5-HT and mechanisms of defence

Adverse early life experience and social stress during adulthood interact to increase serotonin transporter mRNA expression

Anxiety disorders, depression and animal models of vulnerability to a depression-like syndrome have been associated with dysregulation of serotonergic systems in the brain. To evaluate the effects of early life experience, adverse experiences during adulthood, and potential interactions between these factors on serotonin transporter (slc6a4) mRNA expression, we investigated in rats the effects of maternal separation (180 min/day from days 2 to 14 of life; MS180), neonatal handing (15 min/day from days 2 to 14 of life; MS15), or normal animal facility rearing (AFR) control conditions with or without subsequent exposure to adult social defeat on slc6a4 mRNA expression in the dorsal raphe nucleus (DR) and caudal linear nucleus. At the level of specific subdivisions of the DR, there were no differences in slc6a4 mRNA expression between MS15 and AFR rats. Among rats exposed to a novel cage control condition, increased slc6a4 mRNA expression was observed in the dorsal part of the DR in MS180 rats, relative to AFR control rats. In contrast, MS180 rats exposed to social defeat as adults had increased slc6a4 mRNA expression throughout the DR compared to both MS15 and AFR controls. Social defeat increased slc6a4 mRNA expression, but only in MS180 rats and only in the "lateral wings" of the DR. Overall these data demonstrate that early life experience and stressful experience during adulthood interact to determine slc6a4 mRNA expression. These data support the hypothesis that early life experience and major stressful life events contribute to dysregulation of serotonergic systems in stress-related neuropsychiatric disorders.

Effects of Lorazepam and citalopram on human defensive reactions: ethopharmacological differentiation of fear and anxiety

Drugs that are clinically effective against generalized anxiety disorder preferentially alter rodent risk assessment behavior, whereas drugs that are clinically effective against panic disorder preferentially alter rodent flight behavior. The theoretical principle of "defensive direction" explains the pattern of associations between emotion and defensive behavior in terms of the differing functional demands arising from cautious approach to threat (anxiety) versus departure from threat (fear), offering the prospect that clinically important emotions may be explained using a single rubric of defense. We used a within-subjects, placebo-controlled, design to test this theory, measuring the effects of citalopram and lorazepam on the defensive behavior of 30 healthy adult male humans. We indexed human defensive behavior with a translation of an active avoidance task used to measure rodent defense and found that lorazepam significantly reduced the intensity of defensive behavior during approach to threat (hypothetically anxiety-related) but not departure from threat (hypothetically fear-related). Contrary to prediction, citalopram did not affect either form of defensive reaction. Since lorazepam is a drug with well established anxiety reducing properties, these data support the hypothesis that anxiety is an emotion elicited by threat stimuli that require approach. These data also contribute to the validation of a novel human analog of an established experimental model of rodent fear and anxiety.

Improved stress response in bipolar affective disorder with adjunctive spironolactone (mineralocorticoid receptor antagonist): case series

The psychopathologies underlying affective disorders are thought to involve persistent changes in the expression and function of both mineralocorticoid receptors and glucocorticoid receptors in the hippocampus. In addition, exposure to stressful stimuli can precipitate episodes in vulnerable individuals. The aim of this study is to determine if spironolactone as an adjunctive therapy is effective in improving residual symptoms in bipolar disorder. Four cases of euthymic bipolar disorder (BD) patients were treated with spironolactone as an adjunctive therapy in a private treatment sector. All patients met the DSM-IV diagnosis criteria for bipolar disorder. Clinical response was assessed retrospectively using the Clinical Global Impression Scale for Improvement. Spironolactone was effective in all patients. The four cases illustrate a clinical response to residual symptoms and improvement in stress response after use of spironolactone as an adjunctive therapy in BD. This pilot case series suggests reducing in residual symptoms, with spironolactone as an adjunctive therapy in these DSM-IV BD patients. Mineralocorticoid receptors antagonists' role in reducing stress-induced symptoms deserves further investigation through placebo-controlled trials.

Antimanic potency of typical neuroleptic drugs and affinity for dopamine D2 and serotonin 5-HT2A receptors--a new analysis of data from the archives and implications for improved antimanic treatments

Datasets of antimanic potency ratings and receptor-binding affinities [inhibition constants (K(i))] at dopamine D2 and serotonin 5-HT2A brain receptors were accessed from published literature for a large series (n = 24) of typical neuroleptic drugs, many of which are now obsolete and unobtainable. There was a strong positive association between antimanic potency and affinity for D2 receptors, in support of a 'dopamine-blockade hypothesis' of antimanic drug action. Taking the series of neuroleptics as a whole, there was no association between antimanic potency and affinity for 5-HT2A receptors. Despite this, within a subsample of typical neuroleptics with low affinity for D2 receptors resembling new generation atypical antipsychotics, a positive association between antimanic potency and affinity for 5-HT2A receptors emerged. This suggests that blockade of brain 5-HT2A receptors plays at least a subsidiary role in the antimanic effects of some typical neuroleptics. Other considerations also suggest that combining drugs to achieve high affinity for and blockade of both dopamine D2 receptors and serotonin 5-HT2A receptors, possibly with additional direct or indirect stimulation of postsynaptic 5-HT1A receptors, might maximize antimanic efficacy.

From threat to fear: the neural organization of defensive fear systems in humans

Postencounter and circa-strike defensive contexts represent two adaptive responses to potential and imminent danger. In the context of a predator, the postencounter reflects the initial detection of the potential threat, whereas the circa-strike is associated with direct predatory attack. We used functional magnetic resonance imaging to investigate the neural organization of anticipation and avoidance of artificial predators with high or low probability of capturing the subject across analogous postencounter and circa-strike contexts of threat. Consistent with defense systems models, postencounter threat elicited activity in forebrain areas, including subgenual anterior cingulate cortex (sgACC), hippocampus, and amygdala. Conversely, active avoidance during circa-strike threat increased activity in mid-dorsal ACC and midbrain areas. During the circa-strike condition, subjects showed increased coupling between the midbrain and mid-dorsal ACC and decreased coupling with the sgACC, amygdala, and hippocampus. Greater activity was observed in the right pregenual ACC for high compared with low probability of capture during circa-strike threat. This region showed decreased coupling with the amygdala, insula, and ventromedial prefrontal cortex. Finally, we found that locomotor errors correlated with subjective reports of panic for the high compared with low probability of capture during the circa-strike threat, and these panic-related locomotor errors were correlated with midbrain activity. These findings support models suggesting that higher forebrain areas are involved in early-threat responses, including the assignment and control of fear, whereas imminent danger results in fast, likely "hard-wired," defensive reactions mediated by the midbrain.

Implication of the 5-HT2A and 5-HT2C (but not 5HT1A) receptors located within the periaqueductal gray in the elevated plus-maze test-retest paradigm in mice

A single exposure to the elevated plus-maze test (EPM) increases open arms avoidance and reduces or abolishes the anxiolytic-like effect of benzodiazepines assessed during a second trial, a phenomenon defined as "one-trial tolerance" (OTT). It has been emphasized that the dorsal portion of the midbrain periaqueductal gray (dPAG) plays a role on this enhanced aversion phenomenon in maze-experienced rodents. Given that intra-dPAG injections of a wide range of serotonergic 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor agonists produce anxiolytic-like effects in maze-naïve rodents, the present study examined the effects of the 5-HT(1A) receptor agonist 8-OH-DPAT (5.6 and 10.0nmol in 0.15microl) the preferential 5-HT(2A) receptor agonist DOI (2.0 and 8.0nmol in 0.1microl) and the preferential 5-HT(2C) receptor agonist MK-212 (21.2 and 63.6nmol in 0.1microl) microinjected into the dPAG prior to Trial 1 and Trial 2 on the behaviour of mice in the EPM. Test sessions were recorded and subsequently scored for anxiety-like behaviour (percentage of open arms entries and time) as well as general locomotor activity (closed arm entries). The results showed a lack of 8-OH-DPAT (5.6 and 10.0nmol) effect on the behaviour of maze-naïve and maze-experienced mice, while intra-dPAG microinfusions of DOI (8nmol) reduced anxiety-like behaviour only in maze-experienced mice that had received a similar treatment prior to Trial 1. Furthermore, intra-dPAG MK-212 (63.6nmol) showed an anxiolytic-like effect on both Trial 1 and Trial 2. Importantly, these effects were observed in the absence of any significant change in closed arm entries, the parameter considered to be a valid index of locomotor activity in the plus-maze. These results support the dPAG as a crucial structure involved in the neurobiology of the OTT phenomenon as well as accounting the role of the 5-HT(2A) and 5-HT(2C) receptors located within this midbrain structure on the emotional state induced by EPM test and retest paradigm mice.

From threat to fear: the neural organization of defensive fear systems in humans

Postencounter and circa-strike defensive contexts represent two adaptive responses to potential and imminent danger. In the context of a predator, the postencounter reflects the initial detection of the potential threat, whereas the circa-strike is associated with direct predatory attack. We used functional magnetic resonance imaging to investigate the neural organization of anticipation and avoidance of artificial predators with high or low probability of capturing the subject across analogous postencounter and circa-strike contexts of threat. Consistent with defense systems models, postencounter threat elicited activity in forebrain areas, including subgenual anterior cingulate cortex (sgACC), hippocampus, and amygdala. Conversely, active avoidance during circa-strike threat increased activity in mid-dorsal ACC and midbrain areas. During the circa-strike condition, subjects showed increased coupling between the midbrain and mid-dorsal ACC and decreased coupling with the sgACC, amygdala, and hippocampus. Greater activity was observed in the right pregenual ACC for high compared with low probability of capture during circa-strike threat. This region showed decreased coupling with the amygdala, insula, and ventromedial prefrontal cortex. Finally, we found that locomotor errors correlated with subjective reports of panic for the high compared with low probability of capture during the circa-strike threat, and these panic-related locomotor errors were correlated with midbrain activity. These findings support models suggesting that higher forebrain areas are involved in early-threat responses, including the assignment and control of fear, whereas imminent danger results in fast, likely "hard-wired," defensive reactions mediated by the midbrain.

Affective Neuroscience: Past, Present, and Future

The discipline of affective neuroscience is concerned with the underlying neural substrates of emotion and mood. This review presents an historical overview of the pioneering work in affective neuroscience of James and Lange, Cannon and Bard, and Hess, Papez, and MacLean before summarizing the current state of research on the brain regions identified by these seminal researchers. We also discuss the more recent strides made in the field of affective neuroscience. A final section considers different hypothetical organizations of affective neuroanatomy and highlights future directions for the discipline.

Transient firing of dorsal raphe neurons encodes diverse and specific sensory, motor, and reward events

Serotonin (5-hydroxytryptamine [5-HT]) is known to influence a wide range of behaviors and physiological processes, but relatively little is known about events that trigger 5-HT release. To address this issue, we recorded from neurons in the dorsal raphe nucleus (DRN) in rats performing an odor-guided spatial decision task. A large fraction of DRN neurons showed transient firing time locked to behavioral events on timescales as little as 20 ms. DRN transients were sometimes correlated with reward parameters, but also encoded specific sensorimotor events, including stimulus identity and response direction. These behavioral correlates were diverse but showed no apparent relationship with waveform or other firing properties indicative of neurochemical identity. These results suggest that the 5-HT system does not encode a unitary signal and that it will broadcast specific information to the forebrain with speed and precision sufficient not only to modulate but also to dynamically sculpt ongoing information processing.

Potential benefits of recombinant human growth hormone (rhGH) to athletes

Athletes have enjoyed almost a thirty year amnesty of rhGH abuse, which they consider has contributed to the winning of medals and the breaking of world records. Such a reprieve is almost at an end, since WADA have identified a method to detect rhGH abuse. Or have they? The anecdotal word "on the street" is that rhGH is still undetectable and athletes believe that the benefits, at the dosages they administer, far outweigh the risks! Scientists are aware that in a hormone deficiency condition, replacement can halt and in certain situations reverse some of the adverse effects. Growth hormone deficiency can lead to a loss of skeletal muscle mass and an increase in abdomino-visceral obesity, which is reversed on replacement with rhGH. Since the availability of GH, athletes have been trying to extrapolate these effects from the deficiency state to the healthy corpus and increase their sporting prowess. Past confessions from athletes, such as Ben Johnson, Kelly White, Tim Montgomery, Marion Jones and currently Dwain Chambers have demonstrated that they are prepared to tread the very fine lines that separate the "men from the boys". Rewards are so great, that anonymous surveys have identified that athletes will risk ill health, if they believe they can cheat, win and not get caught. The question that still needs to be answered is, "does growth hormone enhance performance"? Recent research suggests that it could. There is also a suspicion that in "cycled" low supraphysiological doses, it is no where near as harmful as WADA claim it to be.

Serotonin and sensitivity to trauma-related exposure in selective serotonin reuptake inhibitors-recovered posttraumatic stress disorder

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are first-line treatments for posttraumatic stress disorder (PTSD). Serotonergic (5HT) attenuation of stress sensitivity is postulated from SSRIs' effects in other anxiety disorders, and we studied this in PTSD.

METHODS

Ten patients with PTSD fully recovered on SSRIs (Clinical Global Impression Scale-I 1 and 2) were enrolled in the study. Patients were tested on two occasions 1 week apart; in each session, they received a drink containing large neutral amino acids (LNAAs) either with (sham tryptophan depletion [STD], control) or without (acute tryptophan depletion [ATD]) tryptophan. At 5.5 hours after the drink, subjects were exposed to a trauma-related exposure challenge. Self-reports of PTSD (visual analogue scales [VAS] and the Davidson Trauma Scale [DTS]), anxiety (Spielberger State Inventory [STAI] Form Y-1), and mood (Profile of Mood States [POMS]) were obtained. Heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressure were also measured.

RESULTS

The trauma-related exposure challenge induced anxiety on both days, with more marked responses on the ATD day according to VAS, DTS, POMS, and DBP (p < .05). A trend of significance (.1 > p > .05) was observed for STAI Form Y-1, HR, and SBP.

CONCLUSIONS

These data demonstrate that ATD accentuates responses to trauma-related stimuli in SSRI-recovered PTSD. They also suggest that SSRI-induced increases in serotonin function restrain PTSD symptoms, especially under provocation, supporting a role for serotonin in mediating stress resilience.

Early effects of mirtazapine on emotional processing

BACKGROUND

Acute administration of selective serotonin and noradrenaline re-uptake blockers to healthy volunteers affects the processing of emotional information but it is not known if similar effects occur with antidepressants acting through other pharmacological mechanisms. Mirtazapine is a clinically established antidepressant with complex actions involving blockade of noradrenaline alpha(2)-adrenoceptors as well as a number of 5-HT receptor subtypes. The aim of the present study was to test whether, like monoamine re-uptake inhibitors, mirtazapine would also produce positive biases in emotional processing.

METHODS

We studied 30 healthy volunteers who received either a single dose of mirtazapine (15 mg) or placebo in a parallel group, double-blind study. Two hours following medication administration, participants completed a battery of tasks testing various aspects of emotional processing including facial expression recognition, emotion potentiated startle, and emotional categorization and memory.

RESULTS

Compared to placebo, mirtazapine significantly impaired the recognition of fearful facial expressions and reduced eye-blink responses in the emotion potentiated startle task. Participants receiving mirtazapine were also significantly quicker to respond to emotional self-relevant information in the categorization task and showed a positive bias in memory recall compared to those receiving placebo.

CONCLUSIONS

Our findings indicate that mirtazapine reduces fear processing in healthy volunteers, an effect similar to that produced by repeated administration of selective serotonin re-uptake inhibitors. In addition, mirtazapine increased memory for likeable versus dislikeable self-relevant information suggesting an induction of positive bias in emotional memory. Such effects may be important for our understanding of the neuropsychological mechanisms of antidepressant action in both anxiety and depressive disorders.

Mood and cortisol responses following tryptophan-rich hydrolyzed protein and acute stress in healthy subjects with high and low cognitive reactivity to depression

BACKGROUND & AIMS

It is suggested that stress particularly in subjects with high cognitive reactivity (CR), a psychological vulnerability marker of depression, may increase or even induce serotonergic vulnerability, which in turn may lead to reduced serotonin (5-HT) function, decreased stress coping and an increased risk to develop depressive symptoms. The aim of the present study was to investigate the beneficial effects of 5-HT augmentation through a tryptophan-rich hydrolyzed protein (HP) on mood and stress coping in subjects with high and low CR. We hypothesized that subjects with high CR are more responsive to the beneficial effects of HP than subjects with low CR particularly after acute stress exposure.

METHODS

In a double-blind, crossover study, participants' mood and cortisol was assessed before and after acute stress exposure either following intake of HP or a standard casein protein (CP) as control condition.

RESULTS

HP significantly increased positive mood in all subjects and dampened the cortisol response to acute stress. No differences were found between high and low CR subjects.

CONCLUSIONS

To conclude, because dietary treatment with HP has beneficial effects on mood and physiological stress coping in both high and low CR subjects, HP may be a good dietary method for augmenting brain TRP and 5-HT and thus 5-HT linked stress resilience.

The 5-HT1A C(-1019)G polymorphism, personality and electrodermal reactivity in a reward/punishment paradigm

During past years the 5-HT(1A) C(-1019)G polymorphism has been associated with vulnerability to depression, anxiety-disorder and personality traits related to negative emotionality (e.g. neuroticism). Many of these studies focused on case-control comparisons or associations between genetic markers and personality traits assessed by the use of questionnaires. In contrast, overt behaviour and physiological measures in experimental paradigms, although very promising, have seldom been the focus of studies investigating the role of the 5-HT(1A) polymorphism for behaviour and psychopathology. To fill this gap, we examined the relationship between the 5-HT(1A) C(-1019)G polymorphism and reaction times (in a reward/punishment paradigm) as well as electrodermal activity, as a marker of autonomic arousal, in 123 healthy subjects. This paradigm seems very promising, as sensitivity to punishment in particular, is strongly associated to traits related to negative emotionality. Carriers of the GG genotype, which is related to increased expression of 5-HT(1A) autoreceptors, exhibited increased reaction times when they were able to win money (reward condition). In direct contrast to the reward condition, these subjects show faster reaction times in the punishment condition (losing money). Moreover, GG carriers are characterized by an enhanced electrodermal activity in all experimental conditions (win, lose and verbal feedback). Finally, the reaction-time pattern mentioned was related to higher scores on negative emotionality as revealed by self-reports. These findings demonstrate for the first time that the 5-HT(1A) polymorphism is related to personality on the level of a triadic approach including behaviour, physiology and self-reports.

Goal-directed control and its antipodes

In instrumental conditioning, there is a rather precise definition of goal-directed control, and therefore an acute boundary between it and the somewhat more amorphous category comprising its opposites. Here, we review this division in terms of the various distinctions that accompany it in the fields of reinforcement learning and cognitive architectures, considering issues such as declarative and procedural control, the effect of prior distributions over environments, the neural substrates involved, and the differing views about the relative rationality of the various forms of control. Our overall aim is to reconnect some presently far-flung relations.

Panic disorder: is the PAG involved?

Data from studies with humans have suggested that abnormalities of midbrain structures, including the periaqueductal gray matter (PAG), could be involved in the neurobiology of panic disorder (PD). The electrical stimulation of the PAG in neurosurgical patients induces panic-like symptoms and the effect of drugs that are effective in the treatment of PD in the simulation of public speaking model of anxiety is in agreement with data from animal models of PD. Structural neuroimaging studies have shown increases in gray matter volume of midbrain and pons of PD patients. There is also evidence of lower serotonin transporter and receptor binding, and increases of metabolism in the midbrain of PD patients. Nevertheless, these midbrain abnormalities can not be considered as specific findings, since neuroimaging data indicate that PD patients have abnormalities in other brain structures that process fear and anxiety.

Short-term full kindling of the amygdala dissociates natural and periaqueductal gray-evoked flight behaviors of the rat

Panic attacks present a high comorbidity with agoraphobia, separation anxiety and generalized anxiety disorder. Nevertheless, while panic attacks have been frequently equated to fear, the relationship of panic disorder with specific phobias remains uncertain. The combination of experimental models of panic and phobias could but afford valuable information about both the comorbidity and causation of these disorders. As it regards, while the defensive behaviors produced by stimulation of dorsal periaqueductal gray matter (DPAG) resemble a panic attack, resistance to capture (RC) behaviors that ensue the kindling of the amygdala (AMY) are reminiscent of a phobic reaction. Therefore, this study examined the thresholds of DPAG-evoked panic-like behaviors in rats showing RC behaviors. Rats bearing electrodes in the DPAG and right AMY were subjected to either the full- (Kin-F) or sham- (Kin-S) kindling of AMY. RC behaviors were evaluated throughout both kindling procedures. Thresholds of DPAG-evoked defensive behaviors were recorded before and after the kindling. Moreover, performances of Kin-F and Kin-S rats in the elevated plus-maze were compared to those of intact controls. Data showed that Kin-F and Kin-S rats perform similarly in the elevated plus-maze, thereby making AMY-kindled rats unlikely as a model of generalized anxiety disorder. On the other hand, whereas the RC is characterized by a marked facilitation of natural freezing and flight behaviors, DPAG-evoked freezing (immobility and exophthalmus) and flight (galloping) behaviors were unexpectedly attenuated. Data suggest that RC is mediated by circuits distinct from those of DPAG-evoked defensive behaviors. Consequently, panic and phobic attacks may be mediated by different mechanisms as well.

Role of Serotonin and Dopamine System Interactions in the Neurobiology of Impulsive Aggression and its Comorbidity with other Clinical Disorders

Impulsive aggression is characterized by an inability to regulate affect as well as aggressive impulses, and is highly comorbid with other mental disorders including depression, suicidal behavior, and substance abuse. In an effort to elucidate the neurobiological underpinnings of impulsive aggression and to help account for its connections with these other disorders, this paper reviews relevant biochemical, brain imaging, and genetic studies. The review suggests that dysfunctional interactions between serotonin and dopamine systems in the prefrontal cortex may be an important mechanism underlying the link between impulsive aggression and its comorbid disorders. Specifically, serotonin hypofunction may represent a biochemical trait that predisposes individuals to impulsive aggression, with dopamine hyperfunction contributing in an additive fashion to the serotonergic deficit. The current paper proposes a modified diathesis-stress model of impulsive aggression in which the underlying biological diathesis may be deficient serotonergic function in the ventral prefrontal cortex. This underlying disposition can be manifested behaviorally as impulsive aggression towards oneself and others, and as depression under precipitating life stressors. Substance abuse associated with impulsive aggression is understood in the context of dopamine dysregulation resulting from serotonergic deficiency. Also discussed are future research directions in the neurobiology of impulsive aggression and its comorbid disorders.

Serotonergic modulation of face-emotion recognition

Facial expressions of basic emotions have been widely used to investigate the neural substrates of emotion processing, but little is known about the exact meaning of subjective changes provoked by perceiving facial expressions. Our assumption was that fearful faces would be related to the processing of potential threats, whereas angry faces would be related to the processing of proximal threats. Experimental studies have suggested that serotonin modulates the brain processes underlying defensive responses to environmental threats, facilitating risk assessment behavior elicited by potential threats and inhibiting fight or flight responses to proximal threats. In order to test these predictions about the relationship between fearful and angry faces and defensive behaviors, we carried out a review of the literature about the effects of pharmacological probes that affect 5-HT-mediated neurotransmission on the perception of emotional faces. The hypothesis that angry faces would be processed as a proximal threat and that, as a consequence, their recognition would be impaired by an increase in 5-HT function was not supported by the results reviewed. In contrast, most of the studies that evaluated the behavioral effects of serotonin challenges showed that increased 5-HT neurotransmission facilitates the recognition of fearful faces, whereas its decrease impairs the same performance. These results agree with the hypothesis that fearful faces are processed as potential threats and that 5-HT enhances this brain processing.

Tryptophan research in panic disorder

A considerable body of evidence suggests the involvement of serotonin neurotransmission in the pathogenesis of panic disorder. Research on pathways and functions of tryptophan, an essential amino acid converted into serotonin, may advance our understanding of serotonergic actions in panic disorder and related phenomena. The investigative approaches in this field include manipulations of tryptophan availability as well as genetic association and functional brain imaging studies. In this review we examine the principle findings of these studies and propose further research directions.

Emotion-induced retrograde amnesia is determined by a 5-HTT genetic polymorphism

A polymorphism in the human serotonin transporter (5-HTT) gene is implicated in susceptibility to anxiety and depression and in enhanced emotion-induced activation in the amygdala. A role for 5-HTT polymorphism in the emotional modulation of human episodic memory has yet to be demonstrated. Here, we demonstrate that whereas emotional memory for aversive events per se is not influenced by 5-HTT polymorphism, an emotion-induced retrograde amnesia is expressed solely in the presence of the short allele. The findings indicate a critical role for the serotonin system in emotion-mediated memory disruption.

New insights on the subcortical representation of reward

Reward information is represented by many subcortical areas and neuron types, which constitute a complex network. Its output is usually mediated by the basal ganglia where behaviors leading to rewards are disinhibited and behaviors leading to no reward are suppressed. Midbrain dopamine neurons modulate these basal ganglia neurons differentially using signals related to reward-prediction error. Recent studies suggest that other types of subcortical neurons assist, instruct, or work in parallel with dopamine neurons. Such reward-related neurons are found in the areas which have been associated with stress, pain, mood, emotion, memory, and arousal. These results suggest that reward needs to be understood in a larger framework of animal behavior.

Acute tryptophan depletion in healthy volunteers enhances punishment prediction but does not affect reward prediction

Central serotonin (5-HT) has been implicated in emotional and behavioral control processes for many decades, but its precise contribution is not well understood. We used the acute tryptophan depletion procedure in young healthy volunteers to test the hypothesis that central 5-HT is critical for predicting punishment. An observational reversal-learning task was employed that provided separate measures of punishment and reward prediction. Under baseline, subjects made more prediction errors for punishment-associated stimuli than for reward-associated stimuli. This bias was abolished after central 5-HT depletion, which enhanced the ability to predict punishment while not affecting reward prediction. The selective potentiation of punishment prediction concurs with recent theorizing, suggesting that central 5-HT carries a prediction error for future punishment, but not for future reward (Daw et al, 2002). Furthermore, the finding highlights the importance of central 5-HT in resilience to adversity and may have implications for a variety of neuropsychiatric disorders including depression and anxiety.

High-frequency stimulation of the dorsolateral periaqueductal gray and ventromedial hypothalamus fails to inhibit panic-like behaviour

Electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) and one of its target structures, the ventromedial hypothalamus (VMH), produces a typical behaviour in rats consisting of vigorous running and jumping which is known as "escape behaviour". Escape behaviour in rodents closely mimics panic attacks in humans. Since electrical stimulation at higher frequencies generally inhibits the stimulated region, we tested in this study the hypothesis that deep brain stimulation (DBS) of the dlPAG and VMH at higher frequencies (> 100 Hz) would not induce escape behaviour. More specifically, we evaluated whether experimental DBS could be used to inhibit panic-like behaviour. Rats underwent implantation of DBS-electrodes at the level of the dlPAG and VMH and the effects of various stimulation parameters were assessed. In addition, we studied the neural activation pattern resulting from DBS of the dlPAG and VMH using c-Fos immunohistochemistry. We found that stimulation amplitude is the most important stimulation parameter in the induction of escape behaviour. Remarkably, stimulation frequency (1-300 Hz) had no effect on stimulation-induced escape behaviour and therefore it was not possible to prevent the induction of escape behaviour with higher frequencies. The neuronal activation pattern resulting from dlPAG and VMH DBS was similar. These findings suggest that DBS of the dlPAG and VMH induces panic-related behaviours even at higher frequencies.

The endocannabinoid system: emotion, learning and addiction

The identification of the cannabinoid receptor type 1 (CB1 receptor) was the milestone discovery in the elucidation of the behavioural and emotional responses induced by the Cannabis sativa constituent Delta(9)-tetrahydrocannabinol. The subsequent years have established the existence of the endocannabinoid system. The early view relating this system to emotional responses is reflected by the fact that N-arachidonoyl ethanolamine, the pioneer endocannabinoid, was named anandamide after the Sanskrit word 'ananda', meaning 'bliss'. However, the emotional responses to cannabinoids are not always pleasant and delightful. Rather, anxiety and panic may also occur after activation of CB1 receptors. The present review discusses three properties of the endocannabinoid system as an attempt to understand these diverse effects. First, this system typically functions 'on-demand', depending on environmental stimuli and on the emotional state of the organism. Second, it has a wide neuro-anatomical distribution, modulating brain regions with different functions in responses to aversive stimuli. Third, endocannabinoids regulate the release of other neurotransmitters that may have even opposing functions, such as GABA and glutamate. Further understanding of the temporal, spatial and functional characteristics of this system is necessary to clarify its role in emotional responses and will promote advances in its therapeutic exploitation.

Regional gray matter abnormalities in panic disorder: a voxel-based morphometry study

Although abnormalities in brain structures involved in the neurobiology of fear and anxiety have been implicated in the pathophysiology of panic disorder (PD), relatively few studies have made use of voxel-based morphometry (VBM) magnetic resonance imaging (MRI) to determine structural brain abnormalities in PD. We have assessed gray matter volume in 19 PD patients and 20 healthy volunteers using VBM. Images were acquired using a 1.5 T MRI scanner, and were spatially normalized and segmented using optimized VBM. Statistical comparisons were performed using the general linear model. A relative increase in gray matter volume was found in the left insula of PD patients compared with controls. Additional structures showing differential increases were the left superior temporal gyrus, the midbrain, and the pons. A relative gray matter deficit was found in the right anterior cingulate cortex. The insula and anterior cingulate abnormalities may be relevant to the pathophysiology of PD, since these structures participate in the evaluation process that ascribes negative emotional meaning to potentially distressing cognitive and interoceptive sensory information. The abnormal brain stem structures may be involved in the generation of panic attacks.

Serotonin, inhibition, and negative mood

Pavlovian predictions of future aversive outcomes lead to behavioral inhibition, suppression, and withdrawal. There is considerable evidence for the involvement of serotonin in both the learning of these predictions and the inhibitory consequences that ensue, although less for a causal relationship between the two. In the context of a highly simplified model of chains of affectively charged thoughts, we interpret the combined effects of serotonin in terms of pruning a tree of possible decisions, (i.e., eliminating those choices that have low or negative expected outcomes). We show how a drop in behavioral inhibition, putatively resulting from an experimentally or psychiatrically influenced drop in serotonin, could result in unexpectedly large negative prediction errors and a significant aversive shift in reinforcement statistics. We suggest an interpretation of this finding that helps dissolve the apparent contradiction between the fact that inhibition of serotonin reuptake is the first-line treatment of depression, although serotonin itself is most strongly linked with aversive rather than appetitive outcomes and predictions.

Serotonin is an evolutionarily ancient neuromodulator probably best known for its role in psychiatric disorders. However, that role has long appeared contradictory to its role in normal function, and indeed its various roles in normal affective behaviors have been hard to reconcile. Here, we model two predominant functions of normal serotonin function in a highly simplified reinforcement learning model and show how these may explain some of its complex roles in depression and anxiety.

Activation of raphe efferents to the medial prefrontal cortex by corticotropin-releasing factor: correlation with anxiety-like behavior

BACKGROUND

Parallel lines of research suggest that dysfunction affecting both corticotropin-releasing factor (CRF) and serotonin (5-HT) systems is involved in the pathophysiology of psychiatric illnesses such as anxiety and depression. The effect of CRF on behavior and on the accompanying change in activity of 5-HT neurons in the dorsal and median raphe nuclei (DR and MR) that project to the ventral medial prefrontal cortex (mPFC), a brain area implicated in mood and anxiety disorders, was studied.

METHODS

Male Sprague-Dawley rats with intra-mPFC deposits of fluorescent microspheres received injections of CRF (1 microg, intracerebroventricular [i.c.v.]) and were tested for CRF-enhanced startle, a behavioral assay believed to reflect stress- or anxiety-like states. C-Fos immunohistochemistry was used to measure CRF-induced activity in retrogradely labeled neurons in the DR and MR and correlate this level of activity with the level of CRF-enhanced startle.

RESULTS

The CRF-enhanced startle was accompanied by an increased c-Fos expression in retrogradely labeled cells in the raphe. In the DR and MR, there was a clear topography of activation, with a higher-percent activation in retrogradely labeled neurons in caudal sections. In the caudal DR, this effect was positively correlated with the level of CRF-enhanced startle. Co-expression of retrogradely labeled cells with tryptophan hydroxylase showed that the majority (> 90%) of raphe efferents to the mPFC were from serotonergic neurons.

CONCLUSIONS

These data indicate that CRF activates a subpopulation of cortical-projecting 5-HT raphe neurons and suggest that increased 5-HT release in the mPFC might be an important component driving some types of anxiety-like behaviors.

Anxiolytic and panicolytic effects of escitalopram in the elevated T-maze

Escitalopram is a highly selective inhibitor of serotonin re-uptake that is used to treat anxiety disorders. In the present study, we investigated the effects of acute, sub-chronic (14 days) and chronic (21 days) administration of escitalopram (2, 4 and 8 mg/kg, PO) on the performance of rats in the elevated T-maze. For comparison, imipramine (15 mg/kg, PO) was also studied. The apparatus is made of three elevated arms of equal dimension, one enclosed transversal to the two open arms. Inhibitory avoidance of the open arms, trained in the enclosed arm, has been related to generalised anxiety disorder, while one-way escape from one open arm, to panic disorder. After acute administration, the three doses of escitalopram impaired avoidance (anxiolytic effect), while imipramine was ineffective. Escape was unaffected by either drug. With subchronic administration, both drugs were ineffective on either avoidance or escape. After chronic treatment, avoidance was impaired by imipramine and by the two highest doses of escitalopram. In addition, escape was impaired (panicolytic effect) by imipramine and by the highest dose of escitalopram. Locomotion measured in a square arena was increased by the three doses of escitalopram, given chronically. Therefore, both imipramine and escitalopram had anxiolytic and panicolytic-like effects after chronic administration, but acutely only escitalopram decreased anxiety. Since no such effect was observed following subchronic administration, it is likely that the mechanisms of the early and late anxiolytic actions of escitalopram are different.

Modulation of defensive responses and anxiety-like behaviors by group I metabotropic glutamate receptors located in the dorsolateral periaqueductal gray

Glutamatergic neurotransmission in the dorsolateral periaqueductal gray (dlPAG) is related to defensive responses. However, the role of group I glutamate metabotropic receptors (mGluR) in these responses has been poorly investigated. The objective of the present study, therefore, was to test the hypothesis that interference with group I mGluR-mediated neurotransmission in dlPAG could modulate defensive responses. Male Wistar rats with cannulae aimed at the dlPAG were submitted to the following experiments: 1. intra dlPAG injections of vehicle (veh, 0.2 microL) or (RS)1-aminoindan-1,5-dicarboxylic acid (AIDA, 30-100 nmol, an mGluR1 receptor competitive antagonist) followed, 5 min later, by veh or trans-(+)-1-amino-1,3-ciclopentanedicarboxylic acid (tACPD, a group I and II mGluR agonist, 30 nmol); 2. intra-dlPAG injections of veh, AIDA (30 nmol) or 2-methyl-6-(phenylethynyl)-pyridine (MPEP, an mGluR5 receptor non-competitive antagonist, 50 nmol) followed by trans-azetidine-2,4-dicarboxylic acid (tADA, a group I mGluR agonist, 10 nmol); 3. and 4. intra-dlPAG injections of vehicle, AIDA (10-30 nmol) or MPEP (10-50 nmol) before the elevated plus maze (EPM) test; 5. intra-dlPAG injections of vehicle, AIDA (30 nmol) or MPEP (50 nmol) before the Vogel punished licking test. tACPD induced defensive responses characterized by jumps and an increased number of crossings in the observation box. These responses were attenuated by AIDA (30 nmol). tADA produced similar responses, although of lower intensity. tADA effects were prevented by AIDA and MPEP. Both drugs also produced anxiolytic-like effects in the EPM and Vogel tests when injected alone. The results suggest that group I metabotropic glutamate receptors in the dlPAG facilitate defensive responses and may also be involved in regulating anxiety-like behavior.