Locomotion and self-administration induced by cocaine in 129/OlaHsd mice lacking galanin

microRNA expression levels in the nucleus accumbens correlate with morphine-taking but not morphine-seeking behaviour in male rats

A powerful motivation to seek opioids remains after drug cessation and intensifies during extended periods of abstinence. Unfortunately, biomarkers associated with continued drug seeking have not been described. Moreover, previous studies have focused on the effects of early abstinence with little exploration into the long-term drug-induced mechanisms that occur after extended abstinence. Here we demonstrated that 30 days (D) of forced abstinence results in a time-dependent increase in morphine seeking in a rat model of morphine self-administration (SA). We measured expression of known drug-responsive microRNAs (miRNAs) in the nucleus accumbens, an area critical for reward-related plasticity, during early or late abstinence in animals that underwent either a cue-induced relapse test or no relapse test. miRNAs are small noncoding RNAs that represent suitable biomarker candidates due to their long-lasting nature. mir-32-5p levels during early abstinence negatively correlated with active lever pressing in both cue-exposed and cue-naïve animals. mir-1298-5p positively correlated with drug SA history after a relapse test during late abstinence. When animals underwent acute abstinence with no relapse test, mir-1298-5p correlated with drug infusions and active lever pressing during SA. In late abstinence with no relapse test, mir-137-3p negatively correlated with drug infusions. Regulation of mir-32-5p target genes and significant correlation of target gene mRNA with mir-32-5p was observed after abstinence. These results indicate that lasting regulation of miRNA expression is associated with drug intake following morphine SA. In addition, we conclude that the miRNA profile undergoes regulation from early to late abstinence and miRNA expression may indicate past drug history.

Drug-associated cues and drug dosage contribute to increased opioid seeking after abstinence

Patients with opioid use disorder experience high rates of relapse during recovery, despite successful completion of rehabilitation programs. A key factor contributing to this problem is the long-lasting nature of drug-seeking behavior associated with opioid use. We modeled this behavior in a rat drug self-administration paradigm in which drug-seeking is higher after extended abstinence than during the acute abstinence phase. The goal of this study was to determine the contribution of discrete or discriminative drug cues and drug dosage to time-dependent increases in drug-seeking. We examined heroin-seeking after 2 or 21 days of abstinence from two different self-administration cue-context environments using high or low doses of heroin and matched animals for their drug intake history. When lower dosages of heroin are used in discriminative or discrete cue protocols, drug intake history contributed to drug-seeking after abstinence, regardless of abstinence length. Incubation of opioid craving at higher dosages paired with discrete drug cues was not dependent on drug intake. Thus, interactions between drug cues and drug dosage uniquely determined conditions permissible for incubation of heroin craving. Understanding factors that contribute to long-lasting opioid-seeking can provide essential insight into environmental stimuli and drug-taking patterns that promote relapse after periods of successful abstinence.

Pitolisant and intravenous cocaine self-administration in mice

Pitolisant, a selective inverse agonist for the histamine H3 receptor, is a new treatment for adults suffering from narcolepsy. Numerous studies have shown that striatal H3 receptors can modulate the activity of the dopamine mesolimbic system, a neuronal pathway that plays a crucial role in drug addiction. Therefore, it is important to guarantee that pitolisant has no abuse potential and does not potentiate the behavioral effects of psychostimulants. The present study tested the effects of pitolisant on cocaine reinforcement in C57BL/6J mice using the intravenous self-administration technique. Mice were trained to self-administer cocaine intravenously. After the acquisition of cocaine self-administration, pitolisant was tested on cocaine self-administration under different schedules of reinforcement (fixed ratio and progressive ratio). In another group of mice, cocaine was replaced with pitolisant after the acquisition of cocaine self-administration. Finally, a group of mice was trained to self-administer pitolisant intravenously and directly compared to mice trained to self-administer cocaine under the same conditions. Our results indicate that pitolisant does not influence the reinforcing effects of cocaine under any of the experimental conditions used in this study. Moreover, pitolisant has no reinforcing properties alone when tested in the self-administration paradigm. Our results offer more evidence to support the hypothesis that pitolisant is not addictive. In addition, pitolisant does not alter the reinforcing effects of cocaine. Finally, the present study provides no evidence for a significant involvement of histamine H3 receptors in cocaine dependence.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

The galanin receptor agonist, galnon, attenuates cocaine-induced reinstatement and dopamine overflow in the frontal cortex

Relapse represents one of the most significant problems in the long-term treatment of drug addiction. Cocaine blocks plasma membrane monoamine transporters and increases dopamine (DA) overflow in the brain, and DA is critical for the motivational and primary reinforcing effects of the drug as well as cocaine-primed reinstatement of cocaine seeking in rats, a model of relapse. Thus, modulators of the DA system may be effective for the treatment of cocaine dependence. The endogenous neuropeptide galanin inhibits DA transmission, and both galanin and the synthetic galanin receptor agonist, galnon, interfere with some rewarding properties of cocaine. The purpose of this study was to further assess the effects of galnon on cocaine-induced behaviors and neurochemistry in rats. We found that galnon attenuated cocaine-induced motor activity, reinstatement and DA overflow in the frontal cortex at a dose that did not reduce baseline motor activity, stable self-administration of cocaine, baseline extracellular DA levels or cocaine-induced DA overflow in the nucleus accumbens (NAc). Similar to cocaine, galnon had no effect on stable food self-administration but reduced food-primed reinstatement. These results indicate that galnon can diminish cocaine-induced hyperactivity and relapse-like behavior, possibly in part by modulating DA transmission in the frontal cortex.

Galanin-induced decreases in nucleus accumbens/striatum excitatory postsynaptic potentials and morphine conditioned place preference require both galanin receptor 1 and galanin receptor 2

The neuropeptide galanin has been shown to alter the rewarding properties of morphine. To identify potential cellular mechanisms that might be involved in the ability of galanin to modulate opiate reward, we measured excitatory postsynaptic potentials (EPSPs), using both field and whole-cell recordings from striatal brain slices extracted from wild-type mice and mice lacking specific galanin receptor (GalR) subtypes. We found that galanin decreased the amplitude of EPSPs in both the dorsal striatum and nucleus accumbens. We then performed recordings in slices from knockout mice lacking either the GalR1 or GalR2 gene, and found that the ability of galanin to decrease EPSP amplitude was absent from both mouse lines, suggesting that both receptor subtypes are required for this effect. In order to determine whether behavioral responses to opiates were dependent on the same receptor subtypes, we tested GalR1 and GalR2 knockout mice for morphine conditioned place preference (CPP). Morphine CPP was significantly attenuated in both GalR1 and GalR2 knockout mice. These data suggest that mesolimbic excitatory signaling is significantly modulated by galanin in a GalR1-dependent and GalR2-dependent manner, and that morphine CPP is dependent on the same receptor subtypes.

The drive to eat: comparisons and distinctions between mechanisms of food reward and drug addiction

The growing rates of obesity have prompted comparisons between the uncontrolled intake of food and drugs; however, an evaluation of the equivalence of food- and drug-related behaviors requires a thorough understanding of the underlying neural circuits driving each behavior. Although it has been attractive to borrow neurobiological concepts from addiction to explore compulsive food seeking, a more integrated model is needed to understand how food and drugs differ in their ability to drive behavior. In this Review, we will examine the commonalities and differences in the systems-level and behavioral responses to food and to drugs of abuse, with the goal of identifying areas of research that would address gaps in our understanding and ultimately identify new treatments for obesity or drug addiction.

Galanin negatively modulates opiate withdrawal via galanin receptor 1

RATIONALE

The neuropeptide galanin has been shown to modulate opiate dependence and withdrawal. These effects could be mediated via activation of one or more of the three distinct G protein-coupled receptors, namely galanin receptors 1 (GalR1), 2 (GalR2), and 3 (GalR3).

OBJECTIVES

In this study, we used several transgenic mouse lines to further define the mechanisms underlying the role played by galanin and its receptors in the modulation of morphine dependence. First, transgenic mice expressing β-galactosidase under the control of the galanin promoter were used to assess the regulation of galanin expression in response to chronic morphine administration and withdrawal. Next, the behavioral responses to chronic morphine administration and withdrawal were tested in mice that over-express galanin, lack the GalR1 gene, or lack the GalR2 gene.

METHODS

Transgenic and matched wild-type mice were given increasing doses of morphine followed by precipitation of withdrawal by naloxone and behavioral responses to withdrawal were assessed.

RESULTS

Both morphine administration and withdrawal increased galanin gene transcription in the locus coeruleus (LC). Increasing galanin levels in the brain reduced signs of opiate withdrawal. Mice lacking GalR1 undergo more severe opiate withdrawal, whereas mice lacking GalR2 show no significant difference in withdrawal signs, compare with matched wild-type controls.

CONCLUSIONS

Opiate administration and withdrawal increase galanin expression in the LC. Galanin opposes the actions of morphine which leads to opiate dependence and withdrawal, an effect that is mediated via GalR1.

Mice lacking the galanin gene show decreased sensitivity to nicotine conditioned place preference

Previous work has indicated that the neuropeptide galanin decreases sensitivity to the rewarding effects of morphine and cocaine, but increases alcohol drinking. The aim of the current study was to examine the role of galanin signaling in nicotine reward by testing the effects of nicotine in mice lacking galanin peptide (GAL-/-) as compared to wild-type (GAL+/+) controls. Using an unbiased, three-chamber conditioned place preference (CPP) paradigm the dose-response function for nicotine CPP was tested in GAL-/- and GAL+/+ mice. Since activation of extracellular signal-related kinase (ERK2) is involved in the rewarding effects of several classes of drugs of abuse, we then measured the level of ERK2 phosphorylation in the nucleus accumbens shell (NACsh) and core (NACco) of GAL-/- and GAL+/+ mice following re-exposure to the CPP chamber previously paired with nicotine as a marker of mesolimbic system activation. Finally, we examined whether acute nicotine administration affects ERK2 activity in GAL-/- and GAL+/+ mice. GAL-/- mice required a higher dose of nicotine to induce a significant CPP compared to GAL+/+ mice. In the conditioning groups showing significant expression of nicotine CPP, only GAL+/+ mice showed ERK2 activation in the NACsh. This suggests that the nicotine CPP observed in GAL+/+ mice resulted in differential recruitment of ERK signaling in the NACsh compared to GAL-/- mice. In addition, no activation of ERK2 was observed following acute nicotine administration in either genotype. These data, along with prior results, suggest that galanin alters sensitivity to drugs of abuse differentially, with morphine, cocaine and amphetamine place preference suppressed, and nicotine and alcohol preference increased, by galanin signaling.