Ethanol-induced conditioned place preference is expressed through a ventral tegmental area dependent mechanism

The role of nucleus accumbens shell GABA receptors on ventral tegmental area intracranial self-stimulation and a potential role for the 5-HT(2C) receptor

Brain γ-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT)(2C) receptors are implicated in the neuronal regulation of reward- and aversion-related behaviour. Within the mesocorticolimbic pathways of the brain, relationships between GABA containing neurons and 5-HT(2C) receptor activity may be important in this context. The primary aim of this study was to investigate the role of NAc shell GABA receptors on ventral tegmental area intracranial self-stimulation (ICSS) and to examine the systemic effects of GABAergic ligands in this context. The second aim was to investigate the relationship between GABA receptor- and 5-HT(2C) receptor-related ICSS behaviour, using systemic administration of the selective agonist WAY 161503. Locomotor activity was assessed to compare the potential motor effects of drugs; feeding behaviour and intra-NAc injections of amphetamine (1.0 µg/side) were used as positive controls. When administered systemically the GABA(A) receptor agonist muscimol and antagonist picrotoxin did not selectively change ICSS reward thresholds, although the 5-HT(2C) receptor agonist WAY 161503 (1.0 mg/kg) decreased reward measures. Intra-NAc shell administration of muscimol (225 ng/side) and picrotoxin (125 ng/side), respectively, decreased and increased measures of reward. Intra-NAc shell baclofen (0-225 ng/side; GABA(B) receptor agonist) did not affect any ICSS measures although it increased feeding. Combining picrotoxin and WAY 161503 attenuated the effects of each. These results suggest that a 5-HT(2C) and GABA(A) receptor-mediated neuronal relationship in the NAc shell may be relevant for the regulation of brain reward pathways.

Involvement of the beta-endorphin neurons of the hypothalamic arcuate nucleus in ethanol-induced place preference conditioning in mice

BACKGROUND

Increasing evidence indicates that mu- and delta-opioid receptors are decisively involved in the retrieval of memories underlying conditioned effects of ethanol. The precise mechanism by which these receptors participate in such effects remains unclear. Given the important role of the proopiomelanocortin (POMc)-derived opioid peptide beta-endorphin, an endogenous mu- and delta-opioid receptor agonist, in some of the behavioral effects of ethanol, we hypothesized that beta-endorphin would also be involved in ethanol conditioning.

METHODS

In this study, we treated female Swiss mice with estradiol valerate (EV), which induces a neurotoxic lesion of the beta-endorphin neurons of the hypothalamic arcuate nucleus (ArcN). These mice were compared to saline-treated controls to investigate the role of beta-endorphin in the acquisition, extinction, and reinstatement of ethanol (0 or 2 g/kg; intraperitoneally)-induced conditioned place preference (CPP).

RESULTS

Immunohistochemical analyses confirmed a decreased number of POMc-containing neurons of the ArcN with EV treatment. EV did not affect the acquisition or reinstatement of ethanol-induced CPP, but facilitated its extinction. Behavioral sensitization to ethanol, seen during the conditioning days, was not present in EV-treated animals.

CONCLUSIONS

The present data suggest that ArcN beta-endorphins are involved in the retrieval of conditioned memories of ethanol and are implicated in the processes that underlie extinction of ethanol-cue associations. Results also reveal a dissociated neurobiology supporting behavioral sensitization to ethanol and its conditioning properties, as a beta-endorphin deficit affected sensitization to ethanol, while leaving acquisition and reinstatement of ethanol-induced CPP unaffected.

Acquisition and expression of ethanol-induced conditioned place preference in mice is inhibited by naloxone

The effects of opioid antagonists on conditioned reward produced by ethanol provide variable and sometimes conflicting results, especially in mice. In the present set of experiments, male C57BL/6 mice received 4 vehicle and 4 ethanol conditionings, and the rewarding effects of ethanol were assessed in an unbiased version of the conditioned place preference (CPP) apparatus and an unbiased stimulus assignment procedure. Intraperitoneal (ip) administration of ethanol (2 g/kg, but not 1 g/kg) resulted in the conditioned reward when conditionings lasted for 6 min but not when conditioning lasted for 20 min. Administration of the non-selective opioid receptor antagonist naloxone (1 and 5 mg/kg) before the conditionings attenuated the acquisition of ethanol-induced place preference. Naloxone (1 mg/kg) also inhibited expression of the CPP response, but it did not alter the preference of vehicle-conditioned mice, suggesting the lack of its own motivational effects in this experimental setting. Taken together, the present results suggest that an unbiased version of ethanol-induced CPP in C57BL/6 mice could be a valid model for the study of the motivational effects of ethanol, confirming and expanding previous findings that have demonstrated inhibitory effects of opioid receptor antagonist on alcohol conditioned reward.

Involvement of the orexin/hypocretin system in ethanol conditioned place preference

RATIONALE

Recent studies suggest that orexin/hypocretin is involved in drug reward and drug-seeking behaviors, including ethanol self-administration. However, orexin's role in ethanol-induced seeking behaviors remains unclear.

OBJECTIVE

These studies examined the role of orexin in the acquisition and expression of ethanol conditioned place preference (CPP) using the orexin 1 receptor (OX1R) antagonist SB-334867.

METHODS

Effects of SB-334867 (0-30 mg/kg) on locomotor activity were determined in DBA/2J mice (Experiment 1). SB-334867 (0-30 mg/kg) was administered during acquisition of ethanol (2 g/kg) CPP to determine whether orexin signaling is required (Experiment 2). Blood ethanol concentrations (BECs) were measured after ethanol (2 g/kg) injection to determine whether SB-334867 (30 mg/kg) pretreatment altered ethanol pharmacokinetics (Experiment 3). Finally, SB-334867 (0-40 mg/kg) was given before ethanol-free preference testing (Experiments 4 and 5).

RESULTS

SB-334867 did not alter basal locomotor activity (Experiment 1). SB-334867 (30 mg/kg) reduced ethanol-induced locomotor stimulation, but did not affect the acquisition of ethanol CPP (Experiment 2) or BEC, suggesting no alteration in ethanol pharmacokinetics (Experiment 3). Although OX1R antagonism blocked expression of a weak ethanol CPP (Experiment 4), it did not affect expression of a moderate to strong CPP (Experiment 5).

CONCLUSIONS

Blockade of OX1R by systemic administration of SB-334867 reduced ethanol-stimulated activity, but did not affect acquisition or expression of ethanol-induced CPP, suggesting that orexin does not influence ethanol's primary or conditioned rewarding effects. Other neurotransmitter systems may be sufficient to support acquisition and expression of CPP despite alterations in orexin signaling.

Assessment of GABA-B, metabotropic glutamate, and opioid receptor involvement in an animal model of binge drinking

Drinking to intoxication or binge drinking is a hallmark characteristic of alcohol abuse. Although hard to model in rodents, the scheduled high alcohol consumption (SHAC) procedure generates high, stable ethanol intake and blood ethanol concentrations in mice to levels consistent with definitions of binge drinking. The purpose of the present studies was to determine the effects of pharmacological manipulation of the opioidergic, glutamatergic, and γ-aminobutyric acid (GABA)ergic systems on binge drinking with the SHAC procedure. Parallel manipulations were conducted in mice trained in operant self-administration of either sucrose or ethanol. For the SHAC procedure, genetically heterogeneous Withdrawal Seizure Control mice were given varying periods of fluid access, with a 30-min ethanol session every third day (total of seven). Mice were pretreated intraperitoneally with naltrexone (0, 0.6, or 1.25 mg/kg), baclofen (0, 2.5, or 5.0 mg/kg), or 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 0, 3.0, or 10.0 mg/kg) before each ethanol session. For the operant self-administration procedure, separate groups of C57BL/6 mice were trained to complete a single response requirement (16 presses on the active lever) to gain 30 min of access to an ethanol or a sucrose solution. Mice received pretreatments of the same doses of naltrexone, MPEP, or baclofen before the self-administration sessions, with saline injections on intervening days. Naltrexone produced a dose-dependent decrease in binge drinking, and the highest dose also significantly decreased operant self-administration of ethanol and sucrose. Both doses of baclofen significantly decreased binge alcohol consumption, but the higher dose also tended to decrease water intake. The highest dose of baclofen also significantly decreased operant self-administration of sucrose. MPEP (10 mg/kg) significantly decreased binge alcohol consumption and sucrose self-administration. These results indicate that manipulation of the opioidergic, glutamatergic, and GABAergic systems significantly decreased binge drinking.

Mediodorsal nucleus of the thalamus is critical for the expression of memory of methamphetamine-produced conditioned place preference in rats

Methamphetamine (MA) is a powerful and highly addictive psychostimulant. However, the neural substrate mediating MA-induced conditioned effects, an essential part of addiction, remain unclear. The present study investigated the involvement of the anterior cingulate cortex (ACC), the lateral nucleus of amygdala (LNA), and the mediodorsal nucleus of the thalamus (MD) in MA-conditioned place preference (CPP). Rats underwent bilateral radio-frequency lesions of the ACC, LNA, or MD followed by MA CPP training. Lesions of the MD, but not the ACC or LNA, disrupted MA CPP learning. To clarify the role of the MD on the different stages of the MA CPP memory process, bilateral microinfusions of lidocaine into the MD were performed 5 min prior to each conditioning trial, immediately after the conditioning trial, or 5 min before the testing phase. Pretesting, but not pre- or post-conditioning, infusions of lidocaine into the MD impaired MA CPP. Furthermore, a clear preference for the previously conditioned MA paired cues was expressed when the rats were tested again 24 h after infusions of lidocaine. These results are interpreted as indicating that the MD is specifically involved in the memory retrieval process of MA associated memory which suggests the MD could have an important role in relapse in individuals suffering from MA addiction.

Blockade of opioid receptors in anterior cingulate cortex disrupts ethanol-seeking behavior in mice

The anterior cingulate cortex (ACC) and opioid receptors have been suggested to play a role in attributing incentive motivational properties to drug-related cues. We examined whether blockade of ACC opioid receptors would reduce cue-induced ethanol-seeking behavior in mice. We show that intra-ACC opioid receptor blockade disrupted expression of an ethanol-induced conditioned place preference, suggesting that endogenous opioid modulation in the ACC may be critical for maintaining the cue's conditioned rewarding effects.

Effects of the novel endocannabinoid uptake inhibitor, LY2183240, on fear-potentiated startle and alcohol-seeking behaviors in mice selectively bred for high alcohol preference

RATIONALE

Alcohol-use disorders often occur together with anxiety disorders in humans which may be partly due to common inherited genetic factors. Evidence suggests that the endocannabinoid system (ECS) is a promising therapeutic target for the treatment of individuals with anxiety and/or alcohol-use disorders.

OBJECTIVES

The present study assessed the effects of a novel endocannabinoid uptake inhibitor, LY2183240, on anxiety- and alcohol-seeking behaviors in a unique animal model that may represent increased genetic risk to develop co-morbid anxiety and alcohol-use disorders in humans. Mice selectively bred for high alcohol preference (HAP) show greater fear-potentiated startle (FPS) than mice selectively bred for low alcohol preference (LAP). We examined the effects of LY2183240 on the expression of FPS in HAP and LAP mice and on alcohol-induced conditioned place preference (CPP) and limited-access alcohol drinking behavior in HAP mice.

RESULTS

Repeated administration of LY2183240 (30 mg/kg) reduced the expression of FPS in HAP but not LAP mice when given prior to a second FPS test 48 h after fear conditioning. Both the 10 and 30 mg/kg doses of LY2183240 enhanced the expression of alcohol-induced CPP and this effect persisted in the absence of the drug. LY2183240 did not alter limited-access alcohol drinking behavior, unconditioned startle responding, or locomotor activity.

CONCLUSIONS

These findings suggest that ECS modulation influences both conditioned fear and conditioned alcohol reward behavior. LY2183240 may be an effective pharmacotherapy for individuals with anxiety disorders, such as post-traumatic stress disorder, but may not be appropriate for individuals with co-morbid anxiety and alcohol-use disorders.

Inhibition of extracellular signal-regulated kinase (ERK) activity with SL327 does not prevent acquisition, expression, and extinction of ethanol-seeking behavior in mice

Although extracellular signal-regulated kinase (ERK) activity is essential for the acquisition of a variety of associative learning tasks, its involvement in the acquisition and extinction of ethanol (EtOH)-induced conditioned place preference (CPP) remains unknown. Therefore, in these experiments we examined the effects of the ERK-kinase (MEK)-inhibitor SL327 on acquisition and expression of EtOH-CPP as well as the dose- and time-dependent effects of SL327 on CPP extinction. The parametric findings of Experiment 1 showed that three 30-min (but not 15- or 5-min) non-reinforced trials were required to completely extinguish EtOH-CPP in male, DBA/2J mice. In Experiments 2 and 3, SL327 (30 and 50mg/kg), administered 30 or 90min prior to extinction trials, was unable to impair EtOH-CPP extinction. Experiment 4 showed that SL327 (50mg/kg) had no effect on acquisition of EtOH-CPP or the development of EtOH-induced sensitization during conditioning. When administered prior to testing in Experiments 5 and 6, SL327 did not alter expression of EtOH-CPP but did reduce test activity. Importantly, SL327 significantly reduced pERK protein levels when assessed in the dorsal striatum and motor cortex (Experiment 7). Together, these data suggest that EtOH-related learning and EtOH reward in mice, as assessed with CPP, are not impaired by the systemically administered MEK-inhibitor SL327.

GABAB receptors in reward processes

gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the brain which acts through different receptor subtypes. Metabotropic GABA(B) receptors are widely distributed throughout the brain. Alterations in GABA signaling through pharmacological activation or deactivation of the GABA(B) receptor regulate behavior and brain reward processes. GABA(B) receptor agonists and, most recently, positive modulators have been found to inhibit the reinforcing effects of drugs of abuse, such as cocaine, amphetamine, nicotine, ethanol, and opiates. This converging evidence of the effects of GABA(B) compounds on the reinforcing properties of addictive drugs is based on behavioral studies that used a variety of procedures with relevance to reward processes and drug abuse liability, including intracranial self-stimulation, intravenous self-administration under both fixed- and progressive-ratio schedules of reinforcement, reinstatement, and conditioned place preference. GABA(B) receptor agonists and positive modulators block the reinforcing effects of drugs of abuse in these animal models. However, GABA(B) receptor agonists also have undesirable side-effects. GABA(B) receptor modulators have potential advantages as medications for drug addiction. These compounds have a better side-effect profile than GABA(B) agonists because they are devoid of intrinsic agonistic activity in the absence of GABA. They only exert their modulatory actions in concert with endogenous GABAergic activity. Thus, GABA(B) receptor positive modulators are promising therapeutics for the treatment of various aspects of dependence (e.g., initiation, maintenance, and relapse) on various drugs of abuse, such as cocaine, nicotine, heroin, and alcohol.

Effects of histamine H3 receptor ligands on the rewarding, stimulant and motor-impairing effects of ethanol in DBA/2J mice

Histamine H3 receptor (H3R) antagonists are currently being investigated for the possible therapeutic use in various cognitive deficits such as those in schizophrenia, attention deficit hyperactivity disorder and Alzheimer's disease. Our previous studies suggest a role for H3Rs in ethanol-related behaviors in rat and mice. Here we have examined the role of different H3R ligands on the effects of ethanol in conditioned place preference (CPP) paradigm, stimulation of locomotor activity and motor impairment in rotarod and balance beam in male DBA/2J mice. We found that H3R antagonists ciproxifan and JNJ-10181457 inhibited the ethanol-evoked CPP whereas H3R agonist immepip did not alter ethanol-induced place preference. Acute stimulatory response by ethanol was also modulated by H3R ligands. Ciproxifan increased ethanol activation when ethanol was given 1g/kg but not at 1.5g/kg dose. Immepip pretreatment diminished ethanol stimulation and increased motor-impairing effects of ethanol on the balance beam. In conclusion, these findings give further evidence of the involvement of H3R in the regulation of the effects of ethanol. The inhibition of ethanol reward by H3R antagonism implies that H3R might be a possible target to suppress compulsory ethanol seeking. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

An acetaldehyde-sequestering agent inhibits appetitive reinforcement and behavioral stimulation induced by ethanol in preweanling rats

Ethanol's motivational consequences have been related to the actions of acetaldehyde, a metabolic product of ethanol oxidation. The present study assessed the role of acetaldehyde in the motivational effects of ethanol on preweanling rats. In Experiment 1 pups (postnatal days 13-14, PD 13-14) were given systemic administration of D-penicillamine (DP, a drug that sequesters acetaldehyde: 0, 25, 50 or 75 mg/kg) before pairings of 1.0 g/kg ethanol and a rough surface (sandpaper, conditioned stimulus, CS). At test, pups given sandpaper-ethanol pairings exhibited greater preference for the CS than unpaired controls, but this preference was not expressed by pups given DP. Pre-training administration of 25 or 50 mg/kg DP completely blocked the expression of ethanol-mediated appetitive conditioning. D-penicillamine did not alter blood ethanol levels. Subsequent experiments revealed that ethanol-induced activation was blocked by central (intra-cisterna magna injections, volume: 1 μl, dose: 0 or 75 μg) but not systemic treatment with DP (0, 25, 50 or 75 mg/kg; ip). These results indicate that: (a) preweanling rats are sensitive to the reinforcing effect of ethanol, and (b) that this effect is associated with the motor activating effect of the drug. These effects seem to be mediated by the first metabolite of ethanol, acetaldehyde.

Acupuncture inhibits GABA neuron activity in the ventral tegmental area and reduces ethanol self-administration

BACKGROUND

Withdrawal from chronic ethanol enhances ventral tegmental area (VTA) GABA neuron excitability and reduces mesolimbic dopamine (DA) neurotransmission, which is suppressed by acupuncture at Shenmen (HT7) points (Zhao et al., 2006). The aim of this study was to evaluate the effects of HT7 acupuncture on VTA GABA neuron excitability, ethanol inhibition of VTA GABA neuron firing rate, and ethanol self-administration. A role for opioid receptors (ORs) in ethanol and acupuncture effects is also explored.

METHODS

Using electrophysiological methods in mature rats, we evaluated the effects of HT7 stimulation and opioid antagonists on VTA GABA neuron firing rate. Using behavioral paradigms in rats, we evaluated the effects of HT7 stimulation and opioid antagonists on ethanol self-administration using a modification of the sucrose-fading procedure.

RESULTS

HT7 stimulation produced a biphasic modulation of VTA GABA neuron firing rate characterized by transient enhancement followed by inhibition and subsequent recovery in 5 minutes. HT7 inhibition of VTA GABA neuron firing rate was blocked by systemic administration of the nonselective μ-opioid receptor antagonist naloxone. HT7 stimulation significantly reduced ethanol suppression of VTA GABA neuron firing rate, which was also blocked by naloxone. HT7 acupuncture reduced ethanol self-administration without affecting sucrose consumption. Systemic administration of the δ-opioid receptor (DOR) antagonist naltrindole blocked ethanol suppression of VTA GABA neuron firing rate and significantly reduced ethanol self-administration without affecting sucrose consumption.

CONCLUSIONS

These findings suggest that DOR-mediated opioid modulation of VTA GABA neurons may mediate acupuncture's role in modulating mesolimbic DA release and suppressing the reinforcing effects of ethanol.

Naloxone blocks ethanol-mediated appetitive conditioning and locomotor activation in adolescent rats

Age-related differences in ethanol sensitivity could put adolescents at risk for developing alcohol-related problems. Little information exists, however, about adolescent sensitivity to ethanol's appetitive effects and the neurobiological mechanisms underlying ethanol reinforcement during this developmental stage. The present study assessed the role of the opioid system in adolescent rats in an appetitive second-order schedule of ethanol reinforcement and ethanol-induced locomotor stimulation. On postnatal day 32 (PD32), animals were pretreated with the general opioid antagonist naloxone (0.0, 0.75, 1.50, or 2.5 mg/kg) and then given pairings of ethanol (0.0 or 2.0 g/kg, intragastrically) with intraoral pulses of water (conditioned stimulus 1 [CS₁], first-order conditioning phase). CS₁ delivery occurred 30-45 min after ethanol administration when the effect of ethanol was assumed to be appetitive. On PD33, adolescents were exposed to CS₁ (second-order conditioning phase) while in a chamber featuring distinctive exteroceptive cues (CS₂). Preference for CS₂ was then tested. Adolescents given CS₁-ethanol pairings exhibited greater preference for CS₂ than controls, indicating ethanol-mediated reinforcement, but only when not pretreated with naloxone. Blood alcohol levels during conditioning were not altered by naloxone. Experiment 2 revealed that ethanol-induced locomotor activation soon after administration, and naloxone dose-dependently suppressed this stimulating effect. The present study indicates that adolescent rats are sensitive to ethanol's reinforcing and locomotor-stimulating effects. Both effects of ethanol appear to be mediated by endogenous opioid system activation.

Social isolation and stress related hormones modulate the stimulating effect of ethanol in preweanling rats

Preweanling rats are highly sensitive to the locomotor stimulation induced by relatively high ethanol doses. In adult mice this ethanol effect is modulated by stress. The goal of the present study was to analyze the role of stress and corticosterone in the stimulating effect of ethanol in preweanling rats. In Experiment 1 15-day-old rats were separated from the mother during a period of 4h in which subjects remained isolated or paired with a littermate. In a third condition pups remained in the home-cage with the dam. After this isolation period pups were given ethanol (0 or 2.5 g/kg) and were tested in a novel environment. Previous data have shown that a similar period of isolation is enough to increase corticosterone levels in preweanling rats. Experiment 2 evaluated the effect of exogenous administration of corticosterone (0, 3 or 6 mg/kg) along with ethanol, and Experiment 3 tested ethanol-mediated locomotor activation in adrenalectomized preweanling rats. The last experiment aimed to test the role of corticotropic releasing factor 1 (CRF1) receptors in locomotion induced by ethanol in isolated pups. According to our results there is a synergism between stress or corticosterone and ethanol in preweanling rats. The interaction between stress (induced by social isolation) and ethanol seems to be mediated by CRF, since blockade of CRF1 receptors cancelled the effect of ethanol in isolated pups. This study highlights the importance of considering stress as a possible intervening variable in studies evaluating ethanol effects in developing animals when maternal separation is used in the experimental procedure.

Opioid receptors in the basolateral amygdala but not dorsal hippocampus mediate context-induced alcohol seeking

Contexts associated with the availability of alcohol can induce craving in humans and alcohol seeking in rats. The opioid antagonist naltrexone attenuates context-induced reinstatement (renewal) of alcohol seeking and suppresses neuronal activation in the basolateral amygdaloid complex and dorsal hippocampus induced by such reinstatement. The objective of this study was to determine whether pharmacological blockade of opioid receptors in the basolateral amygdala or dorsal hippocampus would attenuate the context-induced reinstatement of alcohol seeking. Rats were trained to self-administer alcohol in one context (Context A), extinguished in a distinct context (Context B) and then tested for reinstatement of alcohol seeking in A and B contexts. Prior to the test session, rats were bilaterally microinjected with 0, 333 or 1000ng (total) naloxone methiodide into the basolateral amygdala or dorsal hippocampus. Naloxone methiodide in the amygdala, but not the hippocampus, dose dependently suppressed context-induced reinstatement. This suggests that opioid transmission in the basolateral amygdaloid complex is an important mediator of context-induced alcohol seeking.

Ethanol action on dopaminergic neurons in the ventral tegmental area: interaction with intrinsic ion channels and neurotransmitter inputs

The dopaminergic system originating in the midbrain ventral tegmental area (VTA) has been extensively studied over the past decades as a critical neural substrate involved in the development of alcoholism and addiction to other drugs of abuse. Accumulating evidence indicates that ethanol modulates the functional output of this system by directly affecting the firing activity of VTA dopamine neurons, whereas withdrawal from chronic ethanol exposure leads to a reduction in the functional output of these neurons. This chapter will provide an update on the mechanistic investigations of the acute ethanol action on dopamine neuron activity and the neuroadaptations/plasticities in the VTA produced by previous ethanol experience.

Effects of disconnection of amygdala dopamine and nucleus accumbens N-methyl-d-aspartate receptors on ethanol-seeking behavior in mice

There is a strong interest in harnessing the genetic manipulations that are possible in mice to investigate the functional neural mechanisms modulating the associative processes that control drug-seeking behavior. However, it is unknown whether intracranial techniques, such as the disconnection procedure commonly used in rats to examine serial connectivity between implicated areas, can be successfully applied to mice. We have previously demonstrated that the expression of ethanol-seeking behavior in mice is dependent upon amygdala (Amy) dopamine and nucleus accumbens (Acb) N-methyl-d-aspartate (NMDA) receptor activation (Gremel & Cunningham, 2009). Here, we used a neuropharmacological disconnection procedure to investigate whether dopamine activation of the Amy directly leading to increases in Acb glutamate release and binding of NMDA receptors modulates the expression of ethanol-seeking behavior. Immediately before testing the expression of an ethanol-induced conditioned place preference, mice were given an Amy infusion of flupenthixol and either an ipsilateral or contralateral Acb infusion of AP-5. Although both ipsilateral and contralateral manipulations reduced the expression of ethanol conditioned place preference, in a separate experiment we demonstrated that a unilateral Acb infusion of AP-5, but not Amy flupenthixol, is sufficient to disrupt preference. The finding of a significant blockade by unilateral AP-5 into the Acb precludes any conclusions about a unique role for the Amy/Acb neuroanatomical connection in this model of ethanol-seeking behavior. Further, the current results suggest potential limitations in transferring techniques from rats to mice in order to study serial interactions between neural areas underlying motivated behaviors. Nevertheless, these findings provide evidence showing that Acb NMDA receptors play an important role in the expression of ethanol-conditioned behavior.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Role of the GABA(B) receptor in alcohol-seeking and drinking behavior

The present paper summarizes experimental data demonstrating the reducing effect of direct agonists and positive allosteric modulators (PAMs) of the gamma-aminobutyric acid(B) (GABA(B)) receptor on different alcohol-related behaviors. Different lines of evidence indicate that direct agonists, including baclofen, effectively suppress acquisition and maintenance of alcohol drinking behavior, relapse-like drinking, and alcohol's reinforcing, rewarding, stimulating, and motivational properties in rats and mice. More recently, the discovery of a positive allosteric modulatory binding site, together with the synthesis of in vivo effective ligands, opened a new avenue of research in GABA(B) pharmacology. Accumulating lines of evidence suggest that PAMs retain baclofen's capcity to suppress alcohol consumption and alcohol's reinforcing and motivational properties in rats; these effects occur at doses far from those producing behavioral toxicity.

Opioid antagonists block the acquisition of ethanol-mediated conditioned tactile preference in infant rats

It has been difficult to find conditioned preference for tactile cues paired with ethanol intoxication in rats. Toward understanding the ontogeny of ethanol reinforcement, we aimed at establishing a simple and reliable procedure for (1) assessing primary appetitive conditioning to ethanol in infant rats and (2) discerning the role the opioid system plays in ethanol-mediated conditioning at this age. Experiment 1 determined the parameters (i.e., dose, interval of conditioning) for assessing ethanol-mediated conditioning. Pups were then trained with differential Pavlovian conditioning (Experiments 2 and 3) in which ethanol intoxication (1.0-2.0 g/kg, intragastrically or intraperitoneally delivered) was paired with a tactile stimulus (sandpaper) while an alternative texture signaled the absence of ethanol's effects. Unpaired control conditions were also used. Tactile preferences were assessed after two conditioning sessions. Paired rats spent significantly more time on sandpaper than unpaired controls, an effect that was greater after intragastric administration of 1.0 than 2.0 g/kg ethanol. This effect was replicated in Experiments 4a and 4c and found to be inhibited by pretreatment with general (naloxone [NAL]) or specific (d-Pen-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 [CTOP] and naltrindole) opioid antagonists. Blood ethanol levels at conditioning were not altered by NAL (Experiment 4b). The study outlines a procedure that reveals appetitive conditioning to ethanol by infant rats. The results are discussed in terms of a potential ethanol-induced activation of the endogenous opioid system during the onset of the intoxication process.

Regulation of alcohol-seeking by orexin (hypocretin) neurons

Orexins (hypocretins) are found primarily within a restricted portion of neurons within the hypothalamus, but provide innervation across the neuraxis. Orexin A (hypocretin 1) has been implicated in drug and food reward. Not surprisingly therefore, interest has come to bear on whether orexins are implicated in aspects of alcohol consumption and/or seeking. This mini-review provides a concise, but timely, discussion on this issue. The evidence to date would suggest a role for orexins in alcohol use, and integration of orexin-containing neurons in reward-seeking circuitry. There are however still many unanswered questions, some of which are canvassed herein.

Baclofen prevents drug-induced reinstatement of extinguished nicotine-seeking behaviour and nicotine place preference in rodents

The gamma-aminobutyric acid(GABA)-B receptor agonist baclofen is known to reduce drug intake in both animals and humans and to prevent reinstatement of cocaine-, opioid-, and alcohol-seeking in rats after a period of extinction, but its effect on nicotine reinstatement is unknown. This study investigated the effect of baclofen on nicotine-seeking reinstatement both using the extinction/reinstatement model of nicotine self-administration and conditioned place preference (CPP). Results showed that in rats previously trained to intravenously self-administer nicotine (30 microg/kg/inf) under a FR-1 schedule of reinforcement, acute nicotine (0.15 mg/kg) priming effectively reinstates nicotine-seeking behaviour following extinction. At doses used in this study (up to 2.5 mg/kg) baclofen alone did not affect locomotor activity and did not reinstate responding. However, baclofen dose-dependently attenuated drug-induced reinstatement of nicotine-seeking in rats. Moreover, baclofen (1.25 mg/kg) completely blocked nicotine-induced reinstatement of extinguished nicotine (0.3 mg/kg) CPP in mice. Altogether, our results showed that baclofen is able to antagonise reinstatement of nicotine-seeking and CPP triggered by nicotine primings, suggesting its potential clinical utility as an anti-relapse agent.

Assessing appetitive, aversive, and negative ethanol-mediated reinforcement through an immature rat model

The motivational effects of drugs play a key role during the transition from casual use to abuse and dependence. Ethanol reinforcement has been successfully studied through Pavlovian and operant conditioning in adult rats and mice genetically selected for their ready acceptance of ethanol. Another model for studying ethanol reinforcement is the immature (preweanling) rat, which consumes ethanol and exhibits the capacity to process tactile, odor and taste cues and transfer information between different sensorial modalities. This review describes the motivational effects of ethanol in preweanling, heterogeneous non-selected rats. Preweanlings exhibit ethanol-mediated conditioned taste avoidance and conditioned place aversion. Ethanol's appetitive effects, however, are evident when using first- and second-order conditioning and operant procedures. Ethanol also devalues the motivational representation of aversive stimuli, suggesting early negative reinforcement. It seems that preweanlings are highly sensitive not only to the aversive motivational effects of ethanol but also to its positive and negative (anti-anxiety) reinforcement potential. The review underscores the advantages of using a developing rat to evaluate alcohol's motivational effects.

Involvement of amygdala dopamine and nucleus accumbens NMDA receptors in ethanol-seeking behavior in mice

Although progress has been made identifying neural mechanisms underlying ethanol's primary reinforcing effects, few studies have examined the mechanisms mediating ethanol-induced conditioned effects. A recent lesion study suggests that expression of ethanol-conditioned behaviors depends upon an intact amygdala and nucleus accumbens core. However, specific mechanisms within these nuclei are unknown. In the present experiments, we used site-specific microinfusions of dopamine and NMDA receptor antagonists to examine the roles of accumbens and amygdala in the expression of ethanol conditioned place preference (CPP) in mice. In experiments 1 and 2, a D1/D2/D3 receptor antagonist (flupenthixol) was infused into accumbens or amygdala before testing, whereas experiment 3 used pretest infusions of an NMDA antagonist (AP-5) to examine the role of intra-accumbens NMDA receptors. Dopamine antagonism of accumbens was without effect, but intra-amygdala infusions of flupenthixol blocked CPP expression. Moreover, this effect was dependent upon dopamine antagonism within the basolateral nucleus but not the central nucleus of the amygdala. Antagonism of NMDA receptors in accumbens also blocked CPP expression. The present findings suggest that expression of the ethanol-conditioned response depends upon amygdala dopamine and accumbens NMDA receptors. These are the first studies in any species to show a role for amygdala dopamine receptors and the first studies in mice to implicate accumbens NMDA receptors in ethanol-induced conditioned effects.

Strain specific synaptic modifications on ventral tegmental area dopamine neurons after ethanol exposure

BACKGROUND

Genetic factors and previous alcohol experience influence alcohol consumption in both humans and rodents. Specifically, a prior experience with ethanol increases ethanol intake in both ethanol-preferring C57BL/6 (C57) and ethanol non-preferring DBA/2 (DBA) mice. Whereas the ventral tegmental area (VTA) importantly regulates dopamine levels and ethanol intake, it is unknown whether ethanol experience differentially alters synaptic properties of VTA dopamine neurons in ethanol-preferring and non-preferring mice.

METHODS

The properties of excitatory and inhibitory inputs and the ability to elicit long-term potentiation (LTP) were assessed with whole-cell patch-clamp recordings in VTA dopamine neurons from C57 and DBA mice 24 hours after a single ethanol (2 g/kg, IP) or equivalent saline injection.

RESULTS

Ethanol exposure increased gamma-aminobutyric acid (GABA) release onto VTA dopamine neurons in DBA mice, as previously observed in C57 mice. However, a single ethanol exposure reduced alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) and N-methyl-D-aspartate receptor (NMDAR) function and LTP in VTA dopamine neurons from DBA but not C57 mice.

CONCLUSIONS

A single ethanol exposure selectively reduced glutamate receptor function in VTA dopamine neurons from the ethanol non-preferring DBA strain but enhanced GABA signaling in both C57 and DBA strains. These results support the notion that VTA dopamine neurons are a central target of ethanol-induced neural plasticity, which could contribute to ethanol consumption. Furthermore, these findings highlight the possible need for specialized therapeutic interventions for alcoholism based on individual intrinsic differences.

Delta-opioid receptor expression in the ventral tegmental area protects against elevated alcohol consumption

Alcoholism is a complex and debilitating syndrome affecting approximately 140 million people worldwide. However, not everyone who consumes ethanol develops abuse, raising the possibility that some individuals have a protective mechanism that inhibits elevated alcohol consumption. We tested the hypothesis that the delta-opioid receptor (DOR) plays such a protective role. Here we show that DOR activity in the ventral tegmental area (VTA) robustly decreases ethanol consumption in rats and that these effects depend on baseline ethanol consumption. Intra-VTA microinjection of the DOR agonist DPDPE decreases drinking, particularly in low-drinking animals. Furthermore, VTA microinjection of the DOR selective antagonist TIPP-Psi increases drinking in low, but not high, drinkers and this increase is blocked by comicroinjection of the GABA(A) antagonist bicuculline. Using electrophysiological techniques we found that in VTA brain slices from drinking rats DPDPE presynaptically inhibits GABA(A) receptor mediated IPSCs in low drinkers, but not in high drinkers or naive animals, most likely through activation of DORs on GABA terminals. This DOR-mediated inhibition of IPSCs also correlates inversely with behavioral correlates of anxiety measured in the elevated plus maze. In contrast, presynaptic inhibition of VTA GABA(A) IPSCs by the mu-opioid receptor agonist DAMGO is significantly reduced in both high- and low-drinking rats (<30%) compared with age-matched nondrinking controls (>70%). Together, our findings demonstrate the protective nature of VTA DORs and identify an important new target for therapeutic intervention for alcoholism.

Mice selectively bred for high- or low-alcohol-induced locomotion exhibit differences in dopamine neuron function

Elevated sensitivity to the euphoric or stimulant effects of ethanol is associated with higher levels of alcohol use in some human populations. Midbrain dopamine neurons are thought to be important mediators of both ethanol reward and locomotor stimulation. Patch-clamp recordings were used to examine the electrical properties of dopamine neurons in a genetic model of heightened (FAST) and reduced (SLOW) sensitivity to the locomotor-activating effects of ethanol. Pacemaker firing of dopamine neurons was faster in FAST than SLOW mice, as was the current density through I(H) channels. Acute administration of ethanol accelerated the firing of dopamine neurons to a greater extent in recordings from FAST than SLOW mice. Dopamine neurons from FAST mice also exhibited reduced GABA(A) receptor-mediated synaptic input, compared with SLOW mice. The results suggest that dopamine neuron I(H) channels, firing rate, and GABAergic input may play a role in sensitivity to the locomotor activation observed at early time points after ethanol administration and could underlie differences in sensitivity to alcohol relevant to risk for alcohol abuse.

Acute effects of acamprosate and MPEP on ethanol Drinking-in-the-Dark in male C57BL/6J mice

BACKGROUND

Recently, a simple procedure in mice, Drinking-in-the-Dark (DID), was hypothesized to have value for medication development for human alcoholism. In DID, mice are offered intermittent, limited access to ethanol over a series of days during the dark phase that results in rapid drinking to intoxication in predisposed genotypes.

METHODS

We measured the effects of acamprosate or MPEP, metabotropic glutamate 5 receptor (mGluR5) antagonist, on intake of 20% ethanol, plain tap water or 10% sugar water using the DID procedure in male C57BL/6J mice.

RESULTS

Acamprosate (100, 200, 300, or 400 mg/kg) dose dependently decreased ethanol drinking with 300 mg/kg reducing ethanol intake by approximately 20% without affecting intake of plain water or 10% sugar water. MPEP (1, 3, 5, 10, 20, or 40 mg/kg) was more potent than acamprosate with 20 mg/kg reducing ethanol intake by approximately 20% and for longer duration without affecting intake of plain water or 10% sugar water.

CONCLUSIONS

These results support the hypothesis that mGluR5 signaling plays a role in excessive ethanol intake in DID and suggest DID may have value for screening novel compounds that reduce overactive glutamate signaling for potential pharmaceutical treatment of excessive ethanol drinking behavior.

Baclofen attenuates cue-induced reinstatement of alcohol-seeking behavior in Sardinian alcohol-preferring (sP) rats

The GABA(B) receptor agonist, baclofen, suppressed alcohol deprivation effect (a proposed experimental model of alcohol relapse) in Sardinian alcohol-preferring rats. The present study was designed to extend the characterization of the "anti-relapse" properties of baclofen to the reinstatement of alcohol-seeking behavior (another proposed model of alcohol relapse). Rats of the sP line were first trained to lever press for alcohol under a fixed ratio 4 schedule of reinforcement. Subsequently, rats were exposed to two within-session 70-min extinction/reinstatement tests with saline or baclofen administered in a counterbalanced, within-subject design. After a 60-min extinction phase, an alcohol-associated stimulus complex was presented (reinstatement phase). Saline or baclofen (3 mg/kg) were administered via a permanent intraperitoneal catheter, 30 min before the reinstatement phase. During the reinstatement phase, baclofen administration: (a) reduced by approximately 60% responses on the previously active lever, (b) increased latency to the first response and (c) decreased the response rate. These results indicate that baclofen reduced cue-induced reinstatement of alcohol-seeking behavior in sP rats.

Glycyl-glutamine reduces ethanol intake at three reward sites in P rats

beta-endorphin, implicated in modulation of ethyl alcohol reward, has neuron terminals in several reward sites. Alcohol consumption was reduced after ventricular or site-specific injections into the nucleus accumbens of an opioid-derived dipeptide, glycyl-glutamine. The current study examined the effects of this dipeptide after site-specific injections into additional reward sites. Alcohol-preferring (P) rats, stereotaxically implanted with bilateral guide cannulae into the nucleus accumbens, ventral tegmental area, and the central nucleus of the amygdala were given 30% alcohol and water in a 24h voluntary two-bottle choice paradigm. Upon achieving stable baseline intakes, glycyl-glutamine (GQ) doses were injected bilaterally, and the alcohol and water intakes and body weight recorded for the response and recovery. The data show reduced alcohol intake by 32-49.5% after 100-pmol glycyl-glutamine into reward sites (nucleus accumbens, ventral tegmental area, and central nucleus of the amygdala), but not after injections into control sites dorsal to reward sites. The order of sensitivity to the 1-fmol dose was amygdala > or = ventral tegmental area > accumbens. GQ was effective in reducing ethanol intake at reported beta-endorphin terminal regions in each of the three reward sites tested. The effective doses were similar to reported endogenous GQ levels, consistent with the notion that it may function as part of an endogenous counter regulatory mechanism and represent a "stop drinking" signal in the high drinking, P rats at these three reward sites.

Roles of the nucleus accumbens and amygdala in the acquisition and expression of ethanol-conditioned behavior in mice

Although progress has been made identifying the neural areas underlying the primary reinforcing effects of ethanol, few studies have examined the neural areas mediating ethanol-induced conditioned effects. Previous work using the conditioned place preference (CPP) procedure implicates the ventral tegmental area (VTA) (Bechtholt and Cunningham, 2005), but the downstream neural areas modulating the conditioned rewarding effects of ethanol have not been identified. Although the nucleus accumbens (Acb) and the amygdala (Amy), which both receive dopamine innervation from the VTA, have been implicated in the primary reinforcing effects of ethanol, the roles these areas play in ethanol-conditioned behaviors are unknown. In the present set of experiments, we use the CPP procedure along with selective electrolytic lesions to examine the neural areas underlying the acquisition and expression of ethanol conditioned behavior. In the acquisition experiment, male DBA/2J mice received bilateral lesions of the Acb or Amy before CPP training. In the expression experiments, mice received bilateral lesions of the Acb, Acb shell, Acb core, and Amy, or unilateral lesions of the Amy after training but before testing. Lesions of the Acb and Amy before training disrupted acquisition and expression of ethanol CPP. However, when given after training, only lesions of the Amy disrupted expression, whereas lesions of the Acb core facilitated loss of responding, of ethanol CPP. For the first time, these results demonstrate the role of the Acb and Amy in the acquisition and expression of ethanol-induced conditioned reward.

Cellular learning theory: theoretical comment on Cole and McNally (2007)

The idea that learning proceeds as a function of the discrepancy (or error) between expected and obtained outcomes is central to many theories of associative learning. However, remarkably little is known about the neurobiological mechanisms that underlie this learning of predictive errors in fear conditioning, a widely used preparation in studies of cellular and molecular mechanisms of memory. In this issue of Behavioral Neuroscience, S. Cole and G. P. McNally demonstrate an important dissociation between the establishment and regulation of predictive error at the cellular level. Their findings have added a level of complexity to currently established views of the function of NMDA and opioid receptors in learning and memory. This commentary discusses some of the implications of these findings for theoretical and neurobiological approaches to memory, as well as current thinking about the cellular circuitry involved in reward learning and drug abuse.

Drug-induced conditioned place preference and aversion in mice

This protocol describes the equipment and methods used to establish conditioned place preference (CPP) or aversion (CPA). Place conditioning is a form of Pavlovian conditioning routinely used to measure the rewarding or aversive motivational effects of objects or experiences (e.g., abused drugs). Here, we present a place conditioning procedure that has been used extensively to study the motivational effects of ethanol and other abused drugs in mice. This protocol involves three phases: (i) habituation (or a pretest), (ii) conditioning of an association between the drug and a tactile or visual stimulus and (iii) a test that offers a choice between the drug-associated cue and a neutral cue. If the drug has motivational significance, mice will spend significantly more time (CPP) or less time (CPA) in proximity to the drug-associated cue. Potential problems in the design and interpretation of place conditioning studies are discussed. A typical experiment lasts 2 weeks.

Differential effects of ethanol and midazolam upon the devaluation of an aversive memory in infant rats

In infant rats, low doses of ethanol (EtOH) have been found to attenuate the aversive representation of an unconditioned stimulus (US) as assessed through a revaluation paradigm. This may be explained by early anxiolytic properties of EtOH. The present set of experiments was aimed at analyzing possible mechanisms of these putative antianxiety effects of EtOH. In the first experiment, EtOH's effects upon the expression of citric acid-induced distress calls were compared with varying doses of midazolam (MDZ), a fast-acting gamma-aminobutyric acid type A (GABA(A)) agonist. Similar calming effects of 0.5 g/kg EtOH and 0.09 mg/kg MDZ were observed. Both drugs were then assessed in their capability to alter the expression of a conditioned aversion by devaluing the US. Aversive conditioning was conducted on postnatal day 14 (PD14) by pairing a lemon odor (conditioned stimulus, CS) with intraoral stimulation of citric acid (US). Control animals experienced both stimuli in an explicitly unrelated fashion. On PD15, pups were briefly exposed to the citric acid solution under the effects of 0.5 g/kg EtOH, 0.09 mg/kg MDZ, or the respective vehicle for each drug. Pups were then tested in a two-way odor preference test (lemon vs. cineole). Both vehicle- and MDZ-treated animals spent significantly less time near the lemon CS, thus expressing a citric acid-mediated odor aversion. This conditioned response was completely inhibited in pups that received 0.5 g/kg EtOH. Locomotor patterns at test were not affected by either EtOH or MDZ administration. A higher dose of MDZ (0.18 mg/kg, intraperitoneal) was also ineffective in attenuating the aversive memory. In summary, EtOH's devaluating capabilities are not shared by MDZ, indicating that these effects of EtOH may not be GABA mediated. Appetitive motivational properties of EtOH or non-GABA(A)-mediated antianxiety effects [i.e., N-methyl-D-aspartic acid (NMDA) related] could underlie this devaluation effect of EtOH.

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.

FOS expression induced by an ethanol-paired conditioned stimulus

To identify brain areas involved in ethanol-induced Pavlovian conditioning, brains of male DBA/2J mice were immunohistochemically analyzed for FOS expression after exposure to a conditioned stimulus (CS) previously paired with ethanol (2 g/kg) in two experiments. Mice were trained with a procedure that normally produces place preference (Before: ethanol before the CS) or one that normally produces place aversion (After: ethanol after the CS). Control groups received unpaired ethanol injections in the home cage (Delay) or saline only (Naïve). On the test day, mice were exposed to the 5-min CS 90 min before sacrifice. Before groups showed a conditioned increase in activity, whereas the After group showed a conditioned decrease in activity. FOS expression after a drug-free CS exposure was significantly higher in Before-group mice than in control mice in the bed nucleus of the stria terminalis (Experiment 1) and anterior ventral tegmental area (Experiments 1-2). Conditioned FOS responses were also seen in areas of the extended amygdala and hippocampus (Experiment 2). However, no conditioned FOS changes were seen in any brain area examined in After-group mice. Overall, these data suggest an important role for the mesolimbic dopamine pathway, extended amygdala and hippocampus in ethanol-induced conditioning.

Differential motivational properties of ethanol during early ontogeny as a function of dose and postadministration time

While appetitive reinforcement effects of ethanol are easily detected in rat neonates, such phenomena rarely have been observed in older infants. Recently, Molina et al. [Molina, J. C., Ponce L. F., Truxell, E., & Spear N. E. (2006). Infantile sensitivity to ethanol's motivational effects: ethanol reinforcement during the third postnatal week. Alcohol Clin Exp Res 30, 1506-1519] reported such effects of ethanol in 14-day-olds using a second-order conditioning procedure. Infants also appear to be sensitive to biphasic reinforcement or general motivational effects of ethanol, with appetitive effects seeming to occur early in the state of intoxication and aversive effects predominant during late stages, but tests have been inconclusive. The present study examined the possibility of biphasic motivational effects of ethanol during infancy through the use of second-order conditioning procedures. Preweanling rats (14 days old) experienced intraoral water infusions (conditioned stimulus, CS) either 5-20 or 30-45 min after administration of 0.5 or 2.0 g/kg i.g. ethanol. Pups were then exposed to the CS while over a novel texture (second-order phase). Tests of tactile preference for that texture followed. Locomotive, thermal, hormonal (corticosterone release), and pharmacokinetic patterns likely to underlie the acquisition of ethanol-mediated conditioning were also examined in subsequent experiments. Intraoral CSs paired with either early or late effects of low-dose ethanol (0.5 g/kg, blood ethanol concentration: 40 mg%) became positive second-order reinforcers. Appetitive effects were also exhibited by pups exposed to the CS during commencement of the toxic episode induced by a 2.0 g/kg ethanol dose, 5-20 min after administration of ethanol, whereas aversions emerged when CS presentation occurred 30-45 min postadministration time (blood ethanol concentrations: 157 and 200 mg%, respectively). Overall, the results indicate that infants rapidly detect differential motivational properties of ethanol as a function of dose or drug postadministration time. Relatively neutral stimuli associated with these properties are later capable of acting as either positive or aversive reinforcers. Thermal and motor responses that accompany ethanol intoxication do not seem to be directly associated with differential hedonic properties of the drug at this stage of development.

GABAB Receptor-Positive Modulation-Induced Blockade of the Rewarding Properties of Nicotine Is Associated with a Reduction in Nucleus Accumbens ΔFosB Accumulation

There is an increasing demand for a novel non-nicotinic, nondopaminergic therapeutic approach to nicotine addiction. GABAergic mechanisms have been implicated in drug dependence. Recently, a novel GABAB receptor allosteric-positive modulator, GS39783, was characterized. There are no investigations to date on the effects of GABAB receptor-positive modulators in animal models of nicotine reinforcement. Conditioned place preference (CPP) paradigms are based on the principle that animals, like humans, would learn to seek environmental stimuli that have been previously associated with rewarding events. Here we show that nicotine (0.06 mg/kg s.c.) induced a robust CPP response. Furthermore, GS39783 (30-100 mg/kg p.o.) during the conditioning phase blocked the rewarding effects of nicotine in the CPP paradigm in rats. However, GS39783 did not significantly alter the CPP effects of nicotine when given only immediately before the CPP test. A growing body of evidence suggests that repeated administration of drugs of abuse induced long-term molecular changes in brain plasticity, most notably an accumulation of DeltaFosB, in the striatal complex that contribute to the manifestation of dependence. There was a significant accumulation of DeltaFosB in the nucleus accumbens, but not in the dorsal striatum, of rats treated daily for 5 days with nicotine (0.06 mg/kg i.p.). GS39783 completely (30-100 mg/kg p.o.) counteracted these nicotine-induced molecular adaptations when given before the CPP acquisition phase but not when administered immediately before the test phase. Taken together, the behavioral and molecular changes induced by nicotine occur in concert and are concomitantly amenable to reversal by GABAB receptor-positive modulators.

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).

The orexin system regulates alcohol-seeking in rats

1. Orexin-containing neurons have been implicated in feeding, sleep-wake cycles and more recently in drug-seeking behaviour. 2. Pretreatment of alcohol-preferring (iP) rats with an orexin1 receptor antagonist (SB-334867, 20 mg kg(-1), intraperitoneally) completely abolished an olfactory cue-induced reinstatement of alcohol-seeking behaviour, and also attenuated alcohol responding under an operant fixed ratio regimen without affecting water responding. 3. The mRNA encoding orexin within the hypothalamus was expressed at a similar density in iP and non-preferring (NP) rats; chronic consumption of ethanol in iP rats did not significantly regulate the density of this expression, but did increase the area of expression within the lateral, but not medial, hypothalamus. 4. These data indicate that while orexin may not be implicated in the development of an alcohol preference, re-exposure of cues previously associated with alcohol availability is sufficient and adequate to activate orexin-containing neurons and drive reinstatement of alcohol-seeking.

The ethanol self-administration context as a reinstatement cue: acute effects of naltrexone

A major difficulty in treating alcohol addiction is the high rate of relapse even after prolonged abstinence. Relapse can be triggered by several factors, including stress, re-exposure to the drug, conditioned discrete stimuli and exposure to the context in which alcohol consumption occurred. The present study investigated the role played by the environmental context on ethanol relapse using an extinction/reinstatement animal model: rats were trained to self-administer ethanol in a distinctive context, and extinction occurred in a setting that differed by visual, tactile and olfactory properties; reinstatement was tested by placing the animals into the ethanol-associated context in the absence of ethanol. We found that re-exposure to the ethanol-associated context significantly increased responses on the ethanol-paired lever. The increase in responding required the presence of the complete configuration of the multimodal context. The non-selective opioid receptor antagonist naltrexone (0.3 mg/kg) administered 20 min prior to the reinstatement test significantly attenuated context-induced reinstatement of lever press responding, compared with saline-treated subjects. These data indicate that the environmental context associated with ethanol availability influences ethanol-seeking behavior in the rat, and that endogenous opioids are involved in this process. Our findings are in accordance with clinical reports demonstrating naltrexone efficacy in the treatment of alcohol relapse in humans, and indicate that the context-induced reinstatement model described here may be useful to investigate the biological mechanisms underlying alcohol relapse.