Dopamine receptor supersensitivity

Reinstatement of nicotine conditioned place preference in a transgenerational model of drug abuse vulnerability in psychosis: Impact of BDNF on the saliency of drug associations

RATIONALE

Psychotic disorders such as schizophrenia are often accompanied by high rates of cigarette smoking, reduced quit success, and high relapse rates, negatively affecting patient outcomes. However, the mechanisms underlying altered relapse-like behaviors in psychosis are poorly understood.

OBJECTIVES

The present study analyzed changes in extinction and reinstatement of nicotine conditioned place preference (CPP) and resulting changes in brain-derived neurotrophic factor (BDNF) in a novel heritable rodent model of psychosis, demonstrating increased dopamine D₂ receptor sensitivity, to explore mechanisms contributing to changes in relapse-like behaviors.

METHODS

Male and female offspring of two neonatal quinpirole-treated (1 mg/kg quinpirole from postnatal day (P)1-21; QQ) and two neonatal saline-treated (SS) Sprague-Dawley rats (F1 generation) were tested on an extended CPP paradigm to analyze extinction and nicotine-primed reinstatement. Brain tissue was analyzed 60 min after the last nicotine injection for BDNF response in the ventral tegmental area (VTA), the infralimbic (IfL) and prelimbic (PrL) cortices.

RESULTS

F1 generation QQ offspring demonstrated delayed extinction and more robust reinstatement compared to SS control animals. In addition, QQ animals demonstrated an enhanced BDNF response to nicotine in the VTA, IfL and Prl cortices compared to SS offspring.

CONCLUSIONS

This study is the first to demonstrate altered relapse-like behavior in a heritable rodent model with relevance to comorbid drug abuse and psychosis. This altered pattern of behavior is hypothesized to be related to elevated activity-dependent BDNF in brain areas associated with drug reinforcement during conditioning that persists through the extinction phase, rendering aberrantly salient drug associations resistant to extinction and enhancing relapse vulnerability.

Long-lasting behavioral effects of quinpirole exposure on zebrafish

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

Transgenerational evidence of increases in dopamine D2 receptor sensitivity in rodents: Impact on sensorimotor gating, the behavioral response to nicotine and BDNF

BACKGROUND/AIMS

Neonatal quinpirole (NQ) treatment to rats increases dopamine D₂ (DAD₂) receptor sensitivity in adult animals. We investigated if increased DAD₂ sensitivity would be passed to the next (F1) generation, and if these animals demonstrated sensorimotor gating deficits and enhanced behavioral responses to nicotine.

METHODS

Male and female rats were intraperitoneal (IP) administered quinpirole (1 mg/kg) or saline (NS) from postnatal day (P)1-21. Animals were either behaviorally tested (F0) or raised to P60 and mated, creating F1 offspring.

RESULTS

Experiment 1 revealed that F1 generation animals that were the offspring of at least one NQ-treated founder increased yawning behavior, a DAD₂-mediated behavioral event, in response to acute quinpirole (0.1 mg/kg). F1 generation rats also demonstrated increased striatal β arrestin-2 and decreased phospho-AKT signaling, consistent with increased G-protein independent DAD₂ signaling, which was equal to F0 NQ-treated founders, although this was not observed in all groups. RNA-Seq analysis revealed significant gene expression changes in the F1 generation that were offspring of both NQ-treated founders compared to F0 NQ founders and controls, with enrichment in sensitivity to stress hormones and cell signaling pathways. In Experiment 2, all F1 generation offspring demonstrated sensorimotor gating deficits compared to controls, which were equivalent to F0 NQ-treated founders. In Experiment 3, all F1 generation animals demonstrated enhanced nicotine behavioral sensitization and nucleus accumbens (NAcc) brain-derived neurotrophic factor (BDNF) protein. Further, F1 generation rats demonstrated enhanced adolescent nicotine conditioned place preference equivalent to NQ-treated founders conditioned with nicotine.

CONCLUSIONS

This represents the first demonstration of transgenerational effects of increased DAD₂ sensitivity in a rodent model.

The effects of a novel inhibitor of tumor necrosis factor (TNF) alpha on prepulse inhibition and microglial activation in two distinct rodent models of schizophrenia

Increased neuroinflammation has been shown in individuals diagnosed with schizophrenia (SCHZ). This study evaluated a novel immune modulator (PD2024) that targets the pro-inflammatory cytokine tumor necrosis factor-alpha (TNFα) to alleviate sensorimotor gating deficits and microglial activation employing two different rodent models of SCHZ. In Experiment 1, rats were neonatally treated with saline or the dopamine D₂-like agonist quinpirole (NQ; 1 mg/kg) from postnatal day (P) 1-21 which produces increases of dopamine D₂ receptor sensitivity throughout the animal's lifetime. In Experiment 2, rats were neonatally treated with saline or the immune system stimulant polyinosinic:polycytidylic acid (Poly I:C) from P5-7. Neonatal Poly I:C treatment mimics immune system activation associated with SCHZ. In both experiments, rats were raised to P30 and administered a control diet or a novel TNFα inhibitor PD2024 (10 mg/kg) in the diet from P30 until P67. At P45-46 and from P60-67, animals were behaviorally tested on auditory sensorimotor gating as measured through prepulse inhibition (PPI). NQ or Poly I:C treatment resulted in PPI deficits, and PD2024 treatment alleviated PPI deficits in both models. Results also revealed that increased hippocampal and prefrontal cortex microglial activation produced by neonatal Poly I:C was significantly reduced to control levels by PD2024. In addition, a separate group of animals neonatally treated with saline or Poly I:C from P5-7 demonstrated increased TNFα protein levels in the hippocampus but not prefrontal cortex, verifying increased TNFα in the brain produced by Poly I:C. Results from this study suggests that that brain TNFα is a viable pharmacological target to treat the neuroinflammation known to be associated with SCHZ.

Effects of antipsychotics on rumination in patients with first-episode psychosis

BACKGROUND

Rumination is a well-known risk factor for depression. It is also associated with negative and positive symptoms and suicidality in patients suffering from psychosis. However, no studies have addressed the effect of antipsychotics on rumination.

METHODS

Using the Brooding Scale (BS), we investigated the effect of antipsychotics on rumination at the 6-month follow up in patients with first-episode psychosis (n = 257). The relationship between rumination and other clinical variables was explored by conducting a correlation analysis and structural equation modeling (SEM). The clinical characteristics and short-term outcomes were compared between high and low ruminators at 6 months.

RESULTS

Significant reductions in rumination and various clinical variables were observed at the 6-month follow-up. A significant correlation was observed between rumination and the score on the positive subscale of the Positive and Negative Syndrome Scale (PANSS). A direct path between the PANSS score and rumination was identified by SEM. High ruminators had more severe psychopathology, experienced more childhood traumas, and took less exercise than low ruminators. The recovery rate at 6 months was higher in low ruminators than in high ruminators.

CONCLUSIONS

Our findings suggest that antipsychotics are beneficial for reducing rumination in patients with first-episode psychosis. The outcomes at the 6-month follow-up were better in low ruminators than high ruminators.

New roles for dopamine D2 and D3 receptors in pancreatic beta cell insulin secretion

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) has important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic β-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D₂-like receptors including D₂ (D2R) and D₃ (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic β-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in β-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA's inhibition of GSIS. Our results show that the uptake of L-DOPA is essential for establishing intracellular DA stores in β-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which β-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D₂-like receptors modify DA release to modulate GSIS. Lastly, β-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo. These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D₂-like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

Animal models for preclinical Parkinson's research: An update and critical appraisal

Animal models of Parkinson's disease (PD) are essential to investigate pathogenic pathways at the whole-organism level. Moreover, they are necessary for a preclinical investigation of potential new therapies. Different pathological features of PD can be induced in a variety of invertebrate and vertebrate species using toxins, drugs, or genetic perturbations. Each model has a particular utility and range of applicability. Invertebrate PD models are particularly useful for high throughput-screening applications, whereas mammalian models are needed to explore complex motor and non-motor features of the human disease. Here, we provide a comprehensive review and critical appraisal of the most commonly used mammalian models of PD, which are produced in rats and mice. A substantial loss of nigrostriatal dopamine neurons is necessary for the animal to exhibit a hypokinetic motor phenotype responsive to dopaminergic agents, thus resembling clinical PD. This level of dopaminergic neurodegeneration can be induced using specific neurotoxins, environmental toxicants, or proteasome inhibitors. Alternatively, nigrostriatal dopamine degeneration can be induced via overexpression of α-synuclein using viral vectors or transgenic techniques. In addition, protein aggregation pathology can be triggered by inoculating preformed fibrils of α-synuclein in the substantia nigra or the striatum. Thanks to the conceptual and technical progress made in the past few years a vast repertoire of well-characterized animal models are currently available to address different aspects of PD in the laboratory.

p-Chloroamphetamine-Enhanced Neostriatal Dopamine Exocytosis in Rats Neonatally Co-lesioned with 6-OHDA and 5,7-DHT: Relevance to Parkinson's Disease

Serotoninergic nerves are known to modulate sensitization of dopamine receptors (DA-R) in a rodent model of Parkinson's disease (PD). However, serotoninergic nerves are not known to have a prominent role on DA exocytosis in intact rats. The current study was undertaken to explore the possible influence of serotoninergic nerves on DA exocytosis in Parkinsonian rats. Rat pups were treated at 3 days after birth with the neurotoxin 6-hydroxydopamine (6-OHDA; 134 μg icv, half into each lateral ventricle; desipramine, 1 h pretreatment), in order to produce marked long-lasting destruction of neostriatal dopaminergic innervation, as evidenced by the 90-95% depletion of DA (p < 0.001) [HPLC/ED] into adulthood. Controls received vehicle/desipramine in place of 6-OHDA. Other groups received the serotoninergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 25 μg base, icv, half in each lateral ventricle; desipramine, 1 h; 75 mg/kg pargyline HCl, 30 min) at 3 days post-birth; or both 6-OHDA+5,7-DHT treatments. In adulthood, an in vivo microdialysis study was undertaken to ascertain that p-chloroamphetamine (PCA, 1 mM in the microdialysate)-evoked DA release in the neostriatum was reduced approximately 50% in the 6-OHDA group, while PCA-evoked DA release in the 6-OHDA+5,7-DHT group was substantially increased, to a level equivalent to that of the vehicle control. The baseline neostriatal microdialysate level of 3,4-dihydroxyphenylacetic acid (DOPAC) was also higher in the 6-OHDA+5,7-DHT group vs 6-OHDA group; also, during the 2nd hour of PCA infusion. PCA-enhanced DA exocytosis occurred in the absence of changes in hydroxyl radical (HO·) in the microdialysate (i.e., assay of 2,3- and 2,5-dihydroxybenzoic acid, 2,3-DHBA; 2,5-DHBA). The overall findings demonstrate that an adulthood serotoninergic nerve lesion enhanced PCA-evoked DA exocytosis in a rodent model of severe PD, while susceptibility to oxidative stress was unchanged. The implication is that serotoninergic nerves may normally suppress the release of DA and/or act as an uptake site and storage sink for accumulated DA in parkinsonian-like neostriatum. Potentially, serotoninergic agonists or antagonists, targeting subtype-selective serotonin receptors, may be viable therapeutic adjuncts in PD.

Effects of an adenosine A2A agonist on the rewarding associative properties of nicotine and neural plasticity in a rodent model of schizophrenia

BACKGROUND

Adenosine A_2a receptors form a mutually inhibitory heteromeric complex with dopamine D₂ receptors such that each receptor exhibits lower sensitivity to its agonist after the opposing receptor agonist is bound. This study analyzed the effects of CGS 21680, an adenosine A_2A agonist, on nicotine conditioned place preference (CPP) in adolescence using a rodent model of schizophrenia (SZ).

METHODS

Rats were treated from postnatal day (P) 1 to P21 with saline or the dopamine D₂/D₃ agonist quinpirole (NQ treatment) and raised to P41. After an initial preference test, rats were conditioned with saline or nicotine (0.6 mg/kg base) from P43 to P51. CGS 21680 (0.03 or 0.09 mg/kg) was given 15 minutes before nicotine was administered. The post-conditioning test was administered on P52. On P53, the nucleus accumbens (NAcc) was analyzed for brain-derived neurotrophic factor (BDNF) and glial cell-lined neurotrophic factor (GDNF).

RESULTS

Results revealed that NQ treatment enhanced nicotine CPP, and both doses of CGS 21680 alleviated this enhancement. Nicotine also resulted in a CPP in controls, which was alleviated by both doses of CGS 21680. BDNF closely followed the behavioral results: CGS 21680 alleviated the enhancement in NAcc BDNF in NQ-treated animals, and eliminated the increase in NAcc BDNF produced by nicotine in controls. NQ-treated animals conditioned to nicotine resulted in an increase of NAcc GDNF, but this was eliminated by CGS 21680. Both BDNF and GDNF correlated with CPP performance.

CONCLUSIONS

Results revealed that an adenosine A_2A agonist decreased the rewarding aspects of nicotine and its accompanying neural plasticity changes in a model of SZ.

Dopamine D2 Receptor Supersensitivity as a Spectrum of Neurotoxicity and Status in Psychiatric Disorders

Abnormality of dopamine D₂ receptor (D₂R) function, often observed as D₂R supersensitivity (D₂RSS), is a commonality of schizophrenia and related psychiatric disorders in humans. Moreover, virtually all psychotherapeutic agents for schizophrenia target D₂R in brain. Permanent D₂RSS as a feature of a new animal model of schizophrenia was first reported in 1991, and then behaviorally and biochemically characterized over the next 15-20 years. In this model of schizophrenia characterized by production of D₂RSS in ontogeny, there are demonstrated alterations of signaling processes, as well as functional links between the biologic template of the animal model and ability of pharmacotherapeutics to modulate or reverse biologic and behavioral modalities toward normality. Another such animal model, featuring knockout of trace amine-associated receptor 1 (TAAR1), demonstrates D₂RSS with an increase in the proportion of D₂R in the high-affinity state. Currently, TAAR1 agonists are being explored as a therapeutic option for schizophrenia. There is likewise an overlay of D₂RSS with substance use disorder. The aspect of adenosine A_2A-D₂ heteroreceptor complexes in substance use disorder is highlighted, and the association of adenosine A_2A receptor antagonists in discriminative and rewarding effects of psychostimulants is outlined. In summary, these new animal models of schizophrenia have face, construct, and predictive validity, and distinct advantages over earlier models. While the review summarizes elements of D₂RSS in schizophrenia per se, and its interplay with substance use disorder, a major focus is on presumed new molecular targets attending D₂RSS in schizophrenia and related clinical entities.

An analysis of the rewarding and aversive associative properties of nicotine in the neonatal quinpirole model: Effects on glial cell line-derived neurotrophic factor (GDNF)

This study analyzed the associative properties of nicotine in a conditioned place preference (CPP) paradigm in adolescent rats neonatally treated with quinpirole (NQ) or saline (NS). NQ produces dopamine D₂ receptor supersensitivity that persists throughout the animal's lifetime, and therefore has relevance towards schizophrenia. In two experiments, rats were ip administered quinpirole (1mg/kg) or saline from postnatal day (P)1-21. After an initial preference test at P42-43, animals were conditioned for eight consecutive days with saline or nicotine (0.6mg/kg free base) in Experiment 1 or saline or nicotine (1.8mg/kg free base) in Experiment 2. In addition, there were NQ and NS groups in each experiment given the antipsychotic haloperidol (0.05mg/kg) or clozapine (2.5mg/kg) before nicotine conditioning. A drug free post-conditioning test was administered at P52. At P53, the nucleus accumbens (NAc) was analyzed for glial cell-line derived neurotrophic factor (GDNF). Results revealed that NQ enhanced nicotine CPP, but blunted the aversive properties of nicotine. Haloperidol was more effective than clozapine at blocking nicotine CPP in Experiment 1, but neither antipsychotic affected nicotine conditioned place aversion in Experiment 2. NQ increased accumbal GDNF which was sensitized in NQ rats conditioned to nicotine in Experiment 1, but the aversive dose of nicotine reduced GDNF in NQ animals in Experiment 2. Both antipsychotics in combination with the aversive dose of nicotine decreased accumbal GDNF. In sum, increased D₂ receptor sensitivity influenced the associative properties and GDNF response to nicotine which has implications towards pharmacological targets for smoking cessation in schizophrenia.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Neuroteratology and Animal Modeling of Brain Disorders

Over the past 60 years, a large number of selective neurotoxins were discovered and developed, making it possible to animal-model a broad range of human neuropsychiatric and neurodevelopmental disorders. In this paper, we highlight those neurotoxins that are most commonly used as neuroteratologic agents, to either produce lifelong destruction of neurons of a particular phenotype, or a group of neurons linked by a specific class of transporter proteins (i.e., dopamine transporter) or body of receptors for a specific neurotransmitter (i.e., NMDA class of glutamate receptors). Actions of a range of neurotoxins are described: 6-hydroxydopamine (6-OHDA), 6-hydroxydopa, DSP-4, MPTP, methamphetamine, IgG-saporin, domoate, NMDA receptor antagonists, and valproate. Their neuroteratologic features are outlined, as well as those of nerve growth factor, epidermal growth factor, and that of stress. The value of each of these neurotoxins in animal modeling of human neurologic, neurodegenerative, and neuropsychiatric disorders is discussed in terms of the respective value as well as limitations of the derived animal model. Neuroteratologic agents have proven to be of immense importance for understanding how associated neural systems in human neural disorders may be better targeted by new therapeutic agents.

Perinatal 6-Hydroxydopamine to Produce a Lifelong Model of Severe Parkinson's Disease

The classic rodent model of Parkinson's disease (PD) is produced by unilateral lesioning of pars compacta substantia nigra (SNpc) in adult rats, producing unilateral motor deficits which can be assessed by dopamine (DA) D₂ receptor (D₂-R) agonist induction of measurable unilateral rotations. Bilateral SNpc lesions in adult rats produce life-threatening aphagia, adipsia, and severe motor disability resembling paralysis-a PD model that is so compromised that it is seldom used. Described in this paper is a PD rodent model in which there is bilateral 99 % loss of striatal dopaminergic innervation, produced by bilateral intracerebroventricular or intracisternal 6-hydroxydopamine (6-OHDA) administration to perinatal rats. This procedure produces no lethality and does not shorten the life span, while rat pups continue to suckle through the pre-weaning period; and eat without impairment post-weaning. There is no obvious motor deficit during or after weaning, except with special testing, so that parkinsonian rats are indistinguishable from control and thus allow for behavioral assessments to be conducted in a blinded manner. L-DOPA (L-3,4-dihydroxyphenylalanine) treatment increases DA content in striatal tissue, also evokes a rise in extraneuronal (i.e., in vivo microdialysate) DA, and is able to evoke dyskinesias. D₂-R agonists produce effects similar to those of L-DOPA. In addition, effects of both D₁- and D₂-R agonist effects on overt or latent receptor supersensitization are amenable to study. Elevated basal levels of reactive oxygen species (ROS), namely hydroxyl radical, occurring in dopaminergic denervated striatum are suppressed by L-DOPA treatment. Striatal serotoninergic hyperinnervation ensuing after perinatal dopaminergic denervation does not appear to interfere with assessments of the dopaminergic system by L-DOPA or D₁- or D₂-R agonist challenge. Partial lesioning of serotonin fibers with a selective neurotoxin either at birth or in adulthood is able to eliminate serotoninergic hyperinnervation and restore the normal level of serotoninergic innervation. Of all the animal models of PD, that produced by perinatal 6-OHDA lesioning provides the most pronounced destruction of nigrostriatal neurons, thus representing a model of severe PD, as the neurochemical outcome resembles the status of severe PD in humans but without obvious motor deficits.

Perinatal 6-Hydroxydopamine Modeling of ADHD

The neonatally 6-hydroxydopamine (n6-OHDA)-lesioned rat has been the standard for 40 years, as an animal model of attention-deficit hyperactivity disorder (ADHD). Rats so lesioned during postnatal ontogeny are characterized by ~99 % destruction of dopaminergic nerves in pars compacta substantia nigra, with comparable destruction of the nigrostriatal tract and lifelong ~99 % dopaminergic denervation of striatum, with lesser destructive effect on the ventral tegmental nucleus and associated lesser dopaminergic denervation of nucleus accumbens and prefrontal cortex. As a consequence of striatal dopaminergic denervation, reactive serotoninergic hyperinnervation of striatum ensues. The striatal extraneuronal milieu of DA and serotonin is markedly altered. Also, a variety of sensitization changes occur for dopaminergic D₁ and D₂ receptors, and for serotoninergic receptors. Behaviorally, these rats in adulthood display spontaneous hyperlocomotor activity, attentional deficits, and cognitive impairment-all of which are acutely attenuated by the psychostimulants amphetamine (AMPH) and methylphenidate (MPH) (i.e., opposite to the acute effects of AMPH and MPH in intact control rats). The acute behavioral effects of AMPH and MPH in intact and lesioned rats are analogous to their respective acute effects in non-ADHD and in ADHD humans. The neurochemical template of brain, and behavioral series of changes in n6-OHDA-lesioned rats, is described in the review. Despite the fact that nigrostriatal damage is not an underlying pathophysiological process of human ADHD (i.e., lacking construct validity), the described animal model has face validity (behavioral profile) and predictive validity (mirror of ADHD/MPH effects, as well as putative and new ADHD treatment effects). Also described in this review is a modification of the n6-OHDA rat, produced by adulthood partial lesioning of the serotoninergic fiber overgrowth. This ADHD model has even more accentuated hyperlocomotor and attentional deficits, counteracted by AMPH-thus providing a more robust means of animal modeling of ADHD. The n6-OHDA rat as a model of ADHD continues to be important in the search for new ADHD treatments.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Perinatal Treatments with the Dopamine D₂-Receptor Agonist Quinpirole Produces Permanent D₂-Receptor Supersensitization: a Model of Schizophrenia

Repeated daily treatments of perinatal rats with the dopamine D2-receptor (D2-R) agonist quinpirole for a week or more produces the phenomenon of 'priming'-gradual but long-term sensitization of D2-R. In fact a daily dose of quinpirole as low as 50 µg/kg/day is adequate for sensitizing D2-R. Primed rats as neonates and in adolescence, when acutely treated with quinpirole display enhanced eating/gnawing/nursing on dams, also horizontal locomotor activity. Between 3 and 5 weeks of age, acute quinpirole treatment of primed rats produces profound vertical jumping with paw treading-a behavior that is not observed in control rats. At later ages acute quinpirole treatment is associated with enhanced yawning, a D2-R-associated behavior. This long-term D2-R supersensitivity is believed to be life-long, despite the relatively brief period of D2-R priming near the time of birth. D2-R supersensitivity is not associated with an increase in the number or affinity of D2-R, as assessed in the striatum of rats; nor is it induced with the D3-R agonist 7-OH-DPAT. However, quinpirole-induced D2-R supersensitivity is associated with cognitive deficits, also a deficit in pre-pulse inhibition and in neurotrophic factors, and low levels of the transcript regulator of G-protein signaling (RGS) RGS9 in brain; and acute reversal of these alterations by the antipsychotic agent olanzapine. In sum, rats ontogenetically D2-R supersensitized have face validity, construct validity and predictive ability for schizophrenia.

Lifelong Rodent Model of Tardive Dyskinesia-Persistence After Antipsychotic Drug Withdrawal

Tardive dyskinesia (TD), first appearing in humans after introduction of the phenothiazine class of antipsychotics in the 1950s, is now recognized as an abnormality resulting predominately by long-term block of dopamine (DA) D₂ receptors (R). TD is thus reproduced in primates and rodents by chronic administration of D₂-R antagonists. Through a series of studies predominately since the 1980s, it has been shown in rodent modeling of TD that when haloperidol or other D₂-R antagonist is added to drinking water, rats develop spontaneous oral dyskinesias, vacuous chewing movements (VCMs), after ~3 months, and this TD is associated with an increase in the number of striatal D₂-R. This TD persists for the duration of haloperidol administration and another ~2 months after haloperidol withdrawal. By neonatally lesioning dopaminergic nerves in brain in neonatal rats with 6-hydroxydopamine (6-OHDA), it has been found that TD develops sooner, at ~2 months, and also is accompanied by a much higher number of VCMs in these haloperidol-treated lesioned rats, and the TD persists lifelong after haloperidol withdrawal, but is not associated with an increased D₂-R number in the haloperidol-withdrawn phase. TD apparently is related in part to supersensitization of both D₁-R and serotoninergic 5-HT₂-R, which is also a typical outcome of neonatal 6-OHDA (n6-OHDA) lesioning. Testing during the haloperidol-withdrawn phase in n6-OHDA rats displaying TD reveals that receptor agonists and antagonists of a host of neuronal phenotypic classes have virtually no effect on spontaneous VCM number, except for 5-HT₂-R antagonists which acutely abate the incidence of VCMs in part. Extrapolating to human TD, it appears that (1) 5-HT₂-R supersensitization is the crucial alteration accounting for persistence of TD, (2) dopaminergic-perhaps age-related partial denervation-is a risk factor for the development of TD, and (3) 5-HT₂-R antagonists have the therapeutic potential to alleviate TD, particularly if/when an antipsychotic D₂-R blocker is withdrawn.

Applications of the Neonatal Quinpirole Model to Psychosis and Convergence upon the Dopamine D2 Receptor

This mini review focuses on the importance of the dopamine D₂-like receptor family and its importance in psychosis. Past findings from this laboratory along with collaborators have been that neonatal quinpirole (a dopamine D₂-like receptor agonist) results in increases in dopamine D₂ receptor sensitivity that persists throughout the animal's lifetime. Findings from this model have been shown to have particular application and validity to schizophrenia, but may have broader implications toward other psychoses, which is reviewed in the present manuscript. In the present review, we also highlight other models of psychoses that have been centered on the subchronic administration of quinpirole to rats in order to model certain psychoses, which has uncovered some interesting and valid behavioral findings. This review highlights the importance of the combination of behavioral findings and neurobiological mechanisms focusing on neural plasticity in discovering underlying pathologies in these disorders that may lead to treatment discoveries, as well as the value of animal models across all psychoses.

Nigral dopamine loss induces a global upregulation of presynaptic dopamine D1 receptor facilitation of the striatonigral GABAergic output

In Parkinson's disease (PD), the dopamine (DA) neuron loss in the substantia nigra and the DA axon loss in the dorsal striatum are severe, but DA neurons in the ventral tegmental area and DA axons in middle and ventral striatal subregions are less affected. Severe DA loss leads to DA receptor supersensitivity, but it was not known whether the supersensitivity of the DA D1 receptors (D1Rs) on the striatonigral axon terminal is determined by the severe striatal or nigral DA loss. This question is important because these two possibilities affect the extent of the striatonigral terminals with supersensitive D1Rs and hence the strength of the direct pathway output. Here we have investigated this question in the transcription factor Pitx3 mutant mice that have a PD-like DA loss pattern. We found that the presynaptic D1R function was upregulated globally: the D1R-mediated facilitation was equally enhanced for the striatonigral GABA output originated in the dorsal striatum where the DA loss is severe and the somatic D1Rs are supersensitive, and for the striatonigral GABA output originated in the middle and ventral striatum where the DA loss is moderate and the somatic D1Rs are not supersensitive. These results suggest that severe nigral DA loss is sufficient to induce functional upregulation of the D1Rs on striatonigral axon terminals. Consequently, in PD, the globally enhanced D1Rs on striatonigral axon terminals originated in broad striatal subregions may strongly enhance the striatonigral GABA output upon D1R stimulation, potentially contributing to D1R agonism's profound motor-stimulating effects.

Serotonin2C Receptors and the Motor Control of Oral Activity

Data from many experiments has shown that serotonin2C (5-HT2C) receptor plays a role in the control of orofacial activity in rodents. Purposeless oral movements can be elicited either by agonists or inverse agonists implying a tight control exerted by the receptor upon oral activity. The effects of agonists has been related to an action of these drugs in the subthalamic nucleus and the striatum, the two input structures for cortical efferents to the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. The oral effects of agonists are dramatically enhanced in case of chronic blockade of central dopaminergic transmission induced by neuroleptics or massive destruction of dopamine neurons. The mechanisms involved in the hypersensitized oral responses to 5-HT2C agonists are not clear and deserve additional studies. Indeed, while the oral behavior triggered by 5-HT2C drugs would barely correspond to the dyskinesia observed in humans, the clinical data have consistently postulated that 5-HT2C receptors could be involved in these aberrant motor manifestations.

Supersensitive presynaptic dopamine D2 receptor inhibition of the striatopallidal projection in nigrostriatal dopamine-deficient mice

The dopamine (DA) D2 receptor (D2R)-expressing medium spiny neurons (D2-MSNs) in the striatum project to and inhibit the GABAergic neurons in the globus pallidus (GP), forming an important link in the indirect pathway of the basal ganglia movement control circuit. These striatopallidal axon terminals express presynaptic D2Rs that inhibit GABA release and thus regulate basal ganglion function. Here we show that in transcription factor Pitx3 gene mutant mice with a severe DA loss in the dorsal striatum mimicking the DA denervation in Parkinson's disease (PD), the striatopallidal GABAergic synaptic transmission displayed a heightened sensitivity to presynaptic D2R-mediated inhibition with the dose-response curve shifted to the left, although the maximal inhibition was not changed. Functionally, low concentrations of DA were able to more efficaciously reduce the striatopallidal inhibition-induced pauses of GP neuron activity in DA-deficient Pitx3 mutant mice than in wild-type mice. These results demonstrate that presynaptic D2R inhibition of the striatopallidal synapse becomes supersensitized after DA loss. These supersensitive D2Rs may compensate for the lost DA in PD and also induce a strong disinhibition of GP neuron activity that may contribute to the motor-stimulating effects of dopaminergic treatments in PD.

Imaging studies in focal dystonias: a systems level approach to studying a systems level disorder

Focal dystonias are dystonias that affect one part of the body, and are sometimes task-specific. Brain imaging and transcranial magnetic stimulation techniques have been valuable in defining the pathophysiology of dystonias in general, and are particularly amenable to studying focal dystonias. Over the past few years, several common themes have emerged in the imaging literature, and this review summarizes these findings and suggests some ways in which these distinct themes might all point to one common systems-level mechanism for dystonia. These themes include (1) the role of premotor regions in focal dystonia, (2) the role of the sensory system and sensorimotor integration in focal dystonia, (3) the role of decreased inhibition/increased excitation in focal dystonia, and (4) the role of brain imaging in evaluating and guiding treatment of focal dystonias. The data across these themes, together with the features of dystonia itself, are consistent with a hypothesis that all dystonias reflect excessive output of postural control/stabilization systems in the brain, and that the mechanisms for dystonia reflect amplification of an existing functional system, rather than recruitment of the wrong motor programs. Imaging is currently being used to test treatment effectiveness, and to visually guide treatment of dystonia, such as placement of deep brain stimulation electrodes. In the future, it is hoped that imaging may be used to individualize treatments across behavioral, pharmacologic, and surgical domains, thus optimizing both the speed and effectiveness of treatment for any given individual with focal dystonia.

RGS17: an emerging therapeutic target for lung and prostate cancers

Ligands for G-protein-coupled receptors (GPCRs) represent approximately 50% of currently marketed drugs. RGS proteins modulate heterotrimeric G proteins and, thus, GPCR signaling, by accelerating the intrinsic GTPase activity of the Gα subunit. Given the prevalence of GPCR targeted therapeutics and the role RGS proteins play in G protein signaling, some RGS proteins are emerging as targets in their own right. One such RGS protein is RGS17. Increased RGS17 expression in some prostate and lung cancers has been demonstrated to support cancer progression, while reduced expression of RGS17 can lead to development of chemotherapeutic resistance in ovarian cancer. High-throughput screening is a powerful tool for lead compound identification, and utilization of high-throughput technologies has led to the discovery of several RGS inhibitors, thus far. As screening technologies advance, the identification of novel lead compounds the subsequent development of targeted therapeutics appears promising.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

U, R.L.W. C. 2-Hydroxyestradiol enhances binge onset in female rats and reduces prefrontal cortical dopamine in male rats

Women are more likely to suffer from a bingeing-related eating disorder, which is surprising, since estradiol reduces meal size and is associated with reduced binge frequency. This apparent contradiction may involve the estradiol metabolite, 2-hydroxyestradiol. We previously reported that female rats had faster escalations in shortening intake during the development of bingeing than did males, but acute administration of 2-hydroxyestradiol increased the intake of vegetable shortening to a greater extent in male rats once bingeing was established. Here, we report two separate studies that follow up these previous findings. In the first, we hypothesized that chronic exposure to 2-hydroxyestradiol would promote escalation of bingeing during binge development in ovariectomized female rats. In the second, we hypothesized that acute exposure to 2-hydroxyestradiol would enhance dopamine signaling in the prefrontal cortex after bingeing was established in male rats. In study 1, non-food-deprived female rats were separated into 3 groups: ovariectomized (OVX) with chronic 2-hydroxyestradiol supplementation (E), OVX with vehicle supplementation (O), and intact with vehicle (I). Each group was given access to an optional source of dietary fat (shortening) on Mon, Wed, and Fri for 4 weeks. 2-hydroxyestradiol supplementation prevented OVX-induced weight gain and enhanced escalation of shortening intake over the four-week period (ps<0.05). Additionally, in week 4, rats in the E group ate significantly more shortening than I controls, less chow than either the O or I group, and had a higher shortening to chow ratio than O or I (ps<0.05). Study 2 indicated that acute injection of 2-hydroxyestradiol abolished shortening-evoked dopamine efflux in the prefrontal cortex of bingeing male rats (p<0.05). Together, these studies indicate that 2-hydroxyestradiol can exacerbate bingeing as it develops and can suppress dopamine signaling in the prefrontal cortex once bingeing is established.

Amphetamine locomotor sensitization and conditioned place preference in adolescent male and female rats neonatally treated with quinpirole

Neonatal quinpirole treatment has been shown to produce an increase in dopamine D2-like receptor sensitivity that persists throughout the subject's lifetime. The objective was to analyze the effects of neonatal quinpirole treatment on effects of amphetamine in adolescent rats using locomotor sensitization and conditioned place preference procedures. Sprague-Dawley rats were treated with quinpirole (1 mg/kg) or saline from postnatal days (P)1 to P11 and raised to adolescence. For locomotor sensitization, subjects were given amphetamine (1 mg/kg) or saline every second day from P35 to P47 and were placed into a locomotor arena. In female rats, neonatal quinpirole treatment enhanced amphetamine locomotor sensitization compared with quinpirole-free controls sensitized to amphetamine. Male rats demonstrated sensitization to amphetamine, although this was muted compared with female rats, and were unaffected by neonatal quinpirole. For conditioned place preference, subjects were conditioned for 8 consecutive days (P32-39) with amphetamine (1 mg/kg) or saline and a drug-free preference test was conducted at P40. Rats treated with neonatal quinpirole enhanced time spent in the amphetamine-paired context compared with quinpirole-free controls conditioned with amphetamine, but only female controls conditioned with amphetamine spent more time in the drug-paired context compared with saline-treated controls. Increased D₂-like receptor sensitivity appears to have enhanced the behavioral effects of amphetamine, but these effects were more prevalent in adolescent female rats compared with male rats.

Imbalanced Dopaminergic Transmission Mediated by Serotonergic Neurons in L-DOPA-Induced Dyskinesia

L-DOPA-induced dyskinesias (LIDs) are one of the main motor side effects of L-DOPA therapy in Parkinson's disease. The review will consider the biochemical evidence indicating that the serotonergic neurons are involved in the dopaminergic effects of L-DOPA in the brain. The consequences are an ectopic and aberrant release of dopamine that follows the serotonergic innervation of the brain. After mid- to long-term treatment with L-DOPA, the pattern of L-DOPA-induced dopamine release is modified. In several brain regions, its effect is dramatically reduced while, in the striatum, its effect is quite preserved. LIDs could appear when the dopaminergic effects of L-DOPA fall in brain areas such as the cortex, enhancing the subcortical impact of dopamine and promoting aberrant motor responses. The consideration of the serotonergic system in the core mechanism of action of L-DOPA opens an important reserve of possible strategies to limit LIDs.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Neurobehavioral manifestations of developmental impairment of the brain

Individual characteristics of human nature (e.g. introversion, extroversion, mood, activity, adaptability, aggressiveness, social ability, anxiety) do not need to be primarily innate. They can be determined by the action of various influences and their interactions on functional development of the brain. There is ample epidemiological and experimental evidence that chemical and/or physical factors acting during sensitive time windows of the brain development can cause mental, behavioral, emotional and/or cognitive disorders. Environmental pollutants, addictive substances, drugs, malnutrition, excessive stress and/or hypoxia-ischemia were reported to induce functional maldevelopment of the brain with consequent neurobehavioral disorders. The article provides review on most significant neurobehavioral manifestations of developmental impairment of the brain during prenatal, perinatal and early postnatal period. The most known adverse factors causing developmental neurobehavioral dysfunctions in humans as well as in experimental animals are discussed.

Neonatal quinpirole treatment enhances locomotor activation and dopamine release in the nucleus accumbens core in response to amphetamine treatment in adulthood

Neonatal quinpirole treatment to rats produces long-term increases in D(2) receptor sensitivity that persists throughout the animal's lifetime, a phenomenon referred to as D(2) priming. Male and female Sprague-dawley rats were administered quinpirole (1 mg kg(-1)) or saline from postnatal days (P)1-11. At P60, all animals were given an injection of quinpirole (100 microg kg(-1)), and results showed that rats neonatally treated with quinpirole demonstrated enhanced yawning in response to quinprole, verifying D(2) receptor priming because yawning is a D(2) receptor mediated event. Beginning 1-3 days later, locomotor sensitization was tested through administration of d-amphetamine (1 mg kg(-1)) or saline every other day over 14 days, and horizontal activity and turning behavior were analyzed. Findings indicated that D(2)-priming enhanced horizontal activity in response to amphetamine in females compared to males at Days 1 and 4 of locomotor sensitization testing, and D(2)-priming enhanced turning in response to amphetamine. Seven to ten days after sensitization was complete, microdialysis of the NAcc core was performed using a cumulative dosing regimen of amphetamine (0.1-3.0 mg kg(-1)). D(2)-primed rats administered amphetamine demonstrated a 500% increase in accumbal DA overflow compared to control rats administered amphetamine. Additionally, amphetamine produced a significant increase in NE overflow compared to controls, but this was unaffected by D(2) priming. These results indicate that D(2) receptor priming as is produced by neonatal quinpirole treatment robustly enhances behavioral activation and accumbal DA overflow in response to amphetamine, which may underlie increases in psychostimulant use and abuse within the psychotic population where increased D(2) receptor sensitivity is a hallmark.

Ontogenetic exposure of rats to pre- and post-natal manganese enhances behavioral impairments produced by perinatal 6-hydroxydopamine

Rats lesioned shortly after birth with 6-hydroxydopamine (6-OHDA; 134 μg icv) represent a near-ideal model of severe Parkinson's disease because of the near-total destruction of nigrostriatal dopaminergic fibers. The element manganese, an essential cofactor for many enzymatic reactions, itself in toxic amount, replicates some clinical features similar to those of Parkinson's disease. The aim of this study was to examine the effect of neonatal manganese exposure on 6-OHDA modeling of Parkinson's disease in rats. Manganese (MnCl(2)·4H(2)O) 10,000 ppm was included in the drinking water of pregnant Wistar rats from the time of conception until the 21st day after delivery, the age when neonatal rats were weaned. Control rats consumed tap water. Other groups of neonatal rat pups, on the 3rd day after birth, were pretreated with desipramine (20 mg/kg ip 1 h) prior to bilateral icv administration of 6-OHDA (30, 60, or 137 μg) or its vehicle saline-ascorbic (0.1%) (control). At 2 months after birth, in rats lesioned with 30, 60, or 134 μg 6-OHDA, endogenous striatal dopamine (DA) content was reduced, respectively, by 66, 92, and 98% (HPLC/ED), while co-exposure of these groups to perinatal manganese did not magnify the DA depletion. However, there was prominent enhancement of DA D(1) agonist (i.e., SKF 38393)-induced oral activity in the group of rats exposed perinatally to manganese and also treated neonatally with the 30 mg/kg dose of 6-OHDA. The 30 mg/kg 6-OHDA group, demonstrating cataleptogenic responses to SCH 23390 (0.5 mg/kg) and haloperidol (0.5 mg/kg ip), developed resistance if co-exposed to perinatal manganese. In the group exposed to manganese and lesioned with the 60 mg/kg dose of 6-OHDA, there was a reduction in D(2) agonist (i.e., quinpirole, 0.1 mg/kg)-induced yawning. The series of findings demonstrate that ontogenetic exposure to manganese results in an enhancement of behavioral toxicity to a moderate dose of 6-OHDA, despite the fact that there is no enhanced depletion of striatal DA depletion by the manganese treatment.

D2 antagonist during development decreases anxiety and infanticidal behavior in adult female prairie voles (Microtus ochrogaster)

On postnatal day 8, prairie vole pups were randomly assigned a treatment of 1mg/kg SKF38393 (D1 agonist), quinpirole (D2 agonist), SCH23390 (D1 antagonist), eticlopride (D2 antagonist), or saline vehicle. As adults, females treated with eticlopride exhibited reduced anxiety-like behavior in an elevated plus maze and a reduction in infanticidal behavior. These behavioral effects were not seen in males. These data demonstrate that a single exposure to a D2 antagonist during development can have persistent, sex-specific effects on behavior into adulthood.

Neurogenetics of dopaminergic receptor supersensitivity in activation of brain reward circuitry and relapse: proposing "deprivation-amplification relapse therapy" (DART)

BACKGROUND AND HYPOTHESIS

It is well known that after prolonged abstinence, individuals who use their drug of choice experience a powerful euphoria that often precipitates relapse. While a biological explanation for this conundrum has remained elusive, we hypothesize that this clinically observed "supersensitivity" might be tied to genetic dopaminergic polymorphisms. Another therapeutic conundrum relates to the paradoxical finding that the dopaminergic agonist bromocriptine induces stronger activation of brain reward circuitry in individuals who carry the DRD2 A1 allele compared with DRD2 A2 allele carriers. Because carriers of the A1 allele relative to the A2 allele of the DRD2 gene have significantly lower D2 receptor density, a reduced sensitivity to dopamine agonist activity would be expected in the former. Thus, it is perplexing that with low D2 density there is an increase in reward sensitivity with the dopamine D2 agonist bromocriptine. Moreover, under chronic or long-term therapy with D2 agonists, such as bromocriptine, it has been shown in vitro that there is a proliferation of D2 receptors. One explanation for this relates to the demonstration that the A1 allele of the DRD2 gene is associated with increased striatal activity of L-amino acid decarboxylase, the final step in the biosynthesis of dopamine. This appears to be a protective mechanism against low receptor density and would favor the utilization of an amino acid neurotransmitter precursor like L-tyrosine for preferential synthesis of dopamine. This seems to lead to receptor proliferation to normal levels and results in significantly better treatment compliance only in A1 carriers.

PROPOSAL AND CONCLUSION

We propose that low D2 receptor density and polymorphisms of the D2 gene are associated with risk for relapse of substance abuse, including alcohol dependence, heroin craving, cocaine dependence, methamphetamine abuse, nicotine sensitization, and glucose craving. With this in mind, we suggest a putative physiological mechanism that may help to explain the enhanced sensitivity following intense acute dopaminergic D2 receptor activation: "denervation supersensitivity." Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D2 receptor proliferation occurs (20%-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. Thus, the administration of dopamine D2 agonists would target D2 sensitization and attenuate relapse, especially in D2 receptor A1 allele carriers. This hypothesized mechanism is supported by clinical trials utilizing amino acid neurotransmitter precursors, enkephalinase, and catechol-O-methyltransferase (COMT) enzyme inhibition, which have resulted in attenuated relapse rates in reward deficiency syndrome (RDS) probands. If future translational research reveals that dopamine agonist therapy reduces relapse in RDS, it would support the proposed concept, which we term "deprivation-amplification relapse therapy" (DART). This term couples the mechanism for relapse, which is "deprivation-amplification," especially in DRD2 A1 allele carriers with natural D2 agonist therapy utilizing amino acid precursors and COMT and enkepalinase inhibition therapy.

High-frequency stimulation of the nucleus accumbens core and shell reduces quinpirole-induced compulsive checking in rats

Electrical deep brain stimulation (DBS) is currently studied in the treatment of therapy-refractory obsessive compulsive disorders (OCDs). The variety of targeted brain areas and the inconsistency in demonstrating anti-compulsive effects, however, highlight the need for better mapping of brain regions in which stimulation may produce beneficial effects in OCD. Such a goal may be advanced by the assessment of DBS in appropriate animal models of OCD. Currently available data on DBS of the nucleus accumbens (NAc) on OCD-like behavior in rat models of OCD are contradictory and partly in contrast to clinical data and theoretical hypotheses about how the NAc might be pathophysiologically involved in the manifestation of OCD. Consequently, the present study investigates the effects of DBS of the NAc core and shell in a quinpirole rat model of OCD. The study demonstrates that electrical modulation of NAc core and shell activity via DBS reduces quinpirole-induced compulsive checking behavior in rats. We therefore conclude that both, the NAc core and shell constitute potential target structures in the treatment of OCD.

Dopamine receptor supersensitivity: development, mechanisms, presentation, and clinical applicability

The process of receptor supersensitivity (RSS) has a long history and is an epiphenomenon of neuronal denervation. Dopamine (DA) RSS (DARSS) similarly occurs after DA denervation, and this process is invoked in neuropsychiatric and neurodegenerative disorders. From studies largely over the past 25 years, much has been learned regarding DARSS. For example, overt D1 DARSS occurs after perinatal destruction of nigrostriatal DA fibers. However, following perinatal destruction of DA innervation, the most-prominent behavioral effects of a D1 agonist are observed after a series of D1 agonist treatments--a process known as priming of D1 DA receptors. Moreover, perinatal lesioning of DA fibers produces prominent serotonin (5-HT) RSS, and in fact 5-HT RSS appears to modulate D1 DA RSS. In rodents, receptor supersensitization by these means appears to be irreversible. In contrast to the observed D1 DARSS, D2 DARSS apparently does not occur after perinatal DA denervation. Also, while repeated D1 agonist treatment of intact rats has no observable effect, repeated D2 agonist treatments, during or after the ontogenetic phase, produces prominent life-long D2 RSS. The process may have an association with substance abuse. Therefore, production of D1 and D2 DARSS occurs by different means and under different circumstances, and in association with perhaps different neuronal phenotypes, and with greater incidence in either intact (D2) or DA-lesioned counterparts (D1). The physiological consequence of RSS are multiple.

Proclivity to self-injurious behavior in MRL-lpr mice: implications for autoimmunity-induced damage in the dopaminergic system

Systemic lupus erythematosus is frequently accompanied by psychiatric manifestations of unknown origin. Although damage of central neurons had been documented, little is known about neurotransmitter systems affected by the autoimmune/inflammatory process. Recent studies on lupus-prone MRL-lpr mice point to imbalanced dopamine function and neurodegeneration in dopamine-rich brain regions. We follow up on anecdotal observations of singly housed mice developing chest wounds. Compulsive grooming and/or skin biting accounted for open lesions, lending itself to the operational term 'self-injurious behavior' (SIB). Low incidence of spontaneous SIB increased significantly after repeated injections of dopamine-2/3 receptor (D2/D3R) agonist quinpirole (QNP). To further probe the dopaminergic circuitry and examine whether SIB is associated with development of lupus-like disease, we compared behavioral responses among cohorts that differed in the immune status. Two-week treatment with QNP (intraperitoneal, 0.5 mg kg(-1) body weight per day) induced SIB in 60% of diseased MRL-lpr mice, and exacerbated their splenomegaly. Although increased grooming and stereotypy were observed in less symptomatic MRL+/+ controls, only one mouse (10%) developed SIB. Similarly, SIB was not seen in young, asymptomatic groups despite dissimilar ambulatory responses to QNP. In situ hybridization revealed treatment-independent upregulation of D2R mRNA in substantia nigra of diseased MRL-lpr mice. The above results suggest that development of systemic autoimmunity alters sensitivity of the dopaminergic system and renders MRL-lpr mice prone to SIB. Although pathogenic factors were not examined, we hypothesize that immune and endocrine mechanisms jointly contribute to early neuronal damage, which underlies behavioral deficiency in the adulthood.

Nicotine sensitization in adult male and female rats quinpirole-primed as neonates

RATIONALE

Increases in dopamine D2-like receptor function are common in several psychological disorders that demonstrate a four to five fold increase in nicotine abuse compared to the general population.

OBJECTIVE

The objective of this study was to analyze the interaction of sex differences and sensitization to nicotine in rats D2 receptor primed as neonates.

MATERIALS AND METHODS

A total of 32 male and 32 female Sprague-Dawley rats derived from eight litters were ontogenetically treated with quinpirole (1 mg/kg) or saline from postnatal days (P) 1-21 and raised to adulthood. At P60, all animals were given an acute injection of quinpirole HCl (100 microg/kg) and yawns were counted for 1 h. Yawning has been shown to be a behavioral event mediated by D2-like receptors. Beginning on P61-65, animals were habituated to a locomotor arena and subsequently administered either nicotine (0.5 mg/kg free base) or saline (intraperitoneal) every second day for 3 weeks. Approximately 15 min after each injection, animals were placed into the arena and horizontal activity and vertical rears were recorded.

RESULTS

A robust increase of yawning was observed at P60 in D2 primed as compared to saline controls. Priming of D2-like receptors increased the locomotor response to nicotine in horizontal activity in both males and females, but females demonstrated a more robust hypoactive locomotor response to initial nicotine treatment when compared to saline-treated females. Nicotine also produced a significant decrease of vertical rearing in both males and females.

CONCLUSIONS

It appears that D2 receptor priming enhances sensitization to nicotine in adult rats, and females may be more behaviorally sensitive to nicotine than males.