Neuroanatomical boundaries of the reward-relevant opiate-receptor field in the ventral tegmental area as mapped by the conditioned place preference method in rats

Neural correlates and potential targets for the contribution of orexin to addiction in cortical and subcortical areas

The orexin (hypocretin) is one of the hypothalamic neuropeptides that plays a critical role in some behaviors including feeding, sleep, arousal, reward processing, and drug addiction. This variety of functions can be described by a united function for orexins in translating states of heightened motivation, for example during physiological requirement states or following exposure to reward opportunities, into planned goal-directed behaviors. An addicted state is characterized by robust activation of orexin neurons from the environment, which triggers downstream circuits to facilitate behavior directed towards obtaining the drug. Two orexin receptors 1 (OX1R) and 2 (OX2R) are widely distributed in the brain. Here, we will introduce and describe the cortical and subcortical brain areas involved in addictive-like behaviors and the impact of orexin on addiction.

Assessing dairy calf response to long-distance transportation using conditioned place aversion

Surplus neonatal dairy calves are routinely transported long distances from the dairy farm of birth to calf raisers. Most of the research assessing the effect of transportation on young calves has focused on physiology and health, without considering calf affective states. The aim of this study was to assess the affective response of young male dairy calves to transportation using conditioned place aversion (CPA) testing. Three bouts of transportation exposed dairy calves (n = 95) to transportation for 6 (n = 31), 12 (n = 32), or 16 h (n = 30). During transportation, the trailer interior was made visually distinct using blue horizontal (n = 2 groups) or red vertical (n = 1 group) stripes. Distinct color and pattern combinations were used to create a visual association with transportation. A subset of calves (6 h = 14; 12 h = 14; 16 h = 14) were selected for CPA testing if they were ≥7 and <28 d of age at the time of testing after transport. A group of sham calves that were not transported (n = 13) and were ≥7 and <28 d of age at the time of testing also were enrolled. Wall coverings with the same color and pattern of those in the trailer were fixed in individual sham calf stalls for 6 h on the morning of transportation. Before transportation, calves were placed in a 3-compartment test apparatus with neutral walls and were able to move about freely for 15 min to habituate them to the test apparatus. Calves were CPA tested on d 2 after transportation. During the posttest, calves were placed in the test apparatus with 3 compartments including a neutral center pen (center), blue horizontal stripe compartment (side), and red vertical stripe compartment (side) for 30 min; calves were able to move throughout the apparatus freely. Time spent in each compartment was recorded using continuous observation and analyzed as a percentage of total time in the apparatus. Calves spent less time in the compartment associated with transportation compared with the compartment with no prior association (22% vs. 42% ± 3% of time in the transportation-association and no-association compartments, respectively); however, this behavior was not affected by transportation treatment. Results from this study suggest that calves may perceive transportation to be aversive regardless of duration but may also be attracted to exploring a novel stimulus.

Early Life Stress and Risks for Opioid Misuse: Review of Data Supporting Neurobiological Underpinnings

A robust body of research has shown that traumatic experiences occurring during critical developmental periods of childhood when neuronal plasticity is high increase risks for a spectrum of physical and mental health problems in adulthood, including substance use disorders. However, until recently, relatively few studies had specifically examined the relationships between early life stress (ELS) and opioid use disorder (OUD). Associations with opioid use initiation, injection drug use, overdose, and poor treatment outcome have now been demonstrated. In rodents, ELS has also been shown to increase the euphoric and decrease antinociceptive effects of opioids, but little is known about these processes in humans or about the neurobiological mechanisms that may underlie these relationships. This review aims to establish a theoretical model that highlights the mechanisms by which ELS may alter opioid sensitivity, thereby contributing to future risks for OUD. Alterations induced by ELS in mesocorticolimbic brain circuits, and endogenous opioid and dopamine neurotransmitter systems are described. The limited but provocative evidence linking these alterations with opioid sensitivity and risks for OUD is presented. Overall, the findings suggest that better understanding of these mechanisms holds promise for reducing vulnerability, improving prevention strategies, and prescribing guidelines for high-risk individuals.

Reward-related limbic memory and stimulation of the cannabinoid system: An upgrade in value attribution?

While a lot is known about the mechanisms promoting aversive learning, the impact of rewarding factors on memory has received comparatively less attention. This research investigates reward-related explicit memory in male rats, by taking advantage of the emotional-object recognition test. This is based on the prior association, during conditioned learning, between a rewarding experience (the encounter with a receptive female rat) and an object; afterwards rat discrimination and recognition of the 'emotional object' is recorded in the presence of a novel object, as a measure of positive limbic memory formation. Since endocannabinoids are critical for processing reward and motivation, the consequences of the stimulation of cannabinoid signalling are also assessed by the administration of WIN 55,212-2 at pre- and post-conditioning time. Our results show that rats encode the association between object and rewarding experience, form positive limbic memory of the emotional object, and retrieve this information in the face of novelty. Stimulation of the cannabinoid system at pre-conditioning time is able to strengthen reward-related explicit memory in the presence of novelty, whereas post-conditioning activation increases approach behaviour to novel stimuli. The assessment of limbic memory by the emotional-object recognition test can help unveiling the addictive and confounding properties of psychotropic drugs.

Two delta opioid receptor subtypes are functional in single ventral tegmental area neurons, and can interact with the mu opioid receptor

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K⁺ dependent hyperpolarization or a Ca_v2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes. However, in a subset of VTA neurons the DOR antagonist TIPP-Ψ augmented DAMGO responses; we also observed DPDPE or deltorphin II responses augmented by the MOR selective antagonist CTAP. These findings directly support the existence of two independent, stable forms of the DOR, and show that MOR and DOR can interact in some neurons to alter downstream signaling.

Pain in an era of armed conflicts: Prevention and treatment for warfighters and civilian casualties

Chronic pain is a common squealae of military- and terror-related injuries. While its pathophysiology has not yet been fully elucidated, it may be potentially related to premorbid neuropsychobiological status, as well as to the type of injury and to the neural alterations that it may evoke. Accordingly, optimized approaches for wounded individuals should integrate primary, secondary and tertiary prevention in the form of thorough evaluation of risk factors along with specific interventions to contravene and mitigate the ensuing chronicity. Thus, Premorbid Events phase may encompass assessments of psychological and neurobiological vulnerability factors in conjunction with fostering preparedness and resilience in both military and civilian populations at risk. Injuries per se phase calls for immediate treatment of acute pain in the field by pharmacological agents that spare and even enhance coping and adaptive capabilities. The key objective of the Post Injury Events is to prevent and/or reverse maladaptive peripheral- and central neural system's processes that mediate transformation of acute to chronic pain and to incorporate timely interventions for concomitant mental health problems including post-traumatic stress disorder and addiction We suggest that the proposed continuum of care may avert more disability and suffering than the currently employed less integrated strategies. While the requirements of the armed forces present a pressing need for this integrated continuum and a framework in which it can be most readily implemented, this approach may be also instrumental for the care of civilian casualties.

Ventral Tegmental Area Afferents and Drug-Dependent Behaviors

Drug-related behaviors in both humans and rodents are commonly thought to arise from aberrant learning processes. Preclinical studies demonstrate that the acquisition and expression of many drug-dependent behaviors involves the ventral tegmental area (VTA), a midbrain structure comprised of dopamine, GABA, and glutamate neurons. Drug experience alters the excitatory and inhibitory synaptic input onto VTA dopamine neurons, suggesting a critical role for VTA afferents in mediating the effects of drugs. In this review, we present evidence implicating the VTA in drug-related behaviors, highlight the diversity of neuronal populations in the VTA, and discuss the behavioral effects of selectively manipulating VTA afferents. Future experiments are needed to determine which VTA afferents and what neuronal populations in the VTA mediate specific drug-dependent behaviors. Further studies are also necessary for identifying the afferent-specific synaptic alterations onto dopamine and non-dopamine neurons in the VTA following drug administration. The identification of neural circuits and adaptations involved with drug-dependent behaviors can highlight potential neural targets for pharmacological and deep brain stimulation interventions to treat substance abuse disorders.

Exposure to opiates in female adolescents alters mu opiate receptor expression and increases the rewarding effects of morphine in future offspring

Prescription opiate use and abuse has increased dramatically over the past two decades, including increased use in adolescent populations. Recently, it has been proposed that use during this critical period may affect future offspring even when use is discontinued prior to conception. Here, we utilize a rodent model to examine the effects of adolescent morphine exposure on the reward functioning of the offspring. Female Sprague Dawley rats were administered morphine for 10 days during early adolescence (post-natal day 30-39) using an escalating dosing regimen. Animals then remained drug free until adulthood at which point they were mated with naïve males. Adult offspring (F1 animals) were tested for their response to morphine-induced (0, 1, 2.5, 5, and 10 mg/kg, s.c.) conditioned place preference (CPP) and context-independent morphine-induced sensitization. Naïve littermates were used to examine mu opiate receptor expression in the nucleus accumbens and ventral tegmental area. Results indicate that F1 females whose mothers were exposed to morphine during adolescence (Mor-F1) demonstrate significantly enhanced CPP to the lowest doses of morphine compared with Sal-F1 females. There were no differences in context-independent sensitization between maternal treatment groups. Protein expression analysis showed significantly increased levels of accumbal mu opiate receptor in Mor-F1 offspring and decreased levels in the VTA. Taken together, these findings demonstrate a shift in the dose response curve with regard to the rewarding effects of morphine in Mor-F1 females which may in part be due to altered mu opiate receptor expression in the nucleus accumbens and VTA.

It's MORe exciting than mu: crosstalk between mu opioid receptors and glutamatergic transmission in the mesolimbic dopamine system

Opioids selective for the G protein-coupled mu opioid receptor (MOR) produce potent analgesia and euphoria. Heroin, a synthetic opioid, is considered one of the most addictive substances, and the recent exponential rise in opioid addiction and overdose deaths has made treatment development a national public health priority. Existing medications (methadone, buprenorphine, and naltrexone), when combined with psychosocial therapies, have proven efficacy in reducing aspects of opioid addiction. Unfortunately, these medications have critical limitations including those associated with opioid agonist therapies (e.g., sustained physiological dependence and opioid withdrawal leading to high relapse rates upon discontinuation), non-adherence to daily dosing, and non-renewal of monthly injection with extended-release naltrexone. Furthermore, current medications fail to ameliorate key aspects of addiction such as powerful conditioned associations that trigger relapse (e.g., cues, stress, the drug itself). Thus, there is a need for developing novel treatments that target neural processes corrupted with chronic opioid use. This requires a basic understanding of molecular and cellular mechanisms underlying effects of opioids on synaptic transmission and plasticity within reward-related neural circuits. The focus of this review is to discuss how crosstalk between MOR-associated G protein signaling and glutamatergic neurotransmission leads to immediate and long-term effects on emotional states (e.g., euphoria, depression) and motivated behavior (e.g., drug-seeking, relapse). Our goal is to integrate findings on how opioids modulate synaptic release of glutamate and postsynaptic transmission via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in the nucleus accumbens and ventral tegmental area with the clinical (neurobehavioral) progression of opioid dependence, as well as to identify gaps in knowledge that can be addressed in future studies.

Behavioral characterization of mice lacking Trek channels

Two-pore domain K⁺ (K_2P) channels are thought to underlie background K⁺ conductance in many cell types. The Trek subfamily of K_2P channels consists of three members, Trek1/Kcnk2, Trek2/Kcnk10, and Traak/Kcnk4, all three of which are expressed in the rodent CNS. Constitutive ablation of the Trek1 gene in mice correlates with enhanced sensitivity to ischemia and epilepsy, decreased sensitivity to the effects of inhaled anesthetics, increased sensitivity to thermal and mechanical pain, and resistance to depression. While the distribution of Trek2 mRNA in the CNS is broad, little is known about the relevance of this Trek family member to neurobiology and behavior. Here, we probed the effect of constitutive Trek2 ablation, as well as the simultaneous constitutive ablation of all three Trek family genes, in paradigms that assess motor activity, coordination, anxiety-related behavior, learning and memory, and drug-induced reward-related behavior. No differences were observed between Trek2^−/− and Trek1/2/Traak^−/− mice in coordination or total distance traveled in an open-field. A gender-dependent impact of Trek ablation on open-field anxiety-related behavior was observed, as female but not male Trek2^−/− and Trek1/2/Traak^−/− mice spent more time in, and made a greater number of entries into, the center of the open-field than wild-type counterparts. Further evaluation of anxiety-related behavior in the elevated plus maze and light/dark box, however, did not reveal a significant influence of genotype on performance for either gender. Furthermore, Trek^−/− mice behaved normally in tests of learning and memory, including contextual fear conditioning and novel object recognition, and with respect to opioid-induced motor stimulation and conditioned place preference (CPP). Collectively, these data argue that despite their broad distribution in the CNS, Trek channels exert a minimal influence on a wide-range of behaviors.

Disrupting effect of drug-induced reward on spatial but not cue-guided learning: implication of the striatal protein kinase A/cAMP response element-binding protein pathway

The multiple memory systems hypothesis posits that different neural circuits function in parallel and may compete for information processing and storage. For example, instrumental conditioning would depend on the striatum, whereas spatial memory may be mediated by a circuit centered on the hippocampus. However, the nature of the task itself is not sufficient to select durably one system over the other. In this study, we investigated the effects of natural and pharmacological rewards on the selection of a particular memory system during learning. We compared the effects of food- or drug-induced activation of the reward system on cue-guided versus spatial learning using a Y-maze discrimination task. Drug-induced reward severely impaired the acquisition of a spatial discrimination task but spared the cued version of the task. Immunohistochemical analysis of the phosphorylated form of the cAMP response element binding (CREB) protein and c-Fos expression induced by behavioral testing revealed that the spatial deficit was associated with a decrease of both markers within the hippocampus and the prefrontal cortex. In contrast, drug reward potentiated the cued learning-induced CREB phosphorylation within the dorsal striatum. Administration of the protein kinase A inhibitor 8-Bromo-adenosine-3',5'-cyclic monophosphorothioate Rp isomer (Rp-cAMPS) into the dorsal striatum before training completely reversed the drug-induced spatial deficit and restored CREB phosphorylation levels within the hippocampus and the prefrontal cortex. Therefore, drug-induced striatal hyperactivity may underlie the declarative memory deficit reported here. This mechanism could represent an important early step toward the development of addictive behaviors by promoting conditioning to the detriment of more flexible forms of memory.

Opioid-Induced GABA potentiation after chronic morphine attenuates the rewarding effects of opioids in the ventral tegmental area

GABA transmission in the ventral tegmental area (VTA) is critical for fine tuning the activity of dopamine neurons in response to opioids. However, the precise mechanism by which GABA input shapes opioid reward is poorly understood. We observed a reduction of conditioned place preference for low doses of the opioid [d-Ala2, N-MePhe4, Gly5-ol]-enkephalin (DAMGO) and a switch in the functional effects of μ-opioid receptor modulation of GABA postsynaptic currents in the mouse VTA 1 d after chronic morphine treatment. Specifically, whereas in naive mice DAMGO inhibits GABA postsynaptic currents, GABAergic currents are potentiated by DAMGO after chronic morphine treatment. Importantly, pretreatment with the cAMP signaling inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium both restored DAMGO reward and reversed the DAMGO-mediated potentiation, thereby reestablishing the inhibitory effects of opioids on GABA currents. Thus, a paradoxical bidirectionality in μ-receptor-mediated control of GABA transmission following chronic morphine treatment is a critical mechanism that determines the expression of opioid reward in the VTA.

Pentylenetetrazol produces a state-dependent conditioned place aversion to alcohol withdrawal in mice

The purpose of this study was to determine if aversive effects of alcohol withdrawal could be detected in mice using the place conditioning procedure and whether the GABA(A) receptor antagonist, pentylenetetrazol (PTZ), would increase the aversive effects of alcohol withdrawal and increase the probability of detecting conditioned place aversion. Subjects were alcohol-naïve mice from a specific line selectively bred for low alcohol preference (LAP1; n=91) and were assigned to three groups: alcohol withdrawal, PTZ alone, and PTZ+alcohol withdrawal. On four trials, mice received either a 4.0 g/kg intraperitoneal (i.p.) injection of alcohol (alcohol withdrawal, PTZ+alcohol withdrawal groups) or saline (PTZ group) 8 h prior to being placed on a distinctive floor texture for a 30-min conditioning session. Five minutes before these sessions, mice in the PTZ and PTZ+alcohol withdrawal groups received PTZ (5.0 mg/kg; i.p.) and the alcohol withdrawal group received saline. On intervening days mice received two saline injections at the same time points prior to being placed on a different floor texture. Post-conditioning floor preference was assessed in two 60-min tests; the first test was drug-free and the second test was state-dependent. Neither alcohol withdrawal nor PTZ produced significant place conditioning. The PTZ+alcohol withdrawal group showed a significant place aversion during the state-dependent test. These data suggest that the combined stimulus properties of PTZ and alcohol withdrawal facilitated the expression of conditioned place aversion to alcohol withdrawal.

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.

Anatomically dissociable effects of dopamine D1 receptor agonists on reward and relief of withdrawal in morphine-dependent rats

RATIONALE

Chronic opiate administration induces neuroadaptations within the nucleus accumbens (NAc) and ventral tegmental area (VTA) that can contribute to dependence. We have shown that morphine dependence shifts the behavioral consequences of D1 dopamine (DA) receptor signaling: systemic administration of a D1 receptor agonist is rewarding and blocks naloxone-precipitated withdrawal signs in morphine-dependent rats, but has minimal effects in nondependent rats. These data suggest that D1 receptors acquire the ability to regulate reward and withdrawal in morphine-dependent rats. The brain regions involved in these effects are not known.

OBJECTIVE

Studies were designed to test the hypothesis that the nucleus accumbens shell (NASh) and the ventral tegmental area (VTA) are important sites for mediating the behavioral effects of D1 receptor activation in morphine-dependent rats.

MATERIALS AND METHODS

The effects of microinjecting the D1 receptor agonist SKF 82958 into the NASh or the VTA on place conditioning and somatic withdrawal signs were studied in morphine-dependent and nondependent rats.

RESULTS

Intra-NASh microinjection of SKF 82958 (1 microg/side) established conditioned place preferences in morphine-dependent but not nondependent rats, but had no effect on naloxone-induced place aversions or somatic withdrawal signs. Intra-VTA microinjection of SKF 82958 (2 microg) did not establish place preferences under any conditions, but blocked naloxone-induced place aversions without effects on somatic withdrawal signs.

CONCLUSIONS

There is an anatomical dissociation between D1 receptor-mediated reward and relief of withdrawal in morphine-dependent rats. When combined, the individual effects of D1 receptor activation in the NASh and VTA on the affective signs of precipitated morphine withdrawal resemble those seen with systemic administration.

Neuropeptide S inhibits the acquisition and the expression of conditioned place preference to morphine in mice

Neuropeptide S (NPS), a recently identified bioactive peptide, was reported to regulate arousal, anxiety, motoring and feeding behaviors. NPS precursor and NPS receptor mRNA were found in the amygdala, the ventral tegmental area (VTA) and the substantia nigra, the area thought to modulate rewarding properties of drugs. In the present study, we examined the influence of NPS on the rewarding action of morphine, using the unbiased conditioned place preference (CPP) paradigm. Morphine (1, 3 and 6 nmol, i.c.v.) induced a significant place preference. For testing the effect of NPS on the acquisition of morphine CPP, mice were given the combination of NPS and morphine on the conditioning days, and without drug treatment on the followed test day. To study the effect of NPS on the expression of morphine CPP, mice received the treatment of saline/morphine on the conditioning days, and NPS on the test day, 15 min before the placement in the CPP apparatus. Our results showed that NPS (0.3-10 nmol) alone neither induced place preference nor aversion, however, NPS (1 and 3 nmol) blocked the acquisition of CPP induced by 3 nmol morphine, and acquisition of 6 nmol morphine-induced CPP was also reduced by NPS (6 and 10 nmol). Moreover, the expression of CPP induced by 6 nmol morphine was also inhibited by NPS (0.1, 1 and 10 nmol). These results revealed the involvement of NPS in rewarding activities of morphine, and demonstrated the interaction between NPS system and opioid system for the first time.

Ingestion of amniotic fluid enhances the facilitative effect of VTA morphine on the onset of maternal behavior in virgin rats

Previous research has shown that injection of morphine into the ventral tegmental area (VTA) facilitates the onset of maternal behavior in virgin female rats, and injection of the opioid antagonist naltrexone into the VTA disrupts the onset of maternal behavior in parturient rats. Placentophagia -- ingestion of placenta and amniotic fluid, usually at parturition -- modifies central opioid processes. Ingestion of the active substance in placenta and amniotic fluid, Placental Opioid-Enhancing Factor (POEF), enhances the hypoalgesic effect of centrally administered morphine, and more specifically, enhances delta- and kappa-opioid-receptor-mediated hypoalgesia and attenuates mu-opioid-receptor-mediated hypoalgesia. POEF (in placenta or amniotic fluid) ingestion does not, by itself, produce hypoalgesia. In the present study, we tested the hypothesis that ingestion of amniotic fluid enhances the facilitative effect of opioid activity (unilateral morphine injection) in the VTA on the rate of onset of maternal behavior. Virgin female Long-Evans rats were given one intra-VTA injection of morphine sulfate (0.0, 0.01, or 0.03 microg, in saline) and an orogastric infusion of 0.25 ml amniotic fluid or saline once each day of the first three days of the 10-day testing period. Subjects were continuously exposed to foster pups that were replaced every 12 h; replacement of pups was followed by a 15-min observation period. Maternal behavior latency was determined by the first of two consecutive tests wherein the subject displayed pup retrieval, pup licking in the nest, and crouching over all foster pups, during the 15-min observation. We confirmed the previous finding that the VTA injection, alone, of 0.03 microg morphine shortened the latency to show maternal behavior and that 0.0 microg and 0.01 microg morphine did not. Ingestion of amniotic fluid (and therefore POEF) facilitated the onset of maternal behavior in rats receiving an intra-VTA microinjection of an otherwise subthreshold dose of morphine (0.01 microg).

Brain regional Fos expression elicited by the activation of mu- but not delta-opioid receptors of the ventral tegmental area: evidence for an implication of the ventral thalamus in opiate reward

Both mu-opioid receptors (MORs) and delta-opioid receptors (DORs) are expressed in the ventral tegmental area (VTA) and are thought to be involved in the addictive properties of opiates. However, their respective contributions to opiate reward remain unclear. We used intracranial self-administration (ICSA) to study the rewarding effects of morphine microinjections into the VTA of male and female MOR-/- and DOR-/- mice. In brains of mice tested for intra-VTA morphine self-administration, we analyzed regional Fos protein expression to investigate the neural circuitry underlying this behavior. Male and female WT and DOR-/- mice exhibited similar self-administration performances, whereas knockout of the MOR gene abolished intra-VTA morphine self-administration at all doses tested. Naloxone (4 mg/kg) disrupted this behavior in WT and DOR mutants, without triggering physical signs of withdrawal. Morphine ICSA was associated with an increase in Fos within the nucleus accumbens, striatum, limbic cortices, amygdala, hippocampus, the lateral mammillary nucleus (LM), and the ventral posteromedial thalamus (VPM). This latter structure was found to express high levels of Fos exclusively in self-administering WT and DOR-/- mice. Abolition of morphine reward in MOR-/- mice was associated with a decrease in Fos-positive neurons in the mesocorticolimbic dopamine system, amygdala, hippocampus (CA1), LM, and a complete absence within the VPM. We conclude that (i) VTA MORs, but not DORs, are critical for morphine reward and (ii) the role of VTA-thalamic projections in opiate reward deserves to be further explored.

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.

Post-treatment of dextromethorphan reverses morphine effect on conditioned place preference in rats

Previously we showed that coadministration of dextromethorphan (DM) with morphine attenuates morphine-rewarding effect. Here we further investigated if DM is effective in reversing or treating drug-seeking effect when given after subchronic morphine treatment. The conditioned place preference (CPP) test was used to investigate the rewarding and drug-seeking effects of morphine. Rats were injected and conditioned with morphine for 6 days and then withdrawn from morphine for 4 days and treated with saline or DM during this period. Subchronic morphine induced a significant place preference for the drug-paired compartment. DM treatment during the morphine withdrawal period significantly reversed the preference from the drug-paired compartment to saline-paired compartment. Both nucleus accumbens (NAc) and VTA were proven to be the sites involved in the action of DM. Behavioral sensitization occurred in both morphine group and DM treatment group determined by the locomotor activity before and after subchronic morphine treatment. The dopamine (DA) turnover rate in the NAc, dorsal striatum (DS) and medial prefrontal cortex (mPFC) was increased after subchronic morphine treatment. DM treatment reversed the increase of DA turnover rate in the NAc but not in the DS or mPFC. These data suggest that DM might have potential in the treatment of morphine addiction.

Reinforcing effects of nicotine microinjections into the ventral tegmental area of mice: Dependence on cholinergic nicotinic and dopaminergic D1 receptors

We used an intracranial self-administration (ICSA) procedure to assess the involvement of the ventral tegmental area (VTA) nicotinic receptors in the rewarding effects of nicotine. We then challenged intra-VTA nicotine self-administration via systemic or local injections of dopamine (DA)-D1 and nicotinic receptor antagonists. C57BL/6J mice were stereotaxically implanted unilaterally with a guide cannula above the VTA. After 1 week of recovery, mice were allowed to discriminate between two arms of a Y-maze over seven daily sessions, one arm being reinforced by intracranial nicotine microinjection. Mice exhibited nicotine self-administration at both doses tested, i.e. 10 ng (21.6 pmol) and 100 ng (216 pmol)/50-nl injection. In contrast, mice receiving a 216-pmol nicotine dose 0.8 mm above VTA performed at chance level. Once the ICSA response was acquired, systemic pretreatment with the DA-D1 receptor antagonist SCH 23390 (25 microg/kg i.p.) or co-infusion of the nAChR antagonist DHbetaE with nicotine disrupted ICSA. Replacement of SCH 23390 by vehicle, or withdrawal of DHbetaE from nicotine/DHbetaE mixed solutions led to recovery of intra-VTA nicotine self-administration. We conclude that nicotinic receptors in the VTA, presumably alpha4beta2 nAChRs are critically to mediate the rewarding effects of nicotine and that DA-D1 receptors are also directly implicated.

Morphine reward in dopamine-deficient mice

Dopamine has been widely implicated as a mediator of many of the behavioural responses to drugs of abuse. To test the hypothesis that dopamine is an essential mediator of various opiate-induced responses, we administered morphine to mice unable to synthesize dopamine. We found that dopamine-deficient mice are unable to mount a normal locomotor response to morphine, but a small dopamine-independent increase in locomotion remains. Dopamine-deficient mice have a rightward shift in the dose-response curve to morphine on the tail-flick test (a pain sensitivity assay), suggesting either a decreased sensitivity to the analgesic effects of morphine and/or basal hyperalgesia. In contrast, dopamine-deficient mice display a robust conditioned place preference for morphine when given either caffeine or l-dihydroxyphenylalanine (a dopamine precursor that restores dopamine throughout the brain) during the testing phases. Together, these data demonstrate that dopamine is a crucial component of morphine-induced locomotion, dopamine may contribute to morphine analgesia, but that dopamine is not required for morphine-induced reward as measured by conditioned place preference.

Intra-ventral tegmental area heroin-induced place preferences in rats are potentiated by peripherally administered alprazolam

The present experiment was designed to replicate and extend previous results of an opiate+benzodiazepine interaction in which peripherally administered alprazolam was observed to modulate behavior resulting from intravenous injections of heroin. As a first step in determining the role of central sites in this drug interaction, changes in drug reward (measured by conditioned place preference; CPP) were assessed in rats given systemic administration of alprazolam coupled with intracranially infused heroin (into the ventral tegmental area; VTA). Sprague-Dawley rats were implanted with guide cannula targeting the VTA, after which a heroin-induced CPP dose-response curve was determined (2.5-40 ng administered bilaterally in 0.5 microl/side). In other animals, intra-VTA heroin-induced place preferences were challenged with systemically applied alprazolam (0.125 mg/kg i.p.). The data confirm that rats dose-dependently develop reliable place preferences for a distinct environment paired with bilateral VTA-infusions of heroin. Additionally, when a non-rewarding dose of alprazolam was combined with a non-rewarding bilateral intra-VTA heroin dose (5 ng), a significant CPP was produced. These data extend earlier results by demonstrating that a systemically applied benzodiazepine can enhance the rewarding effects produced by central opiate administration. The results suggest that the VTA might be a site where this opiate+benzodiazepine interaction occurs.

Intracranial self-administration of cocaine within the posterior ventral tegmental area of Wistar rats: evidence for involvement of serotonin-3 receptors and dopamine neurons

The rewarding properties of cocaine have been postulated to be regulated, in part, by the mesolimbic dopamine (DA) system. The present study assessed whether adult female Wistar rats would self-administer cocaine directly into the ventral tegmental area (VTA). Following guide cannulae surgery aimed at either the posterior or anterior VTA, subjects were placed in an operant box equipped with an active lever that caused the delivery of the infusate and an inactive lever that did not. Posterior and anterior VTA subjects were randomly assigned to one of six groups that self-administered either artificial cerebrospinal fluid (aCSF) or 25 to 400 pmol cocaine/100 nl in aCSF for the first four sessions, aCSF in sessions 5 and 6, and the acquisition dose of infusate during session 7. Additionally, the effects of increasing the time-out period, higher concentrations of cocaine, coadministration of a 5HT3 antagonist, and coadministration of a D2/3 agonist on self-infusion of cocaine were determined. Self-infusions were maintained when the time-out period was extended from 5 to 25 s. Coinfusion of a 5HT3 antagonist or D2/3 agonist blocked the self-infusion of cocaine. In contrast, rats did not self-administer 25 to 400 pmol/100 nl cocaine into the anterior VTA. Additionally, rats did not self-administer either 800 or 1600 pmol/100 nl cocaine into the posterior or anterior VTA. Overall, the data indicate that the VTA is functionally heterogeneous with regard to the rewarding actions of cocaine and that the reinforcing effects of cocaine within the posterior VTA are mediated by activation 5-HT3 receptors and DA neurons.

Midbrain muscarinic receptors modulate morphine-induced accumbal and striatal dopamine efflux in the rat

Midbrain dopamine neurons are critical in mediating the rewarding effects of opiates in dependent rats, as well as modulating some manifestations of opiate withdrawal. Morphine is known to excite dopamine neurons and thereby facilitate forebrain dopamine transmission through inhibition of GABA neurons. Cholinergic neurons in the mesopontine laterodorsal and pedunculopontine tegmental nuclei provide the principal source of excitatory cholinergic input to ventral tegmental area and substantia nigra pars compacta dopamine-containing neurons, via actions on midbrain muscarinic and nicotinic acetylcholine receptors. The present study hypothesized that a reduction in tonic cholinergic input via blockade of midbrain muscarinic receptors would reduce the pharmacological effects of morphine on forebrain dopamine release. Using in vivo chronoamperometry, alterations in morphine-evoked dopamine efflux were monitored at stearate-graphite paste electrodes implanted unilaterally in the nucleus accumbens and striatum of urethane (1.5 g/kg) anesthetized rats, following the pharmacological inhibition of ventral tegmental area/substantia nigra pars compacta muscarinic receptors. The facilitatory effects of morphine (2.0 mg/kg, i.v.) on accumbens and striatal dopamine efflux were markedly reduced by prior infusion of the non-selective muscarinic receptor antagonist scopolamine (200 microg/microl) into the ventral tegmental area or substantia nigra pars compacta, respectively. These findings demonstrate that decreased activation of midbrain muscarinic receptors attenuates the excitatory effects of morphine on mesoaccumbens and nigrostriatal dopaminergic transmission.

Androgen receptor blockade in the MPOA or VMN: effects on male sociosexual behaviors

Previously, our laboratory has shown that androgen receptors in the medial preoptic area (MPOA) and ventromedial nucleus (VMN) are necessary for copulation in male rats. The present study examined whether these receptors are required for other sociosexual behaviors. In Experiment 1, different regions of the VMN were implanted with the antiandrogen hydroxyflutamide (OHF). We found that implants located in anterodorsal portions of the VMN were more effective at inhibiting the restoration of copulation than implants in the posteroventral VMN. In Experiment 2, a second set of male rats was pretested for copulation and other sociosexual behaviors and was castrated. Experimental animals then received Silastic capsules filled with testosterone (T) plus intracranial (IC) implants filled with OHF to selectively block androgen receptors in either the MPOA or VMN. We found that androgen receptor blockade in the MPOA inhibited the restoration of copulation but had no effect on other sociosexual behaviors. OHF directed at the VMN inhibited the restoration of copulation and 50-kHz vocalizations but had no effect on scent marking. Two tests were used to assay sexual motivation: partner preference and conditioned place preference (CPP). Both methods revealed impairments in sexual motivation in the VMN group but not in animals receiving OHF in the MPOA. Taken together, these data suggest that androgen receptors in the MPOA are essential for copulatory performance, while androgen receptors in the VMN are important for copulation, sexual motivation, and androgen-dependent vocalizations.

Endogenous opioids mediate basal hedonic tone independent of dopamine D-1 or D-2 receptor activation

Exogenously administered opiates are recognized as rewarding and the involvement of dopamine systems in mediating their apparent pleasurable effects is contentious. The aversive response to naloxone administration observed in animal studies suggests the presence of an endogenous opioid tone regulating hedonic state. We sought evidence for the requirement for dopamine systems in mediating this action of endogenous opioids by determining whether mice deficient in dopamine D-1 or D-2 receptors were able to display conditioned place aversion to naloxone. Mice received saline in the morning in one chamber and either saline or naloxone (10 mg/kg, s.c.) in the afternoon in another chamber, each day for 3 days. On the test day they were given free access to the testing chambers in the afternoon. Similar to their wild-type littermates, D-1 and D-2 receptor knockout mice receiving naloxone in the afternoon spent significantly less time on the test day in the compartment in which they previously received naloxone, compared with animals receiving saline in the afternoon. The persistence of naloxone-conditioned place aversion in D-1 and D-2 knockout mice suggests that endogenous opioid peptides maintain a basal level of positive affect that is not dependent on downstream activation of dopamine systems involving D-1 or D-2 receptors.

Systemic and intracerebroventricular administration of sodium barbital induced a place preference in rats

We have shown previously that 15 mg/kg pentobarbital induces a conditioned place preference (CPP), but it is unsuitable for intracranial administration. Since the long-acting barbiturate, sodium barbital, is soluble at a neutral pH, we tested whether it would induce a CPP when administered centrally. Furthermore, because barbital has a long duration of action, and because we obtained a significant CPP to systemically administered barbital using 30-minute conditioning trials, we tested whether longer conditioning trials would produce a more robust CPP. Using a three-compartment apparatus and an unbiased procedure, we found that systemic administration of barbital induced a significant CPP at 8 and 24 mg/kg, but not 2.7 or 72 mg/kg (i.p.). When rats were conditioned to 24 mg/kg barbital for conditioning trials of (1/2), 1, 3, or 6 hours, only the 30-min conditioning trial produced a CPP. Finally, 240 and 480 microg intracerebroventricular (ICV) barbital induced a significant CPP, but 60 or 120 microg did not. These findings suggest that: (1) like pentobarbital, barbital has reinforcing properties measured in the CPP test; (2) the CPP is impaired, rather than enhanced, by increasing the duration of drug-context pairing; and (3) the reinforcing effects of barbiturates are centrally mediated.

The effects of alprazolam on conditioned place preferences produced by intravenous heroin

Case studies reveal that opiate addicts often self-medicate with benzodiazepine (BDZ) tranquilizers prior to taking their opiate. Our laboratory has previously utilized the conditioned place preference paradigm to confirm that BDZs can augment the affective response to heroin in laboratory animals. The combination of alprazolam and varying doses of intravenous heroin resulted in a leftward shift of the heroin dose-response curve. The present experiment was devised to extend the previous findings by examining the ability of varying alprazolam doses (0.125, 0.25, or 0.5 mg/kg ip) to potentiate the reward of a single challenge dose of heroin (0.025 mg/kg iv). The results demonstrate that a nonrewarding dose of alprazolam (0.125 mg/kg) and intravenous heroin can interact to produce reliable place preferences. The data thereby support prior work from our laboratory regarding the synergistic actions of BDZs and opiates.

Endomorphin-1 and -2 immunoreactive cells in the hypothalamus are labeled by fluoro-gold injections to the ventral tegmental area

Endomorphin-1 and -2 (EM1, EM2) are endogenous opioids with high affinity and selectivity for the mu-opioid receptor. Cells expressing EM-like immunoreactivity (EM-LI) are present in the hypothalamus, and fibers containing EM-LI project to many brain regions, including the ventral tegmental area (VTA). The VTA is one of the most sensitive brain regions for the rewarding and locomotor effects of opioids. It contains mu-opioid receptors, which are thought to mediate gamma-aminobutyric acid-dependent disinhibition of dopamine transmission to the nucleus accumbens. We investigated whether hypothalamic EM-LI cells project to the VTA, where they could play a natural role in this circuitry. The retrograde tracer Fluoro-Gold (FG) was microinjected into the anterior or posterior VTA in rats. Nine days later, colchicine was injected, and 24 hours later, the animals were perfused and processed for fluorescence immunocytochemistry. Numerous FG-labeled cells were detected in the hypothalamus. Both EM1-LI and EM2-LI cells were present in the periventricular nucleus, between the dorsomedial and ventromedial hypothalamus and between the ventromedial and arcuate nuclei. Subpopulations of EM1-LI and EM2-LI cells were labeled by FG. Injections of FG to the anterior and posterior VTA were both effective in producing double-labeled cells, and an anterior-posterior topographical organization between the VTA and hypothalamus was observed. The results support the idea that some endomorphin-containing neurons in the hypothalamus project to the VTA, where they may modulate reward and locomotor circuitry.

Rewarding and psychomotor stimulant effects of endomorphin-1: anteroposterior differences within the ventral tegmental area and lack of effect in nucleus accumbens

Endomorphin-1 (EM-1) is a recently isolated endogenous peptide having potent analgesic activity and high affinity and selectivity for the mu-opioid receptor. The present study was designed to investigate the rewarding and psychomotor stimulant effects of EM-1 in specific brain regions. We found that rats would learn without priming or response shaping to lever-press for microinjections of EM-1 into the ventral tegmental area (VTA); responding was most vigorous for high-dose injections into the posterior VTA. Rats did not learn to lever-press for microinjections of EM-1 into the nucleus accumbens (NAS) or regions just dorsal to the VTA. Lever-pressing for EM-1 in the VTA was extinguished when vehicle was substituted for the peptide and was reinstated when EM-1 reinforcement was re-established. Conditioned place preference was established by EM-1 injections into the posterior but not the anterior VTA or the NAS. Injection of EM-1 (0.1-1.0 nmol) into the posterior VTA induced robust increases in locomotor activity, whereas injections into the anterior VTA had very weak locomotor-stimulating effects. When injected into the NAS, EM-1 (0.1-10.0 nmol) did not affect locomotor activity. The present findings implicate the posterior VTA as a highly specific and sensitive site for opioid reward and suggest a role for EM-1-containing projections to the posterior VTA in the rewarding effects of other reinforcers.

Chronic morphine treatment modulates the extracellular levels of endogenous enkephalins in rat brain structures involved in opiate dependence: a microdialysis study

The endogenous opioid system is often assumed to play a role in vulnerability to drug abuse. However, controversial results have been reported regarding the levels of enkephalins or preproenkephalin in neurons of rodent brains after opiate administration. The present study was performed to determine the extracellular levels of enkephalins and its physiological antagonist cholecystokinin (CCK), using in vivo microdialysis in freely moving rats after morphine-induced physical dependence or positive place conditioning. A large increase (340%) of Met-enkephalin was observed in the periaqueductal gray matter, a structure involved in morphine withdrawal syndrome, in morphine-dependent rats. No change in CCK immunoreactivity occurred in these conditions. Moreover, using the conditioning place preference paradigm, we observed for the first time opposite changes of enkephalin outflow in the nucleus accumbens (NAc). Thus, an increase in enkephalin levels was observed in rats placed in the drug-associated compartment and a decrease in the saline-paired side. These changes in opioid peptides in the NAc may reflect an "emotional state" of the animals in relation to the expectation of drug reward (reinforcing effects of morphine). Moreover, the lack of regulation in CCK outflow suggests that CCK-opioid interactions in morphine dependence involve probably post-receptor events.

Oxytocin mediates the inhibitory action of acute lithium on the morphine dependence in rats

The role of central oxytocin in inhibitory action of lithium on the development of morphine dependence was behavioral investigated in rats. Acute lithium could enhance the morphine-induced analgesia in rats with or without chronic morphine treatment; this effect could be inhibited by intraventricular injection of oxytocin antagonist d (CH(2))(5)-Tyr (Me)-[Orn(8)]-Vasotocin (OVT). Lithium could attenuate naloxone-precipitated withdrawal signs in morphine dependent rats. The reduction of the expression of naloxone-precipitated withdrawal signs by lithium was reversed by ICV of OVT. The lithium significantly inhibited the conditioned place preference (CPP) induced by morphine, which inhibitory action of lithium could also reverse by ICV injection of OVT. These results suggested that lithium might inhibit the physical dependence on morphine as well as psychological dependence in rats, and that this inhibitory effect of lithium on the development of morphine dependence might be associated with oxytocin systems in the central nervous system.

A concept of welfare based on reward evaluating mechanisms in the brain: anticipatory behaviour as an indicator for the state of reward systems

In this review we attempt to link the efficiency by which animals behave (economy of animal behaviour) to a neuronal substrate and subjective states to arrive at a definition of animal welfare which broadens the scope of its study. Welfare is defined as the balance between positive (reward, satisfaction) and negative (stress) experiences or affective states. The state of this balance may range from positive (good welfare) to negative (poor welfare). These affective states are momentary or transient states which occur against the background of and are integrated with the state of this balancing system. As will be argued the efficiency in behaviour requires that, for instance, satisfaction is like a moving target: reward provides the necessary feedback to guide behaviour; it is a not steady-state which can be maintained for long. This balancing system is reflected in the brain by the concerted action of opioid and mesolimbic dopaminergic systems. The state of this system reflects the coping capacity of the animal and is determined by previous events. In other words, this integrative approach of behavioural biology and neurobiology aims at understanding how the coping capacity of animals may be affected and measured. We argue that this balancing system underlies the economy of behaviour. Furthermore we argue that among other techniques anticipation in Pavlovian conditioning is an easy and useful tool to assess the state of this balancing system: for estimating the state of an animal in terms of welfare we focus on the conditions when an animal is facing a challenge.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

Potentiation of opioid-induced conditioned place preference by the selective serotonin reuptake inhibitor fluoxetine

The ability of the selective serotonin reuptake inhibitor, fluoxetine, to modify the effects of morphine, N-((S)-2-benzyl-3[(S) 2-amino-4-methylthio)butyldithio-]-1-oxopropyl)-L-alanine benzylester (RB 120; mixed inhibitor of enkephalin metabolism), and 4-¿[2-[[3-(1H-indol-3-yl))-2-methyl-1-oxo-2-[[(tricyclo[3,3,1,1] dec-2-yloxy) carbonyl] amino¿ propyl] amino]-1-phenylethyl] amino¿-4-oxo-[R-(R*,R*)]-butanoate N-methyl-D-glucamine (CI 988; cholecystokinin receptor subtype [CCK(2)] antagonist), was assessed using conditioned place preference. RB 120 and morphine both induced significant, dose-dependent conditioned place preference, whilst CI 988 failed to elicit conditioned place preference. A subthreshold dose of fluoxetine (2.5 mg/kg) potentiated the morphine submaximal response. Notably, the combination of a subthreshold dose of fluoxetine (2.5 mg/kg) with RB 120 (5 mg/kg) or CI 988 (3 mg/kg) was devoid of any significant conditioned place preference properties. Fluoxetine may act via enhanced serotonergic activity to modulate enkephalinergic tone. Agents that increase enkephalinergic tone more directly such as RB 120 and CI 988, at submaximal doses, did not induce conditioned place preference when co-administered with fluoxetine. These data suggest that fluoxetine, in combination with CI 988 or RB 120, might prove to be a beneficial treatment strategy for opioid drug addiction, though further studies are necessary.

The dopamine hypothesis of reward: past and current status

Mesolimbic dopaminergic neurons are thought to serve as a final common neural pathway for mediating reinforcement processes. However, several recent findings have challenged the view that mesolimbic dopamine has a crucial role in the maintenance of reinforcement processes, or the subjective rewarding actions of natural rewards and drugs of abuse. Instead, there is growing evidence that dopamine is involved in the formation of associations between salient contextual stimuli and internal rewarding or aversive events. This evidence suggests that dopaminergic-neuron activation aids the organism in learning to recognize stimuli associated with such events. Thus, mesolimbic dopaminergic neurons have an important function in the acquisition of behavior reinforced by natural reward and drug stimuli. Furthermore, long-lasting neuroadaptive changes in mesolimbic dopamine-mediated transmission that develop during chronic drug use might contribute to compulsive drug-seeking behavior and relapse.

Localization of brain reinforcement mechanisms: intracranial self-administration and intracranial place-conditioning studies

Intracranial self-administration (ICSA) and intracranial place conditioning (ICPC) methodologies have been mainly used to study drug reward mechanisms, but they have also been applied toward examining brain reward mechanisms. ICSA studies in rodents have established that the ventral tegmental area (VTA) is a site supporting morphine and ethanol reinforcement. ICPC studies confirmed that injection of morphine into the VTA produces conditioned place preference (CPP). Further confirmation that activation of opioid receptors within the VTA is reinforcing comes from the findings that the endogenous opioid peptide met-enkephalin injected into the VTA produces CPP, and that the mu- and delta-opioid agonists, DAMGO and DPDPE, are self-infused into the VTA. Activation of the VTA dopamine (DA) system may produce reinforcing effects in general because (a) neurotensin is self-administered into the VTA, and injection of neurotensin into the VTA produces CPP and enhances DA release in the nucleus accumbens (NAC), and (b) GABA(A) antagonists are self-administered into the anterior VTA and injections of GABA(A) antagonists into the anterior VTA enhance DA release in the NAC. The NAC also appears to have a major role in brain reward mechanisms, whereas most data from ICSA and ICPC studies do not support an involvement of the caudate-putamen in reinforcement processes. Rodents will self-infuse a variety of drugs of abuse (e.g. amphetamine, morphine, phencyclidine and cocaine) into the NAC, and this occurs primarily in the shell region. ICPC studies also indicate that injection of amphetamine into the shell portion of the NAC produces CPP. Activation of the DA system within the shell subregion of the NAC appears to play a key role in brain reward mechanisms. Rats will ICSA the DA uptake blocker, nomifensine, into the NAC shell; co-infusion with a D2 antagonist can block this behavior. In addition, rats will self-administer a mixture of a D1 plus a D2 agonist into the shell, but not the core, region of the NAC. The ICSA of this mixture can be blocked with the co-infusion of either a D1 or a D2 antagonist. However, the interactions of other transmitter systems within the NAC may also play key roles because NMDA antagonists and the muscarinic agonist carbachol are self-infused into the NAC. The medial prefrontal (MPF) cortex supports the ICSA of cocaine and phencyclidine. The DA system also seems to play a role in this behavior since cocaine self-infusion into the MPF cortex can be blocked by co-infusing a D2 antagonist, or with 6-OHDA lesions of the MPF cortex. Limited studies have been conducted on other CNS regions to elucidate their role in brain and drug reward mechanisms using ICSA or ICPC procedures. Among these regions, ICPC findings suggest that cocaine and amphetamine are rewarding in the rostral ventral pallidum (VP); ICSA and ICPC studies indicate that morphine is rewarding in the dorsal hippocampus, central gray and lateral hypothalamus. Finally, substance P mediated systems within the caudal VP (nucleus basalis magnocellularis) and serotonin systems of the dorsal and median raphe nuclei may also be important anatomical components involved in brain reward mechanisms. Overall, the ICSA and ICPC studies indicate that there are a number of receptors, neuronal pathways, and discrete CNS sites involved in brain reward mechanisms.

Naltrexone affects cocaine self-administration in naïve rats through the ventral tegmental area rather than dopaminergic target regions

Behavioural studies have shown an involvement of central endogenous opioid systems in experimental cocaine addiction. Seeking to further localize the attenuating effect of opioid blockade on the reinforcing effects of cocaine, naltrexone was administered locally to different regions of the mesocorticolimbic system, which are thought to be critically involved in cocaine self-administration behaviour. Both cell body and nerve terminal regions of this system were targeted. Using a model for the initiation of cocaine self-administration behaviour, no effect of naltrexone was found in caudate, amygdaloid or accumbens nuclei, nor in the medial prefrontal cortex. However, blockade of endogenous opioid receptors in the ventral tegmental area region attenuated cocaine self-administration. With the initiation model, this finding reflects an attenuating effect on the reinforcing effects of cocaine. The attenuation of self-administration was dependent on the naltrexone dose. The present findings suggest that endogenous opioid systems in the ventral tegmental area modulate the reinforcing efficacy of cocaine.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

Effects of beta-funaltrexamine on dose-effect curves for heroin self-administration in rats: comparison with alteration of [3H]DAMGO binding to rat brain sections

These studies were undertaken to determine the effects of mu-opioid receptor depletion through irreversible alkylation on the dose-effect curve for heroin self-administration. Heroin maintained responding in rats with an inverted U-shaped dose-effect curve and administration of 10 nmol of beta-funaltrexamine i.c.v. (beta-FNA) significantly increased the ED50 on the ascending limb from 1.9 to 5.3 micrograms/infusion, and from 24.3 to 211.8 micrograms/infusion on the descending limb. Administration of saline i.c.v. produced no effect on heroin self-administration. Administration of 40 nmol of beta-FNA increased the ED50S from 5.1 to 33.9 and from 14.4 to 502.8 micrograms/infusion on the ascending and descending portions of heroin's dose-effect curve, respectively. beta-FNA (40 nmol, i.c.v.) had no effect on cocaine self-administration. [3H]DAMGO binding density was decreased in the caudate and nucleus accumbens by 29 or 54% 24 h after administration of 10 or 40 nmol of beta-FNA i.c.v., respectively. The effects of beta-FNA on heroin self-administration were completely overcome by increasing the dose of heroin however, as the shape and slope of the self-administration dose-effect curve was not different when higher doses of heroin were made available for self-administration compared to control data or saline administration. Therefore, there appear to be spare mu-opioid receptors for heroin for the production of its reinforcing effects in rats. Furthermore, the self-administration dose-effect curves returned to control values prior to the return of [3H]DAMGO binding, further suggesting that the full complement of mu-opioid receptors is not necessary for heroin to produce its reinforcing effects. These findings support the existence of spare mu-opioid receptors for heroin in maintaining self-administration in rats.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Rewarding effects elicited by the microinjection of either AMPA or NMDA glutamatergic antagonists into the ventral tegmental area revealed by an intracranial self-administration paradigm in mice

In order to study the functional role of the trans-synaptic neuronal interaction between glutamatergic afferents and mesolimbic dopaminergic neurons in internal reward processes, BALB/c male mice were unilaterally implanted with a guide-cannula, the tip of which was positioned 1.5 mm above the ventral tegmental area (VTA). On each day of the following experimental period, a stainless steel injection cannula was inserted into the VTA in order to study the eventual self-administration behaviour of either the competitive N-methyl-D-aspartate antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (AP-7) or the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (3 ng/50 nL) using a spatial discrimination task in a Y maze. Mice rapidly discriminated between the arm enabling a microinjection of either of these glutamatergic antagonists and the neutral arm of the maze, and a robust self-administration of either of these compounds was observed from the first session of acquisition. These data provide strong evidence that the intra-VTA microinjection of either of these subclasses of glutamatergic antagonist produces an effect which is interpreted centrally by the experimental subjects as being highly rewarding. Once the self-administration response had been fully acquired by the experimental subjects, preinjection of the dopaminergic D2 antagonist, sulpiride (50 mg/kg i.p.), 30 min before the test, produced a rapid extinction of the self-administration response. This latter result demonstrates the dopaminergic D2 receptor dependence of this intra-VTA self-administration of both of these subclasses of glutamatergic antagonist. We conclude that the different glutamatergic afferent neuronal inputs to the VTA globally exert, in vivo, via the mediation of interposed endogenous GABAergic interneurons, a tonic trans-synaptic inhibitory regulation of neuronal activity in the mesolimbic dopaminergic pathway and that this complex neuronal interaction in the VTA plays a significant functional part in the modulation of internal reward processes.

Antisense oligodeoxynucleotide to delta opioid receptors attenuates morphine dependence in mice

The effect of intracerebroventricular (i.c.v.) treatment with antisense oligodeoxynucleotide (A-oligo) to delta opioid receptor mRNA on the morphine-induced place preference and naloxone-precipitated jumping was examined in morphine-dependent mice. Morphine (5 mg/kg, s.c.) produced a significant place preference. I.c.v. pretreatment with A-oligo (0.01-1 microg/mouse) dose-dependently attenuated this morphine (5 mg/kg, s.c.)-induced place preference, while mismatched oligodeoxynucleotide (M-oligo; 1 microg/mouse, i.c.v.) was ineffective. Naloxone (3 mg/kg, s.c.) precipitated jumping in morphine-dependent mice. I.c.v. pretreatment with A-oligo (1 microg/mouse) attenuated this naloxone (3 mg/kg, s.c.)-precipitated jumping in morphine-dependent mice, while M-oligo (1 microg/mouse, i.c.v.) was ineffective. These data demonstrate that the selective reduction in supraspinal delta opioid receptor function caused by pretreatment with A-oligo attenuated the morphine-induced place preference and naloxone-precipitated jumping in morphine-dependent mice, suggesting that the rewarding effect of and physical dependence on morphine may be modulated by central delta opioid receptors.