The ventral pallidum area is involved in the acquisition but not expression of the amphetamine conditioned place preference

Ventral pallidal regulation of motivated behaviors and reinforcement

The interconnected nuclei of the ventral basal ganglia have long been identified as key regulators of motivated behavior, and dysfunction of this circuit is strongly implicated in mood and substance use disorders. The ventral pallidum (VP) is a central node of the ventral basal ganglia, and recent studies have revealed complex VP cellular heterogeneity and cell- and circuit-specific regulation of reward, aversion, motivation, and drug-seeking behaviors. Although the VP is canonically considered a relay and output structure for this circuit, emerging data indicate that the VP is a central hub in an extensive network for reward processing and the regulation of motivation that extends beyond classically defined basal ganglia borders. VP neurons respond temporally faster and show more advanced reward coding and prediction error processing than neurons in the upstream nucleus accumbens, and regulate the activity of the ventral mesencephalon dopamine system. This review will summarize recent findings in the literature and provide an update on the complex cellular heterogeneity and cell- and circuit-specific regulation of motivated behaviors and reinforcement by the VP with a specific focus on mood and substance use disorders. In addition, we will discuss mechanisms by which stress and drug exposure alter the functioning of the VP and produce susceptibility to neuropsychiatric disorders. Lastly, we will outline unanswered questions and identify future directions for studies necessary to further clarify the central role of VP neurons in the regulation of motivated behaviors. Significance: Research in the last decade has revealed a complex cell- and circuit-specific role for the VP in reward processing and the regulation of motivated behaviors. Novel insights obtained using cell- and circuit-specific interrogation strategies have led to a major shift in our understanding of this region. Here, we provide a comprehensive review of the VP in which we integrate novel findings with the existing literature and highlight the emerging role of the VP as a linchpin of the neural systems that regulate motivation, reward, and aversion. In addition, we discuss the dysfunction of the VP in animal models of neuropsychiatric disorders.

Effect of D1- and D2-like Dopamine Receptor Antagonists on the Rewarding and Anxiolytic Effects of Neurotensin in the Ventral Pallidum

Background: Neurotensin (NT) acts as a neurotransmitter and neuromodulator in the central nervous system. It was shown previously that NT in the ventral pallidum (VP) has rewarding and anxiolytic effects. NT exerts its effect in interaction with dopamine (DA) receptors in numerous brain areas; however, this has not yet been investigated in the VP. The aim of this study was to examine whether the inhibition of D1-like and D2-like DA receptors of the VP can modify the above mentioned effects of NT. Methods: Microinjection cannulas were implanted by means of stereotaxic operations into the VP of male Wistar rats. The rewarding effect of NT was examined by means of a conditioned place preference test. Anxiety was investigated with an elevated plus maze test. To investigate the possible interaction, D1-like DA receptor antagonist SCH23390 or D2-like DA receptor antagonist sulpiride were microinjected prior to NT. All of the drugs were also injected independently to analyze their effects alone. Results: In the present experiments, both the rewarding and anxiolytic effects of NT in the VP were prevented by both D1-like and D2-like DA receptor antagonists. Administered on their own, the antagonists did not influence reward and anxiety. Conclusion: Our present results show that the activity of the D1-like and D2-like DA receptors of the VP is a necessary requirement for both the rewarding and anxiolytic effects of NT.

Dichotomous Dopaminergic Control of Ventral Pallidum Neurons

The ventral pallidum (VP) is crucially involved in reward processing. Dopaminergic afferents reach the VP from the ventral tegmental area (VTA). Recent in vivo studies suggest dopamine application increase the firing in the VP. However, little is known about the cellular effects of dopamine within the VP. We aimed to address this paucity of data using brain slices containing the VP and multi-electrode array recordings. Dopamine significantly affected firing in 86% of spontaneously active VP neurons. Among the affected neurons, 84% were excited, while 16% were inhibited. The selective D1-like receptor agonist SKF81297 also had modulatory effects on the majority of VP neurons, but its effects were universally excitatory. On the other hand, the D2-like receptor agonist quinpirole had modulatory effects on 87% of VP neurons studied. It caused significant inhibitory effects in 33% of the cases and excitatory effects in the remaining 67%. The effects of D1-like receptor activation were presynaptic as blocking synaptic transmission with low Ca²⁺ abolished the effects of SKF81297 application. Furthermore, SKF81297 effects were abolished by blocking ionotropic glutamate receptors, suggesting that D1-like receptors boost glutamate release, which in turn excites VP neurons through postsynaptic glutamate receptors. Effects caused by D2-like receptor activation were found to involve pre and postsynaptic mechanisms, as low Ca²⁺ abolished the excitatory effects of quinpirole but not the inhibitory ones. Increases in firing frequency (ff) to quinpirole application were abolished by a group 2/3 mGluR antagonist, suggesting that D2-like receptors cause presynaptic inhibition of glutamate release, resulting in reduced postsynaptic activation of inhibitory mGluRs. Conversely, the inhibitory effects of quinpirole persisted in low Ca²⁺ and therefore can be attributed to postsynaptic D2-like receptor activation. VP neurons excited by dopamine had shorter spike half-widths and are excited by D1-like receptors (presynaptically) and by D2-like receptors (postsynaptically). VP neurons inhibited by dopamine have longer spike half-widths and while D1-like receptor activation has a presynaptic excitatory influence on them, D2-like receptor activation has a postsynaptic inhibitory effect that prevails, on balance. These data provide novel insights into the cellular mechanisms by which dopamine controls information processing within the VP.

Optogenetic Inhibition of Ventral Pallidum Neurons Impairs Context-Driven Salt Seeking

Salt appetite, in which animals can immediately seek out salt when under a novel state of sodium deprivation, is a classic example of how homeostatic systems interface with learned associations to produce an on-the-fly updating of motivated behavior. Neural activity in the ventral pallidum (VP) has been shown to encode changes in the value of salt under such conditions, both the value of salt itself (Tindell et al., 2006) and the motivational value of its predictive cues (Tindell et al., 2009; Robinson and Berridge, 2013). However, it is not known whether the VP is necessary for salt appetite in terms of seeking out salt or consuming salt following sodium depletion. Here, we used a conditioned place-preference procedure to investigate the effects of optogenetically inhibiting the VP on context-driven salt seeking and the consumption of salt following deprivation. Male rats learned to associate one context with sucrose and another context with less-desirable salt. Following sodium depletion, and in the absence of either sucrose or salt, we found that inhibiting the VP selectively reduced the elevation in time spent in the salt-paired context. VP inhibition had minimal effects on the consumption of salt once it was made available. To our knowledge, this is the first evidence that the VP or any brain region is necessary for the ability to use contextual cues to guide salt seeking. These results highlight a dissociation between deficit-driven reward seeking and reward consumption to replenish those deficits, with the former process being particularly sensitive to on-line VP activity.SIGNIFICANCE STATEMENT Salt appetite, in which rats will immediately seek out a once-undesirable concentrated salt solution after being depleted of bodily sodium despite never having tasted salt as a positive reward, is a phenomenon showing how animals can update their motivational goals without any new learning or conditioning. This salt-seeking behavior is also observed when the animal is presented with salt-paired cues. The neural circuitry necessary for context-driven salt-seeking behavior is unknown. We used a novel conditioned place preference procedure to show that optogenetic inhibition of the ventral pallidum (VP), a region known for processing reward, impairs context-driven salt seeking and has minimal effects on the consumption of salt itself following sodium depletion. These results highlight the importance of the VP in context-driven reward-seeking behavior.

AMN082, a metabotropic glutamate receptor 7 allosteric agonist, attenuates locomotor sensitization and cross-sensitization induced by cocaine and morphine in mice

Previous studies have indicated that metabotropic glutamate receptors 7 (mGluR7s) are involved in drug addiction. However, the role of these receptors in drug-induced behavioral sensitization is unknown. The aim of the present study was to determine whether systemic injection of AMN082, a selective mGluR7 allosteric agonist, reduces the cocaine- and morphine-induced hyperactivity and the development and expression of locomotor sensitization, and also affects the reciprocal cross-sensitization to the stimulant effect of cocaine and morphine in mice. AMN082 (1.25-10.0 mg/kg, i.p.) did not have an impact on locomotion of naive mice and did not affect the acute cocaine- or morphine-induced hyperactivity, except the dose of 10 mg/kg that suppressed the locomotor effect of both drugs. Repeated exposure to cocaine or morphine (10 mg/kg, 5× every 3 days) gradually increased locomotion during induction of sensitization and after 4 (cocaine) or 7 day (morphine) withdrawal phase when challenged with cocaine (10 mg/kg, i.p.) or morphine (10 mg/kg, i.p.) on day 17 or 20, respectively. Pretreatment of animals with the lower doses of AMN082 (1.25-5.0 mg/kg, i.p.), 30 min before every cocaine or morphine injection during repeated drug administration or before cocaine or morphine challenge, dose-dependently attenuated the development, as well as the expression of cocaine or morphine locomotor sensitization. AMN082 also inhibited the reciprocal cross-sensitization between these drugs. Prior to administration of MMPIP (10 mg/kg, i.p.), a selective mGluR7 antagonist reversed the inhibitory effect of AMN082 on the development or expression of cocaine or morphine sensitization. These data indicate that AMN082 attenuated the development and expression of cocaine and morphine sensitization, and the reciprocal cross-sensitization via a mechanism that involves mGluR7s. Thus, AMN082 might have therapeutic implications not only in the treatment of cocaine or opioid addiction but also in the treatment of cocaine/opioid polydrug-abusers.

Cocaine dysregulates opioid gating of GABA neurotransmission in the ventral pallidum

The ventral pallidum (VP) is a target of dense nucleus accumbens projections. Many of these projections coexpress GABA and the neuropeptide enkephalin, a δ and μ opioid receptor (MOR) ligand. Of these two, the MOR in the VP is known to be involved in reward-related behaviors, such as hedonic responses to palatable food, alcohol intake, and reinstatement of cocaine seeking. Stimulating MORs in the VP decreases extracellular GABA, indicating that the effects of MORs in the VP on cocaine seeking are via modulating GABA neurotransmission. Here, we use whole-cell patch-clamp on a rat model of withdrawal from cocaine self-administration to test the hypothesis that MORs presynaptically regulate GABA transmission in the VP and that cocaine withdrawal changes the interaction between MORs and GABA. We found that in cocaine-extinguished rats pharmacological activation of MORs no longer presynaptically inhibited GABA release, whereas blocking the MORs disinhibited GABA release. Moreover, MOR-dependent long-term depression of GABA neurotransmission in the VP was lost in cocaine-extinguished rats. Last, GABA neurotransmission was found to be tonically suppressed in cocaine-extinguished rats. These substantial synaptic changes indicated that cocaine was increasing tone on MOR receptors. Accordingly, increasing endogenous tone by blocking the enzymatic degradation of enkephalin inhibited GABA neurotransmission in yoked saline rats but not in cocaine-extinguished rats. In conclusion, our results indicate that following withdrawal from cocaine self-administration enkephalin levels in the VP are elevated and the opioid modulation of GABA neurotransmission is impaired. This may contribute to the difficulties withdrawn addicts experience when trying to resist relapse.

Stability and dynamics of forebrain vasopressin receptor and oxytocin receptor during pregnancy in prairie voles

During pregnancy, females undergo several physiologically driven changes that facilitate adaptive behaviours and prepare the mother to care for her developing offspring. The nonapeptide hormone oxytocin is best recognised for its involvement in mammalian pregnancy and has been tightly associated with maternal care, in addition to its roles in pregnancy, parturition and lactation. A closely-related nonapeptide hormone, arganine vasopressin, has received considerably less attention for its role in pregnancy, although it has recently been implicated in modulating maternal care and aggression. In the present study, we examined the expression patterns of receptors for oxytocin (OXTR) and vasopressin (V1aR) over the course of pregnancy, ranging from non-mated virgin to immediately postpartum female prairie voles (Microtus ochrogaster). Unexpectedly, we found that OXTR was highly stable in all measured structures in the forebrain. V1aR was also stable throughout most of the brain. Two exceptions to this were found in the ventral pallidum (VPall) and the paraventricular nucleus of the hypothalamus (PVN); both significantly correlated with the length of time that females were pregnant. Changes in the PVN may reflect functional feedback in vasopressin release, or preparatory changes for ensuing maternal behaviour. The results also indicate an unappreciated role for VPall V1aR in pregnancy, which may relate to the function of the VPall in hedonic 'liking' and motivational 'wanting.' Taken together, our data indicate that, with a few compelling exceptions, nonapeptide dynamics during prairie vole pregnancy are largely limited to changes in the synthesis and release of oxytocin and vasopressin, and not the receptors to which they bind.

Neural substrates of amphetamine-induced behavioral sensitization: unconditioned (zero context) and conditioned (switch versus same context) components in c-fos overexpression

The neural substrates of the unconditioned and conditioned components of amphetamine (AMPH)-induced behavioral sensitization remain unknown. The present study examines the brain activation of rats in response to an AMPH challenge with augmented locomotion in groups receiving chronic AMPH under chloral hydrate anesthetization (i.e., the 'zero context') or when tested in the 'same context' as a chronic treatment, or when tested in a 'different context'. The neural activations of the three groups reveal fairly consistent patterns: (a) The substantia nigra is activated in the same context condition and the pure AMPH effect (i.e., the zero context with the unconditioned component), but not in the switch context condition. (b) The ventral pallidum showed Fos expression in the switch context and the same context, but not in the zero context condition. (c) The other nuclei, including the medial prefrontal cortex, nucleus accumbens, caudate putamen, medial thalamus, hippocampus, amygdala, and ventral tegmental area, are activated in all contextual conditions and the pure AMPH effect (the zero context). The context exerts definable effects on the mesocorticolimbic dopamine system on AMPH-induced behavioral sensitization. (d) The ventral pallidum and the substantia nigra activations dissociate the unconditioned component from the conditioned component in behavioral sensitization. Further studies are needed to determine how these two nuclei mediate the effect in terms of primary and conditioned rewards.

The rostral subcommissural ventral pallidum is a mix of ventral pallidal neurons and neurons from adjacent areas: an electrophysiological study

The ventral pallidum (VP) is a part of the ventral striatopallidal system and is involved in reward-related behaviors. The VP is composed of a ventromedial (VPvm) and a dorsolateral (VPdl) subregion, and some rostral-caudal differences are reported. Study of the VP often focuses on the subcommissural VP, typically considered homogenous in spite of known subdivisions. In this work, we used slice electrophysiology combined with immunohistochemistry for marker neuropeptides to test whether the subcommissural VP is functionally homogenous. Using sagittal slices, we show that more lateral levels (2.40 mm) of the subcommissural VP are homogenous but that a more medial slice (1.90 mm) contains two types of neurons. One type, located more caudally, resembles neurons in the lateral subcommissural VP, with long aspiny dendrites, primarily GABAergic input, and characteristic electrophysiological properties, such as depolarized membrane potential and spontaneous action potential discharge. The second type of neuron, located mostly in the rostral subcommissural VP, shows properties that are akin to medium spiny neurons of adjacent regions, including spiny dendrites, major glutamatergic input, hyperpolarized membrane potential, and no spontaneous action potentials. The two types of neurons were present in both the VPvm and VPdl, implying that the mix is not a characteristic of histologically defined subregions. We conclude that at medial levels the rostral subcommissural VP contains a mix of typical ventral pallidal neurons and spiny neurons similar to those in adjacent regions. This observation needs to be considered when interpreting past experiments and designing future experiments in the subcommissural VP.

The behavioral, anatomical and pharmacological parallels between social attachment, love and addiction

RATIONALE

Love has long been referred to as an addiction in literature and poetry. Scientists have often made comparisons between social attachment processes and drug addiction, and it has been suggested that the two may share a common neurobiological mechanism. Brain systems that evolved to govern attachments between parents and children and between monogamous partners may be the targets of drugs of abuse and serve as the basis for addiction processes.

OBJECTIVES

Here, we review research on drug addiction in parallel with research on social attachments, including parent-offspring attachments and social bonds between mating partners. This review focuses on the brain regions and neurochemicals with the greatest overlap between addiction and attachment and, in particular, the mesolimbic dopamine (DA) pathway.

RESULTS

Significant overlap exists between these two behavioral processes. In addition to conceptual overlap in symptomatology, there is a strong commonality between the two domains regarding the roles and sites of action of DA, opioids, and corticotropin-releasing factor. The neuropeptides oxytocin and vasopressin are hypothesized to integrate social information into attachment processes that is not present in drug addiction.

CONCLUSIONS

Social attachment may be understood as a behavioral addiction, whereby the subject becomes addicted to another individual and the cues that predict social reward. Understandings from both fields may enlighten future research on addiction and attachment processes.

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.

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.

Positron emission tomography measures of endogenous opioid neurotransmission and impulsiveness traits in humans

CONTEXT

The endogenous opioid system and opioid mu receptors (mu-receptors) are known to interface environmental events, positive (eg, relevant emotional stimuli) and negative (eg, stressors), with pertinent behavioral responses and to regulate motivated behavior.

OBJECTIVE

To examine the degree to which trait impulsiveness (the tendency to act on cravings and urges rather than to delay gratification) is predicted by baseline mu-receptor availability or the response of this system to a standardized, experientially matched stressor.

DESIGN, SETTING, AND PATIENTS

Nineteen young healthy male volunteers completed a personality questionnaire (NEO Personality Inventory, Revised) and underwent positron emission tomography scans with the mu-receptor-selective radiotracer carfentanil labeled with carbon 11. Measures of receptor concentrations were obtained at rest and during receipt of an experimentally maintained pain stressor of matched intensity between subjects.

MAIN OUTCOME MEASURES

Baseline receptor levels and stress-induced activation of mu-opioid system neurotransmission compared between subjects scoring above and below the population median on the NEO Personality Inventory, Revised, impulsiveness subscale and the orthogonal dimension (deliberation) expected to interact with it.

RESULTS

High impulsiveness and low deliberation scores were associated with significantly higher regional mu-receptor concentrations and greater stress-induced endogenous opioid system activation. Effects were obtained in the prefrontal and orbitofrontal cortices, anterior cingulate, thalamus, nucleus accumbens, and basolateral amygdala-all regions involved in motivated behavior and the effects of drugs of abuse. Availability of the mu-receptor and the magnitude of stress-induced endogenous opioid activation in these regions accounted for 17% to 49% of the variance in these personality traits.

CONCLUSIONS

Individual differences in the function of the endogenous mu-receptor system predict personality traits that confer vulnerability to or resiliency against risky behaviors such as the predisposition to develop substance use disorders. These personality traits are also implicated in psychopathological states (eg, personality disorders) in which variations in the function of this neurotransmitter system also may play a role.

18-Methoxycoronaridine blocks acquisition but enhances reinstatement of a cocaine place preference

The iboga alkaloid congener, 18-methoxycoronaridine (18-MC), decreases self-administration of multiple drugs of abuse. Here, in a biased procedure, we investigated whether 18-MC would have a similar effect on the acquisition, expression and reinstatement of a cocaine conditioned place preference (CPP) in male Sprague-Dawley rats. While 18-MC attenuated acquisition of a cocaine CPP, it had no effect on CPP expression, and enhanced the reinstatement of cocaine CPP following extinction. Our results are consistent with those obtained using ibogaine, but reinforce the notion that acquisition, expression and reinstatement of a CPP likely involve separate mechanisms.

Ventral pallidum roles in reward and motivation

In recent years the ventral pallidum has become a focus of great research interest as a mechanism of reward and incentive motivation. As a major output for limbic signals, the ventral pallidum was once associated primarily with motor functions rather than regarded as a reward structure in its own right. However, ample evidence now suggests that ventral pallidum function is a major mechanism of reward in the brain. We review data indicating that (1) an intact ventral pallidum is necessary for normal reward and motivation, (2) stimulated activation of ventral pallidum is sufficient to cause reward and motivation enhancements, and (3) activation patterns in ventral pallidum neurons specifically encode reward and motivation signals via phasic bursts of excitation to incentive and hedonic stimuli. We conclude that the ventral pallidum may serve as an important 'limbic final common pathway' for mesocorticolimbic processing of many rewards.

Behavioral functions of the mesolimbic dopaminergic system: an affective neuroethological perspective

The mesolimbic dopaminergic (ML-DA) system has been recognized for its central role in motivated behaviors, various types of reward, and, more recently, in cognitive processes. Functional theories have emphasized DA's involvement in the orchestration of goal-directed behaviors and in the promotion and reinforcement of learning. The affective neuroethological perspective presented here views the ML-DA system in terms of its ability to activate an instinctual emotional appetitive state (SEEKING) evolved to induce organisms to search for all varieties of life-supporting stimuli and to avoid harms. A description of the anatomical framework in which the ML system is embedded is followed by the argument that the SEEKING disposition emerges through functional integration of ventral basal ganglia (BG) into thalamocortical activities. Filtering cortical and limbic input that spreads into BG, DA transmission promotes the "release" of neural activity patterns that induce active SEEKING behaviors when expressed at the motor level. Reverberation of these patterns constitutes a neurodynamic process for the inclusion of cognitive and perceptual representations within the extended networks of the SEEKING urge. In this way, the SEEKING disposition influences attention, incentive salience, associative learning, and anticipatory predictions. In our view, the rewarding properties of drugs of abuse are, in part, caused by the activation of the SEEKING disposition, ranging from appetitive drive to persistent craving depending on the intensity of the affect. The implications of such a view for understanding addiction are considered, with particular emphasis on factors predisposing individuals to develop compulsive drug seeking behaviors.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

The role of opioid receptor subtypes in the development of behavioral sensitization to ethanol

Nonspecific blockade of opioid receptors has been found to prevent development of behavioral sensitization to ethanol. Whether this effect is achieved through a specific opioid receptor subtype, however, is not clear. The present study investigated, for the first time, the role of specific opioid receptor subtypes in the development of ethanol-(2.5 g/kg/day; six sessions) induced locomotor sensitization in mice. We confirmed previous results showing that the nonspecific antagonism of opioid receptors (naltrexone; 0-2 mg/kg) prevented the development of behavioral sensitization to ethanol, an effect attained at doses presumed to occupy only mu opioid receptors. This was confirmed by using the selective mu opioid receptor antagonist CTOP (0-1.5 mg/kg), which also blocked sensitization to ethanol. The selective delta receptor antagonist, naltrindole (0-10 mg/kg), however, did not alter sensitization. We further assessed the role of mu opioid receptors in sensitization to ethanol by exploring the involvement of mu(1), mu(1+2), and mu(3) opioid receptor subtypes. Results of these experiments revealed that the blockade of mu(1) (naloxonazine; 0-30 mg/kg) or mu(3) opioid receptors (3-methoxynaltrexone; 0-6 mg/kg) did not prevent locomotor sensitization to ethanol. Using naloxonazine under treatment conditions that block mu(1+2) opioid receptor subtypes we observed a retarded sensitization. The present data suggest that the concurrent inactivation of all mu opioid receptor subtypes may be required to prevent the neural adaptations underlying the development of behavioral sensitization to ethanol. In addition, these results support previous data suggesting a putative role for the mu opioid receptor endogenous ligand, beta-endorphin, and the hypothalamic arcuate nucleus in ethanol sensitization.

Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: Roles in a fear-based context discrimination task

The goal of the present study was to evaluate the contributions of various brain structures anatomically and functionally linked to the hippocampus and amygdala in a fear-based context discrimination task. The brain areas of interest included the fornix, medial prefrontal cortex, mediodorsal (MD) thalamic nucleus, and nucleus accumbens. Damage to the MD thalamic nucleus and medial prefrontal cortex produced the largest impairment in context-specific fear responses. Damage to the fornix impaired some fear responses (freezing, ultrasonic vocalizations, defecation, and approach/avoidance) while leaving conditioned fear expression of heart rate and urination unaltered. Damage to the nucleus accumbens was also coupled with deficits in the discriminative expression of some (heart rate, urination, and ultrasonic vocalizations) but sparing of context-appropriate freezing, defecation, and approach/avoidance behaviors.

Cross-sensitization to morphine in cocaine-sensitized rats: behavioral assessments correlate with enhanced responding of ventral pallidal neurons to morphine and glutamate, with diminished effects of GABA

Common neurobiological substrates contribute to the progressively increased behavioral effects (i.e., sensitization) that occur with repeated intermittent treatments of cocaine and morphine. Consequently, repeated exposure to cocaine can augment responding to morphine (termed cross-sensitization). Drug-induced sensitization in rats may model aspects of the dysfunction in motivation that are imposed by addiction. The ventral pallidum (VP) is involved in motivated behaviors and its function is altered by acute administration of cocaine and morphine, but the effects of repeated drug exposure remain unknown. Targeting this paucity, the present study evaluated electrophysiological changes in the VP of rats exposed to five once-daily cocaine treatments (15 mg/kg i.p.). This regimen also induced behavioral-sensitization that was expressed 3 days later when the rats received either an acute injection of cocaine (15 mg/kg i.p.) or morphine (10 mg/kg i.p.). VP neurons recorded in vivo 3 days after the repeated cocaine treatment regimen demonstrated increased excitatory responding to microiontophoretic applications of morphine and glutamate. The maximal effect (E(max)) was increased without altering potency, suggesting a change in the functional efficacy of the respective receptor systems. This did not represent a potentiation in transmission in general, for the effects of GABA were diminished. The results provide the first evidence for cellular adaptation in the VP after a sensitizing drug treatment paradigm and reveal that cross-sensitization of drug-induced behaviors temporally correlates with changes in VP neuronal responding. These findings advance an emerging theme that alterations in the VP may contribute to the increased motivation for drug seeking that occurs in drug-withdrawn addicts.

Intravenous heroin self-administration decreases GABA efflux in the ventral pallidum: an in vivo microdialysis study in rats

Several lines of evidence suggest that opiate-induced disinhibition of the ventral pallidum participates in the mediation of opiate reward, though direct in vivo evidence to support this hypothesis has been lacking. The present experiment tested this hypothesis by investigating alterations in ventral pallidal amino acid efflux using in vivo microdialysis during ongoing intravenous heroin self-administration in rats. Concentrations of the inhibitory amino acid GABA in ventral pallidal dialysates were significantly reduced within the first 10 min of heroin self-administration (0.02 mg per infusion; FR-1), and remained approximately 65% of presession baseline levels for the remainder of the 3-h self-administration session. Dialysate glutamate levels were unaltered during the first hour of heroin intake but significantly increased to a stable level of approximately 120% presession values during the subsequent 2 h of self-administration. Thus, heroin self-administration is associated with both decreased GABA efflux and a late phase increase in glutamate efflux in the ventral pallidum. These observations are consistent with the hypothesis that heroin self-administration results in a disinhibition and/or excitation of the ventral pallidum.

Ventral pallidal representation of pavlovian cues and reward: population and rate codes

We recorded neural activity in the ventral pallidum (VP) while rats learned a pavlovian reward association. Rats learned to distinguish a tone that predicted sucrose pellets (CS+) from a different tone that predicted nothing (CS-). Many VP units became responsive to CS+, but few units responded to CS-. When two CS+ were encountered sequentially, the earliest predictor of reward became most potent. Many VP units were also activated when the sucrose reward was received [unconditioned stimulus (UCS)]. These VP units for UCS remained responsive to sucrose reward after learning, even when sucrose was already predicted by CS+. Neural representation of reward learning and reward itself was characterized by population codes. The population of units that responded to CS+ increased with learning, whereas the population that responded to UCS did not change. A relative firing rate code also represented the identities of conditioned stimuli and UCS. Firing rate differences among stimuli were acquired early and remained stable during subsequent training, whereas population codes and behavioral conditioned responses continued to develop during subsequent training. Thus, the VP makes use of dynamic CS population and rate codes to encode pavlovian reward cues in reward learning and uses stable UCS population and firing codes to encode sucrose reward itself.

The reinforcing properties of alcohol are mediated by GABA(A1) receptors in the ventral pallidum

It has been hypothesized that alcohol addiction is mediated, at least in part, by specific gamma-aminobutyric acid(A) (GABA(A)) receptors within the ventral pallidum (VP). Among the potential GABA(A) receptor isoforms regulating alcohol-seeking behaviors within the VP, the GABA(A) alpha1 receptor subtype (GABA(A1)) appears pre-eminent. In the present study, we developed beta-carboline-3-carboxylate-t-butyl ester (betaCCt), a mixed agonist-antagonist benzodiazepine (BDZ) site ligand, with binding selectivity at the A1 receptor to explore the functional role of VP(A1) receptors in the euphoric properties of alcohol. The in vivo actions of betaCCt were then determined following microinfusion into the VP, a novel alcohol reward substrate that primarily expresses the A1 receptor. In two selectively bred rodent models of chronic alcohol drinking (HAD-1, P rats), bilateral microinfusion of betaCCt (0.5-40 microg) produced marked reductions in alcohol-reinforced behaviors. Further, VP infusions of betaCCt exhibited both neuroanatomical and reinforcer specificity. Thus, no effects on alcohol-reinforced behaviors were observed following infusion in the nucleus accumbens (NACC)/caudate putamen (CPu), or on response maintained by saccharin. Parenteral-administered betaCCt (1-40 mg/kg) was equally effective and selective in reducing alcohol-reinforced behaviors in P and HAD-1 rats. Additional tests of locomotor activity revealed that betaCCt reversed the locomotor sedation produced by both chlordiazepoxide (10 mg/kg) and EtOH (1.25 g/kg), but was devoid of intrinsic effects when given alone. Studies in recombinant receptors expressed in Xenopus oocytes revealed that betaCCt acted as a low-efficacy partial agonist at alpha3beta3gamma2 and alpha4beta3gamma2 receptors and as a low-efficacy inverse agonist at alpha1beta3gamma2, alpha2beta3gamma2, and alpha5beta3gamma2 receptors. The present study indicates that betaCCt is capable of antagonizing the reinforcing and the sedative properties of alcohol. These anti-alcohol properties of betaCCt are primarily mediated via the GABA(A1) receptor. betaCCt may represent a prototype of a pharmacotherapeutic agent to effectively reduce alcohol drinking behavior in human alcoholics.

The effects of ventral tegmental administration of GABA(A), GABA(B), NMDA and AMPA receptor agonists on ventral pallidum self-stimulation

The ventral pallidum (VP) is a basal forebrain structure that is interconnected with motor and limbic structures and may be considered as an interface between motivational and effector neural signals. Results from a considerable number of studies suggest that this structure is critically involved in reward-related behavior. The VP shares reciprocal connections with other reward-implicated regions, such as the ventral tegmental area (VTA). This anatomy predicts that drug-induced neuronal alterations in the VTA could profoundly alter the function of the VP. Here, using the curve-shift intracranial self-stimulation method, we studied the effects of muscimol (GABA(A) agonist), baclofen (GABA(B) agonist), NMDA and AMPA, microinjected bilaterally into the VTA on the rewarding efficacy of VP self-stimulation. Central injections of the highest dose of muscimol (0.128 microg) resulted in significant elevations in VP self-stimulation thresholds, indicating a reduction in the rewarding efficacy of the stimulation. Elevations in VP self-stimulation thresholds were also evident after intrategmental injections of higher doses of baclofen (0.12, 0.48 microg). By contrast, intrategmental activation of NMDA and AMPA receptors did not affect reward thresholds. These findings suggest that GABAergic and glutamatergic transmission in the VTA activate different circuits that may mediate different functions. Thus, the VTA--VP projection activated by GABA modulates VP stimulation reward, while the projection activated by glutamate may be involved in reward-unrelated effects, rather than in the processing of reward. The decreased rewarding efficacy of VP self-stimulation following intrategmental injections of muscimol and baclofen may be due to GABAergic modulation of ventral tegmental dopaminergic and nondopaminergic neurons projecting to the VP.

Mu and kappa opioid agonists modulate ventral tegmental area input to the ventral pallidum

The ventral pallidum (VP) is situated at the convergence of midbrain dopamine and accumbal opioid efferent projections. Using in vivo electrophysiological procedures in chloral hydrate-anaesthetized rats, we examined whether discrete application of mu- [D-Ala2,N-Me-Phe4,Gly-ol5 (DAMGO)] or kappa- (U50488) opioid receptor agonists could alter VP responses to electrical stimulation of ventral tegmental area. Rate suppressions occurred frequently following ventral tegmental area stimulation. Consistent with an involvement of dopamine in this effect, none of the 12 spontaneously active ventral pallidal neurons recorded in rats that had monoamines depleted by reserpine responded to electrical stimulation of ventral tegmental area. Moreover, in intact rats, the dopamine antagonist flupenthixol attenuated evoked suppression in 100% of the neurons tested; however, the GABAA antagonist bicuculline was able to slightly attenuate the response in 50% of the neurons tested. These observations concur with our previous studies in indicating that ventral tegmental area stimulation releases dopamine (and sometimes GABA) onto ventral pallidal neurons. Both DAMGO and U50488 decreased the inhibitory effects of ventral tegmental area stimulation. These effects on the endogenously released transmitter differed from those seen with exogenously applied dopamine, for DAMGO did not alter the efficacy or potency of microiontophoretically applied dopamine. Taken together, these observations suggest that the interaction between DAMGO and dopamine does not occur at a site that is immediately postsynaptic to the dopaminergic input within the VP, but rather that opioid modulation involves mechanisms governing presynaptically released dopamine. These modulatory processes would enable ventral pallidal opioids to gate the influence of ventral tegmental area dopamine transmission on limbic system outputs at the level of the VP.

Amphetamine-stimulated cortical acetylcholine release: role of the basal forebrain

Systemic administration of amphetamine results in increases in the release of acetylcholine in the cortex. Basal forebrain mediation of this effect was examined in three experiments using microdialysis in freely-moving rats. Experiment 1 examined whether dopamine receptor activity within the basal forebrain was necessary for amphetamine-induced increase in cortical acetylcholine by examining whether intra-basalis perfusion of dopamine antagonists attenuates this increase. Systemic administration of 2.0 mg/kg amphetamine increased dopamine efflux within the basal forebrain nearly 700% above basal levels. However, the increase in cortical acetylcholine efflux following amphetamine administration was unaffected by intra-basalis perfusions of high concentrations of D1- (100 microM SCH 23390) or D2-like (100 microM sulpiride) dopamine receptor antagonists. Experiments 2 and 3 determined whether glutamatergic or GABAergic local modulation of the excitability of the basal forebrain cholinergic neurons influences the ability of systemic amphetamine to increase cortical acetylcholine efflux. In Experiment 2, perfusion of kynurenate (1.0 mM), a non-selective glutamate receptor antagonist, into the basal forebrain attenuated the increase in cortical acetylcholine produced by amphetamine. Experiment 3 revealed that positive modulation of GABAergic transmission by bilateral intra-basalis infusion of the benzodiazepine receptor agonist chlordiazepoxide (40 microg/hemisphere) also attenuated the amphetamine-stimulated increase in cortical acetylcholine efflux. These data suggest that amphetamine increases cortical acetylcholine release via a complex neuronal network rather than simply increasing basal forebrain D1 or D2 receptor activity.

Diazepam modifies the effect of pedunculopontine lesions on morphine but not on amphetamine conditioned place preference

We have previously shown that T-maze learning impairments caused by lesions to the pedunculopontine tegmental nucleus (PPTg) can be reversed by the anxiolytic diazepam. We now report that diazepam also reverses the effect of PPTg lesions on conditioned place preference (CPP) to morphine but not to amphetamine. Rats with bilateral sham or N-methyl-D-aspartate lesions (0.1 or 0.05 M) to the PPTg were trained in a unbiased CPP paradigm with 2 mg/kg morphine or 2 mg/kg D-amphetamine associated with one compartment of the apparatus and vehicle injections in the alternative compartment. After three drug/saline-compartment pairings, the preference of the animals was assessed by allowing them to explore the entire apparatus for 20 min. In contrast to sham-lesioned subjects, the rats with PPTg lesions did not show a preference for the compartment paired with morphine or amphetamine. In two experiments the expression of a morphine CPP was restored by injecting the lesioned animals with 1 mg/kg of diazepam 30 min before the test session. Diazepam pre-treatment did not restore the expression of amphetamine CPP.

Effect of intracerebral administration of NMDA and AMPA on dopamine and glutamate release in the ventral pallidum and on motor behavior

The present study investigates the modulation of the ventral tegmental area (VTA)-ventral pallidum (VP) dopaminergic system by glutamate agonists in rats. The glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were infused via reversed microdialysis into the VTA, and dopamine (DA), glutamate, and aspartate levels in the VTA and ipsilateral VP were monitored together with motor behavior screened in an open field. NMDA (750 microM) infusion, as well as AMPA (50 microM) infusion, induced an increase of DA and glutamate levels in the VTA, followed by an increase of DA levels in the ipsilateral VP and by enhanced locomotor activity. The increase of DA in the VP was similar after administration of these two glutamate agonists, although motor activity was more pronounced and showed an earlier onset after NMDA infusion. Glutamate levels in the VP were not increased by the stimulation of DA release. It is concluded that DA is released from mesencephalic DA neurons projecting to the VP and that these neurons are controlled by glutamatergic systems, via NMDA and AMPA receptors. Thus, DA in the VP has to be considered as a substantial modulator. Dysregulation of the mesopallidal DA neurons, as well as their glutamatergic control, may play an additional or distinct role in disorders like schizophrenia and drug addiction.

NMDA receptor antagonist-induced dopamine release in the ventral pallidum does not correlate with motor activation

The ventral pallidum is the output structure of the nucleus accumbens in the ventral corticostriato-thalamocortical loop. Information processing in this loop is critically involved in motor behavior and reinforcement. The ventral pallidum receives a direct dopaminergic input from the ventral tegmental area, but also glutamatergic input from cortical and limbic areas. It has been assumed that dopamine release in the VP is indeed modulated by glutamate. The present study investigated the effects of NMDA receptor blockade on motor behavior and dopamine release in the ventral pallidum. In a first experiment, rats were implanted with microdialysis probes in the ventral pallidum and were systemically injected or locally perfused via the microdialysis probe with dizocilpine (0.32 mg/kg, 10 and 100 microM, respectively). Effects on dopamine and on locomotion were simultaneously monitored. In a second experiment, ventral pallidum was lesioned by quinolinic acid and the effects of systemic dizocilpine (0.08 and 0.16 mg/kg) on locomotion and stereotyped sniffing behavior were determined. It was found that systemic and local dizocilpine administration increased dopamine release in the ventral pallidum to a similar extent whereas only systemic treatment was accompanied by locomotor stimulation. Lesion of the ventral pallidum did not affect locomotion and stereotyped sniffing behavior induced by systemic dizocilpine treatment. Thus, DA release in the ventral pallidum that is elevated by blockade of NMDA receptors is not relevant for activation of motor behavior.

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 intraaccumbens administration of SCH-23390 on cocaine-induced locomotion and conditioned place preference

The effects of systemic (0-1.0 mg/kg) or intraaccumbens (0-1.0 microg/side) administration of SCH-23390 on cocaine-induced (0 or 4.2 mg/kg, i.v.) locomotion, sniffing, and conditioned place preference (CPP) were investigated in rats. After behavioral testing was completed, animals were injected with their respective dose of SCH-23390 into the nucleus accumbens (NAc), followed by a systemic injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Receptors occupied by intraaccumbens SCH-23390, and therefore protected from EEDQ-induced inactivation, were then quantified from autoradiograms of sections labeled with 3H-SCH-23390. Systemic administration of 0.5 and 1.0 mg/kg SCH-23390 reversed cocaine-induced locomotion, sniffing, and CPP, suggesting that stimulation of D1-like receptors is necessary for these behavioral changes. Intraaccumbens administration of 1.0 microg/side SCH-23390 reversed cocaine-CPP, and this dose occupied D1-like receptors primarily in the rostral pole of the NAc. Intraaccumbens administration of 0.5 microg/side SCH-23390 reversed cocaine-induced locomotion. However, this dose occupied a similar number of D1-like receptors in the NAc as a lower and behaviorally ineffective dose of 0.1 microg/side, but occupied more receptors in the caudate-putamen relative to both the 0.1 and 1.0 microg/side doses. These findings suggest that stimulation of D1-like receptors in the NAc is necessary for cocaine-CPP, but not for cocaine-induced locomotion.

Continued trends in the conditioned place preference literature from 1992 to 1996, inclusive, with a cross-indexed bibliography

In light of the overwhelming response to the previous publication in Neuroscience and Biobehavioral Reviews (1993, 17, 21-41) regarding trends in place conditioning (either preference or aversion), the present work constitutes a five-year follow-up to review the empirical research in this behavioral paradigm from 1992 to 1996, inclusively. The behavioral technique has grown as indicated by the number of publications over the last five years which equals those authored over the 35 years covered by our last survey. The previous work used descriptive statistics to explore topical issues, whereas the present work discusses trends since that time and hopes to provide an exhaustive bibliography of the CPP literature, including articles, published abstracts, book chapters and reviews, as well as providing a cross-index of identified key words/drugs tested.

Injections of D-amphetamine into the ventral pallidum increase locomotor activity and responding for conditioned reward: a comparison with injections into the nucleus accumbens

The nucleus accumbens and ventral pallidum receive dopamine (DA) projections from the mesencephalon. Although DA inputs to the nucleus accumbens are implicated in both locomotion and reward processes, little is known of the behavioural significance of DA in the ventral pallidum. These studies examined the effects of D-amphetamine injected into the nucleus accumbens or ventral pallidum on locomotor activity and responding for a conditioned reward (CR). In the nucleus accumbens D-amphetamine dose dependently (1, 3 and 10 microg) increased locomotion within 5-10 min of injection. Intra-ventral pallidum microinjections of D-amphetamine also increased activity in this dose range, but the effect occurred with a longer latency (5-20 min). The magnitude of the response evoked by ventral pallidum injections was lower than that evoked by nucleus accumbens injections. The GABAA antagonist picrotoxin (0.1 microg) stimulated activity when injected into the ventral pallidum but not the nucleus accumbens, providing a pharmacological dissociation between the two injection sites. In the CR studies, D-amphetamine injected into both sites potentiated responding for a CR previously paired with food delivery, without altering responding on an inactive lever. Picrotoxin injected into the ventral pallidum reduced responding and abolished the selectivity of responding for CR. The results show that DA release in the ventral pallidum enhances locomotion and responding for a CR, providing evidence that DA in the ventral pallidum plays a significant role in the mediation of the effects of D-amphetamine. The failure of picrotoxin to elevate responding for CR despite increasing locomotor activity indicates that pharmacologically-induced blockade of GABAA receptors in the ventral pallidum disrupts goal-directed responding.

Amphetamine infused into the ventrolateral striatum produces oral stereotypies and conditioned place preference

The effects of amphetamine infused into the ventrolateral striatum (VLS) on locomotion, stereotypies, and conditioned place preference (CPP) were investigated. Five 2-day conditioning trials were conducted over 10 consecutive days. On 1 day of each trial, animals received an infusion of amphetamine (0, 2.5, 5, 10, or 20 mg/0.5 ml/side) and were placed into a distinct compartment for 30 min. On the other day, animals received sham intracranial infusions and were placed into a different compartment for 30 min. Locomotion and stereotypies were assessed following the first and last amphetamine infusions. CPP was assessed the day following the last conditioning trial. Intra-VLS infusions of amphetamine did not alter sniffing or locomotion. Acute administration of amphetamine into the VLS dose dependently produced oral stereotypies, however, tolerance developed to this effect following repeated administrations. Also, intra-VLS infusions of amphetamine dose dependently produced CPP. These results suggest that the VLS is involved in amphetamine-induced oral stereotypies and reward.

Dissociation of locomotor and conditioned place preference responses following manipulation of GABA-A and AMPA receptors in ventral pallidum

1. This study examined the roles of GABAergic and glutamatergic neurotransmission in ventral pallidum (VP) in conditioned place preference and locomotor activity. 2. Picrotoxin (0.1 microgram), a GABA antagonist, and (+/-)alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 0.14 microgram), a non-NMDA glutamatergic agonist, were injected bilaterally into VP through implanted cannulae. 3. Both drugs produced a robust increase in locomotion, but neither produced conditioned place preference. 4. These results suggest a dissociation of locomotor activity and reward at the level of ventral pallidum. In addition, it was argued that the GABAergic projection from nucleus accumbens to ventral pallidum may not be involved in the processing of reward initiated from dopaminergic activation in nucleus accumbens.

Persistent increases in basal cerebral metabolic activity induced by morphine sensitization

To characterize the underlying neuroanatomic substrate of morphine (MS) sensitization, changes in the local cerebral metabolic rate for glucose (LCMRglu) were examined in 95 brain regions of male F-344 rats using the 2-deoxy-D-[1-14C]glucose method. The results of these experiments demonstrate that MS-induced sensitization is manifested by increases in basal metabolic activity that last for at least 6 days. Although changes in basal metabolic rate were found to be more extensive in the presence of conditioned cues, the increases in LCMRglu in nonconditioned sensitized rats indicate a basic underlying pharmacologic effect of MS sensitization on basal brain activity. Regions in which MS sensitization had a lasting pharmacologic effect include the shell of the nucleus accumbens, the prelimbic area of the prefrontal cortex, and the dorsolateral prefrontal cortex. Interestingly, the core of the nucleus accumbens and regions of the caudate were found to have an increased LCMRglu only in the presence of conditioned cues, indicating conditioned brain activity without observable changes in behavior. The previous administration of an MS-sensitizing treatment was also found to alter the cerebral metabolic response to a subsequent acute MS challenge (0.5 mg/kg, subcutaneously), most notably in forebrain systems. The more widespread activation of brain structures in the basal state in the presence of conditioned cues suggests that these MS-sensitized rats may model an altered brain state related to craving in the abstinent opiate addict.

6-Hydroxydopamine lesion of ventral pallidum blocks acquisition of place preference conditioning to cocaine

In parallel with nucleus accumbens (NAS), ventral pallidum (VP) also receives a dopaminergic projection from the ventral tegmental area (VTA). The present study examined the involvement of this mesopallidal dopaminergic system in the action of cocaine. In the first experiment, the effect of cocaine injections on VP dopamine was examined by microdialysis. Intraperitoneal (i.p.) injections of cocaine 5-20 mg/kg dose-dependently increased the extracellular dopamine level in VP 2.5-4.5-fold. In addition, intra-VP perfusion of 20 microM cocaine induced a 12-fold increase of dopamine locally. The second experiment examined the role of VP dopamine in cocaine-induced conditioned place preference (CPP) and locomotor activation. Rats received bilateral intra-VP injections of 3-4 microg 6-OHDA or ascorbic acid vehicle in 0.5 microl volume. Tissue assays indicated that the 6-OHDA-lesioned rats had significantly lowered dopamine concentration in VP, but not in NAS or striatum. As a group, 6-OHDA lesions blocked the development of CPP to 5 mg/kg cocaine but not to 10 mg/kg cocaine. However, rats with more than 60% depletion in VP dopamine did not develop CPP to cocaine at either dose. Preference for the cocaine-paired side correlated significantly with dopamine concentration in VP, but not in NAS or striatum. It was concluded that VP dopamine may play a critical role in the initial rewarding effect of cocaine. 6-OHDA lesions also blocked locomotor activation induced by 5 mg/kg cocaine but had no effect on 10 mg/kg cocaine-induced locomotion. Dopamine concentration in VP did not correlate with the locomotor activation response to cocaine at either dose. These findings further establish the involvement of the mesopallidal dopaminergic system in the action of cocaine.

Ventral pallidum self-stimulation induces stimulus dependent increase in c-fos expression in reward-related brain regions

Neuronal expression of Fos, the protein product of the immediate early gene c-fos has been used as a high resolution metabolic marker for mapping polysynaptic pathways in the brain. We used Fos immunohistochemistry to reveal neuronal activation following self-stimulation of the ventral pallidum. Four groups of rats were allowed to self-stimulate for 30 min with 0.4 s trains of cathodal rectangular pulses of constant intensity (0.4 mA) and duration (0.1 ms). Each group was assigned a different pulse frequency, (3, 17, 24 and 50 pulses/stimulation train). Which was preselected from within each animal's rate-frequency function. The subjects that were assigned three pulses failed to self-stimulate and were considered as controls. The subjects that were assigned 17 pulses self-stimulated at half-maximal rate, whereas those that were assigned 24 and 50 pulses self-stimulated at maximal rates. The animals were sacrificed 90 min after the self-stimulation session and their brains were processed for Fos-like immunoreactivity. Fos-like immunoreactivity was found to increase as a function of pulse frequency in several brain regions known to be involved in drug and/or brain stimulation reward (medial prefrontal cortex, lateral septum, nucleus accumbens; lateral hypothalamus and ventral tegmental area), whereas it was not affected in structures devoid of such involvement (substantia nigra reticulata and dorsolateral striatum). The level of Fos expression induced by trains of 50 pulses was considerably higher than that produced by 24 pulses although both frequencies supported the same (maximal) self-stimulation rate. This finding indicates that Fos expression correlated with reward magnitude (known to increase between these frequencies), not with bar-pressing rate, thus suggesting the presence of a reward-specific effect. The finding of a frequency-dependent Fos expression in a behavioural paradigm can be considered analogous to a pharmacological dose-response curve and, as such, our results may open new avenues for the use of Fos immunohistochemistry in quantitative neurobehavioural studies.

Neuropharmacological evidence for the role of dopamine in ventral pallidum self-stimulation

The present study examines the role of dopaminergic neurotransmission in modulating the reinforcing effect of ventral pallidum (VP) intracranial self-stimulation (ICSS). Fifty four adult rats were implanted with a monopolar moveable stimulating electrode in the VP. Rate-frequency functions were determined by logarithmically decreasing the number of pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding. After the ICSS thresholds stabilized, the animals received treatments with several doses of cocaine and of various selective drugs acting at the level of DA receptor subtypes. Their effects on threshold and asymptotic rate were analyzed. Cocaine produced a significant decrease in ICSS threshold but had no significant effect on the asymptotic rate. A significant decrease in ICSS threshold was also seen with the D3 agonist 7-OH-DPAT. This was associated with a decrease rather than an increase in performance. D1 and D2 DA receptor blockers (haloperidol, SCH-23390, raclopride and sulpiride) produced a dose dependent increase in ICSS threshold and a decrease in the maximal rate. The results suggest that DA plays a modulatory role in VP intracranial self-stimulation, and that D1, D2 and D3 receptors are involved in the mediation of this effect, although to different extents.

Conditioned place preference and locomotor activation produced by injection of psychostimulants into ventral pallidum

The ventral pallidum (VP) is often viewed as an output structure of the nucleus accumbens septi (NAS). However, VP, like NAS, receives a dopaminergic input from the ventral tegmental area. These experiments investigated some behavioral effects of microinjection into VP of drugs which enhance dopaminergic transmission. Injection of 25 micrograms dopamine or 5-10 micrograms amphetamine into VP produced hypermotility. In contrast, injection of 12.5-50 micrograms cocaine initially suppressed, then increased, activity. Injection of 100 micrograms cocaine only produced hypomotility in the 1-h period examined. The hypomotility following cocaine seemed to be a local anesthetic effect, because it was mimicked by 50-200 micrograms procaine. Procaine did not, however, produce subsequent hypermotility. Conditioned place preference (CPP) was produced by 10 micrograms amphetamine and 50 micrograms cocaine but not 100 micrograms procaine. We conclude that injection of cocaine into VP unlike similar injections into NAS, produces CPP. These results support the idea of an involvement of dopamine in VP in reward and locomotor activation, independent of dopamine in NAS. The use of intracerebral injections of cocaine is complicated, however, by an apparent local anesthetic effect of the drug.

Memory for the changing cost of a reward is mediated by the sublenticular extended amygdala

The aim of this study was to examine the role of the sublenticular extended amygdala (SEA) in processes of reward and reinforcement. Previous studies have examined the effects of ibotenate lesions in this area on motivation for cocaine reward. In this study, animals were trained to work for sucrose pellets, rather than a drug, on a progressive-ratio schedule of reinforcement. Bilateral intracerebral infusions of ibotenic acid (lesion group) or vehicle (control group) were made into the SEA, following the same procedures as used in previous studies. After recovery from surgery, animals were tested for six sessions on the progressive ratio schedule. The lesion did not result in motivational impairments of the kind that have previously been reported: rather than decreases in breaking point (a measure of motivational strength), the lesion resulted in greater variability of breaking points, with a tendency for lesioned animals to work harder for reward than controls. The SEA-lesioned rats did not show the increase in postreinforcement pause that usually accompanies the increase in perceived work as the number of bar presses for a reward increases. Histological analyses showed that the ibotenate lesions had successfully destroyed the SEA and that damage was also present in adjacent structures. The results are interpreted in terms of a mnemonic, rather than a motivational, deficit.

Discriminative cues indicating reward magnitude continue to determine reaction time of rats following lesions of the nucleus accumbens

The role of the nucleus accumbens in incentive motivation is accepted but poorly understood. In this study, we examined in the rat one aspect of motivated behaviour which might be mediated by the nucleus accumbens, namely the translation of a motivational signal (the expected value of a reward) into motor output (responding for the reward). Rats were trained in a reaction time task in which on each trial they received one, two or three pellets. The number of pellets for each trial was randomly determined in advance and signalled to the rats by cue lights. Rats responded with faster reaction times as the size of the expected reward increased. Following ibotenic acid lesions of the nucleus accumbens, there was no difference in the pattern or the speed of reaction times. Although lesions of the nucleus accumbens did not disconnect the motivational system from the motor system, it is possible that the nucleus accumbens is involved in the learning of the incentive salience of external stimuli. Therefore, after postoperative testing the cue contingencies were reversed. Initially, the cues continued to be interpreted according to their prior significance, but eventually both the lesioned rats and the control group acquired the new relationship and did so in equivalent times. We conclude that the nucleus accumbens is not involved in the acquisition or expression of the processes whereby the expectation of rewards of different value is translated into a motor initiation signal.

Ventral pallidum self-stimulation: a moveable electrode mapping study

The distribution of electrical self-stimulation (ESS) foci within the ventral pallidum (VP) was mapped using moveable electrodes in rats. The function relating ESS bar-pressing rate to the frequency of cathodal rectangular pulses (0.4 mA and 0.1 ms) was obtained for several positions of a moveable electrode in the VP and in the various adjacent to VP nuclei. The rate-frequency functions were fitted to a sigmoid model to obtain the asymptotic rate and threshold frequency. ESS was found in almost all (98%) VP sites tested and to a lesser degree (66%) in the surrounding areas (namely globus pallidus and caudate). Depending on the VP site, maximum rates varied from 14 to 85 bar presses/min, whereas threshold frequencies varied from 10.2 to 36.4 pulses/train; no correlation between these two aspects of ESS was found. Extra-pallidal areas contained less low-frequency threshold sites compared to VP. The lowest threshold found in the VP was slightly higher than that usually obtained for the most rewarding brain areas (VTA, dorsal raphé, LH, amygdala), which suggests that the VP represents an important structure for reward. Furthermore the threshold frequencies were found to decline along the rostrocaudal axis of the VP which supports the view that the VP is heterogeneous in regard to reward related functions.