Dopamine release in the nucleus accumbens by hypothalamic stimulation-escape behavior

Dopamine-mediated formation of a memory module in the nucleus accumbens for goal-directed navigation

Spatial memories guide navigation efficiently toward desired destinations. However, the neuronal and circuit mechanisms underlying the encoding of goal locations and its translation into goal-directed navigation remain unclear. Here we demonstrate that mice rapidly form a spatial memory of a shelter during shelter experiences, guiding escape behavior toward the goal location-a shelter-when under threat. Dopaminergic neurons in the ventral tegmental area and their projection to the nucleus accumbens (NAc) encode safety signals associated with the shelter. Optogenetically induced phasic dopamine signals are sufficient to create a place memory that directs escape navigation. Converging dopaminergic and hippocampal glutamatergic inputs to the NAc mediate the formation of a goal-related memory within a subpopulation of NAc neurons during shelter experiences. Artificial co-activation of this goal-related NAc ensemble with neurons in the dorsal periaqueductal gray was sufficient to trigger memory-guided, rather than random, escape behavior. These findings provide causal evidence of cognitive circuit modules linking memory with goal-directed action.

Neuromodulation, Emotional Feelings and Affective Disorders

Affective disorders such as anxiety, phobia and depression are a leading cause of disabilities worldwide. Monoamine neuromodulators are used to treat most of them, with variable degrees of efficacy. Here, we review and interpret experimental findings about the relation of neuromodulation and emotional feelings, in pursuit of two goals: (a) to improve the conceptualisation of affective/emotional states, and (b) to develop a descriptive model of basic emotional feelings related to the actions of neuromodulators. In this model, we hypothesize that specific neuromodulators are effective for basic emotions. The model can be helpful for mental health professionals to better understand the affective dynamics of persons and the actions of neuromodulators - and respective psychoactive drugs - on this dynamics.

Modularity of mind and the role of incentive motivation in representing novelty

Animal and human brains contain a myriad of mental representations that have to be successfully tracked within fractions of a second in a large number of situations. This retrieval process is hard to explain without postulating the massive modularity of cognition. Assuming that the mind is massively modular, it is then necessary to understand how cognitive modules can efficiently represent dynamic environments-in which some modules may have to deal with change-induced novelty and uncertainty. Novelty of a stimulus is a problem for a module when unknown, significant stimuli do not satisfy the module's processing criteria-or domain specificity-and cannot therefore be included in its database. It is suggested that the brain mechanisms of incentive motivation, recruited when faced with novelty and uncertainty, induce transient variations in the domain specificity of cognitive modules in order to allow them to process information they were not prepared to learn. It is hypothesised that the behavioural transitions leading from exploratory activity to habit formation are correlated with (and possibly caused by) the organism's ability to counter novelty-induced uncertainty.

The mesoaccumbens dopamine in coping with stress

Mesoaccumbens dopamine (DA) is involved in the stress response. Although neural mechanisms involved in stress are of paramount importance for both clinical and preclinical research, the results of studies on the stress response by mesoaccumbens DA have received little attention. Therefore, we aimed to review these results and propose a role for mesoaccumbens DA in coping with stress. The data reviewed support the view that fluctuations of tonic levels characterize the mesoaccumbens DA stress response. Stress-induced increase of tonic DA levels in nucleus accumbens (NAc) supports expression of responses aimed at removing and avoiding the stressor through activation of DA D2 receptors, whereas inhibition of DA is associated with cessation of active defensive responses. In novel unescapable/uncontrollable stressful conditions tonic levels of DA in NAc show an initial increase followed by a decrease below pre-stress levels that lasts as long as the stressful situation. This biphasic response fits with the dynamics of the primary and secondary appraisal of a stressor that cannot be removed, escaped or controlled by the organism. In fact, NAc DA fluctuations are controlled by the medial pre-frontal cortex, which is involved in stress appraisal. We propose that enhanced mesoaccumbens DA supports expression of active coping strategies against an event appraised as a stressor and that inhibition of DA is required for passive coping with stressful situations appraised as unescapable/uncontrollable.

Cholinergic modulation of mesolimbic dopamine function and reward

The substantial health risk posed by obesity and compulsive drug use has compelled a serious research effort to identify the neurobiological substrates that underlie the development these pathological conditions. Despite substantial progress, an understanding of the neurochemical systems that mediate the motivational aspects of drug-seeking and craving remains incomplete. Important work from the laboratory of Bart Hoebel has provided key information on neurochemical systems that interact with dopamine (DA) as potentially important components in both the development of addiction and the expression of compulsive behaviors such as binge eating. One such modulatory system appears to be cholinergic pathways that interact with DA systems at all levels of the reward circuit. Cholinergic cells in the pons project to DA-rich cell body regions in the ventral tegmental area (VTA) and substantial nigra (SN) where they modulate the activity of dopaminergic neurons and reward processing. The DA terminal region of the nucleus accumbens (NAc) contains a small but particularly important group of cholinergic interneurons, which have extensive dendritic arbors that make synapses with a vast majority of NAc neurons and afferents. Together with acetylcholine (ACh) input onto DA cell bodies, cholinergic systems could serve a vital role in gating information flow concerning the motivational value of stimuli through the mesolimbic system. In this report we highlight evidence that CNS cholinergic systems play a pivotal role in behaviors that are motivated by both natural and drug rewards. We argue that the search for underlying neurochemical substrates of compulsive behaviors, as well as attempts to identify potential pharmacotherapeutic targets to combat them, must include a consideration of central cholinergic systems.

Neurobiology of aversive states

Hoebel and colleagues are often known as students of reward and how it is coded in the CNS. This article, however, attempts to focus on the significant advances by Hoebel and others in dissecting out behavioral components of distinct aversive states and in understanding the neurobiology of aversion and the link between aversive states and addictive behaviors. Reward and aversion are not necessarily dichotomous and may reflect an affective continuum contingent upon environmental conditions. Descriptive and mechanistic studies pioneered by Bart Hoebel have demonstrated that the shift in the reward-aversion spectrum may be, in part, a result of changes in central dopamine/acetylcholine ratio, particularly in the nucleus accumbens. The path to aversion appears to include a specific neurochemical signature: reduced dopamine release and increased acetylcholine release in "reward centers" of the brain. Opioid receptors may have a neuromodulatory role on both of these neurotransmitters.

The uncertainty processing theory of motivation

Most theories describe motivation using basic terminology (drive, 'wanting', goal, pleasure, etc.) that fails to inform well about the psychological mechanisms controlling its expression. This leads to a conception of motivation as a mere psychological state 'emerging' from neurophysiological substrates. However, the involvement of motivation in a large number of behavioural parameters (triggering, intensity, duration, and directedness) and cognitive abilities (learning, memory, decision, etc.) suggest that it should be viewed as an information processing system. The uncertainty processing theory (UPT) presented here suggests that motivation is the set of cognitive processes allowing organisms to extract information from the environment by reducing uncertainty about the occurrence of psychologically significant events. This processing of information is shown to naturally result in the highlighting of specific stimuli. The UPT attempts to solve three major problems: (i) how motivations can affect behaviour and cognition so widely, (ii) how motivational specificity for objects and events can result from nonspecific neuropharmacological causal factors (such as mesolimbic dopamine), and (iii) how motivational interactions can be conceived in psychological terms, irrespective of their biological correlates. The UPT is in keeping with the conceptual tradition of the incentive salience hypothesis while trying to overcome the shortcomings inherent to this view.

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

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

Dynamic changes in dopamine tone during self-stimulation of the ventral tegmental area in rats

In a prior study, phasic release of dopamine (DA) in the nucleus accumbens (NAc) was only transiently and rarely detected by means of fast-scan cyclic voltammetry (FCSV) in rats already trained to work for electrical stimulation of the ventral tegmental area (VTA) on a continuous reinforcement schedule. However, in rats receiving rewarding electrical stimulation via lateral hypothalamic (LH) electrodes, elevated DA tone in the NAc terminal field was detected via microdialysis for up to 2h, even when short (1.5s) inter-train intervals were employed. To better characterize the similarities and differences between the FSCV and microdialysis measurements, we trained rats to self-administer VTA stimulation under conditions similar to those employed in the initial FSCV study. The results resemble those obtained by means of microdialysis in rats receiving LH stimulation but differed from the prior FSCV data. Although the concentration of DA in dialysate obtained from NAc probes did fall after having peaked at the 30 min mark, this decline set in much later than in the FSCV studies, and elevated DA tone could still be detected after 110 min of self-stimulation. The stimulation-induced peak in DA tone could be restored by a 30 min rest period, a manipulation that was ineffective previously in restoring the FSCV measure of phasic release. These findings are discussed in terms of the differential sensitivity of the FSCV and microdialysis methods to phasic and tonic signaling by DA neurons and to different transitions between their activity states.

Overconsumption of dietary fat and alcohol: mechanisms involving lipids and hypothalamic peptides

The studies described in this report provide interesting animal models for exploring some of the metabolic and neural antecedents to the over-consumption of fat and alcohol. The results provide strong support for the existence of positive feedback loops that involve a close relation between circulating lipids and orexigenic peptides in dorsal regions of the hypothalamus. The peptides involved in these circuits include galanin, enkephalin, dynorphin and orexin. These peptides are expressed in the paraventricular nucleus and perifornical lateral hypothalamus, and they have very different functions from peptides expressed in the arcuate nucleus. Through mechanisms involving circulating lipids that rise on energy-dense diets, these peptides in the dorsal hypothalamus are each increased by the consumption of fat and ethanol; these nutrients, in turn, stimulate further production of these same peptides that promote overeating and excess drinking. These mechanisms involving non-homeostatic, positive feedback circuits may be required under conditions when food supplies are scarce and periods of gorging are essential to survival. However, they have pathological and sometimes life-threatening consequences in modern society, where fat-rich foods and alcoholic drinks are abundantly available and are contributing to the marked rise over the past 25 years in obesity and diabetes in both children and adults.

Prolonged rewarding stimulation of the rat medial forebrain bundle: neurochemical and behavioral consequences

Extracellular dopamine levels were measured in the rat nucleus accumbens by means of in vivo microdialysis. Delivery of rewarding medial forebrain bundle stimulation at a low rate (5 trains/min) produced a sustained elevation of dopamine levels, regardless of whether train onset was predictable. When the rate of train delivery was increased to 40 trains/min, dopamine levels rose rapidly during the first 40 min but then declined toward the baseline range. The rewarding impact of the stimulation was reduced following prior delivery of stimulation at the high, but not the low, rate. These results support the idea that dopamine tone plays an enabling role in brain stimulation reward and is elevated similarly by predictable and unpredictable stimulation.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Alterations in blood glucose levels under hyperinsulinemia affect accumbens dopamine

Dopaminergic systems have been implicated in diabetes and obesity. Notwithstanding, the most basic relationship between dopamine and plasma insulin as well as glucose levels yet remains unknown. The present experiments were designed to investigate the effects of acute hyperinsulinemia on basal dopamine levels in the nucleus accumbens of the rat under chloral hydrate anesthesia using acute microdialysis in combination with the hyperinsulinemic-glycemic clamping procedure. In Experiment 1, each rat was infused with one of the three concentrations of insulin (2.4, 4.8, or 9.6 mU/kg per min) while plasma glucose levels were maintained at euglycemia (approximately 5.5 mmol/L). Dopamine, dihydroxyphenylacetic acid and homovanillic acid were not significantly different from baseline during either the clamp or post-clamp periods for all insulin concentrations. In Experiment 2, rats were infused with the highest concentration of insulin (9.6 mU/kg per min) and plasma glucose levels were maintained at either hypoglycemia (approximately 3 mmol/L) or hyperglycemia (approximately 14 mmol/L). Dopamine was elevated at 100 min (+113% above basal levels) and 120 min (+117%) in the hypoglycemic condition and at 120 min (+121%) in the hyperglycemic condition. In the hyperglycemic post-clamp period, homovanillic acid was decreased below basal levels (approximately -32%). These results together suggest that short-term blood glucose deviations coupled with acute hyperinsulinemia affect the mesoaccumbens dopamine system.

Critical assessment of how to study addiction and its treatment: human and non-human animal models

Laboratory models, both animal and human, have made enormous contributions to our understanding of addiction. For addictive disorders, animal models have the great advantage of possessing both face validity and a significant degree of predictive validity, already demonstrated. Another important advantage to this field is the ability of reciprocal interplay between preclinical and clinical experiments. These models have made important contributions to the development of medications to treat addictive disorders and will likely result in even more advances in the future. Human laboratory models have gone beyond data obtained from patient histories and enabled investigators to make direct observations of human drug self-administration and test the effects of putative medications on this behavior. This review examines in detail some animal and human models that have led not only to important theories of addiction mechanisms but also to medications shown to be effective in the clinic.

The effects of systemic administration of selective antagonists of dopamine D1 and D2/D3 receptors on food-related and defensive (escape responses) conditioned paw-placing responses in cats

Experiments were performed on cats to study the effects of systemic administration of antagonists of dopaminergic transmission on food-related and defensive (an escape response) operant conditioned reflexes acquired on the basis of the innate response of placing the forepaw on a support. Selective blockade of D1 receptors with SCH23390 (0.005-0.1 mg/kg) completely and selective blockade of D2/D3 receptors with raclopride (0.1-0.25 mg/kg) partially suppressed both reflexes. At these doses, both blockers had stronger actions on the defensive conditioned escape reflex than the food-related reflex: SCH23390 had significantly stronger inhibitory effects on both reflexes than raclopride.

Acetylcholine in the accumbens is decreased by diazepam and increased by benzodiazepine withdrawal: a possible mechanism for dependency

Diazepam is a benzodiazepine used in the treatment of anxiety, insomnia and seizures, but with the potential for abuse. Like the other benzodiazepine anxiolytics, diazepam does not increase dopamine in the nucleus accumbens. This raises the question as to which other neurotransmitter systems are involved in diazepam dependence. The goal was to monitor dopamine and acetylcholine simultaneously following acute and chronic diazepam treatment and after flumazenil-induced withdrawal. Rats were prepared with microdialysis probes in the nucleus accumbens and given diazepam (2, 5 and 7.5 mg/kg) acutely and again after chronic treatment. Accumbens dopamine and acetylcholine decreased, with signs of tolerance to the dopamine effect. When these animals were put into the withdrawal state with flumazenil, there was a significant rise in acetylcholine (145%, P<0.001) with a smaller significant rise in dopamine (124%, P<0.01). It is suggested that the increase in acetylcholine release, relative to dopamine, is a neural component of the withdrawal state that is aversive.

Neural Substrates of Coping Behavior in the Rat: Possible Importance of Mesocorticolimbic Dopamine System

This study measured expression of Fos protein, an indicator of neural activation, in 116 brain regions of rats that were able to control a stressor (i.e., avoid and/or escape an electric shock), and compared the changes with those observed in yoked rats that received the same shocks but without having control over them. The authors' interest was to find brain regions where elevated activity occurs in conjunction with control. Activity in these brain regions might be responsible for the consequences of having control, such as reduction of stress responses. Eleven brain regions were found in which rats with control showed significantly more Fos expression than was seen in yoked rats that did not have control. Six of these brain regions were part of the mesocorticolimbic dopamine system. These results point to the mesocorticolimbic dopamine system as being importantly involved in the mediation and/or the consequences of coping behavior.

The dopamine receptor antagonist SCH 23390 attenuates feeding induced by Δ9-tetrahydrocannabinol

A large body of evidence supports the notion that Delta9-tetrahydrocannabinol (THC) stimulates food intake by its actions on CB1 cannabinoid receptors. Indirect evidence also suggests a role for dopamine (DA) receptors in mediating THC-induced feeding. In the present study, a series of experiments involving intraperitoneal drug administration in rats were conducted to further investigate the interaction between cannabinoid and dopamine receptors in feeding behaviour. Male Wistar rats were habituated to the test environment and injection procedure, and then were injected with vehicle alone, the dopamine D1-like receptor antagonist SCH 23390 (0.005, 0.01, 0.5 or 0.1 mg/kg), THC (0.1, 0.5 or 1.0 mg/kg) or SCH 23390 and THC combined. Food intake and locomotor activity were then measured for 120 min. Results revealed that administration of SCH 23390 dose-dependently decreased food intake while THC dose-dependently increased feeding. Furthermore, SCH 23390 attenuated feeding induced by THC at a dose that did not affect feeding on its own. These findings provide direct evidence for the existence of cannabinoid-dopamine interactions in feeding behaviour and suggest that dopamine D1 signalling is necessary for cannabinoids to stimulate food intake.

The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition?

Psychopharmacological studies have implicated the dopaminergic innervation of the nucleus accumbens (NAC) in reward and reinforcement, in the actions of addictive drugs, and in the control of the symptoms of schizophrenia. Recent developments in in vivo dialysis, and other in vivo neurochemical techniques have permitted a more direct analysis of the behavioural correlates of increased dopamine release in rats, and have largely confirmed these findings in relation to reward, and drugs of abuse potential. However, dopamine release has also been found to be increased by many other stimuli/situations including aversive stimuli, stimuli conditioned to aversive stimuli, complex novel stimuli, and in the process of conditioning itself. These results contrast with electrophysiological data obtained in the behaving monkey, where rewarding stimuli, or stimuli predictive of reward are associated with increased firing of presumptive dopamine neurones projecting to the NAC (and indeed to the striatum), but mild aversive stimuli are not, leading to the suggestion that this system subserves a more purely reward function, or indeed that it provides a reward error signal. Further exploration of these issues will depend upon a comparison of increased dopamine cell firing and increased dopamine release, and an analysis of the behavioural effects of blocking these increases in dopamine transmission. One suggestion, deriving from work on latent inhibition, is that the significance of dopamine release by salient stimuli is to allow learning about stimuli which would otherwise be excluded on the basis of familiarity. This suggests that in addition to a role in some types of learning about salient stimuli, dopamine release in NAC may have a role in controlling the attention paid to familiar stimuli. Since it is difficult to see a connection between simple learning about rewards, and the symptoms of schizophrenia, this provides a more convincing link between the dopamine theory of schizophrenia, and the attentional difficulties held by many theorists to underlie schizophrenic symptoms.

Genetic susceptibility of mesocortical dopamine to stress determines liability to inhibition of mesoaccumbens dopamine and to behavioral 'despair' in a mouse model of depression

Clinical and preclinical research suggests a major role of mesocortical dopamine (DA) in psychopathology through regulation of subcortical, especially mesoaccumbens, DA functioning. In these experiments we demonstrate that the high vulnerability to stress-induced 'despair' and mesoaccumbens DA inhibition, exhibited by mice of the inbred strain C57BL/6 (C57) in a common animal model of depression, depends on their being highly susceptible to stress-induced mesocortical DA activation. Thus, C57 mice but not mice of the DBA/2 strain showed an extremely high level of immobility on their first experience with the forced swimming test (FST) as well as immediate and strong activation of mesocortical DA metabolism and inhibition of mesoaccumbens DA metabolism and release. In addition, the behavioral and the mesoaccumbens DA responses to FST in C57 mice were reduced and reversed, respectively, by bilateral mesocortical DA depletion. Finally, chronic treatment with the antidepressant clomipramine reduced immobility and eliminated both mesocortical DA activation and mesoaccumbens DA inhibition in response to FST. These results suggest that a genetically determined susceptibility to stress by the mesocortical DA system may favor the development of pathological behavioral responses through inhibition of subcortical DA transmission.

Dopamine and glutamate release in the nucleus accumbens and ventral tegmental area of rat following lateral hypothalamic self-stimulation

Rewarding hypothalamic brain stimulation is thought to depend on trans-synaptic activation of high-threshold (and thus rarely directly depolarized by rewarding stimulation) dopaminergic fibers of the medial forebrain bundle. We used in vivo microdialysis and high-performance liquid chromatography coupled with electrochemical or fluorometric detection to investigate the concurrent release of dopamine and glutamate in the nucleus accumbens septi and in the ventral tegmental area, as a function of lateral hypothalamic self-stimulation.Self-stimulation at a variety of stimulation frequencies and pulse widths increased levels of dopamine and its primary metabolites, dihydroxyphenylacetic acid and homovanillic acid in the nucleus accumbens. Lateral hypothalamic self-stimulation also induced significant increases in ventral tegmental area dopamine and metabolite levels, and the percentage increase of dopamine was higher in this region than in the nucleus accumbens. Local perfusion with the dopamine uptake inhibitor nomifensine (10 microM) increased dopamine levels in the nucleus accumbens about three-fold and potentiated the increase of dopamine levels induced by self-stimulation. Nomifensine perfusion also induced a delayed decrease in nucleus accumbens glutamate levels, and self-stimulation did not modify this effect of the drug. Local perfusion with the D2-type dopamine receptor antagonist raclopride significantly increased both basal and self-stimulation induced dopamine release in the nucleus accumbens. Neither nomifensine nor raclopride perfusion significantly affected the maximal rates of self-stimulation. Perfusion with tetrodotoxin (2 microM) into nucleus accumbens significantly decreased basal and prevented stimulation-induced increases in accumbens dopamine levels but only slightly decreased the rate of self-stimulation. In contrast, perfusion of tetrodotoxin (0.5 microM) into the ventral tegmental area decreased basal and blocked stimulation-induced increases in both nucleus accumbens and ventral tegmental area dopamine levels; this treatment also blocked or strongly inhibited self-stimulation. While it had no effect on glutamate levels in the nucleus accumbens, lateral hypothalamic self-stimulation induced a significant and tetrodotoxin-sensitive increase in glutamate levels in the ventral tegmental area. Taken together, the present results indicate that, across a broad range of stimulation parameters, rewarding lateral hypothalamus stimulation causes major and persistent activation of the mesolimbic dopamine system, and suggest descending glutamatergic fibers in the medial forebrain bundle as a candidate for the directly activated descending pathway in lateral hypothalamus brain stimulation reward.

Predictable stress promotes place preference and low mesoaccumbens dopamine response

Aversive stimuli that are signaled, and therefore predictable, are preferred to unsignaled ones and promote less severe stress-related disturbances. Since stressful events are known to activate mesoaccumbens dopamine (DA) transmission, in the present experiments, we evaluated possible differences in mesoaccumbens DA response to predictable and unpredictable footshocks. Mice of the inbred strain DBA/2 were trained for conditioned place preference (CPP) in shuttle boxes. The procedure promoted significant preference for the compartment previously paired with predictable shocks (PR) to that paired with unpredictable shocks (NP). Mesoaccumbens levels of DA and its metabolites were therefore evaluated either after the first or the last (third) training session. A significant increase of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels were observed in animals exposed for the first time to the apparatus without shock delivery (SHAM) or to the PR and NP conditions compared with unhandled mice. There was no difference between PR and NP values, and DOPAC and HVA levels in both groups differed from those observable in the SHAM-exposed group. However, trained mice exposed to NP showed a significant elevation of DOPAC and HVA levels in comparison with those exposed to PR. These results show that information about predictability of aversive stimuli reduces central stress responses and suggest a possible relationship between reduced stressfulness and preference for predictable aversive experiences.

Opposite genotype-dependent mesocorticolimbic dopamine response to stress

Identification of relevant phenotypes related to neural functioning has yet to receive the needed attention, although behavioral phenotyping, through comparative studies in inbred strains of mice, has produced some major findings (Cabib et al., 2000; Crabbe, 1999; Gerlai, 1996; Lathe, 1996). Central responses to stress play a major psychopathogenic role in the presence of a genetic liability (Fowels, 1992), and mesocortical and mesoacumbens dopamine metabolism and release are the most relevant among these responses (Abercrombie et al., 1989; Cabib and Puglisi-Allegra, 1994; Chrapusta et al., 1997; Di Chiara et al., 1999; Hervé et al., 1979; Imperato et al., 1991). Therefore, in the present study, we assessed strain-dependent differences in mesocortical and mesoaccumbens dopamine responses to a widely utilized stressful procedure (restraint), by comparing mice of the oldest and most studied inbred strains (Cabib et al., 2000): the C57BL/6JIco and DBA/2JIco. We found that stress produced inhibition of mesoaccumbens dopamine release accompanied by a very fast and strong activation of mesocortical dopamine metabolism in C57BL/6JIco mice, and the opposite in mice of the DBA/2JIco strain. These results suggest a genetic control over the balance between mesocortical and mesoaccumbens dopamine responses to stress, and provide a model for pre-clinical studies on molecular genetics of depression.

Aversive hypothalamic stimulation releases acetylcholine in the nucleus accumbens, and stimulation-escape decreases it

Hypothalamic electrodes can generate positive reinforcement, as shown by self-stimulation, and negative reinforcement shown by stimulation-escape. It was hypothesized that acetylcholine (ACh) is released in the nucleus accumbens during the aversive state that underlies stimulation-escape. If this is correct, escape behavior should lower extracellular ACh. Rats were prepared with microdialysis probes in the accumbens (posterior shell region) and electrodes in the perifornical lateral hypothalamus. Animals learned to press a lever for 0.5 s trains of stimulation (typically 3600 responses/h). Then they were given automatic stimulation to determine which animals would also learn to press a lever to turn stimulation off for 5 s at a time (typically 75 responses/h). Accumbens microdialysis showed that automatic stimulation caused extracellular ACh to double, but only in the rats that were motivated to learn stimulation-escape. When allowed to escape stimulation, these animals lowered extracellular ACh significantly. It is concluded that ACh release in the accumbens is related to the neural state that animals work to escape.

The role of nucleus accumbens dopamine in motivated behavior: a unifying interpretation with special reference to reward-seeking

Studies addressing behavioral functions of dopamine (DA) in the nucleus accumbens septi (NAS) are reviewed. A role of NAS DA in reward has long been suggested. However, some investigators have questioned the role of NAS DA in rewarding effects because of its role in aversive contexts. As findings supporting the role of NAS DA in mediating aversively motivated behaviors accumulate, it is necessary to accommodate such data for understanding the role of NAS DA in behavior. The aim of the present paper is to provide a unifying interpretation that can account for the functions of NAS DA in a variety of behavioral contexts: (1) its role in appetitive behavioral arousal, (2) its role as a facilitator as well as an inducer of reward processes, and (3) its presently undefined role in aversive contexts. The present analysis suggests that NAS DA plays an important role in sensorimotor integrations that facilitate flexible approach responses. Flexible approach responses are contrasted with fixed instrumental approach responses (habits), which may involve the nigro-striatal DA system more than the meso-accumbens DA system. Functional properties of NAS DA transmission are considered in two stages: unconditioned behavioral invigoration effects and incentive learning effects. (1) When organisms are presented with salient stimuli (e.g., novel stimuli and incentive stimuli), NAS DA is released and invigorates flexible approach responses (invigoration effects). (2) When proximal exteroceptive receptors are stimulated by unconditioned stimuli, NAS DA is released and enables stimulus representations to acquire incentive properties within specific environmental context. It is important to make a distinction that NAS DA is a critical component for the conditional formation of incentive representations but not the retrieval of incentive stimuli or behavioral expressions based on over-learned incentive responses (i.e., habits). Nor is NAS DA essential for the cognitive perception of environmental stimuli. Therefore, even without normal NAS DA transmission, the habit response system still allows animals to perform instrumental responses given that the tasks take place in fixed environment. Such a role of NAS DA as an incentive-property constructor is not limited to appetitive contexts but also aversive contexts. This dual action of NAS DA in invigoration and incentive learning may explain the rewarding effects of NAS DA as well as other effects of NAS DA in a variety of contexts including avoidance and unconditioned/conditioned increases in open-field locomotor activity. Particularly, the present hypothesis offers the following interpretation for the finding that both conditioned and unconditioned aversive stimuli stimulate DA release in the NAS: NAS DA invigorates approach responses toward 'safety'. Moreover, NAS DA modulates incentive properties of the environment so that organisms emit approach responses toward 'safety' (i.e., avoidance responses) when animals later encounter similar environmental contexts. There may be no obligatory relationship between NAS DA release and positive subjective effects, even though these systems probably interact with other brain systems which can mediate such effects. The present conceptual framework may be valuable in understanding the dynamic interplay of NAS DA neurochemistry and behavior, both normal and pathophysiological.

Diazepam-induced hyperphagia in mice is sensitive to quinpirole

The present trial examined the possibility that diazepam (DZP, 1 mg/kg) induces hyperphagia by acting on the dopaminergic system. Quinpirole (QP), dopamine D-2 receptor agonist, was used for this purpose. Mice fasted for 24 hr were treated with QP 1 (QP-1) or 2 (QP-2) mg/kg 30 min prior to termination of the starvation. DZP was given to untreated mice and half of the QP-1 and QP-2 treated mice 10 min before the termination of the starvation. Food consumed during six 30 min intervals (30 min-feeding), food consumed for 3 hr (total feeding), time required to enter the room containing food by passing through a maze with four multiple routes (time to banquet), latent period to commencement of eating food after entering the banquet room (latent period), and feeding frequency for the 30 min intervals as well as for 3 hr were measured. DZP stimulated feeding, shortened the latent period without affecting the time to banquet and increased the feeding frequency. The hyperphagic effect was restricted to the first 30 min interval only. Both QP-1 and QP-2 first reduced, then progressively stimulated, and finally reduced feeding without modifying total feeding, thus making a bell-shaped profile. They also prolonged both the time to banquet and the latent period, and reduced the feeding frequency of the first 30 min interval but not that for 3 hr. Both QP-1 and QP-2 canceled all the effects of DZP. These results imply that dopamine D2 receptor is involved in the induction of hyperphagia by DZP.

In vivo neurochemical correlates of cognitive processes: methodological and conceptual challenges

The advent of the use of in vivo microdialysis and voltammetry techniques in behaving animals has ushered in a great deal of research on the neurochemistry of cognition. While studies exploring the relationship between neurotransmitter release and cognitive processing are quite feasible, a number of methodological and conceptual issues pose challenges to the interpretation of experimental results. These challenges include: 1) a demonstration that the behavioral task highlights the particular cognitive construct under study; 2) a determination of the role of non-cognitive variables (i.e. transfer effects, sensory stimulation, motivational variables, and motor activity) in affecting transmitter release, and 3) a recognition of the value of a distributed systems approach to studying the neurochemistry of cognition. This review summarizes the data on the validity of microdialysis and voltammetry as correlates of neurotransmitter release and then illustrates the impact that the above challenges can have on the conclusions drawn from various studies.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Galanin in the hypothalamus raises dopamine and lowers acetylcholine release in the nucleus accumbens: a possible mechanism for hypothalamic initiation of feeding behavior

Rats were prepared with two implanted guide shafts, one for microdialysis to measure extracellular dopamine (DA) and acetylcholine (ACh) in the posterior, medial nucleus accumbens (NAc), and the other for microinjection of galanin, neuropeptide Y or saline in the hypothalamic paraventricular nucleus (PVN). There was an increase in DA release and a decrease in ACh in the NAc following microinjections of galanin into the PVN. The effect was observed only in rats for which identical galanin injections induced feeding in separate tests. Ringer injections had no effects. Unlike galanin, neuropeptide Y in the PVN induced eating without altering DA/ACh; whereas earlier results showed that norepinephrine in the PVN works like galanin. These results suggest that galanin initiates feeding, in part, by activating the mesolimbic DA system and suppressing intrinsic cholinergic activity in the NAc. This may prime instrumental behavior with DA while disinhibiting behavior by lowering ACh.