Attenuation of morphine tolerance and dependence in scopolamine-treated rats

The Cholinergic System as a Treatment Target for Opioid Use Disorder

Opioid overdoses recently became the leading cause of accidental death in the US, marking an increase in the severity of the opioid use disorder (OUD) epidemic that is impacting global health. Current treatment protocols for OUD are limited to opioid medications, including methadone, buprenorphine, and naltrexone. While these medications are effective in many cases, new treatments are required to more effectively address the rising societal and interpersonal costs associated with OUD. In this article, we review the opioid and cholinergic systems, and examine the potential of acetylcholine (ACh) as a treatment target for OUD. The cholinergic system includes enzymes that synthesize and degrade ACh and receptors that mediate the effects of ACh. ACh is involved in many central nervous system functions that are critical to the development and maintenance of OUD, such as reward and cognition. Medications that target the cholinergic system have been approved for the treatment of Alzheimer's disease, tobacco use disorder, and nausea. Clinical and preclinical studies suggest that medications such as cholinesterase inhibitors and scopolamine, which target components of the cholinergic system, show promise for the treatment of OUD and further investigations are warranted.

Cholinergic activation affects the acute and chronic antinociceptive effects of morphine

Current studies indicate that the cholinergic and opioid systems interact to modulate pain. In the present work, we investigated the influence of the cholinesterase inhibitors, donepezil (0.5; 1 or 3mg/kg, i.p.) and rivastigmine (0.03; 0.5 or 1mg/kg, i.p.), on the acute antinociceptive effects of morphine (5mg/kg, i.p.) in the hot plate test in mice. Herein, both inhibitors were found to enhance and prolong the analgesic effects of morphine without affecting latencies themselves. In an extension of this work, we determined which cholinergic receptors subtype mediates the enhancement of analgesic effects of morphine, following inhibition of cholinesterases. In this part of the study, scopolamine (0.5mg/kg, i.p.), a muscarinic cholinergic receptors antagonist, but not mecamylamine (3mg/kg, i.p.), a nicotinic cholinergic receptors antagonist, reversed the enhancing effects of donepezil (3mg/kg, i.p.) and rivastigmine (1mg/kg, i.p.) on the morphine antinociception. Moreover, both cholinesterase inhibitors attenuated the development of tolerance to the antinociceptive effects of morphine. In contrast, acute administration of donepezil (3mg/kg, i.p.) or rivastigmine (1mg/kg, i.p.) on the day of expression of tolerance, had no effect on the already developed morphine tolerance. What is more, in both set of experiments, rivastigmine was slightly more potent than donepezil due to the broader inhibitory spectrum of this drug on acetylcholine degradation. Thus, our results suggest that the cholinesterase inhibitors, donepezil and rivastigmine, may be administered with morphine in order to enhance the latter's analgesic effects for the treatment of acute and chronic pain.

Aβ damages learning and memory in Alzheimer's disease rats with kidney-yang deficiency

Previous studies demonstrated that Alzheimer's disease was considered as the consequence produced by deficiency of Kidney essence. However, the mechanism underlying the symptoms also remains elusive. Here we report that spatial learning and memory, escape, and swimming capacities were damaged significantly in Kidney-yang deficiency rats. Indeed, both hippocampal Aβ(40) and 42 increases in Kidney-yang deficiency contribute to the learning and memory impairments. Specifically, damage of synaptic plasticity is involved in the learning and memory impairment of Kidney-yang deficiency rats. We determined that the learning and memory damage in Kidney-yang deficiency due to synaptic plasticity impairment and increases of Aβ(40) and 42 was not caused via NMDA receptor internalization induced by Aβ increase. β-Adrenergic receptor agonist can rescue the impaired long-term potential (LTP) in Kidney-yang rats. Taken together, our results suggest that spatial learning and memory inhibited in Kidney-yang deficiency might be induced by Aβ increase and the decrease of β(2) receptor function in glia.

Effects of Brugmansia arborea Extract and Its Secondary Metabolites on Morphine Tolerance and Dependence in Mice

The aim of the present study was to investigate, in vivo, the effect of a Brugmansia arborea extract (BRU), chromatographic fractions (FA and FNA), and isolated alkaloids on the expression and the acquisition of morphine tolerance and dependence. Substances were acutely (for expression) or repeatedly (for acquisition) administered in mice treated with morphine twice daily for 5 or 6 days, in order to make them tolerant or dependent. Morphine tolerance was assessed using the tail-flick test at 1st and 5th days. Morphine dependence was evaluated through the manifestation of withdrawal symptoms induced by naloxone injection at 6th day. Results showed that BRU significantly reduced the expression of morphine tolerance, while it was ineffective to modulate its acquisition. Chromatographic fractions and pure alkaloids failed to reduce morphine tolerance. Conversely BRU, FA, and pure alkaloids administrations significantly attenuated both development and expression of morphine dependence. These data suggest that Brugmansia arborea Lagerh might have human therapeutic potential for treatment of opioid addiction.

Interactions between morphine, scopolamine and nicotine: schedule-controlled responding in rats

Functional interactions between drugs acting on either opioid or cholinergic systems have been demonstrated for both neurochemical and behavioral measures. This study used schedule-controlled responding and isobolographic analyses to examine interactions between the micro opioid receptor agonist morphine and the muscarinic acetylcholine receptor antagonist scopolamine as well as the nicotinic acetylcholine receptor agonist nicotine. In 8 rats responding under a fixed ratio 5 schedule of food presentation, morphine (3.2-10mg/kg), scopolamine (0.032-1.0mg/kg), and nicotine (0.1-1mg/kg) each dose-dependently decreased responding. Acute injection of scopolamine shifted the morphine dose-response curved leftward and downward and acute injection of morphine shifted the scopolamine and nicotine dose-response curves leftward and downward. The interaction between morphine and nicotine was additive; however, the interaction between morphine and scopolamine was infra-additive or supra-additive, depending on whether scopolamine or morphine was administered first. These results provide quantitative evidence regarding potentially important interactions between drugs acting on either opioid or cholinergic systems, although these interactions are modest and appear to depend on the specific conditions of drug administration.

Ginsenoside Rg1 restores the impairment of learning induced by chronic morphine administration in rats

Rg1, as a ginsenoside extracted from Panax ginseng, could ameliorate spatial learning impairment. Previous studies have demonstrated that Rg1 might be a useful agent for the prevention and treatment of the adverse effects of morphine. The aim of this study was to investigate the effect of Rg1 on learning impairment by chronic morphine administration and the mechanism responsible for this effect. Male rats were subcutaneously injected with morphine (10 mg/kg) twice a day at 12 hour intervals for 10 days, and Rg1 (30 mg/kg) was intraperitoneally injected 2 hours after the second injection of morphine once a day for 10 days. Spatial learning capacity was assessed in the Morris water maze. The results showed that rats treated with Morphine/Rg1 decreased escape latency and increased the time spent in platform quadrant and entering frequency. By implantation of electrodes and electrophysiological recording in vivo, the results showed that Rg1 restored the long-term potentiation (LTP) impaired by morphine in both freely moving and anaesthetised rats. The electrophysiological recording in vitro showed that Rg1 restored the LTP in slices from the rats treated with morphine, but not changed LTP in the slices from normal saline- or morphine/Rg1-treated rats; this restoration could be inhibited by N-methyl-D-aspartate (NMDA) receptor antagonist MK801. We conclude that Rg1 may significantly improve the spatial learning capacity impaired by chonic morphine administration and restore the morphine-inhibited LTP. This effect is NMDA receptor dependent.

Combined scopolamine and ethanol treatment results in a locomotor stimulant response suggestive of synergism that is not blocked by dopamine receptor antagonists

BACKGROUND

Muscarinic acetylcholine receptors (mAChRs) are well positioned to mediate ethanol's stimulant effects. To investigate this possibility, we examined the effects of scopolamine, a receptor subtype nonselective mAChR antagonist, on ethanol-induced stimulation in genotypes highly sensitive to this effect of ethanol. We also investigated whether the dopamine D1-like receptor antagonist, SCH-23390 or the dopamine D2-like receptor antagonist, haloperidol, could block the extreme stimulant response found following co-administration of scopolamine and ethanol.

METHODS

Scopolamine (0, 0.0625, 0.125, 0.25, or 0.5 mg/kg) was given 10 minutes prior to saline or ethanol (0.75 to 2 g/kg) to female FAST (Experiment I) or DBA/2J (Experiment II) mice that were then tested for locomotion for 30 minutes. In Experiments III and IV, respectively, SCH-23390 (0, 0.015, or 0.03 mg/kg) was given 10 minutes prior, and haloperidol (0, 0.08, or 0.16 mg/kg) was given 2 minutes prior, to scopolamine (0 or 0.5 mg/kg), followed 10 minutes later by saline or ethanol (1.5 g/kg) and female DBA/2J mice were tested for locomotion for 30 minutes.

RESULTS

FAST and DBA/2J mice displayed a robust enhancement of the locomotor effects of ethanol following pretreatment with scopolamine that was suggestive of synergism. SCH-23390 had no effect on the response to the scopolamine + ethanol drug combination, nor did it attenuate ethanol- or scopolamine-induced locomotor activity. Haloperidol, while attenuating the effects of ethanol, was not able to block the effects of scopolamine or the robust response to the scopolamine-ethanol drug combination.

CONCLUSIONS

These results suggest that while muscarinic receptor antagonism robustly enhances acute locomotor stimulation to ethanol, dopamine receptors are not involved in the super-additive interaction of scopolamine and ethanol treatment. They also suggest that in addition to cautions regarding the use of alcohol when scopolamine is clinically prescribed due to enhanced sedative effects, enhanced stimulation may also be a concern.

Effects of scopolamine and ketamine on reconsolidation of morphine conditioned place preference in rats

Persistent memory associated with addictive drugs contributes to the relapse of drug abuse. The current study was conducted to examine the effects of scopolamine and ketamine on reconsolidation of morphine-induced conditioned place preference (CPP). In experiment 1, after morphine CPP was acquired, rats were injected with ketamine (60 mg/kg, intraperitoneally) and scopolamine (2 mg/kg, intraperitoneally), respectively, after reexposure to an earlier morphine-paired context or in their home cages. The CPP was reassessed 24 and 48 h after reexposure. An additional group of rats received saline following reexposure to the earlier morphine-paired context. In experiment 2, two groups of rats were only given saline during the CPP training and subsequent administration of ketamine or scopolamine during the reexposure. In experiment 1, rats failed to exhibit morphine CPP when ketamine and scopolamine were administered only after reexposure to a morphine-paired context. CPP was not abolished by ketamine or scopolamine administration in the animals' home cages. Also, the animals receiving only saline injections showed strong morphine CPP 24 h after a short exposure to the morphine-paired context. In experiment 2, ketamine or scopolamine treatment alone did not induce CPP or aversion. Administration of scopolamine and ketamine, after reexposure to a drug-paired context, resulted in the disruption of morphine CPP, suggesting the potential effects of scopolamine and ketamine in disrupting memory associated with environmental cues and addictive drugs.

Antagonism of muscarinic M1 receptors by dicyclomine inhibits the consolidation of morphine-associated contextual memory

M1 muscarinic receptor has been shown to be involved in cognitive functions of the brain. Conditioned place preference (CPP) paradigm involves memory for the association between environmental stimuli and the rewarding properties produced by a treatment. Using a balanced CPP design, we studied the possible involvement of M1 muscarinic receptors on the acquisition, expression and consolidation of morphine place conditioning in male mice. Subcutaneous administration of morphine sulphate-induced CPP in a dose-dependent manner. Using a 6-day schedule of conditioning, it was found that dicyclomine, an M1 muscarinic antagonist, significantly reduced the time spent by mice in the morphine compartment when given immediately, but not 6h, after each conditioning session (consolidation). It had no effect when administered 30 min before each conditioning session during CPP training period (acquisition) or 30 min before testing for place preference in the absence of morphine (expression). It is concluded that M1 muscarinic receptors may play a time-dependent role in the consolidation of reward-related memory of morphine.

Effects of scopolamine on morphine-induced conditioned place preference in mice

It is well known that the cholinergic system plays a crucial role in learning and memory. Psychopharmacological studies in humans and animals have shown that a systemic cholinergic blockade may induce deficits in learning and memory. Accumulated studies have indicated that learning and memory play an important role in drug addition. In the present study, in order to get a further understanding about the functions of the cholinergic system in drug-related learning and memory, we examined the effects of scopolamine (0.5, 1.0 and 2.0 mg/kg) on morphine-induced conditioned place preference (CPP). Two kinds of morphine exposure durations (4 days and 12 days) were used. The main finding was that all doses of scopolamine enhanced the extinction of morphine-induced CPP in mice treated with morphine for 12 days. However, in mice treated with morphine for 4 days, all doses of scopolamine did not inhibit morphine-induced CPP. The highest dose (2.0 mg/kg) of scopolamine even significantly delayed the extinction of morphine-induced CPP. Our results suggest that the effects of a systemic cholinergic blockade on morphine-induced CPP depend on the morphine exposure time.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Behavioral effects of morphine and cocaine in M1 muscarinic acetylcholine receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors (M1-M5) modulate the activity of the central nervous system and an array of physiological functions. Recent evidence has also implicated muscarinic receptors in behavioral effects of drugs of abuse such as morphine and cocaine. However, the genetic similarity between muscarinic receptors and the coexpression of multiple subtypes in most cells has impeded the development of selective antagonists and the determination of the role of each muscarinic receptor subtype in morphine's and cocaine's behavioral effects.

OBJECTIVE

The present studies employ mice deficient in the M1 receptor subtype (M1 KO) to assess morphine antinociception (2.5, 5.0, 10, or 20 mg/kg) and the conditioned rewarding effects of morphine and cocaine (2.5, 5.0, or 10 mg/kg).

METHODS

M1 KO and their wild-type (WT) littermates were tested using a 56 degrees C hotplate assay and a conditioned place preference procedure. Parallel studies using the M1 receptor antagonist, pirenzepine, were also conducted in the background strain C57BL6 mice.

RESULTS

The results demonstrate that M1 KO mice display a greater antinociceptive effect of morphine in the hotplate assay; however, the effects of morphine as well as cocaine were attenuated in the conditioned place preference procedure. Comparable results were obtained with the pharmacological antagonism of the M1 receptor by pirenzepine.

CONCLUSIONS

These results suggest a modulatory role of the M1 muscarinic receptor in opioid antinociception and conditioned drug reward, and demonstrate the utility of M1 receptor knockout models for the determination of the role of the M1 subtype in the behavioral effects of morphine and cocaine.

Ethological analysis of scopolamine treatment or pretreatment in morphine dependent rats

Although scopolamine is currently used to treat morphine addiction in humans, its extensive actions on behaviors have not been systematically analyzed yet, and the underlying mechanisms of its effects still remain ambiguous. The present study was carried out to clarify the possible mechanisms by evaluating the effects of scopolamine pretreatment and treatment on naloxone-precipitated withdrawal signs and some of other general behaviors in morphine dependent rats. Our results showed that scopolamine pretreatment and treatment attenuated naloxone-precipitated withdrawal signs including jumping, writhing posture, weight loss, genital grooming, teeth-chattering, ptosis, diarrhea and irritability, except for wet dog shakes, while general behaviors such as water intake, urine volume and morphine excretion in urine were increased. Our findings suggest that scopolamine has significant actions in the treatment of opiate addiction, which might result from increasing morphine excretion from urine.

Periodic abstinence enhances nociception without significantly altering the antinociceptive efficacy of spinal morphine in the rat

Naloxone administration in the opioid dependent rat is associated with spinal glutamate release and NMDA receptor activation which reportedly is also responsible for opioid tolerance. We hypothesized that episodic withdrawal during chronic infusion of spinal morphine might paradoxically enhance tolerance. Rats (24/group) infused with intrathecal morphine (M) for 4 days (20 nmol/microl per h) were given a daily subcutaneous (s.c.) injection of naloxone 0.6 mg/kg per 0.2 ml (MN) or saline 0.2 ml (MS). A third saline infused group was given daily s.c. saline 0.2 ml (SS). Latencies (rear paw hot box) were tested immediately prior to each daily injection. After termination of each infusion, the dose effect of spinal morphine (0.1, 1, 10, and 100 nmol) was examined. The MN group showed a significantly greater decline in daily latencies compared with the MS group, but also had greater withdrawal hyperalgesia upon termination of the infusion. Dose response to spinal morphine was not significantly different in either MS or MN groups. Periodic abstinence thus enhanced nociception without significantly altering the antinociceptive effect of spinal morphine in this group.

Posttraining intra-basolateral amygdala scopolamine impairs food- and amphetamine-induced conditioned place preferences

The present study investigated the role of cholinergic muscarinic receptor function within the basolateral amygdala memory in the consolidation of conditioned place preference (CPP) memory. Adult male Long-Evans rats were confined to treatment- or nontreatment-paired compartments for 30 min on 4 alternating days. After training, rats received intrabasolateral amygdala infusions of scopolamine (2.5 microg or 5.0 microg/0.5 microl) or saline. The rats were then given a 20-min test session, and the time spent in each of the compartments was recorded. Immediate posttraining (but not delayed 2 hr) scopolamine (5.0 microg) blocked acquisition of food- and amphetamine-induced CPPs. The findings indicate a time-dependent role for basolateral amygdala muscarinic receptors in memory consolidation underlying CPPs for natural and drug rewards.

Effects of acamprosate and scopolamine on the working memory of rats in a three-panel runway task

The aim of this study was to evaluate the effect of treatment with single (1x) and multiple (10x) doses of the anti-craving compound acamprosate (AC, calcium acetyl homotaurinate) on working memory in rats, using in a three-panel runway test. We measured tasks after the animals were treated with AC (500 mg/kg/d, i.p.); scopolamine (SC, 0.5 mg/kg/d, i.p.), a cholinergic muscarinic receptor antagonist; or both drugs concomitantly (ACSC), either for 1 day (1x) or daily for 10 consecutive days (10x). Neither 1x not 10x AC alone had a significant effect on working memory task performance, whereas treatment with SC alone had a significantly negative effect on the ability of the rats to complete the tasks. Rats receiving ACSC performed better than those receiving SC alone, making fewer errors and displaying shorter latency, similar to the performance of the control group. These observations support the hypothesis of an indirect involvement of AC in the cholinergic system.

Hippocampal long-term potentiation is reduced by chronic opiate treatment and can be restored by re-exposure to opiates

Chronic exposure to opiates eventually leads to drug addiction, which is believed to involve maladaptive changes in brain function, but the underlying neuronal mechanisms remain primarily unknown. Given the known effects of opiates such as morphine and heroin on hippocampal function, we investigated the potential effect of chronic opiate treatment on long-term potentiation (LTP) at CA1 synapses in rat hippocampus, a leading experimental model for studying synaptic plasticity. Our results revealed that chronic exposure of rats to morphine or heroin, which induced severe drug tolerance and dependence, markedly reduced the capacity of hippocampal CA1 LTP during the period of drug withdrawal (from approximately 190% in control to approximately 120%). More interestingly, the capacity of LTP could be restored to the normal level by re-exposure of the animals to opiates, indicating that the synaptic function was already adapted to opiates. Morris water maze test, which measures behavioral consequences of synaptic plasticity, showed parallel learning deficits after chronic exposure to opiates. Moreover, the opiate-reduced LTP could also be restored by inhibitors of cAMP-dependent protein kinase A (PKA), suggesting that upregulation of cAMP pathway was likely one of the underlying mechanisms of the observed phenomena. These findings demonstrated that chronic opiate treatment can significantly modulate synaptic plasticity in the hippocampus, leading to an opiate dependence of the plasticity.

Spatial learning and morphine-rewarded place preference negatively correlates in mice

Accumulating evidence has indicated that there might exist some correlation between opiate reward and certain kinds of learning and memory processes. The present study attempted to investigate the correlation between individual differences in morphine reward and capacities in spatial learning and spontaneous alternation. In the present studies, good-response (GR) and poor-response (PR) mice were respectively selected according to their performance in a spatial learning test involving the Morris water maze or in a spontaneous alternation task using the Y-maze. In a place preference conditioning procedure, morphine (3.0 mg/kg) produced significant conditioned place preference (CPP) in both GR and PR mice selected by using either the Morris water maze or the Y-maze. The PR mice selected with the Morris water maze showed significantly more CPP induced by morphine than the GR mice. However, no detectable difference was observed in morphine-induced CPP between the GR and PR mice selected with the Y-maze. These results suggested that the variation in morphine-induced CPP in mice is somehow differentially related to that of spatial learning but unlikely to that of spontaneous alternation.