Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Dopamine D1 receptor-induced signaling through TrkB receptors in striatal neurons

In addition to its role as a neurotransmitter, dopamine can stimulate neurite outgrowth and morphological effects upon primary neurons. To investigate the signal transduction mechanisms used by dopamine in developing striatal neurons, we focused upon the effects of activating the dopamine D1 receptor. Using the D1 receptor agonist SKF38393, we found that Trk neurotrophin receptors were activated in embryonic day 18 striatal neurons. K-252a, a Trk tyrosine kinase inhibitor, and a dopamine D1 receptor antagonist could block the effects of SKF38393. The increase in TrkB phosphorylation was not the result of increased neurotrophin production. Induction of TrkB activity by SKF38393 was accompanied by the phosphorylation of several Trk signaling proteins, including phospholipase Cgamma, Akt, and MAPK. Biotinylation experiments followed by immunostaining by phospho-TrkB-specific antibodies indicated that the mechanism involved increased TrkB surface expression by dopamine D1 receptor activation. This increase in cell surface TrkB expression was dependent upon an increase in intracellular Ca(2+). These results indicate that stimulation of dopamine D1 receptors can be coupled to the neurotrophin receptor signaling to mediate the effects of dopamine upon striatal neurons.

Stereological estimates of dopaminergic, GABAergic and glutamatergic neurons in the ventral tegmental area, substantia nigra and retrorubral field in the rat

Midbrain dopamine neurons in the ventral tegmental area, substantia nigra and retrorubral field play key roles in reward processing, learning and memory, and movement. Within these midbrain regions and admixed with the dopamine neurons, are also substantial populations of GABAergic neurons that regulate dopamine neuron activity and have projection targets similar to those of dopamine neurons. Additionally, there is a small group of putative glutamatergic neurons within the ventral tegmental area whose function remains unclear. Although dopamine neurons have been intensively studied and quantified, there is little quantitative information regarding the GABAergic and glutamatergic neurons. We therefore used unbiased stereological methods to estimate the number of dopaminergic, GABAergic and glutamatergic cells in these regions in the rat. Neurons were identified using a combination of immunohistochemistry (tyrosine hydroxylase) and in situ hybridization (glutamic acid decarboxylase mRNA and vesicular glutamate transporter 2 mRNA). In substantia nigra pars compacta 29% of cells were glutamic acid decarboxylase mRNA-positive, 58% in the retrorubral field and 35% in the ventral tegmental area. There were further differences in the relative sizes of the GABAergic populations in subnuclei of the ventral tegmental area. Thus, glutamic acid decarboxylase mRNA-positive neurons represented 12% of cells in the interfascicular nucleus, 30% in the parabrachial nucleus, and 45% in the parainterfascicular nucleus. Vesicular glutamate transporter 2 mRNA-positive neurons were present in the ventral tegmental area, but not substantia nigra or retrorubral field. They were mainly confined to the rostro-medial region of the ventral tegmental area, and represented approximately 2-3% of the total neurons counted ( approximately 1600 cells). These results demonstrate that GABAergic and glutamatergic neurons represent large proportions of the neurons in what are traditionally considered as dopamine nuclei and that there are considerable heterogeneities in the proportions of cell types in the different dopaminergic midbrain regions.

Dopamine alters AMPA receptor synaptic expression and subunit composition in dopamine neurons of the ventral tegmental area cultured with prefrontal cortex neurons

Excitatory synapses onto dopamine (DA) neurons of the ventral tegmental area (VTA) represent a critical site of psychostimulant-induced synaptic plasticity. This plasticity involves alterations in synaptic strength through AMPA receptor (AMPAR) redistribution. Here, we report an in vitro model for studying regulation of AMPAR trafficking in DA neurons under control conditions and after elevation of DA levels, mimicking cocaine exposure. We used cocultures containing rat VTA neurons and prefrontal cortex (PFC) neurons from enhanced cyan fluorescent protein-expressing mice. In VTA-PFC cocultures, D1 receptor activation (10 min) increased synaptic and nonsynaptic glutamate receptor subunit 1 (GluR1) and GluR2 surface expression on DA neurons. NMDA or AMPA receptor antagonists blocked this effect, and it was not observed in pure VTA cultures, suggesting that DA agonists acted on D1 receptors on PFC neurons, altering their excitatory transmission onto VTA DA neurons and, thus, influencing AMPARs. To mimic the longer elevation in extracellular DA levels produced by systemic cocaine, cocultures were incubated with DA for 1 h. Synaptic GluR1 was increased 24 h later, reminiscent of the increased AMPA/NMDA ratio at excitatory synapses onto VTA DA neurons 24 h after cocaine injection (Ungless et al., 2001). In contrast, GluR2 was unchanged. Analysis of colocalization of surface GluR1-3 labeling suggested that control DA neurons express a substantial number of GluR1/2, GluR2/3, and homomeric GluR1 receptors and that the increase in surface AMPARs 24 h after DA exposure may in part reflect increased GluR1/3-containing receptors. These results help define the cellular basis for plasticity underlying the development of behavioral sensitization.

Nanomolar propofol stimulates glutamate transmission to dopamine neurons: a possible mechanism of abuse potential?

Anesthesiologists among physicians are on the top of the drug abuse list, and the mechanism is unclear. Recent studies suggest occupation-related second-hand exposure to i.v. drugs, including propofol, may play a role. Growing evidence indicates that propofol is one of the choices of drugs being abused. In this study, we show that propofol at minute concentrations increases glutamatergic excitatory synaptic transmission and discharges of dopamine neurons in the ventral tegmental area (VTA). We found that acute application of propofol (0.1-10 nM) to the VTA in midbrain slices of rats increased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (EPSCs) mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors. We observed that propofol increased the amplitude but decreased the paired-pulse ratio of EPSCs evoked by stimulation in the absence and the presence of gabazine (SR 95531), a GABA(A) receptor antagonist. Moreover, the propofol-induced facilitation of EPSCs was mimicked by 6-phenyl-4-azabicyclo[5.4.0]undeca-7,9,11-triene-9,10-diol (SKF38393), an agonist of dopamine D(1) receptor, and by 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine dihydrochloride (GBR 12935), a dopamine reuptake inhibitor, but blocked by (+/-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride (SKF83566), a D(1) antagonist, or by depleting dopamine stores with reserpine. Finally, 1 nM propofol increased the spontaneous discharge rate of dopamine neurons. These findings suggest that propofol at minute concentrations enhances presynaptic D(1) receptor-mediated facilitation of glutamatergic synaptic transmission and the excitability of VTA dopamine neurons, probably by increasing extracellular dopamine levels. These changes in synaptic plasticity in the VTA, an addiction-related brain area might contribute to the development of propofol abuse and the increased susceptibility to addiction of other drugs.

Corticotropin-releasing factor 1 and 2 receptors in the dorsal raphé differentially affect serotonin release in the nucleus accumbens

Corticotropin-releasing factor (CRF) is a neurohormone that mediates stress, anxiety, and affects serotonergic activity. Studies have shown that CRF has dose-dependent opposing effects on serotonergic activity. This effect has been hypothesized to be differentially mediated by CRF(1) and CRF(2) receptors in the dorsal raphé nucleus. We directly tested this hypothesis by using in vivo microdialysis to determine the effects of CRF and CRF receptor antagonists in the dorsal raphé nucleus on serotonin (5-HT) release in the nucleus accumbens, a brain region implicated in the neuropathology of stress-related psychiatric disorders. Male urethane-anesthetized rats were implanted with a microdialysis probe into the nucleus accumbens, and CRF (0, 100 or 500 ng) was infused into the dorsal raphé. Infusion of CRF into the dorsal raphé nucleus had dose-dependent opposite effects, with 100 ng of CRF significantly decreasing 5-HT levels in the nucleus accumbens and 500 ng CRF significantly increasing accumbal 5-HT levels. In subsequent experiments, the raphé was pre-treated with the CRF(1) receptor antagonist antalarmin (0.25 microg) or the CRF(2) receptor antagonist antisauvagine-30 (ASV-30; 2 microg) prior to CRF infusion. Antagonism of CRF(1) receptors in the dorsal raphé nucleus abolished the decrease in accumbal 5-HT levels elicited by 100 ng CRF, and CRF(2) receptor antagonism in the raphé blocked the increase in accumbal 5-HT levels elicited by 500 ng CRF. These results suggest that the opposing effects of dorsal raphé CRF on 5-HT release in the nucleus accumbens are dependent on differential activation of CRF(1) and CRF(2) receptors in the dorsal raphé nucleus.

Neurobiological processes in adolescent addictive disorders

The purpose of this review is to summarize the neurobiological factors involved in the etiology of adolescent addiction and present evidence implicating various mechanisms in its development. Adolescents are at heightened risk for experimentation with substances, and early experimentation is associated with higher rates of SUD in adulthood. Both normative (e.g., immature frontal-limbic connections, immature frontal lobe development) and non-normative (e.g., lowered serotonergic function, abnormal hypothalamic-pituitary-adrenal axis function) neurobiological developmental factors can predispose adolescents to a heightened risk for SUD. In addition, a normative imbalance in the adolescent neurobiological motivational system may be caused by the relative underdevelopment of suppressive mechanisms when compared to stimulatory systems. These neurobiological liabilities may correspond to neurobehavioral impairments in decision-making, affiliation with deviant peers and externalizing behavior; these and other cognitive and behavioral traits converge with neurobiological factors to increase SUD risk. The progression to SUD acts as an amplifying feedback loop, where the development of SUD results in reciprocal impairments in neurobehavioral and neurobiological processes. A clearer understanding of adolescent neurobiology is a necessary step in the development of prevention and treatment interventions for adolescent SUD.

Functional implications of glutamatergic projections to the ventral tegmental area

Glutamatergic afferents of the ventral tegmental area (VTA) play an important role in the functioning of the VTA and are involved in the pathophysiology of drug addiction. It has recently been demonstrated that the VTA is densely innervated by glutamatergic axons and that glutamatergic neurons projecting to the VTA are situated in almost all structures that project there. While the projection from the prefrontal cortex is essentially entirely glutamatergic, subcortical glutamatergic neurons innervating the VTA intermingle with non-glutamatergic, most likely GABAergic and/or peptidergic VTA-projecting neurons. The first part of this review focuses on the origins and putative functional implications of various glutamatergic projections to the VTA. In the second part we consider how different neuropeptides via different mechanisms modulate glutamatergic actions in the VTA. We conclude by developing a model of how the glutamatergic afferents might together contribute to the functions of the VTA.

Neonatal basolateral amygdala lesions affect monoamine and cannabinoid brain systems in adult rats

There is evidence for neurodevelopment disturbances in schizophrenia. In rats, a neonatal basolateral amygdala lesion induces behavioural features in adults reminiscent of the symptomatology of schizophrenia. Dopamine plays a key role in the pathogenesis of schizophrenia, and cannabis use has been implicated in the risk for developing schizophrenia. The effects of an excitotoxic, bilateral basolateral amygdala lesion on postnatal days 7 or 21 were compared when the rats were adult. The behavioural response to a novelty challenge and the level of dopamine receptors and cannabinoid receptors in the brain using in-vitro autoradiography was determined. In brain tissue punches concentrations of monoamines and metabolites were determined by high-performance liquid chromatography. The neonatal lesion, but not the later lesion induced behavioural hyperactivity and biochemical effects. The neonatal lesion reduced the density of dopamine D2-like, but not D3-, and less markely D1-like receptors and increased dopamine turnover. These effects were observed in the mesolimbic, but not in the striatal regions. In contrast, density of cannabinoid receptors was increased in the striatal, but not the mesolimbic regions of these animals. Noradrenergic neurotransmission was reduced in both regions. The present findings contribute to the idea that the neonatal basolateral amygdala lesion induces features in adults reminiscent of the neurodevelopmental disturbances in schizophrenia, with a focus on the amygdala-prefrontal cortex-nucleus accumbens circuit.

Contribution of ventral tegmental GABA receptors to cocaine self-administration in rats

Recent evidence has suggested that compounds affecting GABAergic transmission may provide useful pharmacological tools for the treatment of cocaine addiction. Using a rat model of self-administration, the present study examined the effects of GABA agonists and antagonists injected directly into the ventral tegmental area (VTA) on cocaine intake in rats trained to self-administer cocaine (0, 125, 250 and 500 microg/infusion) under an FR5 schedule of reinforcement. Separate groups of rats received bilateral intra-VTA injections of the GABA-A antagonist picrotoxin (34 ng/side, n = 7; 68 ng/side, n = 8), GABA-A agonist muscimol (14 ng/side, n = 8), GABA-B agonist baclofen (56 ng/side, n = 7; 100 ng/side, n = 6), picrotoxin (68 ng/side) co-injected with the GABA-B antagonist 2-hydroxysaclofen (100 ng/side, n = 7; 2 microg/side, n = 8) or artificial cerebrospinal fluid (aCSF, n = 6) to assess the effects of the various compounds on the cocaine self-administration dose-response curve. Both picrotoxin and baclofen reduced responding maintained by cocaine, whereas muscimol had no effect on responding. In contrast, neither picrotoxin (n = 6) nor baclofen (n = 8) affected responding maintained by food. Interestingly, 2-hydroxysaclofen effectively blocked the suppression of responding produced by picrotoxin, suggesting that both picrotoxin and baclofen exert their effects via activation of GABA-B receptors. Additionally, these effects appear to be specific to cocaine reinforcement, supporting current investigation of baclofen as a treatment for cocaine addiction.

Neonatal immune system activation with lipopolysaccharide enhances behavioural sensitization to the dopamine agonist, quinpirole, in adult female but not male rats

Administration of the bacterial cell wall component, lipopolysaccharide (LPS), stimulates the immune and endocrine systems inducing an acute phase of sickness and stress responses in adult and neonatal rats. Neonatal LPS exposure has been shown to alter a variety of behavioural and physiological processes in the adult animal. Early developmental stress, such as maternal separation, causes similar acute as well as long-term behavioural changes in adults, including altered sensitivity to drugs of abuse. Moreover, results of studies have shown evidence of a direct link between immune activation and sensitivity to dopamine-based drugs of abuse. The current study examined the effects of neonatal LPS treatment on subsequent locomotor sensitization to the dopamine (D(2)/D(3)) agonist, quinpirole, in adult rats as an index of drug sensitivity. Male and female Long-Evans rats were treated systemically with either LPS (50microg/kg) or saline (0.9%) on postnatal days 3 and 5. Locomotor sensitization was then examined in the adult rats (postnatal day 70). Animals were injected with quinpirole (0.5mg/kg, s.c.) or saline every other day for a total of 10 injections and locomotor activity was assessed for 60min immediately following injections 1, 2, 4, 6, and 10. Animals also received a 'challenge' injection of 0.5mg/kg quinpirole 28 days after injection 10, to assess persistence of behavioural sensitization. Locomotor activity progressively increased with repeated administration of quinpirole, indicating locomotor sensitization in all of the drug-treated groups. There was an overall sex difference, with females showing significantly greater sensitization than males. Moreover, neonatal LPS treatment potentiated both the level and the rate of development of locomotor sensitization to quinpirole administration in females, but not in males. Thus, the current study revealed that neonatal exposure to bacterial infection increases dopamine (D(2)/D(3)) agonist sensitivity in a sex-specific manner. These findings have important implications for the sexually dimorphic development of addictions to both natural and artificial rewards.

Metabotropic glutamate 2/3 receptors in the ventral tegmental area and the nucleus accumbens shell are involved in behaviors relating to nicotine dependence

The motivation to maintain nicotine self-administration and dependence may involve alterations in glutamatergic neurotransmission. Metabotropic glutamate (mGlu) 2/3 receptors regulate glutamate and dopamine release in the ventral tegmental area (VTA) and the nucleus accumbens (NAc) shell, two brain areas critically involved in reward and motivational processes. We found that acute systemic, as well as intra-VTA or intra-NAc, administration of the mGlu2/3 receptor agonist LY379268 [(-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate] decreased nicotine, but not food, self-administration in rats. In addition, nicotine self-administration downregulated mGlu2/3 receptor function in corticolimbic rat brain sites including the VTA and the NAc, demonstrated by decreased coupling of mGlu2/3 receptors to G-proteins in the [35S]GTPgammaS binding assay. Furthermore, repeated treatment with LY379268 reduced nicotine self-administration at the beginning of a 14 d treatment period; however, the number of nicotine infusions earned gradually returned to baseline levels, indicating tolerance to the effects of repeated LY379268 treatment. Finally, LY379268 administration decreased both cue-induced reinstatement of nicotine- and food-seeking behavior. Together, these findings indicate an important role for mGlu2/3 receptors in the posterior VTA and the NAc shell in the mediation of the rewarding effects of nicotine and potentially in cue-induced nicotine-seeking behavior.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Effects of chronic environmental and social stimuli during adolescence on mesolimbic dopaminergic circuitry markers

Previously, we have shown that chronic exposure to environmental and social stimuli (ESS) during adolescence prevents the development of behavioral sensitization to amphetamine in adult rats. At the onset of the peripubertal-juvenile period (28-d) male rats were subjected to a 28-d long intermittent ESS protocol or handled as controls (NO-ESS). Twenty-four hours after the last session of ESS or NO-ESS, all rats started a regimen of behavioral sensitization to amphetamine (1mg/kg, i.p.), in which rats were injected every third day with amphetamine or saline on four occasions. Then following one week abstinence all rats were challenged with a lower dose of amphetamine (0.5mg/kg, i.p.) and their locomotor activity monitored for 2h. Our results showed that while NO-ESS rats developed behavioral sensitization to amphetamine, ESS rats did not develop this behavior. All rats were then sacrificed 3 days following the challenge to allow for amphetamine clearance. Since mesolimbic dopamine has been implicated in behavioral sensitization to amphetamine we compared messenger RNA (mRNA) expression of key dopamine-related molecules in the mesolimbic circuitry in ESS and NO-ESS rats. A decrease in dopaminergic D1 receptor (D1R) gene expression in the caudate-putamen (CPu) was associated with amphetamine sensitization in the controls, possibly as a result of a chronic increase in DA release. In contrast, amphetamine treatment did not modulate D1R mRNA levels in ESS rats. No change has been detected in any other dopaminergic markers [D2R, D3R, tyrosine hydroxylase (TH) or dopamine transporter (DAT) mRNAs]. Consequently, we conclude that ESS may inhibit the development of behavioral sensitization to amphetamine through preventing the decrease in CPu D1R mRNA levels.

A scale-free systems theory of motivation and addiction

Scale-free organizations, characterized by uneven distributions of linkages between nodal elements, describe the structure and function of many life-based complex systems developing under evolutionary pressures. We explore motivated behavior as a scale-free map toward a comprehensive translational theory of addiction. Motivational and behavioral repertoires are reframed as link and nodal element sets, respectively, comprising a scale-free structure. These sets are generated by semi-independent information-processing streams within cortical-striatal circuits that cooperatively provide decision-making and sequential processing functions necessary for traversing maps of motivational links connecting behavioral nodes. Dopamine modulation of cortical-striatal plasticity serves a central-hierarchical mechanism for survival-adaptive sculpting and development of motivational-behavioral repertoires by guiding a scale-free design. Drug-induced dopamine activity promotes drug taking as a highly connected behavioral hub at the expense of natural-adaptive motivational links and behavioral nodes. Conceptualizing addiction as pathological alteration of scale-free motivational-behavioral repertoires unifies neurobiological, neurocomputational and behavioral research while addressing addiction vulnerability in adolescence and psychiatric illness. This model may inform integrative research in defining more effective prevention and treatment strategies for addiction.

The effects of a single session of inescapable tailshock on the subsequent locomotor response to brief footshock and cocaine administration in rats

RATIONALE

We have previously shown that exposure to a single session of inescapable (IS), but not escapable (ES), tailshock can sensitize the subsequent conditioned place preference and locomotor responses to opioids, but not other drug classes. However, prior work suggests that IS might sensitize nonopioid drug responding if the drug were to be preceded by a mild stressor.

OBJECTIVES

In the following experiments, we examined the effects of IS and ES on the subsequent locomotor response to brief footshock and/or cocaine administration.

METHODS

First, we measured the locomotor response to cocaine (0, 1, 5, 10 mg/kg, intraperitoneally) 48 h after a single session of IS in adult, male Sprague-Dawley rats. Then, this procedure was repeated with 10 mg/kg cocaine, except that half of the rats received two footshocks immediately before drug administration. Finally, we manipulated the escapability of the initial stressor, as rats received either ES or yoked IS 48 h prior to footshock and cocaine administration.

RESULTS

IS did not affect the subsequent locomotor response to cocaine, but did enhance this response when cocaine administration was immediately preceded by two footshocks. The footshocks alone were without effect. This sensitizing effect was dependent on the escapability of the initial stressor, as ES did not alter the locomotor response to footshock and cocaine administration.

CONCLUSIONS

These results indicate that acute exposure to IS, but not ES, can sensitize the locomotor response to cocaine 48 h later, but only when cocaine administration is immediately preceded by a brief stressor.

The effects of a single exposure to uncontrollable stress on the subsequent conditioned place preference responses to oxycodone, cocaine, and ethanol in rats

RATIONALE

Acute stress has been shown to facilitate the rewarding effects of a number of commonly abused drugs, although the stressor typically must be administered either immediately before or during drug administration and often in the same environment. We have previously reported that a single session of an uncontrollable (inescapable tailshock, IS), but not controllable (escapable tailshock, ES), stressor can enhance the conditioned place preference (CPP) response to morphine, even when stressor and drug administration are separated temporally and spatially. However, this persistent, trans-situational enhancement did not occur to amphetamine CPP.

OBJECTIVES

The following experiments were conducted to determine whether the long-term effects of IS on drug reward are specific to opioids.

MATERIALS AND METHODS

Adult, male Sprague-Dawley rats were exposed to a single session of IS or remained in their home cages (HC). Twenty-four hours later, using an unbiased procedure, CPP conditioning was conducted with either oxycodone (0, 2, or 5 mg/kg, sc), cocaine (0, 1, 5, or 10 mg/kg, ip), or ethanol (0.3, 1, or 2 g/kg, ip). Another group of rats were exposed to IS, ES, or HC treatment and conditioned with oxycodone (5 mg/kg, sc) 24 h later.

RESULTS

IS enhanced the subsequent CPP response to oxycodone, but not cocaine or ethanol. This enhancement was dependent on the controllability of the stressor, as ES did not affect oxycodone CPP.

CONCLUSIONS

These results indicate that the long-term, trans-situational enhancing effect of uncontrollable stress on drug reward is specific to opioids.

Association analysis of the protein phosphatase 1 regulatory subunit 1B (PPP1R1B) gene with nicotine dependence in European- and African-American smokers

The protein phosphatase 1 regulatory subunit 1B gene (PPP1R1B; also known as dopamine and cAMP-regulated phosphoprotein; DARPP32) is a target for the actions of dopamine. Because the mesolimbic dopaminergic system is implicated in the reinforcing effects of many drugs, including nicotine, PPP1R1B is considered a plausible candidate for involvement in the development of vulnerability to nicotine dependence (ND). Further, this gene is located within a region on chromosome 17 that demonstrated "suggestive linkage" to ND in our previous genome-wide scan. In the present study, we analyzed six single nucleotide polymorphisms (SNPs) within PPP1R1B for association with three ND measures: smoking quantity (SQ), the heaviness of smoking index (HSI), and the Fagerström Test for ND (FTND) score. Our sample consisted of 602 nuclear families of African-American (AA) or European-American (EA) origin. No significant associations were found for single SNPs after correction for multiple testing. However, haplotype analysis indicated that in the EA sample, the C-T-G-C haplotype formed by rs2271309-rs907094-rs3764352-rs3817160 with a frequency of 32.0% was significantly associated with SQ (Z = 2.50; P = 0.01), even after Bonferroni correction. No significant associations with haplotypes were found in the AA sample. In summary, our findings provide the first evidence for the potential involvement of PPP1R1B in the etiology of ND and further investigation is thus warranted.

Mesolimbic dopamine and cortico-accumbens glutamate afferents as major targets for the regulation of the ventral striato-pallidal GABA pathways by neurotensin peptides

The tridecapeptide neurotensin (NT) acts in the mammalian brain as a primary neurotransmitter or neuromodulator of classical neurotransmitters. Morphological and functional in vitro and in vivo studies have demonstrated the existence of close interactions between NT and dopamine both in limbic and in striatal brain regions. Additionally, biochemical and neurochemical evidence indicates that in these brain regions NT plays also a crucial role in the regulation of the aminoacidergic signalling. It is suggested that in the nucleus accumbens the regulation of prejunctional dopaminergic transmission induced by NT may be primarily due to indirect mechanism(s) involving mediation via the aminoacidergic neuronal systems with increased glutamate release followed by increased GABA release in the nucleus accumbens rather than a direct action of the peptide on accumbens dopaminergic terminals. The neurochemical profile of action of NT in the control of the pattern of dopamine, glutamate and GABA release in the nucleus accumbens differs to a substantial degree from that shown by the peptide in the dorsal striatum. The neuromodulatory NT mechanisms in the regulation of the ventral striato-pallidal GABA pathways are discussed and their relevance for schizophrenia is underlined.

Blockade of GABA(A) receptors within the extended amygdala attenuates D(2) regulation of alcohol-motivated behaviors in the ventral tegmental area of alcohol-preferring (P) rats

The dopamine (DA) mesolimbic pathway, which originates from DA cell bodies within the ventral tegmental area (VTA), has been shown by various studies to play a role in the mediation of various drugs of abuse including alcohol (EtOH). It has been suggested that the VTA's control of EtOH reward is mediated in part by the D2 receptors within the VTA. These receptors may be under the regulation of reciprocal GABAergic inputs from forebrain components of the mesolimbic path such as the nucleus accumbens (NAcc), a classic EtOH reward substrate, and the bed nucleus of the stria terminalis, a substrate recently implicated in EtOH reinforcement, forming a self-regulating feedback loop. To test this hypothesis, D2 regulation of EtOH self-administration (SA) was evaluated by the microinfusion of the D2 antagonist eticlopride into the VTA of P rats, which produced profound reductions in EtOH SA in the highest (20.0 and 40.0microg) doses tested in both BST/VTA and NAcc/VTA implanted P rats. To determine the role of GABA in the mediation of EtOH SA, a 32.0ng dose the non-selective GABA antagonist SR 95531 was microinfused into the BST producing no effect on responding for EtOH and into the NAcc which lead to a reduction in EtOH responding. Finally, the hypothesis that GABA innervation of the VTA from the mesolimbic forebrain may influence EtOH SA was examined by the simultaneous infusion of eticlopride (40.0microg) into the VTA and SR 95531 (32.0ng) into either the BST or NAcc. This combination infusion completely attenuated the reduction in EtOH SA observed with the 40.0microg dose of eticlopride alone in both groups of animals. These results suggest that while the D2 receptors within the VTA regulate EtOH-motivated behaviors, this is modulated by GABAergic input from the mesolimbic forebrain, specifically from the BST and NAcc.

Topographical organization of GABAergic neurons within the ventral tegmental area of the rat

The ventral tegmental area (VTA), the origin of dopaminergic cell bodies that comprise the mesocorticolimibic DA system, is widely implicated in drug and natural reward, cognition, and several psychiatric disorders. In addition to dopaminergic neurons, this region is populated by GABAergic neurons, which both regulate the firing of their dopaminergic counterparts and send projections throughout the brain. Although the dopaminergic neurons of the VTA have been extensively characterized neuroanatomically, much less is known about the GABAergic neurons in this region. Recent data suggest that the rostro-caudal topographic organization of these GABAergic neurons may correspond to their ability to regulate drug reward. In the present study, we used immunohistochemical techniques to examine the frequency and topography of GABAergic neurons throughout the rostro-caudal axis of the VTA and the extent to which they coexpress other proteins, including tyrosine hydroxylase (a marker of DA neurons), cholecystokinin, parvalbumin, calretinin, and calbindin d 28k.

Rostral-caudal differences in the effects of intra-VTA muscimol on cocaine self-administration

We have found that dopamine (DA) in the ventral tegmental area (VTA) plays an important role in cocaine self-administration. DA in the VTA acts at D1-type receptors on the terminals of GABA afferents causing release of this neurotransmitter. Thus, the neurochemical pathways whereby VTA DA might be involved in cocaine self-administration may include GABA neurotransmission. In the present study, we investigated this possibility. Rats were prepared with intravenous catheters and bilateral guide cannulae positioned to allow microinjections directly into the VTA or a site 1 mm dorsal to it. The rats were then trained to self-administer cocaine (1.0 mg/kg/injection) under a fixed-ratio 1 schedule of reinforcement and tested with microinjections of muscimol (0, 0.05 and 0.1 microg/0.25 microl) or picrotoxin (0, 0.025 and 0.05 microg/0.25 microl) or trained under a progressive ratio (PR) schedule and tested with vehicle and 0.05 microg/0.25 microl muscimol. Muscimol in the VTA, but not immediately dorsal to it, significantly reduced cocaine intake under the FR1 schedule. Furthermore, when analyzed by rostral/caudal site of injection, it was found that rostral injections of muscimol significantly reduced cocaine self-administration whereas caudal injections produced non-significant decreases in self-administration. Inspection of individual records revealed no signs of non-specific behavioral effects of the muscimol treatments. Muscimol in the rostral VTA also significantly increased break points in responding under the PR schedule. Intra-VTA picrotoxin did not significantly affect cocaine self-administration. These data suggest that stimulation of GABA-A receptors in the VTA is involved in cocaine self-administration and reward and that this involvement is more pronounced in the rostral than in the caudal VTA.

Difference in magnitude of psychostimulant-induced extracellular norepinephrine in the ventral tegmental area contributes to discrepant prefrontal dopamine outflow

The dopamine (DA) efflux in the medial prefrontal cortex (mPFC) can be modulated by the interaction between afferent norepinephrine (NE) and somatodendritic DA in the ventral tegmental area (VTA). However, it is unclear how locally administered amphetamine (AMPH) or cocaine in the VTA results in discrepant response of DA efflux in the mPFC. In this study, intra-VTA infusion of AMPH (1000 microM) or cocaine (200 microM) in anesthetized rats was employed to study the concurrent profile of extracellular DA level in the VTA and mPFC. In addition, the extracellular NE levels during the intra-VTA infusion of these two psychostimulants were analyzed to compare their effects on prefrontal DA efflux. During the intra-VTA infusion of AMPH, both extracellular DA and NE increased significantly in the VTA (270 +/- 12% and 819 +/- 40%, respectively). Meanwhile, the DA efflux in the mPFC elevated significantly. During the intra-VTA infusion of cocaine, the extracellular DA and NE in the VTA also increased (271 +/- 21% and 150 +/- 15%, respectively). However, the DA efflux decreased significantly in the mPFC. Noteworthy, the increase of extracellular NE in the VTA was much more robust via AMPH infusion, as compared with cocaine. It is suggested that AMPH and cocaine enhance the extracellular NE concentrations in the VTA in different magnitudes, which in turn contribute to discrepant profiles of distal DA efflux in the mPFC.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.

GABA(B) receptor-positive modulation decreases selective molecular and behavioral effects of cocaine

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

Dopamine and monogamy

Social attachments play a central role in human society. In fact, such attachments are so important that deficits in the ability to form meaningful social bonds are associated with a variety of psychological disorders. Although mother-infant bonding has been studied for many years, we only recently have begun to examine the processes that underlie social bonds between adults. Over the past decade, central dopamine has become a focus of such research, especially its role in pair bonding between mates in species that display monogamous life strategies. Neuroanatomical and pharmacological studies in rodents have firmly established central dopamine systems, especially the mesocorticolimbic dopamine circuitry, in the formation, expression, and maintenance of monogamous pair bonds. As this research has progressed, it has become apparent that there is considerable overlap between the processes that underlie pair bonding and those that mediate responses to abused substances. This suggests that social bonding and substance abuse each may affect the other. Herein we review the current state of knowledge of central dopamine involvement in pair bond formation, expression, and maintenance. We first describe the neuroanatomical substrate within which dopamine exerts its effects on social bonding. We then describe dopamine receptor subtype-specific influences on pair bonding and how dopamine receptor activation may interact with activation of other neurochemical systems. Finally, we describe possible interactions between social bonding and substance abuse.

Different roles of dopamine receptor subtypes in footshock stress-induced enhancement of morphine conditioned place preference

The present study investigated the involvement of dopamine mechanism in the effect of intermittent footshock stress on the morphine-induced place preference. A single intermittent footshock session significantly enhanced the place preference induced by 3.0mg/kg morphine. This enhancing effect was inhibited by selective D(1) receptor antagonist SCH23390 and selective D(2) receptor antagonist sulpiride pretreatment 20min before footshock session, suggesting dopamine D(1) and D(2) receptors are required for the development of intermittent footshock stress-induced enhancement of morphine-associated place preference. However, different from D(1) and D(2) receptors this enhancing effect was blocked by stimulation of dopamine D(3) receptor with selective D(3) receptor agonist 7-OH-DPAT pretreatment 20min before footshock session which suggest dopamine D(3) receptor play a negative mediation effect on the intermittent footshock stress-induced this enhancement. These results indicate that dopamine D(1), D(2), and D(3) receptor subtypes play different roles in footshock stress-induced enhancement of morphine conditioned place preference.

Interactive effects of the mGlu5 receptor antagonist MPEP and the mGlu2/3 receptor antagonist LY341495 on nicotine self-administration and reward deficits associated with nicotine withdrawal in rats

Stimulatory actions of nicotine on mesocorticolimbic dopamine transmission are partly mediated by nicotine-induced glutamate release acting on ionotropic and metabotropic glutamate (mGlu) receptors. Because both presynaptic inhibitory mGlu2/3 and postsynaptic excitatory mGlu5 receptors provide potential targets for treatment of aspects of nicotine dependence, we examined interacting effects of mGlu5 (2-methyl-6-(phenylethynyl)-pyridine, MPEP) and mGlu2/3 (LY341495) receptor antagonists on nicotine self-administration and brain reward threshold elevations associated with spontaneous nicotine withdrawal in rats. We hypothesized that increasing glutamate transmission by blocking presynaptic inhibitory mGlu2/3 autoreceptors would antagonize MPEP-induced decreases in nicotine self-administration. We also hypothesized that blocking postsynaptic actions of glutamate on mGlu5 receptors would exacerbate nicotine withdrawal-induced reward deficits, and that this effect would be attenuated by co-administration of the mGlu2/3 receptor antagonist LY341495. MPEP selectively decreased nicotine, but not food, self-administration in rats. LY341495 slightly decreased both nicotine and food self-administration. Co-administration of LY341495 with MPEP attenuated the effectiveness of MPEP in decreasing nicotine intake, although MPEP was still effective. Spontaneous nicotine withdrawal induced somatic signs of withdrawal and reward threshold elevations indicating reward deficits. MPEP increased somatic signs and reward deficits in both nicotine- and saline-withdrawing rats. Thus, while mGlu5 receptor antagonists may be therapeutically useful in decreasing tobacco smoking, they worsen nicotine withdrawal. Co-administration of LY341495 reduced MPEP-induced reward deficits in both nicotine- and saline-withdrawing rats. Thus, increasing glutamate transmission via mGlu2/3 autoreceptor blockade reduces the effects of mGlu5 receptor blockade on nicotine self-administration and MPEP-induced exacerbation of brain reward deficits associated with nicotine withdrawal.

Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission

The neurotransmitter dopamine (DA) has a long association with normal functions such as motor control, cognition, and reward, as well as a number of syndromes including drug abuse, schizophrenia, and Parkinson's disease. Studies show that serotonin (5-HT) acts through several 5-HT receptors in the brain to modulate DA neurons in all 3 major dopaminergic pathways. There are at least fourteen 5-HT receptor subtypes, many of which have been shown to play some role in mediating 5-HT/DA interactions. Several subtypes, including the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT3 and 5-HT4 receptors, act to facilitate DA release, while the 5-HT2C receptor mediates an inhibitory effect of 5-HT on DA release. Most 5-HT receptor subtypes only modulate DA release when 5-HT and/or DA neurons are stimulated, but the 5-HT2C receptor, characterized by high levels of constitutive activity, inhibits tonic as well as evoked DA release. This review summarizes the anatomical evidence for the presence of each 5-HT receptor subtype in dopaminergic regions of the brain and the neuropharmacological evidence demonstrating regulation of each DA pathway. The relevance of 5-HT receptor modulation of DA systems to the development of therapeutics used to treat schizophrenia, depression, and drug abuse is discussed. Lastly, areas are highlighted in which future research would be maximally beneficial to the treatment of these disorders.

Effects of Prenatal Ethanol Exposure on the Excitability of Ventral Tegmental Area Dopamine Neurons in Vitro

Prenatal ethanol exposure leads to a persistent reduction in the number of spontaneously active dopaminergic (DA) neurons (DA neuron population activity) in the ventral tegmental area (VTA) in developing and adult animals. This effect might contribute to the dysfunction of the mesolimbic/cortical DA system and attention problems in children with fetal alcohol spectrum disorders. To characterize the underlying cellular mechanism for prenatal ethanol exposure-induced reduction in VTA DA neuron population activity, we used the whole-cell patch-clamp technique to study the membrane properties of putative VTA DA neurons in brain slices in 2- to 3-week-old control and prenatal ethanol-exposed animals. The results show that prenatal ethanol exposure did not impair the spontaneous pacemaker activity in putative VTA DA neurons but reduced the frequency of evoked action potentials. In addition, prenatal ethanol exposure led to a reduction in hyperpolarization-induced cation current (I(h)) and an up-regulation of somatodendritic DA autoreceptors. The above prenatal ethanol exposure-induced changes could decrease the excitability of VTA DA neurons. However, they do not seem to play a role in reduced VTA DA neuron population activity in vivo, an effect thought to be mediated by excessive excitation leading to depolarization inactivation. Taken together, the above results indicate that prenatal ethanol exposure-induced reduction in VTA DA neuron population activity in vivo is not caused by changes in the intrinsic pacemaker activity or other membrane properties and could instead be caused by altered inputs to VTA DA neurons.

In the ventral tegmental area, progestins have actions at D1 receptors for lordosis of hamsters and rats that involve GABA A receptors

In the ventral tegmental area (VTA), progestins facilitate lordosis via actions at gamma-aminobutyric acid (GABA)(A)/benzodiazepine receptor complexes (GBRs) and dopamine type 1 receptors (D1). The relationship between progestins' actions at GBRs and D1 in the VTA for facilitating sexual behavior of hamsters and rats was examined. Ovariectomized (ovx), estradiol (E(2); 10 microg)+progesterone (P; 250 microg; SC)-primed hamsters, with bilateral guide cannulae to the VTA, were pre-tested for sexual and motor behavior and infused with the GBR antagonist bicuculline (100 ng/side) or vehicle. Thirty minutes later, hamsters were re-tested and then infused with the D1 agonist SKF38393 (100 ng/side) or vehicle. Hamsters were post-tested 30 min later. Ovx, E(2) (10 microg)-primed rats were pre-tested, infused first with bicuculline or vehicle, second with SKF38393 or vehicle, third with 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP; 0, 100, or 200 ng) and were post-tested 10 and 60 min after 3alpha,5alpha-THP infusions. VTA infusions of SKF38393 increased lordosis of hamsters or rats. Bicuculline pretreatment reduced SKF38393- and/or progesterone-mediated increases in lordosis of E2-primed hamsters. In E2-primed rats, bicuculline blocked SKF38393- and/or 3alpha,5alpha-THP-mediated increases in lordosis. There were no effects on motor behavior. Thus, in the VTA, GBR activity modulates D1-mediated actions for lordosis of hamsters and rats.

Neurotensin afferents of the ventral tegmental area in the rat: [1] re-examination of their origins and [2] responses to acute psychostimulant and antipsychotic drug administration

The ventral tegmental area (VTA) is involved in reward-related behaviours and the actions of psychostimulant drugs. It is influenced by afferents expressing a variety of neurotransmitters and neuromodulators; the innervation containing neurotensin is among the densest of these. Intra-VTA neurotensin activates dopaminergic neurons and plays an important role in the development of behavioural sensitization to psychostimulant drugs and possibly in schizophrenia. Using gold-coupled wheatgerm agglutinin as retrograde tracer in combination with nonisotopic in situ hybridization for neurotensin mRNA or neurotensin antibodies after colchicine treatment, the present study was undertaken to demonstrate the neurotensinergic neurons projecting to the VTA and determine whether (and in which subpopulations) neurotensin expression is regulated in VTA-projecting neurons after administrations of the psychostimulant drug methamphetamine or the antipsychotic haloperidol. This study reveals the lateral preoptico-rostral lateral hypothalamic continuum and the medial preoptic area as main sources for the neurotensin afferents of the VTA. Fewer neurotensinergic, VTA-projecting neurons are situated in the dorsal raphe, pedunculopontine and laterodorsal tegmental nuclei, lateral hypothalamic area, ventral endopiriform area, lateral septum, accumbens shell, parabrachial nucleus and different parts of the extended amygdala. The number of neurotensinergic VTA-projecting neurons increased significantly only after methamphetamine administration and exclusively in the accumbens shell. It is concluded that the widespread neurotensinergic VTA-projecting neurons, situated in areas involved in different reward-related behaviours, are well suited to convey distinct reward information to the VTA. The up-regulation of neurotensin expression selectively in VTA-projecting neurons in the accumbens shell following methamphetamine administration may be an important factor in the development of behavioural sensitization.

Degeneration of dopaminergic mesocortical neurons and activation of compensatory processes induced by a long-term paraquat administration in rats: implications for Parkinson's disease

A deficiency of the dopaminergic transmission in the mesocortical system has been suggested to contribute to cognitive disturbances in Parkinson's disease. Therefore, the aim of the present study was to examine whether the long-term administration of a commonly used herbicide, paraquat, which has already been found to induce a slowly progressing degeneration of the nigrostriatal neurons, influences mesocortical dopaminergic neurons in rats. Paraquat at a dose of 10 mg/kg i.p. was injected either acutely or once a week for 4, 8, 12 and 24 weeks. Acute treatment with this pesticide increased the level of homovanillic acid (HVA) and HVA/dopamine ratio in the prefrontal cortex. After 8 weeks of administration paraquat increased the number of stereologically counted tyrosine hydroxylase-immunoreactive (TH-ir) neurons and their staining intensity in the ventral tegmental area (VTA), which is a source of the mesocortical dopaminergic projection. At the same time, few TH-ir neurons appeared in different regions of the cerebral cortex: in the frontal, cingulate, retrosplenial and parietal cortices. Chronic paraquat administration did not influence the level of dopamine in the prefrontal cortex but increased the levels of its metabolites: 3,4-dihydroxyphenylacetic acid (after 8-12 weeks), HVA (after 4 and 12 weeks) and HVA/dopamine ratio (4 weeks). After 24 weeks this pesticide reduced the number of TH-ir neurons in the VTA by 42% and of the Nissl-stained neurons by 26%, and induced shrinkage of this structure by ca. 25%. Moreover, TH-ir neurons in the cortex were no more visible after such a long period of administration and levels of dopamine metabolites returned to control values. The present results suggest that the long-term paraquat administration destroys dopaminergic neurons of the VTA. However, compensatory activation of the VTA neurons and cortex overcomes progressing degeneration and maintains cortical dopaminergic transmission.

Reduced psychostimulant effects on dopamine dynamics in the nucleus accumbens of mu-opioid receptor knockout mice

Dopamine neurotransmission in the nucleus accumbens plays a pivotal role in the reinforcing properties of drugs of abuse. Two interacting processes regulate nucleus accumbens dopamine overflow: release of dopamine from presynaptic terminals and the subsequent reuptake by dopamine transporters. Opioid neurotransmission, primarily through mu-opioid receptors has also been strongly implicated in drug reward. We have previously shown that mice lacking the mu-opioid receptor display decreased cocaine self-administration. In addition, we found decreased impulse activity of midbrain dopaminergic neurons and an increased GABAergic input to these neurons in mu-opioid receptor knockout mice. In the present study we investigated whether these changes in dopaminergic cell bodies are accompanied by altered dopamine dynamics at the terminal level. To that aim, we measured nucleus accumbens dopamine overflow using fast scan cyclic voltammetry. Our data demonstrate that in mu-opioid receptor knockout mice 1) the reuptake of dopamine in the nucleus accumbens is slower, and 2) the relative effect of cocaine and amphetamine on the reuptake of dopamine is smaller compared with wild type mice. These data provide a mechanism for the decreased reinforcing properties of cocaine observed in mu-opioid receptor knockout mice.

Effects of chronic alcohol and repeated deprivations on dopamine D1 and D2 receptor levels in the extended amygdala of inbred alcohol-preferring rats

BACKGROUND

Dopaminergic (DA) activity in the extended amygdala (EA) has been known to play a pivotal role in mediating drug and alcohol addiction. Alterations of DA activity within the EA after chronic exposure to alcohol or substances of abuse are considered a major mechanism for the development of alcoholism and addiction. To date, it is not clear how different patterns of chronic alcohol drinking affect DA receptor levels. Therefore, the current studies investigated the effects of chronic ethanol consumption, with or without deprivations, on D1 and D2 receptor densities within the EA.

METHODS

Inbred alcohol-preferring (iP) rats were divided into 3 groups with the following treatments: (1) water for 14 weeks; (2) continuous alcohol (C-Alc) for 14 weeks [24-hour concurrent access to 15 and 30% (v/v) ethanol]; or (3) repeatedly deprived of alcohol (RD-Alc) (24-hour concurrent access to 15 and 30% ethanol for 6 weeks, followed by 2 cycles of 2 weeks of deprivation of and 2 weeks of reexposure to ethanol access). At the end of 14 weeks, the rats were killed for autoradiographic labeling of D1 and D2 receptors.

RESULTS

Compared with the water control group, both the C-Alc and the RD-Alc groups displayed increases in D1 receptor binding density in the anterior region of the Acb core, whereas the RD-Alc group displayed additional increases in D1 receptor binding density in anterior regions of the lateral and intercalated nuclei of the amygdala. Additionally, both C-Alc and RD-Alc rats displayed increases in D2 receptor binding density in anterior regions of the Acb shell and core, whereas RD-Alc rats displayed additional increases in D2 receptor binding density in the dorsal striatum.

CONCLUSION

The results of this study indicate that 14-week extended alcohol drinking with continuous chronic or repeated deprivations increase binding sites of D1 and D2 receptors in specific regions of the EA with greater sensitivity in the anterior regions. The repeated deprivation has greater effect on altering D1 and D2 receptor binding sites in the Acb, dorsal striatum, and subamygdala regions. The current result indicates that the two drinking paradigms may have common as well as differential mechanisms on alteration of dopamine receptor-binding sites in specific regions of the EA.

Significant association of catechol-O-methyltransferase (COMT) haplotypes with nicotine dependence in male and female smokers of two ethnic populations

The catechol-O-methyltransferase (COMT) gene plays a prominent role in dopaminergic circuits central to drug reward. Allelic variants within the COMT gene are therefore potential candidates for examining interindividual differences in vulnerability to nicotine dependence (ND). We analyzed five single nucleotide polymorphisms (SNPs), including the Val/Met variant (rs4680), which results in a three- to four-fold difference in enzyme activity within COMT, for association with the three ND measures, SQ, HSI, and FTND, in 602 nuclear families of African-American (AA) or European-American (EA) origin. The Val/Met variant showed a significant association with the three ND measures in the pooled and EA samples and with FTND in the AA sample. Haplotype analysis revealed a major protective A-G-T haplotype (frequency 23.6%) for rs740603-rs4680-rs174699 in the AA sample (minimum Z=-3.35; P=0.0005 for FTND), a major protective T-G-T haplotype (frequency 15.2%; minimum Z=-2.92; P=0.003 for FTND) in the EA sample, and a high-risk C-A-T haplotype (frequency 16.9%; minimum Z=3.16; P=0.002 for SQ) in the AA sample for rs933271-rs4680-rs174699. Furthermore, we found that the significant haplotypes within COMT were gender-specific and the significantly associated A-G-T is protective in AA females only, whereas T-G-T is protective in EA males only. Moreover, we found a major high-risk T-A-T haplotype (frequency 56.7%) that showed significant association with the three ND measures in EA males. Further examination of two protective haplotypes, A-G-T in AAs and T-G-T in EAs, indicated that the low COMT enzyme activity Met allele is protective to become nicotine dependent. In summary, our results provide evidence for a role of COMT in the susceptibility to ND and further confirm that its effect is ethnic and gender specific.

Treatment for psychological dependence on morphine: usefulness of inhibiting NMDA receptor and its associated protein kinase in the nucleus accumbens

A growing body of evidence indicates that the mesolimbic dopaminergic (DAergic) pathway projecting from the ventral tegmental area (VTA) to the nucleus accumbens (N.Acc.) play a critical role in the initiation of psychological dependence on morphine. As well as DAergic system, the involvement of non-DAergic neurotransmitter and neuromodulator systems in rewarding effects induced by morphine has been recently documented. We previously demonstrated that the morphine-induced rewarding effect was dramatically suppressed by co-treatment with NMDA receptor antagonists, such as dizocilpine (MK-801), ketamine and ifenprodil. Therefore, we propose here that inhibiting the N-methyl-D-aspartate (NMDA) receptor and its associated protein kinase in the N.Acc. is useful for the treatment for psychological dependence on morphine. The following review provides a summary of recent our findings regarding the role of NMDA receptor and its associated protein kinase in the development of psychological dependence on morphine.

Increased mRNA levels of tyrosine hydroxylase and dopamine transporter in the VTA of male rats after chronic food restriction

Dieting as a strategy to reduce body weight often fails as it causes food cravings leading to bingeing and weight regain. Evidence from several lines of research suggests the presence of shared elements for neural regulation of food and drug craving. We quantified the expression of eight genes involved in dopamine signalling in brain regions related to the mesolimbic and nigrostriatal dopamine system in male rats subjected to chronic food restriction using quantitative real-time polymerase chain reaction. Food restriction strongly increased mRNA levels of tyrosine hydroxylase and the dopamine transporter in the ventral tegmental area. Quantitative autoradiography indicated that the dopamine transporter was also upregulated at the protein level in the shell of the nucleus accumbens. However, these effects were not observed after acute food deprivation. We suggest that the results reflect a sensitization of the mesolimbic dopamine pathway characterized by increased clearance of extracellular dopamine in the nucleus accumbens shell. Such sensitization of the mesolimbic dopamine system may be one of the underlying causes for the food cravings that interfere with dietary compliance.

Ionotropic glutamatergic neurotransmission in the ventral tegmental area modulates ΔFosB expression in the nucleus accumbens and abstinence syndrome in morphine withdrawal rats

The present study sought to assess whether the blockade of ionotropic glutamate receptors in the ventral tegmental area could modulate morphine withdrawal in morphine-dependent rats and the expression of stable DeltaFosB isoforms in the nucleus accumbens during morphine withdrawal. Rats were injected (i.p.) with increasing doses of morphine for 1 week to develop physical dependence, and withdrawal was then precipitated by one injection of naloxone (2 mg/kg, i.p.). Abstinence signs such as jumping, wet-dog shake, writhing posture, weight loss, and Gellert-Holtzman scale score were recorded to evaluate naloxone-induced morphine withdrawal. Two ionotropic glutamate receptor antagonists, dizocilpine (MK-801) and 6, 7-dinitroquinnoxaline-2, 3-dione (DNQX), were microinjected unilaterally into the ventral tegmental area 30 min before naloxone precipitation. A second injection of naloxone (2 mg/kg i.p.) was given 1 h after the first naloxone injection to sustain a maximal level of withdrawal so that the expression of stable DeltaFosB isoforms in the nucleus accumbens could be measured. This would enable determination of the correlation between the MK-801 or DNQX-induced decrease in somatic withdrawal signs and the change in neuronal activity in the nucleus accumbens. The results showed that both MK-801 and DNQX significantly alleviated all symptoms of morphine withdrawal except for weight loss and reduced the expression of stable DeltaFosB isoforms within the nucleus accumbens. These data suggest that ionotropic glutamatergic neurotransmission in the ventral tegmental area regulates the levels of stable DeltaFosB isoforms in the nucleus accumbens, which play a very important role in modulating opiate withdrawal.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

3alpha-hydroxy-5alpha-pregnan-20-one in the midbrain ventral tegmental area mediates social, sexual, and affective behaviors

Progestins mediate the onset and duration of lordosis, the mating posture of female rodents, through actions in the hypothalamus and ventral tegmental area. In the hypothalamus, progesterone has traditional, "genomic" actions via intracellular progestin receptors. In the ventral tegmental area, 3alpha-hydroxy-5alpha-pregnan-20-one has "non-genomic" actions independent of progestin receptors to facilitate lordosis that involve GABA(A)/benzodiazepine receptors, NMDA type glutamate receptors, and/or dopamine receptors. 3alpha-Hydroxy-5alpha-pregnan-20-one levels also change with behavioral and/or environmental stimuli and may have a role in other reproductively-relevant behaviors, such as affiliation, exploration, and anxiety (socio-sexual behaviors). Data are reviewed that support the notion that: 1) effects of 3alpha-hydroxy-5alpha-pregnan-20-one in the midbrain ventral tegmental area facilitate lordosis and other reproductively-relevant behaviors. 2) 3alpha-Hydroxy-5alpha-pregnan-20-one, formed in the ventral tegmental area from metabolism of progestins, produced peripherally by endocrine glands, or centrally from biosynthesis in glial cells mediates socio-sexual behaviors. 3) 3alpha-Hydroxy-5alpha-pregnan-20-one's actions at GABA(A)/benzodiazepine receptors, NMDA type glutamate receptors, and dopamine receptors in the ventral tegmental area are important for lordosis; however, effects at these substrates on socio-sexual behaviors have not been elucidated. Given 3alpha-hydroxy-5alpha-pregnan-20-one's involvement in stress responses, its putative role as a homeostatic regulator and in the pathophysiology and treatment of neuropsychiatric disorders is discussed.

Overeating after restraint stress in cholecystokinin-a receptor-deficient mice

In mammals, including humans, a brain-gut hormone, cholecystokinin (CCK) mediates the satiety effect via CCK-A receptor (R). We generated CCK-AR gene-deficient (-/-) mice and found that the daily food intake, energy expenditure, and gastric emptying of a liquid meal did not change compared with those of wild-type mice. Because CCK-AR(-/-) mice show anxiolytic status, we examined the effects of restraint stress. Seven hours of restraint stress was found to significantly decrease both body weight and food intake during the subsequent 3 days in all tested animals. On the fourth day after restraint stress, the CCK-AR(-/-) mice showed a significantly higher level of daily food intake than prior to stress, and food intake recovered to prestress levels in the wild-type mice. Since peripheral CCK-AR has been known to mediate gastric emptying, both gastric emptying and gastric acid secretion were determined to examine the mechanism of overeating in CCK-AR(-/-) mice. Neither gastric emptying nor gastric acid secretion differed between CCK-AR(-/-) and wild-type mice on the fourth day after stress. In contrast, however, the contents of dopamine and its metabolites in the cerebral cortex of CCK-AR(-/-) mice were increased by stress, but were rather decreased in wild-type mice. Changes in 5-hydroxytryptamine (5-HT) and its metabolite 5HIAA did not differ between the genotypes. In conclusion, CCK-AR(-/-) mice showed overeating after restraint stress, and dopaminergic hyperfunction in the brain of these mice was observed. The present evidence suggests that the CCK-AR function, possibly via altering the dopaminergic function, might be involved in overeating after stress.

Different effects of GABAergic receptors located in the ventral tegmental area on the expression of morphine-induced conditioned place preference in rat

In the present study, an unbiased conditioned place preference paradigm was used to study the effects of intra-ventral tegmental area injections of Gama-amino-butyric acid (GABA)-A and B (GABA(A) and GABA(B)) receptor agonists and antagonists on the expression of morphine-induced conditioned place preference (CPP) in rats. Subcutaneous (s.c.) injections of morphine sulfate (5 mg/kg) induced CPP. Intra-ventral tegmental area administration of the GABA(A) receptor agonist, muscimol (6 microg/rat) reduced the expression of morphine-induced CPP. Muscimol (25 microg/rat) increased the expression of CPP induced by morphine. A reduction of the expression of morphine-induced CPP was observed on intra-ventral tegmental area injection of GABA(A) receptor antagonist bicuculline (25 microg/rat). Bicuculline (10 microg/rat) increased the expression of CPP induced by morphine. Baclofen (12 microg/rat) increased where as (19 and 25 microg/rat) reduced the expression of morphine-induced CPP. Injection of CGP38345 (10, 19, 25 and 50 microg/rat) into the ventral tegmental area significantly reduced the expression of CPP induced by morphine. It is concluded that GABA(A) and GABA(B) receptor subtypes within the ventral tegmental area may have different effects on the expression of morphine-induced CPP.

Intracerebroventricular d-Pen2, d-Pen5-enkephalin administration soon after stressor imposition influences behavioral responsivity to a subsequent stressor encounter in CD-1 mice

Endogenous opioid peptide systems diminish stress-induced autonomic nervous system, neuroendocrine (hypothalamic-pituitary-adrenal axis) and behavioral responses, attenuating a collection of physiological symptoms basic to emotional and affective states. Neurogenic stressors may incite specific central changes in opioid peptide availability as well as changes in mu and delta-opioid receptor function. The present investigation evaluated the proactive influence of an intracerebroventricular injection of the opioid receptor agonist D-Pen2, D-Pen5-enkephalin (DPDPE) (0 microg, 0.005 microg, 1.0 microg or 2.5 microg) on locomotor behavior of mice following uncontrollable footshock (Shock) or novel shock chamber exposure (No Shock). It was expected that DPDPE administration following Shock on Day 1 would restore locomotor activity up to 1 week and prevent shock-associated behavior of mice encountering a brief session of footshock 18 days later. Exposure to Shock reduced horizontal locomotor and vertical locomotor (rearing) activity of mice while 2.5 microg DPDPE restored behavior. Eighteen days following Shock and DPDPE challenge, mice were exposed to either an abbreviated session of footshock (Mild Stress) or the shock chamber (Cues). Mice in the No Shock and Shock groups administered 2.5 microg DPDPE on Day 1 did not exhibit any locomotor deficits in response to Mild Stress on Day 18. Mice in the Shock group administered 0.005 microg DPDPE on Day 1, did not exhibit exaggerated rearing deficits following ensuing Mild Stressor encounter relative to mice reexposed to Cues on Day 18. Taken together, these data show that (a) footshock differentially affects rearing and locomotor activity, (b) DPDPE administration increases locomotor activity for up to 1 week following footshock and DPDPE administration, (c) reexposure to Mild Stress affects rearing and locomotor performance differently depending on previous stressor history and DPDPE dose, (d) DPDPE affords long-lasting protection to previously non-stressed mice against the deleterious effects of subsequent mild stress on locomotor activity, while a low dose of DPDE is sufficient to prevent shock-induced sensitization of rearing deficits, 18 days following original stressor and drug presentation. Finally, our investigation demonstrates that DPDPE administration alters the behavioral impact of future stressful encounters and emphasizes the importance of investigating opioid mechanisms in chronic stress disorders.

Prefrontal GABA levels in cocaine-dependent subjects increase with pramipexole and venlafaxine treatment

RATIONALE

There is evidence that prefrontal lobe GABA levels are low in cocaine-dependent (CD) individuals, and treatment with GABA agonists decreases cocaine self-administration.

OBJECTIVES

The aim of the study is to measure changes in GABA levels in CD subjects at baseline and after 8 weeks of treatment with pramipexole, venlafaxine, or placebo.

METHODS

CD subjects enrolled in a treatment trial for cocaine dependence were recruited for this proton (1H) magnetic resonance spectroscopy (MRS) study. GABA levels in the prefrontal lobe were measured before and after treatment.

RESULTS

Mean percentage changes in GABA levels were as follows: pramipexole +17.0+/-28.0%, venlafaxine +13.0+/-11.0%, and placebo -2.1+/-19.5%. Pramipexole-treated subjects had significantly increased brain GABA levels compared to placebo (p=0.031). Venlafaxine treatment was nonsignificantly associated with increased GABA levels compared to placebo (p=0.16). The overall statistical model for the effect of drug treatment vs placebo on brain GABA levels, including adjustment for baseline levels, was highly significant (p=0.002). Despite significant changes in GABA levels, there were no significant differences in the number of urine samples positive for cocaine metabolites.

CONCLUSIONS

This study demonstrates that 1H MRS can measure changes in GABA levels following pharmacologic treatment. The increase in GABA levels, although significant, is modest compared to other MRS studies of depression or epilepsy associated with clinical improvements. The failure to see larger increases in GABA levels and an associated reduction in cocaine consumption may reflect the relatively low doses of medication used.

Restraint increases dopaminergic burst firing in awake rats

In anesthetized animals, dopamine neurons fire in tonic and phasic firing modes hypothesized to be regulated by dissociable circuit mechanisms. Salient events critical to learning, reward processing, and attentional selection elicit transient phasic bursts. It is unclear, however, how burst activity contributes to sustained firing patterns in awake animals and if behavioral conditions known to affect dopaminergic neurotransmission change impulse activity levels. Acute stress is known to increase extracellular dopamine in the striatum and the prefrontal cortex. In this study, we have used multiunit recording to define and follow activity patterns in single dopaminergic neurons across days and to determine how restraint, a model of acute stress, changes tonic and phasic firing patterns. Long-term recording shows that a population of 23 putative dopamine neurons has heterogeneous firing profiles under baseline conditions. In all, 62% showed significant burst activity under resting conditions, while others showed predominantly regular (17%) or random (21%) activity patterns. Restraint increased mean firing rate in all dopamine neurons, but preferentially increased burst firing in neurons with higher burst rates under resting conditions. Finally, we show that increased burst firing can persist 24 h after a single exposure to stress. These data indicate that subsets of dopamine neurons may be sensitive to circuit mechanisms activated by stress and that persistent changes in burst firing may be evidence of synaptic plasticity. Furthermore, increased burst firing may be a mechanism through which stress augments extracellular dopamine in selected terminal regions.