Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.

Post-training intra-basolateral complex of the amygdala infusions of clenbuterol enhance memory for conditioned place preference and increase ARC protein expression in dorsal hippocampal synaptic fractions

The basolateral complex of the amygdala (BLA) is critically involved in modulation of memory by stress hormones. Noradrenergic activation of the BLA enhances memory consolidation and plays a necessary role in the enhancing or impairing effects of stress hormones on memory. The BLA is not only involved in the consolidation of aversive memories but can regulate appetitive memory formation as well. Extensive evidence suggests that the BLA is a modulatory structure that influences consolidation of arousing memories through modulation of plasticity and expression of plasticity-related genes, such as the activity regulated cytoskeletal-associated (Arc/Arg 3.1) protein, in efferent brain regions. ARC is an immediate early gene whose mRNA is localized to the dendrites and is necessary for hippocampus-dependent long-term potentiation and long-term memory formation. Post-training intra-BLA infusions of the β-adrenoceptor agonist, clenbuterol, enhances memory for an aversive task and increases dorsal hippocampus ARC protein expression following training on that task. To examine whether this function of BLA noradrenergic signaling extends to the consolidation of appetitive memories, the present studies test the effect of post-training intra-BLA infusions of clenbuterol on memory for the appetitive conditioned place preference (CPP) task and for effects on ARC protein expression in hippocampal synapses. Additionally, the necessity of increased hippocampal ARC protein expression was also examined for long-term memory formation of the CPP task. Immediate post-training intra-BLA infusions of clenbuterol (4 ng/0.2 µL) significantly enhanced memory for the CPP task. This same memory enhancing treatment significantly increased ARC protein expression in dorsal, but not ventral, hippocampal synaptic fractions. Furthermore, immediate post-training intra-dorsal hippocampal infusions of Arc antisense oligodeoxynucleotides (ODNs), which reduce ARC protein expression, prevented long-term memory formation for the CPP task. These results suggest that noradrenergic activity in the BLA influences long-term memory for aversive and appetitive events in a similar manner and the role of the BLA is conserved across classes of memory. It also suggests that the influence of the BLA on hippocampal ARC protein expression and the role of hippocampal ARC protein expression are conserved across classes of emotionally arousing memories.

Interactive effects of morphine and nicotine on memory function depend on the central amygdala cannabinoid CB1 receptor function in rats

The present study investigated the possible involvement of the central amygdala (CeA) cannabinoid receptors type-1 (CB1Rs) in the interactive effects of morphine and nicotine on memory formation in a passive avoidance learning task. Our results showed that systemic administration of morphine (3 and 6mg/kg, s.c.) immediately after training phase impaired memory consolidation and induced amnesia. Administration of nicotine (0.3 and 0.6mg/kg, s.c.) before testing phase significantly restored morphine-induced amnesia, suggesting a cross state-dependent learning between morphine and nicotine. The results showed that while the administration of the lower dose of nicotine (0.1mg/kg, s.c.) per se did not induce a significant effect on morphine-induced amnesia, intra-CeA injection of arachidonylcyclopropylamide (ACPA), a cannabinoid CB1 receptor agonist (3 and 4ng/rat), significantly potentiated the nicotine response. Furthermore, the blockade of the CeA cannabinoid CB1 receptors by the injection of AM251 (0.75 and 1ng/rat) reversed the potentiative effect of nicotine (0.6mg/kg, s.c.) on morphine-induced amnesia. It should be considered that bilateral injection of the same doses of ACPA or AM251 (0.5-1ng/rat) into the CeA by itself had no effect on morphine response in a passive avoidance learning task. Confirmed by the cubic interpolation planes, the dose-response data revealed a cross-state-dependent learning between morphine and nicotine which may be mediated by the CeA endocannabinoid system via CB1 receptors.

Interactions between nicotine and drugs of abuse: a review of preclinical findings

Polysubstance abuse is common among substance-use disorder patients, and nicotine is one of the most commonly co-used substances. Epidemiological and clinical laboratory studies suggest that nicotine, when combined with other drugs of abuse, increases intake of one or both substances. This review focuses on the preclinical literature regarding nicotine's interaction with alcohol, stimulants (i.e., cocaine, amphetamines), opioids (i.e., morphine, heroin), and Δ⁹-tetrahydrocannabinol (THC). The current understanding of how these various classes of abused drugs may interact with nicotine on behavioral, physiological, and pharmacological indices that may be important in maintaining co-use of one or both substances in human populations are highlighted. Suggestions as to future areas of research and gaps in knowledge are offered.

Basolateral amygdala CB1 cannabinoid receptors mediate nicotine-induced place preference

In the present study, the effects of bilateral microinjections of cannabinoid CB1 receptor agonist and antagonist into the basolateral amygdala (intra-BLA) on nicotine-induced place preference were examined in rats. A conditioned place preference (CPP) apparatus was used for the assessment of rewarding effects of the drugs in adult male Wistar rats. Subcutaneous (s.c.) administration of nicotine (0.2mg/kg) induced a significant CPP, without any effect on the locomotor activity during the testing phase. Intra-BLA microinjection of a non-selective cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.1-0.5 μg/rat) with an ineffective dose of nicotine (0.1mg/kg, s.c.) induced a significant place preference. On the other hand, intra-BLA administration of AM251 (20-60 ng/rat), a selective cannabinoid CB1 receptor antagonist inhibited the acquisition of nicotine-induced place preference. It should be considered that the microinjection of the same doses of WIN 55,212-2 or AM251 into the BLA, by itself had no effect on the CPP score. The administration of a higher dose of AM251 (60 ng/rat) during the acquisition decreased the locomotor activity of animals on the testing phase. Interestingly, the microinjection of AM251 (20 and 40 ng/rat), but not WIN55,212-2 (0.1-0.5 μg/rat), into the BLA inhibited the expression of nicotine-induced place preference without any effect on the locomotor activity. Taken together, these findings support the possible role of endogenous cannabinoid system of the BLA in the acquisition and the expression of nicotine-induced place preference. Furthermore, it seems that there is a functional interaction between the BLA cannabinoid receptors and nicotine in producing the rewarding effects.

Role of amygdala in drug memory

Drug addiction is a chronic brain disorder with the hallmark of a high rate of relapse to compulsive drug seeking and drug taking even after long-term abstinence. Addiction has been considered as an aberrant memory that has been termed "addiction memory." Drug-related memory plays a critical role in the maintenance of learned addictive behaviors and emergence of relapse. Disrupting these long-lasting memories by administering amnestic agents or other manipulations during specific phases of drug memory is a promising strategy for relapse prevention. Recent studies on the processes of drug addiction and relapse have demonstrated that the amygdala is involved in associative drug addiction learning processes. In this review, we focus on preclinical studies that used conditioned place preference and self-administration models to investigate the differential roles of the amygdala in each phase of drug-related memory, including acquisition, consolidation, retrieval, reconsolidation, and extinction. These studies indicate that the amygdala plays a critical role in both cue-associative learning and the expression of cue-induced relapse to drug-seeking behavior.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

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.

Nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala are involved in ethanol-induced conditioned place preference

The purpose of this study was to evaluate whether nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala (BLA) can potentiate ethanol response in the conditioned place preference (CPP) paradigm. I.p. administration of different doses of ethanol (0.25-1 g/kg) did not induce CPP. However, the higher dose of the drug (1.5 g/kg i.p.) induced place aversion. Furthermore, microinjection of nicotine (0.5-1 microg/rat) into both CA1 regions (intra-CA1) and the BLA (intra-BLA) did not produce a significant CPP. Interestingly, intra-CA1 or -BLA administration of nicotine plus ethanol (0.5 g/kg) during conditioning phase significantly induced a strong CPP. Microinjection of mecamylamine, the nicotinic acetylcholine receptor antagonist, into the CA1 regions or into the BLA did not alter CPP. However, intra-CA1 or -BLA microinjection of mecamylamine (1-4 microg/rat) reversed the response induced by the microinjection of nicotine (1 microg/rat, intra-CA1 or -BLA) plus ethanol (0.5 g/kg i.p.) in the CPP paradigm. On the other hand, the microinjection of nicotine (0.5-1.5 microg/rat) into the BLA, but not into the CA1 regions before the testing phase potentiated the response of ethanol on the expression of conditioned place preference. Moreover, intra-CA1 administration of nicotine plus ethanol increased the locomotor activity on the test day which was reversed by pretreatment with mecamylamine, while other treatments had no effect on locomotor activity. It can be concluded that the activation of nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala can potentiate the ethanol response in the CPP paradigm.

Galanin and addiction

There has been increasing interest in the ability of neuropeptides involved in feeding to modulate circuits important for responses to drugs of abuse. A number of peptides with effects on hypothalamic function also modulate the mesolimbic dopamine system (ventral tegmental area and nucleus accumbens). Similarly, common stress-related pathways can modulate food intake, drug reward and symptoms of drug withdrawal. Galanin promotes food intake and the analgesic properties of opiates, thus it initially seemed possible that galanin might potentiate opiate reinforcement. Instead, galanin agonists decrease opiate reward, measured by conditioned place preference, and opiate withdrawal signs, whereas opiate reward and withdrawal are increased in knockout mice lacking galanin. This is consistent with studies showing that galanin decreases activity-evoked dopamine release in striatal slices and decreases the firing rate of noradrenergic neurons in locus coeruleus, areas involved in drug reward and withdrawal, respectively. These data suggest that polymorphisms in genes encoding galanin or galanin receptors might be associated with susceptibility to opiate abuse. Further, galanin receptors might be potential targets for development of novel treatments for addiction.

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.

Morphine-nicotine interaction in conditioned place preference in mice after chronic nicotine exposure

Previously we found that morphine's effects on locomotor activity and brain dopamine metabolism were enhanced in mice after cessation of 7-week oral nicotine treatment. In the present experiments we show that such chronic nicotine exposure cross-sensitizes NMRI mice to the reinforcing effect of morphine in the conditioned place preference paradigm. The nicotine-treated mice developed conditioned place preference after being conditioned twice with morphine 5 mg/kg s.c. whereas in control mice a higher dose (10 mg/kg) of morphine was required. Since the reinforcing effect of morphine is mediated via micro-opioid receptors we used [3H]DAMGO autoradiography to study whether the number (B(max)) or affinity (K(D)) of mu-opioid receptors in the mouse brain are affected following chronic nicotine exposure. However, no changes were found in the number or affinity of micro-opioid receptors in any of the brain areas studied. Neither did we find alterations in the functional activity of mu-opioid receptors studied by [35S]GTPgammaS-binding. In conclusion, chronic oral nicotine treatment augments the reinforcing effects of morphine in mice, and this cross-sensitization does not seem to be mediated by micro-opioid receptors.

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.

Involvement of dorsal hippocampal nicotinic receptors in the effect of morphine on memory retrieval in passive avoidance task

The present study evaluated the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus on morphine-induced amnesia and morphine state-dependent memory in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas in the CA1 regions of the dorsal hippocampi, trained in a step-through type passive avoidance task, and tested 24 h after training to measure step-through latency. Results indicate that post-training subcutaneous (s.c.) administration of morphine (2.5-7.5 mg/kg) dose-dependently reduced the step-through latency, showing an amnestic response. Post-training intra-CA1 microinjection of nicotine (0.5-1 microg/rat) decreased significantly the amnesia induced by post-training morphine (7.5 mg/kg). Moreover, co-treatment of mecamylamine (0.5 and 1 microg/rat, intra-CA1) with an ineffective dose of morphine (2.5 mg/kg), immediately after training, caused inhibition of memory retrieval. On the other hand, amnesia produced by post-training morphine (7.5 mg/kg) was reversed by pre-test administration of the opioid that is due to a state-dependent effect. Interestingly, pre-test intra-CA1 microinjection of nicotine (0.25 and 0.5 microg/rat) improved post-training morphine (7.5 mg/kg)-induced retrieval impairment. Moreover, pre-test administration of the same doses of nicotine in combination with a lower dose of morphine (0.5 mg/kg), which had no effects alone, synergistically improved memory performance impaired by post-training morphine. Pre-test injection of mecamylamine (0.5-2 microg/rat) prevented the restoration of memory by pre-test morphine. It is important to note that post-training or pre-test intra-CA1 administration of the same doses of nicotine or mecamylamine, alone did not affect memory retrieval. These results suggest that nicotinic acetylcholine receptors of the hippocampal CA1 regions may play an important role in morphine-induced amnesia and morphine state-dependent memory.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

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.

NMDA receptors of dorsal hippocampus are involved in the acquisition, but not in the expression of morphine-induced place preference

In the present study, involvement of the N-methyl-d-aspartate (NMDA) receptors of the CA1 region of dorsal hippocampus (intra-CA1) in the acquisition or expression of morphine-induced conditioned place preference in rats was studied. Male Wistar rats were used in these experiments. NMDA-receptor agonist (NMDA) and antagonist (MK-801) were injected into the CA1 region of the dorsal hippocampus (intra-CA1) and morphine was injected subcutaneously. An unbiased conditioned place preference paradigm was used to study the effect of these agents. In the first set of experiments, the drugs were used during the development of conditioned place preference by morphine or they were used alone in order to see if they induce conditioned place preference or conditioned place aversion. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (2.5-10 mg/kg) induced conditioned place preference in rat. NMDA (0.1-1 microg/rat) or MK-801 (1-4 microg/rat) did not induce conditioned place preference or conditioned place aversion. Intra-CA1 administration of different doses of NMDA (0.1-1 microg/rat) increased, while MK-801 (1-4 microg/rat) decreased morphine-induced place preference. MK-801 reversed the effect of NMDA on morphine response. In the second set of experiments, when the drugs were used before testing on Day 5, in order to test their effects on the expression of morphine (7.5 mg/kg)-induced place preference, intra-CA1 administration of NMDA or MK-801 did not alter the morphine response. None of the drugs influenced locomotion. It is concluded that NMDA receptor of the CA1 region of hippocampus are involved in the acquisition but not expression of morphine-induced place preference.

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.

Conditioned effects of heroin on the expression of inducible nitric oxide synthase in the rat are susceptible to extinction and latent inhibition

RATIONALE

The administration of heroin has been shown to inhibit the induction of nitric oxide, a molecule known to play a critical role in immune function. Previous research has shown that this alteration can be conditioned to environmental stimuli that have been associated with drug administration. However, it remains unknown whether the conditioned effects of heroin on nitric oxide formation follow accepted principles of learning.

OBJECTIVE

This study sought to determine whether manipulations that induce extinction and latent inhibition, two learning paradigms known to reduce the expression of conditioned responses, would alter heroin's conditioned effects on the expression of inducible nitric oxide synthase (iNOS).

MATERIALS AND METHODS

The conditioning procedure involved repeated pairing of heroin administration with placement into a standard conditioning chamber. Rats were repeatedly exposed to the chambers without heroin reinforcement to determine whether the conditioned response would extinguish. To induce latent inhibition, rats received repeated exposure to the chamber before the start of conditioning to inhibit the acquisition of the conditioned response. Ten days after the final conditioning session, all rats were injected with lipopolysaccharide (LPS) to induce iNOS expression. Spleen and liver tissue were removed to determine iNOS expression using reverse transcriptase polymerase chain reaction. Blood was collected to determine the concentration of nitrite/nitrate.

RESULTS

The results showed that both extinction and latent inhibition reduced the conditioned effects of heroin on the production of nitric oxide.

CONCLUSION

This study provides the first evidence that the conditioned effects of heroin on nitric oxide production follow accepted principles of learning.

Morphine-induced place preference: Involvement of cholinergic receptors of the ventral tegmental area

In the present study, the effects of intra-ventral tegmental area injections of cholinergic agents on morphine-induced conditioned place preference were investigated by using an unbiased 3-day schedule of place conditioning design in rats. The conditioning treatments with subcutaneous injections of morphine (0.5-7.5 mg/kg) induced a significant dose-dependent conditioned place preference for the drug-associated place. Intra-ventral tegmental area injection of an anticholinesterase, physostigmine (2.5 and 5 microg/rat) or nicotinic acetylcholine receptor agonist, nicotine (0.5 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant conditioned place preference. Furthermore, intra-ventral tegmental area administration of muscarinic acetylcholine receptor antagonist, atropine (1-4 microg/rat) or nicotinic acetylcholine receptor antagonist, mecamylamine (5 and 7.5 microg/rat) dose-dependently inhibited the morphine (5 mg/kg)-induced place preference. Atropine or mecamylamine reversed the effect of physostigmine or nicotine on morphine response respectively. The injection of physostigmine, but not atropine, nicotine or mecamylamine, into the ventral tegmental area alone produced a significant place aversion. Moreover, intra-ventral tegmental area administration of the higher doses of physostigmine or atropine, but not nicotine or mecamylamine decreased the locomotor activity. We conclude that muscarinic and nicotinic acetylcholine receptors in the ventral tegmental area may critically mediate the rewarding effects of morphine.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).