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

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.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use 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.

Ventral pallidum cellular and pathway specificity in drug seeking

The ventral pallidum (VP) is central to the reinforcing effects across a variety of drugs and relapse to drug seeking. Emerging studies from animal models of reinstatement reveal a complex neurobiology of the VP that contributes to different aspects of relapse to drug seeking. This review builds on classical understanding of the VP as part of the final common pathway of relapse but also discusses the properties of the VP as an independent structure. These include VP neural anatomical subregions, cellular heterogeneity, circuitry, neurotransmitters and peptides. Collectively, this review provides a current understanding of the VP from molecular to circuit level architecture that contributes to both the appetitive and aversive symptoms of drug addiction. We show the complex neurobiology of the VP in drug seeking, emphasizing its critical role in addiction, and review strategic approaches that target the VP to reduce relapse rates.

DRD3-dependent plasticity within the VP drives subcircuit activity critical for cocaine seeking

The ventral pallidum (VP) is a key hub within the reward system that mediates drug-seeking behaviors. However, molecular and cellular adaptations within the VP following drug use are not fully elucidated. In this issue of Neuron, Pribiag et al. (2021) demonstrate how cocaine induces circuit-specific changes within the VP via dopamine-receptor-D3-dependent processes to promote cocaine seeking.

Dissociable roles of ventral pallidum neurons in the basal ganglia reinforcement learning network

Reinforcement learning models treat the basal ganglia (BG) as an actor-critic network. The ventral pallidum (VP) is a major component of the BG limbic system. However, its precise functional roles within the BG circuitry, particularly in comparison to the adjacent external segment of the globus pallidus (GPe), remain unexplored. We recorded the spiking activity of VP neurons, GPe cells (actor) and striatal cholinergic interneurons (critic) while monkeys performed a classical conditioning task. Here, we report that VP neurons can be classified into two distinct populations. The persistent population displayed sustained activation following visual cue presentation, was correlated with monkeys' behavior and showed uncorrelated spiking activity. The transient population displayed phasic synchronized responses that were correlated with the rate of learning and the reinforcement learning model's prediction error. Our results suggest that the VP is physiologically different from the GPe and identify the transient VP neurons as a BG critic.

Glutamatergic Ventral Pallidal Neurons Modulate Activity of the Habenula-Tegmental Circuitry and Constrain Reward Seeking

BACKGROUND

The ability to appropriately integrate and respond to rewarding and aversive stimuli is essential for survival. The ventral pallidum (VP) plays a critical role in processing both rewarding and aversive stimuli. However, the VP is a heterogeneous structure, and how VP subpopulations integrate into larger reward networks to ultimately modulate these behaviors is not known. We identify a noncanonical population of glutamatergic VP neurons that play a unique role in responding to aversive stimuli and constraining inappropriate reward seeking.

METHODS

Using neurochemical, genetic, and electrophysiological approaches, we characterized glutamatergic VP neurons (n = 4-8 mice/group). We performed patch clamp and in vivo electrophysiology recordings in the lateral habenula, rostromedial tegmental nucleus, and ventral tegmental area to determine the effect of glutamatergic VP neuron activation in these target regions (n = 6-10 mice/group). Finally, we selectively optogenetically stimulated glutamatergic VP neurons in a real-time place preference task and ablated these neurons using a virally expressed caspase to determine their necessity for reward seeking.

RESULTS

Glutamatergic VP neurons exhibit little overlap with cholinergic or gamma-aminobutyric acidergic markers, the canonical VP subtypes, and exhibit distinct membrane properties. Glutamatergic VP neurons innervate and increase firing activity of the lateral habenula, rostromedial tegmental nucleus, and gamma-aminobutyric acidergic ventral tegmental area neurons. While nonselective optogenetic stimulation of the VP induced a robust place preference, selective activation of glutamatergic VP neurons induced a place avoidance. Viral ablation of glutamatergic VP neurons increased reward responding and abolished taste aversion to sucrose.

CONCLUSIONS

Glutamatergic VP neurons constitute a noncanonical subpopulation of VP neurons. These glutamatergic VP neurons increase activity of the lateral habenula, rostromedial tegmental nucleus, and gamma-aminobutyric acidergic ventral tegmental area neurons and adaptively constrain reward seeking.

The group II metabotropic glutamate receptor agonist LY379268 reduces toluene-induced enhancement of brain-stimulation reward and behavioral disturbances

RATIONALE

Toluene, a widely abused solvent with demonstrated addictive potential in humans, hasbeen reported to negatively modulate N-methyl-D-aspartate receptors (NMDARs) and alter glutamatergicneurotransmission. The group II metabotropic glutamate receptor (mGluR) agonist LY379268 has beenshown to regulate glutamate release transmission and NMDAR function and block toluene-induced locomotorhyperactivity. However, remaining unknown is whether group II mGluRs are involved in the toluene-induced reward-facilitating effect and other behavioral manifestations.

OBJECTIVES

The present study evaluated the effects of LY379268 on toluene-induced reward enhancement, motor incoordination, recognition memory impairment, and social interaction deficits.

RESULTS

Our data demonstrated that LY379268 significantly reversed the toluene-induced lowering of intracranial self-stimulation (ICSS) thresholds and impairments in novel object recognition, rotarod performance, and social interaction with different potencies.

CONCLUSIONS

These results indicate a negative modulatory role of group II mGluRs in acute toluene-induced reward-facilitating and behavioral effects and suggest that group II mGluR agonists may have therapeutic potential for toluene addiction and the prevention of toluene intoxication caused by occupational or intentional exposure.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Intra-ventral tegmental area HIV-1 Tat1-86 attenuates nicotine-mediated locomotor sensitization and alters mesocorticolimbic ERK and CREB signaling in rats

Cigarette smoking prevalence in the HIV-positive individuals is profoundly higher than that in the HIV-negative individuals. We have demonstrated that HIV-1 transgenic rats exhibit attenuated nicotine-mediated locomotor activity, altered cAMP response element binding protein (CREB) and extracellular regulated kinase (ERK1/2) signaling in the mesocorticolimbic regions. This study investigated the role of HIV-1 transactivator of transcription (Tat) protein in the alterations of nicotine-mediated behavior and the signaling pathway observed in the HIV-1 transgenic rats. Rats received bilateral microinjection of recombinant Tat1-86 (25 μg/side) or vehicle directed at ventral tegmental area (VTA) followed by locomotor testing in response to 13 daily intravenous injections of nicotine (0.05 mg/kg, freebase, once/day) or saline. Further, we examined the phosphorylated levels of CREB (pCREB) and ERK1/2 (pERK1/2) in the prefrontal cortex (PFC), nucleus accumbens (NAc) and VTA. Tat diminished baseline activity in saline control rats, and attenuated nicotine-induced behavioral sensitization. Following repeated saline injection, the basal levels of pERK1 in the NAc and VTA and pERK2 in VTA were lower in the vehicle control group, relative to the Tat group. After repeated nicotine injection, pERK1 in NAc and VTA and pERK2 in VTA were increased in the vehicle group, but not in the Tat group. Moreover, repeated nicotine injections decreased pCREB in the PFC and VTA in the Tat group but not in the vehicle group. Thus, these findings indicate that the direct injection of Tat at the VTA may mediate CREB and ERK activity in response to nicotine-induced locomotor activity.

Ropinirole regulates emotionality and neuronal activity markers in the limbic forebrain

Restless legs syndrome (RLS) and Parkinson's disease (PD) are movement disorders usually accompanied by emotional and cognitive deficits. Although D3/D2 receptor agonists are effective against motor and non-motor deficits in RLS and PD, the exact behavioral and neurochemical effects of these drugs are not clearly defined. This study aimed to evaluate the effects of acute ropinirole (0, 0.1, 1 or 10 mg/kg, i.p.), a preferential D3/D2 receptor agonist, on intracranial self-stimulation (ICSS), spontaneous motor activity, anxiety- and depression-like behaviors, spatial reference and working memory in rats as well as on certain markers of neuronal activity, i.e. induction of immediate early genes, such as c-fos and arc, and crucial phosphorylations on GluA1 subunit of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and NA1, NA2A and NA2B subunits of N-methyl-D-aspartate (NMDA) receptors. Ropinirole decreased ICSS thresholds and induced anxiolytic- and antidepressive-like effects without affecting motor activity or spatial memory. The effects on emotionality were associated with a decrease in p-Ser897-NA1 and an increase in p-Tyr1472-NA2B in the ventral striatum as well as an increased induction of c-fos messenger RNA (mRNA) in the prefrontal cortex (PFC) and decreased expression of arc mRNA in the striatum and the shell of the nucleus accumbens. Our data indicate that ropinirole significantly affects emotionality at doses (1-10 mg/kg, i.p.) that exert no robust effects on locomotion or cognition. The data reinforce the use of D3/D2 receptor agonists in the treatment of RLS and PD patients characterized by emotional deficits and suggest that altered NMDA-mediated neurotransmission in the limbic forebrain may underlie some of ropinirole's therapeutic actions.

Different contributions of dopamine D1 and D2 receptor activity to alcohol potentiation of brain stimulation reward in C57BL/6J and DBA/2J mice

C57BL/6J (C57) and DBA/2J (DBA) mice respond differently to drugs that affect dopamine systems, including alcohol. The current study compared effects of D1 and D2 receptor agonists and antagonists, and the interaction between D1/D2 antagonists and alcohol, on intracranial self-stimulation in male C57 and DBA mice to determine the role of dopamine receptors in the effects of alcohol on brain stimulation reward (BSR). In the initial strain comparison, dose effects on BSR thresholds and maximum operant response rates were determined for the D1 receptor agonist SKF-82958 (±-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; 0.1-0.56 mg/kg) and antagonist SCH 23390 (+-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepinehydrochloride; 0.003-0.056 mg/kg), and the D2 receptor agonist quinpirole (0.1-3.0 mg/kg) and antagonist raclopride (0.01-0.56 mg/kg). For the alcohol interaction, SCH 23390 (0.003 mg/kg) or raclopride (0.03 mg/kg) was given before alcohol (0.6-2.4 g/kg p.o.). D1 antagonism dose-dependently elevated and SKF-82958 dose-dependently lowered BSR threshold in both strains; DBA mice were more sensitive to SKF-82958 effects. D2 antagonism dose-dependently elevated BSR threshold only in C57 mice. Low doses of quinpirole elevated BSR threshold equally in both strains, whereas higher doses of quinpirole lowered BSR threshold only in C57 mice. SCH 23390, but not raclopride, prevented lowering of BSR threshold by alcohol in DBA mice. Conversely, raclopride, but not SCH 23390, prevented alcohol potentiation of BSR in C57 mice. These results extend C57 and DBA strain differences to D1/D2 sensitivity of BSR, and suggest differential involvement of D1 and D2 receptors in the acute rewarding effects of alcohol in these two mouse strains.

Genetically expressed HIV-1 viral proteins attenuate nicotine-induced behavioral sensitization and alter mesocorticolimbic ERK and CREB signaling in rats

The prevalence of tobacco smoking in HIV-1 positive individuals is 3-fold greater than that in the HIV-1 negative population; however, whether HIV-1 viral proteins and nicotine together produce molecular changes in mesolimbic structures that mediate psychomotor behavior has not been studied. This study determined whether HIV-1 viral proteins changed nicotine-induced behavioral sensitization in HIV-1 transgenic (HIV-1Tg) rats. Further, we examined cAMP response element binding protein (CREB) and extracellular regulated kinase (ERK1/2) signaling in the prefrontal cortex (PFC), nucleus accumbens (NAc) and ventral tegmental area (VTA). HIV-1Tg rats exhibited a transient decrease of activity during habituation, but showed attenuated nicotine (0.35mg/kg, s.c.)-induced behavioral sensitization compared to Fisher 344 (F344) rats. The basal levels of phosphorylated CREB and ERK2 were lower in the PFC of HIV-1Tg rats, but not in the NAc and VTA, relative to the controls. In the nicotine-treated groups, the levels of phosphorylated CREB and ERK2 in the PFC were increased in HIV-1Tg rats, but decreased in F344 animals. Moreover, repeated nicotine administration reduced phosphorylated ERK2 in the VTA of HIV-1Tg rats and in the NAc of F344 rats, but had no effect on phosphorylated CREB, indicating a region-specific change of intracellular signaling. These results demonstrate that HIV-1 viral proteins produce differences in basal and nicotine-induced alterations in CREB and ERK signaling that may contribute to the alteration in psychomotor sensitization. Thus, HIV-1 positive smokers are possibly more vulnerable to alterations in CREB and ERK signaling and this has implications for motivated behavior, including tobacco smoking, in HIV-1 positive individuals who self-administer nicotine.

Role of serotonin 5-HT2A and 5-HT2C receptors on brain stimulation reward and the reward-facilitating effect of cocaine

RATIONALE

The serotonin 5-HT(2A) and 5-HT(2C) receptors, which are found in abundance in the mesolimbocortical dopaminergic system, appear to modulate the behavioral effects of cocaine.

OBJECTIVES

The present series of studies set out to investigate the role of 5-HT(2A) and 5-HT(2C) receptors on brain reward and on the reward-facilitating effect of cocaine and localize the neural substrates within the mesolimbocortical dopaminergic system that are responsible for these effects.

METHODS

Male Sprague-Dawley rats were implanted with stimulating electrodes and bilateral cannulae for the experiments involving microinjections and were trained to respond to electrical stimulation. In the first study, we examined the effects of systemic administration of selective 5-HT(2A) and 5-HT(2C) receptor agonists (TCB-2 and WAY-161503) and antagonists (R-96544 and SB-242084) on intracranial self-stimulation (ICSS). In the second study, we examined the effectiveness of TCB-2, WAY-161503, R-96544, and SB-242084 in blocking the reward-facilitating effect of cocaine. In the third study, we examined the effects of intra-medial prefrontal cortex (mPFC), intra-nucleus accumbens (NAC), and intra-ventral tegmental area (VTA) injection of WAY-161503 on the reward-facilitating effect of cocaine.

RESULTS

Acute systemic administration of TCB-2 and WAY-161503 increased ICSS threshold. Systemic WAY-161503 attenuated the reward-facilitating effect of cocaine. This effect was reversed by pretreatment with SB-242084. Intracranial microinjections of WAY-161503 into the mPFC and the NAC shell/core, but not the VTA, attenuated the reward-facilitating effect of cocaine.

CONCLUSION

These data indicate that 5-HT(2C) receptors within the mPFC and the NAC modulate the reinforcing effects of cocaine and provide evidence that 5-HT(2C) receptor agonists could be a possible drug discovery target for the treatment of psychostimulant addiction.

Cocaine and amphetamine-regulated transcript-containing neurons in the nucleus accumbens project to the ventral pallidum in the rat and may inhibit cocaine-induced locomotion

We have previously demonstrated that cocaine- and amphetamine-regulated transcript (CART) peptide colocalizes with GABA, dynorphin, D1 receptors, and substance P in some neurons in the nucleus accumbens (NAcc). One of the main nuclei that receive accumbal efferents is the ventral pallidum (VP), and both dynorphin and substance P have been shown to be present in the cell bodies and terminals of this projection. Thus, we investigated whether CART peptide is also present in the VP in terminals that originate in the accumbens. The anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L) colocalized with CART in neuronal processes in the VP when injected into the NAcc. Also, CART colocalized with the retrograde tracer r-BDA in accumbens cell bodies after the tracer was injected into the VP. Using electron microscopic immunocytochemistry, we examined CART terminals in the VP and found that CART-immunoreactive terminals formed symmetric synapses consistent with inhibitory GABAergic synapses. These synapses closely resemble GABAergic synapses in the substantia nigra pars reticulata (SNr), another nucleus that receives some CART-containing accumbal efferents. Lastly, we found that intra-pallidal injection of CART 55-102 inhibited cocaine-induced locomotion, indicating that CART peptide in the VP can have functional effects.

The neuropharmacological substrates of nicotine reward: reinforcing versus reinforcement-enhancing effects of nicotine

Compulsive nicotine use is thought to be maintained by the acute reinforcing effects of nicotine and the reinforcement-enhancing effects of nicotine, in addition to the negative consequences of nicotine abstinence. Nicotine self-administration and nicotine-induced enhancement of non-nicotine reinforcers such as intracranial self-stimulation provide measures of these dual rewarding properties of nicotine. First, pharmacological manipulations that modulate the reinforcing and reinforcement-enhancing effects of nicotine are identified and discussed. Second, the interpretation and implications of data that identified shared and specific pharmacological substrates underlying the dual rewarding effects of nicotine are discussed, including implications for the preclinical testing of putative antismoking medications. In conclusion, reinforcement-related behaviors that are mediated by central reinforcement processes are likely to, and generally do, exhibit a number of common pharmacological substrates. Interestingly, however, a few pharmacological classes of compounds seem to exert selective effects on components of the dual nicotine reward mechanisms, indicating differences in the pharmacological substrates of the reinforcing and reinforcement-enhancing effects of nicotine. Further characterization of such compounds may ultimately lead to the identification of novel medications for nicotine dependence in humans.

Effects of mood stabilizers on brain reward processes in rats: studies using the intracranial self-stimulation paradigm

Bipolar disorder is characterized by dysregulated motivation and increased hedonistic drive. d-Amphetamine induces manic symptoms in humans and exacerbates mania in bipolar disorder patients, effects that are counteracted by mood stabilizers. We utilized intracranial self-stimulation (ICSS) to examine how lithium (LiCl), valproate (VPA) or their combination that is commonly used in the clinic affect brain reward function in rats, and how these drugs affect d-amphetamine's reward-facilitating effects. Acute intraperitoneal (i.p.) administration of LiCl (100, 200 mg/kg), VPA (400 mg/kg) or combined administration of subthreshold doses of LiCl (50 mg/kg) and VPA (200 mg/kg) increased ICSS thresholds. LiCl (100 mg/kg) and combined administration of LiCl and VPA (50 and 200 mg/kg), but not VPA alone (200, 400 mg/kg), attenuated d-amphetamine's reward-facilitating effects. These results suggest that ICSS combined with d-amphetamine constitutes a useful model to explore the elation and increased hedonistic drive observed in bipolar patients and ultimately help to identify novel pharmacotherapies for bipolar disorder.

Enhancement of endocannabinoid neurotransmission through CB1 cannabinoid receptors counteracts the reinforcing and psychostimulant effects of cocaine

Cannabinoids, in contrast to typical drugs of abuse, have been shown to exert complex effects on behavioural reinforcement and psychomotor function. We have shown that cannabinoid agonists lack reinforcing/rewarding properties in the intracranial self-stimulation (ICSS) paradigm and that the CB1 receptor (CB1R) agonist WIN55,212-2 attenuates the reward-facilitating actions of cocaine. We sought to determine the effects of the endocannabinoid neurotransmission enhancer AM-404 (1, 3, 10, 30 mg/kg) on the changes in ICSS threshold and locomotion elicited by cocaine and extend the study of the effects of WIN55,212-2 (0.3, 1, 3 mg/kg) on cocaine-induced hyperlocomotion. AM-404 did not exhibit reward-facilitating properties, and actually increased self-stimulation threshold at the highest dose. Cocaine significantly reduced self-stimulation threshold, without altering maximal rates of responding. AM-404 (10 mg/kg) attenuated this action of cocaine, an effect which was reversed by pretreatment with the selective CB1R antagonist SR141716A. WIN55,212-2 decreased locomotion at the two highest doses, an effect that was blocked by SR141716A; AM-404 had no effect on locomotion. Cocaine caused a significant, dose-dependent increase in locomotion, which was reduced by WIN55,212-2 and AM-404. SR141716A blocked the effects of WIN55,212-2 and AM-404 on cocaine-induced hyperlocomotion. SR141716A alone had no effect on ICSS threshold or locomotion. These results indicate that cannabinoids may interfere with brain reward systems responsible for the expression of acute reinforcing/rewarding properties of cocaine, and provide further evidence that the cannabinoid system could be explored as a potential drug discovery target for the treatment of psychostimulant addiction and pathological states associated with psychomotor overexcitability.

Lack of evidence for appetitive effects of Delta 9-tetrahydrocannabinol in the intracranial self-stimulation and conditioned place preference procedures in rodents

Data on the ability of Delta 9-tetrahydrocannabinol (THC) to modify reward processes in experimental animals are inconsistent. This study examined the effects of Delta 9-THC on brain reward function using the rate-frequency curve shift paradigm of intracranial self-stimulation (ICSS) and the conditioned place preference (CPP) paradigm. In ICSS tests, rats were implanted with electrodes into the medial forebrain bundle. After brain stimulation reward thresholds stabilized, rats received intraperitoneal injections of Delta 9-THC (0, 0.5, 1 and 2 mg/kg) or the CB1 receptor antagonist SR141716A (0, 0.02 mg/kg) and Delta 9-THC (0, 2 mg/kg). The two highest doses of Delta 9-THC significantly increased the threshold ICSS frequency. SR141716A reversed the action of Delta 9-THC (2 mg/kg), without affecting reward thresholds by itself. In the CPP test, mice received intraperitoneal injections of Delta 9-THC (0, 1 or 3 mg/kg). Delta 9-THC showed neither statistically significant preference nor aversion in either of the doses tested. These findings indicate that Delta 9-THC, in contrast to other drugs of abuse, does not facilitate ICSS or support CPP under the present experimental conditions, but rather has a dose-dependent inhibitory influence on ICSS.

Anti-human placental antigen complex X-P2 (hPAX-P2) anti-serum recognizes C-terminus of huntingtin-associated protein 1A common to 1B as a determinant marker for the stigmoid body

The anti-serum against an unknown human placental antigen complex X-P2 (hPAX-P2) immunohistochemically recognizes three putative molecules (hPAX-P2S, hPAX-P2N, and hPAX-P2R), each of which is associated with the stigmoid bodies (STBs), necklace olfactory glomeruli (NOGs), or reticulo-filamentous structures (RFs) in the rat brain. The STBs also contain huntingtin-associated protein 1 (HAP1), and the HAP1-cDNA transfection induces STB-like inclusions in cultured cells. In order to clarify the relationship between hPAX-P2S and HAP1 isoforms (A/B), we performed Western blotting, immuno-histo/cytochemistry for light- and electron-microscopy and pre-adsorption tests with HAP1 deletion fragments. The results showed that the anti-hPAX-P2 anti-serum recognizes HAP1(474-577) of HAP1A/B in Western blotting and strongly immunostains HAP1A-induced STB-like inclusions but far weakly detects HAP1B-induced diffuse structures in HAP1-transfected HEK 293 cells. In the rat brain, immunoreactivity of the anti-hPAX-P2 anti-serum for the STBs was eliminated by pre-adsorption with HAP1(474-577), whereas no pre-adsorption with any different HAP1 fragments can suppress immunoreactivity for the NOGs and RFs, which were not immunoreactive to anti-HAP1 anti-serum. These findings indicate that hPAX-P2S, which is distinct from hPAX-P2N and hPAX-P2R, is identical with STB-constituted HAP1 and that the HAP1-induced/immunoreactive inclusions correspond to the hPAX-P2-immunoreactive STBs previously identified in the brain.

The selective dopamine D3 receptor antagonist SB-277011A reduces nicotine-enhanced brain reward and nicotine-paired environmental cue functions

Increasing evidence suggests that enhanced dopamine (DA) neurotransmission in the nucleus accumbens (NAc) may play a role in mediating the reward and reinforcement produced by addictive drugs and in the attentional processing of drug-associated environmental cues. The meso-accumbens DA system is selectively enriched with DA D3 receptors, a DA receptor subtype increasingly implicated in reward-related brain and behavioural processes. From a variety of evidence, it has been suggested that selective DA D3 receptor antagonism may be a useful pharmacotherapeutic approach for treating addiction. The present experiments tested the efficacy of SB-277011A, a selective DA D3 receptor antagonist, in rat models of nicotine-enhanced electrical brain-stimulation reward (BSR), nicotine-induced conditioned locomotor activity (LMA), and nicotine-induced conditioned place preference (CPP). Nicotine was given subcutaneously within the dose range of 0.25-0.6 mg/kg (nicotine-free base). SB-277011A, given intraperitoneally within the dose range of 1-12 mg/kg, dose-dependently reduced nicotine-enhanced BSR, nicotine-induced conditioned LMA, and nicotine-induced CPP. The results suggest that selective D3 receptor antagonism constitutes a new and promising pharmacotherapeutic approach to the treatment of nicotine dependence.

Amphetamine lowers brain stimulation reward (BSR) threshold in alcohol-preferring (P) and -nonpreferring (NP) rats: regulation by D-sub-1 and D-sub-2 receptors in the nucleus accumbens

Differences in the mesolimbic dopamine (DA) pathway that regulates alcohol preference may also increase sensitivity to the reinforcing effects of other drugs of abuse. In the present study, the curve-shift (rate-frequency) paradigm was used to quantify the interaction of amphetamine with the rewarding effects of lateral hypothalamic brain stimulation reward (BSR) in alcohol-preferring (P) and -nonpreferring (NP) rats. The role of D-sub-1 and D-sub-2 DA receptors of the nucleus accumbens (NAcc) in mediating the reward-potentiating effects of amphetamine was also determined. Animals were tested with randomly administered amphetamine (0.25, 0.75, 1.25 mg/kg ip), DA-receptor antagonists (SCH 23390 [2.0 microg, 5.0 microg]; eticlopride [2.0 microg, 5.0 microg]), or a combination of the 2 (SCH 23390 [2.0 microg, 5.0 microg] + 0.75 mg/kg amphetamine; eticlopride [2.0 microg, 5.0 microg] + 0.75 mg/kg amphetamine). Amphetamine produced comparable dose-related leftward shifts in the rate-frequency function for both P and NP rats, with a greater than 60% reduction observed in BSR threshold. On intervening days, baseline threshold was unaltered between tests and similar between rat lines. Unilateral infusion in the NAcc of either the D-sub-1 or D-sub-2 receptor antagonist produced rightward shifts in the rate-frequency function of amphetamine, completely reversing-attenuating its reward-enhancing effects. The results demonstrate that amphetamine produces similar threshold-lowering effects in both P and NP rats and that the reward-potentiating effects of amphetamine do not correlate with alcohol preference under the conditions of the present study.

The novel dopamine D3 receptor antagonist NGB 2904 inhibits cocaine's rewarding effects and cocaine-induced reinstatement of drug-seeking behavior in rats

Accumulating evidence indicates that dopamine (DA) D(3) receptor antagonists appear highly promising in attenuating cocaine reward and relapse in preclinical models of addiction. In the present study, we investigated the effects of the novel D(3)-selective antagonist NGB 2904 (N-(4-[4-{2,3-dichlorophenyl}-1-piperazinyl]butyl)-3-fluorenylcarboxamide) on cocaine self-administration, cocaine-enhanced brain stimulation reward (BSR), and cocaine-triggered reinstatement of drug-seeking behavior in male Long-Evans rats. We found that: (1) acute intraperitoneal (i.p.) administration of NGB 2904 (0.1-10 mg/kg) failed to alter cocaine self-administration (0.5 mg/kg/infusion) under fixed-ratio 2 (FR2) reinforcement, but 1 or 5 mg/kg NGB 2904 significantly lowered the break-point for cocaine self-administration under progressive-ratio (PR) reinforcement; (2) cocaine (1, 2, and 10 mg/kg) significantly enhanced electrical BSR (decreased brain reward thresholds), while NGB 2904 significantly inhibited the enhancement of BSR elicited by 2 mg/kg, but not 10 mg/kg of cocaine; (3) NGB 2904 alone neither maintained self-administration behavior nor altered brain reward thresholds; and (4) NGB 2904 significantly inhibited cocaine-triggered reinstatement of extinguished drug-seeking behavior, but not sucrose-plus-sucrose-cue-triggered reinstatement of sucrose-seeking behavior. Overall, these data show that the novel D(3)-selective antagonist NGB 2904 attenuates cocaine's rewarding effects as assessed by PR self-administration, BSR, and cocaine-triggered reinstatement of cocaine-seeking behavior. Owing to these properties and to its lack of rewarding effects (as assessed by BSR and by substitution during drug self-administration), NGB 2904 merits further investigation as a potential agent for treatment of cocaine addiction.

Neurochemical modulation of ingestive behavior in the ventral pallidum

The nucleus accumbens and its related circuitry have been shown to play an important role in promoting the intake of hedonically desirable food. A previous report has demonstrated that the blockade of GABAA receptors in the ventral pallidum (VP), a target of GABAergic projection from the nucleus accumbens, greatly increases food, but not water, intake in satiated rats [Stratford et al. (1999)Brain Research, 825, 199-203]. The present study examined which neurotransmission in the VP is specifically involved in the intake of normally preferred taste stimuli. Microinjections of the GABAA antagonist bicuculline selectively increased the intake of saccharin solution but not that of water and quinine solution in water-deprived rats. In contrast, the facilitation of GABAA receptors by microinjections of muscimol in the VP generally suppressed the intake of saccharin, water and quinine. The same injections induced strong aversive taste reactivity responses to oral stimulation with not only quinine but also water and saccharin. The local administration of D-Ala2,N-Me-Phe4,Glyol5-enkephalin, a selective micro-opioid receptor agonist, into the VP had time-dependent effects, decreasing saccharine intake early and increasing intake late. Microinjections of SCH-23390, a dopamine D1 receptor antagonist, in the VP suppressed the intake of saccharin but not water or quinine. Microinjections of sulpiride, the dopamine D2 receptor antagonist, and 6-cyano-7-nitroquinoxaline-2,3-dione, the AMPA/kainate glutamate receptor antagonist, had no effect on fluid intake. These results reveal that GABA, opioid and D1 receptors in the VP are involved in the consumption of hedonically positive taste stimuli.

Behavioral pharmacological properties of a novel cannabinoid 1???,1???-dithiolane ??8-THC analog, AMG-3

Newly developed cannabinoids may hold the promise of the development of useful and safe drugs. This study aimed to investigate the behavioral effects of the novel 1',1'-dithiolane delta8-HC analogue AMG-3, a cannabinomimetic molecule with high affinity for CB1/CB2 receptors. This analog was chosen for its binding affinity to these receptors, which is higher than that reported for delta8-tetrahydrocannabinol (delta8-THC). Behavioral responses were assessed after the administration of AMG-3 (1, 2, 4, 8 mg/kg, i.p.) in the open field, on the bar test, on the hot plate and in the intracranial self-stimulation procedure. AMG-3 increased the reactivity time on the hot plate in a dose- and time-dependent manner, indicating a long-lasting analgesic effect (at least 24 h). The substance was found dose-dependently to decrease spontaneous motor activity and to induce catalepsy, particularly at the highest dose (8 mg/kg). AMG-3 did not affect the rewarding value of intracranial self-stimulation, except to increase the reward threshold at the highest dose (8 mg/kg). The effects of the highest dose of AMG-3 on spontaneous activity and on the self-stimulation paradigm were completely reversed by pre-treatment with the CB1 receptor antagonist AM-251. These findings indicate that the administration of AMG-3 to rats elicits a specific behavioral profile, most probably associated with the activation of CB1 receptors and without effects indicating abuse potential.

Acute administration of SB-277011A, NGB 2904, or BP 897 inhibits cocaine cue-induced reinstatement of drug-seeking behavior in rats: role of dopamine D3 receptors

Recent studies have shown that the novel dopamine (DA) D3 receptor antagonists SB-277011A and NGB 2904 inhibit cocaine- and/or stress-induced reinstatement of drug-seeking behavior. The present study sought to determine if SB-277011A, NGB 2904, or BP-897 (a mixed D3 agonist/antagonist) similarly inhibit cocaine-associated cue-induced reinstatement of drug-seeking behavior. Long-Evans rats were allowed to self-administer cocaine. Each cocaine infusion was paired with discrete conditioned cue-light and tone. Subsequently, drug-seeking (i.e., lever-pressing) behavior was extinguished in the absence of cocaine and cocaine-associated cues. Rats were then tested for cue-induced reinstatement of drug-seeking. We found that cocaine-associated cues evoked robust reinstatement of lever-pressing. Acute intraperitoneal (i.p.) administration of SB-277011A (6, 12, or 24 mg/kg) produced a dose-dependent inhibition of cue-induced reinstatement of drug-seeking behavior by 35, 65, and 85%, respectively, compared to vehicle-treated animals. Acute i.p. administration of NGB 2904 (0.1, 1.0, or 5.0 mg/kg) produced a 45, 30, and 70% inhibition of cue-induced reinstatement, respectively, compared to vehicle-treated animals. Acute i.p. administration of either 0.1 or 1 mg/kg of BP 897 did not produce a significant effect on cue-induced reinstatement, whereas a dose of 3 mg/kg produced a 70% inhibition of cue-induced reinstatement. These findings, combined with previous data, suggest that DA D3 receptor antagonism may underlie the inhibitory effects of SB-277011A and NGB 2904 on cocaine cue-induced reinstatement, while the effects of BP 897 may involve D3 and non-D3 receptor mechanisms.

Brain stimulation reward performance and sucrose maintained behaviors in alcohol-preferring and -nonpreferring rats

BACKGROUND

The relation between ethanol (EtOH) preference and sensitivity to brain stimulation reward (BSR) was examined under multiples schedules of reinforcement in the current study. For comparison, the study also examined the relation between EtOH preference and motivation for a sweet, palatable sucrose solution under similar schedules of reinforcement.

METHODS

To investigate sensitivity to BSR performance, alcohol-preferring and -nonpreferring rats were tested using the curve-shift (rate-frequency) paradigm under several intensity levels during a 20-min session. Animals were first trained under an optimal current intensity, which produced maximal responding (i.e., 100%) across a series of descending frequencies (i.e., 300-20 Hz). BSR was then evaluated at 100%, 75%, and 50% of the optimal current. The sensitivity of the curve-shift method was further evaluated under the animal's optimal current using the FR1, FR6, and FR12 schedules. To examine responding for the sucrose solution, a separate group of alcohol-preferring and -nonpreferring rats was initially stabilized on an FR1 schedule and then subsequently on FR6 and FR12 schedules.

RESULTS

The results demonstrated that reducing the reinforcing efficacy of BSR via reduction in current intensity/reinforcement schedule produced marked orderly rightward shifts in the rate-frequency curves relating responding to stimulation frequency in both rat lines. However, no differences were found between the lines with either manipulation. Specifically, both lines demonstrated orderly reductions in response rate and increases in BSR threshold parameters (i.e., half maximal frequency/responding, minimum and maximum frequencies). In contrast to BSR, genetic selection for EtOH preference was highly associated with responding for the sweet, palatable sucrose solution. The association was even more salient as the reinforcement schedule increased (i.e., reward cost).

CONCLUSION

The results demonstrate that responding for BSR is not associated with EtOH preference, insofar as alcohol-preferring and -nonpreferring rats respond similarly under an array of reinforcement schedules and current intensities. In contrast, genetic selection for EtOH preference is highly associated with responding for a palatable sucrose reward, and the relation increases as the reward cost for the sucrose increases. These findings suggest that similar/overlapping mechanisms of action regulate the reinforcing properties of EtOH and sucrose but that overlapping yet distinct neuronal mechanism may modulate the reward characteristics of BSR and EtOH preference.

Selective dopamine D3 receptor antagonism by SB-277011A attenuates cocaine reinforcement as assessed by progressive-ratio and variable-cost-variable-payoff fixed-ratio cocaine self-administration in rats

In rats, acute administration of SB-277011A, a highly selective dopamine (DA) D(3) receptor antagonist, blocks cocaine-enhanced brain stimulation reward, cocaine-seeking behaviour and reinstatement of cocaine-seeking behaviour. Here, we investigated whether SB-277011A attenuates cocaine reinforcement as assessed by cocaine self-administration under variable-cost-variable-payoff fixed-ratio (FR) and progressive-ratio (PR) reinforcement schedules. Acute i.p. administration of SB-277011A (3-24 mg/kg) did not significantly alter cocaine (0.75 mg/kg/infusion) self-administration reinforced under FR1 (one lever press for one cocaine infusion) conditions. However, acute administration of SB-277011A (24 mg/kg, i.p.) progressively attenuated cocaine self-administration when: (a) the unit dose of self-administered cocaine was lowered from 0.75 to 0.125-0.5 mg/kg, and (b) the work demand for cocaine reinforcement was increased from FR1 to FR10. Under PR (increasing number of lever presses for each successive cocaine infusion) cocaine reinforcement, acute administration of SB-277011A (6-24 mg/kg i.p.) lowered the PR break point for cocaine self-administration in a dose-dependent manner. The reduction in the cocaine (0.25-1.0 mg/kg) dose-response break-point curve produced by 24 mg/kg SB-277011A is consistent with a reduction in cocaine's reinforcing efficacy. When substituted for cocaine, SB-277011A alone did not sustain self-administration behaviour. In contrast with the mixed DA D(2)/D(3) receptor antagonist haloperidol (1 mg/kg), SB-277011A (3, 12 or 24 mg/kg) failed to impede locomotor activity, failed to impair rearing behaviour, failed to produce catalepsy and failed to impair rotarod performance. These results show that SB-277011A significantly inhibits acute cocaine-induced reinforcement except at high cocaine doses and low work requirement for cocaine. If these results extrapolate to humans, SB-277011A or similar selective DA D(3) receptor antagonists may be useful in the treatment of cocaine addiction.

CB1 cannabinoid receptor agonists increase intracranial self-stimulation thresholds in the rat

RATIONALE

Addictive drugs have a number of commonalities in animal behavioral models. They lower intracranial self-stimulation (ICSS) thresholds, support self-administration, and produce conditioned place preference (CPP). However, cannabinoids appear atypical as drugs of abuse, since there are controversial data in the literature concerning their reinforcing properties.

OBJECTIVES

The aim of the present study was to examine the effects of cannabinoids on brain reward using the rate-frequency curve shift paradigm of ICSS.

METHODS

Male Sprague-Dawley rats were implanted with electrodes into the medial forebrain bundle (MFB). Rate-frequency functions were determined by logarithmically decreasing the number of cathodal pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding. After brain stimulation reward thresholds stabilized rats received intraperitoneal (IP) injections of the potent CB1 receptor agonists WIN 55,212-2 (graded doses 0.1, 0.3, 1 and 3 mg/kg), CP 55,940 (graded doses 10, 30, 56 and 100 microg/kg), or HU-210 (graded doses 10, 30, 100 microg/kg).

RESULTS

With the exception of the highest dose of all cannabinoid agonists tested, which significantly increased the threshold frequency required for MFB ICSS, all other doses of the tested drugs did not affect ICSS thresholds. The CB1 receptor antagonist SR141716A reversed the actions of WIN 55,212-2 and CP 55,940, but not HU-210. However, the selective CB1 cannabinoid receptor antagonist AM 251 counteracted the effect of HU-210. Both CB1 receptor antagonists, at the doses used in the present study, did not affect reward thresholds by themselves.

CONCLUSIONS

The present results indicate that cannabinoid agonists do not exhibit reinforcing properties in the ICSS paradigm, but rather have an inhibitory influence on reward mechanisms. The results suggest that the anhedonic effects of cannabinoids are probably mediated by cannabinoid CB1 receptors.

WIN 55,212-2 decreases the reinforcing actions of cocaine through CB1 cannabinoid receptor stimulation

CB(1) cannabinoid receptor agonists show a different profile compared to other drugs of abuse on the basis of experimental data that reveal their reinforcing properties. Thus, there are controversial data in the literature concerning the ability of CB(1) receptor agonists to reinforce behavioral responses in experimental animals, i.e. to lower self-stimulation thresholds, and to support self-administration or conditioned place preference. The aim of the present study was to examine the effects of WIN 55,212-2, a potent CB(1) receptor agonist (graded doses 0.1, 0.3, 1 mg/kg, i.p.), on the rewarding efficacy of lateral hypothalamic self-stimulation and on the systemic cocaine-induced potentiation of brain-stimulation reward. WIN 55,212-2 did not affect lateral hypothalamic self-stimulation thresholds both in drug nai;ve rats and in rats pretreated with the drug, whereas it produced a significant, dose-dependent decrease in the maximal rate of responding, i.e. in the performance of the animals. Cocaine (5.0 mg/kg, i.p.) produced a significant reduction in self-stimulation threshold, without altering maximal rates of responding. Importantly, WIN 55,212-2 attenuated the effect of cocaine at the two higher doses tested. The effects of the CB(1) receptor agonist were reversed by pretreatment with the selective CB(1) receptor antagonist SR 141716A (0.02 mg/kg, i.p.) that did not by itself affect cocaine's action. These results indicate that acute stimulation of CB(1) receptors per se does not affect baseline self-stimulation, but reduces the reinforcing effects induced by cocaine. Taken together these findings suggest that cannabinoids may interfere with brain-reward systems responsible for the expression of acute reinforcing properties of drugs of abuse, such as cocaine, and provide evidence that the cannabinoid system could be an interesting drug discovery and development target for the treatment of drug addiction.

Neural correlates of "hot" and "cold" emotional processing: a multilevel approach to the functional anatomy of emotion

The neural correlates of two hypothesized emotional processing modes, i.e., schematic and propositional modes, were investigated with positron emission tomography. Nineteen subjects performed an emotional mental imagery task while mentally repeating sentences linked to the meaning of the imagery script. In the schematic conditions, participants repeated metaphoric sentences, whereas in the propositional conditions, the sentences were explicit questions about specific emotional appraisals of the imagery scenario. Five types of emotional scripts were proposed to the subjects (happiness, anger, affection, sadness, and a neutral scenario). The results supported the hypothesized distinction between schematic and propositional emotional processing modes. Specifically, schematic mode was associated with increased activity in the ventromedial prefrontal cortex whereas propositional mode was associated with activation of the anterolateral prefrontal cortex. In addition, interaction analyses showed that schematic versus propositional processing of happiness (compared with the neutral scenario) was associated with increased activity in the ventral striatum whereas "schematic anger" was tentatively associated with activation of the ventral pallidum.

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.

Reduced dopamine D1 receptor binding in the ventral striatum of cigarette smokers

Several drugs of abuse, including nicotine, are thought to exert their reinforcing effects through actions on the mesolimbic dopamine system. Animal and human studies suggest that chronic administration of addictive drugs may lead to impaired dopamine neurotransmission in the nucleus accumbens. We measured D1 receptor density in 11 smokers and 18 nonsmokers using positron emission tomography and the D1 receptor ligand [11C]SCH 23390. Ten of the smokers were scanned twice, once after overnight abstinence from cigarettes, and once while smoking at their usual rate, to account for possible acute effects of cigarette smoking on D1 receptor binding. In addition, eight control subjects were scanned twice to assess the reproducibility of the method. We used compartmental modeling to measure [11C]SCH 23390 binding potential, a measure of D1 receptor density. There were no differences in binding between abstinent and nonabstinent scans in smokers or in the two scans in controls. However, there was a significant reduction in [11C]SCH 23390 binding potential in smokers compared to nonsmokers in the striatum, most prominently in the ventral striatum. This suggests that there is a reduction in dopamine D1 receptor density in the ventral striatum of human cigarette smokers relative to nonsmokers, which implies that the postsynaptic mesolimbic dopamine system may be chronically underactive in smokers, either as an antecedent or consequence of addiction to cigarettes. Such a hypodopaminergic state may play an important role in sustaining nicotine-seeking behavior. Alternatively, an inherited reduction in dopamine receptors in the striatum may be associated with an increased risk of addictive behavior.

Using intracranial electrical stimulation to study the timing of prepulse inhibition of the startle reflex

Due to the short latency and briefness of the startle reflex, event-related inhibition of startle has high temporal resolution and is useful for studying the hierarchical organization of sensorimotor gating and motive-motor gating. In this article, we describe methods for measuring the inhibitory effects of electrically stimulating each of the following four brain structures on startle in awake rats: the inferior colliculus (IC), the deeper layers of the superior colliculus (SC), the pedunculopontine tegmental nucleus (PPTg), and the ventral pallidum (VP). These four brain structures have been reported to be important in mediating sensorimotor or motive-motor gating. Startle responses are elicited by either intense noise bursts or electrical stimulation of the principal trigeminal nucleus. The time course of the IC-inhibited startle reflex is used as a standard for estimating timing of the neural transfer of startle-inhibitory information to motor outputs. We also discuss how these methods can be used in combination with neuropharmacology.

Effects of bilateral electrical stimulation of the ventral pallidum on acoustic startle

The ventral pallidum (VP) is believed to occupy a critical position between the limbic and the motor systems, for transferring motive information into motor commands. To estimate the time course of signaling from the VP to motor outputs, in the present study we examined the effects of bilateral electrical stimulation of the VP on the acoustic startle reflex in awake rats. When the interstimulus interval (ISI) between VP stimulation and acoustic stimulation was shorter than 5 ms, VP stimulation potentiated acoustic startle. When the ISI was longer than 5 ms, VP stimulation inhibited acoustic startle over a large range of ISIs with the maximum inhibition at ISIs between 15 and 25 ms. In contrast, bilateral electrical stimulation of the amygdala did not have a significant inhibitory effect on acoustic startle, but strongly augmented acoustic startle at shorter ISIs (0-10 ms). Compared to unilateral electrical stimulation of the inferior colliculus (IC), bilateral stimulation of the VP gave rise to a rightward shift of the ISI curve, indicating that the neural pathways conveying the inhibitory influence from the VP to the acoustic startle circuit are longer than those from the IC.

Opioid modulation of ventral pallidal inputs

While the ventral pallidum (VP) is known to be important in relaying information between the nucleus accumbens and target structures, it has become clear that substantial information processing occurs within the VP. We evaluated the possibility that opioid modulation of other transmitters contained in VP afferents is involved in this process. Initially, we demonstrated that opioids hyperpolarized VP neurons in vitro and suppressed spontaneous firing in vivo. The ability of opioids to modulate other transmitters was determined using microiontophoretically applied ligands and extracellular recordings of VP neurons from chloral hydrate-anesthetized rats. With neurons that responded to iontophoresed opioid agonists, the ejection current was reduced to a level that was below that necessary to alter spontaneous firing. This "subthreshold" current was used to determine the ability of mu opioid receptor (microR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. microR agonists decreased the variability and enhanced the acuity (e.g., "signal-to-noise" relationship) of VP responses to activation of glutamatergic inputs from the prefrontal cortex and amygdala. By contrast, microR agonists attenuated both the slow excitatory responses to substance P and GABA-induced inhibitions that resulted from activating the nucleus accumbens. Subthreshold opioids also attenuated inhibitory responses to stimulating midbrain dopaminergic cells. These results suggest that a consequence of opioid transmission in the VP is to negate the influence of some afferents (e.g., midbrain dopamine and accumbal GABA and substance P) while selectively potentiating the efficacy of others (e.g., cortical and amygdaloid glutamate). Interpreted in the context of opiate abuse, microR opioids in the VP may serve to diminish the influence of reinforcement (ventral tegmental area and nucleus accumbens) in the transduction of cognition (prefrontal cortex) and affect (amygdala) into behavior. This may contribute to drug craving that occurs even in the absence of reward.

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.