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

Chronic escalating-dose and acute binge cocaine treatments change the hippocampal cholinergic muscarinic system on drug presence and after withdrawal

Cocaine addiction is a relapsing disorder with loss of control in limiting drug intake. Considering the involvement of acetylcholine in the neurobiology of the disease, our aim was to evaluate whether cocaine induces plastic changes in the hippocampal cholinergic muscarinic system. Male Swiss-Webster mice received saline or cocaine (ip) three times daily (60-min intervals) either acutely or in an escalating-dose binge paradigm for 14 days. Locomotor activity was measured in all treatment days. Dopaminergic and cholinergic muscarinic receptors (D₁R, D₂R, M₁-M_5, mAChRs), choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT) and acetylcholinesterase (AChE) were quantified in the hippocampus by immunoblotting one hour after the last injection (on drug) or after 14 days of abstinence (withdrawal). Escalating-dose group showed cocaine-induced locomotor sensitization from day 2. M₃ mAChR and ChAT significantly increased after the on-drug acute binge treatment. Escalating-dose on-drug group showed increased ChAT, M₁, M₅ mAChR and D₂R; and decreased D₁R. Acute-binge withdrawal group showed increased VAChT, M₂ mAChR, D₁R, and D₂R; and decreased M₁ mAChR. Escalating-dose withdrawal group presented increased D₁R and VAChT and decreased M₁ mAChR and D₂R. Locomotor activity was negatively correlated with M₁ mAChR and AChE in on-drug group and positively correlated with VAChT in withdrawal group. M₁ mAChR was positively correlated with M₂ mAChR and ChAT in on-drug group, whereas ChAT was positively correlated with M₅ mAChR in withdrawal group. The results indicate that cocaine induced an increase in the hippocampal cholinergic tone in the presence of the drug, whereas withdrawal causes a resetting in the system.

M1 muscarinic receptor activation decreases alcohol consumption via a reduction in consummatory behavior

Muscarinic acetylcholine receptors (mAChRs) have been shown to mediate alcohol consumption and seeking. Both M₄ and M₅ mAChRs have been highlighted as potential novel treatment targets for alcohol use disorders (AUD). Similarly, M₁ mAChRs are expressed throughout reward circuitry, and their signaling has been implicated in cocaine consumption. However, whether the same effects are seen for alcohol consumption, or whether natural reward intake is inadvertently impacted is still unknown. To determine the role of M₁ mAChRs in alcohol consumption, we tested operant self-administration of alcohol under both fixed ratio (FR3) and progressive ratio (PR3-4) schedules. Enhancing M₁ mAChR signaling (via the M₁ PAM-Agonist PF-06767832, 1 mg/kg, i.p.) reduced operant alcohol consumption on a fixed schedule but had no effect on motivation to acquire alcohol. To determine whether these actions were specific to alcohol, we examined the effects of M₁ enhancement on natural reward (sucrose) self-administration. Systemic administration of PF-06767832 (1 mg/kg, i.p.) also reduced operant sucrose self-administration, suggesting the actions of the M₁ receptor may be non-selective across drug and natural rewards. Finally, to understand whether this reduction extended to natural consummatory behaviors, we assessed home cage standard chow and water consumption. M₁ enhancement via systemic PF-06767832 administration reduced food and water consumption. Together our results suggest the M₁ PAM-agonist, PF-06767832, non-specifically reduces consummatory behaviors that are not associated with motivational strength for the reward. These data highlight the need to further characterize M₁ agonists, PAMs, and PAM-agonists, which may have varying degrees of utility in the treatment of neuropsychiatric disorders including AUD.

SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner

The serum and glucocorticoid-regulated kinase 1 (SGK1) is a widely expressed protein in the Central Nervous System (CNS), involved in regulating the activity of a wide variety of ion channels and transporters and physiological functions, such as neuronal excitability. SGK1.1 is a neuronal splice isoform of SGK1, expressed exclusively in the CNS, distributed in brain and cerebellum, that decreases neuronal excitability via up-regulation of M-current, linked to Kv7.2/3 potassium channels. Strategies to maintain increased SGK1.1 activity could be helpful in decreasing neuronal hyperexcitability, as occurs in neuropathic pain. Transgenic mice overexpressing SGK1.1 (B6.Tg.sgk1) offer a particularly relevant opportunity to assess the physiological involvement of this protein in nociception. Behavior and physiological nociception were evaluated in male and female B6.Tg.sgk1 and wild-type mice (B6.WT), characterizing nociceptive thresholds to different nociceptive stimuli (thermal, chemical and mechanical), as well as the electrophysiological properties of cutaneous sensory Aδ-fibres isolated from the saphenous nerve. The acute antinociceptive effect of morphine was also evaluated. Compared with B6.WT animals, male and female B6.Tg.sgk1 mice showed increased spontaneous locomotor activity. Regarding nociception, there were no differences between transgenic and wild-type mice in heat, chemical and mechanical thresholds, but interestingly, male B6.Tg.sgk1 mice were less sensitive to cold stimulus; B6.Tg.sgk1 animals showed lower sensitivity to morphine. Electrophysiological properties of cutaneous primary afferent fibres were maintained. This is the first demonstration that the SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner. B6.Tg.sgk1 mice offer a particularly relevant opportunity to further analyze the involvement of this protein in nociception, and studies in models of chronic, neuropathic pain are warranted.

Effects of biperiden (cholinergic muscarinic m1/m4 receptor antagonist) on ethanol conditioned place preference in mice

BACKGROUND

Previous studies suggest that muscarinic cholinergic receptors might act upon the dopamine release in the mesolimbic system and alter drug-reinforcing values related to drug craving.

AIMS

We examined the effects of systemic biperiden administration, a muscarinic cholinergic (M1/M4) receptor antagonist, on ethanol (dose of 2 g/Kg) conditioned place preference (CPP), neuronal activation, dopamine and its metabolites levels in the nucleus accumbens.

METHODS

Thirty minutes before the ethanol-induced CPP test, mice received saline or biperiden at doses of 1.0, 5.0, or 10.0 mg/kg. The time spent in each compartment was recorded for 15 min. After the CPP protocol, animals were euthanized, and we investigated the activation of the nucleus accumbens by immunohistochemistry for Fos. We also quantified dopamine, homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC) levels in the nucleus accumbens by high-performance liquid chromatography (HPLC). Additionally, the rotarod was employed to evaluate the effects of biperiden on motor coordination.

RESULTS

Biperiden at different doses (1.0, 5.0, and 10.0 mg/kg) blocked the expression of ethanol-induced CPP. These biperiden doses increased the number of Fos-positive cells and the dopamine turnover in the nucleus accumbens. None of the doses affected the motor coordination evaluated by the rotarod.

CONCLUSIONS

Our results show that biperiden can modulate the effect of alcohol reward, and its mechanism of action may involve a change in dopamine and cholinergic mesolimbic neurotransmission.

Genetic Architecture and Molecular Neuropathology of Human Cocaine Addiction

We integrated genomic and bioinformatic analyses, using data from the largest genome-wide association study of cocaine dependence (CD; n = 6546; 82.37% with CD; 57.39% male) and the largest postmortem gene-expression sample of individuals with cocaine use disorder (CUD; n = 36; 51.35% with CUD; 100% male). Our genome-wide analyses identified one novel gene (NDUFB9) associated with the genetic predisposition to CD in African-Americans. The genetic architecture of CD was similar across ancestries. Individual genes associated with CD demonstrated modest overlap across European-Americans and African-Americans, but the genetic liability for CD converged on many similar tissue types (brain, heart, blood, liver) across ancestries. In a separate sample, we investigated the neuronal gene expression associated with CUD by using RNA sequencing of dorsal-lateral prefrontal cortex neurons. We identified 133 genes differentially expressed between CUD case patients and cocaine-free control subjects, including previously implicated candidates for cocaine use/addiction (FOSB, ARC, KCNJ9/GIRK3, NR4A2, JUNB, and MECP2). Differential expression analyses significantly correlated across European-Americans and African-Americans. While genes significantly associated with CD via genome-wide methods were not differentially expressed, two of these genes (NDUFB9 and C1qL2) were part of a robust gene coexpression network associated with CUD involved in neurotransmission (GABA, acetylcholine, serotonin, and dopamine) and drug addiction. We then used a "guilt-by-association" approach to unravel the biological relevance of NDUFB9 and C1qL2 in the context of CD. In sum, our study furthers the understanding of the genetic architecture and molecular neuropathology of human cocaine addiction and provides a framework for translating biological meaning into otherwise obscure genome-wide associations.SIGNIFICANCE STATEMENT Our study further clarifies the genetic and neurobiological contributions to cocaine addiction, provides a rapid approach for generating testable hypotheses for specific candidates identified by genome-wide research, and investigates the cross-ancestral biological contributions to cocaine use disorder/dependence for individuals of European-American and African-American ancestries.

Computational Systems Pharmacology-Target Mapping for Fentanyl-Laced Cocaine Overdose

The United States of America is fighting against one of its worst-ever drug crises. Over 900 people a week die from opioid- or heroin-related overdoses, while millions more suffer from opioid prescription addiction. Recently, drug overdoses caused by fentanyl-laced cocaine specifically are on the rise. Due to drug synergy and an increase in side effects, polydrug addiction can cause more risk than addiction to a single drug. In the present work, we systematically analyzed the overdose and addiction mechanism of cocaine and fentanyl. First, we applied our established chemogenomics knowledgebase and machine-learning-based methods to map out the potential and known proteins, transporters, and metabolic enzymes and the potential therapeutic target(s) for cocaine and fentanyl. Sequentially, we looked into the detail of (1) the addiction to cocaine and fentanyl by binding to the dopamine transporter and the μ opioid receptor (DAT and μOR, respectively), (2) the potential drug-drug interaction of cocaine and fentanyl via p-glycoprotein (P-gp) efflux, (3) the metabolism of cocaine and fentanyl in CYP3A4, and (4) the physiologically based pharmacokinetic (PBPK) model for two drugs and their drug-drug interaction at the absorption, distribution, metabolism, and excretion (ADME) level. Finally, we looked into the detail of JWH133, an agonist of cannabinoid 2-receptor (CB2) with potential as a therapy for cocaine and fentanyl overdose. All these results provide a better understanding of fentanyl and cocaine polydrug addiction and future drug abuse prevention.

Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Muscarinic acetylcholine receptor (mAChRs) subtypes represent exciting new targets for the treatment of schizophrenia and substance use disorder (SUD). Recent advances in the development of subtype-selective allosteric modulators have revealed promising effects in preclinical models targeting the different symptoms observed in schizophrenia and SUD. M₁ PAMs display potential for addressing the negative and cognitive symptoms of schizophrenia, while M₄ PAMs exhibit promise in treating preclinical models predictive of antipsychotic-like activity. In SUD, there is increasing support for modulation of mesocorticolimbic dopaminergic circuitry involved in SUD with selective M₄ mAChR PAMs or M₅ mAChR NAMs. Allosteric modulators of these mAChR subtypes have demonstrated efficacy in rodent models of cocaine and ethanol seeking, with indications that these ligand may also be useful for other substances of abuse, as well as in various stages in the cycle of addiction. Importantly, allosteric modulators of the different mAChR subtypes may provide viable treatment options, while conferring greater subtype specificity and corresponding enhanced therapeutic index than orthosteric muscarinic ligands and maintaining endogenous temporo-spatial ACh signaling. Overall, subtype specific mAChR allosteric modulators represent important novel therapeutic mechanisms for schizophrenia and SUD.

Physiological roles of CNS muscarinic receptors gained from knockout mice

Because the five muscarinic acetylcholine receptor subtypes have overlapping distributions in many CNS tissues, and because ligands with a high degree of selectivity for a given subtype long remained elusive, it has been difficult to determine the physiological functions of each receptor. Genetically engineered knockout mice, in which one or more muscarinic acetylcholine receptor subtype has been inactivated, have been instrumental in identifying muscarinic receptor functions in the CNS, at the neuronal, circuit, and behavioral level. These studies revealed important functions of muscarinic receptors modulating neuronal activity and neurotransmitter release in many brain regions, shaping neuronal plasticity, and affecting functions ranging from motor and sensory function to cognitive processes. As gene targeting technology evolves including the use of conditional, cell type specific strains, knockout mice are likely to continue to provide valuable insights into brain physiology and pathophysiology, and advance the development of new medications for a range of conditions such as Alzheimer's disease, Parkinson's disease, schizophrenia, and addictions, as well as non-opioid analgesics. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Cross State-dependent Learning Interaction Between Scopolamine and Morphine in Mice: The Role of Dorsal Hippocampus

INTRODUCTION

The current study aimed at investigating the existence of the cross state-dependent learning between morphine and scopolamine (SCO) in mice by passive avoidance method, pointing to the role of CA1 area.

METHODS

The effects of pre-training SCO (0.75, 1.5, and 3 μg, Intra-CA1), or morphine (1, 3, and 6 mg/kg, intraperitoneal (i.p.) was evaluated on the retrieval of passive avoidance learning using step-down task in mice (n=10). Then, the effect of pretest administration of morphine (1.5, 3, and 6 mg/kg, i.p.) was examined on passive avoidance retrieval impairment induced by pre-training SCO (3 μg/mice, Intra-CA1). Next, the effect of pretest Intra-CA1 injection of scopolamine (0.75, 1.5, and 3 μg/mice) was evaluated on morphine (6 mg/kg, i.p.) pre-training deficits in this task in mice.

RESULTS

The pre-training Intra-CA1 injection of scopolamine (1.5 and 3 μg/mouse), or morphine (3 and 6 mg/kg, i.p.) impaired the avoidance memory retrieval when it was tested 24 hours later. Pretest injection of both drugs improved its pre-training impairing effects on mice memory. Moreover, the amnesia induced by the pre-training injections of scopolamine (3 μg/mice) was restored significantly (P<0.01) by pretest injections of morphine (3 and 6 mg/kg, i.p.). Similarly, pretest injection of scopolamine (3 μg/mice) restored amnesia induced by the pre-training injections of morphine (6 mg/kg, i.p.), significantly (P<0.01).

CONCLUSION

The current study findings indicated a cross state-dependent learning between SCO and morphine at CA1 level. Therefore, it seems that muscarinic and opioid receptors may act reciprocally on modulation of passive avoidance memory retrieval, at the level of dorsal hippocampus, in mice.

A threshold model for opposing actions of acetylcholine on reward behavior: Molecular mechanisms and implications for treatment of substance abuse disorders

The cholinergic system plays important roles in both learning and addiction. Medications that modify cholinergic tone can have pronounced effects on behaviors reinforced by natural and drug reinforcers. Importantly, enhancing the action of acetylcholine (ACh) in the nucleus accumbens and ventral tegmental area (VTA) dopamine system can either augment or diminish these behaviors. A threshold model is presented that can explain these seemingly contradictory results. Relatively low levels of ACh rise above a lower threshold, facilitating behaviors supported by drugs or natural reinforcers. Further increases in cholinergic tone that rise above a second upper threshold oppose the same behaviors. Accordingly, cholinesterase inhibitors, or agonists for nicotinic or muscarinic receptors, each have the potential to produce biphasic effects on reward behaviors. Pretreatment with either nicotinic or muscarinic antagonists can block drug- or food- reinforced behavior by maintaining cholinergic tone below its lower threshold. Potential threshold mediators include desensitization of nicotinic receptors and biphasic effects of ACh on the firing of medium spiny neurons. Nicotinic receptors with high- and low- affinity appear to play greater roles in reward enhancement and inhibition, respectively. Cholinergic inhibition of natural and drug rewards may serve as mediators of previously described opponent processes. Future studies should evaluate cholinergic agents across a broader range of doses, and include a variety of reinforced behaviors.

Allosteric modulation of cholinergic system: Potential approach to treating cognitive deficits of schizophrenia

Schizophrenia is a psychiatric disorder affecting approximately 1% of the population worldwide and is characterised by the presence of positive and negative symptoms and cognitive deficits. Whilst current therapeutics ameliorate positive symptoms, they are largely ineffective in improving negative symptoms and cognitive deficits. The cholinergic neurotransmitter system heavily influences cognitive function and there is evidence that implicates disruption of the central cholinergic system in schizophrenia. Historically, targeting the cholinergic system has been impeded by poor selectivity leading to intolerable side effects warranting the need to develop more targeted therapeutic compounds. In this review we will summarise evidence supporting the roles of the cholinergic system, particularly the muscarinic M₁ receptor, in the pathophysiology of schizophrenia and discuss the potential of a promising new class of candidate compounds, allosteric ligands, for addressing the difficulties involved in targeting this system. The body of evidence presented here highlights the dysfunction of the cholinergic system in schizophrenia and that targeting this system by taking advantage of allosteric ligands is having clinically meaningful effect on cognitive deficits.

A Methanol Extract of Brugmansia arborea Affects the Reinforcing and Motor Effects of Morphine and Cocaine in Mice

Previous reports have shown that several of the effects of morphine, including the development of tolerance and physical withdrawal symptoms, are reduced by extracts of Brugmansia arborea (L.) Lagerheim (Solanaceae) (B. arborea). In the present study we evaluate the action of the methanol extract of B. arborea (7.5–60 mg/kg) on the motor and reinforcing effects of morphine (20 and 40 mg/kg) and cocaine (25 mg/kg) using the conditioned place preference (CPP) procedure. At the doses employed, B. arborea did not affect motor activity or induce any effect on CPP. The extract partially counteracted morphine-induced motor activity and completely blocked the CPP induced by 20 mg/kg morphine. On the other hand, B. arborea blocked cocaine-induced hyperactivity but did not block cocaine-induced CPP. Reinstatement of extinguished preference with a priming dose of morphine or cocaine was also inhibited by B. arborea. The complex mechanism of action of B. arborea, which affects the dopaminergic and the cholinergic systems, seems to provide a neurobiological substrate for the effects observed. Considered as a whole, these results point to B. arborea as a useful tool for the treatment of morphine or cocaine abuse.

Contribution of both M1 and M4 receptors to muscarinic agonist-mediated attenuation of the cocaine discriminative stimulus in mice

RATIONALE

We previously showed that muscarinic agonists with M(1) and/or M(4) receptor affinities attenuated cocaine discrimination and self-administration in wild-type mice but not in M(1)/M(4) double-knockout mice.

OBJECTIVE

This study aims to elucidate the respective contributions of M(1) and M(4) receptors to this effect.

METHODS

Knockout mice lacking either the M(1) subtype (M (1) (-/-) ) or the M(4) subtype (M (4) (-/-) ) and wild-type mice were trained to discriminate 10 mg/kg cocaine from saline. Muscarinic ligands were tested for modulation of cocaine discrimination: xanomeline (M(1)/M(4)-preferring agonist), VU0357017 (M(1)-selective partial agonist), 77-LH-28-1 (M(1) agonist), and BQCA (M(1)-selective positive allosteric modulator).

RESULTS

Xanomeline produced rightward shifts in the cocaine dose-effect curve in all three genotypes, but most robustly in wild-type mice. VU0357017 produced rightward shifts in the cocaine dose-effect curve in wild-type and M (4) (-/-) mice, but not in M (1) (-/-) mice. Response rates were suppressed by xanomeline in wild-type and M (1) (-/-) but not in M (4) (-/-) mice and were unaltered by VU0357017. 77-LH-28-1 and BQCA also showed evidence of attenuating cocaine's discriminative stimulus, but at doses that suppressed responding or had other undesirable effects. Intriguingly, both VU0357017 and 77-LH-28-1 exhibited U-shaped dose-effect functions in attenuating cocaine discrimination. None of the drugs substituted for the cocaine stimulus.

CONCLUSIONS

Attenuation of the cocaine stimulus by VU0357017 depended upon M(1) receptors, and full effects of xanomeline depended upon both M(1) and M(4) receptors. Therefore M(1)-selective agonists and mixed M(1)/M(4) agonists may be promising leads for developing medications that block cocaine's effects.

Muscarinic Acetylcholine Receptor Subtypes as Potential Drug Targets for the Treatment of Schizophrenia, Drug Abuse and Parkinson's Disease

The neurotransmitter dopamine plays important roles in modulating cognitive, affective, and motor functions. Dysregulation of dopaminergic neurotransmission is thought to be involved in the pathophysiology of several psychiatric and neurological disorders, including schizophrenia, Parkinson's disease and drug abuse. Dopaminergic systems are regulated by cholinergic, especially muscarinic, input. Not surprisingly, increasing evidence implicates muscarinic acetylcholine receptor-mediated pathways as potential targets for the treatment of these disorders classically viewed as "dopamine based". There are five known muscarinic receptor subtypes (M(1) to M(5)). Due to their overlapping expression patterns and the lack of receptor subtype-specific ligands, the roles of the individual muscarinic receptors have long remained elusive. During the past decade, studies with knock-out mice lacking specific muscarinic receptor subtypes have greatly advanced our knowledge of the physiological roles of the M(1)-M(5) receptors. Recently, new ligands have been developed that can interact with allosteric sites on different muscarinic receptor subtypes, rather than the conventional (orthosteric) acetylcholine binding site. Such agents may lead to the development of novel classes of drugs useful for the treatment of psychosis, drug abuse and Parkinson's disease. The present review highlights recent studies carried out using muscarinic receptor knock-out mice and new subtype-selective allosteric ligands to assess the roles of M(1), M(4), and M(5) receptors in various central processes that are under strong dopaminergic control. The outcome of these studies opens new perspectives for the use of novel muscarinic drugs for several severe disorders of the CNS.

Muscarinic acetylcholine receptors in the nucleus accumbens core and shell contribute to cocaine priming-induced reinstatement of drug seeking

Muscarinic acetylcholine receptors in the nucleus accumbens play an important role in mediating the reinforcing effects of cocaine. However, there is a paucity of data regarding the role of accumbal muscarinic acetylcholine receptors in the reinstatement of cocaine-seeking behavior. The goal of these experiments was to assess the role of muscarinic acetylcholine receptors in the nucleus accumbens core and shell in cocaine and sucrose priming-induced reinstatement. Rats were initially trained to self-administer cocaine or sucrose on a fixed-ratio schedule of reinforcement. Lever-pressing behavior was then extinguished and followed by a subsequent reinstatement phase during which operant responding was induced by either a systemic injection of cocaine in cocaine-experienced rats or non-contingent delivery of sucrose pellets in subjects with a history of sucrose self-administration. Results indicated that systemic administration of the muscarinic acetylcholine receptor antagonist scopolamine (5.0 mg/kg, i.p.) dose-dependently attenuated cocaine, but not sucrose, reinstatement. Furthermore, administration of scopolamine (36.0 μg) directly into the nucleus accumbens shell or core attenuated cocaine priming-induced reinstatement. In contrast, infusion of scopolamine (36.0 μg) directly into the accumbens core, but not shell, attenuated sucrose reinstatement, which suggests that muscarinic acetylcholine receptors in these two subregions of the nucleus accumbens have differential roles in sucrose seeking. Taken together, these results indicate that cocaine priming-induced reinstatement is mediated, in part, by increased signaling through muscarinic acetylcholine receptors in the shell subregion of the nucleus accumbens. Muscarinic acetylcholine receptors in the core of the accumbens, in contrast, appear to play a more general (i.e. not cocaine specific) role in motivated behaviors.

A new nosology of psychosis and the pharmacological basis of affective and negative symptom dimensions in schizophrenia

Although first rank symptoms focus on positive symptoms of psychosis they are shared by a number of psychiatric conditions. The difficulty in differentiating bipolar disorder from schizophrenia with affective features has led to a third category of patients often loosely labeled as schizoaffective. Research in schizophrenia has attempted to render the presence or absence of negative symptoms and their relation to etiology and prognosis more explicit. A dichotomous population is a recurring theme in experimental paradigms. Thus, schizophrenia is defined as process or reactive, deficit or non-deficit and by the presence or absence of affective symptoms. Laboratory tests confirm the clinical impression showing conflicting responses to dexamethasone suppression and clearly defined differences in autonomic responsiveness, but their patho-physiological significance eludes mainstream theory. Added to this is the difficulty in agreeing to what exactly constitutes useful clinical features differentiating, for example, negative symptoms of a true deficit syndrome from features of depression. Two recent papers proposed that the general and specific cognitive features of schizophrenia and major depression result from a monoamine-cholinergic imbalance, the former due to a relative muscarinic receptor hypofunction and the latter, in contrast, to a muscarinic hypersensitivity exacerbated by monoamine depletion. Further development of these ideas will provide pharmacological principles for what is currently an incomplete and largely, descriptive nosology of psychosis. It will propose a dimensional view of affective and negative symptoms based on relative muscarinic integrity and is supported by several exciting intracellular signaling and gene expression studies. Bipolar disorder manifests both muscarinic and dopaminergic hypersensitivity. The greater the imbalance between these two receptor signaling systems, the more the clinical picture will resemble schizophrenia with bizarre, incongruent delusions and increasingly disorganized thought. The capacity for affective expression, by definition a non-deficit syndrome, will remain contingent on the degree of preservation of muscarinic signaling, which itself may be unstable and vary between trait and state examinations. At the extreme end of muscarinic impairment, a deficit schizophrenia subpopulation is proposed with a primary and fixed muscarinic receptor hypofunction.

The genomic profile of bipolar disorder and schizophrenia overlap and both have a common dopaminergic intracellular signaling which is hypersensitive to various stressors. It is proposed that the concomitant muscarinic receptor upregulation differentiates the syndromes, being marked in bipolar disorder and rather less so in schizophrenia. From a behavioral point of view non-deficit syndromes and bipolar disorder appear most proximate and could be reclassified as a spectrum of affective psychosis or schizoaffective disorders. Because of a profound malfunction of the muscarinic receptor, the deficit subgroup cannot express a comparable stress response. Nonetheless, a convergent principle of psychotic features across psychiatric disorders is a relative monoaminergic-muscarinic imbalance in signal transduction.

Differences between nicotine and cocaine-induced conditioned place preferences

In previous studies, we found differences between nicotine and cocaine-induced changes in the levels of neurotransmitters in various brain areas, which suggested differences in their reward - preference mechanisms. The present study was based on the idea that drug preference is modulated by a number of different factors, among them several neurotransmitters and their receptors, and antagonists of specific receptors will influence preference. We also assumed that the factors (components of reward mechanisms) involved are different in the case of different drugs. We compared the inhibition of nicotine preference with cocaine preference. We assayed preference as conditioned place preference (CPP) and measured CPP inhibition by receptor subtype antagonists using mice. In general, induced CPP of cocaine was stronger than of nicotine as shown by more time spent in the nonpreferred area after conditioning with cocaine. We measured inhibition by four antagonists: mecamylamine, atropine, SCH23390, and phentolamine: antagonists respectively of nicotinic, and muscarinic acetylcholine, dopamine D1, and alpha noradrenergic receptors. The inhibition by the antagonists of cocaine CPP was lower in most instances than that of nicotine CPP. Atropine and SCH23390 inhibited nicotine and cocaine CPP approximately to the same degree, while the inhibition by mecamylamine and phentolamine of nicotine CPP was 100%; that of cocaine was 20% and 0, respectively. We conclude that several receptor systems and transmitters play a role in drug preference, some represent essential elements or circuits, some may be only required partially or their role can be partially substituted. The composition of such systems is different for different drugs - in the present study, some of the components influencing CPP are different for nicotine as opposed to cocaine.

Attenuation of cocaine's reinforcing and discriminative stimulus effects via muscarinic M1 acetylcholine receptor stimulation

Muscarinic cholinergic receptors modulate dopaminergic function in brain pathways thought to mediate cocaine's abuse-related effects. Here, we sought to confirm and extend in the mouse species findings that nonselective muscarinic receptor antagonists can enhance cocaine's discriminative stimulus. More importantly, we tested the hypothesis that muscarinic receptor agonists with varied receptor subtype selectivity can blunt cocaine's discriminative stimulus and reinforcing effects; we hypothesized a critical role for the M(1) and/or M(4) receptor subtypes in this modulation. Mice were trained to discriminate cocaine from saline, or to self-administer intravenous cocaine chronically. The nonselective muscarinic antagonists scopolamine and methylscopolamine, the nonselective muscarinic agonists oxotremorine and pilocarpine, the M(1)/M(4)-preferring agonist xanomeline, the putative M(1)-selective agonist (4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride (McN-A-343), and the novel M(1)-selective agonist 1-(1-2-methylbenzyl)-1,4-bipiperidin-4-yl)-1H benzo[d]imidazol-2(3H)-one (TBPB) were tested as substitution and/or pretreatment to cocaine. Both muscarinic antagonists partially substituted for cocaine and enhanced its discriminative stimulus. Conversely, muscarinic agonists blunted cocaine discrimination and abolished cocaine self-administration with varying effects on food-maintained behavior. Specifically, increasing selectivity for the M(1) subtype (oxotremorine < xanomeline < TBPB) conferred lesser nonspecific rate-suppressing effects, with no rate suppression for TBPB. In mutant mice lacking M(1) and M(4) receptors, xanomeline failed to diminish cocaine discrimination while rate-decreasing effects were intact. Our data suggest that central M(1) receptor activation attenuates cocaine's abuse-related effects, whereas non-M(1)/M(4) receptors probably contribute to undesirable effects of muscarinic stimulation. These data provide the first demonstration of anticocaine effects of systemically applied, M(1) receptor agonists and suggest the possibility of a new approach to pharmacotherapy for cocaine addiction.

A dopamine transport inhibitor with markedly low abuse liability suppresses cocaine self-administration in the rat

RATIONALE

N-substituted benztropine analogs are potent dopamine uptake inhibitors that display pharmacokinetic/dynamic properties consistent with the profile of a substitute medication for cocaine addiction.

OBJECTIVES

The purpose of the present experiments was to characterize in rats the addictive-like properties of one such analog, 3 alpha-[bis(4'-fluorophenyl)methoxy]-tropane (AHN-1055), incorporating probes of its stimulant and incentive/motivational effects and of its ability to influence cocaine self-administration.

METHODS

We used open field activity and drug self-administration assays. To examine the effects of AHN-1055 on locomotor behavior, the analog was administered alone (0, 1, 3, and 10 mg/kg intraperitoneally) and in combination with cocaine (15 mg/kg i.p.). The influence of AHN-1055 on cocaine's intake was studied by administering the analog (0, 3, and 10 mg/kg i.p.) before the start of the self-administration sessions. To compare the addictive-like properties of AHN-1055 and cocaine, progressive ratio performance and abstinence-induced context-conditioned relapse were evaluated.

RESULTS

AHN-1055 evoked robust and sustained locomotor activity when administered alone and increased cocaine-induced locomotor stimulation. Notably, the analog showed by comparison to cocaine weak reinforcing efficacy in a modified progressive ratio schedule of drug reinforcement, and contrary to cocaine, it showed no ability to promote context-conditioned relapse to drug seeking following stable self-administration and abstinence. Further, AHN-1055 treatment blocked cocaine intake dose-dependently in rats with a steady history of cocaine self-administration without reducing responding for sucrose, a natural reward.

CONCLUSIONS

These findings demonstrate essential psychopharmacological differences between AHN-1055 and cocaine and highlight important properties of the analog as a possible pharmacotherapy in cocaine addiction.

Cholinergic functioning in stimulant addiction: implications for medications development

Acetylcholine, the first neurotransmitter discovered, participates in many CNS functions, including sensory and motor processing, sleep, nociception, mood, stress response, attention, arousal, memory, motivation and reward. These diverse cholinergic effects are mediated by nicotinic- and muscarinic-type cholinergic receptors (nAChR and mAChR, respectively). The goal of this review is to synthesize a growing literature that supports the potential role of acetylcholine as a treatment target for stimulant addiction. Acetylcholine interacts with the dopaminergic reward system in the ventral tegmental area, nucleus accumbens and prefrontal cortex. In the ventral tegmental area, both nAChR and mAChR stimulate the dopaminergic system. In the nucleus accumbens, cholinergic interneurons integrate cortical and subcortical information related to reward. In the prefrontal cortex, the cholinergic system contributes to the cognitive aspects of addiction. Preclinical studies support a facilitative role of nicotinic receptor agonists in the development of stimulant addiction. In contrast, nonselective muscarinic receptor agonists seem to have an inhibitory role. In human studies, acetylcholinesterase inhibitors, which increase synaptic acetylcholine levels, have shown promise for the treatment of stimulant addiction. Further studies testing the efficacy of cholinergic medications for stimulant addiction are warranted.

The dopamine uptake inhibitor 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane reduces cocaine-induced early-gene expression, locomotor activity, and conditioned reward

Benztropine (BZT) analogs, a family of high-affinity dopamine transporter ligands, are molecules that exhibit pharmacological and behavioral characteristics predictive of significant therapeutic potential in cocaine addiction. Here, we examined in mice the effects of 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane (AHN-1055) on motor activity, conditioned place preference (CPP) and c-Fos expression in the striatum. AHN-1055 produced mild attenuation of spontaneous locomotor activity at a low dose (1 mg/kg) and weak stimulation at a higher dose (10 mg/kg). In parallel, the BZT analog significantly increased c-Fos expression in the dorsolateral caudoputamen at the high dose, whereas producing marginal decreases at low and moderate doses (1, 3 mg/kg) in both dorsal and ventral striatum. Interaction assays showed that cocaine's ability to stimulate locomotor activity was decreased by AHN-1055 treatment, but not by treatment with D-amphetamine. Such reduced ability did not result from an increase in stereotyped behavior. Another dopamine uptake inhibitor, nomifensine, decreased cocaine-induced locomotor activity but evoked by itself intense motor stereotypies. Remarkably, the BZT analog dose-dependently blocked cocaine-induced CPP without producing CPP when given alone, and blocked in conditioned mice cocaine-stimulated early-gene activation in the nucleus accumbens and dorsomedial striatum. These observations provide evidence that AHN-1055 does not behave as a classical psychomotor stimulant and that some of its properties, including attenuation of cocaine-induced striatal c-Fos expression, locomotor stimulation, and CPP, support its candidacy, and that of structurally related molecules, as possible pharmacotherapies in cocaine addiction.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Comparison of systemic and local methamphetamine treatment on acetylcholine and dopamine levels in the ventral tegmental area in the mouse

Acetylcholine (ACh) is an important mediator of dopamine (DA) release and the behavioral reinforcing characteristics of drugs of abuse in the mesocorticolimbic pathway. Within the ventral tegmental area (VTA), the interaction of DA with ACh appears to be integral in mediating motivated behaviors. However, the effects of methamphetamine on VTA ACh and DA release remain poorly characterized. The current investigation performed microdialysis to evaluate the effects of methamphetamine on extracellular levels of ACh and DA. Male C57BL/6J mice received an i.p. injection (saline, 2 mg/kg, or 5 mg/kg) and an intra-VTA infusion (vehicle, 100 microM or 1 mM) of methamphetamine. Locally perfused methamphetamine resulted in no change in extracellular ACh compared with vehicle, but caused a strong, immediate and dose-dependent increase in extrasynaptic DA levels (1240% and 2473% of baseline, respectively) during the 20-min pulse perfusion. An i.p. injection of methamphetamine increased extrasynaptic DA to 275% and 941% of baseline (2 mg/kg and 5 mg/kg, respectively). Systemic methamphetamine significantly increased ACh levels up to 275% of baseline for 40-60 min (2 mg/kg) and 397% of baseline for 40-160 min (5 mg/kg) after injection. ACh remained elevated above baseline for 2-3 h post injection, depending on the methamphetamine dose. Methamphetamine-induced locomotor activity was dose-dependently correlated with extrasynaptic VTA ACh, but not DA levels. These data suggest that methamphetamine acts in the VTA to induce a robust and short-lived increase in extracellular DA release but acts in an area upstream from the VTA to produce a prolonged increase in ACh release in the VTA. We conclude that methamphetamine may activate a recurrent loop in the mesocorticolimbic DA system to stimulate pontine cholinergic nuclei and produce a prolonged ACh release in the VTA.

Galanin protects against behavioral and neurochemical correlates of opiate reward

The mechanisms underlying responses to drugs of abuse have been widely investigated; however, less is known about pathways normally protective against the development of drug reinforcement. These pathways are also important since they may regulate individual differences in vulnerability to addiction. The neuropeptide galanin and its binding sites are expressed in brain areas important for drug reward. Previous studies have shown that centrally infused galanin attenuates morphine place preference and peripheral injection of galnon, a galanin agonist, decreases opiate withdrawal signs. The current studies in galanin knockout (GKO) mice examined the hypothesis that galanin is an endogenous negative regulator of opiate reward and identified downstream signaling pathways regulated by galanin. We show that GKO mice demonstrate increased locomotor activation following morphine administration, which is inhibited by acute administration of galnon. GKO mice also show enhanced morphine place preference, supporting the idea that galanin normally antagonizes opiate reward. In addition, morphine-induced ERK1/2 phosphorylation was increased in the VTA of both wild-type and GKO mice, but only the GKO mice showed increases in ERK1/2 and CREB phosphorylation in the amygdala or nucleus accumbens. Furthermore, a single systemic injection of galnon in GKO mice was sufficient to reverse some of the biochemical changes brought about by morphine administration. These data suggest that galanin normally attenuates behavioral and neurochemical effects of opiates; thus, galanin agonists may represent a new class of therapeutic targets for opiate addiction.

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.