sugars, and sugar effects on brain morphine

The mouse locomotor activation test of opiate action in a 2+2 dose parallel line assay was used in a repeated testing paradigm to determine the test, opiate and hexose specificities of a previously reported antagonism of morphine-induced antinocociception by hyperglycemia. In opiate specificity studies, fructose (5 g/kg, i.p.) significantly reduced the potency ratio for morphine and methadone, but not for levorphanol, meperidine or phenazocine when intragroup comparisons were made. In intergroup comparisons, fructose significantly reduced the potencies of levorphanol and phenazocine, but not methadone or meperidine. In hexose/polyol specificity studies, tagatose and fructose significantly reduced the potency ratio for morphine, whereas glucose, galactose, mannose and the polyols, sorbitol and xylitol, caused no significant decrease in potency. Fructose, tagatose, glucose and mannose (5 g/kg, i.p.) were tested for effects on brain morphine levels 30 min after morphine (60 min after sugar), and all four sugars significantly increased brain morphine relative to saline-pretreated controls. It is concluded that the antagonism of morphine by acute sugar administration shows specificity for certain sugars and occurs despite sugar-induced increases in the distribution of morphine to the brain. Furthermore, the effects of fructose show an opiate specificity similar to that of glucose on antinociception observed previously in our laboratory, except that methadone was also significantly inhibited in the present study, when a repeated-testing experimental design was used.

Dexamethasone induces biphasic effect on morphine hypermotility in mice: a dose-related phenomenon

The present study examined interaction between dexamethasone (DEX) and morphine on the locomotor activity in groups of mice by using the activity cage test. Morphine administration (30-75-150 mg/kg, ip) induced a dose-related increase of the locomotor activity of mice, whereas DEX per se (0.1-1.0-10 mg/kg, ip) did not modify the activity of control mice. Pretreatment of mice with DEX 0.1 mg did not alter the hyperactivity produced by the three doses of morphine. In contrast, DEX administered at 1.0 mg reduced the morphine effects on locomotor activity, whereas DEX at 10 mg potentiated the morphine hypermotility. Our results suggest that DEX may play an important regulatory role on the central effects of morphine through a differential modulation of brain excitability systems.