Roles of forced nicotine exposure and Comt gene disruption in the development of addiction-related behavioural and neurochemical changes in mice

Activation of midbrain dopamine systems is thought to be critically involved in the addictive properties of abused substances. Drugs of abuse increase dopamine release in the nucleus accumbens and dorsal striatum, which are the target areas of mesolimbic and nigrostriatal dopamine pathways, respectively. Dopamine release in the nucleus accumbens is thought to mediate the attribution of incentive salience to rewards, and dorsal striatal dopamine release is involved in habit formation. In addition, changes in the function of prefrontal cortex (PFC), the target area of mesocortical dopamine pathway, may skew information processing and memory formation such that the addict pays an abnormal amount of attention to drug-related cues. In this study, we wanted to explore how long-term forced oral nicotine exposure or the lack of catechol-O-methyltransferase (COMT), one of the dopamine metabolizing enzymes, would affect the functioning of these pathways. We also wanted to find out how the forced nicotine exposure or the lack of COMT would affect the consumption of nicotine, alcohol, or cocaine. First, we studied the effect of forced chronic nicotine exposure on the sensitivity of dopamine D2-like autoreceptors in microdialysis and locomotor activity experiments. We found that the sensitivity of these receptors was unchanged after forced oral nicotine exposure, although an increase in the sensitivity was observed in mice treated with intermittent nicotine injections twice daily for 10 days. Thus, the effect of nicotine treatment on dopamine autoreceptor sensitivity depends on the route, frequency, and time course of drug administration. Second, we investigated whether the forced oral nicotine exposure would affect the reinforcing properties of nicotine injections. The chronic nicotine exposure did not significantly affect the development of conditioned place preference to nicotine. In the intravenous self-administration paradigm, however, the nicotine-exposed animals self-administered nicotine at a lower unit dose than the control animals, indicating that their sensitivity to the reinforcing effects of nicotine was enhanced. Next, we wanted to study whether the Comt gene knock-out animals would be a suitable model to study alcohol and cocaine consumption or addiction. Although previous work had shown male Comt knock-out mice to be less sensitive to the locomotor-activating effects of cocaine, the present study found that the lack of COMT did not affect the consumption of cocaine solutions or the development of cocaine-induced place preference. However, the present work did find that male Comt knock-out mice, but not female knock-out mice, consumed ethanol more avidly than their wild-type littermates. This finding suggests that COMT may be one of the factors, albeit not a primary one, contributing to the risk of alcoholism. Last, we explored the effect of COMT deficiency on dorsal striatal, accumbal, and prefrontal cortical dopamine metabolism under no-net-flux conditions and under levodopa load in freely-moving mice. The lack of COMT did not affect the extracellular dopamine concentrations under baseline conditions in any of the brain areas studied. In the prefrontal cortex, the dopamine levels remained high for a prolonged time after levodopa treatment in male, but not female, Comt knock-out mice. COMT deficiency induced accumulation of 3,4-dihydroxyphenylacetic acid, which increased further under levodopa load. Homovanillic acid was not detectable in Comt knock-out animals either under baseline conditions or after levodopa treatment. Taken together, the present results show that although forced chronic oral nicotine exposure affects the reinforcing properties of self-administered nicotine, it is not an addiction model itself. COMT seems to play a minor role in dopamine metabolism and in the development of addiction under baseline conditions, indicating that dopamine function in the brain is well-protected from perturbation. However, the role of COMT becomes more important when the dopaminergic system is challenged, such as by pharmacological manipulation.

The Effects of Nicotine on the Regulation of Neuronal Alpha7 Nicotinic Acetylcholine Receptors and Intracellular Signalling Pathways

Nicotine, the addictive compound of tobacco products, exerts its effects in the brain by binding to neuronal acetylcholine nicotinic receptors (nAChRs). The aim of the present study was to increase the knowledge of nicotine s complex effects, the focus being on homomeric alpha7-nAChRs that are widely expressed in the brain. Nicotinic regulation of differential signalling molecules including transcriptional regulators was also studied. We found that the number of alpha7-nAChRs is increased in specific brain regions in mice, in a time-dependent manner after chronic oral nicotine administration. Our results suggest that in addition to alpha4beta2-nAChRs, the other major nAChR subtype expressed in the brain, the number of alpha7-nAChRs is affected by chronic presence of nicotine. We suggest that when studying the long-term effects of nicotine, the duration on administration is of great importance. Next, we observed that nicotine exposure induces accumulation of cAMP in cell cultures expressing nAChRs. Furthermore, nicotine-induced alpha7-nAChR upregulation was potentiated by treatments enhancing cAMP-signalling, suggesting a role for cAMP in the upregulation process. Protein kinase C (PKC) was found essential for the basal regulation of alpha7-nAChR number. The nicotine-evoked alpha7-nAChR upregulation could be further increased by PKC overexpression. Thirdly, the effects of nicotine on dopamine and cAMP regulated phosphoprotein (DARPP-32) were characterised in rat brain. The results show that DARPP-32 is regulated by both acute and long-term nicotine treatment in the striatal subdivisions. The effect of acute nicotine is dose-dependent and the three striatal regions display differential sensitivities to nicotine. Chronic nicotine is also able to regulate DARPP-32 signalling with prominent effect seen in the nucleus accumbens (NAc), suggesting a role for DARPP-32 in the mediation of long-term effects of nicotine. Finally, the regulation of transcription factors Elk-1 and FosB/deltaFosB by nicotine was investigated. We found that Elk-1 is activated by acute nicotine selectively in the NAc core and hippocampal area CA1, whereas acute nicotine does not affect FosB/deltaFosB. Long-term intermittent or continuous nicotine increases the level of total Elk-1 in the same brain regions as acute nicotine. FosB/deltaFosB is also affected by chronic nicotine. Thus, similarly to other drugs of abuse, nicotine regulates transcriptional regulators Elk-1 and FosB/deltaFosB. These results bring further support for a common mechanism underlying the development of addiction. Nicotine s positive effects on learning and memory might involve the transcription factor Elk-1 based on the changes seen in the hippocampus, the key area in cognitive functions.

Glutamatergic Modulation of Cue-Induced Drug-Seeking Behavior in the Rat

The characteristics of drug addiction include compulsive drug use despite negative consequences and re-occurring relapses, returns to drug use after a period of abstinence. Therefore, relapse prevention is one of the major challenges for the treatment of drug addiction. There are three main factors capable of inducing craving for drugs and triggering relapse long after cessation of drug use and dissipation of physical withdrawal signs: stress, re-exposure to the drug, and environmental stimuli (cues) that have been previously associated with drug use. The neurotransmitters dopamine and glutamate have been implicated in the modulation of drug-seeking behavior. The aim of this project was to examine the role of glutamatergic neurotransmission in relapse triggered by conditioned drug-associated stimuli. The focus was on clarifying whether relapse to drug seeking can be attenuated by blockade of glutamate receptors. In addition, as the nucleus accumbens has been proposed to participate in the modulation of drug-seeking behavior, the effects of glutamate receptor blockade in this brain structure on cue-induced relapse were investigated. The studies employed animals models in which rats were trained to press a lever in a test cage to obtain alcohol or intravenous cocaine. Drug availability was paired with distinct olfactory, auditory, or visual stimuli. This phase was followed by extinction training, during which lever presses did not result in the presentation of the drug or the drug-associated stimuli. Extinction training led to a gradual decrease in the number of lever presses during test sessions. Relapse was triggered by presenting the rats with the drug-associated stimuli in the absence of alcohol or cocaine. The drug-associated stimuli were alone capable of inducing resumption of lever pressing and maintaining this behavior during repeated testing. The number of lever presses during a session represented the intensity of drug-seeking and relapse behavior. The results suggest that glutamatergic neurotransmission is involved in the modulation of drug-seeking behavior. Both alcohol and cocaine relapse were attenuated by systemic pretreatment with glutamate receptor antagonists. However, differences were found in the ability of ionotropic AMPA/kainate and NMDA receptor antagonists to regulate drug-seeking behavior. The AMPA/kainate antagonists CNQX and NBQX, and L-701,324, an antagonist with affinity for the glycine site of the NMDA receptor, attenuated cue-induced drug seeking, whereas the competitive NMDA antagonist CGP39551 and the NMDA channel blocker MK-801 were without effect. MPEP, an antagonist at metabotropic mGlu5 glutamate receptors, also decreased drug seeking, but its administration was found to lead to conditioned suppression of behavior during subsequent treatment sessions, suggesting that MPEP may have undesirable side effects. The mGluR2/3 agonist LY379268 and the mGluR8 agonist (S)-3,4-DCPG decreased both cue-induced relapse to alcohol drinking and alcohol consumption. Control experiments showed however that administration of the agonists was accompanied by motor suppression limiting their usefulness. Administration of the AMPA/kainate antagonist CNQX, the NMDA antagonist D-AP5, and the mGluR5 antagonist MPEP into the nucleus accumbens resulted also in a decrease in drug-seeking behavior, suggesting that the nucleus accumbens is at least one of the anatomical sites regulating drug seeking and mediating the effects of glutamate receptor antagonists on this behavior.

Importance of COMT in the regulation of prefrontal dopamine

The prefrontal cortex (PFC), located in the anterior region of the frontal lobe, is considered to have several key roles in higher cognitive and executive functions. In general, the PFC can be seen as a coordinator of thought and action allowing subjects to behave in a goal-directed manner. Due to its anatomical connections with a variety of cortical and subcortical structures, several neurotransmitters, including dopamine, are involved in the regulation of PFC activity. In general, the majority of released dopamine is cleared by the dopamine transporter (DAT). In the PFC however, the number of presynaptic DAT is diminished, emphasizing the relative importance of catechol-O-methyltransferase (COMT) in dopamine metabolism. As a result, the role of COMT in the etiology of psychotic disorders is under constant debate. The present study investigated the role of COMT in prefrontal cortical dopamine metabolism by different neurochemical methods in COMT knockout (COMT-KO) mice. Pharmacological tools to inhibit other dopamine clearing mechanisms were also used for a more comprehensive and collective picture. In addition, this study investigated how a lack of the soluble (S-) COMT isoform affects the total COMT activity as well as the pharmacokinetics of orally administered L-dopa using mutant mice expressing only the membrane-bound (MB-) COMT isoform. Also the role of COMT in striatal and accumbal dopamine turnover during Δ9-tetrahydrocannabinol (THC) challenge was studied. We found markedly increased basal dopamine concentrations in the PFC, but not the striatum or nucleus accumbens (NAcc), of mice lacking COMT. Pharmacological inhibition of the noradrenaline transporter (NET) and monoamine oxidase (MAO) elevated prefrontal cortical dopamine levels several-fold, whereas inhibition of DAT did not. The lack of COMT doubled the dopamine raising effects of NET and MAO inhibition. No compensatory expression of either DAT or NET was found in the COMT-KO mice. The lack of S-COMT decreased the total COMT activity by 50-70 % and modified dopamine transmission and the pharmacokinetics of exogenous Ldopa in a sex and tissue specific manner. Finally, we found that subsequent tolcapone and THC increased dopamine levels in the NAcc, but not in the striatum. Conclusively, this study presents neurochemical evidence for the important role of COMT in the PFC and shows that COMT is responsible for about half of prefrontal cortical dopamine metabolism. This study also highlights the previously underestimated proportional role of MB-COMT and supports the clinical evidence of a gene x environment interaction between COMT and cannabis.

Tonic current through GABAA receptors and hyperpolarization-activated cyclic nucleotide-gated channels modulate resonance properties of rat subicular pyramidal neurons

The subiculum, considered to be the output structure of the hippocampus, modulates information flow from the hippocampus to various cortical and sub-cortical areas such as the nucleus accumbens, lateral septal region, thalamus, nucleus gelatinosus, medial nucleus and mammillary nuclei. Tonic inhibitory current plays an important role in neuronal physiology and pathophysiology by modulating the electrophysiological properties of neurons. While the alterations of various electrical properties due to tonic inhibition have been studied in neurons from different regions, its influence on intrinsic subthreshold resonance in pyramidal excitatory neurons expressing hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is not known. Using pharmacological agents, we show the involvement of alpha 5 beta gamma GABA(A) receptors in the picrotoxin-sensitive tonic current in subicular pyramidal neurons. We further investigated the contribution of tonic conductance in regulating subthreshold electrophysiological properties using current clamp and dynamic clamp experiments. We demonstrate that tonic GABAergic inhibition can actively modulate subthreshold properties, including resonance due to HCN channels, which can potentially alter the response dynamics of subicular pyramidal neurons in an oscillating neuronal network.

Efeitos da exposição à nicotina e à fumaça de cigarro durante a lactação sobre o sistema de recompensa cerebral em ratos

Crianças de mães fumantes são mais suscetíveis a se tornarem adultos obesos e se viciarem em drogas ou alimentos palatáveis. Drogas e alimentos ativam a via mesolímbica de recompensa, causando sensação de prazer que induz ainda mais o consumo. Assim, avaliamos a relação entre a exposição apenas à nicotina ou à fumaça do cigarro durante a lactação com a preferência alimentar e sistema dopaminérgico de recompensa cerebral das proles, em dois modelos de programação: Modelo I: no 2o dia pós-natal (PN), lactantes receberam implante de minibombas osmóticas que liberam nicotina (NIC) ou salina (C), durante 14 dias. Em PN150 e novamente em PN160, as proles foram divididas em 4 grupos para um desafio alimentar: N-SC e C-SC que receberam ração padrão; N-SSD e C-SSD que podiam escolher livremente entre as dietas hiperlipídica e hiperglicídica. A ingestão alimentar foi avaliada após 12 h. As mães foram sacrificadas apenas na 21 da lactação (desmame) e as proles em PN15 (com nicotina), PN21 e PN170 (ausência da NIC). Ao desmame, as ratas lactantes NIC apresentaram menor conteúdo de tirosina hidroxilase (TH), maior OBRb e SOCS3 na area tegmentar ventral (VTA); menor TH, maior receptor de dopamina 1 (D1R), receptor de dopamina 2 (D2R) e transportador de dopamina (DAT) no núcleo accumbens (NAc); maior conteúdo de TH no estriado dorsal (DS); e maior D2R e SOCS3 no núcleo arqueado (ARC). Em PN15, os filhotes NIC apresentaram maior conteúdo de D1R, D2R e menor DAT no NAc, enquanto em PN21, apresentaram apenas menor DAT no DS, e menor conteúdo de pSTAT3 em ARC. Aos 170 dias, as proles SSD demonstraram maior preferência para a ração hiperlipídica. No entanto, os animais N-SSD consumiram mais ração hiperglicidica do que as proles C-SSD. A prole N apresentou menor conteúdo de D2R e DAT no NAc e menor D2R no ARC. Modelo II: as mães e suas proles foram divididas em: expostos à fumaça do cigarro (grupo S: 4 vezes / dia, do 3 ao 21 dia de lactação), e expostos ao ar filtrado (grupo C). Em PN175, as proles foram divididas em 4 grupos para o desafio alimentar S-SC, C-SC, S-SSD e C-SSD. A ingestão alimentar foi avaliada após 30 min e 12 h. Em PN180, as proles foram sacrificadas. O grupo S-SSD ingeriu mais das rações palatáveis do que o grupo C-SSD em 30 min e 12 h. Ambos os grupos preferiram a ração hiperlipídica. No entanto, os animais S-SSD consumiram mais ração hiperlipídica do que C-SSD em 30 min. A prole S apresentou menor conteúdo de TH no VTA, menor conteúdo de TH, D2R e maior conteúdo de D1R no NAc e menor OBRb no ARC. Demonstramos que tanto a nicotina isolada como a exposição à fumaça do cigarro durante a lactação resultaram em mudanças no sistema dopaminérgico das proles, programando o comportamento alimentar devido à diminuição da dopamina no NAc.

Dopamine and performance in a reinforcement learning task: evidence from Parkinson's disease.

The role dopamine plays in decision-making has important theoretical, empirical and clinical implications. Here, we examined its precise contribution by exploiting the lesion deficit model afforded by Parkinson's disease. We studied patients in a two-stage reinforcement learning task, while they were ON and OFF dopamine replacement medication. Contrary to expectation, we found that dopaminergic drug state (ON or OFF) did not impact learning. Instead, the critical factor was drug state during the performance phase, with patients ON medication choosing correctly significantly more frequently than those OFF medication. This effect was independent of drug state during initial learning and appears to reflect a facilitation of generalization for learnt information. This inference is bolstered by our observation that neural activity in nucleus accumbens and ventromedial prefrontal cortex, measured during simultaneously acquired functional magnetic resonance imaging, represented learnt stimulus values during performance. This effect was expressed solely during the ON state with activity in these regions correlating with better performance. Our data indicate that dopamine modulation of nucleus accumbens and ventromedial prefrontal cortex exerts a specific effect on choice behaviour distinct from pure learning. The findings are in keeping with the substantial other evidence that certain aspects of learning are unaffected by dopamine lesions or depletion, and that dopamine plays a key role in performance that may be distinct from its role in learning.

Human pavlovian-instrumental transfer.

The vigor with which a participant performs actions that produce valuable outcomes is subject to a complex set of motivational influences. Many of these are believed to involve the amygdala and the nucleus accumbens, which act as an interface between limbic and motor systems. One prominent class of influences is called pavlovian-instrumental transfer (PIT), in which the motivational characteristics of a predictor influence the vigor of an action with respect to which it is formally completely independent. We provide a demonstration of behavioral PIT in humans, with an audiovisual predictor of the noncontingent delivery of money inducing participants to perform more avidly an action involving squeezing a handgrip to earn money. Furthermore, using functional magnetic resonance imaging, we show that this enhanced motivation was associated with a trial-by-trial correlation with the blood oxygenation level-dependent (BOLD) signal in the nucleus accumbens and a subject-by-subject correlation with the BOLD signal in the amygdala. Our data dovetails well with the animal literature and sheds light on the neural control of vigor.

Dopamine and performance in a reinforcement learning task: Evidence from Parkinson's disease

The role dopamine plays in decision-making has important theoretical, empirical and clinical implications. Here, we examined its precise contribution by exploiting the lesion deficit model afforded by Parkinson's disease. We studied patients in a two-stage reinforcement learning task, while they were ON and OFF dopamine replacement medication. Contrary to expectation, we found that dopaminergic drug state (ON or OFF) did not impact learning. Instead, the critical factor was drug state during the performance phase, with patients ON medication choosing correctly significantly more frequently than those OFF medication. This effect was independent of drug state during initial learning and appears to reflect a facilitation of generalization for learnt information. This inference is bolstered by our observation that neural activity in nucleus accumbens and ventromedial prefrontal cortex, measured during simultaneously acquired functional magnetic resonance imaging, represented learnt stimulus values during performance. This effect was expressed solely during the ON state with activity in these regions correlating with better performance. Our data indicate that dopamine modulation of nucleus accumbens and ventromedial prefrontal cortex exerts a specific effect on choice behaviour distinct from pure learning. The findings are in keeping with the substantial other evidence that certain aspects of learning are unaffected by dopamine lesions or depletion, and that dopamine plays a key role in performance that may be distinct from its role in learning. © 2012 The Author.

去甲肾上腺素参与吗啡神经适应性改变的中枢机制

That relapse still exists even after prolonged withdrawal is a difficult issue in the medical cure of drug addiction. Neuro-adaptation induced by prolonged exposure to addictive drugs is the neural mechanisms of both compulsive drug seeking and relapse.Neuro-adaptation caused by addictive drugs increases the individuals’ response to drugs and on the other hand, it reduces the response to natural reward in withdrawn individuals.There must be common neural mechanisms between the co-existing phenomena, and there must also be unique neural mechanisms in the drugs.To reveal the neuro-adaptation arising in the process from random, controllable drug-use to uncontrollable compulsive drug seeking is of great significance both theoretically and practically.Based on the above hypothesis, in order to reveal the function of alpha adrenergic receptor in compulsive drug-seeking motivation during the process of drug addiction, using sensitization of morphine-induced psychomotor activity as behavioral model, through the method of behavioral pharmacology, the neural mechanisms of alpha adrenergic receptor’s involvement in the process of addiction has been studied.The adjustment function caused by alpha receptors in medial prefrontal cortex and nucleus accumbens to morphine-induced psychomotor activity has been compared in the period of first use of drugs and in repetitive-use period. Furthermore, the effect on novelty seeking caused by alpha-receptors in relevant brain areas has also been compared. Major results are as follow: 1 After prolonged morphine exposure, rats’ response to morphine-induced psychomotor activity is strengthened and response to novel object induced reward weakened. 2 Injection of prazosin in medial prefrontal cortex will block morphine-induced psychomotor activity of naïve rats, however, it will not block that of morphine-withdrawn rats, but it will block the novelty seeking behavior of morphine-withdrawn rats. 3 Injection of clonidine in medial prefrontal cortex will block morphine-induced psychomotor effect of both naïve rats and morphine-withdrawn rats, and will block the novelty seeking behavior of morphine-withdrawn rats. 4 Injection of prazosin in nucleus accumbens will not affect the morphine-induced psychomotor effect of either naïve rats or morphine-withdrawn rats, nor will it affect the novelty seeking behavior of morphine-withdrawn rats. 5 Injection of clonidine in nucleus accumbens will block morphine-induced psychomotor effect of naïve rats, however, it will not block that of morphine-withdrawn rats, nor will it affect the novelty seeking behavior of morphine-withdrawn rats. These results show: 1 The weakening of the function of alpha1 receptors in medial prefrontal cortex and alpha2 receptors in nucleus accumbens caused by repetitive exposure to morphine is probably the cause of compulsive drug-seeking activity. 2 Blocking alpha1 receptors in medial prefrontal cortex accelerates the loss of interest in natural reward after morphine withdrawal. 3 Blocking alpha2 receptors in medial prefrontal cortex not only restrains drug-seeking motivation, but also blocks the individual’s seeking motivation for novelty stimulus, which suggests that, while selecting medicine for curing addiction, it should be considered to reduce the influence on natural reward as much as possible and to avoid major side-effect.

Novel insights into the role of the GABAB receptor in depression and anxiety

The GABAB receptor is a functional heterodimer comprising the GABAB1 and GABAB2 subunits, with the GABAB1 subunit displaying two major isoforms, GABAB(1a) and GABAB(1b). Preclinical findings have strongly implicated the GABAB receptor in stress-related psychiatric disorders, however, the precise contribution of the GABAB receptor in depression and anxiety disorders remains unknown. Emerging data suggest that the interaction between adverse environmental conditions, such as early life stress, and a specific genetic composition can increase the risk to develop psychiatric disorders in adulthood. This thesis investigated the role of the GABAB receptor alone or in combination with early-life stress (maternal separation), in modulating antidepressant like and anxiety-related behaviours. Pharmacological blockade of the GABAB receptor with CGP52432 had antidepressant-like behavioural effects. Moreover, mice lacking the GABAB(1b) receptor subunit isoform exhibited antidepressant-like behaviours in adulthood but anxiety-like behaviour in early-life. In response to maternal separation, GABAB(1a)-/- mice exhibited early-life stress-induced anhedonia, a core symptom of depression, while GABAB(1b)-/- mice exhibited a more resilient phenotype. Moreover, when compared with wildtype or GABAB(1a)-/- mice, GABAB(1b)-/- mice that underwent maternal separation exhibited enhanced stressinduced neuronal activation in the hippocampus and in the nucleus accumbens (NAcc), a critical area for anhedonia thus suggesting that enhanced stress-induced neuronal activation in the hippocampus and NAcc in GABAB(1b)-/- mice may be important for their antidepressant-like phenotype and their resilience to stress-induced anhedonia. Pharmacological blockade of GABAB receptor and GABAB(1b) receptor subunit isoform loss of function increased adult hippocampal cell proliferation, thus suggesting that increased hippocampal neurogenesis could be a potential mechanism for the antidepressant-like effects of GABAB receptor antagonists and GABAB(1b) receptor subunit isoform disruption. Finally, this thesis investigated whether the expression of several genes involved in hippocampal neurogenesis or the antidepressant response were altered in the mouse hippocampus following chronic treatment with a GABAB receptor antagonist.

Identifying novel molecular mechanisms of ghrelin receptor signalling underlying neural control of food intake: interaction with stress and impulsivity

The gut-hormone, ghrelin, activates the centrally expressed growth hormone secretagogue 1a (GHS-R1a) receptor, or ghrelin receptor. The ghrelin receptor is a G-protein coupled receptor (GPCR) expressed in several brain regions, including the arcuate nucleus (Arc), lateral hypothalamus (LH), ventral tegmental area (VTA), nucleus accumbens (NAcc) and amygdala. Activation of the GHS-R1a mediates a multitude of biological activities, including release of growth hormone and food intake. The ghrelin signalling system also plays a key role in the hedonic aspects of food intake and activates the dopaminergic mesolimbic circuit involved in reward signalling. Recently, ghrelin has been shown to be involved in mediating a stress response and to mediate stress-induced food reward behaviour via its interaction with the HPA-axis at the level of the anterior pituitary. Here, we focus on the role of the GHS-R1a receptor in reward behaviour, including the motivation to eat, its anxiogenic effects, and its role in impulsive behaviour. We investigate the functional selectivity and pharmacology of GHS-R1a receptor ligands as well as crosstalk of the GHS-R1a receptor with the serotonin 2C (5-HT2C) receptor, which represent another major target in the regulation of eating behaviour, stress-sensitivity and impulse control disorders. We demonstrate, to our knowledge for the first time, the direct impact of GHS-R1a signalling on impulsive responding in a 2-choice serial reaction time task (2CSRTT) and show a role for the 5-HT2C receptor in modulating amphetamine-associated impulsive action. Finally, we investigate differential gene expression patterns in the mesocorticolimbic pathway, specifically in the NAcc and PFC, between innate low- and high-impulsive rats. Together, these findings are poised to have important implications in the development of novel treatment strategies to combat eating disorders, including obesity and binge eating disorders as well as impulse control disorders, including, substance abuse and addiction, attention deficit hyperactivity disorder (ADHD) and mood disorders.

Effect of angiotensin-related antihypertensives on brain neurotransmitter levels in rats.

The extensive clinical experience of angiotensin converting enzyme inhibitors and angiotensin AT(1) receptor antagonists as antihypertensive agents provide numerous examples of anecdotal evidence of improvements in cognition and mood. This study aimed to determine the effect of chronic treatment with the angiotensin converting enzyme inhibitor, perindopril, and the angiotensin AT(1) receptor antagonist, candesartan, on central neurotransmitter levels in the rat. Perindopril (1.0mg/kg/day) or candesartan (10mg/kg/day) was administered via the drinking water at for 1 week, while controls received water alone. At the end of treatment rats were sacrificed, brains removed and discrete regions dissected and analysed for noradrenaline, dopamine and its major metabolites, and serotonin content. As shown previously we found an increase in striatal dopamine levels after perindopril treatment, though this did not extend to the mesolimbic system with neurotransmitter levels unchanged in the hippocampus, nucleus accumbens and frontal cortex. Conversely, candesartan administration produced no change in dopamine, but significant decreases in both DOPAC and HVA in the striatum. In addition chronic candesartan infusion produced a significant increase in the levels of hippocampal noradrenaline and serotonin; and frontal cortex serotonin content. These results demonstrate that while angiotensin converting enzyme inhibitors and angiotensin AT(1) receptor antagonists act as antihypertensives by affecting the renin-angiotensin system, they have divergent actions on brain neurochemistry.

D-amphetamine fails to increase extracellular dopamine levels in mice lacking alpha 1b-adrenergic receptors: relationship between functional and nonfunctional dopamine release.

It was found recently that locomotor and rewarding effects of psychostimulants and opiates were dramatically decreased or suppressed in mice lacking alpha1b-adrenergic receptors [alpha1b-adrenergic receptor knock-outs (alpha1bAR-KOs)] (Drouin et al., 2002). Here we show that blunted locomotor responses induced by 3 and 6 mg/kg d-amphetamine in alpha1bAR-KO mice [-84 and -74%, respectively, when compared with wild-type (WT) mice] are correlated with an absence of d-amphetamine-induced increase in extracellular dopamine (DA) levels in the nucleus accumbens of alpha1bAR-KO mice. Moreover, basal extracellular DA levels in the nucleus accumbens are lower in alpha1bAR-KO than in WT littermates (-28%; p < 0.001). In rats however, prazosin, an alpha1-adrenergic antagonist, decreases d-amphetamine-induced locomotor hyperactivity without affecting extracellular DA levels in the nucleus accumbens, a finding related to the presence of an important nonfunctional release of DA (Darracq et al., 1998). We show here that local d-amphetamine releases nonfunctional DA with the same affinity but a more than threefold lower amplitude in C57BL6/J mice than in Sprague Dawley rats. Altogether, this suggests that a trans-synaptic mechanism amplifies functional DA into nonfunctional DA release. Our data confirm the presence of a powerful coupling between noradrenergic and dopaminergic neurons through the stimulation of alpha1b-adrenergic receptors and indicate that nonfunctional DA release is critical in the interpretation of changes in extracellular DA levels. These results suggest that alpha1b-adrenergic receptors may be important therapeutic pharmacological targets not only in addiction but also in psychosis because most neuroleptics possess anti-alpha1-adrenergic properties.

Effects of low doses of quinpirole on production of 50 kHz vocalizations in Wistar rats

Rats emit two distinct types of ultrasonic vocalizations in adulthood: 22 kHz (aversive situation), and 50 kHz calls (appetitive situation). The present project is focussed on pharmacological studies of 50 kHz vocalizations. The 50 kHz calls are elicited from dopaminergic activation in the meso limbic pathway and are emitted in such appetitive situations as social contact(s), sexual encounters, food reward, etc. Eighty-five male rats were stereotaxically implanted with bilateral guide cannulae in the nucleus accumbens shell (A= 9.7, L= 1.2, V= 6.7). Quinpirole, a D2/D3 dopaminergic agonist, was injected in low doses to the nucleus accumbens shell in an attempt to elicit 50 kHz vocalizations. A dose response was obtained for the low dose range of quinpirole for six doses: 0.025 Jlg, 0.06 Jlg, 0.12 Jlg, 0.25 Jlg, 0.5 Jlg, and 1.0 Jlg. It was found that only application of the 0.25 Jlg dose of quinpirole and the 7 Jlg dose of amphetamine (positive control) significantly increased the total number of 50 kHz calls (p < 0.006 and p < 0.004 respectively); and particularly significantly increased the frequency modulated type of these calls (p < 0.01, and p < 0.006 respectively). In a double injection procedure, the dose of 0.25 Jlg quinpirole was antagonized with raclopride (D2 antagonist) or U99194A maleate (D3 antagonist) in an attempt to antagonize the response. The 0.25 Jlg dose of quinpirole was successfully antagonized by pre-treatment with an equimolar dose of U99194A maleate (p < 0.008) but not with raclopride. The 7Jlg amphetamine response was also antagonized with an equimolar dose of raclopride. Based on these results, it seems that low doses of quinpirole, particularly the 0.25 Jlg dose, are capable of increasing 50 kHz vocalizations in rats and do so by activation of the D3 dopamine receptor. This is not a biphasic response as seen with locomotor studies. Also noteworthy is the increase in frequency modulated 50 kHz calls elicited by the 0.25 Jlg dose of quinpirole indicating a possible increase in positive affect.