Anxiogenic effect of median raphe nucleus lesion in stressed rats

5-HT1A receptors within the intermediate lateral septum modulate stress vulnerability in male mice

Chronic stress is a major risk factor for psychiatric disorders. However, certain individuals may be at higher risk due to greater stress susceptibility. Elucidating the neurobiology of stress resilience and susceptibility may facilitate the development of novel strategies to prevent and treat stress-related disorders such as depression. Mounting evidence suggests that the serotonin (5-HT) system is a major regulator of stress sensitivity. In this study, we assessed the functions of 5-HT1A and 5-HT2A receptors within the lateral septum (LS) in regulating stress vulnerability. Among a group of male mice exposed to chronic social defeat stress (CSDS), 47.2% were classified as stress-susceptible, and these mice employed more passive coping strategies during the defeat and exhibited more severe anxiety- and depression-like behaviors during the following behavioral tests. These stress-susceptible mice also exhibited elevated neuronal activity in the LS as evidenced by greater c-Fos expression, greater activity of 5-HT neurons in both the dorsal and median raphe nucleus, and downregulated expression of the 5-HT1A receptor in the intermediate LS (LSi). Finally, we found the stress-induced social withdrawal symptoms could be rapidly relieved by LSi administration of 8-OH-DPAT, a 5-HT1A receptor agonist. These results indicate that 5-HT1A receptors within the LSi play an important role in stress vulnerability in mice. Therefore, modulation of stress vulnerable via 5-HT1A receptor activation in the LSi is a potential strategy to treat stress-related psychiatric disorders.

Dorsal and median raphe neuronal firing dynamics characterized by nonlinear measures

The dorsal (DRN) and median (MRN) raphe are important nuclei involved in similar functions, including mood and sleep, but playing distinct roles. These nuclei have a different composition of neuronal types and set of neuronal connections, which among other factors, determine their neuronal dynamics. Most works characterize the neuronal dynamics using classic measures, such as using the average spiking frequency (FR), the coefficient of variation (CV), and action potential duration (APD). In the current study, to refine the characterization of neuronal firing profiles, we examined the neurons within the raphe nuclei. Through the utilization of nonlinear measures, our objective was to discern the redundancy and complementarity of these measures, particularly in comparison with classic methods. To do this, we analyzed the neuronal basal firing profile in both nuclei of urethane-anesthetized rats using the Shannon entropy (Bins Entropy) of the inter-spike intervals, permutation entropy of ordinal patterns (OP Entropy), and Permutation Lempel-Ziv Complexity (PLZC). Firstly, we found that classic (i.e., FR, CV, and APD) and nonlinear measures fail to distinguish between the dynamics of DRN and MRN neurons, except for the OP Entropy. We also found strong relationships between measures, including the CV with FR, CV with Bins entropy, and FR with PLZC, which imply redundant information. However, APD and OP Entropy have either a weak or no relationship with the rest of the measures tested, suggesting that they provide complementary information to the characterization of the neuronal firing profiles. Secondly, we studied how these measures are affected by the oscillatory properties of the firing patterns, including rhythmicity, bursting patterns, and clock-like behavior. We found that all measures are sensitive to rhythmicity, except for the OP Entropy. Overall, our work highlights OP Entropy as a powerful and useful quantity for the characterization of neuronal discharge patterns.

The role of serotonin in declarative memory: A systematic review of animal and human research

The serotonergic system is involved in diverse cognitive functions including memory. Of particular importance to daily life are declarative memories that contain information about personal experiences, general facts, and events. Several psychiatric or neurological diseases, such as depression, attention-deficit-hyperactivity disorder (ADHD), and dementia, show alterations in serotonergic signalling and attendant memory disorders. Nevertheless, understanding serotonergic neurotransmission and its influence on memory remained a challenge until today. In this systematic review, we summarize recent psychopharmacological studies in animals and humans from a psychological memory perspective, in consideration of task-specific requirements. This approach has the advantage that comparisons between serotonin (5-HT)-related neurochemical mechanisms and manipulations are each addressing specific mnemonic circuits. We conclude that applications of the same 5-HT-related treatments can differentially affect unrelated tasks of declarative memories. Moreover, the analysis of specific mnemonic phases (e.g., encoding vs. consolidation) reveals opposing impacts of increased or decreased 5-HT tones, with low 5-HT supporting spatial encoding but impairing the consolidation of objects and verbal memories. Promising targets for protein synthesis-dependent consolidation enhancements include 5-HT₄ receptor agonists and 5-HT₆ receptor antagonists, with the latter being of special interest for the treatment of age-related decline. Further implications are pointed out as base for the development of novel therapeutic targets for memory impairment of neuropsychiatric disorders.

Pharmacogenetic excitation of the median raphe region affects social and depressive-like behavior and core body temperature in male mice

AIMS

Median raphe region (MRR) is an important bottom-up regulatory center for various behaviors as well as vegetative functions, but detailed descriptions and links between the two are still largely unexplored.

METHODS

Pharmacogenetics was used to study the role of MRR in social (sociability, social interaction, resident intruder test) and emotional behavior (forced swim test) parallel with some vegetative changes (biotelemetry: core body temperature). Additionally, to validate pharmacogenetics, the effect of clozapine-N-oxide (CNO), the ligand of the artificial receptor, was studied by measuring (i) serum and brainstem concentrations of CNO and clozapine; (ii) MRR stimulation induced neurotransmitter release in hippocampus; (iii) CNO induced changes in body temperature and locomotor activity.

KEY FINDINGS

MRR stimulation decreased locomotion, increased friendly social behavior in the resident intruder test and enhanced depressive-like behavior. The latter was accompanied by diminished decrease in core body temperature. Thirty minutes after CNO injection clozapine was predominant in the brainstem. Nonetheless, peripheral CNO injection was able to induce glutamate release in the hippocampus. CNO had no immediate (<30 min) or chronic (repeated injections) effect on the body temperature or locomotion.

SIGNIFICANCE

We confirmed the role of MRR in locomotion, social and depressive-like behavior. Most interestingly, only depressive-like behavior was accompanied by changed body temperature regulation, which was also observed in human depressive disorders previously. This indicates clinical relevance of our findings. Despite low penetration, CNO acts centrally, but does not influence the examined basic parameters, being suitable for repeated behavioral testing.

Anti-anxiety properties of selected medicinal plants

Exploration of new drugs targeting anxiety treatment is a major concern worldwide. Medicinal plants are being used as a potential source of novel drugs for anxiety disorders. The objective of this review is to provide information about the healing outcomes of anxiety treatment with natural products. Valeriana officinalis, Citrus aurantium, Commelina benghalensis, Achyranthes aspera, Mimosa pudica, Achillea millefolium, Nymphaea alba, Leonurus cardiac, Camellia sinensis, Turnera aphrodisiaca, Crataegus oxyacantha and Piper methysticum showed promising effects on anxiety in animal models. In clinical studies, passion flower, kava, valerian, St John's wort, and ashwagandha showed the most positive results. More studies are needed for the exploration of the anti-anxiety of medicinal plants. In drugs derived from natural sources have explored many components that are playing an essential role in curing anxiety disorders and associated complications.

Attenuation of stress-induced behavioral changes by activation of serotonin type 7 receptors in the median raphe nucleus of rats

BACKGROUND

Exposure to stressful aversive situations induces physiological and behavioral changes. Serotonin has been suggested to mediate such changes, as well as adaptation to stressful events. Serotoninergic projections arising from the median raphe nucleus to the dorsal hippocampus have been suggested to promote adaptation to chronic aversive stimuli. Such pathway may involve serotonin type 1a receptor-mediated neurotransmission. However, the serotonin 7 receptor can also be found in the median raphe nucleus and may be involved in mechanisms underlying response to stress.

AIMS

In this work we sought to investigate if activation of serotonin type 7 receptors would attenuate stress-induced deficits in different animal models of depression.

METHODS

Male Wistar rats with a guide-cannula aimed to the median raphe nucleus were submitted to restraint or forced swim stress and were tested in an elevated plus maze or forced swim test, respectively, 24 h later. SB 258741 (serotonin type 7 receptor antagonist) and/or LP 44 (serotonin type 7 receptor agonist) were administered intra-median raphe nucleus immediately before or after exposure to stress or before test. Control groups received intra-median raphe nucleus treatment 24 h or immediately before test in the elevated plus maze or forced swim test.

RESULTS

LP 44 attenuated restraint-induced exploratory deficits independently of the moment it was administered. Similar results were observed in the forced swim test, with the exception on post-stress condition. These effects on adaptation to stress induced by serotonin type 7 receptor activation were prevented by previous treatment with SB 258741.

CONCLUSIONS

Our data support the idea that activation of median raphe nucleus serotonin 7 receptor is important to the development of adaptation to stress.

Median raphe serotonin neurons promote anxiety-like behavior via inputs to the dorsal hippocampus

Anxiety disorders may be mediated in part by disruptions in serotonin (5-hydroxytryptamine, 5-HT) system function. Behavioral measures of approach-avoidance conflict suggest that serotonin neurons within the median raphe nucleus (MRN) promote an anxiogenic state, and some evidence indicates this may be mediated by serotonergic signaling within the dorsal hippocampus. Here, we test this hypothesis using an optogenetic approach to examine the contribution of MRN 5-HT neurons and 5-HT innervation of the dorsal hippocampus (dHC) to anxiety-like behaviours in female mice. Mice expressing the excitatory opsin ChR2 were generated by crossing the ePet-cre serotonergic cre-driver line with the conditional Ai32 ChR2 reporter line, resulting in selective expression of ChR2 in 5-HT neurons. Electrophysiological recordings confirmed that this approach enabled reliable optogenetic stimulation of MRN 5-HT neurons, and this stimulation produced downstream 5-HT release in the dHC as measured by in vivo microdialysis. Optogenetic stimulation of the MRN elicited behavioral responses indicative of an anxiogenic effect in three behavioural tests: novelty-suppressed feeding, marble burying and exploration on the elevated-plus maze. These effects were shown to be behaviourally-specific. Stimulation of 5-HT terminals in the dHC recapitulated the anxiety-like behaviour in the novelty-suppressed feeding and marble burying tests. These results show that activation of 5-HT efferents from the MRN rapidly induces expression of anxiety-like behaviour, in part via projections to the dHC. These findings reveal an important neural circuit implicated in the expression of anxiety in female mice.

Chronic stress dysregulates amygdalar output to the prefrontal cortex

Chronic stress contributes to the neuropathology of mental health disorders, including those associated with anxiety. The basolateral amygdala (BLA) coordinates emotional behavioral responses through glutamatergic outputs to downstream regions such as the prefrontal cortex (PFC), nucleus accumbens core (NAcc) and bed nucleus of the stria terminalis (BNST). We explored the effects of chronic stress on BLA outputs to the PFC, NAcc and BNST using slice electrophysiology combined with optogenetics in two inbred mouse strains with distinct stress-induced anxiety responses. We found that ten consecutive days of chronic restraint stress enhanced pre-synaptic glutamate release at BLA-to-PFC synapses in C57BL/6J mice, but reduced pre-synaptic glutamate release at these synapses in DBA/2J mice. To assess the behavioral relevance of enhanced glutamate output at BLA-to-PFC synapses, we approximated the effects of chronic stress on the BLA-PFC circuit using optogenetics. We found that photostimulation of the BLA-PFC circuit in unstressed C57BL/6J mice produced persistent (i.e., post-stimulation) increased anxiety-like behavior and hyperactivity in the elevated plus-maze - a profile consistent with prototypical behavioral responses of stressed C57BL/6J mice. These data demonstrate that chronic stress dysregulates the BLA-PFC circuit by altering pre-synaptic glutamate release from BLA outputs, and provide a mechanism by which chronic stress can lead to increased anxiety.

Modulation of hippocampal neuronal activity by So-ochim-tang-gamibang in mice subjected to chronic restraint stress

Background

So-ochim-tang-gamibang (SOCG) is a decoction formula which has been used to improve mental activity in traditional Korean medicine. The present study was performed to evaluate whether the treatment of SOCG was involved in activating hippocampal neurons in mice which were subjected to chronic restraint stress (CRS).

Methods

Mice were subjected to CRS for 2 weeks to induce depressive-like behaviors. SOCG was orally administered for the same period. mRNA expression in the hippocampus was analyzed by RT-PCR. Levels of serotonin receptor 5-HT1AR in the hippocampus were determined by western blotting and by immunofluorescence staining in coronal brain sections. Cultured neurons were prepared from the dorsal root ganglia (DRG) in mice to examine the effects of CRS and SOCG treatment on neurite outgrowth. Depressive-like behaviors of experimental animals were measured by open field test (OFT) and forced swimming test (FST).

Results

mRNA levels of serotonin 1A and 1B receptors (5-HT1AR and 5-HT1BR) were decreased in the hippocampus of CRS animals and increased by SOCG treatment. Signals of 5-HT1AR protein in CA3 pyramidal cells were decreased by CRS but elevated back to levels in control animals after SOCG treatment. Phospho-Erk1/2 protein in CA3 cells showed similar pattern of changes as in 5-HT1AR, suggesting coordinated regulation after SOCG treatment in CRS animals. Axonal growth-associated protein GAP-43 levels were also decreased by CRS and then increased by SOCG treatment. In vivo administration of SOCG improved neurite outgrowth of primary DRG neurons from CRS animals and also increased 5-HT1AR protein signals. Behavioral tests of open field and forced swimming showed that immobility time periods were significantly decreased by SOCG treatment.

Conclusions

Our data suggest that SOCG treatment may increase synaptic responsiveness to serotonergic neuronal inputs by upregulating 5-HT1AR in the hippocampal neurons.

Both serotonergic and noradrenergic systems modulate the development of tolerance to chronic stress in rats with lesions of the serotonergic neurons of the median raphe nucleus

Acute exposure to stress induces significant behavioural changes, while repeated exposure to the same stressor leads to the development of tolerance to stress. The development of tolerance appears to involve the serotonergic projections from the Median Raphe Nucleus (MnRN) to the dorsal Hippocampus (dH), since rats with lesions of this pathway does not develop tolerance to stress. Previous data from our laboratory showed that treatment with imipramine, a serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor, lead to the development of tolerance. However, it remains to be elucidated whether such tolerance involves the participation of the noradrenergic system, apart from the serotonergic projections. Therefore, the aim of this work was to investigate the behavioural and neurochemical effects of chronic treatment with desipramine (NA reuptake inhibitor) or fluoxetine (5-HT reuptake inhibitor) in chronically stressed rats with lesions of the serotonergic neurons of the MnRN. Male Wistar rats with or without lesion in the MnRN were submitted or not to acute (2 h) or chronic restraint (2 h/seven days) stress and tested in the elevated pus maze (EPM). Treatment with fluoxetine, desipramine (10 mg/kg) or saline was performed twice daily (12-12 h interval), for 7 consecutive days. EPM test was conducted 24 h after the treatment. Fluoxetine attenuated the anxiogenic-induced effect of lesion in chronically restrained rats, without changing serotonin and noradrenaline levels in the hippocampus of lesioned rats. A similar profile was also observed after treatment with desipramine. These results suggest that both the serotonergic and the noradrenergic systems are involved in the development of tolerance to chronic stress. Additionally, the integrity of the serotonergic pathway of the MnRN-dH is not essential for the anxiolytic-like effects of these drugs.

Critical neuropsychobiological analysis of panic attack- and anticipatory anxiety-like behaviors in rodents confronted with snakes in polygonal arenas and complex labyrinths: a comparison to the elevated plus- and T-maze behavioral tests

Objective:

To compare prey and snake paradigms performed in complex environments to the elevated plus-maze (EPM) and T-maze (ETM) tests for the study of panic attack- and anticipatory anxiety-like behaviors in rodents.

Methods:

PubMed was reviewed in search of articles focusing on the plus maze test, EPM, and ETM, as well as on defensive behaviors displayed by threatened rodents. In addition, the authors' research with polygonal arenas and complex labyrinth (designed by the first author for confrontation between snakes and small rodents) was examined.

Results:

The EPM and ETM tests evoke anxiety/fear-related defensive responses that are pharmacologically validated, whereas the confrontation between rodents and snakes in polygonal arenas with or without shelters or in the complex labyrinth offers ethological conditions for studying more complex defensive behaviors and the effects of anxiolytic and panicolytic drugs. Prey vs. predator paradigms also allow discrimination between non-oriented and oriented escape behavior.

Conclusions:

Both EPM and ETM simple labyrinths are excellent apparatuses for the study of anxiety- and instinctive fear-related responses, respectively. The confrontation between rodents and snakes in polygonal arenas, however, offers a more ethological environment for addressing both unconditioned and conditioned fear-induced behaviors and the effects of anxiolytic and panicolytic drugs.

Severely impaired hippocampal neurogenesis associates with an early serotonergic deficit in a BAC α-synuclein transgenic rat model of Parkinson's disease

Parkinson's disease (PD) is a multisystem disorder, involving several monoaminergic neurotransmitter systems resulting in a broad range of motor and non-motor symptoms. Pathological hallmarks of PD are the loss of dopaminergic neurons and the accumulation of alpha-synuclein, however also being present in the serotonergic raphe nuclei early in the disease course. The dysfunction of the serotonergic system projecting to the hippocampus may contribute to early non-motor symptoms such as anxiety and depression. The adult hippocampal dentate gyrus (DG), a unique niche of the forebrain continuously generating new neurons, may particularly present enhanced susceptibility towards accumulating alpha-synuclein levels. The underlying molecular mechanisms in the context of neuronal maturation and survival of new-born neurons are yet not well understood. To characterize the effects of overexpression of human full-length alpha-synuclein on hippocampal cellular and synaptic plasticity, we used a recently generated BAC alpha-synuclein transgenic rat model showing important features of PD such as widespread and progressive alpha-synuclein aggregation pathology, dopamine loss and age-dependent motor decline. At the age of four months, thus prior to the occurrence of the motor phenotype, we observed a profoundly impaired dendritogenesis of neuroblasts in the hippocampal DG resulting in severely reduced survival of adult new-born neurons. Diminished neurogenesis concurred with a serotonergic deficit in the hippocampus as defined by reduced levels of serotonin (5-HT) 1B receptor, decreased 5-HT neurotransmitter levels, and a loss of serotonergic nerve terminals innervating the DG/CA3 subfield, while the number of serotonergic neurons in the raphe nuclei remained unchanged. Moreover, alpha-synuclein overexpression reduced proteins involved in vesicle release, in particular synapsin-1 and Rab3 interacting molecule (RIM3), in conjunction with an altered ultrastructural architecture of hippocampal synapses. Importantly, BAC alpha-synuclein rats showed an early anxiety-like phenotype consisting of reduced exploratory behavior and feeding. Taken together, these findings imply that accumulating alpha-synuclein severely affects hippocampal neurogenesis paralleled by impaired 5-HT neurotransmission prior to the onset of aggregation pathology and overt motor deficits in this transgenic rat model of PD.

Comparison of the effects of single and daily repeated immobilization stress on resting activity and heterotypic sensitization of the hypothalamic-pituitary-adrenal axis

Acute exposure to severe stressors causes marked activation of the hypothalamic-pituitary-adrenal (HPA) axis that is reflected on the day after higher resting levels of HPA hormones and sensitization of the HPA response to novel (heterotypic) stressors. However, whether a single exposure to a severe stressor or daily repeated exposure to the same (homotypic) stressor modifies these responses to the same extent has not been studied. In this experiment, we studied this issue in adult male Sprague-Dawley rats daily exposed for seven days to a severe stressor such as immobilization on boards (IMO). A first exposure to 1 h IMO resulted in a marked activation of the HPA axis as reflected in plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, and such activation was significantly reduced after the seventh IMO. On the day after the first IMO, higher resting levels of ACTH and corticosterone and sensitization of their responses to a short exposure to an open-field (OF) were observed, together with a marked hypoactivity in this environment. Repeated exposure to IMO partially reduced hypoactivity, the increase in resting levels of HPA hormones and the ACTH responsiveness to the OF on the day after the last exposure to IMO. In contrast, corticosterone response was gradually increased, suggesting partial dissociation from ACTH. These results indicate that daily repeated exposure to the same stressor partially reduced the HPA response to the homotypic stressor as well as the sensitization of HPA axis activity observed the day after chronic stress cessation.

Role of serotonin 1A receptors in the median raphe nucleus on the behavioral consequences of forced swim stress

Despite the intense research on the neurobiology of stress, the role of serotonin (5-HT)1A receptors still remains to be elucidated. In the hippocampus, post-synaptic 5-HT1A receptors activation induces anxiolytic effects in animals previously exposed to stressful situations. However, little is known about somatodendritic 5-HT1A receptors in the median raphe nucleus (MRN). Therefore, the aim of this study was to investigate the role of 5-HT1A receptors located in the MRN in rats exposed to forced swim stress. After recovering from surgery, rats were forced to swim for 15 min in a cylinder. Intra-MRN injections of saline, 8-OH-DPAT (3 nmol/0.2 µL) and/or WAY-100635 (0.3 nmol/0.2 µL) were performed immediately before or after pre-exposure or 24 h later (immediately before test). Non-stressed rats received the same treatment 24 h or 10 min before test. Our data showed that 8-OH-DPAT increased latency to display immobility while decreasing time spent immobile in almost all experimental conditions. These effects were not prevented by previous treatment with WAY-100635. No effects of different treatments were described in non-stressed animals. Taken together, our data suggest that in addition to activation of 5-HT1A, 5-HT7 receptors may also be involved in the behavioural consequences of exposure to swim stress.

The median raphe nucleus in anxiety revisited

Although the role of the median raphe nucleus (MRN) in the regulation of anxiety has received less attention than that of the dorsal raphe nucleus (DRN) there is substantial evidence supporting this function. Reported results with different animal models of anxiety in rats show that whereas inactivation of serotonergic neurons in the MRN causes anxiolysis, the stimulation of the same neurons is anxiogenic. In particular, studies using the elevated T-maze comparing serotonergic interventions in the MRN and in the DRN indicate that the former affect only the inhibitory avoidance task, which has been related to generalized anxiety. In contrast, similar operations in the DRN change both the inhibitory avoidance and the one-way escape task, the latter being representative of panic disorder. Simultaneous injections of 5-HT-acting drugs in the MRN and in the dorsal hippocampus (DH) suggest that the MRN-DH pathway mediates the regulatory function of the MRN in anxiety. Overall, the results discussed in this review point to a relevant role of the MRN in the regulation of anxiety, but not panic, through the 5-HT pathway that innervates the DH.

Targeting brain serotonin synthesis: insights into neurodevelopmental disorders with long-term outcomes related to negative emotionality, aggression and antisocial behaviour

Aggression, which comprises multi-faceted traits ranging from negative emotionality to antisocial behaviour, is influenced by an interaction of biological, psychological and social variables. Failure in social adjustment, aggressiveness and violence represent the most detrimental long-term outcome of neurodevelopmental disorders. With the exception of brain-specific tryptophan hydroxylase-2 (Tph2), which generates serotonin (5-HT) in raphe neurons, the contribution of gene variation to aggression-related behaviour in genetically modified mouse models has been previously appraised (Lesch 2005 Novartis Found Symp. 268, 111-140; Lesch & Merschdorf 2000 Behav. Sci. Law 18, 581-604). Genetic inactivation of Tph2 function in mice led to the identification of phenotypic changes, ranging from growth retardation and late-onset obesity, to enhanced conditioned fear response, increased aggression and depression-like behaviour. This spectrum of consequences, which are amplified by stress-related epigenetic interactions, are attributable to deficient brain 5-HT synthesis during development and adulthood. Human data relating altered TPH2 function to personality traits of negative emotionality and neurodevelopmental disorders characterized by deficits in cognitive control and emotion regulation are based on genetic association and are therefore not as robust as the experimental mouse results. Mouse models in conjunction with approaches focusing on TPH2 variants in humans provide unexpected views of 5-HT's role in brain development and in disorders related to negative emotionality, aggression and antisocial behaviour.

Serotonergic neural links from the dorsal raphe nucleus modulate defensive behaviours organised by the dorsomedial hypothalamus and the elaboration of fear-induced antinociception via locus coeruleus pathways

Decrease of γ-aminobutyric acid (GABA)-mediated neurotransmission in the dorsomedial hypothalamus (DMH) evokes instinctive fear-like responses. The aim of the present study was to investigate the involvement of the serotonin (5-HT)- and norepinephrine-mediated pathways of the endogenous pain inhibitory system, including the dorsal raphe nucleus (DRN) and the locus coeruleus (LC), in the defensive responses and antinociceptive processes triggered by the blockade of GABAergic receptors in the DMH. The intra-hypothalamic microinjection of the GABA(A) receptor antagonist bicuculline (40 ng/200 nL) elicited elaborate defensive behaviours interspersed with exploratory responses. This escape behaviour was followed by significantly increased pain thresholds, a phenomenon known as fear-induced antinociception. Furthermore, at 5 and 14 days after DRN serotonin-containing neurons were damaged using the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), the frequency and duration of alertness and escape behaviour evoked by the GABA(A) receptor blockade in the DMH decreased, as well as fear-induced antinociception. Pre-treatment with the non-selective 5-HT receptor antagonist methysergide, the 5-HT(2A/2C) receptor antagonist ketanserin and the 5-HT(2A) receptor selective antagonist R-96544 in the LC also decreased fear-induced antinociception, without significant changes in the expression of defensive behaviours. These data suggest that the serotonergic neurons of the DRN are directly involved in the organisation of defensive responses as well as in the elaboration of the innate fear-induced antinociception. However, serotonin-mediated inputs from the NDR to the LC modulate only fear-induced antinociception and not the defensive behaviours evoked by GABA(A) receptor blockade in the DMH.

Serotonergic and noradrenergic pathways are required for the anxiolytic-like and antidepressant-like behavioral effects of repeated vagal nerve stimulation in rats

BACKGROUND

Vagal nerve stimulation (VNS) is used for treatment-refractory depression, but there are few preclinical studies of its effects when administered repeatedly over time using clinically relevant stimulation parameters in nonanesthetized animals.

METHODS

The novelty-suppressed feeding test (NSFT) and forced swim test (FST) were used to evaluate the anxiolytic- and antidepressant-like potential of VNS in rats, respectively. The behavioral effects of VNS were compared with those of desipramine (DMI; 10 mg/kg/day) and sertraline (7.5 mg/kg/day) administered via osmotic minipump. Such experiments were carried out in intact rats as well as those that had selective destruction of either serotonin or noradrenergic neurons in brain caused by the neurotoxins, 5,7-dihyroxytryptamine (5,7-DHT), or 6-hydroxydopamine (6-OHDA).

RESULTS

Repeated administration of VNS, DMI, and sertraline decreased latency to feed in the NSFT. In the FST, repeated VNS, DMI, and sertraline caused decreased immobility; the VNS-induced decrease in immobility resulted from increases in both swimming and climbing behaviors. Effects of VNS and sertraline, but not DMI, in both the NSFT and the FST were abolished in rats treated with 5,7-DHT. Effects of DMI in both behavioral tests, but not those of sertraline, were abolished in 6-OHDA treated rats. VNS effects on immobility and climbing in the FST were not blocked in the 6-OHDA-treated rats. There was no significant difference in locomotor activity caused by any of the treatments or by the lesions.

CONCLUSIONS

Serotonergic nerves are required for repeated VNS-induced anxiolytic- and antidepressant-like effects. Noradrenergic nerves can also be activated by VNS to cause its anxiolytic-like effect.

Phenylephrine into the median raphe nucleus evokes an anxiolytic-like effect in free-feeding rats but does not alter food intake in free feeding rats

This study investigated the role of MnR α₁-adrenergic receptors in the control of anxiety-like and feeding behaviors and attempted to reveal a possible functional association between both behaviors. The α₁-adrenergic agonist phenylephrine (PHE) (at doses of 0.2, 2, 6, 20 nmol) or saline was injected into the MnR or into the pontine nucleus (Pn) of free-feeding rats. The animals were exposed to the elevated plus maze to analyse spatial-temporal and ethological variables. Subsequently, the ingestive and non-ingestive behaviors were recorded during 30 min and feeding and drinking behaviors were measured. Both in the elevated plus-maze and in the feeding chamber, all PHE doses injected into the MnR decreased the risk assessment frequency, an ethological parameter of anxiolytic-like effect. The spatial-temporal variables remained unchanged after PHE treatment. Feeding behavior was not affected by PHE into the MnR. The anxiety-like or ingestive behaviors were not affected by PHE treatment in the Pn, an area adjacent to the MnR. These data indicate that α₁-adrenergic receptors within MnR participate in the control of anxiety-like behaviors. The absence of effects on feeding behavior after MnR α₁-adrenergic activation could be due to an elevated α₁-adrenergic tonus and its possible strong facilitatory influence on 5-HT neurons within MnR. Furthermore, the present results suggest that anxiety-like and feeding behaviors controled by MnR adrenergic circuits operate by independent neural pathways.

Effects of single and simultaneous lesions of serotonergic and noradrenergic pathways on open-space and bright-space anxiety-like behavior in two animal models

The objective of the present study is to investigate the effects of single and simultaneous lesions of the noradrenergic and serotonergic pathways (NA-X, 5-HT-X and XX, respectively) by intracerebroventricular administration of selective neurotoxins N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-HCl (DSP-4) and 5,7-dihydroxytryptamine (5,7-DHT) on anxiety-like behavior in rats. To evaluate the effects of the various lesions, animals were tested in elevated plus-maze (EPM) and light-dark (LD) paradigms. In EPM, single lesions produced strong, statistically significant increase (p<0.001) of both time spent in the open arms (OT) and number of entries into the open arms (OE) compared to sham-lesioned animals. Simultaneous lesion further strengthened this anxiolytic effect causing an approximate 500% elevation of OT compared to sham-lesioned animals. In LD, 5-HT lesion caused a significant (p<0.05) increase in both light movement time and light horizontal activity parameters compared to intact, sham, and NA-lesioned groups. Neither of the lesions caused any change in the spontaneous locomotor activity of the animals up to 15min as measured in activity meter. These findings suggest that single and simultaneous lesions of 5-HT- and NA-pathways modify anxiety-related state of experimental animals to different extents and these modifications alter the behavior of animals differently in the two models used: NA-X and 5-HT-X reduce open space anxiety-like behavior and XX further strengthens this effect in the EPM, while only 5-HT-X is resulting in reduced bright-space anxiety-like behavior leaving the performance of NA-X and XX animals unchanged.

Social and non-social anxiety in adolescent and adult rats after repeated restraint

Adolescence is associated with potentially stressful challenges, and adolescents may differ from adults in their stress responsivity. To investigate possible age-related differences in stress responsiveness, the consequences of repeated restraint stress (90 min/day for 5 days) on anxiety, as indexed using the elevated plus-maze (EPM) and modified social interaction (SI) tests, were assessed in adolescent and adult Sprague-Dawley male and female rats. Control groups at each age included non-stressed and socially deprived animals, with plasma corticosterone (CORT) levels also measured in another group of rats on days 1 and 5 of stress (sampled 0, 30, 60, 90, and 120 min following restraint onset). While repeatedly restrained animals exhibited similar anxiety levels compared to non-stressed controls in the EPM, restraint stress increased anxiety at both ages in the SI test (as indexed by reduced social investigation and social preference). Daily weight gain measurements, however, revealed more marked stress-related suppression of body weight in adolescents versus adults. Analysis of stress-induced increases in CORT likewise showed that adolescents demonstrated less habituation than adults, embedded within typical sex differences in CORT magnitude (females greater than males) and age differences in CORT recovery (adolescents slower than adults). Despite no observable age-related differences in the behavioral response to restraint, adolescents were more sensitive to the repeated stressor in terms of physiological indices of attenuated weight gain and habituation of stress-induced CORT.

Corticotropin-releasing factor in the dorsal raphe nucleus increases medial prefrontal cortical serotonin via type 2 receptors and median raphe nucleus activity

Interactions between central corticotropin-releasing factor (CRF) and serotonergic systems are believed to be important for mediating fear and anxiety behaviors. Recently we demonstrated that infusions of CRF into the rat dorsal raphe nucleus result in a delayed increase in serotonin release within the medial prefrontal cortex that coincided with a reduction in fear behavior. The current studies were designed to study the CRF receptor mechanisms and pathways involved in this serotonergic response. Infusions of CRF (0.5 microg/0.5 microL) were made into the dorsal raphe nucleus of urethane-anesthetized rats following either inactivation of the median raphe nucleus by muscimol (25 ng/0.25 microL) or antagonism of CRF receptor type 1 or CRF receptor type 2 in the dorsal raphe nucleus with antalarmin (25-50 ng/0.5 microL) or antisauvagine-30 (2 microg/0.5 microL), respectively. Medial prefrontal cortex serotonin levels were measured using in-vivo microdialysis and high-performance liquid chromatography with electrochemical detection. Increased medial prefrontal cortex serotonin release elicited by CRF infusion into the dorsal raphe nucleus was abolished by inactivation of the median raphe nucleus. Furthermore, antagonism of CRF receptor type 2 but not CRF receptor type 1 in the dorsal raphe nucleus abolished CRF-induced increases in medial prefrontal cortex serotonin. Follow-up studies involved electrical stimulation of the central nucleus of the amygdala, a source of CRF afferents to the dorsal raphe nucleus. Activation of the central nucleus increased medial prefrontal cortex serotonin release. This response was blocked by CRF receptor type 2 antagonism in the dorsal raphe. Overall, these results highlight complex CRF modulation of medial prefrontal cortex serotonergic activity at the level of the raphe nuclei.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Rat behavior after chronic variable stress and partial lesioning of 5-HT-ergic neurotransmission: effects of citalopram

Deficits in serotonergic (5-HT-ergic) neurotransmission and stressful life events have been implicated in affective disorders, and chronic variable stress (CVS) can elicit behavioral changes reminiscent of increased emotionality, anxiety and atypical depression after partial 5-HT depletion. This study examined the effect of chronic citalopram treatment (10 mg/kg daily) on these changes. Parachloroamphetamine (PCA) (2 mg/kg) reduced the levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex, increased anxiety in the social interaction test, and increased activity in the open field. CVS reduced social activity in the social interaction test and immobility time in the forced swimming test. Reduction of excrements left during immobilization indicated partial adaptation with the CVS. Specific stressors had different effects on body weight gain, shorter lasting stressors having a smaller effect in general than those that lasted longer. Combination of CVS and PCA increased sucrose intake after two weeks of stress. In addition, combination of the two treatments reduced diving in the forced swimming test. Citalopram prevented the increase in sucrose consumption in the PCA+CVS rats, and in 5-HT-depleted animals blocked the increase in struggling and reduced the number of defecations in the forced swim test. In conclusion, citalopram treatment prevented several effects of either 5-HT depletion or combined PCA+CVS treatment, suggesting that these behavioral changes could be used in studies on the neural mechanisms underlying emotional behavior that may have relevance to the neurobiology of depression.

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.

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.

Increased sensitivity to restraint stress and novelty-induced emotionality following long-term, high dose cannabinoid exposure

Cannabinoids have long been known to affect anxiety, emotionality and stress responsivity; however, the direction of this effect has been controversial. This research aimed to compare the effects of chronic administration of both a very low dose (5 microg/kg) and a very high dose (100 microg/kg) of the potent cannabinoid CB(1) receptor agonist HU-210 on emotionality and stress responsivity. Twelve day administration of the high dose of HU-210 increased anxiety-like behavior as indicated by a significant reduction in time spent in the central quadrant and an increase in fecal boli in the open field test. However, neither dose of HU-210 elicited any significant behavioral effect in the elevated plus maze. Stress responsivity appeared to be sensitized in animals that had received high dose HU-210 treatment, as 30 min of restraint resulted in significantly higher plasma corticosterone levels in these subjects than in stressed controls or those receiving the low dose treatment. Furthermore, restraint stress elicited a non-significant increase in c-fos induction in the central amygdala, which was significantly potentiated following high-dose treatment with HU-210. This interaction was not seen in the basolateral amygdala. Together, these findings suggest that chronic high dose, but not low dose, cannabinoid administration may result in increased emotionality and sensitization of the stress axis.

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.

Fos-like immunoreactivity in the brain associated with freezing or escape induced by inhibition of either glutamic acid decarboxylase or GABAA receptors in the dorsal periaqueductal gray

GABAergic neurons exert tonic control over the neural substrates of aversion in the dorsal periaqueductal gray (dPAG). It has been shown that electrical stimulation of this region at freezing or escape thresholds activates different neural circuits in the brain. Since electrical stimulation activates cell bodies and fibers of passage, it is necessary to use chemical stimulation that activates only post-synaptic receptors. To investigate this issue further, reduction of GABA transmission was performed with local injections of either the GABA-A receptor antagonist bicuculline or the glutamic acid decarboxylase (GAD) inhibitor semicarbazide into the dorsolateral periaqueductal gray (dlPAG). Local infusions of semicarbazide (5.0 microg/0.2 microl) or bicuculline (40 ng/0.2 microl) into this region caused freezing and escape, respectively. The results obtained showed that freezing behavior induced by semicarbazide was associated with an increase in Fos expression in the laterodorsal nucleus of the thalamus (LD) and ventrolateral periaqueductal gray (vlPAG), while bicuculline-induced escape was related to widespread increase in Fos labeling, notably in the columns of the periaqueductal gray, hypothalamus nuclei, the central amygdaloid nucleus (Ce), the LD, the cuneiform nucleus (CnF) and the locus coeruleus (LC). Thus, the present data support the notion that freezing and escape behaviors induced by GABA blockade in the dlPAG are neurally segregated: freezing activates only structures that are mainly involved in sensory processing, and bicuculline-induced escape activates structures involved in both sensory processing and motor output of defensive behavior. Therefore, the freezing elicited by activation of dlPAG appears to be related to the acquisition of aversive information, whereas most brain structures involved in the defense reaction are recruited during escape.

Regulation of corticosteroid receptors in the rat brain: the role of serotonin and stress

It has been suggested that physiological resistance to repeated stress is associated with increased 5-hydroxytryptamine (5-HT) release in the dorsal hippocampus and that dysregulation of this neuroadaptation may be implicated in the psychopathology of depression. This study used 5,7-dihydroxytryptamine lesions to investigate the role of 5-HT projections to the hippocampus in physiological responses to repeated stress and putative changes in corticosteroid receptor immunoreactivity in the brain. Repeated exposure to elevated open platform stress (1 h/day) caused regionally selective changes in glucocorticoid and mineralocorticoid receptor immunoreactivity in the dorsal hippocampus that were not observed in ventral hippocampus, frontal cortex, hypothalamus or parietal cortex. Glucocorticoid receptor immunoreactivity in the dorsal hippocampus was decreased after 5 days but increased after 20 days of stress. Mineralocorticoid receptor immunoreactivity was increased after 5 or 10 days of stress. The increases in glucocorticoid and mineralocorticoid receptor immunoreactivity, evoked by repeated stress, were abolished by lesions of the principal 5-HT projections to the hippocampus. The lesions abolished the increased defecation observed in stressed animals, but had no effects on the plasma corticosterone response to the stressor or the habituation of this response observed following repeated stress. The experiments have revealed a dissociation in the regulation of corticosteroid receptor expression in the dorsal and ventral hippocampus by repeated stress and 5-HT. The data suggest that adaptation to inescapable stress is associated with regionally selective changes in corticosteroid receptor expression in dorsal hippocampus that are largely 5-HT-dependent, although these changes do not mediate habituation of the pituitary adrenocortical response to the stressor.

Effect of acute tryptophan depletion on the response to controllable and uncontrollable noise stress

BACKGROUND

Previous research provides evidence linking serotonin (5-hydroxytryptamine, 5-HT) with stress and depression. The controllable/uncontrollable (C/UC) stress paradigm aims to generate a state/condition, namely a feeling of lack of control in the context of a stressor, which might be an important factor in precipitating a negative mood state. Acute tryptophan depletion (ATD) is a technique that produces a decrease in central 5-HT levels in vivo. This study investigated the role of 5-HT in the behavioral response to a C/UC stress paradigm with ATD.

METHODS

Healthy adult volunteers were randomly assigned to receive either a TRP-supplemented (n = 15) or TRP-deficient (n = 13) amino acid drink. At 5 hours postdrink, volunteers were subjected to sessions of controllable and uncontrollable noise stress (100-dB white noise). Subjective ratings of mood were obtained before and after the interventions.

RESULTS

Participants who received the tryptophan-depleting drink had greater self-report ratings of negative mood on visual analogue scales and the Profile of Mood States after the uncontrollable stress than did participants who received the balanced drink.

CONCLUSIONS

The results suggest that 5-HT might play a role in providing resilience to uncontrollable stress. Additional studies with specific 5-HT pharmacologic probes will further clarify the results.

Effects of acute and chronic fluoxetine and diazepam on freezing behavior induced by electrical stimulation of dorsolateral and lateral columns of the periaqueductal gray matter

The defensive responses induced by electrical stimulation of the dorsal periaqueductal gray matter (dPAG) of the rat have been proposed as a model of panic attacks in humans. In the present study we investigated the acute and chronic effects of fluoxetine and diazepam on freezing and escape reactions elicited by electrical stimulation of the dorsolateral (dlPAG) and lateral (lPAG) columns of the periaqueductal gray matter (PAG). The frequencies of crossing, rearing, bouts of micturition and fecal boli were also recorded. Electrodes were unilaterally implanted in the brainstem aimed at the PAG. Drug treatments were given daily for 2 weeks with fluoxetine (5, 10 and 20 mg/kg ip), a selective inhibitor of serotonin reuptake, diazepam (1, 2 and 4 mg/kg ip), or saline. Drug effects were assessed acutely (15 min after the first injection) and chronically (15 min after the 14th injection). Chronic, but not acute, administration of fluoxetine caused a significant increase in the threshold of freezing without affecting the escape response elicited by dlPAG/lPAG stimulation. This characteristic pattern of effects could not be attributed to motor deficit, since this drug did not change the number of crossings and rearings. In contrast, no significant threshold changes were observed following acute and chronic treatment with diazepam. These data give further evidence for (a) an antiaversive effect of chronic treatment with fluoxetine, which caused a selective reduction in freezing behavior and neurovegetative responses associated with fearlike reaction elicited by dlPAG/lPAG electrical stimulation; (b) the involvement of the dlPAG and lPAG in the generation and organization of defensive responses and that freezing may probably be associated with panic attacks; and (c) the lack of effect of diazepam in this model is in line with its inefficacy as a panicolytic drug. The study of the unconditioned freezing behavior evoked by dlPAG/lPAG stimulation may constitute a new and interesting model for the study of panic disorder.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.