Activation of the mesocortical dopamine system by feeding: lack of a selective response to stress

Unanticipated Stressful and Rewarding Experiences Engage the Same Prefrontal Cortex and Ventral Tegmental Area Neuronal Populations

Brain networks that mediate motivated behavior in the context of aversive and rewarding experiences involve the prefrontal cortex (PFC) and ventral tegmental area (VTA). Neurons in both regions are activated by stress and reward, and by learned cues that predict aversive or appetitive outcomes. Recent studies have proposed that separate neuronal populations and circuits in these regions encode learned aversive versus appetitive contexts. But how about the actual experience? Do the same or different PFC and VTA neurons encode unanticipated aversive and appetitive experiences? To address this, we recorded unit activity and local field potentials (LFPs) in the dorsomedial PFC (dmPFC) and VTA of male rats as they were exposed, in the same recording session, to reward (sucrose) or stress (tail pinch) spaced 1 h apart. As expected, experience-specific neuronal responses were observed. Approximately 15–25% of single units in each region responded by excitation or inhibition to either stress or reward, and only stress increased LFP theta oscillation power in both regions and coherence between regions. But the largest number of responses (29% dmPFC and 30% VTA units) involved dual-valence neurons that responded to both stress and reward exposure. Moreover, the temporal profile of neuronal population activity in dmPFC and VTA as assessed by principal component analysis (PCA) were similar during both types of experiences. These results reveal that aversive and rewarding experiences engage overlapping neuronal populations in the dmPFC and the VTA. These populations may provide a locus of vulnerability for stress-related disorders, which are often associated with anhedonia.

Food reward and gut-brain signalling

The increasing availability of ultra-processed, energy dense food is contributing to the spread of the obesity pandemic, which is a serious health threat in today’s world. One possible cause for this association arises from the fact that the brain is wired to derive pleasure from eating. Specifically, food intake activates reward pathways involving dopamine receptor signalling. The reinforcing value of specific food items results from the interplay between taste and nutritional properties. Increasing evidence suggests that nutritional value is sensed in the gut by chemoreceptors in the intestinal tract and the hepatic portal vein, and conveyed to the brain through neuronal and endocrine pathways to guide food selection behaviour. Ultra-processed food is designed to potentiate the reward response through a combination of high fat and high sugar, therebye seeming highly appetizing. There is increasing evidence that overconsumption of processed food distorts normal reward signalling, leading to compulsive eating behaviour and obesity. Hence, it is essential to understand food reward and gut-brain signalling to find an effective strategy to combat the obesity pandemic.

Food Intake Recruits Orosensory and Post-ingestive Dopaminergic Circuits to Affect Eating Desire in Humans

Pleasant taste and nutritional value guide food selection behavior. Here, orosensory features of food may be secondary to its nutritional value in underlying reinforcement, but it is unclear how the brain encodes the reward value of food. Orosensory and peripheral physiological signals may act together on dopaminergic circuits to drive food intake. We combined fMRI and a novel [11C]raclopride PET method to assess systems-level activation and dopamine release in response to palatable food intake in humans. We identified immediate orosensory and delayed post-ingestive dopamine release. Both responses recruit segregated brain regions: specialized integrative pathways and higher cognitive centers. Furthermore, we identified brain areas where dopamine release reflected the subjective desire to eat. Immediate dopamine release in these wanting-related regions was inversely correlated with, and presumably inhibited, post-ingestive release in the dorsal striatum. Our results highlight the role of brain and periphery in interacting to reinforce food intake in humans.

Destruction of noradrenergic terminals increases dopamine concentration and reduces dopamine metabolism in the medial prefrontal cortex

Effects of destroyed noradrenergic (NE) innervation in the medial prefrontal cortex (mPFC) were examined on dopamine (DA) content and metabolism. Six-hydroxy-DOPA (6-OHDOPA) or 6-hydroxy-dopamine (6-OHDA) in combination with a potent DA reuptake inhibitor GBR 12935 or 6-OHDA were injected bilaterally into the mPFC in separate groups of animals. In addition, GBR 12935 or vehicle was injected into the mPFC in two other groups of animals as control experiments. NE and DA concentrations from postmortem tissue of the mPFC were measured using HPLC with electrochemical detection. In addition, extracellular NE, DA and DOPAC levels were determined using in vivo microdialysis after the 6-OHDA lesion in combination with GBR 12935 pretreatment in the mPFC. Using reverse microdialysis of alpha-2-adrenoreceptor antagonist yohimbine, we tested the remaining activity of NE innervation and the extracellular concentration of DA and DOPAC. NE and DA concentrations from postmortem tissue of the mPFC showed that 6-OHDOPA lesion reduced NE concentration to 76%, which was a non-significant alteration, however it enhanced significantly DA concentration to 186% compared to vehicle. After 6-OHDA lesion with GBR 12935 pretreatment, concentration of NE significantly decreased to 51% and DA level increased to 180%. 6-OHDA lesion without GBR 12635 pretreatment decreased NE concentration to 23% and DA concentration to 67%. In the microdialysis experiment, after 6-OHDA lesion with GBR 12935 pretreatment, extracellular NE levels were not detectable, whereas extracellular DA levels were increased and DOPAC levels were decreased compared to controls. Reverse microdialysis of yohimbine demonstrated that the residual NE innervation was able to increase NE level and DA levels, but DOPAC concentration remained low after lesion of the NE terminals. These findings suggest that the damage of NE innervation in the mPFC may alter extracellular DA level due to a reduced DA clearance.

Reactive Oxygen Species: Physiological and Physiopathological Effects on Synaptic Plasticity

In the mammalian central nervous system, reactive oxygen species (ROS) generation is counterbalanced by antioxidant defenses. When large amounts of ROS accumulate, antioxidant mechanisms become overwhelmed and oxidative cellular stress may occur. Therefore, ROS are typically characterized as toxic molecules, oxidizing membrane lipids, changing the conformation of proteins, damaging nucleic acids, and causing deficits in synaptic plasticity. High ROS concentrations are associated with a decline in cognitive functions, as observed in some neurodegenerative disorders and age-dependent decay of neuroplasticity. Nevertheless, controlled ROS production provides the optimal redox state for the activation of transductional pathways involved in synaptic changes. Since ROS may regulate neuronal activity and elicit negative effects at the same time, the distinction between beneficial and deleterious consequences is unclear. In this regard, this review assesses current research and describes the main sources of ROS in neurons, specifying their involvement in synaptic plasticity and distinguishing between physiological and pathological processes implicated.

Acute stress impairs set-shifting but not reversal learning

The ability to update and modify previously learned behavioral responses in a changing environment is essential for successful utilization of promising opportunities and for coping with adverse events. Valid models of cognitive flexibility that contribute to behavioral flexibility include set-shifting and reversal learning. One immediate effect of acute stress is the selective impairment of performance on higher-order cognitive control tasks mediated by the medial prefrontal cortex (mPFC) but not the hippocampus. Previous studies show that the mPFC is required for set-shifting but not for reversal learning, therefore the aim of the present experiment is to assess whether exposure to acute stress (15 min of mild tail-pinch stress) given immediately before testing on either a set-shifting or reversal learning tasks would impair performance selectively on the set-shifting task. An automated operant chamber-based task, confirmed that exposure to acute stress significantly disrupts set-shifting but has no effect on reversal learning. Rats exposed to an acute stressor require significantly more trials to reach criterion and make significantly more perseverative errors. Thus, these data reveal that an immediate effect of acute stress is to impair mPFC-dependent cognition selectively by disrupting the ability to inhibit the use of a previously relevant cognitive strategy.

Abrupt changes in the patterns and complexity of anterior cingulate cortex activity when food is introduced into an environment

Foraging typically involves two distinct phases, an exploration phase where an organism explores its local environment in search of needed resources and an exploitation phase where a discovered resource is consumed. The behavior and cognitive requirements of exploration and exploitation are quite different and yet organisms can quickly and efficiently switch between them many times during a foraging bout. The present study investigated neural activity state dynamics in the anterior cingulate sub-region of the rat medial prefrontal cortex (mPFC) when a reliable food source was introduced into an environment. Distinct and largely independent states were detected using a Hidden Markov Model (HMM) when food was present or absent in the environment. Measures of neural entropy or complexity decreased when rats went from exploring the environment to exploiting a reliable food source. Exploration in the absence of food was associated with many weak activity states, while bouts of food consumption were characterized by fewer stronger states. Widespread activity state changes in the mPFC may help to inform foraging decisions and focus behavior on what is currently most prominent or valuable in the environment.

From disordered eating to addiction: the "food drug" in bulimia nervosa

The high prevalence of substance abuse in individuals with bulimia nervosa (BN) and the pervasive symptom substitution in many types of drug addiction suggest that a number of substances--including food--can impair an individual's self-control, even in the presence of negative consequences. Nonetheless, the neurobiological similarities between BN and drug addiction are not clearly established. This review explores how the specific eating patterns seen in BN (binge eating and purging, with intermittent dietary restriction) are particularly addictive and differentiate BN from other eating disorders and obesity. A number of peripheral and central biological aberrations seen in BN may result in altered reward sensitivity in these individuals, particularly through effects on the dopaminergic system. Neurobiological findings support the notion that BN is an addictive disorder, which has treatment implications for therapy and pharmacological manipulations.

Strong somatic stimulation differentially regulates the firing properties of prefrontal cortex neurons

Among the brain structures involved in processing affective stimuli, the roles of the prefrontal cortex (PFC) and the mesocorticolimbic dopaminergic (DA) innervation are well established. In contrast to our understanding of the reward stimuli, less is known about how strong somatic stimulation is processed within the PFC. Here, we examined the effects of a strong pinch delivered to the rat posterior paw on spontaneous and current-evoked activity of PFC neurons using intracellular recordings in anesthetized rats. Following the paw pinch, pyramidal cells exhibited a significant decrease in spontaneous activity along with a significant increase in the current-evoked firing. The increase in current-evoked firing elicited by the paw pinch was inversely correlated with the baseline firing rate. Systemic administration of a selective dopamine D2 receptor antagonist partially blocked the effects elicited by the paw pinch on cortical excitability, whereas systemic administration of a D1 antagonist seems to facilitate paw-mediated increases in evoked firing. These results suggest that strong somatic stimuli decrease spontaneous firing while increasing depolarization-evoked firing in a DA receptor dependent manner. These mechanisms may help in the control of the signal to noise ratio or the salience of information processing in the PFC following strong somatic stimulation.

A top-down perspective on dopamine, motivation and memory

Dopamine (DA) activity, in the form of increased neural firing or enhanced release of transmitter from nerve terminals and varicosities, is linked to a number of important psychological processes including: movement; hedonic reactions to positive reward; provision of an error detection signal during the acquisition of new learning; response to novel stimuli; provision of reinforcement signals essential for acquisition of new action patterns; and incentive motivation. This review focuses primarily on our research linking dynamic changes in DA efflux on the timescale of minutes, with incentive motivation, as revealed by brain dialysis experiments in behaving animals. Recent experiments on sensory-specific satiety and successive positive and negative contrast are discussed along with the distinction between preparatory behaviors that precede contact with biologically significant stimuli and subsequent consummatory behaviors. The relationship between DA efflux in the medial prefrontal cortex (mPFC) and foraging for food based on working memory is also discussed in support of the conjecture that DA may serve as a link between motivation and memory functions. Evidence in support of 'top-down' regulation of dopaminergic activity in the mesocorticolimbic DA pathways is reviewed briefly to introduce a mechanism by which activation of ascending DA projections in this manner might optimize dopaminergic modulation of executive function within regions such as the mPFC. Collectively, these processes could ensure coordination between cognitive processes that assess current opportunities and the motivational systems that select and engage patterns of approach behavior that bring organisms into contact with the essentials for survival.

From prediction error to psychosis: ketamine as a pharmacological model of delusions

Recent cognitive neuropsychiatric models of psychosis emphasize the role of attentional disturbances and inappropriate incentive learning in the development of delusions. These models highlight a pre-psychotic period in which the patient experiences perceptual and attentional disruptions. Irrelevant details and numerous associations between stimuli, thoughts and percepts are imbued with inappropriate significance and the attempt to rationalize and account for these bizarre experiences results in the formation of delusions. The present paper discusses delusion formation in terms of basic associative learning processes. Such processes are driven by prediction error signals. Prediction error refers to mismatches between an organism's expectation in a given environment and what actually happens and it is signalled by both dopaminergic and glutamatergic mechanisms. Disruption of these neurobiological systems may underlie delusion formation. We review similarities between acute psychosis and the psychotic state induced by the NMDA receptor antagonist drug ketamine, which impacts upon both dopaminergic and glutamatergic function. We conclude by suggesting that ketamine may provide an appropriate model to investigate the formative stages of symptom evolution in schizophrenia, and thereby provide a window into the earliest and otherwise inaccessible aspects of the disease process.

Glutamate-dopamine cotransmission and reward processing in addiction

While Dale's principle of "one neuron, one neurotransmitter" has undergone revisions to incorporate evidence of the corelease of atypical neurotransmitters such as neuropeptides, the corelease of classical neurotransmitters has only recently been realized. Surprisingly, numerous studies now indicate that the corelease of neurotransmitters in the mammalian central nervous system is not an obscure and rare phenomenon but is widespread and involves most classical neurotransmitters systems. However, the suggestion that glutamate can be coreleased with dopamine (DA) has remained controversial. Furthermore, glutamate-DA cotransmission has not yet been seriously considered in the context of the neurocircuitry of addiction. If glutamate is in fact coreleased with DA as some evidence now suggests, this may have significant implications for advancing our understanding of the interactive role that these 2 neurotransmitters play in cognitive and reward processes. In this commentary, we review the evidence for and against glutamate as a cotransmitter and discuss the potential role of glutamate-DA corelease in addiction. In particular, we describe a recently proposed model in which coreleased glutamate transmits a temporally precise prediction error signal of reward described by Schultz et al., whereas the function of coreleased DA is to exert prolonged modulatory influences on neuronal activity. In addition, we suggest that as alcohol consumption transitions from recreational use to addiction, there is a corresponding transition in the reward valence signal from better than predicted to worse than predicted.

Noradrenaline and dopamine elevations in the rat prefrontal cortex in spatial working memory

The role of prefrontal cortical dopamine (DA) in the modulation of working memory functions is well documented, but substantial evidence indicates that the locus ceruleus noradrenergic system also modulates working memory via actions within the prefrontal cortex (PFC). This study shows that PFC noradrenaline (NA) and DA dialysate levels phasically increase when rats perform correctly in a delayed alternation task in a T-maze, a test of spatial working memory. However, NA levels were markedly enhanced in animals trained to alternate compared with rats that acquired the spatial information about the location of food in the maze but were untrained to make a choice to obtain the reward. In contrast, PFC DA elevations occurred independently of whether the animal had acquired the trial-specific information for correct task execution. The contribution of anticipatory responses to catecholamine efflux was also evaluated by exposing rats to an environment signaling the presence of the reward in the successive alternation task. No conditioned NA efflux was observed in either group. In contrast, in both groups, DA efflux increased in the anticipatory phase of the test to the same levels of those reached during the task. These data provide the first direct evidence for a selective activation of PFC NA transmission during a spatial working memory task. We propose that, in the working memory task, DA is primarily associated with reward expectancy, whereas NA is involved in the active maintenance of the information about a goal and the rules to achieve it.

Mesocortical dopamine neurons operate in distinct temporal domains using multimodal signaling

In vivo extracellular recording studies have traditionally shown that dopamine (DA) transiently inhibits prefrontal cortex (PFC) neurons, yet recent biophysical measurements in vitro indicate that DA enhances the evoked excitability of PFC neurons for prolonged periods. Moreover, although DA neurons apparently encode stimulus salience by transient alterations in firing, the temporal properties of the PFC DA signal associated with various behaviors is often extraordinarily prolonged. The present study used in vivo electrophysiological and electrochemical measures to show that the mesocortical system produces a fast non-DA-mediated postsynaptic response in the PFC that appears to be initiated by glutamate. In contrast, short burst stimulation of mesocortical DA neurons that produced transient (<4 s) DA release in the PFC caused a simultaneous reduction in spontaneous firing (consistent with extracellular in vivo recordings) and a form of DA-induced potentiation in which evoked firing was increased for tens of minutes (consistent with in vitro measurements). We suggest that the mesocortical system might transmit fast signals about reward or salience via corelease of glutamate, whereas the simultaneous prolonged DA-mediated modulation of firing biases the long-term processing dynamics of PFC networks.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

Pavlovian autoshaping procedures increase plasma corticosterone and levels of norepinephrine and serotonin in prefrontal cortex in rats

Pavlovian autoshaping procedures provide for pairings of a small object conditioned stimulus (CS) with a rewarding substance unconditioned stimulus (US), resulting in the acquisition of complex sequences of CS-directed skeletal-motor responses or autoshaping conditioned responses (CRs). Autoshaping procedures induce higher post-session levels of corticosterone than in controls receiving CS and US randomly, and the enhanced post-session corticosterone levels have been attributed to the appetitive or arousal-inducing effects of autoshaping procedures. Enhanced corticosterone release can be induced by aversive stimulation or stressful situations, where it is often accompanied by higher levels of norepinephrine (NE) and serotonin (5-HT) in prefrontal cortex (PFC) but not in striatum (ST). Effects of autoshaping procedures on post-session corticosterone levels, NE contents in PFC, and 5-HT contents in PFC and ST were investigated in male Long-Evans rats. Post-session blood samples revealed higher corticosterone levels in the CS-US Paired group (n = 46) than in the CS-US Random control group (n = 21), and brain samples revealed higher levels of PFC NE and 5-HT in CS-US Paired group. Striatal 5-HT levels were unaltered by the autoshaping procedures. Autoshaping procedures provide for appetitive stimulation and induce an arousal-like state, as well as simultaneous stress-like changes in plasma corticosterone and monoamine levels in PFC. Autoshaping, therefore, may be useful for the study of endocrine and central processes associated with appetitive conditions.

Sucrose sham feeding decreases accumbens norepinephrine in the rat

Noradrenergic projections from the dorsomedial medulla reach the shell of the nucleus accumbens (NAcc), a structure implicated in both reward and feeding behavior. Despite this relationship, the effect of food reward on accumbens norepinephrine (NE) remains uninvestigated. In the course of assessing dopamine (DA) in the NAcc during sucrose ingestion [0.03, 0.1, and 0.3 M; Am. J. Physiol., Regul. Integr. Comp. Physiol., 286 (2004) R31], we also analyzed NE in the microdialysis samples from 14 ad-libitum-fed male rats. In contrast to DA, which increased with sucrose concentration (+20-47%) during sham feeding, in the same animals, NE levels were reduced (approximately -20%), regardless of sucrose concentration. These results demonstrate a novel relationship between accumbens DA and NE during orosensory stimulation with a preferred nutrient.

Differential involvement of amygdaloid CRH system(s) in the salience and valence of the stimuli

Anxiety is a heterogeneous term encompassing not only state or trait characteristics but also a wide range of pathologies such as generalized anxiety disorders, phobias, panic and obsessive-compulsive disorders, acute stress disorder, and posttraumatic stress disorder. Given that diverse forms of anxiety exist, numerous animal models have been developed, which are considered to be useful in identifying mechanisms underlying anxiety states. Examples of such animal models include paradigms that assess the behavioral response to neurogenic (or painful stimuli) or psychogenic stressors or to cues that had previously been associated with painful stimuli. The present report presents data regarding the impact of stressors on corticotropin-releasing hormone (CRH), and relates these to changes in anxiety-like states. Specifically, we demonstrate that (1) psychogenic stressors influence the in vivo release of CRH at the central nucleus of the amygdala (CeA); (2) although CRH changes within the CeA are exquisitely sensitive to stressors, they are also elicited by positive stimuli; and (3) while treatment with diazepam attenuates behavioral signs of anxiety, the CRH release associated with a stressor is unaffected by the treatment. The position is offered that although release of CRH within the CeA is increased under stressful conditions, it is not a necessary condition for the consequent behavioral expression of anxiety-like reactions, at least not in minimally threatening situations. We suggest that the CRH responses at the CeA may be involved in a preparatory capacity and, as such, may accompany a range of emotionally significant stimuli, be they appetitive or aversive.

Asymmetric modulation of a catecholamine-regulated protein in the rat brain, following quinpirole administration

We previously reported a brain-specific 40 kDa catecholamine-regulated protein (CRP40) that binds dopamine (DA) and related catecholamines. CPR40 shares significant sequence homology with human heat shock protein (Hsp70), GRP78/BIP, and human #BQ24193 protein. Recent studies with the DA D(2) receptor antagonist, haloperidol, demonstrated a significant increase in expression of CRP40 in the striatum (STR). The objective of the present study was to investigate CRP40 expression in various brain regions following treatments with the DA D(2)/D(3) receptor agonist quinpirole (QNP) in rats and examine possible relationships between neurochemical parameters and locomotor activity. Rats received injections of either QNP (0.5 mg/kg, for 27 days every third day) or saline (SAL) and their locomotor activities were measured for 90 min after each injection. At injection 9, QNP-treated rats showed locomotor activity that was significantly greater than SAL controls (F(2,28) = 3.88, P < 0.05, Duncan's multiple range test, P < 0.05). Neurochemically, acute QNP-treated rats demonstrated significant differential expression of CRP40 in the left/right prefrontal cortex (PFC) relative to SAL-treated rats (-17.76 +/- 2.10%, -10.35 +/- 1.23%, P < 0.001). Chronic QNP significantly decreased CRP40 expression in the STR, ventral tegmental area (VTA), and left/right PFC (-24.85+/- 2.10%, -18.15 +/- 5.64%, -49.13 +/- 7.05%, -25 +/- 3.63%, P < 0.001). Finally, chronic QNP treatment resulted in a significant increase in CRP40 levels in the nucleus accumbens (NA) (+39.32 +/- 7.00%, P < 0.001). Heat shock protein (i.e., Hsp70 or Hsc70) expression remained unaltered following QNP treatment. Since QNP is a DA D(2)/D(3) agonist, alterations in CRP40 expression following QNP treatment suggest the protein's function in dopaminergic neurotransmission.

Induction of the learning and plasticity-associated gene Zif268 following exposure to a discrete cocaine-associated stimulus

We investigated whether the expression of the plasticity-associated gene, zif268, was associated with memories retrieved by exposure to a discrete stimulus that had been associated with cocaine, either self-administered or administered noncontingently. In the absence of drug, passive presentation of a cocaine-associated light stimulus induced changes in the expression of zif268 measured by in situ hybridization within a limbic cortical-ventral striatal circuit that was not only regionally selective but related to whether the rats had originally received response-contingent or noncontingent stimulus-drug pairings. In rats that had self-administered drug, the cocaine-conditioned stimulus (CS) increased zif268 expression in neurons of the ventral tegmental area, nucleus accumbens core and shell, and basal nucleus of the amygdala but not hippocampus, prelimbic area of the medial prefrontal cortex or amygdala central nucleus. The same CS that had been associated with cocaine administered noncontingently additionally increased zif268 mRNA levels in area Cg1 of the anterior cingulate cortex, ventral and lateral regions of the orbitofrontal cortex and lateral nucleus of the amygdala. Zif268 induction was related to the predictive relationship between the stimulus and cocaine as no changes were seen in cocaine-experienced rats that had received unpaired light and drug presentations during training. Thus, zif268 expression is increased by appetitively (drug) conditioned stimuli after Pavlovian learning. Zif268 may participate in the molecular mechanisms underlying the reconsolidation or re-encoding of Pavlovian stimulus-drug associations across a distributed limbic cortical-ventral striatal neural network and that may contribute to the basis of the enduring drug-seeking behaviour produced by environmental cues.

Attenuation of sucrose reinforcement in dopamine D1 receptor deficient mice

Dopaminergic systems are thought to mediate the rewarding and reinforcing effects of palatable food. However, the relative contribution of different dopamine receptor subtypes is not clear. We used dopamine D1 receptor deficient mice (D1 -/-) and their wild-type and heterozygous littermates to study the role of the D1 receptor in palatable food reinforced behaviour using operant responding and free access paradigms. Non-deprived mice were trained to press a lever for sucrose pellets under three schedules of reinforcement including fixed ratios (FR-1 and FR-4) and a progressive ratio (PR). Responding on one lever was reinforced by the delivery of a sucrose pellet or solution while responding on a second lever had no programmed consequences. Initially, D1 mutant mice took longer to learn to discriminate between the two levers and had significantly lower operant responding for sucrose pellets and solution than wild-type and heterozygous mice under all schedules of reinforcement. Food deprivation enhanced responding on the active lever in all mice although it remained significantly lower in D1 -/- mice than in control mice. Following extinction of sucrose reinforcement and reversal of the levers, D1 -/- mice showed deficits in extinguishing and reversing previously learned responses. Home cage intake and preference of sucrose pellets and solutions when given under free-choice access paradigms were similar among the groups. These results suggest that the dopamine D1 receptor plays a role in the motivation to work for reward (palatable food) but not in reward perception and is critical in learning new but relevant information and discontinuing previously learned responses.

Differential activation of orexin neurons by antipsychotic drugs associated with weight gain

Weight gain is one side effect of many antipsychotic drugs (APDs). A small number of lateral hypothalamic/perifornical area (LH/PFA) neurons express the orexins, peptides that are critically involved in body weight regulation and arousal. We examined the ability of APDs to activate orexin neurons, as reflected by induction of Fos. APDs with significant weight gain liability increased Fos expression in orexin neurons, but APDs with low or absent weight gain liability did not. The weight gain liability of APDs was correlated with the degree of Fos induction in orexin neurons of the lateral LH/PFA. In contrast, amphetamine, which causes weight loss, increased Fos expression in orexin neurons of the medial but not lateral LH/PFA. We compared the effects of amphetamine and clozapine, an APD with weight gain liability, on orexin neurons innervating the prefrontal cortex. Clozapine induced Fos in 75% of the orexin neurons that project to the cortex, but amphetamine induced Fos in less than a third of these cells. These data suggest that APD-induced weight gain is associated with activation of distinct orexin neurons and emphasize the presence of anatomically and functionally heterogeneous populations of orexin neurons.

Stimulation of 5-hydroxytryptamine (5-HT(2C) ) receptors in the ventrotegmental area inhibits stress-induced but not basal dopamine release in the rat prefrontal cortex

The present study investigated whether 5-HT(2C) receptors in the ventrotegmental area and prefrontal cortex regulate basal and stimulus-evoked dopamine release in the prefrontal cortex. Using the in vivo microdialysis technique in conscious rats, we studied the effect of a selective 5-HT(2C) receptor agonist, Ro60-0175, on basal and immobilization stress-induced dopamine release in the prefrontal cortex. Ro60-0175 intraperitoneally (2.5 mg/kg) and into the ventrotegmental area (10 microg/0.5 microL) completely antagonized the effect of stress on extracellular dopamine without altering basal levels. Infusion of 10 microm Ro60-0175 through the cortical probe had no significant effect on basal and stress-induced dopamine release. SB242084 (10 mg/kg), a selective antagonist of 5-HT(2C) receptors, significantly increased basal extracellular dopamine and completely prevented the effect of intraperitoneal and intraventrotegmental Ro60-0175 on the stress-induced rise of extracellular dopamine, but had no effect itself in stressed rats. The results show that Ro60-0175 suppresses cortical dopamine release induced by immobilization stress through the stimulation of 5-HT(2C) receptors in the ventrotegmental area. While confirming that endogenous 5-HT acting on 5-HT(2C) receptors tonically inhibit basal dopamine release in the prefrontal cortex, the present findings suggest that the stimulation of 5-HT(2C) receptors with an exogenous agonist preferentially inhibit stimulated release.

In vivo regulation of dopamine and noradrenaline release by alpha2A-adrenoceptors in the mouse prefrontal cortex

The present study investigated the role of alpha2A-adrenoceptor subtype in the regulation of noradrenaline and dopamine release in the medial prefrontal cortex. The effect of local introduction of the alpha2-adrenoceptor agonist dexmedetomidine (10-9-10-8 m) on noradrenaline and dopamine release was investigated in alpha2A-adrenoceptor knockout and control mice by using in vivo microdialysis. Furthermore, to reveal a possible distinction between regulation of baseline and peak release, we sampled the dialysate during both rest and handling-induced mild stress. Baseline noradrenaline and dopamine concentrations did not differ between alpha2A-adrenoceptor knockout and control mice. Dexmedetomidine decreased, in a concentration-dependent manner, noradrenaline and dopamine levels in both genotypes. However, the effect of dexmedetomidine on noradrenaline release was attenuated in the alpha2A-adrenoceptor knockout mice, whereas the effect on dopamine release did not differ between the genotypes. The first handling episode increased noradrenaline and dopamine levels to the same extent in both genotypes. However, in alpha2A-adrenoceptor knockout mice the noradrenaline and dopamine levels remained elevated in the samples following the first handling whilst, in the control mice, transmitter levels returned to baseline levels. In control mice the handling-induced peak noradrenaline and dopamine levels were lower after the administration of dexmedetomidine than during the first handling episode, but in alpha2A-adrenoceptor knockout mice no drug effect on handling-induced peak noradrenaline and dopamine levels was found. Our results suggest that the release of noradrenaline in the medial prefrontal cortex is mainly regulated via alpha2A-adrenoceptors, whilst other alpha-adrenoceptor subtypes play a significant role in the regulation of dopamine release.

Altered forebrain neurotransmitter responses to immobilization stress following 3,4-methylenedioxymethamphetamine

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is an increasingly popular drug of abuse that acts as a neurotoxin to forebrain serotonin neurons. The neurochemical effects of the serotonin depletion following high doses of MDMA were investigated in response to acute immobilization stress. Male rats were treated with a neurotoxic dosing regimen of MDMA (10 mg/kg, i.p. every 2 h for four injections) or equivalent doses of saline. Seven days after treatment, in vivo microdialysis was used to assess extracellular dopamine and serotonin in the dorsal hippocampus and prefrontal cortex during 1 h of immobilization stress. In saline treated control rats, serotonin in the hippocampus and serotonin and dopamine in the prefrontal cortex were increased during immobilization stress. Rats pretreated with MDMA, however, showed blunted neurotransmitter responses in the hippocampus and the prefrontal cortex. In the drug pretreated rats, basal serotonin levels in the hippocampus, but not the prefrontal cortex, were lower compared to saline pretreated controls. Stress-induced increases in plasma corticosterone and body temperature were not affected by the pretreatment condition. From these studies we suggest that depletion of serotonin stores in terminal regions with the neurotoxin MDMA compromises the ability of the serotonergic neurons to activate central systems that respond to stressful stimuli. This altered responsiveness may have implications for long-term functional consequences of MDMA abuse as well as the interactions between the serotonergic system and stress.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Synchronized release of dopamine and serotonin in the medial and lateral hypothalamus of rats

A positive linear correlation between dopamine and serotonin release was found in the ventromedial hypothalamus and in the lateral hypothalamic area in fasting rats and in fed rats during intermeal intervals. Dopamine release in the ventromedial hypothalamus positively correlated with dopamine and serotonin release in the lateral hypothalamic area, which occurred only during intermeal intervals and was non-significant during the meal consumption periods or during fasting. Meal size correlated significantly only with a decrease in serotonin release in the lateral hypothalamic area. The study was designed to evaluate the relationship between dopamine and serotonin release in these hypothalamic areas and their dependence on feeding status. Microdialysis was performed simultaneously via two probes, one in the ventromedial hypothalamus and the other in the contralateral lateral hypothalamic area, of freely moving male lean Zucker rats over 24h with preserved light and dark phase, either with ad libitum access to food and water, or when no food was available. Dopamine and serotonin concentrations were measured by high-performance liquid chromatography with electrochemical detection in 20-min dialysis samples. Time-series analysis was applied to determine linear correlations between monoamines and in relation to food intake. Data showed that release of dopamine and serotonin is synchronized within the ventromedial hypothalamus and lateral hypothalamic area, particularly in the dark phase and when no food was ingested. However, synchronized release of monoamines between these nuclei occurred only during intermeal intervals: the periods of satiety. These findings suggest a tight relationship between dopaminergic and serotonergic systems of the lateral hypothalamic area and ventromedial hypothalamus, which is influenced by the feeding state and which may be involved in maintaining the balance within and between the centers of the parasympathetic and sympathetic nervous systems. The data also illustate that food intake is coupled unequivocally to the release of dopamine and serotonin in the hypothalamus, suggesting it as a mechanism of activation of postsynaptic neurons associated with new metabolic status.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Mesolimbocortical and nigrostriatal dopamine responses to salient non-reward events

While it has previously been assumed that mesolimbic dopamine neurons carry a reward signal, recent data from single-unit, microdialysis and voltammetry studies suggest that these neurons respond to a large category of salient and arousing events, including appetitive, aversive, high intensity, and novel stimuli. Elevations in dopamine release within mesolimbic, mesocortical and nigrostriatal target sites coincide with arousal, and the increase in dopamine activity within target sites modulates a number of behavioral functions. However, because dopamine neurons respond to a category of salient events that extend beyond that of reward stimuli, dopamine levels are not likely to code for the reward value of encountered events. The paper (i) examines evidence showing that dopamine neurons respond to salient and arousing change in environmental conditions, regardless of the motivational valence of that change, and (ii) asks how this might shape our thinking about the role of dopamine systems in goal-directed behavior.

Differential profiles of nitric oxide and norepinephrine releases in the paraventricular nucleus region in response to mild footshock in rats

The purpose of this study was to determine whether the application of mild intermittent footshock stress can cause changes in the nitric oxide (NO) and norepinephrine (NE) releases in the hypothalamic paraventricular nucleus (PVN) region and medial prefrontal cortex (mPFC). Extracellular levels of NO metabolites and NE in the PVN region and mPFC were determined using an in vivo brain microdialysis technique in conscious rats. In the PVN region, we demonstrated that perfusion of N-methyl-D-aspartate through a microdialysis probe resulted in a dose-dependent increase in NO metabolite levels, whereas intraperitoneal administration of N(G)-nitro-L-arginine methyl ester produced a dose-dependent reduction in the levels of NO metabolites. The levels of NO metabolites in the PVN region increased after intraperitoneal administration of interleukin-1beta in a dose-dependent manner, as we previously reported. This increase in NO metabolite levels was abolished 60 min after systemic administration of N(G)-nitro-L-arginine methyl ester compared to the vehicle-treated control group. Twenty minutes of intermittent footshock induced NE release but did not induce NO release in the PVN region. On the contrary, in the mPFC, 20 min of intermittent footshock induced both NO and NE releases. The present results reveal different patterns and time courses in NO and NE releases between the PVN region and the mPFC in response to mild intermittent footshock stress. These findings are likely to have helpful suggestions for our understanding of the hypothalamic-pituitary-adrenal axis and the limbic forebrain system response to different kinds of stress.

Dissociative effects of apomorphine infusions into the medial prefrontal cortex of rats on latent inhibition, prepulse inhibition and amphetamine-induced locomotion

Impaired ability to "gate out" sensory and cognitive information is considered to be a central feature of schizophrenia and is manifested, among others, in disrupted prepulse inhibition and latent inhibition. The present study investigated, in rats, the effects of increasing dopamine receptor activation within the medial prefrontal cortex by local administration of the dopamine receptor agonist apomorphine (9 microg/side) on prepulse inhibition and latent inhibition, as well as on spontaneous and amphetamine-induced activity. Apomorphine infusions decreased spontaneous locomotor activity and blocked amphetamine-induced increase in locomotor activity in the open field, which is in line with the suggestion that dopamine receptor activation in the medial prefrontal cortex inhibits mesolimbic dopamine activity. However, apomorphine infusions induced a disruption of prepulse inhibition, an effect associated with increased dopaminergic activity in the nucleus accumbens, and left the latent inhibition effect intact. While these results support previous evidence that the medial prefrontal cortex is a component of the neural circuitry mediating prepulse inhibition but plays no role in latent inhibition, they show that dopamine receptor activation in the medial prefrontal cortex of the rat produces behavioural outcomes that cannot be explained by postulating a simple reciprocal relationship between the mesocortical and mesolimbic dopamine systems.

Differential effect of immobilization stress on in vivo synthesis rate of monoamines in medial prefrontal cortex and nucleus accumbens of conscious rats

We have used microdialysis to measure the in vivo hydroxylation level of tyrosine and tryptophan in the medial prefrontal cortex and nucleus accumbens of conscious rats that were subjected to immobilization. The brain was perfused with an inhibitor of aromatic L-amino acid decarboxylase, 3-hydroxybenzylhydrazine, and the amount of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in the dialysate was measured as an index of the in vivo hydroxylation rate of tyrosine and tryptophan. One hour of immobilization caused a significant increase in extracellular DOPAin the medial prefrontal cortex but not nucleus accumbens. The same manipulation produced a significant and more prolonged elevation in extracellular 5-HTP in the nucleus accumbens as well as medial prefrontal cortex. The observed profile of stress-induced 5-HTP response was comparable in two brain regions. The results suggest that in vivo catecholamine synthesis is heterogenous, whereas in vivo serotonin synthesis is homogenous, with respect to responsiveness to stress in the medial prefrontal cortex and nucleus accumbens.

High and low responders to novelty: effects of a catecholamine synthesis inhibitor on novelty-induced changes in behaviour and release of accumbal dopamine

The purpose of the present study was two-fold: (i) to investigate to what extent novelty, i.e. a novel cage with slightly larger dimensions than the home cage and lacking the floor covering that was originally present, produced behavioural effects in high responders to novelty and low responders to novelty that could be correlated with the extracellular amount of accumbal dopamine, using the microdialysis technique, and (ii) to establish the ability of the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine to inhibit the novelty-induced responses in high and low responders. The difference in the behavioural response to novelty between the high and low responders was limited to walking, which lasted significantly longer in high responders than in low responders. The novelty-induced increase in extracellular concentration of accumbal dopamine was significantly greater in high responders than in low responders; moreover, the shape of the growth curves differed between high and low responders. The behavioural changes did not correlate with the neurochemical effects, which outlasted the duration of the novelty-induced behavioural arousal. It is hypothesized that this long-lasting increase in accumbal dopamine produces "adaptive changes" that help and/or allow the animal to incorporate knowledge about the condition that it experienced. When the nucleus accumbens was perfused with alpha-methyl-p-tyrosine for a period of 40 min, given at the same time as the transfer of the rat to the novel cage, it reduced the novelty-induced increase in walking in the high responders, but did not alter the novelty-induced behaviour of low responders. Finally, alpha-methyl-p-tyrosine reduced the novelty-induced increase in the release of accumbal dopamine in high responders, but enhanced it in low responders. The present neurochemical data are discussed in view of the outcome of earlier reported pharmacobehavioural studies on the neurochemical state of the nucleus accumbens of non-challenged versus challenged high and low responders. It is hypothesized that, in the high responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-resistant, alpha-methyl-p-tyrosine-sensitive pools that are under the stimulatory control of beta-adrenergic receptors in the nucleus accumbens, and that, in the low responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-sensitive, alpha-methyl-p-tyrosine-resistant pools that are under the inhibitory control of alpha-adrenergic receptors in the nucleus accumbens.

Aversive and appetitive events evoke the release of corticotropin-releasing hormone and bombesin-like peptides at the central nucleus of the amygdala

There is wide agreement that corticotropin-releasing hormone (CRH) systems within the brain are activated by stressful stimuli. There is also mounting evidence for the role of bombesin (BN)-like peptides in the mediation of the stress response. To date, however, the extent to which other stimuli increase the activity of these peptidergic systems has received little attention. In the present investigation we validated and used in vivo microdialysis sampling followed by ex vivo radioimmunoassays to monitor the release of CRH and BN-like peptides during appetitive (food intake) and stressful (restraint) events. It is demonstrated for the first time that the in vivo release of CRH and BN-like peptides at the central nucleus of the amygdala was markedly increased by both stressor exposure and food ingestion. In fact, the meal-elicited rise of CRH release was as great as that associated with 20 min of restraint stress. Paralleling these findings, circulating ACTH and corticosterone levels were also increased in response to both food intake and restraint. Contrary to the current views, these results indicate that either food ingestion is interpreted as a "stressful" event by certain neural circuits involving the central amygdala or that the CRH- and BN-related peptidergic systems may serve a much broader role than previously envisioned. Rather than evoking feelings of fear and anxiety, these systems may serve to draw attention to events or cues of biological significance, such as those associated with food availability as well as those posing a threat to survival.

Dissociations in dopamine release in medial prefrontal cortex and ventral striatum during the acquisition and extinction of classical aversive conditioning in the rat

Dual perfusion in vivo brain microdialysis was used to monitor extracellular levels of dopamine in the medial prefrontal cortex and ventral striatum during the acquisition and extinction of a classical aversive conditioning paradigm in rats. The main finding was a dissociation in the pattern of release in the two brain areas. The first stimulus-footshock pairing elicited large increases in cortical dopamine over baseline levels that were much greater than the increases elicited by different stimuli of equivalent salience that were unpaired with footshock. In contrast, dopamine levels in ventral striatum were unchanged under these conditions. Over the next two pairings, there was a decline in the cortical response and an increase in the response in ventral striatum. The first presentation of the aversive conditioned stimulus in a separate context elicited the largest response in ventral striatum. Post-conditioning, the cortical response to the conditioned stimulus was smaller than that elicited by the initial stimulus-footshock pairing and was equivalent in magnitude to that elicited by stimuli unpaired with footshock. Over the final two conditioned stimuli presentations, in the absence of the footshock reinforcer (extinction), responses declined in both brain areas. Simultaneous monitoring of behaviour indicated that the neurochemical events were accompanied by effective aversive learning, as indexed by conditioned freezing responses. The data are discussed in terms of the hypothesis that medial prefrontal cortex is especially engaged during novel circumstances which may, potentially, require new learning, whilst ventral striatal dopamine more closely follows the expression of conditioned responding during learning and extinction.

Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

Mesolimbocortical dopamine (DA) neurotransmission is important in the mediation of the dependence-producing actions of nicotine and other drugs of abuse. Withdrawal from chronic treatment with various types of addictive drugs, including amphetamine, cocaine, ethanol and morphine is associated with a decrease in dopaminergic output in the nucleus accumbens (NAC), whereas the effects of withdrawal from these drugs on dopaminergic output in the medial prefrontal cortex (PFC), as yet, remain largely unknown. This study examined putative changes in the extracellular levels of dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC and in the PFC of rats displaying behavioral signs of nicotine withdrawal. Rats were infused for 7 days with nicotine via subcutaneously implanted minipumps, whereas control animals carried saline-containing pumps. On the fifth day of infusion a microdialysis probe was implanted in the NAC or the PFC of the rats. Forty-eight hours later the levels of DA and the monoamine metabolites were assessed in the dialysate. The behavioral and biochemical effects of a saline injection and a subsequent challenge with the nicotinic receptor antagonist mecamylamine (1 mg/kg s.c.) were determined. Following mecamylamine challenge in nicotine-treated animals, the levels of DA, DOPAC and HVA in the NAC, but not in the PFC, decreased below pre-injection levels and in relation to control animals. The score of abstinence signs increased in the nicotine-treated rats, as compared both to the score after saline and to that in control animals. The decreased DA output in the NAC in animals displaying nicotine withdrawal signs is similar to that seen after withdrawal of several other drugs of abuse, and may have bearing on motivational deficits associated with the abstinence reactions.