5-HT and mechanisms of defence

A temporal difference account of avoidance learning

Aversive processing plays a central role in human phobic fears and may also be important in some symptoms of psychosis. We developed a temporal-difference model of the conditioned avoidance response, an important experimental model for aversive learning which is also a central pharmacological model of psychosis. In the model, dopamine neurons reported outcomes that were better than the learner expected, typically coming from reaching safety states, and thus controlled the acquisition of a suitable policy. The model accounts for normal conditioned avoidance learning, the persistence of responding in extinction, and critical effects of dopamine blockade, notably that subjects experiencing shocks under dopamine blockade, and hence failing to avoid them, nevertheless develop avoidance responses when both shocks and dopamine blockade are subsequently removed. These postulated roles of dopamine in aversive learning can thus account for many of the effects of dopaminergic modulation seen in laboratory models of psychopathological processes.

CO2 challenge induced HPA axis activation in panic

The hypothalamic-pituitary-adrenal axis (HPA axis) plays a critical role in stress management. Involvement of this physiological axis in the underlying mechanisms of panic disorder (PD) has been suggested. Studies using 35% CO(2) inhalation to provoke panic found no evidence for robust increases in cortisol levels in PD. However, cortisol levels alone may not be conclusive, as this hormone is merely the end product of a complex physiological axis. Sixteen PD patients and 16 healthy control subjects underwent a 35% CO(2) inhalation and a placebo inhalation on separate days according to a fixed order, double-blind design. Both serum and salivary cortisol, as well as adrenocorticotropic hormone (ACTH) were measured at regular time intervals. Cortisol and ACTH levels increased in the patient and control groups following 35% CO(2) inhalation. The magnitude of the increase was similar in patients and controls despite marked differences in anxiety. This study is the first to document a clear HPA response following 35% CO(2) inhalation in both PD patients and controls. This effect occurs independently of the specific panicogenic properties of the CO(2) challenge. It remains to be clarified whether panic is initially accompanied by major HPA axis activation or whether other stress-responsive systems underlie panic.

Repeated cortisol administration attenuates the EEG response to buspirone in healthy volunteers: evidence for desensitization of the 5-HT1A autoreceptor

It has previously been postulated that the therapeutic effect of antidepressants, particularly selective serotonin re-uptake inhibitors (SSRIs), is mediated by a down-regulation of somatodendritic (presynaptic) 5-HT(1A) autoreceptors with chronic treatment. Animal studies have revealed that repeated administration of corticosteroids similarly down-regulate this receptor. However, it has previously been difficult to explore if this receptor is similarly modulated in man in vivo. The objective of this study was to explore the effect of repeated administration of cortisol to healthy volunteers utilising a novel putative index of somatodendritic 5-HT(1A) autoreceptor function. This method involves the administration of the 5-HT(1A) agonist buspirone and observing the subsequent negative shift in the frequency spectrum of the electroencephalogram (EEG). Healthy male volunteers were treated with cortisol 20 mg, or placebo, orally twice daily for 7 days in a double-blind random-order crossover study. After each treatment period volunteers were administered buspirone 30 mg orally prior to EEG recordings. Following a week's treatment with placebo, buspirone led to a negative shift in the EEG frequency spectrum as previously reported. However, following treatment with cortisol, the effect of buspirone was significantly attenuated. This is consistent with corticosteroids having a similar effect on somatodendritic 5-HT(1A) autoreceptors in man as seen in rodents.

Serotonergic mechanisms in the basolateral amygdala differentially regulate the conditioned and unconditioned fear organized in the periaqueductal gray

The amygdala is an important filter for unconditioned and conditioned aversive information. The amygdala synthesizes the stimuli input from the environment and then signals the degree of threat that they represent to the dorsal periaqueductal gray (dPAG), which would be in charge of selecting, organizing and executing the appropriate defense reaction. In this study, we examined the influence of fluoxetine microinjections (1.75 and 3.5 nmol/0.2 microL) into the lateral (LaA) and basolateral (BLA) amygdaloid nuclei on the freezing and escape responses induced by electrical stimulation of the dPAG. Freezing behavior was also measured after the interruption of the electrical stimulation of the dPAG. On the following day, these rats were also submitted to a contextual fear paradigm to examine whether these microinjections would affect the conditioned freezing to contextual cues previously associated with foot shocks. Fluoxetine injections into both amygdaloid nuclei did not change the freezing and escape thresholds, but disrupted the dPAG-post-stimulation freezing. Moreover, the conditioned freezing was enhanced by fluoxetine. Whereas 5-HT mechanisms in the amygdala facilitate the acquisition of conditioned fear they inhibit the dPAG-post-stimulation freezing. However, the unconditioned fear triggered by activation of the dPAG is produced downstream of the amygdala. These findings have important implications for the understanding of the neurochemical substrates that underlie panic and generalized anxiety disorders.

Different patterns of freezing behavior organized in the periaqueductal gray of rats: association with different types of anxiety

Freezing defined as the complete absence of body movements is a normal response of animals to unavoidable fear stimuli. The present review presents a series of evidence relating different defensive patterns with specific anxiety disorders. There are at least four different kinds of freezing with specific neural substrates. The immobility induced by stimulation of the ventral column of the periaqueductal gray (vPAG) has been considered a quiescence characteristic of the recovery component of defense-recuperative processes. There is an isomorphism between freezing response to contextual stimuli paired with electrical shocks and generalized anxiety disorder. Besides, two types of freezing emerge with the electrical stimulation of the dorsal aspects of the periaqueductal gray (dPAG): the dPAG-evoked freezing and the dPAG post-stimulation freezing. Evidence is presented in support of the hypothesis that whereas dPAG-evoked freezing would serve as a model of panic attacks, the dPAG post-stimulation freezing appears to be a model of panic disorder. It is also proposed that conditioned freezing plus dPAG electrical stimulation might also mimic panic disorder with agoraphobia. A model of serotoninergic modulation through on- and off-cells of the defense reaction generated in the dPAG is also presented. The understanding of how the periaqueductal gray generates and elaborates different types of freezing is of relevance for our better knowledge of distinct types of anxiety such as panic disorder or generalized anxiety disorder.

Exercício aeróbio como terapia de exposição a estímulos interoceptivos no tratamento do transtorno de pânico

Os ataques de pânico são representados por um período distinto no qual há o início súbito de intensa apreensão, temor ou terror, freqüentemente associados com sentimentos de catástrofe iminente, diagnosticado em aproximadamente 10% da população. O Transtorno de Pânico é um transtorno de ansiedade que se caracteriza pela recorrência de ataques de pânico: crises súbitas de mal-estar e sensação de perigo ou morte iminente, acompanhadas de diversos sintomas físicos e cognitivos. Os indivíduos com Transtorno de Pânico apresentam, caracteristicamente, preocupações acerca das implicações ou conseqüências dos ataques de pânico. É uma condição clínica complexa que envolve diferentes modalidades ou conglomerados de sintomas. Assim, o foco nas sensações físicas erroneamente interpretadas no transtorno de pânico e na hipocondria centraliza-se basicamente nas manifestações autonômicas, como taquicardia e dispnéia. Há poucos estudos sobre atividade física e transtorno de pânico. O principal objetivo do estudo visa identificar com diferentes descrições se há uma população "nuclear" com sintomas predominantemente respiratórios apresentando esquiva de atividade física e a influência do exercício nesta população.

When fear is near: threat imminence elicits prefrontal-periaqueductal gray shifts in humans

Humans, like other animals, alter their behavior depending on whether a threat is close or distant. We investigated spatial imminence of threat by developing an active avoidance paradigm in which volunteers were pursued through a maze by a virtual predator endowed with an ability to chase, capture, and inflict pain. Using functional magnetic resonance imaging, we found that as the virtual predator grew closer, brain activity shifted from the ventromedial prefrontal cortex to the periaqueductal gray. This shift showed maximal expression when a high degree of pain was anticipated. Moreover, imminence-driven periaqueductal gray activity correlated with increased subjective degree of dread and decreased confidence of escape. Our findings cast light on the neural dynamics of threat anticipation and have implications for the neurobiology of human anxiety-related disorders.

GABA/benzodiazepine receptors in the ventromedial hypothalamic nucleus regulate both anxiety and panic-related defensive responses in the elevated T-maze

It has been shown that facilitation of GABA-mediated neurotransmission in the medial nucleus of the amygdala and the dorsal periaqueductal gray (dPAG) inhibits the escape, but not the inhibitory avoidance response generated in the elevated T-maze test of anxiety (ETM). These defensive behaviors have been associated with panic and generalized anxiety, respectively. Previous evidence indicates that the dorsomedial part of the ventromedial hypothalamus (VMHdm), which is interconnected with these two brain areas, is also part of the neurobiological substrate controlling escape behavior. In the present study, we investigated in male Wistar rats whether the intra-VMHdm injection of GABA-modulating drugs differently affect the two defensive tasks measured in the ETM. The results showed that the microinjection of the benzodiazepine (BZD) receptor agonist midazolam (10, 20 and 40 nmol), the GABA(A) receptor agonist muscimol (2, 4 and 8 nmol) or the GABA(B) receptor agonist baclofen (2, 4 and 8 nmol) impaired inhibitory avoidance and escape performance, an anxiolytic and panicolytic-like effect, respectively. On the other hand, local administration of the BZD inverse agonist FG 7142 (20, 40 and 80 pmol) facilitated both behaviors, suggesting anxiogenic and panicogenic-like effects. These results were not due to motor alterations, since the drugs did not affect exploratory behavior in an open field. The data suggest that GABA(A)/BZD and GABA(B) receptors within the VMHdm are involved not only in the control of panic-related, but also of anxiety-related behaviors.

Candidate genes for panic disorder: insight from human and mouse genetic studies

Panic disorder is a major cause of medical attention with substantial social and health service cost. Based on pharmacological studies, research on its etiopathogenesis has been focused on the possible dysfunction of specific neurotransmitter systems. However, recent work has related the genes involved in development, synaptic plasticity and synaptic remodeling to anxiety disorders. This implies that learning processes and changes in perception, interpretation and behavioral responses to environmental stimuli are essential for development of complex anxiety responses secondary to the building of specific brain neural circuits and to adult plasticity. The focus of this review is on progress achieved in identifying genes that confer increased risk for panic disorder through genetic epidemiology and the use of genetically modified mouse models. The integration of human and animal studies targeting behavioral, systems-level, cellular and molecular levels will most probably help identify new molecules with potential impact on the pathogenetic aspects of the disease.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Effects of ayahuasca on psychometric measures of anxiety, panic-like and hopelessness in Santo Daime members

The use of the hallucinogenic brew ayahuasca, obtained from infusing the shredded stalk of the malpighiaceous plant Banisteriopsis caapi with the leaves of other plants such as Psychotria viridis, is growing in urban centers of Europe, South and North America in the last several decades. Despite this diffusion, little is known about its effects on emotional states. The present study investigated the effects of ayahuasca on psychometric measures of anxiety, panic-like and hopelessness in members of the Santo Daime, an ayahuasca-using religion. Standard questionnaires were used to evaluate state-anxiety (STAI-state), trait-anxiety (STAI-trait), panic-like (ASI-R) and hopelessness (BHS) in participants that ingested ayahuasca for at least 10 consecutive years. The study was done in the Santo Daime church, where the questionnaires were administered 1h after the ingestion of the brew, in a double-blind, placebo-controlled procedure. While under the acute effects of ayahuasca, participants scored lower on the scales for panic and hopelessness related states. Ayahuasca ingestion did not modify state- or trait-anxiety. The results are discussed in terms of the possible use of ayahuasca in alleviating signs of hopelessness and panic-like related symptoms.

Ansiedade, pânico e o eixo hipotálamo-pituitária-adrenal

OBJETIVO: Este artigo discute a ativação diferencial do eixo hipotálamo-pituitária-adrenal no transtorno de ansiedade generalizada e no transtorno de pânico. MÉTODO: Resultados de recentes revisões da literatura são resumidos e discutidos. RESULTADOS: Os resultados de estudos experimentais que dosaram o hormônio adrenocorticotrópico, o cortisol e a prolactina mostram que ataques de pânico naturais, bem como os provocados por agentes panicogênicos seletivos - como lactato de sódio e dióxido de carbono -, não ativam o eixo hipotálamo-pituitária-adrenal. Agonistas do receptor de colecistocinina do tipo B, como o peptídeo colecistocinina-4 e a pentagastrina, elevam os hormônios de estresse, independentemente da ocorrência de um ataque de pânico, parecendo ativar diretamente o eixo hipotálamo-pituitária-adrenal. O antagonista benzodiazepínico flumazenil não eleva o nível dos hormônios de estresse; porém, este agente farmacológico não induz ataques de pânico de modo consistente. Agentes farmacológicos que aumentam a ansiedade em pacientes de pânico (cafeína, ioimbina, agonistas serotonérgicos), assim como em pessoas saudáveis, elevam o nível dos hormônios de estresse. CONCLUSÕES: Além das diferenças na sintomatologia e na resposta farmacológica, o transtorno de ansiedade generalizada e o transtorno de pânico afetam os hormônios de estresse de modo distinto. Enquanto a ansiedade antecipatória e o transtorno de ansiedade generalizada ativam tanto o eixo hipotálamo-pituitária-adrenal como o simpático-adrenal, o ataque de pânico causa acentuada ativação simpática; porém, afeta pouco o eixo hipotálamo-pituitária-adrenal.

Carbohydrate intake improves cognitive performance of stress-prone individuals under controllable laboratory stress

Cognitive performance has been found to decline after exposure to stress, particularly in stress-prone subjects. The present study investigated whether a carbohydrate-rich, protein-poor (CR/PP) diet, which may enhance cerebral serotonin function in stress-prone subjects due to increases in the available tryptophan, improves the performance of stress-prone subjects after exposure to acute laboratory stress. Twenty-two high-stress-prone (HS) subjects and twenty-one low-stress-prone (LS) subjects aged between 19 and 26 years performed a memory scanning task after controllable and uncontrollable stress, following either a CR/PP diet or a protein-rich, carbohydrate-poor (PR/CP) isoenergetic diet. Uncontrollable stress reduced feelings of control (F(1,38) 9·30; P = 0·004), whereas pulse rate and skin conductance increased after both stress tasks (F(1,38) 78·34; P = 0·0005 and F(1,37) 83·16; P = 0·0004). Diet, stress-proneness and stress-controllability interacted (F(1,36) 9·46; P = 0·004) in such a way that performance in HS subjects was better with the CR/PP diet than with the PR/CP diet, but only after controllable stress. As the CR/PP diet has been found to increase the plasma tryptophan: large neutral amino acids ratio, indicating an increased availability of cerebral tryptophan and, thus, higher serotonin levels, it appears that there may be an increased availability of brain serotonin in HS subjects after controllable laboratory stress.

Exploratory behaviour of rats in the elevated plus-maze is differentially sensitive to inactivation of the basolateral and central amygdaloid nuclei

The amygdala has a crucial role in detecting motivationally significant inputs and in communicating relevant information to other limbic structures. Behavioural studies have shown that the central (CeA) and basolateral (BLA) nuclei of amygdala differentially regulate conditioned and unconditioned fear. Indeed, much evidence has accumulated suggesting that regulatory mechanisms in the BLA serve as a filter for unconditioned and conditioned aversive information that ascends to higher structures from the brainstem, whereas the CeA is the main output for the autonomic and somatic components of fear reaction through major projections to other limbic regions. It is still unclear, however, how amygdaloid nuclei function in high and open spaces so as to determine the characteristic exploratory behaviour of rats submitted to the elevated plus-maze test (EPM). In the present study, we carried out an ethopharmacological analysis of the behaviour of rats submitted to the elevated plus-maze test together with analysis of the tissue content of monoamine dopamine (DA) and serotonin (5-HT) and their metabolites in the dorsal hippocampus (DH), nucleus accumbens (NAC) and dorsal striatum (DS) of animals injected with saline or muscimol (1.0 nmol/0.2 microL) into the BLA or CeA. The data obtained show that injections of muscimol into the CeA, but not into the BLA, caused anxiolytic-like effects in the EPM. Such effects of muscimol into the CeA were accompanied by increases in 5-HT content of the DH, whereas corresponding injections into the BLA caused a reduction in the DA content of the NAC. There was no change in the turnover rates of these monoamines. These data suggest that the BLA and CeA have distinct roles in the exploratory behaviour of rodents in the EPM. While BLA appears to be related to the detection and validation of threatening stimuli, the CeA appears to be involved in the expression of fear behaviours in the EPM.

Animal defense strategies and anxiety disorders

Anxiety disorders are classified according to symptoms, time course and therapeutic response. Concurrently, the experimental analysis of defensive behavior has identified three strategies of defense that are shared by different animal species, triggered by situations of potential, distal and proximal predatory threat, respectively. The first one consists of cautious exploration of the environment for risk assessment. The associated emotion is supposed to be anxiety and its pathology, Generalized Anxiety Disorder. The second is manifested by oriented escape or by behavioral inhibition, being related to normal fear and to Specific Phobias, as disorders. The third consists of disorganized flight or complete immobility, associated to dread and Panic Disorder. Among conspecific interactions lies a forth defense strategy, submission, that has been related to normal social anxiety (shyness) and to Social Anxiety Disorder. In turn, Posttraumatic Stress Disorder and Obsessive-Compulsive Disorder do not seem to be directly related to innate defense reactions. Such evolutionary approach offers a reliable theoretical framework for the study of the biological determinants of anxiety disorders, and a sound basis for psychiatric classification.

Elevated mazes as animal models of anxiety: effects of serotonergic agents

This article reviews reported results about the effects of drugs that act upon the serotonergic neurotransmission measured in three elevated mazes that are animal models of anxiety. A bibliographic search has been performed in MEDLINE using different combinations of the key words X-maze, plus-maze, T-maze, serotonin and 5-HT, present in the title and/or the abstract, with no time limit. From the obtained abstracts, several publications were excluded on the basis of the following criteria: review articles that did not report original results, species other than the rat, intracerebral drug administration alone, genetically manipulated rats, and animals having any kind of experimental pathology. The reported results indicate that the effect of drugs on the inhibitory avoidance task performed in the elevated T-maze and on the spatio temporal indexes of anxiety measured in the X and plus mazes correlate with their effect in patients diagnosed with generalized anxiety disorder. In contrast, the drug effects on the one-way escape task in the elevated T-maze predict the drug response of panic disorder patients. Overall, the drug effects assessed with the avoidance task in the T-maze are more consistent than those measured through the anxiety indexes of the X and plus mazes. Therefore, the elevated T-maze is a promising animal model of generalized anxiety and panic disorder.

5-HT2- and D1-mechanisms of the basolateral nucleus of the amygdala enhance conditioned fear and impair unconditioned fear

The inferior colliculus (IC) is involved in processing of auditory information, but also integrates acoustic information of aversive nature. In fact, chemical stimulation of the IC with semicarbazide (SMC) - an inhibitor of the GABA synthesizing enzyme glutamic acid decarboxylase - has been found to cause defensive behavior in an open-field test and functions as an unconditioned stimulus in the place conditioned aversion test (PCA). A question has arisen regarding whether the basolateral nucleus of the amygdala (BLA) is involved in the acquisition of the aversive information ascending from the IC and whether dopaminergic and serotoninergic mechanisms of the BLA regulate this process. Recent evidence has shown that inactivation of the BLA with muscimol inhibits the PCA and causes an increase in the aversiveness of the chemical stimulation of the IC. Based on this, we examined the effects of ketanserin and SCH-23390, antagonists of the 5HT(2) and D(1) receptors, respectively, on the conditioned and unconditioned fear elicited by IC stimulation with SMC. The results obtained confirm the crucial role of 5-HT(2)- and D(1)-mechanisms of the BLA on conditioned fear in that ketanserin and SCH-23390 injections into the BLA caused a reduction in the PCA. On the other hand, ketanserin and SCH-23390 injections into the BLA enhanced the aversiveness of the IC injections of SMC. These findings suggest that while 5-HT(2) and DA(1) mechanisms in the BLA appear to facilitate the conditioned fear they inhibit the unconditioned fear triggered by IC activation.

Neurochemical and behavioral consequences of striatal injection of 5,7-dihydroxytryptamine

It is known that central serotonin (5HT) is involved in anxiety, but the behavioral results of many studies have been inconsistent. A prevalent research approach is to destroy 5HT neurotoxically. Such lesions were mostly generated by injecting 5,7-dihydroxytryptamine into ventricles or raphé nuclei, leading to rather global losses of 5HT in the brain. However, there is evidence for differential effects of 5HT in different brain structures regarding anxiety. Therefore, we decided to study the effects of injecting 5,7-dihydroxytryptamine into the forebrain. We chose the ventral striatum as the site of injection, since there is evidence that 5HT may be involved in anxiety there. We administered the neurotoxin bilaterally in adult rats, and analyzed neurochemical and behavioral consequences in three experiments. The first one showed that the toxin dose-dependently (10-50 microg) depleted 5HT in the ventral striatum, neostriatum, frontal cortex, and amygdala. Besides 5HT, dopamine was also partly depleted there. This dopaminergic lesion was prevented in a second experiment, where rats were pre-treated systemically with the dopamine reuptake inhibitor nomifensine. In the final experiment, the functional consequences of such 5HT lesions were tested, which yielded moderate anxiogenic effects in the elevated plus maze and in the open field. Also, there were lesion effects on aversively motivated ultrasonic vocalization during an active avoidance test. In contrast, active avoidance performance itself and general activity in the open field were not affected. Lesion effects became discernible there when challenging rats with MDMA. The psycho-stimulatory effectiveness of this drug, which acts largely via the availability of 5HT in the brain, was reduced to degrees that depended on the size of 5HT lesion. These results are discussed with respect to factors such as severity of lesion, anatomical specificity, and the role of 5HT in anxiety.

Effects of intra-PAG infusion of ovine CRF on defensive behaviors in Swiss-Webster mice

The midbrain dorsal periaqueductal gray (DPAG) is part of the brain defensive system involved in active defense reactions to threatening stimuli. Corticotrophin releasing factor (CRF) is a peptidergic neurotransmitter that has been strongly implicated in the control of both behavioral and endocrine responses to threat and stress. We investigated the effect of the nonspecific CRF receptor agonist, ovine CRF (oCRF), injected into the DPAG of mice, in two predator-stress situations, the mouse defense test battery (MDTB), and the rat exposure test (RET). In the MDTB, oCRF weakly modified defensive behaviors in mice confronted by the predator (rat); e.g. it increased avoidance distance when the rat was approached and escape attempts (jump escapes) in forced contact. In the RET, drug infusion enhanced duration in the chamber while reduced tunnel and surface time, and reduced contact with the screen which divides the subject and the predator. oCRF also reduced both frequency and duration of risk assessment (stretch attend posture: SAP) in the tunnel and tended to increase freezing. These findings suggest that patterns of defensiveness in response to low intensity threat (RET) are more sensitive to intra-DPAG oCRF than those triggered by high intensity threats (MDTB). Our data indicate that CRF systems may be functionally involved in unconditioned defenses to a predator, consonant with a role for DPAG CRF systems in the regulation of emotionality.

Reversible inactivation of the dorsal raphe nucleus blocked the antipanic-like effect of chronic imipramine in the elevated T-maze

Antidepressant drugs are effective in the treatment of panic in human panic disorder patients. One hypothesis is that the anti-panic effects of antidepressant drugs are mediated by an increase in the activity of serotonergic neurons within dorsal raphe nucleus (DRN) leading to an increase in serotonin-mediated inhibition of the dorsal periaqueductal gray (DPAG). In order to test this hypothesis, we investigated the effects of reversible inhibition of the DRN on behavior in the elevated T-maze (ETM) in control rats and rats chronically treated with imipramine. Rats were injected daily with imipramine (15 mg/kg i.p.) or saline for 24 days. A guide cannula was implanted in the DRN on day 14. Lidocaine (4%, 0.2 microL) or saline was injected into the DRN 10 min before the test in the ETM followed immediately by the open-field test (day 21). Three days later, the infusions were crossed-over, rats microinjected into the DRN with saline in the first trial received lidocaine and vice versa, and the behavioral tests were repeated (day 24). Chronic saline plus lidocaine in the DRN and chronic imipramine (plus saline or lidocaine in the DRN) impaired inhibitory avoidance, indicating an anxiolytic effect. In the one-way escape, lidocaine facilitated it, suggesting a panicogenic effect, while chronic imipramine impaired it, which is indicative of a panicolytic effect. Moreover, lidocaine blocked the facilitatory effect of chronic imipramine. The locomotor activity in the open field was not changed by any treatment compared to the control group. These effects were congruent with the hypothesis that the DRN has a dual effect on anxiety: increasing learned anxiety and decreasing innate anxiety. Moreover, they suggest that the DRN exerts a crucial role in the antipanic-like effect of chronic imipramine in the ETM.

A comparison of the effects of citalopram and moclobemide on resting brain perfusion in social anxiety disorder

INTRODUCTION

The serotonin specific reuptake inhibitor (SSRI) citalopram and the reversible mono-amine oxidase-A inhibitor (RIMA) moclobemide have both been used successfully for the treatment of social anxiety disorder (SAD). In this study we investigate the effects of these compounds on resting brain function using single photon emission computed tomography (SPECT).

METHODS

Subjects meeting DSM-IV criteria for SAD underwent regional cerebral blood flow (rCBF) SPECT using Tc-HMPAO at baseline and after 8 weeks of treatment with either citalopram or moclobemide. Using statistical parametric mapping brain SPECT studies were analysed to determine the effects of treatment on rCBF, to compare the effects of citalopram and moclobemide, and to detect correlations between changes in rCBF and clinical response.

RESULTS

Subjects received citalopram (n=17) or moclobemide (n=14) as therapy. Subjects in both treatment groups demonstrated a significant improvement of SAD symptoms as measured by the Liebowitz Social Anxiety Scale total score. All subjects demonstrated a decrease in rCBF in the insulae post therapy. Subjects receiving citalopram had decreased superior cingulate rCBF after therapy compared to those receiving moclobemide.

CONCLUSION

Both SSRI's and RIMA's decreased rCBF in the insulae during treatment of SAD; an effect that may be consistent with the role of these regions in processing internal somatic cues evoked by emotional stimuli. Citalopram had a greater effect on superior cingulate perfusion, an effect that is consistent with evidence of high levels of 5-HT transporters in this region.

Tiermodelle für Angst und Angststörungen

Zusammenfassung. Zahlreiche Tiermodelle haben entscheidend zur Aufklärung der neurobiopsychologischen Grundlagen von Angst und damit auch zum Verständnis von Angststörungen beigetragen. An zwei unterschiedlichen Tiermodellen, dem Elevated Plus-Maze und der potenzierten Schreckreaktion, sollen die verschiedenen Ansätze exemplarisch veranschaulicht werden. Ansätze zur Untersuchung individueller Differenzen habitueller Angstmerkmale ergeben sich bei der Betrachtung natürlich vorkommender Variation innerhalb einer Stammesgeneration, bei selektiven Züchtungen und bei genetisch manipulierten Modellen. Schließlich bestehen auch Möglichkeiten zur Untersuchung kognitiver Prozesse in Tieren. Auch zukünftig werden tierexperimentelle Ansätze wesentlich zu unserem Wissen um die behavioralen und neurobiologischen Grundlagen der Angst im Humanbereich beitragen.

Tryptophan depletion does not modify response to CCK-4 challenge in patients with panic disorder after treatment with citalopram

RATIONALE

Data by [Bell et al. J Psychopharmacol (2002) 16:5-14] suggest that a decrease in 5-HT neurotransmission predisposes to panic attacks and that the antipanic effect of SSRIs depends upon the availability of 5-HT in the brain.

OBJECTIVES

Our aim was to assess the effect of acute tryptophan depletion (TD) on cholecystokinin-tetrapeptide (CCK-4)- induced symptoms in patients with panic disorder (PD) who had responded to a 10-week treatment with a selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram.

MATERIALS AND METHODS

A total of 18 patients (6 males and 12 females, mean age 34.5 years) received a tryptophan-free amino acid drink and a control drink, each followed by a CCK-4 challenge (25 microg), 1 week apart in a double-blind crossover design.

RESULTS

The results showed no significant differences in response to the CCK-4 challenge between the TD and the control conditions. Panic rate after the CCK-4 challenge was 27.8% after depletion and 33.3% after control drink (chi2=0.13, p=0.72). No significant effects of TD were observed in panic intensity scores, subjective anxiety, or cardiovascular indices.

CONCLUSIONS

This study demonstrates that an acute lowering of brain 5-HT availability with TD does not affect response to a CCK-4 challenge in PD patients successfully treated with citalopram. Thus, the reduction of CCK-4 sensitivity following SSRI-treatment in patients with PD may be related to mechanisms other than 5-HT availability in the brain, possibly to a reduction in brain cholecystokinin receptor sensitivity.

Serotonergic modulation of neuronal responses to behavioural inhibition and reinforcing stimuli: an fMRI study in healthy volunteers

Serotonin (5-HT) has been implicated in the aetiology of a number of psychiatric conditions, including depression, anxiety and antisocial personality disorder. The development of these disorders may arise from alterations in underlying motivational and cognitive processes such as emotional recognition, reinforcement processing and central inhibitory control. This study aimed to localize where in the brain 5-HT modulates neuropsychological processes relevant to putative 5-HT disorders, using functional magnetic resonance imaging. We examined the effect of the antidepressant mirtazapine on brain activations associated with behavioural inhibition and reinforcement processing in healthy subjects. Forty-five men were randomly allocated to receiving mirtazapine or placebo in a double-blind fashion. A Go/No-Go, Reward/No-Reward and Loss/No-loss task were performed during functional magnetic resonance imaging using a 1.5 Tesla Philips Gyroscan scanner. Blood oxygenation level dependent (BOLD) responses were analysed using SPM2. Task activations were largely consistent with previous findings. Mirtazapine modulated brain activations in the Go/No-Go and Reward/No-Reward task. During behavioural inhibition, enhanced activations were observed in the right orbitofrontal cortex (BA47). Increased activations in bilateral parietal cortex were found during the Reward task while no significant interaction was observed in the Loss task. Our results support the suggestion of an important role of serotonin in modulating basic processes involved in psychiatric disorders. Combining drug challenge with fMRI (pharmacoMRI; pMRI) is a promising tool for investigating these processes in healthy as well as patient groups.

Post-stress facilitation of serotonergic, but not noradrenergic, neurotransmission in the dorsal hippocampus prevents learned helplessness development in rats

Recent pieces of evidence suggest that the dorsal hippocampus may mediate adaptation to severe and inescapable stress, possibly by the facilitation of serotonergic and/or noradrenergic neurotransmission. Chronic social stress and high corticosteroid levels would impair this coping mechanism, predisposing animals to learned helplessness. To test the hypothesis that increasing serotonin or noradrenaline levels in the dorsal hippocampus would attenuate the development of learned helplessness (LH), rats received inescapable foot shock (IS) and were tested in a shuttle box 24 h latter. Prestressed animals showed impairment of escape responses. This effect was prevented by bilateral intrahippocampal injections of zimelidine (100 nmol/0.5 microl), a serotonin reuptake blocker, immediately after IS. This effect was not observed when zimelidine was administered before or 2 h after IS. Bilateral intrahippocampal injections of desipramine (3 or 30 nmol/0.5 microl), a noradrenaline reuptake blocker, before IS or immediately after it did not prevent LH development. Desipramine (30 nmol) enhanced LH development when injected before IS. These data suggest that poststress facilitation of hippocampal serotonergic, but not noradrenergic, neurotransmission in the dorsal hippocampus facilitates adaptation to severe inescapable stress. Antidepressant effects of noradrenaline-selective drugs seem to depend on other structures than the dorsal hippocampus.

5,7-DHT lesion of the dorsal raphe nuclei impairs object recognition but not affective behavior and corticosterone response to stressor in the rat

Previous studies with acute tryptophan depletion, leading to transient central 5-HT reductions, showed no effects on affective behavior but impaired object memory. In the present study, the behavioral effects of a 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsal raphe were evaluated in animal models of anxiety (open field test), depression (forced swimming test), behavioral inhibition (discrete fixed interval test) and cognition (object recognition task). The corticosterone response to a stress condition was examined at several intervals after 5,7-DHT treatment. The substantial reduction in neuronal 5-HT markers in the dorsal raphe did not affect anxiety-related, depressive-like or impulsive behavior. Compared to the SHAM group, the lesioned rats showed a lower response latency to obtain a reward, indicating a quick and accurate reaction to a stimulus. No differences were found in the progressive ratio test for food motivation. A marked impairment in object recognition was found. The 5,7-DHT treatment did not affect the corticosterone response to a stressful situation. Overall, these results corroborate studies with acute tryptophan depletion suggesting a role of 5-HT in object memory, but not affective behavior.

Anxiolytic property of estrogen related to the changes of the monoamine levels in various brain regions of ovariectomized rats

Anxiety is a symptom reflecting the dysregulation of monoaminergic neurotransmitters which may be modulated by estrogen. In our current study, we investigated the effects of chronic estrogen administration (10 microg/kg, s.c. for 4 weeks) on anxiety-like behavior using the elevated plus-maze with the corresponding changes of monoamines in the brain regions contributing to anxiety. The behavioral test revealed that estrogen-treated rats (Ovx+E(2)) spent more time in the open arm of the maze as well as a higher time/entry ratio in open arms than ovariectomized (Ovx) rats, indicating an anxiolytic property of estrogen. The increase in open arm time corresponded to an increase in uterine weight, indicated a correlation between the function of estrogen and its anxiolytic effect. Measurements of brain monoamines following estrogen treatment revealed decreases in norepinephrine, dopamine and serotonin in all of the brain regions studied, which also lead to an increase in turnover rates. The concentrations of norepinephrine in caudate putamen, of dopamine in nucleus accumbens, of serotonin in frontal cortex, hippocampus, caudate putamen, nucleus accumbens, and substantia nigra and of the serotonin metabolite, the 5-hydroxyindolacetic acid in substantia nigra of Ovx+E(2) rats were significantly lower than those of Ovx rats. Interestingly, the uterine weight was negatively correlated with the changes of dopamine and serotonin (with the exception of the hippocampus), suggesting a regulatory role of estrogen on these systems. From these data, we concluded that, in fact, there is a relationship between estrogen and monoamines (i.e. serotonin, dopamine) in modulating the anxiety-like behaviors in female rats.

Flight reactions induced by injection of glutamate N-methyl-d-aspartate receptor agonist into the rat dorsolateral periaqueductal gray are not dependent on endogenous nitric oxide

Glutamate N-methyl-d-aspartate (NMDA) receptors and the enzyme neuronal nitric oxide synthase (nNOS) are significantly expressed in the midbrain dorsolateral periaqueductal gray (dlPAG). Local injections of either NMDA-receptor agonists or nitric oxide (NO) donors induce flight reactions in rats. Since the activation of NMDA receptors in the brain increases the synthesis of NO, the present work was conducted to test the hypothesis that the flight reaction induced by intra-dlPAG administration of NMDA would be mediated by endogenous NO. Male Wistar rats with cannulas aimed at the dlPAG received intracerebral injections of l-NAME (NOS inhibitor, 100-200 nmol), carboxy-PTIO (NO scavenger, 1-3 nmol) or ODQ (guanylate cyclase inhibitor, 1-3 nmol). Saline or NMDA (0.1 nmol) was injected 10 min later and the behavioral changes were recorded for 2 min in the injection box. Intra-dlPAG injection of NMDA produced flight reactions characterized by crossings and jumps. Contrary to the initial hypothesis, these effects were not prevented by pretreatment with l-NAME, carboxy-PTIO or ODQ. Although the NO pathway may mediate some effects induced by NMDA receptor activation in the brain, the present results suggest that the administration of NMDA into the dlPAG induces flight reactions by mechanisms that are independent of endogenous NO.

Effects of fluoxetine and buspirone on the panicolytic-like response induced by the activation of 5-HT1A and 5-HT2A receptors in the rat dorsal periaqueductal gray

RATIONALE

Administration of 5-hydroxytryptamine (5-HT)1A and 5-HT2A receptor agonists into the dorsal periaqueductal gray (DPAG) inhibits escape, a defensive behavior associated with panic attacks. Long-term treatment with the antipanic compound imipramine enhances the DPAG 5-HT1A- and 5-HT2A-receptor-mediated inhibition of escape, implicating these receptors in the mode of action of panicolytic drugs.

OBJECTIVES

In the present study, we investigated whether the inhibitory effect on escape elicited by the intra-DPAG injection of 5-HT1A and 5-HT2A receptor agonists is also enhanced after treatment with fluoxetine, another widely used antipanic drug. The effects of fluoxetine were compared to those of buspirone, an anxiolytic drug without major effect on panic disorder.

METHODS

Male Wistar rats, subchronically (3-6 days) or chronically (21-24 days) treated with fluoxetine (10 mg/kg i.p.) or chronically treated with buspirone (0.3 mg/kg i.p.), were intra-DPAG injected with 5-HT (20 nmol), the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 8 nmol) or the preferential 5-HT2A receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl) piperazine dihydrochloride (DOI; 16 nmol). The intensity of electrical current that applied to the DPAG-evoked escape behavior was measured before and after the microinjection of these agonists.

RESULTS

The electrical current necessary to produce escape was increased after the microinjection of the three 5-HT receptor agonists in all groups of animals tested. However, this panicolytic-like effect was significantly higher in animals receiving long-term treatment with fluoxetine.

CONCLUSIONS

The results suggest that facilitation of the 5-HT1A- and 5-HT2A-receptor-mediated inhibition of DPAG neuronal activity is implicated in the beneficial effect of antidepressants in panic disorder.

Serotonin function in panic disorder: important, but why?

The essential role of serotonin (5-hydroxytryptamine (5-HT)) system in the neurobiology and pharmacotherapy of panic disorder (PD) continues to be a topic of intensive interdisciplinary research. Interest in the involvement of 5-HT in PD has been fuelled by clinical studies demonstrating that medications increasing the synaptic availability of 5-HT, such as selective 5-HT re-uptake inhibitors, are effective in the treatment of PD. Rival theories of 5-HT deficiency vs excess have attempted to explain the impact of 5-HT function in PD. In the past decade, knowledge of the role of 5-HT in the neurobiology of PD has expanded dramatically due to much new research including experimental, treatment, brain-imaging, and genetic studies. The current review attempts to summarize the new data and their implications. The challenge and treatment studies generally confirm the specific inhibitory influence of 5-HT on panicogenesis. The brain-imaging studies in PD patients demonstrate functional and clinically relevant alterations in various elements of 5-HT system affecting the neurocircuitry of panic. The findings of genetic association studies suggest that certain 5-HT-related genes may contribute to the susceptibility to PD; however, these data are rather limited and inconsistent. It appears that, even if not the primary etiological factor in PD, the 5-HT function conveys important vulnerability, as well as adaptive factors. A better understanding of these processes may be critical in achieving progress in the treatment of patients suffering from PD.

Subjective and neurovegetative changes in healthy volunteers and panic patients performing simulated public speaking

Drug-free symptomatic panic patients, drug-treated nonsymptomatic patients and healthy controls were submitted to simulated public speaking. Subjective anxiety, cognitive impairment and discomfort measured by the visual analog mood scale as well as skin conductance level were higher in symptomatic patients than in controls at the beginning of the experimental session, nonsymptomatic patients lying in between. Subjective sedation, spontaneous fluctuations of skin conductance, heart rate and blood pressure were similar in the three groups. Preparation and performance of speech decreased sedation while increasing anxiety, cognitive impairment, level and fluctuations of skin conductance, heart rate and blood pressure. Anxiety, cognitive impairment and conductance level were less increased in symptomatic patients than in controls. Electrodermal activity, but not cardiovascular measures of sympathetic arousal correlated with anticipatory anxiety. Chronic treatment with serotonin uptake inhibitors attenuated the differences between panic patients and controls, supporting the participation of serotonin in panic disorder.

5-HT(1A) and 5-HT(2A) receptors in the rat dorsal periaqueductal gray mediate the antipanic-like effect induced by the stimulation of serotonergic neurons in the dorsal raphe nucleus

RATIONALE

It has been proposed that the serotonergic pathway that connects the dorsal raphe nucleus (DRN) to the dorsal periaqueductal gray (DPAG) is implicated in the regulation of escape, a behavior that has been related to panic.

OBJECTIVES

We further evaluated this hypothesis by investigating whether intra-DRN injection of the 5-HT(1A) receptor antagonist WAY-100635 changes the escape response of rats submitted to the elevated T-maze. This test also measures inhibitory avoidance, which has been associated with generalized anxiety disorder. We also investigated whether the 5-HT(1A) and 5-HT(2A) receptors in the DPAG mediate the behavioral consequences induced by the injection of WAY-100635 into the DRN.

RESULTS

Intra-DRN injection of WAY-100635 facilitated inhibitory avoidance, while impairing escape. The same effect was obtained after intra-DRN injection of the glutamate receptor agonist kainic acid. Preadministration of WAY-100635 into the DPAG counteracted the effect induced by intra-DRN injection of WAY-100635 and of kainic acid on escape, but not on inhibitory avoidance. Preadministration of the preferential 5-HT(2A) receptor antagonist ketanserin into the DPAG abolished the effects of intra-DRN injection of WAY-100635 on both elevated T-maze tasks.

CONCLUSION

The results are indicative that 5-HT(1A) autoreceptors in the DRN are under tonic inhibitory influence by endogenous 5-HT. The effects of 5-HT release in the DPAG after intra-DRN injection of WAY-100635 and kainic acid on inhibitory avoidance and escape involve different 5-HT receptor subtypes. Whereas 5-HT(2A) receptors in the DPAG seem to mediate the effect of 5-HT on both behaviors, 5-HT(1A) receptors are only involved in the regulation of escape.

Antidepressant treatment and rodent aggressive behaviour

This review examines two 'ethologically relevant' rodent models, the resident-intruder and social hierarchy paradigms, that are sensitive to chronic antidepressant treatment (including repeated electroconvulsive shock). These models of rodent social and agonistic behaviour demonstrate that acute and chronic treatment with antidepressant drugs (regardless of their acute pharmacological activity) induce diametrically opposite changes in rodent aggressive behaviour. The common ability of chronic antidepressant treatment to increase rodent aggression (which in turn results in increased hierarchical status in closed social groups) most likely reflects the increased assertiveness and associated externalization of emotions (indicative of increased extrapunitive aggression) expressed during recovery from depressive illness. Finally, findings that relate observed behavioural changes to underlying neurochemical changes are briefly reviewed in terms of adaptive mechanisms in the rodent central nervous system induced by antidepressants, and also with respect to suicide ideation and panicogenic responses observed in some patients at the onset of treatment with selective serotonin reuptake inhibitors for affective disorders.

Changes in the biogenic amine content of the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens of rats submitted to single and repeated sessions of the elevated plus-maze test

It has been demonstrated that exposure to a variety of stressful experiences enhances fearful reactions when behavior is tested in current animal models of anxiety. Until now, no study has examined the neurochemical changes during the test and retest sessions of rats submitted to the elevated plus maze (EPM). The present study uses a new approach (HPLC) by looking at the changes in dopamine and serotonin levels in the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens in animals upon single or double exposure to the EPM (one-trial tolerance). The study involved two experiments: i) saline or midazolam (0.5 mg/kg) before the first trial, and ii) saline or midazolam before the second trial. For the biochemical analysis a control group injected with saline and not tested in the EPM was included. Stressful stimuli in the EPM were able to elicit one-trial tolerance to midazolam on re-exposure (61.01%). Significant decreases in serotonin contents occurred in the prefrontal cortex (38.74%), amygdala (78.96%), dorsal hippocampus (70.33%), and nucleus accumbens (73.58%) of the animals tested in the EPM (P < 0.05 in all cases in relation to controls not exposed to the EPM). A significant decrease in dopamine content was also observed in the amygdala (54.74%, P < 0.05). These changes were maintained across trials. There was no change in the turnover rates of these monoamines. We suggest that exposure to the EPM causes reduced monoaminergic neurotransmission activity in limbic structures, which appears to underlie the "one-trial tolerance" phenomenon.

Chronic imipramine treatment sensitizes 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal gray matter: evidence from the elevated T-maze test of anxiety

The dorsal periaqueductal gray matter (DPAG) has been implicated in the mediation of escape, a defensive behavior associated with panic disorder (PD). Chronic treatment with the anti-panic agent imipramine enhances the inhibitory effect on escape evoked by DPAG electrical stimulation of intra-DPAG administration of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the preferential 5-HT 2 A receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI). In the present study we further explore the hypothesis that sensitization of 5-HT1A and 5-HT 2 A receptors in the DPAG is involved in the anti-panic effect of imipramine. To this end, Wistar rats, subchronically or chronically treated with imipramine, were intra-DPAG injected with 8-OH-DPAT (0.4 or 3.2 nmoles) or DOI (16 nmoles), and tested in the elevated T-maze. In addition to its possible relevance to panic disorder, this test also measures inhibitory avoidance, a behavior that has been associated with generalized anxiety disorder (GAD). The effects of these 5-HT agonists in the DPAG were also investigated in animals chronically injected with buspirone, a drug clinically effective in treating GAD, but not PD. The results showed that intra-DPAG administration of the highest dose of 8-OH-DPAT and of DOI inhibited escape, and this panicolytic-like effect was significantly higher in animals previously treated chronically, but not subchronically, with imipramine. 8-OH-DPAT (0.4 nmole), although not affecting escape in animals systemically treated with saline, had a panicolytic-like effect in those receiving long-term treatment with imipramine. Microinjection of 8-OH-DPAT (3.2 nmoles), but not of DOI, impaired inhibitory avoidance, and this anxiolytic effect did not differ between animals treated with saline or imipramine. Chronic buspirone did not change the effect of 8-OH-DPAT and DOI on inhibitory avoidance and escape. Therefore, chronic imipramine seems to sensitize both 5-HT1A and 5-HT 2 A receptors in the DPAG, strengthening the view that these receptors are involved in the mode of action of anti-panic drugs.

Behavioral effects of systemically administered MK-212 are prevented by ritanserin microinfusion into the basolateral amygdala of rats exposed to the elevated plus-maze

RATIONALE

Although 5-HT2 receptors seem to play an important role in anxiety, results from numerous studies are still highly variable. Moreover, little is known about the behavioral effects of centrally administered 5-HT2 compounds in animal models of anxiety.

OBJECTIVE

The current study was performed to: (1) further investigate the effects of 5-HT2 receptor activation in rats exposed to the elevated plus-maze (EPM) and the open-field arena, two widely used animal models for studying anxiety and locomotor activity; and (2) evaluate the involvement of the 5-HT2 receptors within the basolateral nucleus of the amygdala (BLA) in the modulation of such effects.

METHODS

In the first experiment, male Wistar rats were exposed for 5 min to the EPM 27 min following intraperitoneal (i.p.) (1.0 ml/kg) injections of the preferential 5-HT2C receptor agonist 6-chloro-2[1-piperazinyl]pyrazine (MK-212) at doses of 1.0, 2.0, or 4.0 mg/kg. Control animals were injected with saline. The percentage of open-arm entries and the percentage of time spent in these arms were employed as anxiety indexes, whereas the number of closed-arm entries was calculated as indicative of locomotor activity. In the second experiment, rats were exposed for 10 min in an open-field arena to further assess the interference of the same MK-212 doses upon locomotor activity. In Experiment 3, rats were microinjected (0.2 microl) either with the mixed 5-HT 2A/2C receptor antagonist ritanserin (0.5, 1.25, 2.5, and 5.0 microg) or its vehicle into the BLA 12 min following i.p. injections of saline or the intermediate dose of MK-212 (2.0 mg/kg). Fifteen minutes later, each animal was exposed to the EPM as before.

RESULTS

Whereas the highest dose of MK-212 (4.0 mg/kg) induced motor-suppressant effects in both EPM and open-field arena, the intermediate dose of the drug (2.0 mg/kg) reduced open-arm exploration without significantly affecting the number of closed-arm entries. This behavioral profile, consistent with selective anxiogenic effect in the EPM, was dose-dependently prevented by ritanserin microinfusion into the BLA. In saline-pretreated animals, however, ritanserin (all doses) was ineffective.

CONCLUSIONS

MK-212 increases anxiety and decreases locomotor activity. The anxiogenic-like profile of 5-HT2 receptor activation is prevented by the blockade of 5-HT2 receptors within the BLA, which does not have an effect by itself upon basal anxiety levels triggered by the EPM.

Role of serotonin receptors in panic-like behavior induced by nitric oxide in the rat dorsolateral periaqueductal gray: effects of chronic clomipramine treatment

Local administration of serotonin (5-HT) receptor agonists inhibits panic-like reactions induced by electrical stimulation of the rat dorsolateral periaqueductal grey (dlPAG). This anti-aversive effect is enhanced by chronic treatment with anti-panic drugs such as clomipramine. Since nitric oxide (NO) may mediate panic-like behavior in the dlPAG, we tested the hypothesis that chronic clomipramine treatment would also potentiate the effects of locally injected 5-HT-receptor agonists on panic-like reactions induced by intra-dlPAG injection of an NO-donor (SIN). After 21 days of daily i.p. injections of saline or clomipramine (10 mg/kg) male Wistar rats received local injections of saline, the 5-HT(1A)-receptor agonist 8-OH-DPAT (8 nmol) or the 5-HT2A/2C-receptor agonist DOI (16 nmol) followed by saline or SIN (150 nmol). NO-induced panic-like reactions were inhibited by DOI, but not by 8-OH-DPAT. Chronic clomipramine did not modify these effects but tended to produce anti-aversive effect by itself. In chronically clomipramine treated animals 8-OH-DPAT potentiated NO-induced panic-like reactions. The results indicate that the panic-like effects of NO in the dlPAG may be attenuated by 5-HT2A/2C-, but not by 5-HT1A-receptors. The anti-aversive effect of DOI is not modified by chronic clomipramine treatment.

Involvement of dorsolateral periaqueductal gray cholecystokinin-2 receptors in the regulation of a panic-related behavior in rats

Cholecystokinin (CCK) has been implicated in anxiety disorders. The midbrain periaqueductal gray (PAG), which modulates anxiety and panic reactions, contains CCK-immunoreactive fibers and CCK(2) receptors. The present study investigated the involvement of CCK(2) receptors of the PAG dorsolateral subdivision (dlPAG) in the regulation of inhibitory avoidance and escape, two defensive behaviors that have been related in terms of psychopathology to generalized-anxiety and panic disorders, respectively. Male Wistar rats were microinjected in the dlPAG with the CCK(2) receptor agonist cholecystokinin-tetrapeptide (CCK-4; 0.08-0.32 nmol/0.2 microL), the CCK(2) receptor antagonist LY-225910 (0.05-0.20 nmol/0.2 microL) or LY-225910 prior to CCK-4. Inhibitory avoidance and escape behaviors were evaluated in the elevated T-maze. Whereas CCK-4 facilitated escape, indicating a panic-like action, LY-225910 had the opposite effect. Pretreatment with a non-effective dose of LY-225910 prevented the panic-eliciting action of CCK-4. Neither CCK-4 nor LY-225910 affected inhibitory avoidance acquisition. The present results substantiate the view that dlPAG CCK(2) receptors modulate panic-related behaviors.

Does the panic attack activate the hypothalamic-pituitary-adrenal axis?

A bibliographic search has been performed in MEDLINE using cortisol and panic as key-words, occurring in the title and/or in the abstract. Human studies were selected, with no time limit. The following publications were excluded: reviewarticles, case reports, panic attacks in disorders other than panic disorder, and studies on changes that occurred in-between panic attacks. The results showed that real-life panic attacks as well as those induced by selective panicogenic agents such as lactate and carbon dioxide do not activate the hypothalamicpituitary- adrenal (HPA) axis. Agonists of the colecystokinin receptor B, such as the colecystokinin-4 peptide and pentagastrin, increase stress hormones regardless of the occurrence of a panic attack and thus, seem to activate the HPA axis directly. The benzodiazepine antagonist flumazenil does not increase stress hormones, but this agent does not reliably induce panic attacks. Pharmacological agents that increased anxiety in both normal subjects and panic patients raised stress hormone levels; among them are the alpha2-adrenergic antagonist yohimbine, the serotonergic agents 1-(m-chlorophenyl) piperazine (mCPP) and fenfluramine, as well as the psychostimulant agent caffeine. Therefore, the panic attack does not seem to activate the HPAaxis, in contrast to anticipatory anxiety.

The effect of citalopram pretreatment on neuronal responses to neuropsychological tasks in normal volunteers: an FMRI study

Changes in serotonin neurotransmission have also been implicated in the etiology and treatment of impulse control disorders, depression, and anxiety. We have investigated the effect of enhancing serotonin function on fundamental brain processes that we have proposed are abnormal in these disorders. In all, 12 male volunteers received citalopram (7.5 mg intravenously) and placebo pretreatment in a single-blind crossover design before undertaking Go/No-go, Loss/No-loss, and covert (aversive) face emotion recognition tasks during functional magnetic resonance imaging (fMRI). Blood oxygenation level dependent responses were analyzed using Statistical Parametric Mapping (SPM2). The tasks activated prefrontal and subcortical regions generally consistent with literature with lateral orbitofrontal cortex (BA47) common to the three tasks. Citalopram pretreatment enhanced the right BA47 responses to the No-go condition, but attenuated this response to aversive faces. Attenuations were seen following citalopram in the medial orbitofrontal (BA11) responses to the No-go and No-loss (ie relative reward compared with Loss) conditions. The right amygdala response to aversive faces was attenuated by citalopram. These results support the involvement of serotonin in modulating basic processes involved in psychiatric disorders but argue for a process-specific, rather than general effect. The technique of combining drug challenge with fMRI (pharmacoMRI) has promise for investigating human psychiatric disorders.

Panicolytic-like effect induced by the stimulation of GABAA and GABAB receptors in the dorsal periaqueductal grey of rats

Activation of GABA(A) and benzodiazepine receptors within the dorsal periaqueductal grey inhibits the escape behaviour evoked by the electrical stimulation of this midbrain area, a defensive reaction that has been related to panic. Nevertheless, there is no evidence indicating whether the same antiaversive effect is also observed in escape responses evoked by species-specific threatening stimuli. In the present study, male Wistar rats were injected intra-dorsal periaqueductal grey with the benzodiazepine receptor agonist midazolam (10, 20 and 40 nmol), the GABA(A) receptor agonist muscimol (2, 4 and 8 nmol), the GABA(B) receptor agonist baclofen (2, 4 and 8 nmol), or with the benzodiazepine inverse agonist FG 7142 (20, 40 and 80 pmol) and tested in an ethologically-based animal model of anxiety, the elevated T-maze. Besides escape, this test also allows the measurement of inhibitory avoidance which has been related to generalised anxiety disorder. Midazolam, muscimol and baclofen impaired escape, a panicolytic-like effect, without altering inhibitory avoidance. FG 7142, on the other hand, facilitated both avoidance and escape reactions, suggesting an anxiogenic and panicogenic-like effect, respectively. The data suggest that GABA(A)/benzodiazepine and GABA(B) receptors within the dorsal periaqueductal grey are involved in the control of escape behaviour and that a failure in this regulatory mechanism may be of importance in panic disorder.

Anxiolytic effect of estradiol in the median raphe nucleus mediated by 5-HT1A receptors

Estrogen deficiency has been associated with stress, anxiety and depression. Estrogen receptors have been identified in the median raphe nucleus (MRN). This structure is the main source of serotonergic projections to the hippocampus, a forebrain area implicated in the regulation of defensive responses and in the resistance to chronic stress. There is evidence showing that estrogen modulates 5-HT1A receptor functions. In the MRN, somatodendritic 5-HT1A receptors control the activity of serotonergic neurones by negative feedback. The present study evaluated the effect of intra-MRN injection of estradiol benzoate (EB) (600 or 1200ng/0.2microl) on the performance of ovariectomised rats submitted to the elevated plus-maze test of anxiety and to the open-field test. Additionally, the same effect was evaluated with a previous intra-MRN injection of WAY 100635(100ng/0.2microl), an antagonist of 5-HT1A receptors. The results showed that both doses of EB increased the percentage of entries and the percentage of time spent into the open arms, suggestive of an anxiolytic effect. The highest dose of the drug also increased the number of entries into the enclosed arm and locomotion in the open field, indicating a stimulatory motor effect. WAY 100635 antagonised the effect of estradiol in the elevated plus-maze and in the open-field. The results show that estrogen receptors of the MRN are implicated in the regulation of anxiety-related behaviour. The results also support claims that the effect of estrogen involves a change in 5-HT1A receptor function.

Acute hypercarbic gas exposure reveals functionally distinct subpopulations of serotonergic neurons in rats

Although increasing evidence suggests that anatomically defined subpopulations of serotonergic neurons have unique stress-related functional properties, the topographical distribution of the serotonergic neurons involved in responses to stress-related stimuli have not been well-defined. Inspiration of air containing elevated concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) at high concentrations activates both hypothalamic-pituitary-adrenal axis and sympathetic responses in rats and humans. In order to determine the effects of acute hypercarbic gas exposure on subpopulations of topographically organized serotonergic neurons, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO2 concentrations (from baseline concentrations up to 20% CO2) for 5min. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure, at the single cell level, of functional cellular responses within subpopulations of serotonergic, noradrenergic and adrenergic neurons. Rats exposed to hypercarbic gas had increased numbers of c-Fos/tryptophan hydroxylase immunoreactive (ir) and c-Fos/tyrosine hydroxylase-ir neurons in specific topographically organized subdivisions of brainstem nuclei, compared to control rats. Within serotonergic cell groups (B1-B9), the most striking effects occurred in a subpopulation of large, multipolar serotonergic neurons within the ventrolateral periaqueductal grey and ventrolateral part of the dorsal raphe nucleus, a region implicated in serotonin-dependent suppression of stress-induced sympathetic outflow and serotonin-dependent inhibition of 'fight or flight' behaviour. These findings have important implications for understanding the role of serotonergic systems in the modulation of stress-related physiology and behaviour and stress-related neuropsychiatric disorders.

Serotonergic neurons in the median raphe nucleus regulate inhibitory avoidance but not escape behavior in the rat elevated T-maze test of anxiety

RATIONALE

A wealth of evidence supports the involvement of the serotonergic neurons of the median raphe nucleus (MRN) in anxiety. However, it is presently unclear whether serotonergic pathways arising from this nucleus play distinguishing regulatory roles in defensive behaviors that have been associated with specific subtypes of anxiety disorders.

OBJECTIVES

To evaluate the role of the MRN serotonergic neurons in the regulation of two defensive behaviors, inhibitory avoidance and escape, which have been related, respectively, to generalized anxiety and panic disorders.

METHODS

Male Wistar rats were submitted to the elevated T-maze test of anxiety after intra-MRN administration of drugs that either non-selectively or selectively change the activity of the serotonergic neurons.

RESULTS

Intra-MRN injection of FG 7142 (0.04 and 0.08 nmol) and kainic acid (0.03 and 0.06 nmol), drugs that non-selectively stimulate the MRN serotonergic neurons, facilitated inhibitory avoidance acquisition, but impaired escape performance. Microinjection of muscimol (0.11 and 0.22 nmol), a compound that non-selectively inhibits the activity of the MRN serotonergic neurons, impaired inhibitory avoidance and facilitated escape performance. Both kainic acid and muscimol also changed rat locomotion in the open-field test. Intra-MRN injection of 8-OH-DPAT (0.6-15 nmol) and WAY-100635 (0.18-0.74 nmol), respectively an agonist and an antagonist of somatodendritic 5-HT(1A) receptors located on serotonergic neurons of the MRN, only affected inhibitory avoidance-while the former inhibited the acquisition of this behavior, the latter facilitated it.

CONCLUSION

MRN serotonergic neurons seem to be selectively involved in the regulation of inhibitory avoidance in the elevated T-maze. This result supports the proposal that 5-HT pathways departing from this nucleus play an important role in anxiety processing, with implications for pathologies such as generalized anxiety disorder.

A comparative study with two types of elevated plus-maze (transparent vs. opaque walls) on the anxiolytic effects of midazolam, one-trial tolerance and fear-induced analgesia

The phenomenon known as one-trial tolerance (OTT) to the anxiolytic effects of benzodiazepines observed in rats submitted to the elevated plus-maze test (EPM) is considered to be due to the emergence of phobic states across the test/retest sessions. Antinociception is a usual component of the defense reaction. Until now, no study has examined antinociception and OTT together in freely behaving rats in the EPM. This work is a new approach looking at the sensorimotor gatings underlying OTT through the examination of the changes in reactivity to noxious stimuli during OTT development. We used the tail-flick test to assess the reactivity of rats to noxious stimulus during the effects of midazolam in test/retest sessions using two types of EPM, one with opaque (standard EPM) and another one with transparent walls (modified EPM). The authors had previously shown that this modified test caused an overall stressful situation more related to anxiety while the standard test coursed with a mixture of anxiety and high fear levels. In both plus mazes, the study was conducted in two experiments: (i) midazolam before the first trial, and (ii) midazolam before the second trial. In each experimental condition the effects of midazolam were tested under two doses (0.5 and 1.0 mg/kg) against a control group that received injections of saline. The anxiolytic effects of midazolam were more pronounced in animals tested in the modified EPM than in the standard EPM. Stressful stimuli present in both types of maze were able to elicit one-trial tolerance to midazolam on re-exposure. However, anxiolytic-insensitive behaviors in the first and the reduction in exploratory activity in the second trial are more pronounced in the standard EPM indicating that this test is more prone to transfer fear-related states across trials than the modified maze test. Antinociception is not present upon the re-exposure of rats to the EPM. These findings show that animals tested in the modified EPM showed higher sensitivity to the anxiolytic effects of midazolam than the standard EPM. Antinociception was not a concomitant of the shift in the emotional state present in the retest sessions of the EPM. These results are in agreement with the premises that repeated stressful experience leads to anxiolytic-insensitive fear state different from anxiety.

Novel hypothesis for the cause of panic disorder via the neuroepithelial bodies in the lung

Panic disorder (PD) is a complex condition that is further complicated by its numerous inducers, which include hypercapnia, hypoxia, sodium lactate, caffeine and cholecystokinin. It seems unlikely that there are specific suffocation receptors for each of these inducers in the brain. The pulmonary neuroepithelial bodies (NEBs), which are situated at the bifurcation point of the small bronchi, act as storage cells for 5-hydroxytryptamine (5-HT) and sensors for suffocation. If we suppose that PD might represent an inflammation of the NEBs, bradykinin (BK) which augments the airway hyper-response to diverse indcers might cause these cells to release 5-HT along with peptides and panneuroendcrine markers from their dence-core secretory granules. It was revealed that BK with 5-HT could cross the blood-brain barrier (BBB). When 5-HT released from these cells along with BK cross the BBB, the release of 5-HT at the axonal terminals in the serotonergic neurons in the brain will be inhibited, since the 5-HT1 autoreceptor have a higher affinity for 5-HT than do the 5-HT2 receptors. The inhibition of 5-HT at the axonal terminal causes to suppress the periaqueductal gray matter, which inhibits flight reactions to impending danger, pain or asphyxia. In short, this serotonergic situation might bring about PD. According to this theory, the type of inducer that the PD patient is exposed to is unimportant as long as it stimulates the NEBs, and through the effect of 5-HT and BK, PD would be revaluated as a somatic disease that directly and reversibly affects the brain.