Chronic morphine increases hippocampal acetylcholine release: possible relevance in drug dependence

Involvement of hippocampal phosphatidylethanolamine-binding protein in morphine dependence and withdrawal

Drug addiction is thought to result from an intractable and aberrant learning and memory in response to drug-related stimulation, and cholinergic neurotransmission plays an important role in this process. Phosphatidylethanolamine-binding protein (PEBP) is the precursor of the hippocampal cholinergic neurostimulating peptide (HCNP), an 11 amino acid peptide that enhances the production of choline acetyltransferase (ChAT) and assists in the development of cholinergic projections from the medial septal nuclei to the hippocampus. However, whether PEBP is involved in drug addiction remains unclear. In the present study, PEBP expression in the hippocampus, as detected by proteomics analysis, was found to be dramatically up-regulated after rats received chronic morphine treatment. Western blotting analysis revealed a specific up-regulation of PEBP expression in the hippocampus but not in any other brain regions assessed. A down-regulation of hippocampal PEBP levels induced by antisense oligodeoxynucleotides resulted in aggravated morphine dependence. Together, these findings indicate that PEBP is involved in morphine dependence. Moreover, the time course of PEBP expression changes and ChAT activity was investigated during chronic morphine treatment and withdrawal. The results showed that the hippocampal PEBP levels were up-regulated during chronic morphine treatment and returned to the baseline 3 days after withdrawal, after which PEBP levels were persistently up-regulated for 28 days after withdrawal. The changes in hippocampal ChAT activity followed a pattern that was similar to that of the PEBP levels. Taken together, these results suggest that hippocampal PEBP is involved in morphine dependence and withdrawal, perhaps through modulating cholinergic transmission in the hippocampus.

The contribution of brain reward circuits to the obesity epidemic

One of the defining characteristics of the research of Ann E. Kelley was her recognition that the neuroscience underlying basic learning and motivation processes also shed significant light upon mechanisms underlying drug addiction and maladaptive eating patterns. In this review, we examine the parallels that exist in the neural pathways that process both food and drug reward, as determined by recent studies in animal models and human neuroimaging experiments. We discuss contemporary research that suggests that hyperphagia leading to obesity is associated with substantial neurochemical changes in the brain. These findings verify the relevance of reward pathways for promoting consumption of palatable, calorically dense foods, and lead to the important question of whether changes in reward circuitry in response to intake of such foods serve a causal role in the development and maintenance of some cases of obesity. Finally, we discuss the potential value for future studies at the intersection of the obesity epidemic and the neuroscience of motivation, as well as the potential concerns that arise from viewing excessive food intake as an "addiction". We suggest that it might be more useful to focus on overeating that results in frank obesity, and multiple health, interpersonal, and occupational negative consequences as a form of food "abuse".

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.

Morphine-induced changes in acetylcholine release in the interpeduncular nucleus and relationship to changes in motor behavior in rats

Owing to multiple anatomical connections and functional interactions between the habenulo-interpeduncular and the mesolimbic pathways, it has been proposed that these systems could together mediate the reinforcing properties of addictive drugs. 18-Methoxycoronaridine, an agent that reduces morphine self-administration and attenuates dopamine sensitization in the nucleus accumbens in response to repeated morphine, has been shown to produce these effects by acting in the medial habenula and interpeduncular nucleus. Acetylcholine, one of the predominant neurotransmitters in the interpeduncular nucleus, may be a major determinant of these interactions. To determine if and how morphine acts in the interpeduncular nucleus, the effects of acute and repeated administration of morphine on extracellular acetylcholine levels in this brain area were assessed. In addition, the motor behavior of rats receiving repeated morphine administration was monitored during microdialysis sessions. Acutely, morphine produced a biphasic effect on extracellular acetylcholine levels in the interpeduncular nucleus such that low and high doses of morphine (i.e., 5 and 20mg/kg i.p.) significantly increased and decreased acetylcholine levels, respectively. Repeated administration of the same doses of morphine resulted in tolerance to the inhibitory but not to the stimulatory effects; tolerance was accompanied by sensitization to morphine-induced changes in locomotor activity and stereotypic behavior. The latter results suggest that tolerance to morphine's effect on the cholinergic habenulo-interpeduncular pathway is related to its sensitizing effects on the mesostriatal dopaminergic pathways.

Sensitization to the conditioned rewarding effects of morphine modulates gene expression in rat hippocampus

Opiates addiction is characterized by its long-term persistence. In order to study the enduring changes in long-term memory in hippocampus, a pivotal region for this process, we used suppression subtractive hybridization to compare hippocampal gene expression in morphine and saline-treated rats. Animals were subjected to an extended place preference paradigm consisting of four conditioning phases. Sensitization to the reinforcing effects of the drug occurred after three conditioning phases. After 25 days of treatment rats were euthanized and the complementary DNA (cDNA) from the hippocampus of morphine-dependent and saline-treated animals were then screened for differentially expressed cDNAs. The selected 177 clones were then subjected to a microarray procedure and 20 clones were found differentially regulated. The pattern of regulated genes suggests impairments in neurotransmitter release and the activation of neuroprotective pathways.

The influence of central administration of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice

In the present study, effects of intracerebroventricular (i.c.v.) injections of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice were investigated by using a one-trial passive avoidance task. Amnesia induced by pre-training morphine was significantly reversed in morphine-sensitized mice, which had previously received once daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days. Three daily injections of SKF 38393 (1, 2 and 4 g/mouse, i.c.v.) or SCH 23390 (0.25, 0.5, 0.75 and 1 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of quinpirole (0.3, 1 and 3 g/mouse, i.c.v.) or sulpiride (0.03, 0.1, 0.3 and 1 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. Morphine-sensitized mice received similar injections of cholinergic agents. Three daily injections of physostigmine (1, 3 and 5 g/mouse, i.c.v.) or atropine (1, 4 and 7 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of nicotine (0.75, 1 and 2 g/mouse, i.c.v.) or mecamylamine (1, 3 and 6 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. The results suggest that morphine sensitization affects the impairment of memory formation and thus it is postulated that central dopaminergic and cholinergic systems may play an important role in this effect.

Opiates, psychostimulants, and adult hippocampal neurogenesis: Insights for addiction and stem cell biology

Once thought to produce global, nonspecific brain injury, drugs of abuse are now known to produce selective neuro-adaptations in particular brain regions. These neuro-adaptations are being closely examined for clues to the development, maintenance, and treatment of addiction. The hippocampus is an area of particular interest, as it is central to many aspects of the addictive process, including relapse to drug taking. A recently appreciated hippocampal neuro-adaptation produced by drugs as diverse as opiates and psychostimulants is decreased neurogenesis in the sub-granular zone (SGZ). While the role of adult-generated neurons is not clear, their functional integration into hippocampal circuitry raises the possibility that decreased adult SGZ neurogenesis may alter hippocampal function in such a way as to maintain addictive behavior or contribute to relapse. Here, we review the impact of opiates and psychostimulants on the different stages of cell development in the adult brain, as well as the different stages of the addictive process. We discuss how examination of drug-induced alterations of adult neurogenesis advances our understanding of the complex mechanisms by which opiates and psychostimulants affect brain function while also opening avenues for novel ways of assessing the functional role of adult-generated neurons. In addition, we highlight key discrepancies in the field and underscore the necessity to move "beyond BrdU"--beyond merely counting new hippocampal cells labeled with the S phase marker bromodeoxyuridine--so as to probe mechanistic questions about how drug-induced alterations in adult hippocampal neurogenesis occur and what the functional ramifications of alterations in neurogenesis are for addiction.

Virtual screen for ligands of orphan G protein-coupled receptors

This paper describes a virtual screening methodology that generates a ranked list of high-binding small molecule ligands for orphan G protein-coupled receptors (oGPCRs), circumventing the requirement for receptor three-dimensional structure determination. Features representing the receptor are based only on physicochemical properties of primary amino acid sequence, and ligand features use the two-dimensional atomic connection topology and atomic properties. An experimental screen comprised nearly 2 million hypothetical oGPCR-ligand complexes, from which it was observed that the top 1.96% predicted affinity scores corresponded to "highly active" ligands against orphan receptors. Results representing predicted high-scoring novel ligands for many oGPCRs are presented here. Validation of the method was carried out in several ways: (1) A random permutation of the structure-activity relationship of the training data was carried out; by comparing test statistic values of the randomized and nonshuffled data, we conclude that the value obtained with nonshuffled data is unlikely to have been encountered by chance. (2) Biological activities linked to the compounds with high cross-target binding affinity were analyzed using computed log-odds from a structure-based program. This information was correlated with literature citations where GPCR-related pathways or processes were linked to the bioactivity in question. (3) Anecdotal, out-of-sample predictions for nicotinic targets and known ligands were performed, with good accuracy in the low-to-high "active" binding range. (4) An out-of-sample consistency check using the commercial antipsychotic drug olanzapine produced "active" to "highly-active" predicted affinities for all oGPCRs in our study, an observation that is consistent with documented findings of cross-target affinity of this compound for many different GPCRs. It is suggested that this virtual screening approach may be used in support of the functional characterization of oGPCRs by identifying potential cognate ligands. Ultimately, this approach may have implications for pharmaceutical therapies to modulate the activity of faulty or disease-related cellular signaling pathways. In addition to application to cell surface receptors, this approach is a generalized strategy for discovery of small molecules that may bind intracellular enzymes and involve protein-protein interactions.

ROLES OF .MU.-OPIOID RECEPTORS IN DEVELOPMENT OF TOLERANCE TO DIISOPROPYLFLUOROPHOSPHATE (DFP)

Anatomical evidence indicates that cholinergic and opioidergic systems are co-localized and acting on the same neuron. However, the regulatory mechanisms between cholinergic and opioidergic system have not been well characterized. In the present study, the potential involvement of mu-opioid receptors in mediating the changes of toxic signs and muscarinic receptor binding after administration of irreversible anti-acetylcholinesterase diisopropylfluorophosphate (DFP) was investigated. DFP (1 mg/kg/day, subcutaneous injection, s.c.)-induced tremors and chewing movements were monitored during the 28-day treatment period in mu-opioid receptor knockout and wild type mice. Autoradiographic studies of total, M1, and M2 muscarinic receptors were conducted using [(3)H]-quinuclidinyl benzilate, [(3)H]-pirenzepine, and [(3)H]-AF-DX384 as ligands, respectively. DFP-induced tremors in both mu-opioid receptor knockout and wild type mice showed tolerance development. However, DFP-induced tremors in mu-opioid receptor knockout mice showed delayed tolerance development than that of DFP-treated wild type controls. DFP-induced chewing movements in both mu-opioid receptor knockout and wild type mice failed to show development of tolerance after four weeks of treatment. M2 muscarinic receptor binding of DFP-treated mu-opioid receptor knockout mice was significantly decreased than that of the DFP-treated wild type controls in the striatum, but not in the cortex and hippocampus. However, there were no significant differences in total and M1 muscarinic receptor binding between DFP-treated mu-opioid receptor knockout and wild type mice in the cortex, striatum and hippocampus. These studies indicate that mu-opioid receptors play an important role through the striatal M2 muscarinic receptors to regulate the development of tolerance to DFP-induced tremors.

Increased diisopropylfluorophosphate-induced toxicity in ?-opioid receptor knockout mice

The potential involvement of mu-opioid receptors in mediating the changes of toxic signs and muscarinic receptor bindings after acute administration of irreversible antiacetylcholinesterase diisopropylfluorophosphate (DFP) was investigated. DFP-induced chewing movement and tremors were monitored for a period of 180 min in mu-opioid receptor knockout and wild-type mice. The autoradiographic studies of total, M1, and M2 muscarinic receptors were conducted using [(3)H]quinuclidinyl benzilate, [(3)H]pirenzepine, and [(3)H]AF-DX384 as ligands, respectively. Saline-treated mu-opioid receptor knockout and wild-type mice did not show chewing movement or tremors. Although DFP (1, 2, or 3 mg/kg, subcutaneous injection, s.c.)-induced chewing movement and tremors were shown in a dose-dependent manner, there were no significant differences in tremors induced by 1 or 2 mg/kg of DFP between mu-opioid receptor knockout and wild-type mice. There were also no significant differences in chewing movement induced by all doses of DFP between mu-opioid receptor knockout and wild-type mice. However, DFP (3 mg/kg)-induced tremors in mu-opioid receptor knockout mice were significantly increased over those in wild-type controls. Acetylcholinesterase activity in the striatum of saline-treated mu-opioid receptor knockout mice was significantly higher than that of the wild-type controls. After administration of DFP, acetylcholinesterase activity in the striatum of both mu-opioid receptor knockout and wild-type mice was significantly decreased (more than 36%, 58%, and 94% reduced at the doses of 1, 2, and 3 mg/kg, respectively) than that of their respective saline controls. M2 muscarinic receptor binding in saline-treated mu-opioid receptor knockout mice was significantly lower than that of the wild-type controls in the striatum. However, there were no significant differences in total, M1, or M2 muscarinic receptor binding in the cortex, striatum, or hippocampus of mu-opioid receptor knockout and wild-type mice after DFP administration. Our data show increased DFP-induced tremors, compensatory up-regulation of acetylcholinesterase activity, and compensatory down-regulation of M2 muscarinic receptors in the striatum of mice lacking mu-opioid receptor gene. These results suggest that the enhancement of DFP-induced tremors may be associated with the compensatory up-regulation of acetylcholinesterase activity and compensatory down-regulation of M2 muscarinic receptors in the striatum of mu-opioid receptor knockout mice.

Impaired water maze learning performance in μ-opioid receptor knockout mice

Previous study has demonstrated that the lack of mu-opioid receptor decreased LTP in the dentate gyrus of the hippocampus, suggesting the possibility that the lack of mu-opioid receptor may accompany a change in learning and memory. However, no behavioral study has been undertaken to correlate LTP deficits with spatial memory impairment in mu-opioid receptor knockout mice. Therefore, the present study investigated the hypothesis that mu-opioid receptors contribute to learning and memory by using the Morris water maze, and comparing responses in wild type and mu-opioid receptor gene knockout mice. Our results indicated that mu-opioid receptor knockout mice showed a significant spatial memory impairment compared to wild type in the Morris water maze. This result suggests that the expression of mu-opioid receptor plays an important role in spatial learning and memory examined by Morris water maze.

Drug dependence: neuropharmacology and management

This overview has attempted to highlight the brain regions associated with reward, and the pathways and neurotransmitters responsible for communication between these regions. Work conducted in this field has shown that stimulants and opioids, despite interactions with different receptor types and different neurotransmitter reuptake transporters, appear to share a common action on brain reward pathways. Their effects on these pathways (the distinct brain regions making up the mesocorticolimbic dopaminergic system) are predominantly mediated through changes in dopamine neurotransmission, and compounds aimed at selectively modulating these effects may form the basis of drugs to treat addiction. Other transmitters such as GABA, acetylcholine and serotonin inevitably have a role to play in reward, although at present the exact nature of their effects remains unclear. Diverging from manipulating the CNS directly as a management strategy for dependence, it might be possible to exploit the immune system to prevent administered psychostimulants penetrating the brain, but antibody saturation and specificity are problematic.

Psychostimulant-induced behavioral sensitization depends on nicotinic receptor activation

Animal studies have shown that nicotine and psychostimulant drugs (amphetamine and cocaine) share the property of inducing long-lasting behavioral and neurochemical sensitization, which is thought to contribute to their addictive properties. Neuroplasticity subserving learning and memory mechanisms is considered to be involved in psychostimulant-induced sensitization and addiction behavior. Because nicotinic receptors in the brain play a role in the storage of drug-related information underlying reinforcement learning, we evaluated the possibility that activation of central nicotinic receptors may underlie psychostimulant-induced sensitization. Repeated exposure of rats to nicotine profoundly enhanced the psychomotor effects of nicotine and amphetamine 3 weeks after nicotine pretreatment. Moreover, the nicotinic receptor antagonist mecamylamine completely blocked the induction, but not the long-term expression, of behavioral sensitization to amphetamine in amphetamine-pretreated rats. Mecamylamine also prevented the development of cocaine-induced behavioral sensitization. Behavioral sensitization induced by nicotine, amphetamine, or cocaine was associated with an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens slices. Coadministration of mecamylamine during pretreatment with nicotine, amphetamine, or cocaine prevented the development of this long-term hyperreactivity of nucleus accumbens dopamine neurons. Similarly, the high-affinity non-alpha7 subtype nicotinic receptor antagonist dihydro-beta-erythroidine prevented the development of amphetamine-induced behavioral and neurochemical sensitization. These data indicate that nicotinic receptor activation (by endogenously released acetylcholine) is a common denominator initiating neuroplasticity involved in the development of amphetamine, as well as cocaine-induced sensitization.

Reversal of morphine-induced memory impairment in mice by withdrawal in Morris water maze: possible involvement of cholinergic system

The effects of morphine and morphine withdrawal on memory performance were examined in mice by using Morris water maze task. Morphine-induced memory impairment at the doses of 5 and 10 mg/kg recovered after repeated administration. Oxotremorine, a muscarinic receptor agonist, at the dose of 0.1 mg/kg ip, and physostigmine, a cholinesterase inhibitor, at the dose of 0.1 mg/kg ip, significantly antagonized morphine (10 mg/kg sc)-induced memory impairment in mice. Furthermore, repeated naloxone (0.5 mg/kg ip) attenuated scopolamine (0.2 mg/kg ip)-induced memory impairment. By using escalating doses of morphine for 13 days, morphine-induced memory impairment was continuously maintained. When withdrawal was precipitated by naloxone (5 mg/kg ip), or administration of oxotremorine (0.1 and 0.2 mg/kg ip) or physostigmine (0.05 and 0.1 mg/kg ip), the impairment was completely reversed. These results suggest that morphine-induced memory impairment could be partially due to the inhibition of the central cholinergic activity.

Adaptive changes in M1 muscarinic receptors localized to specific rostral brain regions during and after morphine withdrawal

Morphine-dependent rats were allowed to undergo withdrawal by abrupt discontinuation of the drug. The regional expression of brain M1 muscarinic receptors was measured directly by autoradiographic determination with [(3)H] pirenzepine, and indirectly by quantifying the relative levels of M1 mRNA encoding the receptor protein. Patterns of receptor changes after morphine treatment were in general agreement using the two methods. Frontal cortical samples derived from morphine-dependent rats exhibited a 28% increase in M1 receptor mRNA measured at the end of the infusion. At the peak of the withdrawal, M1 mRNA levels for dependent rats were much lower (33.4%) than those for control rats. Hippocampal samples derived from morphine-dependent rats exhibited no changes in M1 mRNA levels after the morphine infusion. During the peak of withdrawal, however, hippocampal M1 mRNA levels were reduced (57%) compared with levels for controls. The M1 mRNA levels remained at this reduced degree of expression even after withdrawal symptoms had subsided. Addition of diisopropylflurophophate (DFP) to the morphine infusion schedule inhibited the adaptive changes in M1 mRNA levels induced by morphine. During the peak period of withdrawal, M1 mRNA levels in the hippocampus declined by only 18% as compared with 57% for the morphine control group. The adaptive decrease in hippocampal M1 receptors after withdrawal subsided may reflect prolonged heightened cholinergic activity in an area where such cholinergic innervation plays an important role in memory.

Lack of specific effects of selective D(1) and D(2) dopamine antagonists vs. risperidone on morphine-induced hyperactivity

In the present study, three different dopamine antagonists were challenged in order to counteract hyperactivity induced by 50 mg/kg of morphine. A wide range of doses of morphine (50, 25, 12.5, 6.25, or 3.12 mg/kg) were evaluated on spontaneous locomotor activity. A significant increase was observed only with the two higher doses tested (25 and 50 mg/kg). No decrease was found with any of the doses used at any period of time. After analyzing doses of SCH 23390 (0.5, 0.1, and 0.05 mg/kg), raclopride (0.5, 0.25, and 0.125 mg/kg) and risperidone (0.1, 0.05, and 0.025 mg/kg) administered alone, only the 0.5 mg/kg dose of SCH 23390 decreased locomotor activity. The three compounds counteracted morphine-induced hyperactivity, but with SCH 23390 it was only achieved with the dose of 0.5 mg/kg, which also decreased spontaneous locomotor activity and induced catalepsy. On the other hand, raclopride and risperidone neutralized morphine-induced hyperactivity at doses that did not affect locomotor activity, although the former induced catalepsy when administered with morphine. It is concluded that although the blockade of D(1) and D(2) DA receptors decreases morphine-induced hyperactivity, this action is not specific, contrary to the action of risperidone, which counteracts this hyperactivity without any other motor effects.

Effects of lesions of the nucleus basalis magnocellularis on the acquisition of cocaine self-administration in rats

The nucleus basalis magnocellularis (NBM) is one element in the limbic cortical-ventral striatal circuitry that has been implicated in reinforcement processes. The present study examined the involvement of the cholinergic neurons of the NBM in mediating aspects of cocaine reinforcement. Lesions of the NBM were made by injecting 0.01 M AMPA into the subpallidal basal forebrain. Following 4 days' recovery, rats were implanted chronically with catheters in the jugular vein. In three separate experiments, rats were trained to acquire cocaine self-administration under a FR1 schedule of reinforcement at doses of 0.25, 0.083 and 0.028 mg/injection. A dose-effect function was also determined at the end of the acquisition experiments using five different doses of cocaine (0.009, 0.028, 0.083, 0.25, 0.50 mg/injection) and saline which were presented once daily in a Latin square design. There were no significant differences between groups in the acquisition of cocaine self-administration at any of the three doses studied (0.028, 0.083 and 0.25 mg/injection), although at the lowest dose, lesioned animals responded at greater levels on both active and inactive levers. However, a shift to the left in the cocaine dose-response function was observed revealing that the lesioned group self-administered significantly higher amounts of low doses of cocaine than control rats. These data suggest that the integrity of the NBM is not a critical determinant of the reinforcing effects of cocaine during the acquisition of self-administration of the drug, but that NBM-dependent cholinergic mechanisms may nevertheless interact with the neural substrates mediating the reinforcing properties of cocaine. The data are relevant to recent hypotheses of functional interactions between the dopaminergic system and the cholinergic NBM.