Petri Hyytiä

mGluR5 Antagonist MPEP Reduces Ethanol-Seeking and Relapse Behavior

The glutamatergic system plays an important role in mediating neurobehavioral effects of ethanol. Metabotropic glutamate receptors subtype 5 (mGluR5) are modulators of glutamatergic neurotransmission and are abundant in brain regions known to be involved in ethanol self-administration. Here, we studied the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a highly potent, noncompetitive mGlu5 receptor antagonist, on voluntary ethanol consumption and relapse behavior. For this purpose, we used two models for the measurement of relapse behavior: (i) reinstatement of ethanol-seeking behavior by drug-associated cues and (ii) the alcohol deprivation effect in long-term ethanol-consuming rats. In the first set of experiments, rats were trained to lever press for ethanol in the presence of a distinct set of cues. After extinction, the animals were exposed to the respective cues that initiated reinstatement of responding. A response-contingent ethanol prime further enhanced responding compared to the conditioned cues alone. Under these conditions, MPEP (0, 1, 3, and 10 mg/kg) attenuated ethanol seeking significantly and in a dose-related manner. However, at the highest dose, MPEP also decreased the number of inactive lever responses. In the second set of experiments, rats with 1 year of ethanol experience and repeated deprivation phases were used. A subchronic treatment with MPEP (twice daily; 0, 3, and 10 mg/kg) resulted in a significant and dose-dependent reduction of the alcohol deprivation effect (ADE). Although the same MPEP treatment regimen decreased baseline drinking, this effect was not as pronounced as on the ADE. These results show in two commonly used models of relapse to ethanol that pharmacological targeting of mGlu5 receptors may be a promising approach for the treatment of alcoholism.

Ionotropic and metabotropic glutamate receptor antagonism attenuates cue-induced cocaine seeking.

Neuroanatomical and pharmacological evidence implicates glutamate transmission in drug-environment conditioning that partly controls drug seeking and relapse. Glutamate receptors could be targets for pharmacological attenuation of the motivational properties of drug-paired cues and for relapse prevention. The purpose of the present study was therefore to investigate the involvement of ionotropic and metabotropic glutamate receptor subtypes in cue-induced reinstatement of cocaine-seeking behavior. Rats were trained to self-administer cocaine using a second-order schedule of reinforcement (FR4(FR5:S)) under which a compound stimulus (light and tone) associated with cocaine infusions was presented contingently. Following extinction, the effects of the competitive NMDA receptor antagonist CGP 39551 (0, 2.5, 5, 10 mg/kg intraperitoneally (i.p.)), two competitive AMPA/kainate antagonists, CNQX (0, 0.75, 1.5, 3 mg/kg i.p.) and NBQX (0, 1.25, 2.5, 5 mg/kg i.p.), the NMDA/glycine site antagonist L-701,324 (0, 0.63, 1.25, 2.5 mg/kg i.p.), and the mGluR5 antagonist MPEP (0, 1.25, 2.5, 5 mg/kg i.p.) on cue-induced reinstatement of cocaine seeking were examined. The AMPA/kainate receptor antagonists CNQX and NBQX, the NMDA/glycine site antagonist L-701,324, and the mGluR5 antagonist MPEP attenuated significantly cue-induced reinstatement. The NMDA antagonist CGP 39551 failed to affect reinstatement. Additional control experiments indicated that attenuation of cue-induced reinstatement by CNQX, NBQX, L-701,324, and MPEP was not accompanied by significant suppression of spontaneous locomotor activity. These results suggest that conditioned influences on cocaine seeking depend on glutamate transmission. Accordingly, drugs with antagonist properties at various glutamate receptor subtypes could be useful in prevention of relapse induced by conditioned stimuli.

Genetic Impairment of Frontocortical Endocannabinoid Degradation and High Alcohol Preference

Endocannabinoid signaling has recently been implicated in ethanol-seeking behavior. We analyzed the expression of endocannabinoid-related genes in key brain regions of reward and dependence, and compared them between the alcohol-preferring AA (Alko Alcohol) and nonpreferring ANA (Alko Non-Alcohol) rat lines. A decreased expression of fatty acid amidohydrolase (FAAH), the main endocannabinoid-degrading enzyme, was found in prefrontal cortex (PFC) of AA rats, and was accompanied by decreased enzyme activity in this region. Binding of the endocannabinoid-cannabinoid 1 (CB1) receptor ligand 3[H]SR141716A, and [35S]GTPγS incorporation stimulated by the CB1 agonist WIN 55,212-2 were downregulated in the same area. Together, this suggests an overactive endocannabinoid transmission in the PFC of AA animals, and a compensatory downregulation of CB1 signaling. The functional role of impaired FAAH function for alcohol self-administration was validated in two independent ways. The CB1 antagonist SR141716A potently and dose-dependently suppressed self-administration in AA rats when given systemically, or locally into the PFC, but not in the striatum. Conversely, intra-PFC injections of the competitive FAAH inhibitor URB597 increased ethanol self-administration in nonselected Wistar rats. These results show for the first time that impaired FAAH function may confer a phenotype of high voluntary alcohol intake, and point to a FAAH both as a potential susceptibility factor and a therapeutic target.

The alcohol-preferring AA and alcohol-avoiding ANA rats: neurobiology of the regulation of alcohol drinking.

The AA (alko, alcohol) and ANA (alko, non‐alcohol) rat lines were among the earliest rodent lines produced by bidirectional selection for ethanol preference. The purpose of this review is to highlight the strategies for understanding the neurobiological factors underlying differential alcohol‐drinking behavior in these lines. Most early work evaluated functioning of the major neurotransmitter systems implicated in drug reward in the lines. No consistent line differences were found in the dopaminergic system either under baseline conditions or after ethanol challenges. However, increased opioidergic tone in the ventral striatum and a deficiency in endocannabinoid signaling in the prefrontal cortex of AA rats may comprise mechanisms leading to increased ethanol consumption. Because complex behaviors, such as ethanol drinking, are not likely to be controlled by single factors, system‐oriented molecular‐profiling strategies have been used recently. Microarray based expression analysis of AA and ANA brains and novel data‐mining strategies provide a system biological view that allows us to formulate a hypothesis on the mechanism underlying selection for ethanol preference. Two main factors appear active in the selection: a recruitment of signal transduction networks, including mitogen‐activated protein kinases and calcium pathways and involving transcription factors such as Creb, Myc and Max, to mediate ethanol reinforcement and plasticity. The second factor acts on the mitochondrion and most likely provides metabolic flexibility for alternative substrate utilization in the presence of low amounts of ethanol.

Ionotropic Glutamate Receptor Antagonists Modulate Cue‐Induced Reinstatement of Ethanol‐Seeking Behavior

BACKGROUND Glutamatergic neurotransmission has been implicated in drug-environment conditioning, but little is known about the role of glutamate in alcohol seeking maintained by alcohol-associated cues. Therefore, we examined the effects of ionotropic glutamate receptor antagonists on cue-induced ethanol-seeking behavior in the extinction/reinstatement model. METHODS Rats were trained to orally self-administer ethanol (10% w/v) and a nonrewarding (80 microM) quinine solution on randomly alternating days. Ethanol and quinine availability were signaled by olfactory discriminative stimuli (S+/S-). In addition, ethanol delivery was accompanied by a light stimulus (CS+) and quinine delivery by an auditory stimulus (CS-). Thereafter, rats were subjected to extinction training during which responding had no programmed consequences. Reinstatement of responding was tested under three conditions: in the presence of the S-/CS-, S+/CS+, and S+/CS+ together with a small (0.2 ml) response-contingent oral ethanol dose at the beginning of the reinstatement session (S+/CS+/priming). We examined the effects of the noncompetitive NMDA receptor antagonist MK-801 (0, 0.05, 0.15 mg/kg intraperitoneally), the competitive NMDA antagonist CGP39551 (0, 5, 10 mg/kg intraperitoneally), the NMDA/glycine receptor antagonist L-701,324 (0, 2, 4 mg/kg intraperitoneally), the AMPA/kainate receptor antagonist CNQX (0, 0.5, 1.5 mg/kg intraperitoneally), and the opioid receptor antagonist naltrexone (0, 0.3, 1 mg/kg subcutaneously) on ethanol seeking under the S+/CS+/priming condition. RESULTS Presentation of the S+/CS+ stimulus condition reinstated extinguished responding, whereas presentation of the S-/CS- condition did not. Response-contingent ethanol priming enhanced reinstatement further. Under these reinstatement conditions, L-701,324, CNQX, and naltrexone inhibited ethanol-seeking behavior significantly. In contrast, MK-801 and CGP39551 failed to affect reinstated responding. CONCLUSIONS These results show that glutamate antagonism suppresses ethanol-seeking behavior induced by ethanol-paired stimuli. Furthermore, the data suggest that ionotropic glutamate receptors may have differential roles in mediation of this behavior.

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity.

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

Alcohol drinking is reduced by a μ1- but not by a δ-opioid receptor antagonist in alcohol-preferring rats

To assess the roles of opioid receptor subtypes in voluntary alcohol drinking, alcohol-preferring AA (Alko, Alcohol) rats, non-deprived of food or water, were used in a paradigm where access to 10% alcohol solution was limited to 1–4-h sessions on every 2nd working day. The δ-opioid receptor antagonist naltrindole (1–5 mg/kg i.p. 15 min before the session) had no effect on alcohol drinking, while it attenuated the δ-opioid receptor agonist [d-Pen2,d-Pen5]enkephalin-induced locomotor stimulation. The μ1-opioid receptor antagonist naloxonazine (1–15 mg/kg i.p. 20 h before the session), at the largest dose, decreased alcohol drinking. It also decreased food intake. When naltrindole (1 mg/kg) and naloxonazine (15 mg/kg) were given prior to 3 consecutive sessions, the former had no effects at any session. Naloxonazine decreased alcohol consumption only in the 1st session, although the reduction of daily water intake became stronger during repeated administration. 4 days after the last drug administration, naloxonazine-treated animals consumed alcohol nearly twice as much as in the control session before any drug treatment. These data suggest that δ-opioid receptors are not involved in the regulation of alcohol drinking in AA rats. μ1-Opioid receptors may be involved in alcohol drinking, although the data suggest that even their prolonged blockade alone is insufficient to induce a sustained decrease in alcohol drinking.

Ghrelin receptor (GHS-R1A) antagonism suppresses both operant alcohol self-administration and high alcohol consumption in rats.

The mechanisms involved in alcohol use disorders are complex. It has been shown that ghrelin is an important signal for the control of body weight homeostasis, preferably by interacting with hypothalamic circuits, as well as for drug reward by activating the mesolimbic dopamine system. The ghrelin receptor (GHS‐R1A) has been shown to be required for alcohol‐induced reward. Additionally, ghrelin increases and GHR‐R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS‐R1A gene has been associated with high alcohol consumption in humans. However, the role of central ghrelin signaling in high alcohol consumption is not known. Therefore, the role of GHS‐R1A in operant self‐administration of alcohol in rats as well as for high alcohol consumption in Long‐Evans rats and in alcohol preferring [Alko alcohol (AA)] rats was studied here. In the present study, the GHS‐R1A antagonist, JMV2959, was found to reduce the operant self‐administration of alcohol in rats and to decrease high alcohol intake in Long‐Evans rats as well as in AA rats. These results suggest that the ghrelin receptor signaling system, specifically GHS‐R1A, is required for operant self‐administration of alcohol and for high alcohol intake in rats. Therefore, the GHS‐R1A may be a therapeutic target for treatment of addictive behaviors, such as alcohol dependence.

Site-specific NMDA receptor antagonists produce differential effects on cocaine self-administration in rats

The effects of site-specific NMDA receptor antagonists on intravenous cocaine self-administration were examined in rats trained to self-administer cocaine (0.25 mg/infusion) on a fixed ratio (FR) 5 schedule with a 20-s time-out (TO) after each reinforcer. The non-competitive NMDA receptor antagonists, dizocilpine (MK-801, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (0.05-0.2 mg/kg i.p.) and memantine (1,3-dimethyl-5-amino-adamantane hydrochloride) (2.5-20 mg/kg i.p.), dose-dependently decreased cocaine self-administration, while the competitive NMDA receptor antagonist, CGP 39551 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid carboxyethylester) (2.5-15 mg/kg i.p.), and the NMDA/glycine receptor antagonist, L-701,324 (7-chloro-4-hydroxy-3(3-phenoxy)-phenyl-2(H)quinolone) (1.25-10 mg/kg p.o.), were without effect. Under a progressive ratio (PR) schedule, dizocilpine (0.15 mg/kg i.p.) increased the number of cocaine infusions in a manner similar to increasing the unit dose of cocaine, suggestive of potentiation of cocaine reward. Conversely, memantine (10 mg/kg i.p.) produced rate-decreasing effects on the PR schedule. These results demonstrate that NMDA receptor antagonists acting at different modulatory sites of the NMDA receptor do not share dizocilpines cocaine reward enhancing effects although they are all known to be effective blockers of NMDA receptor activity.

Ethanol Self-Administration Is Regulated by CB1 Receptors in the Nucleus Accumbens and Ventral Tegmental Area in Alcohol-Preferring AA Rats

BACKGROUND Endogenous cannabinoids and their receptors, CB1 receptors in particular, have been implicated in mediation of ethanol reinforcement. Previously, suppression of ethanol drinking by CB1 antagonists has been demonstrated in many experimental paradigms. However, the exact mechanism by which CB1 antagonists modulate ethanol drinking remains elusive. In the present study, we assessed the role of CB1 receptors within the key regions of the mesolimbic dopamine pathway, the nucleus accumbens (NAcc) and ventral tegmental area (VTA), in regulation of ethanol self-administration. METHODS Adult male alcohol-prefer AA rats were trained to self-administer either 10% (w/v) ethanol or 0.1% (w/v) saccharin under an FR1 schedule during daily 30-minute sessions. Following stable baseline responding, rats were tested after systemic administration of the CB1 antagonist SR141716A (0 to 10 mg/kg) and the agonist WIN55,212-2 (0 to 2 mg/kg). Separate groups of rats were implanted with bilateral cannulas aimed at the NAcc or VTA, and tested after microinjections of SR141716A (0 to 3 microg) and WIN55,212-2 (0 to 5 microg) into the NAcc or VTA. The highest intracerebral doses were tested also in rats responding for a 0.1% saccharin solution. RESULTS SR141617A dose-dependently suppressed ethanol responding after systemic administration. Microinjections of SR141617A both into NAcc and VTA attenuated ethanol responding. In addition, intra-NAcc injections of SR141617A suppressed saccharin intake. Although low doses of systemically given WIN55,212-2 increased ethanol responding, no effects were seen after WIN55,212-2 microinjections into NAcc or VTA. CONCLUSIONS Bidirectional changes in ethanol self-administration by the systematically administered CB1 agonist and antagonist show that ethanol reinforcement is controlled by CB1 receptors in alcohol-preferring AA rats. Replication of the suppressive effects by CB1 antagonism in the NAcc and VTA suggests that endocannabinoids and their receptors mediate ethanol reinforcement through interaction with the mesolimbic dopamine pathway.