Previous stress increases in vivo biogenic amine response to swim stress

Affective disorders and the loudness dependence of the auditory evoked potential: Serotonin and beyond

Identifying additional noninvasive biomarkers for affective disorders, such as unipolar major depressive disorder (MDD) and bipolar disorder (BD), could aid in the diagnosis and treatment of these prevalent and debilitating neuropsychiatric conditions. One such candidate biomarker is the loudness dependence of the auditory evoked potential (LDAEP), an event-related potential that measures responsiveness of the auditory cortex to different intensities of sound. The LDAEP has been associated with MDD and BD, including therapeutic response to particular classes of antidepressant drugs, while also correlating with several other neuropsychiatric disorders. It has been suggested that increased values of the LDAEP indicate low central serotonergic neurotransmission, further implicating this EEG measure in depression. Here, we briefly review the literature on the LDAEP in affective disorders, including its association with serotonergic signaling, as well as with that of other neurotransmitters such as dopamine. We summarize key findings on the LDAEP and the genetics of these neurotransmitters, as well as prediction of response to particular classes of antidepressants in MDD, including SSRIs versus noradrenergic agents. The possible relationship between this EEG measure and suicidality is addressed. We also briefly analyze acute pharmacologic studies of serotonin and/or dopamine precursor depletion and the LDAEP. In conclusion, the existing literature suggests that serotonin and norepinephrine may modulate the LDAEP in an opposing manner, and that this event-related marker may be of use in predicting response to chronic treatment with particular pharmacologic agents in the context of affective disorders, such as MDD and BD, including in the presence of suicidality.

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

Neurodining: Common dietary factors may be substrates in novel biosynthetic pathways for monoaminergic neurotransmitters

It is not established that there are multiple endogenous mechanisms for synthesizing each of the three major monoamine neurotransmitters: serotonin, norepinephrine, and dopamine. Having multiple biosynthetic pathways for each of these important signaling molecules would provide greater assurance that they are available in sufficient quantities for their various physiological roles in the body. This paper puts forth the hypothesis that a number of common dietary factors-including sucrose and glucose, fats, plant components, and even ethanol-are substrates in novel biosynthetic pathways for the monoamines. A major aspect of this hypothesis is that in a range of multicellular organisms, D-glucose in particular may participate in novel biosynthetic pathways for the monoamines, where this sugar has already been linked with synthesis of the neurotransmitters acetylcholine, glutamate, and GABA through the tricarboxylic acid cycle. Another major aspect of the hypothesis is that phenol or polyphenol molecules, found in various plants, may combine with particular fats or even ethanol to form dopamine, which can then be converted to norepinephrine through the already established step involving the enzyme dopamine beta-hydroxylase. If such a biosynthetic pathway exists for converting ethanol to dopamine in humans, it could be a major factor in substance abuse, including early onset alcoholism. Further, if the above biosynthetic pathways exist in a range of organisms, they may be associated with appetitive processes regulating consumption of particular dietary factors, such as fruits and vegetables, to maintain internal "set points" of, for example, elevated noradrenergic signaling. In this scenario, exposure to psychological stress, which could eventually deplete neurotransmitters such as norepinephrine, may result in craving for sucrose, fats, or alcohol to help replenish the depleted cellular levels of this signaling molecule. An alternative to the overall biosynthetic hypothesis put forth here is that animal cells do not possess these pathways, but the animal microbiome harbors bacteria that do carry out these reactions and helps supply the body with monoamines and other signaling molecules.

Mechanisms of Shared Vulnerability to Post-traumatic Stress Disorder and Substance Use Disorders

Psychoactive substance use is a nearly universal human behavior, but a significant minority of people who use addictive substances will go on to develop an addictive disorder. Similarly, though ~90% of people experience traumatic events in their lifetime, only ~10% ever develop post-traumatic stress disorder (PTSD). Substance use disorders (SUD) and PTSD are highly comorbid, occurring in the same individual far more often than would be predicted by chance given the respective prevalence of each disorder. Some possible reasons that have been proposed for the relationship between PTSD and SUD are self-medication of anxiety with drugs or alcohol, increased exposure to traumatic events due to activities involved in acquiring illegal substances, or addictive substances altering the brain's stress response systems to make users more vulnerable to PTSD. Yet another possibility is that some people have an intrinsic vulnerability that predisposes them to both PTSD and SUD. In this review, we integrate clinical and animal data to explore these possible etiological links between SUD and PTSD, with an emphasis on interactions between dopaminergic, adrenocorticotropic, GABAergic, and glutamatergic neurobehavioral mechanisms that underlie different emotional learning styles.

Stress augments the rewarding memory of cocaine via the activation of brainstem-reward circuitry

Effects of stress on the reward system are well established in the literature. Although previous studies have revealed that stress can reinstate extinguished addictive behaviors related to cocaine, the effects of stress on the rewarding memory of cocaine are not fully understood. Here, we provide evidence that stress potentiates the expression of rewarding memory of cocaine via the activation of brainstem-reward circuitry using a cocaine-induced conditioned place preference (CPP) paradigm combined with restraint stress in rats. The rats exposed to 30-minute restraint stress immediately before posttest exhibited significantly larger CPP scores compared with non-stressed rats. Intra-laterodorsal tegmental nucleus (LDT) microinjection of a β or α2 adrenoceptor antagonist attenuated the stress-induced enhancement of cocaine CPP. Consistent with this observation, intra-LDT microinjection of a β or α2 adrenoceptor agonist before posttest increased cocaine CPP. Additionally, intra-ventral tegmental area (VTA) microinjection of antagonists for the muscarinic acetylcholine, nicotinic acetylcholine or glutamate receptors attenuated the stress-induced enhancement of cocaine CPP. Finally, intra-medial prefrontal cortex (mPFC) microinjection of a D1 receptor antagonist also reduced the stress-induced enhancement of cocaine CPP. These findings suggest a mechanism wherein the LDT is activated by noradrenergic input from the locus coeruleus, leading to the activation of VTA dopamine neurons via both cholinergic and glutamatergic transmission and the subsequent excitation of the mPFC to enhance the memory of cocaine-induced reward value.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

Night eating model shows time-specific depression-like behavior in the forced swimming test

The circadian clock system is associated with feeding and mood. Patients with night eating syndrome (NES) delay their eating rhythm and their mood declines during the evening and night, manifesting as time-specific depression. Therefore, we hypothesized that the NES feeding pattern might cause time-specific depression. We established new NES model by restricted feeding with high-fat diet during the inactive period under normal-fat diet ad libitum. The FST (forced swimming test) immobility time in the NES model group was prolonged only after lights-on, corresponding to evening and early night for humans. We examined the effect of the NES feeding pattern on peripheral clocks using PER2::LUCIFERASE knock-in mice and an in vivo monitoring system. Caloric intake during the inactive period would shift the peripheral clock, and might be an important factor in causing the time-specific depression-like behavior. In the NES model group, synthesis of serotonin and norepinephrine were increased, but utilization and metabolism of these monoamines were decreased under stress. Desipramine shortened some mice’s FST immobility time in the NES model group. The present study suggests that the NES feeding pattern causes phase shift of peripheral clocks and malfunction of the monoamine system, which may contribute to the development of time-specific depression.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

Increased mesocorticolimbic dopamine during acute and repeated social defeat stress: modulation by corticotropin releasing factor receptors in the ventral tegmental area

RATIONALE

Stress activates a subset of dopamine neurons in the ventral tegmental area (VTA), increasing extracellular dopamine in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAcSh). The stress neuropeptide corticotropin releasing factor (CRF) and its receptors (CRF-R1 and CRF-R2) are located within the VTA and directly and indirectly influence dopaminergic activity. However, it has yet to be shown in vivo whether VTA CRF receptor activation is necessary for acute and repeated stress-induced dopamine efflux.

OBJECTIVE

With intra-VTA CRF-R1 and CRF-R2 antagonism during social defeat, we assessed whether blockade of these receptors could prevent stress-induced dopamine increases in the mPFC and NAcSh using in vivo microdialysis.

METHODS

Rats were microinjected with a CRF-R1 or CRF-R2 antagonist into the VTA prior to social defeat stress on days 1, 4, 7, and 10. In vivo microdialysis for dopamine in the mPFC and NAcSh was performed during stress on days 1 and 10.

RESULTS

During the first social defeat, extracellular dopamine was significantly elevated in both the mPFC and NAcSh, and this increase in the NAcSh was blocked by intra-VTA CRF-R2, but not CRF-R1, antagonism. During the final social defeat, the dopaminergic increase was neither sensitized nor habituated in the mPFC and NAcSh, and intra-VTA CRF-R2, but not CRF-R1, antagonism prevented the dopamine increase in both brain regions.

CONCLUSION

These findings show that CRF-R2 in the VTA is necessary for acute and repeated stress-induced dopamine efflux in the NAcSh, but is only recruited into mPFC-projecting dopamine neurons with repeated stress exposure.

Predator stress engages corticotropin-releasing factor and opioid systems to alter the operating mode of locus coeruleus norepinephrine neurons

The norepinephrine nucleus, locus coeruleus (LC), has been implicated in cognitive aspects of the stress response, in part through its regulation by the stress-related neuropeptide, corticotropin-releasing factor (CRF). LC neurons discharge in tonic and phasic modes that differentially modulate attention and behavior. Here, the effects of exposure to an ethologically relevant stressor, predator odor, on spontaneous (tonic) and auditory-evoked (phasic) LC discharge were characterized in unanesthetized rats. Similar to the effects of CRF, stressor presentation increased tonic LC discharge and decreased phasic auditory-evoked discharge, thereby decreasing the signal-to-noise ratio of the sensory response. This stress-induced shift in LC discharge toward a high tonic mode was prevented by a CRF antagonist. Moreover, CRF antagonism during stress unmasked a large decrease in tonic discharge rate that was opioid mediated because it was prevented by pretreatment with the opiate antagonist, naloxone. Elimination of both CRF and opioid influences with an antagonist combination rendered LC activity unaffected by the stressor. These results demonstrate that both CRF and opioid afferents are engaged during stress to fine-tune LC activity. The predominant CRF influence shifts the operational mode of LC activity toward a high tonic state that is thought to facilitate behavioral flexibility and may be adaptive in coping with the stressor. Simultaneously, stress engages an opposing opioid influence that restrains the CRF influence and may facilitate recovery toward pre-stress levels of activity. Changes in the balance of CRF:opioid regulation of the LC could have consequences for stress vulnerability.

Dopamine and serotonin interactively modulate prefrontal cortex neurons in vitro

BACKGROUND

Dopamine (DA) and serotonin (5-HT) are released in cortex under similar circumstances, and many psychiatric drugs bind to both types of receptors, yet little is known about how they interact.

METHODS

To characterize the nature of these interactions, the current study used in vitro patch-clamp recordings to measure the effects of DA and/or 5-HT on pyramidal cells in layer V of the medial prefrontal cortex.

RESULTS

Either DA or 5-HT applied in isolation increased the evoked excitability of prefrontal cortex neurons, as shown previously. Coapplication of DA and 5-HT produced either a larger increase in excitability than when either was given alone or a significant decrease that was never observed when either was given alone. Dopamine or 5-HT also "primed" neurons to respond in an exaggerated manner to the subsequent application of the other monoamine.

CONCLUSIONS

These data reveal the unappreciated interactive nature of neuromodulation in cortex by showing that the combined effects of DA and 5-HT can be different from their effects recorded in isolation. On the basis of these findings, we present a theory of how DA and 5-HT might synergistically modulate cortical circuits during various tasks.

Stress-dependent impairment of passive-avoidance memory by propranolol or naloxone

Previous work has shown that the effect of opioid-receptor blockade on memory modulation is critically dependent upon the intensity of stress. The current study determined the effect of adrenergic-receptor blockade on memory modulation under varied levels of stress and then compared the effect of adrenergic-receptor blockade under intense stress to that of a) opioid-receptor blockade and b) concurrent opioid- and adrenergic-receptor blockade. In the first experiment, the β-adrenergic-receptor blocker propranolol impaired retention in the passive-avoidance procedure when administered immediately after exposure to intense stress (passive-avoidance training followed by swim stress) but not mild stress (passive-avoidance training alone). In the second experiment, while separate administration of either propranolol or the opioid-receptor blocker naloxone immediately after exposure to intense stress impaired retention, the combined administration of propranolol and naloxone failed to do so. These findings demonstrate that the effect of β-adrenergic-receptor blockade or opioid-receptor blockade on memory modulation in the passive-avoidance procedure is dependent upon the intensity of stress, and suggest that concurrent inactivation of endogenous adrenergic- and opioid-based memory modulation systems under stressful conditions is protective of memory.

Galanin, galanin receptor subtypes and depression-like behaviour

The pathophysiology of depression remains unclear, but involves disturbances in brain monoaminergic transmission. Current antidepressant drugs, which act by enhancing this type of neurotransmission, have limited therapeutic efficacy in a number of patients, and also cause serious side-effects, which limits their compliance. Increasing evidence suggests that neuropeptides, including galanin, can be of relevance in mood disorders. Galanin is co-expressed with and modulates noradrenaline and serotonin transmission, both implicated in depression. Pharmacological and genetic studies suggest a role for galanin in depression-like behaviour in rodents, involving specific receptor subtypes. Thus, stimulation of GalR1 and/or GalR3 receptors results in depression-like phenotype, while activation of the GalR2 receptor reduces depression-like behaviour in the rat. These findings suggest that galanin receptor subtypes may represent novel targets for the development of antidepressant drugs.

Stress-dependent enhancement and impairment of retention by naloxone: evidence for an endogenous opioid-based modulatory system protective of memory

The opiate-receptor antagonist naloxone was administered to rats after passive-avoidance training either alone or in combination with forced-swim stress. A retention test revealed that while naloxone enhanced retention when administered alone, it impaired retention when administered in combination with forced-swim stress. The findings provide evidence for a "protective" endogenous opioid-based system that, when not blocked pharmacologically, limits enhancement or impairment of retention under conditions of mild and intense stress, respectively.

Evidence for sex-specific shifting of neural processes underlying learning and memory following stress

Recent human research has been focused upon determining whether there is evidence that stress responses cause qualitative changes in neural activity such that people change their learning strategies from a spatial/contextual memory process through the hippocampus to a procedural stimulus-response process through the caudate nucleus. Moreover, interest has shifted to determining whether males and females exhibit the same type of stress-induced change in neural processing of associations. Presented is a select review of 2 different animal models that have examined how acute or chronic stressors change learning in a sex-specific manner. This is followed by a brief review of recent human studies documenting how learning and memory functions change following stressor exposure. In both cases, it is clear that ovarian hormones have a significant influence on how stress affects learning processes in females. We then examine the evidence for a role of acetylcholine, dopamine, norepinephrine, or serotonin in modulating this shifting of processing and how that may differ across sex. Conclusions drawn suggest that there may be evidence for sex-specific changes in amygdala and hippocampus neuromodulation; however, the behavioral data are still not conclusive as to whether this represents a common or sex-specific shift in how males and females process associations after stressor exposure.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

The immobility produced by intermittent swim stress is not mediated by serotonin

Exposure to uncontrollable stressors such as intermittent swim stress (ISS) produces a behavioral syndrome that resembles behavioral depression including immobility in a Forced Swim Test (FST) and escape learning deficits. The results of previous studies suggest that stress causes a temporary sensitization of the brain serotonin (5-HT) system that is necessary and sufficient for producing behavioral depression. If this hypothesis is true in the ISS paradigm, then enhancing or inhibiting 5-HT transmission during stress should exacerbate or block the development of behavioral depression, respectively. The selective 5-HT uptake inhibitor fluoxetine (FLX) was administered prior to ISS or confinement; 24 h later the FST was used to detect behavioral immobility. ISS, but not FLX, significantly increased immobility in the FST. The purported 5-HT uptake enhancer tianeptine (TPT) was administered in place of FLX. Again ISS increased immobility in the FST, but TPT had no effect. These results suggested that 5-HT is not a critical mediator of ISS induced behavioral depression. However, some authors have raised concern that TPT does not act directly on 5-HT. Therefore, the 5-HT synthesis inhibitor, para-chlorophenylaline (PCPA) was administered to deplete central 5-HT before stress. PCPA did not alter immobility in the FST. Finally, a sub-chronic regimen of FLX given after ISS, but before the FST, was without effect on reversing the ISS-induced immobility. Taken together, these experiments indicate that ISS produces a significant behavioral depression manifested as increased immobility but offer no support of the hypothesis that 5-HT is a critical mediator of these effects.

NAN-190 potentiates the impairment of retention produced by swim stress

Exposing rats to stress in the form of forced swim immediately after passive-avoidance training impaired retention. In contrast, exposure to the same stressor 2 h after training failed to impair retention. Systemic administration of the 5-HT1A receptor antagonist NAN-190 (1 mg/kg) immediately after forced swim markedly potentiated the stress-induced impairment of retention. In contrast, NAN-190 failed to affect retention when administered 2 h after forced swim or in forced swim's absence. These findings provide evidence for a NAN-190-sensitive system modulating retention that is 1) activated during a critical period shortly after exposure to swim stress, and 2) protective of memory, thereby limiting the extent to which retention is impaired by experiential stress.

Behavioural characterisation of transgenic mice overexpressing galanin under the PDGF-B promoter

The behavioural phenotype of transgenic mice (3-5-months old) overexpressing galanin (GalOE mice) under the platelet-derived growth factor B (PDGF-B) promoter was evaluated in a battery of tests, including locomotor cages, light-dark exploration test, elevated plus-maze and the Porsolt forced swim test. Learning and memory were assessed in the Morris water maze task. GalOE mice showed a slight increase in spontaneous locomotor activity assessed in the locomotor cages, but the amphetamine-induced increase in locomotor activity was somewhat lower in GalOE mice. Anxiety-like behaviour in light-dark exploration and elevated plus-maze tests did not differ between genotypes. In the Porsolt forced swim test, GalOE mice displayed an increased time of immobility, indicative of increased learned helplessness possibly reflecting increased stress-susceptibility and/or depression-like behaviour. GalOE mice showed normal learning and memory retention in the Morris water maze tasks. These data support the hypothesis that galanin may have a role in functions related to mood states, including affective disorders.

Behavioural characterisation of young adult transgenic mice overexpressing galanin under the PDGF-B promoter

The behavioural phenotype of transgenic mice (3- to 5-months old) overexpressing galanin (GalOE) under the platelet-derived growth factor B (PDGF-B) promoter was evaluated in a battery of tests, including open field, locomotor cages, light-dark exploration test, elevated plus-maze and the Porsolt forced swim test. Learning and memory were assessed in the passive avoidance and the Morris water maze tasks. No difference between genotypes was found in exploratory activity in the open field. GalOE mice showed a slight increase in spontaneous locomotor activity assessed in the locomotor cages, but the amphetamine-induced increase in locomotor activity was somewhat lower in GalOE mice. Anxiety-like behaviour in the three different tests including open field, light-dark exploration and elevated plus-maze did not differ between genotypes. In the Porsolt forced swim test, GalOE mice displayed an increased time of immobility, indicative of increased learned helplessness possibly reflecting increased stress-susceptibility and/or depression-like behaviour. GalOE mice showed normal learning and memory retention in the passive avoidance and the Morris water maze tasks. These data support the hypothesis that galanin may have a role in functions related to mood states including affective disorders.

A neural network underlying individual differences in emotion and aggression in male golden hamsters

In rodents, aggressive behavior can be altered by experimental manipulations of emotional responsiveness. The goal of this study was to identify characteristics of emotional reactivity associated with individual differences in aggressive behavior and their integration within a common neural network. Male golden hamsters were first screened for offensive aggression. Then, the animals were trained through immediate reinforcement and tested for their adaptation to a delayed reward. Similar protocols have been used to test behaviors associated with frustration. At first, all hamsters showed increased frequency of bar pressing per reward during delayed reinforcement. However, Low-Aggression animals were able to adapt to the delay and showed a decreased rate of bar pressing per reward within 5 days. In contrast, High-Aggression animals maintained a high rate of bar pressing per reward. In addition, brains were collected after immediate reward training or delayed reward testing, and labeled for pCREB-immunoreactivity as a marker of trans-synaptic activity. In High-Aggression individuals, elevated density of cyclic AMP response element binding protein, phosphorylated (pCREB) immunostaining was found within the anterior hypothalamus, an area critical to the control of aggression. Delayed reinforcement was associated with enhanced pCREB immunostaining within the central amygdala, medial amygdala and preoptic area/hypothalamus continuum. Further analysis of the data also showed a positive correlation in labeling density between the lateral septum and the anterior hypothalamus, specifically in Low-Aggression animals exposed to delayed reward. Therefore, as High-Aggression individuals lack control of their emotional reactivity, they are also characterized by a de-synchronization between the inhibitory output of the septum and the aggression areas of the hypothalamus. Finally, our data also show that frustration is associated with an extensive activation of the preoptic area/hypothalamus continuum and amygdala.

Enhanced hippocampal noradrenaline and serotonin release in galanin-overexpressing mice after repeated forced swimming test

Basal and forced swimming (FS) stress-induced release of noradrenaline (NA) and serotonin (5-HT) were determined by in vivo microdialysis in the ventral hippocampus of mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE/P) or under the dopamine beta-hydroxylase promoter (GalOE/D) (only NA). WT mice served as controls. Intraventricular infusion of galanin significantly reduced basal extracellular NA in WT mice and in GalOE/P mice (albeit less so). Microdialysis sampling during a 10-min FS showed that NA and 5-HT release were elevated to 213% and 156%, respectively, in the GalOE/P group, whereas in the WT group the increases were only 127% and 119%, respectively. The second (repeated) 10-min FS (RFS) caused a marked enhancement of NA and 5-HT release in the GalOE/P mice to 344% and 275%, respectively. However, the RFS caused only a 192% increase of extracellular NA levels in the GalOE/D mice. Pretreatment with the putative peptidergic galanin receptor antagonist M35 almost completely blocked the elevation of NA and 5-HT levels in the GalOE/P after RFS. These results suggest that the NA and 5-HT hippocampal afferents in GalOE/P mice are hypersensitive to both conditioned and unconditioned stressful stimuli, such as FS, and that this effect is mediated by galanin receptors. The present findings support a role of galanin in the regulation of release of NA and 5-HT, two neurotransmitters involved in mood control.

Behavioral, neuroendocrine and neurochemical effects of the imidazoline I2 receptor selective ligand BU224 in naive rats and rats exposed to the stress of the forced swim test

RATIONALE

There is evidence for alterations in imidazoline(2) (I(2)) receptor density in depressed patients. Selective I(2) receptor ligands modulate central monoamine levels and activate the hypothalamo-pituitary-adrenal (HPA) axis and may have potential as antidepressants.

OBJECTIVES

To study the behavioral effects of the selective I(2) receptor ligand BU224 in the rat forced swim test (FST) and its effects on the HPA axis and central monoaminergic responses.

METHODS

Rats received saline or BU224 (10 mg/kg IP) 24, 18 and 1 h prior to 15 min exposure to the FST. Saline- and BU224-treated non-stressed groups were included. Time spent immobile, struggling and swimming calmly was measured. Plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels 90 min post-BU224 were measured in addition to tissue levels of monoamines and metabolites in the frontal cortex, hippocampus and hypothalamus.

RESULTS

Administration of BU224 significantly reduced immobility and increased mild swimming without affecting struggling. Exposure to the FST significantly increased plasma ACTH and corticosterone levels. BU224 administration also increased ACTH and potentiated the ACTH response to FST with no effect on corticosterone. BU224 administration significantly increased frontal cortex 5-hydroxytryptamine (5-HT) levels and decreased 5-HT turnover in the frontal cortex and hypothalamus of rats exposed to FST. In non-stressed rats, BU224 decreased 5-HT turnover in the hippocampus and hypothalamus and decreased norepinephrine turnover in the frontal cortex.

CONCLUSIONS

The selective I(2) receptor ligand BU224 reduces immobility of rats in the FST, indicative of antidepressant-like activity. This effect is accompanied by alterations in HPA axis and central monoaminergic activity.

Increased extracellular serotonin level in rat hippocampus induced by chronic citalopram is augmented by subchronic lithium: neurochemical and behavioural studies in the rat

RATIONALE

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentation strategy. However, the neurochemical rationale behind this strategy needs to be further clarified.

OBJECTIVES

We examined the effect of chronic citalopram and subchronic lithium, alone or in combination, on (a) serum levels of citalopram and lithium, (b) animal behaviour and (c) hippocampal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels. Furthermore, we examined the serum level of citalopram and hippocampal 5-HT following one acute citalopram injection.

METHODS

Microdialysis in the freely moving animals was used to determine hippocampal 5-HT and 5-HIAA. The animal behaviour was examined in the open field and forced swim test. RESULTS. We found that chronic administration of citalopram (20 mg/kg/24 h s.c.) significantly increased the 5-HT baseline relative to vehicle-treated rats. Addition of subchronic lithium (60 mmol/kg chow pellet p.o.) to chronic citalopram therapy further elevated the 5-HT levels. Moreover, we found acute citalopram (5 mg/kg s.c.) to increase the 5-HT level. The immobility time in the FST and the locomotion in the OF were unaffected by any treatments.

CONCLUSIONS

The present results support the assumption that increases in hippocampal 5-HT neurotransmission may be important in the augmentatory effect of lithium.

Rapid habituation of hippocampal serotonin and norepinephrine release and anxiety-related behaviors, but not plasma corticosterone levels, to repeated footshock stress in rats

Prior stress exposure is known to alter the activation response to a subsequent stressor. In the present study, we examined neurochemical, neuroendocrinological, and behavioral correlates of short-term adaptation to homotypic stressors administered 60 min apart. An initial electric footshock significantly induced extracellular levels of both serotonin (5-HT) and norepinephrine (NE) in the rat hippocampus (650% and 200% above baseline, respectively), as measured by in vivo microdialysis. A rapid habituation in this response was evident in the inability of a second footshock to evoke similar increases. In contrast, the hypothalamic-pituitary-adrenal (HPA) response was augmented further after the second shock session: plasma corticosterone (CORT) levels were 18.1, 316.5, and 441.6 mg/ml in nonstressed, one-footshock-, or two-footshock-treated rats, respectively. In a social interaction paradigm, rats subjected to a single footshock showed several fear- and anxiety-related behaviors such as increases in freezing and decreases in rearing and active approach for social interaction. Exposure to a second footshock completely blocked the freezing response and restored rearing behavior without affecting the disruption in social interactions. Taken together, these data raise the possibility that neurochemical and neuroendocrine adaptations to short-term homotypic stressors differentially contribute to expression of different fear and anxiety-like responses in the rat.

Chronic social stress during puberty enhances tyrosine hydroxylase immunoreactivity within the limbic system in golden hamsters

The present study was carried out to determine the effects of chronic exposure to social stress during puberty on the dopamine system in male golden hamsters. Experimental animals were socially subjugated between postnatal days 28 (P28) and 42. All animals were sacrificed on P46 and their brains processed for immunocytochemistry to tyrosine hydroxylase (TH). A large increase in the number of TH-immunoreactive (TH-ir) neurons was noted within the posterior portion of the medial amygdaloid nucleus and the posterior portion of the medial division of the bed nucleus of the stria terminalis in subjugated animals as compared to controls. This effect appeared to be site-specific as no difference was seen between groups in the periventricular nucleus, another steroid receptor-rich area. The data suggest that these dopamine neurons may play an important role in the behavioral changes associated with chronic social stress during puberty.

Increased serotonin release in mice frontal cortex and hippocampus induced by acute physiological stressors

The effects of acute physiological stressors (5 s tail pinch, handling and forced swimming at +25 and +5 degrees C for 3 min each) on serotonin (5-HT) release in the mouse brain were investigated using in vivo microdialysis. The extracellular 5-HT levels were determined by a newly developed highly-sensitive and selective high-performance liquid chromatography method based on derivatization with benzylamine and fluorescence detection. The basal levels of 5-HT in 3 min microdialysates from the ventral hippocampus and frontal cortex were 0.68+/-0.21 and 0.75+/-0.28 fmol/6 microl (n=24), respectively. All three stressors caused an immediate, significant and reversible increase (handling: 150%; swimming: 240%) of extracellular 5-HT levels in both brain structures, suggesting a more dynamic role played by the serotonergic system in response to acute stress.

Prolonged effect of an anesthetic dose of ketamine on behavioral despair

The present study investigated the effect of a single, anesthetic dose of ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, on behavioral despair, an animal model of depression. Separate groups of male Wistar rats injected with an anesthetic dose of ketamine (160 mg/kg ip) and tested 3, 7, or 10 days later showed significantly less immobility in the second of two forced-swim tests compared to saline-injected controls. Ketamine- and saline-treated animals did not differ significantly in the swim tests with respect to other behavioral measures, namely diving, jumping, and head shakes. The present findings point to an ameliorative effect of ketamine on behavioral despair and support the view that NMDA antagonists may have a beneficial effect on depression.

Microdialysis in freely moving mice: determination of acetylcholine, serotonin and noradrenaline release in galanin transgenic mice

In the present study, we describe micro-surgical methods for simultaneous implantation of a microdialysis probe and an intraventricular injection cannula via their respective guide cannulas into the mouse brain. Basal and stimulated release of acetylcholine (ACh), serotonin (5-HT) and noradrenaline (NA) was determined in the ventral hippocampus of freely moving mice. NA and 5-HT were determined in one run by a newly developed HPLC method based on precolumn derivatization with benzylamine and fluorescence detection. The mice with a loss-of-function mutation of the galanin gene (KO) and the mice that over-expressed galanin (OE) were studied. No significant differences in basal, potassium-stimulated or scopolamine-induced extracellular ACh levels were observed in 4-month-old wild-type (WT) and KO mice. In the aged, 10-month-old animals, the basal extracellular ACh levels were significantly reduced in both WT and KO groups. Galanin (1 nmol i.c.v.) caused a significant reduction of basal extracellular NA by about 40% in both WT and galanin OE mice, however, in the latter group the effect was delayed by almost 2 h. A 10-min forced swimming stress caused a higher increase in release of NA and 5-HT in the OE group than in the corresponding WT mice. Finally, venlafaxin (10 mg/kg i.p.) increased extracellular NA to 400% of the control values in the CBA mice, but only to 250% in the C57BL mice. It is concluded that galanin may play an important role in the cholinergic mechanisms underlying cognitive disorders. Furthermore, modulation by galanin and by behavioral activation, of NA and 5-HT neurotransmission in galanin over-expressing mice indicates its possible role in the aetiology of mood disorders.

Extracellular dopamine in the rat prefrontal cortex during reward-, punishment- and novelty-associated behaviour. Effects of diazepam

Variations of extracellular dopamine (DA(ext)) levels in prefrontal cortex were assessed by in vivo microdialysis. In rats trained in an operant fixed interval (FI(30s)) schedule of food delivery, acute exposure to contingent foot shocks resulted in a suppression of responding that was reversed by diazepam (4 mg/kg, ip). No changes in cortical DA(ext) levels occurred during this period in both control and treated rats. By contrast, in control rats, cortical DA(ext) levels increased (+25-40%) during the nonpunished component of the operant session, and during noncontingent food delivery (+25%). Control rats placed into an unfamiliar brightly lit openfield exhibited a marked increase in cortical DA(ext) levels (+100%). This effect occurred neither in rats given diazepam at a dose (2 mg/kg) which stimulated motor activity, nor during a second exposure to the openfield. In conclusion, a benzodiazepine-sensitive activation of mesoprefrontal DA neurones is induced by exposure to novel stressful surroundings and by food availability and consumption. The fact that cortical DA(ext) levels remained unchanged in rats that exerted complete control upon negative stimuli indicates that an activation of the mesoprefrontal DA system is not required for punishment-induced behavioural blockade.

Tyrosine improves behavioral and neurochemical deficits caused by cold exposure

The effects of acute cold stress were assessed behaviorally and neurochemically. The norepinephrine (NE) precursor, tyrosine (TYR), the catecholamine-releasing compound, amphetamine (AMPH), and the adrenoceptor agonist, phenylpropanolamine (PPA), were administered systemically either alone or in conjunction with TYR 30 min prior to cold exposure. All three sympathomimetic treatments dose-dependently improved performance in a forced swim test following hypothermia (T(c)=30 degrees C). AMPH/TYR or PPA/TYR combinations further improved performance vs. either agent given alone. Microdialysis showed elevated hippocampal NE concentrations in response to hypothermia. TYR further elevated NE concentration in cold/restrained rats vs. saline (SAL)-treated controls. These results suggest that sympathomimetic agents, including the nutrient TYR, which enhance noradrenergic function, improve performance in animals acutely stressed by hypothermia.

Extracellular serotonin is enhanced in the striatum, but not in the dorsal hippocampus or prefrontal cortex, in rats subjected to an operant conflict procedure

In rats trained in an operant fixed-interval-30-s schedule of food reward (FI-30s), acute exposure to contingent footshock resulted in a response suppression that was released by diazepam (DZP; 4 mg/kg ip) but not by buspirone (0.25 or 0.50 mg/kg ip). Compared with baseline, hippocampal and cortical extracellular levels of serotonin (5-HText) did not change, regardless of operant period (punished or nonpunished) and drug. In contrast, in the striatum, an increase of 5-HText levels (535%) occurred during the punished period, counteracted by DZP. This effect was observed only in rats that were low responders during both nonpunished and punished periods, that is, those that exerted an efficacious control over responding. Uncontrollable shocks or exposure to an unfamiliar open field did not modify striatal 5-HText. Together, these results suggest that an acute activation of 5-HT neurons afferent to the striatum allows the rats to efficiently block responses that are negatively reinforced.

Previous exposure to footshock stress attenuates nicotine-induced serotonin release in rat striatum during the subsequent stress

We have analyzed the effects of chronic or repeated footshock stress on the release of serotonin (5-hydroxytryptamine: 5-HT) in the striatum of rats that received nicotine by using a microdialysis technique. Neither local infusion of nicotine alone nor stress application alone changed 5-HT release. Local infusion of 1 mM nicotine to the striatum, however, significantly increased 5-HT release in the striatum to 145.9 +/- 30.8 pg/dialysate during simultaneous stress application. These increases of extracellular 5-HT release induced by the combination of nicotine and stress application were also observed in rats that had received daily chronic footshock. However, the previously administered footshock induced the reduced release of 5-HT from the striatum to 33.5 +/- 8. 6 (repeated footshock) and 10.0 +/- 3.6 pg/dialysate (daily footshock) when footshock was given together with nicotine infusion. These results suggest that previous exposure to stress attenuated the nicotine-induced 5-HT release in the striatum during the subsequent stress.

Serotonin, stress and corticoids

There is evidence for stressor- and brain region-specific selectivity in serotonergic transmission responses to aversive stimuli. The aim of the present review is to provide an overview of the effects of different acute and repeated/chronic stressors on serotonin (5-HT) release and reuptake, extracellular 5-HT levels, and 5-HT pre- and postsynaptic receptors in areas tightly linked to the control of fear and anxiety, namely the dorsal and median raphe nuclei, the frontal cortex, the amygdala and the hippocampus. In addition, our knowledge of the impacts of corticoids on serotonergic systems in these brain areas is also briefly provided to examine whether the hypothalamo-pituitary-adrenal axis may play a role in stress-induced alterations in 5-HT neurotransmission. Taken together, the data presented reinforce the hypothesis that stress affects such a transmission, partly through the actions of corticoids. However, we are still left with unanswered, albeit crucial questions. First, the question of the specificity of the serotonergic responses to stress, with regard to the site of action and the nature of the stressor still remains open due to the heterogeneity of the results obtained so far. This could indicate that environmental factors, other than the stressor itself, may have enduring consequences on 5-HT sensitivity to stress. Second, the question regarding the role of stress-elicited changes in 5-HT transmission within coping processes finds in most cases no clearcut answer. In keeping with human symptomatology, the need to consider the environment (including the early one) and the genetic status when assessing the effects of stress on 5-HT neurotransmission is underlined. Such a consideration could help to answer the questions raised.

Long lasting increase in nociceptive threshold induced in mice by forced swimming: involvement of an endorphinergic mechanism

Mice submitted to forced swimming session(s) displayed a long lasting modification in their nociceptive threshold, assessed through their jump latency from a hot plate (55 degrees C). Thus two forced swimming sessions (6 min each, 8h apart), in water at 33 degrees C, increased by about 50% the jump latency when the hot plate test was performed 14 hours, 3 days or 6 days thereafter. The water temperature (16 degrees C vs 33 degrees C) had no critical influence in this respect. To be clearly effective (at 33 degrees C) the swimming session had to be performed twice (when performed only once it was irregularly effective); it apparently culminated for a 6 min duration, since its effectiveness was not significantly increased by extending the swimming time to 12 min or 18 min. Performing 2 forced swimming sessions (6 min each, 8h apart), 5 consecutive days, resulted in a suppression of the increase in jump latency in the hot plate test. The two forced swimming episodes-induced analgesia was prevented by the s.c. administration of diazepam (from 0.125 mg/kg) or morphine (from 5 mg/kg) or scopolamine (1 mg/kg) before each forced swimming episode. Morphine (7.5 mg/kg) was uneffective to prevent the induction of two forced swimming episodes-induced analgesia when it was administered immediately after each forced swimming session. Finally this analgesia was dose dependently reversed by naloxone (ID(50) = 0.14 mg/kg, s.c., 30 min before the hot plate test). It is hypothesized that the handling of mice immediately before the hot plate test induces the remembrance of the stress induced by previous forced swimming episodes, triggering a fear reaction which increases the nociceptive threshold.

Effect of subchronic antidepressant treatments on behavioral, neurochemical, and endocrine changes in the forced-swim test

The purpose of the present study was to examine the effect of subchronic treatment (24 days) with antidepressants displaying differential effects on noradrenaline and serotonin reuptake, on behavior, neurochemistry, and hypothalamic-pituitary-adrenal (HPA) axis activity following FST exposure in the rat. Desipramine (7.5 mg/kg, IP) significantly decreased immobility in the FST, whilst paroxetine (7.5 mg/kg IP) and venlafaxine (10 mg/kg, IP) were without effect. Nonetheless, treatment with all three antidepressants significantly attenuated stress-related increases in amygdaloid and cortical serotonin turnover. Of the three antidepressants examined, only desipramine attenuated the stress-associated elevation in serum corticosterone. In conclusion, although FST-induced increases in serotonin turnover in the frontal cortex and amygdala were attenuated following treatment with all three antidepressants, FST-induced behavioral changes and increased HPA axis activity were normalized only following desipramine treatment. In addition, these results suggest that neurochemical mechanisms independent of increased serotonergic activity subserve the normalization of behavior and HPA axis responses in the FST. These data also add to our understanding of the interactions between antidepressants and stress-induced behavioral, neurochemical, and endocrine alterations, and illustrates important differences between classes of antidepressants.

The role of stress in the pathophysiology of the dopaminergic system

In this review, we will examine the most recent preclinical evidence in support of the fact that both acute and chronic stress may have a detrimental impact on the normal function of the dopaminergic system. In recent decades, the term stress has changed its meaning from that of a 'non-specific body response' to a 'monitoring system of internal and external cues'; that is a modality of reaction of the mammalian central nervous system (CNS) which is critical to the adaptation of the organism to its environment. Compelling results have demonstrated that the dopaminergic system is important not only for hedonic impact or reward learning but also, in a broader sense, for reactivity to perturbation in environmental conditions, for selective information processing, and for general emotional responses, which are essential functions in the ability (or failure) to cope with the external world. In this, stress directly influences several basic behaviors which are mediated by the dopaminergic system such as locomotor activity, sexual activity, appetite, and cross sensitization with drugs of abuse. Studies using rat lines which are genetically different in dopamine (DA) physiology, have shown that even small alterations in the birth procedure or early life stress events may contribute to the pathophysiology of psychiatric disorders-in particular those involving central DA dysfunction-and may cause depression or psychotic derangement in the offspring. Finally, the fact that the dopaminergic system after stress responds, preferentially, in the medial prefrontal cortex (MFC), is thought to serve, in humans, as a protection against positive psychotic symptoms, since the increased DA activity in the MFC suppresses limbic DA transmission. However, excessive MFC dopaminergic activity has a negative impact on the cognitive functions of primates, making them unable to select and process significant environmental stimuli. Thus it appears that a critical range of DA turnover is necessary for optimal cognitive functioning after stress, in the response of the CNS to ever-changing environmental demands. Molecular Psychiatry (2000) 5, 14-21.

Reboxetine attenuates forced swim test-induced behavioural and neurochemical alterations in the rat

The forced swim test is a behavioural paradigm that is predicative of antidepressant activity in rodents. Until recently, research has focused on the ability of antidepressant drugs to decrease immobility in the forced swim test paradigm, but the neurochemical sequelae induced by swim stress, or the neurochemical basis of antidepressant-induced behavioural changes have received little attention. In this regard, we have recently demonstrated that forced swim test exposure increases serotonergic activity in the amygdala, frontal cortex and hippocampus and dopamine turnover in the striatum. In addition, forced swim test-exposure activates the hypothalamic pituitary adrenal axis. The purpose of the present study was to examine the effect of treatment with the selective noradrenaline reuptake inhibitor reboxetine (3, 10 and 30 mg/kg; i.p.) on immobility and defaecation scores in the forced swim test, and on forced swim test-induced neurochemical and hypothalamic pituitary adrenal axis changes in the rat. Reboxetine treatment (10 and 30 mg/kg) significantly decreased immobility and defaecation in the forced swim test in dose dependent manner. Furthermore, reboxetine produced a dose dependent attenuation of forced swim test-induced increases in serotonin turnover in the amygdala and frontal cortex and dopamine turnover in the striatum. Reboxetine (30 mg/kg) produced a modest, but non-significant, attenuation of forced swim test-induced increases in serum corticosterone concentrations. These data demonstrate that, in addition to the behavioural activity of reboxetine in the rat forced swim test paradigm, a dose-dependent attenuation of swim stress-induced increases in serotonergic and dopaminergic activity occurred in a region specific manner. These are the first data to demonstrate that treatment with the selective noradrenaline reuptake inhibitor, reboxetine can impact on the activity of other neurotransmitter systems in response to stress. Moreover, these data further demonstrate that this paradigm is a valuable tool in studying the effect of antidepressants, on both behaviour and swim stress-related alterations in central neurotransmitter function and hypothalamic pituitary adrenal axis activity in the rat.

Prefrontocortical dopamine depletion induces antidepressant-like effects in rats and alters the profile of desipramine during Porsolt's test

The objective of this study was to investigate whether bilateral dopamine depletion within the medial prefrontal cortex affects depression state, as well as the antidepressant efficacy of desipramine, in the forced swimming test. The rat's behaviour was evaluated by quantifying duration of immobility, climbing, swimming and diving. Immobility latency was also quantified and proved to be a suitable novel parameter. Monoamine levels within the medial prefrontal cortex were measured by high-performance liquid chromatography during Porsolt's test, as well as one week after it. While Porsolt's test was followed by a typical depression-like profile in sham rats, depletion of prefrontocortical dopamine (86% vs sham controls) reduced immobility and enhanced swimming, which is consistent with a diminished depression tonus. The observed enhancement of swimming was correlated with a high prefrontocortical serotonergic neurotransmission. On the other hand, desipramine induced antidepression-like effects in sham rats by increasing prefrontocortical noradrenaline and serotonin neurotransmisson, but also by blocking the normal increase in dopamine activity during the swimming test. Interestingly, desipramine behaved in a quite different manner in lesioned rats. Thus, immobility duration was not further reduced and only climbing, but not swimming, was enhanced. These effects were correlated with a preferential enhancement of noradrenaline neurotransmission. In conclusion, the results indicate that: (i) dopamine neurotransmission within the medial prefrontal cortex is a factor involved in depression, since dopamine reduction led to a low depression tonus; (ii) desipramine induces antidepression not only by enhancing prefrontocortical noradrenaline and serotonin neurotransmission, but also by blocking the normal increase in dopamine neurotransmission during a depressant situation; (iii) a selective enhancement of prefrontocortical serotonin neurotransmission mediates swimming; and (iv) a selectively augmented prefrontocortical noradrenaline activity mediates climbing during Porsolt's test.

The effect of repeated exposure to forced swimming on extracellular levels of 5-hydroxytryptamine in the rat

The effects of repeated exposure to forced swimming was examined on extracellular concentrations of 5-hydroxytryptamine (5-HT), behavioral, and endocrine responses in rats. Animals were exposed to a 15-min swimming session on two consecutive days. On the first day, the swimming session increased extracellular 5-HT by 80 % over baseline in the striatum and reduced 5-HT to 40 % below baseline in the lateral septum. On the second day, however, the swimming session produced no effect on 5-HT in either brain region. Ratings of behavior showed that rats demonstrated climbing and swimming behaviors and developed immobility during the initial swimming session and that ratings of immobility increased and swimming decreased during the second swimming session. Immobility was positively correlated and swimming was negatively correlated with changes in extracellular 5-HT in the lateral septum but not in the striatum. Plasma corticosterone was equally elevated by 950 % after either 1 or 2 days of swimming exposure. These results show that there is rapid adaptation to the effects of repeated forced swimming on the regionally-specific, bi-directional response of extracellular 5-HT. In addition, changes in extracellular 5-HT in the lateral septum may be related to the behaviors produced during the forced swimming test that underlie its utility as an animal model of depression.

The relationship between plasma MHPG and NE: employing regression models in estimating centrally derived MHPG and peripheral NE turnover rate in panic disorder

Studies investigating the role of the noradrenergic system in the pathophysiology of anxiety have focused on measuring plasma 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) levels. Fewer studies have examined norepinephrine levels. Basal plasma norepinephrine and free MHPG levels were simultaneously measured in 33 normal controls and 20 panic disorder (PD) patients. Norepinephrine levels were similar in patients and controls, but MHPG levels were significantly lower in patients (13.34 +/- 3.22 vs 18.37 +/- 4.49 pmol ml-1, p < 0.0001). Norepinephrine correlated significantly with plasma MHPG levels in controls (r = 0.538, p < 0.0001) and patients (r = 0.645, p < 0.002). Patients had a trend toward a lower y-intercept than controls, suggesting a lower contribution by the CNS to MHPG pool plasma levels (9.18 vs 12.51, p < 0.08). Norepinephrine turnover rate was similar in patients and controls. We propose that the dysregulation in the noradrenergic system in PD may be akin to animal studies of acute-on-top-of-chronic stress paradigms, whereby chronic stress results in normal or decreased basal NE turnover and sensitized responses to recurrent stresses.

Forced swim test-induced neurochemical endocrine, and immune changes in the rat

The forced swim test (FST) is a behavioral paradigm that is widely used as a screening test for antidepressant activity in rodents. The objectives of the present study were to characterize the corticosterone and immune responses and in addition to examine neurotransmitter levels, in five brain regions at intervals (15, 30, 60, 90, and 120 min) following the second exposure to the FST. There was a significant but transient reduction in noradrenaline and 5-HT concentrations, in the hypothalamus 15 min post-FST exposure. 5-HT turnover in the frontal cortex and amygdala was significantly increased between 20-120 min post-FST exposure. The FST elicited a robust corticosterone response that peaked significantly at 30 min and had almost returned to baseline 120 min after exposure. There was a significant reduction in total white blood cell count 120 min after the FST, which was accompanied by a significantly reduced percentage of lymphocytes 90 and 120 min post-FST exposure. In addition, there was a significant but transient suppression of both PHA and Con A-induced lymphocyte proliferation 15 min following FST exposure. This study demonstrates that there are neurochemical changes that are coincident with the endocrine and immune changes associated with FST exposure in rats. Furthermore, this model could be used to examine the effects of manipulation of this stress response by antidepressant drugs. Such an investigation could add to our understanding of the interactions between antidepressants, stress and the neuroendocrine and immune systems.

A critical review of 5-HT brain microdialysis and behavior

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

Release of serotonin in the rat locus coeruleus: effects of cardiovascular, stressful and noxious stimuli

To investigate the function of serotonergic neurons within the locus coeruleus, this brain nucleus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the extracellular concentration of serotonin was determined in the superfusate. Serotonin release was increased by depolarization with veratridine (5 microM) or 80 mM K+, while superfusion with tetrodotoxin (1 microM) or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin substantially diminished the release rate of serotonin in the locus coeruleus. The pressor response to intravenous infusion of noradrenaline (4 micrograms/kg/min) was associated with a pronounced increase in the release rate of serotonin. Superfusion of the locus coeruleus with tetrodotoxin (1 microM) abolished the increase in serotonin release evoked by the pressor response. A fall of blood pressure produced by intravenous administration of nitroprusside (150 micrograms/kg/min) or chlorisondamine (3 mg/kg) diminished the release rate of serotonin. Immobilization, noise (95 dB) or tail pinch increased the release of serotonin in the locus coeruleus and slightly elevated blood pressure. Chlorisondamine abolished the rise in blood pressure elicited by tail pinch without influencing the increased serotonin release. Tail pinch-induced serotonin release was abolished by superfusion with tetrodotoxin. The findings demonstrate that neuronal serotonin release in the locus coeruleus responds to cardiovascular and sensory stimuli, suggesting a function of serotonergic neurons in central blood pressure regulation, as well as in the modulation of locus coeruleus activity by stress and noxious stimuli.

Catecholamines, stress, and disease: a psychobiological perspective

OBJECTIVE

Research on the relationship between physiological responses to stressful stimulation and the onset of psychosomatic illnesses has been an area of intense interest for many years. Studies using animal models have contributed significantly to this field of inquiry by taking several complementary approaches.

METHOD

Three specific research strategies taken in our laboratory will be highlighted here. Each involves studies in conscious, freely behaving animals.

RESULTS

Genetically selected animals have been exposed to acute stressors to unmask neuroendocrine and autonomic abnormalities related to disease susceptibility. In addition, studies of aged animals suggest that exaggerated physiological responses to acute stress may underlie some age-related pathologies. Finally, a series of studies has revealed that exposure of laboratory animals to stressful stimulation may exert long-lasting influences on the ways in which these subjects respond in the future to the same or novel stressors.

CONCLUSIONS

These findings illustrate how studies with laboratory animals have the potential for refining the questions that are posed in research with clinical populations and for providing insight into the underlying physiological mechanisms of individual variability in disease susceptibility and the development of appropriate therapeutic interventions.